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Network Working Group C. Bormann, Editor, TZI/Uni Bremen
Request for Comments: 3095 C. Burmeister, Matsushita
Category: Standards Track M. Degermark, Univ. of Arizona
H. Fukushima, Matsushita
H. Hannu, Ericsson
L-E. Jonsson, Ericsson
R. Hakenberg, Matsushita
T. Koren, Cisco
K. Le, Nokia
Z. Liu, Nokia
A. Martensson, Ericsson
A. Miyazaki, Matsushita
K. Svanbro, Ericsson
T. Wiebke, Matsushita
T. Yoshimura, NTT DoCoMo
H. Zheng, Nokia
July 2001
RObust Header Compression (ROHC):
Framework and four profiles: RTP, UDP, ESP, and uncompressed
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2001). All Rights Reserved.
Abstract
This document specifies a highly robust and efficient header
compression scheme for RTP/UDP/IP (Real-Time Transport Protocol, User
Datagram Protocol, Internet Protocol), UDP/IP, and ESP/IP
(Encapsulating Security Payload) headers.
Existing header compression schemes do not work well when used over
links with significant error rates and long round-trip times. For
many bandwidth limited links where header compression is essential,
such characteristics are common.
Bormann, et al. Standards Track [Page 1]
RFC 3095 Robust Header Compression July 2001
This is done in a framework designed to be extensible. For example,
a scheme for compressing TCP/IP headers will be simple to add, and is
in development. Headers specific to Mobile IPv4 are not subject to
special treatment, but are expected to be compressed sufficiently
well by the provided methods for compression of sequences of
extension headers and tunneling headers. For the most part, the same
will apply to work in progress on Mobile IPv6, but future work might
be required to handle some extension headers, when a standards track
Mobile IPv6 has been completed.
Table of Contents
1. Introduction....................................................6
2. Terminology.....................................................8
2.1. Acronyms.....................................................13
3. Background.....................................................14
3.1. Header compression fundamentals..............................14
3.2. Existing header compression schemes..........................14
3.3. Requirements on a new header compression scheme..............16
3.4. Classification of header fields..............................17
4. Header compression framework...................................18
4.1. Operating assumptions........................................18
4.2. Dynamicity...................................................19
4.3. Compression and decompression states.........................21
4.3.1. Compressor states..........................................21
4.3.1.1. Initialization and Refresh (IR) State....................22
4.3.1.2. First Order (FO) State...................................22
4.3.1.3. Second Order (SO) State..................................22
4.3.2. Decompressor states........................................23
4.4. Modes of operation...........................................23
4.4.1. Unidirectional mode -- U-mode..............................24
4.4.2. Bidirectional Optimistic mode -- O-mode....................25
4.4.3. Bidirectional Reliable mode -- R-mode......................25
4.5. Encoding methods.............................................25
4.5.1. Least Significant Bits (LSB) encoding .....................25
4.5.2. Window-based LSB encoding (W-LSB encoding).................28
4.5.3. Scaled RTP Timestamp encoding .............................28
4.5.4. Timer-based compression of RTP Timestamp...................31
4.5.5. Offset IP-ID encoding......................................34
4.5.6. Self-describing variable-length values ....................35
4.5.7. Encoded values across several fields in compressed headers 36
4.6. Errors caused by residual errors.............................36
4.7. Impairment considerations....................................37
5. The protocol...................................................39
5.1. Data structures..............................................39
5.1.1. Per-channel parameters.....................................39
5.1.2. Per-context parameters, profiles...........................40
5.1.3. Contexts and context identifiers ..........................41
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RFC 3095 Robust Header Compression July 2001
5.2. ROHC packets and packet types................................41
5.2.1. ROHC feedback .............................................43
5.2.2. ROHC feedback format ......................................45
5.2.3. ROHC IR packet type .......................................47
5.2.4. ROHC IR-DYN packet type ...................................48
5.2.5. ROHC segmentation..........................................49
5.2.5.1. Segmentation usage considerations........................49
5.2.5.2. Segmentation protocol....................................50
5.2.6. ROHC initial decompressor processing.......................51
5.2.7. ROHC RTP packet formats from compressor to decompressor....53
5.2.8. Parameters needed for mode transition in ROHC RTP..........54
5.3. Operation in Unidirectional mode.............................55
5.3.1. Compressor states and logic (U-mode).......................55
5.3.1.1. State transition logic (U-mode)..........................55
5.3.1.1.1. Optimistic approach, upwards transition................55
5.3.1.1.2. Timeouts, downward transition..........................56
5.3.1.1.3. Need for updates, downward transition..................56
5.3.1.2. Compression logic and packets used (U-mode)..............56
5.3.1.3. Feedback in Unidirectional mode..........................56
5.3.2. Decompressor states and logic (U-mode).....................56
5.3.2.1. State transition logic (U-mode)..........................57
5.3.2.2. Decompression logic (U-mode).............................57
5.3.2.2.1. Decide whether decompression is allowed................57
5.3.2.2.2. Reconstruct and verify the header......................57
5.3.2.2.3. Actions upon CRC failure...............................58
5.3.2.2.4. Correction of SN LSB wraparound........................60
5.3.2.2.5. Repair of incorrect SN updates.........................61
5.3.2.3. Feedback in Unidirectional mode..........................62
5.4. Operation in Bidirectional Optimistic mode...................62
5.4.1. Compressor states and logic (O-mode).......................62
5.4.1.1. State transition logic...................................63
5.4.1.1.1. Negative acknowledgments (NACKs), downward transition..63
5.4.1.1.2. Optional acknowledgments, upwards transition...........63
5.4.1.2. Compression logic and packets used.......................63
5.4.2. Decompressor states and logic (O-mode).....................64
5.4.2.1. Decompression logic, timer-based timestamp decompression.64
5.4.2.2. Feedback logic (O-mode)..................................64
5.5. Operation in Bidirectional Reliable mode.....................65
5.5.1. Compressor states and logic (R-mode).......................65
5.5.1.1. State transition logic (R-mode)..........................65
5.5.1.1.1. Upwards transition.....................................65
5.5.1.1.2. Downward transition....................................66
5.5.1.2. Compression logic and packets used (R-mode)..............66
5.5.2. Decompressor states and logic (R-mode).....................68
5.5.2.1. Decompression logic (R-mode).............................68
5.5.2.2. Feedback logic (R-mode)..................................68
5.6. Mode transitions.............................................69
5.6.1. Compression and decompression during mode transitions......70
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RFC 3095 Robust Header Compression July 2001
5.6.2. Transition from Unidirectional to Optimistic mode..........71
5.6.3. From Optimistic to Reliable mode...........................72
5.6.4. From Unidirectional to Reliable mode.......................72
5.6.5. From Reliable to Optimistic mode...........................72
5.6.6. Transition to Unidirectional mode..........................73
5.7. Packet formats...............................................74
5.7.1. Packet type 0: UO-0, R-0, R-0-CRC .........................78
5.7.2. Packet type 1 (R-mode): R-1, R-1-TS, R-1-ID ...............79
5.7.3. Packet type 1 (U/O-mode): UO-1, UO-1-ID, UO-1-TS ..........80
5.7.4. Packet type 2: UOR-2 ......................................82
5.7.5. Extension formats..........................................83
5.7.5.1. RND flags and packet types...............................88
5.7.5.2. Flags/Fields in context..................................89
5.7.6. Feedback packets and formats...............................90
5.7.6.1. Feedback formats for ROHC RTP............................90
5.7.6.2. ROHC RTP Feedback options................................91
5.7.6.3. The CRC option...........................................92
5.7.6.4. The REJECT option........................................92
5.7.6.5. The SN-NOT-VALID option..................................92
5.7.6.6. The SN option............................................93
5.7.6.7. The CLOCK option.........................................93
5.7.6.8. The JITTER option........................................93
5.7.6.9. The LOSS option..........................................94
5.7.6.10. Unknown option types....................................94
5.7.6.11. RTP feedback example....................................94
5.7.7. RTP IR and IR-DYN packets..................................96
5.7.7.1. Basic structure of the IR packet.........................96
5.7.7.2. Basic structure of the IR-DYN packet.....................98
5.7.7.3. Initialization of IPv6 Header [IPv6].....................99
5.7.7.4. Initialization of IPv4 Header [IPv4, section 3.1].......100
5.7.7.5. Initialization of UDP Header [RFC-768]..................101
5.7.7.6. Initialization of RTP Header [RTP]......................102
5.7.7.7. Initialization of ESP Header [ESP, section 2]...........103
5.7.7.8. Initialization of Other Headers.........................104
5.8. List compression............................................104
5.8.1. Table-based item compression..............................105
5.8.1.1. Translation table in R-mode.............................105
5.8.1.2. Translation table in U/O-modes..........................106
5.8.2. Reference list determination..............................106
5.8.2.1. Reference list in R-mode and U/O-mode...................107
5.8.3. Encoding schemes for the compressed list..................109
5.8.4. Special handling of IP extension headers..................112
5.8.4.1. Next Header field.......................................112
5.8.4.2. Authentication Header (AH)..............................114
5.8.4.3. Encapsulating Security Payload Header (ESP).............115
5.8.4.4. GRE Header [RFC 2784, RFC 2890].........................117
5.8.5. Format of compressed lists in Extension 3.................119
5.8.5.1. Format of IP Extension Header(s) field..................119
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RFC 3095 Robust Header Compression July 2001
5.8.5.2. Format of Compressed CSRC List..........................120
5.8.6. Compressed list formats...................................120
5.8.6.1. Encoding Type 0 (generic scheme)........................120
5.8.6.2. Encoding Type 1 (insertion only scheme).................122
5.8.6.3. Encoding Type 2 (removal only scheme)...................123
5.8.6.4. Encoding Type 3 (remove then insert scheme).............124
5.8.7. CRC coverage for extension headers........................124
5.9. Header compression CRCs, coverage and polynomials...........125
5.9.1. IR and IR-DYN packet CRCs.................................125
5.9.2. CRCs in compressed headers................................125
5.10. ROHC UNCOMPRESSED -- no compression (Profile 0x0000).......126
5.10.1. IR packet................................................126
5.10.2. Normal packet............................................127
5.10.3. States and modes.........................................128
5.10.4. Feedback.................................................129
5.11. ROHC UDP -- non-RTP UDP/IP compression (Profile 0x0002)....129
5.11.1. Initialization...........................................130
5.11.2. States and modes.........................................130
5.11.3. Packet types.............................................131
5.11.4. Extensions...............................................132
5.11.5. IP-ID....................................................133
5.11.6. Feedback.................................................133
5.12. ROHC ESP -- ESP/IP compression (Profile 0x0003)............133
5.12.1. Initialization...........................................133
5.12.2. Packet types.............................................134
6. Implementation issues.........................................134
6.1. Reverse decompression.......................................134
6.2. RTCP........................................................135
6.3. Implementation parameters and signals.......................136
6.3.1. ROHC implementation parameters at compressor..............137
6.3.2. ROHC implementation parameters at decompressor............138
6.4. Handling of resource limitations at the decompressor........139
6.5. Implementation structures...................................139
6.5.1. Compressor context........................................139
6.5.2. Decompressor context......................................141
6.5.3. List compression: Sliding windows in R-mode and U/O-mode..142
7. Security Considerations.......................................143
8. IANA Considerations...........................................144
9. Acknowledgments...............................................145
10. Intellectual Property Right Claim Considerations.............145
11. References...................................................146
11.1. Normative References.......................................146
11.2. Informative References.....................................147
12. Authors' Addresses...........................................148
Appendix A. Detailed classification of header fields.............152
A.1. General classification......................................153
A.1.1. IPv6 header fields........................................153
A.1.2. IPv4 header fields........................................155
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RFC 3095 Robust Header Compression July 2001
A.1.3. UDP header fields.........................................157
A.1.4. RTP header fields.........................................157
A.1.5. Summary for IP/UDP/RTP....................................159
A.2. Analysis of change patterns of header fields................159
A.2.1. IPv4 Identification.......................................162
A.2.2. IP Traffic-Class / Type-Of-Service........................163
A.2.3. IP Hop-Limit / Time-To-Live...............................163
A.2.4. UDP Checksum..............................................163
A.2.5. RTP CSRC Counter..........................................164
A.2.6. RTP Marker................................................164
A.2.7. RTP Payload Type..........................................164
A.2.8. RTP Sequence Number.......................................164
A.2.9. RTP Timestamp.............................................164
A.2.10. RTP Contributing Sources (CSRC)..........................165
A.3. Header compression strategies...............................165
A.3.1. Do not send at all........................................165
A.3.2. Transmit only initially...................................165
A.3.3. Transmit initially, but be prepared to update.............166
A.3.4. Be prepared to update or send as-is frequently............166
A.3.5. Guarantee continuous robustness...........................166
A.3.6. Transmit as-is in all packets.............................167
A.3.7. Establish and be prepared to update delta.................167
Full Copyright Statement..........................................168
1. Introduction
During the last five years, two communication technologies in
particular have become commonly used by the general public: cellular
telephony and the Internet. Cellular telephony has provided its
users with the revolutionary possibility of always being reachable
with reasonable service quality no matter where they are. The main
service provided by the dedicated terminals has been speech. The
Internet, on the other hand, has from the beginning been designed for
multiple services and its flexibility for all kinds of usage has been
one of its strengths. Internet terminals have usually been general-
purpose and have been attached over fixed connections. The
experienced quality of some services (such as Internet telephony) has
sometimes been low.
Today, IP telephony is gaining momentum thanks to improved technical
solutions. It seems reasonable to believe that in the years to come,
IP will become a commonly used way to carry telephony. Some future
cellular telephony links might also be based on IP and IP telephony.
Cellular phones may have become more general-purpose, and may have IP
stacks supporting not only audio and video, but also web browsing,
email, gaming, etc.
Bormann, et al. Standards Track [Page 6]
RFC 3095 Robust Header Compression July 2001
One of the scenarios we are envisioning might then be the one in
Figure 1.1, where two mobile terminals are communicating with each
other. Both are connected to base stations over cellular links, and
the base stations are connected to each other through a wired (or
possibly wireless) network. Instead of two mobile terminals, there
could of course be one mobile and one wired terminal, but the case
with two cellular links is technically more demanding.
Mobile Base Base Mobile
Terminal Station Station Terminal
| ~ ~ ~ \ / \ / ~ ~ ~ ~ |
| | | |
+--+ | | +--+
| | | | | |
| | | | | |
+--+ | | +--+
| |
|=========================|
Cellular Wired Cellular
Link Network Link
Figure 1.1 : Scenario for IP telephony over cellular links
It is obvious that the wired network can be IP-based. With the
cellular links, the situation is less clear. IP could be terminated
in the fixed network, and special solutions implemented for each
supported service over the cellular link. However, this would limit
the flexibility of the services supported. If technically and
economically feasible, a solution with pure IP all the way from
terminal to terminal would have certain advantages. However, to make
this a viable alternative, a number of problems have to be addressed,
in particular problems regarding bandwidth efficiency.
For cellular phone systems, it is of vital importance to use the
scarce radio resources in an efficient way. A sufficient number of
users per cell is crucial, otherwise deployment costs will be
prohibitive. The quality of the voice service should also be as good
as in today's cellular systems. It is likely that even with support
for new services, lower quality of the voice service is acceptable
only if costs are significantly reduced.
Bormann, et al. Standards Track [Page 7]
RFC 3095 Robust Header Compression July 2001
A problem with IP over cellular links when used for interactive voice
conversations is the large header overhead. Speech data for IP
telephony will most likely be carried by RTP [RTP]. A packet will
then, in addition to link layer framing, have an IP [IPv4] header (20
octets), a UDP [UDP] header (8 octets), and an RTP header (12 octets)
for a total of 40 octets. With IPv6 [IPv6], the IP header is 40
octets for a total of 60 octets. The size of the payload depends on
the speech coding and frame sizes being used and may be as low as
15-20 octets.
From these numbers, the need for reducing header sizes for efficiency
reasons is obvious. However, cellular links have characteristics
that make header compression as defined in [IPHC,CRTP] perform less
than well. The most important characteristic is the lossy behavior
of cellular links, where a bit error rate (BER) as high as 1e-3 must
be accepted to keep the radio resources efficiently utilized. In
severe operating situations, the BER can be as high as 1e-2. The
other problematic characteristic is the long round-trip time (RTT) of
the cellular link, which can be as high as 100-200 milliseconds. An
additional problem is that the residual BER is nontrivial, i.e.,
lower layers can sometimes deliver frames containing undetected
errors. A viable header compression scheme for cellular links must
be able to handle loss on the link between the compression and
decompression point as well as loss before the compression point.
Bandwidth is the most costly resource in cellular links. Processing
power is very cheap in comparison. Implementation or computational
simplicity of a header compression scheme is therefore of less
importance than its compression ratio and robustness.
2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119.
BER
Bit Error Rate. Cellular radio links can have a fairly high BER.
In this document BER is usually given as a probability, but one
also needs to consider the error distribution as bit errors are
not independent.
Bormann, et al. Standards Track [Page 8]
RFC 3095 Robust Header Compression July 2001
Cellular links
Wireless links between mobile terminals and base stations.
Compression efficiency
The performance of a header compression scheme can be described
with three parameters: compression efficiency, robustness and
compression transparency. The compression efficiency is
determined by how much the header sizes are reduced by the
compression scheme.
Compression transparency
The performance of a header compression scheme can be described
with three parameters: compression efficiency, robustness, and
compression transparency. The compression transparency is a
measure of the extent to which the scheme ensures that the
decompressed headers are semantically identical to the original
headers. If all decompressed headers are semantically identical
to the corresponding original headers, the transparency is 100
percent. Compression transparency is high when damage propagation
is low.
Context
The context of the compressor is the state it uses to compress a
header. The context of the decompressor is the state it uses to
decompress a header. Either of these or the two in combination
are usually referred to as "context", when it is clear which is
intended. The context contains relevant information from previous
headers in the packet stream, such as static fields and possible
reference values for compression and decompression. Moreover,
additional information describing the packet stream is also part
of the context, for example information about how the IP
Identifier field changes and the typical inter-packet increase in
sequence numbers or timestamps.
Context damage
When the context of the decompressor is not consistent with the
context of the compressor, decompression may fail to reproduce the
original header. This situation can occur when the context of the
decompressor has not been initialized properly or when packets
have been lost or damaged between compressor and decompressor.
Bormann, et al. Standards Track [Page 9]
RFC 3095 Robust Header Compression July 2001
Packets which cannot be decompressed due to inconsistent contexts
are said to be lost due to context damage. Packets that are
decompressed but contain errors due to inconsistent contexts are
said to be damaged due to context damage.
Context repair mechanism
Context repair mechanisms are mechanisms that bring the contexts
in sync when they were not. This is needed to avoid excessive
loss due to context damage. Examples are the context request
mechanism of CRTP, the NACK mechanisms of O- and R-mode, and the
periodic refreshes of U-mode.
Note that there are also mechanisms that prevent (some) context
inconsistencies from occurring, for example the ACK-based updates
of the context in R-mode, the repetitions after change in U- and
O-mode, and the CRCs which protect context updating information.
CRC-DYNAMIC
Opposite of CRC-STATIC.
CRC-STATIC
A CRC over the original header is the primary mechanism used by
ROHC to detect incorrect decompression. In order to decrease
computational complexity, the fields of the header are
conceptually rearranged when the CRC is computed, so that it is
first computed over octets which are static (called CRC-STATIC in
this document) and then over octets whose values are expected to
change between packets (CRC-DYNAMIC). In this manner, the
intermediate result of the CRC computation, after it has covered
the CRC-STATIC fields, can be reused for several packets. The
restarted CRC computation only covers the CRC-DYNAMIC octets. See
section 5.9.
Damage propagation
Delivery of incorrect decompressed headers, due to errors in
(i.e., loss of or damage to) previous header(s) or feedback.
Loss propagation
Loss of headers, due to errors in (i.e., loss of or damage to)
previous header(s)or feedback.
Bormann, et al. Standards Track [Page 10]
RFC 3095 Robust Header Compression July 2001
Error detection
Detection of errors. If error detection is not perfect, there
will be residual errors.
Error propagation
Damage propagation or loss propagation.
Header compression profile
A header compression profile is a specification of how to compress
the headers of a certain kind of packet stream over a certain kind
of link. Compression profiles provide the details of the header
compression framework introduced in this document. The profile
concept makes use of profile identifiers to separate different
profiles which are used when setting up the compression scheme.
All variations and parameters of the header compression scheme
that are not part of the context state are handled by different
profile identifiers.
Packet
Generally, a unit of transmission and reception (protocol data
unit). Specifically, when contrasted with "frame", the packet
compressed and then decompressed by ROHC. Also called
"uncompressed packet".
Packet Stream
A sequence of packets where the field values and change patterns
of field values are such that the headers can be compressed using
the same context.
Pre-HC links
The Pre-HC links are all links that a packet has traversed before
the header compression point. If we consider a path with cellular
links as first and last hops, the Pre-HC links for the compressor
at the last link are the first cellular link plus the wired links
in between.
Residual error
Error introduced during transmission and not detected by lower-
layer error detection schemes.
Bormann, et al. Standards Track [Page 11]
RFC 3095 Robust Header Compression July 2001
Robustness
The performance of a header compression scheme can be described
with three parameters: compression efficiency, robustness, and
compression transparency. A robust scheme tolerates loss and
residual errors on the link over which header compression takes
place without losing additional packets or introducing additional
errors in decompressed headers.
RTT
The RTT (round-trip time) is the time elapsing from the moment the
compressor sends a packet until it receives feedback related to
that packet (when such feedback is sent).
Spectrum efficiency
Radio resources are limited and expensive. Therefore they must be
used efficiently to make the system economically feasible. In
cellular systems this is achieved by maximizing the number of
users served within each cell, while the quality of the provided
services is kept at an acceptable level. A consequence of
efficient spectrum use is a high rate of errors (frame loss and
residual bit errors), even after channel coding with error
correction.
String
A sequence of headers in which the values of all fields being
compressed change according to a pattern which is fixed with
respect to a sequence number. Each header in a string can be
compressed by representing it with a ROHC header which essentially
only carries an encoded sequence number. Fields not being
compressed (e.g., random IP-ID, UDP Checksum) are irrelevant to
this definition.
Timestamp stride
The timestamp stride (TS_STRIDE) is the expected increase in the
timestamp value between two RTP packets with consecutive sequence
numbers.
Bormann, et al. Standards Track [Page 12]
RFC 3095 Robust Header Compression July 2001
2.1. Acronyms
This section lists most acronyms used for reference.
AH Authentication Header.
CID Context Identifier.
CRC Cyclic Redundancy Check. Error detection mechanism.
CRTP Compressed RTP. RFC 2508.
CTCP Compressed TCP. Also called VJ header compression. RFC 1144.
ESP Encapsulating Security Payload.
FC Full Context state (decompressor).
FO First Order state (compressor).
GRE Generic Routing Encapsulation. RFC 2784, RFC 2890.
HC Header Compression.
IPHC IP Header Compression. RFC 2507.
IPX Flag in Extension 2.
IR Initiation and Refresh state (compressor). Also IR packet.
IR-DYN IR-DYN packet.
LSB Least Significant Bits.
MRRU Maximum Reconstructed Reception Unit.
MTU Maximum Transmission Unit.
MSB Most Significant Bits.
NBO Flag indicating whether the IP-ID is in Network Byte Order.
NC No Context state (decompressor).
O-mode Bidirectional Optimistic mode.
PPP Point-to-Point Protocol.
R-mode Bidirectional Reliable mode.
RND Flag indicating whether the IP-ID behaves randomly.
ROHC RObust Header Compression.
RTCP Real-Time Control Protocol. See RTP.
RTP Real-Time Protocol. RFC 1889.
RTT Round Trip Time (see section 2).
SC Static Context state (decompressor).
SN (compressed) Sequence Number. Usually RTP Sequence Number.
SO Second Order state (compressor).
SPI Security Parameters Index.
SSRC Sending source. Field in RTP header.
CSRC Contributing source. Optional list of CSRCs in RTP header.
TC Traffic Class. Octet in IPv6 header. See also TOS.
TOS Type Of Service. Octet in IPv4 header. See also TC.
TS (compressed) RTP Timestamp.
U-mode Unidirectional mode.
W-LSB Window based LSB encoding. See section 4.5.2.
Bormann, et al. Standards Track [Page 13]
RFC 3095 Robust Header Compression July 2001
3. Background
This chapter provides a background to the subject of header
compression. The fundamental ideas are described together with
existing header compression schemes. Their drawbacks and
requirements are then discussed, providing motivation for new header
compression solutions.
3.1. Header compression fundamentals
The main reason why header compression can be done at all is the fact
that there is significant redundancy between header fields, both
within the same packet header but in particular between consecutive
packets belonging to the same packet stream. By sending static field
information only initially and utilizing dependencies and
predictability for other fields, the header size can be significantly
reduced for most packets.
Relevant information from past packets is maintained in a context.
The context information is used to compress (decompress) subsequent
packets. The compressor and decompressor update their contexts upon
certain events. Impairment events may lead to inconsistencies
between the contexts of the compressor and decompressor, which in
turn may cause incorrect decompression. A robust header compression
scheme needs mechanisms for avoiding context inconsistencies and also
needs mechanisms for making the contexts consistent when they were
not.
3.2. Existing header compression schemes
The original header compression scheme, CTCP [VJHC], was invented by
Van Jacobson. CTCP compresses the 40 octet IP+TCP header to 4
octets. The CTCP compressor detects transport-level retransmissions
and sends a header that updates the context completely when they
occur. This repair mechanism does not require any explicit signaling
between compressor and decompressor.
A general IP header compression scheme, IP header compression [IPHC],
improves somewhat on CTCP and can compress arbitrary IP, TCP, and UDP
headers. When compressing non-TCP headers, IPHC does not use delta
encoding and is robust. When compressing TCP, the repair mechanism
of CTCP is augmented with a link-level nacking scheme which speeds up
the repair. IPHC does not compress RTP headers.
CRTP [CRTP, IPHC] by Casner and Jacobson is a header compression
scheme that compresses 40 octets of IPv4/UDP/RTP headers to a minimum
of 2 octets when the UDP Checksum is not enabled. If the UDP
Checksum is enabled, the minimum CRTP header is 4 octets. CRTP
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cannot use the same repair mechanism as CTCP since UDP/RTP does not
retransmit. Instead, CRTP uses explicit signaling messages from
decompressor to compressor, called CONTEXT_STATE messages, to
indicate that the context is out of sync. The link round-trip time
will thus limit the speed of this context repair mechanism.
On lossy links with long round-trip times, such as most cellular
links, CRTP does not perform well. Each lost packet over the link
causes several subsequent packets to be lost since the context is out
of sync during at least one link round-trip time. This behavior is
documented in [CRTPC]. For voice conversations such long loss events
will degrade the voice quality. Moreover, bandwidth is wasted by the
large headers sent by CRTP when updating the context. [CRTPC] found
that CRTP did not perform well enough for a lossy cellular link. It
is clear that CRTP alone is not a viable header compression scheme
for IP telephony over cellular links.
To avoid losing packets due to the context being out of sync, CRTP
decompressors can attempt to repair the context locally by using a
mechanism known as TWICE. Each CRTP packet contains a counter which
is incremented by one for each packet sent out by the CRTP
compressor. If the counter increases by more than one, at least one
packet was lost over the link. The decompressor then attempts to
repair the context by guessing how the lost packet(s) would have
updated it. The guess is then verified by decompressing the packet
and checking the UDP Checksum -- if it succeeds, the repair is deemed
successful and the packet can be forwarded or delivered. TWICE
derives its name from the observation that when the compressed packet
stream is regular, the correct guess is to apply the update in the
current packet twice. [CRTPC] found that even with TWICE, CRTP
doubled the number of lost packets. TWICE improves CRTP performance
significantly. However, there are several problems with using TWICE:
1) It becomes mandatory to use the UDP Checksum:
- the minimal compressed header size increases by 100% to 4
octets.
- most speech codecs developed for cellular links tolerate errors
in the encoded data. Such codecs will not want to enable the
UDP Checksum, since they do want damaged packets to be
delivered.
- errors in the payload will make the UDP Checksum fail when the
guess is correct (and might make it succeed when the guess is
wrong).
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2) Loss in an RTP stream that occurs before the compression point
will make updates in CRTP headers less regular. Simple-minded
versions of TWICE will then perform badly. More sophisticated
versions would need more repair attempts to succeed.
3.3. Requirements on a new header compression scheme
The major problem with CRTP is that it is not sufficiently robust
against packets being damaged between compressor and decompressor. A
viable header compression scheme must be less fragile. This
increased robustness must be obtained without increasing the
compressed header size; a larger header would make IP telephony over
cellular links economically unattractive.
A major cause of the bad performance of CRTP over cellular links is
the long link round-trip time, during which many packets are lost
when the context is out of sync. This problem can be attacked
directly by finding ways to reduce the link round-trip time. Future
generations of cellular technologies may indeed achieve lower link
round-trip times. However, these will probably always be fairly
high. The benefits in terms of lower loss and smaller bandwidth
demands if the context can be repaired locally will be present even
if the link round-trip time is decreased. A reliable way to detect a
successful context repair is then needed.
One might argue that a better way to solve the problem is to improve
the cellular link so that packet loss is less likely to occur. Such
modifications do not appear to come for free, however. If links were
made (almost) error free, the system might not be able to support a
sufficiently large number of users per cell and might thus be
economically infeasible.
One might also argue that the speech codecs should be able to deal
with the kind of packet loss induced by CRTP, in particular since the
speech codecs probably must be able to deal with packet loss anyway
if the RTP stream crosses the Internet. While the latter is true,
the kind of loss induced by CRTP is difficult to deal with. It is
usually not possible to completely hide a loss event where well over
100 ms worth of sound is completely lost. If such loss occurs
frequently at both ends of the end-to-end path, the speech quality
will suffer.
A detailed description of the requirements specified for ROHC may be
found in [REQ].
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3.4. Classification of header fields
As mentioned earlier, header compression is possible due to the fact
that there is much redundancy between header field values within
packets, but especially between consecutive packets. To utilize
these properties for header compression, it is important to
understand the change patterns of the various header fields.
All header fields have been classified in detail in appendix A. The
fields are first classified at a high level and then some of them are
studied more in detail. Finally, the appendix concludes with
recommendations on how the various fields should be handled by header
compression algorithms. The main conclusion that can be drawn is
that most of the header fields can easily be compressed away since
they never or seldom change. Only 5 fields, with a combined size of
about 10 octets, need more sophisticated mechanisms. These fields
are:
- IPv4 Identification (16 bits) - IP-ID
- UDP Checksum (16 bits)
- RTP Marker (1 bit) - M-bit
- RTP Sequence Number (16 bits) - SN
- RTP Timestamp (32 bits) - TS
The analysis in Appendix A reveals that the values of the TS and IP-
ID fields can usually be predicted from the RTP Sequence Number,
which increments by one for each packet emitted by an RTP source.
The M-bit is also usually the same, but needs to be communicated
explicitly occasionally. The UDP Checksum should not be predicted
and is sent as-is when enabled.
The way ROHC RTP compression operates, then, is to first establish
functions from SN to the other fields, and then reliably communicate
the SN. Whenever a function from SN to another field changes, i.e.,
the existing function gives a result which is different from the
field in the header to be compressed, additional information is sent
to update the parameters of that function.
Headers specific to Mobile IP (for IPv4 or IPv6) do not receive any
special treatment in this document. They are compressible, however,
and it is expected that the compression efficiency for Mobile IP
headers will be good enough due to the handling of extension header
lists and tunneling headers. It would be relatively painless to
introduce a new ROHC profile with special treatment for Mobile IPv6
specific headers should the completed work on the Mobile IPv6
protocols (work in progress in the IETF) make that necessary.
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4. Header compression framework
4.1. Operating assumptions
Cellular links, which are a primary target for ROHC, have a number of
characteristics that are described briefly here. ROHC requires
functionality from lower layers that is outlined here and more
thoroughly described in the lower layer guidelines document [LLG].
Channels
ROHC header-compressed packets flow on channels. Unlike many
fixed links, some cellular radio links can have several channels
connecting the same pair of nodes. Each channel can have
different characteristics in terms of error rate, bandwidth, etc.
Context identifiers
On some channels, the ability to transport multiple packet streams
is required. It can also be feasible to have channels dedicated
to individual packet streams. Therefore, ROHC uses a distinct
context identifier space per channel and can eliminate context
identifiers completely for one of the streams when few streams
share a channel.
Packet type indication
Packet type indication is done in the header compression scheme
itself. Unless the link already has a way of indicating packet
types which can be used, such as PPP, this provides smaller
compressed headers overall. It may also be less difficult to
allocate a single packet type, rather than many, in order to run
ROHC over links such as PPP.
Reordering
The channel between compressor and decompressor is required to
maintain packet ordering, i.e., the decompressor must receive
packets in the same order as the compressor sent them.
(Reordering before the compression point, however, is dealt with,
i.e., there is no assumption that the compressor will only receive
packets in sequence.)
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Duplication
The channel between compressor and decompressor is required to not
duplicate packets. (Duplication before the compression point,
however, is dealt with, i.e., there is no assumption that the
compressor will receive only one copy of each packet.)
Packet length
ROHC is designed under the assumption that lower layers indicate
the length of a compressed packet. ROHC packets do not contain
length information for the payload.
Framing
The link layer must provide framing that makes it possible to
distinguish frame boundaries and individual frames.
Error detection/protection
The ROHC scheme has been designed to cope with residual errors in
the headers delivered to the decompressor. CRCs and sanity checks
are used to prevent or reduce damage propagation. However, it is
RECOMMENDED that lower layers deploy error detection for ROHC
headers and do not deliver ROHC headers with high residual error
rates.
Without giving a hard limit on the residual error rate acceptable
to ROHC, it is noted that for a residual bit error rate of at most
1E-5, the ROHC scheme has been designed not to increase the number
of damaged headers, i.e., the number of damaged headers due to
damage propagation is designed to be less than the number of
damaged headers caught by the ROHC error detection scheme.
Negotiation
In addition to the packet handling mechanisms above, the link
layer MUST provide a way to negotiate header compression
parameters, see also section 5.1.1. (For unidirectional links,
this negotiation may be performed out-of-band or even a priori.)
4.2. Dynamicity
The ROHC protocol achieves its compression gain by establishing state
information at both ends of the link, i.e., at the compressor and at
the decompressor. Different parts of the state are established at
different times and with different frequency; hence, it can be said
that some of the state information is more dynamic than the rest.
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Some state information is established at the time a channel is
established; ROHC assumes the existence of an out-of-band negotiation
protocol (such as PPP), or predefined channel state (most useful for
unidirectional links). In both cases, we speak of "negotiated
channel state". ROHC does not assume that this state can change
dynamically during the channel lifetime (and does not explicitly
support such changes, although some changes may be innocuous from a
protocol point of view). An example of negotiated channel state is
the highest context ID number to be used by the compressor (MAX_CID).
Other state information is associated with the individual packet
streams in the channel; this state is said to be part of the context.
Using context identifiers (CIDs), multiple packet streams with
different contexts can share a channel. The negotiated channel state
indicates the highest context identifier to be used, as well as the
selection of one of two ways to indicate the CID in the compressed
header.
It is up to the compressor to decide which packets to associate with
a context (or, equivalently, which packets constitute a single
stream); however, ROHC is efficient only when all packets of a stream
share certain properties, such as having the same values for fields
that are described as "static" in this document (e.g., the IP
addresses, port numbers, and RTP parameters such as the payload
type). The efficiency of ROHC RTP also depends on the compressor
seeing most RTP Sequence Numbers.
Streams need not share all characteristics important for compression.
ROHC has a notion of compression profiles: a compression profile
denotes a predefined set of such characteristics. To provide
extensibility, the negotiated channel state includes the set of
profiles acceptable to the decompressor. The context state includes
the profile currently in use for the context.
Other elements of the context state may include the current values of
all header fields (from these one can deduce whether an IPv4 header
is present in the header chain, and whether UDP Checksums are
enabled), as well as additional compression context that is not part
of an uncompressed header, e.g., TS_STRIDE, IP-ID characteristics
(incrementing as a 16-bit value in network byte order? random?), a
number of old reference headers, and the compressor/decompressor
state machines (see next section).
This document actually defines four ROHC profiles: One uncompressed
profile, the main ROHC RTP compression profile, and two variants of
this profile for compression of packets with header chains that end
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in UDP and ESP, respectively, but where RTP compression is not
applicable. The descriptive text in the rest of this section is
referring to the main ROHC RTP compression profile.
4.3. Compression and decompression states
Header compression with ROHC can be characterized as an interaction
between two state machines, one compressor machine and one
decompressor machine, each instantiated once per context. The
compressor and the decompressor have three states each, which in many
ways are related to each other even if the meaning of the states are
slightly different for the two parties. Both machines start in the
lowest compression state and transit gradually to higher states.
Transitions need not be synchronized between the two machines. In
normal operation it is only the compressor that temporarily transits
back to lower states. The decompressor will transit back only when
context damage is detected.
Subsequent sections present an overview of the state machines and
their corresponding states, respectively, starting with the
compressor.
4.3.1. Compressor states
For ROHC compression, the three compressor states are the
Initialization and Refresh (IR), First Order (FO), and Second Order
(SO) states. The compressor starts in the lowest compression state
(IR) and transits gradually to higher compression states. The
compressor will always operate in the highest possible compression
state, under the constraint that the compressor is sufficiently
confident that the decompressor has the information necessary to
decompress a header compressed according to that state.
+----------+ +----------+ +----------+
| IR State | <--------> | FO State | <--------> | SO State |
+----------+ +----------+ +----------+
Decisions about transitions between the various compression states
are taken by the compressor on the basis of:
- variations in packet headers
- positive feedback from decompressor (Acknowledgments -- ACKs)
- negative feedback from decompressor (Negative ACKs -- NACKs)
- periodic timeouts (when operating in unidirectional mode, i.e.,
over simplex channels or when feedback is not enabled)
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How transitions are performed is explained in detail in chapter 5 for
each mode of operation.
4.3.1.1. Initialization and Refresh (IR) State
The purpose of the IR state is to initialize the static parts of the
context at the decompressor or to recover after failure. In this
state, the compressor sends complete header information. This
includes all static and nonstatic fields in uncompressed form plus
some additional information.
The compressor stays in the IR state until it is fairly confident
that the decompressor has received the static information correctly.
4.3.1.2. First Order (FO) State
The purpose of the FO state is to efficiently communicate
irregularities in the packet stream. When operating in this state,
the compressor rarely sends information about all dynamic fields, and
the information sent is usually compressed at least partially. Only
a few static fields can be updated. The difference between IR and FO
should therefore be clear.
The compressor enters this state from the IR state, and from the SO
state whenever the headers of the packet stream do not conform to
their previous pattern. It stays in the FO state until it is
confident that the decompressor has acquired all the parameters of
the new pattern. Changes in fields that are always irregular are
communicated in all packets and are therefore part of what is a
uniform pattern.
Some or all packets sent in the FO state carry context updating
information. It is very important to detect corruption of such
packets to avoid erroneous updates and context inconsistencies.
4.3.1.3. Second Order (SO) State
This is the state where compression is optimal. The compressor
enters the SO state when the header to be compressed is completely
predictable given the SN (RTP Sequence Number) and the compressor is
sufficiently confident that the decompressor has acquired all
parameters of the functions from SN to other fields. Correct
decompression of packets sent in the SO state only hinges on correct
decompression of the SN. However, successful decompression also
requires that the information sent in the preceding FO state packets
has been successfully received by the decompressor.
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The compressor leaves this state and goes back to the FO state when
the header no longer conforms to the uniform pattern and cannot be
independently compressed on the basis of previous context
information.
4.3.2. Decompressor states
The decompressor starts in its lowest compression state, "No Context"
and gradually transits to higher states. The decompressor state
machine normally never leaves the "Full Context" state once it has
entered this state.
+--------------+ +----------------+ +--------------+
| No Context | <---> | Static Context | <---> | Full Context |
+--------------+ +----------------+ +--------------+
Initially, while working in the "No Context" state, the decompressor
has not yet successfully decompressed a packet. Once a packet has
been decompressed correctly (for example, upon reception of an
initialization packet with static and dynamic information), the
decompressor can transit all the way to the "Full Context" state, and
only upon repeated failures will it transit back to lower states.
However, when that happens it first transits back to the "Static
Context" state. There, reception of any packet sent in the FO state
is normally sufficient to enable transition to the "Full Context"
state again. Only when decompression of several packets sent in the
FO state fails in the "Static Context" state will the decompressor go
all the way back to the "No Context" state.
When state transitions are performed is explained in detail in
chapter 5.
4.4. Modes of operation
The ROHC scheme has three modes of operation, called Unidirectional,
Bidirectional Optimistic, and Bidirectional Reliable mode.
It is important to understand the difference between states, as
described in the previous chapter, and modes. These abstractions are
orthogonal to each other. The state abstraction is the same for all
modes of operation, while the mode controls the logic of state
transitions and what actions to perform in each state.
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+----------------------+
| Unidirectional Mode |
| +--+ +--+ +--+ |
| |IR| |FO| |SO| |
| +--+ +--+ +--+ |
+----------------------+
^ ^
/ \
/ \
v v
+----------------------+ +----------------------+
| Optimistic Mode | | Reliable Mode |
| +--+ +--+ +--+ | | +--+ +--+ +--+ |
| |IR| |FO| |SO| | <--------------> | |IR| |FO| |SO| |
| +--+ +--+ +--+ | | +--+ +--+ +--+ |
+----------------------+ +----------------------+
The optimal mode to operate in depends on the characteristics of the
environment of the compression protocol, such as feedback abilities,
error probabilities and distributions, effects of header size
variation, etc. All ROHC implementations MUST implement and support
all three modes of operation. The three modes are briefly described
in the following subsections.
Detailed descriptions of the three modes of operation regarding
compression and decompression logic are given in chapter 5. The mode
transition mechanisms, too, are described in chapter 5.
4.4.1. Unidirectional mode -- U-mode
When in the Unidirectional mode of operation, packets are sent in one
direction only: from compressor to decompressor. This mode therefore
makes ROHC usable over links where a return path from decompressor to
compressor is unavailable or undesirable.
In U-mode, transitions between compressor states are performed only
on account of periodic timeouts and irregularities in the header
field change patterns in the compressed packet stream. Due to the
periodic refreshes and the lack of feedback for initiation of error
recovery, compression in the Unidirectional mode will be less
efficient and have a slightly higher probability of loss propagation
compared to any of the Bidirectional modes.
Compression with ROHC MUST start in the Unidirectional mode.
Transition to any of the Bidirectional modes can be performed as soon
as a packet has reached the decompressor and it has replied with a
feedback packet indicating that a mode transition is desired (see
chapter 5).
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4.4.2. Bidirectional Optimistic mode -- O-mode
The Bidirectional Optimistic mode is similar to the Unidirectional
mode. The difference is that a feedback channel is used to send
error recovery requests and (optionally) acknowledgments of
significant context updates from decompressor to compressor (not,
however, for pure sequence number updates). Periodic refreshes are
not used in the Bidirectional Optimistic mode.
O-mode aims to maximize compression efficiency and sparse usage of
the feedback channel. It reduces the number of damaged headers
delivered to the upper layers due to residual errors or context
invalidation. The frequency of context invalidation may be higher
than for R-mode, in particular when long loss/error bursts occur.
Refer to section 4.7 for more details.
4.4.3. Bidirectional Reliable mode -- R-mode
The Bidirectional Reliable mode differs in many ways from the
previous two. The most important differences are a more intensive
usage of the feedback channel and a stricter logic at both the
compressor and the decompressor that prevents loss of context
synchronization between compressor and decompressor except for very
high residual bit error rates. Feedback is sent to acknowledge all
context updates, including updates of the sequence number field.
However, not every packet updates the context in Reliable mode.
R-mode aims to maximize robustness against loss propagation and
damage propagation, i.e., minimize the probability of context
invalidation, even under header loss/error burst conditions. It may
have a lower probability of context invalidation than O-mode, but a
larger number of damaged headers may be delivered when the context
actually is invalidated. Refer to section 4.7 for more details.
4.5. Encoding methods
This chapter describes the encoding methods used for header fields.
How the methods are applied to each field (e.g., values of associated
parameters) is specified in section 5.7.
4.5.1. Least Significant Bits (LSB) encoding
Least Significant Bits (LSB) encoding is used for header fields whose
values are usually subject to small changes. With LSB encoding, the
k least significant bits of the field value are transmitted instead
of the original field value, where k is a positive integer. After
receiving k bits, the decompressor derives the original value using a
previously received value as reference (v_ref).
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The scheme is guaranteed to be correct if the compressor and the
decompressor each use interpretation intervals
1) in which the original value resides, and
2) in which the original value is the only value that has the
exact same k least significant bits as those transmitted.
The interpretation interval can be described as a function f(v_ref,
k). Let
f(v_ref, k) = [v_ref - p, v_ref + (2^k - 1) - p]
where p is an integer.
<------- interpretation interval (size is 2^k) ------->
|-------------+---------------------------------------|
v_ref - p v_ref v_ref + (2^k-1) - p
The function f has the following property: for any value k, the k
least significant bits will uniquely identify a value in f(v_ref, k).
The parameter p is introduced so that the interpretation interval can
be shifted with respect to v_ref. Choosing a good value for p will
yield a more efficient encoding for fields with certain
characteristics. Below are some examples:
a) For field values that are expected always to increase, p can be
set to -1. The interpretation interval becomes
[v_ref + 1, v_ref + 2^k].
b) For field values that stay the same or increase, p can be set to
0. The interpretation interval becomes [v_ref, v_ref + 2^k - 1].
c) For field values that are expected to deviate only slightly from a
constant value, p can be set to 2^(k-1) - 1. The interpretation
interval becomes [v_ref - 2^(k-1) + 1, v_ref + 2^(k-1)].
d) For field values that are expected to undergo small negative
changes and larger positive changes, such as the RTP TS for video,
or RTP SN when there is misordering, p can be set to 2^(k-2) - 1.
The interval becomes [v_ref - 2^(k-2) + 1, v_ref + 3 * 2^(k-2)],
i.e., 3/4 of the interval is used for positive changes.
The following is a simplified procedure for LSB compression and
decompression; it is modified for robustness and damage propagation
protection in the next subsection:
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1) The compressor (decompressor) always uses v_ref_c (v_ref_d), the
last value that has been compressed (decompressed), as v_ref;
2) When compressing a value v, the compressor finds the minimum value
of k such that v falls into the interval f(v_ref_c, k). Call this
function k = g(v_ref_c, v). When only a few distinct values of k
are possible, for example due to limitations imposed by packet
formats (see section 5.7), the compressor will instead pick the
smallest k that puts v in the interval f(v_ref_c, k).
3) When receiving m LSBs, the decompressor uses the interpretation
interval f(v_ref_d, m), called interval_d. It picks as the
decompressed value the one in interval_d whose LSBs match the
received m bits.
Note that the values to be encoded have a finite range; for example,
the RTP SN ranges from 0 to 0xFFFF. When the SN value is close to 0
or 0xFFFF, the interpretation interval can straddle the wraparound
boundary between 0 and 0xFFFF.
The scheme is complicated by two factors: packet loss between the
compressor and decompressor, and transmission errors undetected by
the lower layer. In the former case, the compressor and decompressor
will lose the synchronization of v_ref, and thus also of the
interpretation interval. If v is still covered by the
intersection(interval_c, interval_d), the decompression will be
correct. Otherwise, incorrect decompression will result. The next
section will address this issue further.
In the case of undetected transmission errors, the corrupted LSBs
will give an incorrectly decompressed value that will later be used
as v_ref_d, which in turn is likely to lead to damage propagation.
This problem is addressed by using a secure reference, i.e., a
reference value whose correctness is verified by a protecting CRC.
Consequently, the procedure 1) above is modified as follows:
1) a) the compressor always uses as v_ref_c the last value that has
been compressed and sent with a protecting CRC.
b) the decompressor always uses as v_ref_d the last correct
value, as verified by a successful CRC.
Note that in U/O-mode, 1) b) is modified so that if decompression of
the SN fails using the last verified SN reference, another
decompression attempt is made using the last but one verified SN
reference. This procedure mitigates damage propagation when a small
CRC fails to detect a damaged value. See section 5.3.2.2.3 for
further details.
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4.5.2. Window-based LSB encoding (W-LSB encoding)
This section describes how to modify the simplified algorithm in
4.5.1 to achieve robustness.
The compressor may not be able to determine the exact value of
v_ref_d that will be used by the decompressor for a particular value
v, since some candidates for v_ref_d may have been lost or damaged.
However, by using feedback or by making reasonable assumptions, the
compressor can limit the candidate set. The compressor then
calculates k such that no matter which v_ref_d in the candidate set
the decompressor uses, v is covered by the resulting interval_d.
Since the decompressor always uses as the reference the last received
value where the CRC succeeded, the compressor maintains a sliding
window containing the candidates for v_ref_d. The sliding window is
initially empty. The following operations are performed on the
sliding window by the compressor:
1) After sending a value v (compressed or uncompressed) protected by
a CRC, the compressor adds v to the sliding window.
2) For each value v being compressed, the compressor chooses k =
max(g(v_min, v), g(v_max, v)), where v_min and v_max are the
minimum and maximum values in the sliding window, and g is the
function defined in the previous section.
3) When the compressor is sufficiently confident that a certain value
v and all values older than v will not be used as reference by the
decompressor, the window is advanced by removing those values
(including v). The confidence may be obtained by various means.
In R-mode, an ACK from the decompressor implies that values older
than the ACKed one can be removed from the sliding window. In
U/O-mode there is always a CRC to verify correct decompression,
and a sliding window with a limited maximum width is used. The
window width is an implementation dependent optimization
parameter.
Note that the decompressor follows the procedure described in the
previous section, except that in R-mode it MUST ACK each header
received with a succeeding CRC (see also section 5.5).
4.5.3. Scaled RTP Timestamp encoding
The RTP Timestamp (TS) will usually not increase by an arbitrary
number from packet to packet. Instead, the increase is normally an
integral multiple of some unit (TS_STRIDE). For example, in the case
of audio, the sample rate is normally 8 kHz and one voice frame may
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cover 20 ms. Furthermore, each voice frame is often carried in one
RTP packet. In this case, the RTP increment is always n * 160 (=
8000 * 0.02), for some integer n. Note that silence periods have no
impact on this, as the sample clock at the source normally keeps
running without changing either frame rate or frame boundaries.
In the case of video, there is usually a TS_STRIDE as well when the
video frame level is considered. The sample rate for most video
codecs is 90 kHz. If the video frame rate is fixed, say, to 30
frames/second, the TS will increase by n * 3000 (= n * 90000 / 30)
between video frames. Note that a video frame is often divided into
several RTP packets to increase robustness against packet loss. In
this case several RTP packets will carry the same TS.
When using scaled RTP Timestamp encoding, the TS is downscaled by a
factor of TS_STRIDE before compression. This saves
floor(log2(TS_STRIDE))
bits for each compressed TS. TS and TS_SCALED satisfy the following
equality:
TS = TS_SCALED * TS_STRIDE + TS_OFFSET
TS_STRIDE is explicitly, and TS_OFFSET implicitly, communicated to
the decompressor. The following algorithm is used:
1. Initialization: The compressor sends to the decompressor the value
of TS_STRIDE and the absolute value of one or several TS fields.
The latter are used by the decompressor to initialize TS_OFFSET to
(absolute value) modulo TS_STRIDE. Note that TS_OFFSET is the
same regardless of which absolute value is used, as long as the
unscaled TS value does not wrap around; see 4) below.
2. Compression: After initialization, the compressor no longer
compresses the original TS values. Instead, it compresses the
downscaled values: TS_SCALED = TS / TS_STRIDE. The compression
method could be either W-LSB encoding or the timer-based encoding
described in the next section.
3. Decompression: When receiving the compressed value of TS_SCALED,
the decompressor first derives the value of the original
TS_SCALED. The original RTP TS is then calculated as TS =
TS_SCALED * TS_STRIDE + TS_OFFSET.
4. Offset at wraparound: Wraparound of the unscaled 32-bit TS will
invalidate the current value of TS_OFFSET used in the equation
above. For example, let us assume TS_STRIDE = 160 = 0xA0 and the
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current TS = 0xFFFFFFF0. TS_OFFSET is then 0x50 = 80. Then if
the next RTP TS = 0x00000130 (i.e., the increment is 160 * 2 =
320), the new TS_OFFSET should be 0x00000130 modulo 0xA0 = 0x90 =
144. The compressor is not required to re-initialize TS_OFFSET at
wraparound. Instead, the decompressor MUST detect wraparound of
the unscaled TS (which is trivial) and update TS_OFFSET to
TS_OFFSET = (Wrapped around unscaled TS) modulo TS_STRIDE
5. Interpretation interval at wraparound: Special rules are needed
for the interpretation interval of the scaled TS at wraparound,
since the maximum scaled TS, TSS_MAX, (0xFFFFFFFF / TS_STRIDE) may
not have the form 2^m - 1. For example, when TS_STRIDE is 160,
the scaled TS is at most 26843545 which has LSBs 10011001. The
wraparound boundary between the TSS_MAX may thus not correspond to
a natural boundary between LSBs.
interpretation interval
|<------------------------------>|
unused scaled TS
------------|--------------|---------------------->
TSS_MAX zero
When TSS_MAX is part of the interpretation interval, a number of
unused values are inserted into it after TSS_MAX such that their
LSBs follow naturally upon each other. For example, for TS_STRIDE
= 160 and k = 4, values corresponding to the LSBs 1010 through
1111 are inserted. The number of inserted values depends on k and
the LSBs of the maximum scaled TS. The number of valid values in
the interpretation interval should be high enough to maintain
robustness. This can be ensured by the following rule:
Let a be the number of LSBs needed if there was no
wraparound, and let b be the number of LSBs needed to
disambiguate between TSS_MAX and zero where the a LSBs of
TSS_MAX are set to zero. The number of LSB bits to send
while TSS_MAX or zero is part of the interpretation interval
is b.
This scaling method can be applied to many frame-based codecs.
However, the value of TS_STRIDE might change during a session, for
example as a result of adaptation strategies. If that happens, the
unscaled TS is compressed until re-initialization of the new
TS_STRIDE and TS_OFFSET is completed.
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4.5.4. Timer-based compression of RTP Timestamp
The RTP Timestamp [RFC 1889] is defined to identify the number of the
first sample used to generate the payload. When 1) RTP packets carry
payloads corresponding to a fixed sampling interval, 2) the sampling
is done at a constant rate, and 3) packets are generated in lock-step
with sampling, then the timestamp value will closely approximate a
linear function of the time of day. This is the case for
conversational media, such as interactive speech. The linear ratio
is determined by the source sample rate. The linear pattern can be
complicated by packetization (e.g., in the case of video where a
video frame usually corresponds to several RTP packets) or frame
rearrangement (e.g., B-frames are sent out-of-order by some video
codecs).
With a fixed sample rate of 8 kHz, 20 ms in the time domain is
equivalent to an increment of 160 in the unscaled TS domain, and to
an increment of 1 in the scaled TS domain with TS_STRIDE = 160.
As a consequence, the (scaled) TS of headers arriving at the
decompressor will be a linear function of time of day, with some
deviation due to the delay jitter (and the clock inaccuracies)
between the source and the decompressor. In normal operation, i.e.,
no crashes or failures, the delay jitter will be bounded to meet the
requirements of conversational real-time traffic. Hence, by using a
local clock the decompressor can obtain an approximation of the
(scaled) TS in the header to be decompressed by considering its
arrival time. The approximation can then be refined with the k LSBs
of the (scaled) TS carried in the header. The value of k required to
ensure correct decompression is a function of the jitter between the
source and the decompressor.
If the compressor knows the potential jitter introduced between
compressor and decompressor, it can determine k by using a local
clock to estimate jitter in packet arrival times, or alternatively it
can use a fixed k and discard packets arriving too much out of time.
The advantages of this scheme include:
a) The size of the compressed TS is constant and small. In
particular, it does NOT depend on the length of silence intervals.
This is in contrast to other TS compression techniques, which at
the beginning of a talkspurt require sending a number of bits
dependent on the duration of the preceding silence interval.
b) No synchronization is required between the clock local to the
compressor and the clock local to the decompressor.
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Note that although this scheme can be made to work using both scaled
and unscaled TS, in practice it is always combined with scaled TS
encoding because of the less demanding requirement on the clock
resolution, e.g., 20 ms instead of 1/8 ms. Therefore, the algorithm
described below assumes that the clock-based encoding scheme operates
on the scaled TS. The case of unscaled TS would be similar, with
changes to scale factors.
The major task of the compressor is to determine the value of k. Its
sliding window now contains not only potential reference values for
the TS but also their times of arrival at the compressor.
1) The compressor maintains a sliding window
{(T_j, a_j), for each header j that can be used as a reference},
where T_j is the scaled TS for header j, and a_j is the arrival
time of header j. The sliding window serves the same purpose as
the W-LSB sliding window of section 4.5.2.
2) When a new header n arrives with T_n as the scaled TS, the
compressor notes the arrival time a_n. It then calculates
Max_Jitter_BC =
max {|(T_n - T_j) - ((a_n - a_j) / TIME_STRIDE)|,
for all headers j in the sliding window},
where TIME_STRIDE is the time interval equivalent to one
TS_STRIDE, e.g., 20 ms. Max_Jitter_BC is the maximum observed
jitter before the compressor, in units of TS_STRIDE, for the
headers in the sliding window.
3) k is calculated as
k = ceiling(log2(2 * J + 1),
where J = Max_Jitter_BC + Max_Jitter_CD + 2.
Max_Jitter_CD is the upper bound of jitter expected on the
communication channel between compressor and decompressor (CD-CC).
It depends only on the characteristics of CD-CC.
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The constant 2 accounts for the quantization error introduced by
the clocks at the compressor and decompressor, which can be +/-1.
Note that the calculation of k follows the compression algorithm
described in section 4.5.1, with p = 2^(k-1) - 1.
4) The sliding window is subject to the same window operations as in
section 4.5.2, 1) and 3), except that the values added and removed
are paired with their arrival times.
Decompressor:
1) The decompressor uses as its reference header the last correctly
(as verified by CRC) decompressed header. It maintains the pair
(T_ref, a_ref), where T_ref is the scaled TS of the reference
header, and a_ref is the arrival time of the reference header.
2) When receiving a compressed header n at time a_n, the
approximation of the original scaled TS is calculated as:
T_approx = T_ref + (a_n - a_ref) / TIME_STRIDE.
3) The approximation is then refined by the k least significant bits
carried in header n, following the decompression algorithm of
section 4.5.1, with p = 2^(k-1) - 1.
Note: The algorithm does not assume any particular pattern in the
packets arriving at the compressor, i.e., it tolerates reordering
before the compressor and nonincreasing RTP Timestamp behavior.
Note: Integer arithmetic is used in all equations above. If
TIME_STRIDE is not equal to an integral number of clock ticks,
time must be normalized such that TIME_STRIDE is an integral
number of clock ticks. For example, if a clock tick is 20 ms and
TIME_STRIDE is 30 ms, (a_n - a_ref) in 2) can be multiplied by 3
and TIME_STRIDE can have the value 2.
Note: The clock resolution of the compressor or decompressor can
be worse than TIME_STRIDE, in which case the difference, i.e.,
actual resolution - TIME_STRIDE, is treated as additional jitter
in the calculation of k.
Note: The clock resolution of the decompressor may be communicated
to the compressor using the CLOCK feedback option.
Note: The decompressor may observe the jitter and report this to
the compressor using the JITTER feedback option. The compressor
may use this information to refine its estimate of Max_Jitter_CD.
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4.5.5. Offset IP-ID encoding
As all IPv4 packets have an IP Identifier to allow for fragmentation,
ROHC provides for transparent compression of this ID. There is no
explicit support in ROHC for the IPv6 fragmentation header, so there
is never a need to discuss IP IDs outside the context of IPv4.
This section assumes (initially) that the IPv4 stack at the source
host assigns IP-ID according to the value of a 2-byte counter which
is increased by one after each assignment to an outgoing packet.
Therefore, the IP-ID field of a particular IPv4 packet flow will
increment by 1 from packet to packet except when the source has
emitted intermediate packets not belonging to that flow.
For such IPv4 stacks, the RTP SN will increase by 1 for each packet
emitted and the IP-ID will increase by at least the same amount.
Thus, it is more efficient to compress the offset, i.e., (IP-ID - RTP
SN), instead of IP-ID itself.
The remainder of section 4.5.5 describes how to compress/decompress
the sequence of offsets using W-LSB encoding/decoding, with p = 0
(see section 4.5.1). All IP-ID arithmetic is done using unsigned
16-bit quantities, i.e., modulo 2^16.
Compressor:
The compressor uses W-LSB encoding (section 4.5.2) to compress a
sequence of offsets
Offset_i = ID_i - SN_i,
where ID_i and SN_i are the values of the IP-ID and RTP SN of
header i. The sliding window contains such offsets and not the
values of header fields, but the rules for adding and deleting
offsets from the window otherwise follow section 4.5.2.
Decompressor:
The reference header is the last correctly (as verified by CRC)
decompressed header.
When receiving a compressed packet m, the decompressor calculates
Offset_ref = ID_ref - SN_ref, where ID_ref and SN_ref are the
values of IP-ID and RTP SN in the reference header, respectively.
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Then W-LSB decoding is used to decompress Offset_m, using the
received LSBs in packet m and Offset_ref. Note that m may contain
zero LSBs for Offset_m, in which case Offset_m = Offset_ref.
Finally, the IP-ID for packet m is regenerated as
IP-ID for m = decompressed SN of packet m + Offset_m
Network byte order:
Some IPv4 stacks do use a counter to generate IP ID values as
described, but do not transmit the contents of this counter in
network byte order, but instead send the two octets reversed. In
this case, the compressor can compress the IP-ID field after
swapping the bytes. Consequently, the decompressor also swaps the
bytes of the IP-ID after decompression to regenerate the original
IP-ID. This requires that the compressor and the decompressor
synchronize on the byte order of the IP-ID field using the NBO or
NBO2 flag (see section 5.7).
Random IP Identifier:
Some IPv4 stacks generate the IP Identifier values using a
pseudo-random number generator. While this may provide some
security benefits, it makes it pointless to attempt compressing
the field. Therefore, the compressor should detect such random
behavior of the field. After detection and synchronization with
the decompressor using the RND or RND2 flag, the field is sent
as-is in its entirety as additional octets after the compressed
header.
4.5.6. Self-describing variable-length values
The values of TS_STRIDE and a few other compression parameters can
vary widely. TS_STRIDE can be 160 for voice and 90 000 for 1 f/s
video. To optimize the transfer of such values, a variable number of
octets is used to encode them. The number of octets used is
determined by the first few bits of the first octet:
First bit is 0: 1 octet.
7 bits transferred.
Up to 127 decimal.
Encoded octets in hexadecimal: 00 to 7F
First bits are 10: 2 octets.
14 bits transferred.
Up to 16 383 decimal.
Encoded octets in hexadecimal: 80 00 to BF FF
Bormann, et al. Standards Track [Page 35]
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First bits are 110: 3 octets.
21 bits transferred.
Up to 2 097 151 decimal.
Encoded octets in hexadecimal: C0 00 00 to DF FF FF
First bits are 111: 4 octets.
29 bits transferred.
Up to 536 870 911 decimal.
Encoded octets in hexadecimal: E0 00 00 00 to FF FF FF FF
4.5.7. Encoded values across several fields in compressed headers
When a compressed header has an extension, pieces of an encoded value
can be present in more than one field. When an encoded value is
split over several fields in this manner, the more significant bits
of the value are closer to the beginning of the header. If the
number of bits available in compressed header fields exceeds the
number of bits in the value, the most significant field is padded
with zeroes in its most significant bits.
For example, an unscaled TS value can be transferred using an UOR-2
header (see section 5.7) with an extension of type 3. The Tsc bit of
the extension is then unset (zero) and the variable length TS field
of the extension is 4 octets, with 29 bits available for the TS (see
section 4.5.6). The UOR-2 TS field will contain the three most
significant bits of the unscaled TS, and the 4-octet TS field in the
extension will contain the remaining 29 bits.
4.6. Errors caused by residual errors
ROHC is designed under the assumption that packets can be damaged
between the compressor and decompressor, and that such damaged
packets can be delivered to the decompressor ("residual errors").
Residual errors may damage the SN in compressed headers. Such damage
will cause generation of a header which upper layers may not be able
to distinguish from a correct header. When the compressed header
contains a CRC, the CRC will catch the bad header with a probability
dependent on the size of the CRC. When ROHC does not detect the bad
header, it will be delivered to upper layers.
Damage is not confined to the SN:
a) Damage to packet type indication bits can cause a header to be
interpreted as having a different packet type.
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b) Damage to CID information may cause a packet to be interpreted
according to another context and possibly also according to
another profile. Damage to CIDs will be more harmful when a large
part of the CID space is being used, so that it is likely that the
damaged CID corresponds to an active context.
c) Feedback information can also be subject to residual errors, both
when feedback is piggybacked and when it is sent in separate ROHC
packets. ROHC uses sanity checks and adds CRCs to vital feedback
information to allow detection of some damaged feedback.
Note that context damage can also result in generation of
incorrect headers; section 4.7 elaborates further on this.
4.7. Impairment considerations
Impairments to headers can be classified into the following types:
(1) the lower layer was not able to decode the packet and did not
deliver it to ROHC,
(2) the lower layer was able to decode the packet, but discarded
it because of a detected error,
(3) ROHC detected an error in the generated header and discarded
the packet, or
(4) ROHC did not detect that the regenerated header was damaged
and delivered it to upper layers.
Impairments cause loss or damage of individual headers. Some
impairment scenarios also cause context invalidation, which in turn
results in loss propagation and damage propagation. Damage
propagation and undetected residual errors both contribute to the
number of damaged headers delivered to upper layers. Loss
propagation and impairments resulting in loss or discarding of single
packets both contribute to the packet loss seen by upper layers.
Examples of context invalidating scenarios are:
(a) Impairment of type (4) on the forward channel, causing the
decompressor to update its context with incorrect information;
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(b) Loss/error burst of pattern update headers: Impairments of
types (1),(2) and (3) on consecutive pattern update headers; a
pattern update header is a header carrying a new pattern
information, e.g., at the beginning of a new talk spurt; this
causes the decompressor to lose the pattern update
information;
(c) Loss/error burst of headers: Impairments of types (1),(2) and
(3) on a number of consecutive headers that is large enough to
cause the decompressor to lose the SN synchronization;
(d) Impairment of type (4) on the feedback channel which mimics a
valid ACK and makes the compressor update its context;
(e) a burst of damaged headers (3) erroneously triggers the "k-
out-of-n" rule for detecting context invalidation, which
results in a NACK/update sequence during which headers are
discarded.
Scenario (a) is mitigated by the CRC carried in all context updating
headers. The larger the CRC, the lower the chance of context
invalidation caused by (a). In R-mode, the CRC of context updating
headers is always 7 bits or more. In U/O-mode, it is usually 3 bits
and sometimes 7 or 8 bits.
Scenario (b) is almost completely eliminated when the compressor
ensures through ACKs that no context updating headers are lost, as in
R-mode.
Scenario (c) is almost completely eliminated when the compressor
ensures through ACKs that the decompressor will always detect the SN
wraparound, as in R-mode. It is also mitigated by the SN repair
mechanisms in U/O-mode.
Scenario (d) happens only when the compressor receives a damaged
header that mimics an ACK of some header present in the W-LSB window,
say ACK of header 2, while in reality header 2 was never received or
accepted by the decompressor, i.e., header 2 was subject to
impairment (1), (2) or (3). The damaged header must mimic the
feedback packet type, the ACK feedback type, and the SN LSBs of some
header in the W-LSB window.
Scenario (e) happens when a burst of residual errors causes the CRC
check to fail in k out of the last n headers carrying CRCs. Large k
and n reduces the probability of scenario (e), but also increases the
number of headers lost or damaged as a consequence of any context
invalidation.
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ROHC detects damaged headers using CRCs over the original headers.
The smallest headers in this document either include a 3-bit CRC
(U/O-mode) or do not include a CRC (R-mode). For the smallest
headers, damage is thus detected with a probability of roughly 7/8
for U/O-mode. For R-mode, damage to the smallest headers is not
detected.
All other things (coding scheme at lower layers, etc.) being equal,
the rate of headers damaged by residual errors will be lower when
headers are compressed compared when they are not, since fewer bits
are transmitted. Consequently, for a given ROHC CRC setup the rate
of incorrect headers delivered to applications will also be reduced.
The above analysis suggests that U/O-mode may be more prone than R-
mode to context invalidation. On the other hand, the CRC present in
all U/O-mode headers continuously screens out residual errors coming
from lower layers, reduces the number of damaged headers delivered to
upper layers when context is invalidated, and permits quick detection
of context invalidation.
R-mode always uses a stronger CRC on context updating headers, but no
CRC in other headers. A residual error on a header which carries no
CRC will result in a damaged header being delivered to upper layers
(4). The number of damaged headers delivered to the upper layers
depends on the ratio of headers with CRC vs. headers without CRC,
which is a compressor parameter.
5. The protocol
5.1. Data structures
The ROHC protocol is based on a number of parameters that form part
of the negotiated channel state and the per-context state. This
section describes some of this state information in an abstract way.
Implementations can use a different structure for and representation
of this state. In particular, negotiation protocols that set up the
per-channel state need to establish the information that constitutes
the negotiated channel state, but it is not necessary to exchange it
in the form described here.
5.1.1. Per-channel parameters
MAX_CID: Nonnegative integer; highest context ID number to be used by
the compressor (note that this parameter is not coupled to, but in
effect further constrained by, LARGE_CIDS).
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LARGE_CIDS: Boolean; if false, the short CID representation (0 bytes
or 1 prefix byte, covering CID 0 to 15) is used; if true, the
embedded CID representation (1 or 2 embedded CID bytes covering CID 0
to 16383) is used.
PROFILES: Set of nonnegative integers, each integer indicating a
profile supported by the decompressor. The compressor MUST NOT
compress using a profile not in PROFILES.
FEEDBACK_FOR: Optional reference to a channel in the reverse
direction. If provided, this parameter indicates which channel any
feedback sent on this channel refers to (see 5.7.6.1).
MRRU: Maximum reconstructed reception unit. This is the size of the
largest reconstructed unit in octets that the decompressor is
expected to reassemble from segments (see 5.2.5). Note that this
size includes the CRC. If MRRU is negotiated to be 0, no segment
headers are allowed on the channel.
5.1.2. Per-context parameters, profiles
Per-context parameters are established with IR headers (see section
5.2.3). An IR header contains a profile identifier, which determines
how the rest of the header is to be interpreted. Note that the
profile parameter determines the syntax and semantics of the packet
type identifiers and packet types used in conjunction with a specific
context. This document describes profiles 0x0000, 0x0001, 0x0002,
and 0x0003; further profiles may be defined when ROHC is extended in
the future.
Profile 0x0000 is for sending uncompressed IP packets. See section
5.10.
Profile 0x0001 is for RTP/UDP/IP compression, see sections 5.3
through 5.9.
Profile 0x0002 is for UDP/IP compression, i.e., compression of the
first 12 octets of the UDP payload is not attempted. See section
5.11.
Profile 0x0003 is for ESP/IP compression, i.e., compression of the
header chain up to and including the first ESP header, but not
subsequent subheaders. See section 5.12.
Initially, all contexts are in no context state, i.e., all packets
referencing this context except IR packets are discarded. If defined
by a "ROHC over X" document, per-channel negotiation can be used to
pre-establish state information for a context (e.g., negotiating
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profile 0x0000 for CID 15). Such state information can also be
marked read-only in the negotiation, which would cause the
decompressor to discard any IR packet attempting to modify it.
5.1.3. Contexts and context identifiers
Associated with each compressed flow is a context, which is the state
compressor and decompressor maintain in order to correctly compress
or decompress the headers of the packet stream. Contexts are
identified by a context identifier, CID, which is sent along with
compressed headers and feedback information.
The CID space is distinct for each channel, i.e., CID 3 over channel
A and CID 3 over channel B do not refer to the same context, even if
the endpoints of A and B are the same nodes. In particular, CIDs for
any pairs of forward and reverse channels are not related (forward
and reverse channels need not even have CID spaces of the same size).
Context information is conceptually kept in a table. The context
table is indexed using the CID which is sent along with compressed
headers and feedback information. The CID space can be negotiated to
be either small, which means that CIDs can take the values 0 through
15, or large, which means that CIDs take values between 0 and 2^14 -
1 = 16383. Whether the CID space is large or small is negotiated no
later than when a channel is established.
A small CID with the value 0 is represented using zero bits. A small
CID with a value from 1 to 15 is represented by a four-bit field in
place of a packet type field (Add-CID) plus four more bits. A large
CID is represented using the encoding scheme of section 4.5.6,
limited to two octets.
5.2. ROHC packets and packet types
The packet type indication scheme for ROHC has been designed under
the following constraints:
a) it must be possible to use only a limited number of packet sizes;
b) it must be possible to send feedback information in separate ROHC
packets as well as piggybacked on forward packets;
c) it is desirable to allow elimination of the CID for one packet
stream when few packet streams share a channel;
d) it is anticipated that some packets with large headers may be
larger than the MTU of very constrained lower layers.
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These constraints have led to a design which includes
- optional padding,
- a feedback packet type,
- an optional Add-CID octet which provides 4 bits of CID, and
- a simple segmentation and reassembly mechanism.
A ROHC packet has the following general format (in the diagram,
colons ":" indicate that the part is optional):
--- --- --- --- --- --- --- ---
: Padding : variable length
--- --- --- --- --- --- --- ---
: Feedback : 0 or more feedback elements
--- --- --- --- --- --- --- ---
: Header : variable, with CID information
--- --- --- --- --- --- --- ---
: Payload :
--- --- --- --- --- --- --- ---
Padding is any number (zero or more) of padding octets. Either of
Feedback or Header must be present.
Feedback elements always start with a packet type indication.
Feedback elements carry internal CID information. Feedback is
described in section 5.2.2.
Header is either a profile-specific header or an IR or IR-DYN header
(see sections 5.2.3 and 5.2.4). Header either
1) does not carry any CID information (indicating CID zero), or
2) includes one Add-CID Octet (see below), or
3) contains embedded CID information of length one or two octets.
Alternatives 1) and 2) apply only to compressed headers in channels
where the CID space is small. Alternative 3) applies only to
compressed headers in channels where the CID space is large.
Padding Octet
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 1 1 0 0 0 0 0 |
+---+---+---+---+---+---+---+---+
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RFC 3095 Robust Header Compression July 2001
Add-CID Octet
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 1 1 0 | CID |
+---+---+---+---+---+---+---+---+
CID: 0x1 through 0xF indicates CIDs 1 through 15.
Note: The Padding Octet looks like an Add-CID octet for CID 0.
Header either starts with a packet type indication or has a packet
type indication immediately following an Add-CID Octet. All Header
packet types have the following general format (in the diagram,
slashes "/" indicate variable length):
0 x-1 x 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if (CID 1-15) and (small CIDs)
+---+--- --- --- ---+--- --- ---+
| type indication | body | 1 octet (8-x bits of body)
+---+--- ---+---+---+--- --- ---+
: :
/ 0, 1, or 2 octets of CID / 1 or 2 octets if (large CIDs)
: :
+---+---+---+---+---+---+---+---+
/ body / variable length
+---+---+---+---+---+---+---+---+
The large CID, if present, is encoded according to section 4.5.6.
5.2.1. ROHC feedback
Feedback carries information from decompressor to compressor. The
following principal kinds of feedback are supported. In addition to
the kind of feedback, other information may be included in profile-
specific feedback information.
ACK : Acknowledges successful decompression of a packet,
which means that the context is up-to-date with a high
probability.
NACK : Indicates that the dynamic context of the
decompressor is out of sync. Generated when several
successive packets have failed to be decompressed
correctly.
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STATIC-NACK : Indicates that the static context of the decompressor
is not valid or has not been established.
It is anticipated that feedback to the compressor can be realized in
many ways, depending on the properties of the particular lower layer.
The exact details of how feedback is realized is to be specified in a
"ROHC over X" document, for each lower layer X in question. For
example, feedback might be realized using
1) lower-layer specific mechanisms
2) a dedicated feedback-only channel, realized for example by the
lower layer providing a way to indicate that a packet is a
feedback packet
3) a dedicated feedback-only channel, where the timing of the
feedback provides information about which compressed packet caused
the feedback
4) interspersing of feedback packets among normal compressed packets
going in the same direction as the feedback (lower layers do not
indicate feedback)
5) piggybacking of feedback information in compressed packets going
in the same direction as the feedback (this technique may reduce
the per-feedback overhead)
6) interspersing and piggybacking on the same channel, i.e., both 4)
and 5).
Alternatives 1-3 do not place any particular requirements on the ROHC
packet type scheme. Alternatives 4-6 do, however. The ROHC packet
type scheme has been designed to allow alternatives 4-6 (these may be
used for example over PPP):
a) The ROHC scheme provides a feedback packet type. The packet type
is able to carry variable-length feedback information.
b) The feedback information sent on a particular channel is passed
to, and interpreted by, the compressor associated with feedback on
that channel. Thus, the feedback information must contain CID
information if the associated compressor can use more than one
context. The ROHC feedback scheme requires that a channel carries
feedback to at most one compressor. How a compressor is
associated with feedback on a particular channel needs to be
defined in a "ROHC over X" document.
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c) The ROHC feedback information format is octet-aligned, i.e.,
starts at an octet boundary, to allow using the format over a
dedicated feedback channel, 2).
d) To allow piggybacking, 5), it is possible to deduce the length of
feedback information by examining the first few octets of the
feedback. This allows the decompressor to pass piggybacked
feedback information to the associated same-side compressor
without understanding its format. The length information
decouples the decompressor from the compressor in the sense that
the decompressor can process the compressed header immediately
without waiting for the compressor to hand it back after parsing
the feedback information.
5.2.2. ROHC feedback format
Feedback sent on a ROHC channel consists of one or more concatenated
feedback elements, where each feedback element has the following
format:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 1 1 1 0 | Code | feedback type octet
+---+---+---+---+---+---+---+---+
: Size : if Code = 0
+---+---+---+---+---+---+---+---+
/ feedback data / variable length
+---+---+---+---+---+---+---+---+
Code: 0 indicates that a Size octet is present.
1-7 indicates the size of the feedback data field in
octets.
Size: Optional octet indicating the size of the feedback data
field in octets.
feedback data: Profile-specific feedback information. Includes
CID information.
The total size of the feedback data field is determinable upon
reception by the decompressor, by inspection of the Code field and
possibly the Size field. This explicit length information allows
piggybacking and also sending more than one feedback element in a
packet.
When the decompressor has determined the size of the feedback data
field, it removes the feedback type octet and the Size field (if
present) and hands the rest to the same-side associated compressor
Bormann, et al. Standards Track [Page 45]
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together with an indication of the size. The feedback data received
by the compressor has the following structure (feedback sent on a
dedicated feedback channel MAY also use this format):
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
: Add-CID octet : if for small CIDs and (CID != 0)
+---+---+---+---+---+---+---+---+
: :
/ large CID (4.5.6 encoding) / 1-2 octets if for large CIDs
: :
+---+---+---+---+---+---+---+---+
/ feedback /
+---+---+---+---+---+---+---+---+
The large CID, if present, is encoded according to section 4.5.6.
CID information in feedback data indicates the CID of the packet
stream for which feedback is sent. Note that the LARGE_CIDS
parameter that controls whether a large CID is present is taken from
the channel state of the receiving compressor's channel, NOT from
that of the channel carrying the feedback.
It is REQUIRED that the feedback field have either of the following
two formats:
FEEDBACK-1
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| profile specific information | 1 octet
+---+---+---+---+---+---+---+---+
FEEDBACK-2
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
|Acktype| |
+---+---+ profile specific / at least 2 octets
/ information |
+---+---+---+---+---+---+---+---+
Acktype: 0 = ACK
1 = NACK
2 = STATIC-NACK
3 is reserved (MUST NOT be used. Otherwise unparseable.)
The compressor can use the following logic to parse the feedback
field.
Bormann, et al. Standards Track [Page 46]
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1) If for large CIDs, the feedback will always start with a CID
encoded according to section 4.5.6. If the first bit is 0, the
CID uses one octet. If the first bit is 1, the CID uses two
octets.
2) If for small CIDs, and the size is one octet, the feedback is a
FEEDBACK-1.
3) If for small CIDs, and the size is larger than one octet, and the
feedback starts with the two bits 11, the feedback starts with an
Add-CID octet. If the size is 2, it is followed by FEEDBACK-1.
If the size is larger than 2, the Add-CID is followed by
FEEDBACK-2.
4) Otherwise, there is no Add-CID octet, and the feedback starts with
a FEEDBACK-2.
5.2.3. ROHC IR packet type
The IR header associates a CID with a profile, and typically also
initializes the context. It can typically also refresh (parts of)
the context. It has the following general format.
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for small CIDs and (CID != 0)
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 1 0 | x | IR type octet
+---+---+---+---+---+---+---+---+
: :
/ 0-2 octets of CID / 1-2 octets if for large CIDs
: :
+---+---+---+---+---+---+---+---+
| Profile | 1 octet
+---+---+---+---+---+---+---+---+
| CRC | 1 octet
+---+---+---+---+---+---+---+---+
| |
/ profile specific information / variable length
| |
+---+---+---+---+---+---+---+---+
x: Profile specific information. Interpreted according to the
profile indicated in the Profile field.
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Profile: The profile to be associated with the CID. In the IR
packet, the profile identifier is abbreviated to the 8 least
significant bits. It selects the highest-number profile in the
channel state parameter PROFILES that matches the 8 LSBs given.
CRC: 8-bit CRC computed using the polynomial of section 5.9.1. Its
coverage is profile-dependent, but it MUST cover at least the
initial part of the packet ending with the Profile field. Any
information which initializes the context of the decompressor
should be protected by the CRC.
Profile specific information: The contents of this part of the IR
packet are defined by the individual profiles. Interpreted
according to the profile indicated in the Profile field.
5.2.4. ROHC IR-DYN packet type
In contrast to the IR header, the IR-DYN header can never initialize
an uninitialized context. However, it can redefine what profile is
associated with a context, see for example 5.11 (ROHC UDP) and 5.12
(ROHC ESP). Thus the type needs to be reserved at the framework
level. The IR-DYN header typically also initializes or refreshes
parts of a context, typically the dynamic part. It has the following
general format:
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for small CIDs and (CID != 0)
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 0 0 0 | IR-DYN type octet
+---+---+---+---+---+---+---+---+
: :
/ 0-2 octets of CID / 1-2 octets if for large CIDs
: :
+---+---+---+---+---+---+---+---+
| Profile | 1 octet
+---+---+---+---+---+---+---+---+
| CRC | 1 octet
+---+---+---+---+---+---+---+---+
| |
/ profile specific information / variable length
| |
+---+---+---+---+---+---+---+---+
Profile: The profile to be associated with the CID. This is
abbreviated in the same way as with IR packets.
Bormann, et al. Standards Track [Page 48]
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CRC: 8-bit CRC computed using the polynomial of section 5.9.1.
Its coverage is profile-dependent, but it MUST cover at least
the initial part of the packet ending with the Profile field.
Any information which initializes the context of the
decompressor should be protected by the CRC.
Profile specific information: This part of the IR packet is
defined by individual profiles. It is interpreted according
to the profile indicated in the Profile field.
5.2.5. ROHC segmentation
Some link layers may provide a much more efficient service if the set
of different packet sizes to be transported is kept small. For such
link layers, these sizes will normally be chosen to transport
frequently occurring packets efficiently, with less frequently
occurring packets possibly adapted to the next larger size by the
addition of padding. The link layer may, however, be limited in the
size of packets it can offer in this efficient mode, or it may be
desirable to request only a limited largest size. To accommodate the
occasional packet that is larger than that largest size negotiated,
ROHC defines a simple segmentation protocol.
5.2.5.1. Segmentation usage considerations
The segmentation protocol defined in ROHC is not particularly
efficient. It is not intended to replace link layer segmentation
functions; these SHOULD be used whenever available and efficient for
the task at hand.
ROHC segmentation should only be used for occasional packets with
sizes larger than what is efficient to accommodate, e.g., due to
exceptionally large ROHC headers. The segmentation scheme was
designed to reduce packet size variations that may occur due to
outliers in the header size distribution. In other cases,
segmentation should be done at lower layers. The segmentation scheme
should only be used for packet sizes that are larger than the maximum
size in the allowed set of sizes from the lower layers.
In summary, ROHC segmentation should be used with a relatively low
frequency in the packet flow. If this cannot be ensured,
segmentation should be performed at lower layers.
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5.2.5.2. Segmentation protocol
Segment Packet
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 1 1 | F |
+---+---+---+---+---+---+---+---+
/ Segment / variable length
+---+---+---+---+---+---+---+---+
F: Final bit. If set, it indicates that this is the last segment of
a reconstructed unit.
The segment header may be preceded by padding octets and/or feedback.
It never carries a CID.
All segment header packets for one reconstructed unit have to be sent
consecutively on a channel, i.e., any non-segment-header packet
following a nonfinal segment header aborts the reassembly of the
current reconstructed unit and causes the decompressor to discard the
nonfinal segments received on this channel so far. When a final
segment header is received, the decompressor reassembles the segment
carried in this packet and any nonfinal segments that immediately
preceded it into a single reconstructed unit, in the order they were
received. The reconstructed unit has the format:
Reconstructed Unit
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| |
/ Reconstructed ROHC packet / variable length
| |
+---+---+---+---+---+---+---+---+
/ CRC / 4 octets
+---+---+---+---+---+---+---+---+
The CRC is used by the decompressor to validate the reconstructed
unit. It uses the FCS-32 algorithm with the following generator
polynomial: x^0 + x^1 + x^2 + x^4 + x^5 + x^7 + x^8 + x^10 + x^11 +
x^12 + x^16 + x^22 + x^23 + x^26 + x^32 [HDLC]. If the reconstructed
unit is 4 octets or less, or if the CRC fails, or if it is larger
than the channel parameter MRRU (see 5.1.1), the reconstructed unit
MUST be discarded by the decompressor.
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If the CRC succeeds, the reconstructed ROHC packet is interpreted as
a ROHC Header, optionally followed by a payload. Note that this
means that there can be no padding and no feedback in the
reconstructed unit, and that the CID is derived from the initial
octets of the reconstructed unit.
(It should be noted that the ROHC segmentation protocol was inspired
by SEAL by Steve Deering et al., which later became ATM AAL5. The
same arguments for not having sequence numbers in the segments but
instead providing a strong CRC in the reconstructed unit apply here
as well. Note that, as a result of this protocol, there is no way in
ROHC to make any use of a segment that has residual bit errors.)
5.2.6. ROHC initial decompressor processing
The following packet types are reserved at the framework level in the
ROHC scheme:
1110: Padding or Add-CID octet
11110: Feedback
11111000: IR-DYN packet
1111110: IR packet
1111111: Segment
Other packet types can be used at will by individual profiles.
The following steps is an outline of initial decompressor processing
which upon reception of a ROHC packet can determine its contents.
1) If the first octet is a Padding Octet (11100000),
strip away all initial Padding Octets and goto next step.
2) If the first remaining octet starts with 1110, it is an Add-CID
octet:
remember the Add-CID octet; remove the octet.
3) If the first remaining octet starts with 11110, and an Add-CID
octet was found in step 2),
an error has occurred; the header MUST be discarded without
further action.
4) If the first remaining octet starts with 11110, and an Add-CID
octet was not found in step 2), this is feedback:
find the size of the feedback data, call it s;
remove the feedback type octet;
Bormann, et al. Standards Track [Page 51]
RFC 3095 Robust Header Compression July 2001
remove the Size octet if Code is 0;
send feedback data of length s to the same-side associated
compressor;
if packet exhausted, stop; otherwise goto 2).
5) If the first remaining octet starts with 1111111, this is a
segment:
attempt reconstruction using the segmentation protocol
(5.2.5). If a reconstructed packet is not produced, this
finishes the processing of the original packet. If a
reconstructed packet is produced, it is fed into step 1)
above. Padding, segments, and feedback are not allowed in
reconstructed packets, so when processing them, steps 1),
4), and 5) are modified so that the packet is discarded
without further action when their conditions match.
6) Here, it is known that the rest is forward information (unless the
header is damaged).
7) If the forward traffic uses small CIDs, there is no large CID in
the packet. If an Add-CID immediately preceded the packet type
(step 2), it has the CID of the Add-CID; otherwise it has CID 0.
8) If the forward traffic uses large CIDs, the CID starts with the
second remaining octet. If the first bit(s) of that octet are not
0 or 10, the packet MUST be discarded without further action. If
an Add-CID octet immediately preceded the packet type (step 2),
the packet MUST be discarded without further action.
9) Use the CID to find the context.
10) If the packet type is IR, the profile indicated in the IR packet
determines how it is to be processed. If the CRC fails to verify
the packet, it MUST be discarded. If a profile is indicated in
the context, the logic of that profile determines what, if any,
feedback is to be sent. If no profile is noted in the context,
no further action is taken.
11) If the packet type is IR-DYN, the profile indicated in the IR-DYN
packet determines how it is to be processed.
a) If the CRC fails to verify the packet, it MUST be discarded.
If a profile is indicated in the context, the logic of that
profile determines what, if any, feedback is to be sent. If no
profile is noted in the context, no further action is taken.
Bormann, et al. Standards Track [Page 52]
RFC 3095 Robust Header Compression July 2001
b) If the context has not been initialized by an IR packet, the
packet MUST be discarded. The logic of the profile indicated
in the IR-DYN header (if verified by the CRC), determines what,
if any, feedback is to be sent.
12) Otherwise, the profile noted in the context determines how the
rest of the packet is to be processed. If the context has not
been initialized by an IR packet, the packet MUST be discarded
without further action.
The procedure for finding the size of the feedback data is as
follows:
Examine the three bits which immediately follow the feedback packet
type. When these bits are
1-7, the size of the feedback data is given by the bits;
0, a Size octet, which explicitly gives the size of the
feedback data, is present after the feedback type octet.
5.2.7. ROHC RTP packet formats from compressor to decompressor
ROHC RTP uses three packet types to identify compressed headers, and
two for initialization/refresh. The format of a compressed packet
can depend on the mode. Therefore a naming scheme of the form
<modes format is used in>-<packet type number>-<some property>
is used to uniquely identify the format when necessary, e.g., UOR-2,
R-1. For exact formats of the packet types, see section 5.7.
Packet type zero: R-0, R-0-CRC, UO-0.
This, the minimal, packet type is used when parameters of all SN-
functions are known by the decompressor, and the header to be
compressed adheres to these functions. Thus, only the W-LSB
encoded RTP SN needs to be communicated.
R-mode: Only if a CRC is present (packet type R-0-CRC) may the
header be used as a reference for subsequent decompression.
U-mode and O-mode: A small CRC is present in the UO-0 packet.
Packet type 1: R-1, R-1-ID, R-1-TS, UO-1, UO-1-ID, UO-1-TS.
This packet type is used when the number of bits needed for the SN
exceeds those available in packet type zero, or when the
parameters of the SN-functions for RTP TS or IP-ID change.
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R-mode: R-1-* packets are not used as references for subsequent
decompression. Values for other fields than the RTP TS or IP-ID
can be communicated using an extension, but they do not update the
context.
U-mode and O-mode: Only the values of RTP SN, RTP TS and IP-ID can
be used as references for future compression. Nonupdating values
can be provided for other fields using an extension (UO-1-ID).
Packet type 2: UOR-2, UOR-2-ID, UOR-2-TS
This packet type can be used to change the parameters of any SN-
function, except those for most static fields. Headers of packets
transferred using packet type 2 can be used as references for
subsequent decompression.
Packet type: IR
This packet type communicates the static part of the context,
i.e., the value of the constant SN-functions. It can optionally
also communicate the dynamic part of the context, i.e., the
parameters of the nonconstant SN-functions.
Packet type: IR-DYN
This packet type communicates the dynamic part of the context,
i.e., the parameters of nonconstant SN-functions.
5.2.8. Parameters needed for mode transition in ROHC RTP
The packet types IR (with dynamic information), IR-DYN, and UOR-2 are
common for all modes. They can carry a mode parameter which can take
the values U = Unidirectional, O = Bidirectional Optimistic, and R =
Bidirectional Reliable.
Feedback of types ACK, NACK, and STATIC-NACK carry sequence numbers,
and feedback packets can also carry a mode parameter indicating the
desired compression mode: U, O, or R.
As a shorthand, the notation PACKET(mode) is used to indicate which
mode value a packet carries. For example, an ACK with mode parameter
R is written ACK(R), and an UOR-2 with mode parameter O is written
UOR-2(O).
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RFC 3095 Robust Header Compression July 2001
5.3. Operation in Unidirectional mode
5.3.1. Compressor states and logic (U-mode)
Below is the state machine for the compressor in Unidirectional mode.
Details of the transitions between states and compression logic are
given subsequent to the figure.
Optimistic approach
+------>------>------>------>------>------>------>------>------+
| |
| Optimistic approach Optimistic approach |
| +------>------>------+ +------>------>------+ |
| | | | | |
| | v | v v
+----------+ +----------+ +----------+
| IR State | | FO State | | SO State |
+----------+ +----------+ +----------+
^ ^ | ^ | |
| | Timeout | | Timeout / Update | |
| +------<------<------+ +------<------<------+ |
| |
| Timeout |
+------<------<------<------<------<------<------<------<------+
5.3.1.1. State transition logic (U-mode)
The transition logic for compression states in Unidirectional mode is
based on three principles: the optimistic approach principle,
timeouts, and the need for updates.
5.3.1.1.1. Optimistic approach, upwards transition
Transition to a higher compression state in Unidirectional mode is
carried out according to the optimistic approach principle. This
means that the compressor transits to a higher compression state when
it is fairly confident that the decompressor has received enough
information to correctly decompress packets sent according to the
higher compression state.
When the compressor is in the IR state, it will stay there until it
assumes that the decompressor has correctly received the static
context information. For transition from the FO to the SO state, the
compressor should be confident that the decompressor has all
parameters needed to decompress according to a fixed pattern.
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The compressor normally obtains its confidence about decompressor
status by sending several packets with the same information according
to the lower compression state. If the decompressor receives any of
these packets, it will be in sync with the compressor. The number of
consecutive packets to send for confidence is not defined in this
document.
5.3.1.1.2. Timeouts, downward transition
When the optimistic approach is taken as described above, there will
always be a possibility of failure since the decompressor may not
have received sufficient information for correct decompression.
Therefore, the compressor MUST periodically transit to lower
compression states. Periodic transition to the IR state SHOULD be
carried out less often than transition to the FO state. Two
different timeouts SHOULD therefore be used for these transitions.
For an example of how to implement periodic refreshes, see [IPHC]
chapters 3.3.1-3.3.2.
5.3.1.1.3. Need for updates, downward transition
In addition to the downward state transitions carried out due to
periodic timeouts, the compressor must also immediately transit back
to the FO state when the header to be compressed does not conform to
the established pattern.
5.3.1.2. Compression logic and packets used (U-mode)
The compressor chooses the smallest possible packet format that can
communicate the desired changes, and has the required number of bits
for W-LSB encoded values.
5.3.1.3. Feedback in Unidirectional mode
The Unidirectional mode of operation is designed to operate over
links where a feedback channel is not available. If a feedback
channel is available, however, the decompressor MAY send an
acknowledgment of successful decompression with the mode parameter
set to U (send an ACK(U)). When the compressor receives such a
message, it MAY disable (or increase the interval between) periodic
IR refreshes.
5.3.2. Decompressor states and logic (U-mode)
Below is the state machine for the decompressor in Unidirectional
mode. Details of the transitions between states and decompression
logic are given subsequent to the figure.
Bormann, et al. Standards Track [Page 56]
RFC 3095 Robust Header Compression July 2001
Success
+-->------>------>------>------>------>--+
| |
No Static | No Dynamic Success | Success
+-->--+ | +-->--+ +--->----->---+ +-->--+
| | | | | | | | |
| v | | v | v | v
+--------------+ +----------------+ +--------------+
| No Context | | Static Context | | Full Context |
+--------------+ +----------------+ +--------------+
^ | ^ |
| k_2 out of n_2 failures | | k_1 out of n_1 failures |
+-----<------<------<-----+ +-----<------<------<-----+
5.3.2.1. State transition logic (U-mode)
Successful decompression will always move the decompressor to the
Full Context state. Repeated failed decompression will force the
decompressor to transit downwards to a lower state. The decompressor
does not attempt to decompress headers at all in the No Context and
Static Context states unless sufficient information is included in
the packet itself.
5.3.2.2. Decompression logic (U-mode)
Decompression in Unidirectional mode is carried out following three
steps which are described in subsequent sections.
5.3.2.2.1. Decide whether decompression is allowed
In Full Context state, decompression may be attempted regardless of
what kind of packet is received. However, for the other states
decompression is not always allowed. In the No Context state only IR
packets, which carry the static information fields, may be
decompressed. Further, when in the Static Context state, only
packets carrying a 7- or 8-bit CRC can be decompressed (i.e., IR,
IR-DYN, or UOR-2 packets). If decompression may not be performed the
packet is discarded, unless the optional delayed decompression
mechanism is used, see section 6.1.
5.3.2.2.2. Reconstruct and verify the header
When reconstructing the header, the decompressor takes the header
information already stored in the context and updates it with the
information received in the current header. (If the reconstructed
header fails the CRC check, these updates MUST be undone.)
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RFC 3095 Robust Header Compression July 2001
The sequence number is reconstructed by replacing the sequence number
LSBs in the context with those received in the header. The resulting
value is then verified to be within the interpretation interval by
comparison with a previously reconstructed reference value v_ref (see
section 4.5.1). If it is not within this interval, an adjustment is
applied by adding N x interval_size to the reconstructed value so
that the result is brought within the interpretation interval. Note
that N can be negative.
If RTP Timestamp and IP Identification fields are not included in the
received header, they are supposed to be calculated from the sequence
number. The IP Identifier usually increases by the same delta as the
sequence number and the timestamp by the same delta times a fixed
value. See chapters 4.5.3 and 4.5.5 for details about how these
fields are encoded in compressed headers.
When working in Unidirectional mode, all compressed headers carry a
CRC which MUST be used to verify decompression.
5.3.2.2.3. Actions upon CRC failure
This section is written so that it is applicable to all modes.
A mismatch in the CRC can be caused by one or more of:
1. residual bit errors in the current header
2. a damaged context due to residual bit errors in previous headers
3. many consecutive packets being lost between compressor and
decompressor (this may cause the LSBs of the SN in compressed
packets to be interpreted wrongly, because the decompressor has
not moved the interpretation interval for lack of input -- in
essence, a kind of context damage).
(Cases 2 and 3 do not apply to IR packets; case 3 does not apply to
IR-DYN packets.) The 3-bit CRC present in some header formats will
eventually detect context damage reliably, since the probability of
undetected context damage decreases exponentially with each new
header processed. However, residual bit errors in the current header
are only detected with good probability, not reliably.
When a CRC mismatch is caused by residual bit errors in the current
header (case 1 above), the decompressor should stay in its current
state to avoid unnecessary loss of subsequent packets. On the other
hand, when the mismatch is caused by a damaged context (case 2), the
decompressor should attempt to repair the context locally. If the
local repair attempt fails, it must move to a lower state to avoid
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delivering incorrect headers. When the mismatch is caused by
prolonged loss (case 3), the decompressor might attempt additional
decompression attempts. Note that case 3 does not occur in R-mode.
The following actions MUST be taken when a CRC check fails:
First, attempt to determine whether SN LSB wraparound (case 3) is
likely, and if so, attempt a correction. For this, the algorithm of
section 5.3.2.2.4 MAY be used. If another algorithm is used, it MUST
have at least as high a rate of correct repairs as the one in
5.3.2.2.4. (This step is not applicable to R-mode.)
Second, if the previous step did not attempt a correction, a repair
should be attempted under the assumption that the reference SN has
been incorrectly updated. For this, the algorithm of section
5.3.2.2.5 MAY be used. If another algorithm is used, it MUST have at
least as high a rate of correct repairs as the one in 5.3.2.2.5.
(This step is not applicable to R-mode.)
If both the above steps fail, additional decompression attempts
SHOULD NOT be made. There are two possible reasons for the CRC
failure: case 1 or unrecoverable context damage. It is impossible to
know for certain which of these is the actual cause. The following
rules are to be used:
a. When CRC checks fail only occasionally, assume residual errors in
the current header and simply discard the packet. NACKs SHOULD
NOT be sent at this time.
b. In the Full Context state: When the CRC check of k_1 out of the
last n_1 decompressed packets have failed, context damage SHOULD
be assumed and a NACK SHOULD be sent in O- and R-mode. The
decompressor moves to the Static Context state and discards all
packets until an update (IR, IR-DYN, UOR-2) which passes the CRC
check is received.
c. In the Static Context state: When the CRC check of k_2 out of the
last n_2 updates (IR, IR-DYN, UOR-2) have failed, static context
damage SHOULD be assumed and a STATIC-NACK is sent in O- and R-
mode. The decompressor moves to the No Context state.
d. In the No Context state: The decompressor discards all packets
until a static update (IR) which passes the CRC check is received.
(In O-mode and R-mode, feedback is sent according to sections
5.4.2.2 and 5.5.2.2, respectively.)
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Note that appropriate values for k_1, n_1, k_2, and n_2, are related
to the residual error rate of the link. When the residual error rate
is close to zero, k_1 = n_1 = k_2 = n_2 = 1 may be appropriate.
5.3.2.2.4. Correction of SN LSB wraparound
When many consecutive packets are lost there will be a risk of
sequence number LSB wraparound, i.e., the SN LSBs being interpreted
wrongly because the interpretation interval has not moved for lack of
input. The decompressor might be able to detect this situation and
avoid context damage by using a local clock. The following algorithm
MAY be used:
a. The decompressor notes the arrival time, a(i), of each incoming
packet i. Arrival times of packets where decompression fails are
discarded.
b. When decompression fails, the decompressor computes INTERVAL =
a(i) - a(i - 1), i.e., the time elapsed between the arrival of the
previous, correctly decompressed packet and the current packet.
c. If wraparound has occurred, INTERVAL will correspond to at least
2^k inter-packet times, where k is the number of SN bits in the
current header. On the basis of an estimate of the packet inter-
arrival time, obtained for example using a moving average of
arrival times, TS_STRIDE, or TS_TIME, the decompressor judges if
INTERVAL can correspond to 2^k inter-packet times.
d. If INTERVAL is judged to be at least 2^k packet inter-arrival
times, the decompressor adds 2^k to the reference SN and attempts
to decompress the packet using the new reference SN.
e. If this decompression succeeds, the decompressor updates the
context but SHOULD NOT deliver the packet to upper layers. The
following packet is also decompressed and updates the context if
its CRC succeeds, but SHOULD be discarded. If decompression of
the third packet using the new context also succeeds, the context
repair is deemed successful and this and subsequent decompressed
packets are delivered to the upper layers.
f. If any of the three decompression attempts in d. and e. fails, the
decompressor discards the packets and acts according to rules a)
through c) of section 5.3.2.2.3.
Using this mechanism, the decompressor may be able to repair the
context after excessive loss, at the expense of discarding two
packets.
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5.3.2.2.5. Repair of incorrect SN updates
The CRC can fail to detect residual errors in the compressed header
because of its limited length, i.e., the incorrectly decompressed
packet can happen to have the same CRC as the original uncompressed
packet. The incorrect decompressed header will then update the
context. This can lead to an erroneous reference SN being used in
W-LSB decoding, as the reference SN is updated for each successfully
decompressed header of certain types.
In this situation, the decompressor will detect the incorrect
decompression of the following packet with high probability, but it
does not know the reason for the failure. The following mechanism
allows the decompressor to judge if the context was updated
incorrectly by an earlier packet and, if so, to attempt a repair.
a. The decompressor maintains two decompressed sequence numbers: the
last one (ref 0) and the one before that (ref -1).
b. When receiving a compressed header the SN (SN curr1) is
decompressed using ref 0 as the reference. The other header
fields are decompressed using this decompressed SN curr1. (This
is part of the normal decompression procedure prior to any CRC
test failures.)
c. If the decompressed header generated in b. passes the CRC test,
the references are shifted as follows:
ref -1 = ref 0
ref 0 = SN curr1.
d. If the header generated in b. does not pass the CRC test, and the
SN (SN curr2) generated when using ref -1 as the reference is
different from SN curr1, an additional decompression attempt is
performed based on SN curr2 as the decompressed SN.
e. If the decompressed header generated in b. does not pass the CRC
test and SN curr2 is the same as SN curr1, an additional
decompression attempt is not useful and is not attempted.
f. If the decompressed header generated in d. passes the CRC test,
ref -1 is not changed while ref 0 is set to SN curr2.
g. If the decompressed header generated in d. does not pass the CRC
test, the decompressor acts according to rules a) through c) of
section 5.3.2.2.3.
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The purpose of this algorithm is to repair the context. If the
header generated in d. passes the CRC test, the references are
updated according to f., but two more headers MUST also be
successfully decompressed before the repair is deemed successful. Of
the three successful headers, the first two SHOULD be discarded and
only the third delivered to upper layers. If decompression of any of
the three headers fails, the decompressor MUST discard that header
and the previously generated headers, and act according to rules a)
through c) of section 5.3.2.2.3.
5.3.2.3. Feedback in Unidirectional mode
To improve performance for the Unidirectional mode over a link that
does have a feedback channel, the decompressor MAY send an
acknowledgment when decompression succeeds. Setting the mode
parameter in the ACK packet to U indicates that the compressor is to
stay in Unidirectional mode. When receiving an ACK(U), the
compressor should reduce the frequency of IR packets since the static
information has been correctly received, but it is not required to
stop sending IR packets. If IR packets continue to arrive, the
decompressor MAY repeat the ACK(U), but it SHOULD NOT repeat the
ACK(U) continuously.
5.4. Operation in Bidirectional Optimistic mode
5.4.1. Compressor states and logic (O-mode)
Below is the state machine for the compressor in Bidirectional
Optimistic mode. The details of each state, state transitions, and
compression logic are given subsequent to the figure.
Optimistic approach / ACK
+------>------>------>------>------>------>------>------>------+
| |
| Optimistic appr. / ACK Optimistic appr. /ACK ACK |
| +------>------>------+ +------>--- -->-----+ +->--+
| | | | | | |
| | v | v | v
+----------+ +----------+ +----------+
| IR State | | FO State | | SO State |
+----------+ +----------+ +----------+
^ ^ | ^ | |
| | STATIC-NACK | | NACK / Update | |
| +------<------<------+ +------<------<------+ |
| |
| STATIC-NACK |
+------<------<------<------<------<------<------<------<------+
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5.4.1.1. State transition logic
The transition logic for compression states in Bidirectional
Optimistic mode has much in common with the logic of the
Unidirectional mode. The optimistic approach principle and
transitions occasioned by the need for updates work in the same way
as described in chapter 5.3.1. However, in Optimistic mode there are
no timeouts. Instead, the Optimistic mode makes use of feedback from
decompressor to compressor for transitions in the backward direction
and for OPTIONAL improved forward transition.
5.4.1.1.1. Negative acknowledgments (NACKs), downward transition
Negative acknowledgments (NACKs), also called context requests,
obviate the periodic updates needed in Unidirectional mode. Upon
reception of a NACK the compressor transits back to the FO state and
sends updates (IR-DYN, UOR-2, or possibly IR) to the decompressor.
NACKs carry the SN of the latest packet successfully decompressed,
and this information MAY be used by the compressor to determine what
fields need to be updated.
Similarly, reception of a STATIC-NACK packet makes the compressor
transit back to the IR state.
5.4.1.1.2. Optional acknowledgments, upwards transition
In addition to NACKs, positive feedback (ACKs) MAY also be used for
UOR-2 packets in the Bidirectional Optimistic mode. Upon reception
of an ACK for an updating packet, the compressor knows that the
decompressor has received the acknowledged packet and the transition
to a higher compression state can be carried out immediately. This
functionality is optional, so a compressor MUST NOT expect to get
such ACKs initially.
The compressor MAY use the following algorithm to determine when to
expect ACKs for UOR-2 packets. Let an update event be when a
sequence of UOR-2 headers are sent to communicate an irregularity in
the packet stream. When ACKs have been received for k_3 out of the
last n_3 update events, the compressor will expect ACKs. A
compressor which expects ACKs will repeat updates (possibly not in
every packet) until an ACK is received.
5.4.1.2. Compression logic and packets used
The compression logic is the same for the Bidirectional Optimistic
mode as for the Unidirectional mode (see section 5.3.1.2).
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5.4.2. Decompressor states and logic (O-mode)
The decompression states and the state transition logic are the same
as for the Unidirectional case (see section 5.3.2). What differs is
the decompression and feedback logic.
5.4.2.1. Decompression logic, timer-based timestamp decompression
In Bidirectional mode (or if there is some other way for the
compressor to obtain the decompressor's clock resolution and the
link's jitter), timer-based timestamp decompression may be used to
improve compression efficiency when RTP Timestamp values are
proportional to wall-clock time. The mechanisms used are those
described in 4.5.4.
5.4.2.2. Feedback logic (O-mode)
The feedback logic defines what feedback to send due to different
events when operating in the various states. As mentioned above,
there are three principal kinds of feedback; ACK, NACK and STATIC-
NACK. Further, the logic described below will refer to different
kinds of packets that can be received by the decompressor;
Initialization and Refresh (IR) packets, IR packets without static
information (IR-DYN) and type 2 packets (UOR-2), or type 1 (UO-1) and
type 0 packets (UO-0). A type 0 packet carries a packet header
compressed according to a fixed pattern, while type 1, 2 and IR-DYN
packets are used when this pattern is broken.
Below, rules are defined stating which feedback to use when. If the
optional feedback is used once, the decompressor is REQUIRED to
continue to send optional feedback for the lifetime of the packet
stream.
State Actions
NC: - When an IR packet passes the CRC check, send an ACK(O).
- When receiving a type 0, 1, 2 or IR-DYN packet, or an IR
packet has failed the CRC check, send a STATIC-NACK(O),
subject to the considerations at the beginning of section
5.7.6.
SC: - When an IR packet is correctly decompressed, send an ACK(O).
- When a type 2 or an IR-DYN packet is correctly decompressed,
optionally send an ACK(O).
- When a type 0 or 1 packet is received, treat it as a
mismatching CRC and use the logic of section 5.3.2.2.3 to
decide if a NACK(O) should be sent.
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- When decompression of a type 2 packet, an IR-DYN packet or an
IR packet has failed, use the logic of section 5.3.2.2.3 to
decide if a STATIC-NACK(O) should be sent.
FC: - When an IR packet is correctly decompressed, send an ACK(O).
- When a type 2 or an IR-DYN packet is correctly decompressed,
optionally send an ACK(O).
- When a type 0 or 1 packet is correctly decompressed, no
feedback is sent.
- When any packet fails the CRC check, use the logic of
5.3.2.2.3 to decide if a NACK(O) should be sent.
5.5. Operation in Bidirectional Reliable mode
5.5.1. Compressor states and logic (R-mode)
Below is the state machine for the compressor in Bidirectional
Reliable mode. The details of each state, state transitions, and
compression logic are given subsequent to the figure.
ACK
+------>------>------>------>------>------>------>------+
| |
| ACK ACK | ACK
| +------>------>------+ +------>------>------+ +->-+
| | | | | | |
| | v | v | v
+----------+ +----------+ +----------+
| IR State | | FO State | | SO State |
+----------+ +----------+ +----------+
^ ^ | ^ | |
| | STATIC-NACK | | NACK / Update | |
| +------<------<------+ +------<------<------+ |
| |
| STATIC-NACK |
+------<------<------<------<------<------<------<------<------+
5.5.1.1. State transition logic (R-mode)
The transition logic for compression states in Reliable mode is based
on three principles: the secure reference principle, the need for
updates, and negative acknowledgments.
5.5.1.1.1. Upwards transition
The upwards transition is determined by the secure reference
principle. The transition procedure is similar to the one described
in section 5.3.1.1.1, with one important difference: the compressor
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bases its confidence only on acknowledgments received from the
decompressor. This ensures that the synchronization between the
compression context and decompression context will never be lost due
to packet losses.
5.5.1.1.2. Downward transition
Downward transitions are triggered by the need for updates or by
negative acknowledgment (NACKs and STATIC_NACKs), as described in
section 5.3.1.1.3 and 5.4.1.1.1, respectively. Note that NACKs
should rarely occur in R-mode because of the secure reference used
(see fourth paragraph of next section).
5.5.1.2. Compression logic and packets used (R-mode)
The compressor starts in the IR state by sending IR packets. It
transits to the FO state once it receives a valid ACK for an IR
packet sent (an ACK can only be valid if it refers to an SN sent
earlier). In the FO state, it sends the smallest packets that can
communicate the changes, according to W-LSB or other encoding rules.
Those packets could be of type R-1*, UOR-2, or even IR-DYN.
The compressor will transit to the SO state after it has determined
the presence of a string (see section 2), while also being confident
that the decompressor has the string parameters. The confidence can
be based on ACKs. For example, in a typical case where the string
pattern has the form of non-SN-field = SN * slope + offset, one ACK
is enough if the slope has been previously established by the
decompressor (i.e., only the new offset needs to be synchronized).
Otherwise, two ACKs are required since the decompressor needs two
headers to learn both the new slope and the new offset. In the SO
state, R-0* packets will be sent.
Note that a direct transition from the IR state to the SO state is
possible.
The secure reference principle is enforced in both compression and
decompression logic. The principle means that only a packet carrying
a 7- or 8-bit CRC can update the decompression context and be used as
a reference for subsequent decompression. Consequently, only field
values of update packets need to be added to the encoding sliding
windows (see 4.5) maintained by the compressor.
Reasons for the compressor to send update packets include:
1) The update may lead to a transition to higher compression
efficiency (meaning either a higher compression state or smaller
packets in the same state).
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2) It is desirable to shrink sliding windows. Windows are only
shrunk when an ACK is received.
The generation of a CRC is infrequent since it is only needed for
an update packet.
One algorithm for sending update packets could be:
* Let pRTT be the number of packets that are sent during one
round-trip time. In the SO state, when (64 - pRTT) headers have
been sent since the last acked reference, the compressor will
send m1 consecutive R-0-CRC headers, then send (pRTT - m1) R-0
headers. After these headers have been sent, if the compressor
has not received an ACK to at least one of the previously sent
R0-CRC, it sends R-0-CRC headers continuously until it receives a
corresponding ACK. m1 is an implementation parameter, which can
be as large as pRTT.
* In the FO state, m2 UOR-2 headers are sent when there is a
pattern change, after which the compressor sends (pRTT - m2)
R-1-* headers. m2 is an implementation parameter, which can be
as large as pRTT. At that time, if the compressor has not
received enough ACKs to the previously sent UOR-2 packets in
order to transit to SO state, it can repeat the cycle with the
same m2, or repeat the cycle with a larger m2, or send UOR-2
headers continuously (m2 = pRTT). The operation stops when the
compressor has received enough ACKs to make the transition.
An algorithm for processing ACKs could be:
* Upon reception of an ACK, the compressor first derives the
complete SN (see section 5.7.6.1). Then it searches the sliding
window for an update packet that has the same SN. If found, that
packet is the one being ACKed. Otherwise, the ACK is invalid and
MUST be discarded.
* It is possible, although unlikely, that residual errors on the
reverse channel could cause a packet to mimic a valid ACK
feedback. The compressor may use a local clock to reduce the
probability of processing such a mistaken ACK. After finding the
update packet as described above, the compressor can check the
time elapsed since the packet was sent. If the time is longer
than RTT_U, or shorter than RTT_L, the compressor may choose to
discard the ACK. RTT_U and RTT_L correspond to an upper bound
and lower bound of the RTT, respectively. (These bounds should
be chosen appropriately to allow some variation of RTT.) Note
that the only side effect of discarding a good ACK is slightly
reduced compression efficiency.
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5.5.2. Decompressor states and logic (R-mode)
The decompression states and the state transition logic are the same
as for the Unidirectional case (see section 5.3.2). What differs is
the decompression and feedback logic.
5.5.2.1. Decompression logic (R-mode)
The rules for when decompression is allowed are the same as for U-
mode. Although the acking scheme in R-mode guarantees that non-
decompressible packets are never sent by the compressor, residual
errors can cause delivery of unexpected packets for which
decompression should not be attempted.
Decompression MUST follow the secure reference principle as described
in 5.5.1.2.
CRC verification is infrequent since only update packets carry CRCs.
A CRC mismatch can only occur due to 1) residual bit errors in the
current header, and/or 2) a damaged context due to residual bit
errors in previous headers or feedback. Although it is impossible to
determine which is the actual cause, case 1 is more likely, as a
previous header reconstructed according to a damaged packet is
unlikely to pass the 7- or 8-bit CRC, and damaged packets are
unlikely to result in feedback that damages the context. The
decompressor SHOULD act according to section 5.3.2.2.3 when CRCs
fail, except that no local repair is performed. Note that all the
parameter numbers, k_1, n_1, k_2, and n_2, are applied to the update
packets only (i.e., exclude R-0, R-1*).
5.5.2.2. Feedback logic (R-mode)
The feedback logic for the Bidirectional Reliable mode is as follows:
- When an updating packet (i.e., a packet carrying a 7- or 8-bit CRC)
is correctly decompressed, send an ACK(R), subject to the sparse
ACK mechanism described below.
- When context damage is detected, send a NACK(R) if in Full Context
state, or a STATIC-NACK(R) if in Static Context state.
- In No Context state, send a STATIC-NACK(R) when receiving a non-IR
packet, subject to the considerations at the beginning of section
5.7.6. The decompressor SHOULD NOT send STATIC-NACK(R) when
receiving an IR packet that fails the CRC check, as the compressor
will stay in IR state and thus continue sending IR packets until a
valid ACK is received (see section 5.5.1.2).
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- Feedback is never sent for packets not updating the context (i.e.,
packets that do not carry a CRC)
A mechanism called "Sparse ACK" can be applied to reduce the feedback
overhead caused by a large RTT. For a sequence of ACK-triggering
events, a minimal set of ACKs MUST be sent:
1) For a sequence of R-0-CRC packets, the first one MUST be ACKed.
2) For a sequence of UOR-2, IR, or IR-DYN packets, the first N of
them MUST be ACKEd, where N is the number of ACKs needed to give
the compressor confidence that the decompressor has acquired the
new string parameters (see second paragraph of 5.5.1.2). In case
the decompressor cannot determine the value of N, the default
value 2 SHOULD be used. If the subsequently received packets
continue the same change pattern of header fields, sparse ACK can
be applied. Otherwise, each new pattern MUST be treated as a new
sequence, i.e., the first N packets that exhibit a new pattern
MUST be ACKed.
After sending these minimal ACKs, the decompressor MAY choose to ACK
only k subsequent packets per RTT ("Sparse ACKs"), where k is an
implementation parameter. To achieve robustness against loss of
ACKs, k SHOULD be at least 1.
To avoid ambiguity at the compressor, the decompressor MUST use the
feedback format whose SN field length is equal to or larger than the
one in the compressed packet that triggered the feedback.
Context damage is detected according to the principles in 5.3.2.2.3.
When the decompressor is capable of timer-based compression of the
RTP Timestamp (e.g., it has access to a clock with sufficient
resolution, and the jitter introduced internally in the receiving
node is sufficiently small) it SHOULD signal that it is ready to do
timer-based compression of the RTP Timestamp. The compressor will
then make a decision based on its knowledge of the channel and the
observed properties of the packet stream.
5.6. Mode transitions
The decision to move from one compression mode to another is taken by
the decompressor and the possible mode transitions are shown in the
figure below. Subsequent chapters describe how the transitions are
performed together with exceptions for the compression and
decompression functionality during transitions.
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+-------------------------+
| Unidirectional (U) mode |
+-------------------------+
/ ^ \ ^
/ / Feedback(U) \ \ Feedback(U)
/ / \ \
/ / \ \
Feedback(O) / / Feedback(R) \ \
v / v \
+---------------------+ Feedback(R) +-------------------+
| Optimistic (O) mode | ----------------> | Reliable (R) mode |
| | <---------------- | |
+---------------------+ Feedback(O) +-------------------+
5.6.1. Compression and decompression during mode transitions
The following sections assume that, for each context, the compressor
and decompressor maintain a variable whose value is the current
compression mode for that context. The value of the variable
controls, for the context in question, which packet types to use,
which actions to be taken, etc.
As a safeguard against residual errors, all feedback sent during a
mode transition MUST be protected by a CRC, i.e., the CRC option MUST
be used. A mode transition MUST NOT be initiated by feedback which
is not protected by a CRC.
The subsequent subsections define exactly when to change the value of
the MODE variable. When ROHC transits between compression modes,
there are several cases where the behavior of compressor or
decompressor must be restricted during the transition phase. These
restrictions are defined by exception parameters that specify which
restrictions to apply. The transition descriptions in subsequent
chapters refer to these exception parameters and defines when they
are set and to what values. All mode related parameters are listed
below together with their possible values, with explanations and
restrictions:
Parameters for the compressor side:
- C_MODE:
Possible values for the C_MODE parameter are (U)NIDIRECTIONAL,
(O)PTIMISTIC and (R)ELIABLE. C_MODE MUST be initialized to U.
- C_TRANS:
Possible values for the C_TRANS parameter are (P)ENDING and
(D)ONE. C_TRANS MUST be initialized to D. When C_TRANS is P,
it is REQUIRED
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1) that the compressor only use packet formats common to all
modes,
2) that mode information is included in packets sent, at least
periodically,
3) that the compressor not transit to the SO state,
4) that new mode transition requests be ignored.
Parameters for the decompressor side:
- D_MODE:
Possible values for the D_MODE parameter are (U)NIDIRECTIONAL,
(O)PTIMISTIC and (R)ELIABLE. D_MODE MUST be initialized to U.
- D_TRANS:
Possible values for the D_TRANS parameter are (I)NITIATED,
(P)ENDING and (D)ONE. D_TRANS MUST be initialized to D. A
mode transition can be initiated only when D_TRANS is D. While
D_TRANS is I, the decompressor sends a NACK or ACK carrying a
CRC option for each received packet.
5.6.2. Transition from Unidirectional to Optimistic mode
When there is a feedback channel available, the decompressor may at
any moment decide to initiate transition from Unidirectional to
Bidirectional Optimistic mode. Any feedback packet carrying a CRC
can be used with the mode parameter set to O. The decompressor can
then directly start working in Optimistic mode. The compressor
transits from Unidirectional to Optimistic mode as soon as it
receives any feedback packet that has the mode parameter set to O and
that passes the CRC check. The transition procedure is described
below:
Compressor Decompressor
----------------------------------------------
| |
| ACK(O)/NACK(O) +-<-<-<-| D_MODE = O
| +-<-<-<-<-<-<-<-+ |
C_MODE = O |-<-<-<-+ |
| |
If the feedback packet is lost, the compressor will continue to work
in Unidirectional mode, but as soon as any feedback packet reaches
the compressor it will transit to Optimistic mode.
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5.6.3. From Optimistic to Reliable mode
Transition from Optimistic to Reliable mode is permitted only after
at least one packet has been correctly decompressed, which means that
at least the static part of the context is established. An ACK(R) or
a NACK(R) feedback packet carrying a CRC is sent to initiate the mode
transition. The compressor MUST NOT use packet types 0 or 1 during
transition. The transition procedure is described below:
Compressor Decompressor
----------------------------------------------
| |
| ACK(R)/NACK(R) +-<-<-<-| D_TRANS = I
| +-<-<-<-<-<-<-<-+ |
C_TRANS = P |-<-<-<-+ |
C_MODE = R | |
|->->->-+ IR/IR-DYN/UOR-2(SN,R) |
| +->->->->->->->-+ |
|->-.. +->->->-| D_TRANS = P
|->-.. | D_MODE = R
| ACK(SN,R) +-<-<-<-|
| +-<-<-<-<-<-<-<-+ |
C_TRANS = D |-<-<-<-+ |
| |
|->->->-+ R-0*, R-1* |
| +->->->->->->->-+ |
| +->->->-| D_TRANS = D
| |
As long as the decompressor has not received an UOR-2, IR-DYN, or IR
packet with the mode transition parameter set to R, it must stay in
Optimistic mode. The compressor must not send packet types 1 or 0
while C_TRANS is P, i.e., not until it has received an ACK for a
UOR-2, IR-DYN, or IR packet sent with the mode transition parameter
set to R. When the decompressor receives packet types 0 or 1, after
having ACKed an UOR-2, IR-DYN, or IR packet, it sets D_TRANS to D.
5.6.4. From Unidirectional to Reliable mode
The transition from Unidirectional to Reliable mode follows the same
transition procedure as defined in section 5.6.3 above.
5.6.5. From Reliable to Optimistic mode
Either the ACK(O) or the NACK(O) feedback packet is used to initiate
the transition from Reliable to Optimistic mode and the compressor
MUST always run in the FO state during transition. The transition
procedure is described below:
Bormann, et al. Standards Track [Page 72]
RFC 3095 Robust Header Compression July 2001
Compressor Decompressor
----------------------------------------------
| |
| ACK(O)/NACK(O) +-<-<-<-| D_TRANS = I
| +-<-<-<-<-<-<-<-+ |
C_TRANS = P |-<-<-<-+ |
C_MODE = O | |
|->->->-+ IR/IR-DYN/UOR-2(SN,O) |
| +->->->->->->->-+ |
|->-.. +->->->-| D_MODE = O
|->-.. |
| ACK(SN,O) +-<-<-<-|
| +-<-<-<-<-<-<-<-+ |
C_TRANS = D |-<-<-<-+ |
| |
|->->->-+ UO-0, UO-1* |
| +->->->->->->->-+ |
| +->->->-| D_TRANS = D
| |
As long as the decompressor has not received an UOR-2, IR-DYN, or IR
packet with the mode transition parameter set to O, it must stay in
Reliable mode. The compressor must not send packet types 0 or 1
while C_TRANS is P, i.e., not until it has received an ACK for an
UOR-2, IR-DYN, or IR packet sent with the mode transition parameter
set to O. When the decompressor receives packet types 0 or 1, after
having ACKed the UOR-2, IR-DYN, or IR packet, it sets D_TRANS to D.
5.6.6. Transition to Unidirectional mode
The decompressor can force a transition back to Unidirectional mode
if it desires to do so. Regardless of which mode this transition
starts from, a three-way handshake MUST be carried out to ensure
correct transition on the compressor side. The transition procedure
is described below:
Bormann, et al. Standards Track [Page 73]
RFC 3095 Robust Header Compression July 2001
Compressor Decompressor
----------------------------------------------
| |
| ACK(U)/NACK(U) +-<-<-<-| D_TRANS = I
| +-<-<-<-<-<-<-<-+ |
C_TRANS = P |-<-<-<-+ |
C_MODE = U | |
|->->->-+ IR/IR-DYN/UOR-2(SN,U) |
| +->->->->->->->-+ |
|->-.. +->->->-|
|->-.. |
| ACK(SN,U) +-<-<-<-|
| +-<-<-<-<-<-<-<-+ |
C_TRANS = D |-<-<-<-+ |
| |
|->->->-+ UO-0, UO-1* |
| +->->->->->->->-+ |
| +->->->-| D_TRANS = D, D_MODE= U
After ACKing the first UOR-2(U), IR-DYN(U), or IR(U), the
decompressor MUST continue to send feedback with the Mode parameter
set to U until it receives packet types 0 or 1.
5.7. Packet formats
The following notation is used in this section:
bits(X) = the number of bits for field X present in the compressed
header (including extension).
field(X) = the value of field X in the compressed header.
context(X) = the value of field X as established in the context.
value(X) = field(X) if X is present in the compressed header;
= context(X) otherwise.
hdr(X) = the value of field X in the uncompressed or
decompressed header.
Updating properties: Lists the fields in the context that are
directly updated by processing the compressed header. Note
that there may be dependent fields that are implicitly also
updated (e.g., an update to context(SN) often updates
context(TS) as well). See also section 5.2.7.
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The following fields occur in several headers and extensions:
SN: The compressed RTP Sequence Number.
Compressed with W-LSB. The interpretation intervals, see section
4.5.1, are defined as follows:
p = 1 if bits(SN) <= 4
p = 2^(bits(SN)-5) - 1 if bits(SN) > 4
IP-ID: A compressed IP-ID field.
IP-ID fields in compressed base headers carry the compressed IP-ID
of the innermost IPv4 header whose corresponding RND flag is not
1. The rules below assume that the IP-ID is for the innermost IP
header. If it is for an outer IP header, the RND2 and NBO2 flags
are used instead of RND and NBO.
If value(RND) = 0, hdr(IP-ID) is compressed using Offset IP-ID
encoding (see section 4.5.5) using p = 0 and default-slope(IP-ID
offset) = 0.
If value(RND) = 1, IP-ID is the uncompressed hdr(IP-ID). IP-ID is
then passed as additional octets at the end of the compressed
header, after any extensions.
If value(NBO) = 0, the octets of hdr(IP-ID) are swapped before
compression and after decompression. The value of NBO is ignored
when value(RND) = 1.
TS: The compressed RTP Timestamp value.
If value(TIME_STRIDE) > 0, timer-based compression of the RTP
Timestamp is used (see section 4.5.4).
If value(Tsc) = 1, Scaled RTP Timestamp encoding is used before
compression (see section 4.5.3), and default-slope(TS) = 1.
If value(Tsc) = 0, the Timestamp value is compressed as-is, and
default-slope(TS) = value(TS_STRIDE).
The interpretation intervals, see section 4.5.1, are defined as
follows:
p = 2^(bits(TS)-2) - 1
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CRC: The CRC over the original, uncompressed, header.
For 3-bit CRCs, the polynomial of section 5.9.2 is used.
For 7-bit CRCs, the polynomial of section 5.9.2 is used.
For 8-bit CRCs, the polynomial of section 5.9.1 is used.
M: RTP Marker bit.
Context(M) is initially zero and is never updated. value(M) = 1
only when field(M) = 1.
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The general format for a compressed RTP header is as follows:
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for small CIDs and CID 1-15
+---+---+---+---+---+---+---+---+
| first octet of base header | (with type indication)
+---+---+---+---+---+---+---+---+
: :
/ 0, 1, or 2 octets of CID / 1-2 octets if large CIDs
: :
+---+---+---+---+---+---+---+---+
/ remainder of base header / variable number of bits
+---+---+---+---+---+---+---+---+
: :
/ Extension (see 5.7.5) / extension, if X = 1 in base header
: :
--- --- --- --- --- --- --- ---
: :
+ IP-ID of outer IPv4 header + 2 octets, if value(RND2) = 1
: :
--- --- --- --- --- --- --- ---
/ AH data for outer list / variable (see 5.8.4.2)
--- --- --- --- --- --- --- ---
: :
+ GRE checksum (see 5.8.4.4) + 2 octets, if GRE flag C = 1
: :
--- --- --- --- --- --- --- ---
: :
+ IP-ID of inner IPv4 header + 2 octets, if value(RND) = 1
: :
--- --- --- --- --- --- --- ---
/ AH data for inner list / variable (see 5.8.4.2)
--- --- --- --- --- --- --- ---
: :
+ GRE checksum (see 5.8.4.4) + 2 octets, if GRE flag C = 1
: :
--- --- --- --- --- --- --- ---
: :
+ UDP Checksum + 2 octets,
: : if context(UDP Checksum) != 0
--- --- --- --- --- --- --- ---
Note that the order of the fields following the optional extension is
the same as the order between the fields in an uncompressed header.
In subsequent sections, the position of the large CID in the diagrams
is indicated using this notation:
Bormann, et al. Standards Track [Page 77]
RFC 3095 Robust Header Compression July 2001
+===+===+===+===+===+===+===+===+
Whether the UDP Checksum field is present or not is controlled by the
value of the UDP Checksum in the context. If nonzero, the UDP
Checksum is enabled and sent along with each packet. If zero, the
UDP Checksum is disabled and not sent. Should hdr(UDP Checksum) be
nonzero when context(UDP Checksum) is zero, the header cannot be
compressed. It must be sent uncompressed or the context
reinitialized using an IR packet. Context(UDP Checksum) is updated
only by IR or IR-DYN headers, never by UDP checksums sent in headers
of type 2, 1, or 0.
When an IPv4 header is present in the static context, for which the
corresponding RND flag has not been established to be 1, the packet
types R-1 and UO-1 MUST NOT be used.
When no IPv4 header is present in the static context, or the RND
flags for all IPv4 headers in the context have been established to be
1, the packet types R-1-ID, R-1-TS, UO-1-ID, and UO-1-TS MUST NOT be
used.
While in the transient state in which an RND flag is being
established, the packet types R-1-ID, R-1-TS, UO-1-ID, and UO-1-TS
MUST NOT be used. This implies that the RND flag(s) of the Extension
3 may have to be inspected before the format of a base header
carrying an Extension 3 can be determined.
5.7.1. Packet type 0: UO-0, R-0, R-0-CRC
Packet type 0 is indicated by the first bit being 0:
R-0
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 0 | SN |
+===+===+===+===+===+===+===+===+
Updating properties: R-0 packets do not update any part of the
context.
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RFC 3095 Robust Header Compression July 2001
R-0-CRC
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 1 | SN |
+===+===+===+===+===+===+===+===+
|SN | CRC |
+---+---+---+---+---+---+---+---+
Note: The SN field straddles the CID field.
Updating properties: R-0-CRC packets update context(RTP Sequence
Number).
UO-0
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | SN | CRC |
+===+===+===+===+===+===+===+===+
Updating properties: UO-0 packets update the current value of
context(RTP Sequence Number).
5.7.2. Packet type 1 (R-mode): R-1, R-1-TS, R-1-ID
Packet type 1 is indicated by the first bits being 10:
R-1
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 0 | SN |
+===+===+===+===+===+===+===+===+
| M | X | TS |
+---+---+---+---+---+---+---+---+
Note: R-1 cannot be used if the context contains at least one IPv4
header with value(RND) = 0. This disambiguates it from R-1-ID and
R-1-TS.
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RFC 3095 Robust Header Compression July 2001
R-1-ID
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 0 | SN |
+===+===+===+===+===+===+===+===+
| M | X |T=0| IP-ID |
+---+---+---+---+---+---+---+---+
Note: R-1-ID cannot be used if there is no IPv4 header in the
context or if value(RND) and value(RND2) are both 1.
R-1-TS
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 0 | SN |
+===+===+===+===+===+===+===+===+
| M | X |T=1| TS |
+---+---+---+---+---+---+---+---+
Note: R-1-TS cannot be used if there is no IPv4 header in the
context or if value(RND) and value(RND2) are both 1.
X: X = 0 indicates that no extension is present;
X = 1 indicates that an extension is present.
T: T = 0 indicates format R-1-ID;
T = 1 indicates format R-1-TS.
Updating properties: R-1* headers do not update any part of the
context.
5.7.3. Packet type 1 (U/O-mode): UO-1, UO-1-ID, UO-1-TS
UO-1
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 0 | TS |
+===+===+===+===+===+===+===+===+
| M | SN | CRC |
+---+---+---+---+---+---+---+---+
Note: UO-1 cannot be used if the context contains at least one
IPv4 header with value(RND) = 0. This disambiguates it from UO-
1-ID and UO-1-TS.
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RFC 3095 Robust Header Compression July 2001
UO-1-ID
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 0 |T=0| IP-ID |
+===+===+===+===+===+===+===+===+
| X | SN | CRC |
+---+---+---+---+---+---+---+---+
Note: UO-1-ID cannot be used if there is no IPv4 header in the
context or if value(RND) and value(RND2) are both 1.
UO-1-TS
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 0 |T=1| TS |
+===+===+===+===+===+===+===+===+
| M | SN | CRC |
+---+---+---+---+---+---+---+---+
Note: UO-1-TS cannot be used if there is no IPv4 header in the
context or if value(RND) and value(RND2) are both 1.
X: X = 0 indicates that no extension is present;
X = 1 indicates that an extension is present.
T: T = 0 indicates format UO-1-ID;
T = 1 indicates format UO-1-TS.
Updating properties: UO-1* packets update context(RTP Sequence
Number). UO-1 and UO-1-TS packets update context(RTP Timestamp).
UO-1-ID packets update context(IP-ID). Values provided in
extensions, except those in other SN, TS, or IP-ID fields, do not
update the context.
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5.7.4. Packet type 2: UOR-2
Packet type 2 is indicated by the first bits being 110:
UOR-2
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 1 0 | TS |
+===+===+===+===+===+===+===+===+
|TS | M | SN |
+---+---+---+---+---+---+---+---+
| X | CRC |
+---+---+---+---+---+---+---+---+
Note: UOR-2 cannot be used if the context contains at least one
IPv4 header with value(RND) = 0. This disambiguates it from UOR-
2-ID and UOR-2-TS.
Note: The TS field straddles the CID field.
UOR-2-ID
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 1 0 | IP-ID |
+===+===+===+===+===+===+===+===+
|T=0| M | SN |
+---+---+---+---+---+---+---+---+
| X | CRC |
+---+---+---+---+---+---+---+---+
Note: UOR-2-ID cannot be used if there is no IPv4 header in the
context or if value(RND) and value(RND2) are both 1.
UOR-2-TS
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 1 0 | TS |
+===+===+===+===+===+===+===+===+
|T=1| M | SN |
+---+---+---+---+---+---+---+---+
| X | CRC |
+---+---+---+---+---+---+---+---+
Note: UOR-2-TS cannot be used if there is no IPv4 header in the
context or if value(RND) and value(RND2) are both 1.
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X: X = 0 indicates that no extension is present;
X = 1 indicates that an extension is present.
T: T = 0 indicates format UOR-2-ID;
T = 1 indicates format UOR-2-TS.
Updating properties: All values provided in UOR-2* packets update
the context, unless explicitly stated otherwise.
5.7.5. Extension formats
(Note: the term extension as used for additional information
contained in the ROHC headers does not bear any relationship to the
term extension header used in IP.)
Fields in extensions are concatenated with the corresponding field in
the base compressed header, if there is one. Bits in an extension
are less significant than bits in the base compressed header (see
section 4.5.7).
The TS field is scaled in all extensions, as it is in the base
header, except optionally when using Extension 3 where the Tsc flag
can indicate that the TS field is not scaled. Value(TS_STRIDE) is
used as the scale factor when scaling the TS field.
In the following three extensions, the interpretation of the fields
depends on whether there is a T-bit in the base compressed header,
and if so, on the value of that field. When there is no T-bit, +T
and -T both mean TS. This is the case when there are no IPv4 headers
in the static context, and when all IPv4 headers in the static
context have their corresponding RND flag set (i.e., RND = 1).
If there is a T-bit,
T = 1 indicates that +T is TS, and
-T is IP-ID;
T = 0 indicates that +T is IP-ID, and
-T is TS.
Extension 0:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 0 | SN | +T |
+---+---+---+---+---+---+---+---+
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RFC 3095 Robust Header Compression July 2001
Extension 1:
+---+---+---+---+---+---+---+---+
| 0 1 | SN | +T |
+---+---+---+---+---+---+---+---+
| -T |
+---+---+---+---+---+---+---+---+
Extension 2:
+---+---+---+---+---+---+---+---+
| 1 0 | SN | +T |
+---+---+---+---+---+---+---+---+
| +T |
+---+---+---+---+---+---+---+---+
| -T |
+---+---+---+---+---+---+---+---+
Extension 3 is a more elaborate extension which can give values for
fields other than SN, TS, and IP-ID. Three optional flag octets
indicate changes to IP header(s) and RTP header, respectively.
Bormann, et al. Standards Track [Page 84]
RFC 3095 Robust Header Compression July 2001
Extension 3:
0 1 2 3 4 5 6 7
+-----+-----+-----+-----+-----+-----+-----+-----+
| 1 1 | S |R-TS | Tsc | I | ip | rtp | (FLAGS)
+-----+-----+-----+-----+-----+-----+-----+-----+
| Inner IP header flags | ip2 | if ip = 1
..... ..... ..... ..... ..... ..... ..... .....
| Outer IP header flags | if ip2 = 1
..... ..... ..... ..... ..... ..... ..... .....
| SN | if S = 1
..... ..... ..... ..... ..... ..... ..... .....
/ TS (encoded as in section 4.5.6) / 1-4 octets,
..... ..... ..... ..... ..... ..... ..... ..... if R-TS = 1
| |
/ Inner IP header fields / variable,
| | if ip = 1
..... ..... ..... ..... ..... ..... ..... .....
| IP-ID | 2 octets, if I = 1
..... ..... ..... ..... ..... ..... ..... .....
| |
/ Outer IP header fields / variable,
| | if ip2 = 1
..... ..... ..... ..... ..... ..... ..... .....
| |
/ RTP header flags and fields / variable,
| | if rtp = 1
..... ..... ..... ..... ..... ..... ..... .....
S, R-TS, I, ip, rtp, ip2: Indicate presence of fields as shown to
the right of each field above.
Tsc: Tsc = 0 indicates that TS is not scaled;
Tsc = 1 indicates that TS is scaled according to section
4.5.3, using value(TS_STRIDE).
Context(Tsc) is always 1. If scaling is not desired, the
compressor will establish TS_STRIDE = 1.
SN: See the beginning of section 5.7.
TS: Variable number of bits of TS, encoded according to
section 4.5.6. See the beginning of section 5.7.
IP-ID: See the beginning of section 5.7.
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Inner IP header flags
These correspond to the inner IP header if there are two, and the
single IP header otherwise.
0 1 2 3 4 5 6 7
..... ..... ..... ..... ..... ..... ..... .....
| TOS | TTL | DF | PR | IPX | NBO | RND | ip2 | if ip = 1
..... ..... ..... ..... ..... ..... ..... .....
TOS, TTL, PR, IPX: Indicates presence of fields as shown to the
right of the field in question below.
DF: Don't Fragment bit of IP header.
NBO: Indicates whether the octets of hdr(IP identifier) of this IP
header are swapped before compression and after decompression.
NBO = 1 indicates that the octets need not be swapped. NBO = 0
indicates that the octets are to be swapped. See section 4.5.5.
RND: Indicates whether hdr(IP identifier) is not to be compressed
but instead sent as-is in compressed headers.
IP2: Indicates presence of Outer IP header fields. Unless the
static context contains two IP headers, IP2 is always zero.
Inner IP header fields
..... ..... ..... ..... ..... ..... ..... .....
| Type of Service/Traffic Class | if TOS = 1
..... ..... ..... ..... ..... ..... ..... .....
| Time to Live/Hop Limit | if TTL = 1
..... ..... ..... ..... ..... ..... ..... .....
| Protocol/Next Header | if PR = 1
..... ..... ..... ..... ..... ..... ..... .....
/ IP extension headers / variable,
..... ..... ..... ..... ..... ..... ..... ..... if IPX = 1
Type of Service/Traffic Class: That field in the uncompressed IP
header (absolute value).
Time to Live/Hop Limit: That field in the uncompressed IP header.
Protocol/Next Header: That field in the uncompressed IP header.
IP extension header(s): According to section 5.8.5.
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Outer IP header flags
The fields in this part of the Extension 3 header refer to the
outermost IP header:
0 1 2 3 4 5 6 7
..... ..... ..... ..... ..... ..... ..... ..... | TOS2| TTL2|
DF2 | PR2 |IPX2 |NBO2 |RND2 | I2 | if ip2 = 1
..... ..... ..... ..... ..... ..... ..... .....
These flags are the same as the Inner IP header flags, but refer
to the outer IP header instead of the inner IP header. The
following flag, however, has no counterpart in the Inner IP header
flags:
I2: Indicates presence of the IP-ID field.
Outer IP header fields
..... ..... ..... ..... ..... ..... ..... .....
| Type of Service/Traffic Class | if TOS2 = 1
..... ..... ..... ..... ..... ..... ..... .....
| Time to Live/Hop Limit | if TTL2 = 1
..... ..... ..... ..... ..... ..... ..... .....
| Protocol/Next Header | if PR2 = 1
..... ..... ..... ..... ..... ..... ..... .....
/ IP extension header(s) / variable,
..... ..... ..... ..... ..... ..... ..... ..... if IPX2 = 1
| IP-ID | 2 octets,
..... ..... ..... ..... ..... ..... ..... ..... if I2 = 1
The fields in this part of Extension 3 are as for the Inner IP
header fields, but they refer to the outer IP header instead of
the inner IP header. The following field, however, has no
counterpart among the Inner IP header fields:
IP-ID: The IP Identifier field of the outer IP header, unless
the inner header is an IPv6 header, in which case I2 is always
zero.
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RFC 3095 Robust Header Compression July 2001
RTP header flags and fields
0 1 2 3 4 5 6 7
..... ..... ..... ..... ..... ..... ..... .....
| Mode |R-PT | M | R-X |CSRC | TSS | TIS | if rtp = 1
..... ..... ..... ..... ..... ..... ..... .....
| R-P | RTP PT | if R-PT = 1
..... ..... ..... ..... ..... ..... ..... .....
/ Compressed CSRC list / if CSRC = 1
..... ..... ..... ..... ..... ..... ..... .....
/ TS_STRIDE / 1-4 oct if TSS = 1
..... ..... ..... ..... ..... ..... ..... ....
/ TIME_STRIDE (milliseconds) / 1-4 oct if TIS = 1
..... ..... ..... ..... ..... ..... ..... .....
Mode: Compression mode. 0 = Reserved,
1 = Unidirectional,
2 = Bidirectional Optimistic,
3 = Bidirectional Reliable.
R-PT, CSRC, TSS, TIS: Indicate presence of fields as shown to the
right of each field above.
R-P: RTP Padding bit, absolute value (presumed zero if absent).
R-X: RTP eXtension bit, absolute value.
M: See the beginning of section 5.7.
RTP PT: Absolute value of RTP Payload type field.
Compressed CSRC list: See section 5.8.1.
TS_STRIDE: Predicted increment/decrement of the RTP Timestamp
field when it changes. Encoded as in section 4.5.6.
TIME_STRIDE: Predicted time interval in milliseconds between
changes in the RTP Timestamp. Also an indication that the
compressor desires to perform timer-based compression of the RTP
Timestamp field: see section 4.5.4. Encoded as in section 4.5.6.
5.7.5.1. RND flags and packet types
The values of the RND and RND2 flags are changed by sending UOR-2
headers with Extension 3, or IR-DYN headers, where the flag(s) have
their new values. The establishment procedure of the flags is the
normal one for the current mode, i.e., in U-mode and O-mode the
values are repeated several times to ensure that the decompressor
Bormann, et al. Standards Track [Page 88]
RFC 3095 Robust Header Compression July 2001
receives at least one. In R-mode, the flags are sent until an
acknowledgment for a packet with the new RND flag values is received.
The decompressor updates the values of its RND and RND2 flags
whenever it receives an UOR-2 with Extension 3 carrying values for
RND or RND2, and the UOR-2 CRC verifies successful decompression.
When an IPv4 header for which the corresponding RND flag has not been
established to be 1 is present in the static context, the packet
types R-1 and UO-1 MUST NOT be used.
When no IPv4 header is present in the static context, or the RND
flags for all IPv4 headers in the context have been established to be
1, the packet types R-1-ID, R-1-TS, UO-1-ID, and UO-1-TS MUST NOT be
used.
While in the transient state in which an RND flag is being
established, the packet types R-1-ID, R-1-TS, UO-1-ID, and UO-1-TS
MUST NOT be used. This implies that the RND flag(s) of Extension 3
may have to be inspected before the exact format of a base header
carrying an Extension 3 can be determined, i.e., whether a T-bit is
present or not.
5.7.5.2. Flags/Fields in context
Some flags and fields in Extension 3 need to be maintained in the
context of the decompressor. Their values are established using the
mechanism appropriate to the compression mode, unless otherwise
indicated in the table below and in referred sections.
Flag/Field Initial value Comment
---------------------------------------------------------------------
Mode Unidirectional See section 5.6
NBO 1 See section 4.5.5
RND 0 See sections 4.5.5, 5.7.5.1
NBO2 1 As NBO, but for outer header
RND2 0 As RND, but for outer header
TS_STRIDE 1 See section 4.5.3
TIME_STRIDE 0 See section 4.5.4
Tsc 1 Tsc is always 1 in context;
can be 0 only when an Extension 3
is present. See the discussion of the
TS field in the beginning of section
5.7.
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RFC 3095 Robust Header Compression July 2001
5.7.6. Feedback packets and formats
When the round-trip time between compressor and decompressor is
large, several packets can be in flight concurrently. Therefore,
several packets may be received by the decompressor after feedback
has been sent and before the compressor has reacted to feedback.
Moreover, decompression may fail due to residual errors in the
compressed header.
Therefore,
a) in O-mode, the decompressor SHOULD limit the rate at which
feedback on successful decompression is sent (if it is sent at
all);
b) when decompression fails, feedback SHOULD be sent only when
decompression of several consecutive packets has failed, and when
this occurs, the feedback rate SHOULD be limited;
c) when packets are received which belong to a rejected packet
stream, the feedback rate SHOULD be limited.
A decompressor MAY limit the feedback rate by sending feedback only
for one out of every k packets provoking the same (kind of) feedback.
The appropriate value of k is implementation dependent; k might be
chosen such that feedback is sent 1-3 times per link round-trip time.
See section 5.2.2 for a discussion concerning ways to provide
feedback information to the compressor.
5.7.6.1. Feedback formats for ROHC RTP
This section describes the format for feedback information in ROHC
RTP. See also 5.2.2.
Several feedback formats carry a field labeled SN. The SN field
contains LSBs of an RTP Sequence Number. The sequence number to use
is the sequence number of the header which caused the feedback
information to be sent. If that sequence number cannot be
determined, for example when decompression fails, the sequence number
to use is that of the last successfully decompressed header. If no
sequence number is available, the feedback MUST carry a SN-NOT-VALID
option. Upon reception, the compressor matches valid SN LSBs with
the most recent header sent with a SN with matching LSBs. The
decompressor must ensure that it sends enough SN LSBs in its feedback
that this correlation does not become ambiguous; e.g., if an 8-bit SN
LSB field could wrap around within a round-trip time, the FEEDBACK-1
format cannot be used.
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RFC 3095 Robust Header Compression July 2001
FEEDBACK-1
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| SN |
+---+---+---+---+---+---+---+---+
A FEEDBACK-1 is an ACK. In order to send a NACK or a STATIC-NACK,
FEEDBACK-2 must be used. FEEDBACK-1 does not contain any mode
information; FEEDBACK-2 must be used when mode information is
required.
FEEDBACK-2
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
|Acktype| Mode | SN |
+---+---+---+---+---+---+---+---+
| SN |
+---+---+---+---+---+---+---+---+
/ Feedback options /
+---+---+---+---+---+---+---+---+
Acktype: 0 = ACK
1 = NACK
2 = STATIC-NACK
3 is reserved (MUST NOT be used for parseability)
Mode: 0 is reserved
1 = Unidirectional mode
2 = Bidirectional Optimistic mode
3 = Bidirectional Reliable mode
Feedback options: A variable number of feedback options, see
section 5.7.6.2. Options may appear in any order.
5.7.6.2. ROHC RTP Feedback options
A ROHC RTP Feedback option has variable length and the following
general format:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Opt Type | Opt Len |
+---+---+---+---+---+---+---+---+
/ option data / Opt Len octets
+---+---+---+---+---+---+---+---+
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RFC 3095 Robust Header Compression July 2001
Sections 5.7.6.3-9 describe the currently defined ROHC RTP feedback
options.
5.7.6.3. The CRC option
The CRC option contains an 8-bit CRC computed over the entire
feedback payload, without the packet type and code octet, but
including any CID fields, using the polynomial of section 5.9.1. If
the CID is given with an Add-CID octet, the Add-CID octet immediately
precedes the FEEDBACK-1 or FEEDBACK-2 format. For purposes of
computing the CRC, the CRC fields of all CRC options are zero.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| Opt Type = 1 | Opt Len = 1 |
+---+---+---+---+---+---+---+---+
| CRC |
+---+---+---+---+---+---+---+---+
When receiving feedback information with a CRC option, the compressor
MUST verify the information by computing the CRC and comparing the
result with the CRC carried in the CRC option. If the two are not
identical, the feedback information MUST be ignored.
5.7.6.4. The REJECT option
The REJECT option informs the compressor that the decompressor does
not have sufficient resources to handle the flow.
+---+---+---+---+---+---+---+---+
| Opt Type = 2 | Opt Len = 0 |
+---+---+---+---+---+---+---+---+
When receiving a REJECT option, the compressor stops compressing the
packet stream, and should refrain from attempting to increase the
number of compressed packet streams for some time. Any FEEDBACK
packet carrying a REJECT option MUST also carry a CRC option.
5.7.6.5. The SN-NOT-VALID option
The SN-NOT-VALID option indicates that the SN of the feedback is not
valid. A compressor MUST NOT use the SN of the feedback to find the
corresponding sent header when this option is present.
+---+---+---+---+---+---+---+---+
| Opt Type = 3 | Opt Len = 0 |
+---+---+---+---+---+---+---+---+
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RFC 3095 Robust Header Compression July 2001
5.7.6.6. The SN option
The SN option provides 8 additional bits of SN.
+---+---+---+---+---+---+---+---+
| Opt Type = 4 | Opt Len = 1 |
+---+---+---+---+---+---+---+---+
| SN |
+---+---+---+---+---+---+---+---+
5.7.6.7. The CLOCK option
The CLOCK option informs the compressor of the clock resolution of
the decompressor. This is needed to allow the compressor to estimate
the jitter introduced by the clock of the decompressor when doing
timer-based compression of the RTP Timestamp.
+---+---+---+---+---+---+---+---+
| Opt Type = 5 | Opt Len = 1 |
+---+---+---+---+---+---+---+---+
| clock resolution (ms) |
+---+---+---+---+---+---+---+---+
The smallest clock resolution which can be indicated is 1
millisecond. The value zero has a special meaning: it indicates that
the decompressor cannot do timer-based compression of the RTP
Timestamp. Any FEEDBACK packet carrying a CLOCK option SHOULD also
carry a CRC option.
5.7.6.8. The JITTER option
The JITTER option allows the decompressor to report the maximum
jitter it has observed lately, using the following formula which is
very similar to the formula for Max_Jitter_BC in section 4.5.4.
Let observation window i contain the decompressor's best
approximation of the sliding window of the compressor (see section
4.5.4) when header i is received.
Max_Jitter_i =
max {|(T_i - T_j) - ((a_i - a_j) / TIME_STRIDE)|,
for all headers j in observation window i}
Max_Jitter =
max { Max_Jitter_i, for a large number of recent headers i }
Bormann, et al. Standards Track [Page 93]
RFC 3095 Robust Header Compression July 2001
This information may be used by the compressor to refine the formula
for determining k when doing timer-based compression of the RTP
Timestamp.
+---+---+---+---+---+---+---+---+
| Opt Type = 6 | Opt Len = 1 |
+---+---+---+---+---+---+---+---+
| Max_Jitter |
+---+---+---+---+---+---+---+---+
The decompressor MAY ignore the oldest observed values of
Max_Jitter_i. Thus, the reported Max_Jitter may decrease.
Robustness will be reduced if the compressor uses a jitter estimate
which is too small. Therefore, a FEEDBACK packet carrying a JITTER
option SHOULD also carry a CRC option. Moreover, the compressor MAY
ignore decreasing Max_Jitter values.
5.7.6.9. The LOSS option
The LOSS option allows the decompressor to report the largest
observed number of packets lost in sequence. This information MAY be
used by the compressor to adjust the size of the reference window
used in U- and O-mode.
+---+---+---+---+---+---+---+---+
| Opt Type = 7 | Opt Len = 1 |
+---+---+---+---+---+---+---+---+
| longest loss event (packets) |
+---+---+---+---+---+---+---+---+
The decompressor MAY choose to ignore the oldest loss events. Thus,
the value reported may decrease. Since setting the reference window
too small can reduce robustness, a FEEDBACK packet carrying a LOSS
option SHOULD also carry a CRC option. The compressor MAY choose to
ignore decreasing loss values.
5.7.6.10. Unknown option types
If an option type unknown to the compressor is encountered, it must
continue parsing the rest of the FEEDBACK packet, which is possible
since the length of the option is explicit, but MUST otherwise ignore
the unknown option.
5.7.6.11. RTP feedback example
Feedback for CID 8 indicating an ACK for SN 17 and Bidirectional
Reliable mode can have the following formats.
Bormann, et al. Standards Track [Page 94]
RFC 3095 Robust Header Compression July 2001
Assuming small CIDs:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 1 1 1 0 | 0 1 1 | feedback packet type, Code = 3
+---+---+---+---+---+---+---+---+
| 1 1 1 0 | 1 0 0 0 | Add-CID octet with CID = 8
+---+---+---+---+---+---+---+---+
| 0 0 | 1 1 | SN MSB = 0 | AckType = ACK, Mode = Reliable
+---+---+---+---+---+---+---+---+
| SN LSB = 17 |
+---+---+---+---+---+---+---+---+
The second, third, and fourth octet are handed to the compressor.
The FEEDBACK-1 format may also be used. Assuming large CIDs:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 1 1 1 0 | 0 1 0 | feedback packet type, Code = 2
+---+---+---+---+---+---+---+---+
| 0 0 0 0 1 0 0 0 | large CID with value 8
+---+---+---+---+---+---+---+---+
| SN LSB = 17 |
+---+---+---+---+---+---+---+---+
The second and third octet are handed to the compressor.
Assuming small CIDs:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 1 1 1 0 | 0 1 0 | feedback packet type, Code = 2
+---+---+---+---+---+---+---+---+
| 1 1 1 0 | 1 0 0 0 | Add-CID octet with CID = 8
+---+---+---+---+---+---+---+---+
| SN LSB = 17 |
+---+---+---+---+---+---+---+---+
The second and third octet are handed to the compressor.
Bormann, et al. Standards Track [Page 95]
RFC 3095 Robust Header Compression July 2001
Assuming small CIDs and CID 0 instead of CID 8:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 1 1 1 0 | 0 0 1 | feedback packet type, Code = 1
+---+---+---+---+---+---+---+---+
| SN LSB = 17 |
+---+---+---+---+---+---+---+---+
The second octet is handed to the compressor.
5.7.7. RTP IR and IR-DYN packets
The subheaders which are compressible are split into a STATIC part
and a DYNAMIC part. These parts are defined in sections 5.7.7.3
through 5.7.7.7.
The structure of a chain of subheaders is determined by each header
having a Next Header, or Protocol, field. This field identifies the
type of the following header. Each Static part below that is
followed by another Static part contains the Next Header/Protocol
field and allows parsing of the Static chain; the Dynamic chain, if
present, is structured analogously.
IR and IR-DYN packets will cause a packet to be delivered to upper
layers if and only if the payload is non-empty. This means that an
IP/UDP/RTP packet where the UDP length indicates a UDP payload of
size 12 octets cannot be represented by an IR or IR-DYN packet. Such
packets can instead be represented using the UNCOMPRESSED profile
(section 5.10).
5.7.7.1. Basic structure of the IR packet
This packet type communicates the static part of the context, i.e.,
the values of the constant SN functions. It can optionally also
communicate the dynamic part of the context, i.e., the parameters of
nonconstant SN functions. It can also optionally communicate the
payload of an original packet, if any.
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RFC 3095 Robust Header Compression July 2001
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
| Add-CID octet | if for small CIDs and CID != 0
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 1 0 | D |
+---+---+---+---+---+---+---+---+
| |
/ 0-2 octets of CID info / 1-2 octets if for large CIDs
| |
+---+---+---+---+---+---+---+---+
| Profile | 1 octet
+---+---+---+---+---+---+---+---+
| CRC | 1 octet
+---+---+---+---+---+---+---+---+
| |
| Static chain | variable length
| |
+---+---+---+---+---+---+---+---+
| |
| Dynamic chain | present if D = 1, variable length
| |
- - - - - - - - - - - - - - - -
| |
| Payload | variable length
| |
- - - - - - - - - - - - - - - -
D: D = 1 indicates that the dynamic chain is present.
Profile: Profile identifier, abbreviated as defined in section
5.2.3.
CRC: 8-bit CRC, computed according to section 5.9.1.
Static chain: A chain of static subheader information.
Dynamic chain: A chain of dynamic subheader information. What
dynamic information is present is inferred from the Static
chain.
Payload: The payload of the corresponding original packet, if any.
The presence of a payload is inferred from the packet length.
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RFC 3095 Robust Header Compression July 2001
5.7.7.2. Basic structure of the IR-DYN packet
This packet type communicates the dynamic part of the context, i.e.,
the parameters of nonconstant SN functions.
0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for small CIDs and CID != 0
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 0 0 0 | IR-DYN packet type
+---+---+---+---+---+---+---+---+
: :
/ 0-2 octets of CID info / 1-2 octets if for large CIDs
: :
+---+---+---+---+---+---+---+---+
| Profile | 1 octet
+---+---+---+---+---+---+---+---+
| CRC | 1 octet
+---+---+---+---+---+---+---+---+
| |
/ Dynamic chain / variable length
| |
+---+---+---+---+---+---+---+---+
: :
/ Payload / variable length
: :
- - - - - - - - - - - - - - - -
Profile: Profile identifier, abbreviated as defined in section 5.2.3.
CRC: 8-bit CRC, computed according to section 5.9.1.
NOTE: As the CRC checks only the integrity of the header
itself, an acknowledgment of this header does not signify that
previous changes to the static chain in the context are also
acknowledged. In particular, care should be taken when IR
packets that update an existing context are followed by IR-DYN
packets.
Dynamic chain: A chain of dynamic subheader information. What
dynamic information is present is inferred from the Static chain of
the context.
Payload: The payload of the corresponding original packet, if any.
The presence of a payload is inferred from the packet length.
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RFC 3095 Robust Header Compression July 2001
Note: The static and dynamic chains of IR or IR-DYN packets for
profile 0x0001 (ROHC RTP) MUST end with the static and dynamic parts
of an RTP header. If not, the packet MUST be discarded and the
context MUST NOT be updated.
Note: The static or dynamic chains of IR or IR-DYN packets for
profile 0x0002 (ROHC UDP) MUST end with the static and dynamic parts
of a UDP header. If not, the packet MUST be discarded and the
context MUST NOT be updated.
Note: The static or dynamic chains of IR or IR-DYN packets for
profile 0x0003 (ROHC ESP) MUST end with the static and dynamic parts
of an ESP header. If not, the packet MUST be discarded and the
context MUST NOT be updated.
5.7.7.3. Initialization of IPv6 Header [IPv6]
Static part:
+---+---+---+---+---+---+---+---+
| Version = 6 |Flow Label(msb)| 1 octet
+---+---+---+---+---+---+---+---+
/ Flow Label (lsb) / 2 octets
+---+---+---+---+---+---+---+---+
| Next Header | 1 octet
+---+---+---+---+---+---+---+---+
/ Source Address / 16 octets
+---+---+---+---+---+---+---+---+
/ Destination Address / 16 octets
+---+---+---+---+---+---+---+---+
Dynamic part:
+---+---+---+---+---+---+---+---+
| Traffic Class | 1 octet
+---+---+---+---+---+---+---+---+
| Hop Limit | 1 octet
+---+---+---+---+---+---+---+---+
/ Generic extension header list / variable length
+---+---+---+---+---+---+---+---+
Eliminated:
Payload Length
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RFC 3095 Robust Header Compression July 2001
Extras:
Generic extension header list: Encoded according to section
5.8.6.1, with all header items present in uncompressed form.
CRC-DYNAMIC: Payload Length field (octets 5-6).
CRC-STATIC: All other fields (octets 1-4, 7-40).
CRC coverage for extension headers is defined in section 5.8.7.
Note: The Next Header field indicates the type of the following
header in the static chain, rather than being a copy of the Next
Header field of the original IPv6 header. See also section 5.7.7.8.
5.7.7.4. Initialization of IPv4 Header [IPv4, section 3.1].
Static part:
Version, Protocol, Source Address, Destination Address.
+---+---+---+---+---+---+---+---+
| Version = 4 | 0 |
+---+---+---+---+---+---+---+---+
| Protocol |
+---+---+---+---+---+---+---+---+
/ Source Address / 4 octets
+---+---+---+---+---+---+---+---+
/ Destination Address / 4 octets
+---+---+---+---+---+---+---+---+
Dynamic part:
Type of Service, Time to Live, Identification, DF, RND, NBO,
extension header list.
+---+---+---+---+---+---+---+---+
| Type of Service |
+---+---+---+---+---+---+---+---+
| Time to Live |
+---+---+---+---+---+---+---+---+
/ Identification / 2 octets
+---+---+---+---+---+---+---+---+
| DF|RND|NBO| 0 |
+---+---+---+---+---+---+---+---+
/ Generic extension header list / variable length
+---+---+---+---+---+---+---+---+
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RFC 3095 Robust Header Compression July 2001
Eliminated:
IHL (IP Header Length, must be 5)
Total Length (inferred in decompressed packets)
MF flag (More Fragments flag, must be 0)
Fragment Offset (must be 0)
Header Checksum (inferred in decompressed packets)
Options, Padding (must not be present)
Extras:
RND, NBO See section 5.7.
Generic extension header list: Encoded according to section
5.8.6.1, with all header items present in uncompressed form.
CRC-DYNAMIC: Total Length, Identification, Header Checksum
(octets 3-4, 5-6, 11-12).
CRC-STATIC: All other fields (octets 1-2, 7-10, 13-20)
CRC coverage for extension headers is defined in section 5.8.7.
Note: The Protocol field indicates the type of the following header
in the static chain, rather than being a copy of the Protocol field
of the original IPv4 header. See also section 5.7.7.8.
5.7.7.5. Initialization of UDP Header [RFC-768].
Static part:
+---+---+---+---+---+---+---+---+
/ Source Port / 2 octets
+---+---+---+---+---+---+---+---+
/ Destination Port / 2 octets
+---+---+---+---+---+---+---+---+
Dynamic part:
+---+---+---+---+---+---+---+---+
/ Checksum / 2 octets
+---+---+---+---+---+---+---+---+
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RFC 3095 Robust Header Compression July 2001
Eliminated:
Length
The Length field of the UDP header MUST match the Length field(s)
of the preceding subheaders, i.e., there must not be any padding
after the UDP payload that is covered by the IP Length.
CRC-DYNAMIC: Length field, Checksum (octets 5-8).
CRC-STATIC: All other fields (octets 1-4).
5.7.7.6. Initialization of RTP Header [RTP].
Static part:
SSRC.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
/ SSRC / 4 octets
+---+---+---+---+---+---+---+---+
Dynamic part:
P, X, CC, PT, M, sequence number, timestamp, timestamp stride,
CSRC identifiers.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| V=2 | P | RX| CC | (RX is NOT the RTP X bit)
+---+---+---+---+---+---+---+---+
| M | PT |
+---+---+---+---+---+---+---+---+
/ RTP Sequence Number / 2 octets
+---+---+---+---+---+---+---+---+
/ RTP Timestamp (absolute) / 4 octets
+---+---+---+---+---+---+---+---+
/ Generic CSRC list / variable length
+---+---+---+---+---+---+---+---+
: Reserved | X | Mode |TIS|TSS: if RX = 1
+---+---+---+---+---+---+---+---+
: TS_Stride : 1-4 octets, if TSS = 1
+---+---+---+---+---+---+---+---+
: Time_Stride : 1-4 octets, if TIS = 1
+---+---+---+---+---+---+---+---+
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RFC 3095 Robust Header Compression July 2001
Eliminated:
Nothing.
Extras:
RX: Controls presence of extension.
Mode: Compression mode. 0 = Reserved,
1 = Unidirectional,
2 = Bidirectional Optimistic,
3 = Bidirectional Reliable.
X: Copy of X bit from RTP header (presumed 0 if RX = 0)
Reserved: Set to zero when sending, ignored when received.
Generic CSRC list: CSRC list encoded according to section
5.8.6.1, with all CSRC items present.
CRC-DYNAMIC: Octets containing M-bit, sequence number field,
and timestamp (octets 2-8).
CRC-STATIC: All other fields (octets 1, 9-12, original CSRC list).
5.7.7.7. Initialization of ESP Header [ESP, section 2]
This is for the case when the NULL encryption algorithm [NULL] is NOT
being used with ESP, so that subheaders after the ESP header are
encrypted (see 5.12). See 5.8.4.3 for compression of the ESP header
when NULL encryption is being used.
Static part:
+---+---+---+---+---+---+---+---+
/ SPI / 4 octets
+---+---+---+---+---+---+---+---+
Dynamic part:
+---+---+---+---+---+---+---+---+
/ Sequence Number / 4 octets
+---+---+---+---+---+---+---+---+
Eliminated:
Other fields are encrypted, and can neither be located nor
compressed.
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CRC-DYNAMIC: Sequence number (octets 5-8)
CRC-STATIC: All other octets.
Note: No encrypted data is considered to be part of the header for
purposes of computing the CRC, i.e., octets after the eight octet are
not considered part of the header.
5.7.7.8. Initialization of Other Headers
Headers not explicitly listed in previous subsections can be
compressed only by making them part of an extension header chain
following an IPv4 or IPv6 header, see section 5.8.
5.8. List compression
Header information from the packet stream to be compressed can be
structured as an ordered list, which is largely constant between
packets. The generic structure of such a list is as follows.
+--------+--------+--...--+--------+
list: | item 1 | item 2 | | item n |
+--------+--------+--...--+--------+
This section describes the compression scheme for such information.
The basic principles of list-based compression are the following:
1) While the list is constant, no information about the list is sent
in compressed headers.
2) Small changes in the list are represented as additions (Insertion
scheme), or deletions (Removal scheme), or both (Remove Then
Insert scheme).
3) The list can also be sent in its entirety (Generic scheme).
There are two kinds of lists: CSRC lists in RTP packets, and
extension header chains in IP packets (both IPv4 and IPv6).
IPv6 base headers and IPv4 headers cannot be part of an extension
header chain. Headers which can be part of extension header chains
include
a) the AH header
b) the null ESP header
c) the minimal encapsulation header [RFC2004, section 3.1]
d) the GRE header [GRE1, GRE2]
e) IPv6 extension headers.
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The table-based item compression scheme (5.8.1), which reduces the
size of each item, is described first. Then it is defined which
reference list to use in the insertion and removal schemes (5.8.2).
List encoding schemes are described in section 5.8.3, and a few
special cases in section 5.8.4. Finally, exact formats are described
in sections 5.8.5-5.8.6.
5.8.1. Table-based item compression
The Table-based item compression scheme is a way to compress
individual items sent in compressed lists. The compressor assigns
each item in a list a unique identifier Index. The compressor
conceptually maintains a table with all items, indexed by Index. The
(Index, item) pair is sent together in compressed lists until the
compressor gains enough confidence that the decompressor has observed
the mapping between the item and its Index. Such confidence is
obtained by receiving an acknowledgment from the decompressor in R-
mode, and in U/O-mode by sending L (Index, item) pairs (not
necessarily consecutively). After that, the Index alone is sent in
compressed lists to indicate the corresponding item. The compressor
may reassign an existing Index to a new item, and then needs to re-
establish the mapping in the same manner as above.
The decompressor conceptually maintains a table that contains all
(Index, item) pairs it knows about. The table is updated whenever an
(Index, item) pair is received (and decompression is verified by a
CRC). The decompressor retrieves the item from the table whenever an
Index without an accompanying item is received.
5.8.1.1. Translation table in R-mode
At the compressor side, an entry in the Translation Table has the
following structure.
+-------+------+---------------+
Index i | Known | item | SN1, SN2, ... |
+-------+------+---------------+
The Known flag indicates whether the mapping between Index i and item
has been established, i.e., if Index i alone can be sent in
compressed lists. Known is initially zero. It is also set to zero
whenever Index i is assigned to a new item. Known is set to one when
the corresponding (Index, item) pair is acknowledged.
Acknowledgments are based on the RTP Sequence Number, so a list of
RTP Sequence Numbers of all packets which contain the (Index, item)
pair is included in the translation table. When a packet with a
sequence number in the sequence number list is acknowledged, the
Known flag is set, and the sequence number list can be discarded.
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Each entry in the Translation Table at the decompressor side has the
following structure:
+-------+------+
Index i | Known | item |
+-------+------+
All Known fields are initialized to zero. Whenever the decompressor
receives an (Index, item) pair, it inserts item into the table at
position Index and sets the Known flag in that entry to one. If an
index without an accompanying item is received for which the Known
flag is zero, the header MUST be discarded and a NACK SHOULD be sent.
5.8.1.2. Translation table in U/O-modes
At the compressor side, each entry in the Translation Table has the
following structure:
+-------+------+---------+
Index | Known | item | Counter |
+-------+------+---------+
The Index, Known, and item fields have the same meaning as in section
5.8.1.1.
Known is set when the (Index, item) pair has been sent in L
compressed lists (not necessarily consecutively). The Counter field
keeps track of how many times the pair has been sent. Counter is set
to 0 for each new entry added to the table, and whenever Index is
assigned to a new item. Counter is incremented by 1 whenever an
(Index, item) pair is sent. When the counter reaches L, the Known
field is set and after that only the Index needs to be sent in
compressed lists.
At the decompressor side, the Translation Table is the same as the
Translation Table defined in R-mode.
5.8.2. Reference list determination
In reference based compression schemes (i.e., addition or deletion
based schemes), compression and decompression of a list (curr_list)
are based on a reference list (ref_list) which is assumed to be
present in the context of both compressor and decompressor. The
compressed list is an encoding of the differences between curr_list
and ref_list. Upon reception of a compressed list, the decompressor
applies the differences to its reference list in order to obtain the
original list.
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To identify the reference list (to be) used, each compressed list
carries an identifier (ref_id). The reference list is established by
different methods in R-mode and U/O-mode.
5.8.2.1. Reference list in R-mode and U/O-mode
In R-mode, the choice of reference list is based on acknowledgments,
i.e., the compressor uses as ref_list the latest list which has been
acknowledged by the decompressor. The ref_list is updated only upon
receiving an acknowledgment. The least significant bits of the RTP
Sequence Number of the acknowledged packet are used as the ref_id.
In U/O-mode, a sequence of identical lists are considered as
belonging to the same generation and are all assigned the same
generation identifier (gen_id). Gen_id increases by 1 each time the
list changes and is carried in compressed and uncompressed lists that
are candidates for being used as reference lists. Normally, Gen_id
must have been repeated in at least L headers before the list can be
used as a ref_list. However, some acknowledgments may be sent in O-
mode (and also in U-mode), and whenever an acknowledgment for a
header is received, the list of that header is considered known and
need not be repeated further. The least significant bits of the
Gen_id is used as the ref_id in U/O-mode.
The logic of the compressor and decompressor for reference based list
compression is similar to that for SN and TS. The principal
difference is that the decompressor maintains a sliding window with
candidates for ref_list, and retrieves ref_list from the sliding
window using the ref_id of the compressed list.
Logic of compressor:
a) In the IR state, the compressor sends Generic lists (see 5.8.5)
containing all items of the current list in order to establish or
refresh the context of the decompressor.
In R-mode, such Generic lists are sent until a header is
acknowledged. The list of that header can be used as a reference
list to compress subsequent lists.
In U/O-mode, the compressor sends generation identifiers with the
Generic lists until
1) a generation identifier has been repeated L times, or
2) an acknowledgment for a header carrying a generation identifier
has been received.
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The repeated (1) or acknowledged (2) list can be used as a
reference list to compress subsequent lists and is kept together
with its generation identifier.
b) When not in the IR state, the compressor moves to the FO state
when it observes a difference between curr_list and the previous
list. It sends compressed lists based on ref_list to update the
context of the decompressor. (However, see d).)
In R-mode, the compressor keeps sending compressed lists using the
same reference until it receives an acknowledgment for a packet
containing the newest list. The compressor may then move to the
SO state with regard to the list.
In U/O-mode, the compressor keeps sending compressed lists with
generation identifiers until
1) a generation identifier has been repeated L times, or
2) an acknowledgment for a header carrying the latest generation
identifier has been received.
The repeated or acknowledged list is used as the future reference
list. The compressor may move to the SO state with regard to the
list.
c) In R-mode, the compressor maintains a sliding window containing
the lists which have been sent to update the context of the
decompressor and have not yet been acknowledged. The sliding
window shrinks when an acknowledgment arrives: all lists sent
before the acknowledged list are removed. The compressor may use
the Index to represent items of lists in the sliding window.
In U/O-mode, the compressor needs to store
1) the reference list and its generation identifier, and
2) if the current generation identifier is different from the
reference generation, the current list and the sequence
numbers with which the current list has been sent.
(2) is needed to determine if an acknowledgment concerns the
latest generation. It is not needed in U-mode.
d) In U/O-mode, the compressor may choose to not send a generation
identifier with a compressed list. Such lists without generation
identifiers are not assigned a new generation identifier and must
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not be used as future reference lists. They do not update the
context. This feature is useful when a new list is repeated few
times and the list then reverts back to its old value.
Logic of decompressor:
e) In R-mode, the decompressor acknowledges all received uncompressed
or compressed lists which establish or update the context. (Such
compressed headers contain a CRC.)
In O-mode, the decompressor MAY acknowledge a list with a new
generation identifier, see section 5.4.2.2.
In U-mode, the decompressor MAY acknowledge a list sent in an IR
packet, see section 5.3.2.3.
f) The decompressor maintains a sliding window which contains the
lists that may be used as reference lists.
In R-mode, the sliding window contains lists which have been
acknowledged but not yet used as reference lists.
In U/O-mode, the sliding window contains at most one list per
generation. It contains all generations seen by the decompressor
newer than the last generation used as a reference.
g) When the decompressor receives a compressed list, it retrieves the
proper ref_list from the sliding window based on the ref_id, and
decompresses the compressed list obtaining curr_list.
In R-mode, curr_list is inserted into the sliding window if an
acknowledgment is sent for it. The sliding window is shrunk by
removing all lists received before ref_list.
In U/O-mode, curr_list is inserted into the sliding window
together with its generation identifier if the compressed list had
a generation identifier and the sliding window does not contain a
list with that generation identifier. All lists with generations
older than ref_id are removed from the sliding window.
5.8.3. Encoding schemes for the compressed list
Four encoding schemes for the compressed list are described here.
The exact formats of the compressed CSRC list and compressed IP
extension header list using these encoding schemes are described in
sections 5.8.5-5.8.6.
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Generic scheme
In contrast to subsequent schemes, this scheme does not rely on a
reference list having been established. The entire list is sent,
using table based compression for each individual item. The
generic scheme is always used when establishing the context of the
decompressor and may also be used at other times, as the
compressor sees fit.
Insertion Only scheme
When the new list can be constructed from ref_list by adding
items, a list of the added items is sent (using table based
compression), along with the positions in ref_list where the new
items will be inserted. An insertion bit mask indicates the
insertion positions in ref_list.
Upon reception of a list compressed according to the Insertion
Only scheme, curr_list is obtained by scanning the insertion bit
mask from left to right. When a '0' is observed, an item is
copied from the ref_list. When a '1' is observed, an item is
copied from the list of added items. If a '1' is observed when
the list of added items has been exhausted, an error has occurred
and decompression fails: The header MUST NOT be delivered to upper
layers; it should be discarded, and MUST NOT be acknowledged nor
used as a reference.
To construct the insertion bit mask and the list of added items,
the compressor MAY use the following algorithm:
1) An empty bit list and an empty Inserted Item list are generated
as the starting point.
2) Start by considering the first item of curr_list and ref_list.
3) If curr_list has a different item than ref_list,
a set bit (1) is appended to the bit list;
the first item in curr_list (represented using table-based
item compression) is appended to the Inserted Item list;
advance to the next item of curr_list;
otherwise,
a zero bit (0) is appended to the bit list;
advance to the next item of curr_list;
advance to the next item of ref_list.
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4) Repeat 3) until curr_list has been exhausted.
5) If the length of the bit list is less than the required bit
mask length, append additional zeroes.
Removal Only scheme
This scheme can be used when curr_list can be obtained by removing
some items in ref_list. The positions of the items which are in
ref_list, but not in curr_list, are sent as a removal bit mask.
Upon reception of the compressed list, the decompressor obtains
curr_list by scanning the removal bit mask from left to right.
When a '0' is observed, the next item of ref_list is copied into
curr_list. When a '1' is observed, the next item of ref_list is
skipped over without being copied. If a '0' is observed when
ref_list has been exhausted, an error has occurred and
decompression fails: The header MUST NOT be delivered to upper
layers; it should be discarded, and MUST NOT be acknowledged nor
used as a reference.
To construct the removal bit mask and the list of added items, the
compressor MAY use the following algorithm:
1) An empty bit list is generated as the starting point.
2) Start by considering the first item of curr_list and ref_list.
3) If curr_list has a different item than ref_list,
a set bit (1) is appended to the bit list;
advance to the next item of ref_list;
otherwise,
a zero bit (0) is appended to the bit list;
advance to the next item of curr_list;
advance to the next item of ref_list.
4) Repeat 3) until curr_list has been exhausted.
5) If the length of the bit list is less than the required bit
mask length, append additional ones.
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Remove Then Insert scheme
In this scheme, curr_list is obtained by first removing items from
ref_list, and then inserting items into the resulting list. A
removal bit mask, an insertion bit mask, and a list of added items
are sent.
Upon reception of the compressed list, the decompressor processes
the removal bit mask as in the Removal Only scheme. The resulting
list is then used as the reference list when the insertion bit
mask and the list of added items are processed, as in the
Insertion Only scheme.
5.8.4. Special handling of IP extension headers
In CSRC list compression, each CSRC is assigned an index. In
contrast, in IP extension header list compression an index is usually
associated with a type of extension header. When there is more than
one IP header, there is more than one list of extension headers. An
index per type per list is then used.
The association with a type means that a new index need not always be
used each time a field in an IP extension header changes. However,
when a field in an extension header changes, the mapping between the
index and the new value of the extension header needs to be
established, except in the special handling cases defined in the
following subsections.
5.8.4.1. Next Header field
The next header field in an IP header or extension header changes
whenever the type of the immediately following header changes, e.g.,
when a new extension header is inserted after it, when the immediate
subsequent extension header is removed from the list, or when the
order of extension headers is changed. Thus it may not be uncommon
that, for a given header, the next header field changes while the
remaining fields do not change.
Therefore, in the case that only the next header field changes, the
extension header is considered to be unchanged and rules for special
treatment of the change in the next header field are defined below.
All communicated uncompressed extension header items indicate their
own type in their Next Header field. Note that the rules below
explain how to treat the Next Header fields while showing the
conceptual reference list as an exact recreation of the original
uncompressed extension header list.
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a) When a subsequent extension header is removed from the list, the
new value of the next header field is obtained from the reference
extension header list. For example, assume that the reference
header list (ref_list) consists of headers A, B and C (ref_ext_hdr
A, B, C), and the current extension header list (curr_list) only
consists of extension headers A and C (curr_ext_hdr A, C). The
order and value of the next header fields of these extension
headers are as follows.
ref_list:
+--------+-----+ +--------+-----+ +--------+-----+
| type B | | | type C | | | type D | |
+--------+ | +--------+ | +--------+ |
| | | | | |
+--------------+ +--------------+ +--------------+
ref_ext_hdr A ref_ext_hdr B ref_ext_hdr C
curr_list:
+--------+-----+ +--------+-----+
| type C | | | type D | |
+--------+ | +--------+ |
| | | |
+--------------+ +--------------+
curr_ext_hdr A curr_ext_hdr C
Comparing the curr_ext_hdr A in curr_list and the ref_ext_hdr A in
ref_list, the value of next header field is changed from "type B"
to "type C" because of the removal of extension header B. The new
value of the next header field in curr_ext_hdr A, i.e., "type C",
does not need to be sent to the decompressor. Instead, it is
retrieved from the next header field of the removed ref_ext_hdr B.
b) When a new extension header is inserted after an existing
extension header, the next header field in the communicated item
will carry the type of itself, rather than the type of the header
that follows. For example, assume that the reference header list
(ref_list) consists of headers A and C (ref_ext_hdr A, C), and the
current header list (curr_list) consists of headers A, B and C
(curr_ext_hdr A, B, C). The order and the value of the next
header fields of these extension headers are as follows.
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ref_list:
+--------+-----+ +--------+-----+
| type C | | | type D | |
+--------+ | +--------+ |
| | | |
+--------------+ +--------------+
ref_ext_hdr A ref_ext_hdr C
curr_list:
+--------+-----+ +--------+-----+ +--------+-----+
| type B | | | type C | | | type D | |
+--------+ | +--------+ | +--------+ |
| | | | | |
+--------------+ +--------------+ +--------------+
curr_ext_hdr A curr_ext_hdr B curr_ext_hdr C
Comparing the curr_list and the ref_list, the value of the next
header field in extension header A is changed from "type C" to
"type B".
The uncompressed curr_ext_hdr B is carried in the compressed
header list. However, it carries "type B" instead of "type C" in
its next header field. When the decompressor inserts a new header
after curr_ext_hdr A, the next header field of A is taken from the
new header, and the next header field of the new header is taken
from ref_ext_hdr A.
c) Some headers whose compression is defined in this document do not
contain Next Header fields or do not have their Next Header field
in the standard position (first octet of the header). The GRE and
ESP headers are such headers. When sent as uncompressed items in
lists, these headers are modified so that they do have a Next
Header field as their first octet (see 5.8.4.3 and 5.8.4.4). This
is necessary to enable the decompressor to decode the item.
5.8.4.2. Authentication Header (AH)
The sequence number field in the AH [AH] contains a monotonically
increasing counter value for a security association. Therefore, when
comparing curr_list with ref_list, if the sequence number in AH
changes and SPI field does not change, the AH is not considered as
changed.
If the sequence number in the AH linearly increases as the RTP
Sequence Number increases, and the compressor is confident that the
decompressor has obtained the pattern, the sequence number in AH need
not be sent. The decompressor applies linear extrapolation to
reconstruct the sequence number in the AH.
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Otherwise, a compressed sequence number is included in the IPX
compression field in an Extension 3 of an UOR-2 header.
The authentication data field in AH changes from packet to packet and
is sent as-is. If the uncompressed AH is sent, the authentication
data field is sent inside the uncompressed AH; otherwise, it is sent
after the compressed IP/UDP/RTP and IPv6 extension headers and before
the payload. See beginning of section 5.7.
Note: The payload length field of the AH uses a different notion of
length than other IPv6 extension headers.
5.8.4.3. Encapsulating Security Payload Header (ESP)
When the Encapsulating Security Payload Header (ESP) [ESP] is present
and an encryption algorithm other than NULL is being used, the UDP
and RTP headers are both encrypted and cannot be compressed. The ESP
header thus ends the compressible header chain. The ROHC ESP profile
defined in section 5.12 MAY be used for the stream in this case.
A special case is when the NULL encryption algorithm is used. This
is the case when the ESP header is used for authentication only, and
not for encryption. The payload is not encrypted by the NULL
encryption algorithm, so compression of the rest of the header chain
is possible. The rest of this section describes compression of the
ESP header when the NULL encryption algorithm is used with ESP.
It is not possible to determine whether NULL encryption is used by
inspecting a header in the stream, this information is present only
at the encryption endpoints. However, a compressor may attempt
compression under the assumption that the NULL encryption algorithm
is being used, and later abort compression when the assumption proves
to be false.
The compressor may, for example, inspect the Next Header fields and
the header fields supposed to be static in subsequent headers in
order to determine if NULL encryption is being used. If these change
unpredictably, an encryption algorithm other than NULL is probably
being used and compression of subsequent headers SHOULD be aborted.
Compression of the stream is then either discontinued, or a profile
that compresses only up to the ESP header may be used (see 5.12).
While attempting to compress the header, the compressor should use
the SPI of the ESP header together with the destination IP address as
the defining fields for determining which packets belong to the
stream.
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In the ESP header [ESP, section 2], the fields that can be compressed
are the SPI, the sequence number, the Next Header, and the padding
bytes if they are in the standard format defined in [ESP]. (As
always, the decompressor reinserts these fields based on the
information in the context. Care must be taken to correctly reinsert
all the information as the Authentication Data must be verified over
the exact same information it was computed over.)
ESP header [ESP, section 2]:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Security Parameters Index (SPI) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Payload Data (variable) |
~ ~
| |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Padding (0-255 octets) |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Pad Length | Next Header |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Authentication Data |
+ (variable length, but assumed to be 12 octets) +
| |
+ +
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
SPI: Static. If it changes, it needs to be reestablished.
Sequence Number: Not sent when the offset from the sequence number
of the compressed header is constant. When the offset is not
constant, the sequence number may be compressed by sending
LSBs. See 5.8.4.
Payload Data: This is where subsequent headers are to be found.
Parsed according to the Next Header field.
Padding: The padding octets are assumed to be as defined in [ESP],
i.e., to take the values 1, 2, ..., k, where k = Pad Length.
If the padding in the static context has this pattern, padding
in compressed headers is assumed to have this pattern as well
and is removed. If padding in the static context does not
have this pattern, the padding is not removed.
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Pad Length: Dynamic. Always sent. 14th octet from end of packet.
Next Header: Static. 13th octet from end of packet.
Authentication Data: Can have variable length, but when compression
of NULL-encryption ESP header is attempted, it is assumed to have
length 12 octets.
The sequence number in ESP has the same behavior as the sequence
number field in AH. When it increases linearly, it can be compressed
to zero bits. When it does not increase linearly, a compressed
sequence number is included in the IPX compression field in an
Extension 3 of an UOR-2 header.
The information which is part of an uncompressed item of a compressed
list is the Next Header field, followed by the SPI and the Sequence
Number. Padding, Pad Length, Next Header, and Authentication Data
are sent as-is at the end of the packet. This means that the Next
Header occurs in two places.
Uncompressed ESP list item:
+---+---+---+---+---+---+---+---+
| Next Header ! 1 octet (see section 5.8.4.1)
+---+---+---+---+---+---+---+---+
/ SPI / 4 octets
+---+---+---+---+---+---+---+---+
/ Sequence Number / 4 octets
+---+---+---+---+---+---+---+---+
When sending Uncompressed ESP list items, all ESP fields near the
the end of the packet are left untouched (Padding, Pad Length,
Next Header, Authentication Data).
A compressed item consists of a compressed sequence number. When an
item is compressed, Padding (if it follows the 1, 2, ..., k pattern)
and Next Header are removed near the end of the packet.
Authentication Data and Pad Length remain as-is near the end of the
packet.
5.8.4.4. GRE Header [RFC 2784, RFC 2890]
The GRE header is a set of flags, followed by a mandatory Protocol
Type and optional parts as indicated by the flags.
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The sequence number field in the GRE header contains a counter value
for a GRE tunnel. Therefore, when comparing curr_list with ref_list,
if the sequence number in GRE changes, the GRE is not considered as
changed.
If the sequence number in the GRE header linearly increases as the
RTP Sequence Number increases and the compressor is confident that
the decompressor has received the pattern, the sequence number in GRE
need not be sent. The decompressor applies linear extrapolation to
reconstruct the sequence number in the GRE header.
Otherwise, a compressed sequence number is included in the IPX
compression field in an Extension 3 of an UOR-2 header.
The checksum data field in GRE, if present, changes from packet to
packet and is sent as-is. If the uncompressed GRE header is sent,
the checksum data field is sent inside the uncompressed GRE header;
otherwise, if present, it is sent after the compressed IP/UDP/RTP and
IPv6 extension headers and before the payload. See beginning of
section 5.7.
In order to allow simple parsing of lists of items, an uncompressed
GRE header sent as an item in a list is modified from the original
GRE header in the following manner: 1) the 16-bit Protocol Type field
that encodes the type of the subsequent header using Ether types (see
Ether types section in [ASSIGNED]) is removed. 2) A one-octet Next
Header field is inserted as the first octet of the header. The value
of the Next Header field corresponds to GRE (this value is 47
according to the Assigned Internet Protocol Number section of
[ASSIGNED]) when the uncompressed item is to be inserted in a list,
and to the type of the subsequent header when the uncompressed item
is in a Generic list. Note that this implies that only GRE headers
with Ether types that correspond to an IP protocol number can be
compressed.
Uncompressed GRE list item:
+---+---+---+---+---+---+---+---+
| Next Header ! 1 octet (see section 5.8.4.1)
+---+---+---+---+---+---+---+---+
/ C | | K | S | | Ver | 1 octet
+---+---+---+---+---+---+---+---+
/ Checksum / 2 octets, if C=1
+---+---+---+---+---+---+---+---+
/ Key / 4 octets, if K=1
+---+---+---+---+---+---+---+---+
/ Sequence Number / 4 octets, if S=1
+---+---+---+---+---+---+---+---+
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The bits left blank in the second octet are set to zero when
sending and ignored when received.
The fields Reserved0 and Reserved1 of the GRE header [GRE2] must
be all zeroes; otherwise, the packet cannot be compressed by this
profile.
5.8.5. Format of compressed lists in Extension 3
5.8.5.1. Format of IP Extension Header(s) field
In Extension 3 (section 5.7.5), there is a field called IP extension
header(s). This section describes the format of that field.
0 1 2 3 4 5 6 7
+-----+-----+-----+-----+-----+-----+-----+-----+
| CL | ASeq| ESeq| Gseq| res | 1 octet
+-----+-----+-----+-----+-----+-----+-----+-----+
: compressed AH Seq Number, 1 or 4 octets : if ASeq = 1
----- ----- ----- ----- ----- ----- ----- -----
: compressed ESP Seq Number, 1 or 4 octets : if Eseq = 1
----- ----- ----- ----- ----- ----- ----- -----
: compressed GRE Seq Number, 1 or 4 octets : if Gseq = 1
----- ----- ----- ----- ----- ----- ----- -----
: compressed header list, variable length : if CL = 1
----- ----- ----- ----- ----- ----- ----- -----
ASeq: indicates presence of compressed AH Seq Number
ESeq: indicates presence of compressed ESP Seq Number
GSeq: indicates presence of compressed GRE Seq Number
CL: indicates presence of compressed header list
res: reserved; set to zero when sending, ignored when received
When Aseq, Eseq, or Gseq is set, the corresponding header item (AH,
ESP, or GRE header) is compressed. When not set, the corresponding
header item is sent uncompressed or is not present.
The format of compressed AH, ESP and GRE Sequence Numbers can each be
either of the following:
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0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+
| 0 | LSB of sequence number | | 1 | |
+---+---+---+---+---+---+---+---+ +---+ +
| |
+ LSB of sequence number +
| |
+ +
| |
+---+---+---+---+---+---+---+---+
The format of the compressed header list field is described in
section 5.8.6.
5.8.5.2. Format of Compressed CSRC List
The Compressed CSRC List field in the RTP header part of an Extension
3 (section 5.7.5) is as in section 5.8.6.
5.8.6. Compressed list formats
This section describes the format of compressed lists. The format is
the same for CSRC lists and header lists. In CSRC lists, the items
are CSRC identifiers; in header lists, they are uncompressed or
compressed headers, as described in 5.8.4.2-4.
5.8.6.1. Encoding Type 0 (generic scheme)
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| ET=0 |GP |PS | CC = m |
+---+---+---+---+---+---+---+---+
: gen_id : 1 octet, if GP = 1
+---+---+---+---+---+---+---+---+
| XI 1, ..., XI m | m octets, or m * 4 bits
/ --- --- --- ---/
| : Padding : if PS = 0 and m is odd
+---+---+---+---+---+---+---+---+
| |
/ item 1, ..., item n / variable
| |
+---+---+---+---+---+---+---+---+
ET: Encoding type is zero.
PS: Indicates size of XI fields:
PS = 0 indicates 4-bit XI fields;
PS = 1 indicates 8-bit XI fields.
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GP: Indicates presence of gen_id field.
CC: CSRC counter from original RTP header.
gen_id: Identifier for a sequence of identical lists. It is
present in U/O-mode when the compressor decides that it may use
this list as a future reference list.
XI 1, ..., XI m: m XI items. The format of an XI item is as
follows:
+---+---+---+---+
PS = 0: | X | Index |
+---+---+---+---+
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
PS = 1: | X | Index |
+---+---+---+---+---+---+---+---+
X = 1 indicates that the item corresponding to the Index
is sent in the item 0, ..., item n list.
X = 0 indicates that the item corresponding to the Index is
not sent.
When 4-bit XI items are used and m > 1, the XI items are placed in
octets in the following manner:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| XI k | XI k + 1 |
+---+---+---+---+---+---+---+---+
Padding: A 4-bit padding field is present when PS = 0 and m is
odd. The Padding field is set to zero when sending and ignored
when receiving.
Item 1, ..., item n:
Each item corresponds to an XI with X = 1 in XI 1, ..., XI m.
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5.8.6.2. Encoding Type 1 (insertion only scheme)
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| ET=1 |GP |PS | XI 1 |
+---+---+---+---+---+---+---+---+
: gen_id : 1 octet, if GP = 1
+---+---+---+---+---+---+---+---+
| ref_id |
+---+---+---+---+---+---+---+---+
/ insertion bit mask / 1-2 octets
+---+---+---+---+---+---+---+---+
| XI list | k octets, or (k - 1) * 4 bits
/ --- --- --- ---/
| : Padding : if PS = 0 and k is even
+---+---+---+---+---+---+---+---+
| |
/ item 1, ..., item n / variable
| |
+---+---+---+---+---+---+---+---+
Unless explicitly stated otherwise, fields have the same meaning and
values as for encoding type 0.
ET: Encoding type is one (1).
XI 1: When PS = 0, the first 4-bit XI item is placed here.
When PS = 1, the field is set to zero when sending, and
ignored when receiving.
ref_id: The identifier of the reference CSRC list used when the
list was compressed. It is the 8 least significant bits of
the RTP Sequence Number in R-mode and gen_id (see section
5.8.2) in U/O-mode.
insertion bit mask: Bit mask indicating the positions where new
items are to be inserted. See Insertion Only scheme in
section 5.8.3. The bit mask can have either of the
following two formats:
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0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 0 | 7-bit mask | bit 1 is the first bit
+---+---+---+---+---+---+---+---+
+---+---+---+---+---+---+---+---+
| 1 | | bit 1 is the first bit
+---+ 15-bit mask +
| | bit 7 is the last bit
+---+---+---+---+---+---+---+---+
XI list: XI fields for items to be inserted. When the insertion
bit mask has k ones, the total number of XI fields is k. When
PS = 1, all XI fields are in the XI list. When PS = 0, the
first XI field is in the XI 1 field, and the remaining k - 1
XI fields are in the XI list.
Padding: Present when PS = 0 and k is even.
item 1, ..., item n: One item for each XI field with the X bit
set.
5.8.6.3. Encoding Type 2 (removal only scheme)
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| ET=2 |GP |res| Count |
+---+---+---+---+---+---+---+---+
: gen_id : 1 octet, if GP = 1
+---+---+---+---+---+---+---+---+
| ref_id |
+---+---+---+---+---+---+---+---+
/ removal bit mask / 1-2 octets
+---+---+---+---+---+---+---+---+
Unless explicitly stated otherwise, fields have the same meaning
and values as in section 5.8.5.2.
ET: Encoding type is 2.
res: Reserved. Set to zero when sending, ignored when
received.
Count: Number of elements in ref_list.
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removal bit mask: Indicates the elements in ref_list to be
removed in order to obtain the current list. See section
5.8.3. The removal bit mask has the same format as the
insertion bit mask of section 5.8.6.3.
5.8.6.4. Encoding Type 3 (remove then insert scheme)
See section 5.8.3 for a description of the Remove then insert
scheme.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| ET=3 |GP |PS | XI 1 |
+---+---+---+---+---+---+---+---+
: gen_id : 1 octet, if GP = 1
+---+---+---+---+---+---+---+---+
| ref_id |
+---+---+---+---+---+---+---+---+
/ removal bit mask / 1-2 octets
+---+---+---+---+---+---+---+---+
/ insertion bit mask / 1-2 octets
+---+---+---+---+---+---+---+---+
| XI list | k octets, or (k - 1) * 4 bits
/ --- --- --- ---/
| : Padding : if PS = 0 and k is even
+---+---+---+---+---+---+---+---+
| |
/ item 1, ..., item n / variable
| |
+---+---+---+---+---+---+---+---+
The fields in this header have the same meaning and formats as in
section 5.8.5.2, except when explicitly stated otherwise below.
ET: Encoding type is 3.
removal bit mask: See section 5.8.6.3.
5.8.7. CRC coverage for extension headers
All fields of extension headers are CRC-STATIC, with the following
exceptions which are CRC-DYNAMIC.
1) Entire AH header.
2) Entire ESP header.
3) Sequence number in GRE, Checksum in GRE
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5.9. Header compression CRCs, coverage and polynomials
This chapter describes how to calculate the CRCs used in packet
headers defined in this document. (Note that another type of CRC is
defined for reconstructed units in section 5.2.5.)
5.9.1. IR and IR-DYN packet CRCs
The CRC in the IR and IR-DYN packet is calculated over the entire IR
or IR-DYN packet, excluding Payload and including CID or any Add-CID
octet. For purposes of computing the CRC, the CRC field in the
header is set to zero.
The initial content of the CRC register is to be preset to all 1's.
The CRC polynomial to be used for the 8-bit CRC is:
C(x) = 1 + x + x^2 + x^8
5.9.2. CRCs in compressed headers
The CRC in compressed headers is calculated over all octets of the
entire original header, before compression, in the following manner.
The octets of the header are classified as either CRC-STATIC or CRC-
DYNAMIC, and the CRC is calculated over:
1) the concatenated CRC-STATIC octets of the original header, placed
in the same order as they appear in the original header, followed
by
2) the concatenated CRC-DYNAMIC octets of the original header, placed
in the same order as they appear in the original header.
The intention is that the state of the CRC computation after 1) will
be saved. As long as the CRC-STATIC octets do not change, the CRC
calculation will then only need to process the CRC-DYNAMIC octets.
In a typical RTP/UDP/IPv4 header, 25 octets are CRC-STATIC and 15 are
CRC-DYNAMIC. In a typical RTP/UDP/IPv6 header, 49 octets are CRC-
STATIC and 11 are CRC-DYNAMIC. This technique will thus reduce the
computational complexity of the CRC calculation by roughly 60% for
RTP/UDP/IPv4 and by roughly 80% for RTP/UDP/IPv6.
Note: Whenever the CRC-STATIC fields change, the new saved CRC state
after 1) is compared with the old state. If the states are
identical, the CRC cannot catch the error consisting in the
decompressor not having updated the static context. In U/O-mode the
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RFC 3095 Robust Header Compression July 2001
compressor SHOULD then for a while use packet types with another CRC
length, for which there is a difference in CRC state, to ensure error
detection.
The initial content of the CRC register is preset to all 1's.
The polynomial to be used for the 3 bit CRC is:
C(x) = 1 + x + x^3
The polynomial to be used for the 7 bit CRC is:
C(x) = 1 + x + x^2 + x^3 + x^6 + x^7
The CRC in compressed headers is calculated over the entire original
header, before compression.
5.10. ROHC UNCOMPRESSED -- no compression (Profile 0x0000)
In ROHC, compression has not been defined for all kinds of IP
headers. Profile 0x0000 provides a way to send IP packets without
compressing them. This can be used for IP fragments, RTCP packets,
and in general for any packet for which compression of the header has
not been defined, is not possible due to resource constraints, or is
not desirable for some other reason.
After initialization, the only overhead for sending packets using
Profile 0x0000 is the size of the CID. When uncompressed packets are
frequent, Profile 0x0000 should be associated with a CID with size
zero or one octet. There is no need to associate Profile 0x0000 with
more than one CID.
5.10.1. IR packet
The initialization packet (IR packet) for Profile 0x0000 has the
following format:
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0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for small CIDs and (CID != 0)
+---+---+---+---+---+---+---+---+
| 1 1 1 1 1 1 0 |res|
+---+---+---+---+---+---+---+---+
: :
/ 0-2 octets of CID info / 1-2 octets if for large CIDs
: :
+---+---+---+---+---+---+---+---+
| Profile = 0 | 1 octet
+---+---+---+---+---+---+---+---+
| CRC | 1 octet
+---+---+---+---+---+---+---+---+
: : (optional)
/ IP packet / variable length
: :
--- --- --- --- --- --- --- ---
res: Always zero.
Profile: 0.
CRC: 8-bit CRC, computed using the polynomial of section 5.9.1.
The CRC covers the first octet of the IR packet through the
Profile octet of the IR packet, i.e., it does not cover the
CRC itself or the IP packet.
IP packet: An uncompressed IP packet may be included in the IR
packet. The decompressor determines if the IP packet is
present by considering the length of the IR packet.
5.10.2. Normal packet
A Normal packet is a normal IP packet plus CID information. When the
channel uses small CIDs, and profile 0x0000 is associated with a CID
> 0, an Add-CID octet is prepended to the IP packet. When the
channel uses large CIDs, the CID is placed so that it starts at the
second octet of the Normal packet.
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0 1 2 3 4 5 6 7
--- --- --- --- --- --- --- ---
: Add-CID octet : if for small CIDs and (CID != 0)
+---+---+---+---+---+---+---+---+
| first octet of IP packet |
+---+---+---+---+---+---+---+---+
: :
/ 0-2 octets of CID info / 1-2 octets if for large CIDs
: :
+---+---+---+---+---+---+---+---+
| |
/ rest of IP packet / variable length
| |
+---+---+---+---+---+---+---+---+
Note that the first octet of the IP packet starts with the bit
pattern 0100 (IPv4) or 0110 (IPv6). This does not conflict with any
reserved packet types. Hence, no bits in addition to the CID are
needed. The profile is reasonably future-proof since problems do not
occur until IP version 14.
5.10.3. States and modes
There are two modes in Profile 0x0000: Unidirectional mode and
Bidirectional mode. In Unidirectional mode, the compressor repeats
the IR packet periodically. In Bidirectional mode, the compressor
never repeats the IR packet. The compressor and decompressor always
start in Unidirectional mode. Whenever feedback is received, the
compressor switches to Bidirectional mode.
The compressor can be in either of two states: the IR state or the
Normal state. It starts in the IR state.
a) IR state: Only IR packets can be sent. After sending a small
number of IR packets (only one when refreshing), the compressor
switches to the Normal state.
b) Normal state: Only Normal packets can be sent. When in
Unidirectional mode, the compressor periodically transits back to
the IR state. The length of the period is implementation
dependent, but should be fairly long. Exponential backoff may be
used.
c) When feedback is received in any state, the compressor switches to
Bidirectional mode.
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The decompressor can be in either of two states: NO_CONTEXT or
FULL_CONTEXT. It starts in NO_CONTEXT.
d) When an IR packet is received in the NO_CONTEXT state, the
decompressor first verifies the packet using the CRC. If the
packet is OK, the decompressor 1) moves to the FULL_CONTEXT state,
2) delivers the IP packet to upper layers if present, 3) MAY send
an ACK. If the packet is not OK, it is discarded without further
action.
e) When any other packet is received in the NO_CONTEXT state, it is
discarded without further action.
f) When an IR packet is received in the FULL_CONTEXT state, the
packet is first verified using the CRC. If OK, the decompressor
1) delivers the IP packet to upper layers if present, 2) MAY send
an ACK. If the packet is not OK, no action is taken.
g) When a Normal packet is received in the FULL_CONTEXT state, the
CID information is removed and the IP packet is delivered to upper
layers.
5.10.4. Feedback
The only kind of feedback in Profile 0x0000 is ACKs. Profile 0x0000
MUST NOT be rejected. Profile 0x0000 SHOULD be associated with at
most one CID. ACKs use the FEEDBACK-1 format of section 5.2. The
value of the profile-specific octet in the FEEDBACK-1 ACK is 0
(zero).
5.11. ROHC UDP -- non-RTP UDP/IP compression (Profile 0x0002)
UDP/IP headers do not have a sequence number which is as well-behaved
as the RTP Sequence Number. For UDP/IPv4, there is an IP-ID field
which may be echoed in feedback information, but when no IPv4 header
is present such feedback identification becomes problematic.
Therefore, in the ROHC UDP profile, the compressor generates a 16-bit
sequence number SN which increases by one for each packet received in
the packet stream. This sequence number is thus relatively well-
behaved and can serve as the basis for most mechanisms described for
ROHC RTP. It is called SN or UDP SN below. Unless stated otherwise,
the mechanisms of ROHC RTP are used also for ROHC UDP, with the UDP
SN taking the role of the RTP Sequence Number.
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The ROHC UDP profile always uses p = -1 when interpreting the SN,
since there will be no repetitions or reordering of the compressor-
generated SN. The interpretation interval thus always starts with
(ref_SN + 1).
5.11.1. Initialization
The static context for ROHC UDP streams can be initialized in either
of two ways:
1) By using an IR packet as in section 5.7.7.1, where the profile is
two (2) and the static chain ends with the static part of an UDP
packet. At the compressor, UDP SN is initialized to a random
value when the IR packet is sent.
2) By reusing an existing context where the existing static chain
contains the static part of a UDP packet, e.g., the context of a
stream compressed using ROHC RTP (profile 0x0001). This is done
with an IR-DYN packet (section 5.7.7.2) identifying profile
0x0002, where the dynamic chain corresponds to the prefix of the
existing static chain that ends with the UDP header. UDP SN is
initialized to the RTP Sequence Number if the earlier profile was
profile 0x0001, and to a random number otherwise.
For ROHC UDP, the dynamic part of a UDP packet is different from
section 5.7.7.5: a two-octet field containing the UDP SN is added
after the Checksum field. This affects the format of dynamic chains
in IR and IR-DYN packets.
Note: 2) can be used for packet streams which were initially assumed
to be RTP streams, so that compression started with profile 0x0001,
but were later found evidently not to be RTP streams.
5.11.2. States and modes
ROHC UDP uses the same states and modes as ROHC RTP. Mode
transitions and state logic are the same except when explicitly
stated otherwise. Mechanisms dealing with fields in the RTP header
(except the RTP SN) are not used. The decompressed UDP SN is never
included in any header delivered to upper layers. The UDP SN is used
in place of the RTP SN in feedback.
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5.11.3. Packet types
The general format of a ROHC UDP packet is the same as for ROHC RTP
(see beginning of section 5.7). Padding and CIDs are the same, as is
the feedback packet type (5.7.6.1) and the feedback. IR and IR-DYN
packets (5.7.7) are changed as described in 5.11.1.
The general format of compressed packets is also the same, but there
are differences in specific formats and extensions as detailed below.
The differences are caused by removal of all RTP specific information
except the RTP SN, which is replaced by the UDP SN.
Unless explicitly stated below, the packet formats are as in sections
5.7.1-6.
R-1
The TS field is replaced by an IP-ID field. The M flag has become
part of IP-ID. The X bit has moved. The formats R-1-ID and R-1-
TS are not used.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 0 | SN |
+===+===+===+===+===+===+===+===+
| X | IP-ID |
+---+---+---+---+---+---+---+---+
UO-1
The TS field is replaced by an IP-ID field. The M flag has become
part of SN. Formats UO-1-ID and UO-1-TS are not used.
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 0 | IP-ID |
+===+===+===+===+===+===+===+===+
| SN | CRC |
+---+---+---+---+---+---+---+---+
UOR-2
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New format:
0 1 2 3 4 5 6 7
+---+---+---+---+---+---+---+---+
| 1 1 0 | SN |
+===+===+===+===+===+===+===+===+
| X | CRC |
+---+---+---+---+---+---+---+---+
5.11.4. Extensions
Extensions are as in 5.7.5, with the following exceptions:
Extension 0:
+---+---+---+---+---+---+---+---+
| 0 0 | SN | IP-ID |
+---+---+---+---+---+---+---+---+
Extension 1:
+---+---+---+---+---+---+---+---+
| 0 1 | SN | IP-ID |
+---+---+---+---+---+---+---+---+
| IP-ID |
+---+---+---+---+---+---+---+---+
Extension 2:
+---+---+---+---+---+---+---+---+
| 1 0 | SN | IP-ID2 |
+---+---+---+---+---+---+---+---+
| IP-ID2 |
+---+---+---+---+---+---+---+---+
| IP-ID |
+---+---+---+---+---+---+---+---+
IP-ID2: For outer IP-ID field.
Extension 3 is the same as Extension 3 in section 5.7.5, with the
following exceptions.
1) The initial flag octet has the following format:
0 1 2 3 4 5 6 7
+-----+-----+-----+-----+-----+-----+-----+-----+
| 1 1 | S | Mode | I | ip | ip2 |
+-----+-----+-----+-----+-----+-----+-----+-----+
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RFC 3095 Robust Header Compression July 2001
Mode: Replaces R-TS and Tsc of 5.7.5. Provides mode information
as was earlier done in RTP header flags and fields.
ip2: Replaces rtp bit of 5.7.5. Moved here from the Inner IP
header flags octet.
2) The bit which was the ip2 flag in the Inner IP header flags in
5.7.5 is reserved. It is set to zero when sending and ignored
when receiving.
5.11.5. IP-ID
Treated as in ROHC RTP, but the offset is from UDP SN.
5.11.6. Feedback
Feedback is as for ROHC RTP with the following exceptions:
1) UDP SN replaces RTP SN in feedback.
2) The CLOCK option (5.7.6.6) is not used.
3) The JITTER option (5.7.6.7) is not used.
5.12. ROHC ESP -- ESP/IP compression (Profile 0x0003)
When the ESP header is being used with an encryption algorithm other
than NULL, subheaders after the ESP header are encrypted and cannot
be compressed. Profile 0x0003 is for compression of the chain of
headers up to and including the ESP header in this case. When the
NULL encryption algorithm is being used, other profiles can be used
and could give higher compression rates. See section 5.8.4.3.
This profile is very similar to the ROHC UDP profile. It uses the
ESP sequence number as the basis for compression instead of a
generated number, but is otherwise very similar to ROHC UDP. The
interpretation interval (value of p) for the ESP-based SN is as with
ROHC RTP (profile 0x0001). Apart from this, unless stated explicitly
below, mechanisms and formats are as for ROHC UDP.
5.12.1. Initialization
The static context for ROHC ESP streams can be initialized in either
of two ways:
1) by using an IR packet as in section 5.7.7.1, where the profile is
three (3) and the static chain ends with the static part of an ESP
header.
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2) by reusing an existing context, where the existing static chain
contains the static part of an ESP header. This is done with an
IR-DYN packet (section 5.7.7.2) identifying profile 0x0003, where
the dynamic chain corresponds to the prefix of the existing static
chain that ends with the ESP header.
In contrast to ROHC UDP, no extra sequence number is added to the
dynamic part of the ESP header: the ESP sequence number is the only
element.
Note: 2) can be used for streams where compression has been initiated
under the assumption that NULL encryption was being used with ESP.
When it becomes obvious that an encryption algorithm other than NULL
is being used, the compressor may send an IR-DYN according to 2) to
switch to profile 0x0003 without having to send an IR packet.
5.12.2. Packet types
The packet types for ROHC ESP are the same as for ROHC UDP, except
that the ESP sequence number is used instead of the generated
sequence number of ROHC UDP. The ESP header is not part of any
compressed list in ROHC ESP.
6. Implementation issues
This document specifies mechanisms for the protocol and leaves many
details on the use of these mechanisms to the implementers. This
chapter is aimed to give guidelines, ideas and suggestions for
implementing the scheme.
6.1. Reverse decompression
This section describes an OPTIONAL decompressor operation to reduce
the number of packets discarded due to an invalid context.
Once a context becomes invalid (e.g., when more consecutive packet
losses than expected have occurred), subsequent compressed packets
cannot immediately be decompressed correctly. Reverse decompression
aims at decompressing such packets later instead of discarding them,
by storing them until the context has been updated and validated and
then attempting decompression.
Let the sequence of stored packets be i, i + 1, ..., i + k, where i
is the first packet and i + k is the last packet before the context
was updated. The decompressor will attempt to recover the stored
packets in reverse order, i.e., starting with i + k, and working back
toward i. When a stored packet has been reconstructed, its
correctness is verified using its CRC. Packets not carrying a CRC
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must not be delivered to upper layers. Packets where the CRC
succeeds are delivered to upper layers in their original order, i.e.,
i, i + 1, ..., i + k.
Note that this reverse decompression introduces buffering while
waiting for the context to be validated and thereby introduces
additional delay. Thus, it should be used only when some amount of
delay is acceptable. For example, for video packets belonging to the
same video frame, the delay in packet arrivals does not cause
presentation time delay. Delay-insensitive streaming applications
can also be tolerant of such delay. If the decompressor cannot
determine whether the application can tolerate delay, it should not
perform reverse decompression.
The following illustrates the decompression procedure in some detail:
1. The decompressor stores compressed packets that cannot be
decompressed correctly due to an invalid context.
2. When the decompressor has received a context updating packet and
the context has been validated, it proceeds to recover the last
packet stored. After decompression, the decompressor checks the
correctness of the reconstructed header using the CRC.
3. If the CRC indicates successful decompression, the decompressor
stores the complete packet and attempts to decompress the
preceding packet. In this way, the stored packets are recovered
in reverse order until no compressed packets are left. For each
packet, the decompressor checks the correctness of the
decompressed headers using the header compression CRC.
4. If the CRC indicates an incorrectly decompressed packet, the
reverse decompression attempt MUST be terminated and all remaining
uncompressed packets MUST be discarded.
5. Finally, the decompressor forwards all the correctly decompressed
packets to upper layers in their original order.
6.2. RTCP
RTCP is the RTP Control Protocol [RTP]. RTCP is based on periodic
transmission of control packets to all participants in a session,
using the same distribution mechanism as for data packets. Its
primary function is to provide feedback from the data receivers on
the quality of the data distribution. The feedback information may
be used for issues related to congestion control functions, and
directly useful for control of adaptive encodings.
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In an RTP session there will be two types of packet streams: one with
the RTP header and application data, and one with the RTCP control
information. The difference between the streams at the transport
level is in the UDP port numbers: the RTP port number is always even,
the RTCP port number is that number plus one and therefore always odd
[RTP, section 10]. The ROHC header compressor implementation has
several ways at hand to handle the RTCP stream:
1. One compressor/decompressor entity carrying both types of streams
on the same channel, using CIDs to distinguish between them. For
sending a single RTP stream together with its RTCP packets on one
channel, it is most efficient to set LARGE_CIDS to false, send the
RTP packets with the implied CID 0 and use the Add-CID mechanism
to send the RTCP packets.
2. Two compressor/decompressor entities, one for RTP and another one
for RTCP, carrying the two types of streams on separate channels.
This means that they will not share the same CID number space.
RTCP headers may simply be sent uncompressed using profile 0x0000.
More efficiently, ROHC UDP compression (profile 0x0002) can be used.
6.3. Implementation parameters and signals
A ROHC implementation may have two kinds of parameters: configuration
parameters that are mandatory and must be negotiated between
compressor and decompressor peers, and implementation parameters that
are optional and, when used, stipulate how a ROHC implementation is
to operate.
Configuration parameters are mandatory and must be negotiated between
compressor and decompressor, so that they have the same values at
both compressor and decompressor, see section 5.1.1.
Implementation parameters make it possible for an external entity to
stipulate how an implementation of a ROHC compressor or decompressor
should operate. Implementation parameters have local significance,
are optional to use and are thus not necessary to negotiate between
compressor and decompressor. Note that this does not preclude
signaling or negotiating implementation parameters using lower layer
functionality in order to set the way a ROHC implementation should
operate. Some implementation parameters are valid only at either of
compressor or decompressor. Implementation parameters may further be
divided into parameters that allow an external entity to describe the
way the implementation should operate and parameters that allow an
external entity to trigger a specific event, i.e., signals.
Bormann, et al. Standards Track [Page 136]
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6.3.1. ROHC implementation parameters at compressor
CONTEXT_REINITIALIZATION -- signal
This parameter triggers a reinitialization of the entire context at
the decompressor, both the static and the dynamic part. The
compressor MUST, when CONTEXT_REINITIALIZATION is triggered, back off
to the IR state and fully reinitialize the context by sending IR
packets with both the static and dynamic chains covering the entire
uncompressed headers until it is reasonably confident that the
decompressor contexts are reinitialized. The context
reinitialization MUST be done for all contexts at the compressor.
This parameter may for instance be used to do context relocation at,
e.g., a cellular handover that results in a change of compression
point in the radio access network.
NO_OF_PACKET_SIZES_ALLOWED -- value: positive integer
This parameter may be set by an external entity to specify the number
of packet sizes a ROHC implementation may use. However, the
parameter may be used only if PACKET_SIZES is not used by an external
entity. With this parameter set, the ROHC implementation at the
compressor MUST NOT use more different packet sizes than the value
this parameter stipulates. The ROHC implementation must itself be
able to determine which packet sizes will be used and describe these
to an external entity using PACKET_SIZES_USED. It should be noted
that one packet size might be used for several header formats, and
that the number of packet sizes can be reduced by employing padding
and segmentation.
NO_OF_PACKET_SIZES_USED _- value: positive integer
This parameter is set by the ROHC implementation to indicate how many
packet sizes it will actually use. It can be set to a large value to
indicate that no particular attempt is made to minimize that number.
PACKET_SIZES_ALLOWED -- value: list of positive integers (bytes)
This parameter, if set, governs which packet sizes in bytes may be
used by the ROHC implementation. Thus, packet sizes not in the set
of values for this parameter MUST NOT be used. Hence, an external
entity can mandate a ROHC implementation to produce packet sizes that
fit pre-configured lower layers better. If this parameter is used to
stipulate which packet sizes a ROHC implementation can use, the
following rules apply:
- A packet large enough to hold the entire IR header (both static and
dynamic chain) MUST be part of the set of sizes, unless MRRU is set
to a large enough value to allow segmentation.
- The packet size likely to be used most frequently in the SO state
SHOULD be part of the set.
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- The packet size likely to be used most frequently in the FO state
SHOULD be part of the set.
PACKET_SIZES_USED -- values: set of positive integers (bytes)
This parameter describes which packet sizes a ROHC implementation
uses if NO_OF_PACKET_SIZES_ALLOWED or PACKET_SIZES_ALLOWED is used by
an external entity to stipulate how many packet sizes a ROHC
implementation should use. The information about used packet sizes
(bytes) in this parameter, may then be used to configure lower
layers.
PAYLOAD_SIZES -_ values: set of positive integer values (bytes)
This parameter is set by an external entity that wants to make use of
the PACKET_SIZES_USED parameter to indicate which payload sizes can
be expected.
When a ROHC implementation has a limited set of allowed packet sizes,
and the most preferable header format has a size that is not part of
the set, it has the following options:
- Choose the next larger header format from the allowed set. This is
probably the most efficient choice.
- Use the most preferable header format as if there were no
restrictions on size, and then add padding octets to complete a
packet of the next larger size in the allowed set.
- Use segmentation to fragment the packet into pieces that would make
up packets of sizes that are permissible (possibly after the
addition of padding to the last segment).
It should be noted that even if the two last parameters introduce the
possibility of restricting the number of packet sizes used, such
restrictions will have a negative impact on compression performance.
6.3.2. ROHC implementation parameters at decompressor
MODE -- values: [U-mode, O-mode, R-mode]
This parameter triggers a mode transition using the mechanism
described in chapter 5 when the parameter changes value, i.e., to U-
mode (Unidirectional mode), O-mode (Bidirectional Optimistic mode) or
R-mode (Bidirectional Reliable mode). The mode transition is made
from the current mode to the new mode as signaled by the
implementation parameter. For example, if the current mode is
Bidirectional Optimistic mode, MODE should have the value O-mode. If
the MODE is changed to R-mode, a mode transition MUST be made from
Bidirectional Optimistic mode to Bidirectional Reliable mode. MODE
should not only serve as a trigger for mode transitions, but also
make it visible which mode ROHC operates in.
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CLOCK_RESOLUTION -- value: nonnegative integer
This parameter indicates the system clock resolution in units of
milliseconds. A zero (0) value means that there is no clock
available. If nonzero, this parameter allows the decompressor to use
timer-based TS compression (section 4.5.4) and SN wraparound
detection (section 5.3.2.2.4). In this case, its specific value is
also significant for correctness of the algorithms.
REVERSE_DECOMPRESSION_DEPTH -- value: nonnegative integer
This parameter determines whether reverse decompression as described
in section 6.1 should be used or not, and if used, to what extent.
The value indicates the maximum number of packets that can be
buffered, and thus possibly be reverse decompressed by the
decompressor. A zero (0) value means that reverse decompression MUST
NOT be used.
6.4. Handling of resource limitations at the decompressor
In a point-to-point link, the two nodes can agree on the number of
compressed sessions they are prepared to support for this link. It
may, however, not be possible for the decompressor to accurately
predict when it will run out of resources. ROHC allows the
negotiated number of contexts to be larger than could be accommodated
in the worst case. Then, as context resources are consumed, an
attempt to set up a new context may be rejected by the decompressor,
using the REJECT option of the feedback payload.
Upon reception of a REJECT option, the compressor SHOULD wait for a
while before attempting to compress additional streams destined for
the rejecting node.
6.5. Implementation structures
This section provides some explanatory material on data structures
that a ROHC implementation will have to maintain in one form or
another. It is not intended to constrain the implementations.
6.5.1. Compressor context
The compressor context consists of a static part and a dynamic part.
The content of the static part is the same as the static chain
defined in section 5.7.7. The dynamic part consists of multiple
elements which can be categorized into four types.
a) Sliding Window (SW)
b) Translation Table (TT)
c) Flag
d) Field
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These elements may be common to all modes or mode specific. The
following table summarizes all these elements.
+--------+---------------------------+-------------+----------------+
| | Common to | Specific to | Specific to |
| | all modes | R-mode | U/O-mode |
+--------+---------------------------+-------------+----------------+
| SWs | GSW | R_CSW | UO_CSW |
| | | R_IESW | UO_IESW |
+--------+---------------------------+-------------+----------------+
| TTs | | R_CTT | UO_CTT |
| | | R_IETT | UO_IETT |
+--------+---------------------------+-------------+----------------+
| Flags | UDP Chksum | | ACKED |
| | TSS, TIS | | |
| | RND, RND2 | | |
| | NBO, NBO2 | | |
+--------+---------------------------+-------------+----------------+
| Fields | Profile | | CSRC_REF_ID |
| | C_MODE | | CSRC_GEN_ID |
| | C_STATE | | CSRC_GEN_COUNT |
| | C_TRANS | | IPEH_REF_ID |
| | TS_STRIDE (if TSS = 1) | | IPEH_GEN_ID |
| | TS_OFFSET (if TSS = 1) | | IPEH_GEN_COUNT |
| | TIME_STRIDE (if TIS = 1) | | |
| | CURR_TIME (if TIS = 1) | | |
| | MAX_JITTER_CD (if TIS = 1)| | |
| | LONGEST_LOSS_EVENT(O) | | |
| | CLOCK_RESOLUTION(O) | | |
| | MAX_JITTER(O) | | |
+--------+---------------------------+-------------+----------------+
1) GSW: Generic W_LSB Sliding Window
Each element in GSW consists of all the dynamic fields in the
dynamic chain (defined in section 5.7.7) plus the fields specified
in a) but excluding the fields specified in b).
a) Packet Arrival Time (if TIS = 1)
Scaled RTP Time Stamp (if TSS = 1) (optional)
Offset_i (if RND = 0) (optional)
b) UDP Checksum, TS Stride, CSRC list, IPv6 Extension Headers
2) R_CSW: CSRC Sliding Window in R-mode
R_IESW: IPv6 Extension Header Sliding Window in R-mode
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UO_CSW: CSRC Sliding Window in U/O-mode
UO_IESW: IPv6 Extension Header Sliding Window in U/O-mode
Each element in R_CSW, R_IESW, UO_CSW and UO_IESW is defined in
section 6.5.3.
3) R_CTT: CSRC Translation Table in R-mode
R_IETT: IPv6 Extension Header Translation Table in U/O-mode
UO_CTT: CSRC Translation Table in U/O-mode
UO_IETT: IPv6 Extension Header Translation Table in U/O-mode
Each element in R_CTT and R_IETT is defined in section 5.8.1.1.
Each element in UO_CTT and UO_IETT is defined in section 5.8.1.2.
4) ACKED: Indicates whether or not the decompressor has ever acked
5) CURR_TIME: The current time value (used for context relocation
when timer-based timestamp compression is used)
6) All the other flags and fields are defined elsewhere in the ROHC
document.
6.5.2. Decompressor context
The decompressor context consists of a static part and a dynamic
part. The content of the static part is the same as the static chain
defined in section 5.7.7. The dynamic part consists of multiple
elements, one of which is the nonstatic reference header that
includes all the nonstatic fields. These nonstatic fields are the
fields in the dynamic chain defined in section 5.7.7, excluding UDP
Checksum and TS_Stride. All the remaining elements can be
categorized into four types:
a) Sliding Window (SW)
b) Translation Table (TT)
d) Flag
e) Field
These elements may be mode specific or common to all modes. The
following table summarizes all these elements.
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+--------+---------------------------+-------------+----------------+
| | Common to | Specific to | Specific to |
| | all modes | R-mode | U/O-mode |
+--------+---------------------------+-------------+----------------+
| SWs | | R_CSW | UO_CSW |
| | | R_IESW | UO_IESW |
+--------+---------------------------+-------------+----------------+
| TTs | | R_CTT | UO_CTT |
| | | R_IETT | UO_IETT |
+--------+---------------------------+-------------+----------------+
| Flags | UDP Checksum | | ACKED |
| | TSS, TIS | | |
| | RND, RND2 | | |
| | NBO, NBO2 | | |
+--------+---------------------------+-------------+----------------+
| Fields | Profile | | CSRC_GEN_ID |
| | D_MODE | | IPEH_GEN_ID |
| | D_STATE | | PRE_SN_V_REF |
| | D_TRANS | | |
| | TS_STRIDE (if TSS = 1) | | |
| | TS_OFFSET (if TSS = 1) | | |
| | TIME_STRIDE (if TIS = 1) | | |
| | PKT_ARR_TIME (if TIS = 1) | | |
| | LONGEST_LOSS_EVENT(O) | | |
| | CLOCK_RESOLUTION(O) | | |
| | MAX_JITTER(O) | | |
+--------+---------------------------+-------------+----------------+
1) ACKED: Indicates whether or not ACK has ever been sent.
2) PKT_ARR_TIME: The arrival time of the packet that most recently
decompressed and verified using CRC.
PRE_SN_V_REF: The sequence number of the packet verified before
the most recently verified packet.
CSRC_GEN_ID: The CSRC gen_id of the most recently received packet.
IPEH_GEN_ID: The IPv6 Extension Header gen_id of the most recently
received packet.
3) The remaining elements are as defined in the compressor context.
6.5.3. List compression: Sliding windows in R-mode and U/O-mode
In R-mode list compression (see section 5.8.2.1), each entry in the
sliding window, both at the compressor side and at the decompressor
side, has the following structure:
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+---------------------+--------+------------+
| RTP Sequence Number | icount | index list |
+---------------------+--------+------------+
The table index list contains a list of index. Each of these index
corresponds to the item in the original list carried in the packet
identified by the RTP Sequence Number. The mapping between the index
and the item is identified in the translation table. The icount
field carries the number of index in the following index list.
In U/O-mode list compression, each entry in the sliding window at
both the compressor side and decompressor side has the following
structure.
+--------+--------+------------+
| Gen_id | icount | index list |
+--------+--------+------------+
The icount and index list fields are the same as defined in R-mode.
Instead of using the RTP Sequence Number to identify each entry, the
Gen_id is included in the sliding window in U/O-mode.
7. Security Considerations
Because encryption eliminates the redundancy that header compression
schemes try to exploit, there is some inducement to forego encryption
of headers in order to enable operation over low-bandwidth links.
However, for those cases where encryption of data (and not headers)
is sufficient, RTP does specify an alternative encryption method in
which only the RTP payload is encrypted and the headers are left in
the clear. That would still allow header compression to be applied.
ROHC compression is transparent with regard to the RTP Sequence
Number and RTP Timestamp fields, so the values of those fields can be
used as the basis of payload encryption schemes (e.g., for
computation of an initialization vector).
A malfunctioning or malicious header compressor could cause the
header decompressor to reconstitute packets that do not match the
original packets but still have valid IP, UDP and RTP headers and
possibly also valid UDP checksums. Such corruption may be detected
with end-to-end authentication and integrity mechanisms which will
not be affected by the compression. Moreover, this header
compression scheme uses an internal checksum for verification of
reconstructed headers. This reduces the probability of producing
decompressed headers not matching the original ones without this
being noticed.
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Denial-of-service attacks are possible if an intruder can introduce
(for example) bogus STATIC, DYNAMIC or FEEDBACK packets onto the link
and thereby cause compression efficiency to be reduced. However, an
intruder having the ability to inject arbitrary packets at the link
layer in this manner raises additional security issues that dwarf
those related to the use of header compression.
8. IANA Considerations
The ROHC profile identifier is a non-negative integer. In many
negotiation protocols, it will be represented as a 16-bit value. Due
to the way the profile identifier is abbreviated in ROHC packets, the
8 least significant bits of the profile identifier have a special
significance: Two profile identifiers with identical 8 LSBs should be
assigned only if the higher-numbered one is intended to supersede the
lower-numbered one. To highlight this relationship, profile
identifiers should be given in hexadecimal (as in 0x1234, which would
for example supersede 0x0A34).
Following the policies outlined in [IANA-CONSIDERATIONS], the IANA
policy for assigning new values for the profile identifier shall be
Specification Required: values and their meanings must be documented
in an RFC or in some other permanent and readily available reference,
in sufficient detail that interoperability between independent
implementations is possible. In the 8 LSBs, the range 0 to 127 is
reserved for IETF standard-track specifications; the range 128 to 254
is available for other specifications that meet this requirement
(such as Informational RFCs). The LSB value 255 is reserved for
future extensibility of the present specification.
The following profile identifiers are already allocated:
Profile Document Usage
identifier
0x0000 RFCthis ROHC uncompressed
0x0001 RFCthis ROHC RTP
0x0002 RFCthis ROHC UDP
0x0003 RFCthis ROHC ESP
Bormann, et al. Standards Track [Page 144]
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9. Acknowledgments
Earlier header compression schemes described in [CJHC], [IPHC], and
[CRTP] have been important sources of ideas and knowledge.
The editor would like to extend his warmest thanks to Mikael
Degermark, who actually did a lot of the editing work, and Peter
Eriksson, who made a copy editing pass through the document,
significantly increasing its editorial consistency. Of course, all
remaining editorial problems have then been inserted by the editor.
Thanks to Andreas Jonsson (Lulea University), who supported this work
by his study of header field change patterns.
Finally, this work would not have succeeded without the continual
advice in navigating the IETF standards track, garnished with both
editorial and technical comments, from the IETF transport area
directors, Allison Mankin and Scott Bradner.
10. Intellectual Property Right Claim Considerations
The IETF has been notified of intellectual property rights claimed in
regard to some or all of the specification contained in this
document. For more information consult the online list of claimed
rights.
The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.
Bormann, et al. Standards Track [Page 145]
RFC 3095 Robust Header Compression July 2001
11. References
11.1. Normative References
[UDP] Postel, J., "User Datagram Protocol", STD 6,
RFC 768, August 1980.
[IPv4] Postel, J., "Internet Protocol", STD 5, RFC
791, September 1981.
[IPv6] Deering, S. and R. Hinden, "Internet Protocol,
Version 6 (IPv6) Specification", RFC 2460,
December 1998.
[RTP] Schulzrinne, H., Casner, S., Frederick, R. and
V. Jacobson, "RTP: A Transport Protocol for
Real-Time Applications", RFC 1889, January
1996.
[HDLC] Simpson, W., "PPP in HDLC-like framing", STD
51, RFC 1662, July 1994.
[ESP] Kent, S. and R. Atkinson, "IP Encapsulating
Security Payload", RFC 2406, November 1998.
[NULL] Glenn, R. and S. Kent, "The NULL Encryption
Algorithm and Its Use With Ipsec", RFC 2410,
November 1998.
[AH] Kent, S. and R. Atkinson, "IP Authentication
Header", RFC 2402, November 1998.
[MINE] Perkins, C., "Minimal Encapsulation within IP",
RFC 2004, October 1996.
[GRE1] Farinacci, D., Li, T., Hanks, S., Meyer, D. and
P. Traina, "Generic Routing Encapsulation
(GRE)", RFC 2784, March 2000.
[GRE2] Dommety, G., "Key and Sequence Number
Extensions to GRE", RFC 2890, August 2000.
[ASSIGNED] Reynolds, J. and J. Postel, "Assigned Numbers",
STD 2, RFC 1700, October 1994.
Bormann, et al. Standards Track [Page 146]
RFC 3095 Robust Header Compression July 2001
11.2. Informative References
[VJHC] Jacobson, V., "Compressing TCP/IP Headers for
Low-Speed Serial Links", RFC 1144, February
1990.
[IPHC] Degermark, M., Nordgren, B. and S. Pink, "IP
Header Compression", RFC 2507, February 1999.
[CRTP] Casner, S. and V. Jacobson, "Compressing
IP/UDP/RTP Headers for Low-Speed Serial Links",
RFC 2508, February 1999.
[CRTPC] Degermark, M., Hannu, H., Jonsson, L.E.,
Svanbro, K., "Evaluation of CRTP Performance
over Cellular Radio Networks", IEEE Personal
Communication Magazine, Volume 7, number 4, pp.
20-25, August 2000.
[REQ] Degermark, M., "Requirements for robust
IP/UDP/RTP header compression", RFC 3096, June
2001.
[LLG] Svanbro, K., "Lower Layer Guidelines for Robust
RTP/UDP/IP Header Compression", Work in
Progress.
[IANA-CONSIDERATIONS] Alvestrand, H. and T. Narten, "Guidelines for
Writing an IANA Considerations Section in
RFCs", BCP 26, RFC 2434, October 1998.
Bormann, et al. Standards Track [Page 147]
RFC 3095 Robust Header Compression July 2001
12. Authors' Addresses
Carsten Bormann, Editor
Universitaet Bremen TZI
Postfach 330440
D-28334 Bremen, Germany
Phone: +49 421 218 7024
Fax: +49 421 218 7000
EMail: cabo@tzi.org
Carsten Burmeister
Panasonic European Laboratories GmbH
Monzastr. 4c
63225 Langen, Germany
Phone: +49-6103-766-263
Fax: +49-6103-766-166
EMail: burmeister@panasonic.de
Mikael Degermark
The University of Arizona
Dept of Computer Science
P.O. Box 210077
Tucson, AZ 85721-0077, USA
Phone: +1 520 621-3498
Fax: +1 520 621-4642
EMail: micke@cs.arizona.edu
Hideaki Fukushima
Matsushita Electric Industrial Co.,
Ltd006, Kadoma, Kadoma City,
Osaka, Japan
Phone: +81-6-6900-9192
Fax: +81-6-6900-9193
EMail: fukusima@isl.mei.co.jp
Bormann, et al. Standards Track [Page 148]
RFC 3095 Robust Header Compression July 2001
Hans Hannu
Box 920
Ericsson Erisoft AB
SE-971 28 Lulea, Sweden
Phone: +46 920 20 21 84
Fax: +46 920 20 20 99
EMail: hans.hannu@ericsson.com
Lars-Erik Jonsson
Box 920
Ericsson Erisoft AB
SE-971 28 Lulea, Sweden
Phone: +46 920 20 21 07
Fax: +46 920 20 20 99
EMail: lars-erik.jonsson@ericsson.com
Rolf Hakenberg
Panasonic European Laboratories GmbH
Monzastr. 4c
63225 Langen, Germany
Phone: +49-6103-766-162
Fax: +49-6103-766-166
EMail: hakenberg@panasonic.de
Tmima Koren
Cisco Systems, Inc.
170 West Tasman Drive
San Jose, CA 95134, USA
Phone: +1 408-527-6169
EMail: tmima@cisco.com
Bormann, et al. Standards Track [Page 149]
RFC 3095 Robust Header Compression July 2001
Khiem Le
2-700
Mobile Networks Laboratory
Nokia Research Center
6000 Connection Drive
Irving, TX 75039, USA
Phone: +1-972-894-4882
Fax: +1 972 894-4589
EMail: khiem.le@nokia.com
Zhigang Liu
2-700
Mobile Networks Laboratory
Nokia Research Center
6000 Connection Drive
Irving, TX 75039, USA
Phone: +1 972 894-5935
Fax: +1 972 894-4589
EMail: zhigang.liu@nokia.com
Anton Martensson
Ericsson Radio Systems AB
Torshamnsgatan 23
SE-164 80 Stockholm, Sweden
Phone: +46 8 404 3881
Fax: +46 8 757 5550
EMail: anton.martensson@era.ericsson.se
Akihiro Miyazaki
Matsushita Electric Industrial Co., Ltd
1006, Kadoma, Kadoma City, Osaka, Japan
Phone: +81-6-6900-9192
Fax: +81-6-6900-9193
EMail: akihiro@isl.mei.co.jp
Bormann, et al. Standards Track [Page 150]
RFC 3095 Robust Header Compression July 2001
Krister Svanbro
Box 920
Ericsson Erisoft AB
SE-971 28 Lulea, Sweden
Phone: +46 920 20 20 77
Fax: +46 920 20 20 99
EMail: krister.svanbro@ericsson.com
Thomas Wiebke
Panasonic European Laboratories GmbH
Monzastr. 4c
63225 Langen, Germany
Phone: +49-6103-766-161
Fax: +49-6103-766-166
EMail: wiebke@panasonic.de
Takeshi Yoshimura
NTT DoCoMo, Inc.
3-5, Hikarinooka
Yokosuka, Kanagawa, 239-8536, Japan
Phone: +81-468-40-3515
Fax: +81-468-40-3788
EMail: yoshi@spg.yrp.nttdocomo.co.jp
Haihong Zheng
2-700
Mobile Networks Laboratory
Nokia Research Center
6000 Connection Drive
Irving, TX 75039, USA
Phone: +1 972 894-4232
Fax: +1 972 894-4589
EMail: haihong.zheng@nokia.com
Bormann, et al. Standards Track [Page 151]
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Appendix A. Detailed classification of header fields
Header compression is possible thanks to the fact that most header
fields do not vary randomly from packet to packet. Many of the
fields exhibit static behavior or change in a more or less
predictable way. When designing a header compression scheme, it is
of fundamental importance to understand the behavior of the fields in
detail.
In this appendix, all IP, UDP and RTP header fields are classified
and analyzed in two steps. First, we have a general classification
in A.1 where the fields are classified on the basis of stable
knowledge and assumptions. The general classification does not take
into account the change characteristics of changing fields because
those will vary more or less depending on the implementation and on
the application used. A less stable but more detailed analysis of
the change characteristics is then done in A.2. Finally, A.3
summarizes this appendix with conclusions about how the various
header fields should be handled by the header compression scheme to
optimize compression and functionality.
Bormann, et al. Standards Track [Page 152]
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A.1. General classification
At a general level, the header fields are separated into 5 classes:
INFERRED These fields contain values that can be inferred from
other values, for example the size of the frame
carrying the packet, and thus do not have to be
handled at all by the compression scheme.
STATIC These fields are expected to be constant throughout
the lifetime of the packet stream. Static information
must in some way be communicated once.
STATIC-DEF STATIC fields whose values define a packet stream.
They are in general handled as STATIC.
STATIC-KNOWN These STATIC fields are expected to have well-known
values and therefore do not need to be communicated
at all.
CHANGING These fields are expected to vary in some way:
randomly, within a limited value set or range, or in
some other manner.
In this section, each of the IP, UDP and RTP header fields is
assigned to one of these classes. For all fields except those
classified as CHANGING, the motives for the classification are also
stated. In section A.2, CHANGING fields are further examined and
classified on the basis of their expected change behavior.
A.1.1. IPv6 header fields
+---------------------+-------------+----------------+
| Field | Size (bits) | Class |
+---------------------+-------------+----------------+
| Version | 4 | STATIC |
| Traffic Class | 8 | CHANGING |
| Flow Label | 20 | STATIC-DEF |
| Payload Length | 16 | INFERRED |
| Next Header | 8 | STATIC |
| Hop Limit | 8 | CHANGING |
| Source Address | 128 | STATIC-DEF |
| Destination Address | 128 | STATIC-DEF |
+---------------------+-------------+----------------+
Bormann, et al. Standards Track [Page 153]
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Version
The version field states which IP version is used. Packets with
different values in this field must be handled by different IP
stacks. All packets of a packet stream must therefore be of the
same IP version. Accordingly, the field is classified as STATIC.
Flow Label
This field may be used to identify packets belonging to a specific
packet stream. If not used, the value should be set to zero.
Otherwise, all packets belonging to the same stream must have the
same value in this field, it being one of the fields that define
the stream. The field is therefore classified as STATIC-DEF.
Payload Length
Information about packet length (and, consequently, payload
length) is expected to be provided by the link layer. The field
is therefore classified as INFERRED.
Next Header
This field will usually have the same value in all packets of a
packet stream. It encodes the type of the subsequent header.
Only when extension headers are sometimes present and sometimes
not, will the field change its value during the lifetime of the
stream. The field is therefore classified as STATIC.
Source and Destination addresses
These fields are part of the definition of a stream and must thus
be constant for all packets in the stream. The fields are
therefore classified as STATIC-DEF.
Total size of the fields in each class:
+--------------+--------------+
| Class | Size (octets)|
+--------------+--------------+
| INFERRED | 2 |
| STATIC | 1.5 |
| STATIC-DEF | 34.5 |
| CHANGING | 2 |
+--------------+--------------+
Bormann, et al. Standards Track [Page 154]
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A.1.2. IPv4 header fields
+---------------------+-------------+----------------+
| Field | Size (bits) | Class |
+---------------------+-------------+----------------+
| Version | 4 | STATIC |
| Header Length | 4 | STATIC-KNOWN |
| Type Of Service | 8 | CHANGING |
| Packet Length | 16 | INFERRED |
| Identification | 16 | CHANGING |
| Reserved flag | 1 | STATIC-KNOWN |
| Don't Fragment flag | 1 | STATIC |
| More Fragments flag | 1 | STATIC-KNOWN |
| Fragment Offset | 13 | STATIC-KNOWN |
| Time To Live | 8 | CHANGING |
| Protocol | 8 | STATIC |
| Header Checksum | 16 | INFERRED |
| Source Address | 32 | STATIC-DEF |
| Destination Address | 32 | STATIC-DEF |
+---------------------+-------------+----------------+
Version
The version field states which IP version is used. Packets with
different values in this field must be handled by different IP
stacks. All packets of a packet stream must therefore be of the
same IP version. Accordingly, the field is classified as STATIC.
Header Length
As long no options are present in the IP header, the header length
is constant and well known. If there are options, the fields
would be STATIC, but it is assumed here that there are no options.
The field is therefore classified as STATIC-KNOWN.
Packet Length
Information about packet length is expected to be provided by the
link layer. The field is therefore classified as INFERRED.
Flags
The Reserved flag must be set to zero and is therefore classified
as STATIC-KNOWN. The Don't Fragment (DF) flag will be constant
for all packets in a stream and is therefore classified as STATIC.
Bormann, et al. Standards Track [Page 155]
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Finally, the More Fragments (MF) flag is expected to be zero
because fragmentation is NOT expected, due to the small packet
size expected. The More Fragments flag is therefore classified as
STATIC-KNOWN.
Fragment Offset
Under the assumption that no fragmentation occurs, the fragment
offset is always zero. The field is therefore classified as
STATIC-KNOWN.
Protocol
This field will usually have the same value in all packets of a
packet stream. It encodes the type of the subsequent header.
Only when extension headers are sometimes present and sometimes
not, will the field change its value during the lifetime of a
stream. The field is therefore classified as STATIC.
Header Checksum
The header checksum protects individual hops from processing a
corrupted header. When almost all IP header information is
compressed away, there is no point in having this additional
checksum; instead it can be regenerated at the decompressor side.
The field is therefore classified as INFERRED.
Source and Destination addresses
These fields are part of the definition of a stream and must thus
be constant for all packets in the stream. The fields are
therefore classified as STATIC-DEF.
Total size of the fields in each class:
+--------------+----------------+
| Class | Size (octets) |
+--------------+----------------+
| INFERRED | 4 |
| STATIC | 1 oct + 5 bits |
| STATIC-DEF | 8 |
| STATIC-KNOWN | 2 oct + 3 bits |
| CHANGING | 4 |
+--------------+----------------+
Bormann, et al. Standards Track [Page 156]
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A.1.3. UDP header fields
+------------------+-------------+-------------+
| Field | Size (bits) | Class |
+------------------+-------------+-------------+
| Source Port | 16 | STATIC-DEF |
| Destination Port | 16 | STATIC-DEF |
| Length | 16 | INFERRED |
| Checksum | 16 | CHANGING |
+------------------+-------------+-------------+
Source and Destination ports
These fields are part of the definition of a stream and must thus
be constant for all packets in the stream. The fields are
therefore classified as STATIC-DEF.
Length
This field is redundant and is therefore classified as INFERRED.
Total size of the fields in each class:
+------------+---------------+
| Class | Size (octets) |
+------------+---------------+
| INFERRED | 2 |
| STATIC-DEF | 4 |
| CHANGING | 2 |
+------------+---------------+
A.1.4. RTP header fields
+-----------------+-------------+----------------+
| Field | Size (bits) | Class |
+-----------------+-------------+----------------+
| Version | 2 | STATIC-KNOWN |
| Padding | 1 | STATIC |
| Extension | 1 | STATIC |
| CSRC Counter | 4 | CHANGING |
| Marker | 1 | CHANGING |
| Payload Type | 7 | CHANGING |
| Sequence Number | 16 | CHANGING |
| Timestamp | 32 | CHANGING |
| SSRC | 32 | STATIC-DEF |
| CSRC | 0(-480) | CHANGING |
+-----------------+-------------+----------------+
Bormann, et al. Standards Track [Page 157]
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Version
Only one working RTP version exists, namely version 2. The field
is therefore classified as STATIC-KNOWN.
Padding
The use of this field is application-dependent, but when payload
padding is used it is likely to be present in all packets. The
field is therefore classified as STATIC.
Extension
If RTP extensions are used by the application, these extensions
are likely to be present in all packets (but the use of extensions
is very uncommon). However, for safety's sake this field is
classified as STATIC and not STATIC-KNOWN.
SSRC
This field is part of the definition of a stream and must thus be
constant for all packets in the stream. The field is therefore
classified as STATIC-DEF.
Total size of the fields in each class:
+--------------+---------------+
| Class | Size (octets) |
+--------------+---------------+
| STATIC | 2 bits |
| STATIC-DEF | 4 |
| STATIC-KNOWN | 2 bits |
| CHANGING | 7.5(-67.5) |
+--------------+---------------+
Bormann, et al. Standards Track [Page 158]
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A.1.5. Summary for IP/UDP/RTP
Summarizing this for IP/UDP/RTP one obtains
+----------------+----------------+----------------+
| Class \ IP ver | IPv6 (octets) | IPv4 (octets) |
+----------------+----------------+----------------+
| INFERRED | 4 | 6 |
| STATIC | 1 oct + 6 bits | 1 oct + 7 bits |
| STATIC-DEF | 42.5 | 16 |
| STATIC-KNOWN | 2 bits | 2 oct + 5 bits |
| CHANGING | 11.5(-71.5) | 13.5(-73.5) |
+----------------+----------------+----------------+
| Total | 60(-120) | 40(-100) |
+----------------+----------------+----------------+
A.2. Analysis of change patterns of header fields
To design suitable mechanisms for efficient compression of all header
fields, their change patterns must be analyzed. For this reason, an
extended classification is done based on the general classification
in A.1, considering the fields which were labeled CHANGING in that
classification. Different applications will use the fields in
different ways, which may affect their behavior. For the fields
whose behavior is variable, typical behavior for conversational audio
and video will be discussed.
The CHANGING fields are separated into five different subclasses:
STATIC These are fields that were classified as
CHANGING on a general basis, but are classified
as STATIC here due to certain additional
assumptions.
SEMISTATIC These fields are STATIC most of the time.
However, occasionally the value changes but
reverts to its original value after a known
number of packets.
RARELY-CHANGING (RC) These are fields that change their values
occasionally and then keep their new values.
ALTERNATING These fields alternate between a small number
of different values.
IRREGULAR These, finally, are the fields for which no
useful change pattern can be identified.
Bormann, et al. Standards Track [Page 159]
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To further expand the classification possibilities without increasing
complexity, the classification can be done either according to the
values of the field and/or according to the values of the deltas for
the field.
When the classification is done, other details are also stated
regarding possible additional knowledge about the field values and/or
field deltas, according to the classification. For fields classified
as STATIC or SEMISTATIC, the case could be that the value of the
field is not only STATIC but also well KNOWN a priori (two states for
SEMISTATIC fields). For fields with non-irregular change behavior,
it could be known that changes usually are within a LIMITED range
compared to the maximal change for the field. For other fields, the
values are completely UNKNOWN.
Bormann, et al. Standards Track [Page 160]
RFC 3095 Robust Header Compression July 2001
Table A.1 classifies all the CHANGING fields on the basis of their
expected change patterns, especially for conversational audio and
video.
+------------------------+-------------+-------------+-------------+
| Field | Value/Delta | Class | Knowledge |
+========================+=============+=============+=============+
| Sequential | Delta | STATIC | KNOWN |
| -----------+-------------+-------------+-------------+
| IPv4 Id: Seq. jump | Delta | RC | LIMITED |
| -----------+-------------+-------------+-------------+
| Random | Value | IRREGULAR | UNKNOWN |
+------------------------+-------------+-------------+-------------+
| IP TOS / Tr. Class | Value | RC | UNKNOWN |
+------------------------+-------------+-------------+-------------+
| IP TTL / Hop Limit | Value | ALTERNATING | LIMITED |
+------------------------+-------------+-------------+-------------+
| Disabled | Value | STATIC | KNOWN |
| UDP Checksum: ---------+-------------+-------------+-------------+
| Enabled | Value | IRREGULAR | UNKNOWN |
+------------------------+-------------+-------------+-------------+
| No mix | Value | STATIC | KNOWN |
| RTP CSRC Count: -------+-------------+-------------+-------------+
| Mixed | Value | RC | LIMITED |
+------------------------+-------------+-------------+-------------+
| RTP Marker | Value | SEMISTATIC | KNOWN/KNOWN |
+------------------------+-------------+-------------+-------------+
| RTP Payload Type | Value | RC | UNKNOWN |
+------------------------+-------------+-------------+-------------+
| RTP Sequence Number | Delta | STATIC | KNOWN |
+------------------------+-------------+-------------+-------------+
| RTP Timestamp | Delta | RC | LIMITED |
+------------------------+-------------+-------------+-------------+
| No mix | - | - | - |
| RTP CSRC List: -------+-------------+-------------+-------------+
| Mixed | Value | RC | UNKNOWN |
+------------------------+-------------+-------------+-------------+
Table A.1 : Classification of CHANGING header fields
The following subsections discuss the various header fields in
detail. Note that table A.1 and the discussions below do not
consider changes caused by loss or reordering before the compression
point.
Bormann, et al. Standards Track [Page 161]
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A.2.1. IPv4 Identification
The Identification field (IP ID) of the IPv4 header is there to
identify which fragments constitute a datagram when reassembling
fragmented datagrams. The IPv4 specification does not specify
exactly how this field is to be assigned values, only that each
packet should get an IP ID that is unique for the source-destination
pair and protocol for the time the datagram (or any of its fragments)
could be alive in the network. This means that assignment of IP ID
values can be done in various ways, which we have separated into
three classes.
Sequential jump
This is the most common assignment policy in today's IP stacks. A
single IP ID counter is used for all packet streams. When the
sender is running more than one packet stream simultaneously, the
IP ID can increase by more than one between packets in a stream.
The IP ID values will be much more predictable and require less
bits to transfer than random values, and the packet-to-packet
increment (determined by the number of active outgoing packet
streams and sending frequencies) will usually be limited.
Random
Some IP stacks assign IP ID values using a pseudo-random number
generator. There is thus no correlation between the ID values of
subsequent datagrams. Therefore there is no way to predict the IP
ID value for the next datagram. For header compression purposes,
this means that the IP ID field needs to be sent uncompressed
with each datagram, resulting in two extra octets of header. IP
stacks in cellular terminals SHOULD NOT use this IP ID assignment
policy.
Sequential
This assignment policy keeps a separate counter for each outgoing
packet stream and thus the IP ID value will increment by one for
each packet in the stream, except at wrap around. Therefore, the
delta value of the field is constant and well known a priori.
When RTP is used on top of UDP and IP, the IP ID value follows
the RTP Sequence Number. This assignment policy is the most
desirable for header compression purposes. However, its usage is
not as common as it perhaps should be. The reason may be that it
can be realized only when UDP and IP are implemented together so
that UDP, which separates packet streams by the Port
identification fields, can make IP use separate ID counters for
each packet stream.
Bormann, et al. Standards Track [Page 162]
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In order to avoid violating [IPv4], packets sharing the same IP
address pair and IP protocol number cannot use the same IP ID
values. Therefore, implementations of sequential policies must
make the ID number spaces disjoint for packet streams of the same
IP protocol going between the same pair of nodes. This can be
done in a number of ways, all of which introduce occasional
jumps, and thus makes the policy less than perfectly sequential.
For header compression purposes less frequent jumps are
preferred.
It should be noted that the ID is an IPv4 mechanism and is therefore
not a problem for IPv6. For IPv4 the ID could be handled in three
different ways. First, we have the inefficient but reliable solution
where the ID field is sent as-is in all packets, increasing the
compressed headers by two octets. This is the best way to handle the
ID field if the sender uses random assignment of the ID field.
Second, there can be solutions with more flexible mechanisms
requiring less bits for the ID handling as long as sequential jump
assignment is used. Such solutions will probably require even more
bits if random assignment is used by the sender. Knowledge about the
sender's assignment policy could therefore be useful when choosing
between the two solutions above. Finally, even for IPv4, header
compression could be designed without any additional information for
the ID field included in compressed headers. To use such schemes, it
must be known which assignment policy for the ID field is being used
by the sender. That might not be possible to know, which implies
that the applicability of such solutions is very uncertain. However,
designers of IPv4 stacks for cellular terminals SHOULD use an
assignment policy close to sequential.
A.2.2. IP Traffic-Class / Type-Of-Service
The Traffic-Class (IPv6) or Type-Of-Service (IPv4) field is expected
to be constant during the lifetime of a packet stream or to change
relatively seldom.
A.2.3. IP Hop-Limit / Time-To-Live
The Hop-Limit (IPv6) or Time-To-Live (IPv4) field is expected to be
constant during the lifetime of a packet stream or to alternate
between a limited number of values due to route changes.
A.2.4. UDP Checksum
The UDP checksum is optional. If disabled, its value is constantly
zero and could be compressed away. If enabled, its value depends on
the payload, which for compression purposes is equivalent to it
changing randomly with every packet.
Bormann, et al. Standards Track [Page 163]
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A.2.5. RTP CSRC Counter
This is a counter indicating the number of CSRC items present in the
CSRC list. This number is expected to be almost constant on a
packet- to-packet basis and change by small amounts. As long as no
RTP mixer is used, the value of this field is zero.
A.2.6. RTP Marker
For audio the marker bit should be set only in the first packet of a
talkspurt, while for video it should be set in the last packet of
every picture. This means that in both cases the RTP marker is
classified as SEMISTATIC with well-known values for both states.
A.2.7. RTP Payload Type
Changes of the RTP payload type within a packet stream are expected
to be rare. Applications could adapt to congestion by changing
payload type and/or frame sizes, but that is not expected to happen
frequently.
A.2.8. RTP Sequence Number
The RTP Sequence Number will be incremented by one for each packet
sent.
A.2.9. RTP Timestamp
In the audio case:
As long as there are no pauses in the audio stream, the RTP
Timestamp will be incremented by a constant delta, corresponding
to the number of samples in the speech frame. It will thus mostly
follow the RTP Sequence Number. When there has been a silent
period and a new talkspurt begins, the timestamp will jump in
proportion to the length of the silent period. However, the
increment will probably be within a relatively limited range.
In the video case:
Between two consecutive packets, the timestamp will either be
unchanged or increase by a multiple of a fixed value corresponding
to the picture clock frequency. The timestamp can also decrease
by a multiple of the fixed value if B-pictures are used. The
delta interval, expressed as a multiple of the picture clock
frequency, is in most cases very limited.
Bormann, et al. Standards Track [Page 164]
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A.2.10. RTP Contributing Sources (CSRC)
The participants in a session, which are identified by the CSRC
fields, are expected to be almost the same on a packet-to-packet
basis with relatively few additions and removals. As long as RTP
mixers are not used, no CSRC fields are present at all.
A.3. Header compression strategies
This section elaborates on what has been done in previous sections.
On the basis of the classifications, recommendations are given on how
to handle the various fields in the header compression process.
Seven different actions are possible; these are listed together with
the fields to which each action applies.
A.3.1. Do not send at all
The fields that have well known values a priori do not have to be
sent at all. These are:
- IPv6 Payload Length
- IPv4 Header Length
- IPv4 Reserved Flag
- IPv4 Last Fragment Flag
- IPv4 Fragment Offset
- UDP Checksum (if disabled)
- RTP Version
A.3.2. Transmit only initially
The fields that are constant throughout the lifetime of the packet
stream have to be transmitted and correctly delivered to the
decompressor only once. These are:
- IP Version
- IP Source Address
- IP Destination Address
- IPv6 Flow Label
- IPv4 May Fragment Flag
- UDP Source Port
- UDP Destination Port
- RTP Padding Flag
- RTP Extension Flag
- RTP SSRC
Bormann, et al. Standards Track [Page 165]
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A.3.3. Transmit initially, but be prepared to update
The fields that are changing only occasionally must be transmitted
initially but there must also be a way to update these fields with
new values if they change. These fields are:
- IPv6 Next Header
- IPv6 Traffic Class
- IPv6 Hop Limit
- IPv4 Protocol
- IPv4 Type Of Service (TOS)
- IPv4 Time To Live (TTL)
- RTP CSRC Counter
- RTP Payload Type
- RTP CSRC List
Since the values of the IPv4 Protocol and the IPv6 Next Header fields
are in effect linked to the type of the subsequent header, they
deserve special treatment when subheaders are inserted or removed.
A.3.4. Be prepared to update or send as-is frequently
For fields that normally either are constant or have values deducible
from some other field, but that frequently diverge from that
behavior, there must be an efficient way to update the field value or
send it as-is in some packets. These fields are:
- IPv4 Identification (if not sequentially assigned)
- RTP Marker
- RTP Timestamp
A.3.5. Guarantee continuous robustness
For fields that behave like a counter with a fixed delta for ALL
packets, the only requirement on the transmission encoding is that
packet losses between compressor and decompressor must be tolerable.
If several such fields exist, all these can be communicated together.
Such fields can also be used to interpret the values for fields
listed in the previous section. Fields that have this counter
behavior are:
- IPv4 Identification (if sequentially assigned)
- RTP Sequence Number
Bormann, et al. Standards Track [Page 166]
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A.3.6. Transmit as-is in all packets
Fields that have completely random values for each packet must be
included as-is in all compressed headers. Those fields are:
- IPv4 Identification (if randomly assigned)
- UDP Checksum (if enabled)
A.3.7. Establish and be prepared to update delta
Finally, there is a field that is usually increasing by a fixed delta
and is correlated to another field. For this field it would make
sense to make that delta part of the context state. The delta must
then be initiated and updated in the same way as the fields listed in
A.3.3. The field to which this applies is:
- RTP Timestamp
Bormann, et al. Standards Track [Page 167]
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Full Copyright Statement
Copyright (C) The Internet Society (2001). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
Bormann, et al. Standards Track [Page 168]