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Versions: (draft-ietf-rohc-rtp-rocco) 00 01 02 03 04 05 06 07 08 RFC 3095

Network Working Group              Carsten Bormann (ed.), TZI/Uni Bremen
INTERNET-DRAFT                                                        xx
Expires: March 2000                                                   xx
                                          Carsten Burmeister, Matsushita
                                              Christopher Clanton, Nokia
                                          Mikael Degermark, U of Arizona
                                           Hideaki Fukushima, Matsushita
                                                    Hans Hannu, Ericsson
                                             Lars-Erik Jonsson, Ericsson
                                              Rolf Hakenberg, Matsushita
                                                      Tmima Koren, Cisco
                                                         Khiem Le, Nokia
                                                      Zhigang Liu, Nokia
                                            Akihiro Miyazaki, Matsushita
                                               Krister Svanbro, Ericsson
                                               Thomas Wiebke, Matsushita
                                                    Haihong Zheng, Nokia

                                                      September 18, 2000



                     RObust Header Compression (ROHC)
                       <draft-ietf-rohc-rtp-02.txt>




Status of this memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or cite them other than as "work in progress".

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/lid-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html

   This document is a product of the IETF ROHC WG. Comments should be
   directed to its mailing list, rohc@cdt.luth.se.



Bormann (ed.)                                                   [Page 1]

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Abstract

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

   A header compression framework and a highly robust and efficient
   header compression scheme is introduced in this document, adaptable
   to the characteristics of the link over which it is used and also to
   the properties of the packet streams it compresses.

Revision History


   -02: Major changes after 48th IETF
   -01: Minor editorial changes for 48th IETF
   -00: Document created from ROHC submissions



































Bormann (ed.)                                                   [Page 2]

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Table of contents

   Status of this memo.................................................1
   Abstract............................................................2
   Revision History....................................................2
   Table of contents...................................................3
   0.  ROHC WG internal short-term time plan...........................7
   1.  Introduction....................................................8
   2.  Terminology....................................................10
   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..............................16
   4.  Header compression framework...................................18
   4.1.  Operating assumptions .......................................18
   4.2.  Dynamicity...................................................19
   4.3.  Compression and decompression states.........................20
   4.3.1.  Compressor states..........................................20
   4.3.1.1.  Initiation and Refresh (IR) State........................21
   4.3.1.2.  First Order (FO) State...................................21
   4.3.1.3.  Second Order (SO) State..................................21
   4.3.2.  Decompressor states........................................22
   4.4.  Modes of operation...........................................22
   4.4.1.  Unidirectional mode - U-mode...............................23
   4.4.2.  Bi-directional optimistic mode - O-mode....................24
   4.4.3.  Bi-directional reliable mode - R-mode......................24
   4.5.  Encoding methods.............................................24
   4.5.1.  Least Significant Bits (LSB) encoding .....................24
   4.5.2  Window-based LSB encoding (W-LSB encoding)..................26
   4.5.3 Scaled RTP Timestamp encoding ...............................27
   4.5.4 Timer-Based Compression of RTP Timestamp ....................29
   4.5.5  Offset IP-ID encoding.......................................31
   4.5.6.  Self-contained variable-length values. ....................32
   4.5.7.  Encoded values across several fields in compressed headers.32
   5.  The protocol...................................................34
   5.1.  Data structures..............................................34
   5.1.1.  Per-channel parameters.....................................34
   5.1.2.  Per-CID parameters, profiles...............................34
   5.1.3.  Contexts...................................................34
   5.2.  Packet types.................................................34
   5.2.1.  Packet formats from compressor to decompressor ............35
   5.2.2.  Feedback packets from decompressor to compressor...........36
   5.2.3.  Parameters needed for mode transition......................36
   5.3.  Operation in unidirectional mode.............................37
   5.3.1.  Compressor states and logic (U-mode).......................37
   5.3.1.1.  State transition logic (U-mode)..........................37
   5.3.1.1.1.  Optimistic approach, upwards transition................37
   5.3.1.1.2.  Timeouts, downward transition..........................38
   5.3.1.1.3.  Need for updates, downward transition..................38
   5.3.1.2.  Compression logic and packets used (U-mode)..............38



Bormann (ed.)                                                   [Page 3]

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   5.3.1.3.  Feedback in unidirectional mode..........................38
   5.3.2.  Decompressor states and logic (U-mode).....................39
   5.3.2.1.  State transition logic (U-mode)..........................39
   5.3.2.2.  Decompression logic (U-mode).............................39
   5.3.2.2.1.  Decide whether decompression is allowed................39
   5.3.2.2.2.  Reconstruct and verify the header......................40
   5.3.2.2.3.  Causes for CRC mismatches..............................40
   5.3.2.2.4. Detection of context damage ............................41
   5.3.2.2.5. Repair of SN wrap-around................................41
   5.3.2.2.6.  Repair of incorrect SN updates.........................42
   5.3.2.3.  Feedback in unidirectional mode..........................43
   5.4.  Operation in bi-directional optimistic mode..................44
   5.4.1.  Compressor states and logic (O-mode).......................44
   5.4.1.1.  State transition logic...................................44
   5.4.1.1.1.  Context requests, negative acknowledgements (NACKs)....44
   5.4.1.1.2.  Optional acknowledgements..............................44
   5.4.1.2.  Compression logic and packets used.......................44
   5.4.2.  Decompressor states and logic..............................45
   5.4.2.1.  Decompression logic......................................45
   5.4.2.1.1.  Timer-based timestamp decompression....................45
   5.4.2.2.  Feedback logic...........................................45
   5.5.  Operation in bi-directional reliable mode....................46
   5.5.1.  Compressor states and logic................................46
   5.5.2.  Decompressor states and logic..............................46
   5.5.2.1.  Decompression logic......................................46
   5.5.2.2.  Feedback logic...........................................46
   5.6.  Mode transitions.............................................48
   5.6.1.  Compression and decompression during mode transitions......48
   5.6.2.  Transition from Unidirectional to Optimistic mode..........49
   5.6.3.  From Optimistic to Reliable mode...........................50
   5.6.4.  From Unidirectional to Reliable mode.......................50
   5.6.5.  From Reliable to Optimistic mode...........................50
   5.6.6.  Transition to Unidirectional mode..........................51
   5.7.  Packet formats...............................................53
   5.7.1.  Packet type 0: UO-0, R-0, R-0-CRC .........................53
   5.7.2.  Packet type 1 (R-mode): R-1, R-1-TS, R-1-ID ...............53
   5.7.3.  Packet type 1 (UO-modes): UO-1, UO-1-ID, UO-1-TS ..........55
   5.7.4.  Packet type 2: UOR-2 ......................................56
   5.7.5.  Extension formats .........................................57
   5.7.6.  Feedback packets: FEEDBACK-3 and FEEDBACK-4 ...............60
   5.7.7  IR and IR-DYN packets.......................................62
   5.7.7.1 Basic structure of the IR packet...........................62
   5.7.7.2. Basic structure of the IR-DYN packet......................63
   5.7.7.3.  Initialization of IPv6 Header [IPv6].....................64
   5.7.7.4. Initialization of IPv4 Header [IPv4, section 3.1].........65
   5.7.7.5. Initialization of UDP Header [RFC-768]....................65
   5.7.7.6. Initialization of RTP Header [RTP]........................66
   5.7.7.7.  Minimal Encapsulation header [RFC-2004, section 3.1].....67
   5.8.  List-Based Compression ......................................68
   5.8.1.  CSRC compression...........................................68
   5.8.1.1.  Transformation Classification for CSRC List..............68



Bormann (ed.)                                                   [Page 4]

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   5.8.1.2.  Encoding Schemes.........................................68
   5.8.1.3. Format of compressed CSRC list............................69
   5.8.1.3.1 Insertion Only Scheme....................................69
   5.8.1.3.1.1 R-mode.................................................69
   5.8.1.3.1.2 UO-modes...............................................70
   5.8.1.3.2 Removal Only Scheme......................................71
   5.8.1.3.2.1 R-mode.................................................71
   5.8.1.3.2.2 UO-mode................................................71
   5.8.1.3.3 Generic Scheme...........................................72
   5.8.1.3.3.1 R-mode.................................................72
   5.8.1.3.3.2 UO-mode................................................73
   5.8.2.  Header Compression for IPv6 Extension Headers..............73
   5.8.2.1. Terminology...............................................74
   5.8.2.2. Transformation Classification and Encoding Schemes........74
   5.8.2.2.1 Transformation Classification............................74
   5.8.2.2.2 Encoding Schemes.........................................75
   5.8.2.2.3 Special Handling.........................................75
   5.8.2.2.3.1 Special Handling of AH.................................75
   5.8.2.2.3.2 Encapsulating Security Payload Header..................76
   5.8.2.2.3.3 Special Handling of Next Header Field..................76
   5.8.2.3. Packet Format.............................................78
   5.8.2.3.1. Format in extension "11" header.........................78
   5.8.2.3.2 Format of compressed IPv6 extension header...............79
   5.8.2.3.2.1 Insertion Only Scheme..................................79
   5.8.2.3.2.1.1 R-mode...............................................79
   5.8.2.3.2.2 Removal Only Scheme....................................80
   5.8.2.3.2.2.1 R-mode...............................................80
   5.8.2.3.2.3 Content Change Only Scheme.............................81
   5.8.2.3.2.3.1 R-mode...............................................81
   5.8.2.3.2.3.2 UO-mode..............................................82
   5.8.2.3.2.4 Uncompressed Scheme (common to R-mode and UO-modes)....82
   5.9.  Header compression CRCs, coverage and polynomials............83
   5.9.1.  IR & IR-DYN packet CRCs....................................83
   5.9.2.  CRCs in compressed packets.................................83
   6.  Implementation issues..........................................84
   6.1.  Reverse decompression........................................84
   6.2.  RTCP.........................................................85
   7.  Further work...................................................86
   7.3.  Tunneling....................................................86
   7.3.1.  Header Compression for IPv4 Tunneling Header...............86
   7.3.1.1.  Mobile IPv4 Tunneling Header Fields Type.................86
   7.3.1.2.  Compression of Tunneling Headers in MIPv4................87
   7.3.1.2.1.  IP in IP Encapsulation in IPv4.........................87
   7.3.1.2.2.  Minimum Encapsulation in IPv4..........................87
   7.3.1.2.3.  Generic Routing Encapsulation in IPv4..................87
   7.4.  non-RTP UDP traffic..........................................88
   8.  Section 8 has been removed.....................................90
   9.  Security considerations........................................90
   10.  Acknowledgements..............................................90
   11.  Intellectual property considerations..........................91
   12.  References....................................................92



Bormann (ed.)                                                   [Page 5]

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   13.  Authors' addresses............................................93
   Appendix A.  Detailed classification of header fields..............94
   A.1.  General classification.......................................94
   A.1.1.  IPv6 header fields.........................................95
   A.1.2.  IPv4 header fields.........................................96
   A.1.3.  UDP header fields..........................................98
   A.1.4.  RTP header fields..........................................99
   A.1.5.  Summary for IP/UDP/RTP....................................100
   A.2.  Analysis of change patterns of header fields................100
   A.2.1.  IPv4 Identification.......................................102
   A.2.2.  IP Traffic-Class / Type-Of-Service........................103
   A.2.3.  IP Hop-Limit / Time-To-Live...............................104
   A.2.4.  UDP Checksum..............................................104
   A.2.5.  RTP CSRC Counter..........................................104
   A.2.6.  RTP Marker................................................104
   A.2.7.  RTP Payload Type..........................................104
   A.2.8.  RTP Sequence Number.......................................104
   A.2.9.  RTP Timestamp.............................................104
   A.2.10.  RTP Contributing Sources (CSRC)..........................105
   A.3.  Header compression strategies...............................105
   A.3.1.  Do not send at all........................................105
   A.3.2.  Transmit only initially...................................106
   A.3.3.  Transmit initially, but be prepared to update.............106
   A.3.4.  Be prepared to update or send as-is frequently............106
   A.3.5.  Guarantee continuous robustness...........................106
   A.3.6.  Transmit as-is in all packets.............................107
   A.3.7.  Establish and be prepared to update delta.................107
   Appendix E - Encoding Examples....................................108
   E.1.  Basic VLE...................................................108
   E.2.  Timer-Based VLE.............................................109



   (Editor's note: The TOC has not necessarily been updated.

   I have marked text I consider questionable by making it italic, and
   text that I think simply should be deleted by striking it through.)

















Bormann (ed.)                                                   [Page 6]

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0.  ROHC WG internal short-term time plan

   This document captures the state of the ROHC RTP specification as on
   September 18, 2000.  For information, the ROHC WG internal short-term
   time plan is as follows:

       18 September    ROHC-02 completed (this document)

               Draft review and discussion

               Complementary contributions created

       29 September    Cutoff for draft review and discussion

       02 October      ROHC-03 completed

       04 October      Judge whether draft ready for last call


   If not judged ready for WG last call at this point:

       04 October      Identify what is missing/incorrect

               Further discussion and contributions

       13 October      Cutoff for "second" draft review and discussion

       16 October      ROHC-04 Completed


   WG Last call at completion of ROHC-03 or ROHC-04, respectively
   (depending on progress as indicated above)






















Bormann (ed.)                                                   [Page 7]

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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.

   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




Bormann (ed.)                                                   [Page 8]

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   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 [CELL]. 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.

   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,PPPHC] 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 [CELL]. 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
   [CELL]. 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.







Bormann (ed.)                                                   [Page 9]

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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 rather 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.

   Cellular links

    Wireless links between mobile terminals and base stations. The BER
    and the RTT are rather high in order to achieve an efficient system
    overall.

   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
    for how well 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 per cent. Compression transparency
    is high when damage propagation is low.

   Context

    The context is the state which the compressor uses to compress a
    header and which the decompressor uses to decompress a header. The
    context basically contains the uncompressed version of the last
    header sent (compressor) or received (decompressor) over the link,
    except for fields in the header that are included "as-is" in
    compressed headers or can be inferred from, e.g., the size of the
    link-level frame. The context can also contain additional
    information describing the packet stream, for example the typical
    inter-packet increase in sequence numbers or timestamps.

   Context damage




Bormann (ed.)                                                  [Page 10]

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    When the context of the decompressor is not consistent with the
    context of the compressor, header 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. Packets for which the decompressor detects they
    cannot be decompressed due to inconsistent contexts are said to be
    lost due to context damage.

   Context repair mechanism

    To avoid excessive context damage, a context repair mechanism is
    needed. Context repair mechanisms can be based on explicit requests
    for context updates, periodic updates sent by the compressor, or
    methods for local repair at the decompressor side.

   Damage propagation

     Generation of incorrect decompressed headers due to damage to
     previous packet(s).

   Loss propagation

     Failure to decompress headers due to loss of previous frame(s).

   Error detection

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

   Error propagation

     Damage propagation or loss propagation.

   FLR

    Frame Loss Rate, given as a probability that a frame is lost on the
    channel between compressor and decompressor.  (In contrast, frames
    lost due to context damage contribute to the packet loss rate.)

   Frame

     Packet emitted by the compressor/received by the decompressor.
     Note that, in this document, there is no relationship to other
     (e.g. physical layer) frame concepts such as radio frames.

   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



Bormann (ed.)                                                  [Page 11]

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    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.

    (Editor's note: the profile concept is not finalized yet -- this
    text is based on the current state of the present document.)

   Packet

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

   Pre-HC links

    Pre-HC links are all links 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.

   Robustness

    The performance of a header compression scheme can be described
    with three parameters, compression efficiency, robustness and
    compression transparency. A robust scheme tolerates errors on the
    link over which header compression takes place (including both
    frame losses and residual bit errors) without losing additional
    packets, introducing additional errors, or using more bandwidth.

   RTT

    Round-trip time -- The time it takes to send a packet from
    compressor to decompressor and back again from decompressor to
    compressor.

   Simplex link

    A simplex (or unidirectional) link is a point to point link without
    a return channel. Over simplex links, header compression must rely
    on periodic refreshes since feedback from the decompressor can not
    be sent to the compressor.



Bormann (ed.)                                                  [Page 12]

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   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.

   Timestamp stride

    The timestamp stride (TS STRIDE) is the increase in the timestamp
    value between two consecutive packets.







































Bormann (ed.)                                                  [Page 13]

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3.  Background

   This chapter provides a background to the subject of header
   compression. The fundamental ideas are described together with
   descriptions of existing header compression schemes, their drawbacks
   and requirements and 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.

   In general, header compression methods maintain a context, which is
   essentially the uncompressed version of the last header sent over the
   link, plus some additional information, at both compressor and
   decompressor. Compression and decompression are done relative to the
   context. When compressed headers carry differences from the previous
   header, each compressed header will update the context of the
   decompressor. In this case, when a packet is lost between compressor
   and decompressor, the context of the decompressor will be brought out
   of sync since it is not updated correctly. A header compression
   method must have a way to repair the context, i.e., bring it into
   sync, after such events.


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 no UDP checksum is present. If the UDP checksum is
   present, the minimum CRTP header is 4 octets. CRTP cannot use the



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   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 roundtrip time will thus limit the
   speed of this context repair mechanism.

   On lossy links with long roundtrip 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 roundtrip 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 has
   got 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 it is wrong).

   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.





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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 roundtrip 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 roundtrip time. Future generations of
   cellular technologies may indeed achieve lower link roundtrip times.
   However, these will probably always be rather high [CELL]. 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
   roundtrip 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 [CELL].

   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].


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.





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   All header fields have been classified in detail in appendix A. The
   fields are first classified on a high level and then some of them are
   studied more in detail. Finally, the appendix concludes with
   recommendations about 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. Those 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. 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 functions 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.

























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4.  Header compression framework


4.1.  Operating assumptions

   The cellular links which are a primary target for ROHC have a number
   of characteristics that are briefly described here. ROHC requires
   functionality from lower layers which is outlined here and more
   thoroughly described in the lower layer requirements document [LLG].

   Channels

     ROHC header-compressed packets flow on channels. Unlike most fixed
     links, some cellular radio links can have several channels
     connecting the same pair of nodes. Each channel can have different
     characteristics in terms of error rate, bandwidth, etc.

   Context identifiers

     On some channels, the ability to transport multiple packet streams
     is required.  It can also be feasible to have channels dedicated to
     individual packet streams. Therefore, ROHC uses a distinct context
     identifier space per channel and eliminates context identifiers
     completely in channels where only a single packet stream is
     compressed.

   Packet type indication

     Packet type indication is done in the header compression scheme
     itself. Unless the link already has a way of indicating packet
     types which can be used, such as PPP, this provides smaller
     compressed headers, overall. It may also be less difficult to
     allocate a single packet type, rather than many, in order to run
     ROHC over links such as PPP.

   Reordering

     The channel between compressor and decompressor is not assumed to
     reorder packets, i.e., the decompressor receives packets in the
     same order as the compressor sends them.  Reordering before the
     compression point, however, is dealt with.

   Packet length


     ROHC is designed under the assumption that lower layers can provide
     the length of a compressed packet. ROHC packets do not contain
     length information for the payload.

   Framing



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     The link layer must provide framing, which makes it possible to
     distinguish frame boundaries and individual frames.

   Error detection/protection

     The ROHC scheme has been designed to cope with residual errors in
     the headers delivered to the decompressor. CRCs and sanity checks
     are used to prevent or reduce damage propagation. However, it is
     RECOMMENDED that lower layers deploy error detection for ROHC
     headers and do not deliver ROHC headers with a high residual error
     rate.

     Without giving a hard limit on the residual error rate acceptable
     to ROHC, it is noted that for a residual bit error rate of at most
     1E-5, the ROHC scheme has been designed not to increase the number
     of damaged headers, i.e., the number of damaged headers due to
     damage propagation is designed to be less than the number of
     damaged headers caught by the ROHC error detection scheme.

   Negotiation

     In addition to the packet handling mechanisms above, the link
     layer MUST provide a way to negotiate header compression
     parameters.  (For unidirectional links, this negotiation may be
     performed out-of-band or even a-priori.)


4.2.  Dynamicity

   [[This section will contain introductory text to the dynamic state
   information maintained in the protocol entities running the ROHC
   protocol, e.g., which parameters are established in per-channel
   negotiation, which constitute the stream (i.e., changing them creates
   a different stream), and which are part of the per-stream context and
   thus changeable during the lifetime of the stream.]]

   Channel setup.
     (configured or negotiated at or before channel setup time):

      Set of acceptable profiles.
      MAX-HEADER.
      Size of CID space.

   Stream setup:
      (negotiated at stream setup time.)

      Profile.  Change of profile implies new stream.

   Per-stream context:
      (changeable during lifetime of a stream)



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      Context in :

          Current value of all header fields.
             (from this one can deduce
                 IPv4 header present,
                 UDP checksum enabled)

          Additional context not in header:

             TS STRIDE
             IP-ID in network byte order?
             IP-ID is random?
             A number of old reference headers.

             Compressor & decompressor state.


4.3.  Compression and decompression states

   Header compression with ROHC can be characterized as an interaction
   between two state machines, one compressor machine and one
   decompressor machine. The compressor and the decompressor have three
   states each, which in many ways are related to each other even if the
   meaning of the states are slightly different for the two parties.
   Both machines start in the lowest compression state and transits
   gradually to higher states. Transitions need not be synchronized
   between the two machines and normally the compressor is the only one
   that temporarily may transit back to lower states.

   Subsequent sections present an overview of the state machines and
   their corresponding states respectively, starting with the
   compressor.


4.3.1.  Compressor states

   For ROHC compression, the three compressor states are the Initiation
   and Refresh (IR), First Order (FO), and Second Order (SO) states. The
   compressor starts in the lowest compression state (IR) and transits
   gradually to higher compression states. The compressor will always
   operate in the highest possible compression state, under the
   constraint that the compressor is sufficiently confident that the
   decompressor has the information necessary to decompress a header
   compressed according to that state.

    +----------+                +----------+                +----------+
    | IR State |   <-------->   | FO State |   <-------->   | SO State |
    +----------+                +----------+                +----------+

   Decisions about transitions between the various compression states
   are taken by the compressor based on:



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      - variations in packet headers.
      - positive feedback from decompressor (Acknowledgements - ACKs)
      - negative feedback from decompressor (Negative ACKs - NACKs)
      - periodic timeouts (when feedback is not present)

   How transitions are performed is explained in detail in chapter 5 for
   each mode of operation.


4.3.1.1.  Initiation and Refresh (IR) State

   The purpose of the IR state is to initialize the static parts of the
   context at the decompressor or to recover after failure. In this
   state, the compressor sends complete header information. This
   includes all static and non-static fields in uncompressed form plus
   some additional information. Refreshes may also be performed on non-
   static information only.

   The compressor should stay in IR state until it is rather confident
   that the decompressor has received the information correctly.

4.3.1.2.  First Order (FO) State

   The purpose of the FO state is to efficiently communicate
   irregularities in the packet stream. When operating in this state,
   the compressor rarely sends complete information and the information
   sent is usually compressed at least partially. The difference between
   IR and FO should therefore be clear.

   The compressor enters this state when the headers of the packet
   stream does not conform to their previous pattern, and it stays there
   until it is confident that the decompressor has acquired all the
   parameters of the new pattern. Fields that are always irregular do
   not require FO since they must be communicated in all packets and is
   therefore part of what is a uniform pattern.

   Since packets sent in the FO state usually carry context updating
   information, successful transmission of the FO information may be of
   vital importance for successful decompression of subsequent packets.
   The decompression process is sensitive to loss of, or damage of, FO
   packets in transit.

4.3.1.3.  Second Order (SO) State

   This is the state where compression is optimal. The compressor enters
   SO state when the header to be compressed is completely predictable
   given the SN, and the compressor is sufficiently confident that the
   decompressor has acquired all parameters of the functions from SN to
   other fields. Packets sent in SO state are almost independent of each
   other and error sensitivity is therefore low. However, successful



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   decompression of packets sent in SO state requires that the
   information sent in preceding FO state operations has been
   successfully received by the decompressor.

   The amount of information sent in the SO state is usually fixed.
   However, it may vary on a long term basis since the compressor can
   decide to increase the information amount to send in the SO state by
   using a different, larger header format. The basic requirement is
   however that packets sent in the SO state must be almost independent
   of each other for decompression.

   The compressor leaves this state and goes back to the FO state when
   the header no longer conforms to the uniform pattern and can not be
   independently compressed based on previous context information.


4.3.2.  Decompressor states

   The decompressor starts in its lowest compression state, "No Context"
   and gradually transits to higher states. The decompressor state
   machine normally never leaves the "Full Context" state when it once
   has started to work in that state.

    +--------------+         +----------------+         +--------------+
    |  No Context  |  <--->  | Static Context |  <--->  | Full Context |
    +--------------+         +----------------+         +--------------+

   When initially working in the "No Context" state, the decompressor
   has never successfully decompressed any packet. When a packet once
   has been decompressed correctly (upon reception of an initialization
   packet with static and dynamic information, for example), the
   decompressor can transit all the way to "Full Context" state, and
   only upon repeated failures will it transit back to lower states.
   However, when that happens it first transits back to "Static Context"
   state. There, reception of an FO packet is normally sufficient to
   enable transition to "Full Context" state again. Only when
   decompression of several FO packet fails in the "Static Context"
   state will the decompressor go all the way back to the "No Context"
   state.

   When state transitions are performed is explained in detail in
   chapter 5.


4.4.  Modes of operation.

   The ROHC scheme has three modes of operation, called Unidirectional,
   Bi-directional Optimistic, and Bi-directional Reliable mode.

   It is important to understand the difference between states, as
   described in previous chapter, and modes, since they are abstractions



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   orthogonal to each other. The state abstraction is the same for all
   modes of operation, while each mode has its specific ways to operate
   in the various states and to make decisions about transitions between
   states.

                          +----------------------+
                          |  Unidirectional Mode |
                          |   +--+  +--+  +--+   |
                          |   |IR|  |FO|  |SO|   |
                          |   +--+  +--+  +--+   |
                          +----------------------+
                            ^                  ^
                           /                    \
                          /                      \
                         v                        v
     +----------------------+                  +----------------------+
     |   Optimistic Mode    |                  |    Reliable Mode     |
     |   +--+  +--+  +--+   |                  |   +--+  +--+  +--+   |
     |   |IR|  |FO|  |SO|   | <--------------> |   |IR|  |FO|  |SO|   |
     |   +--+  +--+  +--+   |                  |   +--+  +--+  +--+   |
     +----------------------+                  +----------------------+

   Which mode to operate in depends on the environment the compression
   is performed in considering link parameters such as feedback
   abilities, error probabilities and distributions, header size
   variation effects, etc. All ROHC implementations MUST implement and
   support all the three modes of operation. The three modes are briefly
   described in the following subsections.

   Detailed descriptions of the three modes of operation regarding
   compression and decompression logic are given in chapter 5. The mode
   transition mechanisms are also described in chapter 5.

4.4.1.  Unidirectional mode - U-mode

   When in the unidirectional mode of operation, packets are sent in one
   direction only; from compressor to decompressor. This mode therefore
   makes ROHC usable over links where a return path from decompressor to
   compressor is not available or is undesirable.

   In U-mode, transitions between compressor states are performed based
   only on periodic timeouts and irregularities in the header field
   change patterns in the compressed packet stream. Due to the periodic
   refreshes and the lack of feedback for initiation of error recovery,
   compression in the unidirectional mode will be less efficient and
   have a slightly higher probability for loss propagation compared to
   any of the bi-directional modes.

   Compression with ROHC MUST start in the unidirectional mode.
   Transition to any of the bi-directional modes can be performed as
   soon as a packet has reached the decompressor and it has replied with



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   a feedback packet indicating that a mode transition is desired (see
   chapter 5).


4.4.2.  Bi-directional optimistic mode - O-mode

   The bi-directional optimistic mode is similar to the unidirectional
   mode. The difference is that a feedback channel is used to send error
   recovery requests and (optionally) acknowledgments of significant
   context updates from decompressor to compressor (not for sequence
   number updates only). Periodic refreshes are not used in the bi-
   directional optimistic mode.

   O-mode is designed for optimal compression efficiency and sparse
   usage of the return channel while maintaining reasonable robustness.
   Loss of compressor-decompressor synchronization and introduction of
   loss propagation is rare even under high error rates. When loss
   propagation does occur, the amount is rather insignificant.
   Nevertheless, this mode is not completely robust and with extremely
   high error rates loss propagation can occur.


4.4.3.  Bi-directional reliable mode - R-mode

   The bi-directional reliable mode differs in many ways from the
   previous two. The most important differences regarding implementation
   and functionality is a more intensive usage of the feedback channel
   and a stricter logic at both the compressor and the decompressor.
   Feedback is sent to acknowledge all context updates, including
   updates of the sequence number field. However, not every packet
   updates the context in reliable mode.

   The chief advantage of R-mode is almost complete robustness against
   packet loss between compressor and decompressor. Loss propagation can
   never occur due to header compression when operating in this mode.
   The price is slightly higher overhead in some cases and the
   additional feedback traffic introduced.


4.5.  Encoding methods

   This chapter describes the encoding methods that are used for
   different header fields. How the methods are applied to each field
   (e.g. values of associated parameters) is specified in the packet
   format chapter.

4.5.1. Least Significant Bits (LSB) encoding

   Least Significant Bits (LSB) encoding is used for header fields whose
   values are usually subject to small changes. With LSB encoding, the k
   least significant bits of the field value are transmitted instead of



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   the original field value, where k is a positive integer. After
   receiving the k LSBs, the decompressor derives the original value
   using a previously received value as reference (v ref).

   The scheme is guaranteed to be correct if the compressor and the
   decompressor agree on an interpretation interval

     1) in which the original value resides, and
     2) in which the original value is the only value that has the exact
        same k LSBs as those transmitted.

   The interpretation interval can be described as a function f(v_ref,
   k). Let

      f(v_ref, k) = [v_ref - p, v_ref + (2^k - 1) - p]

   where p is an integer.


      <------- interpretation interval (size is 2^k) ------->
      |-------------+---------------------------------------|
   v_ref - p        v_ref                        v_ref + (2^k-1) - p


   The function f has the following property: for any value k, k LSBs
   will uniquely identify a value in f(v ref, k). This satisfies
   condition 2) above.

   The parameter p is introduced so that the interpretation interval can
   be shifted with respect to v_ref. Choosing a good value for p will
   yield more efficient encoding for fields with certain
   characteristics. For example, if a field value observed by the
   compressor always increases, p should be set to 0, and the interval
   becomes [v_ref, v_ref + 2^k - 1]. Because LSB encoding in ROHC will
   be applied to header fields whose values increase except in uncommon
   situations (such as packet mis-ordering or RTP TS for video), only
   two values will be assigned to p: p1 = 0 or p2 = 2^(k-2) - 1. The
   latter gives the interpretation interval [v_ref - (2^(k-2) - 1),
   v_ref + 3*2^(k-2)], which can handle small negative changes while
   using 3/4 of the interval for positive changes. See section 5.7 for
   further details.

   The following is a simplified procedure for LSB compression and
   decompression, it is modified for robustness and damage propagation
   protection in the next subsection:

     1) The compressor (decompressor) always uses v_ref_c (v ref d), the
        last value that has been compressed (decompressed), as v ref;

     2) When compressing a value v, the compressor calculates the
        minimal (for efficiency) but sufficient (for correctness) value



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        of k such that v falls into the interval f(v_ref_c, k). Let
        this procedure be represented as a function k = g(v, v_ref).
        Note that more than k LSBs may be sent to fit the packet format
        (see packet format section);

     3)When receiving m LSBs, the decompressor uses the interpretation
       interval f(v ref d, m), called interval d. It picks as the
       decompressed value the one in interval_d whose LSBs match the
       received m bits.

   The scheme is complicated by two factors: packet loss between the
   compressor and decompressor, and transmission errors undetected by
   the lower layer. In the former case, the compressor and decompressor
   will lose the synchronization of v_ref, and thus also of the
   interpretation interval. If v is still covered by the
   intersection(interval_c, interval_d), decompression will be correct.
   Otherwise, incorrect decompression will happen. The next section will
   address this issue further.

   In the case of undetected transmission errors, the corrupted LSBs
   will give an incorrectly decompressed value that will later be used
   as v_ref d, which in turn is likely to lead to damage propagation.
   This problem is addressed by using a secure reference, i.e., a
   reference value whose correctness is verified by a protecting CRC.
   Consequently, the procedure 1) above is modified as follows:

    1) a) the compressor always uses as v_ref_c the last value that has
          been compressed and sent with a protecting CRC.
       b) the decompressor always uses as v_ref_d the last correct
          value, as verified by a succeeding CRC.

   Note that in U/O mode, 1) b) is modified so that if decompression
   fails using the last correct value, another decompression attempt is
   made using the last but one correct value. This procedure dampens
   damage propagation when a small CRC fails to detect a damaged value.


4.5.2  Window-based LSB encoding (W-LSB encoding)

   This section describes how to modify the simplified algorithm in
   4.5.1 to achieve robustness.

   The compressor may not be able to determine the exact value of
   v_ref_d that will be used by the decompressor for a particular value
   v, since some candidates for v ref d may have been lost or damaged.
   However, by using feedback or by making reasonable assumptions the
   compressor can limit the candidate set. The compressor then
   calculates k such that no matter which v_ref_d in the candidate set
   the decompressor uses, v is covered by the resulting interval_d.





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   Since the decompressor always uses the last received value where the
   CRC succeeded as the reference, the compressor  maintains a sliding
   window (VSW) containing the candidates for v ref d. VSW is initially
   empty. The following operations are performed on VSW by the
   compressor:

     1) After sending a value v (compressed or uncompressed) protected
        by a CRC, the compressor adds v to the VSW;
     2) For each value v being compressed, the compressor chooses k =
        max(g(v, v_min), g(v, v_max)), where v_min and v_max are the
        minimal and maximal value in VSW, and g is the function defined
        in the previous section;
     3) When the compressor has sufficient confidence that a certain
        value v will not be used as a reference by the decompressor, the
        window is advanced by removing v and all values older than v.
        The confidence may be obtained by various means. In R mode an
        ACK from the decompressor implies that values older than the
        ACKed one can be removed from VSW. In U/O mode there is always a
        CRC to verify correct decompression, and a VSW with a limited
        maximum width is used. The window width is an optimization
        parameter determined during implementation.

   Note that the decompressor follows the procedure described in the
   previous section, except that in R mode it MUST ACK each value
   received with a CRC (refer to compression/decompression logic section
   for details).


4.5.3 Scaled RTP Timestamp encoding

   The RTP Timestamp (TS) will usually not increase by an arbitrary
   number from packet to packet. Instead, the increase is normally an
   integral multiple of some unit (TS_STRIDE). For example, in the case
   of audio, the sample rate is normally 8Khz and one voice frame may
   cover 20 ms. Furthermore, each voice frame is usually carried in one
   RTP packet. In this case, the RTP increment is always n * 160 (= 8000
   * 0.02), for some integer n. Note that silence periods has no impact
   on this as the sample clock at the source normally keeps running
   without changing either frame rate or frame boundaries.

   For the case of video, there is usually a TS_STRIDE as well when we
   consider the video frame level. The sample rate for most video codecs
   is 90Khz. If the video frame rate is fixed, say to 30 frames/second,
   the TS will increase by n * 3000 (= n * 90000 / 30) between video
   frames. Note that a video frame is normally divided into several RTP
   packets to achieve robustness against packet loss. In this case
   several RTP packets will carry the same TS.

   When using scaled RTP Timestamp encoding, the TS is downscaled by a
   factor of TS_STRIDE before compression. This saves




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       floor(log2(TS_STRIDE))

   bits for each compressed TS. The following equality holds between TS
   and TS SCALED:

       TS = TS SCALED * TS STRIDE + TS OFFSET

   TS STRIDE is explicitly, and TS OFFSET implicitly, communicated to
   the decompressor. The following algorithm is used:

     1. Initialization: The compressor sends to the decompressor the
        value of TS_STRIDE (e.g., via in-band signaling, see packet
        format section) and the absolute value of one or several TS. The
        latter are used by the decompressor to initialize TS_OFFSET to
        (absolute value) modulo TS_STRIDE. Note that TS OFFSET is the
        same regardless of which absolute value is used, as long as the
        unscaled TS value does not wrap around, see 4) below.

     2. Compression: After initialization, the compressor no longer
        compresses the original TS values. Instead, it compresses the
        down-scaled values: TS_SCALED = TS / TS_STRIDE. The compression
        method could be either W-LSB encoding or the timer-based
        encoding described in the next section.

     3. Decompression: When receiving the compressed value of TS_SCALED,
        the decompressor first derives the value of the original
        TS_SCALED. The original RTP TS is then calculated as TS =
        TS_SCALED * TS_STRIDE + TS_OFFSET.

     4. Wrap around: Wrap around of the unscaled 32-bit TS will
        invalidate the current value of TS_OFFSET used in the equation
        above. For example, let's assume TS_STRIDE = 160 = 0xA0 and the
        current TS = 0xFFFFFFF0. TS_OFFSET is then 0x50 = 80. Then if
        the next RTP TS = 0x00000130 (i.e., the increment is 160 * 2 =
        320), the new TS_OFFSET should be 0x00000130 modulo 0xA0 = 0x90
        = 144. The compressor is not required to re-initialize TS OFFSET
        at wrap around. Instead, the decompressor MUST detect wrap
        around of the unscaled TS (which is trivial) and update
        TS_OFFSET to

           TS OFFSET = (Wrapped around unscaled TS) modulo TS STRIDE

   This scaling method can be applied to many frame-based codecs.
   However, the value of TS_STRIDE might change during a session, for
   example due to adaptation strategies. If that happens, the unscaled
   TS is compressed until re-initialization of the new TS_STRIDE and
   TS_OFFSET is completed.







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4.5.4 Timer-Based Compression of RTP Timestamp

   The RTP timestamp [RFC 1889] is defined to identify the number of the
   first sample used to generate the payload. When RTP packets carry
   payloads corresponding to a fixed sampling interval, the sampling is
   done at a constant rate, and packets are generated in lock-step with
   sampling, the timestamp will closely follow a linear pattern as a
   function of the time of day. This is the case for interactive speech.
   The linear ratio is determined by the source sample rate. The linear
   pattern can be complicated by packetization (e.g., in the case of
   video where a video frame usually corresponds to several RTP packets)
   or frame re-arrangement (e.g., MPEG B-frames are sent out-of-order by
   some video codecs).

   With a fixed sample rate of 8KHz, 20 ms in time domain is equivalent
   to an increment of 160 in the unscaled TS domain, and to an increment
   of 1 in the scaled TS domain with TS STRIDE=160.

   As a consequence, the (scaled) TS of headers coming to the
   decompressor will follow a linear pattern as a function of time of
   day, with some deviation due to the delay jitter between the source
   and the decompressor. In normal operation, i.e., no crashes or
   failures, the delay jitter will be bounded to meet the requirements
   of conversational real-time traffic.  Hence, by using a local clock
   the decompressor can obtain an approximation of the (scaled) TS in
   the header to be decompressed by considering its arrival time. The
   approximation can then be refined with the k LSBs of the (scaled) TS
   carried in the header. The required value of k to ensure correct
   decompression is a function of the jitter between the source and
   decompressor.

   If the compressor knows the potential jitter introduced between
   compressor and decompressor, it can determine k by using a local
   clock to estimate jitter in packet arrival times, or alternatively it
   can use a fixed k and discard packets arriving too much out of time.

   The advantages of this scheme include:

   a) The size of the compressed TS is constant and small. In
      particular, it does NOT depend on the length of silence intervals.
      This is in contrast to other TS compression techniques, which at
      the beginning of a talk-spurt requires sending a number of bits
      dependent on the duration of the preceding silence interval.

   b) No synchronization is required between the clock local to the
      compressor and the clock local to the decompressor.

   Note that although this scheme can be made to work using both scaled
   and unscaled TS, it is preferable in practice to combine it with



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   scaled TS encoding because of the less demanding requirement on the
   clock resolution, e.g., 20 ms instead of 1/8 ms. Therefore, the
   algorithm described below assumes that the clock-based encoding
   scheme operates on the scaled TS. The case of unscaled TS will be
   similar, with changes to the scale factors.

   Compressor: its major task is to determine the value of k. Its
   sliding window, TSW, now contains not only potential reference values
   for the TS, but also their times of arrival at the compressor.

     1) The compressor maintains a sliding window TSW = {(T_j, a_j),
        for each header j that can be used as a reference}, where T_j is
        the scaled TS for header j, and a_j is the arrival time of
        header j. The TSW fills the same purpose as the VSW of section
        4.5.2.

     2) When a new header n arrives with T_n as the scaled TS, the
        compressor notes the arrival time a_n. It then calculates

        Max_Jitter_BC =

            max {|(T_n - T_j) - ((a_n - a_j) / TIME_STRIDE)|,
                 for all headers j in TSW},

        where TIME_STRIDE is the time interval equivalent to one
        TS_STRIDE, e.g., 20 ms. Max_Jitter_BC is the maximum observed
        jitter before the compressor, in units of TS_STRIDE, for the
        headers in TSW.

     3) k is calculated as: k = ceiling(log2(2 * J + 1), where J =
        Max_Jitter_BC + Max_Jitter_CD + 2.

        Max_Jitter_CD is the upper bound of jitter expected on the
        communication channel between compressor and decompressor (CD-
        CC). It depends only on the characteristics of CD-CC.

        The factor 2 accounts for the quantization error introduced by
        the clocks at the compressor and decompressor, which can be +/-
        1.

        Note that the calculation of k follows the compression algorithm
        described in section 4.5.1, with p = 2^(k-1) - 1.

     4) TSW is subject to the same window operations as in section
        4.5.2, 1) and 3), except that the values added and removed are
        paired with their arrival times.

   Decompressor:

     1) The decompressor always uses as reference header the last
        correctly (as verified by CRC) decompressed header. It maintains



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        the pair (T_ref, a_ref), where T_ref is the scaled TS of the
        reference header, and a_ref is the arrival time of the reference
        header.

     2) When receiving a compressed header n at time a_n, the
        approximation of the original scaled TS is calculated as:

            T_approx = T_ref + (a_n - a_ref) / TIME_STRIDE.

     3) The approximation is then refined by the k LSBs carried in
        header n, following the decompression algorithm of section
        4.5.1, with p = 2^(k-1) - 1.

   Note that the algorithm does not assume any particular pattern in the
   packets arriving to the compressor, i.e., it tolerates reordering
   before the compressor and non-increasing RTP timestamp behavior.
   Moreover, the clock resolution can be worse than TIME_STRIDE, in
   which case the difference, i.e., actual resolution - TIME_STRIDE, is
   treated as additional jitter in the calculation of k.


4.5.5  Offset IP-ID encoding

   This section assumes that the Ipv4 stack at the source host assigns
   IP-ID to the value of a 2-byte counter which is increased by one
   after each assignment to an outgoing packet. Therefore, the IP-ID
   field of a particular IPv4 packet flow will increment by 1 from
   packet to packet except when the source have emitted intermediate
   packets not belonging to that flow.

   For such IPv4 stacks, the RTP SN will increase by 1 for each packet
   and the IP-ID will increase by at least the same amount. Thus, it is
   more efficient to compress the offset, i.e., (IP-ID - RTP SN), in
   stead of IP-ID itself.

   The following text describes how to compress/decompress the sequence
   of offsets using W-LSB encoding/decoding, with p = 0 (see section
   4.5.1).

   Compressor:

     The compressor uses W-LSB encoding to compresses a sequence of
     offsets

        Offset i = ID i - SN i,

     where ID i and SN i are the values of the IP-ID and RTP SN of
     header i. The sliding window contains such offsets and not the
     values of header fields, but the rules for adding and deleting
     offsets from the window otherwise follow section 4.5.2.




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   Decompressor:

     The reference header is the last correctly (as verified by CRC)
     decompressed header.

     When receiving a compressed packet m, the decompressor calculates
     Offset_ref = ID_ref - SN_ref, where ID_ref and SN_ref are the
     values of IP-ID and RTP SN in the reference header, respectively.
     Then W-LSB decoding is used to decompress Offset_m, using the
     received LSBs in packet m and Offset_ref. Note that m may contain
     zero LSBs for Offset m, in which case Offset m = Offset ref.

     Finally, the IP-ID for packet m is regenerated as

       IP-ID for m = decompressed SN of packet m + Offset_m

   Note that some Ipv4 stacks do not transmit the IP-ID field in network
   byte order, but instead send the two octets reversed. In this case,
   the compressor can compress the IP-ID field after swapping the bytes.
   Consequently, the decompressor also swaps the bytes of the IP-ID
   after decompression to regenerate the original IP-ID. This requires
   that the compressor and the decompressor synchronize on the byte
   order of the IP-ID field (see header format section).


4.5.6.  Self-contained variable-length values.

   The values of TS STRIDE and a few other compression parameters can
   vary widely. TS STRIDE can be 160 for voice and 90 000 for 1 f/s
   video. To optimize the transfer of such values, a variable number of
   octets is used to encode them. The first few bits of the encoded
   value determines its length.

   1 octet: first bit is zero. 7 bits transferred. Up to 127 decimal.
               Encoded octets in hexadecimal: 00 to 7F

   2 octets: first bits 10. 14 bits. Up to 16 383 decimal.
               Encoded octets in hexadecimal: 80 00 to BF FF

   3 octets: first bits 110. 21 bits. Up to 2 097 152 decimal.
               Encoded octets in hexadecimal: C0 00 00 to DF FF FF

   4 octets: first bits 111. 29 bits. Up to 536 870 912 decimal.
               Encoded octets in hexadecimal: E0 00 00 00 to FF FF FF FF


4.5.7.  Encoded values across several fields in compressed headers.

   When a compressed header has an extension, pieces of an encoded value
   can be present in more than one field. When an encoded value is split
   over several fields in this manner, the more significant bits of the



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   value are closer to the beginning of the header. If the number of
   bits available in compressed header fields exceeds the number of bits
   of the value, the most significant field is padded with zeroes in its
   most significant bits.

   For example, an unscaled TS value can be transferred using an UOR-2
   header (see section 5.7) with an extension of type 3. The Tsc bit of
   the extension is then unset and the variable length TS field of the
   extension is 4 octets (see section 4.5.6). The UOR-2 TS field will
   contain the most significant three bits of the unscaled TS, and the
   4-octet TS field in the extension contains the remaining 29 bits.











































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5.  The protocol


5.1.  Data structures

   [[Editor's note: This section will mainly be a more detailed,
   specification-like version of the introductory information in 4.2.]]

5.1.1.  Per-channel parameters


5.1.2.  Per-CID parameters, profiles


5.1.3.  Contexts


5.2.  Packet types

   The ROHC scheme uses six packet types, indicated by the first few
   bits of the header. The format of a packet type can depend on the
   mode. Therefore a naming scheme of the form

       <modes format is used in>-<packet type number>-<some property>

   is used to uniquely identify the format when necessary. E.g, UOR-2,
   R-1. For exact formats of the packet types, see section 5.7.

   [Note by Mikael Degermark - the rest of this section is an attempt at
   describing what packet types can be used when. It started out as an
   attempt to group packet types together in a way that would make it
   easier to write the following chapters. I don't seem to have
   succeeded too well in finding a small number of packet classes. Can
   we define this table or something similar? It would be very helpful
   to an implementer. The table should perhaps be moved somewhere
   else...]

   The following table summarizes which format can be used when.
   If a packet type is surrounded by parenthesis, using that packet
   type will decrease robustness. If several packet types are indicated,
   the smallest type which can represent the header to be compressed
   should be used.

                                 Header adheres
         Compressor SN-functions to SN-functions Header format/
   Mode  state      established  TS/ID   others  action
   --------------------------------------------------------------------
   UO     SO            Yes       Yes      Yes   UO-0
                        Yes            No        goto FO
          -------------------------------------------------------------
          FO            Yes       Yes      Yes   goto SO



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                        Yes       No       Yes   UO-1, UOR-2
                        Yes       *        No    UO-1 Ext, UOR-2
                        No        Yes      Yes   (UO-0), UO-1, UOR-2
                        No        No       Yes   *UO-1, UOR-2
                        No        *        No    *UO-1, UOR-2,
                                                 IR-DYN

   --------------------------------------------------------------------
   R      SO            Yes       Yes      Yes   R-0, R-0-CRC
                        Yes            No        goto FO
          -------------------------------------------------------------
          FO            Yes            Yes       goto SO
                        Yes            No        *R-1, UOR-2
                        No             Yes       (R-0-CRC), *R-1, UOR-2
                        No             No        *R-1, UOR-2
          -------------------------------------------------------------

      *: robustness will suffer unless an PT-1 header is used which
         gives the correct result also when old SN-functions are used.


5.2.1.  Packet formats from compressor to decompressor

   Four packet types are used from compressor to decompressor. Three for
   compressed headers, and one for initialization/refresh. Context
   identifiers of the appropriate length precede these headers. Exact
   header formats are found in section 5.7.

   Packet type zero: R-0, R-0-CRC, UO-0.

     This, the minimal, packet type is used when parameters of all SN-
     functions are known by decompressor, and the header to be
     compressed adheres to those functions. Thus, only the W-LSB encoded
     RTP SN needs to be communicated.

     R-mode:  Only if a CRC is present (packet type R-0-CRC) may the
     header be used as a reference for subsequent decompression.

     U-mode and O-mode: A small CRC is present in the UO-0 packet.

   Packet type 1: R-1, R-1-ID, R-1-TS, UO-1, UO-1-ID, UO-1-TS.

     This packet type is used when the number of bits needed for the SN
     exceeds those available in packet type zero, or when the parameters
     of the SN-functions for RTP TS or IP-ID change.

     R-mode: R-1-* packets are not used as references for subsequent
     decompression. Values for other fields than the RTP TS or IP-ID can
     be communicated using an extension, but they do not update the
     context.




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     U-mode and O-mode: Only the values of RTP SN, RTP TS and IP-ID can
     be used as references for future compression. Non-updating values
     can be provided for other fields using an extension (UO-1-ID).

   Packet type 2: UOR-2, UOR-2-ID, UOR-2-TS

     This packet type can be used to change the parameters of any SN-
     function, except for those for most static fields. Headers of
     packets transferred using packet type 2 can be used as references
     for subsequent decompression.

   Packet type 5: IR

     This packet type communicates the static part of the context, i.e.,
     the value of the constant SN-functions. It can optionally also
     communicate the dynamic part of the context, i.e., the parameters
     of the non-constant SN-functions.

   Packet type 6: IR-DYN

     This packet type communicates the dynamic part of the context,
     i.e., the parameters of the non-constant SN-functions.


5.2.2.  Feedback packets from decompressor to compressor

   Feedback packets carry information from decompressor to compressor.
   The following kinds of feedback is supported. The first three control
   state and mode transition, and the fourth informs the compressor that
   the decompressor does not have sufficient resources to decompress a
   packet stream.

   ACK         : Acknowledges successful decompression of a packet,
                 which means that the context is up to date with a
                 high probability.

   NACK        : Indicates that the dynamic context of the
                 decompressor is out of sync.

   STATIC-NACK : Indicates that the static context of the decompressor
                 is not valid or has not been established.

   REJECT      : Indicates that compression of this packet stream cannot
                 be supported due to resource constraints.


5.2.3.  Parameters needed for mode transition

   The packet type UOR-2 is common for all modes. It can carry an
   extension with a mode parameter which can take the values U  =




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   Unidirectional, O = Bi-directional Optimistic, and R= Bi-directional
   Reliable.

   Feedback packets of types ACK, NACK, and STATIC-NACK carry sequence
   numbers and can also carry a mode parameter indicating the desired
   compression mode: U, O, or R.

   As a shorthand, the notation PACKET(mode) is used to indicate which
   mode value a packet carries. For example, an ACK with mode parameter
   R is written ACK(R), and an UOR-2 with mode parameter O is written
   UOR-2(O).

5.3.  Operation in unidirectional mode

5.3.1.  Compressor states and logic (U-mode)

   Below is the state machine for the compressor in unidirectional mode.
   Details of the transitions between states and compression logic are
   given subsequent to the figure.

               Optimistic approach         Optimistic approach
             +------>------>------+      +------>------>------+
             |                    |      |                    |
             |                    v      |                    v
    +----------+                +----------+                +----------+
    | IR State |                | FO State |                | SO State |
    +----------+                +----------+                +----------+
      ^      ^                    |      ^                    |      |
      |      |      Timeout       |      |  Timeout / Update  |      |
      |      +------<------<------+      +------<------<------+      |
      |                                                              |
      |                           Timeout                            |
      +------<------<------<------<------<------<------<------<------+


5.3.1.1.  State transition logic (U-mode)

   The transition logic for compression states in unidirectional mode is
   based on three principles; the optimistic approach principle,
   timeouts, and the need for updates.


5.3.1.1.1.  Optimistic approach, upwards transition

   Transition to higher compression state in unidirectional mode is
   carried out according to the optimistic approach principle. This
   means that the compressor transits to a higher compression state when
   it is rather confident that the decompressor has received enough
   information to correctly decompress packets sent according to the
   higher compression state.




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   When the compressor is in IR state, it will stay there until it
   assumes that the decompressor has correctly received the static
   context information. For transition from FO to SO state, the
   compressor should be confident that the decompressor has all
   parameters needed to decompress according to a fixed pattern.

   The compressor normally gets its confidence about decompressor status
   by sending several packets with the same information according to the
   lower compression state. If the decompressor receives any of these
   packets, it will be in sync with the compressor. The number of
   consecutive packets to send for confidence is not defined in this
   document.


5.3.1.1.2.  Timeouts, downward transition

   By using the optimistic approach described above, there will always
   be a possibility for failure since the decompressor may not have
   received sufficient information for correct decompression. Therefore,
   the compressor must periodically transit to lower compression states.
   Periodic transition to IR state should be carried out less often than
   transition to FO state. Two different timeouts should therefore be
   used for these transitions. For an example of how to implement
   periodic refreshes, see [IPHC] chapter 3.3.1-3.3.2.


5.3.1.1.3.  Need for updates, downward transition

   In addition to the backward state transitions carried out due to
   periodic timeouts, the compressor must also immediately transit back
   to FO state when the header to be compressed does not confirm to the
   established pattern.


5.3.1.2.  Compression logic and packets used (U-mode)

   The compressor chooses the smallest possible packet format that can
   communicate the desired changes, and has the required bits for W-LSB
   encoded values. Sliding windows used in W-LSB encoding have a fixed
   width.


5.3.1.3.  Feedback in unidirectional mode

   The unidirectional mode of operation is designed to operate over
   links where a feedback channel is not available. If a feedback
   channel is available, however, the decompressor MAY send an
   acknowledgment of successful decompression with the mode parameter
   set to U (send an ACK(U)). When the compressor receives such a
   message, it MAY disable or increase the interval between periodic IR
   refreshes.



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5.3.2.  Decompressor states and logic (U-mode)

   Below is the state machine for the decompressor in unidirectional
   mode. Details of the transitions between states and decompression
   logic are given subsequent to the figure.

                                  Success
                 +-->------>------>------>------>------>--+
                 |                                        |
     No Static   |            No Dynamic        Success   |    Success
      +-->--+    |             +-->--+      +--->----->---+    +-->--+
      |     |    |             |     |      |             |    |     |
      |     v    |             |     v      |             v    |     v
    +--------------+         +----------------+         +--------------+
    |  No Context  |         | Static Context |         | Full Context |
    +--------------+         +----------------+         +--------------+
       ^                         |        ^                         |
       |    Repeated failure     |        |    Repeated failure     |
       +-----<------<------<-----+        +-----<------<------<-----+


5.3.2.1.  State transition logic (U-mode)

   The state transition logic of the decompressor is much simpler than
   for the compressor side. It is also common for all the three modes of
   operation. Successful decompression will always move the decompressor
   to the Full Context state. Repeated failed decompression will force
   the decompressor to transit backwards to a lower state. The
   decompressor does not attempt to decompress headers at all in the No
   Context and Static Context states unless sufficient information is
   included in the packet itself.


5.3.2.2.  Decompression logic (U-mode)

   Decompression in unidirectional mode is carried out following three
   steps which are described in subsequent sections.


5.3.2.2.1.  Decide whether decompression is allowed

   In Full Context state, decompression may be attempted regardless of
   what kind of packet is received. However, for the other states
   decompression is not always allowed. In the No Context state, only IR
   packets, which carry the static information fields may be
   decompressed. Further, when in the Static Context state, only packets
   carrying a strong CRC can be decompressed (i.e., IR, IR-DYN, or UOR-2
   packets). If decompression may not be performed the packet is
   discarded, unless the optional delayed decompression mecanism is
   used, see section x.x.x.



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5.3.2.2.2.  Reconstruct and verify the header

   // TWB based on chapter 4.5
   LSB interpretation, wrap around correction
   All decompressed headers are verified with CRC


5.3.2.2.3.  Causes for CRC mismatches

   A mismatch in the 3-bit CRC can be caused by one or more of:

     1. residual bit errors in the current header,

     2. a damaged context due to residual bit errors in previous
        headers, or

     3. loss of 16 or more consecutive packets which causes the 4-bit SN
        to wrap around (which is, in essence, another kind of context
        damage).

   The 3-bit CRC present in UO-0 and UO-1 headers will reliably detect
   context damage eventually, since the probability of undetected
   context damage decreases exponentially with each new header
   processed.  However, residual bit errors in the current header are
   only detected with good probability, not reliably.

   When a CRC mismatch is caused by residual bit errors in the current
   header (case 1 above), the decompressor should stay in its current
   state to avoid unnecessary loss of subsequent packets. On the other
   hand, when the mismatch is caused by a damaged context, the
   decompressor can attempt to repair the context but if that fails, it
   must move to a lower state to avoid delivering incorrect headers.
   When the mismatch is caused by long loss, the decompressor might
   attempt additional decompression attempts.

   - Long loss detection (timer)
   - Incorrect wrap around correction
   - Repeated reconstruction attempts in case of long loss

   //  The following text should be compressed and structured so that it
   //  takes care of the issues listed above in a correct order. Clear,
   //  simple and ordered principles should be described.

   [Micke's note: have I achieved that?]

   In the following sections mechanisms are described that might detect
   the cause for a CRC mismatch. If these mechanisms fail in finding the
   reason for the CRC mismatch, additional decompression attempts MUST
   NOT be performed.




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5.3.2.2.4.  Detection of context damage

   Context damage increases the probability of a CRC mismatch. With 3-
   bit CRCs, the probability of detecting the erroneous header is
   roughly 7/8. With 7-bit and 8-bit CRCs, the probability is roughly
   127/128 and 255/256, respectively.

   In the Full Context state, whenever decompression of 4 UO-0 or UO-1
   headers have failed out of the last 6 headers, the decompressor moves
   to the Static Context state. In the Static Context state, whenever
   decompression of 2 out of the last 3 UOR-2 or IR-DYN packets have
   failed, the compressor moves to the No Context state. An
   implementation MAY move to lower states sooner, but SHOULD NOT move
   to a lower state later than specified here.

   [[Editor's note: the exact numbers probably need more discussion.]]

5.3.2.2.5. Repair of SN wrap-around

   At least 16 headers must be lost for the SN of UO-0 or UO-1 headers
   to wrap around. The decompressor might be able to detect this
   situation by using a local clock. The following algorithm can be
   used:

     a.The decompressor notes the arrival time, a_i, of each incoming
       packet i. Arrival times of packets where decompression fails are
       discarded.

     b.When decompression fails, the decompressor computes INTERVAL =
       a_i - a_i-1, i.e., the time elapsed between the arrival of the
       previous correctly decompressed packet and the current packet.

     c.If wrap-around has occurred, INTERVAL will correspond to at
       least 16 inter-packet times. Based on an estimate of the packet
       inter-arrival time, obtained for example using a moving average
       of arrival times, TS STRIDE, or TS TIME, the decompressor judges
       if INTERVAL can correspond to 16 or more inter-packet times.

     d.If INTERVAL is judged to be at least 16 packet inter-arrival
       times, the decompressor adds 16 to the SN of the context and
       attempts to decompress the packet using the new context.

     e.If this decompression succeeds, the decompressor updates the
       context but SHOULD NOT deliver the packet to upper layers. The
       following packet is also decompressed and updates the context if
       its CRC succeeds, but SHOULD be discarded. If decompression of
       the third packet using the new context also succeeds, the
       context repair is deemed successful and this and subsequent
       decompressed packets are delivered to the upper layers.




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     f.If any of the three decompression attempts in d. and e. fails,
       the decompressor discards the packets and moves to the Static
       Context state.

   Using this mechanism, the decompressor may be able to repair the
   context after excessive loss, at the expense of discarding two
   packets.


5.3.2.2.6.  Repair of incorrect SN updates

   The CRC can fail to detect residual errors in the compressed header
   because of its limited length, i.e., the incorrectly decompressed
   packet can happen to have the same CRC as the original uncompressed
   packet, causing the incorrectly decompressed packet to be accepted
   and the context being updated. This can lead to an erroneous
   reference SN being used in W-LSB decoding, as the reference SN is
   updated for each successfully decompressed header of certain types.

   If this happens, the decompressor will detect the incorrect
   decompression of the following packet with high probability, but it
   does not know the reason for the failure. The following mechanism
   allows the decompressor to judge if the context was updated
   incorrectly by an earlier packet.

   1) The decompressor always keeps two decompressed SN: the
   last one (ref 0) and the one before that (ref -1).

   2) When receiving a compressed SN:

      SN curr = decompressed SN using ref 0 as reference value
      decompress the rest of the header using SN curr
      IF (the header passes the CRC test)
        ref -1 = ref 0
        ref 0 = SN curr
   3) ELSE
        SN curr = decompressed SN using ref -1 as reference value
        decompress rest of header using SN curr
        IF (header passes the CRC test)
          ref0 = SN curr;  // note ref-1 unchanged
   4)   ELSE
          // one of two error situations:
          // 1) bit error in current header,
          // 2) decompression context corrupted. Use 5.3.2.2.4 to
          // distinguish, (e.g., assume context corrupted
          // only after k out of n errors)

   The purpose of this algorithm is to repair the context. If the header
   generated in 3) passes the CRC test, two more headers must also be
   successfully decompressed before the repair is deemed successful. Of
   the three successful headers, the first two SHOULD be discarded and



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   only the third delivered to upper layers. If decompression of any of
   the three headers fail, the decompressor MUST discard that header and
   the previously generated headers, and move to the Static Context
   state.


5.3.2.3.  Feedback in unidirectional mode

   To improve performance for the unidirectional mode over a link that
   does have a feedback channel, the decompressor MAY send an
   acknowledgment when decompression succeeds. Setting the mode
   parameter in the ACK packet to U indicates that the compressor is to
   stay in unidirectional mode.  When receiving an ACK(U), the
   compressor SHOULD reduce the frequency of the IR packets since the
   static information has been correctly received, but it is not
   required to stop sending IR packets. If IR packets continue to
   arrive, the decompressor MAY repeat the ACK(U), but it SHOULD not
   repeat the ACK continuously.




































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5.4.  Operation in bi-directional optimistic mode

5.4.1.  Compressor states and logic (O-mode)

   Below is the state machine for the compressor in bi-directional
   optimistic mode. Details of each state, the transitions between
   states and compression logic are given subsequent to the figure.

                         Optimistic approach / ACK
      +------>------>------>------>------>------>------>------>------+
      |                                                              |
      |      Optimistic appr. / ACK      Optimistic appr. / ACK      |
      |      +------>------>------+      +------>------>------+      |
      |      |                    |      |                    |      |
      |      |                    v      |                    v      v
    +----------+                +----------+                +----------+
    | IR State |                | FO State |                | SO State |
    +----------+                +----------+                +----------+
      ^      ^                    |      ^                    |      |
      |      |    STATIC-NACK     |      |    NACK / Update   |      |
      |      +------<------<------+      +------<------<------+      |
      |                                                              |
      |                         STATIC-NACK                          |
      +------<------<------<------<------<------<------<------<------+


5.4.1.1.  State transition logic

   The transition logic for compression states in bi-directional
   optimistic mode has much in common with the logic of the
   unidirectional mode. The optimistic approach principle and
   transitions because of the need for updates works in the same way as
   described in chapter 5.3.1. However, in optimistic mode there are no
   timeouts. Instead, the optimistic mode makes use of feedback from
   decompressor to compressor both for transitions in the backward
   direction and for improved forward transition.


5.4.1.1.1.  Context requests, negative acknowledgements (NACKs)


5.4.1.1.2.  Optional acknowledgements


5.4.1.2.  Compression logic and packets used

   // Is there a difference from unidirectional mode??







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5.4.2.  Decompressor states and logic

   The decompression states and the state transition logic are the same
   as for the unidirectional case (see section 5.3.2). What differs is
   the decompression and feedback logic.


5.4.2.1.  Decompression logic


5.4.2.1.1.  Timer-based timestamp decompression

   NOTE: This mechanism must be described here since it can not be used
   in unidirectional mode because no indication can be given to the
   compressor if timer-based method fails.


5.4.2.2.  Feedback logic

   The feedback logic defines what feedback messages to send due to
   different events when operating in the various states.

   // NOTE: Must be rewritten and not refer to FH, FO and IR packets

   In NC state: - When a FH packet is correctly decompressed, send an
                  ACK with the mode parameter set to O
                - When an FO or SO packet is received or decompression
                  of a FH packet has failed, send a STATIC-NACK with
                  the mode parameter set to O

   In SC state: - When a FH packet is correctly decompressed, send an
                  ACK with the mode parameter set to O
                - When an FO packet is correctly decompressed,
                  optionally send an ACK with the mode parameter set to
                  O
                - When a SO packet is received, send a NACK with the
                  mode parameter set to O
                - When decompression of an FO or FH packet has failed,
                  send a STATIC-NACK with the mode parameter set to O

   In FC state: - When a FH packet is correctly decompressed, send an
                  ACK with the mode parameter set to O
                - When an FO packet is correctly decompressed,
                  optionally send an ACK with the mode parameter set to
                  O
                - When an SO packet is correctly decompressed, no
                  feedback should be sent
                - When decompression of an SO, FO or FH packet has
                  failed, send a NACK with the mode parameter set to O





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5.5.  Operation in bi-directional reliable mode

5.5.1.  Compressor states and logic

   Below is the state machine for the compressor in bi-directional
   reliable mode. Details of each state, the transitions between states
   and compression logic are given subsequent to the figure.

                                    ACK
      +------>------>------>------>------>------>------>------+
      |                                                       |
      |               ACK                         ACK         |   ACK
      |      +------>------>------+      +------>------>------+  +->-+
      |      |                    |      |                    |  |   |
      |      |                    v      |                    v  |   v
    +----------+                +----------+                +----------+
    | IR State |                | FO State |                | SO State |
    +----------+                +----------+                +----------+
      ^      ^                    |      ^                    |      |
      |      |    STATIC-NACK     |      |    NACK / Update   |      |
      |      +------<------<------+      +------<------<------+      |
      |                                                              |
      |                         STATIC-NACK                          |
      +------<------<------<------<------<------<------<------<------+


   // This chapter should be structured with subchapters in a way
   // consistent with 5.3 and 5.4


5.5.2.  Decompressor states and logic

   The decompression states and the state transition logic are the same
   as for the unidirectional case (see section 5.3.2). What differs is
   the decompression and feedback logic.


5.5.2.1.  Decompression logic


5.5.2.2.  Feedback logic

   The feedback logic defines what feedback messages to send due to
   different events when operating in the various states.

   // NOTE: Must be rewritten and not refer to FH, FO and IR packets


   In NC state: - When a FH packet is correctly decompressed, send an
                  ACK with the mode parameter set to R




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                - When an FO or SO packet is received or decompression
                  of a FH packet has failed, send a STATIC-NACK with
                  the mode parameter set to R

   In SC state: - When an FO or FH packet is correctly decompressed,
                  send an ACK with the mode parameter set to R
                - When an SO packet is received, send a NACK with the
                  mode parameter set to R
                - When decompression of an FO or FH packet has failed,
                  send a STATIC-NACK with the mode parameter set to R

   In FC state: - When an FO or FH packet is correctly decompressed,
                  send an ACK with the mode parameter set to R
                - When an updating SO packet is correctly decompressed,
                  periodically send an ACK with the mode parameter set
                  to R
                - When decompression of an SO, FO or FH packet has
                  failed, send a NACK with the mode parameter set to R




































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5.6.  Mode transitions

   The decision to move from one compression mode to another is taken by
   the decompressor and the possible mode transitions are shown in the
   figure below. Subsequent chapters describe how the transitions are
   performed together with exceptions for the compression and
   decompression functionality during transitions.

                         +-------------------------+
                         | Unidirectional (U) mode |
                         +-------------------------+
                           / ^                 \ ^
                          / /Feedback(U)        \ \Feedback(U)
                         / /                     \ \
                        / /                       \ \
            Feedback(O)/ /              Feedback(R)\ \
                      v /                           v \
      +---------------------+    Feedback(R)    +-------------------+
      | Optimistic (O) mode | ----------------> | Reliable (R) mode |
      |                     | <---------------- |                   |
      +---------------------+    Feedback(O)    +-------------------+


5.6.1.  Compression and decompression during mode transitions

   To be able to work properly, both compressor and decompressor must
   have a variable for which mode it should be working according to.
   Subsequent chapters define exactly when to change the value of this
   mode variable and thereby change the mode of operation. Further, when
   ROHC transits from one mode to another, there are several cases with
   special rules for what is allowed to do at the compressor and
   decompressor sides during the transition phase. These special rules
   are defined by exception parameters that sets various exception rules
   to follow. The transition descriptions in subsequent chapters refer
   to these exception parameters and defines when and to what values
   they are set. All mode related parameters are listed below with
   possible values, explanations and rules:

   Parameters for the compressor side:

      - C_MODE
                Possible values for the C_MODE parameter are
                (U)NIDIRECTIONAL, (O)PTIMISTIC and (R)ELIABLE.
                Initially the value MUST be set to U.

      - C_TRANS
                Possible values for the C_TRANS parameter are (P)ENDING
                and (D)ONE. Initially the value MUST be set to D. When
                the parameter is set to P, it is REQUIRED that the
                compressor only uses packet formats common to all modes



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                and SHALL NOT allow transition to SO state. New mode
                transition request MUST also be ignored when an already
                initiated transition is pending.

   Parameters for the decompressor side:

      - D_MODE
                Possible values for the D_MODE parameter are
                (U)NIDIRECTIONAL, (O)PTIMISTIC and (R)ELIABLE.
                Initially the value MUST be set to U.


      - D_TRANS
                Possible values for the D_TRANS parameter are
                (I)NITIATED, (P)ENDING and (D)ONE. Initially the value
                MUST be set to D. New mode transitions are only allowed
                when the parameter is set to D. When set to I, the
                decompressor should send a NACK or ACK for all received
                packets, which it can stop doing when the parameter is
                set to P or D.


5.6.2.  Transition from Unidirectional to Optimistic mode

   As long as there is a feedback channel available, the decompressor
   can at any moment decide to initiate transition from unidirectional
   to bi-directional Optimistic mode. All feedback packets can be used
   with the mode parameter set to O and the decompressor can then
   directly start working in Optimistic mode. The compressor transits
   from unidirectional to optimistic mode as soon as it receives any
   feedback packet with the mode parameter set to O. The transition
   procedure is described below:

               Comnpressor                     Decompressor
              ----------------------------------------------
                    |                               |
                    |        ACK(R)/NACK(R) +-<-<-<-|  D_MODE = O
                    |       +-<-<-<-<-<-<-<-+       |
    C_MODE = O      |-<-<-<-+                       |
                    |                               |

   If the feedback packet is lost, the compressor will continue to work
   in unidirectional mode with periodic refreshes but as soon as the
   decompressor send another feedback packet, also the compressor will
   transit to optimistic mode.









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5.6.3.  From Optimistic to Reliable mode

   Transition from Optimistic to Reliable mode is only allowed after at
   least one packet has been correctly decompressed, which means that
   the static part of the context is established. Either the ACK(R) or
   the NACK(R) feedback packet is used to initiate the transition and
   the compressor MUST always run in FO state during transition. The
   transition procedure is described below:

               Comnpressor                     Decompressor
              ----------------------------------------------
                    |                               |
                    |        ACK(R)/NACK(R) +-<-<-<-|  D_TRANS = I
                    |       +-<-<-<-<-<-<-<-+       |
    C_TRANS = P     |-<-<-<-+                       |
    C_MODE = R      |                               |
                    |->->->-+   FO(SN0,R)           |
                    |       +->->->->->->->-+       |
                    |->-..                  +->->->-|  D TRANS = P
                    |->-..                          |  D_MODE = R
                    |           ACK(SN0,R)  +-<-<-<-|
                    |       +-<-<-<-<-<-<-<-+       |
    C_TRANS = D     |-<-<-<-+                       |
                    |                               |
                    |->->->-+ SO (Reliable mode)    |
                    |       +->->->->->->->-+       |
                    |                       +->->->-|  D_TRANS = D
                    |                               |

   As long as the decompressor has not received an FO packet with the
   mode transition parameter set to R, it must stay in Optimistic mode.
   The compressor must stay in FO state until it has received an ACK for
   an FO packet sent with the mode transition parameter set to R
   (indicated by the sequence number).


5.6.4.  From Unidirectional to Reliable mode

   Since transition from Unidirectional to Optimistic mode do not
   require any handshakes, it is possible to transit directly from
   Unidirectional to Reliable mode, following the same transition
   procedure in 5.6.3 above.


5.6.5.  From Reliable to Optimistic mode

   Either the ACK(O) or the NACK(O) feedback packet is used to initiate
   the transition from Reliable to Optimistic mode and the compressor
   MUST always run in FO state during transition. The transition
   procedure is described below:




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               Comnpressor                     Decompressor
              ----------------------------------------------
                    |                               |
                    |        ACK(O)/NACK(O) +-<-<-<-|  D_TRANS = I
                    |       +-<-<-<-<-<-<-<-+       |
    C_TRANS = P     |-<-<-<-+                       |
    C_MODE = O      |                               |
                    |->->->-+   FO(SN0,O)           |
                    |       +->->->->->->->-+       |
                    |->-..                  +->->->-|  D_MODE = O
                    |->-..                          |
                    |           ACK(SN0,O)  +-<-<-<-|
                    |       +-<-<-<-<-<-<-<-+       |
    C_TRANS = D     |-<-<-<-+                       |
                    |                               |
                    |->->->-+ SO (Optimistic mode)  |
                    |       +->->->->->->->-+       |
                    |                       +->->->-|  D_TRANS = D
                    |                               |

   As long as the decompressor has not received an FO packet with the
   mode transition parameter set to O, it must stay in Reliable mode.
   The compressor must stay in FO state until it has received an ACK for
   a FO packet sent with the mode transition parameter set to O
   (indicated by the sequence number).


5.6.6.  Transition to Unidirectional mode

   It is possible to force transition back to unidirectional mode if the
   decompressor desires to do so. Independent of which mode it starts
   from, a three way handshake MUST be carried out to ensure correct
   transition on the compressor side. The transition procedure is
   described below:

               Compressor                     Decompressor
              ----------------------------------------------
                    |                               |
                    |        ACK(U)/NACK(U) +-<-<-<-|  D_TRANS = I
                    |       +-<-<-<-<-<-<-<-+       |
    C_TRANS = P     |-<-<-<-+                       |
    C_MODE = U      |                               |
                    |->->->-+   FO(SN0,U)           |
                    |       +->->->->->->->-+       |
                    |->-..                  +->->->-|
                    |->-..                          |
                    |           ACK(SN0,U)  +-<-<-<-|
                    |       +-<-<-<-<-<-<-<-+       |
    C_TRANS = D     |-<-<-<-+                       |
                    |                               |
                    |->->->-+ SO (Uni. mode)        |



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                    |       +->->->->->->->-+       |
                    |                       +->->->-|  D_TRANS = D
                    |                               |  D_MODE = U

   The decompressor must continue to send feedback until it knows that
   the compressor is ready with the transition.
















































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5.7.  Packet formats

5.7.1. Packet type 0: UO-0, R-0, R-0-CRC

   R-0

      0   1   2   3   4   5   6   7
    +---+---+---+---+---+---+---+---+
    | 0   0 |        SN LSB         |
    +---+---+---+---+---+---+---+---+

   R-0-CRC

      0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
    +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
    | 0   1 |         SN  LSB           |            CRC            |
    +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

     SN LSB: Least significant bits of RTP SN.  (Note that, in R-0-CRC,
     the SN LSB straddles a byte boundary.)

     CRC: 7-bit CRC, computed according to section 5.9.2.

     Note: R-0 headers MUST NOT update any part of the context.
     R-0-CRC headers MUST NOT update any part of the context not
     directly related to the RTP SN.


   UO-0

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0 |     SN LSB    |    CRC    |
   +---+---+---+---+---+---+---+---+


     SN LSB: Least significant bits of RTP SN (see section 4.5.1)

     CRC: 3-bit CRC  (see section 5.9.2)

     Note: The UO-0 header MUST update the current value of the RTP SN.
     The UO-0 header MUST NOT update any part of the context not
     directly related to the RTP SN.


5.7.2. Packet type 1 (R-mode): R-1, R-1-TS, R-1-ID

   This packet type is used in R-mode when the RTP SN of the header to
   be compressed differs so much from the context that the LSB of packet
   type 0 is too small, or when the functions from RTP SN to RTP



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   timestamp or IP ID change and need to be communicated to the
   decompressor. It is also possible to give values for other fields by
   using an Extension.

   R-1

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 1   0 |        SN LSB         |
   +---+---+---+---+---+---+---+---+
   | M | X |        TS LSB         |
   +---+---+---+---+---+---+---+---+
   | Extension present if X=1
    - - - - - - - - -


   R-1-ID

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 1   0 |        SN LSB         |
   +---+---+---+---+---+---+---+---+
   | M | X |T=0|     IP-ID LSB     |
   +---+---+---+---+---+---+---+---+
   | Extension present if X=1
    - - - - - - - - -


   R-1-TS

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 1   0 |        SN LSB         |
   +---+---+---+---+---+---+---+---+
   | M | X |T=1|      TS LSB       |
   +---+---+---+---+---+---+---+---+
   | Extension present if X=1
    - - - - - - - - -


     SN LSB: Least significant bits of RTP SN, see section 4.5.1.

     M: RTP Marker bit, absolute value.

     X: X=0 indicates that no Extension is present,
        X=1 indicates that an Extension is present.

     T: T=0 indicates format R-1-ID, T=1 indicates format R-1-TS.

     TS LSB: Least significant bits of scaled RTP TS,
     see section 5.x.x.



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     IP-ID LSB: Least significant bits of IP-ID offset from SN, see
        Section 5.x.x.

     Extension: See section 5.7.5.

   Note: R-1-* headers MUST NOT update any part of the context.


5.7.3. Packet type 1 (UO-modes): UO-1, UO-1-ID, UO-1-TS

   This packet type is used in U-mode and O-mode when the RTP SN of the
   header to be compressed differs so much from the context that the RTP
   LSB of packet type 0 is too small, or when the functions from RTP SN
   to RTP timestamp or IP ID change and need to be communicated to the
   decompressor. It is also possible to give non-updating values for
   other fields by using an Extension.


   UO-1

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 1   0 |         TS LSB        |
   +---+---+---+---+---+---+---+---+
   | M |    SN LSB     |    CRC    |
   +---+---+---+---+---+---+---+---+

   UO-1-ID

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 1   0 |T=0|     IP-ID LSB     |
   +---+---+---+---+---+---+---+---+
   | X |    SN  LSB    |    CRC    |
   +---+---+---+---+---+---+---+---+
   | Extension present if X=1
    - - - - - - - - - - - -

   UO-1-TS

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 1   0 |T=1|       TS LSB      |
   +---+---+---+---+---+---+---+---+
   | M |     SN LSB    |    CRC    |
   +---+---+---+---+---+---+---+---+


     SN LSB: Least significant bits of RTP SN, see section 4.5.1.




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     M: RTP Marker bit, absolute value.

     TS LSB: Least significant bits of scaled RTP TS,
     see section 5.x.x.

          CRC: 3-bit CRC  (see section 5.9.2)

     IP-ID LSB: Least significant bits of IP-ID offset from SN, see
          Section 5.x.x.

     X: X=0 indicates that no Extension is present,
        X=1 indicates that an Extension is present.

     T: T=0 indicates format UO-1-ID, T=1 indicates format UO-1-TS.

     Extension: See section 5.7.5.

   Note: UO-1-* headers MUST NOT update any part of the context except
   parts directly related to the RTP SN, RTP TS, and IP-ID. Values
   provided in Extensions, other than those directly related to RTP SN,
   RTP TS, and IP-ID, MUST NOT update the context.


5.7.4. Packet type 2: UOR-2

   This packet type is used in all modes when the changes that need to
   be communicated, or the RTP SN, cannot fit in the earlier formats.
   The UOR-2 format always updates (or refreshes) the context.

   UOR-2

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 1   1   0 |      TS LSB       |
   +---+---+---+---+---+---+---+---+
   |msb| M |        SN LSB         |
   +---+---+---+---+---+---+---+---+
   | X |            CRC            |
   +---+---+---+---+---+---+---+---+
   | Extension present if X=1
    - - - - - - - - - - -


   UOR-2-ID

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 1   1   0 |     IP-ID LSB     |
   +---+---+---+---+---+---+---+---+
   |T=0| M |        SN LSB         |



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   +---+---+---+---+---+---+---+---+
   | X |            CRC            |
   +---+---+---+---+---+---+---+---+
   | Extension present if X=1
    - - - - - - - - - - -


   UOR-2-TS

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 1   1   0 |       TS LSB      |
   +---+---+---+---+---+---+---+---+
   |T=1| M |        SN LSB         |
   +---+---+---+---+---+---+---+---+
   | X |            CRC            |
   +---+---+---+---+---+---+---+---+
   | Extension present if X=1
    - - - - - - - - - - -

     TS LSB: Least significant bits of scaled RTP TS,
     see section 5.x.x.

     msb: One-bit extension of TS LSB field, so that the total size
          of the LSB of the scaled TS is 6 bits. This is the most
          significant bit.

     M: RTP Marker bit, absolute value.

     SN LSB: Least significant bits of RTP SN, see section 4.5.1.

     IP-ID LSB: Least significant bits of IP-ID offset from SN, see
        Section 5.x.x.

     X: X=0 indicates that no Extension is present,
        X=1 indicates that an Extension is present.

     CRC: 7-bit CRC  (see section 5.9.2)

     T: T=0 indicates format UOR-2-ID, T=1 indicates format UOR-2-TS.

     Extension: See section 5.7.5.


5.7.5. Extension formats

   Fields in extensions are concatenated with the corresponding field in
   the base compressed header, if any. Bits in an extension are more
   significant than bits in the base compressed header.





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   The TS LSB field is scaled in all extensions, as it is in the base
   header, except optionally when using extension 3 where the Tsc flag
   can indicate that the RTP TS LSB field is not scaled. When an
   extension carries a TS-Stride field, the value of that field is used
   when scaling the TS LSB field.

   In the following three extensions, the interpretation of the fields
   depend on whether there is a T-bit in the base compressed header, and
   if so, on the value of that field. If there is no T-bit, T LSB and -T
   LSB both mean TS LSB. If there is a T-bit,

     T = 1   indicates that T is TS, and
                           -T is IP-ID.

     T = 0   indicates that T is IP-ID, and
                           -T is TS.


   Extension 0:

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 0   0 |  SN LSB   |   T LSB   |
   +---+---+---+---+---+---+---+---+


   Extension 1:

   +---+---+---+---+---+---+---+---+
   | 0   1 |  SN LSB   |   T LSB   |
   +---+---+---+---+---+---+---+---+
   |            -T LSB             |
   +---+---+---+---+---+---+---+---+


   Extension 2:

   +---+---+---+---+---+---+---+---+
   | 1   0 |  SN LSB   |   T LSB   |
   +---+---+---+---+---+---+---+---+
   |            T LSB              |
   +---+---+---+---+---+---+---+---+
   |           -T LSB              |  only if T-bit in base comp header.
    --- --- --- --- --- --- --- ---

   Extension 3 is a more elaborate extension which can give values for
   fields other than RTP SN, RTP TS, and IP-ID. Three optional flag
   octets indicates changes to IP header(s) and RTP header,
   respectively.





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   Extension 3:

   +-----+-----+-----+-----+-----+-----+-----+-----+
   |  1     1  |  S  |R-TS | Tsc |  I  | ip  | rtp |
   +-----+-----+-----+-----+-----+-----+-----+-----+
   |                    SN LSB                     |  if S=1
    ..... ..... ..... ..... ..... ..... ..... .....
   |  L  |            TS LSB (msb)                 |  if R-TS=1
   /.....                                          /
   |                  TS LSB (lsb)                 |  if R-TS=1 AND L=1
    ..... ..... ..... ..... ..... ..... ..... .....
   |                    IP-ID LSB                  |  2 octets if I=1
    ..... ..... ..... ..... ..... ..... ..... .....
   | TOS | TTL | DF  | PR  |IPXT |NBO  | RND | IP2 |  if ip=1
    ..... ..... ..... ..... ..... ..... ..... .....
   |         Type of Service/ Traffic Class        |  if TOS=1
    ..... ..... ..... ..... ..... ..... ..... .....
   |         Time to live/ Hop Limit               |  if TTL=1
    ..... ..... ..... ..... ..... ..... ..... .....
   |         Protocol/ Next Header                 |  if PR=1
    ..... ..... ..... ..... ..... ..... ..... .....
   /         compressed IP extension hdr(s)        /  if IPXT=1
    ..... ..... ..... ..... ..... ..... ..... .....
   | TOS2| TTL2| DF2 | PR2 |IPXT2|NBO2 |RND2 |  I2 |  if IP2=1
    ..... ..... ..... ..... ..... ..... ..... .....
   |         Type of Service/ Traffic Class        |  if TOS2=1
    ..... ..... ..... ..... ..... ..... ..... .....
   |         Time to live/ Hop Limit               |  if TTL2=1
    ..... ..... ..... ..... ..... ..... ..... .....
   |         Protocol/ Next Header                 |  if PR2=1
    ..... ..... ..... ..... ..... ..... ..... .....
   /         compressed IP extension hdr(s)        /  if IPXT2=1
    ..... ..... ..... ..... ..... ..... ..... .....
   |                                               |
   |                    IP-ID LSB                  |  2 octets if I2=1
    ..... ..... ..... ..... ..... ..... ..... .....
   |   Mode    |R-PT |R-PT | R-X |CSRC | TSS | IPT |  if rtp=1
    ..... ..... ..... ..... ..... ..... ..... .....
   |  M  |             RTP PT                      |  if R-PT=1
    ..... ..... ..... ..... ..... ..... ..... .....
   |           compressed CSRC-list                |  if CSRC=1
    ..... ..... ..... ..... ..... ..... ..... .....
   |                                               |
   |                 TS Stride                     |  2 oct, if TSS=1
    ..... ..... ..... ..... ..... ..... ..... ....
   |         Inter-packet time (milliseconds)      |  if IPT=1
    ..... ..... ..... ..... ..... ..... ..... .....


   [Note from Micke D: This format can deal with two IP headers, which
   is what we can expect from Mobile IP. More is overkill, I think.]



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5.7.6. Feedback packets: FEEDBACK-3 and FEEDBACK-4

   When the roundtrip time between compressor and decompressor is large,
   several packets can be in flight. Therefore, several packets can be
   received by the decompressor before the compressor can react to a
   FEEDBACK packet.

   The decompressor SHOULD NOT send FEEDBACK packets for each successful
   decompression, unsuccessful decompression, or packet received in a
   rejected packet stream. Instead, a compressor SHOULD limit the rate
   at which FEEDBACK packets are sent.

   The following are the formats of the FEEDBACK packets. They are to be
   preceded with a CID that identifies which context they refer to.

   It is anticipated that FEEDBACK packets may be sent interleaved with
   non-FEEDBACK packets in a channel going from decompressor to
   compressor. In that case the formats below will be preceded by a CID
   with whatever size has been negotiated for that channel. If the CIDs
   of that channel is smaller than the CIDs needed in the FEEDBACK
   packets, additional bytes of CID MUST be added in the FEEDBACK
   formats. How many bytes to add is known at channel setup time, and is
   not explicitly indicated. If the CIDs of the FEEDBACK channel are
   larger than those of the forward channel, the numerical value of the
   CID is used to identify the context.

   FEEDBACK-3

     0   1   2   3   4   5   6   7
   +---+---+---+---+---+---+---+---+
   | 1   1   1   0 | Type  |  Mode |
   +---+---+---+---+---+---+---+---+
   |          RTP SN LSB           |
   +---+---+---+---+---+---+---+---+
   |    additional CID octet(s)    |  0-2 octets
    - - - - - - - - - - - - - - - -

   Type:  0= Reserved
          1= ACK
          2= NACK
          3= STATIC-NACK

   Mode:  0= Reserved
          1= Unidirectional
          2= Bi-directional Optimistic
          3= Bi-directional Reliable

   RTP SN LSB: LSB of the RTP Sequence number of

          ACK: the successfully decompressed header which



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                 caused the FEEDBACK packet to be emitted,

          NACK, STATIC-NACK: the last successfully decompressed header
               (from context), if any available. Otherwise zero.

   Additional CID octet(s): Additional octets of CID if there is a
          mismatch between the CID spaces of the forward channel and
          the feedback channel.


   FEEDBACK-4-REJECT

   +---+---+---+---+---+---+---+---+
   | 1   1   1   1   0 |  Type=4   |
   +---+---+---+---+---+---+---+---+
   |               0               |
   +---+---+---+---+---+---+---+---+
   | 0 |            CRC            |
   +---+---+---+---+---+---+---+---+
   |    additional CID octet(s)    |  0-2 octets
      - - - - - - - - - - - - - - - -

     Type: 4

     CRC: CRC computed over the entire FEEDBACK-4-REJECT packet,
     including CID, using the polynomial for 7-bit CRCs in section
     5.9.2. For purposes of CRC calculation, the CRC field MUST be set
     to zero. When receiving a FEEDBACK-4-REJECT packet, a compressor
     MUST check the CRC, and check that the zeroed fields of the
     FEEDBACK-4-REJECT packet are indeed zero.  If these checks fail,
     the packet MUST be discarded and no further action taken.

     Additional CID octet(s): Additional octets of CID if there is a
     mismatch between the CID spaces of the forward channel and the
     feedback channel.


   FEEDBACK-4

   +---+---+---+---+---+---+---+---+
   | 1   1   1   1   0 |   Type    |
   +---+---+---+---+---+---+---+---+
   |  Mode |   RTP SN LSB (msb)    |
   +---+---+---+---+---+---+---+---+
   |       RTP SN LSB (lsb)        |
   +---+---+---+---+---+---+---+---+
   |    additional CID octet(s)    |  0-2 octets
    - - - - - - - - - - - - - - - -

     Type:  0, 5-7 = Reserved
            1      = ACK



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            2      = NACK
            3      = STATIC-NACK

     Mode:  0 = Reserved
            1 = Unidirectional
            2 = Bi-directional Optimistic
            3 = Bi-directional Reliable

     RTP SN LSB (msb): Most significant 6 bits of 14-bit RTP SN LSB.

     RTP SN LSB (lsb): Least significant 8 bits of 14-bit RTP SN LSB.

     Additional CID octet(s): Additional octets of CID if there is a
     mismatch between the sizes of the CID spaces of the forward channel
     and the feedback channel.

     Note: RTP sequence number to use is the same as for FEEDBACK-3.


5.7.7  IR and IR-DYN packets

   The subheaders which are compressible are split into a STATIC part
   and a DYNAMIC part. These parts can be found in sections 5.7.6.3-*.

   The structure of a chain of subheaders is determined by each header
   having a Next Header, or Protocol, field. This field identifies the
   type of the following header. Each Static part below contains that
   field and allows parsing of the Static chain.


5.7.7.1 Basic structure of the IR packet

   This packet type communicates the static part of the context, i.e.,
   the value of the constant SN-functions. It can optionally also
   communicate the dynamic part of the context, i.e., the parameters of
   non-constant SN-functions. It can also optionally communicate the
   payload of an original packet, if any.

     0   1   2   3   4   5   6   7
    - - - - - - - - - - - - - - - -
   |              CID              |     0-2 octets
   +---+---+---+---+---+---+---+---+
   | 1   1   1   1   1   0 | D | P |
   +---+---+---+---+---+---+---+---+
   |            Profile            |     1 octet
   +---+---+---+---+---+---+---+---+
   |              CRC              |     1 octet
   +---+---+---+---+---+---+---+---+
   |                               |
   |         Static chain          |     variable length




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   |                               |
   +---+---+---+---+---+---+---+---+
   |                               |
   |         Dynamic chain         |     present if D=1, variable length
   |                               |
    - - - - - - - - - - - - - - - -
   |                               |
   |           Payload             |     present if P=1, variable length
   |                               |
    - - - - - - - - - - - - - - - -

     D:   D=1 indicates that the dynamic chain is present.

     P:   P=1 indicates that a payload is present (could be inferred
          from static chain plus frame length).

     Profile:  Indicates transport/application of this stream.

     CRC: 8-bit checksum covering entire IR header, including CID,
          excluding Payload, computed according to chapter 5.9.x.

     Static chain: A chain of static subheader information.

     Dynamic chain: A chain of dynamic subheader information. What
     dynamic information is present is inferred from the Static chain.

     Payload: payload of corresponding original packet, if any.


5.7.7.2. Basic structure of the IR-DYN packet

   This packet type communicates the dynamic part of the context, i.e.,
   the parameters of non-constant SN-functions.

     0   1   2   3   4   5   6   7
    - - - - - - - - - - - - - - - -
   |              CID              |     0-2 octets
   +---+---+---+---+---+---+---+---+
   | 1   1   1   1   1   1 |res| P |
   +---+---+---+---+---+---+---+---+
   |            Profile            |     1 octet
   +---+---+---+---+---+---+---+---+
   |              CRC              |     1 octet
   +---+---+---+---+---+---+---+---+
   |                               |
   |         Dynamic chain         |     variable length
   |                               |
    - - - - - - - - - - - - - - - -
   |                               |
   |           Payload             |     present if P=1, variable length




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   |                               |
    - - - - - - - - - - - - - - - -

     res: Reserved. Set to zero when sending, ignored when received.

     P:   P=1 indicates that Payload is present.

     Profile:  Indicates transport/application of this stream.

     CRC: 8-bit checksum covering entire IR-DYN header, including CID,
          excluding Payload.

     Dynamic chain: A chain of dynamic subheader information. What
     dynamic information is present is inferred from the Static chain of
     the context.

     Payload: payload of corresponding original packet, if any.


5.7.7.3.  Initialization of IPv6 Header [IPv6]

      Static part:

   +---+---+---+---+---+---+---+---+
   |  Version = 6  |Flow Label(msb)|   1 octet
   +---+---+---+---+---+---+---+---+
   /        Flow Label (lsb)       /   2 octets
   +---+---+---+---+---+---+---+---+
   |         Next Header           |   1 octet
   +---+---+---+---+---+---+---+---+
   |         Hop Limit             |   1 octet
   +---+---+---+---+---+---+---+---+
   /         Source Address        /   16 octets
   +---+---+---+---+---+---+---+---+
   /      Destination Address      /   16 octets
   +---+---+---+---+---+---+---+---+


      Dynamic part:

   +---+---+---+---+---+---+---+---+
   |         Traffic Class         |   1 octet
   +---+---+---+---+---+---+---+---+

      Eliminated:

         Payload Length







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5.7.7.4. Initialization of IPv4 Header [IPv4, section 3.1].

      Static part:

        Version, Flags, Time to Live, Protocol, Source Address,
   Destination Address.

        +---+---+---+---+---+---+---+---+
        |  Version = 4  | 0 | DF|   0   |
        +---+---+---+---+---+---+---+---+
        |         Time to Live          |
        +---+---+---+---+---+---+---+---+
        |           Protocol            |
        +---+---+---+---+---+---+---+---+
        /        Source Address         /   4 octets
        +---+---+---+---+---+---+---+---+
        /      Destination Address      /   4 octets
        +---+---+---+---+---+---+---+---+

      Dynamic part:

         Type of Service, Identification

        +---+---+---+---+---+---+---+---+
        |       Type of Service         |
        +---+---+---+---+---+---+---+---+
        /        Identification         /   2 octets
        +---+---+---+---+---+---+---+---+

      Eliminated:

         IHL               (must be 5)
         Total Length      (inferred in decompressed packets))
         MF flag           (More Fragments flag, must be 0)
         Fragment Offset   (must be 0)
         Header Checksum   (inferred in decompressed packets)
         Options, Padding  (must not be present)


5.7.7.5. Initialization of UDP Header [RFC-768].

    Static part:

        +---+---+---+---+---+---+---+---+
        /          Source Port          /   2 octets
        +---+---+---+---+---+---+---+---+
        /       Destination Port        /   2 octets
        +---+---+---+---+---+---+---+---+

    Dynamic part:




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        +---+---+---+---+---+---+---+---+
        /           Checksum            /   2 octets
        +---+---+---+---+---+---+---+---+

    Eliminated:

         Length

   The Length field of the UDP header MUST match the Length field(s) of
   preceding subheaders, i.e., there must not be any padding after the
   UDP payload that is covered by the IP Length.


5.7.7.6. Initialization of RTP Header [RTP].

      Static part:

         P, X, CC, PT, SSRC, CSRC identifiers.

           0   1   2   3   4   5   6   7
         +---+---+---+---+---+---+---+---+
         |  V=2  | P | X |      CC       |
         +---+---+---+---+---+---+---+---+
         | M |            PT             |
         +---+---+---+---+---+---+---+---+
         /             SSRC              /   4 octets
         +---+---+---+---+---+---+---+---+
         :                               :
         :            CSRC ids           :   multiple of 4 octets
         :                               :
         +---+---+---+---+---+---+---+---+


      Dynamic part:

         M, sequence number, timestamp, timestamp delta.

           0   1   2   3   4   5   6   7
         +---+---+---+---+---+---+---+---+
         /      RTP sequence number      /  2 octets
         +---+---+---+---+---+---+---+---+
         |   RTP timestamp (absolute)    |  4 octets
         +---+---+---+---+---+---+---+---+
         | M |      TS-Stride (msb)      |
         +---+---+---+---+---+---+---+---+
         |         TS-Stride (lsb)       |
         +---+---+---+---+---+---+---+---+






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      Eliminated:

         Nothing.


5.7.7.7.  Minimal Encapsulation header [RFC-2004, section 3.1]


   STATIC Part:

         Entire header.

       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
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |   Protocol    |S|  reserved   |        Header Checksum        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                 Original Destination Address                  |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       :            (if present) Original Source Address               :
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      DYNAMIC Part:

         Empty.

      Eliminated:

         Nothing.

























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5.8.  List-Based Compression

   In two cases, information from the packet to be compressed can be
   described as an ordered list, that is largely constant between
   packets, but to which additions and deletions are made occasionally
   in the course of the packet stream.  This section describes the
   compression scheme for this information, called "list-based
   compression."  The two cases are: CSRC lists in RTP packets, and
   extension header chains in IP packets.

   [[Editor's note: This section still is under active discussion in the
   mailing list.  A somewhat simplified version is believed to emerge
   soon.]]

5.8.1.  CSRC compression

   The Contributing Source (CSRC) List in a RTP header contains the
   Synchronization Source (SSRC) identifiers of the contributing sources
   for the payload in the current packet.

   A CSRC list contains at most 15 identifiers, due to the 4-bit size of
   CSRC Count (CC) field in RTP header.  Each 32-bit identifier is
   chosen randomly by the original synchronization source so that it is
   globally unique within an RTP session.

   The compression scheme introduced here will utilize the facts
   mentioned above. To maintain transparency, the order of identifiers
   is preserved during compression. In other words, the CSRC list is
   really compressed as a list, not as a set.

5.8.1.1.  Transformation Classification for CSRC List

   A given CSRC list (curr_list) can be classified as belonging to
   one of the following transformation cases when compared with a
   reference CSRC list (ref_list).

     -  Transformation Case A: curr_list can be derived from ref_list
        just by adding some CSRCs; the relative positions of the CSRCs
        common to curr_list and ref_list are the same.

     -  Transformation Case B: curr_list can be derived from ref_list
        just by deleting some CSRCs; the relative positions of the CSRCs
        common to curr_list and ref_list are the same

     -  Transformation Case C: All the other transformation cases that
        are not covered by transformation case A and B.

5.8.1.2.  Encoding Schemes

   To address aforementioned 3 transformation cases, four encoding
   schemes are used. Each scheme addresses one or more



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   transformation cases mentioned above. The four encoding schemes
   and the transformation cases they address is listed in
   the following table.

   +----------------------------+--------------------+
   | Encoding Scheme            | Transformaton Case |
   +----------------------------+--------------------+
   | Insertion Only             |         A          |
   +----------------------------+--------------------+
   | Removal Only               |         B          |
   +----------------------------+--------------------+
   | Generic                    |  mixture of A,B,C  |
   +----------------------------+--------------------+
   | Uncompressed               |  mixture of A,B,C  |
   +----------------------------+--------------------+

      - In the insertion only scheme, compared with CSRC list in the
        ref_list, the newly added CSRCs in the curr_list are sent along
        with the positions of the CSRCs in the ref_list, before which
        the new CSRCs will be inserted.

      - In the removal only scheme, the positions of the CSRCs, which
        are in the ref_list, but not in the curr_list, are sent.

      - In the generic scheme, for a given CSRC in the curr_list, it is
        sent compressed only with a position field if the CSRC is also
        in the ref_list, or it is sent uncompressed.

   All the aforementioned 3 schemes generate compressed format of the
   CSRC list. The CSRC list can also be sent in an uncompressed format.

5.8.1.3. Format of compressed CSRC list

   The format of the compressed CSRC list using the four schemes is as
   follows.

5.8.1.3.1 Insertion Only Scheme

5.8.1.3.1.1 R-mode

        2      6 or 14       8 or 16
   +---------+--------+--------------------+
   | ET = 00 | ref_id | insertion bit mask |
   +---------+--------+--------------------+

   +--------+----+--------+
   | CSRC 1 |....| CSRC m |
   +--------+----+--------+

   * ref_id - the LSB of the RTP sequence number of the ref_list




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   6 bit: "0" + 5-bit LSB of RTP sequence number
   14 bit: "1" + 13-bit LSB of RTP sequence number

   * insertion bit mask: the bit mask indicating the position of the
     following CSRCs to be inserted into the ref_list in order to
     reconstruct the curr_list.

     The length of insertion bit mask can be 8 bits or 16 bits.

     8 bits:  "0" + 7-bit insertion bit mask
     16 bits: "1" + 15-bit insertion bit mask

     To construct the insertion bit mask and the following inserted
     CSRC list, the following steps are taken.

     ** A list of '0' and an empty inserted CSRC list are generated as
       the starting point. The number of '0's in the '0' list equals the
       number of CSRCs in the ref_list. The i-th '0' in the '0' list
       corresponds to the i-th CSRC in the ref_list.

     ** Comparing the curr_list with the ref_list, if a new CSRC is
       inserted between the i-th item and the (i+1)-th item in the
       ref_list, a '1' is inserted between the i-th '0' and (i+1)-th '0'
       in the original '0' list. The new CSRC should be added to the end
       of inserted CSRC list. This procedure is repeated until all the m
       new CSRCs have been processed. If the length of new insertion bit
       mask is less than 7 bits or 15 bits, additional '0' should be
       added at the end until it reaches 7 bits or 15 bits.

     When the decompressor receives the insertion bit mask, it scans
     from left to right. When a '0' is observed, the decompressor copies
     the corresponding CSRC in the ref_list into the curr_list; when a
     '1' is observed, the decompressor adds the correspondent CSRC into
     the curr_list.

   * CSRC i (i=1..m): the CSRCs to be inserted; assuming that the
   number of CSRC to be inserted is m.

5.8.1.3.1.2 UO-modes

        2       6       8         8          8 or 16
   +---------+------+--------+--------+--------------------+
   | ET = 00 | resv | Gen_id | ref_id | insertion bit mask |
   +---------+------+--------+--------+--------------------+

   +--------+----+--------+
   | CSRC 1 |....| CSRC m |
   +--------+----+--------+

   * resv: reserved




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   * Gen_id: is used to identify a set of packets that belongs to the
   same generation

   * ref_id: the Gen_id carried in the ref_list

   * insertion bit mask and CSRC i : the same as defined
   for R-mode

5.8.1.3.2 Removal Only Scheme

5.8.1.3.2.1 R-mode

        2      6 or 14       8 or 16
   +---------+--------+------------------+
   | ET = 01 | ref_id | removal bit mask |
   +---------+--------+------------------+

   * ref_id: the same as defined in section 3.1.1.

   * removal bit mask: the bit mask indicating the CSRC in the
   ref_list to be removed in order to reconstruct the curr_list.
   A '1' in the i-th bit in the removal bit mask means
   that the i-th CSRC in the ref_list is not in the curr_list,
   while a '0' means it is still present in the curr_list.

   The length of insertion bit mask can be 8 bits or 16 bits.

   8 bits:  "0" + 7-bit insertion bit mask
   16 bits: "1" + 15-bit insertion bit mask

   Which format to be used depends on the number of CSRCs
   in the ref_list. If it is less than 8, then 8-bit removal
   bit mask can be used; otherwise the 16-bit format is required.

5.8.1.3.2.2 UO-mode

        2        6      8         8          8 or 16
   +---------+------+--------+--------+------------------+
   | ET = 01 | resv | gen_id | ref_id | removal bit mask |
   +---------+------+--------+--------+------------------+

   * Gen_id, resv and ref_id: the same as defined in section 3.1.2.

   * removal bit mask: the same as defined for R mode










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5.8.1.3.3 Generic Scheme

5.8.1.3.3.1 R-mode

        2      6 or 14       4
   +---------+--------+------------+--------------------------+
   | ET = 10 | ref_id | gcount = m | g_CSRC 1 |....| g_CSRC m |
   +---------+--------+------------+--------------------------+

   +:::::::::+
   | padding |
   +:::::::::+

   * ref_id: the same as defined in section 3.1.1.

   * gcount: the number of following g_CSRC field; the length
   is 4 bits.

   * g_CSRC i (i=1..m): corresponds to the i-th CSRC in the
   curr_list. The order of g_CSRC represents the order of
   CSRCs in the curr_list. Two types of g_CSRC is defined.

   ** g_CSRC type 0 is used to compress a CSRC that is in both
   ref_list and curr_list.

   ** g_CSRC type 1 is used to carry a CSRC that is in the
   curr_list but not in the ref_list.

   The format of g_CSRC is defined as follows.

                       1    ceiling(log2(k))
                  +--------+-----+
   g_CSRC type 0: | GC = 0 | pos |
                  +--------+-----+

   - pos: the position of a CSRC in the ref_list; the length
    is ceiling(log2(k)) where k is the number of CSRC in the
    ref_list. Since the value of k is known to both the
    compressor and decompressor, the length of the pos field
    doesn't need to be carried in the compressed list

                       1
                  +--------+------+
   g_CSRC type 1: | GC = 1 | CSRC |
                  +--------+------+

   * padding: padding bits may be required to maintain the byte
   alignment





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5.8.1.3.3.2 UO-mode

       8          2        8         4
   +--------+---------+--------+------------+--------------------------+
   | gen_id | ET = 10 | ref_id | gcount = m | g_CSRC 1 |....| g_CSRC m |
   +--------+---------+--------+------------+--------------------------+

   +:::::::::+
   | padding |
   +:::::::::+

   * Gen_id and ref_id: the same as defined in section 3.2.1.2.

   * gcount, g_CSRC and padding: the same as defined for R mode

   3.4 Uncompressed (common to R-mode and UO-modes)

        2        1    1      8          4
   +---------+------+---+::::::::+------------+----------------------+
   | ET = 11 | resv | G | gen_id | ccount = m | CSRC 1 |....| CSRC m |
   +---------+------+---+::::::::+------------+----------------------+

   * resv: reserved

   * G: indicating the presence of gen_id field

   * Gen_id: the same as defined in section 3.1.2.

   - In R-mode, gen_id is not present.

   - During the transition from any mode to U-mode or O-mode, gen_id
   should not be sent. The list without gen_id should not be used
   as a reference to compress and decompress a CSRC list.

   - In UO-modes, gen_id is present if it may be used as a reference
   to compress or decompress the subsequent CSRC list.

   * ccount: the number of CSRCs in the CSRC list

   * CSRC i (i=1..m): the original CSRC list in the curr_list

5.8.2.  Header Compression for IPv6 Extension Headers

   The IPv6 extension headers are encoded as a list of items. Each item
   is one of the extension headers. The length of each extension header
   may vary from each other. When more than one extension header is used
   in the same packet, the order of these extension headers is
   recommended in RFC 2460, but not mandatory. Thus, although it is
   unlikely to happen, the order of the extension headers may vary
   during the same session. In addition, one or more extension header



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   may be added or removed during the session and the content of each
   extension header may change. Therefore, the IPv6 extension headers
   are classified as a list of items and the item list compression
   mechanism can be applied.

   The compression of IPv6 extension headers at the list level is
   similar to that of the CSRC entries (section 5.8.1). The compressed
   value of the extension header list is referred to as a compressed
   extension header list. The compression of IPv6 extension headers at
   the item level, i.e., the compression scheme used for each type of
   extension header, is defined in this subsection. The reference
   extension header used to compress a given extension header is the
   extension header in the reference list that has the same type. The
   compressed value of an extension header is referred to as a
   compressed extension header.

5.8.2.1. Terminology

      * extension header list: the list of IPv6 extension headers
      * compressed extension header: the compressed value of an IPv6
        extension header
      * compressed extension header list: the list of compressed
        extension headers
      * reference extension header list: the extension header list
        which is used as the reference to compress an extension header
        list
      * reference extension header: an extension header in the
        reference list that has the same type and is used as the
        reference to compress a given extension header

5.8.2.2. Transformation Classification and Encoding Schemes

5.8.2.2.1 Transformation Classification

   A given extension header list (curr_list) can be
   classified as belonging to one of the following transformation
   cases when compared with a reference extension header list
   (ref_list).

      - Transformation Case A: curr_list can be derived from ref_list
        just by adding (inserting) some extension headers; the relative
        positions of the extension headers common to curr_list and
        ref_list are the same.

      - Transformation Case B: curr_list can be derived from ref_list
        just by deleting some extension headers; the relative positions
        of the extension headers common to curr_list and ref_list are
        the same






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      - Transformation Case C: curr_list can be derived from ref_list
        just by modifying the content of some extension headers; the
        relative positions of the extension headers remain the same.

      - Transformation Case D: All the other transformation cases that
        are not covered by transformation case A, B and C.

5.8.2.2.2 Encoding Schemes

   To address aforementioned 4 transformation cases, four encoding
   schemes are used. Each scheme addresses one or more
   transformation cases mentioned above. The four encoding schemes
   and the transformation cases they address is listed in
   the following table.

   +---------------------+----------+--------------------+
   | Encoding Scheme     | ET Value | Transformaton Case |
   +---------------------+----------+--------------------+
   | Insertion Only      |    00    |         A          |
   +---------------------+----------+--------------------+
   | Removal Only        |    01    |         B          |
   +---------------------+----------+--------------------+
   | Content Change Only |    10    |         C          |
   +---------------------+----------+--------------------+
   | Uncompressed        |    11    | mixture of A,B,C,D |
   +---------------------+----------+--------------------+

      * In the insertion only scheme, compared with ref_list, the
        uncompressed value of the newly added extension headers in the
        curr_list are sent along with the positions of the extension
        headers in the ref_list, before which the new extension headers
        will be inserted.

      * In the removal only scheme, the positions of the extension
        headers, which are in the ref_list, but not in the curr_list,
        are sent.

      * In the change only scheme, the positions of the extension
        header whose content is changed as well as its uncompressed or
        compressed value are sent. (Note that in the current stage,
        only uncompressed extension header is used. )

   The aforementioned 3 schemes generate compressed format of
   the extension header list. The curr_list can also be sent
   in an uncompressed format.

5.8.2.2.3 Special Handling

5.8.2.2.3.1 Special Handling of AH





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   The sequence number field in the AH contains a monotonically
   increasing counter value for a security association. Therefore, when
   comparing curr_list with ref_list, if the sequence number in AH
   changes and SPI field doesn't change, the AH is not necessary to be
   classified as changed.

   If the sequence number in the AH linearly increases as RTP sequence
   number increases, it doesn't need to be sent. The decompressor
   applies linear extrapolation to reconstruct the sequence number in
   AH.  Otherwise, a compressed sequence number should be included in
   the IPv6 Extension Headers compression element field in PT2 extension
   "11" header.

   The authentication data field in AH changes from packet to packet and
   should be sent in every packet. If the uncompressed AH is sent, the
   authentication data field is sent inside the uncompressed AH;
   otherwise,
   it is sent after the compressed IP/UDP/RTP and IPv6 extension headers
   and before the payload.

5.8.2.2.3.2 Encapsulating Security Payload Header

   If Encapsulating Security Payload Header (ESP) is used, the UDP and
   RTP headers are both encrypted and cannot be compressed. In this
   case, special compressed packet format needs to be defined in ROHC.

   In ESP, the only fields that can be compressed are the SPI and the
   sequence number.

     *  In the case that the SPI field changes, the uncompressed ESP is
        sent.

     *  In the case that no change happens to the SPI field, the ESP is
        not considered as changed.

   The sequence number in ESP has the same behavior as the same field
   in AH. If it linearly increases, it doesn't need to be sent.
   Otherwise, a compressed sequence number should be sent in the IPv6
   Extension Headers compression element field in PT2 extension "11"
   header.

5.8.2.2.3.3 Special Handling of Next Header Field

   The next header field in an extension header changes whenever the
   type of the immediately following header changes, e.g., a new
   extension header is inserted after it, the immediate subsequent
   extension header is removed from the list, or the order of several
   extension headers is changed. Thus, in particular, it may not be
   uncommon that for a given extension header, only the next header
   field changes but the remaining fields don't change. Therefore,
   the next header field in each extension header needs to be treated



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   in a special way.

   In the case that only the next header field changes, the extension
   header should be considered as unchanged. The special treatment of
   the change of the next header field is defined as follows.

   * In the case that a subsequent extension header is removed from the
   list, the new value of the next header field can be obtained from the
   reference extension header list. For example, assume that the
   reference extension header list (ref_list) consists of extension
   header A, B and C (ref_ext_hdr A, B, C), and the current extension
   header list (curr_list) only consists of extension headers A and C
   (curr_ext_hdr A, C). The order and value of the next header field
   of these extension headers are as follows.

   ref_list:
   +--------+-----+    +--------+-----+    +--------+-----+
   | type B |     |    | type C |     |    | type D |     |
   +--------+     |    +--------+     |    +--------+     |
   |              |    |              |    |              |
   +--------------+    +--------------+    +--------------+
   ref_ext_hdr A        ref_ext_hdr B       ref_ext_hdr C

   curr_list:
   +--------+-----+    +--------+-----+
   | type C |     |    | type D |     |
   +--------+     |    +--------+     |
   |              |    |              |
   +--------------+    +--------------+
   curr_ext_hdr A      curr_ext_hdr C

   Comparing the curr_ext_hdr A in curr_list and the ref_ext_hdr A
   in ref_list, the value of next header field is changed from
   "type B" to "type C" because of removal of extension header B.
   The new value of the next header field in curr_ext_hdr A, i.e.,
   "type C" doesn't need to be sent to the decompressor, because
   when the decompressor detects (by observing the list level encoding)
   that the immediate following extension header B is removed from
   the list, it retrieves the next header field in ref_ext_hdr B and
   use it to replace the next header field in the curr_ext_hdr A.

   * In the case that a new extension header is inserted after an
   existing extension header, the next header field in the new extension
   header carries the type of itself, instead of the type of extension
   header that follows. For example, assume that the reference extension
   header list (ref_list) consists of extension header A and C
   (ref_ext_hdr A, C), and the current extension header list (curr_list)
   consists of extension header A, B and C (curr_ext_hdr A, B, C). The
   order and the value of the next header field of these extension
   headers are as follows.




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   ref_list:
   +--------+-----+    +--------+-----+
   | type C |     |    | type D |     |
   +--------+     |    +--------+     |
   |              |    |              |
   +--------------+    +--------------+
   ref_ext_hdr A        ref_ext_hdr C

   curr_list:
   +--------+-----+    +--------+-----+    +--------+-----+
   | type B |     |    | type C |     |    | type D |     |
   +--------+     |    +--------+     |    +--------+     |
   |              |    |              |    |              |
   +--------------+    +--------------+    +--------------+
   curr_ext_hdr A      curr_ext_hdr B      curr_ext_hdr C

   Comparing the curr_list and the ref_list, the value of the next
   header field in extension header A is changed from "type C" to "type
   B".

   In the compressed extension header list, the uncompressed
   curr_ext_hdr B is carried in the uncompressed data field in c_item
   or u_item depending on the list encoding scheme used.  However,
   instead of carrying the type of the next header (type C)
   in the next header field, the type of curr_ext_hdr B (type B) should
   be carried. When the decompressor detects (by observing the list
   level encoding) that a new extension is inserted after curr_ext_hdr
   A, it will replace the old next header field in ref_ext_hdr A with
   the type of the inserted extension header, i.e., type B, which is
   carried in the next header field in the c_item or u_item for
   extension header B. At the same time, the decompressor also replace
   the next header field in curr_ext_hdr B with the old value of the
   next header field in ref_ext_hdr A, i.e., type C.

5.8.2.3. Packet Format

5.8.2.3.1. Format in extension "11" header

       1      1      1    5
   +------+------+------+-------+
   | ASeq | ESeq | CExt |  resv |
   +------+------+------+-------+

   8 or 32 bits                8 or 32 bits
   +::::::::::::::::::::::::::+:::::::::::::::::::::::::::+
   | compressed AH Seq Number | compressed ESP Seq Number |
   +::::::::::::::::::::::::::+:::::::::::::::::::::::::::+

   variable length
   +:::::::::::::::::::::::::::::::+
   | compressed IPv6 extension hdr |



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   +:::::::::::::::::::::::::::::::+

   * ASeq: presence of compressed AH Seq Number
   * ESeq: presence of compressed ESP Seq Number
   * CExt: presence of compressed IPv6 extension hdr

   * compressed AH Seq Number and compressed ESP Seq Number formats:

   "0"  + 7-bit LSB
   "11" + 30-bit LSB

5.8.2.3.2 Format of compressed IPv6 extension header

5.8.2.3.2.1 Insertion Only Scheme

5.8.2.3.2.1.1 R-mode

        2     6 or 14           8
   +---------+--------+--------------------+
   | ET = 00 | ref_id | insertion bit mask |
   +---------+--------+--------------------+

   +----------+----+----------+
   | u_ehdr 1 |....| u_ehdr m |
   +----------+----+----------+

   * ref_id - the LSB of the RTP sequence number of the ref_list

   6 bit: "0" + 5-bit LSB
   14 bit: "1" + 13-bit LSB

   * u_ehdr i (i=1..m) : the uncompressed value of the new
   extension header in the curr_list; m is the number of
   new extension headers to be added.

   * insertion bit mask: the bit mask indicating the position
   of the following u_ehdrs to be inserted into the ref_list in
   order to reconstruct the curr_list.

   The length of insertion bit mask is 8 bits.

   To construct the insertion bit mask and the following inserted
   u_ehdr list, the following steps are taken.
   ** A list of '0' and an empty inserted u_ehdr list are generated
   as the starting point. The number of '0's in the '0' list equals
   the number of extension headers in the ref_list. The i-th
   '0' in the '0' list corresponds to the i-th extension header
   in the ref_list.
   ** Comparing the curr_list with the ref_list, if a new extension
   header is inserted between the i-th item and the (i+1)-th item
   in the ref_list, a '1' is inserted between the i-th '0' and



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   (i+1)-th '0' in the original '0' list. The new extension header
   should be added to the end of inserted CSRC list. This procedure
   is repeated until all the m new extension headers have been
   processed. If the length of new insertion bit mask is less
   than 7 bits, additional '0' should be added at the
   end until it reaches 7 bits.

   When the decompressor receives the insertion bit mask, it scans
   from left to right. When a '0' is observed, the decompressor
   copies the corresponding extension header in the ref_list into
   the curr_list; when a '1' is observed, the decompressor adds the
   correspondent u_ehdr into the curr_list.


   3.2.1.2 UO-modes

        2       6        8       8              8
   +---------+------+--------+--------+--------------------+
   | ET = 00 | resv | Gen_id | ref_id | insertion bit mask |
   +---------+------+--------+--------+--------------------+

   +----------+----+----------+
   | u_ehdr 1 |....| u_ehdr m |
   +----------+----+----------+

   * resv: reserved

   * Gen_id: is used to identify a set of packets that belongs to the
   same
   generation

   * ref_id: the Gen_id carried in the ref_list

   * insertion bit mask and u_ehdr i: the same as defined for R-mode

5.8.2.3.2.2 Removal Only Scheme

5.8.2.3.2.2.1 R-mode

        2      6 or 14         8
   +---------+--------+------------------+
   | ET = 01 | ref_id | removal bit mask |
   +---------+--------+------------------+

   * ref_id: the same as defined in section 3.2.1.1.

   * removal bit mask: the bit mask indicating the extension
   header in the ref_list to be removed in order to reconstruct
   the curr_list. A '1' in the i-th bit in the removal bit mask means
   that the i-th extension header in the ref_list is not in the
   curr_list, while a '0' means it is still present in the curr_list.



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   The length of insertion bit mask can be 8 bits.

   3.2.2.2 UO-mode

        2        6       8        8            8
   +---------+------+--------+--------+------------------+
   | ET = 01 | resv | gen_id | ref_id | removal bit mask |
   +---------+------+--------+--------+------------------+

   * Gen_id, resv and ref_id: the same as defined in section 3.2.1.2.

   * removal bit mask: the same as defined for R mode

5.8.2.3.2.3 Content Change Only Scheme

5.8.2.3.2.3.1 R-mode

        2     6 or 14           8
   +---------+--------+-----------------+
   | ET = 10 | ref_id | change bit mask |
   +---------+--------+-----------------+

   +-----------+----+-----------+
   | uc_ehdr 1 |....| uc_ehdr m |
   +-----------+----+-----------+

   * ref_id: the same as defined in section 2.3.1.1

   * change bit mask: a bit mask indicating the postion of the
   extension header that is changed. A '1' in the i-th bit in the
   change bit mask means that the i-th extension header in the
   ref_list is not the same as the i-th extension in the curr_list,
   while a '0' means they are the same.

   * uc_ehdr: corresponds to the extension header whose content
   is changed when comparing it with the extension header at
   the same position in the ref_list. Their positions in
   curr_list is indicated in the change bit mask field.

   uc_ehdr:
       1
   +-------+-------------------+
   | C = 0 | uncompressed ehdr |
   +-------+-------------------+

   +-------+-----------------+
   | C = 1 | compressed ehdr |
   +-------+-----------------+

   - uncompressed ehdr: uncompressed extension header



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   - compressed ehdr: compressed extension header using the
   extension header at the same position in ref_list as the
   reference. The compression mechanisms for different types
   of extension headers are different from each other and FFS.

5.8.2.3.2.3.2 UO-mode

        2        6       8        8           8
   +---------+------+--------+--------+-----------------+
   | ET = 00 | resv | gen_id | ref_id | change bit mask |
   +---------+------+--------+--------+-----------------+

   +-----------+----+-----------+
   | uc_ehdr 1 |....| uc_ehdr m |
   +-----------+----+-----------+

   * Gen_id, resv, ref_id: the same as defined in section 3.2.1.2.

   * change bit mask and uc_ehdr i: the same as defined for R mode

5.8.2.3.2.4 Uncompressed Scheme (common to R-mode and UO-modes)

        2        5    1     8
   +---------+------+---+::::::::+------------------------------------+
   | ET = 11 | resv | G | Gen_id | uncompressed IPv6 Extension Header |
   +---------+------+---+::::::::+------------------------------------+

   * resv: reserved

   * G: indicating the presence of Gen_id

   * Gen_id: the same as defined in section 3.2.1.2.

   - In R-mode, gen_id is not present.

   - During the transition from any mode to U-mode or O-mode, gen_id
   should not be sent. The list without gen_id should not be used
   as a reference to compress and decompress an extension header
   list.

   - In UO-modes, gen_id is present if it may be used as a reference
   to compress or decompress the subsequent extension header list.












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5.9.  Header compression CRCs, coverage and polynomials

   This chapter describes how to calculate the CRCs used in packet
   headers defined in this document.


5.9.1.  IR & IR-DYN packet CRCs

   The CRC in the IR and IR-DYN packet is calculated over the entire IR
   or IR-DYN packet, excluding Payload and including CID. For purposes
   of computing the CRC, the CRC field in the header is set to zero.

   The initial content of the CRC register be preset to all 1's.

   The CRC polynomial to be used is:

     C(x) = 1 + x + x^2 + x^8


5.9.2.  CRCs in compressed packets

   The CRC in compressed packets is calculated over the entire original
   header, before compression.

   [[Editor's note: In the Pittsburgh WG meeting, we said that the CRC
   computation overhead can be reduced by computing the CRC over the
   static parts of the packet first and then over the dynamic parts.
   This needs to be specified here in detail.]]

   The polynomial to be used for 3 bit CRC is:

     C(x) = 1 + x + x^3

   The polynomial to be used for 7 bit CRC is:

     C(x) = ???[to be defined]


















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6.  Implementation issues

   This document specifies mechanisms for the protocol, while much of
   the usage of these mechanisms is left to the implementers to decide
   upon. This chapter is aimed to give guidelines, ideas and suggestions
   for implementing the scheme.


6.1.  Reverse decompression

   This chapter describes an optional decompressor operation to reduce
   discarded packets due to an invalid context.

   Once a context becomes invalid (e.g., in the case when more
   consecutive packet losses than expected has occurred), subsequent
   compressed packets cannot be decompressed correctly immediately.
   Reverse decompression aims at decompressing such packets later
   instead of discarding them, by storing them until the context has
   been updated and validated and then attempting decompression.

   Let the sequence of stored packets be i, i+1, ..., i+k, where i is
   the first packet and I+k is the packet before the context was
   updated. The decompressor will attempt to recover the stored packets
   in reverse order, i.e., starting with i+k, and working towards i.
   When a stored packet has been reconstructed, its correctness is
   verified using its CRC. Packets not carrying a CRC must not be
   delivered to upper layers. Packets where the CRC succeeds, are
   delivered to upper layers in the original order, i.e., i, ..., i+k.

   Note that this reverse decompression introduces buffering while
   waiting for the context to be validated and thereby introduces
   additional delay. Thus, it should be used only when some amount of
   delay is acceptable. For example, for video packets belonging to the
   same video frame, the delay of packet arrival time does not cause
   presentation time delay. Delay-insensitive streaming applications can
   also be tolerant to such delay. If the decompressor cannot determine
   if the application can tolerate delay, it should not do reverse
   decompression.

   The following illustrates the decompression procedure in some detail:

   1. The decompressor stores compressed packets that cannot be
      decompressed correctly due to an invalid context.

   2. When the decompressor has received a context updating packet and
      the context has been validated, it starts to recover the stored
      packets in reverse order. Decompression is carried out followed
      by the last decompressed packet to its previous packet as if the
      two packets were reordered. After that, the decompressor checks
      the correctness of the reconstructed header using the CRC.



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   3. If the CRC indicates successful decompression, the decompressor
      stores the complete packet and attempts to decompress the
      preceeding packet. In this way, the stored packets are recovered
      until no compressed packets are left. For each packet, the
      decompressor checks the correctness of the decompressed headers
      using header compression CRC.

   4. If the CRC indicates an incorrectly decompressed packet, the
      reverse decompression attempt must be terminated and all
      remaining uncompressed packets must be discarded.

   5. Finally, the decompressor forwards all the correctly decompressed
      packets to upper layers in the original order.

6.2.  RTCP

   RTCP is the RTP Control Protocol, [RTP]. RTCP is based on periodic
   transmission of control packets to all participants in a session,
   using the same distribution mechanism as for data packets. Its
   primary function is to provide feedback from the data receivers on
   the quality of the data distribution. The feedback information may be
   used for issues related to congestion control functions, and directly
   useful for control of adaptive encodings.

   In an RTP session there will be two types of packet streams; one with
   the RTP-header and application data, and a second stream with the
   RTCP control information. The difference between the streams at the
   transport level is the UDP port numbers, which is plus one for RTCP.
   The ROHC header compressor implementation has several ways at hand to
   handle the RTCP stream.

     1. One compressor/decompressor entity for both streams and carried
        on the same channel using CIDs to distinguish between them. On
        the RTCP stream, basically only IP/UDP compression will be
        utilized.
     2. Two compressor/decompressor entities, one for RTP and another
        one for RTCP, and the streams carried on their own channel. This
        means that they will not share the same CID number space.















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7.  Further work

   (Editor: This section is``further work'' in particular as it needs to
   be integrated into the rest of the document.)

7.3.  Tunneling

7.3.1.  Header Compression for IPv4 Tunneling Header

      In order to route the packets to the mobile node that is on a
   foreign link, the home agent of the mobile node may encapsulate the
   original packet into an IP header and tunnel the packet to the care-
   of address of the mobile node. In the case of foreign agent care-of
   address in Mobile IPv4, the tunneling header in each tunneled packet
   will be removed by the foreign agent before transferring it to the
   mobile node through the air interface; therefore there is no need for
   compression of tunneling header.  In the case that mobile node uses
   collocated care-of address, the tunneled packet will be sent to
   mobile station through air interface, and compression needs to be
   applied to the tunneling header.

7.3.1.1.  Mobile IPv4 Tunneling Header Fields Type

      The table below summarizes classification of the various fields
   defined in different tunneling headers used in Mobile IPv4. In the
   column of Encapsulation Scheme (Enc. Scheme), three encapsulation
   methods are included - IP in IP Encapsulation (IIE), Minimum
   Encapsulation (ME), Generic Routing Encapsulation (GRE).

   (Editor's note: Harmonize with the way this is described in ROHC
   document)

      +------+------+---------+---------------------------------+
      |Enc.  |Header| Static  |          Non-static             |
      |Scheme|type  |         +-----------+---------------------+
      |      |      |         | Essential |    Non-Essential    |
      +------+------+---------+-----------+---------------------+
      | IIE  |inner |   same as in the table in Appendix A      |
      |      |header|          in ACE Internet Draft            |
      |      +------+-------------------------------------------+
      |      |outer |   same as in the table in Appendix A      |
      |      |header|          in ACE Internet Draft            |
      +------+------+-------------------------------------------+
      | ME   |IP    |   same as in the table in Appendix A      |
      |      |header|          in ACE Internet Draft            |
      |      +------+----------+----------+---------------------+
      |      |Mini. | Protocol |          | S bit               |
      |      |Fw.   |          |          | Header Checksum     |
      |      |header|          |          | Original Dest. Addr.|
      |      |      |          |          | Original Src. Addr. |
      +------+------+----------+----------+---------------------+



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      | GRE  |inner |   same as in the table in Appendix A      |
      |      |header|          in ACE Internet Draft            |
      |      +------+-------------------------------------------+
      |      |outer |   same as in the table in Appendix A      |
      |      |header|          in ACE Internet Draft            |
      |      +------+----------+----------+---------------------+
      |      |GRE   | Protocol | Sequence | C, R, K, S, s bits  |
      |      |header| Ver      | number   | Recur               |
      |      |      |          |          | Flags               |
      |      |      |          |          | Checksum            |
      |      |      |          |          | Offset              |
      |      |      |          |          | Key                 |
      |      |      |          |          | Routing             |
      +------+------+----------+----------+---------------------+

7.3.1.2.  Compression of Tunneling Headers in MIPv4

      Three encapsulation schemes have been specified in MIPv4. For
   different encapsulation scheme, the compression methods are different
   from each other.

7.3.1.2.1.  IP in IP Encapsulation in IPv4

      Using IP in IP Encapsulation, the original inner IP header is not
   modified at all and therefore can be compressed as if it is not
   encapsulated. The outer header is compressed at the IP level, while
   the inner header is compressed as defined in ROHC.

7.3.1.2.2.  Minimum Encapsulation in IPv4

      With Minimum Encapsulation, the original IP header is modified and
   the Minimal Forwarding Header is inserted between the modified IP
   header and the original IP payload. The modified IP header plus the
   the UDP/RTP headers is compressed as defined in ROHC.

      The compression scheme for the Minimal Forwarding Header is
   similar to the scheme applied to the IP header. The static and
   changing non- essential fields in the Minimum Forwarding Header are
   sent in the Full Header and Refresh state. When any change happens to
   any non-essential field in the Minimum Forwarding Header, a
   compressed header with a bit mask indicating the change should be
   sent.

7.3.1.2.3.  Generic Routing Encapsulation in IPv4

      With Generic Routing Encapsulation, the original IP packet is
   encapsulated in an outer IP header. A GRE header is inserted between
   the inner header and the outer header. The original IP/UDP/RTP header
   is compressed as if there is no encapsulation. The outer IP header is
   compressed at the IP level.




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      The compression scheme for the GRE header is similar to the scheme
   applied to the IP header.  All the static and changing non-essential
   fields in the GRE header are sent in the Full Header and refresh
   state. When any change happens to any non-essential field in the GRE
   header, a compressed header with a bit mask indicating the change
   should be sent. If the sequence number in the GRE header is present,
   the scheme to compress sequence number could be VLE, as defined in
   ACE draft.



7.4.  non-RTP UDP traffic

   (Editor's note: This is text from draft-koren-avt-crtp-enhance-01.txt
   to be added to rohc -- not yet consistent with the rest of the
   document)

   [Micke's note: this should be a separate profile.]
   [cabo's note: I'm not even sure we want to keep it in ROHC at all.
   The negative cache flag could simply be added to I/R, of course, e.g.
   as a profile.
   Note that streams with odd port numbers should never be RTP streams,
   so this might be considered to simplify RTCP compression.
   I'm not sure the case about e2e CRTP in tunnels applies to ROHC very
   much.  We must explain REJECT packets better above.]

   2.1 The negative cache stream flag

   Certain streams, known or suspected to not be RTP, can be placed in a
   "negative cache" at the compressor, so only the IP and UDP headers
   are compressed. It is beneficial to notify the decompressor that the
   compressed stream is in the negative cache: for such streams the
   context is shorter - there is no need to include the RTP header, and
   all RTP-related calculations can be avoided.

   In this enhancement, a new flag bit "N" is added to the FULL_HEADER
   packet that initializes a context at the decompressor.  The bit
   occupied by the new flag was previously always set to zero.  If the N
   flag is set to 1, this indicates that no COMPRESSED_RTP packets will
   be transmitted in this context.  This flag is only an optimization
   and the decompressor may choose to ignore it.

   [[Editor: The negative cache flag could be part of the profile
   information]]


   2.2 Reject a new compressed stream

   In a point to point link the two nodes can agree on the number of
   compressed sessions they are prepared to support for this link. In an
   end-to-end scheme a host may have compressed sessions with many hosts



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   and eventually may run out of resources.  When the end-to-end tunnel
   is negotiated, the number of contexts needed may not be predictable.
   This enhancement allows the negotiated number of contexts to be
   larger than could be accommodated if many tunnels are established.
   Then, as context resources are consumed, an attempt to set up a new
   context may be rejected.
   The compressor initiates a compression of a stream by sending a
   FULL_HEADER packet. Currently if the decompressor has insufficient
   resources to decompress the new stream, it can send a CONTEXT_STATE
   packet to invalidate the newly compressed stream. The compressor does
   not know the reason for the invalidation: usually this happens when
   the decompressor gets out of synchronization due to packet loss. The
   compressor will most likely reattempt to compress this stream by
   sending another FULL_HEADER.
   This enhancement specifies that the decompressor may reject the
   compression of a stream by sending a REJECT message to the
   compressor. A REJECT message tells the compressor to stop compressing
   this stream.
   The REJECT message is a [[feedback-4 of type REJECT]] with an
   additional flag:

      Type code = 1 :CONTEXT_STATE for 8-bit CID streams
      Type code = 2 :  CONTEXT_STATE for16-bit CID streams

   [[Editor: This will be handled by the REJECT feedback packet]]

   The compressor may decide to wait for a while before attempting to
   compress additional streams destined to the rejecting host.


























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8.  Section 8 has been removed.


9.  Security considerations

   Because encryption eliminates the redundancy that header compression
   schemes try to exploit, there is some inducement to forego encryption
   of headers in order to enable operation over low-bandwidth links.
   However, for those cases where encryption of data (and not headers)
   is sufficient, RTP does specify an alternative encryption method in
   which only the RTP payload is encrypted and the headers are left in
   the clear. That would still allow header compression to be applied.

   ROHC compression is transparent with regards to the RTP sequence
   number and RTP timestamp fields, so the values of those fields can be
   trusted by payload encryption schemes.

   A malfunctioning or malicious header compressor could cause the
   header decompressor to reconstitute packets that do not match the
   original packets but still have valid IP, UDP and RTP headers and
   possibly also valid UDP checksums. Such corruption may be detected
   with end-to-end authentication and integrity mechanisms which will
   not be affected by the compression. Moreover, this header compression
   scheme uses an internal checksum for verification of re-constructed
   headers. This reduces the probability of producing decompressed
   headers not matching the original ones without this being noticed.

   Denial-of-service attacks are possible if an intruder can introduce
   (for example) bogus STATIC, DYNAMIC or FEEDBACK packets onto the link
   and thereby cause compression efficiency to be reduced. However, an
   intruder having the ability to inject arbitrary packets at the link
   layer in this manner raises additional security issues that dwarf
   those related to the use of header compression.


10.  Acknowledgements

   When designing this protocol, earlier header compression ideas
   described in [CJHC], [IPHC] and [CRTP] have been important sources of
   knowledge.

   Thanks to Takeshi Yoshimura at NTT DoCoMo for providing the reverse
   decompression section (6.1). Thanks also to Anton Martensson for many
   valuable draft contributions and to Andreas Jonsson (Lulea
   University), who made a great job supporting this work in his study
   of header field change patterns. Thanks also to all others who have
   given comments.







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11.  Intellectual property considerations

   (Editor's note: this section will go to www.ietf.org/ipr and be
   replaced by the standard reference to that, but for now it is left in
   the draft to simplify working on it.)

   This proposal in is conformity with RFC 2026.

   Telefonaktiebolaget LM Ericsson and its subsidiaries, in accordance
   with corporate policy, will for submissions rightfully made by its
   employees which are adopted or recommended as a standard by the IETF
   offer patent licensing as follows:

   If part(s) of a submission by Ericsson employees is (are) included in
   a standard and Ericsson has patents and/or patent application(s) that
   are essential to implementation of such included part(s) in said
   standard, Ericsson is prepared to grant - on the basis of reciprocity
   (grant-back) - a license on such included part(s) on reasonable, non-
   discriminatory terms and conditions.

   For the avoidance of doubt this general patent licensing undertaking
   applies to this proposal.


   Nokia has filed patent applications that might possibly have
   technical relation to this contribution.


   Matsushita has filed patent applications that might possibly have
   technical relation to this contribution.
   If part(s) of the contribution by Matsushita employee is (are)
   included in a standard and Matsushita has patents and/or patent
   application(s) that are essential to implementation of such included
   part(s) in said standard, Matsushita is prepared to grant - on the
   basis of reciprocity (grantback) - a license on such included part(s)
   on reasonable, non-discriminatory terms and conditions (in according
   with paragraph 10.3.3 of the RFC 2026).


   NTT DoCoMo, Inc. also declares this text may relevant to their
   patent, and offer patent licensing as follows:

   If part(s) of this text provided by NTT DoCoMo employees is (are)
   included in a standard and NTT DoCoMo has patents and/or patent
   application(s) that are essential to implementation of such included
   part(s) in said standard, NTT DoCoMo is prepared to grant - on the
   basis of reciprocity (grant-back) - a license on such included
   part(s) on reasonable, non-discriminatory terms and conditions.





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12.  References

   [UDP]    Jon Postel, "User Datagram Protocol", RFC 768, August 1980.

   [IPv4]   Jon Postel, "Internet Protocol", RFC 791, September 1981.

   [IPv6]   Steven Deering, Robert Hinden, "Internet Protocol, Version 6
            (IPv6) Specification", RFC 2460, December 1998.

   [RTP]    Henning Schulzrinne, Stephen Casner, Ron Frederick, Van
            Jacobson, "RTP: A Transport Protocol for Real-Time
            Applications", RFC 1889, January 1996.

   [HDLC]   William Simpson, "PPP in HDLC-like framing", RFC 1662, 1994.

   [VJHC]   Van Jacobson, "Compressing TCP/IP Headers for Low-Speed
            Serial Links", RFC 1144, February 1990.

   [IPHC]   Mikael Degermark, Bjorn Nordgren, Stephen Pink, "IP Header
            Compression", RFC 2507, February 1999.

   [CRTP]   Steven Casner, Van Jacobson, "Compressing IP/UDP/RTP Headers
            for Low-Speed Serial Links", RFC 2508, February 1999.

   [PPPHC]  Mathias Engan, Steven Casner, Carsten Bormann, "IP Header
            Compression over PPP", RFC 2509, February 1999.

   [CRTPC]  Mikael Degermark, Hans Hannu, Lars-Erik Jonsson, Krister
            Svanbro, "CRTP over cellular radio links", Internet Draft
            (work in progress), December 1999.
            <draft-degermark-crtp-cellular-01.txt>

   [REQ]   Mikael Degermark, "Requirements for robust IP/UDP/RTP header
           compression", Internet Draft (work in progress), June 2000.
           <draft-ietf-rohc-rtp-requirements-01.txt>

   [LLG]   Krister Svanbro, "Lower Layer Guidelines for Robust Header
           Compression", Internet Draft (work in progress), May 2000.
           <draft-ietf-rohc-lower-layer-guidelines-00.txt>

   [CELL]   Lars Westberg, Morgan Lindqvist, "Realtime traffic over
            cellular access networks", Internet Draft
           (work in progress), May 2000.
            <draft-westberg-realtime-cellular-02.txt>

   [WCDMA] "Universal Mobile Telecommunications System (UMTS);
           Selection procedures for the choice of radio transmission
           technologies of the UMTS (UMTS 30.03 version 3.1.0)".
           ETSI TR 101 112 V3.0.1, November 1997.





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13.  Authors' addresses

   Carsten Bormann               Tel: +49 421 218 7024
   Universitaet Bremen TZI       Fax: +49 421 218 7000
   Postfach 330440               EMail: cabo@tzi.org
   D-28334 Bremen, GERMANY

   Mikael Degermark               Tel: +1 520 621-3498
   The University of Arizona      Fax: +1 520 621-4642
   Dept of Computer Science       Email: micke@cs.arizona.edu
   P.O. Box 210077
   Tucson, AZ 85721-0077, USA

   Lars-Erik Jonsson             Tel: +46 920 20 21 07
   Ericsson Erisoft AB           Fax: +46 920 20 20 99
   SE-971 28 Lulea, Sweden       EMail: lars-erik.jonsson@ericsson.com

   Zhigang Liu                    Tel: +1 972 894-5935
   Nokia Research Center          Fax: +1 972 894-4589
   6000 Connection Drive          EMail: zhigang.liu@nokia.com
   Irving, TX 75039, USA
































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Appendix A.  Detailed classification of header fields

   Header compression is possible due to the fact that most header
   fields do not vary randomly from packet to packet. Many of the fields
   exhibit static behavior or changes in a more or less predictable way.
   When designing a header compression scheme, it is of fundamental
   importance to understand the behavior of the fields in detail.

   In this appendix, all IP, UDP and RTP header fields are classified
   and analyzed in two steps. First, we have a general classification in
   A.1 where the fields are classified based on stable knowledge and
   assumptions. The general classification does not take into account
   the change characteristics of changing fields because those will vary
   more or less depending on the implementation and on the application
   used. A less stable but more detailed analysis considering the change
   characteristics is then done in A.2. Finally, A.3 summarizes this
   appendix with conclusions about how the various header fields should
   be handled by the header compression scheme to optimize compression
   and functionality.

A.1.  General classification

   On a general level, the header fields are separated into 5 classes:

   INFERRED       These fields contain values that can be inferred from
                  other values, for example the size of the frame
                  carrying the packet, and thus does not have to be
                  handled at all by the compression scheme.

   STATIC         These fields are expected to be constant throughout
                  the lifetime of the packet stream. Static information
                  must in some way be communicated once.

   STATIC-DEF     STATIC fields whose values define a packet stream.
                  They are in general handled as STATIC.

   STATIC-KNOWN   These STATIC fields are expected to have well-known
                  values and therefore do not need to be communicated
                  at all.

   CHANGING       These fields are expected to vary in some way, either
                  randomly, within a limited value set or range, or in
                  some other manner.


   In this section, each of the IP, UDP and RTP header fields is
   assigned to one of these classes. For all fields except those
   classified as CHANGING, the motives for the classification are also
   stated. CHANGING fields are in A.2 further examined and classified
   based on their expected change behavior.




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A.1.1.  IPv6 header fields

    +---------------------+-------------+----------------+
    | Field               | Size (bits) |    Class       |
    +---------------------+-------------+----------------+
    | Version             |      4      |  STATIC-KNOWN  |
    | Traffic Class       |      8      |    CHANGING    |
    | Flow Label          |     20      |   STATIC-DEF   |
    | Payload Length      |     16      |    INFERRED    |
    | Next Header         |      8      |  STATIC-KNOWN  |
    | Hop Limit           |      8      |    CHANGING    |
    | Source Address      |    128      |   STATIC-DEF   |
    | Destination Address |    128      |   STATIC-DEF   |
    +---------------------+-------------+----------------+


   Version

     The version field states which IP version the packet is based on.
     Packets with different values in this field must be handled by
     different IP stacks. For header compression, different compression
     profiles must also be used. When compressor and decompressor have
     negotiated which profile to use, the IP version is also known to
     both parties. The field is therefore classified as STATIC-KNOWN.


   Flow Label

     This field may be used to identify packets belonging to a specific
     packet stream. If not used, the value should be set to zero.
     Otherwise, all packets belonging to the same stream must have the
     same value in this field, it being one of the fields defining the
     stream. The field is therefore classified as STATIC-DEF.


   Payload Length

     Information about the packet length (and then also payload length)
     is expected to be provided by the link layer. The field is
     therefore classified as INFERRED.


   Next Header

     This field is expected to have the same value in all packets of a
     packet stream. As for the version number, a certain compression
     profile can only handle a specific next header which means that
     this value is known when profile has been negotiated. The field is
     therefore classified as STATIC-KNOWN.





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   Source and Destination addresses

     These fields are part of the definition of a stream and must thus
     be constant for all packets in the stream. The fields are therefore
     classified as STATIC-DEF.


   Summarizing the bits corresponding to the classes gives:

    +--------------+--------------+
    | Class        | Size (octets)|
    +--------------+--------------+
    | INFERRED     |       2      |
    | STATIC-DEF   |     34.5     |
    | STATIC-KNOWN |      1.5     |
    | CHANGING     |       2      |
    +--------------+--------------+


A.1.2.  IPv4 header fields

    +---------------------+-------------+----------------+
    | Field               | Size (bits) |     Class      |
    +---------------------+-------------+----------------+
    | Version             |      4      |  STATIC-KNOWN  |
    | Header Length       |      4      |  STATIC-KNOWN  |
    | Type Of Service     |      8      |    CHANGING    |
    | Packet Length       |     16      |    INFERRED    |
    | Identification      |     16      |    CHANGING    |
    | Reserved flag       |      1      |  STATIC-KNOWN  |
    | May Fragment flag   |      1      |     STATIC     |
    | Last Fragment flag  |      1      |  STATIC-KNOWN  |
    | Fragment Offset     |     13      |  STATIC-KNOWN  |
    | Time To Live        |      8      |    CHANGING    |
    | Protocol            |      8      |  STATIC-KNOWN  |
    | Header Checksum     |     16      |    INFERRED    |
    | Source Address      |     32      |   STATIC-DEF   |
    | Destination Address |     32      |   STATIC-DEF   |
    +---------------------+-------------+----------------+


   Version

     The version field states which IP version the packet is based on
     and packets with different values in this field must be handled by
     different IP stacks. For header compression, different compression
     profiles must also be used. When compressor and decompressor has
     negotiated which profile to use, the IP version is also well known
     to both parties. The field is therefore classified as STATIC-KNOWN.





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   Header Length

     As long as there are no options present in the IP header, the
     header length is constant and well known. If there are options, the
     fields would be STATIC, but we assume no options. The field is
     therefore classified as STATIC-KNOWN.


   Packet Length

     Information about the packet length is expected to be provided by
     the link layer. The field is therefore classified as INFERRED.


   Flags

     The Reserved flag must be set to zero and is therefore classified
     as STATIC-KNOWN. The May Fragment flag will be constant for all
     packets in a stream and is therefore classified as STATIC. Finally,
     the Last Fragment bit is expected to be zero because fragmentation
     is NOT expected, due to the small packet size expected. The Last
     Fragment bit is therefore classified as STATIC-KNOWN.


   Fragment Offset

     With the assumption that no fragmentation occurs, the fragment
     offset is always zero. The field is therefore classified as STATIC-
     KNOWN.


   Protocol

     This field is expected to have the same value in all packets of a
     packet stream. As for the version number, a certain compression
     profile can only handle a specific next header which means that
     this value is well known when profile has been negotiated. The
     field is therefore classified as STATIC-KNOWN.


   Header Checksum

     The header checksum protects individual hops from processing a
     corrupted header. When almost all IP header information is
     compressed away, there is no need to have this additional checksum;
     instead it can be regenerate at the decompressor side. The field is
     therefore classified as INFERRED.


   Source and Destination addresses




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     These fields are part of the definition of a stream and must thus
     be constant for all packets in the stream. The fields are therefore
     classified as STATIC-DEF.


   Summarizing the bits corresponding to the classes gives:

    +--------------+--------------+
    | Class        | Size (octets)|
    +--------------+--------------+
    | INFERRED     |      4       |
    | STATIC       |    1 bit     |
    | STATIC-DEF   |      8       |
    | STATIC-KNOWN |   3 +7 bits  |
    | CHANGING     |      4       |
    +--------------+--------------+


A.1.3.  UDP header fields

    +------------------+-------------+-------------+
    | Field            | Size (bits) |    Class    |
    +------------------+-------------+-------------+
    | Source Port      |     16      | STATIC-DEF  |
    | Destination Port |     16      | STATIC-DEF  |
    | Length           |     16      |  INFERRED   |
    | Checksum         |     16      |  CHANGING   |
    +------------------+-------------+-------------+


   Source and Destination ports

     These fields are part of the definition of a stream and must thus
     be constant for all packets in the stream. The fields are therefore
     classified as STATIC-DEF.


   Length

     This field is redundant and is therefore classified as INFERRED.


   Summarizing the bits corresponding to the classes gives:

    +------------+--------------+
    | Class      | Size (octets)|
    +------------+--------------+
    | INFERRED   |       2      |
    | STATIC-DEF |       4      |
    | CHANGING   |       2      |
    +------------+--------------+



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A.1.4.  RTP header fields

    +-----------------+-------------+----------------+
    | Field           | Size (bits) |     Class      |
    +-----------------+-------------+----------------+
    | Version         |      2      |  STATIC-KNOWN  |
    | Padding         |      1      |     STATIC     |
    | Extension       |      1      |     STATIC     |
    | CSRC Counter    |      4      |    CHANGING    |
    | Marker          |      1      |    CHANGING    |
    | Payload Type    |      7      |    CHANGING    |
    | Sequence Number |     16      |    CHANGING    |
    | Timestamp       |     32      |    CHANGING    |
    | SSRC            |     32      |   STATIC-DEF   |
    | CSRC            |   0(-480)   |    CHANGING    |
    +-----------------+-------------+----------------+


   Version

     There exists only one working RTP version and that is version 2.
     The field is therefore classified as STATIC-KNOWN.


   Padding

     The use of this field depends on the application, but when payload
     padding is used it is likely to be present in all packets. The
     field is therefore classified as STATIC.


   Extension

     If RTP extensions is used by the application, it is likely to be an
     extension present in all packets (but use of extensions is very
     uncommon). However, for safety's sake this field is classified as
     STATIC and not STATIC-KNOWN.


   SSRC

     This field is part of the definition of a stream and must thus be
     constant for all packets in the stream. The field is therefore
     classified as STATIC-DEF.








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   Summarizing the bits corresponding to the classes gives:

    +--------------+--------------+
    | Class        | Size (octets)|
    +--------------+--------------+
    | STATIC       |    2 bits    |
    | STATIC-DEF   |      4       |
    | STATIC-KNOWN |    2 bits    |
    | CHANGING     |  7.5(-67.5)  |
    +--------------+--------------+


A.1.5.  Summary for IP/UDP/RTP

   If we summarize this for IP/UDP/RTP we get:

    +----------------+--------------+--------------+
    | Class \ IP ver | IPv6 (octets)| IPv4 (octets)|
    +----------------+--------------+--------------+
    | INFERRED       |       4      |       6      |
    | STATIC         |    2 bits    |    3 bits    |
    | STATIC-DEF     |     42.5     |      16      |
    | STATIC-KNOWN   |   1 +6 bits  |   4 +1 bit   |
    | CHANGING       |  11.5(-71.5) |  13.5(-73.5) |
    +----------------+--------------+--------------+
    | Total          |   60(-120)   |   40(-100)   |
    +----------------+--------------+--------------+


A.2.  Analysis of change patterns of header fields

   To design suitable mechanisms for efficient compression of all header
   fields, their change patterns must be analyzed. For this reason, an
   extended classification is done based on the general classification
   in A.1, considering the fields which were labeled CHANGING in that
   classification. Different applications will use the fields in
   different ways, which may affect their behavior. When this is the
   case, typical behavior for conversational audio and video will be
   discussed.

   The CHANGING fields are separated into five different subclasses:

   STATIC                These are fields that were classified as
                         CHANGING on a general basis, but are classified
                         as STATIC here due to certain additional
                         assumptions.

   SEMISTATIC            These fields are STATIC most of the time.
                         However, occasionally the value changes but



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                         reverts to its original value after a known
                         number of packets.

   RARELY-CHANGING (RC)  These are fields that change their values
                         occasionally and then keep their new values.

   ALTERNATING           These fields alternate between a small number
                         of different values.

   IRREGULAR             These, finally, are the fields for which no
                         useful change pattern can be identified.

   To further expand the classification possibilities without increasing
   complexity, the classification can be done either according to the
   values of the field and/or according to the values of the deltas for
   the field.

   When the classification is done, other details are also stated
   regarding possible additional knowledge about the field values and/or
   field deltas, according to the classification. For fields classified
   as STATIC or SEMISTATIC, the case could be that the value of the
   field is not only STATIC but also well KNOWN a priori (two states for
   SEMISTATIC fields). For fields with non-irregular change behavior, it
   could be known that changes usually are within a LIMITED range
   compared to the maximal change for the field. For other fields, the
   values are completely UNKNOWN.

   Table A.1 classifies all the CHANGING fields based on their expected
   change patterns, especially for conversational audio and video.



    +------------------------+-------------+-------------+-------------+
    |         Field          | Value/Delta |    Class    |  Knowledge  |
    +========================+=============+=============+=============+
    |             Sequential |    Delta    |    STATIC   |    KNOWN    |
    |             -----------+-------------+-------------+-------------+
    | IPv4 Id:    Seq. jump  |    Delta    |      RC     |   LIMITED   |
    |             -----------+-------------+-------------+-------------+
    |             Random     |    Value    |  IRREGULAR  |   UNKNOWN   |
    +------------------------+-------------+-------------+-------------+
    | IP TOS / Tr. Class     |    Value    |      RC     |   UNKNOWN   |
    +------------------------+-------------+-------------+-------------+
    | IP TTL / Hop Limit     |    Value    | ALTERNATING |   LIMITED   |
    +------------------------+-------------+-------------+-------------+
    |               Disabled |    Value    |    STATIC   |    KNOWN    |
    | UDP Checksum: ---------+-------------+-------------+-------------+
    |               Enabled  |    Value    |  IRREGULAR  |   UNKNOWN   |
    +------------------------+-------------+-------------+-------------+
    |                 No mix |    Value    |    STATIC   |    KNOWN    |
    | RTP CSRC Count: -------+-------------+-------------+-------------+



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    |                 Mixed  |    Value    |      RC     |   LIMITED   |
    +------------------------+-------------+-------------+-------------+
    | RTP Marker             |    Value    |  SEMISTATIC | KNOWN/KNOWN |
    +------------------------+-------------+-------------+-------------+
    | RTP Payload Type       |    Value    |      RC     |   UNKNOWN   |
    +------------------------+-------------+-------------+-------------+
    | RTP Sequence Number    |    Delta    |    STATIC   |    KNOWN    |
    +------------------------+-------------+-------------+-------------+
    | RTP Timestamp          |    Delta    |      RC     |   LIMITED   |
    +------------------------+-------------+-------------+-------------+
    |                 No mix |      -      |      -      |      -      |
    | RTP CSRC List:  -------+-------------+-------------+-------------+
    |                 Mixed  |    Value    |      RC     |   UNKNOWN   |
    +------------------------+-------------+-------------+-------------+

           Table A.1 : Classification of CHANGING header fields

   The following subsections discuss the various header fields in
   detail. Note that table A.1 and the discussions below do not consider
   changes caused by loss or reordering before the compression point.


A.2.1.  IPv4 Identification

   The Identification field (IP ID) of the IPv4 header is there to
   identify which fragments constitute a datagram when reassembling
   fragmented datagrams. The IPv4 specification does not specify exactly
   how this field is to be assigned values, only that each packet should
   get an IP ID that is unique for the source-destination pair and
   protocol for the time the datagram (or any of its fragments) could be
   alive in the network. This means that assignment of IP ID values can
   be done in various ways, which we have separated into three classes.

   Sequential

      This assignment policy keeps a separate counter for each outgoing
      packet stream and thus the IP ID value will increment by one for
      each packet in the stream. Therefore, the delta value of the
      field is constant and well known a priori. When RTP is used on
      top of UDP and IP, the IP ID value follows the RTP sequence
      number. This assignment policy is the most desirable for header
      compression purposes but its usage is not as common as it should
      be. The reason is that it can be realized only if UDP and IP are
      implemented together so that UDP, which separates packet streams
      by the port identification, can make IP use separate ID counters
      for each packet stream.

   Sequential jump

      This is the most common assignment policy in today's IP stacks.
      The difference from the sequential method is that only one



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      counter is used for all connections. When the sender is running
      more than one packet stream simultaneously, the IP ID can
      increase by more than one. The IP ID values will be much more
      predictable and require less bits to transfer than random values,
      and the packet-to-packet increment (determined by the number of
      active outgoing packet streams and sending frequencies) will
      usually be limited.

   Random

      Some IP stacks assign IP ID values using a pseudo-random number
      generator. There is thus no correlation between the ID values of
      subsequent datagrams. Therefore there is no way to predict the IP
      ID value for the next datagram. For header compression purposes,
      this means that the IP ID field needs to be sent uncompressed
      with each datagram, resulting in two extra octets of header. IP
      stacks in cellular terminals SHOULD NOT use this IP ID assignment
      policy.

   It should be noted that the ID is an IPv4 mechanism and is therefore
   not needed at all in IPv6 profiles. For IPv4 the ID could be handled
   in three different ways. Firstly, we have the inefficient but
   reliable solution where the ID field is sent as-is in all packets,
   increasing the compressed headers with two octets. This is the best
   way to handle the ID field if the sender uses random assignment of
   the ID field. Secondly, there can be solutions with more flexible
   mechanisms requiring less bits for the ID handling as long as
   sequential jump assignment is used. Such solutions will probably
   require even more bits if random assignment is used by the sender.
   Knowledge about the sender's assignment policy could therefore be
   useful when choosing between the two solutions above. Finally, even
   for IPv4, header compression could be designed without any additional
   information for the ID field included in compressed headers. To use
   such schemes, it must be known that the sender makes use of the pure
   sequential assignment policy for the ID field. That might not be
   possible to know, which implies that the applicability of such
   solutions is very uncertain. However, designers of IPv4 stacks for
   cellular terminals SHOULD use the sequential policy.


A.2.2.  IP Traffic-Class / Type-Of-Service

   The Traffic-Class (IPv6) or Type-Of-Service (IPv4) field is expected
   to be constant during the lifetime of a packet stream or to change
   relatively seldom.









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A.2.3.  IP Hop-Limit / Time-To-Live

   The Hop-Limit (IPv6) or Time-To-Live (IPv4) field is expected to be
   constant during the lifetime of a packet stream or to alternate
   between a limited number of values due to route changes.


A.2.4.  UDP Checksum

   The UDP checksum is optional. If disabled, its value is constantly
   zero and could be compressed away. If enabled, its value depends on
   the payload, which for compression purposes is equivalent to it
   changing randomly with every packet.


A.2.5.  RTP CSRC Counter

   This is a counter indicating the number of CSRC items present in the
   CSRC list. This number is expected to be almost constant on a packet-
   to-packet basis and change by small amount. As long as no RTP mixer
   is used, the value of this field is zero.

A.2.6.  RTP Marker

   For audio the marker bit should be set only in the first packet of a
   talkspurt while for video it should be set in the last packet of
   every picture. This means that in both cases the RTP marker is
   classified as SEMISTATIC with well-known values for both states.


A.2.7.  RTP Payload Type

   Changes of the RTP payload type within a packet stream are expected
   to be rare. Applications could adapt to congestion by changing
   payload type and/or frame sizes, but that is not expected to happen
   frequently.


A.2.8.  RTP Sequence Number

   The RTP sequence number will be incremented by one for each packet
   sent.


A.2.9.  RTP Timestamp

   In the audio case:

      As long as there are no pauses in the audio stream, the RTP
      timestamp will be incremented by a constant delta, corresponding
      to the number of samples in the speech frame. It will thus mostly



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      follow the RTP sequence number. When there has been a silent
      period and a new talkspurt begins, the timestamp will jump in
      proportion to the length of the silent period. However, the
      increment will probably be within a relatively limited range.

   In the video case:

      The timestamp change between two consecutive packets will either
      be zero or increase by a multiple of a fixed value corresponding
      to the picture clock frequency. The timestamp can also decrease
      by a multiple of the fixed value if B-pictures are used. The
      delta interval, expressed as a multiple of the picture clock
      frequency, is in most cases very limited.


A.2.10.  RTP Contributing Sources (CSRC)

   The participants in a session, which are identified by the CSRC
   fields, are expected to be almost the same on a packet-to-packet
   basis with relatively few additions or removals. As long as RTP
   mixers are not used, no CSRC fields are present at all.

A.3.  Header compression strategies

   This section elaborates on what has been done in previous sections.
   Based in the classifications, recommendations are given on how to
   handle the various fields in the header compression process. Seven
   different actions are possible and these are listed together with the
   fields to which each action applies.


A.3.1.  Do not send at all

   The fields that have well known values a priori do not have to be
   sent at all. These are:

   - IP Version
   - IPv6 Payload Length
   - IPv6 Next Header
   - IPv4 Header Length
   - IPv4 Reserved Flag
   - IPv4 Last Fragment Flag
   - IPv4 Fragment Offset
   - IPv4 Protocol
   - UDP Checksum (if disabled)
   - RTP Version








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A.3.2.  Transmit only initially

   The fields that are constant throughout the lifetime of the packet
   stream have to be transmitted and correctly delivered to the
   decompressor only once. These are:

   - IP Source Address
   - IP Destination Address
   - IPv6 Flow Label
   - IPv4 May Fragment Flag
   - UDP Source Port
   - UDP Destination Port
   - RTP Padding Flag
   - RTP Extension Flag
   - RTP SSRC


A.3.3.  Transmit initially, but be prepared to update

   The fields that are changing only occasionally must be transmitted
   initially but there must also be a way to update these fields with
   new values if they change. These fields are:

   - IPv6 Traffic Class
   - IPv6 Hop Limit
   - IPv4 Type Of Service (TOS)
   - IPv4 Time To Live (TTL)
   - RTP CSRC Counter
   - RTP Payload Type
   - RTP CSRC List


A.3.4.  Be prepared to update or send as-is frequently

   For fields that normally are either constant or whose values can be
   deduced from some other field but frequently diverge from that
   behavior, there must be an efficient way to update the field value or
   send it as-is in some packets. Those fields are:

   - IPv4 Identification (if not sequentially assigned)
   - RTP Marker
   - RTP Timestamp


A.3.5.  Guarantee continuous robustness

   Fields that behave like a counter with a fixed delta for ALL packets,
   the only requirement on the transmission encoding is that packet
   losses between compressor and decompressor must be tolerable. If more
   than one such field exists, all these can be communicated together.
   Such fields can also be used to interpret the values for fields



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   listed in the previous section. Fields that have this counter
   behavior are:

   - IPv4 Identification (if sequentially assigned)
   - RTP Sequence Number


A.3.6.  Transmit as-is in all packets

   Fields that have completely random values for each packet must be
   included as-is in all compressed headers. Those fields are:

   - IPv4 Identification (if randomly assigned)
   - UDP Checksum (if enabled)


A.3.7.  Establish and be prepared to update delta

   Finally, there is a field that is usually increasing by a fixed delta
   and is correlated to another field. For this field it would make
   sense to make that delta part of the context state. The delta must
   then be possible to initiate and update in the same way as the fields
   listed in A.3.3. The field to which this applies is:

   - RTP Timestamp





























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Appendix E - Encoding Examples

   [[Editor's note: Need to update this with current terminology.]]

E.1.  Basic VLE

      The examples below illustrate the operation of VLE under various
   scenarios.  The field values used in the examples could correspond to
   any fields that we wish to compress.  The examples illustrate the
   scenario where the compressed field has resolution of one bit.

      Example 1: Normal operation (no packet loss prior to compressor,
   no reodering prior to compressor).

      Suppose packets with header fields 279, 280, 281, 282, and 283
   have been sent, and 279 and 283 are fields of potential reference
   packets.

      The current VLE window is {279, 283}.

      and a packet with field value = 284 is received next, VLE computes
   the following values

      New Value   VMax    VMin             r                   # LSBs
         284      283     279    max[|284-279|,|284-283|]=5       4

      The window is unmodified if we assuming the new packet {284} is
   not a potential reference.  The field is encoded using 4 bits in this
   case, and the actual encoded value is the 4 least significant bits of
   284 (10011100) which = 1100.

      Example 2:  Packet Loss prior to compressor.

      Suppose packets with header fields 279, 280, 281, 282, and 283
   have been sent, and 279 and 283 are fields of potential reference
   packets such that the VSW is again {279, 283}.

      If a packet with field value = 290 is received next, VLE computes
   the following values

      New Value  VMax  VMin             r                # LSBs
        290      283   279   max[|290-283|,|290-279|]=11    5

      So the field is encoded using 5 bits.  Actual encoded value is the
   5 LSBs of 290 (100100010) which = 00010.

      If we assume the new value is a potential reference, the new VSW
   is {279, 283, 290}.

      Example 3:  Packet Misordering prior to compressor.



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      Suppose packets with header fields 279, 280, 281, 282, and 283
   have been sent, and 279 and 283 are fields of potential reference
   packets such that the VSW is again {279, 283}.

      If a packet with field value = 278 is received next, VLE computes
   the following values

      New Value     VMax    VMin             r                  # LSBs
        278         283     279   max[|278-283|,|278-279|]=5      4

      So the field is encoded using 4 bits.  Actual encoded value is the
   4 LSBs of 278 (10010110) which = 0110.

      If we assume the new value is a potential reference, the new VSW
   is {283, 290, 278}.

      In any case, the VLE encoded fields must be accompanied by some
   bits in order to identify the different possible encoded field sizes.
   Sizes of this bit field can vary depending on the number of different
   sizes one wishes to allow.  The approach in ACE is to allow only a
   few different sizes for byte-aligned header formats.  Huffman coding
   of the length is used to achieve some additional efficiency, based on
   the expected frequency of needing the different sizes.  1 or 2
   additional bits are actually sent in the ACE compressed header.

      The decompressor behavior in all the example cases is the same- it
   uses as a reference a specific decompressed header field value.  The
   header to use might be indicated by the presence of a checksum in the
   compressed header packet, or by other means.  They must by definition
   be one of the values in the compressor's window.

      For example let's assume that the last correctly decompressed
   packet which qualifies as a reference was the packet with header
   field = 291.  Now suppose the encoded field value of 303 (10001111)
   is received and = 01111.  The two values closest values to 291 which
   have LSBs = 01111 are 271 and 303.  303 is closest, therefore it is
   correctly selected as the uncompressed field value.

E.2.  Timer-Based VLE



      As a an example of operation, consider the case of a voice codec
   (20 ms), such that TS_stride = 160.  Assume T_current and
   p_TS_current are 357 and 351, respectively, and that we have sliding
   window TSW which contains the following values 4 entries:

           j           T_j         p_TS_j

           1            9            7



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           2            8            6
           3            7            4
           4            3            1

      j above is the packet number.

      In this case we have

      Network_jitter(1)=|(357-9)-(351-7)|=4 (80 ms Network Jitter)
      Network_jitter(2)=|(357-8)-(351-6)|=4  (80 ms Network Jitter)
      Network_jitter(3)=|(357-7)-(351-4)|=3 (60 ms Network Jitter)
      Network_jitter(4)=|(357-3)-(351-1)|=4  (80 ms Network Jitter)

      So Max_Network_Jitter = 4.

      We assume a maximum CD-CC jitter of 2 (40 ms); the total jitter to
   be handled in this case is then

           J = 4 + 2 + 2 = 8 packets (160 ms)

      and k = 5 bits (since 2 * 5 + 1 < 2^5).  The compressor sends the
   5 LSBs of p_TS_current to the decompressor (351 = 101011111, so the
   encoded TS value = 11111).

      When the decompressor receives this value, it first attempts to
   estimate the timestamp by computing the time difference between the
   last reference established and the current packet

            T_current - T_ref, where T_ref is the value of the wall
   clock time at which the reference headers was received by the
   decompressor


      That value is added to p_TS_ref, the packed RTP TS of the
   reference header, to get the estimate.

      Assume that at the decompressor packet #3 is used as the
   reference:


           - T_current = 359
           - T_ref = 7
           - p_TS_ref = 4

      Note:

      T_current is picked here as any value; the difference between it
   and T_ref represents the length of the silence interval as observed
   at the decompressor.  Then:

           T_current - T_ref = 359 - 7 = 352



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           p_TS_current(estimate) = 352 + 4 = 356


      The decompressor searches for the closest value to 356 which has,
   in this case, LSBs = 11111.  The value in this case is 351, the
   original p_TS.


      If instead the compressor were to send the timestamp jump as
   simply the difference in consecutive packed RTP Timestamps, that
   value would be

      p_TS_current - p_TS_ref = 351-4 = 347 = 101011011

      So over twice as many bits would be sent for a silence interval of

           347 (20 ms) = 6.94 seconds

      Due to basic conversational real-time requirements, the cumulative
   jitter in normal operation is expected to be at most only a few times
   T stride for voice.  For this reason, the FO payload formats in
   section 4.3 are optimized (in terms of representing different k-
   length encoded TS values) for the case of k=4 (handles up to 16
   discrepencies in the timestamp).  The remaining formats allow a wide
   range of jitter conditions (outside of just voice) to be handled as
   well.
























   This Internet-Draft expires March 17, 2001.


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