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RFC 2507 - IP Header Compression


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Network Working Group                                       M. Degermark
Request for Comments: 2507           Lulea University of Technology/SICS
Category: Standards Track                                    B. Nordgren
                        Lulea University of Technology/Telia Research AB
                                                                 S. Pink
                                     Lulea University of Technology/SICS
                                                           February 1999

                         IP Header Compression

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

   This document describes how to compress multiple IP headers and TCP
   and UDP headers per hop over point to point links. The methods can be
   applied to of IPv6 base and extension headers, IPv4 headers, TCP and
   UDP headers, and encapsulated IPv6 and IPv4 headers.

   Headers of typical UDP or TCP packets can be compressed down to 4-7
   octets including the 2 octet UDP or TCP checksum. This largely
   removes the negative impact of large IP headers and allows efficient
   use of bandwidth on low and medium speed links.

   The compression algorithms are specifically designed to work well
   over links with nontrivial packet-loss rates. Several wireless and
   modem technologies result in such links.

TABLE OF CONTENTS

   1.  Introduction..............................................3
   2.  Terminology...............................................5
   3.  Compression method........................................7
        3.1.  Packet types.......................................8
        3.2.  Lost packets in TCP packet streams.................9
        3.3.  Lost packets in UDP and non-TCP packet streams....10
   4.  Grouping packets into packet streams.....................14

        4.1.  Guidelines for grouping packets...................15
   5.  Size Issues..............................................16
        5.1.  Context identifiers...............................16
        5.2.  Size of the context...............................17
        5.3.  Size of full headers..............................18
           5.3.1.  Length fields in full TCP headers............19
           5.3.2.  Length fields in full non-TCP headers........19
   6.  Compressed Header Formats................................20
   7.  Compression of subheaders................................22
        7.1.  IPv6 Header.......................................24
        7.2.  IPv6 Extension Headers............................25
        7.3.  Options...........................................25
        7.4.  Hop-by-hop Options Header.........................26
        7.5.  Routing Header....................................26
        7.6.  Fragment Header...................................27
        7.7.  Destination Options Header........................28
        7.8.  No Next Header....................................29
        7.9.  Authentication Header.............................29
        7.10. Encapsulating Security Payload Header.............29
        7.11. UDP Header........................................30
        7.12. TCP Header........................................30
        7.13. IPv4 Header.......................................33
        7.14  Minimal Encapsulation header......................34
   8.  Changing context identifiers.............................35
   9.  Rules for dropping or temporarily storing packets........35
   10. Low-loss header compression for TCP .....................36
        10.1.  The "twice" algorithm............................37
        10.2.  Header Requests..................................37
   11. Links that reorder packets...............................38
        11.1.  Reordering in non-TCP packet streams.............39
        11.2.  Reordering in TCP packet streams.................39
   12. Hooks for additional header compression..................40
   13. Demultiplexing...........................................41
   14. Configuration Parameters.................................42
   15. Implementation Status....................................43
   16. Acknowledgments..........................................44
   17. Security Considerations..................................44
   18. Authors' Addresses.......................................45
   19. References...............................................46
   20. Full Copyright Statement.................................47

1.  Introduction

   There are several reasons to do header compression on low- or
   medium-speed links. Header compression can

   *  Improve interactive response time

      For very low-speed links, echoing of characters may take longer
      than 100-200 ms because of the time required to transmit large
      headers. 100-200 ms is the maximum time people can tolerate
      without feeling that the system is sluggish.

   * Allow using small packets for bulk data with good line efficiency

      This is important when interactive (for example Telnet) and bulk
      traffic (for example FTP) is mixed because the bulk data should be
      carried in small packets to decrease the waiting time when a
      packet with interactive data is caught behind a bulk data packet.

      Using small packet sizes for the FTP traffic in this case is a
      global solution to a local problem. It will increase the load on
      the network as it has to deal with many small packets. A better
      solution might be to locally fragment the large packets over the
      slow link.

   * Allow using small packets for delay sensitive low data-rate traffic

      For such applications, for example voice, the time to fill a
      packet with data is significant if packets are large.  To get low
      end-to-end delay small packets are preferred.  Without header
      compression, the smallest possible IPv6/UDP headers (48 octets)
      consume 19.2 kbit/s with a packet rate of 50 packets/s.  50
      packets/s is equivalent to having 20 ms worth of voice samples in
      each packet. IPv4/UDP headers consumes 11.2 kbit/s at 50
      packets/s.  Tunneling or routing headers, for example to support
      mobility, will increase the bandwidth consumed by headers by 10-20
      kbit/s.  This should be compared with the bandwidth required for
      the actual sound samples, for example 13 kbit/s with GSM encoding.
      Header compression can reduce the bandwidth needed for headers
      significantly, in the example to about 1.7 kbit/s. This enables
      higher quality voice transmission over 14.4 and 28.8 kbit/s
      modems.

   *  Decrease header overhead.

      A common size of TCP segments for bulk transfers over medium-speed
      links is 512 octets today. When TCP segments are tunneled, for
      example because Mobile IP is used, the IPv6/IPv6/TCP header is 100

      octets.  Header compression will decrease the header overhead for
      IPv6/TCP from 19.5 per cent to less than 1 per cent, and for
      tunneled IPv4/TCP from 11.7 to less than 1 per cent. This is a
      significant gain for line-speeds as high as a few Mbit/s.

      The IPv6 specification prescribes path MTU discovery, so with IPv6
      bulk TCP transfers should use segments larger than 512 octets when
      possible.  Still, with 1400 octet segments (RFC 894 Ethernet
      encapsulation allows 1500 octet payloads, of which 100 octets are
      used for IP headers), header compression reduces IPv6 header
      overhead from 7.1% to 0.4%.

   *  Reduce packet loss rate over lossy links.

      Because fewer bits are sent per packet, the packet loss rate will
      be lower for a given bit-error rate. This results in higher
      throughput for TCP as the sending window can open up more between
      losses, and in fewer lost packets for UDP.

   The mechanisms described here are intended for a point-to-point link.
   However, care has been taken to allow extensions for multi-access
   links and multicast.

   Headers that can be compressed include TCP, UDP, IPv4, and IPv6 base
   and extension headers.  For TCP packets, the mechanisms of Van
   Jacobson [RFC-1144] are used to recover from loss. Two additional
   mechanisms that increase the efficiency of VJ header compression over
   lossy links are also described.  For non-TCP packets, compression
   slow-start and periodic header refreshes allow minimal periods of
   packet discard after loss of a header that changes the context. There
   are hooks for adding header compression schemes on top of UDP, for
   example compression of RTP headers.

   Header compression relies on many fields being constant or changing
   seldomly in consecutive packets belonging to the same packet stream.
   Fields that do not change between packets need not be transmitted at
   all.  Fields that change often with small and/or predictable values,
   e.g., TCP sequence numbers, can be encoded incrementally so that the
   number of bits needed for these fields decrease significantly.  Only
   fields that change often and randomly, e.g., checksums or
   authentication data, need to be transmitted in every header.

   The general principle of header compression is to occasionally send a
   packet with a full header; subsequent compressed headers refer to the
   context established by the full header and may contain incremental
   changes to the context.

   This header compression scheme does not require that all packets in
   the same stream passes over the compressed link. However, for TCP
   streams the difference between subsequent headers can become more
   irregular and the compression rate can decrease. Neither is it
   required that corresponding TCP data and acknowledgment packets
   traverse the link in opposite directions.

   This header compression scheme is useful on first-hop or last-hop
   links as well as links in the middle of the network. When many packet
   streams (several hundred) traverse the link, a phenomenon that could
   be called CID thrashing could occur, where headers seldom can be
   matched with an existing context and have to be sent uncompressed or
   as full headers. It is up to an implementation to use techniques such
   as hysteresis to ensure that the packet streams that give the highest
   compression rates keep their context.  Such techniques are more
   likely to be needed in the middle of the network.

2.  Terminology

   This section explains some terms used in this document.

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

   Subheader

      An IPv6 base header, an IPv6 extension header, an IPv4 header, a
      UDP header, or a TCP header.

   Header

      A chain of subheaders.

   Compress

      The act of reducing the size of a header by removing header fields
      or reducing the size of header fields. This is done in a way such
      that a decompressor can reconstruct the header if its context
      state is identical to the context state used when compressing the
      header.

   Decompress

      The act of reconstructing a compressed header.

   Context identifier (CID)

      A small unique number identifying the context that should be used
      to decompress a compressed header.  Carried in full headers and
      compressed headers.

   Context

      The state which the compressor uses to compress a header and the
      decompressor uses to decompress a header.  The context is 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 for a packet stream is associated with a context
      identifier.  The context for non-TCP packet streams is also
      associated with a generation.

   Generation

      For non-TCP packet streams, each new version of the context for a
      given CID is associated with a generation: a small number that is
      incremented whenever the context associated with that CID changes.
      Carried by full and compressed non-TCP headers.

   Packet stream

      A sequence of packets whose headers are similar and share context.
      For example, headers in a TCP packet stream have the same source
      and final destination address, and the same port numbers in the
      TCP header.  Similarly, headers in a UDP packet stream have the
      same source and destination address, and the same port numbers in
      the UDP header.

   Full header (header refresh)

      An uncompressed header that updates or refreshes the context for a
      packet stream. It carries a CID that will be used to identify the
      context.

      Full headers for non-TCP packet streams also carry the generation
      of the context they update or refresh.

   Regular header

      A normal, uncompressed, header.  Does not carry CID or generation
      association.

   Incorrect decompression

      When a compressed and then decompressed header is different from
      the uncompressed header. Usually due to mismatching context
      between the compressor and decompressor or bit errors during
      transmission of the compressed header.

   Differential coding

      A compression technique where the compressed value of a header
      field is the difference between the current value of the field and
      the value of the same field in the previous header belonging to
      the same packet stream. A decompressor can thus obtain the value
      of the field by adding the value in the compressed header to its
      context.  This technique is used for TCP streams but not for non-
      TCP streams.

3.  Compression method

   Much of the header information stays the same over the life-time of a
   packet stream. For non-TCP packet streams almost all fields of the
   headers are constant. For TCP many fields are constant and others
   change with small and predictable values.

   To initiate compression of the headers of a packet stream, a full
   header carrying a context identifier, CID, is transmitted over the
   link.  The compressor and decompressor store most fields of this full
   header as context.  The context consists of the fields of the header
   whose values are constant and thus need not be sent over the link at
   all, or change little between consecutive headers so that it uses
   fewer bits to send the difference from the previous value compared to
   sending the absolute value.

   Any change in fields that are expected to be constant in a packet
   stream will cause the compressor to send a full header again to
   update the context at the decompressor. As long as the context is the
   same at compressor and decompressor, headers can be decompressed to
   be exactly as they were before compression. However, if a full header
   or compressed header is lost during transmission, the context of the
   decompressor may become obsolete as it is not updated properly.
   Compressed headers will then be decompressed incorrectly.

   IPv6 is not meant to be used over links that can deliver a
   significant fraction of damaged packets to the IPv6 module.  This
   means that links must have a very low bit-error rate or that link-
   level frames must be protected by strong checksums, forward error
   correction or something of that nature.  Header compression SHOULD
   not be used for IPv4 without strong link-level checksums.  Damaged

   frames will thus be discarded by the link layer.  The link layer
   implementation might indicate to the header compression module that a
   frame was damaged, but it cannot say what packet stream it belonged
   to as it might be the CID that is damaged.  Moreover, frames may
   disappear without the link layer implementation's knowledge, for
   example if the link is a multi-hop link where frames can be dropped
   due to congestion at each hop.  The kind of link errors that a header
   compression module should deal with and protect against will thus be
   packet loss.

   So a header compression scheme needs mechanisms to update the context
   at the decompressor and to detect or avoid incorrect decompression.
   These mechanisms are very different for TCP and non-TCP streams, and
   are described in sections 3.2 and 3.3.

   The compression mechanisms in this document assume that packets are
   not reordered between the compressor and decompressor.  If the link

   does reorder, section 11 describes mechanisms for ordering the
   packets before decompression.  It is also assumed that the link-layer
   implementation can provide the length of packets, and that there is
   no padding in UDP packets or tunneled packets.

3.1.  Packet types

   This compression method uses four packet types in addition to the
   IPv4 and IPv6 packet types.  The combination of link-level packet
   type and the value of the first four bits of the packet uniquely
   determines the packet type.  Details on how these packet types are
   represented are in section 13.

       FULL_HEADER - indicates a packet with an uncompressed header,
       including a CID and, if not a TCP packet, a generation.  It
       establishes or refreshes the context for the packet stream
       identified by the CID.

       COMPRESSED_NON_TCP - indicates a non-TCP packet with a compressed
       header. The compressed header consists of a CID identifying what
       context to use for decompression, a generation to detect an
       inconsistent context and the randomly changing fields of the
       header.

       COMPRESSED_TCP - indicates a packet with a compressed TCP header,
       containing a CID, a flag octet indentifying what fields have
       changed, and the changed fields encoded as the difference from
       the previous value.

       COMPRESSED_TCP_NODELTA - indicates a packet with a compressed TCP
       header where all fields that are normally sent as the difference
       to the previous value are instead sent as-is.  This packet type
       is only sent as the response to a header request from the
       decompressor. It must not be sent as the result of a
       retransmission.

   In addition to the packet types used for compression, regular IPv4
   and IPv6 packets are used whenever a compressor decides to not
   compress a packet.  An additional packet type may be used to speed up
   repair of TCP streams over links where the decompressor can send
   packets to the compressor.

       CONTEXT_STATE - indicates a special packet sent from the
       decompressor to the compressor to communicate a list of (TCP)
       CIDs for which synchronization has been lost. This packet is only
       sent over a single link so it requires no IP header. The format
       is shown in section 10.2.

3.2.  Lost packets in TCP packet streams

   Since TCP headers are compressed using the difference from the
   previous TCP header, loss of a packet with a compressed or full
   header will cause subsequent compressed headers to be decompressed
   incorrectly because the context used for decompression was not
   incremented properly.

   Loss of a compressed TCP header will cause the TCP sequence numbers
   of subsequently decompressed TCP headers to be off by k, where k is
   the size of the lost segment.  Such incorrectly decompressed TCP
   headers will be discarded by the TCP receiver as the TCP checksum
   reliably catches "off-by-k" errors in the sequence numbers for
   plausible k.

   TCP's repair mechanisms will eventually retransmit the discarded
   segment and the compressor peeks into the TCP headers to detect when
   TCP retransmits.  When this happens, the compressor sends a full
   header on the assumption that the retransmission was due to
   mismatching compression state at the decompressor.  [RFC-1144] has a
   good explanation of this mechanism.

   The mechanisms of section 10 should be used to speed up the repair of
   the context.  This is important over medium speed links with high
   packet loss rates, for example wireless.  Losing a timeout's worth of
   packets due to inconsistent context after each packet lost over the
   link is not acceptable, especially when the TCP connection is over
   the wide area.

3.3.  Lost packets in UDP and other non-TCP packet streams

   Incorrectly decompressed headers of UDP packets and other non-TCP
   packets are not so well-protected by checksums as TCP packets.  There
   are no sequence numbers that become "off-by-k" and virtually
   guarantees a failed checksum as there are for TCP. The UDP checksum
   only covers payload, UDP header, and pseudo header.  The pseudo
   header includes the source and destination addresses, the transport
   protocol type and the length of the transport packet.  Except for
   those fields, large parts of the IPv6 header are not covered by the
   UDP checksum.  Moreover, other non-TCP headers lack checksums
   altogether, for example fragments.

   In order to safely avoid incorrect decompression of non-TCP headers,
   each version of the context for non-TCP packet streams is identified
   by a generation, a small number that is carried by the full headers
   that establish and refresh the context.  Compressed headers carry the
   generation value of the context that were used to compress them.
   When a decompressor sees that a compressed header carries a
   generation value other than the generation of its context for that
   packet stream, the context is not up to date and the packet must be
   discarded or stored until a full header establishes correct context.

   Differential coding is not used for non-TCP streams, so compressed
   non-TCP headers do not change the context.  Thus, loss of a
   compressed header does not invalidate subsequent packets with
   compressed headers. Moreover, the generation changes only when the
   context of a full header is different from the context of the
   previous full header. This means that losing a full header will make
   the context of the decompressor obsolete only when the full header
   would actually have changed the context.

   The generation field is 6 bits long so the generation value repeats
   itself after 64 changes to the context. To avoid incorrect
   decompression after error bursts or other temporary disruptions, the
   compressor must not reuse the same generation value after a shorter
   time than MIN_WRAP seconds. A decompressor which has been
   disconnected MIN_WRAP seconds or more must wait for the next full
   header before decompressing. A compressor must wait at least MIN_WRAP
   seconds after booting before compressing non-TCP headers. Instead of
   reusing a generation value too soon, a compressor may switch to
   another CID or send regular headers until MIN_WRAP seconds have
   passed.  The value of MIN_WRAP is found in section 14.

3.3.1.  Compression Slow-Start

   To allow the decompressor to recover quickly from loss of a full
   header that would have changed the context, full headers are sent
   periodically with an exponentially increasing period after a change
   in the context. This technique avoids an exchange of messages between
   compressor and decompressor used by other compression schemes, such
   as in [RFC-1553]. Such exchanges can be costly for wireless mobiles
   as more power is consumed by the transmitter and delay can be
   introduced by switching between sending and receiving.  Moreover,
   techniques that require an exchange of messages cannot be used over
   simplex links, such as direct-broadcast satellite channels or cable
   TV systems, and are hard to adapt to multicast over multi-access
   links.

    |.|..|....|........|................|..............................
    ^
    Change   Sent packets: | with full header, . with compressed header

   The picture shows how packets are sent after change.  The compressor
   keeps a variable for each non-TCP packet stream, F_PERIOD, that keeps
   track of how many compressed headers may be sent between full
   headers.  When the headers of a non-TCP packet stream change so that
   its context changes, a full header is sent and F_PERIOD is set to
   one. After sending F_PERIOD compressed headers, a full header is
   sent.  F_PERIOD is doubled each time a full header is sent during
   compression slow-start.

3.3.2.  Periodic Header Refreshes

   To avoid losing too many packets if a receiver has lost its context,
   there is an upper limit, F_MAX_PERIOD, on the number of non-TCP
   packets with compressed headers that may be sent between header
   refreshes. If a packet is to be sent and F_MAX_PERIOD compressed
   headers have been sent since the last full header for this packet
   stream was sent, a full header must be sent.

   To avoid long periods of disconnection for low data rate packet
   streams, there is also an upper bound, F_MAX_TIME, on the time
   between full headers in a non-TCP packet stream. If a packet is to be
   sent and more than F_MAX_TIME seconds have passed since the last full
   header was sent for this packet stream, a full header must be sent.
   The values of F_MAX_PERIOD and F_MAX_TIME are found in section 14.

3.3.3. Rules for sending Full Headers

   The following pseudo code can be used by the compressor to determine
   when to send a full header for a non-TCP packet stream.  The code
   maintains two variables:

         C_NUM       -- a count of the number of compressed headers sent
                        since the last full header was sent.
         F_LAST      -- the time of sending the last full header.

   and uses the functions

         current_time()       return the current time
         min(a,b)             return the smallest of a and b

      the procedures send_full_header(), increment_generation_value(),
      and send_compressed_header()
      do the obvious thing.

         if ( <this header changes the context> )

             C_NUM := 0;
             F_LAST := current_time();
             F_PERIOD := 1;
             increment_generation_value();
             send_full_header();

         elseif ( C_NUM >= F_PERIOD )

             C_NUM := 0;
             F_LAST := current_time();
             F_PERIOD := min(2 * F_PERIOD, F_MAX_PERIOD);
             send_full_header();

         elseif ( current_time() > F_LAST + F_MAX_TIME )

             C_NUM := 0;
             F_LAST := current_time();
             send_full_header();

         else

             C_NUM := C_NUM + 1
             send_compressed_header();

         endif

3.3.4.  Cost of sending Header Refreshes

   If every f'th packet carries a full header, H is the size of a full
   header, and C is the size of a compressed header, the average header
   size is

                 (H-C)/f + C

   For f > 1, the average header size is (H-C)/f larger than a
   compressed header.

   In a diagram where the average header size is plotted for various f
   values, there is a distinct knee in the curve, i.e., there is a limit
   beyond which further increasing f gives diminishing returns.
   F_MAX_PERIOD should be chosen to be a frequency well to the right of
   the knee of the curve.  For typical sizes of H and C, say 48 octets
   for the full header (IPv6/UDP) and 4 octets for the compressed
   header, setting F_MAX_PERIOD > 44 means that full headers will
   contribute less than an octet to the average header size. With a
   four-address routing header, F_MAX_PERIOD > 115 will have the same
   effect.

   The default F_MAX_PERIOD value of 256 (section 14) puts the full
   header frequency well to the right of the knee and means that full
   headers will typically contribute considerably less than an octet to
   the average header size.  For H = 48 and C = 4, full headers
   contribute about 1.4 bits to the average header size after reaching
   the steady-state header refresh frequency determined by the default

   F_MAX_PERIOD. 1.4 bits is a very small overhead.

   After a change in the context, the exponential backoff scheme will
   initially send full headers frequently.  The default F_MAX_PERIOD
   will be reached after nine full headers and 255 compressed headers
   have been sent.  This is equivalent to a little over 5 seconds for a
   typical voice stream with 20 ms worth of voice samples per packet.

   During the whole backoff period, full headers contribute 1.5 octets
   to the average header size when H = 48 and C = 4.  For 20 ms voice
   samples, it takes less than 1.3 seconds until full headers contribute
   less than one octet to the average header size, and during these
   initial 1.3 seconds full headers add less than 4 octets to the
   average header size.  The cost of the exponential backoff is not
   great and as the headers of non-TCP packet streams are expected to
   change seldomly, it will be amortized over a long time.

   The cost of header refreshes in terms of bandwidth are higher than
   similar costs for hard state schemes like [RFC-1553] where full
   headers must be acknowledged by the decompressor before compressed
   headers may be sent. Such schemes typically send one full header plus
   a few control messages when the context changes.  Hard state schemes
   require more types of protocol messages and an exchange of messages
   is necessary.  Hard state schemes also need to deal explicitly with
   various error conditions that soft state handles automatically, for
   instance the case of one party disappearing unexpectedly, a common
   situation on wireless links where mobiles may go out of range of the
   base station.

   The major advantage of the soft state scheme is that no handshakes
   are needed between compressor and decompressor, so the scheme can be
   used over simplex links.  The costs in terms of bandwidth are higher
   than for hard state schemes, but the simplicity of the decompressor,
   the simplicity of the protocol, and the lack of handshakes between
   compressor and decompressor justifies this small cost. Moreover, soft
   state schemes are more easily extended to multicast over multi-access
   links, for example radio links.

4.  Grouping packets into packet streams

   This section explains how packets MAY be grouped together into packet
   streams for compression.  To achieve the best compression rates,
   packets SHOULD be grouped together such that packets in the same
   packet stream have similar headers. If this grouping fails, header
   compression performance will be bad, since the compression algorithm
   can rarely utilize the existing context for the packet stream and
   full headers must be sent frequently.

   Grouping is done by the compressor. A compressor may use whatever
   criterion it finds appropriate to group packets into packet streams.
   To determine what packet stream a packet belongs to, a compressor MAY

   a) examine the compressible chain of subheaders (see section 7),

   b) examine the contents of an upper layer protocol header that
      follows the compressible chain of subheaders, for example ICMP
      headers, DVMRP headers, or tunneled IPX headers,

   c) use information obtained from a resource manager, for example if a
      resource manager requests compression for a particular packet
      stream and provides a way to identify packets belonging to that
      packet stream,

   d) use any other relevant information, for example if routes flap and
      the hop limit (TTL) field in a packet stream changes frequently
      between n and n+k, a compressor may choose to group the packets
      into two different packet streams.

   A compressor is also free not to group packets into packet streams
   for compression, letting some packets keep their regular headers and
   passing them through unmodified.

   As long as the rules for when to send full headers for a non-TCP
   packet stream are followed and subheaders are compressed as specified
   in this document, the decompressor is able to reconstruct a
   compressed header correctly regardless of how packets are grouped
   into packet streams.

4.1  Guidelines for grouping packets

   In this section we give OPTIONAL guidelines for how a compressor may
   group packets into packet streams for compression.

   Defining fields

      The defining fields of a header should be present and identical in
      all packets belonging to the same packet stream.  These fields are
      marked DEF in section 7. The defining fields include the flow
      label, source and destination addresses of IP headers, final
      destination address in routing headers, the next header fields
      (for IPv6), the protocol field (IPv4), port numbers (UDP and TCP),
      and the SPI in authentication and encryption headers.

   Fragmented packets

      Fragmented and unfragmented packets should never be grouped
      together in the same packet stream. The Identification field of
      the Fragment header or IPv4 header should not be used to identify
      the packet stream. If it was, the first fragment of a new packet
      would cause a compression slow-start.

      No field after a Fragment Header, or an IPv4 header for a
      fragment, should be used for grouping purposes.

   Upper protocol identification

      The first next header field identifying a header not described in
      section 7 should be used for identifying packet streams, i.e., all
      packets with the same DEF fields and the same upper protocol
      should be grouped together.

   TTL field (Hop Limit field)

      A sophisticated implementation might monitor the TTL (Hop Limit)
      field and if it changes frequently use it as a DEF field. This can
      occur when there are frequent route flaps so that packets traverse
      different paths through the internet.

   Traffic Class field (IPv6), Type of Service field (IPv4)

      It is possible that the Traffic Class field of the IPv6 header and
      the Type of Service of the IPv4 header will change frequently
      between packets with otherwise identical DEF fields.  A
      sophisticated implementation should watch out for this and be
      prepared to use these fields as defining fields.

   When IP packets are tunneled they are encapsulated with an additional
   IP header at the tunnel entry point and then sent to the tunnel
   endpoint. To group such packets into packet streams, the inner
   headers should also be examined to determine the packet stream.  If
   this is not done, full headers will be sent each time the headers of
   the inner IP packet changes.  So when a packet is tunneled, the
   identifying fields of the inner subheaders should be considered in
   addition to the identifying fields of the initial IP header.

   An implementation can use other fields for identification than the
   ones described here. If too many fields are used for identification,
   performance might suffer because more CIDs will be used and the wrong
   CIDs might be reused when new flows need CIDs. If too few fields are
   used for identification, performance might suffer because there are
   too frequent changes to the context.

   We stress that these guidelines are educated guesses. When IPv6 is
   widely deployed and IPv6 traffic can be analyzed, we might find that
   other grouping algorithms perform better. We also stress that if the
   grouping fails, the result will be bad performance but not incorrect
   decompression. The decompressor can do its task regardless of how the
   grouping algorithm works.

5.  Size Issues

5.1.  Context Identifiers

   Context identifiers can be 8 or 16 bits long.  Their size is not
   relevant for finding the context.  An 8-bit CID with value two and a
   16-bit CID with value two are equivalent.

   The CID spaces for TCP and non-TCP are separate, so a TCP CID and a
   non-TCP CID never identify the same context.  Even if they have the
   same value. This doubles the available CID space while using the same
   number of bits for CIDs.  It is always possible to tell whether a
   full or compressed header is for a TCP or non-TCP packet, so no
   mixups can occur.

   Non-TCP compressed headers encode the size of the CID using one bit
   in the second octet of the compressed header. The 8-bit CID allows a
   minimum compressed header size of 2 octets for non-TCP packets, the
   CID uses the first octet and the size bit and the 6-bit Generation
   value fit in the second octet.

   For TCP the only available CID size is 8 bits as in [RFC-1144].  8
   bits is probably sufficient as TCP connections are always point-to-
   point.

   The 16 bit CID size may not be needed for point-to-point links; it is
   intended for use on multi-access links where a larger CID space may
   be needed for efficient selection of CIDs.

   The major difficulty with multi-access links is that several
   compressors share the CID space of a decompressor.  CIDs can no
   longer be selected independently by the compressors as collisions may
   occur.  This problem may be resolved by letting the decompressors
   have a separate CID space for each compressor.  Having separate CID
   spaces requires that decompressors can identify which compressor sent
   the compressed packet, perhaps by utilizing link-layer information as
   to who sent the link-layer frame.  If such information is not
   available, all compressors on the multi-access link may be
   enumerated, automatically or otherwise, and supply their number as
   part of the CID. This latter method requires a large CID space.

5.2.  Size of the context

   The size of the context SHOULD be limited to simplify implementation
   of compressor and decompressor, and put a limit on their memory
   requirements.  However, there is no upper limit on the size of an
   IPv6 header as the chain of extension headers can be arbitrarily
   long.  This is a problem as the context is essentially a stored
   header.

   The configurable parameter MAX_HEADER (see section 14) represents the
   maximum size of the context, expressed as the maximum sized header
   that can be stored as context. When a header is larger than
   MAX_HEADER, only part of it is stored as context.  An implementation
   MUST NOT compress more than the initial MAX_HEADER octets of a
   header.  An implementation MUST NOT partially compress a subheader.

   Thus, the part of the header that is stored as context and is
   compressed is the longest initial sequence of entire subheaders that
   is not larger than MAX_HEADER octets.

5.3.  Size of full headers

   It is desirable to avoid increasing the size of packets with full
   headers beyond their original size, as their size may be optimized
   for the MTU of the link. Since we assume that the link layer
   implementation provides the length of packets, we can use the length
   fields in full headers to pass the values of the CID and the
   generation to the decompressor.

   This requires that the link-layer must not add padding to the
   payload, at least not padding that can be delivered to the
   destination link user. It is also required that no extra padding is
   added after UDP data or in tunneled packets. This allows values of
   length fields to be calculated from the length of headers and the
   length of the link-layer frame.

   The generation requires one octet and the CID may require up to 2
   octets.  There are length fields of 2 octets in the IPv6 Base Header,
   the IPv4 header, and the UDP header.

   A full TCP header will thus have at least 2 octets available in the
   IP header to pass the 8 bit CID, which is sufficient. There will be
   more than two octets available if there is more than one IP header.

   [RFC-1144] uses the 8 bit Protocol field of the IPv4 header to pass
   the CID. We cannot use the corresponding method as the sequence of
   IPv6 extension headers is not fixed and CID values are not disjoint
   from the legal values of Next Header fields.

   An IPv6/UDP or IPv4/UDP packet will have 4 octets available to pass
   the generation and the CID, so all CID sizes may be used. Fragmented
   or encrypted packet streams may have only 2 octets available to pass
   the generation and CID.  Thus, 8-bit CIDs may be the only CID sizes
   that can be used for such packet streams.  When IPv6/IPv4 or
   IPv4/IPv6 tunneling is used, there will be at least 4 octets
   available, and both CID sizes may be used.

   The generation value is passed in the higher order octet of the first
   length field in the full header. When only one length field is
   available, the 8-bit CID is passed in the low order octet.  When two
   length fields are available, the lowest two octets of the CID are
   passed in the second length field and the low order octet of the
   first length field carries the highest octet of the CID.

5.3.1.  Use of length fields in full TCP headers

   Use of first length field:

                                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                        Length field   | LSB of pkt nr |      CID      |
                                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Use of second length field if available:

                                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Second length field  | MSB of pkt nr |       0       |
                                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Pkt nr is short for packet sequence number, described in section
   11.2.

5.3.2.  Use of length fields in full non-TCP headers

   Full non-TCP headers with 8-bit CID:

                                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  First length field   |0|D| Generation|      CID      |
                                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   Second length field (if avail.) |       0       | Data (if D=1) |
                                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Full non-TCP headers with 16-bit CID:

                                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  First length field   |1|D| Generation| Data (if D=1) |
                                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

                                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                  Second length field  |              CID              |
                                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   The first bit in the first length field indicates the length of the
   CID.  The Data field is zero if D is zero. The use of the D bit and
   Data field is explained in section 12.

6.  Compressed Header Formats

   This section uses some terminology (DELTA, RANDOM) defined in section
   7.

      a) COMPRESSED_TCP format (similar to [RFC 1144]):

            +-+-+-+-+-+-+-+-+
            |      CID      |
            +-+-+-+-+-+-+-+-+
            |R O I P S A W U|
            +-+-+-+-+-+-+-+-+
            |               |
            +  TCP Checksum +
            |               |
            +-+-+-+-+-+-+-+-+
            | RANDOM fields, if any (see section 7)   (implied)
             - - - - - - - -
            | R-octet       |                         (if R=1)
             - - - - - - - -
            | Urgent Pointer Value                    (if U=1)
             - - - - - - - -
            | Window Delta                            (if W=1)
             - - - - - - - -
            | Acknowledgment Number Delta             (if A=1)
             - - - - - - - -
            | Sequence Number Delta                   (if S=1)
             - - - - - - - -
            | IPv4 Identification Delta               (if I=1)
             - - - - - - - -
            |  Options                                (if O=1)
             - - - - - - - -

   The latter flags in the second octet (IPSAWU) have the same meaning
   as in [RFC-1144], regardless of whether the TCP segments are carried
   by IPv6 or IPv4. The C bit has been eliminated because the CID is
   always present. The context associated with the CID keeps track of
   the IP version and what RANDOM fields are present.  The order between
   delta fields specified here is exactly as in [RFC-1144]. An
   implementation will typically scan the context from the beginning and
   insert the RANDOM fields in order. The RANDOM fields are thus placed
   before the DELTA fields of the TCP header in the same order as they
   occur in the original uncompressed header.

   The I flag is zero unless an IPv4 header immediately precedes the TCP
   header. The combined IPv4/TCP header is then compressed as a unit as
   described in [RFC-1144]. Identification fields in IPv4 headers that
   are not immediately followed by a TCP header are RANDOM.

   If the O flag is set, the Options of the TCP header were not the same
   as in the previous header. The entire Option field are placed last in
   the compressed TCP header.

   If the R flag is set, there were differences between the context and
   the Reserved field (6 bits) in the TCP header or bit 6 or 7 of the
   TOS octet (Traffic Class octet) in a IPv4 header (IPv6 header) that
   immediately precedes the TCP header.  An octet with the actual values
   of the Reserved field and bit 6 and 7 of the TOS or Traffic Class
   field is then placed immediately after the RANDOM fields.  Bits 0-5
   of the passed octet is the actual value of the Reserved field, and
   bits 6 and 7 are the actual values of bits 6 and 7 in the TOS or
   Traffic Class field. If there is no preceding IP header, bits 6 and 7
   are 0.  The octet passed with the R flag MUST NOT update the context.

   NOTE: The R-octet does not update the context because if it did, the
   nTCP checksum would not guard the receiving TCP from erroneously
   decompressed headers. Bits 6 and 7 of the TOS octet or Traffic Class
   octet is expected to change frequently due to Explicit Congestion
   Notification.

   See section 7.12 and [RFC-1144] for further information on how to
   compress TCP headers.

      b) COMPRESSED_TCP_NODELTA header format

          +-+-+-+-+-+-+-+-+
          |      CID      |
          +-+-+-+-+-+-+-+-+
          |  RANDOM fields, if any (see section 7)   (implied)
          +-+-+-+-+-+-+-+-+
          |  Whole TCP header except for Port Numbers
          +-+-+-+-+-+-+-+-+

      c) Compressed non-TCP header, 8 bit CID:
           0             7
          +-+-+-+-+-+-+-+-+
          |      CID      |
          +-+-+-+-+-+-+-+-+
          |0|D| Generation|
          +-+-+-+-+-+-+-+-+
          |      data     |                      (if D=1)
           - - - - - - - -
          | RANDOM fields, if any (section 7)    (implied)
           - - - - - - - -

      d) Compressed non-TCP header, 16 bit CID:
           0             7
          +-+-+-+-+-+-+-+-+
          |  msb of CID   |
          +-+-+-+-+-+-+-+-+
          |1|D| Generation|
          +-+-+-+-+-+-+-+-+
          |  lsb of CID   |
          +-+-+-+-+-+-+-+-+
          |      data     |                      (if D=1)
           - - - - - - - -
          | RANDOM fields, if any (section 7)    (implied)
           - - - - - - - -

   The generation, CID and optional one octet data are followed by
   relevant RANDOM fields (see section 7) as implied by the compression
   state, placed in the same order as they occur in the original
   uncompressed header, followed by the payload.

7.  Compression of subheaders

   This section gives rules for how the compressible chain of subheaders
   is compressed. These rules MUST be followed.  Subheaders that may be
   compressed include IPv6 base and extension headers, TCP headers, UDP
   headers, and IPv4 headers.  The compressible chain of subheaders
   extends from the beginning of the header

   a) up to but not including the first header that is not an IPv4
      header, an IPv6 base or extension header, a TCP header, or a UDP
      header, or

   b) up to and including the first TCP header, UDP header, Fragment
      Header, Encapsulating Security Payload Header, or IPv4 header for
      a fragment,

   whichever gives the shorter chain. For example, rules a) and b) both
   fit a chain of subheaders that contain a Fragment Header and ends at
   a tunneled IPX packet. Since rule b) gives a shorter chain, the
   compressible chain of subheaders stops at the Fragment Header.

   The following subsections are a systematic classification of how all
   fields in subheaders are expected to change.

   NOCHANGE    The field is not expected to change. Any change means
               that a full header MUST be sent to update the context.

   DELTA       The field may change often but usually the difference
               from the field in the previous header is small, so that
               it is cheaper to send the change from the previous value
               rather than the current value.  This type of compression
               is only used for TCP packet streams.

   RANDOM      The field must be included "as-is" in compressed headers,
               usually because it changes unpredictably.

   INFERRED    The field contains a value that can be inferred from
               other values, for example the size of the frame carrying
               the packet, and thus must not be included in the
               compressed header.

   The classification implies how a compressed header is constructed. No
   field that is NOCHANGE or INFERRED is present in a compressed header.
   A compressor obtains the values of NOCHANGE fields from the context
   identified by the compression identifier, and obtains the values of
   INFERRED fields from the link-layer implementation, e.g., from the
   size of the link-layer frame, or from other fields, e.g., by
   recalculating the IPv4 header checksum.  DELTA fields are encoded as
   the difference to the value in the previous packet in the same packet
   stream.  The decompressor must update the context by adding the value
   in the compressed header to the value in its context. The result is
   the proper value of the field.  RANDOM fields must be sent "as-is" in
   the compressed header.  RANDOM fields must occur in the same order in
   the compressed header as they occur in the full header.

   Fields that may optionally be used to identify what packet stream a
   packet belongs to according to section 4.1 are marked with the word
   DEF.  To a compressor using the optional guidelines from section 4.1,
   any difference in corresponding DEF fields between two packets
   implies that they belong to different packet streams. Moreover, if a
   DEF field is present in one packet but not in another, the packets
   belong to different packet streams.

7.1.  IPv6 Header [IPv6, section 3]

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |Version| Traffic Class |               Flow Label              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |         Payload Length        |  Next Header  |   Hop Limit   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                                                               |
      +                         Source Address                        +
      |                                                               |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                                               |
      +                                                               +
      |                                                               |
      +                      Destination Address                      +
      |                                                               |
      +                                                               +
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         Version                   NOCHANGE (DEF)
         Traffic Class             NOCHANGE (might be DEF, see sect 4.1)
                                            (see also sect 6 a)
         Flow Label                NOCHANGE (DEF)
         Payload Length            INFERRED
         Next Header               NOCHANGE
         Hop Limit                 NOCHANGE (might be DEF, see sect 4.1)
         Source Address            NOCHANGE (DEF)
         Destination Address       NOCHANGE (DEF)

   The Payload Length field of encapsulated headers must correspond to
   the length value of the encapsulating header. If not, the header
   chain MUST NOT be compressed.

   NOTE: If this the IP header closest to a TCP header, bit 7 of the
   Traffic Class field can be passed using the R-flag of the compressed
   TCP header. See section 6 a).

   This classification implies that the entire IPv6 base header will be
   compressed away.

7.2.  IPv6 Extension Headers [IPv6, section 4]

   What extension headers are present and the relative order of them is
   not expected to change in a packet stream.  Whenever there is a
   change, a full packet header must be sent.  All Next Header fields in
   IPv6 base header and IPv6 extension headers are NOCHANGE.

7.3.  Options [IPv6, section 4.2]

   The contents of Hop-by-hop Options and Destination Options extension
   headers are encoded with TLV "options" (see [IPv6]):

            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
            |  Option Type  |  Opt Data Len |  Option Data
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -

   Option Type and Opt Data Len fields are assumed to be fixed for a
   given packet stream, so they are classified as NOCHANGE.  The Option
   data is RANDOM unless specified otherwise below.

   Padding

       Pad1 option

            +-+-+-+-+-+-+-+-+
            |       0       |
            +-+-+-+-+-+-+-+-+

          Entire option is NOCHANGE.

       PadN option

            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -
            |       1       |  Opt Data Len |  Option Data
            +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- - - - - - - - -

          All fields are NOCHANGE.

7.4.  Hop-by-Hop Options Header [IPv6, section 4.3]

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Next Header  |  Hdr Ext Len  |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
    |                                                               |
    .                                                               .
    .                            Options                            .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Next Header          NOCHANGE
       Hdr Ext Len          NOCHANGE

       Options              TLV coded values and padding.
                            Classified according to 7.3 above, unless
                            being a Jumbo Payload option (see below).

   Jumbo Payload option
                                    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                    |      194      |Opt Data Len=4 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                     Jumbo Payload Length                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   First two fields are NOCHANGE and Jumbo Payload Length INFERRED.
   (frame length must be supplied by link layer implementation).

        NOTE: It is silly to compress the headers of a packet carrying a
        Jumbo Payload Option since the relative header overhead is
        negligible. Moreover, it is usually a bad idea to send such
        large packets over low- and medium-speed links.

7.5.  Routing Header [IPv6, section 4.4]

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Next Header  |  Hdr Ext Len  |  Routing Type | Segments Left |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    .                                                               .
    .                       type-specific data                      .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   All fields of the Routing Header are NOCHANGE.

   If the Routing Type is not recognized, it is impossible to determine
   the final Destination Address unless the Segments Left field has the
   value zero, in which case the Destination Address is the final
   Destination Address in the basic IPv6 header.

   In the Type 0 Routing Header, the last address is DEF if (Segments
   Left > 0).

   Routing Headers are compressed away completely.  This is a big win as
   the maximum size of the Routing Header is 392 octets.  Moreover, Type
   0 Routing Headers with one address, size 24 octets, are used by
   Mobile IP.

7.6.  Fragment Header [IPv6, section 4.5]

   The first fragment of a packet has Fragment Offset = 0 and the chain
   of subheaders extends beyond its Fragment Header. If a fragment is
   not the first (Fragment Offset not 0), there are no subsequent
   subheaders (unless the chain of subheaders in the first fragment
   didn't fit entirely in the first fragment).

   Since packets may be reordered before reaching the compression point,
   and some fragments may follow other routes through the network, a
   compressor cannot rely on seeing the first fragment before other
   fragments. This implies that information in subheaders following the
   Fragment Header of the first fragment cannot be examined to determine
   the proper packet stream for other fragments.

   It is possible to design compression schemes that can compress
   subheaders after the Fragment Header, at least in the first fragment,
   but to avoid complicating the rules for sending full headers and the
   rules for compression and decompression, the chain of subheaders that
   follow a Fragment Header MUST NOT be compressed.

   The fields of the Fragment Header are classified as follows.

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Next Header  |   Reserved    |      Fragment Offset    |Res|M|
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                         Identification                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

          Next Header          NOCHANGE
          Reserved             NOCHANGE
          Res                  RANDOM
          M flag               RANDOM
          Fragment Offset      RANDOM
          Identification       RANDOM

   This classification implies that a Fragment Header is compressed down
   to 6 octets. The minimum IPv6 MTU is 1280 octets so most fragments
   will be at least 1280 octets. Since the 6 octet overhead of the
   compressed fragment header is amortized over a fairly large packet,
   the additional complexity of more sophisticated compression schemes
   is not justifiable.

          NOTE: The Identification field is RANDOM instead of NOCHANGE
          to avoid one compression slow-start per original packet.

   Grouping of fragments according to the optional guidelines in
   section4.1:

       Fragments and unfragmented packets should not be grouped
       together.

       Port numbers cannot be used to identify the packet stream because
       port numbers are not present in every fragment.  To adhere to the
       uniqueness rules for the Identification value, a fragmented
       packet stream is identified by the combination of Source Address
       and (final) Destination Address.

          NOTE: The Identification value is NOT used to identify the
          packet stream. This avoids using a new CID for each packet and
          saves the cost of the associated compression slow-start.  We
          expect that the unfragmentable part of the headers will not
          change too frequently, if it does thrashing may occur.

7.7.  Destination Options Header [IPv6, section 4.6]

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Next Header  |  Hdr Ext Len  |                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
    |                                                               |
    .                                                               .
    .                            Options                            .
    .                                                               .
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Next Header          NOCHANGE
    Hdr Ext Len          NOCHANGE

    Options              TLV coded values and padding.
                           Compressed according to 7.3 above.

   The only Destination Options defined in [IPv6] are the padding
   options.

7.8.  No Next Header [IPv6, section 4.7]

   Covered by rules for IPv6 Header Extensions (7.2).

7.9.  Authentication Header [RFC-2402, section 3.2]

     1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8
    +---------------+---------------+---------------+---------------+
    | Next Header   | Length        |           RESERVED            |
    +---------------+---------------+---------------+---------------+
    |                Security Parameters Index (SPI)                |
    +---------------+---------------+---------------+---------------+
    |                                                               |
    +     Authentication Data (variable number of 32-bit words)     |
    |                                                               |
    +---------------+---------------+---------------+---------------+

    Next Header          NOCHANGE
    Length               NOCHANGE
    Reserved             NOCHANGE
    SPI                  NOCHANGE (DEF)
    Authentication Data  RANDOM

   [RFC-1828] specifies how to do authentication with keyed MD5, the
   authentication method all IPv6 implementations must support.  For
   this method, the Authentication Data is 16 octets.

7.10.  Encapsulating Security Payload Header [RFC-2406, section 3.1]

   This header implies that the subsequent parts of the packet are
   encrypted. Thus, no further header compression is possible on
   subsequent headers as encryption is typically already performed when
   the compressor sees the packet.

   However, when the ESP Header is used in tunnel mode an entire IP
   packet is encrypted, and the headers of that packet MAY be compressed
   before the packet is encrypted at the entry point of the tunnel.
   This means that it must be possible to feed an IP packet and its
   length to the decompressor, as if it came from the link-layer. The
   mechanisms for dealing with reordering described in section 11 MUST
   also be used, as packets can be reordered in a tunnel.

    +---------------+---------------+---------------+---------------+
    |        Security Association Identifier (SPI), 32 bits         |
    +===============+===============+===============+===============+
    |            Opaque Transform Data, variable length             |
    +---------------+---------------+---------------+---------------+

    SPI                          NOCHANGE (DEF)
    Opaque Transform Data        RANDOM

   Everything after the SPI is encrypted and is not compressed.

7.11.  UDP Header

   The UDP header is described in [RFC-768].

   The Next Header field (IPv6) or Protocol field (IPv4) in the
   preceding subheader is DEF.

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          Source Port          |       Destination Port        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |            Length             |           Checksum            |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Source Port          NOCHANGE (DEF)
    Destination Port     NOCHANGE (DEF)
    Length               INFERRED
    Checksum             RANDOM, unless it is zero,
                           in which case it is NOCHANGE.

   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.

   The UDP header is typically compressed down to 2 octets, the UDP
   checksum.  When the UDP checksum is zero (which it cannot be with
   IPv6), it is likely to be so for all packets in the flow and is
   defined to be NOCHANGE. This saves 2 octets in the compressed header.

7.12.  TCP Header

   The TCP header is described in [RFC-793].

   The Next Header field (IPv6) or Protocol field (IPv4) in the
   preceding subheader is DEF.

    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |          Source Port          |       Destination Port        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Sequence Number                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Acknowledgment Number                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Offset| Reserved  |U|A|P|R|S|F|            Window             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |           Checksum            |         Urgent Pointer        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Options                    |    Padding    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    U, A, P, R, S, and F stands for Urg, Ack, Psh, Rst, Syn, and Fin.

   There are two ways to compress the TCP header.

7.12.1. Compressed with differential encoding

   Source Port           NOCHANGE  (DEF)
   Destination Port      NOCHANGE  (DEF)
   Sequence Number       DELTA
   Acknowledgment Number DELTA
   Offset                NOCHANGE
   Reserved              DELTA               (if differs from context,
                                              set R-flag in flag octet
                                              and send absolute value
                                              as described in 6 a.)
   Urg,Psh               RANDOM              (placed in flag octet)
   Ack                   INFERRED to be 1
   Rst,Syn,Fin           INFERRED to be 0
   Window                DELTA               (if change in Window,
                                              set W-flag in flag octet
                                              and send difference)
   Checksum              RANDOM
   Urgent Pointer        DELTA               (if Urg is set, send
                                              absolute value)
   Options, Padding      DELTA               (if change in Options,
                                              set O-flag and send
                                              whole Options, Padding)

   A packet with a TCP header compressed according to the above must be
   indicated to be of type COMPRESSED_TCP.  The compressed header is
   described in section 6.

   This method is essentially the differential encoding techniques of
   Jacobson, described in [RFC-1144], the differences being the placement
   of the compressed TCP header fields (see section 6), the use of the
   O-flag, the use of the R-flag, and elimination of the C-flag.  The
   O-flag allows compression of the TCP header when the Timestamp option
   is used and the Options fields changes with each header.

   DELTA values (except for Reserved field and Options, Padding) MUST be
   coded as in [RFC-1144].  A Reserved field value passed with the R-flag
   MUST NOT update the context at compressor or decompressor.

7.12.2. Without differential encoding

       Source Port           NOCHANGE  (DEF)
       Destination Port      NOCHANGE  (DEF)

       (all the rest)        RANDOM

   The Identification field in a preceding IPv4 header is RANDOM.

   A packet with a TCP header compressed according to the above must be
   indicated to be of type COMPRESSED_TCP_NODELTA.  It uses the same CID
   space as COMPRESSED_TCP packets, and the header MUST be saved as
   context. The compressed header is described in section 6.

   This packet type can be sent as the response to a header request
   instead of sending a full header, can be used over links that reorder
   packets, and can be sent instead of a full header when there are
   changes that cannot be represented by a compressed header. A
   sophisticated compressor can switch to sending only
   COMPRESSED_TCP_NODELTA headers when the packet loss frequency is high.

7.13.  IPv4 header [RFC-791, section 3.1]

     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
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |Version|  IHL  |Type of Service|          Total Length         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |         Identification        |Flags|      Fragment Offset    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Time to Live |    Protocol   |         Header Checksum       |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Source Address                          |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Destination Address                        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Options                    |    Padding    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   There are two ways to compress the IPv4 header

   a) If the IPv4 header is not for a fragment (MF flag is not set and
      Fragment Offset is zero) and there are no options (IHL is 5), it
      is classified as follows

       Version              NOCHANGE   (DEF)
       IHL                  NOCHANGE   (DEF, must be 5)
       Type of Service      NOCHANGE   (might be DEF, see sect 4.1)
                                       (see also 6 a)
       Total Length         INFERRED   (from link-layer implementation
                                        or encapsulating IP header)

       Identification       DELTA/     (If the Protocol field has the
                                       (value corresponding to TCP)
                            RANDOM     (otherwise)

       Flags                NOCHANGE   (MF flag must not be set)
       Fragment Offset      NOCHANGE   (must be zero)
       Time to Live         NOCHANGE   (might be DEF, see sect 4.1)
       Protocol             NOCHANGE
       Header Checksum      INFERRED   (calculated from other fields)
       Source Address       NOCHANGE   (DEF)
       Destination Address  NOCHANGE   (DEF)
       Options, Padding                (not present)

       Note: When a TCP header immediately follows, the IPv4 and TCP
       header MUST be compressed as a unit as described in section 6.
       Bits 6 and 7 of the Type of Service field (bits 14 and 15 of the
       first word) can then be passed using the R-flag (see section 6
       a).

   b) If the IPv4 header is for a fragment (MF bit set or Fragment
       Offset nonzero), or there are options (IHL > 5), all fields are
       RANDOM (i.e., if the header is compressed all fields are sent
       as-is and not compressed). This classification allows compression
       of the tunnel header, but not the fragment header, when fragments
       are tunneled. If the IPv4 header is for a fragment it ends the
       compressible chain of subheaders, i.e., it must be the last
       subheader to be compressed.  If the IPv4 header has options but
       is not for a fragment it does not end the compressible chain of
       subheaders, so subsequent subheaders can be compressed.

   A compressor that follows the optional guidelines of section 4.1 will
   in case a) use the Version, Source Address and Destination Address to
   define the packet stream, together with the fact that there are no
   IPv4 options and that this is not a fragment.

   Case b) can define two kinds of packet streams depending on whether
   the IPv4 header is for a fragment or not.

   If the IPv4 header in case b) is for a fragment, a compressor
   following the optional guidelines will use that fact together with
   the Version, Source Address, and Destination Address to determine the
   packet stream.

   If the IPv4 header in case b) is not for a fragment, it must have
   options. A compressor following the optional guidelines will use that
   fact, but not the size of the options, together with the Version,
   Source Address, and Destination Address to determine the packet
   stream.

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

     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               :
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

    Protocol                             NOCHANGE
    Original Source Address Present (S)  NOCHANGE
    reserved                             NOCHANGE
    Header Checksum                      INFERRED (calculated from
                                           other values)
    Original Destination Address         NOCHANGE
    Original Source Address              NOCHANGE (present only
                                                   if S=1)

   This header is likely to be used by Mobile IP.

8.  Changing context identifiers

   On a point-to-point link, the compressor has total knowledge of what
   CIDs are in use at the decompressor and may change what CID a packet
   stream uses or reuse CIDs at will.

   Each non-TCP CID is associated with a context with a generation
   value. To avoid too rapid generation wrap-around and potential
   incorrect decompression, an implementation MUST avoid wrap-around of
   the generation value in less than MIN_WRAP seconds (see section 14).

   To aid in avoiding wrap-around, the generation value associated with
   a CID MUST NOT be reset when changing to a new packet stream.
   Instead, a compressor MUST increment the generation value by one when
   using the CID for a new non-TCP packet stream.

9.  Rules for dropping or temporarily storing packets

   When a decompressor receives a packet with a compressed TCP header
   with CID C, it MUST be discarded when the context for C has not been
   initialized by a full header.

   When a decompressor receives a packet with a compressed non-TCP
   header with CID C and generation G, the header must not be
   decompressed using the current context when

      a) the decompressor has been disconnected from the compressor for
          more than MIN_WRAP seconds, because the context might be
          obsolete even if it has generation G.

      b) the context for C has a generation other than G.

   In case a) and b) the packet may either be

      i)  discarded immediately, or else

      ii) stored temporarily until the context is updated by a packet
          with a full non-TCP header with CID C and generation G, after
          which the header can be decompressed.

          Packets stored in this manner MUST be discarded when

            *)  receiving full or compressed non-TCP headers with CID C
                and a generation other than G,

            *)  the decompressor has not received packets with CID C in
                the last MIN_WRAP seconds.

   When full headers are lost, a decompressor can receive compressed
   non-TCP headers with a generation value other than the generation of
   its context.  Rule ii) allows the decompressor to store such headers
   until they can be decompressed using the correct context.

10. Low-loss header compression for TCP

   Since fewer bits are transmitted per packet with header compression,
   the packet loss rate is lower with header compression than without,
   for a fixed bit-error rate.  This is beneficial for links with high
   bit-error rates such as wireless links.

   However, since TCP headers are compressed using differential
   encoding, a single lost TCP segment can ruin an entire TCP sending
   window because the context is not incremented properly at the
   decompressor.  Subsequent headers will therefore be decompressed to
   be different than before compression and discarded by the TCP
   receiver because the TCP checksum fails.

   A TCP connection in the wide area where the last hop is over a
   medium-speed lossy link, for example a wireless LAN, will then have
   poor performance with traditional header compression because the
   delay-bandwidth product is relatively large and the bit-error rate
   relatively high. For a 2 Mbit/s wireless LAN and an end-to-end RTT of
   200 ms, the delay-bandwidth product is 50 kbyte.  That is equivalent
   to about 97 512-octet segments with compressed headers.  Each loss
   can thus be multiplied by a factor of 100.

   This section describes two simple mechanisms for quick repair of the
   context. With these mechanisms header compression will improve TCP
   throughput over lossy links as well as links with low bit-error
   rates.

10.1.  The "twice" algorithm

   The decompressor may compute the TCP checksum to determine if its
   context is not updated properly. If the checksum fails, the error is
   assumed to be caused by a lost segment that did not update the
   context properly. The delta of the current segment is then added to
   the context again on the assumption that the lost segment contained
   the same delta as the current. By decompressing and computing the TCP
   checksum again, the decompressor checks if the repair succeeded or if
   the delta should be applied once more.

   Analysis of traces of various TCP bulk transfers show that applying
   the delta of the current segment one or two times will repair the
   context for between 83 and 99 per cent of all single-segment losses
   in the data stream. For the acknowledgment stream, the success rate
   is smaller due to the delayed ack mechanism of TCP. The "twice"
   mechanism repairs the context for 53 to 99 per cent of the losses in
   the acknowledgment stream.  A sophisticated implementation of this
   idea would determine whether the TCP stream is an acknowledgment or
   data stream and determine the segment size by observing the stream of
   full and compressed headers.  Trying deltas that are small multiples
   of the segment size will result in even higher rates of successful
   repairs for acknowledgment streams.

10.2.  Header Requests

   The relatively low success rate for the "twice" algorithm for TCP
   acknowledgment streams calls for an additional mechanism for
   repairing the context at the decompressor. When the decompressor
   fails to repair the context after a loss, the decompressor may
   optionally request a full header from the compressor.  This is
   possible on links where the decompressor can identify the compressor
   and send packets to it.

   On such links, a decompressor may send a CONTEXT_STATE packet back to
   the compressor to indicate that one or more contexts are invalid.  A
   decompressor SHOULD NOT transmit a CONTEXT_STATE packet every time a
   compressed packet refers to an invalid context, but instead should
   limit the rate of transmission of CONTEXT_STATE packets to avoid
   flooding the reverse channel. A CONTEXT_STATE packet can indicate
   that several contexts are out of date, this technique SHOULD be used
   instead of sending several separate packets. The following diagram
   shows the format of a CONTEXT_STATE packet.

                           0   1   2   3   4   5   6   7
                        +---+---+---+---+---+---+---+---+
                        |     TCP header request = 3    |
                        +---+---+---+---+---+---+---+---+
                        |           CID count           |
                        +---+---+---+---+---+---+---+---+
                        |              CID              |
                        +---+---+---+---+---+---+---+---+
                        |              CID              |
                        +---+---+---+---+---+---+---+---+
                                               ...
                        +---+---+---+---+---+---+---+---+
                        |              CID              |
                        +---+---+---+---+---+---+---+---+

   The first octet is a type code to allow the CONTEXT_STATE packet type
   to be shared for other compression protocols that are (see [CRTP]) or
   may be defined in parallel with this one. When used for TCP header
   requests the type code has the value 3, and the remainder of the
   packet is a sequence of CIDs preceded by a one-octet count of the
   number of CIDs.

   On receipt of a CONTEXT_STATE packet, the compressor MUST mark the
   CIDs invalid to ensure that the next packet emitted in those packet
   streams are FULL_HEADER or COMPRESSED_TCP_NODELTA packets.

   Header requests are an optimization, so loss of a CONTEXT_STATE
   packet does not affect the correct operation of TCP header
   compression.  When a CONTEXT_STATE packet is lost, eventually a new
   one will be transmitted or TCP will timeout and retransmit.  The big
   advantage of using header requests is that TCP acknowledgment streams
   can be repaired after a roundtrip-time over the lossy link.  This
   will typically avoid a TCP timeout and unnecessary retransmissions.
   The lower packet loss rate due to smaller packets will then result in
   higher throughput because the TCP window can grow larger between
   losses.

11.  Links that reorder packets

   Some links reorder packets, for example multi-hop radio links that
   use deflection routing to route around congested nodes.  Packets
   routed different ways can then arrive at the destination in a
   different order than they were sent.

11.1.  Reordering in non-TCP packet streams

   Compressed non-TCP headers do not change the context, and neither do
   full headers that refresh it.  There can be problems only when a full
   header that changes the context arrives out of order.  There are two
   cases:

       - A packet with a full header with generation G arrives *after*
         a packet with a compressed header with generation G.  This case
         is covered by rule b) ii) in section 9.

       - A packet with a full header with generation G arrives *before*
         a packet with a compressed header with generation G-1 (modulo
         64).  The decompressor MAY then keep both versions of the
         context around for a while to be able to decompress subsequent
         compressed headers with generation G-1 (modulo 64).  The old
         context MUST be discarded after MIN_WRAP seconds.

11.2.  Reordering in TCP packet streams

   A compressor may avoid sending COMPRESSED_TCP headers and only send
   COMPRESSED_TCP_NODELTA headers when there is reordering over the
   link.  Compressed headers will typically be 17 octets with that
   method, significantly larger than the usual 4-7 octets.

   To achieve better compression rates the following method, adding only
   two octets to the compressed header for a total of 6-9 octets, may be
   used.  A packet sequence number, incremented by one for every packet
   in the TCP stream, is then associated with each compressed and full
   header.  This allows the decompressor to place the packets in the
   correct sequence and apply their deltas to the context in the correct
   order.  A simple sliding window scheme is used to place the packets
   in the correct order.

   Two octets are needed for the packet sequence numbers.  One octet
   gives only 256 sequence numbers.  In a sliding window scheme the
   window should be no larger than half of the sequence number space, so
   packets can not arrive more than 127 positions out-of-sequence. This
   is equivalent to a delay of 260 ms on 2 Mbit/s links with 512 octet
   segments.  Delays of that order are not uncommon over wide-area
   Internet connections.  However, two octets giving 2^16 = 65536 values
   should be sufficient.

   Full TCP/IP headers will only have space for one octet of sequence
   number when there is no tunneling. It is not feasible to increase the
   size of full headers since the packet size might be optimized for the
   MTU of the link. Therefore only the least significant octet of the
   packet sequence number can be placed in such full headers. We believe

   that such full headers can be positioned correctly frequently enough
   with only the least significant octet of the packet sequence number
   available.

   The packet sequence number zero MUST be skipped over.  Avoiding zero
   takes care of a problem that can occur when the TCP window scale
   option is used to enlarge the TCP window. When exactly 2^16 octets of
   TCP data is lost, a compressed header will be decompressed
   incorrectly without being detected by the TCP checksum. TCP segment
   sizes are often a power of two.  So by using a packet sequence number
   space that is not a power of two either the TCP sequence number or
   the packet sequence number will differ when 2^16 octets are lost.
   Whenever a compressor sees the window scale option on a SYN segment,
   it MUST use packet sequence numbers when subsequently compressing
   that packet stream.

   In compressed TCP headers the two octet packet sequence number MUST
   be placed immediately after the TCP Checksum.  See section 5.3 for
   placement of packet sequence numbers in full headers.

12.  Hooks for additional header compression

   The following hook is supplied to allow additional header compression
   schemes for headers on top of UDP. The initial chain of subheaders is
   then compressed as described here, and the other header compression
   scheme is applied to the header above the UDP header. An example of
   such additional header compression is Compressed RTP by Casner and
   Jacobson [CRTP]. To allow some error detection, such schemes
   typically need a sequence number that may need to be passed in full
   headers as well as compressed UDP headers.

   The D-bit and Data octet (see section 6) provides the necessary
   mechanism. When a sequence number, say, needs to be passed in a
   FULL_HEADER or COMPRESSED_NON_TCP header, the D-bit is set and the
   sequence number is placed in the Data field. The decompressor must
   then extract and make the Data field available to the additional
   header compression scheme.

   Use of additional header compression schemes like CRTP must be
   negotiated. The D-bit and Data octet mechanism must automatically be
   enabled whenever use of additional header compression schemes has
   been negotiated.

13.  Demultiplexing

   For each link layer, there must be a document specifying how the
   various packet types used by IP header compression is indicated.
   Such a document exists for PPP [PPP-HC].  This section gives OPTIONAL
   guidelines on how packet types may be indicated by a specific link-
   layer.

   It is necessary to distinguish packets with regular IPv4 headers,
   regular IPv6 headers, full IPv6 packets, full IPv4 packets,
   compressed TCP packets, compressed non-TCP packets, and CONTEXT_STATE
   packets.

   The decision to use a distinct ethertype (or equivalent) for IPv6 has
   already been taken, which means that link-layers must be able to
   indicate that a packet is an IPv6 packet.

   IP header compression requires that the link-layer implementation can
   indicate four kinds of packets: COMPRESSED_TCP for format a) in
   section 6, COMPRESSED_TCP_NODELTA for format b), COMPRESSED_NON_TCP
   for formats c) and d), and CONTEXT_STATE as described in section
   11.2.  It is also desirable to indicate FULL_HEADERS at the link
   layer.

   Full headers can be indicated by setting the first bit of the Version
   field in a packet indicated to be an IPv6 packet.  In addition, one
   bit of the Version field is used to indicate if the first subheader
   is an IPv6 or an IPv4 header, and one bit is used to indicate if this
   full header carries a TCP CID or a non-TCP CID. The first four bits
   are encoded as follows:

      Version  Meaning
      -------  -------

      0110     regular IPv6 header

      1T*0     T=1 indicates a TCP header, T=0 indicates a non-TCP header
      1*V0     V=1 indicates a IPv6 header, V=0 indicates a IPv4 header

   If a link-layer cannot indicate the packet types for the compressed
   headers or CONTEXT_STATE, packet types that cannot be indicated could
   start with an octet indicating the packet type, followed by the
   header.

       First octet  Type of compressed header
       -----------   -------------------------

           0        COMPRESSED_TCP
           1        COMPRESSED_TCP_NODELTA
           2        COMPRESSED_NON_TCP
           3        CONTEXT_STATE

   The currently assigned CONTEXT_STATE type values are

       Value   Type                       Reference
       -----   -----                      ----------
         0     Reserved                   -
         1     IP/UDP/RTP w. 8-bit CID    [CRTP]
         2     IP/UDP/RTP w. 16-bit CID   [CRTP]
         3     TCP header request         Section 10.2

14.  Configuration Parameters

   Header compression parameters are negotiated in a way specific to the
   link-layer implementation. Such procedures for link-layer xxx needs
   to be specified in a document "IP header compression over xxx". Such
   a document exists for PPP [PPP-HC].

   The following parameter is fixed for all implementations of this
   header compression scheme.

     MIN_WRAP     - minimum time of generation value wrap around

          3 seconds.

   The following parameters can be negotiated between the compressor and
   decompressor. If not negotiated their values must be as specified by
   DEFAULT.

     F_MAX_PERIOD - Largest number of compressed non-TCP headers that
                    may be sent without sending a full header.

          DEFAULT is 256

          F_MAX_PERIOD must be at least 1 and at most 65535.

     F_MAX_TIME   - Compressed headers may not be sent more than
                    F_MAX_TIME seconds after sending last full header.

          DEFAULT is 5

          F_MAX_TIME must be at least 1 and at most 255.

          NOTE:  F_MAX_PERIOD and F_MAX_TIME should be lower when it is
                 likely that a decompressor loses its state.

     MAX_HEADER   - The largest header size in octets that may
                    be compressed.

          DEFAULT is 168 octets, which covers

                          - Two IPv6 base headers
                          - A Keyed MD5 Authentication Header
                          - A maximum-sized TCP header

          MAX_HEADER must be at least 60 octets and
                  at most 65535 octets.

     TCP_SPACE    - Maximum CID value for TCP.

          DEFAULT is 15   (which gives 16 CID values)

          TCP_SPACE must be at least 3 and at most 255.

     NON_TCP_SPACE    - Maximum CID value for non-TCP.

          DEFAULT is 15   (which gives 16 CID values)

          NON_TCP_SPACE must be at least 3 and at most 65535.

     EXPECT_REORDERING       - The mechanisms in section 11 are used.

          DEFAULT no.

15. Implementation Status

   A prototype using UDP as the link layer has been operational since
   March 1996. A NetBSD implementation for PPP has been operational
   since October 1996.

16.  Acknowledgments

   This protocol uses many ideas originated by Van Jacobson in the
   design of header compression for TCP/IP over slow-speed links [RFC-
   1144]. It has benefited from discussions with Stephen Casner and
   Carsten Bormann.

   We thank Craig Partridge for pointing out a problem that can occur
   when the TCP window scale option is used.  A solution to this problem
   relying on the packet sequence numbers used for reordering is
   described in section 11.2.

17.  Security Considerations

   The compression protocols in this document run on top of a link-layer
   protocol. The compression protocols themselves introduce no new
   additional vulnerabilities beyond those associated with the specific
   link-layer technology being used.

   Denial-of-service attacks are possible if an intruder can introduce
   (for example) bogus Full Header packets onto the link.  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.

   We advise implementors against identifying packet streams with the
   aid of information that is encrypted, even if such information
   happens to be available to the compressor. Doing so may expose
   traffic patterns.

18.  Authors' Addresses

   Mikael Degermark
   Department of Computer Science and Electrical Engineering
   Lulea University of Technology
   SE-971 87 Lulea, Sweden

   Phone: +46 920 91188
   Fax: +46 920 72831
   Mobile: +46 70 833 8933
   EMail: micke@sm.luth.se

   Bjorn Nordgren
   CDT/Telia Research AB
   Aurorum 6
   S-977 75 Lulea, Sweden

   Phone: +46 920 75400
   Fax: +46 920 75490
   EMail: bcn@lulea.trab.se, bcn@cdt.luth.se

   Stephen Pink
   Department of Computer Science and Electrical Engineering
   Lulea University of Technology
   SE-971 87 Lulea, Sweden

   Phone: +46 920 752 29
   Fax: +46 920 728 31
   Mobile: +46 70 532 0007
   EMail: steve@sm.luth.se

19.  References

   [RFC-768]       Postel, J., "User Datagram Protocol", STD 6, RFC 768,
                   August 1980.

   [RFC-791]       Postel, J., "Internet Protocol", STD 5, RFC 791,
                   September 1981.

   [RFC-793]       Postel, J., "Transmission Control Protocol", STD 7,
                   RFC 793, September 1981.

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

   [RFC-1553]      Mathur, A. and M. Lewis, "Compressing IPX Headers
                   Over WAN Media (CIPX)", RFC 1553, December 1993.

   [RFC-1700]      Reynolds, J. and J. Postel, "Assigned Numbers", STD
                   2, RFC 1700, October 1994.  See also:
                   http://www.iana.org/numbers.html

   [RFC-2402]      Kent, S. and R. Atkinson, "IP Authentication Header",
                   RFC 2402, November 1998.

   [RFC-2406]      Kent, S. and R. Atkinson, "IP Encapsulating Security
                   Protocol (ESP)", RFC 2406, November 1998.

   [RFC-1828]      Metzger, W., "IP Authentication using Keyed MD5", RFC
                   1828, August 1995.

   [IPv6]          Deering, S. and R. Hinden, "Internet Protocol,
                   Version 6 (IPv6) Specification", RFC 2460, December
                   1998.

   [ICMPv6]        Conta, A. and S. Deering, "Internet Control Message
                   Protocol (ICMPv6) for the Internet Protocol Version 6
                   (IPv6) Specification.", RFC 2463, December 1998.

   [RFC-2004]      Perkins, C., "Minimal Encapsulation within IP", RFC
                   2004, October 1996.

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

   [PPP-HC]        Engan, M., Casner, S. and C. Bormann, "IP Header
                   Compression for PPP", RFC 2509, February 1999.

20.  Full Copyright Statement

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
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