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Network Working Group G. Fairhurst
Request for Comments: 5163 University of Aberdeen
Category: Standards Track B. Collini-Nocker
University of Salzburg
April 2008
Extension Formats for Unidirectional Lightweight Encapsulation (ULE)
and the Generic Stream Encapsulation (GSE)
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.
Abstract
This document describes a set of Extension Headers for the
Unidirectional Lightweight Encapsulation (ULE), RFC 4326.
The Extension Header formats specified in this document define
extensions appropriate to both ULE and the Generic Stream
Encapsulation (GSE) for the second-generation framing structure
defined by the Digital Video Broadcasting (DVB) family of
specifications.
Table of Contents
1. Introduction ....................................................2
2. Conventions Used in This Document ...............................3
3. Description of the Method .......................................4
3.1. MPEG-2 TS-Concat Extension .................................5
3.2. PDU-Concat Extension .......................................8
3.3. TimeStamp Extension .......................................12
4. IANA Considerations ............................................13
5. Acknowledgments ................................................13
6. Security Considerations ........................................14
7. References .....................................................14
7.1. Normative References ......................................14
7.2. Informative References ....................................14
Appendix A. The Second-Generation DVB Transmission
Specifications .................................................16
Fairhurst & Collini-Nocker Standards Track [Page 1]
RFC 5163 Extension Formats for the ULE Encapsulation April 2008
1. Introduction
This document describes three Extension Headers that may be used with
both the Unidirectional Lightweight Encapsulation (ULE) [RFC4326] and
the Generic Stream Encapsulation (GSE) [GSE]. ULE is defined for
links that employ the MPEG-2 Transport Stream, and supports a wide
variety of physical-layer bearers [RFC4259].
GSE has been designed for the Generic Mode (also known as the Generic
Stream (GS)), offered by second-generation DVB physical layers, and
in the first instance for DVB-S2 [ETSI-S2]. The requirements for the
Generic Stream are described in [S2-REQ]. The important
characteristics of this encapsulation are described in the appendix
of this document. GSE maintains a design philosophy that presents a
network interface that is common to that presented by ULE and uses a
similar construction for SubNetwork Data Units (SNDUs).
The first Extension Header defines a method that allows one or more
TS Packets [ISO-MPEG2] to be sent within a ULE SNDU. This method may
be used to provide control plane information including the
transmission of MPEG-2 Program Specific Information (PSI) for the
Multiplex. In GSE, there is no native support for Transport Stream
packets and this method is therefore suitable for providing an MPEG-2
control plane.
A second Extension Header allows one or more PDUs to be sent within
the same ULE SNDU. This method is designed for cases where a large
number of small PDUs are directed to the same Network Point of
Attachment (NPA) address. The method may improve transmission
efficiency (by removing duplicated MAC layer overhead). It can also
reduce processing overhead for a receiver that is not configured to
receive the NPA address associated with an SNDU, allowing this
receiver to then skip several PDUs in one operation. The method is
defined as a generic Extension Header and may be used for IPv4 or
IPv6 packets. If, and when, a compression format is defined for ULE
or Ethernet, the method may also be used in combination with this
method.
A third Extension Header provides an optional TimeStamp value for an
SNDU. Examples of the use of this TimeStamp option include
monitoring and benchmarking of ULE and GSE links. Receivers that do
not wish to decode (or do not support) the TimeStamp extension may
discard the extension and process the remaining PDU or Extension
Headers.
The appendix includes a summary of key design issues and
considerations relating to the GSE Specification defined by the DVB
Technical Module [GSE].
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2. Conventions Used in This Document
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].
b: bit. For example, one byte consists of 8b.
B: byte. Groups of bytes are represented in Internet byte order.
BBFrame payload: The data field part of a Baseband frame [ETSI-S2]
that may be used for the communication of data. Typical BBFrames
range in size from 3072 to 58192 bits according to the choice of
modulation format and Forward Error Correction (FEC) in use.
DVB: Digital Video Broadcasting. A framework and set of associated
standards published by the European Telecommunications Standards
Institute (ETSI) for the transmission of video, audio, and data.
E: A one-bit flag field defined in GSE [GSE].
Encapsulator: A network device [RFC4259] that receives PDUs and
formats these into Payload Units (known here as SNDUs) for output in
DVB-S or the Generic Mode of DVB-S2.
GS: Generic Stream. A stream of BBFrames identified by a common
Input Stream Identifier, and which does not use the MPEG-2 TS format
[ETSI-S2]. It represents layer 2 of the ISO/OSI reference model.
GSE: Generic Stream Encapsulation [GSE]. A method for encapsulating
PDUs to form a Generic Stream, which is sent using a sequence of
BBFrames. This encapsulation format shares the same extension format
and basic processing rules of ULE and uses a common IANA Registry.
LT: A two-bit flag field defined in GSE [GSE].
MAC: Medium Access Control [IEEE-802.3]. A link-layer protocol
defined by the IEEE 802.3 standard.
MPEG-2: A set of standards specified by the Motion Picture Experts
Group (MPEG), and standardized by the International Organization for
Standardization (ISO/IEC 113818-1) [ISO-MPEG2], and ITU-T (in H.220).
Next-Header: A Type value indicating an Extension Header [RFC4326].
Fairhurst & Collini-Nocker Standards Track [Page 3]
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NPA: Network Point of Attachment [RFC4326]. In this document, refers
to a destination address (resembling an IEEE MAC address) within the
DVB-S/S2 transmission network that is used to identify individual
Receivers or groups of Receivers.
PID: Packet Identifier [ISO-MPEG2]. A 13-bit field carried in the
header of each TS Packet. This identifies the TS Logical Channel to
which a TS Packet belongs [ISO-MPEG2]. The TS Packets that form the
parts of a Table Section or other Payload Unit must all carry the
same PID value. The all-ones PID value indicates a Null TS Packet
introduced to maintain a constant bit rate of a TS Multiplex. There
is no required relationship between the PID values used for TS
Logical Channels transmitted using different TS Multiplexes.
PDU: Protocol Data Unit [RFC4259]. Examples of a PDU include
Ethernet frames, IPv4 or IPv6 datagrams, and other network packets.
PSI: Program Specific Information [ISO-MPEG2].
S: A one-bit flag field defined in [GSE].
SI Table: Service Information Table [ISO-MPEG2]. In this document,
this term describes a table that is been defined by another standards
body to convey information about the services carried on a DVB
Multiplex.
SNDU: SubNetwork Data Unit [RFC4259]. In this document, this is an
encapsulated PDU sent using ULE or GSE.
Stream: A logical flow from an Encapsulator to a set of Receivers.
TS: Transport Stream [ISO-MPEG2], a method of transmission at the
MPEG-2 level using TS Packets; it represents layer 2 of the ISO/OSI
reference model.
ULE: Unidirectional Lightweight Encapsulation (ULE) [RFC4326]. A
method that encapsulates PDUs into SNDUs that are sent in a series of
TS Packets using a single TS Logical Channel. The encapsulation
defines an extension format and an associated IANA Registry.
3. Description of the Method
In ULE, a Type field value that is less than 1536 in decimal
indicates an Extension Header. This section describes a set of three
extension formats for the ULE encapsulation. [GSE] uses a Type field
that adopts the same semantics as specified by RFC 4326. The
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encapsulation format differs in that GSE does not include a Cyclic
Redundancy Check (CRC) for each SNDU, has different header flags, and
utilizes a different SNDU length calculation [GSE].
There is a natural ordering of Extension Headers, which is determined
by the fields upon which the Extension Header operates. A suitable
ordering for many applications is presented in the list below (from
first to last header within an SNDU). This does not imply that all
types of Extensions should be present in a single SNDU. The
presented ordering may serve as a guideline for optimization of
Receiver processing.
+----------------------------------+-------------------------------+
|Fields related to Extension Header| Example Extension Headers |
+----------------------------------+-------------------------------+
| Link framing and transmission | TimeStamp Extension |
+----------------------------------+-------------------------------+
| Entire remaining SNDU Payload | Encryption Extension |
+----------------------------------+-------------------------------+
| Group of encapsulated PDUs | PDU-Concat or TS-Concat |
+----------------------------------+-------------------------------+
| Specific encapsulated PDU | IEEE-defined type |
| | Test or MAC bridging Extension|
+----------------------------------+-------------------------------+
Table 1: Recommended ordering of Extension Headers
3.1. MPEG-2 TS-Concat Extension
The MPEG-2 TS-Concat Extension Header is specified by an IANA-
assigned H-Type value of 0x0002 in hexadecimal. This is a Mandatory
Extension Header.
The extension is used to transport one or more MPEG-2 TS Packets
within a ULE SNDU. The number of TS Packets carried in a specific
SNDU is determined from the size of the remainder of the payload
following the MPEG-2 TS Extension Header. The number of TS Packets
contained in the SNDU is therefore (Length-N-10+D*6) / 188, where N
is the number of bytes associated with Extension Headers that precede
the MPEG-2 TS-Concat Extension (zero if there are none) and D is the
value of the D-bit.
A Receiver MUST check the validity of the Length value prior to
processing the payload. A valid Length corresponds to an integral
number of TS Packets. An invalid Length (a remainder from the
division by 188) MUST result in the discard of all encapsulated TS
Packets and SHOULD be recorded as TS-Concat size mismatch error.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Length (15b) | Type = 0x0002 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiver Destination NPA Address (6B) |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| TS-Packet 1 |
= =
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TS-Packet 2 (if Length > 2*188) |
= =
| etc. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (CRC-32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: ULE/SNDU Format for a TS-Packet Payload (D=0)
Figure 1 illustrates the format of this Extension Header for ULE with
a value D=0, which indicates the presence of an NPA address
[RFC4326]. In this case, the valid payload Length for a ULE SNDU
with no other extensions is (Length-10) / 188.
The method used to define the Length in GSE differs to that of ULE.
The equivalent case for GSE would result in a payload Length value of
(Length-6) / 188 (Figure 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 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S|E|0 0| Length (12b) | Type = 0x0002 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiver Destination NPA Address (6B) |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |
| TS-Packet 1 |
= =
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TS-Packet 2 (if Length > 2*188) |
= =
| etc. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2: GSE/SNDU Format for a TS-Packet Payload (LT=00)
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Fragmented GSE SNDUs are protected by a CRC-32 carried in the final
fragment. After reassembly, this CRC-32 is removed and the resulting
SNDU carries a Total Length field. The fields labeled S and E are
defined by [GSE] and contain control flags used by the GSE link
layer. The Label Type (LT) field specifies the presence and format
of the GSE label. The LT field is only specified for the first
fragment (or a non-fragmented) GSE SNDU (i.e., when S=1).
In ULE, a value of D=1 is also permitted and indicates the absence of
an NPA address (Figure 3). A similar format is supported in GSE.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| Length (15b) | Type = 0x0002 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TS-Packet 1 |
= =
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TS-Packet 2 (if Length > 2*188) |
= =
| etc. |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (CRC-32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: ULE/SNDU Format for a TS-Packet Payload (D=1)
The TS-Concat extension may be used to transport one or more MPEG-2
TS Packets of arbitrary content, interpreted according to [ISO-
MPEG2]. One expected use is for the transmission of MPEG-2 SI/PSI
signalling [RFC4259].
NULL TS Packets [ISO-MPEG2] SHOULD NOT be sent using this
encapsulation. To reduce transmission overhead and processing, an
Encapsulator SHOULD specify a maximum period of time that it can wait
before sending all queued TS Packets. This is known as the TS
Packing Threshold. This value MUST be bounded and SHOULD be
configurable in the Encapsulator. A larger value can improve
efficiency, but incurs higher jitter and could increase the
probability of corruption. If additional TS Packets are NOT received
within the TS Packing Threshold, the Encapsulator MUST immediately
send any queued TS Packets.
The use of this format to transfer MPEG-2 clock references (e.g., a
Network Clock Reference, NCR) over ULE/GSE framing raises timing
considerations at the encapsulation gateway, including the need to
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update/modify the timing information prior to transmission by the
physical layer. These issues are not considered here, but this
operation may be simplified in GSE by ensuring that all SNDUs that
carry this Extension Header are placed before other data within the
BBFrame DataField [GSE].
This document does not specify how TS Packets are to be handled at
the Receiver. However, it notes:
* A Receiver needs to consistently associate all TS Packets in a
Stream with one TS Logical Channel (Stream). If an Encapsulator
transmits more than one Stream of TS Packets each encapsulated at a
different level or with a different NPA address, a Receiver needs
to ensure that each is independently demultiplexed as a separate
Stream (Section 3.2 [RFC4259]).
* If an Encapsulator transmits service information encapsulated at
different levels or with different NPA addresses, the Receivers
need to ensure each Stream is related to the corresponding SI table
information (if any). A RECOMMENDED way to reduce signaling
interactions is to ensure each PID value uniquely identifies a
Stream within a TS Multiplex carrying ULE and also any TS Packets
encapsulated by a ULE/GSE Stream.
The need for consistency in the use of PIDs and the related service
information is described in section 4.2 of [RFC4947].
3.2. PDU-Concat Extension
The PDU-Concat Extension Header is specified by an IANA-assigned
H-Type value of 0x0003 in hexadecimal. This is a Mandatory Next-
Header Extension. It enables a sequence of (usually short) PDUs to
be sent within a single SNDU Payload.
The base header contains the Length of the entire SNDU. This carries
the value of the combined length of all PDUs to be encapsulated,
including each set of encapsulation headers. The base header MAY be
followed by one or more additional Extension Headers that precede the
PDU-Concat Extension Header. These Extension Headers (e.g., a
TimeStamp Extension) apply to the composite concatenated PDU.
The Extension Header also contains a 16-bit ULE Type field describing
the encapsulated PDU, PDU-Concat-Type. Although any Type value
specified in the ULE Next-Header Registry (including Extension Header
Types) may be assigned to the encapsulated PDU (except the recursive
use of a PDU-Concat type), all concatenated PDUs MUST have a common
ULE Type (i.e., all concatenated PDUs passed by the network layer
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RFC 5163 Extension Formats for the ULE Encapsulation April 2008
must be associated with the same Type value). This simplifies the
receiver design, and reduces the transmission overhead for common use
cases.
Each PDU is prefixed by its length in bytes (shown in the following
diagrams as PDU-x-Length for the xth PDU). Encapsulated PDUs are of
arbitrary length (in bytes) and are not necessarily aligned to 16-bit
or 32-bit boundaries within the SNDU (as shown in the figures 4, 5,
and 6). The most significant bit of the first byte is reserved, R,
and this specification requires that this MUST be set to zero.
Receivers MUST ignore the value of the R bit. The length of each PDU
MUST be less than 32758 bytes, but will generally be much smaller.
When the SNDU header indicates the presence of an SNDU Destination
Address field (i.e., D=0 in ULE), a Network Point of Attachment, NPA,
field directly follows the fourth byte of the SNDU header. NPA
destination addresses are 6 byte numbers, normally expressed in
hexadecimal, used to identify the Receiver(s) in a transmission
network that should process a received SNDU. When present, the
Receiver MUST associate the same specified MAC/NPA address with all
PDUs within the SNDU Payload. This MAC/NPA address MUST also be
forwarded with each PDU, if required by the forwarding interface.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0| Length (15b) | Type = 0x0003 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiver Destination NPA Address (6B) |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | PDU-Concat-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| PDU-1-Length (15b) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
= PDU-1 =
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| PDU-2-Length (15b) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
= PDU-2 =
| |
More PDUs as required
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (CRC-32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: ULE/SNDU Format for a PDU-Concat Payload (D=0)
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S|E|0 0| Length (12b) | Type = 0x0003 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Receiver Destination NPA Address (6B) |
+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | PDU-Concat-Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| PDU-1-Length (15b) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
= PDU-1 =
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| PDU-2-Length (15b) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
= PDU-2 =
| |
More PDUs as required
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: GSE/SNDU Format for a PDU-Concat Payload (LT=00)
When the SNDU header indicates the absence of an SNDU Destination
Address field (i.e., D=1 in ULE), all encapsulated PDUs MUST be
processed as if they had been received without an NPA address.
The value of D in the ULE header indicates whether an NPA/MAC address
is in use [RFC4326]. A similar format is supported in GSE (using the
LT field).
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| Length (15b) | Type = 0x0003 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| PDU-Concat-Type |R| PDU-1-Length (15b) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
= PDU-1 =
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| PDU-2-Length (15b) | |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +
= PDU-2 =
| |
More PDUs as required
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| (CRC-32) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6: ULE/SNDU Format for a PDU-Concat Payload (D=1)
To reduce transmission overhead and processing, an Encapsulator
SHOULD specify a maximum period of time it will wait before sending a
Concatenated PDU. This is known as the PDU Packing Threshold. This
value MUST be bounded and SHOULD be configurable in the Encapsulator.
A larger value can improve efficiency, but incurs higher jitter and
could increase the probability of corruption. If additional PDUs are
NOT received within the PDU Packing Threshold, the Encapsulator MUST
immediately send all queued PDUs.
The Receiver processes this Extension Header by verifying that it
supports the specified PDU-Concat Type (unsupported Types MUST be
discarded, but the receiver SHOULD record a PDU-Type error
[RFC4326]). It then extracts each encapsulated PDU in turn. The
Receiver MUST verify the Length of each PDU. It MUST also ensure
that the sum of the Lengths of all processed PDUs equals the Length
specified in the SNDU base header. A Receiver SHOULD discard the
whole SNDU if the total and PDU sizes are not consistent and this
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RFC 5163 Extension Formats for the ULE Encapsulation April 2008
event SHOULD be recorded as a PDU-Concat size mismatch error. A
receiver MUST NOT forward a partial PDU with an indicated PDU-Length
greater than the number of unprocessed bytes remaining in the SNDU
payload field.
3.3. TimeStamp Extension
The TimeStamp Extension Header is an Optional Extension Header that
permits an Encapsulator to add a TimeStamp field to an SNDU. The
TimeStamp Extension Header is specified by the IANA-assigned H-Type
value of 257 decimal. This extension is an Optional Extension Header
([RFC4326], Section 5).
This extension is designed to support monitoring and measurement of
the performance of a link to indicate the quality of an operational
ULE link. This may be useful for GSE links (e.g., where significant
complexity exists in the scheduling provided by the lower layers).
Possible uses of this extension include:
* Validation of in-sequence ordering per Logical Channel
* Measurement of one-way delay (when synchronized with the sender)
* Measurement of PDU Jitter introduced by the link
* Measurement of PDU loss (with additional information from sender)
Figure 7 shows the format of this extension with a HLEN value of 3
indicating a TimeStamp of length 4B with a Type field (there is no
implied byte-alignment).
0 7 15 23 31
+---------------+---------------+---------------+---------------+
| 0x03 | 0x01 | TimeStamp HI |
+---------------+---------------+---------------+---------------+
| TimeStamp LO | Type |
+---------------+---------------+---------------+---------------+
Figure 7: Format of the 32-bit TimeStamp Extension Header
The extension carries a 32-bit value (TimeStamp HI plus TimeStamp
LO). The specified resolution is 1 microsecond. The value therefore
indicates the number of 1-microsecond ticks past the hour in
Universal Time when the PDU was encapsulated. This value may be
earlier than the time of transmission, due for example to Packing,
queuing, and other Encapsulator processing. The value is right-
justified to the 32-bit field. Systems unable to insert TimeStamps
at the specified resolution MUST pad the unused least-significant
bits with a value of zero.
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The last two bytes carry a 16-bit Type field that indicates the type
of payload carried in the SNDU or the presence of a further Next-
Header ([RFC4326], Section 4.4).
Receivers MAY process the TimeStamp when the PDU encapsulation is
removed. Receivers that do not implement, or do not wish to process,
the TimeStamp Extension MAY skip this Extension Header. Receivers
MUST continue to process the remainder of the SNDU, forwarding the
encapsulated PDU.
4. IANA Considerations
IANA has assigned three new Next-Header Type values from the IANA ULE
Next-Header Registry. These options are defined for specific use
cases envisaged by GSE, but are compatible with ULE.
The following assignments have been made in this document and
registered by IANA:
Type Name Reference
2: TS-Concat Section 3.1
3: PDU-Concat Section 3.2
Type Name H-LEN Reference
257: TimeStamp 3 Section 3.3
The TS-Concat Extension is a Mandatory next-type Extension Header,
specified in Section 3.1 of this document. The value of this next-
header is defined by an IANA assigned H-Type value of 0x0002.
The PDU-Concat Extension is a Mandatory next-type Extension Header
specified in Section 3.2 of this document. The value of this next-
header is defined by an IANA assigned H-Type value of 0x0003.
The TimeStamp Extension is an Optional next-type Extension Header
specified in Section 3.3 of this document. The value of this next-
header is defined by an IANA assigned H-Type value of 257 decimal.
This documents defines the format for an HLEN value of 0x3.
5. Acknowledgments
The authors gratefully acknowledge the inputs, comments, and
assistance offered by the members of the DVB-GBS ad hoc group on
DVB-S2 encapsulation, in particular contributions on DVB-S2
transmission aspects from Rita Rinaldo, Axel Jahn, and Ulrik De Bie.
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Juan Cantillo provided a significant contribution to the informative
appendix. The authors thank Christian Praehauser for his insight and
contribution on Extension Header processing issues.
6. Security Considerations
Security considerations for ULE are described in [RFC4326], and
further information on security aspects of using ULE are described in
the security considerations of [RFC4259] and [Sec-Req].
An attacker that is able to inject arbitrary TS Packets in a ULE or
GSE Stream may modify layer 2 signalling information transmitted by
the MPEG-2 TS-Concat extension. Since this attack requires access to
the link and/or layer 2 equipment, such an attack could also directly
attack signalling information sent as native TS Packets (not
encapsulated by ULE/GSE). Security issues relating to the
transmission and interpretation of layer 2 signalling information
(including Address Resolution) within a TS Multiplex are described in
[RFC4947]. The use of security mechanisms to protect the MPEG-2
signalling information is discussed by [Sec-Req].
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4326] Fairhurst, G. and B. Collini-Nocker, "Unidirectional
Lightweight Encapsulation (ULE) for Transmission of IP
Datagrams over an MPEG-2 Transport Stream (TS)", RFC
4326, December 2005.
[GSE] TS 102 606 "Digital Video Broadcasting (DVB); Generic
Stream Encapsulation (GSE) Protocol, "European
Telecommunication Standards, Institute (ETSI), 2007.
7.2. Informative References
[ETSI-S2] EN 302 307, "Digital Video Broadcasting (DVB); Second
generation framing structure, channel coding and
modulation systems for Broadcasting, Interactive
Services, News Gathering and other broadband satellite
applications", European Telecommunication Standards
Institute (ETSI).
Fairhurst & Collini-Nocker Standards Track [Page 14]
RFC 5163 Extension Formats for the ULE Encapsulation April 2008
[S2-REQ] Cantillo, J. and J. Lacan, "A Design Rationale for
Providing IP Services over DVB-S2 Links", Work in
Progress, December 2006.
[Sec-Req] Cruickshank, H., Iyengar, S., and P. Pillai, "Security
requirements for the Unidirectional Lightweight
Encapsulation (ULE) protocol", Work in Progress,
November 2007.
[IEEE-802.3] "Local and metropolitan area networks - Specific
requirements Part 3: Carrier sense multiple access with
collision detection (CSMA/CD) access method and physical
layer specifications", IEEE 802.3, IEEE Computer
Society, (also ISO/IEC 8802-3), 2002.
[ISO-MPEG2] ISO/IEC DIS 13818-1:2000, "Information Technology;
Generic Coding of Moving Pictures and Associated Audio
Information Systems", International Organization for
Standardization (ISO), 2000.
[RFC4259] Montpetit, M.-J., Fairhurst, G., Clausen, H., Collini-
Nocker, B., and H. Linder, "A Framework for Transmission
of IP Datagrams over MPEG-2 Networks", RFC 4259,
November 2005.
[RFC4947] Fairhurst, G. and M. Montpetit, "Address Resolution
Mechanisms for IP Datagrams over MPEG-2 Networks", RFC
4947, July 2007.
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Appendix A. The Second-Generation DVB Transmission Specifications
This section provides informative background to the network-layer
requirements of the second-generation DVB Transmission
Specifications. The second-generation waveforms specified by the
Digital Video Broadcasting project offer two main enhancements.
First, more efficient physical-layer methods that employ higher-order
modulation with stronger FEC and permit adaptive coding and
modulation response to changes in traffic and propagation conditions.
Second, at the link layer, they offer greater flexibility in framing.
Support is provided for a range of stream formats including the
classical Transport Stream (TS) [RFC4259]. In addition, a new method
called Generic Stream (GS) (or the Generic Mode) is supported. A GS
can be packetized or continuous and is intended to provide native
transport of other network-layer services. One such method is that
provided by the Generic Stream Encapsulation (GSE) [GSE].
For example, the DVB-S2 [ETSI-S2] transmission link sequentially
multiplexes a series of baseband frames (BBFrames). Each BBFrame
comprises a fixed-size 10B header and a payload. The payload carries
a DataField and uses padding to fill any unused space. A stream
comprises a sequence of BBFrames associated with an Input Stream
Identifier (ISI) that is carried in the header of each BBFrame. The
simplest scheme uses a single stream (with just one ISI value), but
multiple streams are permitted. The BBFrames forming a stream may be
of variable size (selected from a set of allowed sizes), and must use
the same stream format (i.e., TS or GSE). Each stream represents an
independent link with independent address resolution [RFC4947].
GSE provides functions that are equivalent to those of the
Unidirectional Lightweight Encapsulation (ULE) [RFC4326]. It
supports the transmission of IP packets and other network-layer
protocols. The network-layer interface resembles that of ULE, where
it adopts common mechanisms for a Length field, a 16-bit Type field,
and support for Extension Headers. As in ULE, GSE permits multiple
address formats, indicated by the LT field (functionally equivalent
to the D field in ULE). The default addressing mode uses a 6-byte
NPA and a suppressed NPA address (functionally equivalent to D=1 in
ULE).
GSE also provides more flexible fragmentation at the interface to the
physical layer (using the S and E flags). This adapts the SNDUs to a
variable-sized link-layer frame, and reflects the more complex
requirements in terms of fragmentation and assembly that arise when
using point-to-multipoint adaptive physical layers. The integrity of
a reassembled SNDU is validated using a CRC-32 in the last fragment
for the corresponding PDU.
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Authors' Addresses
Godred Fairhurst
School of Engineering,
University of Aberdeen,
Aberdeen, AB24 3UE
UK
EMail: gorry@erg.abdn.ac.uk
URI: http://www.erg.abdn.ac.uk/users/gorry
Bernhard Collini-Nocker
Department of Computer Sciences,
University of Salzburg,
Jakob Haringer Str. 2,
5020 Salzburg,
Austria
EMail: bnocker@cosy.sbg.ac.at
URI: http://www.cosy.sbg.ac.at
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