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Keywords: [--------|p], multiprotocol label switching, hc
Network Working Group J. Ash, Ed.
Request for Comments: 4901 J. Hand, Ed.
Category: Standards Track AT&T
A. Malis, Ed.
Verizon Communications
June 2007
Protocol Extensions for Header Compression over MPLS
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 IETF Trust (2007).
Abstract
This specification defines how to use Multi-Protocol Label Switching
(MPLS) to route Header-Compressed (HC) packets over an MPLS label
switched path. HC can significantly reduce packet-header overhead
and, in combination with MPLS, can also increases bandwidth
efficiency and processing scalability in terms of the maximum number
of simultaneous compressed flows that use HC at each router). Here
we define how MPLS pseudowires are used to transport the HC context
and control messages between the ingress and egress MPLS label
switching routers. This is defined for a specific set of existing HC
mechanisms that might be used, for example, to support voice over IP.
This specification also describes extension mechanisms to allow
support for future, as yet to be defined, HC protocols. In this
specification, each HC protocol operates independently over a single
pseudowire instance, very much as it would over a single point-to-
point link.
Ash, et al. Standards Track [Page 1]
RFC 4901 Header Compression over MPLS Protocol June 2007
Table of Contents
1. Introduction ....................................................3
2. Terminology .....................................................3
3. Header Compression over MPLS Protocol Overview ..................6
4. Protocol Specifications ........................................11
4.1. MPLS Pseudowire Setup and Signaling .......................13
4.2. Header Compression Scheme Setup, Negotiation, and
Signaling .................................................14
4.2.1. Configuration Option Format [RFC3544] ..............15
4.2.2. RTP-Compression Suboption [RFC3544] ................17
4.2.3. Enhanced RTP-Compression Suboption [RFC3544] .......18
4.2.4. Negotiating Header Compression for Only TCP
or Only Non-TCP Packets [RFC3544] ..................19
4.2.5. Configuration Option Format [RFC3241] ..............20
4.2.6. PROFILES Suboption [RFC3241] .......................21
4.3. Encapsulation of Header Compressed Packets ................22
4.4. Packet Reordering .........................................23
5. HC Pseudowire Setup Example ....................................24
6. Security Considerations ........................................29
7. Acknowledgements ...............................................29
8. IANA Considerations ............................................29
9. Normative References ...........................................30
10. Informative References ........................................31
11. Contributors ..................................................33
Ash, et al. Standards Track [Page 2]
RFC 4901 Header Compression over MPLS Protocol June 2007
1. Introduction
Voice over IP (VoIP) typically uses the encapsulation
voice/RTP/UDP/IP. When MPLS labels [RFC3031] are added, this becomes
voice/RTP/UDP/IP/MPLS-labels. MPLS VPNs (e.g., [RFC4364]) use label
stacking, and in the simplest case of IPv4 the total packet header is
at least 48 bytes, while the voice payload is often no more than 30
bytes, for example. When IPv6 is used, the relative size of the
header in comparison to the payload is even greater. The interest in
header compression (HC) is to exploit the possibility of
significantly reducing the overhead through various compression
mechanisms, such as with enhanced compressed RTP (ECRTP) [RFC3545]
and robust header compression (ROHC) [RFC3095, RFC3095bis, RFC4815],
and also to increase scalability of HC. MPLS is used to route HC
packets over an MPLS label switched path (LSP) without
compression/decompression cycles at each router. Such an HC over
MPLS capability can increase bandwidth efficiency as well as the
processing scalability of the maximum number of simultaneous
compressed flows that use HC at each router. Goals and requirements
for HC over MPLS are discussed in [RFC4247]. The solution using MPLS
pseudowire (PW) technology put forth in this document has been
designed to address these goals and requirements.
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 [RFC2119].
Context: the state associated with a flow subject to IP header
compression. While the exact nature of the context is specific to a
particular HC protocol (CRTP, ECRTP, ROHC, etc.), this state
typically includes:
- the values of all of the fields in all of the headers (IP, UDP,
TCP, RTP, Encapsulating Security Payload (ESP), etc.) that the
particular header compression protocol operates on for the last
packet of the flow sent (by the compressor) or received (by the
decompressor).
- the change in the value of some of the fields in the IP, UDP,
TCP, etc. headers between the last two consecutive sent packets
(compressor) or received packets (decompressor) of the flow.
Some of the fields in the header change by a constant amount
between subsequent packets in the flow most of the time. Saving
the changes in these fields from packet to packet allows
Ash, et al. Standards Track [Page 3]
RFC 4901 Header Compression over MPLS Protocol June 2007
verification that a constant rate of change is taking place, and
to take appropriate action when a deviation from the normal
changes are encountered.
For most HC protocols, a copy of the context of each compressed flow
is maintained at both the compressor and the decompressor.
compressed Real-time Transport Protocol (CRTP): a particular HC
protocol described in [RFC2508].
Context ID (CID): a small number, typically 8 or 16 bits, used to
identify a particular flow, and the context associated with the flow.
Most HC protocols in essence work by sending the CID across the link
in place of the full header, along with any unexpected changes in the
values in the various fields of the headers.
Enhanced Compressed Real-time Protocol (ECRTP): a particular HC
protocol described in [RFC3545].
Forwarding Equivalence Class (FEC): a group of packets that are
forwarded in the same manner (e.g., over the same LSP, with the same
forwarding treatment)
Header Compression scheme (HC scheme): a particular method of
performing HC and its associated protocol. Multiple methods of HC
have been defined, including Robust Header Compression (ROHC
[RFC3095, RFC3095bis]), compressed RTP (CRTP, [RFC2508]), enhanced
CRTP (ECRTP, [RFC3545]), and IP Header Compression (IPHC, [RFC2507]).
This document explicitly supports all of the HC schemes listed above,
and is intended to be extensible to others that may be developed.
Header Compression channel (HC channel): a session established
between a header compressor and a header decompressor using a single
HC scheme, over which multiple individual flows may be compressed.
From this perspective, every PPP link over which HC is operating
defines a single HC channel, and based on this specification, every
HC PW defines a single HC channel. HC PWs are bi-directional, which
means that a unidirectional leg of the PW is set up in each
direction. One leg of the bi-directional PW may be set up to carry
only compression feedback, not header compressed traffic. An HC
channel should not be confused with the individual traffic flows that
may be compressed using a single Context ID. Each HC channel manages
a set of unique CIDs.
IP Header Compression (IPHC): a particular HC protocol described in
[RFC2507]
Ash, et al. Standards Track [Page 4]
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Label: a short fixed length physically contiguous identifier that is
used to identify a FEC, usually of local significance
Label Stack: an ordered set of labels
Label Switched Path (LSP): the path through one or more LSRs at one
level of the hierarchy followed by a packet in a particular
forwarding equivalence class (FEC)
Label Switching Router (LSR): an MPLS node that is capable of
forwarding native L3 packets
MPLS domain: a contiguous set of nodes that operate MPLS routing and
forwarding and which are also in one Routing or Administrative Domain
MPLS label: a label that is carried in a packet header, and that
represents the packet's FEC
MPLS node: a node that is running MPLS. An MPLS node will be aware
of MPLS control protocols, will operate one or more L3 routing
protocols, and will be capable of forwarding packets based on labels.
An MPLS node may also optionally be capable of forwarding native L3
packets.
Multiprotocol Label Switching (MPLS): an IETF working group and the
effort associated with the working group, including the technology
(signaling, encapsulation, etc.) itself
Packet Switched Network (PSN): Within the context of Pseudowire PWE3,
this is a network using IP or MPLS as the mechanism for packet
forwarding.
Protocol Data Unit (PDU): the unit of data output to, or received
from, the network by a protocol layer.
Pseudowire (PW): a mechanism that carries the essential elements of
an emulated service from one provider edge router to one or more
other provider edge routers over a PSN
Pseudowire Emulation Edge to Edge (PWE3): a mechanism that emulates
the essential attributes of service (such as a T1 leased line or
Frame Relay) over a PSN
Pseudowire PDU (PW-PDU): a PDU sent on the PW that contains all of
the data and control information necessary to emulate the desired
service
Ash, et al. Standards Track [Page 5]
RFC 4901 Header Compression over MPLS Protocol June 2007
PSN Tunnel: a tunnel across a PSN, inside which one or more PWs can
be carried
PSN Tunnel Signaling: a protocol used to set up, maintain, and tear
down the underlying PSN tunnel
PW Demultiplexer: data-plane method of identifying a PW terminating
at a provider edge router
Real Time Transport Protocol (RTP): a protocol for end-to-end network
transport for applications transmitting real-time data, such as audio
or video [RFC3550].
Robust Header Compression (ROHC): a particular HC protocol consisting
of a framework [RFC3095bis] and a number of profiles for different
protocols, e.g., for RTP, UDP, ESP [RFC3095], and IP [RFC3843]
Tunnel: a method of transparently carrying information over a network
3. Header Compression over MPLS Protocol Overview
To implement HC over MPLS, after the ingress router applies the HC
algorithm to the IP packet, the compressed packet is forwarded on an
MPLS LSP using MPLS labels, and then the egress router restores the
uncompressed header. Any of a number of HC algorithms/protocols can
be used. These algorithms have generally been designed for operation
over a single point-to-point link-layer hop. MPLS PWs [RFC3985],
which are used to provide emulation of many point-to-point link layer
services (such as frame relay permanent virtual circuits (PVCs) and
ATM PVCs) are used here to provide emulation of a single, point-to-
point link layer hop over which HC traffic may be transported.
Figure 1 illustrates an HC over MPLS channel established on an LSP
that traverses several LSRs, from R1/HC --> R2 --> R3 --> R4/HD,
where R1/HC is the ingress router performing HC, and R4/HD is the
egress router performing header decompression (HD). This example
assumes that the packet flow being compressed has RTP/UDP/IP headers
and is using a HC scheme such as ROHC, CRTP, or ECRTP. Compression
of the RTP/UDP/IP header is performed at R1/HC, and the compressed
packets are routed using MPLS labels from R1/HC to R2, to R3, and
finally to R4/HD, without further decompression/recompression cycles.
The RTP/UDP/IP header is decompressed at R4/HD and can be forwarded
to other routers, as needed. This example assumes that the
application is VoIP and that the HC algorithm operates on the RTP,
UDP, and IP headers of the VoIP flows. This is an extremely common
application of HC, but need not be the only one. The HC algorithms
supported by the protocol extensions specified in this document may
operate on TCP or IPsec ESP headers as well.
Ash, et al. Standards Track [Page 6]
RFC 4901 Header Compression over MPLS Protocol June 2007
|
| data (e.g., voice)/RTP/UDP/IP/link layer
V
_____
| |
|R1/HC| Header Compression (HC) Performed
|_____|
|
| data (e.g., voice)/compressed-header/MPLS-labels
V
_____
| |
| R2 | Label Switching
|_____| (no compression/decompression)
|
| data (e.g., voice)/compressed-header/MPLS-labels
V
_____
| |
| R3 | Label Switching
|_____| (no compression/decompression)
|
| data (e.g., voice)/compressed-header/MPLS-labels
V
_____
| |
|R4/HD| Header Decompression (HD) Performed
|_____|
|
| data (e.g., voice)/RTP/UDP/IP/link layer
V
Figure 1: Example of HC over MPLS over Routers R1 --> R4
In the example scenario, HC therefore takes place between R1 and R4,
and the MPLS LSP transports data/compressed-header/MPLS-labels
instead of data/RTP/UDP/IP/MPLS-labels, often saving more than 90% of
the RTP/UDP/IP overhead. Typically there are two MPLS labels (8
octets) and a link-layer HC control parameter (2 octets). The MPLS
label stack and link-layer headers are not compressed. Therefore, HC
over MPLS can significantly reduce the header overhead through
compression mechanisms.
HC reduces the IP/UDP/RTP headers to 2-4 bytes for most packets.
Half of the reduction in header size comes from the observation that
half of the bytes in the IP/UDP/RTP headers remain constant over the
life of the flow. After sending the uncompressed header template
once, these fields may be removed from the compressed headers that
Ash, et al. Standards Track [Page 7]
RFC 4901 Header Compression over MPLS Protocol June 2007
follow. The remaining compression comes from the observation that
although several fields change in every packet, the difference from
packet to packet is often constant or at least limited, and therefore
the second-order difference is zero.
The compressor and decompressor both maintain a context for each
compressed flow. The context is the session state shared between the
compressor and decompressor. The details of what is included in the
context may vary between HC schemes. The context at the compressor
would typically include the uncompressed headers of the last packet
sent on the flow, and some measure of the differences in selected
header field values between the last packet transmitted and the
packet(s) transmitted just before it. The context at the
decompressor would include similar information about received
packets. With this information, all that must be communicated across
the wire is an indication of which flow a packet is associated with
(the CID), and some compact encoding of the second order differences
(i.e., the harder to predict differences) between packets.
MPLS PWs [RFC3985] are used to transport the HC packets between the
ingress and egress MPLS LSRs. Each PW acts like a logical point-to-
point link between the compressor and the decompressor. Each PW
supports a single HC channel, which, from the perspective of the HC
scheme operation, is similar to a single PPP link or a single frame
relay PVC. One exception to this general model is that PWs carry
only packets with compressed headers, and do not share the PW with
uncompressed packets.
The PW architecture specifies the use of a label stack with at least
2 levels. The label at the bottom of the stack is called the PW
label. The PW label acts as an identifier for a particular PW. With
HC PWs, the compressor adds the label at the bottom of the stack and
the decompressor removes this label. No LSRs between the compressor
and decompressor inspect or modify this label. Labels higher in the
stack are called the packet switch network (PSN) labels, and are used
to forward the packet through the MPLS network as described in
[RFC3031]. The decompressor uses the incoming MPLS PW label (the
label at the bottom of the stack), along with the CID to locate the
proper decompression context. Standard HC methods (e.g., ECRTP,
ROHC, etc.) are used to determine the contexts. The CIDs are
assigned by the HC as normal, and there would be no problem if
duplicate CIDs are received at the HD for different PWs, which
support different compressed channels. For example, if two different
compressors, HCa and HCb, both assign the same CID to each of 2
separate flows destined to decompressor HDc, HDc can still
differentiate the flows and locate the proper decompression context
for each, because the tuples <PWlabel-HCa, CID> and <PWlabel-HCb,
CID> are still unique.
Ash, et al. Standards Track [Page 8]
RFC 4901 Header Compression over MPLS Protocol June 2007
In addition to the PW label and PSN label(s), HC over MPLS packets
also carry a HC control parameter. The HC control parameter contains
both a packet type field and a packet length field. The packet type
field is needed because each HC scheme supported by this
specification defines multiple packet types, for example, "full
header" packets, which are used to initialize and/or re-synchronize
the context between compressor and decompressor, vs. normal HC
packets. And most of the HC schemes require that the underlying link
layer protocols provide the differentiation between packet types.
Similarly, one of the assumptions that is part of most of the HC
schemes is that the packet length fields in the RTP/UDP/IP, etc.
headers need not be explicitly sent across the network, because the
IP datagram length can be implicitly determined from the lower
layers. This specification assumes that, with one exception, the
length of an HC IP datagram can be determined from the link layers of
the packets transmitted across the MPLS network. The exception is
for packets that traverse an Ethernet link. Ethernet requires
padding for packets whose payload size is less than 46 bytes in
length. So the HC control parameter contains a length field of 6
bits to encode the lengths of any HC packets less than 64 bytes in
length.
HC PWs are set up by the PW signaling protocol [RFC4447]. [RFC4447]
actually defines a set of extensions to the MPLS label distribution
protocol (LDP) [RFC3036]. As defined in [RFC4447], LDP signaling to
set up, tear down, and manage PWs is performed directly between the
PW endpoints, in this case, the compressor and the decompressor. PW
signaling is used only to set up the PW label at the bottom of the
stack, and is used independently of any other signaling that may be
used to set up PSN labels. So, for example, in Figure 1, LDP PW
signaling would be performed directly between R1/HC and R4/HD.
Router R2 and R3 would not participate in PW signaling.
[RFC4447] provides extensions to LDP for PWs, and this document
provides further extensions specific to HC. Since PWs provide a
logical point-to-point connection over which HC can be run, the
extensions specified in this document reuse elements of the protocols
used to negotiate HC over the Point-to-Point Protocol [RFC1661].
[RFC3241] specifies how ROHC is used over PPP and [RFC3544] specifies
how several other HC schemes (CRTP, ECRTP, IPHC) are used over PPP.
Both of these RFCs provide configuration options for negotiating HC
over PPP. The formats of these configuration options are reused here
for setting up HC over PWs. When used in the PPP environment, these
configuration options are used as extensions to PPP's IP Control
Protocol [RFC1332] and the detailed PPP options negotiations process
described in [RFC1661]. This is necessary because a PPP link may
support multiple protocols, each with its own addressing scheme and
options. Achieving interoperability requires a negotiation process
Ash, et al. Standards Track [Page 9]
RFC 4901 Header Compression over MPLS Protocol June 2007
so that the nodes at each end of the link can agree on a set of
protocols and options that both support. However, a single HC PW
supports only HC traffic using a single HC scheme. So while the
formats of configuration options from [RFC3241] and [RFC3544] are
reused here, the detailed PPP negotiation process is not. Instead,
these options are reused here just as descriptors (TLVs in the
specific terminology of LDP and [RFC4447]) of basic parameters of an
HC PW. These parameters are further described in Section 4. The HC
configuration parameters are initially generated by the decompressor
and describe what the decompressor is prepared to receive.
Most HC schemes use a feedback mechanism which requires bi-
directional flow of HC packets, even if the flow of compressed IP
packets is in one direction only. The basic signaling process of
[RFC4447] sets up unidirectional PWs, and must be repeated in each
direction in order to set up the bi-directional flow needed for HC.
Figure 1 illustrates an example data flow set up from R1/HC --> R2
--> R3 --> R4/HD, where R1/HC is the ingress router where header
compression is performed, and R4/HD is the egress router where header
decompression is done. Each router functions as an LSR and supports
signaling of LSP/PWs. See Section 5 for a detailed example of how
the flow depicted in Figure 1 is established.
All the HC schemes used here are built so that if an uncompressible
packet is seen, it should just be sent uncompressed. For some types
of compression (e.g., IPHC-TCP), a non-compressed path is required.
For IPHC-TCP compression, uncompressible packets occur for every TCP
flow. Another way that this kind of issue can occur is if MAX_HEADER
is configured lower than the longest header, in which case,
compression might not be possible in some cases.
The uncompressed packets associated with HC flows (e.g., uncompressed
IPHC-TCP packets) can be sent through the same MPLS tunnel along with
all other non-HC (non-PW) IP packets. MPLS tunnels can transport
many types of packets simultaneously, including non-PW IP packets,
layer 3 VPN packets, and PW (e.g., HC flow) packets. In the
specification, we assume that there is a path for uncompressed
traffic, and it is a compressor decision as to what would or would
not go in the HC-PW.
Ash, et al. Standards Track [Page 10]
RFC 4901 Header Compression over MPLS Protocol June 2007
4. Protocol Specifications
Figure 2 illustrates the PW stack reference model to support PW
emulated services.
+-------------+ +-------------+
| Layer2 | | Layer2 |
| Emulated | | Emulated |
| Services | Emulated Service | Services |
| |<==============================>| |
+-------------+ +-------------+
| HC | Pseudowire | HD |
|Demultiplexer|<==============================>|Demultiplexer|
+-------------+ +-------------+
| PSN | PSN Tunnel | PSN |
| MPLS |<==============================>| MPLS |
+-------------+ +-------------+
| Physical | | Physical |
+-----+-------+ +-----+-------+
Figure 2: Pseudowire Protocol Stack Reference Model
Each HC-HD compressed channel is mapped to a single PW and associated
with 2 PW labels, one in each direction. A single PW label MUST be
used for many HC flows (could be 100's or 1000's) rather than
assigning a different PW label to each flow. The latter approach
would involve a complex mechanism for PW label assignment, freeing up
of labels after a flow terminates, etc., for potentially 1000's of
simultaneous HC flows. On the other hand, the mechanism for CID
assignment, freeing up, etc., is in place and there is no need to
duplicate it with PW assignment/deassignment for individual HC flows.
Multiple PWs SHOULD be established in case different quality of
service (QoS) requirements are needed for different compressed
streams. The QoS received by the flow would be determined by the EXP
bit marking in the PW label. Normally, all RTP packets would get the
same EXP marking [RFC3270], equivalent to expedited forwarding (EF)
treatment [RFC3246] in Diffserv. However, the protocol specified in
this document applies to several different types of streams, not just
RTP streams, and QoS treatment other than EF may be required for
those streams.
Figure 3 shows the HC over MPLS protocol stack (with uncompressed
header):
Ash, et al. Standards Track [Page 11]
RFC 4901 Header Compression over MPLS Protocol June 2007
Media stream
RTP
UDP
IP
HC control parameter
MPLS label stack (at least 2 labels for this application)
Link layer under MPLS (PPP, PoS, Ethernet)
Physical layer (SONET/SDH, fiber, copper)
+--------------+
| Media stream |
+--------------+
\_______ ______/
2-4 octets V
+------+--------------+
Compressed /RTP/UDP/IP/ |header| |
+------+--------------+
\__________ __________/
2 octets V
+------+---------------------+
HC Control Parameter |header| |
+------+---------------------+
\______________ _____________/
8 octets V
+------+----------------------------+
MPLS Labels |header| |
+------+----------------------------+
\_________________ _________________/
V
+------------------------------------------+
Link Layer under MPLS | |
+------------------------------------------+
\____________________ _____________________/
V
+-------------------------------------------------+
Physical Layer | |
+-------------------------------------------------+
Figure 3: Header Compression over MPLS Media Stream Transport
The HC control parameter MUST be used to identify the packet types
for the HC scheme in use. The MPLS labels technically define two
layers: the PW identifier and the MPLS tunnel identifier. The PW
label MUST be used as the demultiplexer field by the HD, where the PW
label appears at the bottom label of an MPLS label stack. The LSR
that will be performing decompression MUST ensure that the label it
distributes (e.g., via LDP) for a channel is unique. There can also
Ash, et al. Standards Track [Page 12]
RFC 4901 Header Compression over MPLS Protocol June 2007
be other MPLS labels, for example, to identify an MPLS VPN. The
IP/UDP/RTP headers are compressed before transmission, leaving the
rest of the stack alone, as shown in Figure 3.
4.1. MPLS Pseudowire Setup and Signaling
PWs MUST be set up in advance for the transport of media streams
using [RFC4447] control messages exchanged by the HC-HD endpoints.
Furthermore, a PW type MUST be used to indicate the HC scheme being
used on the PW. [RFC4447] specifies the MPLS label distribution
protocol (LDP) [RFC3036] extensions to set up and maintain the PWs,
and defines new LDP objects to identify and signal attributes of PWs.
Any acceptable method of MPLS label distribution MAY be used for
distributing the MPLS tunnel label [RFC3031]. These methods include
LDP [RFC3036], RSVP-TE [RFC3209], or configuration.
To assign and distribute the PW labels, an LDP session MUST be set up
between the PW endpoints using the extended discovery mechanism
described in [RFC3036]. The PW label bindings are distributed using
the LDP downstream unsolicited mode described in [RFC3036]. An LDP
label mapping message contains a FEC object, a label object, and
possible other optional objects. The FEC object indicates the
meaning of the label, identifies the PW type, and identifies the PW
that the PW label is bound to. See [RFC4447] for further explanation
of PW signaling.
This specification defines new PW type values to be carried within
the FEC object to identify HC PWs for each HC scheme. The PW type is
a 15-bit parameter assigned by IANA, as specified in the [RFC4446]
registry, and MUST be used to indicate the HC scheme being used on
the PW. IANA has set aside the following PW type values for
assignment according to the registry specified in RFC 4446, Section
3.2:
PW type Description Reference
=============================================================
0x001A ROHC Transport Header-compressed Packets [RFC3095bis]
0x001B ECRTP Transport Header-compressed Packets [RFC3545]
0x001C IPHC Transport Header-compressed Packets [RFC2507]
0x001D CRTP Transport Header-compressed Packets [RFC2508]
The HC control parameter enables distinguishing between various
packets types (e.g., uncompressed, UDP compressed, RTP compressed,
context-state, etc.). However, the HC control parameter indications
are not unique across HC schemes, and therefore the PW type value
allows the HC scheme to be identified.
Ash, et al. Standards Track [Page 13]
RFC 4901 Header Compression over MPLS Protocol June 2007
4.2. Header Compression Scheme Setup, Negotiation, and Signaling
As described in the previous section, the HC PW MUST be used for
compressed packets only, which is configured at PW setup. If a flow
is not compressed, it MUST NOT be placed on the HC PW. HC PWs MUST
be bi-directional, which means that a unidirectional leg of the PW
MUST be set up in each direction. One leg of the bi-directional PW
MAY be set up to carry only compression feedback, not header
compressed traffic. The same PW type MUST be used for PW signaling
in both directions.
HC scheme parameters MAY be manually configured, but if so, manual
configuration MUST be done in both directions. If HC scheme
parameters are signaled, the Interface Parameters Sub-TLV MUST be
used on any unidirectional legs of a PW that will carry HC traffic.
For a unidirectional leg of a PW that will carry only compression
feedback, the components of the Interface Parameters Sub-TLV
described below are not relevant and MUST NOT be used.
The PW HC approach relies on the PW/MPLS layer to convey HC channel
configuration information. The Interface Parameters Sub-TLV [IANA,
RFC4447] must be used to signal HC channel setup and specify HC
parameters. That is, the configuration options specified in
[RFC3241, RFC3544] are reused in this specification to specify PW-
specific parameters, and to configure the HC and HD ports at the
edges of the PW so that they have the necessary capabilities to
interoperate with each other.
Pseudowire Interface Parameter Sub-TLV type values are specified in
[RFC4446]. IANA has set aside the following Pseudowire Interface
Parameter Sub-TLV type values according to the registry specified in
RFC 4446, Section 3.3:
Parameter ID Length Description Reference
--------- --------------- ---------------------------- ---------
0x0D up to 256 bytes ROHC over MPLS configuration RFC 4901
RFC 3241
0x0F up to 256 bytes CRTP/ECRTP/IPHC HC over MPLS RFC 4901
configuration RFC 3544
TLVs identified in [RFC3241] and [RFC3544] MUST be encapsulated in
the PW Interface Parameters Sub-TLV and used to negotiate header
compression session setup and parameter negotiation for their
respective protocols. The TLVs supported in this manner MUST include
the following:
Ash, et al. Standards Track [Page 14]
RFC 4901 Header Compression over MPLS Protocol June 2007
o Configuration Option Format, RTP-Compression Suboption, Enhanced
RTP-Compression Suboption, TCP/non-TCP Compression Suboptions, as
specified in [RFC3544]
o Configuration Option Format, PROFILES Suboption, as specified in
[RFC3241]
These TLVs are now specified in the following sections.
4.2.1. Configuration Option Format [RFC3544]
Both the network control protocol for IPv4, IPCP [RFC1332] and the
IPv6 Network Control Protocol (NCP), IPV6CP [RFC2472] may be used to
negotiate IP HC parameters for their respective controlled protocols.
The format of the configuration option is the same for both IPCP and
IPV6CP. This configuration option MUST be included for ECRTP, CRTP
and IPHC PW types and MUST NOT be included for ROHC PW types. A
decompressor MUST reject this option (if misconfigured) for ROHC PW
types and send an explicit error message to the compressor [RFC3544].
Description
This NCP configuration option is used to negotiate parameters for
IP HC. Successful negotiation of parameters enables the use of
Protocol Identifiers FULL_HEADER, COMPRESSED_TCP,
COMPRESSED_TCP_NODELTA, COMPRESSED_NON_TCP, and CONTEXT_STATE as
specified in [RFC2507]. The option format is summarized below.
The fields are transmitted from left to right.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | IP-Compression-Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TCP_SPACE | NON_TCP_SPACE |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| F_MAX_PERIOD | F_MAX_TIME |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAX_HEADER | suboptions... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
2
Ash, et al. Standards Track [Page 15]
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Length
>= 14
The length may be increased if the presence of additional
parameters is indicated by additional suboptions.
IP-Compression-Protocol
0061 (hex)
TCP_SPACE
The TCP_SPACE field is two octets and indicates the maximum
value of a context identifier in the space of context
identifiers allocated for TCP.
Suggested value: 15
TCP_SPACE must be at least 0 and at most 255 (the value 0
implies having one context). This field is not used for CRTP
(PW type 0x001B) and ECRTP (PW type 0x001B) PWs. For these PW
types, it should be set to its suggested value by the sender
and ignored by the receiver.
NON_TCP_SPACE
The NON_TCP_SPACE field is two octets and indicates the maximum
value of a context identifier in the space of context
identifiers allocated for non-TCP. These context identifiers
are carried in COMPRESSED_NON_TCP, COMPRESSED_UDP and
COMPRESSED_RTP packet headers.
Suggested value: 15
NON_TCP_SPACE must be at least 0 and at most 65535 (the value 0
implies having one context).
F_MAX_PERIOD
Maximum interval between full headers. No more than
F_MAX_PERIOD COMPRESSED_NON_TCP headers may be sent between
FULL_HEADER headers.
Suggested value: 256
A value of zero implies infinity, i.e., there is no limit to
the number of consecutive COMPRESSED_NON_TCP headers. This
field is not used for CRTP (PW type 0x001B) and ECRTP (PW type
0x001B) PWs. For these PW types, it should be set to its
suggested value by the sender and ignored by the receiver.
Ash, et al. Standards Track [Page 16]
RFC 4901 Header Compression over MPLS Protocol June 2007
F_MAX_TIME
Maximum time interval between full headers. COMPRESSED_NON_TCP
headers may not be sent more than F_MAX_TIME seconds after
sending the last FULL_HEADER header.
Suggested value: 5 seconds
A value of zero implies infinity. This field is not used for
CRTP (PW type 0x001B) and ECRTP (PW type 0x001B) PWs. For
these PW types, it should be set to its suggested value by the
sender and ignored by the receiver.
MAX_HEADER
The largest header size in octets that may be compressed.
Suggested value: 168 octets
The value of MAX_HEADER should be large enough so that at least
the outer network layer header can be compressed. To increase
compression efficiency MAX_HEADER should be set to a value
large enough to cover common combinations of network and
transport layer headers.
suboptions
The suboptions field consists of zero or more suboptions. Each
suboption consists of a type field, a length field and zero or
more parameter octets, as defined by the suboption type. The
value of the length field indicates the length of the suboption
in its entirety, including the lengths of the type and length
fields.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Parameters...|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.2.2. RTP-Compression Suboption [RFC3544]
The RTP-Compression suboption is included in the NCP IP-Compression-
Protocol option for IPHC if IP/UDP/RTP compression is to be enabled.
This suboption MUST be included for CRTP PWs (0x001C) and MUST NOT be
included for other PW types.
Inclusion of the RTP-Compression suboption enables use of additional
Protocol Identifiers COMPRESSED_RTP and COMPRESSED_UDP along with
additional forms of CONTEXT_STATE as specified in [RFC2508].
Ash, et al. Standards Track [Page 17]
RFC 4901 Header Compression over MPLS Protocol June 2007
Description
Enables the use of Protocol Identifiers COMPRESSED_RTP,
COMPRESSED_UDP, and CONTEXT_STATE as specified in [RFC2508].
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
1
Length
2
4.2.3. Enhanced RTP-Compression Suboption [RFC3544]
To use the enhanced RTP HC defined in [RFC3545], a new suboption 2 is
added. Suboption 2 is negotiated instead of, not in addition to,
suboption 1. This suboption MUST be included for ECRTP PWs (0x001B)
and MUST NOT be included for other PW types.
Note that suboption 1 refers to the RTP-Compression Suboption, as
specified in Section 4.2.2, and suboption 2 refers to the Enhanced
RTP-Compression Suboption, as specified in Section 4.2.3. These
suboptions MUST NOT occur together. If they do (e.g., if
misconfigured), a decompressor MUST reject this option and send an
explicit error message to the compressor [RFC3544].
Description
Enables the use of Protocol Identifiers COMPRESSED_RTP and
CONTEXT_STATE as specified in [RFC2508]. In addition, it enables
the use of [RFC3545] compliant compression including the use of
Protocol Identifier COMPRESSED_UDP with additional flags and use
of the C flag with the FULL_HEADER Protocol Identifier to indicate
use of HDRCKSUM with COMPRESSED_RTP and COMPRESSED_UDP packets.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
2
Ash, et al. Standards Track [Page 18]
RFC 4901 Header Compression over MPLS Protocol June 2007
Length
2
4.2.4. Negotiating Header Compression for Only TCP or Only Non-TCP
Packets [RFC3544]
In [RFC3544] it was not possible to negotiate only TCP HC or only
non-TCP HC because a value of 0 in the TCP_SPACE or the NON_TCP_SPACE
fields actually means that 1 context is negotiated.
A new suboption 3 is added to allow specifying that the number of
contexts for TCP_SPACE or NON_TCP_SPACE is zero, disabling use of the
corresponding compression. This suboption MUST be included for IPHC
PWs (0x001C) and MUST NOT be included for other PW types.
Description
Enable HC for only TCP or only non-TCP packets.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Parameter |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
3
Length
3
Parameter
The parameter is 1 byte with one of the following values:
1 = the number of contexts for TCP_SPACE is 0
2 = the number of contexts for NON_TCP_SPACE is 0
This suboption overrides the values that were previously assigned to
TCP_SPACE and NON_TCP_SPACE in the IP HC option.
If suboption 3 is included multiple times with parameter 1 and 2,
compression is disabled for all packets.
Ash, et al. Standards Track [Page 19]
RFC 4901 Header Compression over MPLS Protocol June 2007
4.2.5. Configuration Option Format [RFC3241]
Both the network control protocol for IPv4, IPCP [RFC1332] and the
IPv6 NCP, IPV6CP [RFC2472] may be used to negotiate IP HC parameters
for their respective controlled protocols. The format of the
configuration option is the same for both IPCP and IPV6CP. This
configuration option MUST be included for ROHC PW types and MUST NOT
be included for ECRTP, CRTP, and IPHC PW types. A decompressor MUST
reject this option (if misconfigured) for ECRTP, CRTP, and IPHC PW
types, and send an explicit error message to the compressor
[RFC3544].
Description
This NCP configuration option is used to negotiate parameters for
ROHC. The option format is summarized below. The fields are
transmitted from left to right.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | IP-Compression-Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAX_CID | MRRU |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| MAX_HEADER | suboptions... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
2
Length
>= 10
The length may be increased if the presence of additional
parameters is indicated by additional suboptions.
IP-Compression-Protocol
0003 (hex)
MAX_CID
The MAX_CID field is two octets and indicates the maximum value of
a context identifier.
Suggested value: 15
MAX_CID must be at least 0 and at most 16383 (The value 0 implies
having one context).
Ash, et al. Standards Track [Page 20]
RFC 4901 Header Compression over MPLS Protocol June 2007
MRRU
The MRRU field is two octets and indicates the maximum
reconstructed reception unit (see [RFC3095bis], Section 5.1.2).
Suggested value: 0
MAX_HEADER
The largest header size in octets that may be compressed.
Suggested value: 168 octets
The value of MAX_HEADER should be large enough so that at least
the outer network layer header can be compressed. To increase
compression efficiency MAX_HEADER should be set to a value large
enough to cover common combinations of network and transport layer
headers.
NOTE: The four ROHC profiles defined in RFC 3095 do not provide
for a MAX_HEADER parameter. The parameter MAX_HEADER defined by
this document is therefore without consequence in these profiles
because the maximum compressible header size is unspecified.
Other profiles (e.g., ones based on RFC 2507) can make use of the
parameter by explicitly referencing it.
suboptions
The suboptions field consists of zero or more suboptions. Each
suboption consists of a type field, a length field, and zero or
more parameter octets, as defined by the suboption type. The
value of the length field indicates the length of the suboption in
its entirety, including the lengths of the type and length fields.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Parameters...|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.2.6. PROFILES Suboption [RFC3241]
The set of profiles to be enabled is subject to negotiation. Most
initial implementations of ROHC implement profiles 0x0000 to 0x0003.
This option MUST be supplied.
Description
Define the set of profiles supported by the decompressor.
Ash, et al. Standards Track [Page 21]
RFC 4901 Header Compression over MPLS Protocol June 2007
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length | Profiles... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type
1
Length
2n+2
Value
n octet-pairs in ascending order, each octet-pair specifying a
ROHC profile supported.
HC flow identification is being done now in many ways. Since there
are multiple possible approaches to the problem, no specific method
is specified in this document.
4.3. Encapsulation of Header Compressed Packets
The HC control parameter is used to identify the packet types for
IPHC [RFC2507], CRTP [RFC2508], and ECRTP [RFC3545], as shown in
Figure 4:
1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0 0 0 0|Pkt Typ| Length |Res|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: HC Control Parameter
where:
"Packet Type" encoding:
0: ROHC Small-CIDs
1: ROHC Large-CIDs
2: FULL_HEADER
3: COMPRESSED_TCP
4: COMPRESSED_TCP_NODELTA
5: COMPRESSED_NON_TCP
6: COMPRESSED_RTP_8
7: COMPRESSED_RTP_16
8: COMPRESSED_UDP_8
9: COMPRESSED_UDP_16
10: CONTEXT_STATE
Ash, et al. Standards Track [Page 22]
RFC 4901 Header Compression over MPLS Protocol June 2007
11-15: Not yet assigned. (See Section 8, "IANA Considerations",
for discussion of the registration rules.)
As discussed in [ECMP-AVOID], since this MPLS payload type is not IP,
the first nibble is set to 0000 to avoid being mistaken for IP. This
is also consistent with the encoding of the PW MPLS control word
(PWMCW) described in [RFC4385]; however, the HC control parameter is
not intended to be a PWMCW.
Note that ROHC [RFC3095, RFC3095bis] provides its own packet type
within the protocol; however, the HC control parameter MUST still be
used to avoid the problems identified above. Since the "Packet Type"
will be there anyway, it is used to indicate ROHC CID size, in the
same way as with PPP.
The HC control parameter length field is ONLY used for short packets
because padding may be appended by the Ethernet Data Link Layer. If
the length is greater than or equal to 64 octets, the length field
MUST be set to zero. If the MPLS payload is less than 64 bytes, then
the length field MUST be set to the length of the PW payload plus the
length of the HC control parameter. Note that the last 2 bits in the
HC control parameter are reserved.
4.4. Packet Reordering
Packet reordering for ROHC is discussed in [RFC4224], which is a
useful source of information. In case of lossy links and other
reasons for reordering, implementation adaptations are needed to
allow all the schemes to be used in this case. Although CRTP is
viewed as having risks for a number of PW environments due to
reordering and loss, it is still the protocol of choice in many
cases. CRTP was designed for reliable point to point links with
short delays. It does not perform well over links with a high rate
of packet loss, packet reordering, and long delays. In such cases,
ECRTP [RFC3545] may be considered to increase robustness to both
packet loss and misordering between the compressor and the
decompressor. This is achieved by repeating updates and sending of
absolute (uncompressed) values in addition to delta values for
selected context parameters. IPHC should use TCP_NODELTA, ECRTP
should send absolute values, ROHC should be adapted as discussed in
[RFC4224]. An evaluation and simulation of ECRTP and ROHC reordering
is given in [REORDER-EVAL].
Ash, et al. Standards Track [Page 23]
RFC 4901 Header Compression over MPLS Protocol June 2007
5. HC Pseudowire Setup Example
This example will trace the setup of an MPLS PW supporting bi-
directional ECRTP [RFC3545] traffic. The example assumes the
topology shown in Figure 1. The PW will be set up between LSRs R1/HC
and R4/HD. LSRs R2 and R3 have no direct involvement in the
signaling for this PW, other than to transport the signaling traffic.
For this example, it is assumed that R1/HC has already obtained the
IP address of R4/HD used for LDP signaling, and vice versa, that both
R1/HC and R4/HD have been configured with the same 32-bit PW ID, as
described in Section 5.2 of [RFC4447], and that R1/HC has been
configured to initiate the LDP discovery process. Furthermore, we
assume that R1/HC has been configured to receive a maximum of 200
simultaneous ECRTP flows from R4/HD, and R4/HD has been configured to
receive a maximum of 255 ECRTP flows from R1/HC.
Assuming that there is no existing LDP session between R1/HC and
R4/HD, the PW signaling must start by setting up an LDP session
between them. As described earlier in this document, LDP extended
discovery is used between HC over MPLS LSRs. Since R1/HC has been
configured to initiate extended discovery, it will send LDP Targeted
Hello messages to R4/HD's IP address at UDP port 646. The Targeted
Hello messages sent by R1/HC will have the "R" bit set in the Common
Hello Parameters TLV, requesting R4/HD to send Targeted Hello
messages back to R1/HC. Since R4/HD has been configured to set up an
HC PW with R1/HD, R4/HD will do as requested and send LDP Targeted
Hello messages as unicast UDP packets to UDP port 646 of R1/HC's IP
address.
When R1/HC receives a Targeted Hello message from R4/HD, it may begin
establishing an LDP session to R4/HD. It starts this by initiating a
TCP connection on port 646 to R4/HD's signaling IP address. After
successful TCP connection establishment, R1/HC sends an LDP
Initialization message to R4/HD with the following characteristics:
When R1/HC receives a Targeted Hello message from R4/HD, it may begin
establishing an LDP session to R4/HD. The procedure described in
Section 2.5.2 of [RFC3036] is used to determine which LSR is the
active LSR and which is the passive LSR. Assume that R1/HC has the
numerically higher IP address and therefore takes the active role.
R1/HC starts by initiating a TCP connection on port 646 to R4/HD's
signaling IP address. After successful TCP connection establishment,
R1/HC sends an LDP Initialization message to R4/HD with the following
characteristics:
Ash, et al. Standards Track [Page 24]
RFC 4901 Header Compression over MPLS Protocol June 2007
o Common Session Parameters TLV:
- A bit = 0 (Downstream Unsolicited Mode)
- D bit = 0 (Loop Detection Disabled)
- PVLim = 0 (required when D bit = 0)
- Receive LDP identifier (taken from R4/HD's Hello message)
> 4 octets LSR identifier (typically an IP address with IPv4)
> 2 octet Label space identifier (typically 0)
o No Optional Parameters TLV
Following the LDP session initialization state machine of Section
2.5.4 of [RFC3036], R4/HD would send a similar Initialization message
to R1/HD. The primary difference would be that R4/HD would use the
LDP identifier it received in R1/HC's Hello message(s) as the Receive
LDP identifier. Assuming that all other fields in the Common Session
Parameters TLV were acceptable to both sides, R1/HC would send an LDP
Keepalive message to R4/HD, R4/HD would send a LDP Keepalive message
to R1/HC, and the LDP session would become operational.
At this point, either R1/HC or R4/HD may send LDP Label Mapping
messages to configure the PW. The Label Mapping message sent by a
particular router advertises the label that should be used at the
bottom of the MPLS label stack for all packets sent to that router
and associated with the particular PW. The Label Mapping message
sent from R1/HC to R4/HD would have the following characteristics:
o FEC TLV
- FEC Element type 0x80 (PWid FEC Element, as defined in [RFC4447]
- Control Parameter bit = 1 (Control Parameter present)
- PW type = 0x001B (ECRTP [RFC3545])
- Group ID as chosen by R1/HC
- PW ID = the configured value for this PW, which must be the same
as that sent in the Label Mapping message by R4/HD
- Interface Parameter Sub-TLVs
> Interface MTU sub-TLV (Type 0x01)
> CRTP/ECRTP/IPHC HC over MPLS configuration sub-TLV (Type 0x0F)
+ Type = 2 (From RFC 3544)
+ Length = 16
+ TCP_SPACE = Don't Care (leave at suggested value = 15)
+ NON_TCP_SPACE = 200 (configured on R1)
+ F_MAX_PERIOD = Don't Care (leave at suggested value = 256)
+ F_MAX_TIME = Don't Care (leave at suggested value = 5
seconds)
+ MAX_HEADER = 168 (Suggested Value)
+ Enhanced RTP-Compression Suboption
& Type = 2
& Length = 2
o Label TLV - contains label selected by R1, Lr1
o No Optional Parameters
Ash, et al. Standards Track [Page 25]
RFC 4901 Header Compression over MPLS Protocol June 2007
The Label Mapping message sent from R4/HD to R1/HC would be almost
identical to the one sent in the opposite direction, with the
following exceptions:
o R4/HD could select a different Group ID
o The Value of NON_TCP_SPACE in the CRTP/ECRTP/IPHC HC over MPLS
configuration sub-TLV would be 255 instead of 200, as configured
on R4/HD
o R4/HD would choose its own value for the Label TLV, Lr4
As soon as either R1/HC or R4/HD has both transmitted and received
Label Mapping Messages with the same PW Type and PW ID, that HC
endpoint considers the PW established. R1/HC could send ECRTP
packets using the label it received in the Label Mapping Message from
R4/HD, Lr4, and could identify received ECRTP packets by the label it
had sent to R4/HD, Lr1. And vice versa.
In this case, assume that R1/HC has an IPv4 RTP flow to send to R4/HD
that it wishes to compress using the ECRTP PW just set up. The RTP
flow is G.729 media with 20 bytes of payload in each RTP packet. In
this particular case, the IPv4 identifier changes by a small constant
value between consecutive packets in the stream. In the RTP layer of
the flow, the Contributing Source Identifiers count is 0. R1/HC
decides to use 8-bit Context Identifiers for the compressed flow.
Also, R1/HC determines that compression in this particular flow
should be able to recover from the loss of 2 consecutive packets
without requiring re-synchronization of the context (i.e., the "N"
value from [RFC3545] is 2).
The first 3 (N + 1) packets of this flow would be sent as FULL_HEADER
packets. The MPLS and PW headers at the beginning of these packets
would be formatted as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | Exp |S| TTL |
| XX | XX |0| XX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | Exp |S| TTL |
| Lr4 | XX |1| >0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |Pkt Typ| Length |Res|
|0 0 0 0| 2 | 62 |0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
^
|
-- 2 == FULL_HEADER
Ash, et al. Standards Track [Page 26]
RFC 4901 Header Compression over MPLS Protocol June 2007
where XX signifies either
a. value determined by the MPLS routing layer
b. don't care
Immediately following the above header would come the FULL_HEADER
packet as defined in [RFC3545], which basically consists of the
IP/UDP/RTP header, with the IP and UDP length field replaced by
values encoding the CID, sequence number, and "generation", as
defined in [RFC3545]. The length field value of 62 comprises:
o 2 bytes of HC control parameter (included in the above diagram)
o 20 bytes of the IP header portion of the RFC 3545 FULL_HEADER
o 8 bytes of the UDP header portion of the RFC 3545 FULL_HEADER
o 12 bytes of the RTP header portion of the RFC 3545 FULL_HEADER
o 20 bytes of G.729 payload
The next 3 RTP packets from this flow would be sent as
COMPRESSED_UDP_8, to establish the absolute and delta values of the
IPv4 identifier and RTP timestamp fields. These packets would use
the same ECRTP CID as the previous 3 FULL_HEADER packets. The MPLS
and PW headers at the beginning of these packets would be formatted
as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | Exp |S| TTL |
| XX | XX |0| XX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | Exp |S| TTL |
| Lr4 | XX |1| >0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |Pkt Typ| Length |Res|
|0 0 0 0| 8 | 36 |0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
^
|
-- 8 == COMPRESSED_UDP_8
There is no change in the MPLS label stack between the FULL_HEADER
packets and the COMPRESSED_UDP packets. The HC control parameter
changes to reflect another ECRTP packet type following the control
parameter, and a change of packet length. The length changes because
the new packet type more compactly encodes the headers. The length
field value of 36 comprises:
Ash, et al. Standards Track [Page 27]
RFC 4901 Header Compression over MPLS Protocol June 2007
o 2 bytes of HC control parameter (included in the above diagram)
o 1 byte of CID
o 2 bytes of COMPRESSED_UDP fields that are not octet-aligned:
- 4 bits of COMPRESSED_UDP flags
- 4 bits of sequence number
- 5 bits of COMPRESSED UDP extension flags
- 3 bits MUST_BE_ZERO
o 2 bytes of UDP checksum or HDRCKSUM
o 1 byte of delta IPv4 ID
o 2 bytes of delta RTP timestamp (changes by 160 in this case,
differential encoding will encode as 2 bytes)
o 2 bytes of absolute IPv4 ID
o 4 bytes of absolute RTP timestamp
o 20 bytes of G.729 payload
After the context for the IPv4 ID and RTP timestamp is initialized.
Subsequent packets on this flow, at least until the end of the talk
spurt or until there is some other unexpected change in the
IP/UDP/RTP headers, may be sent as COMPRESSED_RTP_8 packets. Again,
the same MPLS stack would be used for these packets, and the same
value of the CID would be used in this case as for the packets
described above. The MPLS and PW headers at the beginning of these
packets would be formatted as follows:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | Exp |S| TTL |
| XX | XX |0| XX |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Label | Exp |S| TTL |
| Lr4 | XX |1| >0 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |Pkt Typ| Length |Res|
|0 0 0 0| 6 | 26 |0 0|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
^
|
-- 6 == COMPRESSED_RTP_8
The HC control parameter again changes to reflect another ECRTP
packet type following the control parameter, and shorter length
associated with an even more compact encoding of headers. The length
field value of 26 comprises:
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RFC 4901 Header Compression over MPLS Protocol June 2007
o 2 bytes of HC control parameter (included in the above diagram)
o 1 byte of CID
o 1 byte COMPRESSED_UDP fields that are not octet-aligned:
- 4 bits of COMPRESSED_RTP flags
- 4 bits of sequence number
o 2 bytes of UDP checksum or HDRCKSUM
o 20 bytes of G.729 payload
Additional flows in the same direction may be compressed using the
same basic encapsulation, including the same PW label. The CID that
is part of the HC protocol is used to differentiate flows. For
traffic in the opposite direction, the primary change would be the PW
label, Lr4, used in the example above would be replaced by the label
Lr1 that R1/HC provides to R4/HD.
6. Security Considerations
MPLS PW security considerations in general are discussed in [RFC3985]
and [RFC4447], and those considerations also apply to this document.
This document specifies an encapsulation and not the protocols that
may be used to carry the encapsulated packets across the PSN, or the
protocols being encapsulated. Each such protocol may have its own
set of security issues, but those issues are not affected by the
encapsulations specified herein.
The security considerations of the supported HC protocols [RFC2507,
RFC2508, RFC3095, RFC3095bis, RFC3545] all apply to this document as
well.
7. Acknowledgements
The authors appreciate valuable inputs and suggestions from Loa
Andersson, Scott Brim, Stewart Bryant, Spencer Dawkins, Adrian
Farrel, Victoria Fineberg, Eric Gray, Allison Mankin, Luca Martini,
Colin Perkins, Kristofer Sandlund, Yaakov Stein, George Swallow, Mark
Townsley, Curtis Villamizar, and Magnus Westerlund.
8. IANA Considerations
As discussed in Section 4.1, PW type values have been assigned by
IANA, as follows:
0x001A ROHC Transport Header-compressed Packets [RFC3095bis]
0x001B ECRTP Transport Header-compressed Packets [RFC3545]
0x001C IPHC Transport Header-compressed Packets [RFC2507]
0x001D CRTP Transport Header-compressed Packets [RFC2508]
Procedures for registering new PW type values are given in [RFC4446].
Ash, et al. Standards Track [Page 29]
RFC 4901 Header Compression over MPLS Protocol June 2007
As discussed in Section 4.2, Pseudowire Interface Parameter Sub-TLV
type values have been specified by IANA, as follows:
Parameter ID Length Description Reference
--------- --------------- ---------------------------- ---------
0x0D up to 256 bytes ROHC over MPLS configuration RFC 4901
RFC 3241
0x0F up to 256 bytes CRTP/ECRTP/IPHC HC over MPLS RFC 4901
configuration RFC 3544
As discussed in Section 4.3, IANA has defined a new registry, "Header
Compression Over MPLS HC Control Parameter Packet Type". This is a
four-bit value. Packet Types 0 through 10 are defined in Section 4.3
of this document. Packet Types 11 to 15 are to be assigned by IANA
using the "Expert Review" policy defined in [RFC2434].
9. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", RFC 2119, March 1997.
[RFC3031] Rosen, E., Viswanathan, A., and R. Callon,
"Multiprotocol Label Switching Architecture", RFC
3031, January 2001.
[RFC3036] Andersson, L., Doolan, P., Feldman, N., Fredette, A.,
and B. Thomas, "LDP Specification", RFC 3036, January
2001.
[RFC3241] Bormann, C., "Robust Header Compression (ROHC) over
PPP", RFC 3241, April 2002.
[RFC3544] Engan, M., Casner, S., Bormann, C., and T. Koren, "IP
Header Compression over PPP", RFC 3544, July 2003.
[RFC4447] Martini, L., Ed., Rosen, E., El-Aawar, N., Smith, T.,
and G. Heron, "Pseudowire Setup and Maintenance Using
the Label Distribution Protocol (LDP)", RFC 4447,
April 2006.
Ash, et al. Standards Track [Page 30]
RFC 4901 Header Compression over MPLS Protocol June 2007
10. Informative References
[ECMP-AVOID] Swallow, G., Bryant, S., and L. Andersson, "Avoiding
Equal Cost Multipath Treatment in MPLS Networks", Work
in Progress, February 2007.
[REORDER-EVAL] Knutsson, C., "Evaluation and Implementation of Header
Compression Algorithm ECRTP", http://epubl.luth.se/
1402-1617/2004/286/LTU-EX-04286-SE.pdf.
[RFC1332] McGregor, G., "The PPP Internet Protocol Control
Protocol (IPCP)", RFC 1332, May 1992.
[RFC1661] Simpson, W., Ed., "The Point-to-Point Protocol (PPP)",
STD 51, RFC 1661, July 1994.
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26, RFC
2434, October 1998.
[RFC2472] Haskin, D. and E. Allen, "IP Version 6 over PPP", RFC
2472, December 1998.
[RFC2507] Degermark, M., Nordgren, B., and S. Pink, "IP Header
Compression", RFC 2507, February 1999.
[RFC2508] Casner, S. and V. Jacobson, "Compressing IP/UDP/RTP
Headers for Low-Speed Serial Links", RFC 2508,
February 1999.
[RFC3095] Bormann, C., et al., "RObust Header Compression
(ROHC): Framework and four profiles: RTP, UDP, ESP,
and uncompressed", RFC 3095, July 2001.
[RFC3095bis] Jonsson, L-E. Pelletier, G., and K. Sandlund, "The
RObust Header Compression (ROHC) Framework", Work in
Progress, November 2006.
[RFC3209] Awduche, D., et al., "RSVP-TE: Extensions to RSVP for
LSP Tunnels," RFC 3209, December 2001.
[RFC3544] Koren, T., et al., "IP Header Compression over PPP,"
RFC 3544, July 2003.
[RFC3545] Koren, T., et al., "Compressing IP/UDP/RTP Headers on
Links with High Delay, Packet Loss, and Reordering,"
RFC 3545, July 2003.
Ash, et al. Standards Track [Page 31]
RFC 4901 Header Compression over MPLS Protocol June 2007
[RFC3246] Davie, B., et al., "An Expedited Forwarding PHB (Per-
Hop Behavior)," RFC 3246, March 2002.
[RFC3270] Le Faucheur, F., et al., "Multi-Protocol Label
Switching (MPLS) Support of Differentiated Services,"
RFC 3270, May 2002.
[RFC3550] Schulzrinne, H., et al., "RTP: A Transport Protocol
for Real-Time Applications," RFC 3550, July 2003.
[RFC3843] Jonsson, L-E. and G. Pelletier, "RObust Header
Compression (ROHC): A Compression Profile for IP", RFC
3843, June 2004.
[RFC3985] Bryant, S., Pate, P., "Pseudo Wire Emulation Edge-to-
Edge (PWE3) Architecture," RFC 3985, March 2005.
[RFC4224] Pelletier, G., et al., "RObust Header Compression
(ROHC): ROHC over Channels that can Reorder Packets,"
RFC 4224, January 2006.
[RFC4247] Ash, G., Goode, B., Hand, J., "Requirements for Header
Compression over MPLS", RFC 4247, November 2005.
[RFC4364] Rosen, E., Rekhter, Y., "BGP/MPLS IP Virtual Private
Networks (VPN)s", RFC 4364, February 2006.
[RFC4385] Bryant, S., et al., "Pseudowire Emulation Edge-to-Edge
(PWE3) Control Word for Use over an MPLS PSN," RFC
4385, February 2006.
[RFC4446] Martini, L., et al., "IANA Allocations for Pseudo Wire
Edge To Edge Emulation (PWE3)," RFC 4446, April 2006.
[RFC4815] Jonsson, L-E., Sandlund, K., Pelletier, G., and P.
Kremer, "RObust Header Compression (ROHC): Corrections
and Clarifications to RFC 3095", RFC 4815, February
2007.
Ash, et al. Standards Track [Page 32]
RFC 4901 Header Compression over MPLS Protocol June 2007
11. Contributors
Besides the editors listed below, the following people contributed to
the document:
Bur Goode
AT&T
Phone: +1 203-341-8705
EMail: bgoode@att.com
Lars-Erik Jonsson
Optand 737
SE-831 92 Ostersund, Sweden
Phone: +46 70 365 20 58
EMail: lars-erik@lejonsson.com
Raymond Zhang
Infonet Services Corporation
2160 E. Grand Ave. El Segundo, CA 90025 USA
EMail: zhangr@bt.infonet.com
Editors' Addresses
Jerry Ash
AT&T
Email: gash5107@yahoo.com
Jim Hand
AT&T
Room MT A2-1A03
200 Laurel Avenue
Middletown, NJ 07748, USA
Phone: +1 732-420-3017
EMail: jameshand@att.com
Andrew G. Malis
Verizon Communications
40 Sylvan Road
Waltham, MA 02451 USA
EMail: andrew.g.malis@verizon.com
Ash, et al. Standards Track [Page 33]
RFC 4901 Header Compression over MPLS Protocol June 2007
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