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Versions: (draft-ietf-rohc-tcp-taroc) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 RFC 4996

Robust Header Compression                                   G. Pelletier
Internet-Draft                                                L. Jonsson
Expires: December 21, 2006                                   K. Sandlund
                                                                Ericsson
                                                                 M. West
                                                      Siemens/Roke Manor
                                                           June 19, 2006


   RObust Header Compression (ROHC): A Profile for TCP/IP (ROHC-TCP)
                       draft-ietf-rohc-tcp-12.txt

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
   aware will be disclosed, in accordance with Section 6 of BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on December 21, 2006.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document specifies a ROHC (Robust Header Compression) profile
   for compression of TCP/IP packets.  The profile, called ROHC-TCP, is
   a robust header compression scheme for TCP/IP that provides high
   compression efficiency combined with high robustness.  ROHC-TCP also
   provides enhanced capabilities for compression of frequently used TCP



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   options (such as SACK (Selective Acknowledgments) and Timestamps
   compared to previous TCP/IP header compression schemes.

   ROHC-TCP works well when used over links with significant error rates
   and long round-trip times.  For many bandwidth-limited links where
   header compression is essential, such characteristics are common.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Background . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Existing TCP/IP Header Compression Schemes . . . . . . . .  6
     3.2.  Classification of TCP/IP Header Fields . . . . . . . . . .  7
   4.  Overview of the TCP/IP Profile (Informative) . . . . . . . . .  9
     4.1.  General Concepts . . . . . . . . . . . . . . . . . . . . .  9
     4.2.  Compressor and Decompressor Interactions . . . . . . . . .  9
       4.2.1.  Compressor Operation . . . . . . . . . . . . . . . . .  9
       4.2.2.  Decompressor Feedback  . . . . . . . . . . . . . . . . 10
     4.3.  Packet Formats and Encoding Methods  . . . . . . . . . . . 11
       4.3.1.  TCP Options  . . . . . . . . . . . . . . . . . . . . . 11
       4.3.2.  Compressing Extension Headers  . . . . . . . . . . . . 11
     4.4.  Expected Compression Ratios with ROHC-TCP  . . . . . . . . 11
   5.  Compressor and Decompressor Logic (Normative)  . . . . . . . . 12
     5.1.  Context Initialization . . . . . . . . . . . . . . . . . . 12
     5.2.  Compressor Operation . . . . . . . . . . . . . . . . . . . 13
       5.2.1.  Compression Logic  . . . . . . . . . . . . . . . . . . 13
       5.2.2.  Feedback Logic . . . . . . . . . . . . . . . . . . . . 14
       5.2.3.  Context Replication  . . . . . . . . . . . . . . . . . 15
     5.3.  Decompressor Operation . . . . . . . . . . . . . . . . . . 15
       5.3.1.  Decompressor States and Logic  . . . . . . . . . . . . 15
       5.3.2.  Reconstruction and Verification  . . . . . . . . . . . 17
       5.3.3.  Feedback Logic . . . . . . . . . . . . . . . . . . . . 18
       5.3.4.  Context Replication  . . . . . . . . . . . . . . . . . 19
   6.  Encodings in ROHC-TCP (Normative)  . . . . . . . . . . . . . . 19
     6.1.  Control Fields in ROHC-TCP . . . . . . . . . . . . . . . . 19
       6.1.1.  Master Sequence Number (MSN) . . . . . . . . . . . . . 19
       6.1.2.  IP-ID Behavior . . . . . . . . . . . . . . . . . . . . 20
       6.1.3.  Explicit Congestion Notification (ECN) . . . . . . . . 20
     6.2.  Compressed Header Chains . . . . . . . . . . . . . . . . . 21
     6.3.  Compressing TCP Options with List Compression  . . . . . . 22
       6.3.1.  List Compression . . . . . . . . . . . . . . . . . . . 22
       6.3.2.  Table-based Item Compression . . . . . . . . . . . . . 23
       6.3.3.  Encoding of Compressed Lists . . . . . . . . . . . . . 24
       6.3.4.  Item Table Mappings  . . . . . . . . . . . . . . . . . 25
       6.3.5.  Compressed Lists in Dynamic Chain  . . . . . . . . . . 27
       6.3.6.  Irregular Chain Items for TCP Options  . . . . . . . . 27



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       6.3.7.  Replication of TCP Options . . . . . . . . . . . . . . 27
     6.4.  Profile-specific Encoding Methods  . . . . . . . . . . . . 28
       6.4.1.  inferred_ip_v4_header_checksum . . . . . . . . . . . . 28
       6.4.2.  inferred_mine_header_checksum  . . . . . . . . . . . . 28
       6.4.3.  inferred_ip_v4_length  . . . . . . . . . . . . . . . . 29
       6.4.4.  inferred_ip_v6_length  . . . . . . . . . . . . . . . . 29
       6.4.5.  inferred_offset  . . . . . . . . . . . . . . . . . . . 30
       6.4.6.  baseheader_extension_headers . . . . . . . . . . . . . 30
       6.4.7.  baseheader_outer_headers . . . . . . . . . . . . . . . 31
       6.4.8.  Scaled TCP Sequence Number Encoding  . . . . . . . . . 31
       6.4.9.  Scaled Acknowledgment Number Encoding  . . . . . . . . 32
     6.5.  CRC Calculations . . . . . . . . . . . . . . . . . . . . . 33
   7.  Packet Types (Normative) . . . . . . . . . . . . . . . . . . . 33
     7.1.  Initialization and Refresh Packets (IR)  . . . . . . . . . 33
     7.2.  Context Replication Packets (IR-CR)  . . . . . . . . . . . 35
     7.3.  Compressed Packets (CO)  . . . . . . . . . . . . . . . . . 37
   8.  Packet Formats (Normative) . . . . . . . . . . . . . . . . . . 38
     8.1.  Design rationale for compressed base headers . . . . . . . 38
     8.2.  Formal Definition in ROHC-FN . . . . . . . . . . . . . . . 41
     8.3.  Feedback Formats and Options . . . . . . . . . . . . . . . 83
       8.3.1.  Feedback Formats . . . . . . . . . . . . . . . . . . . 83
       8.3.2.  Feedback Options . . . . . . . . . . . . . . . . . . . 84
   9.  Security Consideration . . . . . . . . . . . . . . . . . . . . 87
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 87
   11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 87
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 88
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 88
     12.2. Informative References . . . . . . . . . . . . . . . . . . 89
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 90
   Intellectual Property and Copyright Statements . . . . . . . . . . 91





















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1.  Introduction

   There are several reasons to perform header compression on low- or
   medium-speed links for TCP/IP traffic, and these have already been
   discussed in RFC 2507 [RFC2507].  Additional considerations that make
   robustness an important objective for a TCP [RFC0793] compression
   scheme are introduced in [RFC4163].  Finally, existing TCP/IP header
   compression schemes (RFC 1144 [RFC1144], RFC 2507 [RFC2507]) are
   limited in their handling of the TCP options field and cannot
   compress the headers of handshaking packets (SYNs and FINs).

   It is thus desirable for a header compression scheme to be able to
   handle loss on the link between the compression and decompression
   point as well as loss before the compression point.  The header
   compression scheme also needs to consider how to efficiently compress
   short-lived TCP transfers and TCP options, such as SACK (RFC 2018
   [RFC2018], RFC 2883 [RFC2883]) and Timestamps (RFC 1323 [RFC1323]).

   The ROHC WG has developed a header compression framework on top of
   which various profiles can be defined for different protocol sets, or
   for different compression strategies.  This document defines a TCP/IP
   compression profile for the ROHC framework [RFC3095], compliant with
   the requirements on ROHC TCP/IP header compression [RFC4163].

   Specifically, it describes a header compression scheme for TCP/IP
   header compression (ROHC-TCP) that is robust against packet loss and
   that offers enhanced capabilities, in particular for the compression
   of header fields including TCP options.  The profile identifier for
   TCP/IP compression is 0x0006.


2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   This document reuses some of the terminology found in RFC 3095
   [RFC3095].  In addition, this document uses or defines the following
   terms:

   Base context

      The base context is a context that has been validated by both the
      compressor and the decompressor.  A base context can be used as
      the reference when building a new context using replication.

   Base CID



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      The Base Context Identifier is the CID used to identify the Base
      Context, where information needed for context replication can be
      extracted from.

   Chaining of Items

      A chain groups fields based on similar characteristics.  ROHC-TCP
      defines chain items for static, dynamic, replicable or irregular
      fields.  Chaining is done by appending an item for e.g. each
      header to the chain in their order of appearance in the
      uncompressed packet.  Chaining is useful to construct compressed
      headers from an arbitrary number of any of the protocol headers
      for which ROHC-TCP defines a compressed format.

   Context Replication (CR)

      Context replication is the mechanism that establishes and
      initializes a new context based on another existing valid context
      (a base context).  This mechanism is introduced to reduce the
      overhead of the context establishment procedure, and is especially
      useful for compression of multiple short-lived TCP connections
      that may be occurring simultaneously or near-simultaneously.

   CRC-8 validation

      The CRC-8 validation refers to the validation of the integrity
      against bit error(s) of the received IR, the IR-DYN or the IR-CR
      header, using the 8-bit CRC that is included in the header.

   CRC verification

      The CRC verification refers to the verification of the result of a
      decompression attempt, using the 3-bit CRC or 7-bit CRC included
      in the header of a compressed packet format (CO).

   ROHC Context Replication (ROHC-CR)

      "ROHC-CR" in this document normatively refers to the context
      replication mechanism for ROHC profiles defined in [RFC4164].

   ROHC Formal Notation (ROHC-FN)

      "ROHC-FN" in this document normatively refers to the formal
      notation for ROHC profiles, as defined in [ROHC-FN], including the
      library of encoding methods it specifies.

   ROHC-TCP packet types




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      ROHC-TCP uses two different packet types: the Initialization and
      Refresh (IR) packet type, and the Compressed packet type (CO).

   Short-lived TCP transfer

      Short-lived TCP transfers refer to TCP connections transmitting
      only small amounts of packets for each single connection.


3.  Background

   This section provides some background information on TCP/IP header
   compression.  The fundamentals of general header compression can be
   found in [RFC3095].  In the following sub-sections, two existing
   TCP/IP header compression schemes are first described along with a
   discussion of their limitations, followed by the classification of
   TCP/IP header fields.  Finally, some of the characteristics of short-
   lived TCP transfers are summarized.

   The behavior analysis of TCP/IP header fields among multiple short-
   lived connections may be found in [RFC4413].

3.1.  Existing TCP/IP Header Compression Schemes

   Compressed TCP (CTCP) and IP Header Compression (IPHC) are two
   different schemes that may be used to compress TCP/IP headers.  Both
   schemes transmit only the differences from the previous header in
   order to reduce the large overhead of the TCP/IP header.

   The CTCP (RFC 1144 [RFC1144]) compressor detects transport-level
   retransmissions and sends a header that updates the context
   completely when they occur.  While CTCP works well over reliable
   links, it is vulnerable when used over less reliable links as even a
   single packet loss results in loss of synchronization between the
   compressor and the decompressor.  This in turn leads to the TCP
   receiver discarding all remaining packets in the current window
   because of a checksum error.  This effectively prevents the TCP Fast
   Retransmit algorithm (RFC 2001) from being triggered.  In such case,
   the compressor must wait until the TCP timeout to resynchronize.

   To reduce the errors due to the inconsistent contexts between
   compressor and decompressor when compressing TCP, IPHC (RFC 2507
   [RFC2507]) improves somewhat on CTCP by augmenting the repair
   mechanism of CTCP with a local repair mechanism called TWICE and with
   a link layer nacking mechanism to request a header that updates the
   context.

   The TWICE algorithm assumes that only the Sequence number field of



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   TCP segments are changing with the deltas between consecutive packets
   being constant in most cases.  This assumption is however not always
   true, especially when TCP Timestamps and SACK options are used.

   The full header request mechanism requires a feedback channel that
   may be unavailable in some circumstances.  This channel is used to
   explicitly request that the next packet be sent with an uncompressed
   header to allow resynchronization without waiting for a TCP timeout.
   In addition, this mechanism does not perform well on links with long
   round-trip time.

   Both CTCP and IPHC are also limited in their handling of the TCP
   options field.  For IPHC, any change in the options field (caused by
   timestamps or SACK, for example) renders the entire field
   uncompressible, while for CTCP such a change in the options field
   effectively disables TCP/IP header compression altogether.

   Finally, existing TCP/IP compression schemes do not compress the
   headers of handshaking packets (SYNs and FINs).  Compressing these
   packets may greatly improve the overall header compression ratio for
   the cases where many short-lived TCP connections share the same
   channel.

3.2.  Classification of TCP/IP Header Fields

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

   All fields of the TCP/IP packet header have been classified in detail
   in [RFC4413].  The main conclusion is that most of the header fields
   can easily be compressed away since they seldom or never change.  The
   following fields do however require more sophisticated mechanisms:


      *  IPv4 Identification (16 bits) - IP-ID
      *  TCP Sequence number (32 bits) - SN
      *  TCP Acknowledgment number (32 bits)
      *  TCP Reserved (4 bits)
      *  TCP ECN flags (2 bits) - ECN
      *  TCP Window (16 bits)
      *  TCP Options
         +  Maximum Segment Size (32 bits) - MSS
         +  Window Scale (24 bits) - WSCALE





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         +  SACK Permitted (16 bits)
         +  TCP SACK (80, 144, 208 or 272 bits) - SACK
         +  TCP Timestamp (80 bits) - TS

   The assignment of IP-ID values can be done in various ways, most
   usually one of sequential, sequential jump, random or constant to a
   value of zero.  Some IPv4 stacks do use a sequential assignment when
   generating IP-ID values but do not transmit the contents of this
   field in network byte order; instead it is sent with the two octets
   reversed.  In this case, the compressor can compress the IP-ID field
   after swapping the bytes.  Consequently, the decompressor also swaps
   the bytes of the IP-ID after decompression to regenerate the original
   IP-ID.  In RFC 3095 [RFC3095], the IP-ID is generally inferred from
   the RTP Sequence number.  However, with respect to TCP compression,
   the analysis in [RFC4413] reveals that there is no obvious candidate
   to this purpose among the TCP fields.

   The change pattern of several TCP fields (Sequence number,
   Acknowledgment number, Window, etc.) is very hard to predict and
   differs entirely from the behavior of RTP fields discussed in
   [RFC3095].  Of particular importance to a TCP/IP header compression
   scheme is the understanding of the sequence and acknowledgment number
   [RFC4413].

   Specifically, the TCP Sequence number can be anywhere within a range
   defined by the TCP window at any point on the path (i.e. wherever a
   compressor might be deployed).  Missing packets or retransmissions
   can cause the TCP Sequence number to fluctuate within the limits of
   this window.  The TCP window also bounds the jumps in acknowledgment
   number.

   Another important behavior of the TCP/IP header is the dependency
   between the sequence number and the acknowledgment number.  TCP
   connections can be either near-symmetrical or show a strong
   asymmetrical bias with respect to the data traffic.  In the latter
   case, the TCP connections mainly have one-way traffic (web browsing
   and FTP downloading, for example).  This means that on the forward
   path (from server to client), only the sequence number is changing
   while the acknowledgment number remains constant for most packets; on
   the backward path (from client to server), only the acknowledgment
   number is changing and the sequence number remains constant for most
   packets.  A compression scheme for TCP should thus have packet
   formats suitable for either cases, i.e. packet formats that can carry
   either only sequence number bits, only acknowledgment bits, or both.

   In addition, TCP flows can be short-lived transfers.  Short-lived TCP
   transfers will degrade the performance of header compression schemes
   that establish a new context by initially sending full headers.



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   Multiple simultaneous or near simultaneous TCP connections may
   exhibit much similarity in header field values and context values
   among each other, which would make it possible to reuse information
   between flows when initializing a new context.  A mechanism to this
   end, context replication [RFC4164], makes the context establishment
   step faster and more efficient, by replicating part of an existing
   context to a new flow.  All header fields and related context values
   have been classified in detail in [RFC4413]; the conclusion is that
   most part of the IP sub-context, some TCP fields, and some context
   values can easily be replicated since they seldom change or change
   with only a small jump.

   ROHC-TCP also compresses the following headers: IPv6 Destination
   Options header [RFC2460], IPv6 Routing header [RFC2460], IPv6 Hop-by-
   hop Options header [RFC2460], AH [RFC4302], NULL-encrypted ESP header
   [RFC4303], GRE [RFC2784][RFC2890] and the Minimal Encapsulation
   header (MINE) [RFC2004] (i.e. the same headers as those compressed by
   RFC 3095 [RFC3095]).  Headers specific to Mobile IP (for IPv4 or
   IPv6) do not receive any special treatment in this document, for
   reasons similar as those described in [RFC3095].


4.  Overview of the TCP/IP Profile (Informative)

4.1.  General Concepts

   Many of the concepts behind the ROHC-TCP profile are similar to those
   described in RFC 3095 [RFC3095].  Like for other ROHC profiles, ROHC-
   TCP makes use of the ROHC protocol as described in [RFC3095], in
   sections 5.1 to 5.2.6.  This includes data structures, reserved
   packet types, general packet formats, segmentation and initial
   decompressor processing.

   In addition, ROHC-TCP supports context replication as defined in
   ROHC-CR [RFC4164].  Context replication can be particularly useful
   for short-lived TCP flows [RFC4413].

4.2.  Compressor and Decompressor Interactions

4.2.1.  Compressor Operation

   Header compression with ROHC can be conceptually characterized as the
   interaction of a compressor with a decompressor state machine.  The
   compressor's task is to minimally send the information needed to
   successfully decompress a packet, based on a certain confidence
   regarding the state of the decompressor context.

   For ROHC-TCP compression, the compressor normally starts compression



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   with the initial assumption that the decompressor has no useful
   information to process the new flow, and sends Initialization and
   Refresh (IR) packets.  Alternatively, the compressor may also support
   Context Replication (CR) and use IR-CR packets [RFC4164] which
   attempts to reuse context information related to another flow.

   The compressor can then adjust the compression level based on its
   confidence that the decompressor has the necessary information to
   successfully process the compressed packets (CO) that it selects.  In
   other words, the task of the compressor is to ensure that the
   decompressor operates in the state that allows decompression of the
   most efficient CO packet(s), and to allow the decompressor to move to
   that state as soon as possible otherwise.

4.2.2.  Decompressor Feedback

   The ROHC-TCP profile can be used in environments with or without
   feedback capabilities from decompressor to compressor.  ROHC-TCP
   however assumes that if a ROHC feedback channel is available and if
   this channel is used at least once by the decompressor for a specific
   ROHC-TCP context, this channel will be used during the entire
   compression operation for that context.  If the feedback channel
   disappears, compression should be restarted.

   The reception of either positive feedback (ACKs) or negative feedback
   (NACKs) establishes the feedback channel from the decompressor for
   the context for which the feedback was received.  Once there is an
   established feedback channel for a specific context, the compressor
   should make use of this feedback to estimate the current state of the
   decompressor.  This helps increasing the compression efficiency by
   providing the information needed for the compressor to achieve the
   necessary confidence level.

   To parallel RFC 3095 [RFC3095], this is similar to allowing only one
   mode transition per compressor: from the initial unidirectional mode
   to the bi-directional mode of operation, with the transition being
   triggered by the reception of the first packet containing feedback
   from the decompressor.  This effectively means that ROHC-TCP does not
   explicitly define any operational modes.

   The ROHC-TCP feedback mechanism is limited in its applicability by
   the number of MSN (LSB coded) bits used in the FEEDBACK-2 format.  It
   is not suitable for a decompressor to use feedback altogether where
   the MSN bits in the feedback could wraparound under one round-trip
   time (RTT).  Instead, unidirectional operation -- where the
   compressor periodically sends larger context updating packets -- is
   more appropriate.




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4.3.  Packet Formats and Encoding Methods

   The packet formats used for ROHC-TCP are defined using the formal
   notation, ROHC-FN.  The formal notation is used to provide an
   unambiguous representation of the packet formats and a clear
   definition of the encoding methods.  The encoding methods used in the
   packet formats for ROHC-TCP are defined in [ROHC-FN].

4.3.1.  TCP Options

   The TCP options in ROHC-TCP are compressed using a downscaled version
   of the list compression in [RFC3095], allowing option content to be
   established so that TCP options can be added to the context without
   having to send all TCP options uncompressed.

4.3.2.  Compressing Extension Headers

   In RFC 3095 [RFC3095], list compression is used to compress extension
   headers.  ROHC-TCP compresses the same type of extension headers as
   in [RFC3095].  However, these headers are treated exactly as other
   headers and thus have a static chain, a dynamic chain, an irregular
   chain and a chain for context replication Section 6.2.

   The consequence is that headers appearing in or disappearing from the
   flow being compressed will lead to changes to the static chain.
   However, the change pattern of extension headers is not deemed to
   impair compression efficiency with respect to this design strategy.

4.4.  Expected Compression Ratios with ROHC-TCP

   The following table illustrates typical compression ratios that can
   be expected when using ROHC-TCP and IPHC [RFC2507].

   The figures in the table assumes that the compression context has
   already been properly initialized.  For the TS option, the timestamp
   is assumed to change with small values.  All TCP options include a
   suitable number of NOP options for padding and/or alignment.
   Finally, in the examples for IPv4, a sequential IP-ID behavior is
   assumed.












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                             Total Header Size (octets)
                              ROHC-TCP          IPHC
                     Unc.   DATA    ACK      DATA    ACK
   IPv4+TCP+TS       52       8      8        18     18
   IPv4+TCP+TS       52       7      6        16     16   (1)
   IPv6+TCP+TS       72       8      7        18     18
   IPv6+TCP+no opt   60       6      5         6      6
   IPv6+TCP+SACK     80       -     15         -     80   (2)
   IPv6+TCP+SACK     80       -      9         -     26   (3)

   (1) The payload size of the data stream is constant
   (2) The SACK option appears in the header, but was not present
       in the previous packet. Two SACK blocks are assumed.
   (3) The SACK option appears in the header, and was also present
       in the previous packet (with different SACK blocks).
       Two SACK blocks are assumed.

   The table below illustrates the typical initial compression ratios
   for ROHC-TCP and IPHC.  The data stream in the example is assumed to
   be IPv4+TCP, with a sequential behavior for the IP-ID.  The following
   options are assumed present in the SYN packet: TS, MSS and WSCALE,
   with an appropriate number of NOP options.

                     Total Header Size (octets)
                      Unc.   ROHC-TCP   IPHC
   1st packet (SYN)   60      49        60
   2nd packet         52      12        52

   The figures in the table assume that a ROHC ACK has reached the
   compressor before the second packet is being compressed, which can be
   expected when using bidirectional ROHC-TCP operation; this is because
   in the most common case the TCP ACKs are expected to take the same
   return path, and because TCP does not send more packets until the TCP
   SYN packet has been acknowledged.


5.  Compressor and Decompressor Logic (Normative)

   The header compression logic as described in this section is a
   simplified version of the one found in [RFC3095].

5.1.  Context Initialization

   The static context of ROHC TCP flows can be initialized in either of
   two ways:






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   1.  By using an IR packet as in Section 7.1, where the profile is six
       (6) and the static chain ends with the static part of a TCP
       header.
   2.  By replicating an existing context using the mechanism defined by
       ROHC-CR.  This is done with the IR-CR packet defined in
       Section 7.2, where the profile number is six (6).

5.2.  Compressor Operation

5.2.1.  Compression Logic

   The task of the compressor is to determine what data must be sent
   when compressing a TCP/IP packet, so that the decompressor can
   successfully reconstruct the original packet based on its current
   state.  The selection of the type of compressed header to send thus
   depends on a number of factors, including:

   o  The change behavior of header fields in the flow, e.g. conveying
      the necessary information within the restrictions of the set of
      available packet formats;
   o  The compressor's level of confidence regarding decompressor state,
      e.g. by using an optimistic approach through repetition of context
      updates or from the reception of decompressor feedback (ACKs
      and/or NACKs);
   o  Additional robustness required for the flow, e.g. periodic
      repetition of static and dynamic information using IR and IR-DYN
      packets when decompressor feedback is not expected.

   The impact of these factors on the compressor's packet type selection
   is described more in detail in the following subsections.

   In this section, a "higher compression state" means that less data
   will be sent in compressed packets, i.e. smaller compressed headers
   are used, while a lower compression state means that a larger amount
   of data will be sent using larger compressed headers.

5.2.1.1.  Optimistic Approach

   When ROHC-TCP is used over lossy links, all information needs to be
   repeated by the compressor until it is fairly confident that the
   decompressor has received the information contained in the packet.

   Therefore, if field X in the uncompressed packet changes value, the
   compressor MUST use a packet type that contains an encoding for field
   X until it has gained confidence that the decompressor has received
   at least one packet containing the new value for X. The compressor
   SHOULD choose a compressed format with the smallest header that can
   convey the changes needed to fulfill the optimistic approach



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   condition used.

5.2.1.2.  Periodic Context Refreshes

   When the optimistic approach is used, there will always be a
   possibility of decompression failures since the decompressor may not
   have received sufficient information for correct decompression.

   Therefore, until the decompressor has established a feedback channel,
   the compressor SHOULD periodically move to a lower compression state
   and send IR and/or IR-DYN packets.  These refreshes can be based on
   timeouts, on the number of compressed packets sent for the flow or
   any other strategy specific to the implementation.  Once the feedback
   channel is established, the decompressor MAY stop sending periodic
   refreshes.

5.2.2.  Feedback Logic

   The compressor makes use of the feedback from the decompressor to
   move to a lower compression state (NACKs), and optionally to move to
   a higher compression state (ACKs).

5.2.2.1.  Optional Acknowledgments (ACKs)

   The compressor MAY optionally use acknowledgment feedback (ACKs) to
   move to a higher compression state.

   Upon reception of an ACK for a context-updating packet, the
   compressor obtains confidence that the decompressor has received the
   acknowledged packet and that it has observed changes in the packet
   flow up to the acknowledged packet.

   This functionality is optional, so a compressor MUST NOT expect to
   get such ACKs, even if a feedback channel is available and has been
   established for that flow.

5.2.2.2.  Negative Acknowledgments (NACKs)

   Negative acknowledgments (NACKs or STATIC-NACKs) are also called
   error recovery requests and indicate that the all or parts of the
   decompressor context has been invalidated.

   On reception of a NACK feedback, the compressor SHOULD:


   o  assume that only the static part of the decompressor is valid, and





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   o  re-send all dynamic information (via an IR or IR-DYN packet) next
      time it compresses a packet for the indicated flow

   unless it has confidence that information sent after the packet that
   is being acknowledged already provides a suitable response to the
   error recovery request.

   On reception of a STATIC-NACK feedback, the compressor SHOULD:

   o  assume that the decompressor has no valid context, and
   o  re-send all static and all dynamic information (via an IR packet)
      next time it compresses a packet for the indicated flow.

   unless it has confidence that information sent after the packet that
   is being acknowledged already provides a suitable response to the
   error recovery request.

5.2.3.  Context Replication

   A compressor MAY support context replication by implementing the
   additional compression and feedback logic defined in ROHC-CR
   [RFC4164].

5.3.  Decompressor Operation

5.3.1.  Decompressor States and Logic

   The three states of the decompressor are No Context (NC), Static
   Context (SC) and Full Context (FC).  The decompressor starts in its
   lowest compression state, the NC state.  Successful decompression
   will always move the decompressor to the FC state.  The decompressor
   state machine normally never leaves the FC state once it has entered
   this state; only repeated decompression failures will force the
   decompressor to transit downwards to a lower state.

   Below is the state machine for the decompressor.  Details of the
   transitions between states and decompression logic are given in the
   sub-sections following the figure.













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                                    Success
                +-->------>------>------>------>------>--+
                |                                        |
    No Static   |            No Dynamic        Success   |    Success
     +-->--+    |             +-->--+      +--->----->---+    +-->--+
     |     |    |             |     |      |             |    |     |
     |     v    |             |     v      |             v    |     v
   +-----------------+   +---------------------+   +-------------------+
   | No Context (NC) |   | Static Context (SC) |   | Full Context (FC) |
   +-----------------+   +---------------------+   +-------------------+
      ^                         |        ^                         |
      |  Static Context Damage  |        |     Context Damage      |
      +-----<------<------<-----+        +-----<------<------<-----+

5.3.1.1.  No Context (NC) State

   Initially, while working in the No Context (NC) state, the
   decompressor has not yet successfully decompressed a packet.

   Allowing decompression:

      In the NC state, only packets carrying sufficient information on
      the static fields (IR and IR-CR packets) can be decompressed;
      otherwise, the packet MUST be discarded.

   Feedback logic:

      In the NC state, the decompressor SHOULD send a STATIC-NACK if a
      packet of a type other than one for which decompression is allowed
      is received, or if an IR packet has failed the CRC-8 validation.

   Once a packet has been validated and decompressed correctly, the
   decompressor MUST transit to the FC state.

5.3.1.2.  Static Context (SC) State

   When the decompressor is in the Static Context (SC) state, only the
   static part of the decompressor context is valid.

   From the SC state, the decompressor moves back to the NC state if
   static context damage is detected.  How the decompressor detects
   static context damage should be based on the residual error rate,
   where a low error rate should make the decompressor assume damage
   more often than on a link with a higher error rate.

   Allowing decompression:





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      In the SC state, only packets carrying sufficient information on
      the dynamic fields covered by an 8-bit CRC can be decompressed
      (e.g.  IR and IR-DYN); otherwise the packet is of type CO and it
      MUST be discarded.

   Feedback logic:

      In the SC state, the decompressor SHOULD send a STATIC-NACK when
      an IR or an IR-DYN packet fails the CRC-8 validation.  If a CO
      packet type is received, the decompressor SHOULD treat it as a CRC
      mismatch when deciding if a NACK is to be sent.

   Once a packet has been validated and decompressed correctly, the
   decompressor MUST transit to the FC state.

5.3.1.3.  Full Context (FC) State

   In the Full Context (FC) state, both the static and the dynamic parts
   of the decompressor context are valid.  The decompressor moves back
   to the SC state if context damage is detected.  How the decompressor
   detects context damage should be based on the residual error rate,
   where a low error rate should make the decompressor assume damage
   more often than on a link with a higher error rate.  The decompressor
   may send feedback, as described below, when assuming context damage.

   Allowing decompression:

      In the FC state, decompression can be attempted regardless of the
      type of packet received.

   Feedback logic:

      In the FC state, the decompressor SHOULD send a NACK when
      decompression of any packet type fails and if context damage is
      assumed.

5.3.2.  Reconstruction and Verification

   When decompression of an IR or an IR-DYN packet is allowed, the
   decompressor MUST validate the integrity of the received header using
   CRC-8 validation: the decompressor computes the 8-bit CRC according
   to the type of the received header, and then compares the result with
   the 8-bit CRC carried in the header.  If the two are identical, the
   decompressor reconstructs the original header.  Otherwise the packet
   MUST be discarded without further processing.

   Upon receiving an IR-CR packet, the decompressor MUST perform the
   actions as specified in [RFC4164].



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   When decompression of other packet types is allowed (CO packets), the
   decompressor MUST check the outcome of the decompression attempt
   using CRC verification: the decompressor computes the 3-bit CRC or
   the 7-bit CRC over the reconstructed header, and then compares the
   result with the corresponding CRC carried in the received header.  If
   the two are identical, the decompression attempt is successful.  If
   they are not identical, decompressor implementations MAY attempt
   corrective or repair measures on the packet, and the result of any
   attempt MUST be validated using the CRC verification; otherwise, the
   packet MUST be discarded without further processing.

   When the CRC-8 validation or the CRC verification of the received
   header is successful, the decompressor SHOULD update its context with
   the information received in the current header; the decompressor then
   passes the reconstructed packet to the system's network layer.
   Otherwise, the decompressor context MUST NOT be updated.

   If the received packet is older than the current reference packet
   (based on the Master Sequence number (MSN) in the compressed packet),
   the decompressor MAY refrain from updating the context using the
   information received in the current packet, even if the correctness
   of its header was successfully verified.

   If a feedback channel is available, the decompressor MAY use positive
   feedback (ACKs) to acknowledge successful decompression of packets.

5.3.3.  Feedback Logic

   The decompressor MAY send positive feedback (ACKs) to initially
   establish the feedback channel for a particular flow.  Either
   positive feedback (ACKs) or negative feedback (NACKs) establishes
   this channel.  The decompressor MUST then use the feedback channel to
   send error recovery requests and (optionally) acknowledgments of
   significant context updates.

   Once the feedback channel is established, the decompressor is
   REQUIRED to continue sending error recovery requests (i.e.  NACKs or
   STATIC-NACKs) for as long as the context is associated with the same
   profile, in this case with profile 0x0006, as per the logic defined
   for each state in Section 5.3.1.

   The decompressor MAY optionally send ACKs upon successful
   decompression of any packet type.  In particular, when an IR, an IR-
   DYN or any CO packet carrying a significant context update is
   correctly decompressed, the decompressor MAY optionally send an ACK.






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5.3.4.  Context Replication

   ROHC-TCP supports context replication, therefore the decompressor
   MUST implement the additional decompressor and feedback logic defined
   in ROHC-CR [RFC4164].


6.  Encodings in ROHC-TCP (Normative)

   This section describes a ROHC profile for TCP/IP compression.  The
   profile identifier for ROHC-TCP is 0x0006.

6.1.  Control Fields in ROHC-TCP

   In ROHC-TCP, a number of control fields are used by the decompressor
   in its interpretation of the packet formats for packets received from
   the compressor.

   A control field is a field that is transmitted from the compressor to
   the decompressor, but is not part of the uncompressed header.  Values
   for control fields can be set up in the context of both the
   compressor and the decompressor.  Once established at the
   decompressor, the values of these fields should be kept until updated
   by another packet.

6.1.1.  Master Sequence Number (MSN)

   There is no field in the TCP header that can act as the master
   sequence number for TCP compression, as explained in [RFC4413],
   section 5.6.

   To overcome this problem, ROHC-TCP introduces a control field called
   the Master Sequence Number (MSN) field.  The MSN field is created at
   the compressor, rather than using one of the fields already present
   in the uncompressed header.  The compressor increments the value of
   the MSN by one for each packet that it sends.

   The MSN field has the following two functions:

   1.  Differentiating between packets when sending feedback data.
   2.  Inferring the value of incrementing fields such as the IP-ID.

   The MSN field is present in every packet sent by the compressor.  The
   MSN is LSB encoded within the CO packets, and the 16-bit MSN is sent
   in full in IR/IR-DYN packets.  The decompressor always sends the MSN
   as part of the feedback information.  The compressor can later use
   the MSN to infer which packet the decompressor is acknowledging.




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   When the MSN is initialized, it SHOULD be initialized to a random
   value.  The compressor should only initialize a new MSN for the
   initial IR or IR-CR packet sent for a CID that corresponds to a
   context that is not already associated with this profile.  In other
   words, if the compressor reuses the same CID to compress many TCP
   flows one after the other, the MSN is not reinitialized but rather
   continues to increment monotonically.

   For context replication, the compressor does not use the MSN of the
   base context when sending the IR-CR packet, unless the replication
   process overwrites the base context (i.e.  BCID == CID).  Instead,
   the compressor uses the value of the MSN if it already exists in the
   ROHC-TCP context being associated with the new flow (CID); otherwise,
   the MSN is initialized to a new value.

6.1.2.  IP-ID Behavior

   The IP-ID field of the IPv4 header can have different change
   patterns.  RFC 3095 [RFC3095] describes three behaviors: sequential
   (NBO), sequential byte-swapped, and random (RND).  In addition, this
   profile uses a fourth behavior, the constant zero IP-ID behavior as
   defined in RFC 3843 [RFC3843], Section 3.3.

   The compressor monitors changes in the value of the IP-ID field for a
   number of packets, to identify which one of the above listed behavior
   is the closest match to the observed change pattern.  The compressor
   can then select packet formats based on the identified field
   behavior.

   If more than one level of IP headers is present, ROHC-TCP can assign
   a sequential behavior (NBO or byte-swapped) only to the IP-ID of
   innermost IP header.  This is because only this IP-ID can possibly
   have a sufficiently close correlation with the MSN (see also
   Section 6.1.1) to compress it as a sequentially changing field.
   Therefore, a compressor MUST assign either the constant zero IP-ID or
   the random IP-ID behavior to tunneling headers.

   The control field for the IP-ID behavior determines which set of
   packet formats will be used.  Note that these control fields are also
   used to determine the contents of the irregular chain item for each
   IP header.

6.1.3.  Explicit Congestion Notification (ECN)

   When ECN [RFC3168] is used once on a flow, it can be expected that
   the ECN bits will change quite often.  ROHC-TCP maintains a control
   field in the context to indicate if ECN is used or not.  This control
   field is transmitted in the dynamic chain of the TCP header, and its



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   value can be updated using specific compressed headers carrying a
   7-bit CRC.

   When this control field indicates that ECN is being used, items of IP
   and TCP headers in the irregular chain will include bits used for
   ECN.  To preserve octet-alignment, all of the TCP reserved bits are
   transmitted and, for outer IP headers, the entire TOS/TC field is
   included in the irregular chain.

   The design rationale behind this is the possible use of the "full-
   functionality option" of section 9.1 of RFC 3168 [RFC3168].

6.2.  Compressed Header Chains

   Some packet types use one or more chains containing sub-header
   information.  The function of a chain is to group fields based on
   similar characteristics, such as static, dynamic or irregular fields.
   Chaining is done by appending an item for each header to the chain in
   their order of appearance in the uncompressed packet, starting from
   the fields in the outermost header.  Chains are defined for all
   headers compressed by ROHC-TCP (i.e.  TCP [RFC0793], IPv4 [RFC0791],
   IPv6 [RFC2460], AH [RFC4302], GRE [RFC2784][RFC2890], MINE [RFC2004],
   NULL-encrupted ESP [RFC4303], IPv6 Destination Options
   header[RFC2460], IPv6 Hop-by-hop Options header[RFC2460] and IPv6
   Routing header [RFC2460].

   Static chain:

      The static chain consists of one item for each header of the chain
      of protocol headers to be compressed, starting from the outermost
      IP header and ending with a TCP header.  In the formal description
      of the packet formats, this static chain item for each header type
      is labelled <protocol_name>_static.  The static chain is only used
      in IR packets.


   Dynamic chain:

      The dynamic chain consists of one item for each header of the
      chain of protocol headers to be compressed, starting from the
      outermost IP header and ending with a TCP header.  It should be
      noted that the dynamic chain item for the TCP header also contains
      a compressed list of TCP options (see Section 6.3).  In the formal
      description of the packet formats, the dynamic chain item for each
      header type is labelled <protocol_name>_dynamic.  The dynamic
      chain is used both in IR and IR-DYN packets.





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   Replicate chain:

      The replicate chain consists of one item for each header in the
      chain of protocol headers to be compressed, starting from the
      outermost IP header and ending with a TCP header.  It should be
      noted that the replicate chain item for the TCP header also
      contains a compressed list of TCP options (see Section 6.3).  In
      the formal description of the packet formats, this replicate chain
      item for each header type is labelled <protocol_name>_replicate.
      Header fields that are not present in the replicate chain are
      replicated from the base context.  The replicate chain is only
      used in the IR-CR packet.


   Irregular chain:

      The structure of the irregular chain is analogous to the structure
      of the static chain.  For each compressed packet, the irregular
      chain is appended at the specified location in the general format
      of the compressed packets as defined in Section 7.3.  This chain
      also includes the irregular chain items for TCP options as defined
      in Section 6.3.6.  The irregular chain is used for all CO packets.
      The format of the irregular chain for the innermost IP header
      differs from the format of outer IP headers, since this header is
      a part of the compressed base header.  Which of these irregular
      chain items to use is decided by the argument "is_innermost" which
      is passed to the corresponding encoding method (ipv4 or ipv6).
      The format of the irregular chain item for the outer IP headers is
      also determined using a flag for TTL/Hop Limit; this flag is
      defined in the format of some of the compressed base headers.

6.3.  Compressing TCP Options with List Compression

   This section describes in details how list compression is applied to
   the TCP options.  In the definition of the packet formats for ROHC-
   TCP, the most frequent type of TCP options are described.  Each of
   these options has an uncompressed format, a <option_type>_list_item
   format and a <option_type>_irregular format, where <option_type> is
   the name of the option type in the option list.

6.3.1.  List Compression

   The TCP options in the uncompressed packet can be represented as an
   ordered list, whose order and presence are usually constant between
   packets.  The generic structure of such a list is as follows:

            +--------+--------+--...--+--------+
      list: | item 1 | item 2 |       | item n |



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            +--------+--------+--...--+--------+

   The basic principles of list-based compression are the following:

      1) When a context is being initialized, a complete representation
      of the compressed list of options is transmitted.  All options
      that have any content are present in the compressed list of items
      sent by the compressor.

   Then, once the context has been initialized:

      2) When the structure AND the content of the list are unchanged,
      no information about the list is sent in compressed headers.
      3) When the structure of the list is constant, and when only the
      content defined within the irregular format for one or more
      options is changed, no information about the list needs to be sent
      in compressed headers; the irregular content is sent as part of
      the irregular chain (as described in Section 6.3.6 in the general
      compressed packet format (Section 7.3).
      4) When the structure of the list changes, a compressed list is
      sent in the compressed header, including a representation of its
      structure and order.

6.3.2.  Table-based Item Compression

   The Table-based item compression compresses individual items sent in
   compressed lists.  The compressor assigns a unique identifier,
   "Index", to each item "Item" of a list.

   Compressor Logic

      The compressor conceptually maintains an Item Table containing all
      items, indexed using "Index".  The (Index, Item) pair is sent
      together in compressed lists until the compressor gains enough
      confidence that the decompressor has observed the mapping between
      items and their respective index.  Confidence is obtained from the
      reception of an acknowledgment from the decompressor, or by
      sending (Index, Item) pairs using the optimistic approach.  Once
      confidence is obtained, the index alone is sent in compressed
      lists to indicate the presence of the item corresponding to this
      index.

      The compressor may reassign an existing index to a new item, by
      re-establishing the mapping using the procedure described above.

   Decompressor Logic





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      The decompressor conceptually maintains an Item Table that
      contains all (Index, Item) pairs received.  The Item Table is
      updated whenever an (Index, Item) pair is received and
      decompression is successfully verified using the CRC.  The
      decompressor retrieves the item from the table whenever an Index
      without an accompanying Item is received.
      If an index without an accompanying item is received and the
      decompressor does not have any context for this index, the header
      MUST be discarded and a NACK SHOULD be sent.

6.3.3.  Encoding of Compressed Lists

   Each item present in a compressed list is represented by:

   o  an index into the table of items, and
   o  a presence bit indicating if a compressed representation of the
      item is present in the list.
   o  an item (if the presence bit is set)

   If the presence bit is not set, the item must already be known by the
   decompressor.

   A compressed list of TCP options uses the following encoding:

        0   1   2   3   4   5   6   7
      +---+---+---+---+---+---+---+---+
      | Reserved  |PS |       m       |
      +---+---+---+---+---+---+---+---+
      |        XI_1, ..., XI_m        | m octets, or m * 4 bits
      /                --- --- --- ---/
      |               :    Padding    : if PS = 0 and m is odd
      +---+---+---+---+---+---+---+---+
      |                               |
      /      item_1, ..., item_n      / variable
      |                               |
      +---+---+---+---+---+---+---+---+

      Reserved: Must be set to zero.

      PS: Indicates size of XI fields:
         PS = 0 indicates 4-bit XI fields;
         PS = 1 indicates 8-bit XI fields.

      m: Number of XI item(s) in the compressed list.







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      XI_1, ..., XI_m: m XI items.  Each XI represents one TCP option in
      the uncompressed packet, in the same order as they appear in the
      uncompressed packet.



         The format of an XI item is as follows:

                 +---+---+---+---+
         PS = 0: | X |   Index   |
                 +---+---+---+---+

                   0   1   2   3   4   5   6   7
                 +---+---+---+---+---+---+---+---+
         PS = 1: | X | Reserved  |     Index     |
                 +---+---+---+---+---+---+---+---+

         X: Indicates whether the item present in the list:

            X = 1 indicates that the item corresponding to the Index is
            sent in the item_1, ..., item_n list;
            X = 0 indicates that the item corresponding to the Index is
            not sent.

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

         Index: An index into the item table.  See Section 6.3.4


         When 4-bit XI items are used and, the XI items are placed in
         octets in the following manner:

           0   1   2   3   4   5   6   7
         +---+---+---+---+---+---+---+---+
         |     XI_k      |    XI_k + 1   |
         +---+---+---+---+---+---+---+---+

      Padding: A 4-bit padding field is present when PS = 0 and the
      number of XIs is odd.  The Padding field is set to zero when
      sending and ignored when receiving.

      Item 1, ..., item n: Each item corresponds to an XI with X = 1 in
      XI 1, ..., XI m.  Each entry in the item list is formatted as
      expressed by <option_type>_list_item in Section 8.

6.3.4.  Item Table Mappings

   The item table for TCP options list compression is limited to 16



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   different items, since it is unlikely that any packet flow will
   contain a larger number of unique options.

   The mapping between TCP option type and table indexes are listed in
   the table below:

         +-----------------+---------------+
         |   Option name   |  Table index  |
         +-----------------+---------------+
         |      NOP        |       0       |
         |      EOL        |       1       |
         |      MSS        |       2       |
         |  WINDOW SCALE   |       3       |
         |   TIMESTAMP     |       4       |
         | SACK-PERMITTED  |       5       |
         |      SACK       |       6       |
         | Generic options |      7-15     |
         +-----------------+---------------+

   Some TCP options are used more frequently than others.  To simplify
   their compression, a part of the item table is reserved for these
   option types, as shown on the table above.  Both the compressor and
   the decompressor MUST use these mappings between item and indexes to
   (de)compress TCP options compressed using list compression.

   It is expected that the option types for which an index is reserved
   in the item table will only appear once in a list.  However, if an
   option type is detected twice in the same options list and if both
   options have a different content, the compressor should compress the
   second occurrence of the option type by mapping it to a generic
   compressed option.  Otherwise, if the options have the exact same
   content, the compressor can still use the same table index for both.

   The NOP option

      The NOP option can appear more than once in the list.  However,
      since its value is always the same, no context information needs
      to be transmitted.  Multiple NOP options can thus be mapped to the
      same index.  Since the NOP option does not have any content when
      compressed as a list_item, it will never be present in the item
      list.  For consistency, the compressor should still establish an
      entry in the list by setting the presence bit, as done for the
      other type of options.
      List compression always preserves the original order of each item
      in the decompressed list, no matter if the item is present or not
      in the compressed list_item or if multiple items of the same type
      can be mapped to the same index, as for the NOP option.




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   The EOL option

      The size of the compressed format for the EOL option can be larger
      than one octet, and it is defined so that it includes the option
      padding.  This is because the EOL should terminate the parsing of
      the options, but it can also be followed by padding octets that
      all have the value zero.

   The Generic option

      The generic option can be used to compress any type of TCP option
      that does not have a reserved index in the item table.

6.3.5.  Compressed Lists in Dynamic Chain

   A compressed list for TCP options that is part of the dynamic chain
   (e.g. in IR or IR-DYN packets) must have all its list items present,
   i.e. all X-bits in the XI list MUST be set.

6.3.6.  Irregular Chain Items for TCP Options

   The list_item represents the option inside the compressed item list,
   and the irregular format is used for the option fields that are
   expected to change with each packet.  When an item of the specified
   type is present in the current context, these irregular fields are
   present in each compressed packet, as part of the irregular chain.
   Since many of the TCP option types are not expected to change for the
   duration of a flow, many of the irregular formats are empty.

   The irregular chain for TCP options is structured analogously to the
   structure of the TCP options in the uncompressed packet.  If a
   compressed list is present in the compressed packet, then the
   irregular chain for TCP options MUST NOT contain irregular items for
   the list items that are transmitted inside the compressed list (i.e.
   items in the list that have the X-bit set in its XI).  The items that
   are not present in the compressed list, but are present in the
   uncompressed list, MUST have their respective irregular items present
   in the irregular chain.

6.3.7.  Replication of TCP Options

   The entire table of TCP options items is always replicated when using
   the IR-CR packet.  In the IR-CR packet, the list of options for the
   new flow is also transmitted as a compressed list in the IR-CR
   packet.






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6.4.  Profile-specific Encoding Methods

   This section defines encoding methods that are specific to this
   profile.  These methods are used in the formal definition of the
   packet formats in Section 8.

6.4.1.  inferred_ip_v4_header_checksum

   This encoding method compresses the header checksum field of the IPv4
   header.  This checksum is defined in RFC 791 [RFC0791] as follows:

      Header Checksum: 16 bits

         A checksum on the header only.  Since some header fields change
         (e.g., time to live), this is recomputed and verified at each
         point that the internet header is processed.

      The checksum algorithm is:

         The checksum field is the 16 bit one's complement of the one's
         complement sum of all 16 bit words in the header.  For purposes
         of computing the checksum, the value of the checksum field is
         zero.

   As described above, the header checksum protects individual hops from
   processing a corrupted header.  When almost all IP header information
   is compressed away, and when decompression is verified by a CRC
   computed over the original header for every compressed packet, there
   is no point in having this additional checksum; instead it can be
   recomputed at the decompressor side.

   The "inferred_ip_v4_header_checksum" encoding method thus compresses
   the IPv4 header checksum down to a size of zero bit, i.e. no bits are
   transmitted in compressed headers for this field.  Using this
   encoding method, the decompressor infers the value of this field
   using the computation above.

   The compressor MAY use the header checksum to validate the
   correctness of the header before compressing it, to avoid compressing
   a corrupted header.

6.4.2.  inferred_mine_header_checksum

   This encoding method compresses the minimal encapsulation header
   checksum.  This checksum is defined in RFC 2004 [RFC2004] as follows:






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      Header Checksum

         The 16-bit one's complement of the one's complement sum of all
         16-bit words in the minimal forwarding header.  For purposes of
         computing the checksum, the value of the checksum field is 0.
         The IP header and IP payload (after the minimal forwarding
         header) are not included in this checksum computation.

   The "inferred_mine_header_checksum" encoding method compresses the
   minimal encapsulation header checksum down to a size of zero bit,
   i.e. no bits are transmitted in compressed headers for this field.
   Using this encoding method, the decompressor infers the value of this
   field using the above computation.

   The motivations for inferring this checksum are similar to the ones
   explained above in Section 6.4.1.

   The compressor MAY use the minimal encapsulation header checksum to
   validate the correctness of the header before compressing it, to
   avoid compressing a corrupted header.

6.4.3.  inferred_ip_v4_length

   This encoding method compresses the total length field of the IPv4
   header.  The total length field of the IPv4 header is defined in RFC
   791 [RFC0791] as follows:

      Total Length: 16 bits

         Total Length is the length of the datagram, measured in octets,
         including internet header and data.  This field allows the
         length of a datagram to be up to 65,535 octets.

   The "inferred_ip_v4_length" encoding method compresses the IPv4
   header checksum down to a size of zero bit, i.e. no bits are
   transmitted in compressed headers for this field.  Using this
   encoding method, the decompressor infers the value of this field by
   counting in octets the length of the entire packet after
   decompression.

6.4.4.  inferred_ip_v6_length

   This encoding method compresses the payload length field in the IPv6
   header.  This length field is defined in RFC 2460 [RFC2460] as
   follows:






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      Payload Length: 16-bit unsigned integer

         Length of the IPv6 payload, i.e., the rest of the packet
         following this IPv6 header, in octets.  (Note that any
         extension headers present are considered part of the payload,
         i.e., included in the length count.)

   The "inferred_ip_v6_length" encoding method compresses the payload
   length field of the IPv6 header down to a size of zero bit, i.e. no
   bits are transmitted in compressed headers for this field.  Using
   this encoding method, the decompressor infers the value of this field
   by counting in octets the length of the entire packet after
   decompression.

6.4.5.  inferred_offset

   This encoding method compresses the data offset field of the TCP
   header.

   The inferred_offset encoding method is used on the data offset field
   of the TCP header.  This field is defined in RFC 793 as:

      Data Offset: 4 bits

         The number of 32 bit words in the TCP Header.  This indicates
         where the data begins.  The TCP header (even one including
         options) is an integral number of 32 bits long.

   The "inferred_offset" encoding method compresses the data offset
   field of the TCP header down to a size of zero bit, i.e. no bits are
   transmitted in compressed headers for this field.  Using this
   encoding method, the decompressor infers the value of this field by
   first decompressing the TCP options list, and by then setting:

              data offset = (options length / 4) + 5

   Note: The equation above uses integer arithmetic.

6.4.6.  baseheader_extension_headers

   In CO packets (see Section 7.3), the innermost IP header and the TCP
   header are combined to create a compressed base header.  In some
   cases the IP header will have a number of extension headers between
   itself and the TCP header.  In order to remain formally correct, the
   base header must define some representation of these extension
   headers, which is what this encoding method is used for.  This
   encoding method skips over all the extension headers and does not
   encode any of the fields.  Changed fields in these headers are



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   encoded in the irregular chain.

6.4.7.  baseheader_outer_headers

   This encoding method, as well as the baseheader_extension_headers
   encoding method described above, is needed for the specification to
   remain formally correct.  It is used in CO packets (see Section 7.3)
   to describe tunneling IP headers and their respective extension
   headers (i.e. all headers located before the innermost IP header).
   This encoding method skips over all the fields in these headers and
   does not perform any encoding.  Changed fields in outer headers are
   instead handled by the irregular chain.

6.4.8.  Scaled TCP Sequence Number Encoding

   For some TCP flows, such as data transfers, the payload size will be
   constant over periods of time.  For such flows, the TCP Sequence
   number is bound to increase by multiples of the payload size between
   packets.  ROHC-TCP provides a method to use scaled compression of the
   TCP sequence number to improve compression efficiency in such case.

   When scaling the TCP Sequence number, the residue is the sequence
   number offset from a multiple of the payload size.  The precondition
   for the compressor to start using this type of encoding is that the
   compressor must be confident that the decompressor has received a
   number of packets sufficient to establish the value of the residue of
   the scaling function.

   This confidence can be established by sending a number of packets
   that are compressed using an unscaled representation of the sequence
   number, when the payload size is constant.  The compressor can then
   start using the scaled sequence number encoding, where the sequence
   number is first downscaled by the value of the payload size and then
   LSB encoded.

   When the compressor detects that the value of the residue has
   changed, it MUST re-establish the residue value.  This is done by
   sending compressed packets that carry the sequence number compressed
   using its unscaled representation until it is confident that the
   decompressor has received the new value.

   When the sequence number wraps around, the value of the residue of
   the scaling function is likely to change, even when the payload size
   remains constant.  When this occurs, the compressor MUST reestablish
   the new residue value using the unscaled representation of the
   sequence number as described above.

   The scaling function applied to the TCP sequence number does not use



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   an explicit scaling factor, such as the TS_STRIDE used in RFC 3095
   [RFC3095].  Instead, the payload size is used as the scaling factor;
   as this value can be inferred from the length of the packet, there is
   no need to transmit this field explicitly.

   The expressions for compressing and decompressing the scaled sequence
   number are specified in the definitions of the packet format
   Section 8.2.

6.4.9.  Scaled Acknowledgment Number Encoding

   Similar to the pattern exhibited by the TCP Sequence number, the
   expected increase in the TCP Acknowledgment number will often be a
   multiple of the packet size.  For the TCP Sequence number, the
   compression scheme can use the payload size of the packets as a
   scaling factor (see section 6.1.6 above).

   For the TCP Acknowledgment number, the scaling factor depends on the
   size of packets flowing in the opposite direction; this information
   might not be available to the compressor/decompressor pair.  For this
   reason, ROHC-TCP uses an explicit scaling factor to compress the TCP
   Acknowledgment number.

   For the compressor to use the scaled acknowledgment number encoding,
   it MUST first explicitly transmit the value of the scaling factor
   (called ack_stride in the formal notation in section Section 8.2) to
   the decompressor, using one of the packet types that can carry this
   information.  Once the value of the scaling factor is established,
   before using this scaled encoding the compressor must have enough
   confidence that the decompressor has successfully calculated the
   residue of the scaling function for the acknowledgment number.  This
   is done the same way as for the scaled sequence number encoding (see
   Section 6.4.8 above).

   Once the compressor has gained enough confidence that both the value
   of the scaling factor and the value of the residue have been
   established in the decompressor, the compressor can start compressing
   packets using the scaled representation of the acknowledgment number.
   The compressor MUST NOT use the scaled acknowledgment number encoding
   with the value of the scaling factor (ack_stride) set to zero.

   The compressor MAY use the scaled acknowledgment number encoding;
   what value it will use as the scaling factor is up to the compressor
   implementation.  In the case where there is a co-located decompressor
   processing packets of the same TCP flow in the opposite direction,
   the scaling factor for the acknowledgment numbers can be set to the
   same value as the scaling factor of the TCP Sequence number used for
   that flow.



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6.5.  CRC Calculations

   The 3-bit and 7-bit CRCs both cover the entire uncompressed header
   chain.  Note that there is no separation between CRC-STATIC or CRC-
   DYNAMIC fields in ROHC-TCP, as opposed to profiles defined in
   [RFC3095].


7.  Packet Types (Normative)

   ROHC-TCP uses two different packet types: the Initialization and
   Refresh (IR) packet type, and the Compressed packet type (CO).

   Each packet type defines a number of packet formats: three packet
   formats are defined for the IR type, and two sets of eight base
   header formats are defined for the CO type with one additional format
   that is common to both sets.

7.1.  Initialization and Refresh Packets (IR)

   ROHC-TCP uses the basic structure of the ROHC IR and IR-DYN packets
   as defined in [RFC3095] (section 5.2.3. and 5.2.4. respectively).

   Packet type: IR

      This packet type communicates the static part and the dynamic part
      of the context.
























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   For the ROHC-TCP IR packet, the value of the x bit MUST be set to
   one.  It has the following format:

        0   1   2   3   4   5   6   7
       --- --- --- --- --- --- --- ---
      :        Add-CID octet          : if for small CIDs and (CID != 0)
      +---+---+---+---+---+---+---+---+
      | 1   1   1   1   1   1   0   1 | IR type octet
      +---+---+---+---+---+---+---+---+
      :                               :
      /       0-2 octets of CID       / 1-2 octets if for large CIDs
      :                               :
      +---+---+---+---+---+---+---+---+
      |         Profile = 0x06        | 1 octet
      +---+---+---+---+---+---+---+---+
      |              CRC              | 1 octet
      +---+---+---+---+---+---+---+---+
      |                               |
      /         Static chain          / variable length
      |                               |
       - - - - - - - - - - - - - - - -
      |                               |
      /         Dynamic chain         / variable length
      |                               |
       - - - - - - - - - - - - - - - -
      |                               |
      /            Payload            / variable length
      |                               |
       - - - - - - - - - - - - - - - -

      CRC: 8-bit CRC, computed according to section 5.9.1 of [RFC3095].

      Static chain: See Section 6.2.

      Dynamic chain: See Section 6.2.

      Payload: The payload of the corresponding original packet, if any.
      The presence of a payload is inferred from the packet length.

   Packet type: IR-DYN

      This packet type communicates the dynamic part of the context.









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   The ROHC-TCP IR-DYN packet has the following format:

        0   1   2   3   4   5   6   7
       --- --- --- --- --- --- --- ---
      :         Add-CID octet         : if for small CIDs and (CID != 0)
      +---+---+---+---+---+---+---+---+
      | 1   1   1   1   1   0   0   0 | IR-DYN type octet
      +---+---+---+---+---+---+---+---+
      :                               :
      /       0-2 octets of CID       / 1-2 octets if for large CIDs
      :                               :
      +---+---+---+---+---+---+---+---+
      |         Profile = 0x06        | 1 octet
      +---+---+---+---+---+---+---+---+
      |              CRC              | 1 octet
      +---+---+---+---+---+---+---+---+
      |                               |
      /         Dynamic chain         / variable length
      |                               |
       - - - - - - - - - - - - - - - -
      |                               |
      /            Payload            / variable length
      |                               |
       - - - - - - - - - - - - - - - -

      CRC: 8-bit CRC, computed according to section 5.9.1 of [RFC3095].

      Dynamic chain: See Section 6.2.

      Payload: The payload of the corresponding original packet, if any.
      The presence of a payload is inferred from the packet length.

7.2.  Context Replication Packets (IR-CR)

   Context replication requires a dedicated IR packet format that
   uniquely identifies the IR-CR packet for the ROHC-TCP profile.  This
   section defines the profile-specific part of the IR-CR packet
   [RFC4164].

   Packet type: IR-CR

      This packet type communicates a reference to a base context along
      with the static and dynamic parts of the replicated context that
      differs from the base context.

   The ROHC-TCP IR-CR packet follows the general format of the ROHC CR
   packet, as defined in ROHC-CR [RFC4164], section 3.4.2.  With
   consideration to the extensibility of the IR packet type defined in



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   RFC 3095 [RFC3095], the ROHC-TCP profile supports context replication
   through the profile specific part of the IR packet.  This is achieved
   using the bit (x) left in the IR packet header for "Profile specific
   information".  For ROHC-TCP, this bit is defined as a flag indicating
   whether this packet is an IR packet or an IR-CR packet.  For the
   ROHC-TCP IR-CR packet, the value of the x bit MUST be set to zero.

   The ROHC-TCP IR-CR has the following format:

        0   1   2   3   4   5   6   7
       --- --- --- --- --- --- --- ---
      :         Add-CID octet         : if for small CIDs and (CID != 0)
      +---+---+---+---+---+---+---+---+
      | 1   1   1   1   1   1   0   0 | IR-CR type octet
      +---+---+---+---+---+---+---+---+
      :                               :
      /       0-2 octets of CID       / 1-2 octets if for large CIDs
      :                               :
      +---+---+---+---+---+---+---+---+
      |         Profile = 0x06        | 1 octet
      +---+---+---+---+---+---+---+---+
      |              CRC              | 1 octet
      +---+---+---+---+---+---+---+---+
      | B |             CRC7          | 1 octet
      +---+---+---+---+---+---+---+---+
      :   Reserved    |   Base CID    : 1 octet, for small CID, if B=1
      +---+---+---+---+---+---+---+---+
      :                               :
      /           Base CID            / 1-2 octets, for large CIDs,
      :                               : if B=1
      +---+---+---+---+---+---+---+---+
      |                               |
      /        Replicate chain        / variable length
      |                               |
       - - - - - - - - - - - - - - - -
      |                               |
      /            Payload            / variable length
      |                               |
       - - - - - - - - - - - - - - - -

      B: B = 1 indicates that the Base CID field is present.

      CRC: See [RFC4164].








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      CRC7: The CRC over the original, uncompressed, header.  This 7-bit
      CRC is computed according section Section 6.5.

      Reserved: MUST be set to zero when transmitting, decompressor MUST
      verify that the value of this field is zero.

      Base CID: CID of base context.  Encoded according to [RFC4164],
      Section 3.5.3.
      Replicate chain: See Section 6.2.

      Payload: The payload of the corresponding original packet, if any.
      The presence of a payload is inferred from the packet length.

7.3.  Compressed Packets (CO)

   The ROHC-TCP CO packets communicate irregularities in the packet
   header.  All CO packets carry a CRC and can update the context.

   The general format for a compressed TCP header is as follows:

         0   1   2   3   4   5   6   7
        --- --- --- --- --- --- --- ---
       :         Add-CID octet         :  if for small CIDs and CID 1-15
       +---+---+---+---+---+---+---+---+
       |   First octet of base header  |  (with type indication)
       +---+---+---+---+---+---+---+---+
       :                               :
       /   0, 1, or 2 octets of CID    /  1-2 octets if large CIDs
       :                               :
       +---+---+---+---+---+---+---+---+
       /   Remainder of base header    /  variable number of octets
       +---+---+---+---+---+---+---+---+
       :                               :
       /        Irregular chain        /  variable
       :                               :
        --- --- --- --- --- --- --- ---
       :                               :
       /  TCP Options Irregular Part   /  variable
       :                               :
        --- --- --- --- --- --- --- ---
       |                               |
       /            Payload            / variable length
       |                               |
        - - - - - - - - - - - - - - - -

   Base header: The compressed representation of the innermost IP header
   and the TCP header in the uncompressed packet.  The full set of base
   headers are described in Section 8.



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   Irregular chain: See Section 6.2.

   TCP options irregular part: See Section 6.3.6.

   Payload: The payload of the corresponding original packet, if any.
   The presence of a payload is inferred from the packet length.


8.  Packet Formats (Normative)

   This section describes the set of compressed TCP/IP packet formats.
   The normative description of the packet formats is given using a
   formal notation, the ROHC-FN [ROHC-FN].  The formal description of
   the packet formats specifies all of the information needed to
   compress and decompress a header relative to the context.

   In particular, the notation provides a list of all the fields present
   in the uncompressed and compressed TCP/IP headers, and defines how to
   map from each uncompressed packet to its compressed equivalent and
   vice versa.  See the ROHC-FN [ROHC-FN] for an explanation of the
   formal notation itself, and for a description of the encoding methods
   used to compress each of the fields in the TCP/IP header.

8.1.  Design rationale for compressed base headers

   The compressed packet formats are defined as two separate sets: one
   set for the packets where the innermost IP header contains a
   sequential IP-ID (either network byte order or byte swapped), and one
   set for the packets without sequential IP-ID (either random, zero, or
   no IP-ID).

   These two sets of packet formats are referred to as the "sequential"
   and the "random" set of packet format.

   In addition, there is a common compressed packet that can be used
   regardless of the type of IP-ID behavior.  This common packet can
   transmit rarely changing fields and also send the frequently changing
   field coded in variable lengths.  The common packet format can also
   change the value of control fields such as IP-ID behavior and ECN
   behavior.

   All compressed base headers contain a 3-bit CRC, unless they update
   control fields such as "ip_id_behavior" or "ecn_used" that affect the
   interpretation of subsequent packets.  Packets that can modify these
   control fields will carry a 7-bit CRC instead.

   The encoding methods used in the compressed base headers are based on
   the following design criteria:



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   o  MSN
         Since the MSN is a number generated by the compressor, it only
         needs to be large enough to ensure robust operation and to
         accommodate a small amount of reordering [RFC4163].  Therefore,
         each compressed base header contains 4 bits of MSN and the LSB
         offset value is set to p=4 to handle a reordering depth of up
         to 4 packets.  Additional guidance to improve performance when
         a larger amount of reordering is possible can be found in
         [RFC4224]

   o  TCP Sequence number
         ROHC-TCP has the capability to handle bulk data transfers
         efficiently, for which the sequence number is expected to
         increase by about 1460 bytes (which can be represented by 11
         bits).  For the compressed base headers to handle
         retransmissions (i.e. negative delta to the sequence number),
         the LSB interpretation interval must handle negative offsets
         about as large as positive offset, which means that one more
         bit is needed.

         Also, for ROHC-TCP to be robust to losses, two additional bits
         are added to the LSB encoding of the sequence number.  This
         means that the base headers should contain at least 14 bits of
         LSB-encoded sequence number when present.  According to the
         logic above, the LSB offset value p is set to be as large as
         the positive offset, i.e. p = 2^(k-1)-1, where k is the number
         of LSB-encoded bits that are transmitted in the base header.

   o  TCP Acknowledgment number
         The design criterion for the acknowledgment number is similar
         to that of the TCP Sequence number.  However, often only every
         other data packet is acknowledged, which means that the
         expected delta value is twice as large as for sequence numbers.
         Therefore, at least 15 bits of acknowledgment number should be
         used in compressed base headers.  Since the acknowledgment
         number is expected to constantly increase, and the only
         exception to this is packet reordering (either on the ROHC
         channel [RFC3759] or prior to the compression point), the
         negative offset for LSB encoding is set to be 1/4 of the total
         interval, i.e. p = 2^(k-2)-1.

   o  TCP Window
         The TCP Window field is expected to increase in increments of
         similar size as the TCP Sequence number, and therefore the
         design criterion for the TCP window has been to send at least
         14 bits when used.





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   o  IP-ID
         For the "sequential" set of packet formats, all the compressed
         base headers contains LSB encoded IP-ID offset bits.  The
         requirement is that at least 3 bits of IP-ID should always be
         present, but it is preferable to use 4 to 7 bits.  When k=3,
         p=1 and if k>3, then p=3 since the offset is expected to
         increase most of the time.

   Each set of packet formats contains eight different compressed base
   headers.  The reason for having this large number of packets is that
   the TCP Sequence number, TCP Acknowledgment number, TCP Window and
   MSN are frequently changing in a non-linear pattern.

   The design of the packet formats is derived from the field behavior
   analysis found in [RFC4413].

   All of the compressed base headers transmit LSB-encoded MSN bits, the
   push flag and a CRC, and in addition to this, all the base headers in
   the sequential packet format set contains LSB encoded IP-ID bits.

   The following packet formats exist in both the sequential and random
   packet format sets:

   o  Format 1: This packet format transmits changes to the TCP Sequence
      number and is expected to be used on the downstream of a data
      transfer.

   o  Format 2: This packet format transmits the TCP Sequence number in
      scaled form, and is expected to be used on the downstream of a
      data transfer where the payload size is constant for multiple
      packets.

   o  Format 3: This packet format transmits changes in the TCP
      Acknowledgment number, and is expected to be used in the
      acknowledgment direction of data transfer.

   o  Format 4: This packet format is similar to format 3, but sends a
      scaled TCP Acknowledgment number.

   o  Format 5: This packet format transmits both the TCP sequence
      number and the TCP Acknowledgment number, and is expected to be
      used useful for flows that send data in both directions.

   o  Format 6: This packet format is similar to format 5, but sends the
      TCP Sequence number in scaled form, when the payload size is
      static for certain intervals in a data flow.





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   o  Format 7: This packet format transmits changes to both the TCP
      Acknowledgment number and the TCP window, and is expected to be
      used for the acknowledgment flows of data connections.

   o  Format 8: This packet format is used to transmit changes to some
      of the more seldom changing fields in the TCP flow, such as ECN
      behavior, RST/SYN/FIN flags, the TTL/Hop Limit and the TCP options
      list.  This format carries a 7-bit CRC, since it can change the
      contents of the irregular chain in later packets.  Note that this
      can be seen as a reduced form of the common packet format.

   o  Common packet format: The common packet format can be used for all
      kinds of IP-ID behavior, and should be used when some of the more
      rarely changing fields in the IP or TCP header changes.  Since
      this packet format change control fields that decide how the
      decompressor interprets packets, it carries a 7-bit CRC to reduce
      the probability of context corruption.  This packet can basically
      change all the dynamic fields in the IP and TCP header, and it
      uses a large set of flags to provide information about which
      fields are present in the packet format.

8.2.  Formal Definition in ROHC-FN

   ////////////////////////////////////////////
   // Constants
   ////////////////////////////////////////////

   IP_ID_BEHAVIOR_SEQUENTIAL = 0;
   IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED = 1;
   IP_ID_BEHAVIOR_RANDOM = 2;
   IP_ID_BEHAVIOR_ZERO = 3;

   ////////////////////////////////////////////
   // Global control fields
   ////////////////////////////////////////////

   CONTROL {
     ecn_used            [ 1 ];
     msn                 [ 16 ];

   }

   ///////////////////////////////////////////////
   // Encoding methods not specified in FN syntax:
   ///////////////////////////////////////////////

   list_tcp_options               "defined in Section 6.3.3";
   inferred_ip_v4_header_checksum "defined in Section 6.4.1";



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   inferred_mine_header_checksum  "defined in Section 6.4.2";
   inferred_ip_v4_length          "defined in Section 6.4.3";
   inferred_ip_v6_length          "defined in Section 6.4.4";
   inferred_offset                "defined in Section 6.4.5";
   baseheader_extension_headers   "defined in Section 6.4.6";
   baseheader_outer_headers       "defined in Section 6.4.7";

   ////////////////////////////////////////////
   // General encoding methods
   ////////////////////////////////////////////

   static_or_irreg(flag, width)
   {
     UNCOMPRESSED {
       field [ width ];
     }

     COMPRESSED irreg_enc {
       field =:= irregular(width) [ width ];
       ENFORCE(flag == 1);
     }

     COMPRESSED static_enc {
       field =:= static [ 0 ];
       ENFORCE(flag == 0);
     }
   }

   variable_length_32_enc(flag)
   {
     UNCOMPRESSED {
       field [ 32 ];
     }

     COMPRESSED not_present {
       field =:= static [ 0 ];
       ENFORCE(flag == 0);
     }

     COMPRESSED 8_bit {
       field =:= lsb(8, 63) [ 8 ];
       ENFORCE(flag == 1);
     }

     COMPRESSED 16_bit {
       field =:= lsb(16, 16383) [ 16 ];
       ENFORCE(flag == 2);
     }



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     COMPRESSED 32_bit {
       field =:= irregular(32) [ 32 ];
       ENFORCE(flag == 3);
     }
   }

   optional32(flag)
   {
     UNCOMPRESSED {
       item [ 0, 32 ];
     }

     COMPRESSED present {
       item =:= irregular(32) [ 32 ];
       ENFORCE(flag == 1);
     }

     COMPRESSED not_present {
       item =:= compressed_value(0, 0) [ 0 ];
       ENFORCE(flag == 0);
     }
   }

   lsb_7_or_31
   {
     UNCOMPRESSED {
       item [ 32 ];
     }

     COMPRESSED lsb_7 {
       discriminator =:= '0'       [ 1 ];
       item          =:= lsb(7, 8) [ 7 ];
     }

     COMPRESSED lsb_31 {
       discriminator =:= '1'          [ 1 ];
       item          =:= lsb(31, 256) [ 31 ];
     }
   }

   opt_lsb_7_or_31(flag)
   {
     UNCOMPRESSED {
       item [ 0, 32 ];
     }

     COMPRESSED present {
       item =:= lsb_7_or_31 [ 8, 32 ];



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       ENFORCE(flag == 1);
     }

     COMPRESSED not_present {
       item =:= compressed_value(0, 0) [ 0 ];
       ENFORCE(flag == 0);
     }
   }

   crc3(data_value, data_length)
   {
     UNCOMPRESSED {
     }

     COMPRESSED {
       crc_value =:=
         crc(3, 0x06, 0x07, data_value, data_length) [ 3 ];
     }
   }

   crc7(data_value, data_length)
   {
     UNCOMPRESSED {
     }

     COMPRESSED {
       crc_value =:=
         crc(7, 0x79, 0x7f, data_value, data_length) [ 7 ];
     }
   }

   // Encoding method for updating a scaled field and its associated
   // control fields. Should be used both when the value is scaled
   // or unscaled in a compressed format.
   field_scaling(stride_value, scaled_value, unscaled_value)
   {
     UNCOMPRESSED {
       residue_field [ 32 ];
     }

     COMPRESSED no_scaling {
       ENFORCE(stride_value == 0);
       ENFORCE(residue_field.UVALUE == unscaled_value);
       ENFORCE(scaled_value == 0);
     }

     COMPRESSED scaling_used {
       ENFORCE(stride_value != 0);



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       ENFORCE(residue_field.UVALUE == (unscaled_value % stride_value));
       ENFORCE(unscaled_value ==
               scaled_value * stride_value + residue_field.UVALUE);
     }
   }

   ////////////////////////////////////////////
   // IPv6 Destination options header
   ////////////////////////////////////////////

   ip_dest_opt
   {
     UNCOMPRESSED {
       next_header [ 8 ];
       length      [ 8 ];
       value       [ VARIABLE ];
     }

     DEFAULT {
       length      =:= static;
       next_header =:= static;
       value       =:= static;
     }

     COMPRESSED dest_opt_static {
       next_header =:= irregular(8) [ 8 ];
       length      =:= irregular(8) [ 8 ];
     }

     COMPRESSED dest_opt_dynamic {
       value =:= irregular(length.UVALUE * 64 + 48) [ VARIABLE ];
     }

     COMPRESSED dest_opt_replicate_0 {
       discriminator =:= '00000000' [ 8 ];
     }

     COMPRESSED dest_opt_replicate_1 {
       discriminator =:= '10000000'                     [ 8 ];
       length        =:= irregular(8)                   [ 8 ];
       value         =:= irregular(length.UVALUE*64+48) [ VARIABLE ];
     }
   }

   ////////////////////////////////////////////
   // IPv6 Hop-by-Hop options header
   ////////////////////////////////////////////




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   ip_hop_opt
   {
     UNCOMPRESSED {
       next_header [ 8 ];
       length      [ 8 ];
       value       [ VARIABLE ];
     }

     DEFAULT {
       length      =:= static;
       next_header =:= static;
       value       =:= static;
     }

     COMPRESSED hop_opt_static {
       next_header =:= irregular(8) [ 8 ];
       length      =:= irregular(8) [ 8 ];
     }

     COMPRESSED hop_opt_dynamic {
       value =:= irregular(length.UVALUE*64+48) [ VARIABLE ];
     }

     COMPRESSED hop_opt_replicate_0 {
       discriminator =:= '00000000' [ 8 ];
     }

     COMPRESSED hop_opt_replicate_1 {
       discriminator =:= '10000000'                     [ 8 ];
       length        =:= irregular(8)                   [ 8 ];
       value         =:= irregular(length.UVALUE*64+48) [ VARIABLE ];
     }
   }

   ////////////////////////////////////////////
   // IPv6 Routing header
   ////////////////////////////////////////////

   ip_rout_opt
   {
     UNCOMPRESSED {
       next_header [ 8 ];
       length      [ 8 ];
       value       [ VARIABLE ];
     }

     DEFAULT {
       length      =:= static;



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       next_header =:= static;
       value       =:= static;
     }

     COMPRESSED rout_opt_static {
       next_header =:= irregular(8)                   [ 8 ];
       length      =:= irregular(8)                   [ 8 ];
       value       =:= irregular(length.UVALUE*64+48) [ VARIABLE ];
     }

     COMPRESSED rout_opt_dynamic {
     }

     COMPRESSED rout_opt_replicate_0 {
       discriminator =:= '00000000' [ 8 ];
     }

     COMPRESSED rout_opt_replicate_1 {
       discriminator =:= '10000000'                     [ 8 ];
       length        =:= irregular(8)                   [ 8 ];
       value         =:= irregular(length.UVALUE*64+48) [ VARIABLE ];
     }
   }

   ////////////////////////////////////////////
   // GRE Header
   ////////////////////////////////////////////

   optional_checksum(flag_value)
   {
     UNCOMPRESSED {
       value     [ 0, 16 ];
       reserved1 [ 0, 16 ];
     }

     COMPRESSED cs_present {
       value     =:= irregular(16)             [ 16 ];
       reserved1 =:= uncompressed_value(16, 0) [ 0 ];
       ENFORCE(flag_value == 1);
     }

     COMPRESSED not_present {
       value     =:= compressed_value(0, 0) [ 0 ];
       reserved1 =:= compressed_value(0, 0) [ 0 ];
       ENFORCE(flag_value == 0);
     }
   }




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   gre_proto
   {
     UNCOMPRESSED {
       protocol [ 16 ];
     }

     COMPRESSED ether_v4 {
       discriminator =:= compressed_value(1, 0) [ 1 ];
       protocol      =:= uncompressed_value(16, 0x0800);
     }

     COMPRESSED ether_v6 {
       discriminator =:= compressed_value(1, 1) [ 1 ];
       protocol      =:= uncompressed_value(16, 0x86DD);
     }
   }

   gre
   {
     UNCOMPRESSED {
       c_flag                                 [ 1 ];
       r_flag    =:= uncompressed_value(1, 0) [ 1 ];
       k_flag                                 [ 1 ];
       s_flag                                 [ 1 ];
       reserved0 =:= uncompressed_value(9, 0) [ 9 ];
       version   =:= uncompressed_value(3, 0) [ 3 ];
       protocol                               [ 16 ];
       checksum_and_res                       [ 0, 32 ];
       key                                    [ 0, 32 ];
       sequence_number                        [ 0, 32 ];
     }

     DEFAULT {
       c_flag           =:= static;
       k_flag           =:= static;
       s_flag           =:= static;
       protocol         =:= static;
       key              =:= static;
       sequence_number  =:= static;
     }

     COMPRESSED gre_static {
       protocol =:= gre_proto                 [ 1 ];
       c_flag   =:= irregular(1)              [ 1 ];
       k_flag   =:= irregular(1)              [ 1 ];
       s_flag   =:= irregular(1)              [ 1 ];
       padding  =:= compressed_value(4, 0)    [ 4 ];
       key      =:= optional32(k_flag.UVALUE) [ 0, 32 ];



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     }

     COMPRESSED gre_dynamic {
       checksum_and_res =:=
         optional_checksum(c_flag.UVALUE)             [ 0, 16 ];
       sequence_number  =:= optional32(s_flag.UVALUE) [ 0, 32 ];
     }

     COMPRESSED gre_replicate_0 {
       discriminator    =:= '00000000'    [ 8 ];
       checksum_and_res =:=
         optional_checksum(c_flag.UVALUE) [ 0, 16 ];
       sequence_number  =:=
         optional32(s_flag.UVALUE)        [ 0, 8, 32 ];
     }

     COMPRESSED gre_replicate_1 {
       discriminator    =:= '10000'                   [ 5 ];
       c_flag           =:= irregular(1)              [ 1 ];
       k_flag           =:= irregular(1)              [ 1 ];
       s_flag           =:= irregular(1)              [ 1 ];
       checksum_and_res =:=
         optional_checksum(c_flag.UVALUE)             [ 0, 16 ];
       key              =:= optional32(k_flag.UVALUE) [ 0, 32 ];
       sequence_number  =:= optional32(s_flag.UVALUE) [ 0, 32 ];
     }

     COMPRESSED gre_irregular {
       checksum_and_res =:=
         optional_checksum(c_flag.UVALUE) [ 0, 16 ];
       sequence_number  =:=
         opt_lsb_7_or_31(s_flag.UVALUE)   [ 0, 8, 32 ];
     }
   }

   /////////////////////////////////////////////
   // MINE header
   /////////////////////////////////////////////

   mine
   {
     UNCOMPRESSED {
       next_header [ 8 ];
       s_bit       [ 1 ];
       res_bits    [ 7 ];
       checksum    [ 16 ];
       orig_dest   [ 32 ];
       orig_src    [ 0, 32 ];



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     }

     DEFAULT {
       next_header =:= static;
       s_bit       =:= static;
       res_bits    =:= static;
       checksum    =:= inferred_mine_header_checksum;
       orig_dest   =:= static;
       orig_src    =:= static;
     }

     COMPRESSED mine_static {
       next_header =:= irregular(8)             [ 8 ];
       s_bit       =:= irregular(1)             [ 1 ];
       // Reserved are included - no benefit in removing them
       res_bits    =:= irregular(7)             [ 7 ];
       orig_dest   =:= irregular(32)            [ 32 ];
       orig_src    =:= optional32(s_bit.UVALUE) [ 0, 32 ];
     }

     COMPRESSED mine_dynamic {
     }

     COMPRESSED mine_replicate_0 {
       discriminator =:= '00000000' [ 8 ];
     }

     COMPRESSED mine_replicate_1 {
       discriminator =:= '10000000'               [ 8 ];
       s_bit         =:= irregular(1)             [ 1 ];
       res_bits      =:= irregular(7)             [ 7 ];
       orig_dest     =:= irregular(32)            [ 32 ];
       orig_src      =:= optional32(s_bit.UVALUE) [ 0, 32 ];
     }
   }

   /////////////////////////////////////////////
   // Authentication Header (AH)
   /////////////////////////////////////////////

   ah
   {
     UNCOMPRESSED {
       next_header     [ 8 ];
       length          [ 8 ];
       res_bits        [ 16 ];
       spi             [ 32 ];
       sequence_number [ 32 ];



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       auth_data       [ VARIABLE ];
     }

     DEFAULT {
       next_header     =:= static;
       length          =:= static;
       res_bits        =:= static;
       spi             =:= static;
       sequence_number =:= static;
     }

     COMPRESSED ah_static {
       next_header =:= irregular(8)  [ 8 ];
       length      =:= irregular(8)  [ 8 ];
       spi         =:= irregular(32) [ 32 ];
     }

     COMPRESSED ah_dynamic {
       res_bits        =:= irregular(16) [ 16 ];
       sequence_number =:= irregular(32) [ 32 ];
       auth_data       =:=
         irregular(length.UVALUE*32-32)  [ VARIABLE ];
     }

     COMPRESSED ah_replicate_0 {
       discriminator   =:= '00000000'    [ 8 ];
       sequence_number =:= irregular(32) [ 32 ];
       auth_data       =:=
         irregular(length.UVALUE*32-32)  [ VARIABLE ];
     }

     COMPRESSED ah_replicate_1 {
       discriminator   =:= '10000000'    [ 8 ];
       length          =:= irregular(8)  [ 8 ];
       res_bits        =:= irregular(16) [ 16 ];
       spi             =:= irregular(32) [ 32 ];
       sequence_number =:= irregular(32) [ 32 ];
       auth_data       =:=
         irregular(length.UVALUE*32-32)  [ VARIABLE ];
     }

     COMPRESSED ah_irregular {
       sequence_number =:= lsb_7_or_31  [ 8, 32 ];
       auth_data       =:=
         irregular(length.UVALUE*32-32) [ VARIABLE ];
     }
   }




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   /////////////////////////////////////////////
   // ESP header (NULL encrypted)
   /////////////////////////////////////////////

   // Since the "next header" field is located in the packet trailer
   // and ROHC-FN requires all UNCOMPRESSED fields to be contiguous,
   // the values of the next header field is passed as a parameter.
   // To avoid forcing the decompression to access the trailer part of
   // the packet, the next header is istead handled with a control field
   esp_null(next_header_value)
   {
     UNCOMPRESSED {
       spi             [ 32 ];
       sequence_number [ 32 ];
     }

     CONTROL {
       nh_field [ 8 ];
     }

     DEFAULT {
       spi             =:= static;
       sequence_number =:= static;
       nh_field        =:= static;
     }

     COMPRESSED esp_static {
       nh_field =:= compressed_value(8, next_header_value) [ 8 ];
       spi      =:= irregular(32)                          [ 32 ];
     }

     COMPRESSED esp_dynamic {
       sequence_number =:= irregular(32) [ 32 ];
     }

     COMPRESSED esp_replicate_0 {
       discriminator   =:= '00000000'    [ 8 ];
       sequence_number =:= irregular(32) [ 32 ];
     }

     COMPRESSED esp_replicate_1 {
       discriminator   =:= '10000000'    [ 8 ];
       spi             =:= irregular(32) [ 32 ];
       sequence_number =:= irregular(32) [ 32 ];
     }

     COMPRESSED esp_irregular {
       sequence_number =:= lsb_7_or_31 [ 8, 32 ];



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     }
   }

   /////////////////////////////////////////////
   // Encoding methods common for IPv4 and IPv6
   /////////////////////////////////////////////

   irreg_tos_tc
   {
     UNCOMPRESSED {
       tos_tc [ 6 ];
     }

     COMPRESSED tos_tc_present {
       tos_tc =:= irregular(6) [ 6 ];
       ENFORCE(ecn_used.UVALUE == 1);
     }

     COMPRESSED tos_tc_not_present {
       tos_tc =:= static [ 0 ];
       ENFORCE(ecn_used.UVALUE == 0);
     }
   }

   ip_irreg_ecn
   {
     UNCOMPRESSED {
       ip_ecn_flags [ 2 ];
     }

     COMPRESSED tc_present {
       ip_ecn_flags =:= irregular(2) [ 2 ];
       ENFORCE(ecn_used.UVALUE == 1);
     }

     COMPRESSED tc_not_present {
       ip_ecn_flags =:= static [ 0 ];
       ENFORCE(ecn_used.UVALUE == 0);
     }
   }

   /////////////////////////////////////////////
   // IPv6 Header
   /////////////////////////////////////////////

   fl_enc
   {
     UNCOMPRESSED {



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       flow_label [ 20 ];
     }

     COMPRESSED fl_zero {
       discriminator =:= '0'                       [ 1 ];
       flow_label    =:= uncompressed_value(20, 0) [ 0 ];
       reserved      =:= '0000'                    [ 4 ];
     }

     COMPRESSED fl_non_zero {
       discriminator =:= '1'           [ 1 ];
       flow_label    =:= irregular(20) [ 20 ];
     }
   }

   // The is_innermost flag should be true if this is the innermost
   // IP header to be compressed.
   // If extracting the irregular chain for an compressed packet,
   // the TTL argument must have the same value as it had when
   // processing co_baseheader. If extracting any other chain
   // items, this argument is not used.
   // ip_inner_ecn is bound in this encoding method and the value that
   // it gets bound to should be passed to the tcp encoding method
   // when processing the irregular chain for TCP.
   ipv6(is_innermost, ttl_irregular_chain_flag, ip_inner_ecn)
   {
     UNCOMPRESSED {
       version         =:= uncompressed_value(4, 6) [ 4 ];
       tos_tc                                       [ 6 ];
       ip_ecn_flags                                 [ 2 ];
       flow_label                                   [ 20 ];
       payload_length                               [ 16 ];
       next_header                                  [ 8 ];
       ttl_hopl                                     [ 8 ];
       src_addr                                     [ 128 ];
       dst_addr                                     [ 128 ];
     }

     DEFAULT {
       tos_tc         =:= static;
       ip_ecn_flags   =:= static;
       flow_label     =:= static;
       payload_length =:= inferred_ip_v6_length;
       next_header    =:= static;
       ttl_hopl       =:= static;
       src_addr       =:= static;
       dst_addr       =:= static;
     }



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     COMPRESSED ipv6_static {
       version_flag =:= '1'            [ 1 ];
       reserved     =:= '00'           [ 2 ];
       flow_label   =:= fl_enc         [ 5, 21 ];
       next_header  =:= irregular(8)   [ 8 ];
       src_addr     =:= irregular(128) [ 128 ];
       dst_addr     =:= irregular(128) [ 128 ];
     }

     COMPRESSED ipv6_dynamic {
       tos_tc       =:= irregular(6) [ 6 ];
       ip_ecn_flags =:= irregular(2) [ 2 ];
       ttl_hopl     =:= irregular(8) [ 8 ];
     }

     COMPRESSED ipv6_replicate {
       tos_tc       =:= irregular(6) [ 6 ];
       ip_ecn_flags =:= irregular(2) [ 2 ];
       reserved     =:= '000'        [ 3 ];
       flow_label   =:= fl_enc       [ 5, 21 ];
     }

     COMPRESSED ipv6_outer_irregular_without_ttl {
       tos_tc       =:= irreg_tos_tc [ 0, 6 ];
       ip_ecn_flags =:= ip_irreg_ecn [ 0, 2 ];
       ENFORCE(ttl_irregular_chain_flag == 0);
       ENFORCE(is_innermost == false);
     }

     COMPRESSED ipv6_outer_irregular_with_ttl {
       tos_tc       =:= irreg_tos_tc [ 0, 6 ];
       ip_ecn_flags =:= ip_irreg_ecn [ 0, 2 ];
       ttl_hopl     =:= irregular(8) [ 8 ];
       ENFORCE(ttl_irregular_chain_flag == 1);
       ENFORCE(is_innermost == false);
     }

     COMPRESSED ipv6_innermost_irregular {
       ENFORCE(ip_inner_ecn == ip_ecn_flags.UVALUE);
       ENFORCE(is_innermost == true);
     }
   }

   /////////////////////////////////////////////
   // IPv4 Header
   /////////////////////////////////////////////

   ip_id_enc_dyn(behavior)



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   {
     UNCOMPRESSED {
       ip_id [ 16 ];
     }

     COMPRESSED ip_id_seq {
       ip_id =:= irregular(16) [ 16 ];
       ENFORCE((behavior == IP_ID_BEHAVIOR_SEQUENTIAL) ||
               (behavior == IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED) ||
               (behavior == IP_ID_BEHAVIOR_RANDOM));
     }

     COMPRESSED ip_id_zero {
       ip_id =:= uncompressed_value(16, 0) [ 0 ];
       ENFORCE(behavior == IP_ID_BEHAVIOR_ZERO);
     }
   }

   ip_id_enc_irreg(behavior)
   {
     UNCOMPRESSED {
       ip_id [ 16 ];
     }

     COMPRESSED ip_id_seq {
       ENFORCE(behavior == IP_ID_BEHAVIOR_SEQUENTIAL);
     }

     COMPRESSED ip_id_seq_swapped {
       ENFORCE(behavior == IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED);
     }

     COMPRESSED ip_id_rand {
       ip_id =:= irregular(16) [ 16 ];
       ENFORCE(behavior == IP_ID_BEHAVIOR_RANDOM);
     }

     COMPRESSED ip_id_zero {
       ip_id =:= uncompressed_value(16, 0) [ 0 ];
       ENFORCE(behavior == IP_ID_BEHAVIOR_ZERO);
     }
   }

   ip_id_behavior_choice
   {
     UNCOMPRESSED {
       behavior [ 2 ];
     }



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     DEFAULT {
       behavior =:= irregular(2);
     }

     COMPRESSED sequential {
       behavior [ 2 ];
       ENFORCE(behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL);
     }

     COMPRESSED sequential_swapped {
       behavior [ 2 ];
       ENFORCE(behavior.UVALUE ==
               IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED);
     }

     COMPRESSED random {
       behavior [ 2 ];
       ENFORCE(behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM);
     }

     COMPRESSED zero {
       behavior [ 2 ];
       ENFORCE(behavior.UVALUE == IP_ID_BEHAVIOR_ZERO);
     }
   }

   // The is_innermost flag should be true if this is the innermost
   // IP header to be compressed.
   // If extracting the irregular chain for an compressed packet,
   // the TTL argument must have the same value as it had when
   // processing co_baseheader. If extracting any other chain
   // items, this argument is not used.
   // ip_inner_ecn is set in this encoding method and the value that
   // it gets bound to should be passed to the tcp encoding method
   ipv4(is_innermost, ttl_irregular_chain_flag, ip_inner_ecn)
   {
     UNCOMPRESSED {
       version        =:= uncompressed_value(4, 4)  [ 4 ];
       hdr_length     =:= uncompressed_value(4, 5)  [ 4 ];
       tos_tc                                       [ 6 ];
       ip_ecn_flags                                 [ 2 ];
       length                                       [ 16 ];
       ip_id                                        [ 16 ];
       rf             =:= uncompressed_value(1, 0)  [ 1 ];
       df                                           [ 1 ];
       mf             =:= uncompressed_value(1, 0)  [ 1 ];
       frag_offset    =:= uncompressed_value(13, 0) [ 13 ];
       ttl_hopl                                     [ 8 ];



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       protocol                                     [ 8 ];
       checksum                                     [ 16 ];
       src_addr                                     [ 32 ];
       dst_addr                                     [ 32 ];
     }

     CONTROL {
       ip_id_behavior [ 2 ];
     }

     DEFAULT {
       tos_tc         =:= static;
       ip_ecn_flags   =:= static;
       length         =:= inferred_ip_v4_length;
       df             =:= static;
       ttl_hopl       =:= static;
       protocol       =:= static;
       checksum       =:= inferred_ip_v4_header_checksum;
       src_addr       =:= static;
       dst_addr       =:= static;
       ip_id_behavior =:= static;
     }

     COMPRESSED ipv4_static {
       version_flag =:= '0'           [ 1 ];
       reserved     =:= '0000000'     [ 7 ];
       protocol     =:= irregular(8)  [ 8 ];
       src_addr     =:= irregular(32) [ 32 ];
       dst_addr     =:= irregular(32) [ 32 ];
     }

     COMPRESSED ipv4_dynamic {
       reserved       =:= '00000'             [ 5 ];
       df             =:= irregular(1)        [ 1 ];
       // compressor chooses behavior of IP-ID
       ip_id_behavior =:= ip_id_behavior_choice [ 2 ];
       tos_tc         =:= irregular(6)        [ 6 ];
       ip_ecn_flags   =:= irregular(2)        [ 2 ];
       ttl_hopl       =:= irregular(8)        [ 8 ];
       ip_id          =:=
         ip_id_enc_dyn(ip_id_behavior.UVALUE) [ 0, 16 ];
     }

     COMPRESSED ipv4_replicate {
       reserved       =:= '0000'              [ 4 ];
       ip_id_behavior =:= ip_id_behavior_choice [ 2 ];
       ttl_flag       =:= irregular(1)        [ 1 ];
       df             =:= irregular(1)        [ 1 ];



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       tos_tc         =:= irregular(6)        [ 6 ];
       ip_ecn_flags   =:= irregular(2)        [ 2 ];
       ip_id          =:=
         ip_id_enc_dyn(ip_id_behavior.UVALUE) [ 0, 16 ];
       ttl_hopl     =:=
           static_or_irreg(ttl_flag.UVALUE, 8) [ 0, 8 ];
     }

     COMPRESSED ipv4_outer_irregular_without_ttl {
       ip_id          =:=
         ip_id_enc_irreg(ip_id_behavior.UVALUE) [ 0, 16 ];
       tos_tc         =:= irreg_tos_tc          [ 0, 6 ];
       ip_ecn_flags   =:= ip_irreg_ecn          [ 0, 2 ];
       ENFORCE(ttl_irregular_chain_flag == 0);
       ENFORCE(is_innermost == false);
     }

     COMPRESSED ipv4_outer_irregular_with_ttl {
       ip_id          =:=
         ip_id_enc_irreg(ip_id_behavior.UVALUE) [ 0, 16 ];
       tos_tc         =:= irreg_tos_tc          [ 0, 6 ];
       ip_ecn_flags   =:= ip_irreg_ecn          [ 0, 2 ];
       ttl_hopl       =:= irregular(8)          [ 8 ];
       ENFORCE(is_innermost == false);
       ENFORCE(ttl_irregular_chain_flag == 1);
     }

     COMPRESSED ipv4_innermost_irregular {
       ip_id          =:=
         ip_id_enc_irreg(ip_id_behavior.UVALUE) [ 0, 16 ];
       ENFORCE(ip_inner_ecn == ip_ecn_flags.UVALUE);
       ENFORCE(is_innermost == true);
     }
   }

   /////////////////////////////////////////////
   // TCP Options
   /////////////////////////////////////////////

   //
   // EOL marks the end of the option list and, based on
   // the description in RFC 793 and the BSB TCP code,
   // nothing after this should be processed...
   // So, ignore everything after the EOL option
   // (according to 793 it must be 0)
   //
   // The length of the padding needs to be transmitted with the
   // compressed list since the length of the list can be unknown to



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   // the decompressor.
   //
   tcp_opt_eol(nbits)
   {
     UNCOMPRESSED {
       type     =:= uncompressed_value(8, 0) [ 8 ];
       padding  =:=
         uncompressed_value(nbits-8, 0)      [ VARIABLE ];
     }

     CONTROL {
       pad_len [ 8 ];
     }

     DEFAULT {
       pad_len =:= static;
     }

     COMPRESSED eol_list_item {
       pad_len =:= compressed_value(8, nbits-8) [ 8 ];
     }

     COMPRESSED eol_irregular {
       ENFORCE(nbits-8 == pad_len.UVALUE);
     }
   }

   tcp_opt_nop
   {
     UNCOMPRESSED {
       type =:= uncompressed_value(8, 1) [ 8 ];
     }

     COMPRESSED nop_list_item {
     }

     COMPRESSED nop_irregular {
     }
   }

   tcp_opt_mss
   {
     UNCOMPRESSED {
       type   =:= uncompressed_value(8, 2) [ 8 ];
       length =:= uncompressed_value(8, 4) [ 8 ];
       mss                                 [ 16 ];
     }




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     DEFAULT {
       mss    =:= static;
     }

     COMPRESSED mss_list_item {
       mss =:= irregular(16) [ 16 ];
     }

     COMPRESSED mss_irregular {
     }
   }

   tcp_opt_wscale
   {
     UNCOMPRESSED {
       type   =:= uncompressed_value(8, 3) [ 8 ];
       length =:= uncompressed_value(8, 3) [ 8 ];
       wscale                              [ 8 ];
     }

     DEFAULT {
       wscale =:= static;
     }

     COMPRESSED wscale_list_item {
       wscale =:= irregular(8) [ 8 ];
     }

     COMPRESSED wscale_irregular {
     }
   }

   ts_lsb
   {
     UNCOMPRESSED {
       tsval [ 32 ];
     }

     // Few bits (7 and 14) bits can only increase, while the larger
     // formats allow decreasing timestamp to handle reordering before
     // the compression point.

     COMPRESSED tsval_7 {
       discriminator =:= '0'        [ 1 ];
       tsval         =:= lsb(7, -1) [ 7 ];
     }

     COMPRESSED tsval_14 {



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       discriminator =:= '10'        [ 2 ];
       tsval         =:= lsb(14, -1) [ 14 ];
     }

     COMPRESSED tsval_21 {
       discriminator =:= '110'               [ 3 ];
       tsval         =:= lsb(21, 0x00040000) [ 21 ];
     }

     COMPRESSED tsval_29 {
       discriminator =:= '111'               [ 3 ];
       tsval         =:= lsb(29, 0x04000000) [ 29 ];
     }
   }

   tcp_opt_tsopt
   {
     UNCOMPRESSED {
       type   =:= uncompressed_value(8, 8)  [ 8 ];
       length =:= uncompressed_value(8, 10) [ 8 ];
       tsval                                [ 32 ];
       tsecho                               [ 32 ];
     }

     COMPRESSED tsopt_list_item {
       tsval  =:= irregular(32) [ 32 ];
       tsecho =:= irregular(32) [ 32 ];
     }

     COMPRESSED tsopt_irregular {
       tsval  =:= ts_lsb [ 8, 16, 24, 32 ];
       tsecho =:= ts_lsb [ 8, 16, 24, 32 ];
     }
   }

   sack_var_length_enc(base)
   {
     UNCOMPRESSED {
       sack_field [ 32 ];
     }

     CONTROL {
       sack_offset [ 32 ];
       ENFORCE(sack_offset.UVALUE == (sack_field.UVALUE - base));
     }


     COMPRESSED lsb_15 {



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       discriminator =:= '0'         [ 1 ];
       sack_offset   =:= lsb(15, -1) [ 15 ];
     }

     COMPRESSED lsb_22 {
       discriminator =:= '10'        [ 2 ];
       sack_offset   =:= lsb(22, -1) [ 22 ];
     }

     COMPRESSED lsb_30 {
       discriminator =:= '11'        [ 2 ];
       sack_offset   =:= lsb(30, -1) [ 30 ];
     }
   }

   tcp_opt_sack_block(prev_block_end)
   {
     UNCOMPRESSED {
       block_start [ 32 ];
       block_end   [ 32 ];
     }

     COMPRESSED {
       block_start =:=
         sack_var_length_enc(prev_block_end) [ 16, 24, 32 ];
       block_end   =:=
         sack_var_length_enc(block_start)    [ 16, 24, 32 ];
     }
   }

   tcp_opt_sack(ack_value)
   {
     // The ACK value from the TCP header is needed as input parameter.

     UNCOMPRESSED {
       type    =:= uncompressed_value(8, 5) [ 8 ];
       length                               [ 8 ];
       block_1                              [ 64 ];
       block_2                              [ 0, 64 ];
       block_3                              [ 0, 64 ];
       block_4                              [ 0, 64 ];
     }

     DEFAULT {
       length  =:= static;
       block_2 =:= uncompressed_value(0, 0);
       block_3 =:= uncompressed_value(0, 0);
       block_4 =:= uncompressed_value(0, 0);



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     }

     COMPRESSED sack1_list_item {
       discriminator =:= '00000001';
       block_1       =:= tcp_opt_sack_block(ack_value);
       ENFORCE(length.UVALUE == 10);
     }

     COMPRESSED sack2_list_item {
       discriminator =:= '00000010';
       block_1       =:= tcp_opt_sack_block(ack_value);
       block_2       =:= tcp_opt_sack_block(block_1_end.UVALUE);
       ENFORCE(length.UVALUE == 18);
     }

     COMPRESSED sack3_list_item {
       discriminator =:= '00000011';
       block_1       =:= tcp_opt_sack_block(ack_value);
       block_2       =:= tcp_opt_sack_block(block_1_end.UVALUE);
       block_3       =:= tcp_opt_sack_block(block_2_end.UVALUE);
       ENFORCE(length.UVALUE == 26);
     }

     COMPRESSED sack4_list_item {
       discriminator =:= '00000100';
       block_1       =:= tcp_opt_sack_block(ack_value);
       block_2       =:= tcp_opt_sack_block(block_1_end.UVALUE);
       block_3       =:= tcp_opt_sack_block(block_2_end.UVALUE);
       block_4       =:= tcp_opt_sack_block(block_3_end.UVALUE);
       ENFORCE(length.UVALUE == 34);
     }

     COMPRESSED sack_unchanged_irregular {
       discriminator =:= '00000000';
       block_1       =:= static;
       block_2       =:= static;
       block_3       =:= static;
       block_4       =:= static;
     }

     COMPRESSED sack1_irregular {
       discriminator =:= '00000001';
       block_1       =:= tcp_opt_sack_block(ack_value);
       ENFORCE(length.UVALUE == 10);
     }

     COMPRESSED sack2_irregular {
       discriminator =:= '00000010';



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       block_1       =:= tcp_opt_sack_block(ack_value);
       block_2       =:= tcp_opt_sack_block(block_1_end.UVALUE);
       ENFORCE(length.UVALUE == 18);
     }

     COMPRESSED sack3_irregular {
       discriminator =:= '00000011';
       block_1       =:= tcp_opt_sack_block(ack_value);
       block_2       =:= tcp_opt_sack_block(block_1_end.UVALUE);
       block_3       =:= tcp_opt_sack_block(block_2_end.UVALUE);
       ENFORCE(length.UVALUE == 26);
     }

     COMPRESSED sack4_irregular {
       discriminator =:= '00000100';
       block_1       =:= tcp_opt_sack_block(ack_value);
       block_2       =:= tcp_opt_sack_block(block_1_end.UVALUE);
       block_3       =:= tcp_opt_sack_block(block_2_end.UVALUE);
       block_4       =:= tcp_opt_sack_block(block_3_end.UVALUE);
       ENFORCE(length.UVALUE == 34);
     }
   }

   tcp_opt_sack_permitted
   {
     UNCOMPRESSED {
       type   =:= uncompressed_value(8, 4) [ 8 ];
       length =:= uncompressed_value(8, 2) [ 8 ];
     }

     COMPRESSED sack_permitted_list_item {
     }

     COMPRESSED sack_permitted_irregular {
     }
   }

   tcp_opt_generic
   {
     UNCOMPRESSED {
       type                                    [ 8 ];
       length_msb =:= uncompressed_value(1, 0) [ 1 ];
       length_lsb                              [ 7 ];
       contents                                [ VARIABLE ];
     }

     CONTROL {
       option_static [ 1 ];



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     }

     DEFAULT {
       type       =:= static;
       length_lsb =:= static;
       contents   =:= static;
     }

     COMPRESSED generic_list_item {
       type          =:= irregular(8)       [ 8 ];
       option_static =:= irregular(1)      [ 1 ];
       length_lsb    =:= irregular(7)      [ 7 ];
       contents      =:=
         irregular(length_len.UVALUE*8-16) [ VARIABLE ];
     }

     // Used when context of option has option_static set to one
     COMPRESSED generic_irregular_static {
       ENFORCE(option_static.UVALUE == 1);
     }

     // An item that can change, but currently is unchanged
     COMPRESSED generic_irregular_stable {
       discriminator =:= '11111111' [ 8 ];
       ENFORCE(option_static.UVALUE == 0);
     }

     // An item that can change, and has changed compared to context.
     // Length is not allowed to change here, since a length change is
     // most likely to cause new NOPs or an EOL length change.
     COMPRESSED generic_irregular_full {
       discriminator =:= '00000000'        [ 8 ];
       contents      =:=
         irregular(length_lsb.UVALUE*8-16) [ VARIABLE ];
       ENFORCE(option_static.UVALUE == 0);
     }
   }

   tcp_list_presence_enc(presence)
   {
     UNCOMPRESSED {
       tcp_options;
     }

     COMPRESSED list_not_present {
       tcp_options =:= static [ 0 ];
       ENFORCE(presence == 0);
     }



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     COMPRESSED list_present {
       tcp_options =:= list_tcp_options [ VARIABLE ];
       ENFORCE(presence == 1);
     }
   }

   /////////////////////////////////////////////
   // TCP Header
   /////////////////////////////////////////////

   port_replicate(flags)
   {
     UNCOMPRESSED {
       port [ 16 ];
     }

     COMPRESSED port_static_enc {
       port =:= static [ 0 ];
       ENFORCE(flags == 0b00);
     }

     COMPRESSED port_lsb8 {
       port =:= lsb(8, 64) [ 8 ];
       ENFORCE(flags == 0b01);
     }

     COMPRESSED port_irr_enc {
       port =:= irregular(16) [ 16 ];
       ENFORCE(flags == 0b10);
     }
   }

   zero_or_irr16_enc(flag)
   {
     UNCOMPRESSED {
       field [ 16 ];
     }

     COMPRESSED non_zero {
       field =:= irregular(16) [ 16 ];
       ENFORCE(flag == 0);
     }

     COMPRESSED zero {
       field =:= uncompressed_value(16, 0) [ 0 ];
       ENFORCE(flag == 1);
     }
   }



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   ack_enc_dyn(flag)
   {
     UNCOMPRESSED {
       ack_number [ 32 ];
     }

     COMPRESSED ack_non_zero {
       ack_number =:= irregular(32) [ 32 ];
       ENFORCE(flag == 0);
     }

     COMPRESSED ack_zero {
       ack_number =:= uncompressed_value(32, 0) [ 0 ];
       ENFORCE(flag == 1);
     }
   }

   ecn_choice
   {
     UNCOMPRESSED {
       ecn_field [ 1 ];
     }

     COMPRESSED zero {
       ecn_field [ 1 ];
       ENFORCE(ecn_field.UVALUE == 0);
     }

     COMPRESSED nonzero {
       ecn_field [ 1 ];
       ENFORCE(ecn_field.UVALUE == 1);
     }
   }

   tcp_ecn_flags_enc
   {
     UNCOMPRESSED {
       tcp_ecn_flags [ 2 ];
     }

     COMPRESSED irreg {
       tcp_ecn_flags =:= irregular(2) [ 2 ];
       ENFORCE(ecn_used.UVALUE == 1);
     }

     COMPRESSED unused {
       tcp_ecn_flags =:= static [ 0 ];
       ENFORCE(ecn_used.UVALUE == 0);



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     }
   }

   tcp_res_flags_enc
   {
     UNCOMPRESSED {
       tcp_res_flags [ 4 ];
     }

     COMPRESSED irreg {
       tcp_res_flags =:= irregular(4) [ 4 ];
       ENFORCE(ecn_used.UVALUE == 1);
     }

     COMPRESSED unused {
       tcp_res_flags =:= uncompressed_value(4, 0) [ 0 ];
       ENFORCE(ecn_used.UVALUE == 0);
     }
   }

   tcp_irreg_ip_ecn(ip_inner_ecn)
   {
     UNCOMPRESSED {
       ip_ecn_flags [ 2 ];
     }

     COMPRESSED ecn_present {
       ip_ecn_flags =:=
         compressed_value(2, ip_inner_ecn) [ 2 ];
       ENFORCE(ecn_used.UVALUE == 1);
     }

     COMPRESSED ecn_not_present {
       ip_ecn_flags =:= static [ 0 ];
       ENFORCE(ecn_used.UVALUE == 0);
     }
   }

   rsf_index_enc
   {
     UNCOMPRESSED {
       rsf_flag [ 3 ];
     }

     COMPRESSED none {
       rsf_idx  =:= '00' [ 2 ];
       rsf_flag =:= uncompressed_value(3, 0x00);
     }



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     COMPRESSED rst_only {
       rsf_idx  =:= '01' [ 2 ];
       rsf_flag =:= uncompressed_value(3, 0x04);
     }

     COMPRESSED syn_only {
       rsf_idx  =:= '10' [ 2 ];
       rsf_flag =:= uncompressed_value(3, 0x02);
     }

     COMPRESSED fin_only {
       rsf_idx  =:= '11' [ 2 ];
       rsf_flag =:= uncompressed_value(3, 0x01);
     }
   }

   optional_2bit_padding(used_flag)
   {
     UNCOMPRESSED {
     }

     COMPRESSED used {
       padding =:= compressed_value(2, 0x0) [ 2 ];
       ENFORCE(used_flag == 1);
     }

     COMPRESSED unused {
       padding =:= compressed_value(0, 0x0);
       ENFORCE(used_flag == 0);
     }
   }

   // ack_stride_value is the user-selected stride for scaling the
   // TCP ack_number
   // ip_inner_ecn is the value bound when processing the innermost
   // IP header (ipv4 or ipv6 encoding method)
   tcp(payload_size, ack_stride_value, ip_inner_ecn)
   {
     UNCOMPRESSED {
       src_port      [ 16 ];
       dst_port      [ 16 ];
       seq_number    [ 32 ];
       ack_number    [ 32 ];
       data_offset   [ 4 ];
       tcp_res_flags [ 4 ];
       tcp_ecn_flags [ 2 ];
       urg_flag      [ 1 ];
       ack_flag      [ 1 ];



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       psh_flag      [ 1 ];
       rsf_flags     [ 3 ];
       window        [ 16 ];
       checksum      [ 16 ];
       urg_ptr       [ 16 ];
       options       [ n ];
     }

     CONTROL {
       seq_number_scaled                    [ 32 ];
       seq_number_residue =:=
           field_scaling(payload_size, seq_number_scaled.UVALUE,
                         seq_number.UVALUE) [ 32 ];
       ack_stride                           [ 16 ];
       ack_number_scaled                    [ 32 ];
       ack_number_residue =:=
           field_scaling(ack_stride.UVALUE, ack_number_scaled.UVALUE,
                         ack_number.UVALUE) [ 32 ];
       ENFORCE(ack_stride.UVALUE == ack_stride_value);
     }

     DEFAULT {
       src_port      =:= static;
       dst_port      =:= static;
       seq_number    =:= static;
       ack_number    =:= static;
       data_offset   =:= inferred_offset;
       tcp_res_flags =:= static;
       tcp_ecn_flags =:= static;
       urg_flag      =:= static;
       ack_flag      =:= uncompressed_value(1, 1);
       rsf_flags     =:= static;
       window        =:= static;
       urg_ptr       =:= static;
     }

     COMPRESSED tcp_static {
       src_port =:= irregular(16) [ 16 ];
       dst_port =:= irregular(16) [ 16 ];
     }

     COMPRESSED tcp_dynamic {
       ecn_used        =:= ecn_choice              [ 1 ];
       ack_stride_flag =:= irregular(1)            [ 1 ];
       ack_zero        =:= irregular(1)            [ 1 ];
       urp_zero        =:= irregular(1)            [ 1 ];
       tcp_res_flags   =:= irregular(4)            [ 4 ];
       tcp_ecn_flags   =:= irregular(2)            [ 2 ];



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       urg_flag        =:= irregular(1)            [ 1 ];
       ack_flag        =:= irregular(1)            [ 1 ];
       psh_flag        =:= irregular(1)            [ 1 ];
       rsf_flags       =:= irregular(3)            [ 3 ];
       msn             =:= irregular(16)           [ 16 ];
       seq_number      =:= irregular(32)           [ 32 ];
       ack_number      =:=
         ack_enc_dyn(ack_zero.CVALUE)              [ 0, 32 ];
       window          =:= irregular(16)           [ 16 ];
       checksum        =:= irregular(16)           [ 16 ];
       urg_ptr         =:=
         zero_or_irr16_enc(urp_zero.CVALUE)        [ 0, 16 ];
       ack_stride      =:=
         static_or_irr16(ack_stride_flag.CVALUE)   [ 0, 16 ];
       options         =:= list_tcp_options        [ VARIABLE ];
     }

     COMPRESSED tcp_replicate {
       reserved          =:= '0'                      [ 1 ];
       window_presence   =:= irregular(1)             [ 1 ];
       list_present      =:= irregular(1)             [ 1 ];
       src_port_presence =:= irregular(2)             [ 2 ];
       dst_port_presence =:= irregular(2)             [ 2 ];
       ack_stride_flag   =:= irregular(1)             [ 1 ];
       ack_presence      =:= irregular(1)             [ 1 ];
       urp_presence      =:= irregular(1)             [ 1 ];
       urg_flag          =:= irregular(1)             [ 1 ];
       ack_flag          =:= irregular(1)             [ 1 ];
       psh_flag          =:= irregular(1)             [ 1 ];
       rsf_flags         =:= rsf_index_enc            [ 2 ];
       ecn_used          =:= ecn_choice               [ 1 ];
       msn               =:= irregular(16)            [ 16 ];
       seq_number        =:= irregular(32)            [ 32 ];
       src_port          =:=
         port_replicate(src_port_presence)      [ 0, 8, 16 ];
       dst_port          =:=
         port_replicate(dst_port_presence)      [ 0, 8, 16 ];
       window            =:=
         static_or_irreg(window_presence, 16)   [ 0, 16 ];
       urg_point         =:=
         static_or_irreg(urp_presence, 16)    [ 0, 16 ];
       ack_number        =:=
         static_or_irreg(ack_presence, 32)    [ 0, 32 ];
       ecn_padding       =:=
         optional_2bit_padding(ecn_used.CVALUE)       [ 0, 2 ];
       tcp_res_flags     =:= tcp_res_flags_enc        [ 0, 4 ];
       tcp_ecn_flags     =:= tcp_ecn_flags_enc        [ 0, 2 ];
       checksum          =:= irregular(16)            [ 16 ];



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       ack_stride        =:=
         static_or_irr16(ack_stride_flag.CVALUE)    [ 0, 16 ];
       options           =:=
         tcp_list_presence_enc((data_offset.UVALUE-5)*32,
                               list_present.CVALUE,
                               ack_number.UVALUE)     [ VARIABLE ];
     }

     // ECN from innermost IP header is taken from global control field
     COMPRESSED tcp_irregular {
       ip_ecn_flags  =:= tcp_irreg_ip_ecn(ip_inner_ecn)  [ 0, 2 ];
       tcp_res_flags =:= tcp_res_flags_enc               [ 0, 4 ];
       tcp_ecn_flags =:= tcp_ecn_flags_enc               [ 0, 2 ];
       checksum      =:= irregular(16)                   [ 16 ];
     }
   }

   ///////////////////////////////////////////////////
   // Encoding methods used in compressed base headers
   ///////////////////////////////////////////////////

   tos_tc_enc(flag)
   {
     UNCOMPRESSED {
       tos_tc [ 6 ];
     }

     COMPRESSED static_enc {
       tos_tc =:= static [ 0 ];
       ENFORCE(flag == 0);
     }

     COMPRESSED irreg {
       tos_tc  =:= irregular(6)           [ 6 ];
       padding =:= compressed_value(2, 0) [ 2 ];
       ENFORCE(flag == 1);
     }
   }

   ip_id_lsb(behavior, k, p)
   {
     UNCOMPRESSED {
       ip_id [ 16 ];
     }

     CONTROL {
       ip_id_offset [ 16 ];
       ip_id_nbo    [ 16 ];



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     }

     COMPRESSED nbo {
       ip_id_offset =:= lsb(k, p) [ VARIABLE ];
       ENFORCE(behavior == IP_ID_BEHAVIOR_SEQUENTIAL);
       ENFORCE(ip_id_offset.UVALUE == ip_id.UVALUE - msn.UVALUE);
     }

     COMPRESSED non_nbo {
       ip_id_offset =:= lsb(k, p) [ VARIABLE ];
       ENFORCE(behavior == IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED);
       ENFORCE(ip_id_nbo.UVALUE ==
               (ip_id.UVALUE / 256) + (ip_id.UVALUE % 256) * 256);
       ENFORCE(ip_id_nbo.ULENGTH == 16);
       ENFORCE(ip_id_offset.UVALUE == ip_id_nbo.UVALUE - msn.UVALUE);
     }
   }

   optional_ip_id_lsb(behavior, indicator)
   {
     UNCOMPRESSED {
       ip_id [ 16 ];
     }

     COMPRESSED short {
       ip_id =:= ip_id_lsb(behavior, 8, 3) [ 8 ];
       ENFORCE((behavior == IP_ID_BEHAVIOR_SEQUENTIAL) ||
               (behavior == IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
       ENFORCE(indicator == 0);
     }

     COMPRESSED long {
       ip_id =:= irregular(16)  [ 16 ];
       ENFORCE((behavior == IP_ID_BEHAVIOR_SEQUENTIAL) ||
               (behavior == IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
       ENFORCE(indicator == 1);
     }

     COMPRESSED not_present {
       ENFORCE((behavior == IP_ID_BEHAVIOR_RANDOM) ||
               (behavior == IP_ID_BEHAVIOR_ZERO));
     }
   }

   dont_fragment(version)
   {
     UNCOMPRESSED {
       df [ 1 ];



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     }

     COMPRESSED v4 {
       df =:= irregular(1) [ 1 ];
       ENFORCE(version == 4);
     }

     COMPRESSED v6 {
       df =:= compressed_value(1, 0) [ 1 ];
       ENFORCE(version == 6);
     }
   }

   //////////////////////////////////
   // Actual start of compressed packet formats
   // Important note:
   // The base header is the compressed representation
   // of the innermost IP header AND the TCP header.
   //////////////////////////////////

   // ttl_irregular_chain_flag is set by the user if the TTL/Hop Limit
   // of an outer header has changed. The same value must be passed as
   // an argument to the ipv4/ipv6 encoding methods when extracting
   // the irregular chain items.
   co_baseheader(payload_size, ack_stride_value,
                 ttl_irregular_chain_flag)
   {
     UNCOMPRESSED v4 {
       outer_headers  =:= baseheader_outer_headers        [ VARIABLE ];
       version        =:= uncompressed_value(4, 4)        [ 4 ];
       header_length  =:= uncompressed_value(4, 5)        [ 4 ];
       tos_tc                                             [ 6 ];
       ip_ecn_flags                                       [ 2 ];
       length                                             [ 16 ];
       ip_id                                              [ 16 ];
       rf             =:= uncompressed_value(1, 0)        [ 1 ];
       df                                                 [ 1 ];
       mf             =:= uncompressed_value(1, 0)        [ 1 ];
       frag_offset    =:= uncompressed_value(13, 0)       [ 13 ];
       ttl_hopl                                           [ 8 ];
       next_header                                        [ 8 ];
       checksum                                           [ 16 ];
       src_addr                                           [ 32 ];
       dest_addr                                          [ 32 ];
       extension_headers =:= baseheader_extension_headers [ VARIABLE ];
       src_port                                           [ 16 ];
       dest_port                                          [ 16 ];
       seq_number                                         [ 32 ];



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       ack_number                                         [ 32 ];
       data_offset                                        [ 4 ];
       tcp_res_flags                                      [ 4 ];
       tcp_ecn_flags                                      [ 2 ];
       urg_flag                                           [ 1 ];
       ack_flag                                           [ 1 ];
       psh_flag                                           [ 1 ];
       rsf_flags                                          [ 3 ];
       window                                             [ 16 ];
       tcp_checksum                                       [ 16 ];
       urg_ptr                                            [ 16 ];
       options_list                                       [ VARIABLE ];
     }

     UNCOMPRESSED v6 {
       outer_headers  =:= baseheader_outer_headers        [ VARIABLE ];
       version =:= uncompressed_value(4, 6)               [ 4 ];
       tos_tc                                             [ 6 ];
       ip_ecn_flags                                       [ 2 ];
       flow_label                                         [ 20 ];
       payload_length                                     [ 16 ];
       next_header                                        [ 8 ];
       ttl_hopl                                           [ 8 ];
       src_addr                                           [ 128 ];
       dest_addr                                          [ 128 ];
       extension_headers =:= baseheader_extension_headers [ VARIABLE ];
       src_port                                           [ 16 ];
       dest_port                                          [ 16 ];
       seq_number                                         [ 32 ];
       ack_number                                         [ 32 ];
       data_offset                                        [ 4 ];
       tcp_res_flags                                      [ 4 ];
       tcp_ecn_flags                                      [ 2 ];
       urg_flag                                           [ 1 ];
       ack_flag                                           [ 1 ];
       psh_flag                                           [ 1 ];
       rsf_flags                                          [ 3 ];
       window                                             [ 16 ];
       tcp_checksum                                       [ 16 ];
       urg_ptr                                            [ 16 ];
       options_list                                       [ VARIABLE ];
       ENFORCE(ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM);
     }

     CONTROL {
       ip_id_behavior                       [ 2 ];
       seq_number_scaled                    [ 32 ];
       seq_number_residue =:=



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           field_scaling(payload_size, seq_number_scaled.UVALUE,
                         seq_number.UVALUE) [ 32 ];
       ack_stride                           [ 16 ];
       ack_number_scaled                    [ 32 ];
       ack_number_residue =:=
           field_scaling(ack_stride.UVALUE, ack_number_scaled.UVALUE,
                         ack_number.UVALUE) [ 32 ];
       ENFORCE(ack_stride_value == ack_stride.UVALUE);
     }

     DEFAULT {
       tcp_ecn_flags  =:= static;
       data_offset    =:= inferred_offset;
       tcp_res_flags  =:= static;
       rsf_flags      =:= uncompressed_value(3, 0);
       dest_port      =:= static;
       tos_tc         =:= static;
       src_port       =:= static;
       urg_flag       =:= uncompressed_value(1, 0);
       window         =:= static;
       dest_addr      =:= static;
       version        =:= static;
       ttl_hopl       =:= static;
       src_addr       =:= static;
       df             =:= static;
       ack_number     =:= static;
       urg_ptr        =:= static;
       seq_number     =:= static;
       ack_flag       =:= uncompressed_value(1, 1);
       options_list   =:= static;
       payload_length =:= inferred_ip_v6_length;
       checksum       =:= inferred_ip_v4_header_checksum;
       length         =:= inferred_ip_v4_length;
       flow_label     =:= static;
       next_header    =:= static;
       ip_ecn_flags   =:= static;
       // The tcp_checksum has no default,
       // it is considered a part of tcp_irregular
       ip_id_behavior =:= static;
       ecn_used       =:= static;

       // Default is to have no TTL in irregular chain
       // Can only be nonzero if co_common is used
       ENFORCE(ttl_irregular_chain_flag == 0);
     }

     ////////////////////////////////////////////
     // Common compressed packet format



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     ////////////////////////////////////////////

     COMPRESSED co_common {
       discriminator        =:= '1111101'                [ 7 ];
       ttl_hopl_outer_flag  =:=
           compressed_value(1, ttl_irregular_chain_flag) [ 1 ];
       ack_flag             =:= irregular(1)             [ 1 ];
       psh_flag             =:= irregular(1)             [ 1 ];
       rsf_flags            =:= rsf_index_enc            [ 2 ];
       msn                  =:= lsb(4, 4)                [ 4 ];
       seq_indicator        =:= irregular(2)             [ 2 ];
       ack_indicator        =:= irregular(2)             [ 2 ];
       ack_stride_indicator =:= irregular(1)             [ 1 ];
       window_indicator     =:= irregular(1)             [ 1 ];
       ip_id_indicator      =:= irregular(1)             [ 1 ];
       urg_ptr_present      =:= irregular(1)             [ 1 ];
       reserved             =:= compressed_value(1, 0)   [ 1 ];
       ecn_used             =:= ecn_choice               [ 1 ];
       tos_tc_present       =:= irregular(1)             [ 1 ];
       ttl_hopl_present     =:= irregular(1)             [ 1 ];
       list_present         =:= irregular(1)             [ 1 ];
       ip_id_behavior       =:= ip_id_behavior_choice    [ 2 ];
       urg_flag             =:= irregular(1)             [ 1 ];
       df                   =:= dont_fragment(version.UVALUE)   [ 1 ];
       header_crc           =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
       seq_number           =:=
         variable_length_32_enc(seq_indicator.CVALUE) [ 0, 8, 16, 32 ];
       ack_number           =:=
         variable_length_32_enc(ack_indicator.CVALUE) [ 0, 8, 16, 32 ];
       ack_stride           =:=
         static_or_irreg(ack_stride_indicator.CVALUE, 16)  [ 0, 16 ];
       window               =:=
         static_or_irreg(window_indicator.CVALUE, 16)      [ 0, 16 ];
       ip_id                =:=
         optional_ip_id_lsb(ip_id_behavior.UVALUE,
                            ip_id_indicator.CVALUE)      [ 0, 8, 16 ];
       urg_ptr              =:=
         static_or_irreg(urg_ptr_present.CVALUE, 16)     [ 0, 16 ];
       tos_tc               =:=
         tos_tc_enc(tos_tc_present.CVALUE)               [ 0, 8 ];
       ttl_hopl             =:=
         static_or_irreg(ttl_hopl_present.CVALUE, 8)     [ 0, 8 ];
       options_list         =:=
         tcp_list_presence_enc((data_offset.UVALUE - 5) * 32,
                               list_present.CVALUE,
                               ack_number.UVALUE)        [ VARIABLE ];
     }




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     // Send LSBs of sequence number
     COMPRESSED rnd_1 {
       discriminator =:= '101110'                        [ 6 ];
       seq_number    =:= lsb(18, 65535)                  [ 18 ];
       msn           =:= lsb(4, 4)                       [ 4 ];
       psh_flag      =:= irregular(1)                    [ 1 ];
       header_crc    =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
       ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM) ||
               (ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_ZERO));
     }

     // Send scaled sequence number LSBs
     COMPRESSED rnd_2 {
       discriminator      =:= '1100'                          [ 4 ];
       seq_number_scaled  =:= lsb(4, 7)                       [ 4 ];
       msn                =:= lsb(4, 4)                       [ 4 ];
       psh_flag           =:= irregular(1)                    [ 1 ];
       header_crc         =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
       ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM) ||
               (ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_ZERO));
     }

     // Send acknowledgment number LSBs
     COMPRESSED rnd_3 {
       discriminator =:= '0'                             [ 1 ];
       ack_number    =:= lsb(15, 8191)                   [ 15 ];
       msn           =:= lsb(4, 4)                       [ 4 ];
       psh_flag      =:= irregular(1)                    [ 1 ];
       header_crc    =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
       ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM) ||
               (ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_ZERO));
     }

     // Send acknowledgment number scaled
     COMPRESSED rnd_4 {
       discriminator      =:= '1101'                          [ 4 ];
       ack_number_scaled  =:= lsb(4, 3)                       [ 4 ];
       msn                =:= lsb(4, 4)                       [ 4 ];
       psh_flag           =:= irregular(1)                    [ 1 ];
       header_crc         =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
       ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM) ||
               (ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_ZERO));
     }

     // Send ACK and sequence number
     COMPRESSED rnd_5 {
       discriminator =:= '100'                           [ 3 ];
       psh_flag      =:= irregular(1)                    [ 1 ];



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       msn           =:= lsb(4, 4)                       [ 4 ];
       header_crc    =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
       seq_number    =:= lsb(14, 8191)                   [ 14 ];
       ack_number    =:= lsb(15, 8191)                   [ 15 ];
       ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM) ||
               (ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_ZERO));
     }

     // Send both ACK and scaled sequence number LSBs
     COMPRESSED rnd_6 {
       discriminator      =:= '1010'                          [ 4 ];
       header_crc         =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
       psh_flag           =:= irregular(1)                    [ 1 ];
       ack_number         =:= lsb(16, 16383)                  [ 16 ];
       msn                =:= lsb(4, 4)                       [ 4 ];
       seq_number_scaled  =:= lsb(4, 7)                       [ 4 ];
       ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM) ||
               (ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_ZERO));
     }

     // Send ACK and window
     COMPRESSED rnd_7 {
       discriminator =:= '101111'                        [ 6 ];
       ack_number    =:= lsb(18, 65535)                  [ 18 ];
       window        =:= irregular(16)                   [ 16 ];
       msn           =:= lsb(4, 4)                       [ 4 ];
       psh_flag      =:= irregular(1)                    [ 1 ];
       header_crc    =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
       ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM) ||
               (ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_ZERO));
     }

     // An extended packet type for seldom-changing fields
     // Can send LSBs of TTL, RSF flags, change ECN behavior and
     // options list
     COMPRESSED rnd_8 {
       discriminator =:= '10110'                         [ 5 ];
       rsf_flags     =:= rsf_index_enc                   [ 2 ];
       list_present  =:= irregular(1)                    [ 1 ];
       header_crc    =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
       msn           =:= lsb(4, 4)                       [ 4 ];
       psh_flag      =:= irregular(1)                    [ 1 ];
       ttl_hopl      =:= lsb(3, 3)                       [ 3 ];
       ecn_used      =:= ecn_choice                      [ 1 ];
       seq_number    =:= lsb(16, 65535)                  [ 16 ];
       ack_number    =:= lsb(16, 16383)                  [ 16 ];
       options_list  =:=
        tcp_list_presence_enc((data_offset.UVALUE-5)*32,



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                              list_present.CVALUE,
                              ack_number.UVALUE)   [ VARIABLE ];
       ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM) ||
               (ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_ZERO));
     }

     // Send LSBs of sequence number
     COMPRESSED seq_1 {
       discriminator =:= '1010'                                 [ 4 ];
       ip_id         =:= ip_id_lsb(ip_id_behavior.UVALUE, 4, 3) [ 4 ];
       seq_number    =:= lsb(16, 32767)                         [ 16 ];
       msn           =:= lsb(4, 4)                              [ 4 ];
       psh_flag      =:= irregular(1)                           [ 1 ];
       header_crc    =:= crc3(THIS.UVALUE, THIS.ULENGTH)        [ 3 ];
       ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL) ||
               (ip_id_behavior.UVALUE ==
                IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
     }

     // Send scaled sequence number LSBs
     COMPRESSED seq_2 {
       discriminator      =:= '11010'                         [ 5 ];
       ip_id              =:=
         ip_id_lsb(ip_id_behavior.UVALUE, 7, 3)               [ 7 ];
       seq_number_scaled  =:= lsb(4, 7)                       [ 4 ];
       msn                =:= lsb(4, 4)                       [ 4 ];
       psh_flag           =:= irregular(1)                    [ 1 ];
       header_crc         =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
       ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL) ||
               (ip_id_behavior.UVALUE ==
                IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
     }

     // Send acknowledgment number LSBs
     COMPRESSED seq_3 {
       discriminator =:= '1001'                                 [ 4 ];
       ip_id         =:= ip_id_lsb(ip_id_behavior.UVALUE, 4, 3) [ 4 ];
       ack_number    =:= lsb(16, 16383)                         [ 16 ];
       msn           =:= lsb(4, 4)                              [ 4 ];
       psh_flag      =:= irregular(1)                           [ 1 ];
       header_crc    =:= crc3(THIS.UVALUE, THIS.ULENGTH)        [ 3 ];
       ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL) ||
               (ip_id_behavior.UVALUE ==
                IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
     }

     // Send scaled acknowledgment number scaled
     COMPRESSED seq_4 {



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       discriminator     =:= '0'                             [ 1 ];
       ack_number_scaled =:= lsb(4, 3)                       [ 4 ];
       // Due to having very few ip_id bits, no negative offset
       ip_id      =:= ip_id_lsb(ip_id_behavior.UVALUE, 3, 1) [ 3 ];
       msn               =:= lsb(4, 4)                       [ 4 ];
       psh_flag          =:= irregular(1)                    [ 1 ];
       header_crc        =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
       ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL) ||
               (ip_id_behavior.UVALUE ==
                IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
     }

     // Send ACK and sequence number
     COMPRESSED seq_5 {
       discriminator =:= '1000'                                 [ 4 ];
       ip_id         =:= ip_id_lsb(ip_id_behavior.UVALUE, 4, 3) [ 4 ];
       ack_number    =:= lsb(16, 16383)                         [ 16 ];
       seq_number    =:= lsb(16, 32767)                         [ 16 ];
       msn           =:= lsb(4, 4)                              [ 4 ];
       psh_flag      =:= irregular(1)                           [ 1 ];
       header_crc    =:= crc3(THIS.UVALUE, THIS.ULENGTH)        [ 3 ];
       ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL) ||
               (ip_id_behavior.UVALUE ==
                IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
     }

     // Send both ACK and scaled sequence number LSBs
     COMPRESSED seq_6 {
       discriminator      =:= '11011'                          [ 5 ];
       seq_number_scaled  =:= lsb(4, 7)                        [ 4 ];
       ip_id        =:= ip_id_lsb(ip_id_behavior.UVALUE, 7, 3) [ 7 ];
       ack_number         =:= lsb(16, 16383)                   [ 16 ];
       msn                =:= lsb(4, 4)                        [ 4 ];
       psh_flag           =:= irregular(1)                     [ 1 ];
       header_crc         =:= crc3(THIS.UVALUE, THIS.ULENGTH)  [ 3 ];
       ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL) ||
               (ip_id_behavior.UVALUE ==
                IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
     }

     // Send ACK and window
     COMPRESSED seq_7 {
       discriminator =:= '1100'                                 [ 4 ];
       window        =:= lsb(15, 16383)                         [ 15 ];
       ip_id         =:= ip_id_lsb(ip_id_behavior.UVALUE, 5, 3) [ 5 ];
       ack_number    =:= lsb(16, 32767)                         [ 16 ];
       msn           =:= lsb(4, 4)                              [ 4 ];
       psh_flag      =:= irregular(1)                           [ 1 ];



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       header_crc    =:= crc3(THIS.UVALUE, THIS.ULENGTH)        [ 3 ];
       ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL) ||
               (ip_id_behavior.UVALUE ==
                IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
     }

     // An extended packet type for seldom-changing fields
     // Can send LSBs of TTL, RSF flags, change ECN behavior and
     // options list
     COMPRESSED seq_8 {
       discriminator =:= '1011'                                 [ 4 ];
       ip_id         =:= ip_id_lsb(ip_id_behavior.UVALUE, 4, 3) [ 4 ];
       list_present  =:= irregular(1)                           [ 1 ];
       header_crc    =:= crc7(THIS.UVALUE, THIS.ULENGTH)        [ 7 ];
       msn           =:= lsb(4, 4)                              [ 4 ];
       psh_flag      =:= irregular(1)                           [ 1 ];
       ttl_hopl      =:= lsb(3, 3)                              [ 3 ];
       ecn_used      =:= ecn_choice                             [ 1 ];
       ack_number    =:= lsb(15, 8191)                          [ 15 ];
       rsf_flags     =:= rsf_index_enc                          [ 2 ];
       seq_number    =:= lsb(14, 8191)                          [ 14 ];
       options_list  =:=
         tcp_list_presence_enc((data_offset.UVALUE-5)*32,
                               list_present.CVALUE,
                               ack_number.UVALUE)         [ VARIABLE ];
       ENFORCE((ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_SEQUENTIAL) ||
               (ip_id_behavior.UVALUE ==
                IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
     }
   }

8.3.  Feedback Formats and Options

8.3.1.  Feedback Formats

   This section describes the feedback format for ROHC-TCP.  ROHC-TCP
   uses the ROHC feedback format described in section 5.2.2 of
   [RFC3095].

   All feedback formats carry a field labelled SN.  The SN field
   contains LSBs of the Master Sequence Number (MSN) described in
   Section 6.1.1.  The sequence number to use is the MSN corresponding
   to the header that caused the feedback information to be sent.  If
   that MSN cannot be determined, for example when decompression fails,
   the MSN to use is that corresponding to the latest successfully
   decompressed header.





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   FEEDBACK-1

        0   1   2   3   4   5   6   7
      +---+---+---+---+---+---+---+---+
      |              MSN              |
      +---+---+---+---+---+---+---+---+

      MSN: The lsb-encoded master sequence number.

   A FEEDBACK-1 is an ACK.  In order to send a NACK or a STATIC-NACK,
   FEEDBACK-2 must be used.

   FEEDBACK-2

        0   1   2   3   4   5   6   7
      +---+---+---+---+---+---+---+---+
      |Acktype|          MSN          |
      +---+---+---+---+---+---+---+---+
      |              MSN              |
      +---+---+---+---+---+---+---+---+
      /       Feedback options        /
      +---+---+---+---+---+---+---+---+

      Acktype:

         0 = ACK
         1 = NACK
         2 = STATIC-NACK
         3 is reserved (MUST NOT be used for parsability)

      MSN: The lsb-encoded master sequence number.
      Feedback options: A variable number of feedback options, see
      Section 8.3.2.  Options may appear in any order.

8.3.2.  Feedback Options

   A ROHC-TCP Feedback option has variable length and the following
   general format:

        0   1   2   3   4   5   6   7
      +---+---+---+---+---+---+---+---+
      |   Opt Type    |    Opt Len    |
      +---+---+---+---+---+---+---+---+
      /          option data          /  Opt Length (octets)
      +---+---+---+---+---+---+---+---+

   Each ROHC-TCP feedback option can appear at most once within a
   FEEDBACK-2.



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8.3.2.1.  The CRC option

   The CRC option contains an 8-bit CRC computed over the entire
   feedback payload including any CID fields but excluding the packet
   type, the 'Size' field and the 'Code' octet, using the polynomial of
   section 5.9.1 of [RFC3095].  If the CID is given with an Add-CID
   octet, the Add-CID octet immediately precedes the FEEDBACK-1 or
   FEEDBACK-2 format.  For purposes of computing the CRC, the CRC fields
   of all CRC options are zero.

        0   1   2   3   4   5   6   7
      +---+---+---+---+---+---+---+---+
      |  Opt Type = 1 |  Opt Len = 1  |
      +---+---+---+---+---+---+---+---+
      |              CRC              |
      +---+---+---+---+---+---+---+---+

   When receiving feedback information with a CRC option, the compressor
   MUST verify the information by computing the CRC and comparing the
   result with the CRC carried in the CRC option.  If the two are not
   identical, the feedback packet MUST be discarded.

8.3.2.2.  The REJECT option

   The REJECT option informs the compressor that the decompressor does
   not have sufficient resources to handle the flow.

      +---+---+---+---+---+---+---+---+
      |  Opt Type = 2 |  Opt Len = 0  |
      +---+---+---+---+---+---+---+---+

   When receiving a REJECT option, the compressor MUST stop compressing
   the packet flow, and SHOULD refrain from attempting to increase the
   number of compressed packet flows for some time.  Any FEEDBACK packet
   carrying a REJECT option MUST also carry a CRC option.

8.3.2.3.  The MSN-NOT-VALID option

   The MSN-NOT-VALID option indicates that the MSN of the feedback is
   not valid.  A compressor MUST NOT use the MSN of the feedback to find
   the corresponding sent header when this option is present.

      +---+---+---+---+---+---+---+---+
      |  Opt Type = 3 |  Opt Len = 0  |
      +---+---+---+---+---+---+---+---+






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8.3.2.4.  The MSN option

   The MSN option provides 2 additional bits of MSN.

      +---+---+---+---+---+---+---+---+
      |  Opt Type = 4 |  Opt Len = 1  |
      +---+---+---+---+---+---+---+---+
      |  MSN  |        Reserved       |
      +---+---+---+---+---+---+---+---+

8.3.2.5.  The LOSS option

   The LOSS option allows the decompressor to report the largest
   observed number of packets lost in sequence.

      +---+---+---+---+---+---+---+---+
      |  Opt Type = 7 |  Opt Len = 1  |
      +---+---+---+---+---+---+---+---+
      | longest loss event (packets)  |
      +---+---+---+---+---+---+---+---+

   The decompressor MAY choose to ignore the oldest loss events.  Thus,
   the value reported may decrease.  Since setting the reference window
   too small can reduce robustness, a FEEDBACK packet carrying a LOSS
   option SHOULD also carry a CRC option.  The compressor MAY choose to
   ignore decreasing loss values.

8.3.2.6.  The CONTEXT_MEMORY Feedback Option

   The CONTEXT_MEMORY option informs the compressor that the
   decompressor does not have sufficient memory resources to handle the
   context of the packet flow, as the flow is currently compressed.

        0   1   2   3   4   5   6   7
      +---+---+---+---+---+---+---+---+
      |  Opt Type = 9 |  Opt Len = 0  |
      +---+---+---+---+---+---+---+---+

   When receiving a CONTEXT_MEMORY option, the compressor SHOULD take
   actions to compress the packet flow in a way that requires less
   decompressor memory resources, or stop compressing the packet flow.

8.3.2.7.  Unknown option types

   If an option type unknown to the compressor is encountered, it must
   continue parsing the rest of the FEEDBACK packet, which is possible
   since the length of the option is explicit, but MUST otherwise ignore
   the unknown option.



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9.  Security Consideration

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

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


10.  IANA Considerations

   The ROHC profile identifier 0x00XX <# Editor's Note: To be replaced
   before publication #> has been reserved by the IANA for the profile
   defined in this document.

   <# Editor's Note: To be removed before publication #>

   A ROHC profile identifier must be reserved by the IANA for the
   profile defined in this document.  Profiles 0x0000-0x0005 have
   previously been reserved, which means this profile could be 0x0006.
   As for previous ROHC profiles, profile numbers 0xnnXX must also be
   reserved for future updates of this profile.  A suggested
   registration in the "RObust Header Compression (ROHC) Profile
   Identifiers" name space would then be:

        Profile             Usage            Document
        identifier

        0x0006              ROHC TCP         [RFCXXXX (this)]
        0xnn06              Reserved


11.  Acknowledgements

   The authors would like to thank Qian Zhang, Hong Bin Liao, Richard
   Price and Fredrik Lindstroem for their work with early versions of
   this specification.  Thanks also to Robert Finking and Carsten Borman



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   for valuable input.  Finally, thanks to Joe Touch for his thorough
   review and comments.


12.  References

12.1.  Normative References

   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              September 1981.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, September 1981.

   [RFC2004]  Perkins, C., "Minimal Encapsulation within IP", RFC 2004,
              October 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC2784]  Farinacci, D., Li, T., Hanks, S., Meyer, D., and P.
              Traina, "Generic Routing Encapsulation (GRE)", RFC 2784,
              March 2000.

   [RFC2890]  Dommety, G., "Key and Sequence Number Extensions to GRE",
              RFC 2890, September 2000.

   [RFC3095]  Bormann, C., Burmeister, C., Degermark, M., Fukushima, H.,
              Hannu, H., Jonsson, L-E., Hakenberg, R., Koren, T., Le,
              K., Liu, Z., Martensson, A., Miyazaki, A., Svanbro, K.,
              Wiebke, T., Yoshimura, T., and H. Zheng, "RObust Header
              Compression (ROHC): Framework and four profiles: RTP, UDP,
              ESP, and uncompressed", RFC 3095, July 2001.

   [RFC4164]  Pelletier, G., "RObust Header Compression (ROHC): Context
              Replication for ROHC Profiles", RFC 4164, August 2005.

   [RFC4302]  Kent, S., "IP Authentication Header", RFC 4302,
              December 2005.

   [RFC4303]  Kent, S., "IP Encapsulating Security Payload (ESP)",
              RFC 4303, December 2005.

   [ROHC-FN]  Finking, R. and G. Pelletier, "Formal Notation for Robust
              Header Compression (ROHC-FN)",



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              draft-ietf-rohc-formal-notation-10.txt (work in progress),
              May 2006.

12.2.  Informative References

   [RFC1144]  Jacobson, V., "Compressing TCP/IP headers for low-speed
              serial links", RFC 1144, February 1990.

   [RFC1323]  Jacobson, V., Braden, B., and D. Borman, "TCP Extensions
              for High Performance", RFC 1323, May 1992.

   [RFC2018]  Mathis, M., Mahdavi, J., Floyd, S., and A. Romanow, "TCP
              Selective Acknowledgment Options", RFC 2018, October 1996.

   [RFC2507]  Degermark, M., Nordgren, B., and S. Pink, "IP Header
              Compression", RFC 2507, February 1999.

   [RFC2883]  Floyd, S., Mahdavi, J., Mathis, M., and M. Podolsky, "An
              Extension to the Selective Acknowledgement (SACK) Option
              for TCP", RFC 2883, July 2000.

   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, September 2001.

   [RFC3759]  Jonsson, L-E., "RObust Header Compression (ROHC):
              Terminology and Channel Mapping Examples", RFC 3759,
              April 2004.

   [RFC3843]  Jonsson, L-E. and G. Pelletier, "RObust Header Compression
              (ROHC): A Compression Profile for IP", RFC 3843,
              June 2004.

   [RFC4163]  Jonsson, L-E., "RObust Header Compression (ROHC):
              Requirements on TCP/IP Header Compression", RFC 4163,
              August 2005.

   [RFC4224]  Pelletier, G., Jonsson, L-E., and K. Sandlund, "RObust
              Header Compression (ROHC): ROHC over Channels That Can
              Reorder Packets", RFC 4224, January 2006.

   [RFC4413]  West, M. and S. McCann, "TCP/IP Field Behavior", RFC 4413,
              March 2006.








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Authors' Addresses

   Ghyslain Pelletier
   Ericsson
   Box 920
   Lulea  SE-971 28
   Sweden

   Phone: +46 (0) 8 404 29 43
   Email: ghyslain.pelletier@ericsson.com


   Lars-Erik Jonsson
   Ericsson
   Box 920
   Lulea  SE-971 28
   Sweden

   Phone: +46 (0) 8 404 29 61
   Email: lars-erik.jonsson@ericsson.com


   Kristofer Sandlund
   Ericsson
   Box 920
   Lulea  SE-971 28
   Sweden

   Phone: +46 (0) 8 404 41 58
   Email: kristofer.sandlund@ericsson.com


   Mark A West
   Siemens/Roke Manor
   Roke Manor Research Ltd.
   Romsey, Hampshire  SO51 0ZN
   UK

   Phone: +44 1794 833311
   Email: mark.a.west@roke.co.uk
   URI:   http://www.roke.co.uk










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