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Versions: 00 01                                                         
INTERNET-DRAFT                                                Do J. Byun
November 20, 2006                                            John Border
Category: Experimental                                  Roderick Ragland
Expiration: April 23, 2007

  Header Compression over Unidirectional Lightweight Encryption (ULE)
                draft-byun-ipdvb-ule-header-comp-01.txt


Status of This Memo

    This memo defines an Experimental Protocol for the Internet
    community.  It does not specify an Internet standard of any kind.
    Discussion and suggestions for improvement are requested.
    Distribution of this memo is unlimited.

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   Copyright (C) The Internet Society (2006).

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Abstract

   Multi-Protocol Encapsulation (MPE) is widely deployed in DVB-S and
   DVB-S2 networks [DVB-S2].  Replacing MPE with Unidirectional
   Lightweight Encryption (ULE) has been proposed to gain flexibility
   and reduce overhead.  This paper introduces a signaling method for
   sending header-compressed unicast packets over satellite networks
   using ULE, taking advantage of ULE's increased flexibility.

   Ed. Note: This is a quick first draft to get the discussion going.




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                     Header Compression over ULE            October 2006

Table of Contents

   1. Introduction ...................................................1
   2. Terminology ....................................................1
   3. Signaling Method ...............................................3
      3.1. SNDU Format ...............................................3
      3.2. Header Compression Algorithm ..............................4
      3.3. Multicast and Broadcast Traffic ...........................5
   4. Summary ........................................................5
   5. Acknowledgements ...............................................6
   6. Security Considerations ........................................6
   7. IANA Considerations ............................................6
   8. References .....................................................6


1.  Introduction

   Header compression is a mechanism that compresses the header fields
   that do not change or change in predictable ways. RFC 3095
   defines "Robust Header Compression (ROHC)" as a standard for
   compressing RTP/UDP/IP, UDP/IP and ESP/IP headers. [RFC3095].  There
   could be other proprietary compression schemes besides ROHC.

   To support header compression, the link-layer has to be flexible
   enough to indicate whether the payload is header-compressed or not.
   Such indication has been difficult with MPE due to its limited
   flexibility in its header format.

   Unidirectional Lightweight Encryption has been proposed to overcome
   this shortcoming of MPE and there had been numerous proposals to
   standardize one as the link-layer protocol of DVB-S2 [GSE].  This
   document describes how ULE is used to support header compression
   over ISO MPEG-2 transport streams [ISO-MPEG2, RFC4259] for
   peer-to-peer traffic.

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.

   DVB
      Digital Video Broadcast.  A framework and set of associated
      standards published by the European Telecommunications Standards
      Institute (ETSI) for the transmission of video, audio, and data
      using the ISO MPEG-2 Standard [ISO-MPEG2].

   MAC
      Medium Access Control [IEEE-802.3].  A link-layer protocol defined
      by the IEEE 802.3 standard (or by Ethernet v2 [DIX]).



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                     Header Compression over ULE            October 2006
   MPE
      Multiprotocol Encapsulation [ETSI-DAT, ATSC-DAT, ATSC-DATG].  A
      scheme that encapsulates PDUs, forming a DSM-CC Table Section.
      Each Section is sent in a series of TS Packets using a single TS
      Logical Channel.

   MPEG-2
      A set of standards specified by the Motion Picture Experts
      Group (MPEG) and standardized by the International Standards
      Organisation (ISO/IEC 13818-1) [ISO-MPEG2], and ITU-T (in H.222
      [ITU-H222]).

   PSI
      Program Specific Information [ISO-MPEG2].  Tables used to convey
      information about the service carried in a TS Multiplex.  The
      information is carried in one of four specifically identified
      Table Sections defined by MPEG-2 [ISO-MPEG2].  See also SI Table.

   PDU
      Protocol Data Unit.  Examples of a PDU include Ethernet frames,
      IPv4 or IPv6 datagrams, and other network packets.

   Receiver
      Equipment that processes the signal from a TS Multiplex and
      performs filtering and forwarding of encapsulated PDUs to the
      network-layer service (or bridging module when operating at the
      link layer).

   SI Table
      Service Information Table [ISO-MPEG2].  In this document, this
      term describes a table that is defined by another standards body
      to convey information about the services carried in a TS
      Multiplex.  A Table may consist of one or more Table Sections;
      however, all sections of a particular SI Table must be carried
      over a single TS Logical Channel [ISO-MPEG2].

   SNDU
      SubNetwork Data Unit.  An encapsulated PDU sent as an MPEG-2
      Payload Unit.

   TS
      Transport stream (TS) is a format specified in MPEG-2 Part 1,
      Systems (ISO/IEC standard 13818-1). Its design goal is to allow
      multiplexing of digital video and audio and to synchronize the
      output. Transport stream offers features for error correction for
      transportation over unreliable media, and is used in broadcast
      applications such as DVB and ATSC.

   ULE Stream
      An MPEG-2 TS Logical Channel that carries only ULE encapsulated
      PDUs.  ULE Streams may be identified by definition of a
      stream_type in SI/PSI [ISO-MPEG2].


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                     Header Compression over ULE            October 2006

3.  Signaling Method

   Header compression shall be indicated by the EtherType field of the
   ULE header.  When this this field is set to header compression type,
   the payload is header-compressed.  The actual type of header
   compression is determined during the context establishment between
   the two peers (one compressor and one decompressor).  Therefore, the
   method by which the payload is compressed and decompressed is part of
   the compression context. Moreover, compression context control
   messages can also be header-compressed but their context will be
   different from the one for the actual user data.

 Decompressor                                                Compressor

    |                                                              |
    |  <----- (EtherType=IPv4) Uncompressed Payload        ------  |
    |  ------ (EtherType=IPv4) Compression Request         ----->  |
    |  <----- (EtherType=IPv4) Compression Ack             ------  |
    |  <----- (EtherType=Comp) Compressed Header Payload   ------  |
    |  <----- (EtherType=Comp) Compressed Header Payload   ------  |
 Something Bad Happens
    |  ------ (EtherType=IPv4) Compression Error           ----->  |
    |  <----- (EtherType=IPv4) Uncompressed Header Payload ------  |
    |  <----- (EtherType=IPv4) Uncompressed Header Payload ------  |

                    Figure 1: Header compression example

   Figure 1 illustrates an example where control messages (that signal
   and synchronize peers to compress/decompress) are not
   header-compressed but the user data messages are.  When EtherType is
   set to 'Comp' whose hex value is TBD, the ULE payload contains
   header-compressed user data messages.

   The EtherType of TBD will be a newly registered IANA EtherType number
   that indicates a compression algorithm that is agreed by both the
   sender and receiver.  In other words, it could be any proprietary
   header compression algorithm as long as the receiver knows how to
   decompress it.  EtherType of 0x876B (TCP/IP Compression [RFC1144])
   was intentionally not used because it is currently defined to imply
   a specific header-compression algorithm.

3.1.  SNDU Format

   This section describes the SNDU format of the MPEG-2 PDU with ULE
   where headers for PDU are compressed.

   < ----------------------------- SNDU ----------------------------- >
   +-+-------------------------------------------------------+--------+
   |D| Length | Type | Dest Address* |           PDU         | CRC-32 |
   +-+-------------------------------------------------------+--------+

        Figure 2: SNDU Encapsulation (* optional Destination Address)

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                     Header Compression over ULE            October 2006

   The definition of all of the fields in Figure 2 stays the same as the
   definition in [RFC4326].  The 16-bit type field will have a new
   EtherType to indicate the PDU is header-compressed with an algorithm
   that both sender and received agreed on.  The hex value for this type
   is TBD.  Note that the new header-compressed PDU EtherType does not
   indicate a specific header-compression algorithm.  It is the sender
   and receiver's responsibility to make sure the algorithm is
   synchronized.

   Ed. Note: This is one of the main points we want to discuss on the
             mailing list.

3.2.  Header Compression Algorithm

   In order to use the proposed EtherType to indicate the PDU is
   header-compressed, both the sender and receiver have to agree with
   the compression algorithm.  This is not an issue because such
   synchronization is always required in peer-to-peer header compression
   anyway.


  Incapable                                              Incompatible
 Decompressor                                             Compressor

    |                                                              |
    |  <----- (EtherType=IPv4) Uncompressed Header Payload ------  |
    |                                                      Waiting for
    |                                                      Comp Request
    |  <----- (EtherType=IPv4) Uncompressed Header Payload ------  |
    |                                                      Waiting for
    |                                                      Comp Request
    |  <----- (EtherType=IPv4) Uncompressed Header Payload ------  |
    |                                                      Waiting for
    |                                                      Comp Request
    |                                                              |

                     Figure 3: Incapable decompressor

   Figure 3 illustrates a scenario where the decompressor (receiver) is
   not capable of decompressing the packets that the compressor (sender)
   sent.  The decompressor does not send any compression request to the
   compressor and the compressor continues to send uncompressed headers
   to the decompressor with non-header-compression EtherType.










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                     Header Compression over ULE            October 2006

 Incompatible                                               Incompatible
 Decompressor                                                Compressor

   |                                                              |
   |  <----- (EtherType=IPv4) Uncompressed Header Payload ------  |
   |  ------ (EtherType=IPv4) Compression Request         ----->  |
   |                                                           detects
   |                                                        incompatible
   |                                                        decompressor
   |  <----- (EtherType=IPv4) Uncompressed Header Payload ------  |
   |  <----- (EtherType=IPv4) Uncompressed Header Payload ------  |
   |  <----- (EtherType=IPv4) Uncompressed Header Payload ------  |
   |                                                              |

             Figure 4: Incompatible compressor and decompressor

   Figure 4 illustrates a scenario where the compressor is not
   compatible with the decompressor and therefore it continues to send
   uncompressed headers to the decompressor with non-header-compression
   EtherType.

   Specifics of a header compression algorithm may differ widely.  They
   include the way header-compression is initiated and synchronized.
   For example, compression request messages can be initiated by the
   compressor instead of decompressor.  Regardless of the algorithm,
   the header-compression indication method proposed here signals the
   decompressor that the payload is header-compressed with the algorithm
   that it agreed to use.

3.3.  Multicast and Broadcast Traffic

   Since out of band synchronization is also assumed for multicast and
   broadcast, the proposed header-compressed PDU signaling scheme
   supports multicast and broadcast as well.

4.  Summary

   This document defines a mechanism to signal the receiver that the
   payload is header-compressed using ULE as the link layer.  The
   mechansim is compatible with any peer-to-peer header compression
   algorithm.  It uses a newly proposed EtherType to indicate that the
   payload is header-compressed.  The EtherType has the value of TBD
   which is not tied to a specific header compression algorithm.

   The proposed method to indicate header-compressed payload is not for
   multicast and broadcast as there is no gaurantee that the receivers
   are compatible decompressors.






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                     Header Compression over ULE            October 2006

5. Acknowledgements

   TBD

6. Security Considerations

   The proposed header compression signaling method does not introduce
   any additional security concerns.

7. IANA Considerations

   A new EtherType number will be proposed to the IANA EtherType
   registry.  This number will be used to indicate that the ULE PDU is
   header-compressed as described in this document.

8. References:

8.1.  Normative References

   [ISO-MPEG2]    IS 13818-1, "Information technology -- Generic coding
                  of moving pictures and associated audio information --
                  Part 1: Systems", International Standards Organization
                  (ISO), 2000.

   [RFC1144]      Jacobson, V., "Compressing TCP/IP Headers for
                  Low-Speed Serial Links", 1990.

   [RFC3095]      Bormann, C., Burmeister, C., Degermark, M., et al,
                  "Robust Header Compression (ROHC): Framework and four
                  profiles: RTP, UDP, ESP, and uncompressed", RFC 3095,
                  2001.

   [RFC4326]      Fairhurst, G., Collini-Nocker, B., "Unidirectional
                  Lightweight Encapsulation (ULE) for Transmission of IP
                  Datagrams over an MPEG-2 Transport Stream (TS)",
                  RFC 4326, 2005.

   [DVB-S2]       Digital Video Broadcasting (DVB); Second generation
                  framing structure, channel coding and modulation
                  systems for Broadcasting, Interactive Services, News
                  Gathering and other broadband satellite applications,
                  ETSI EN 302 307 V1.1.1, 2005.

8.2.  Informative References

   [GSE]          Fairhurst, G., "A Network-Layer Interface To The
                  Second Generation Standard For DVB Over Satellite",
                  Work in Progress, September 2006.





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                     Header Compression over ULE            October 2006

   [DIX]          Digital Equipment Corp, Intel Corp, Xerox Corp,
                  "Ethernet Local Area Network Specification" Version
                  2.0, November 1982.

   [ITU-H222]     H.222.0, "Information technology - Generic coding of
                  moving pictures and associated audio information:
                  Systems", International Telecommunication Union,
                  (ITU-T), 1995.

Authors' Addresses

   Do J. Byun
   Hughes Network Systems
   11717 Exploration Lane
   Germantown, MD, 20876
   USA

   EMail: dbyun@hns.com

   John Border
   Hughes Network Systems
   11717 Exploration Lane
   Germantown, MD, 20876
   USA

   EMail: border@hns.com

   Roderick Ragland
   Hughes Network Systems
   11717 Exploration Lane
   Germantown, MD, 20876
   USA

   EMail: rragland@hns.com


   Comments are solicited and should be addressed to the authors.
















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                     Header Compression over ULE            October 2006
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