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RFC 6846 - RObust Header Compression (ROHC): A Profile for TCP/I


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Internet Engineering Task Force (IETF)                      G. Pelletier
Request for Comments: 6846                   InterDigital Communications
Obsoletes: 4996                                              K. Sandlund
Category: Standards Track                                       Ericsson
ISSN: 2070-1721                                             L-E. Jonsson

                                                                 M. West
                                                      Siemens/Roke Manor
                                                            January 2013

                   RObust Header Compression (ROHC):
                    A Profile for TCP/IP (ROHC-TCP)

Abstract

   This document specifies a RObust Header Compression (ROHC) profile
   for compression of TCP/IP packets.  The profile, called ROHC-TCP,
   provides efficient and robust compression of TCP headers, including
   frequently used TCP options such as selective acknowledgments (SACKs)
   and Timestamps.

   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.

   This specification obsoletes RFC 4996.  It fixes a technical issue
   with the SACK compression and clarifies other compression methods
   used.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6846.

Copyright Notice

   Copyright (c) 2013 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................5
   2. Terminology .....................................................5
   3. Background ......................................................7
      3.1. Existing TCP/IP Header Compression Schemes .................7
      3.2. Classification of TCP/IP Header Fields .....................8
   4. Overview of the TCP/IP Profile (Informative) ...................10
      4.1. General Concepts ..........................................10
      4.2. Compressor and Decompressor Interactions ..................10
           4.2.1. Compressor Operation ...............................10
           4.2.2. Decompressor Feedback ..............................11
      4.3. Packet Formats and Encoding Methods .......................11
           4.3.1. Compressing TCP Options ............................11
           4.3.2. Compressing Extension Headers ......................11
      4.4. Expected Compression Ratios with ROHC-TCP .................12
   5. Compressor and Decompressor Logic (Normative) ..................13
      5.1. Context Initialization ....................................13
      5.2. Compressor Operation ......................................13
           5.2.1. Compression Logic ..................................13
                  5.2.1.1. Optimistic Approach .......................14
                  5.2.1.2. Periodic Context Refreshes ................14
           5.2.2. Feedback Logic .....................................14
                  5.2.2.1. Optional Acknowledgments (ACKs) ...........14
                  5.2.2.2. Negative Acknowledgments (NACKs) ..........15
           5.2.3. Context Replication ................................15
      5.3. Decompressor Operation ....................................16
           5.3.1. Decompressor States and Logic ......................16
                  5.3.1.1. Reconstruction and Verification ...........16
                  5.3.1.2. Detecting Context Damage ..................17
                  5.3.1.3. No Context (NC) State .....................18
                  5.3.1.4. Static Context (SC) State .................18
                  5.3.1.5. Full Context (FC) State ...................19
           5.3.2. Feedback Logic .....................................19
           5.3.3. Context Replication ................................20
   6. Encodings in ROHC-TCP (Normative) ..............................20
      6.1. Control Fields in ROHC-TCP ................................20
           6.1.1. Master Sequence Number (MSN) .......................20
           6.1.2. IP-ID Behavior .....................................21
           6.1.3. Explicit Congestion Notification (ECN) .............22
      6.2. Compressed Header Chains ..................................22
      6.3. Compressing TCP Options with List Compression .............24
           6.3.1. List Compression ...................................25
           6.3.2. Table-Based Item Compression .......................26
           6.3.3. Encoding of Compressed Lists .......................26
           6.3.4. Item Table Mappings ................................28
           6.3.5. Compressed Lists in Dynamic Chain ..................30
           6.3.6. Irregular Chain Items for TCP Options ..............30

           6.3.7. Replication of TCP Options .........................30
      6.4. Profile-Specific Encoding Methods .........................31
           6.4.1. inferred_ip_v4_header_checksum .....................31
           6.4.2. inferred_mine_header_checksum ......................31
           6.4.3. inferred_ip_v4_length ..............................32
           6.4.4. inferred_ip_v6_length ..............................32
           6.4.5. inferred_offset ....................................33
           6.4.6. baseheader_extension_headers .......................33
           6.4.7. baseheader_outer_headers ...........................34
           6.4.8. Scaled Encoding of Fields ..........................34
                  6.4.8.1. Scaled TCP Sequence Number Encoding .......35
                  6.4.8.2. Scaled Acknowledgment Number Encoding .....35
      6.5. Encoding Methods with External Parameters .................36
   7. Packet Types (Normative) .......................................38
      7.1. Initialization and Refresh (IR) Packets ...................38
      7.2. Context Replication (IR-CR) Packets .......................40
      7.3. Compressed (CO) Packets ...................................42
   8. Header Formats (Normative) .....................................43
      8.1. Design Rationale for Compressed Base Headers ..............44
      8.2. Formal Definition of Header Formats .......................47
      8.3. Feedback Formats and Options ..............................88
           8.3.1. Feedback Formats ...................................88
           8.3.2. Feedback Options ...................................89
                  8.3.2.1. The REJECT Option .........................89
                  8.3.2.2. The MSN-NOT-VALID Option ..................90
                  8.3.2.3. The MSN Option ............................90
                  8.3.2.4. The CONTEXT_MEMORY Feedback Option ........91
                  8.3.2.5. Unknown Option Types ......................91
   9. Changes from RFC 4996 ..........................................91
      9.1. Functional Changes ........................................91
      9.2. Non-functional Changes ....................................92
   10. Security Considerations .......................................92
   11. IANA Considerations ...........................................93
   12. Acknowledgments ...............................................93
   13. References ....................................................93
      13.1. Normative References .....................................93
      13.2. Informative References ...................................94

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 [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 ([RFC1144], [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
   points 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 selective
   acknowledgments (SACK) ([RFC2018], [RFC2883]) and Timestamps
   ([RFC1323]).  TCP options that may be less frequently used do not
   necessarily need to be compressed by the protocol, and instead can be
   passed transparently without reducing the overall compression
   efficiency of other parts of the TCP header.

   The Robust Header Compression (ROHC) Working Group 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 [RFC5795], compliant with the requirements listed
   in [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 [RFC2119].

   This document reuses some of the terminology found in [RFC5795].  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 Context Identifier (Base CID)

      The Base CID is the CID that identifies the base context, from
      which information needed for context replication can be extracted.

   Base header

      The Base header is a compressed representation of the innermost IP
      and TCP headers of the uncompressed packet.

   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 each header,
      e.g., 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.

   ROHC-TCP packet types

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

   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 [RFC5795].  In the following subsections, 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.

   A behavior analysis of TCP/IP header fields is 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 size of the TCP/IP header.

   The CTCP [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 [RFC5681]
   from being triggered.  In such a case, the compressor must wait until
   TCP times out and retransmits a packet to resynchronize.

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

   The TWICE algorithm assumes that only the Sequence Number field of
   TCP segments is 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 times.

   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
       o  Maximum Segment Size   (32 bits) - MSS
       o  Window Scale           (24 bits) - WSCALE
       o  SACK Permitted         (16 bits)
       o  TCP SACK               (80, 144, 208, or 272 bits) - SACK
       o  TCP Timestamp          (80 bits) - TS

   The assignment of IP-ID values can be done in various ways, usually
   one of sequential, sequential jump, or random, as described in
   Section 4.1.3 of [RFC4413].  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.  With respect to TCP compression, the
   analysis in [RFC4413] reveals that there is no obvious candidate
   among the TCP fields suitable to infer the IP-ID.

   The change pattern of several TCP fields (Sequence Number,
   Acknowledgment Number, Window, etc.) is very hard to predict.  Of
   particular importance to a TCP/IP header compression scheme is the
   understanding of the sequence and acknowledgment numbers [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 file 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 number
   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.
   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.  The conclusion from [RFC4413] is that part of
   the IP sub-context, some TCP fields, and some context values can 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], Authentication Header (AH) [RFC4302],
   Generic Routing Encapsulation (GRE) [RFC2784][RFC2890], and the
   Minimal Encapsulation (MINE) header [RFC2004].

   Headers specific to Mobile IP (for IPv4 or IPv6) do not receive any
   special treatment in this document, for reasons similar to those
   described in [RFC3095].

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

4.1.  General Concepts

   ROHC-TCP uses the ROHC protocol as described in [RFC5795].  ROHC-TCP
   supports context replication as defined in [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
   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 (CO) packets 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 acknowledgments (ACKs) or negative
   acknowledgments (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 in increasing the
   compression efficiency by providing the information needed for the
   compressor to achieve the necessary confidence level.

   The ROHC-TCP feedback mechanism is limited in its applicability by
   the number of (least significant bit (LSB) encoded) master sequence
   number (MSN) (see Section 6.1.1) bits used in the FEEDBACK-2 format
   (see Section 8.3).  It is not suitable for a decompressor to use
   feedback altogether where the MSN bits in the feedback could wrap
   around within one round-trip time.  Instead, unidirectional operation
   -- where the compressor periodically sends larger context-updating
   packets -- is more appropriate.

4.3.  Packet Formats and Encoding Methods

   The packet formats and encoding methods used for ROHC-TCP are defined
   using the formal notation [RFC4997].  The formal notation is used to
   provide an unambiguous representation of the packet formats and a
   clear definition of the encoding methods.

4.3.1.  Compressing TCP Options

   The TCP options in ROHC-TCP are compressed using a list compression
   encoding that allows 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

   ROHC-TCP compresses the extension headers as listed in Section 3.2.
   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).

   This means 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 assume 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 No Operation (NOP) options [RFC0793] for padding
   and/or alignment.  Finally, in the examples for IPv4, a sequential
   IP-ID behavior is assumed.

                             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 the compressor has received an
   acknowledgment from the decompressor before compressing the second
   packet, which can be expected when feedback is used in ROHC-TCP.

   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)

5.1.  Context Initialization

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

   1.  By using an IR packet as in Section 7.1, where the profile number
       is 0x06 and the static chain ends with the static part of a TCP
       header.

   2.  By replicating an existing context using the mechanism defined by
       [RFC4164].  This is done with the IR-CR packet defined in
       Section 7.2, where the profile number is 0x06.

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 selecting header formats updating the same type of
      information for a number of consecutive packets or from the
      reception of decompressor feedback (ACKs and/or NACKs).

   o  Additional robustness required for the flow, e.g., periodic
      refreshes 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 in more 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

   The optimistic approach is the principle by which a compressor sends
   the same type of information for a number of packets (consecutively
   or not) until it is fairly confident that the decompressor has
   received the information.  The optimistic approach is useful to
   ensure robustness when ROHC-TCP is used to compress packets over
   lossy links.

   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
   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 performing periodic
   refreshes.

5.2.2.  Feedback Logic

   The semantics of feedback messages, acknowledgments (ACKs) and
   negative acknowledgments (NACKs or STATIC-NACKs), are defined in
   Section 5.2.4.1 of [RFC5795].

5.2.2.1.  Optional Acknowledgments (ACKs)

   The compressor MAY 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)

   The compressor uses feedback from the decompressor to move to a lower
   compression state (NACKs).

   On reception of a NACK feedback, the compressor SHOULD:

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

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

   unless it has confidence that information sent after the packet being
   acknowledged already provides a suitable response to the NACK
   feedback.  In addition, the compressor MAY use a CO packet carrying a
   7-bit Cyclic Redundancy Check (CRC) if it can determine with enough
   confidence what information provides a suitable response to the NACK
   feedback.

   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)
      the 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
   STATIC-NACK feedback.

5.2.3.  Context Replication

   A compressor MAY support context replication by implementing the
   additional compression and feedback logic defined in [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
   subsections following the figure.

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

5.3.1.1.  Reconstruction and Verification

   When decompressing an IR or an IR-DYN packet, the decompressor MUST
   validate the integrity of the received header using CRC-8 validation
   [RFC5795].  If validation fails, the packet MUST NOT be delivered to
   upper layers.

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

   When decompressing other packet types (e.g., CO packets), the
   decompressor MUST validate the outcome of the decompression attempt
   using CRC verification [RFC5795].  If verification fails, a
   decompressor implementation 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 NOT be delivered to
   upper layers.

   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,
   e.g., 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.

5.3.1.2.  Detecting Context Damage

   All header formats carry a CRC and are context updating.  A packet
   for which the CRC succeeds updates the reference values of all header
   fields, either explicitly (from the information about a field carried
   within the compressed header) or implicitly (fields that are inferred
   from other fields).

   The decompressor may assume that some or the entire context is
   invalid, following one or more failures to validate or verify a
   header using the CRC.  Because the decompressor cannot know the exact
   reason(s) for a CRC failure or what field caused it, the validity of
   the context hence does not refer to what exact context entry is
   deemed valid or not.

   Validity of the context rather relates to the detection of a problem
   with the context.  The decompressor first assumes that the type of
   information that most likely caused the failure(s) is the state that
   normally changes for each packet, i.e., context damage of the dynamic
   part of the context.  Upon repeated failures and unsuccessful
   repairs, the decompressor then assumes that the entire context,
   including the static part, needs to be repaired, i.e., static context
   damage.

   Context Damage Detection

      The assumption of context damage means that the decompressor will
      not attempt decompression of a CO header that carries a 3-bit CRC,
      and only attempt decompression of IR, IR-DYN, or IR-CR headers or
      CO headers protected by a CRC-7.

   Static Context Damage Detection

      The assumption of static context damage means that the
      decompressor refrains from attempting decompression of any type of
      header other than the IR header.

   How these assumptions are made, i.e., how context damage is detected,
   is open to implementations.  It can be based on the residual error
   rate, where a low error rate makes the decompressor assume damage
   more often than on a high-rate link.

   The decompressor implements these assumptions by selecting the type
   of compressed header for which it may attempt decompression.  In
   other words, validity of the context refers to the ability of a
   decompressor to attempt or not attempt decompression of specific
   packet types.

5.3.1.3.  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 NOT be decompressed and MUST NOT be
      delivered to upper layers.

   Feedback logic:

      In the NC state, the decompressor should send a STATIC-NACK if a
      packet of a type other than IR is received, or if decompression of
      an IR packet has failed, subject to the feedback rate limitation
      as described in Section 5.3.2.

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

5.3.1.4.  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.

   Allowing decompression:

      In the SC state, packets carrying sufficient information on the
      dynamic fields covered by an 8-bit CRC (e.g., IR and IR-DYN) or CO
      packets covered by a 7-bit CRC can be decompressed; otherwise, the
      packet MUST NOT be decompressed and MUST NOT be delivered to upper
      layers.

   Feedback logic:

      In the SC state, the decompressor should send a STATIC-NACK if CRC
      validation of an IR/IR-DYN/IR-CR fails and static context damage
      is assumed.  If any other packet type is received, the
      decompressor should send a NACK.  Both of the above cases are
      subject to the feedback rate limitation as described in
      Section 5.3.2.

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

5.3.1.5.  Full Context (FC) State

   In the Full Context (FC) state, both the static and the dynamic parts
   of the decompressor context are valid.  From the FC state, the
   decompressor moves back to the SC state if context damage is
   detected.

   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 if the
      decompression of any packet type fails and context damage is
      assumed, subject to the feedback rate limitation as described in
      Section 5.3.2.

5.3.2.  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.

   Once the feedback channel is established, the decompressor is
   REQUIRED to continue sending 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 send ACKs upon successful decompression of any
   packet type.  In particular, when a packet carrying a significant
   context update is correctly decompressed, the decompressor MAY send
   an ACK.

   The decompressor should limit the rate at which it sends feedback,
   for both ACKs and STATIC-NACK/NACKs, and should avoid sending
   unnecessary duplicates of the same type of feedback message that may
   be associated to the same event.

5.3.3.  Context Replication

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

6.  Encodings in ROHC-TCP (Normative)

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 format of the 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.

   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., Base CID == 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.  Conceptually, a compressor monitors changes in the value
   of the IP-ID field and selects encoding methods and packet formats
   that are the closest match to the observed change pattern.

   ROHC-TCP defines different types of compression techniques for the
   IP-ID, to provide the flexibility to compress any of the behaviors it
   may observe for this field: sequential in network byte order (NBO),
   sequential byte-swapped, random (RND), or constant to a value of
   zero.

   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
   compression alternatives is the closest match to the observed change
   pattern.  The compressor can then select packet formats and encoding
   methods 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 the
   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 NOT assign either the sequential (NBO)
   or the sequential byte-swapped behavior to tunneling headers.

   The control field for the IP-ID behavior determines which set of
   packet formats will be used.  These control fields are also used to
   determine the contents of the irregular chain item (see Section 6.2)
   for each IP header.

6.1.3.  Explicit Congestion Notification (ECN)

   When ECN [RFC3168] is used once on a flow, the ECN bits could change
   quite often.  ROHC-TCP maintains a control field in the context to
   indicate whether or not ECN is used.  This control field is
   transmitted in the dynamic chain of the TCP header, and its 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
   all IP and TCP headers in the irregular chain include bits used for
   ECN.  To preserve octet-alignment, all of the TCP reserved bits are
   transmitted and, for outer IP headers, the entire Type of Service/
   Traffic Class (TOS/TC) field is included in the irregular chain.
   When there is only one IP header present in the packet (i.e., no IP
   tunneling is used), this compression behavior allows the compressor
   to handle changes in the ECN bits by adding a single octet to the
   compressed header.

   The reason for including the ECN bits of all IP headers in the
   compressed packet when the control field is set is that the profile
   needs to efficiently compress flows containing IP tunnels using the
   "full-functionality option" of Section 9.1 of [RFC3168].  For these
   flows, a change in the ECN bits of an inner IP header is propagated
   to the outer IP headers.  When the "limited-functionality" option is
   used, the compressor will therefore sometimes send one octet more
   than necessary per tunnel header, but this has been considered a
   reasonable trade-off when designing this profile.

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, as listed
   below.  Also listed are the names of the encoding methods used to
   encode each of these protocol headers.

   o  TCP [RFC0793], encoding method: "tcp"

   o  IPv4 [RFC0791], encoding method: "ipv4"

   o  IPv6 [RFC2460], encoding method: "ipv6"

   o  AH [RFC4302], encoding method: "ah"

   o  GRE [RFC2784][RFC2890], encoding method: "gre"

   o  MINE [RFC2004], encoding method: "mine"

   o  IPv6 Destination Options header [RFC2460], encoding method:
      "ip_dest_opt"

   o  IPv6 Hop-by-Hop Options header [RFC2460], encoding method:
      "ip_hop_opt"

   o  IPv6 Routing header [RFC2460], encoding method: "ip_rout_opt"

   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 is a
      format whose name is suffixed by "_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.  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 a
      format whose name is suffixed by "_dynamic".  The dynamic chain is
      used in both IR and IR-DYN packets.

   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.  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, the replicate chain item for each header type is a

      format whose name is suffixed by "_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, which are placed directly after the irregular
      chain item of the TCP header, and in the same order as the options
      appear in the uncompressed packet.  In the formal description of
      the packet formats, the irregular chain item for each header type
      is a format whose name is suffixed by "_irregular".  The irregular
      chain is used only in 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
      part of the compressed base header.

6.3.  Compressing TCP Options with List Compression

   This section describes in detail how list compression is applied to
   the TCP options.  In the definition of the packet formats for ROHC-
   TCP, the most frequent TCP options have one encoding method each, as
   listed in the table below.

           +-----------------+------------------------+
           |   Option name   |  Encoding method name  |
           +-----------------+------------------------+
           |      NOP        | tcp_opt_nop            |
           |      EOL        | tcp_opt_eol            |
           |      MSS        | tcp_opt_mss            |
           |  WINDOW SCALE   | tcp_opt_wscale         |
           |   TIMESTAMP     | tcp_opt_ts             |
           | SACK-PERMITTED  | tcp_opt_sack_permitted |
           |      SACK       | tcp_opt_sack           |
           | Generic options | tcp_opt_generic        |
           +-----------------+------------------------+

   Each of these encoding methods has an uncompressed format, a format
   suffixed by "_list_item" and a format suffixed by "_irregular".  In
   some cases, a single encoding method may have multiple "_list_item"

   or "_irregular" formats, in which case bindings inside these formats
   determine what format is used.  This is further described in the
   following sections.

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 |
            +--------+--------+--...--+--------+

   To compress this list, ROHC-TCP uses a list compression scheme, which
   compresses each of these items individually and combines them into a
   compressed list.

   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 base headers; the irregular content is sent as part
      of the irregular chain, as described in Section 6.3.6.

      4) When the structure of the list changes, a compressed list is
      sent in the compressed base header, including a representation of
      its structure and order.  Content defined within the irregular
      format of an option can still be sent as part of the irregular
      chain (as described in Section 6.3.6), provided that the item
      content is not part of the compressed list.

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

      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

   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)

   Decompression of an item will fail if the presence bit is not set and
   the decompressor has no entry in the context for that item.

   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; otherwise, the decompressor MUST
      discard the packet.

      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.

      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 is 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 and is instead included in the irregular chain.

         Reserved: MUST be set to zero; otherwise, the decompressor MUST
         discard the packet.

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

         When 4-bit XI items are used, 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 MUST be set to zero;
      otherwise, the decompressor MUST discard the packet.

      Item 1, ..., item n: Each item corresponds to an XI with X = 1 in
      XI 1, ..., XI m.  The format of the entries in the item list is
      described in the table in Section 6.3.  The compressed format(s)
      suffixed by "_list_item" in the encoding methods defines the item
      inside the compressed item list.

6.3.4.  Item Table Mappings

   The item table for TCP options list compression is limited to 16
   different items, since it is unlikely that any packet flow will
   contain a larger number of unique options.

   The mapping between the 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 when 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, whether or not the item is present 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.

   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.

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 [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 bits.  Using this
   encoding method, the decompressor infers the value of this field
   using the computation above.

   This encoding method implicitly assumes that the compressor will not
   process a corrupted header; otherwise, it cannot guarantee that the
   checksum as recomputed by the decompressor will be bitwise identical
   to its original value before compression.

6.4.2.  inferred_mine_header_checksum

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

      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
         zero.  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 bits,
   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 and the assumptions for inferring this checksum are
   similar to the ones explained above in Section 6.4.1.

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
   [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 Total
   Length field down to a size of zero bits.  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 of the IPv6
   header.  This length field is defined in [RFC2460] as follows:

      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 bits.  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 [RFC0793] 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 bits.  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

   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.

   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 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 Encoding of Fields

   Some header fields will exhibit a change pattern where the field
   increases by a constant value or by multiples of the same value.

   Examples of fields that may have this behavior are the TCP Sequence
   Number and the TCP Acknowledgment Number.  For such fields, ROHC-TCP
   provides the means to downscale the field value before applying LSB
   encoding, which allows the compressor to transmit fewer bits.

   To be able to use scaled encoding, the field is required to fulfill
   the following equation:

        unscaled_value = scaling_factor * scaled_value + residue

   To use the scaled encoding, the compressor must be confident that the
   decompressor has established values for the "residue" and the
   "scaling_factor", so that it can correctly decompress the field when
   only an LSB-encoded "scaled_value" is present in the compressed
   packet.

   Once the compressor is confident that the value of the scaling_factor
   and the value of the residue have been established in the
   decompressor, the compressor may send compressed packets using the
   scaled representation of the field.  The compressor MUST NOT use
   scaled encoding with the value of the scaling_factor set to zero.

   If the compressor detects that the value of the residue has changed,
   or if the compressor uses a different value for the scaling factor,
   it MUST NOT use scaled encoding until it is confident that the
   decompressor has received the new value(s) of these fields.

   When the unscaled value of the field wraps around, the value of the
   residue is likely to change, even if the scaling_factor remains
   constant.  In such a case, the compressor must act in the same way as
   for any other change in the residue.

   The following subsections describe how the scaled encoding is applied
   to specific fields in ROHC-TCP, in particular, how the scaling_factor
   and residue values are established for the different fields.

6.4.8.1.  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, which means that this field can be a suitable target for
   scaled encoding.  When using this encoding, the payload size will be
   used as the scaling factor (i.e., as the value for scaling_factor) of
   this encoding.  This means that the scaling factor does not need to
   be explicitly transmitted, but is instead inferred from the length of
   the payload in the compressed packet.

   Establishing scaling_factor:

      The scaling factor is established by sending unscaled TCP Sequence
      Number bits, so that the decompressor can infer the scaling_factor
      from the payload size.

   Establishing residue:

      The residue is established identically as the scaling_factor,
      i.e., by sending unscaled TCP Sequence Number bits.

   A detailed specification of how the TCP Sequence Number uses the
   scaled encoding can be found in the definitions of the packet
   formats, in Section 8.2.

6.4.8.2.  Scaled Acknowledgment Number Encoding

   Similar to the pattern exhibited by the TCP Sequence Number, the
   expected increase in the TCP Acknowledgment Number is often constant
   and is therefore suitable for scaled encoding.

   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 explicitly transmitted scaling factor to
   compress the TCP Acknowledgment Number.

   Establishing scaling_factor:

      The scaling factor is established by explicitly transmitting the
      value of the scaling factor (called ack_stride in the formal
      notation in Section 8.2) to the decompressor, using one of the
      packet types that can carry this information.

   Establishing residue:

      The scaling residue is established by sending unscaled TCP
      Acknowledgment Number bits, so that the decompressor can infer its
      value from the unscaled value and the scaling factor (ack_stride).

   A detailed specification of how the TCP Acknowledgment Number uses
   the scaled encoding can be found in the definitions of the packet
   formats, in Section 8.2.

   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 sequence number used for that flow can be
   used by the compressor to determine a suitable scaling factor for the
   TCP Acknowledgment number for this flow.

6.5.  Encoding Methods with External Parameters

   A number of encoding methods in Section 8.2 have one or more
   arguments for which the derivation of the parameter's value is
   outside the scope of the ROHC-FN specification of the header formats.
   This section lists the encoding methods together with a definition of
   each of their parameters.

   o  ipv6(is_innermost, ttl_irregular_chain_flag, ip_inner_ecn):

         is_innermost: This Boolean flag is set to true when processing
         the innermost IP header; otherwise, it is set to false.

         ttl_irregular_chain_flag: This parameter must be set to the
         value that was used for the corresponding
         "ttl_irregular_chain_flag" parameter of the "co_baseheader"
         encoding method (as defined below) when extracting the
         irregular chain for a compressed header; otherwise, it is set
         to zero and ignored for other types of chains.

         ip_inner_ecn: This parameter is bound by the encoding method;
         therefore, it should be undefined when calling this encoding
         method.  This value is then used to bind the corresponding

         parameter in the "tcp" encoding method, as its value is needed
         when processing the irregular chain for TCP.  See the
         definition of the "ip_inner_ecn" parameter for the "tcp"
         encoding method below.

   o  ipv4(is_innermost, ttl_irregular_chain_flag, ip_inner_ecn,
      ip_id_behavior_value):

         See definition of arguments for "ipv6" above.

         ip_id_behavior_value: Set to a 2-bit integer value, using one
         of the constants whose name begins with the prefix
         IP_ID_BEHAVIOR_ and as defined in Section 8.2.

   o  tcp_opt_eol(nbits):

         nbits: This parameter is set to the length of the padding data
         located after the EOL option type octet to the end of the TCP
         options in the uncompressed header.

   o  tcp_opt_sack(ack_value):

         ack_value: Set to the value of the Acknowledgment Number field
         of the TCP header.

   o  tcp(payload_size, ack_stride_value, ip_inner_ecn):

         payload_size: Set to the length (in octets) of the payload
         following the TCP header.

         ack_stride_value: This parameter is the scaling factor used
         when scaling the TCP Acknowledgment Number.  Its value is set
         by the compressor implementation.  See Section 6.4.8.2 for
         recommendations on how to set this value.

         ip_inner_ecn: This parameter binds with the value given to the
         corresponding "ip_inner_ecn" parameter by the "ipv4" or the
         "ipv6" encoding method when processing the innermost IP header
         of this packet.  See also the definition of the "ip_inner_ecn"
         parameter to the "ipv6" and "ipv4" encoding method above.

   o  co_baseheader(payload_size, ack_stride_value,
      ttl_irregular_chain_flag, ip_id_behavior_value):

         payload_size: Set to the length (in octets) of the payload
         following the TCP header.

         ack_stride_value: This parameter is the scaling factor used
         when scaling the TCP Acknowledgment Number.  Its value is set
         by the compressor implementation.  See Section 6.4.8.2 for
         recommendations on how to set this value.

         ttl_irregular_chain_flag: This parameter is set to one if the
         TTL/Hop Limit of an outer header has changed compared to its
         reference in the context; otherwise, it is set to zero.  The
         value used for this parameter is also used for the
         "ttl_irregular_chain_flag" argument for the "ipv4" and "ipv6"
         encoding methods when processing the irregular chain, as
         defined above for the "ipv6" and "ipv4" encoding methods.

         ip_id_behavior_value: Set to a 2-bit integer value, using one
         of the constants whose name begins with the prefix
         IP_ID_BEHAVIOR_ and as defined in Section 8.2.

7.  Packet Types (Normative)

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

   Each packet type defines a number of packet formats: two packet
   formats are defined for the IR type, one packet format is defined for
   the IR-CR 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.

   The profile identifier for ROHC-TCP is 0x0006.

7.1.  Initialization and Refresh (IR) Packets

   ROHC-TCP uses the basic structure of the ROHC IR and IR-DYN packets
   as defined in [RFC5795] (Sections 5.2.2.1 and 5.2.2.2, respectively).

   Packet type: IR

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

      For the ROHC-TCP IR packet, the value of the x bit MUST be set to
      one.  It has the following format, which corresponds to the
      "Header" and "Payload" fields described in Section 5.2.1 of
      [RFC5795]:

        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 5.3.1.1 of
      [RFC5795].  The CRC covers the entire IR header, thus excluding
      payload, padding, and feedback, if any.

      Static chain: See Section 6.2.

      Dynamic chain: See Section 6.2.

      Payload: The payload of the corresponding original packet, if any.
      The payload consists of all data after the last octet of the TCP
      header to the end of the uncompressed packet.  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.

      The ROHC-TCP IR-DYN packet has the following format, which
      corresponds to the "Header" and "Payload" fields described in
      Section 5.2.1 of [RFC5795]:

        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 5.3.1.1 of
      [RFC5795].  The CRC covers the entire IR-DYN header, thus
      excluding payload, padding, and feedback, if any.

      Dynamic chain: See Section 6.2.

      Payload: The payload of the corresponding original packet, if any.
      The payload consists of all data after the last octet of the TCP
      header to end of the uncompressed packet.  The presence of a
      payload is inferred from the packet length.

7.2.  Context Replication (IR-CR) Packets

   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
   IR-CR packet, as defined in [RFC4164], Section 3.5.2.  With
   consideration to the extensibility of the IR packet type defined in
   [RFC5795], 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 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, which corresponds to the
   "Header" and "Payload" fields described in Section 5.2.1 of
   [RFC5795]:

        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: This CRC covers the entire IR-CR header, thus excluding
      payload, padding, and feedback, if any.  This 8-bit CRC is
      calculated according to Section 5.3.1.1 of [RFC5795].

      CRC7: The CRC over the original, uncompressed, header.  Calculated
      according to Section 3.5.1.1 of [RFC4164].

      Reserved: MUST be set to zero; otherwise, the decompressor MUST
      discard the packet.

      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 (CO) Packets

   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, which
   corresponds to the "Header" and "Payload" fields described in Section
   5.2.1 of [RFC5795]:

         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        :
       /   (including irregular chain  /  variable
       :    items for TCP options)     :
        --- --- --- --- --- --- --- ---
       |                               |
       /            Payload            / variable length
       |                               |
        - - - - - - - - - - - - - - - -

      Base header: The complete set of base headers is defined in
      Section 8.

      Irregular chain: See Sections 6.2 and 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.  Header Formats (Normative)

   This section describes the set of compressed TCP/IP packet formats.
   The normative description of the packet formats is given using the
   formal notation for ROHC profiles defined in [RFC4997].  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.

8.1.  Design Rationale for Compressed Base Headers

   The compressed header 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 header formats are referred to as the "sequential"
   and the "random" set of header formats, respectively.

   In addition, there is one compressed format that is common to both
   sets of header formats and that can thus be used regardless of the
   type of IP-ID behavior.  This format can transmit rarely changing
   fields and also send the frequently changing fields coded in variable
   lengths.  It 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 headers.  Headers that can modify these
   control fields carry a 7-bit CRC instead.

   When discussing LSB-encoded fields below, "p" equals the
   "offset_param" and "k" equals the "num_lsbs_param" in [RFC4997].

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

   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 has an MSN field that is LSB-
         encoded with k=4 and p=4 to handle a reordering depth of up to
         4 packets.  Additional guidance to improve robustness when
         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 octets (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 has to handle negative offsets
         about as large as positive offsets, 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 is set to be as large as the
         positive offset, i.e., p = 2^(k-1)-1.

   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; therefore, the design
         criterion for the TCP window is to send at least 14 bits when
         used.

   o  IP-ID

         For the "sequential" set of packet formats, all the compressed
         base headers contain LSB-encoded IP-ID offset bits, where the
         offset is the difference between the value of the MSN field and
         the value of the IP-ID field.  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 then p=1, and if k>3 then p=3
         since the offset is expected to increase most of the time.

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

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

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

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

   o  Format 1: This header format carries changes to the TCP Sequence
      Number and is expected to be used on the downstream of a data
      transfer.

   o  Format 2: This header format carries the TCP Sequence Number in
      scaled form and is expected to be useful for the downstream of a
      data transfer where the payload size is constant for multiple
      packets.

   o  Format 3: This header format carries changes in the TCP
      Acknowledgment Number and is expected to be useful for the
      acknowledgment direction of a data transfer.

   o  Format 4: This header format is similar to format 3, but carries a
      scaled TCP Acknowledgment Number.

   o  Format 5: This header format carries both the TCP Sequence Number
      and the TCP Acknowledgment Number and is expected to be useful for
      flows that send data in both directions.

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

   o  Format 7: This header format carries changes to both the TCP
      Acknowledgment Number and the TCP Window and is expected to be
      useful for the acknowledgment flows of data connections.

   o  Format 8: This header format is used to convey 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 structure of the contents of the irregular chain for
      subsequent packets.  Note that this can be seen as a reduced form
      of the common packet format.

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

8.2.  Formal Definition of Header Formats

// NOTE: The irregular, static, and dynamic chains (see Section 6.2)
// are defined across multiple encoding methods and are embodied
// in the correspondingly named formats within those encoding
// methods.  In particular, note that the static and dynamic
// chains ordinarily go together.  The uncompressed fields are
// defined across these two formats combined, rather than in one
// or the other of them.  The irregular chain items are likewise
// combined with a baseheader format.

////////////////////////////////////////////
// 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 ];
  // ip_id fields are for innermost IP header only
  ip_id_offset                               [ 16 ];
  ip_id_behavior_innermost                   [  2 ];
  // ACK-related
  ack_stride                                 [ 32 ];
  ack_number_scaled                          [ 32 ];
  ack_number_residue                         [ 32 ];
  seq_number_scaled                          [ 32 ];
  seq_number_residue                         [ 32 ];
}

///////////////////////////////////////////////
// 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";
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);
  }
}

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

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

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

  }
}

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

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

  COMPRESSED lsb_8_bit {
    field =:= lsb(8, 63) [ 8 ];
    ENFORCE(flag == 1);
  }

  COMPRESSED lsb_16_bit {
    field =:= lsb(16, 16383) [ 16 ];
    ENFORCE(flag == 2);
  }

  COMPRESSED irreg_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 ];
    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 ];
  }
}

one_bit_choice
{
  UNCOMPRESSED {
    field [ 1 ];
  }

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

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

// 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.
// Does not have an uncompressed side.
field_scaling(stride_value, scaled_value, unscaled_value, residue_value)
{
  UNCOMPRESSED {
    // Nothing
  }

  COMPRESSED no_scaling {
    ENFORCE(stride_value == 0);
    ENFORCE(residue_value == unscaled_value);
    ENFORCE(scaled_value == 0);
  }

  COMPRESSED scaling_used {
    ENFORCE(stride_value != 0);
    ENFORCE(residue_value == (unscaled_value % stride_value));
    ENFORCE(unscaled_value ==
            scaled_value * stride_value + residue_value);
  }
}

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

ip_dest_opt
{
  UNCOMPRESSED {
    next_header [ 8 ];
    length      [ 8 ];
    value       [ length.UVALUE * 64 + 48 ];
  }

  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) [ length.UVALUE * 64 + 48 ];
  }
  COMPRESSED dest_opt_0_replicate {
    discriminator =:= '00000000' [ 8 ];
  }

  COMPRESSED dest_opt_1_replicate {
    discriminator =:= '10000000'                     [ 8 ];
    length        =:= irregular(8)                   [ 8 ];
    value         =:=
      irregular(length.UVALUE*64+48) [ length.UVALUE * 64 + 48 ];
  }

  COMPRESSED dest_opt_irregular {
  }
}

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

ip_hop_opt
{

  UNCOMPRESSED {
    next_header [ 8 ];
    length      [ 8 ];
    value       [ length.UVALUE * 64 + 48 ];
  }

  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) [ length.UVALUE * 64 + 48 ];
  }

  COMPRESSED hop_opt_0_replicate {
    discriminator =:= '00000000' [ 8 ];
  }

  COMPRESSED hop_opt_1_replicate {
    discriminator =:= '10000000'                     [ 8 ];
    length        =:= irregular(8)                   [ 8 ];
    value         =:=
      irregular(length.UVALUE*64+48) [ length.UVALUE * 64 + 48 ];
  }

  COMPRESSED hop_opt_irregular {
  }
}

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

ip_rout_opt
{
  UNCOMPRESSED {
    next_header [ 8 ];
    length      [ 8 ];
    value       [ length.UVALUE * 64 + 48 ];
  }

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

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

  COMPRESSED rout_opt_dynamic {
  }

  COMPRESSED rout_opt_0_replicate {
    discriminator =:= '00000000' [ 8 ];
  }

  COMPRESSED rout_opt_0_replicate {
    discriminator =:= '10000000'                     [ 8 ];
    length        =:= irregular(8)                   [ 8 ];
    value         =:=
      irregular(length.UVALUE*64+48) [ length.UVALUE * 64 + 48 ];
  }
  COMPRESSED rout_opt_irregular {
  }
}

////////////////////////////////////////////
// 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);
  }
}

gre_proto
{
  UNCOMPRESSED {
    protocol [ 16 ];
  }

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

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

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 {

    ENFORCE((c_flag.UVALUE == 1 && checksum_and_res.ULENGTH == 32)
            || checksum_and_res.ULENGTH == 0);
    ENFORCE((s_flag.UVALUE == 1 && sequence_number.ULENGTH == 32)
            || sequence_number.ULENGTH == 0);
    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 ];
  }

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

  COMPRESSED gre_0_replicate {
    discriminator    =:= '00000000'    [ 8 ];
    checksum_and_res =:=
      optional_checksum(c_flag.UVALUE) [ 0, 16 ];
    sequence_number  =:=
      optional32(s_flag.UVALUE)        [ 0, 8, 32 ];
  }

  COMPRESSED gre_1_replicate {
    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 ];
  }

  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 bits are included to achieve byte-alignment
    res_bits    =:= irregular(7)             [ 7 ];
    orig_dest   =:= irregular(32)            [ 32 ];
    orig_src    =:= optional32(s_bit.UVALUE) [ 0, 32 ];
  }

  COMPRESSED mine_dynamic {
  }

  COMPRESSED mine_0_replicate {
    discriminator =:= '00000000' [ 8 ];
  }

  COMPRESSED mine_1_replicate {
    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 ];
  }

  COMPRESSED mine_irregular {
  }
}

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

ah
{
  UNCOMPRESSED {
    next_header     [ 8 ];
    length          [ 8 ];
    res_bits        [ 16 ];
    spi             [ 32 ];
    sequence_number [ 32 ];
    icv             [ length.UVALUE*32-32 ];
  }

  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 ];
    icv       =:=
      irregular(length.UVALUE*32-32)  [ length.UVALUE*32-32 ];
  }

  COMPRESSED ah_0_replicate {
    discriminator   =:= '00000000'    [ 8 ];
    sequence_number =:= irregular(32) [ 32 ];
    icv             =:=
      irregular(length.UVALUE*32-32)  [ length.UVALUE*32-32 ];
  }

  COMPRESSED ah_1_replicate {
    discriminator   =:= '10000000'    [ 8 ];
    length          =:= irregular(8)  [ 8 ];
    res_bits        =:= irregular(16) [ 16 ];
    spi             =:= irregular(32) [ 32 ];

    sequence_number =:= irregular(32) [ 32 ];
    icv             =:=
      irregular(length.UVALUE*32-32)  [ length.UVALUE*32-32 ];
  }

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

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

fl_enc
{
  UNCOMPRESSED {
    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 is true if this is the innermost IP header
// If extracting the irregular chain for a compressed packet:
//   - ttl_irregular_chain_flag must have the same value as it had when
//     processing co_baseheader.
//   - 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
//   For other formats than the irregular chain, these two are ignored
ipv6(is_innermost, ttl_irregular_chain_flag, ip_inner_ecn)
{
  UNCOMPRESSED {
    version         =:= uncompressed_value(4, 6) [ 4 ];
    dscp                                         [ 6 ];
    ip_ecn_flags                                 [ 2 ];
    flow_label                                   [ 20 ];

    payload_length                               [ 16 ];
    next_header                                  [ 8 ];
    ttl_hopl                                     [ 8 ];
    src_addr                                     [ 128 ];
    dst_addr                                     [ 128 ];
  }

  DEFAULT {
    dscp           =:= 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;
  }

  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 {
    dscp         =:= irregular(6) [ 6 ];
    ip_ecn_flags =:= irregular(2) [ 2 ];
    ttl_hopl     =:= irregular(8) [ 8 ];
  }

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

  COMPRESSED ipv6_outer_without_ttl_irregular {
    dscp         =:= static_or_irreg(ecn_used.UVALUE, 6) [ 0, 6 ];
    ip_ecn_flags =:= static_or_irreg(ecn_used.UVALUE, 2) [ 0, 2 ];
    ENFORCE(ttl_irregular_chain_flag == 0);
    ENFORCE(is_innermost == false);
  }

  COMPRESSED ipv6_outer_with_ttl_irregular {

    dscp         =:= static_or_irreg(ecn_used.UVALUE, 6) [ 0, 6 ];
    ip_ecn_flags =:= static_or_irreg(ecn_used.UVALUE, 2) [ 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)
{
  UNCOMPRESSED {
    ip_id [ 16 ];
  }

  COMPRESSED ip_id_seq {
    ENFORCE((behavior == IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (behavior == IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
    ENFORCE(ip_id_offset.UVALUE == ip_id.UVALUE - msn.UVALUE);
    ip_id =:= irregular(16) [ 16 ];
  }

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

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

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);
  }
}

// The is_innermost flag is true if this is the innermost IP header
// If extracting the irregular chain for a compressed packet:
//   - ttl_irregular_chain_flag must have the same value as it had when
//     processing co_baseheader.
//   - 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
//   For other formats than the irregular chain, these two are ignored
ipv4(is_innermost, ttl_irregular_chain_flag, ip_inner_ecn,
     ip_id_behavior_value)
{
  UNCOMPRESSED {
    version        =:= uncompressed_value(4, 4)  [ 4 ];
    hdr_length     =:= uncompressed_value(4, 5)  [ 4 ];
    dscp                                         [ 6 ];
    ip_ecn_flags                                 [ 2 ];
    length         =:= inferred_ip_v4_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 ];
    protocol                                     [ 8 ];
    checksum    =:= inferred_ip_v4_header_checksum [ 16 ];
    src_addr                                     [ 32 ];
    dst_addr                                     [ 32 ];
  }

  CONTROL {
    ENFORCE(reorder_ratio.UVALUE == reorder_ratio_value);

    ENFORCE(innermost_ip.UVALUE == is_innermost);
    ip_id_behavior_outer [ 2 ];
    innermost_ip [ 1 ];
  }

  DEFAULT {
    dscp           =:= static;
    ip_ecn_flags   =:= static;
    df             =:= static;
    ttl_hopl       =:= static;
    protocol       =:= static;
    src_addr       =:= static;
    dst_addr       =:= static;
    ip_id_behavior_outer =:= 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_innermost_dynamic {
    ENFORCE(is_innermost == 1);
    ENFORCE(ip_id_behavior_innermost.UVALUE == ip_id_behavior_value);
    reserved       =:= '00000'             [ 5 ];
    df             =:= irregular(1)        [ 1 ];
    ip_id_behavior_innermost =:= irregular(2) [ 2 ];
    dscp           =:= irregular(6)        [ 6 ];
    ip_ecn_flags   =:= irregular(2)        [ 2 ];
    ttl_hopl       =:= irregular(8)        [ 8 ];
    ip_id          =:=
      ip_id_enc_dyn(ip_id_behavior_innermost.UVALUE) [ 0, 16 ];
  }

  COMPRESSED ipv4_outer_dynamic {
    ENFORCE(is_innermost == 0);
    ENFORCE(ip_id_behavior_outer.UVALUE == ip_id_behavior_value);
    reserved       =:= '00000'             [ 5 ];
    df             =:= irregular(1)        [ 1 ];
    ip_id_behavior_outer =:=     irregular(2)              [ 2 ];
    dscp           =:= irregular(6)        [ 6 ];
    ip_ecn_flags   =:= irregular(2)        [ 2 ];
    ttl_hopl       =:= irregular(8)        [ 8 ];
    ip_id          =:=
      ip_id_enc_dyn(ip_id_behavior_outer.UVALUE) [ 0, 16 ];

  }

  COMPRESSED ipv4_innermost_replicate {
    ENFORCE(is_innermost == 1);
    ENFORCE(ip_id_behavior_innermost.UVALUE == ip_id_behavior_value);
    reserved       =:= '0000'              [ 4 ];
    ip_id_behavior_innermost =:= irregular(2) [ 2 ];
    ttl_flag       =:= irregular(1)        [ 1 ];
    df             =:= irregular(1)        [ 1 ];
    dscp           =:= irregular(6)        [ 6 ];
    ip_ecn_flags   =:= irregular(2)        [ 2 ];
    ip_id          =:=
      ip_id_enc_dyn(ip_id_behavior_innermost.UVALUE) [ 0, 16 ];
    ttl_hopl     =:=
        static_or_irreg(ttl_flag.UVALUE, 8) [ 0, 8 ];
  }

  COMPRESSED ipv4_outer_replicate {
    ENFORCE(is_innermost == 0);
    ENFORCE(ip_id_behavior_outer.UVALUE == ip_id_behavior_value);
    reserved       =:= '0000'              [ 4 ];
    ip_id_behavior_outer =:= irregular(2)  [ 2 ]
    ttl_flag       =:= irregular(1)        [ 1 ];
    df             =:= irregular(1)        [ 1 ];
    dscp           =:= irregular(6)        [ 6 ];
    ip_ecn_flags   =:= irregular(2)        [ 2 ];
    ip_id          =:=
      ip_id_enc_dyn(ip_id_behavior_outer.UVALUE) [ 0, 16 ];
    ttl_hopl     =:=
        static_or_irreg(ttl_flag.UVALUE, 8) [ 0, 8 ];
  }

  COMPRESSED ipv4_outer_without_ttl_irregular {
    ENFORCE(is_innermost == 0);
    ip_id        =:=
      ip_id_enc_irreg(ip_id_behavior_outer.UVALUE) [ 0, 16 ];
    dscp         =:= static_or_irreg(ecn_used.UVALUE, 6) [ 0, 6 ];
    ip_ecn_flags =:= static_or_irreg(ecn_used.UVALUE, 2) [ 0, 2 ];
    ENFORCE(ttl_irregular_chain_flag == 0);
  }

  COMPRESSED ipv4_outer_with_ttl_irregular {
    ENFORCE(is_innermost == 0);
    ip_id        =:=
      ip_id_enc_irreg(ip_id_behavior_outer.UVALUE)       [ 0, 16 ];
    dscp         =:= static_or_irreg(ecn_used.UVALUE, 6) [ 0, 6 ];
    ip_ecn_flags =:= static_or_irreg(ecn_used.UVALUE, 2) [ 0, 2 ];
    ttl_hopl     =:= irregular(8)                        [ 8 ];

    ENFORCE(ttl_irregular_chain_flag == 1);
  }

  COMPRESSED ipv4_innermost_irregular {
    ENFORCE(is_innermost == 1);
    ip_id          =:=
      ip_id_enc_irreg(ip_id_behavior_innermost.UVALUE) [ 0, 16 ];
    ENFORCE(ip_inner_ecn == ip_ecn_flags.UVALUE);
  }
}

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

// nbits is bound to the remaining length (in bits) of TCP
// options, including the EOL type byte.
tcp_opt_eol(nbits)
{
  UNCOMPRESSED {
    type     =:= uncompressed_value(8, 0) [ 8 ];
    padding  =:=
      uncompressed_value(nbits-8, 0)      [ nbits-8 ];
  }

  CONTROL {
    pad_len [ 8 ];
  }

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

  COMPRESSED eol_irregular {
    pad_len =:= static;
    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 ];
  }

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

  COMPRESSED mss_irregular {
    mss    =:= static;
  }
}

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

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

  COMPRESSED wscale_irregular {
    wscale =:= static;
  }
}

ts_lsb
{
  UNCOMPRESSED {
    tsval [ 32 ];
  }

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

  COMPRESSED tsval_14 {
    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_ts
{
  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_pure_lsb(base)
{
  UNCOMPRESSED {
    sack_field [ 32 ];
  }

  CONTROL {
    ENFORCE(sack_field.CVALUE == (sack_field.UVALUE - base));
  }

  COMPRESSED lsb_15 {
    ENFORCE(sack_field.CVALUE == sack_field.CVALUE <= 0x7fff);
    discriminator =:= '0'           [ 1 ];
    sack_field                      [ 15 ];

  }

  COMPRESSED lsb_22 {
    ENFORCE(sack_field.CVALUE == sack_field.CVALUE <= 0x3fffff);
    discriminator =:= '10'          [ 2 ];
    sack_field                      [ 22 ];
  }

  COMPRESSED lsb_29 {
    ENFORCE(sack_field.CVALUE == sack_field.CVALUE <= 0x1fffffff);
    discriminator =:= '110'         [ 3 ];
    sack_field                      [ 29 ];
  }

  COMPRESSED full_offset {
    discriminator =:= '11111111'    [ 8 ];
    sack_field                      [ 32 ];
  }
}

sack_block(reference)
{
  UNCOMPRESSED {
    block_start [ 32 ];
    block_end   [ 32 ];
  }

  COMPRESSED {
    block_start =:=
      sack_pure_lsb(reference)          [ 16, 24, 32, 40 ];
    block_end   =:=
      sack_pure_lsb(block_start.UVALUE) [ 16, 24, 32, 40 ];
  }
}

// The value of the parameter is set to the ack_number value
// of the TCP header
tcp_opt_sack(ack_value)
{

  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);
  }

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

  COMPRESSED sack2_list_item {
    discriminator =:= '00000010';
    block_1       =:= sack_block(ack_value);
    block_2       =:= sack_block(block_1.UVALUE && 0xFFFFFFFF);
    ENFORCE(length.UVALUE == 18);
  }

COMPRESSED sack3_list_item {
   discriminator =:= '00000011';
   block_1       =:= sack_block(ack_value);
   block_2       =:= sack_block(block_1.UVALUE && 0xFFFFFFFF);
   block_3       =:= sack_block(block_2.UVALUE && 0xFFFFFFFF);
   ENFORCE(length.UVALUE == 26);
 }

 COMPRESSED sack4_list_item {
   discriminator =:= '00000100';
   block_1       =:= sack_block(ack_value);
   block_2       =:= sack_block(block_1.UVALUE && 0xFFFFFFFF);
   block_3       =:= sack_block(block_2.UVALUE && 0xFFFFFFFF);
   block_4       =:= sack_block(block_3.UVALUE && 0xFFFFFFFF);
   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       =:= sack_block(ack_value);

    ENFORCE(length.UVALUE == 10);
  }

 COMPRESSED sack2_irregular {
   discriminator =:= '00000010';
   block_1       =:= sack_block(ack_value);
   block_2       =:= sack_block(block_1.UVALUE && 0xFFFFFFFF);
   ENFORCE(length.UVALUE == 18);
 }

 COMPRESSED sack3_irregular {
   discriminator =:= '00000011';
   block_1       =:= sack_block(ack_value);
   block_2       =:= sack_block(block_1.UVALUE && 0xFFFFFFFF);
   block_3       =:= sack_block(block_1.UVALUE && 0xFFFFFFFF);
   ENFORCE(length.UVALUE == 26);
 }

 COMPRESSED sack4_irregular {
   discriminator =:= '00000100';
   block_1       =:= sack_block(ack_value);
   block_2       =:= sack_block(block_1.UVALUE && 0xFFFFFFFF);
   block_3       =:= sack_block(block_2.UVALUE && 0xFFFFFFFF);
   block_4       =:= sack_block(block_3.UVALUE && 0xFFFFFFFF);
   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                           [ length_lsb.UVALUE*8-16 ];
  }

  CONTROL {
    option_static [ 1 ];
  }

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

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

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

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

  // An item that is assumed to change constantly.
  // 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_full_irregular {
    discriminator =:= '00000000'        [ 8 ];
    contents      =:=
      irregular(length_lsb.UVALUE*8-16) [ length_lsb.UVALUE*8-16 ];
    ENFORCE(option_static.UVALUE == 0);
  }
}

tcp_list_presence_enc(presence)
{
  UNCOMPRESSED {
    tcp_options;
  }

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

  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);
  }
}

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

  COMPRESSED ecn_present {
    // This field does not exist in the uncompressed header
    // and therefore cannot use uncompressed_value.
    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);
  }

  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 ];
    psh_flag      [ 1 ];
    rsf_flags     [ 3 ];
    window        [ 16 ];
    checksum      [ 16 ];
    urg_ptr       [ 16 ];
    options       [ (data_offset.UVALUE-5)*32 ];
  }

  CONTROL {
    dummy_field_s =:= field_scaling(payload_size,
      seq_number_scaled.UVALUE, seq_number.UVALUE,
      seq_number_residue.UVALUE) [ 0 ];
    dummy_field_a =:= field_scaling(ack_stride.UVALUE,
      ack_number_scaled.UVALUE, ack_number.UVALUE,
      ack_number_residue.UVALUE) [ 0 ];
    ENFORCE(ack_stride.UVALUE == ack_stride_value);
  }

  INITIAL {
    ack_stride     =:= uncompressed_value(16, 0);
  }

  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     =:= uncompressed_value(3, 0);
    window        =:= static;
    urg_ptr       =:= static;
    ack_stride    =:= static;
    ack_number_scaled =:= static;
    seq_number_scaled =:= static;
    ack_number_residue =:= static;
    seq_number_residue =:= static;
  }

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

  COMPRESSED tcp_dynamic {
    ecn_used        =:= one_bit_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 ];
    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      =:=
      zero_or_irreg(ack_zero.CVALUE, 32)        [ 0, 32 ];
    window          =:= irregular(16)           [ 16 ];
    checksum        =:= irregular(16)           [ 16 ];
    urg_ptr         =:=
      zero_or_irreg(urp_zero.CVALUE, 16)        [ 0, 16 ];
    ack_stride      =:=
      static_or_irreg(ack_stride_flag.CVALUE, 16) [ 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          =:= one_bit_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 =:=
      static_or_irreg(ecn_used.CVALUE, 4)        [ 0, 4 ];
    tcp_ecn_flags     =:=
      static_or_irreg(ecn_used.CVALUE, 2)        [ 0, 2 ];
    checksum          =:= irregular(16)            [ 16 ];
    ack_stride        =:=
      static_or_irreg(ack_stride_flag.CVALUE, 16)  [ 0, 16 ];
    options           =:=
      tcp_list_presence_enc(list_present.CVALUE) [ VARIABLE ];
  }

  COMPRESSED tcp_irregular {
    ip_ecn_flags  =:= tcp_irreg_ip_ecn(ip_inner_ecn)  [ 0, 2 ];
    tcp_res_flags =:=
      static_or_irreg(ecn_used.CVALUE, 4)            [ 0, 4 ];
    tcp_ecn_flags =:=
      static_or_irreg(ecn_used.CVALUE, 2)             [ 0, 2 ];
    checksum      =:= irregular(16)                   [ 16 ];
  }
}

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

dscp_enc(flag)
{
  UNCOMPRESSED {
    dscp [ 6 ];
  }

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

  COMPRESSED irreg {
    dscp    =:= 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_nbo    [ 16 ];
  }

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

  COMPRESSED not_present {
    ENFORCE((behavior == IP_ID_BEHAVIOR_RANDOM) ||
            (behavior == IP_ID_BEHAVIOR_ZERO));
  }
}
dont_fragment(version)
{
  UNCOMPRESSED {
    df [ 1 ];
  }

  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, ip_id_behavior_value)
{
  UNCOMPRESSED v4 {
    outer_headers  =:= baseheader_outer_headers        [ VARIABLE ];
    version        =:= uncompressed_value(4, 4)        [ 4 ];
    header_length  =:= uncompressed_value(4, 5)        [ 4 ];
    dscp                                               [ 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 ];
    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                           [ (data_offset.UVALUE-5)*32 ];
  }

  UNCOMPRESSED v6 {
    ENFORCE(ip_id_behavior_innermost.UVALUE == IP_ID_BEHAVIOR_RANDOM);
    outer_headers  =:= baseheader_outer_headers        [ VARIABLE ];
    version =:= uncompressed_value(4, 6)               [ 4 ];
    dscp                                               [ 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                           [ (data_offset.UVALUE-5)*32 ];
    df    =:= uncompressed_value(0,0)                  [   0 ];
    ip_id =:= uncompressed_value(0,0)                  [   0 ];
  }

  CONTROL {
    dummy_field_s =:= field_scaling(payload_size,
      seq_number_scaled.UVALUE, seq_number.UVALUE,
      seq_number_residue.UVALUE) [ 0 ];
    dummy_field_a =:= field_scaling(ack_stride.UVALUE,
      ack_number_scaled.UVALUE, ack_number.UVALUE,
      ack_number_residue.UVALUE) [ 0 ];
    ENFORCE(ack_stride.UVALUE == ack_stride_value);
    ENFORCE(ip_id_behavior_innermost.UVALUE == ip_id_behavior_value);
  }

  INITIAL {
    ack_stride     =:= uncompressed_value(16, 0);
  }

  DEFAULT {
    tcp_ecn_flags  =:= static;
    data_offset    =:= inferred_offset;
    tcp_res_flags  =:= static;
    rsf_flags      =:= uncompressed_value(3, 0);
    dest_port      =:= static;
    dscp           =:= 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);
    // The default for "options" is case 2) and 3) from
    // the list in Section 6.3.1 (i.e., nothing present in the
    // baseheader itself).
    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_innermost =:= static;
    ecn_used       =:= static;
    ack_stride    =:= static;
    ack_number_scaled =:= static;
    seq_number_scaled =:= static;
    ack_number_residue =:= static;
    seq_number_residue =:= 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
  ////////////////////////////////////////////

  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             =:= one_bit_choice           [ 1 ];
    dscp_present         =:= irregular(1)             [ 1 ];
    ttl_hopl_present     =:= irregular(1)             [ 1 ];
    list_present         =:= irregular(1)             [ 1 ];
    ip_id_behavior_innermost       =:= irregular(2)             [ 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_innermost.UVALUE,
                         ip_id_indicator.CVALUE)      [ 0, 8, 16 ];
    urg_ptr              =:=
      static_or_irreg(urg_ptr_present.CVALUE, 16)     [ 0, 16 ];
    dscp                 =:=
      dscp_enc(dscp_present.CVALUE)                   [ 0, 8 ];
    ttl_hopl             =:=
      static_or_irreg(ttl_hopl_present.CVALUE, 8)     [ 0, 8 ];
    options              =:=
      tcp_list_presence_enc(list_present.CVALUE)      [ VARIABLE ];
  }

  // 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_innermost.UVALUE ==
             IP_ID_BEHAVIOR_RANDOM) ||
            (ip_id_behavior_innermost.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(payload_size != 0);
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_RANDOM) ||
            (ip_id_behavior_innermost.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_innermost.UVALUE ==
             IP_ID_BEHAVIOR_RANDOM) ||
            (ip_id_behavior_innermost.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(ack_stride.UVALUE != 0);
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_RANDOM) ||
            (ip_id_behavior_innermost.UVALUE == IP_ID_BEHAVIOR_ZERO));
  }

  // Send ACK and sequence number
  COMPRESSED rnd_5 {
    discriminator =:= '100'                           [ 3 ];
    psh_flag      =:= irregular(1)                    [ 1 ];
    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_innermost.UVALUE ==
             IP_ID_BEHAVIOR_RANDOM) ||
            (ip_id_behavior_innermost.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(payload_size != 0);
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_RANDOM) ||
            (ip_id_behavior_innermost.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_innermost.UVALUE ==
             IP_ID_BEHAVIOR_RANDOM) ||
            (ip_id_behavior_innermost.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      =:= one_bit_choice                  [ 1 ];
    seq_number    =:= lsb(16, 65535)                  [ 16 ];
    ack_number    =:= lsb(16, 16383)                  [ 16 ];
    options       =:=
      tcp_list_presence_enc(list_present.CVALUE)      [ VARIABLE ];
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_RANDOM) ||
            (ip_id_behavior_innermost.UVALUE == IP_ID_BEHAVIOR_ZERO));
  }

  // Send LSBs of sequence number
  COMPRESSED seq_1 {
    discriminator =:= '1010'                                 [ 4 ];
    ip_id =:= ip_id_lsb(ip_id_behavior_innermost.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_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.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_innermost.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(payload_size != 0);
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
  }

  // Send acknowledgment number LSBs
  COMPRESSED seq_3 {
    discriminator =:= '1001'                                 [ 4 ];
    ip_id =:= ip_id_lsb(ip_id_behavior_innermost.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_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
  }

  // Send scaled acknowledgment number scaled
  COMPRESSED seq_4 {
    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_innermost.UVALUE, 3, 1) [ 3 ];
    msn               =:= lsb(4, 4)                       [ 4 ];
    psh_flag          =:= irregular(1)                    [ 1 ];
    header_crc        =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
    ENFORCE(ack_stride.UVALUE != 0);
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.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_innermost.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_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.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_innermost.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(payload_size != 0);
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.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_innermost.UVALUE, 5, 3) [ 5 ];

    ack_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_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.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_innermost.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      =:= one_bit_choice                         [ 1 ];
    ack_number    =:= lsb(15, 8191)                          [ 15 ];
    rsf_flags     =:= rsf_index_enc                          [ 2 ];
    seq_number    =:= lsb(14, 8191)                          [ 14 ];
    options       =:=
      tcp_list_presence_enc(list_present.CVALUE)       [ VARIABLE ];
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
  }
}

8.3.  Feedback Formats and Options

8.3.1.  Feedback Formats

   This section describes the feedback formats for the ROHC-TCP profile,
   following the general ROHC feedback format described in Section 5.2.4
   of [RFC5795].

   All feedback formats carry a field labeled MSN.  The MSN field
   contains LSBs of the MSN control field described in Section 6.1.1.
   The sequence number to use is the MSN corresponding to the last
   header that was successfully CRC-8 validated or CRC verified.

   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              |
      +---+---+---+---+---+---+---+---+
      |              CRC              |
      +---+---+---+---+---+---+---+---+
      /       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.

      CRC: 8-bit CRC computed over the entire feedback element (as
      defined in Section 5.3.1.1 of [RFC5795]).  For the purpose of
      computing the CRC, the CRC field is zero.  The CRC is calculated
      using the polynomial defined in [RFC5795].

      Feedback options: A variable number of feedback options, see
      Section 8.3.2.  Options may appear in any order.

   A FEEDBACK-2 of type NACK or STATIC-NACK is always implicitly an
   acknowledgment for a successfully decompressed packet, which packet
   corresponds to the MSN of the feedback element, unless the MSN-NOT-
   VALID option (Section 8.3.2.2) appears in the feedback element.

   The FEEDBACK-2 format always carries a CRC and is thus more robust
   than the FEEDBACK-1 format.  When receiving FEEDBACK-2, the
   compressor MUST verify the information by computing the CRC and by
   comparing the result with the CRC carried in the feedback format.  If
   the two are not identical, the feedback element MUST be discarded.

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.

8.3.2.1.  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.  The REJECT option
   MUST NOT appear more than once in the FEEDBACK-2 format; otherwise,
   the compressor MUST discard the entire feedback element.

8.3.2.2.  The MSN-NOT-VALID Option

   The MSN-NOT-VALID option indicates that the MSN of the feedback is
   not valid.

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

   A compressor MUST ignore the MSN of the feedback element when this
   option is present.  Consequently, a NACK or a STATIC-NACK feedback
   type sent with the MSN-NOT-VALID option is equivalent to a STATIC-
   NACK with respect to the semantics of the feedback message.

   The MSN-NOT-VALID option MUST NOT appear more than once in the
   FEEDBACK-2 format and MUST NOT appear in the same feedback element as
   the MSN option; otherwise, the compressor MUST discard the entire
   feedback element.

8.3.2.3.  The MSN Option

   The MSN option provides 2 additional bits of MSN.

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

   These 2 bits are the least significant bits of the MSN and are thus
   concatenated with the 14 bits already present in the FEEDBACK-2
   format.

   The MSN option MUST NOT appear more than once in the FEEDBACK-2
   format and MUST NOT appear in the same feedback element as the MSN-
   NOT-VALID option; otherwise, the compressor MUST discard the entire
   feedback element.

8.3.2.4.  The CONTEXT_MEMORY Feedback Option

   The CONTEXT_MEMORY option means 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.

   The CONTEXT_MEMORY option MUST NOT appear more than once in the
   FEEDBACK-2 format; otherwise, the compressor MUST discard the entire
   feedback element.

8.3.2.5.  Unknown Option Types

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

9.  Changes from RFC 4996

   This RFC revises RFC 4996.  It is mostly backwards-compatible with
   RFC 4996, except for two cases that did not interoperate as described
   below.

9.1.  Functional Changes

   o  The SACK option compression in [RFC4996] assumed that multiple
      SACK blocks within the same option would be in sorted order so
      that the block starts were LSB-encoded from the end of the
      previous block.  This meant that SACK blocks that are not in
      sorted order could be impossible to compress in some cases.
      Therefore, the SACK compression in the formal notation has changed
      and therefore also the bits-on-the-wire.

   o  The ESP NULL header compression has been deprecated due to
      interoperability problems with needing to know information from
      the trailer.  The ESP NULL compression was already removed from
      ROHCv2 [RFC5225] for the same reason and it was considered better
      to remove it from this profile rather than try to fix the
      interoperability issue.

9.2.  Non-functional Changes

   o  The way sequential IP-ID compression was described in the FN code
      was incorrect and the code used for ROHCv2 [RFC5225] has been
      imported into this specification (e.g., offset is made into a
      global control field).  This does not change the bits-on-the-wire.
      The only change is how this encoding is described in the formal
      notation, not how the compression occurs.

   o  Default encoding for the 'df' and 'ip_id' fields have been added
      for IPv6 with 0-bit uncompressed format to clarify that these
      never appear in IPv6.

   o  The scaled encoding of the Acknowledgment Number and Sequence
      Number were incorrectly described in the FN code in [RFC4996] and
      have been updated in the same style as in ROHCv2 [RFC5225].  This
      does not change the bits-on-the-wire, only the way the compression
      is described in the FN code.

   o  The external arguments to ipv4 and co_baseheader have been
      updated.  This is again only a change for FN correctness and does
      not affect interoperability.

   o  Errata for [RFC4996] related to minor errors in the FN and textual
      errors have also been corrected.

10.  Security Considerations

   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.

11.  IANA Considerations

   The reference for the ROHC profile identifier 0x0006 has been updated
   to reference this document instead of RFC 4996.

   A ROHC profile identifier has been reserved by IANA for the profile
   defined in this document.  Profiles 0x0000-0x0005 have previously
   been reserved; this profile is 0x0006.  As for previous ROHC
   profiles, profile numbers 0xnn06 have been reserved for future
   updates of this profile.

        Profile             Usage            Document
        identifier

        0x0006              ROHC TCP         [RFC6846]
        0xnn06              Reserved

12.  Acknowledgments

   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
   Bormann for valuable input and to Carl Knutsson and Gilbert Strom for
   suggestions and review of the updates made in this document.

   Additional thanks: this document was reviewed during working group
   last-call by committed reviewers Joe Touch and Ted Faber, as well as
   by Sally Floyd, who provided a review at the request of the Transport
   Area Directors.

13.  References

13.1.  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.

   [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.

   [RFC4997]  Finking, R. and G. Pelletier, "Formal Notation for RObust
              Header Compression (ROHC-FN)", RFC 4997, July 2007.

   [RFC5795]  Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
              Header Compression (ROHC) Framework", RFC 5795,
              March 2010.

13.2.  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.

   [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.

   [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.

   [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.

   [RFC4996]  Pelletier, G., Sandlund, K., Jonsson, L-E., and M. West,
              "RObust Header Compression (ROHC): A Profile for TCP/IP
              (ROHC-TCP)", RFC 4996, July 2007.

   [RFC5225]  Pelletier, G. and K. Sandlund, "RObust Header Compression
              Version 2 (ROHCv2): Profiles for RTP, UDP, IP, ESP and
              UDP-Lite", RFC 5225, April 2008.

   [RFC5681]  Allman, M., Paxson, V., and E. Blanton, "TCP Congestion
              Control", RFC 5681, September 2009.

Authors' Addresses

   Ghyslain Pelletier
   InterDigital Communications
   1000, Sherbrooke Street West, 10th floor
   Montreal, Quebec  H3A 3G4
   Canada

   Phone: +46 (0) 70 609 27 73
   EMail: ghyslain.pelletier@interdigital.com

   Kristofer Sandlund
   Ericsson
   Box 920
   Lulea  SE-971 28
   Sweden

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

   Lars-Erik Jonsson
   Optand 737
   Ostersund  SE-831 92
   Sweden

   Phone: +46 70 365 20 58
   EMail: lars-erik@lejonsson.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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