tech-invite   World Map
3GPP     Specs     Glossaries     UICC       IETF     RFCs     Groups     SIP     ABNFs       T+       Search     Home

RFC 3095

Proposed STD
Pages: 168
Top     in Index     Prev     Next
in Group Index     No Prev: Lowest Number in Group     Next in Group     Group: ROHC

RObust Header Compression (ROHC): Framework and four profiles: RTP, UDP, ESP, and uncompressed

Part 1 of 7, p. 1 to 17
None       Next RFC Part

Updated by:    3759    4815


Top       ToC       Page 1 
Network Working Group                 C. Bormann, Editor, TZI/Uni Bremen
Request for Comments: 3095                     C. Burmeister, Matsushita
Category: Standards Track                 M. Degermark, Univ. of Arizona
                                                H. Fukushima, Matsushita
                                                      H. Hannu, Ericsson
                                                  L-E. Jonsson, Ericsson
                                                R. Hakenberg, Matsushita
                                                         T. Koren, Cisco
                                                            K. Le, Nokia
                                                           Z. Liu, Nokia
                                                 A. Martensson, Ericsson
                                                 A. Miyazaki, Matsushita
                                                    K. Svanbro, Ericsson
                                                   T. Wiebke, Matsushita
                                                T. Yoshimura, NTT DoCoMo
                                                         H. Zheng, Nokia
                                                               July 2001


                   RObust Header Compression (ROHC):
      Framework and four profiles: RTP, UDP, ESP, and uncompressed

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   This document specifies a highly robust and efficient header
   compression scheme for RTP/UDP/IP (Real-Time Transport Protocol, User
   Datagram Protocol, Internet Protocol), UDP/IP, and ESP/IP
   (Encapsulating Security Payload) headers.

   Existing header compression schemes do not work well when used over
   links with significant error rates and long round-trip times.  For
   many bandwidth limited links where header compression is essential,
   such characteristics are common.

Page 2 
   This is done in a framework designed to be extensible.  For example,
   a scheme for compressing TCP/IP headers will be simple to add, and is
   in development.  Headers specific to Mobile IPv4 are not subject to
   special treatment, but are expected to be compressed sufficiently
   well by the provided methods for compression of sequences of
   extension headers and tunneling headers.  For the most part, the same
   will apply to work in progress on Mobile IPv6, but future work might
   be required to handle some extension headers, when a standards track
   Mobile IPv6 has been completed.

Table of Contents

   1.  Introduction....................................................6
   2.  Terminology.....................................................8
   2.1.  Acronyms.....................................................13
   3.  Background.....................................................14
   3.1.  Header compression fundamentals..............................14
   3.2.  Existing header compression schemes..........................14
   3.3.  Requirements on a new header compression scheme..............16
   3.4.  Classification of header fields..............................17
   4.  Header compression framework...................................18
   4.1.  Operating assumptions........................................18
   4.2.  Dynamicity...................................................19
   4.3.  Compression and decompression states.........................21
   4.3.1.  Compressor states..........................................21
   4.3.1.1.  Initialization and Refresh (IR) State....................22
   4.3.1.2.  First Order (FO) State...................................22
   4.3.1.3.  Second Order (SO) State..................................22
   4.3.2.  Decompressor states........................................23
   4.4.  Modes of operation...........................................23
   4.4.1.  Unidirectional mode -- U-mode..............................24
   4.4.2.  Bidirectional Optimistic mode -- O-mode....................25
   4.4.3.  Bidirectional Reliable mode -- R-mode......................25
   4.5.  Encoding methods.............................................25
   4.5.1.  Least Significant Bits (LSB) encoding .....................25
   4.5.2.  Window-based LSB encoding (W-LSB encoding).................28
   4.5.3.  Scaled RTP Timestamp encoding .............................28
   4.5.4.  Timer-based compression of RTP Timestamp...................31
   4.5.5.  Offset IP-ID encoding......................................34
   4.5.6.  Self-describing variable-length values ....................35
   4.5.7.  Encoded values across several fields in compressed headers 36
   4.6.  Errors caused by residual errors.............................36
   4.7.  Impairment considerations....................................37
   5.  The protocol...................................................39
   5.1.  Data structures..............................................39
   5.1.1.  Per-channel parameters.....................................39
   5.1.2.  Per-context parameters, profiles...........................40
   5.1.3.  Contexts and context identifiers ..........................41

Top      ToC       Page 3 
   5.2.  ROHC packets and packet types................................41
   5.2.1.  ROHC feedback .............................................43
   5.2.2.  ROHC feedback format ......................................45
   5.2.3.  ROHC IR packet type .......................................47
   5.2.4.  ROHC IR-DYN packet type ...................................48
   5.2.5.  ROHC segmentation..........................................49
   5.2.5.1.  Segmentation usage considerations........................49
   5.2.5.2.  Segmentation protocol....................................50
   5.2.6.  ROHC initial decompressor processing.......................51
   5.2.7.  ROHC RTP packet formats from compressor to decompressor....53
   5.2.8.  Parameters needed for mode transition in ROHC RTP..........54
   5.3.  Operation in Unidirectional mode.............................55
   5.3.1.  Compressor states and logic (U-mode).......................55
   5.3.1.1.  State transition logic (U-mode)..........................55
   5.3.1.1.1.  Optimistic approach, upwards transition................55
   5.3.1.1.2.  Timeouts, downward transition..........................56
   5.3.1.1.3.  Need for updates, downward transition..................56
   5.3.1.2.  Compression logic and packets used (U-mode)..............56
   5.3.1.3.  Feedback in Unidirectional mode..........................56
   5.3.2.  Decompressor states and logic (U-mode).....................56
   5.3.2.1.  State transition logic (U-mode)..........................57
   5.3.2.2.  Decompression logic (U-mode).............................57
   5.3.2.2.1.  Decide whether decompression is allowed................57
   5.3.2.2.2.  Reconstruct and verify the header......................57
   5.3.2.2.3.  Actions upon CRC failure...............................58
   5.3.2.2.4.  Correction of SN LSB wraparound........................60
   5.3.2.2.5.  Repair of incorrect SN updates.........................61
   5.3.2.3.  Feedback in Unidirectional mode..........................62
   5.4.  Operation in Bidirectional Optimistic mode...................62
   5.4.1.  Compressor states and logic (O-mode).......................62
   5.4.1.1.  State transition logic...................................63
   5.4.1.1.1.  Negative acknowledgments (NACKs), downward transition..63
   5.4.1.1.2.  Optional acknowledgments, upwards transition...........63
   5.4.1.2.  Compression logic and packets used.......................63
   5.4.2.  Decompressor states and logic (O-mode).....................64
   5.4.2.1.  Decompression logic, timer-based timestamp decompression.64
   5.4.2.2.  Feedback logic (O-mode)..................................64
   5.5.  Operation in Bidirectional Reliable mode.....................65
   5.5.1.  Compressor states and logic (R-mode).......................65
   5.5.1.1.  State transition logic (R-mode)..........................65
   5.5.1.1.1.  Upwards transition.....................................65
   5.5.1.1.2.  Downward transition....................................66
   5.5.1.2.  Compression logic and packets used (R-mode)..............66
   5.5.2.  Decompressor states and logic (R-mode).....................68
   5.5.2.1.  Decompression logic (R-mode).............................68
   5.5.2.2.  Feedback logic (R-mode)..................................68
   5.6.  Mode transitions.............................................69
   5.6.1.  Compression and decompression during mode transitions......70

Top      ToC       Page 4 
   5.6.2.  Transition from Unidirectional to Optimistic mode..........71
   5.6.3.  From Optimistic to Reliable mode...........................72
   5.6.4.  From Unidirectional to Reliable mode.......................72
   5.6.5.  From Reliable to Optimistic mode...........................72
   5.6.6.  Transition to Unidirectional mode..........................73
   5.7.  Packet formats...............................................74
   5.7.1.  Packet type 0: UO-0, R-0, R-0-CRC .........................78
   5.7.2.  Packet type 1 (R-mode): R-1, R-1-TS, R-1-ID ...............79
   5.7.3.  Packet type 1 (U/O-mode): UO-1, UO-1-ID, UO-1-TS ..........80
   5.7.4.  Packet type 2: UOR-2 ......................................82
   5.7.5.  Extension formats..........................................83
   5.7.5.1.  RND flags and packet types...............................88
   5.7.5.2.  Flags/Fields in context..................................89
   5.7.6.  Feedback packets and formats...............................90
   5.7.6.1.  Feedback formats for ROHC RTP............................90
   5.7.6.2.  ROHC RTP Feedback options................................91
   5.7.6.3.  The CRC option...........................................92
   5.7.6.4.  The REJECT option........................................92
   5.7.6.5.  The SN-NOT-VALID option..................................92
   5.7.6.6.  The SN option............................................93
   5.7.6.7.  The CLOCK option.........................................93
   5.7.6.8.  The JITTER option........................................93
   5.7.6.9.  The LOSS option..........................................94
   5.7.6.10.  Unknown option types....................................94
   5.7.6.11.  RTP feedback example....................................94
   5.7.7.  RTP IR and IR-DYN packets..................................96
   5.7.7.1.  Basic structure of the IR packet.........................96
   5.7.7.2.  Basic structure of the IR-DYN packet.....................98
   5.7.7.3.  Initialization of IPv6 Header [IPv6].....................99
   5.7.7.4.  Initialization of IPv4 Header [IPv4, section 3.1].......100
   5.7.7.5.  Initialization of UDP Header [RFC-768]..................101
   5.7.7.6.  Initialization of RTP Header [RTP]......................102
   5.7.7.7.  Initialization of ESP Header [ESP, section 2]...........103
   5.7.7.8.  Initialization of Other Headers.........................104
   5.8.  List compression............................................104
   5.8.1.  Table-based item compression..............................105
   5.8.1.1.  Translation table in R-mode.............................105
   5.8.1.2.  Translation table in U/O-modes..........................106
   5.8.2.  Reference list determination..............................106
   5.8.2.1.  Reference list in R-mode and U/O-mode...................107
   5.8.3.  Encoding schemes for the compressed list..................109
   5.8.4.  Special handling of IP extension headers..................112
   5.8.4.1.  Next Header field.......................................112
   5.8.4.2.  Authentication Header (AH)..............................114
   5.8.4.3.  Encapsulating Security Payload Header (ESP).............115
   5.8.4.4.  GRE Header [RFC 2784, RFC 2890].........................117
   5.8.5.  Format of compressed lists in Extension 3.................119
   5.8.5.1.  Format of IP Extension Header(s) field..................119

Top      ToC       Page 5 
   5.8.5.2.  Format of Compressed CSRC List..........................120
   5.8.6.  Compressed list formats...................................120
   5.8.6.1.  Encoding Type 0 (generic scheme)........................120
   5.8.6.2.  Encoding Type 1 (insertion only scheme).................122
   5.8.6.3.  Encoding Type 2 (removal only scheme)...................123
   5.8.6.4.  Encoding Type 3 (remove then insert scheme).............124
   5.8.7.  CRC coverage for extension headers........................124
   5.9.  Header compression CRCs, coverage and polynomials...........125
   5.9.1.  IR and IR-DYN packet CRCs.................................125
   5.9.2.  CRCs in compressed headers................................125
   5.10.  ROHC UNCOMPRESSED -- no compression (Profile 0x0000).......126
   5.10.1.  IR packet................................................126
   5.10.2.  Normal packet............................................127
   5.10.3.  States and modes.........................................128
   5.10.4.  Feedback.................................................129
   5.11.  ROHC UDP -- non-RTP UDP/IP compression (Profile 0x0002)....129
   5.11.1.  Initialization...........................................130
   5.11.2.  States and modes.........................................130
   5.11.3.  Packet types.............................................131
   5.11.4.  Extensions...............................................132
   5.11.5.  IP-ID....................................................133
   5.11.6.  Feedback.................................................133
   5.12.  ROHC ESP -- ESP/IP compression (Profile 0x0003)............133
   5.12.1.  Initialization...........................................133
   5.12.2.  Packet types.............................................134
   6.  Implementation issues.........................................134
   6.1.  Reverse decompression.......................................134
   6.2.  RTCP........................................................135
   6.3.  Implementation parameters and signals.......................136
   6.3.1.  ROHC implementation parameters at compressor..............137
   6.3.2.  ROHC implementation parameters at decompressor............138
   6.4.  Handling of resource limitations at the decompressor........139
   6.5.  Implementation structures...................................139
   6.5.1.  Compressor context........................................139
   6.5.2.  Decompressor context......................................141
   6.5.3.  List compression: Sliding windows in R-mode and U/O-mode..142
   7.  Security Considerations.......................................143
   8.  IANA Considerations...........................................144
   9.  Acknowledgments...............................................145
   10.  Intellectual Property Right Claim Considerations.............145
   11.  References...................................................146
   11.1.  Normative References.......................................146
   11.2.  Informative References.....................................147
   12.  Authors' Addresses...........................................148
   Appendix A.  Detailed classification of header fields.............152
   A.1.  General classification......................................153
   A.1.1.  IPv6 header fields........................................153
   A.1.2.  IPv4 header fields........................................155

Top      ToC       Page 6 
   A.1.3.  UDP header fields.........................................157
   A.1.4.  RTP header fields.........................................157
   A.1.5.  Summary for IP/UDP/RTP....................................159
   A.2.  Analysis of change patterns of header fields................159
   A.2.1.  IPv4 Identification.......................................162
   A.2.2.  IP Traffic-Class / Type-Of-Service........................163
   A.2.3.  IP Hop-Limit / Time-To-Live...............................163
   A.2.4.  UDP Checksum..............................................163
   A.2.5.  RTP CSRC Counter..........................................164
   A.2.6.  RTP Marker................................................164
   A.2.7.  RTP Payload Type..........................................164
   A.2.8.  RTP Sequence Number.......................................164
   A.2.9.  RTP Timestamp.............................................164
   A.2.10.  RTP Contributing Sources (CSRC)..........................165
   A.3.  Header compression strategies...............................165
   A.3.1.  Do not send at all........................................165
   A.3.2.  Transmit only initially...................................165
   A.3.3.  Transmit initially, but be prepared to update.............166
   A.3.4.  Be prepared to update or send as-is frequently............166
   A.3.5.  Guarantee continuous robustness...........................166
   A.3.6.  Transmit as-is in all packets.............................167
   A.3.7.  Establish and be prepared to update delta.................167
   Full Copyright Statement..........................................168

1.  Introduction

   During the last five years, two communication technologies in
   particular have become commonly used by the general public: cellular
   telephony and the Internet.  Cellular telephony has provided its
   users with the revolutionary possibility of always being reachable
   with reasonable service quality no matter where they are.  The main
   service provided by the dedicated terminals has been speech.  The
   Internet, on the other hand, has from the beginning been designed for
   multiple services and its flexibility for all kinds of usage has been
   one of its strengths.  Internet terminals have usually been general-
   purpose and have been attached over fixed connections.  The
   experienced quality of some services (such as Internet telephony) has
   sometimes been low.

   Today, IP telephony is gaining momentum thanks to improved technical
   solutions.  It seems reasonable to believe that in the years to come,
   IP will become a commonly used way to carry telephony.  Some future
   cellular telephony links might also be based on IP and IP telephony.
   Cellular phones may have become more general-purpose, and may have IP
   stacks supporting not only audio and video, but also web browsing,
   email, gaming, etc.

Top      ToC       Page 7 
   One of the scenarios we are envisioning might then be the one in
   Figure 1.1, where two mobile terminals are communicating with each
   other.  Both are connected to base stations over cellular links, and
   the base stations are connected to each other through a wired (or
   possibly wireless) network.  Instead of two mobile terminals, there
   could of course be one mobile and one wired terminal, but the case
   with two cellular links is technically more demanding.

   Mobile            Base                      Base            Mobile
   Terminal          Station                   Station         Terminal


         |  ~   ~   ~  \ /                       \ /  ~   ~   ~   ~  |
         |              |                         |                  |
      +--+              |                         |               +--+
      |  |              |                         |               |  |
      |  |              |                         |               |  |
      +--+              |                         |               +--+
                        |                         |
                        |=========================|

            Cellular              Wired               Cellular
            Link                  Network             Link

        Figure 1.1 : Scenario for IP telephony over cellular links

   It is obvious that the wired network can be IP-based.  With the
   cellular links, the situation is less clear.  IP could be terminated
   in the fixed network, and special solutions implemented for each
   supported service over the cellular link.  However, this would limit
   the flexibility of the services supported.  If technically and
   economically feasible, a solution with pure IP all the way from
   terminal to terminal would have certain advantages.  However, to make
   this a viable alternative, a number of problems have to be addressed,
   in particular problems regarding bandwidth efficiency.

   For cellular phone systems, it is of vital importance to use the
   scarce radio resources in an efficient way.  A sufficient number of
   users per cell is crucial, otherwise deployment costs will be
   prohibitive.  The quality of the voice service should also be as good
   as in today's cellular systems.  It is likely that even with support
   for new services, lower quality of the voice service is acceptable
   only if costs are significantly reduced.

Top      ToC       Page 8 
   A problem with IP over cellular links when used for interactive voice
   conversations is the large header overhead.  Speech data for IP
   telephony will most likely be carried by RTP [RTP].  A packet will
   then, in addition to link layer framing, have an IP [IPv4] header (20
   octets), a UDP [UDP] header (8 octets), and an RTP header (12 octets)
   for a total of 40 octets.  With IPv6 [IPv6], the IP header is 40
   octets for a total of 60 octets.  The size of the payload depends on
   the speech coding and frame sizes being used and may be as low as
   15-20 octets.

   From these numbers, the need for reducing header sizes for efficiency
   reasons is obvious.  However, cellular links have characteristics
   that make header compression as defined in [IPHC,CRTP] perform less
   than well.  The most important characteristic is the lossy behavior
   of cellular links, where a bit error rate (BER) as high as 1e-3 must
   be accepted to keep the radio resources efficiently utilized.  In
   severe operating situations, the BER can be as high as 1e-2.  The
   other problematic characteristic is the long round-trip time (RTT) of
   the cellular link, which can be as high as 100-200 milliseconds.  An
   additional problem is that the residual BER is nontrivial, i.e.,
   lower layers can sometimes deliver frames containing undetected
   errors.  A viable header compression scheme for cellular links must
   be able to handle loss on the link between the compression and
   decompression point as well as loss before the compression point.

   Bandwidth is the most costly resource in cellular links.  Processing
   power is very cheap in comparison.  Implementation or computational
   simplicity of a header compression scheme is therefore of less
   importance than its compression ratio and robustness.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119.

   BER

      Bit Error Rate.  Cellular radio links can have a fairly high BER.
      In this document BER is usually given as a probability, but one
      also needs to consider the error distribution as bit errors are
      not independent.

Top      ToC       Page 9 
   Cellular links

      Wireless links between mobile terminals and base stations.

   Compression efficiency

      The performance of a header compression scheme can be described
      with three parameters: compression efficiency, robustness and
      compression transparency.  The compression efficiency is
      determined by how much the header sizes are reduced by the
      compression scheme.

   Compression transparency

      The performance of a header compression scheme can be described
      with three parameters: compression efficiency, robustness, and
      compression transparency.  The compression transparency is a
      measure of the extent to which the scheme ensures that the
      decompressed headers are semantically identical to the original
      headers.  If all decompressed headers are semantically identical
      to the corresponding original headers, the transparency is 100
      percent.  Compression transparency is high when damage propagation
      is low.

   Context

      The context of the compressor is the state it uses to compress a
      header.  The context of the decompressor is the state it uses to
      decompress a header.  Either of these or the two in combination
      are usually referred to as "context", when it is clear which is
      intended.  The context contains relevant information from previous
      headers in the packet stream, such as static fields and possible
      reference values for compression and decompression.  Moreover,
      additional information describing the packet stream is also part
      of the context, for example information about how the IP
      Identifier field changes and the typical inter-packet increase in
      sequence numbers or timestamps.

   Context damage

      When the context of the decompressor is not consistent with the
      context of the compressor, decompression may fail to reproduce the
      original header.  This situation can occur when the context of the
      decompressor has not been initialized properly or when packets
      have been lost or damaged between compressor and decompressor.

Top      ToC       Page 10 
      Packets which cannot be decompressed due to inconsistent contexts
      are said to be lost due to context damage.  Packets that are
      decompressed but contain errors due to inconsistent contexts are
      said to be damaged due to context damage.

   Context repair mechanism

      Context repair mechanisms are mechanisms that bring the contexts
      in sync when they were not.  This is needed to avoid excessive
      loss due to context damage.  Examples are the context request
      mechanism of CRTP, the NACK mechanisms of O- and R-mode, and the
      periodic refreshes of U-mode.

      Note that there are also mechanisms that prevent (some) context
      inconsistencies from occurring, for example the ACK-based updates
      of the context in R-mode, the repetitions after change in U- and
      O-mode, and the CRCs which protect context updating information.

   CRC-DYNAMIC

      Opposite of CRC-STATIC.

   CRC-STATIC

      A CRC over the original header is the primary mechanism used by
      ROHC to detect incorrect decompression.  In order to decrease
      computational complexity, the fields of the header are
      conceptually rearranged when the CRC is computed, so that it is
      first computed over octets which are static (called CRC-STATIC in
      this document) and then over octets whose values are expected to
      change between packets (CRC-DYNAMIC).  In this manner, the
      intermediate result of the CRC computation, after it has covered
      the CRC-STATIC fields, can be reused for several packets.  The
      restarted CRC computation only covers the CRC-DYNAMIC octets.  See
      section 5.9.

   Damage propagation

      Delivery of incorrect decompressed headers, due to errors in
      (i.e., loss of or damage to) previous header(s) or feedback.

   Loss propagation

      Loss of headers, due to errors in (i.e., loss of or damage to)
      previous header(s)or feedback.

Top      ToC       Page 11 
   Error detection

      Detection of errors.  If error detection is not perfect, there
      will be residual errors.

   Error propagation

      Damage propagation or loss propagation.

   Header compression profile

      A header compression profile is a specification of how to compress
      the headers of a certain kind of packet stream over a certain kind
      of link.  Compression profiles provide the details of the header
      compression framework introduced in this document.  The profile
      concept makes use of profile identifiers to separate different
      profiles which are used when setting up the compression scheme.
      All variations and parameters of the header compression scheme
      that are not part of the context state are handled by different
      profile identifiers.

   Packet

      Generally, a unit of transmission and reception (protocol data
      unit).  Specifically, when contrasted with "frame", the packet
      compressed and then decompressed by ROHC.  Also called
      "uncompressed packet".

   Packet Stream

      A sequence of packets where the field values and change patterns
      of field values are such that the headers can be compressed using
      the same context.

   Pre-HC links

      The Pre-HC links are all links that a packet has traversed before
      the header compression point.  If we consider a path with cellular
      links as first and last hops, the Pre-HC links for the compressor
      at the last link are the first cellular link plus the wired links
      in between.

   Residual error

      Error introduced during transmission and not detected by lower-
      layer error detection schemes.

Top      ToC       Page 12 
   Robustness

      The performance of a header compression scheme can be described
      with three parameters: compression efficiency, robustness, and
      compression transparency.  A robust scheme tolerates loss and
      residual errors on the link over which header compression takes
      place without losing additional packets or introducing additional
      errors in decompressed headers.

   RTT

      The RTT (round-trip time) is the time elapsing from the moment the
      compressor sends a packet until it receives feedback related to
      that packet (when such feedback is sent).

   Spectrum efficiency

      Radio resources are limited and expensive.  Therefore they must be
      used efficiently to make the system economically feasible.  In
      cellular systems this is achieved by maximizing the number of
      users served within each cell, while the quality of the provided
      services is kept at an acceptable level.  A consequence of
      efficient spectrum use is a high rate of errors (frame loss and
      residual bit errors), even after channel coding with error
      correction.

   String

      A sequence of headers in which the values of all fields being
      compressed change according to a pattern which is fixed with
      respect to a sequence number.  Each header in a string can be
      compressed by representing it with a ROHC header which essentially
      only carries an encoded sequence number.  Fields not being
      compressed (e.g., random IP-ID, UDP Checksum) are irrelevant to
      this definition.

   Timestamp stride

      The timestamp stride (TS_STRIDE) is the expected increase in the
      timestamp value between two RTP packets with consecutive sequence
      numbers.

Top      ToC       Page 13 
2.1.  Acronyms

   This section lists most acronyms used for reference.

   AH     Authentication Header.
   CID    Context Identifier.
   CRC    Cyclic Redundancy Check.  Error detection mechanism.
   CRTP   Compressed RTP.  RFC 2508.
   CTCP   Compressed TCP.  Also called VJ header compression.  RFC 1144.
   ESP    Encapsulating Security Payload.
   FC     Full Context state (decompressor).
   FO     First Order state (compressor).
   GRE    Generic Routing Encapsulation.  RFC 2784, RFC 2890.
   HC     Header Compression.
   IPHC   IP Header Compression.  RFC 2507.
   IPX    Flag in Extension 2.
   IR     Initiation and Refresh state (compressor).  Also IR packet.
   IR-DYN IR-DYN packet.
   LSB    Least Significant Bits.
   MRRU   Maximum Reconstructed Reception Unit.
   MTU    Maximum Transmission Unit.
   MSB    Most Significant Bits.
   NBO    Flag indicating whether the IP-ID is in Network Byte Order.
   NC     No Context state (decompressor).
   O-mode Bidirectional Optimistic mode.
   PPP    Point-to-Point Protocol.
   R-mode Bidirectional Reliable mode.
   RND    Flag indicating whether the IP-ID behaves randomly.
   ROHC   RObust Header Compression.
   RTCP   Real-Time Control Protocol.  See RTP.
   RTP    Real-Time Protocol.  RFC 1889.
   RTT    Round Trip Time (see section 2).
   SC     Static Context state (decompressor).
   SN     (compressed) Sequence Number.  Usually RTP Sequence Number.
   SO     Second Order state (compressor).
   SPI    Security Parameters Index.
   SSRC   Sending source.  Field in RTP header.
   CSRC   Contributing source.  Optional list of CSRCs in RTP header.
   TC     Traffic Class.  Octet in IPv6 header.  See also TOS.
   TOS    Type Of Service.  Octet in IPv4 header.  See also TC.
   TS     (compressed) RTP Timestamp.
   U-mode Unidirectional mode.
   W-LSB  Window based LSB encoding.  See section 4.5.2.

Top      ToC       Page 14 
3.  Background

   This chapter provides a background to the subject of header
   compression.  The fundamental ideas are described together with
   existing header compression schemes.  Their drawbacks and
   requirements are then discussed, providing motivation for new header
   compression solutions.

3.1.  Header compression fundamentals

   The main reason why header compression can be done at all is the fact
   that there is significant redundancy between header fields, both
   within the same packet header but in particular between consecutive
   packets belonging to the same packet stream.  By sending static field
   information only initially and utilizing dependencies and
   predictability for other fields, the header size can be significantly
   reduced for most packets.

   Relevant information from past packets is maintained in a context.
   The context information is used to compress (decompress) subsequent
   packets.  The compressor and decompressor update their contexts upon
   certain events.  Impairment events may lead to inconsistencies
   between the contexts of the compressor and decompressor, which in
   turn may cause incorrect decompression.  A robust header compression
   scheme needs mechanisms for avoiding context inconsistencies and also
   needs mechanisms for making the contexts consistent when they were
   not.

3.2.  Existing header compression schemes

   The original header compression scheme, CTCP [VJHC], was invented by
   Van Jacobson.  CTCP compresses the 40 octet IP+TCP header to 4
   octets.  The CTCP compressor detects transport-level retransmissions
   and sends a header that updates the context completely when they
   occur.  This repair mechanism does not require any explicit signaling
   between compressor and decompressor.

   A general IP header compression scheme, IP header compression [IPHC],
   improves somewhat on CTCP and can compress arbitrary IP, TCP, and UDP
   headers.  When compressing non-TCP headers, IPHC does not use delta
   encoding and is robust.  When compressing TCP, the repair mechanism
   of CTCP is augmented with a link-level nacking scheme which speeds up
   the repair.  IPHC does not compress RTP headers.

   CRTP [CRTP, IPHC] by Casner and Jacobson is a header compression
   scheme that compresses 40 octets of IPv4/UDP/RTP headers to a minimum
   of 2 octets when the UDP Checksum is not enabled.  If the UDP
   Checksum is enabled, the minimum CRTP header is 4 octets.  CRTP

Top      ToC       Page 15 
   cannot use the same repair mechanism as CTCP since UDP/RTP does not
   retransmit.  Instead, CRTP uses explicit signaling messages from
   decompressor to compressor, called CONTEXT_STATE messages, to
   indicate that the context is out of sync.  The link round-trip time
   will thus limit the speed of this context repair mechanism.

   On lossy links with long round-trip times, such as most cellular
   links, CRTP does not perform well.  Each lost packet over the link
   causes several subsequent packets to be lost since the context is out
   of sync during at least one link round-trip time.  This behavior is
   documented in [CRTPC].  For voice conversations such long loss events
   will degrade the voice quality.  Moreover, bandwidth is wasted by the
   large headers sent by CRTP when updating the context.  [CRTPC] found
   that CRTP did not perform well enough for a lossy cellular link.  It
   is clear that CRTP alone is not a viable header compression scheme
   for IP telephony over cellular links.

   To avoid losing packets due to the context being out of sync, CRTP
   decompressors can attempt to repair the context locally by using a
   mechanism known as TWICE.  Each CRTP packet contains a counter which
   is incremented by one for each packet sent out by the CRTP
   compressor.  If the counter increases by more than one, at least one
   packet was lost over the link.  The decompressor then attempts to
   repair the context by guessing how the lost packet(s) would have
   updated it.  The guess is then verified by decompressing the packet
   and checking the UDP Checksum -- if it succeeds, the repair is deemed
   successful and the packet can be forwarded or delivered.  TWICE
   derives its name from the observation that when the compressed packet
   stream is regular, the correct guess is to apply the update in the
   current packet twice.  [CRTPC] found that even with TWICE, CRTP
   doubled the number of lost packets.  TWICE improves CRTP performance
   significantly.  However, there are several problems with using TWICE:

   1) It becomes mandatory to use the UDP Checksum:

      - the minimal compressed header size increases by 100% to 4
        octets.

      - most speech codecs developed for cellular links tolerate errors
        in the encoded data.  Such codecs will not want to enable the
        UDP Checksum, since they do want damaged packets to be
        delivered.

      - errors in the payload will make the UDP Checksum fail when the
        guess is correct (and might make it succeed when the guess is
        wrong).

Top      ToC       Page 16 
   2) Loss in an RTP stream that occurs before the compression point
      will make updates in CRTP headers less regular.  Simple-minded
      versions of TWICE will then perform badly.  More sophisticated
      versions would need more repair attempts to succeed.

3.3.  Requirements on a new header compression scheme

   The major problem with CRTP is that it is not sufficiently robust
   against packets being damaged between compressor and decompressor.  A
   viable header compression scheme must be less fragile.  This
   increased robustness must be obtained without increasing the
   compressed header size; a larger header would make IP telephony over
   cellular links economically unattractive.

   A major cause of the bad performance of CRTP over cellular links is
   the long link round-trip time, during which many packets are lost
   when the context is out of sync.  This problem can be attacked
   directly by finding ways to reduce the link round-trip time.  Future
   generations of cellular technologies may indeed achieve lower link
   round-trip times.  However, these will probably always be fairly
   high.  The benefits in terms of lower loss and smaller bandwidth
   demands if the context can be repaired locally will be present even
   if the link round-trip time is decreased.  A reliable way to detect a
   successful context repair is then needed.

   One might argue that a better way to solve the problem is to improve
   the cellular link so that packet loss is less likely to occur.  Such
   modifications do not appear to come for free, however.  If links were
   made (almost) error free, the system might not be able to support a
   sufficiently large number of users per cell and might thus be
   economically infeasible.

   One might also argue that the speech codecs should be able to deal
   with the kind of packet loss induced by CRTP, in particular since the
   speech codecs probably must be able to deal with packet loss anyway
   if the RTP stream crosses the Internet.  While the latter is true,
   the kind of loss induced by CRTP is difficult to deal with.  It is
   usually not possible to completely hide a loss event where well over
   100 ms worth of sound is completely lost.  If such loss occurs
   frequently at both ends of the end-to-end path, the speech quality
   will suffer.

   A detailed description of the requirements specified for ROHC may be
   found in [REQ].

Top      ToC       Page 17 
3.4.  Classification of header fields

   As mentioned earlier, header compression is possible due to the fact
   that there is much redundancy between header field values within
   packets, but especially between consecutive packets.  To utilize
   these properties for header compression, it is important to
   understand the change patterns of the various header fields.

   All header fields have been classified in detail in appendix A.  The
   fields are first classified at a high level and then some of them are
   studied more in detail.  Finally, the appendix concludes with
   recommendations on how the various fields should be handled by header
   compression algorithms.  The main conclusion that can be drawn is
   that most of the header fields can easily be compressed away since
   they never or seldom change.  Only 5 fields, with a combined size of
   about 10 octets, need more sophisticated mechanisms.  These fields
   are:

    - IPv4 Identification (16 bits)   - IP-ID
    - UDP Checksum (16 bits)
    - RTP Marker (1 bit)              - M-bit
    - RTP Sequence Number (16 bits)   - SN
    - RTP Timestamp (32 bits)         - TS

   The analysis in Appendix A reveals that the values of the TS and IP-
   ID fields can usually be predicted from the RTP Sequence Number,
   which increments by one for each packet emitted by an RTP source.
   The M-bit is also usually the same, but needs to be communicated
   explicitly occasionally.  The UDP Checksum should not be predicted
   and is sent as-is when enabled.

   The way ROHC RTP compression operates, then, is to first establish
   functions from SN to the other fields, and then reliably communicate
   the SN.  Whenever a function from SN to another field changes, i.e.,
   the existing function gives a result which is different from the
   field in the header to be compressed, additional information is sent
   to update the parameters of that function.

   Headers specific to Mobile IP (for IPv4 or IPv6) do not receive any
   special treatment in this document.  They are compressible, however,
   and it is expected that the compression efficiency for Mobile IP
   headers will be good enough due to the handling of extension header
   lists and tunneling headers.  It would be relatively painless to
   introduce a new ROHC profile with special treatment for Mobile IPv6
   specific headers should the completed work on the Mobile IPv6
   protocols (work in progress in the IETF) make that necessary.


Next RFC Part