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RFC 5163 - Extension Formats for Unidirectional Lightweight Enca


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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

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].

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].

   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

   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.

    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)

   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

   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

   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)

    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).

    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

   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.

   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.

   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).

   [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.

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.

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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