





Network Working Group                                       G. Pelletier
Request for Comments: 5225                                   K. Sandlund
Category: Standards Track                                       Ericsson
                                                              April 2008


             RObust Header Compression Version 2 (ROHCv2):
              Profiles for RTP, UDP, IP, ESP and UDP-Lite

Status of This Memo

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

Abstract

   This document specifies ROHC (Robust Header Compression) profiles
   that efficiently compress RTP/UDP/IP (Real-Time Transport Protocol,
   User Datagram Protocol, Internet Protocol), RTP/UDP-Lite/IP (User
   Datagram Protocol Lite), UDP/IP, UDP-Lite/IP, IP and ESP/IP
   (Encapsulating Security Payload) headers.

   This specification defines a second version of the profiles found in
   RFC 3095, RFC 3843 and RFC 4019; it supersedes their definition, but
   does not obsolete them.

   The ROHCv2 profiles introduce a number of simplifications to the
   rules and algorithms that govern the behavior of the compression
   endpoints.  It also defines robustness mechanisms that may be used by
   a compressor implementation to increase the probability of
   decompression success when packets can be lost and/or reordered on
   the ROHC channel.  Finally, the ROHCv2 profiles define their own
   specific set of header formats, using the ROHC formal notation.















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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Acronyms  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   4.  Background (Informative)  . . . . . . . . . . . . . . . . . .   7
     4.1.  Classification of Header Fields . . . . . . . . . . . . .   7
     4.2.  Improvements of ROHCv2 over RFC 3095 Profiles . . . . . .   8
     4.3.  Operational Characteristics of ROHCv2 Profiles  . . . . .  10
   5.  Overview of the ROHCv2 Profiles (Informative) . . . . . . . .  10
     5.1.  Compressor Concepts . . . . . . . . . . . . . . . . . . .  11
       5.1.1.  Optimistic Approach . . . . . . . . . . . . . . . . .  11
       5.1.2.  Tradeoff between Robustness to Losses and to
               Reordering  . . . . . . . . . . . . . . . . . . . . .  11
       5.1.3.  Interactions with the Decompressor Context  . . . . .  13
     5.2.  Decompressor Concepts . . . . . . . . . . . . . . . . . .  14
       5.2.1.  Decompressor State Machine  . . . . . . . . . . . . .  14
       5.2.2.  Decompressor Context Management . . . . . . . . . . .  17
       5.2.3.  Feedback Logic  . . . . . . . . . . . . . . . . . . .  19
   6.  ROHCv2 Profiles (Normative) . . . . . . . . . . . . . . . . .  19
     6.1.  Channel Parameters, Segmentation, and Reordering  . . . .  19
     6.2.  Profile Operation, Per-context  . . . . . . . . . . . . .  20
     6.3.  Control Fields  . . . . . . . . . . . . . . . . . . . . .  21
       6.3.1.  Master Sequence Number (MSN)  . . . . . . . . . . . .  21
       6.3.2.  Reordering Ratio  . . . . . . . . . . . . . . . . . .  21
       6.3.3.  IP-ID Behavior  . . . . . . . . . . . . . . . . . . .  22
       6.3.4.  UDP-Lite Coverage Behavior  . . . . . . . . . . . . .  22
       6.3.5.  Timestamp Stride  . . . . . . . . . . . . . . . . . .  22
       6.3.6.  Time Stride . . . . . . . . . . . . . . . . . . . . .  22
       6.3.7.  CRC-3 for Control Fields  . . . . . . . . . . . . . .  23
     6.4.  Reconstruction and Verification . . . . . . . . . . . . .  23
     6.5.  Compressed Header Chains  . . . . . . . . . . . . . . . .  24
     6.6.  Header Formats and Encoding Methods . . . . . . . . . . .  25
       6.6.1.  baseheader_extension_headers  . . . . . . . . . . . .  26
       6.6.2.  baseheader_outer_headers  . . . . . . . . . . . . . .  26
       6.6.3.  inferred_udp_length . . . . . . . . . . . . . . . . .  26
       6.6.4.  inferred_ip_v4_header_checksum  . . . . . . . . . . .  26
       6.6.5.  inferred_mine_header_checksum . . . . . . . . . . . .  27
       6.6.6.  inferred_ip_v4_length . . . . . . . . . . . . . . . .  28
       6.6.7.  inferred_ip_v6_length . . . . . . . . . . . . . . . .  28
       6.6.8.  Scaled RTP Timestamp Compression  . . . . . . . . . .  29
       6.6.9.  timer_based_lsb . . . . . . . . . . . . . . . . . . .  30
       6.6.10. inferred_scaled_field . . . . . . . . . . . . . . . .  31
       6.6.11. control_crc3_encoding . . . . . . . . . . . . . . . .  32
       6.6.12. inferred_sequential_ip_id . . . . . . . . . . . . . .  33
       6.6.13. list_csrc(cc_value) . . . . . . . . . . . . . . . . .  34
     6.7.  Encoding Methods with External Parameters as Arguments  .  38
     6.8.  Header Formats  . . . . . . . . . . . . . . . . . . . . .  40



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       6.8.1.  Initialization and Refresh Header Format (IR) . . . .  40
       6.8.2.  Compressed Header Formats (CO)  . . . . . . . . . . .  41
     6.9.  Feedback Formats and Options  . . . . . . . . . . . . . . 100
       6.9.1.  Feedback Formats  . . . . . . . . . . . . . . . . . . 100
       6.9.2.  Feedback Options  . . . . . . . . . . . . . . . . . . 102
   7.  Security Considerations . . . . . . . . . . . . . . . . . . . 104
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . 105
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . 105
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . . 106
     10.1. Normative References  . . . . . . . . . . . . . . . . . . 106
     10.2. Informative References  . . . . . . . . . . . . . . . . . 107
   Appendix A.    Detailed Classification of Header Fields . . . . . 108
     A.1.  IPv4 Header Fields  . . . . . . . . . . . . . . . . . . . 109
     A.2.  IPv6 Header Fields  . . . . . . . . . . . . . . . . . . . 112
     A.3.  UDP Header Fields   . . . . . . . . . . . . . . . . . . . 113
     A.4.  UDP-Lite Header Fields  . . . . . . . . . . . . . . . . . 114
     A.5.  RTP Header Fields . . . . . . . . . . . . . . . . . . . . 115
     A.6.  ESP Header Fields . . . . . . . . . . . . . . . . . . . . 117
     A.7.  IPv6 Extension Header Fields  . . . . . . . . . . . . . . 117
     A.8.  GRE Header Fields . . . . . . . . . . . . . . . . . . . . 118
     A.9.  MINE Header Fields  . . . . . . . . . . . . . . . . . . . 119
     A.10. AH Header Fields  . . . . . . . . . . . . . . . . . . . . 120
   Appendix B.    Compressor Implementation Guidelines . . . . . . . 121
     B.1.  Reference Management  . . . . . . . . . . . . . . . . . . 121
     B.2.  Window-based LSB Encoding (W-LSB)  . . .  . . . . . . . . 121
     B.3.  W-LSB Encoding and Timer-based Compression  . . . . . . . 122

























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1.  Introduction

   The ROHC WG has developed a header compression framework on top of
   which various profiles can be defined for different protocol sets or
   compression requirements.  The ROHC framework was first documented in
   [RFC3095], together with profiles for compression of RTP/UDP/IP
   (Real-Time Transport Protocol, User Datagram Protocol, Internet
   Protocol), UDP/IP, IP and ESP/IP (Encapsulating Security Payload)
   headers.  Additional profiles for compression of IP headers [RFC3843]
   and UDP-Lite (User Datagram Protocol Lite) headers [RFC4019] were
   later specified to complete the initial set of ROHC profiles.

   This document defines an updated version for each of the above
   mentioned profiles, and the definitions depend on the ROHC framework
   as found in [RFC4995].  The framework is required reading to
   understand the profile definitions, rules, and their role.

   Specifically, this document defines header compression schemes for:

   o RTP/UDP/IP      : profile 0x0101
   o UDP/IP          : profile 0x0102
   o ESP/IP          : profile 0x0103
   o IP              : profile 0x0104
   o RTP/UDP-Lite/IP : profile 0x0107
   o UDP-Lite/IP     : profile 0x0108

   Each of the profiles above can compress the following type of
   extension headers:

   o  AH [RFC4302]

   o  GRE [RFC2784][RFC2890]

   o  MINE [RFC2004]

   o  IPv6 Destination Options header[RFC2460]

   o  IPv6 Hop-by-hop Options header[RFC2460]

   o  IPv6 Routing header [RFC2460]

2.  Terminology

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





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   This document is consistent with the terminology found in the ROHC
   framework [RFC4995] and in the formal notation for ROHC [RFC4997].
   In addition, this document uses or defines the following terms:

   Acknowledgment Number

      The Acknowledgment Number identifies what packet is being
      acknowledged in the RoHCv2 feedback element (See Section 6.9).
      The value of this field normally corresponds to the Master
      Sequence Number (MSN) of the header that was last successfully
      decompressed, for the compression context (CID) for which the
      feedback information applies.

   Chaining of Items

      A chain of items groups fields based on similar characteristics.
      ROHCv2 defines chain items for static, dynamic and irregular
      fields.  Chaining is achieved by appending an item to the chain
      for each header in its 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 a
      ROHCv2 profile defines a compressed format.

   CRC-3 Control Fields Validation

      The CRC-3 control fields validation refers to the validation of
      the control fields.  This validation is performed by the
      decompressor when it receives a Compressed (CO) header that
      contains a 3-bit Cyclic Redundancy Check (CRC) calculated over
      control fields.  This 3-bit CRC covers controls fields carried in
      the CO header as well as specific control fields in the context.
      In the formal definition of the header formats, this 3-bit CRC is
      labeled "control_crc3" and uses the control_crc3_encoding (See
      also Section 6.6.11).

   Delta

      The delta refers to the difference in the absolute value of a
      field between two consecutive packets being processed by the same
      compression endpoint.

   Reordering Depth

      The number of packets by which a packet is received late within
      its sequence due to reordering between the compressor and the
      decompressor, i.e., reordering between packets associated with the
      same context (CID).  See the definition of sequentially late
      packet below.



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   ROHCv2 Header Types

      ROHCv2 profiles use two different header types: the Initialization
      and Refresh (IR) header type, and the Compressed (CO) header type.

   Sequentially Early Packet

      A packet that reaches the decompressor before one or several
      packets that were delayed over the channel, where all of the said
      packets belong to the same header-compressed flow and are
      associated to the same compression context (CID).  At the time of
      the arrival of a sequentially early packet, the packet(s) delayed
      on the link cannot be differentiated from lost packet(s).

   Sequentially Late Packet

      A packet is late within its sequence if it reaches the
      decompressor after one or several other packets belonging to the
      same CID have been received, although the sequentially late packet
      was sent from the compressor before the other packet(s).  How the
      decompressor detects a sequentially late packet is outside the
      scope of this specification, but it can for example use the MSN
      for this purpose.

   Timestamp Stride (ts_stride)

      The timestamp stride (ts_stride) is the expected increase in the
      timestamp value between two RTP packets with consecutive sequence
      numbers.  For example, for a media encoding with a sample rate of
      8 kHz producing one frame every 20 ms, the RTP timestamp will
      typically increase by n * 160 (= 8000 * 0.02), for some integer n.

   Time Stride (time_stride)

      The time stride (time_stride) is the time interval equivalent to
      one ts_stride, e.g., 20 ms in the example for the RTP timestamp
      increment above.














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3.  Acronyms

   This section lists most acronyms used for reference, in addition to
   those defined in [RFC4995].

   AH       Authentication Header.
   ESP      Encapsulating Security Payload.
   GRE      Generic Routing Encapsulation.
   FC       Full Context state (decompressor).
   IP       Internet Protocol.
   LSB      Least Significant Bits.
   MINE     Minimal Encapsulation in IP.
   MSB      Most Significant Bits.
   MSN      Master Sequence Number.
   NC       No Context state (decompressor).
   OA       Optimistic Approach.
   RC       Repair Context state (decompressor).
   ROHC     Header compression framework (RFC 4995).
   ROHCv2   Set of header compression profiles defined in this document.
   RTP      Real-time Transport Protocol.
   SSRC     Synchronization source. Field in RTP header.
   CSRC     Contributing source.  The RTP header contains an optional
            list of contributing sources.
   TC       Traffic Class.  Field in the IPv6 header.  See also TOS.
   TOS      Type Of Service.  Field in the IPv4 header.  See also TC.
   TS       RTP Timestamp.
   TTL      Time to Live.  Field in the IPv4 header.
   UDP      User Datagram Protocol.
   UDP-Lite User Datagram Protocol Lite.

4.  Background (Informative)

   This section provides background information on the compression
   profiles defined in this document.  The fundamentals of general
   header compression and of the ROHC framework may be found in sections
   3 and 4 of [RFC4995], respectively.  The fundamentals of the formal
   notation for ROHC are defined in [RFC4997].  [RFC4224] describes the
   impacts of out-of-order delivery on profiles based on [RFC3095].

4.1.  Classification of Header Fields

   Section 3.1 of [RFC4995] explains that header compression is possible
   due to the fact that there is much redundancy between field values
   within the headers of a packet, especially between the headers of
   consecutive packets.

   Appendix A lists and classifies in detail all the header fields
   relevant to this document.  The appendix concludes with



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   recommendations on how the various fields should be handled by header
   compression algorithms.

   The main conclusion is that most of the header fields can easily be
   compressed away since they never or seldom change.  A small number of
   fields however need more sophisticated mechanisms.

   These fields are:

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

   In particular, for RTP, the analysis in Appendix A reveals that the
   values of the RTP Timestamp (TS) field usually have a strong
   correlation to the RTP Sequence Number (SN), which increments by one
   for each packet emitted by an RTP source.  The RTP M-bit is expected
   to have the same value most of the time, but it needs to be
   communicated explicitly on occasion.

   For UDP, the Checksum field cannot be inferred or recalculated at the
   receiving end without violating its end-to-end properties, and is
   thus sent as-is when enabled (mandatory with IPv6).  The same applies
   to the UDP-Lite Checksum (mandatory with both IPv4 and IPv6), while
   the UDP-Lite Checksum Coverage may in some cases be compressible.

   For IPv4, a similar correlation as that of the RTP TS to the RTP SN
   is often observed between the Identifier field (IP-ID) and the master
   sequence number (MSN) used for compression (e.g., the RTP SN when
   compressing RTP headers).

4.2.  Improvements of ROHCv2 over RFC 3095 Profiles

   The ROHCv2 profiles can achieve compression efficiency and robustness
   that are both at least equivalent to RFC 3095 profiles [RFC3095],
   when used under the same operating conditions.  In particular, the
   size and bit layout of the smallest compressed header (i.e., PT-0
   format U/O-0 in RFC 3095, and pt_0_crc3 in ROHCv2) are identical.

   There are a number of differences and improvements between profiles
   defined in this document and their earlier version defined in RFC
   3095.  This section provides an overview of some of the most
   significant improvements:



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   Tolerance to reordering

      Profiles defined in RFC 3095 require that the channel between
      compressor and decompressor provide in-order delivery between
      compression endpoints.  ROHCv2 profiles, however, can handle
      robustly and efficiently a limited amount of reordering after the
      compression point as part of the compression algorithm itself.  In
      addition, this improved support for reordering makes it possible
      for ROHCv2 profiles to handle prelink reordering more efficiently.

   Operational logic

      Profiles in RFC 3095 define multiple operational modes, each with
      different updating logic and compressed header formats.  ROHCv2
      profiles operate in unidirectional operation until feedback is
      first received for a context (CID), at which point bidirectional
      operation is used; the formats are independent of what operational
      logic is used.

   IP extension header

      Profiles in RFC 3095 compress IP Extension headers using list
      compression.  ROHCv2 profiles instead treat extension headers in
      the same manner as other protocol headers, i.e., using the
      chaining mechanism; it thus assumes that extension headers are not
      added or removed during the lifetime of a context (CID), otherwise
      compression has to be restarted for this flow.

   IP encapsulation

      Profiles in RFC 3095 can compress at most two levels of IP
      headers.  ROHCv2 profiles can compress an arbitrary number of IP
      headers.

   List compression

      ROHCv2 profiles do not support reference-based list compression.

   Robustness and repairs

      ROHCv2 profiles do not define a format for the IR-DYN packet;
      instead, each profile defines a compressed header that can be used
      to perform a more robust context repair using a 7-bit CRC
      verification.  This also implies that only the IR header can
      change the association between a CID and the profile it uses.






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   Feedback

      ROHCv2 profiles mandate a CRC in the format of the FEEDBACK-2,
      while this is optional in RFC 3095.  A different set of feedback
      options is also used in ROHCv2 compared to RFC 3095.

4.3.  Operational Characteristics of ROHCv2 Profiles

   Robust header compression can be used over different link
   technologies.  Section 4.4 of [RFC4995] lists the operational
   characteristics of the ROHC channel.  The ROHCv2 profiles address a
   wide range of applications, and this section summarizes some of the
   operational characteristics that are specific to these profiles.

   Packet length

      ROHCv2 profiles assume that the lower layer indicates the length
      of a compressed packet.  ROHCv2 compressed headers do not contain
      length information for the payload.

   Out-of-order delivery between compression endpoints

      The definition of the ROHCv2 profiles places no strict requirement
      on the delivery sequence between the compression endpoints, i.e.,
      packets may be received in a different order than the compressor
      has sent them and still have a fair probability of being
      successfully decompressed.

      However, frequent out-of-order delivery and/or significant
      reordering depth will negatively impact the compression
      efficiency.  More specifically, if the compressor can operate
      using a proper estimate of the reordering characteristics of the
      path between the compression endpoints, larger headers can be sent
      more often to increase the robustness against decompression
      failures due to out-of-order delivery.  Otherwise, the compression
      efficiency will be impaired from an increase in the frequency of
      decompression failures and recovery attempts.

5.  Overview of the ROHCv2 Profiles (Informative)

   This section provides an overview of concepts that are important and
   useful to the ROHCv2 profiles.  These concepts may be used as
   guidelines for implementations but they are not part of the normative
   definition of the profiles, as these concepts relate to the
   compression efficiency of the protocol without impacting the
   interoperability characteristics of an implementation.





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5.1.  Compressor Concepts

   Header compression can be conceptually characterized as the
   interaction of a compressor with a decompressor state machine, one
   per context.  The responsibility of the compressor is to convey the
   information needed to successfully decompress a packet, based on a
   certain confidence regarding the state of the decompressor context.

   This confidence is obtained from the frequency and the type of
   information the compressor sends when updating the decompressor
   context from the optimistic approach (Section 5.1.1), and optionally
   from feedback messages (See Section 6.9), received from the
   decompressor.

5.1.1.  Optimistic Approach

   A compressor always uses the optimistic approach when it performs
   context updates.  The compressor normally repeats the same type of
   update until it is fairly confident that the decompressor has
   successfully received the information.  If the decompressor
   successfully receives any of the headers containing this update, the
   state will be available for the decompressor to process smaller
   compressed headers.

   If field X in the uncompressed header changes value, the compressor
   uses a header type that contains an encoding of field X until it has
   gained confidence that the decompressor has received at least one
   packet containing the new value for X.  The compressor normally
   selects a compressed format with the smallest header that can convey
   the changes needed to achieve confidence.

   The number of repetitions that is needed to obtain this confidence is
   normally related to the packet loss and out-of-order delivery
   characteristics of the link where header compression is used; it is
   thus not defined in this document.  It is outside the scope of this
   specification and is left to implementors to decide.

5.1.2.  Tradeoff between Robustness to Losses and to Reordering

   The ability of a header compression algorithm to handle sequentially
   late packets is mainly limited by two factors: the interpretation
   interval offset of the sliding window used for lsb encoded fields
   [RFC4997], and the optimistic approach (See Section 5.1.1) for seldom
   changing fields.







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   lsb encoded fields:

      The interpretation interval offset specifies an upper limit for
      the maximum reordering depth, by which is it possible for the
      decompressor to recover the original value of a dynamically
      changing (i.e., sequentially incrementing) field that is encoded
      using a window-based lsb encoding.  Its value is typically bound
      to the number of lsb compressed bits in the compressed header
      format, and thus grows with the number of bits transmitted.
      However, the offset and the lsb encoding only provide robustness
      for the field that it compresses, and (implicitly) for other
      sequentially changing fields that are derived from that field.

      This is shown in the figure below:

         <--- interpretation interval (size is 2^k) ---->
         |------------------+---------------------------|
      v_ref-p             v_ref              v_ref + (2^k-1) - p
       Lower                                          Upper
       Bound                                          Bound
         <--- reordering --> <--------- losses --------->

         where p is the maximum negative delta, corresponding to the
         maximum reordering depth for which the lsb encoding can recover
         the original value of the field;

         where (2^k-1) - p is the maximum positive delta, corresponding
         to the maximum number of consecutive losses for which the lsb
         encoding can recover the original value of the field;

         where v_ref is the reference value, as defined in the lsb
         encoding method in [RFC4997].

      There is thus a tradeoff between the robustness against reordering
      and the robustness against packet losses, with respect to the
      number of MSN bits needed and the distribution of the
      interpretation interval between negative and positive deltas in
      the MSN.

   Seldom changing fields

      The optimistic approach (Section 5.1.1) provides the upper limit
      for the maximum reordering depth for seldom changing fields.

   There is thus a tradeoff between compression efficiency and
   robustness.  When only information on the MSN needs to be conveyed to
   the decompressor, the tradeoff relates to the number of compressed




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   MSN bits in the compressed header format.  Otherwise, the tradeoff
   relates to the implementation of the optimistic approach.

   In particular, compressor implementations should adjust their
   optimistic approach strategy to match both packet loss and reordering
   characteristics of the link over which header compression is applied.
   For example, the number of repetitions for each update of a non-lsb
   encoded field can be increased.  The compressor can ensure that each
   update is repeated until it is reasonably confident that at least one
   packet containing the change has reached the decompressor before the
   first packet sent after this sequence.

5.1.3.  Interactions with the Decompressor Context

   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.

   Initially, when sending the first IR packet for a compressed flow,
   the compressor does not expect to receive feedback for that flow,
   until such feedback is first received.  At this point, the compressor
   may then assume that the decompressor will continue to send feedback
   in order to repair its context when necessary.  The former is
   referred to as unidirectional operation, while the latter is called
   bidirectional operation.

   The compressor can then adjust the compression level (i.e., what
   header format it selects) based on its confidence that the
   decompressor has the necessary information to successfully process
   the compressed headers that it selects.

   In other words, the responsibilities of the compressor are to ensure
   that the decompressor operates with state information that is
   sufficient to successfully decompress the type of compressed
   header(s) it receives, and to allow the decompressor to successfully
   recover that state information as soon as possible otherwise.  The
   compressor therefore selects the type of compressed header based on
   the following factors:

   o  the outcome of the encoding method applied to each field;

   o  the optimistic approach, with respect to the characteristics of
      the channel;

   o  the type of operation (unidirectional or bidirectional), and if in
      bidirectional operation, feedback received from the decompressor
      (ACKs, NACKs, STATIC-NACK, and options).




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   Encoding methods normally use previous value(s) from a history of
   packets whose headers it has previously compressed.  The optimistic
   approach is meant to ensure that at least one compressed header
   containing the information to update the state for a field is
   received.  Finally, feedback indicates what actions the decompressor
   has taken with respect to its assumptions regarding the validity of
   its context (Section 5.2.2); it indicates what type of compressed
   header the decompressor can or cannot decompress.

   The decompressor has the means to detect decompression failures for
   any compressed (CO) header format, using the CRC verification.
   Depending on the frequency and/or on the type of the failure, it
   might send a negative acknowledgement (NACK) or an explicit request
   for a complete context update (STATIC-NACK).  However, the
   decompressor does not have the means to identify the cause of the
   failure, and in particular the decompression of what field(s) is
   responsible for the failure.  The compressor is thus always
   responsible for determining the most suitable response to a negative
   acknowledgement, using the confidence it has in the state of the
   decompressor context, when selecting the type of compressed header it
   will use when compressing a header.

5.2.  Decompressor Concepts

   The decompressor normally uses the last received and successfully
   validated (IR packets) or verified (CO packets) header as the
   reference for future decompression.

   The decompressor is responsible for verifying the outcome of every
   decompression attempt, to update its context when successful, and
   finally to request context repairs by making coherent usage of
   feedback once it has started using feedback.

   Specifically, the outcome of every decompression attempt is verified
   using the CRC present in the compressed header; the decompressor
   updates the context information when this outcome is successfully
   verified; finally, if the decompressor uses feedback once for a
   compressed flow, then it will continue to do so for as long as the
   corresponding context is associated with the same profile.

5.2.1.  Decompressor State Machine

   The decompressor operation may be represented as a state machine
   defining three states: No Context (NC), Repair Context (RC), and Full
   Context (FC).

   The decompressor starts without a valid context, the NC state.  Upon
   receiving an IR packet, the decompressor validates the integrity of



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   its header using the CRC-8 validation.  If the IR header is
   successfully validated, the decompressor updates the context and uses
   this header as the reference header, and moves to the FC state.  Once
   the decompressor state machine has entered the FC state, it does not
   normally leave; only repeated decompression failures will force the
   decompressor to transit downwards to a lower state.  When context
   damage is detected, the decompressor moves to the repair context (RC)
   state, where it stays until it successfully verifies a decompression
   attempt for a compressed header with a 7-bit CRC or until it
   successfully validates an IR header.  When static context damage is
   detected, the decompressor moves back to the NC state.

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

  CRC-8(IR) Validation
   +----->----->----->----->----->----->----->----->----->----->----+
   |                                                  CRC-8(IR)     |
   |  !CRC-8(IR) or      CRC-7(CO) or                 or CRC-7(CO)  |
   |  PT not allowed     CRC-8(IR)                    or CRC-3(CO)  |
   |  +--->---+         +--->----->----->----->---+  +--->---->---+ |
   |  |       |         |                         |  |            | |
   |  |       v         |                         v  |            v v
  +-----------------+  +----------------------+  +--------------------+
  | No Context (NC) |  | Repair Context (RC)  |  | Full Context (FC)  |
  +-----------------+  +----------------------+  +--------------------+
    ^ ^ Static Context  | ^ !CRC-7(CO) or  | ^ Context Damage  | |
    | | Damage Detected | | PT not allowed | | Detected        | |
    | +--<-----<-----<--+ +----<------<----+ +--<-----<-----<--+ |
    |                                                            |
    |            Static Context Damage Detected                  |
    +--<-----<-----<-----<-----<-----<-----<-----<-----<---------+

  where:

    CRC-8(IR)        : Successful CRC-8 validation for the IR header.
    !CRC-8(IR)       : Unsuccessful CRC-8 validation for the IR header.
    CRC-7(CO) and/or
    CRC-3(CO)        : Successful CRC verification for the decompression
                       of a CO header, based on the number of CRC bits
                       carried in the CO header.
    !CRC-7(CO)       : Failure to CRC verify the decompression of a CO
                       header carrying a 7-bit CRC.
    PT not allowed   : The decompressor has received a packet type (PT)
                       for which the decompressor's current context does
                       not provide enough valid state information to
                       decompress the packet.



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      Static Context Damage Detected: See definition in Section 5.2.2.

      Context Damage Detected: See definition in Section 5.2.2.

5.2.1.1.  No Context (NC) State

   Initially, while working in the No Context (NC) state, the
   decompressor has not yet successfully validated an IR header.

   Attempting decompression:

      In the NC state, only packets carrying sufficient information on
      the static fields (i.e., IR packets) can be decompressed.

   Upward transition:

      The decompressor can move to the Full Context (FC) state when the
      CRC validation of an 8-bit CRC in an IR header is successful.

   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 an IR header has
      failed the CRC-8 validation, subject to the feedback rate
      limitation as described in Section 5.2.3.

5.2.1.2.  Repair Context (RC) State

   In the Repair Context (RC) state, the decompressor has successfully
   decompressed packets for this context, but does not have confidence
   that the entire context is valid.

   Attempting decompression:

      In the RC state, only headers covered by an 8-bit CRC (i.e., IR)
      or CO headers carrying a 7-bit CRC can be decompressed.

   Upward transition:

      The decompressor can move to the Full Context (FC) state when the
      CRC verification succeeds for a CO header carrying a 7-bit CRC or
      when validation of an 8-bit CRC in an IR header succeeds.

   Downward transition:

      The decompressor moves back to the NC state if it assumes static
      context damage.




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   Feedback logic:

      In the RC state, the decompressor should send a STATIC-NACK when
      CRC-8 validation of an IR header fails, or when a CO header
      carrying a 7-bit CRC fails and static context damage is assumed,
      subject to the feedback rate limitation as described in
      Section 5.2.3.  If any other packet type is received, the
      decompressor should treat it as a CRC verification failure to
      determine if NACK is to be sent.

5.2.1.3.  Full Context (FC) State

   In the Full Context (FC) state, the decompressor assumes that its
   entire context is valid.

   Attempting decompression:

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

   Downward transition:

      The decompressor moves back to the RC state if it assumes context
      damage.  If the decompressor assumes static context damage, it
      moves directly to the NC state.

   Feedback logic:

      In the FC state, the decompressor should send a NACK when CRC-8
      validation or CRC verification of any header type fails and if
      context damage is assumed, or it should send a STATIC-NACK if
      static context damage is assumed; this is subject to the feedback
      rate limitation described in Section 5.2.3.

5.2.2.  Decompressor Context Management

   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 inferred from
   other fields).

   The decompressor may assume that some or the entire context is
   invalid, when it fails to validate or to verify one or more headers
   using the CRC.  Because the decompressor cannot know the exact






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   reason(s) for a CRC failure or what field caused it, the validity of
   the context hence does not refer to what specific part(s) of the
   context 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 decompression 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.  Failure to validate the 3-bit CRC that
   protects control fields should be treated as a decompression failure
   when the decompressor asserts the validity of its context.

   Context Damage Detection

      The assumption of context damage means that the decompressor will
      not attempt decompression of a CO header that carries only a 3-bit
      CRC, and will only attempt decompression of IR 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 will attempt decompression.  In
   other words, validity of the context refers to the ability of a
   decompressor to attempt (or not) decompression of specific packet
   types.

   When ROHCv2 profiles are used over a channel that cannot guarantee
   in-order delivery, the decompressor may refrain from updating its
   context with the content of a sequentially late packet that is
   successfully decompressed.  This is to avoid updating the context
   with information that is older than what the decompressor already has
   in its context.






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5.2.3.  Feedback Logic

   ROHCv2 profiles may be used in environments with or without feedback
   capabilities from decompressor to compressor.  ROHCv2 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 context, this
   channel will be used during the entire compression operation for that
   context (i.e., bidirectional operation).

   The ROHC framework defines 3 types of feedback messages: ACKs, NACKs,
   and STATIC-NACKs.  The semantics of each message is defined in
   Section 5.2.4.1. of [RFC4995].  What feedback to send is coupled with
   the context management of the decompressor, i.e., with the
   implementation of the context damage detection algorithms as
   described in Section 5.2.2.

   The decompressor should send a NACK when it assumes context damage,
   and it should send a STATIC-NACK when it assumes static context
   damage.  The decompressor is not strictly expected to send ACK
   feedback upon successful decompression, other than for the purpose of
   improving compression efficiency.

   When ROHCv2 profiles are used over a channel that cannot guarantee
   in-order delivery, the decompressor may refrain from sending ACK
   feedback for a sequentially late packet that is successfully
   decompressed.

   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 with the same event.

6.  ROHCv2 Profiles (Normative)

6.1.  Channel Parameters, Segmentation, and Reordering

   The compressor MUST NOT use ROHC segmentation (see Section 5.2.5 of
   [RFC4995]), i.e., the Maximum Reconstructed Reception Unit (MRRU)
   MUST be set to 0, if the configuration of the ROHC channel contains
   at least one ROHCv2 profile in the list of supported profiles (i.e.,
   the PROFILES parameter) and if the channel cannot guarantee in-order
   delivery of packets between compression endpoints.









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6.2.  Profile Operation, Per-context

   ROHCv2 profiles operate differently, per context, depending on how
   the decompressor makes use of the feedback channel, if any.  Once the
   decompressor uses the feedback channel for a context, it establishes
   the feedback channel for that CID.

   The compressor always starts with the assumption that the
   decompressor will not send feedback when it initializes a new context
   (see also the definition of a new context in Section 5.1.1. of
   [RFC4995], i.e., there is no established feedback channel for the new
   context.  At this point, despite the use of the optimistic approach,
   decompression failure is still possible because the decompressor may
   not have received sufficient information to correctly decompress the
   packets; therefore, until the decompressor has established a feedback
   channel, the compressor SHOULD periodically send IR packets.  The
   periodicity can be based on timeouts, on the number of compressed
   packets sent for the flow, or any other strategy the implementer
   chooses.

   The reception of either positive feedback (ACKs) or negative feedback
   (NACKs or STATIC-NACKs) from the decompressor establishes the
   feedback channel for the context (CID) for which the feedback was
   received.  Once there is an established feedback channel for a
   specific context, the compressor can make use of this feedback to
   estimate the current state of the decompressor.  This helps to
   increase the compression efficiency by providing the information
   needed for the compressor to achieve the necessary confidence level.
   When the feedback channel is established, it becomes superfluous for
   the compressor to send periodic refreshes, and instead it can rely
   entirely on the optimistic approach and feedback from the
   decompressor.

   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 or STATIC-NACKs)
   establishes this channel.  Once it has established a feedback channel
   for a CID, the decompressor is REQUIRED to continue sending feedback
   for the lifetime of the context (i.e., until it receives an IR packet
   that associates the CID to a different profile), to send error
   recovery requests and (optionally) acknowledgments of significant
   context updates.

   Compression without an established feedback channel will be less
   efficient, because of the periodic refreshes and the lack of feedback
   to trigger error recovery; there will also be a slightly higher
   probability of loss propagation compared to the case where the
   decompressor uses feedback.



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6.3.  Control Fields

   ROHCv2 defines a number of control fields that are used by the
   decompressor in its interpretation of the header formats received
   from the compressor.  The control fields listed in the following
   subsections are defined using the formal notation [RFC4997] in
   Section 6.8.2.4 of this document.

6.3.1.  Master Sequence Number (MSN)

   The Master Sequence Number (MSN) field is either taken from a field
   that already exists in one of the headers of the protocol that the
   profile compresses (e.g., RTP SN), or alternatively it is created at
   the compressor.  There is one MSN space per context.

   The MSN field has the following two functions:

   o  Differentiating between reference headers when receiving feedback
      data;

   o  Inferring the value of incrementing fields (e.g., IPv4
      Identifier).

   There is one MSN field in every ROHCv2 header, i.e., the MSN is
   always present in each header type sent by the compressor.  The MSN
   is sent in full in IR headers, while it can be lsb encoded within CO
   header formats.  The decompressor always includes LSBs of the MSN in
   the Acknowledgment Number field in feedback (see Section 6.9).  The
   compressor can later use this field to infer what packet the
   decompressor is acknowledging.

   For profiles for which the MSN is created by the compressor (i.e.,
   0x0102, 0x0104, and 0x0108), the following applies:

   o  The compressor only initializes the MSN for a context when that
      context is first created or when the profile associated with a
      context changes;

   o  When the MSN is initialized, it is initialized to a random value;

   o  The value of the MSN SHOULD be incremented by one for each packet
      that the compressor sends for a specific CID.

6.3.2.  Reordering Ratio

   The control field reorder_ratio specifies how much reordering is
   handled by the lsb encoding of the MSN.  This is useful when header
   compression is performed over links with varying reordering



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   characteristics.  The reorder_ratio control field provides the means
   for the compressor to adjust the robustness characteristics of the
   lsb encoding method with respect to reordering and consecutive
   losses, as described in Section 5.1.2.

6.3.3.  IP-ID Behavior

   The IP-ID field of the IPv4 header can have different change
   patterns: sequential in network byte order, sequential byte-swapped,
   random or constant (a constant value of zero, although not conformant
   with [RFC0791], has been observed in practice).  There is one IP-ID
   behavior control field per IP header.  The control field for the
   IP-ID behavior of the innermost IP header determines which set of
   header formats is used.  The IP-ID behavior control field is also
   used to determine the contents of the irregular chain item, for each
   IP header.

   ROHCv2 profiles MUST NOT assign a sequential behavior (network byte
   order or byte-swapped) to any IP-ID but the one in the innermost IP
   header when compressing more than one level of IP headers.  This is
   because only the IP-ID of the innermost IP header is likely to have a
   sufficiently close correlation with the MSN to compress it as a
   sequentially changing field.  Therefore, a compressor MUST assign
   either the constant zero IP-ID or the random IP-ID behavior to
   tunneling headers.

6.3.4.  UDP-Lite Coverage Behavior

   The control field coverage_behavior specifies how the checksum
   coverage field of the UDP-Lite header is compressed with RoHCv2.  It
   can indicate one of the following encoding methods: irregular,
   static, or inferred encoding.

6.3.5.  Timestamp Stride

   The ts_stride control field is used in scaled RTP timestamp encoding
   (see Section 6.6.8).  It defines the expected increase in the RTP
   timestamp between consecutive RTP sequence numbers.

6.3.6.  Time Stride

   The time_stride control field is used in timer-based compression
   encoding (see Section 6.6.9).  When timer-based compression is used,
   time_stride should be set to the expected difference in arrival time
   between consecutive RTP packets.






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6.3.7.  CRC-3 for Control Fields

   ROHCv2 profiles define a CRC-3 calculated over a number of control
   fields.  This 3-bit CRC protecting the control fields is present in
   the header format for the co_common and co_repair header types.

   The decompressor MUST always validate the integrity of the control
   fields covered by this 3-bit CRC when processing a co_common or a
   co_repair compressed header.

   Failure to validate the control fields using this CRC should be
   considered as a decompression failure by the decompressor in the
   algorithm that assesses the validity of the context.  However, if the
   decompression attempt can be verified using either the CRC-3 or the
   CRC-7 calculated over the uncompressed header, the decompressor MAY
   still forward the decompressed header to upper layers.  This is
   because the protected control fields are not always used to
   decompress the header (i.e., co_common or co_repair) that updates
   their respective value.

   The CRC polynomial and coverage of this CRC-3 is defined in
   Section 6.6.11.

6.4.  Reconstruction and Verification

   Validation of the IR header (8-bit CRC)

      The decompressor MUST always validate the integrity of the IR
      header using the 8-bit CRC carried within the IR header.  When the
      header is validated, the decompressor updates the context with the
      information in the IR header.  Otherwise, if the IR cannot be
      validated, the context MUST NOT be updated and the IR header MUST
      NOT be delivered to upper layers.

   Verification of CO headers (3-bit CRC or 7-bit CRC)

      The decompressor MUST always verify the decompression of a CO
      header using the CRC carried within the compressed header.  When
      the decompression is verified and successful, the decompressor
      updates the context with the information received in the CO
      header; otherwise, if the reconstructed header fails the CRC
      verification, these updates MUST NOT be performed.

      A packet for which the decompression attempt cannot be verified
      using the CRC MUST NOT be delivered to upper layers.






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      Decompressor implementations may attempt corrective or repair
      measures on CO headers prior to performing the above actions, and
      the result of any decompression attempt MUST be verified using the
      CRC.

6.5.  Compressed Header Chains

   Some header 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.

   In the text below, the term <protocol_name> is used to identify
   formal notation names corresponding to different protocol headers.
   The mapping between these is defined in the following table:

     +----------------------------------+---------------+
     | Protocol                         | protocol_name |
     +----------------------------------+---------------+
     | IPv4                    RFC 0791 | ipv4          |
     | IPv6                    RFC 2460 | ipv6          |
     | UDP                     RFC 0768 | udp           |
     | RTP                     RFC 3550 | rtp           |
     | ESP                     RFC 4303 | esp           |
     | UDP-Lite                RFC 3828 | udp_lite      |
     | AH                      RFC 4302 | ah            |
     | GRE           RFC 2784, RFC 2890 | gre           |
     | MINE                    RFC 2004 | mine          |
     | IPv6 Destination Option RFC 2460 | dest_opt      |
     | IPv6 Hop-by-hop Options RFC 2460 | hop_opt       |
     | IPv6 Routing Header     RFC 2460 | rout_opt      |
     +----------------------------------+---------------+

   Static chain:

      The static chain consists of one item for each header of the chain
      of protocol headers that is compressed, starting from the
      outermost IP header.  In the formal description of the header
      formats, this static chain item for each header type is labeled
      <protocol_name>_static.  The static chain is only used in the IR
      header format.






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   Dynamic chain:

      The dynamic chain consists of one item for each header of the
      chain of protocol headers that is compressed, starting from the
      outermost IP header.  In the formal description of the header
      formats, the dynamic chain item for each header type is labeled
      <protocol_name>_dynamic.  The dynamic chain is only used in the IR
      and co_repair header formats.

   Irregular chain:

      The structure of the irregular chain is analogous to the structure
      of the static chain.  For each compressed header that uses the
      general format of Section 6.8, the irregular chain is appended at
      a specific location in the general format of the compressed
      headers.  In the formal description of the header formats, the
      irregular chain item for each header type is a format whose name
      is suffixed by "_irregular".  The irregular chain is used in all
      CO headers, except for the co_repair format.

      The format of the irregular chain for the innermost IP header
      differs from the format used for the outer IP headers, because the
      innermost IP header is part of the compressed base header.  In the
      definition of the header formats using the formal notation, the
      argument "is_innermost", which is passed to the corresponding
      encoding method (ipv4 or ipv6), determines what irregular chain
      items to use.  The format of the irregular chain item for the
      outer IP headers is also determined using one flag for TTL/Hop
      Limit and TOS/TC.  This flag is defined in the format of some of
      the compressed base headers.

   ROHCv2 profiles compress extension headers as other headers, and thus
   extension headers have a static chain, a dynamic chain, and an
   irregular chain.

   ROHCv2 profiles define chains for all headers that can be compressed,
   i.e., RTP [RFC3550], UDP [RFC0768], ESP [RFC4303], UDP-Lite
   [RFC3828], IPv4 [RFC0791], IPv6 [RFC2460], AH [RFC4302], GRE
   [RFC2784][RFC2890], MINE [RFC2004], IPv6 Destination Options header
   [RFC2460], IPv6 Hop-by-hop Options header [RFC2460], and IPv6 Routing
   header [RFC2460].

6.6.  Header Formats and Encoding Methods

   The header formats are defined using the ROHC formal notation.  Some
   of the encoding methods used in the header formats are defined in
   [RFC4997], while other methods are defined in this section.




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6.6.1.  baseheader_extension_headers

   The baseheader_extension_headers encoding method skips over all
   fields of the extension headers of the innermost IP header, without
   encoding any of them.  Fields in these extension headers are instead
   encoded in the irregular chain.

   This encoding is used in CO headers (see Section 6.8.2).  The
   innermost IP header is combined with other header(s) (i.e., UDP, UDP-
   Lite, RTP) to create the compressed base header.  In this case, there
   may be a number of extension headers between the IP headers and the
   other headers.

   The base header defines a representation of the extension headers, to
   comply with the syntax of the formal notation; this encoding method
   provides this representation.

6.6.2.  baseheader_outer_headers

   The baseheader_outer_headers encoding method skips over all the
   fields of the extension header(s) that do not belong to the innermost
   IP header, without encoding any of them.  Changing fields in outer
   headers are instead handled by the irregular chain.

   This encoding method, similarly to the baseheader_extension_headers
   encoding method above, is necessary to keep the definition of the
   header formats syntactically correct.  It describes tunneling IP
   headers and their respective extension headers (i.e., all headers
   located before the innermost IP header) for CO headers (see
   Section 6.8.2).

6.6.3.  inferred_udp_length

   The decompressor infers the value of the UDP length field as being
   the sum of the UDP header length and the UDP payload length.  The
   compressor must therefore ensure that the UDP length field is
   consistent with the length field(s) of preceding subheaders, i.e.,
   there must not be any padding after the UDP payload that is covered
   by the IP Length.

   This encoding method is also used for the UDP-Lite Checksum Coverage
   field when it behaves in the same manner as the UDP length field
   (i.e., when the checksum always covers the entire UDP-Lite payload).

6.6.4.  inferred_ip_v4_header_checksum

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



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      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.  As the data that this checksum
   protects is mostly compressed away and is instead taken from state
   stored in the context, this checksum becomes cumulative to the ROHC
   CRC.  When using this encoding method, the checksum is recomputed by
   the decompressor.

   The inferred_ip_v4_header_checksum encoding method thus compresses
   the header checksum field of the IPv4 header 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 computation above.

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

6.6.5.  inferred_mine_header_checksum

   This encoding method compresses the minimal encapsulation header
   checksum.  This checksum is defined in RFC 2004 [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 0.
         The IP header and IP payload (after the minimal forwarding
         header) are not included in this checksum computation.

   The inferred_mine_header_checksum encoding method compresses the
   minimal encapsulation header checksum down to a size of zero 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.



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   The motivations for inferring this checksum are similar to the ones
   explained above in Section 6.6.4.

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

6.6.6.  inferred_ip_v4_length

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

      Total Length: 16 bits

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

   The inferred_ip_v4_length encoding method compresses the IPv4 header
   checksum down to a size of zero 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 by counting in octets
   the length of the entire packet after decompression.

6.6.7.  inferred_ip_v6_length

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

      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, i.e., no
   bits are transmitted in compressed headers for this field.  Using
   this encoding method, the decompressor infers the value of this field
   by counting in octets the length of the entire packet after
   decompression.

   IPv6 headers using the jumbo payload option of RFC 2675 [RFC2675]
   will not be compressible with this encoding method since the value of
   the payload length field does not match the length of the packet.



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6.6.8.  Scaled RTP Timestamp Compression

   This section provides additional details on encodings used to scale
   the RTP timestamp, as defined in the formal notation in
   Section 6.8.2.4.

   The RTP timestamp (TS) usually increases by a multiple of the RTP
   Sequence Number's (SN's) increase and is therefore a suitable
   candidate for scaled encoding.  This scaling factor is labeled
   ts_stride in the definition of the profile in the formal notation.
   The compressor sets the scaling factor based on the change in TS with
   respect to the change in the RTP SN.

   The default value of the scaling factor ts_stride is 160, as defined
   in Section 6.8.2.4.  To use a different value for ts_stride, the
   compressor explicitly updates the value of ts_stride to the
   decompressor using one of the header formats that can carry this
   information.

   When the compressor uses a scaling factor that is different than the
   default value of ts_stride, it can only use the new scaling factor
   once it has enough confidence that the decompressor has successfully
   calculated the residue (ts_offset) of the scaling function for the
   timestamp.  The compressor achieves this by sending unscaled
   timestamp values, to allow the decompressor to establish the residue
   based on the current ts_stride.  The compressor MAY send the unscaled
   timestamp in the same compressed header(s) used to establish the
   value of ts_stride.

   Once the compressor has gained enough confidence that both the value
   of the scaling factor and the value of the residue have been
   established in the decompressor, the compressor can start compressing
   packets using the new scaling factor.

   When the compressor detects that the residue (ts_offset) value has
   changed, it MUST NOT select a compressed header format that uses the
   scaled timestamp encoding before it has re-established the residue as
   described above.

   When the value of the timestamp field wraps around, the value of the
   residue of the scaling function is likely to change.  When this
   occurs, the compressor re-establishes the new residue value as
   described above.

   If the decompressor receives a compressed header containing scaled
   timestamp bits while the ts_stride equals zero, it MUST NOT deliver
   the packet to upper layers and it SHOULD treat this as a CRC
   verification failure.



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   Whether or not the scaling is applied to the RTP TS field is up to
   the compressor implementation (i.e., the use of scaling is OPTIONAL),
   and is indicated by the tsc_indicator control field.  In case scaling
   is applied to the RTP TS field, the value of ts_stride used by the
   compressor is up to the implementation.  A value of ts_stride that is
   set to the expected increase in the RTP timestamp between consecutive
   unit increases of the RTP SN will provide the most gain for the
   scaled encoding.  Other values may provide the same gain in some
   situations, but may reduce the gain in others.

   When scaled timestamp encoding is used for header formats that do not
   transmit any lsb-encoded timestamp bits at all, the
   inferred_scaled_field encoding of Section 6.6.10 is used for encoding
   the timestamp.

6.6.9.  timer_based_lsb

   The timer-based compression encoding method, timer_based_lsb,
   compresses a field whose change pattern approximates a linear
   function of the time of day.

   This encoding uses the local clock to obtain an approximation of the
   value that it encodes.  The approximated value is then used as a
   reference value together with the num_lsbs_param least-significant
   bits received as the encoded value, where num_lsbs_param represents a
   number of bits that is sufficient to uniquely represent the encoded
   value in the presence of jitter between compression endpoints.

     ts_scaled =:= timer_based_lsb(<time_stride_param>,
                                   <num_lsbs_param>, <offset_param>)

   The parameters "num_lsbs_param" and "offset_param" are the parameters
   to use for the lsb encoding, i.e., the number of least significant
   bits and the interpretation interval offset, respectively.  The
   parameter "time_stride_param" represents the context value of the
   control field time_stride.

   This encoding method always uses a scaled version of the field it
   compresses.

   The value of the field is decoded by calculating an approximation of
   the scaled value, using:

        tsc_ref_advanced = tsc_ref + (a_n - a_ref) / time_stride.







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      where:

      - tsc_ref is a reference value of the scaled representation
        of the field.
      - a_n is the arrival time associated with the value to decode.
      - a_ref is the arrival time associated with the reference header.
      - tsc_ref_advanced is an approximation of the scaled value
        of the field.

   The lsb encoding is then applied using the num_lsbs_param bits
   received in the compressed header and the tsc_ref_advanced as
   "ref_value" (as per Section 4.11.5 of [RFC4997]).

   Appendix B.3 provides an example of how the compressor can calculate
   jitter.

   The control field time_stride controls whether or not the
   timer_based_lsb method is used in the CO header.  The decompressor
   SHOULD send the CLOCK_RESOLUTION option with a zero value, if:

   o  it receives a non-zero time_stride value, and

   o  it has not previously sent a CLOCK_RESOLUTION feedback with a non-
      zero value.

   This is to allow compression to recover from the case where a
   compressor erroneously activates timer-based compression.

   The support and usage of timer-based compression is OPTIONAL for both
   the compressor and the decompressor; the compressor is not required
   to set the time_stride control field to a non-zero value when it has
   received a non-zero value for the CLOCK_RESOLUTION option.

6.6.10.  inferred_scaled_field

   The inferred_scaled_field encoding method encodes a field that is
   defined as changing in relation to the MSN, and for which the
   increase with respect to the MSN can be scaled by some scaling
   factor.  This encoding method is used in compressed header formats
   that do not contain any bits for the scaled field.  In this case, the
   decompressor infers the unscaled value of the scaled field from the
   MSN field.  The unscaled value is calculated according to the
   following formula:

      unscaled_value = delta_msn * stride + reference_unscaled_value

   where "delta_msn" is the difference in MSN between the reference
   value of the MSN in the context and the value of the MSN decompressed



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   from this packet, "reference_unscaled_value" is the value of the
   field being scaled in the context, and "stride" is the scaling value
   for this field.

   For example, when this encoding method is applied to the RTP
   timestamp in the RTP profile, the calculation above becomes:

      timestamp = delta_msn * ts_stride + reference_timestamp

6.6.11.  control_crc3_encoding

   The control_crc3_encoding method provides a CRC calculated over a
   number of control fields.  The definition of this encoding method is
   the same as for the "crc" encoding method specified in Section 4.11.6
   of [RFC4997], with the difference being that the data covered by the
   CRC is given by a concatenated list of control fields.

   In other words, the definition of the control_crc3_encoding method is
   equivalent to the following definition:

     control_crc_encoding(ctrl_data_value, ctrl_data_length)
     {
       UNCOMPRESSED {
       }

       COMPRESSED {
         control_crc3 =:=
           crc(3, 0x06, 0x07, ctrl_data_value, ctrl_data_length) [ 3 ];
       }
     }

   where the parameter "ctrl_data_value" binds to the concatenated
   values of the following control fields, in the order listed below:

   o  reorder_ratio, 2 bits padded with 6 MSB of zeroes

   o  ts_stride, 32 bits (only for profiles 0x0101 and 0x0107)

   o  time_stride, 32 bits (only for profiles 0x0101 and 0x0107)

   o  msn, 16 bits (not applicable for profiles 0x0101, 0x0103, and
      0x0107)

   o  coverage_behavior, 2 bits padded with 6 MSB of zeroes (only for
      profiles 0x0107 and 0x0108)






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   o  ip_id_behavior, one octet for each IP header in the compressible
      header chain starting from the outermost header.  Each octet
      consists of 2 bits padded with 6 MSBs of zeroes.

   The "ctrl_data_length" binds to the sum of the length of the control
   field(s) that are applicable to the specific profile.

   The decompressor uses the resulting 3-bit CRC to validate the control
   fields that are updated by the co_common and co_repair header
   formats; this CRC cannot be used to verify the outcome of a
   decompression attempt.

   This CRC protects the update of control fields, as the updated values
   are not always used to decompress the header that carries them and
   thus are not protected by the CRC-7 verification.  This prevents
   impairments that could occur if the decompression of a co_common or
   of a co_repair succeeds and the decompressor sends positive feedback,
   while for some reason the control fields are incorrectly updated.

6.6.12.  inferred_sequential_ip_id

   This encoding method is used with a sequential IP-ID behavior
   (sequential or sequential byte-swapped) and when there are no coded
   IP-ID bits in the compressed header.  In this case, the IP-ID offset
   from the MSN is constant, and the IP-ID increases by the same amount
   as the MSN (similar to the inferred_scaled_field encoding method).

   The decompressor calculates the value for the IP-ID according to the
   following formula:

      IP-ID = delta_msn + reference_IP_ID_value

   where "delta_msn" is the difference between the reference value of
   the MSN in the context and the uncompressed value of the MSN
   associated to the compressed header, and where
   "reference_IP_ID_value" is the value of the IP-ID in the context.
   For swapped IP-ID behavior (i.e., when ip_id_behavior_innermost is
   set to IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED), "reference_IP_ID_value"
   and "IP-ID" are byte-swapped with regard to the corresponding fields
   in the context.

   If the IP-ID behavior is random or zero, this encoding method does
   not update any fields.








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6.6.13.  list_csrc(cc_value)

   This encoding method compresses the list of RTP CSRC identifiers
   using list compression.  This encoding establishes a content for the
   different CSRC identifiers (items) and a list describing the order in
   which they appear.

   The compressor passes an argument (cc_value) to this encoding method:
   this is the value of the CC field taken from the RTP header.  The
   decompressor is required to bind the value of this argument to the
   number of items in the list, which will allow the decompressor to
   correctly reconstruct the CC field.

6.6.13.1.  List Compression

   The CSRC identifiers 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 |
            +--------+--------+--...--+--------+

   When performing list compression on a CSRC list, each item is the
   uncompressed value of one CSRC identifier.

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

   When initializing the context:

   1) The complete representation of the list of CSRC identifiers is
      transmitted.

   Then, once the context has been initialized:

   2) When the list is unchanged, a compressed header that does not
      contain information about the list can be used.

   3) When the list changes, a compressed list is sent in the compressed
      header, including a representation of its structure and order.
      Previously unknown items are sent uncompressed in the list, while
      previously known items are only represented by an index pointing
      to the item stored in the context.








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6.6.13.2.  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 reset its item table upon receiving a negative
      acknowledgement.

      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 successful (CRC verification, or CRC-8
      validation).  The decompressor retrieves the item from the table
      whenever an Index is received without an accompanying Item.

      If an index is received without an accompanying Item and the
      decompressor does not have any context for this index, the
      decompressor MUST NOT deliver the packet to upper layers.

6.6.13.3.  Encoding of Compressed Lists

   Each item present in a compressed list is represented by:

   o  an Index into the table of items, and 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).





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

   A compressed list of items 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.  Also, the value
      of the cc_value argument of the list_csrc encoding (see
      Section 6.6.13).

      XI_1, ..., XI_m: m XI items.  Each XI represents one item in the
      list of items of the uncompressed header, in the same order as
      they appear in the uncompressed header.

         The format of an XI item is as follows:

                   0   1   2   3
                 +---+---+---+---+
         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:



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            X = 1 indicates that the item corresponding to the Index is
            sent in the Item_1, ..., Item_n list;

            X = 0 indicates that the item corresponding to the Index is
            not sent.

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

         Index: An index into the item table.  See Section 6.6.13.4

         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.  Each entry in the Item list is the uncompressed
      representation of one CSRC identifier.

6.6.13.4.  Item Table Mappings

   The item table for list compression is limited to 16 different items,
   since the RTP header can only carry at most 15 simultaneous CSRC
   identifiers.  The effect of having more than 16 items in the item
   table will only cause a slight overhead to the compressor when items
   are swapped in/out of the item table.

6.6.13.5.  Compressed Lists in Dynamic Chain

   A compressed list that is part of the dynamic chain must have all of
   its list items present, i.e., all X-bits in the XI list MUST be set.
   All items previously established in the item table that are not
   present in the list decompressed from this packet MUST also be
   retained in the decompressor context.









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6.7.  Encoding Methods with External Parameters as Arguments

   A number of encoding methods in Section 6.8.2.4 have one or more
   arguments for which the derivation of the parameter's value is
   outside the scope of the ROHC-FN [RFC4997] specification of the
   header formats.

   The following is a list of encoding methods with external parameters
   as arguments, from Section 6.8.2.4:

   o  udp(profile_value, reorder_ratio_value)

   o  udp_lite(profile_value, reorder_ratio_value,
      coverage_behavior_value)

   o  esp(profile_value, reorder_ratio_value)

   o  rtp(profile_value, ts_stride_value, time_stride_value,
      reorder_ratio_value)

   o  ipv4(profile_value, is_innermost, outer_ip_flag,
      ip_id_behavior_value, reorder_ratio_value))

   o  ipv6(profile_value, is_innermost, outer_ip_flag,
      reorder_ratio_value))

   o  iponly_baseheader(profile_value, outer_ip_flag,
      ip_id_behavior_value, reorder_ratio_value)

   o  udp_baseheader(profile_value, outer_ip_flag, ip_id_behavior_value,
      reorder_ratio_value)

   o  udplite_baseheader(profile_value, outer_ip_flag,
      ip_id_behavior_value, reorder_ratio_value)

   o  esp_baseheader(profile_value, outer_ip_flag, ip_id_behavior_value,
      reorder_ratio_value)

   o  rtp_baseheader(profile_value, ts_stride_value, time_stride_value,
      outer_ip_flag, ip_id_behavior_value, reorder_ratio_value)

   o  udplite_rtp_baseheader(profile_value, ts_stride_value,
      time_stride_value, outer_ip_flag, ip_id_behavior_value,
      reorder_ratio_value, coverage_behavior_value)

   The following applies for all parameters listed below: At the
   compressor, the value of the parameter is set according to the
   recommendations for each parameter.  At the decompressor, the value



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   of the parameter is set to undefined and will get bound by encoding
   methods, except where otherwise noted.

   The following is a list of external arguments with their respective
   definition:

   o  profile_value:

         Set to the 16-bit number that identifies the profile used to
         compress this packet.  When processing the static chain at the
         decompressor, this parameter is set to the value of the profile
         field in the IR header (see Section 6.8.1).

   o  reorder_ratio_value:

         Set to a 2-bit integer value, using one of the constants whose
         name begins with the prefix REORDERING_ and as defined in
         Section 6.8.2.4.

   o  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 6.8.2.4.

   o  coverage_behavior_value:

         Set to a 2-bit integer value, using one of the constants whose
         name begins with the prefix UDP_LITE_COVERAGE_ and as defined
         in Section 6.8.2.4.

   o  outer_ip_flag:

         This parameter is set to 1 if at least one of the TOS/TC or
         TTL/Hop Limit fields in outer IP headers has changed compared
         to their reference values in the context; otherwise, it is set
         to 0.  This flag may only be set to 1 for the "co_common"
         header format in the different profiles.

   o  is_innermost:

         This boolean flag is set to 1 when processing the innermost of
         the compressible IP headers; otherwise, it is set to 0.








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   o  ts_stride_value

         The value of this parameter should be set to the expected
         increase in the RTP Timestamp between consecutive RTP sequence
         numbers.  The value selected is implementation-specific.  See
         also Section 6.6.8.

   o  time_stride_value

         The value of this parameter should be set to the expected
         inter-arrival time between consecutive packets for the flow.
         The value selected is implementation-specific.  This parameter
         MUST be set to zero, unless the compressor has received a
         feedback message with the CLOCK_RESOLUTION option set to a non-
         zero value.  See also Section 6.6.9.

6.8.  Header Formats

   ROHCv2 profiles use two different header types: the Initialization
   and Refresh (IR) header type, and the Compressed header type (CO).

   The CO header type defines a number of header formats: there are two
   sets of base header formats, with a few additional formats that are
   common to both sets.

6.8.1.  Initialization and Refresh Header Format (IR)

   The IR header format uses the structure of the ROHC IR header as
   defined in Section 5.2.2.1 of [RFC4995].

   Header type: IR

      This header format communicates the static part and the dynamic
      part of the context.

















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   The ROHCv2 IR header has the following format:

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

      CRC: 8-bit CRC over the entire IR-header, including any CID fields
      and up until the end of the dynamic chain, using the polynomial
      defined in [RFC4995].  For purposes of computing the CRC, the CRC
      field is zero.

      Static chain: See Section 6.5.

      Dynamic chain: See Section 6.5.

6.8.2.  Compressed Header Formats (CO)

6.8.2.1.  Design Rationale for Compressed Base Headers

   The compressed header formats are defined as two separate sets for
   each profile: one set for the headers where the innermost IP header
   contains a sequential IP-ID (either network byte order or byte-
   swapped), and one set for the headers without sequential IP-ID
   (either random, zero, or no IP-ID).  There are also a number of
   common header formats shared between both sets.  In the description
   below, the naming convention used for header formats that belong to
   the sequential set is to include "seq" in the name of the format,
   while similarly "rnd" is used for those that belong to the non-
   sequential set.



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   The design of the header formats is derived from the field behavior
   analysis found in Appendix A.

   All of the compressed base headers transmit lsb-encoded MSN bits and
   a CRC.

   The following header formats exist for all profiles defined in this
   document, and are common to both the sequential and the random header
   format sets:

   o  co_common: This format can be used to update the context when the
      established change pattern of a dynamic field changes, for any of
      the dynamic fields.  However, not all dynamic fields are updated
      by conveying their uncompressed value; some fields can only be
      transmitted using a compressed representation.  This format is
      especially useful when a rarely changing field needs to be
      updated.  This format contains a set of flags to indicate what
      fields are present in the header, and its size can vary
      accordingly.  This format is protected by a 7-bit CRC.  It can
      update control fields, and it thus also carries a 3-bit CRC to
      protect those fields.  This format is similar in purpose to the
      UOR-2-extension 3 format of [RFC3095].

   o  co_repair: This format can be used to update the context of all
      the dynamic fields by conveying their uncompressed value.  This is
      especially useful when context damage is assumed (e.g., from the
      reception of a NACK) and a context repair is performed.  This
      format is protected by a 7-bit CRC.  It also carries a 3-bit CRC
      over the control fields that it can update.  This format is
      similar in purpose to the IR-DYN format of [RFC3095] when
      performing context repairs.

   o  pt_0_crc3: This format conveys only the MSN; it can therefore only
      update the MSN and fields that are derived from the MSN, such as
      IP-ID and the RTP Timestamp (for applicable profiles).  It is
      protected by a 3-bit CRC.  This format is equivalent to the UO-0
      header format in [RFC3095].

   o  pt_0_crc7: This format has the same properties as pt_0_crc3, but
      is instead protected by a 7-bit CRC and contains a larger amount
      of lsb-encoded MSN bits.  This format is useful in environments
      where a high amount of reordering or a high-residual error rate
      can occur.








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   The following header format descriptions apply to profiles 0x0101 and
   0x0107.

   o  pt_1_rnd: This format can convey changes to the MSN and to the RTP
      Marker bit, and it can update the RTP timestamp using scaled
      timestamp encoding.  It is protected by a 3-bit CRC.  It is
      similar in purpose to the UO-1 format in [RFC3095].

   o  pt_1_seq_id: This format can convey changes to the MSN and to the
      IP-ID.  It is protected by a 3-bit CRC.  It is similar in purpose
      to the UO-1-ID format in [RFC3095].

   o  pt_1_seq_ts: This format can convey changes to the MSN and to the
      RTP Marker bit, and it can update the RTP Timestamp using scaled
      timestamp encoding.  It is protected by a 3-bit CRC.  It is
      similar in purpose to the UO-1-TS format in [RFC3095].

   o  pt_2_rnd: This format can convey changes to the MSN, to the RTP
      Marker bit, and to the RTP Timestamp.  It is protected by a 7-bit
      CRC.  It is similar in purpose to the UOR-2 format in [RFC3095].

   o  pt_2_seq_id: This format can convey changes to the MSN and to the
      IP-ID.  It is protected by a 7-bit CRC.  It is similar in purpose
      to the UO-2-ID format in [RFC3095].

   o  pt_2_seq_ts: This format can convey changes to the MSN, to the RTP
      Marker bit and it can update the RTP Timestamp using scaled
      timestamp encoding.  It is protected by a 7-bit CRC.  It is
      similar in purpose to the UO-2-TS format in [RFC3095].

   o  pt_2_seq_both: This format can convey changes to both the RTP
      Timestamp and the IP-ID, in addition to the MSN and to the Marker
      bit.  It is protected by a 7-bit CRC.  It is similar in purpose to
      the UOR-2-ID extension 1 format in [RFC3095].

   The following header format descriptions apply to profiles 0x0102,
   0x0103, 0x0104, and 0x0108.

   o  pt_1_seq_id: This format can convey changes to the MSN and to the
      IP-ID.  It is protected by a 7-bit CRC.  It is similar in purpose
      to the UO-1-ID format in [RFC3095].

   o  pt_2_seq_id: This format can convey changes to the MSN and to the
      IP-ID.  It is protected by a 7-bit CRC.  It is similar in purpose
      to the UO-2-ID format in [RFC3095].






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6.8.2.2.  co_repair Header Format

   The ROHCv2 co_repair header has the following format:

        0   1   2   3   4   5   6   7
       --- --- --- --- --- --- --- ---
      :         Add-CID octet         : if for small CIDs and CID 1-15
      +---+---+---+---+---+---+---+---+
      | 1   1   1   1   1   0   1   1 | discriminator
      +---+---+---+---+---+---+---+---+
      :                               :
      /   0, 1, or 2 octets of CID    / 1-2 octets if large CIDs
      :                               :
      +---+---+---+---+---+---+---+---+
      |r1 |         CRC-7             |
      +---+---+---+---+---+---+---+---+
      |        r2         |   CRC-3   |
      +---+---+---+---+---+---+---+---+
      |                               |
      /         Dynamic chain         / variable length
      |                               |
       - - - - - - - - - - - - - - - -

      r1: MUST be set to zero; otherwise, the decompressor MUST discard
      the packet.

      CRC-7: A 7-bit CRC over the entire uncompressed header, computed
      using the crc7 (data_value, data_length) encoding method defined
      in Section 6.8.2.4, where data_value corresponds to the entire
      uncompressed header chain and where data_length corresponds to the
      length of this header chain.

      r2: MUST be set to zero; otherwise, the decompressor MUST discard
      the packet.

      CRC-3: Encoded using the control_crc3_encoding method defined in
      Section 6.6.11.

      Dynamic chain: See Section 6.5.

6.8.2.3.  General CO Header Format

   The CO header format communicates irregularities in the packet
   header.  All CO formats carry a CRC and can update the context.  All
   CO header formats use the general format defined in this section,
   with the exception of the co_repair format, which is defined in
   Section 6.8.2.2.




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   The general format for a compressed header is as follows:

        0   1   2   3   4   5   6   7
       --- --- --- --- --- --- --- ---
      :         Add-CID octet         : if for small CIDs and CID 1-15
      +---+---+---+---+---+---+---+---+
      |  first octet of base header   | (with type indication)
      +---+---+---+---+---+---+---+---+
      :                               :
      /   0, 1, or 2 octets of CID    / 1-2 octets if large CIDs
      :                               :
      +---+---+---+---+---+---+---+---+
      /   remainder of base header    / variable length
      +---+---+---+---+---+---+---+---+
      :                               :
      /        Irregular Chain        / variable length
      :                               :
       --- --- --- --- --- --- --- ---

   The base header in the figure above is the compressed representation
   of the innermost IP header and other header(s), if any, in the
   uncompressed packet.  The base header formats are defined in
   Section 6.8.2.4.  In the formal description of the header formats,
   the base header for each profile is labeled
   <profile_name>_baseheader, where <profile_name> is defined in the
   following table:

      +------------------+----------------+
      | Profile number   | profile_name   |
      +------------------+----------------+
      | 0x0101           | rtp            |
      | 0x0102           | udp            |
      | 0x0103           | esp            |
      | 0x0104           | ip             |
      | 0x0107           | udplite_rtp    |
      | 0x0108           | udplite        |
      +------------------+----------------+

6.8.2.4.  Header Formats in ROHC-FN

   This section defines the complete set of base header formats for
   ROHCv2 profiles.  The base header formats are defined using the ROHC
   Formal Notation [RFC4997].








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// NOTE: The irregular, static, and dynamic chains (see Section 6.5)
// 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 constants
IP_ID_BEHAVIOR_SEQUENTIAL         = 0;
IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED = 1;
IP_ID_BEHAVIOR_RANDOM             = 2;
IP_ID_BEHAVIOR_ZERO               = 3;

// UDP-lite checksum coverage behavior constants
UDP_LITE_COVERAGE_INFERRED  = 0;
UDP_LITE_COVERAGE_STATIC    = 1;
UDP_LITE_COVERAGE_IRREGULAR = 2;
// The value 3 is reserved and cannot be used for coverage behavior

// Variable reordering offset
REORDERING_NONE          = 0;
REORDERING_QUARTER       = 1;
REORDERING_HALF          = 2;
REORDERING_THREEQUARTERS = 3;

// Profile names and versions
PROFILE_RTP_0101     = 0x0101;
PROFILE_UDP_0102     = 0x0102;
PROFILE_ESP_0103     = 0x0103;
PROFILE_IP_0104      = 0x0104;
PROFILE_RTP_0107     = 0x0107; // With UDP-LITE
PROFILE_UDPLITE_0108 = 0x0108; // Without RTP

// Default values for RTP timestamp encoding
TS_STRIDE_DEFAULT    = 160;
TIME_STRIDE_DEFAULT  = 0;

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

CONTROL {



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  profile                                    [ 16 ];
  msn                                        [ 16 ];
  reorder_ratio                              [  2 ];
  // ip_id fields are for innermost IP header only
  ip_id_offset                               [ 16 ];
  ip_id_behavior_innermost                   [  2 ];
  // The following are only used in RTP-based profiles
  ts_stride                                  [ 32 ];
  time_stride                                [ 32 ];
  ts_scaled                                  [ 32 ];
  ts_offset                                  [ 32 ];
  // UDP-lite-based profiles only
  coverage_behavior                          [  2 ];
}

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

baseheader_extension_headers       "defined in Section 6.6.1";
baseheader_outer_headers           "defined in Section 6.6.2";
control_crc3_encoding              "defined in Section 6.6.11";
inferred_ip_v4_header_checksum     "defined in Section 6.6.4";
inferred_ip_v4_length              "defined in Section 6.6.6";
inferred_ip_v6_length              "defined in Section 6.6.7";
inferred_mine_header_checksum      "defined in Section 6.6.5";
inferred_scaled_field              "defined in Section 6.6.10";
inferred_sequential_ip_id          "defined in Section 6.6.12";
inferred_udp_length                "defined in Section 6.6.3";
list_csrc(cc_value)                "defined in Section 6.6.13";
timer_based_lsb(time_stride, k, p) "defined in Section 6.6.9";

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

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

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

  COMPRESSED static_enc {



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

optional_32(flag)
{
  UNCOMPRESSED {
    item [ 0, 32 ];
  }

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

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

// Send the entire value, or keep previous value
sdvl_or_static(flag)
{
  UNCOMPRESSED {
    field [ 32 ];
  }

  COMPRESSED present_7bit {
    ENFORCE(flag == 1);
    ENFORCE(field.UVALUE < 2^7);
    ENFORCE(field.CVALUE == field.UVALUE);
    discriminator =:= '0' [ 1 ];
    field                 [ 7 ];
  }

  COMPRESSED present_14bit {
    ENFORCE(flag == 1);
    ENFORCE(field.UVALUE < 2^14);
    ENFORCE(field.CVALUE == field.UVALUE);
    discriminator =:= '10'   [  2 ];
    field                    [ 14 ];
  }

  COMPRESSED present_21bit {
    ENFORCE(flag == 1);
    ENFORCE(field.UVALUE < 2^21);



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    ENFORCE(field.CVALUE == field.UVALUE);
    discriminator =:= '110'  [  3 ];
    field                    [ 21 ];
  }

  COMPRESSED present_28bit {
    ENFORCE(flag == 1);
    ENFORCE(field.UVALUE < 2^28);
    ENFORCE(field.CVALUE == field.UVALUE);
    discriminator =:= '1110'  [  4 ];
    field                     [ 28 ];
  }

  COMPRESSED present_32bit {
    ENFORCE(flag == 1);
    ENFORCE(field.CVALUE == field.UVALUE);
    discriminator =:= '11111111'  [  8 ];
    field                         [ 32 ];
  }

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

// Send the entire value, or revert to default value
sdvl_or_default(flag, default_value)
{
  UNCOMPRESSED {
    field [ 32 ];
  }

  COMPRESSED present_7bit {
    ENFORCE(flag == 1);
    ENFORCE(field.UVALUE < 2^7);
    ENFORCE(field.CVALUE == field.UVALUE);
    discriminator =:= '0' [ 1 ];
    field                 [ 7 ];
  }

  COMPRESSED present_14bit {
    ENFORCE(flag == 1);
    ENFORCE(field.UVALUE < 2^14);
    ENFORCE(field.CVALUE == field.UVALUE);
    discriminator =:= '10'   [  2 ];
    field                    [ 14 ];
  }



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  COMPRESSED present_21bit {
    ENFORCE(flag == 1);
    ENFORCE(field.UVALUE < 2^21);
    ENFORCE(field.CVALUE == field.UVALUE);
    discriminator =:= '110'  [  3 ];
    field                    [ 21 ];
  }

  COMPRESSED present_28bit {
    ENFORCE(flag == 1);
    ENFORCE(field.UVALUE < 2^28);
    ENFORCE(field.CVALUE == field.UVALUE);
    discriminator =:= '1110'  [  4 ];
    field                     [ 28 ];
  }

  COMPRESSED present_32bit {
    ENFORCE(flag == 1);
    ENFORCE(field.CVALUE == field.UVALUE);
    discriminator =:= '11111111'  [  8 ];
    field                         [ 32 ];
  }

  COMPRESSED not_present {
    ENFORCE(flag == 0);
    field =:= uncompressed_value(32, default_value);
  }
}

lsb_7_or_31
{
  UNCOMPRESSED {
    item [ 32 ];
  }

  COMPRESSED lsb_7 {
    discriminator =:= '0'                       [  1 ];
    item          =:= lsb(7, ((2^7) / 4) - 1)   [  7 ];
  }

  COMPRESSED lsb_31 {
    discriminator =:= '1'                       [  1 ];
    item          =:= lsb(31, ((2^31) / 4) - 1) [ 31 ];
  }
}

crc3(data_value, data_length)
{



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

crc7(data_value, data_length)
{
  UNCOMPRESSED {
  }

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

// Encoding method for updating a scaled field and its associated
// control fields.  Should be used both when the value is scaled
// or unscaled in a compressed format.
// 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 {



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    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_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 ];
  }



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  COMPRESSED hop_opt_dynamic {
    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_irregular {
  }
}

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

optional_lsb_7_or_31(flag)
{



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  UNCOMPRESSED {
    item [ 0, 32 ];
  }

  COMPRESSED present {
    ENFORCE(flag == 1);
    item =:= lsb_7_or_31 [ 8, 32 ];
  }

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

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

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

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

gre_proto
{
  UNCOMPRESSED {
    protocol [ 16 ];
  }

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

  COMPRESSED ether_v6 {
    discriminator =:= '1'                            [ 1 ];



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    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      =:= optional_32(k_flag.UVALUE) [ 0, 32 ];
  }

  COMPRESSED gre_dynamic {
    checksum_and_res =:=
      optional_checksum(c_flag.UVALUE)              [ 0, 16 ];
    sequence_number  =:= optional_32(s_flag.UVALUE) [ 0, 32 ];
  }

  COMPRESSED gre_irregular {



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    checksum_and_res =:= optional_checksum(c_flag.UVALUE) [ 0, 16 ];
    sequence_number  =:=
      optional_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    =:= optional_32(s_bit.UVALUE) [ 0, 32 ];
  }

  COMPRESSED mine_dynamic {
  }

  COMPRESSED mine_irregular {
  }
}

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



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RFC 5225                    ROHCv2 Profiles                   April 2008


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

ah
{
  UNCOMPRESSED {
    next_header                            [  8 ];
    length                                 [  8 ];
    res_bits =:= uncompressed_value(16, 0) [ 16 ];
    spi                                    [ 32 ];
    sequence_number                        [ 32 ];
    icv                   [ length.UVALUE*32-32 ];
  }

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



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RFC 5225                    ROHCv2 Profiles                   April 2008


    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 ];
  }
}

ipv6(profile_value, is_innermost, outer_ip_flag, reorder_ratio_value)
{
  UNCOMPRESSED {
    version         =:= uncompressed_value(4, 6) [   4 ];
    tos_tc                                       [   8 ];
    flow_label                                   [  20 ];
    payload_length                               [  16 ];
    next_header                                  [   8 ];
    ttl_hopl                                     [   8 ];
    src_addr                                     [ 128 ];
    dst_addr                                     [ 128 ];
  }

  CONTROL {
    ENFORCE(profile == profile_value);
    ENFORCE(reorder_ratio.UVALUE == reorder_ratio_value);
    ENFORCE(innermost_ip.UVALUE == is_innermost);
    innermost_ip [ 1 ];
  }

  DEFAULT {
    tos_tc         =:= 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 ];
    innermost_ip        =:= irregular(1)     [   1 ];



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RFC 5225                    ROHCv2 Profiles                   April 2008


    reserved            =:= '0'              [   1 ];
    flow_label          =:= fl_enc           [ 5, 21 ];
    next_header         =:= irregular(8)     [   8 ];
    src_addr            =:= irregular(128)   [ 128 ];
    dst_addr            =:= irregular(128)   [ 128 ];
  }

  COMPRESSED ipv6_endpoint_dynamic {
    ENFORCE((is_innermost == 1) &&
            (profile_value == PROFILE_IP_0104));
    tos_tc        =:= irregular(8)           [  8 ];
    ttl_hopl      =:= irregular(8)           [  8 ];
    reserved      =:= compressed_value(6, 0) [  6 ];
    reorder_ratio =:= irregular(2)           [  2 ];
    msn           =:= irregular(16)          [ 16 ];
  }

  COMPRESSED ipv6_regular_dynamic {
    ENFORCE((is_innermost == 0) ||
            (profile_value != PROFILE_IP_0104));
    tos_tc       =:= irregular(8) [ 8 ];
    ttl_hopl     =:= irregular(8) [ 8 ];
  }

  COMPRESSED ipv6_outer_irregular {
    ENFORCE(is_innermost == 0);
    tos_tc       =:=
        static_or_irreg(outer_ip_flag, 8) [ 0, 8 ];
    ttl_hopl     =:=
        static_or_irreg(outer_ip_flag, 8) [ 0, 8 ];
  }

  COMPRESSED ipv6_innermost_irregular {
    ENFORCE(is_innermost == 1);
  }

}

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

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




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RFC 5225                    ROHCv2 Profiles                   April 2008


  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 {
    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 ];
  }
}

ipv4(profile_value, is_innermost, outer_ip_flag, ip_id_behavior_value,
  reorder_ratio_value)
{
  UNCOMPRESSED {
    version     =:= uncompressed_value(4, 4)       [  4 ];



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RFC 5225                    ROHCv2 Profiles                   April 2008


    hdr_length  =:= uncompressed_value(4, 5)       [  4 ];
    tos_tc                                         [  8 ];
    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(profile == profile_value);
    ENFORCE(reorder_ratio.UVALUE == reorder_ratio_value);
    ENFORCE(innermost_ip.UVALUE == is_innermost);
    ip_id_behavior_outer [ 2 ];
    innermost_ip [ 1 ];
  }

  DEFAULT {
    tos_tc               =:= 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 ];
    innermost_ip        =:= irregular(1)           [  1 ];
    reserved            =:= '000000'               [  6 ];
    protocol            =:= irregular(8)           [  8 ];
    src_addr            =:= irregular(32)          [ 32 ];
    dst_addr            =:= irregular(32)          [ 32 ];
  }

  COMPRESSED ipv4_endpoint_innermost_dynamic {
    ENFORCE((is_innermost == 1) && (profile_value == PROFILE_IP_0104));
    ENFORCE(ip_id_behavior_innermost.UVALUE == ip_id_behavior_value);
    reserved       =:= '000'                                 [  3 ];
    reorder_ratio  =:= irregular(2)                          [  2 ];
    df             =:= irregular(1)                          [  1 ];



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RFC 5225                    ROHCv2 Profiles                   April 2008


    ip_id_behavior_innermost =:= irregular(2)                [  2 ];
    tos_tc         =:= irregular(8)                          [  8 ];
    ttl_hopl       =:= irregular(8)                          [  8 ];
    ip_id =:= ip_id_enc_dyn(ip_id_behavior_innermost.UVALUE) [ 0, 16 ];
    msn            =:= irregular(16)                         [ 16 ];
  }

  COMPRESSED ipv4_regular_innermost_dynamic {
    ENFORCE((is_innermost == 1) && (profile_value != PROFILE_IP_0104));
    ENFORCE(ip_id_behavior_innermost.UVALUE == ip_id_behavior_value);
    reserved       =:= '00000'                               [ 5 ];
    df             =:= irregular(1)                          [ 1 ];
    ip_id_behavior_innermost =:= irregular(2)                [ 2 ];
    tos_tc         =:= irregular(8)                          [ 8 ];
    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 ];
    tos_tc         =:= irregular(8)                        [ 8 ];
    ttl_hopl       =:= irregular(8)                        [ 8 ];
    ip_id =:= ip_id_enc_dyn(ip_id_behavior_outer.UVALUE)   [ 0, 16 ];
  }

  COMPRESSED ipv4_outer_irregular {
    ENFORCE(is_innermost == 0);
    ip_id    =:=
      ip_id_enc_irreg(ip_id_behavior_outer.UVALUE)      [ 0, 16 ];
    tos_tc   =:= static_or_irreg(outer_ip_flag, 8)      [  0, 8 ];
    ttl_hopl =:= static_or_irreg(outer_ip_flag, 8)      [  0, 8 ];
  }

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

}

/////////////////////////////////////////////
// UDP Header
/////////////////////////////////////////////



Pelletier & Sandlund        Standards Track                    [Page 62]

RFC 5225                    ROHCv2 Profiles                   April 2008


udp(profile_value, reorder_ratio_value)
{
  UNCOMPRESSED {
    ENFORCE((profile_value == PROFILE_RTP_0101) ||
            (profile_value == PROFILE_UDP_0102));
    src_port                           [ 16 ];
    dst_port                           [ 16 ];
    udp_length =:= inferred_udp_length [ 16 ];
    checksum                           [ 16 ];
  }

  CONTROL {
    ENFORCE(profile == profile_value);
    ENFORCE(reorder_ratio.UVALUE == reorder_ratio_value);
    checksum_used [ 1 ];
  }

  DEFAULT {
    src_port      =:= static;
    dst_port      =:= static;
    checksum_used =:= static;
  }

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

  COMPRESSED udp_endpoint_dynamic {
    ENFORCE(profile_value == PROFILE_UDP_0102);
    ENFORCE(profile == PROFILE_UDP_0102);
    ENFORCE(checksum_used.UVALUE == (checksum.UVALUE != 0));
    checksum      =:= irregular(16)          [ 16 ];
    msn           =:= irregular(16)          [ 16 ];
    reserved      =:= compressed_value(6, 0) [  6 ];
    reorder_ratio =:= irregular(2)           [  2 ];
  }

  COMPRESSED udp_regular_dynamic {
    ENFORCE(profile_value == PROFILE_RTP_0101);
    ENFORCE(checksum_used.UVALUE == (checksum.UVALUE != 0));
    checksum =:= irregular(16) [ 16 ];
  }

  COMPRESSED udp_zero_checksum_irregular {
    ENFORCE(checksum_used.UVALUE == 0);
    checksum =:= uncompressed_value(16, 0)   [ 0 ];
  }



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RFC 5225                    ROHCv2 Profiles                   April 2008


  COMPRESSED udp_with_checksum_irregular {
    ENFORCE(checksum_used.UVALUE == 1);
    checksum =:= irregular(16) [ 16 ];
  }

}

/////////////////////////////////////////////
// RTP Header
/////////////////////////////////////////////

csrc_list_dynchain(presence, cc_value)
{
  UNCOMPRESSED {
    csrc_list;
  }

  COMPRESSED no_list {
    ENFORCE(cc_value == 0);
    ENFORCE(presence == 0);
    csrc_list =:= uncompressed_value(0, 0) [ 0 ];
  }

  COMPRESSED list_present {
    ENFORCE(presence == 1);
    csrc_list =:= list_csrc(cc_value) [ VARIABLE ];
  }
}

rtp(profile_value, ts_stride_value, time_stride_value,
    reorder_ratio_value)
{
  UNCOMPRESSED {
    ENFORCE((profile_value == PROFILE_RTP_0101) ||
            (profile_value == PROFILE_RTP_0107));
    rtp_version =:= uncompressed_value(2, 0) [  2 ];
    pad_bit                                  [  1 ];
    extension                                [  1 ];
    cc                                       [  4 ];
    marker                                   [  1 ];
    payload_type                             [  7 ];
    sequence_number                          [ 16 ];
    timestamp                                [ 32 ];
    ssrc                                     [ 32 ];
    csrc_list                                [ cc.UVALUE * 32 ];
  }

  CONTROL {



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RFC 5225                    ROHCv2 Profiles                   April 2008


    ENFORCE(profile == profile_value);
    ENFORCE(reorder_ratio.UVALUE == reorder_ratio_value);
    ENFORCE(time_stride_value == time_stride.UVALUE);
    ENFORCE(ts_stride_value == ts_stride.UVALUE);
    dummy_field =:= field_scaling(ts_stride.UVALUE,
      ts_scaled.UVALUE, timestamp.UVALUE, ts_offset.UVALUE) [ 0 ];
  }

  INITIAL {
    ts_stride     =:= uncompressed_value(32, TS_STRIDE_DEFAULT);
    time_stride   =:= uncompressed_value(32, TIME_STRIDE_DEFAULT);
  }

  DEFAULT {
    ENFORCE(msn.UVALUE == sequence_number.UVALUE);
    pad_bit         =:= static;
    extension       =:= static;
    cc              =:= static;
    marker          =:= static;
    payload_type    =:= static;
    sequence_number =:= static;
    timestamp       =:= static;
    ssrc            =:= static;
    csrc_list       =:= static;
    ts_stride       =:= static;
    time_stride     =:= static;
    ts_scaled       =:= static;
    ts_offset       =:= static;
  }

  COMPRESSED rtp_static {
    ssrc            =:= irregular(32)  [ 32 ];
  }

  COMPRESSED rtp_dynamic {
    reserved        =:= compressed_value(1, 0)       [  1 ];
    reorder_ratio   =:= irregular(2)                 [  2 ];
    list_present    =:= irregular(1)                 [  1 ];
    tss_indicator   =:= irregular(1)                 [  1 ];
    tis_indicator   =:= irregular(1)                 [  1 ];
    pad_bit         =:= irregular(1)                 [  1 ];
    extension       =:= irregular(1)                 [  1 ];
    marker          =:= irregular(1)                 [  1 ];
    payload_type    =:= irregular(7)                 [  7 ];
    sequence_number =:= irregular(16)                [ 16 ];
    timestamp       =:= irregular(32)                [ 32 ];
    ts_stride       =:= sdvl_or_default(tss_indicator.CVALUE,
      TS_STRIDE_DEFAULT)                             [ VARIABLE ];



Pelletier & Sandlund        Standards Track                    [Page 65]

RFC 5225                    ROHCv2 Profiles                   April 2008


    time_stride     =:= sdvl_or_default(tis_indicator.CVALUE,
      TIME_STRIDE_DEFAULT)                           [ VARIABLE ];
    csrc_list   =:= csrc_list_dynchain(list_present.CVALUE,
      cc.UVALUE)                                     [ VARIABLE ];
  }

  COMPRESSED rtp_irregular {
  }
}

/////////////////////////////////////////////
// UDP-Lite Header
/////////////////////////////////////////////

checksum_coverage_dynchain(behavior)
{
  UNCOMPRESSED {
    checksum_coverage [ 16 ];
  }

  COMPRESSED inferred_coverage {
    ENFORCE(behavior == UDP_LITE_COVERAGE_INFERRED);
    checksum_coverage =:= inferred_udp_length [  0 ];
  }

  COMPRESSED static_coverage {
    ENFORCE(behavior == UDP_LITE_COVERAGE_STATIC);
    checksum_coverage =:= irregular(16)       [ 16 ];
  }

  COMPRESSED irregular_coverage {
    ENFORCE(behavior == UDP_LITE_COVERAGE_IRREGULAR);
    checksum_coverage =:= irregular(16)       [ 16 ];
  }
}

checksum_coverage_irregular(behavior)
{
  UNCOMPRESSED {
    checksum_coverage [ 16 ];
  }

  COMPRESSED inferred_coverage {
    ENFORCE(behavior == UDP_LITE_COVERAGE_INFERRED);
    checksum_coverage =:= inferred_udp_length [  0 ];
  }

  COMPRESSED static_coverage {



Pelletier & Sandlund        Standards Track                    [Page 66]

RFC 5225                    ROHCv2 Profiles                   April 2008


    ENFORCE(behavior == UDP_LITE_COVERAGE_STATIC);
    checksum_coverage =:= static              [  0 ];
  }

  COMPRESSED irregular_coverage {
    ENFORCE(behavior == UDP_LITE_COVERAGE_IRREGULAR);
    checksum_coverage =:= irregular(16)       [ 16 ];
  }
}

udp_lite(profile_value, reorder_ratio_value, coverage_behavior_value)
{
  UNCOMPRESSED {
    ENFORCE((profile_value == PROFILE_RTP_0107) ||
            (profile_value == PROFILE_UDPLITE_0108));
    src_port          [ 16 ];
    dst_port          [ 16 ];
    checksum_coverage [ 16 ];
    checksum          [ 16 ];
  }

  CONTROL {
    ENFORCE(profile == profile_value);
    ENFORCE(coverage_behavior.UVALUE == coverage_behavior_value);
    ENFORCE(reorder_ratio.UVALUE == reorder_ratio_value);
  }

  DEFAULT {
    src_port          =:= static;
    dst_port          =:= static;
    coverage_behavior =:= static;
  }

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

  COMPRESSED udp_lite_endpoint_dynamic {
    ENFORCE(profile_value == PROFILE_UDPLITE_0108);
    reserved =:= compressed_value(4, 0)                      [  4 ];
    coverage_behavior =:= irregular(2)                       [  2 ];
    reorder_ratio     =:= irregular(2)                       [  2 ];
    checksum_coverage =:=
      checksum_coverage_dynchain(coverage_behavior.UVALUE)   [ 16 ];
    checksum          =:= irregular(16)                      [ 16 ];
    msn               =:= irregular(16)                      [ 16 ];
  }



Pelletier & Sandlund        Standards Track                    [Page 67]

RFC 5225                    ROHCv2 Profiles                   April 2008


  COMPRESSED udp_lite_regular_dynamic {
    ENFORCE(profile_value == PROFILE_RTP_0107);
    coverage_behavior =:= irregular(2)                       [  2 ];
    reserved =:= compressed_value(6, 0)                      [  6 ];
    checksum_coverage =:=
        checksum_coverage_dynchain(coverage_behavior.UVALUE) [ 16 ];
    checksum =:= irregular(16)                               [ 16 ];
  }

  COMPRESSED udp_lite_irregular {
    checksum_coverage =:=
      checksum_coverage_irregular(coverage_behavior.UVALUE) [ 0, 16 ];
    checksum          =:= irregular(16)                     [ 16 ];
  }
}

/////////////////////////////////////////////
// ESP Header
/////////////////////////////////////////////

esp(profile_value, reorder_ratio_value)
{
  UNCOMPRESSED {
    ENFORCE(profile_value == PROFILE_ESP_0103);
    ENFORCE(msn.UVALUE == sequence_number.UVALUE % 65536);
    spi             [ 32 ];
    sequence_number [ 32 ];
  }

  CONTROL {
    ENFORCE(profile == profile_value);
    ENFORCE(reorder_ratio.UVALUE == reorder_ratio_value);
  }

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

  COMPRESSED esp_static {
    spi =:= irregular(32)                         [ 32 ];
  }

  COMPRESSED esp_dynamic {
    sequence_number =:= irregular(32)             [ 32 ];
    reserved        =:= compressed_value(6, 0)    [  6 ];
    reorder_ratio   =:= irregular(2)              [  2 ];
  }



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RFC 5225                    ROHCv2 Profiles                   April 2008


  COMPRESSED esp_irregular {
  }
}

///////////////////////////////////////////////////
// Encoding methods used in the profiles' CO headers
///////////////////////////////////////////////////

// Variable reordering offset used for MSN
msn_lsb(k)
{
  UNCOMPRESSED {
    master [ VARIABLE ];
  }

  COMPRESSED none {
    ENFORCE(reorder_ratio.UVALUE == REORDERING_NONE);
    master =:= lsb(k, 1);
  }

  COMPRESSED quarter {
    ENFORCE(reorder_ratio.UVALUE == REORDERING_QUARTER);
    master =:= lsb(k, ((2^k) / 4) - 1);
  }

  COMPRESSED half {
    ENFORCE(reorder_ratio.UVALUE == REORDERING_HALF);
    master =:= lsb(k, ((2^k) / 2) - 1);
  }

  COMPRESSED threequarters {
    ENFORCE(reorder_ratio.UVALUE == REORDERING_THREEQUARTERS);
    master =:= lsb(k, (((2^k) * 3) / 4) - 1);
  }
}

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

  CONTROL {
    ip_id_nbo    [ 16 ];
  }

  COMPRESSED nbo {
    ENFORCE(behavior == IP_ID_BEHAVIOR_SEQUENTIAL);



Pelletier & Sandlund        Standards Track                    [Page 69]

RFC 5225                    ROHCv2 Profiles                   April 2008


    ENFORCE(ip_id_offset.UVALUE == ip_id.UVALUE - msn.UVALUE);
    ip_id_offset =:= lsb(k, ((2^k) / 4) - 1) [ k ];
  }

  COMPRESSED non_nbo {
    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);
    ip_id_offset =:= lsb(k, ((2^k) / 4) - 1) [ k ];
  }
}

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

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

  COMPRESSED long {
    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);
    ip_id =:= irregular(16)  [ 16 ];
  }

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

dont_fragment(version)
{
  UNCOMPRESSED {
    df [ 0, 1 ];
  }

  COMPRESSED v4 {



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RFC 5225                    ROHCv2 Profiles                   April 2008


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

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

pt_irr_or_static(flag)
{
  UNCOMPRESSED {
    payload_type [ 7 ];
  }

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

  COMPRESSED present {
    ENFORCE(flag == 1);
    reserved     =:= compressed_value(1, 0) [ 1 ];
    payload_type =:= irregular(7)           [ 7 ];
  }
}

csrc_list_presence(presence, cc_value)
{
  UNCOMPRESSED {
    csrc_list;
  }

  COMPRESSED no_list {
    ENFORCE(presence == 0);
    csrc_list =:= static [ 0 ];
  }

  COMPRESSED list_present {
    ENFORCE(presence == 1);
    csrc_list =:= list_csrc(cc_value) [ VARIABLE ];
  }
}

scaled_ts_lsb(time_stride_value, k)
{
  UNCOMPRESSED {



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    timestamp [ 32 ];
  }

  COMPRESSED timerbased {
    ENFORCE(time_stride_value != 0);
    timestamp =:= timer_based_lsb(time_stride_value, k,
                                  ((2^k) / 2) - 1);
  }

  COMPRESSED regular {
    ENFORCE(time_stride_value == 0);
    timestamp =:= lsb(k, ((2^k) / 4) - 1);
  }
}

// Self-describing variable length encoding with reordering offset
sdvl_sn_lsb(field_width)
{
  UNCOMPRESSED {
    field [ field_width ];
  }

  COMPRESSED lsb7 {
    discriminator =:= '0'   [ 1 ];
    field =:= msn_lsb(7)    [ 7 ];
  }

  COMPRESSED lsb14 {
    discriminator =:= '10'  [  2 ];
    field =:= msn_lsb(14)   [ 14 ];
  }

  COMPRESSED lsb21 {
    discriminator =:= '110'  [  3 ];
    field =:= msn_lsb(21)    [ 21 ];
  }

  COMPRESSED lsb28 {
    discriminator =:= '1110' [  4 ];
    field =:= msn_lsb(28)    [ 28 ];
  }

  COMPRESSED lsb32 {
    discriminator =:= '11111111'        [  8 ];
    field =:= irregular(field_width)    [ field_width ];
  }
}




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// Self-describing variable length encoding
sdvl_lsb(field_width)
{
  UNCOMPRESSED {
    field [ field_width ];
  }

  COMPRESSED lsb7 {
    discriminator =:= '0'               [ 1 ];
    field =:= lsb(7, ((2^7) / 4) - 1)   [ 7 ];
  }

  COMPRESSED lsb14 {
    discriminator =:= '10'              [  2 ];
    field =:= lsb(14, ((2^14) / 4) - 1) [ 14 ];
  }

  COMPRESSED lsb21 {
    discriminator =:= '110'             [  3 ];
    field =:= lsb(21, ((2^21) / 4) - 1) [ 21 ];
  }

  COMPRESSED lsb28 {
    discriminator =:= '1110'            [  4 ];
    field =:= lsb(28, ((2^28) / 4) - 1) [ 28 ];
  }

  COMPRESSED lsb32 {
    discriminator =:= '11111111'        [  8 ];
    field =:= irregular(field_width)    [ field_width ];
  }
}

sdvl_scaled_ts_lsb(time_stride)
{
   UNCOMPRESSED {
     field [ 32 ];
   }

   COMPRESSED lsb7 {
     discriminator =:= '0'                     [  1 ];
     field =:= scaled_ts_lsb(time_stride, 7)   [  7 ];
   }

   COMPRESSED lsb14 {
     discriminator =:= '10'                    [  2 ];
     field =:= scaled_ts_lsb(time_stride, 14)  [ 14 ];
   }



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   COMPRESSED lsb21 {
     discriminator =:= '110'                   [  3 ];
     field =:= scaled_ts_lsb(time_stride, 21)  [ 21 ];
   }

   COMPRESSED lsb28 {
     discriminator =:= '1110'                  [  4 ];
     field =:= scaled_ts_lsb(time_stride, 28)  [ 28 ];
   }

   COMPRESSED lsb32 {
     discriminator =:= '11111111'              [  8 ];
     field =:= irregular(32)                   [ 32 ];
   }
}

variable_scaled_timestamp(tss_flag, tsc_flag, ts_stride, time_stride)
{
  UNCOMPRESSED {
    scaled_value [ 32 ];
  }

  COMPRESSED present {
    ENFORCE((tss_flag == 0) && (tsc_flag == 1));
    ENFORCE(ts_stride != 0);
    scaled_value =:= sdvl_scaled_ts_lsb(time_stride) [ VARIABLE ];
  }

  COMPRESSED not_present {
    ENFORCE(((tss_flag == 1) && (tsc_flag == 0)) ||
            ((tss_flag == 0) && (tsc_flag == 0)));
  }
}

variable_unscaled_timestamp(tss_flag, tsc_flag)
{
  UNCOMPRESSED {
    timestamp [ 32 ];
  }

  COMPRESSED present {
    ENFORCE(((tss_flag == 1) && (tsc_flag == 0)) ||
            ((tss_flag == 0) && (tsc_flag == 0)));
    timestamp =:= sdvl_lsb(32);
  }

  COMPRESSED not_present {
    ENFORCE((tss_flag == 0) && (tsc_flag == 1));



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  }
}

profile_1_7_flags1_enc(flag, ip_version)
{
  UNCOMPRESSED {
    ip_outer_indicator  [ 1 ];
    ttl_hopl_indicator  [ 1 ];
    tos_tc_indicator    [ 1 ];
    df                  [ 0, 1 ];
    ip_id_behavior      [ 2 ];
    reorder_ratio       [ 2 ];
  }

  COMPRESSED not_present {
    ENFORCE(flag == 0);
    ENFORCE(ip_outer_indicator.CVALUE == 0);
    ENFORCE(ttl_hopl_indicator.CVALUE == 0);
    ENFORCE(tos_tc_indicator.CVALUE == 0);
    df                   =:= static;
    ip_id_behavior       =:= static;
    reorder_ratio        =:= static;
  }

  COMPRESSED present {
    ENFORCE(flag == 1);
    ip_outer_indicator  =:= irregular(1)                [ 1 ];
    ttl_hopl_indicator  =:= irregular(1)                [ 1 ];
    tos_tc_indicator    =:= irregular(1)                [ 1 ];
    df                  =:= dont_fragment(ip_version)   [ 1 ];
    ip_id_behavior      =:= irregular(2)                [ 2 ];
    reorder_ratio       =:= irregular(2)                [ 2 ];
  }
}

profile_1_flags2_enc(flag)
{
  UNCOMPRESSED {
    list_indicator        [ 1 ];
    pt_indicator          [ 1 ];
    time_stride_indicator [ 1 ];
    pad_bit               [ 1 ];
    extension             [ 1 ];
  }

  COMPRESSED not_present{
    ENFORCE(flag == 0);
    ENFORCE(list_indicator.UVALUE == 0);



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    ENFORCE(pt_indicator.UVALUE == 0);
    ENFORCE(time_stride_indicator.UVALUE == 0);
    pad_bit      =:= static;
    extension    =:= static;
  }

  COMPRESSED present {
    ENFORCE(flag == 1);
    list_indicator =:= irregular(1)                  [ 1 ];
    pt_indicator   =:= irregular(1)                  [ 1 ];
    time_stride_indicator =:= irregular(1)           [ 1 ];
    pad_bit        =:= irregular(1)                  [ 1 ];
    extension      =:= irregular(1)                  [ 1 ];
    reserved       =:= compressed_value(3, 0)        [ 3 ];
  }
}

profile_2_3_4_flags_enc(flag, ip_version)
{
  UNCOMPRESSED {
    ip_outer_indicator [ 1 ];
    df                 [ 0, 1 ];
    ip_id_behavior     [ 2 ];
  }

  COMPRESSED not_present {
    ENFORCE(flag == 0);
    ENFORCE(ip_outer_indicator.CVALUE == 0);
    df                 =:= static;
    ip_id_behavior     =:= static;
  }

  COMPRESSED present {
    ENFORCE(flag == 1);
    ip_outer_indicator =:= irregular(1)              [ 1 ];
    df                 =:= dont_fragment(ip_version) [ 1 ];
    ip_id_behavior     =:= irregular(2)              [ 2 ];
    reserved           =:= compressed_value(4, 0)    [ 4 ];
  }
}

profile_8_flags_enc(flag, ip_version)
{
  UNCOMPRESSED {
    ip_outer_indicator  [ 1 ];
    df                  [ 0, 1 ];
    ip_id_behavior      [ 2 ];
    coverage_behavior   [ 2 ];



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  }

  COMPRESSED not_present {
    ENFORCE(flag == 0);
    ENFORCE(ip_outer_indicator.CVALUE == 0);
    df                  =:= static;
    ip_id_behavior      =:= static;
    coverage_behavior   =:= static;
  }

  COMPRESSED present {
    ENFORCE(flag == 1);
    reserved            =:= compressed_value(2, 0)      [ 2 ];
    ip_outer_indicator  =:= irregular(1)                [ 1 ];
    df                  =:= dont_fragment(ip_version)   [ 1 ];
    ip_id_behavior      =:= irregular(2)                [ 2 ];
    coverage_behavior   =:= irregular(2)                [ 2 ];
  }
}

profile_7_flags2_enc(flag)
{
  UNCOMPRESSED {
    list_indicator          [ 1 ];
    pt_indicator            [ 1 ];
    time_stride_indicator   [ 1 ];
    pad_bit                 [ 1 ];
    extension               [ 1 ];
    coverage_behavior       [ 2 ];
  }

  COMPRESSED not_present{
    ENFORCE(flag == 0);
    ENFORCE(list_indicator.CVALUE == 0);
    ENFORCE(pt_indicator.CVALUE == 0);
    ENFORCE(time_stride_indicator.CVALUE == 0);
    pad_bit             =:= static;
    extension           =:= static;
    coverage_behavior   =:= static;
  }

  COMPRESSED present {
    ENFORCE(flag == 1);
    reserved       =:= compressed_value(1, 0)      [ 1 ];
    list_indicator =:= irregular(1)                [ 1 ];
    pt_indicator   =:= irregular(1)                [ 1 ];
    time_stride_indicator =:= irregular(1)         [ 1 ];
    pad_bit        =:= irregular(1)                [ 1 ];



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    extension      =:= irregular(1)                [ 1 ];
    coverage_behavior =:= irregular(2)             [ 2 ];
  }
}

////////////////////////////////////////////
// RTP profile
////////////////////////////////////////////

rtp_baseheader(profile_value, ts_stride_value, time_stride_value,
               outer_ip_flag, ip_id_behavior_value,
               reorder_ratio_value)
{
  UNCOMPRESSED v4 {
    ENFORCE(msn.UVALUE == sequence_number.UVALUE);
    outer_headers  =:= baseheader_outer_headers        [ VARIABLE ];
    ip_version     =:= uncompressed_value(4, 4)        [  4 ];
    header_length  =:= uncompressed_value(4, 5)        [  4 ];
    tos_tc                                             [  8 ];
    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 ];
    next_header                                        [  8 ];
    ip_checksum =:= inferred_ip_v4_header_checksum     [ 16 ];
    src_addr                                           [ 32 ];
    dest_addr                                          [ 32 ];
    extension_headers =:= baseheader_extension_headers [ VARIABLE ];
    src_port                                           [ 16 ];
    dst_port                                           [ 16 ];
    udp_length  =:= inferred_udp_length                [ 16 ];
    udp_checksum                                       [ 16 ];
    rtp_version =:= uncompressed_value(2, 2)           [  2 ];
    pad_bit                                            [  1 ];
    extension                                          [  1 ];
    cc                                                 [  4 ];
    marker                                             [  1 ];
    payload_type                                       [  7 ];
    sequence_number                                    [ 16 ];
    timestamp                                          [ 32 ];
    ssrc                                               [ 32 ];
    csrc_list                                          [ VARIABLE ];
  }

  UNCOMPRESSED v6 {



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    ENFORCE(ip_id_behavior_innermost.UVALUE == IP_ID_BEHAVIOR_RANDOM);
    ENFORCE(msn.UVALUE == sequence_number.UVALUE);
    outer_headers  =:= baseheader_outer_headers        [ VARIABLE ];
    ip_version     =:= uncompressed_value(4, 6)        [   4 ];
    tos_tc                                             [   8 ];
    flow_label                                         [  20 ];
    payload_length =:= inferred_ip_v6_length           [  16 ];
    next_header                                        [   8 ];
    ttl_hopl                                           [   8 ];
    src_addr                                           [ 128 ];
    dest_addr                                          [ 128 ];
    extension_headers =:= baseheader_extension_headers [ VARIABLE ];
    src_port                                           [  16 ];
    dst_port                                           [  16 ];
    udp_length     =:= inferred_udp_length             [  16 ];
    udp_checksum                                       [  16 ];
    rtp_version    =:= uncompressed_value(2, 2)        [   2 ];
    pad_bit                                            [   1 ];
    extension                                          [   1 ];
    cc                                                 [   4 ];
    marker                                             [   1 ];
    payload_type                                       [   7 ];
    sequence_number                                    [  16 ];
    timestamp                                          [  32 ];
    ssrc                                               [  32 ];
    csrc_list                                          [ VARIABLE ];
    df    =:= uncompressed_value(0,0)                  [   0 ];
    ip_id =:= uncompressed_value(0,0)                  [   0 ];
  }

  CONTROL {
    ENFORCE(profile_value == PROFILE_RTP_0101);
    ENFORCE(profile == profile_value);
    ENFORCE(time_stride.UVALUE == time_stride_value);
    ENFORCE(ts_stride.UVALUE == ts_stride_value);
    ENFORCE(reorder_ratio.UVALUE == reorder_ratio_value);
    ENFORCE(ip_id_behavior_innermost.UVALUE == ip_id_behavior_value);
    dummy_field =:= field_scaling(ts_stride.UVALUE,
      ts_scaled.UVALUE, timestamp.UVALUE, ts_offset.UVALUE) [ 0 ];
  }

  INITIAL {
    ts_stride     =:= uncompressed_value(32, TS_STRIDE_DEFAULT);
    time_stride   =:= uncompressed_value(32, TIME_STRIDE_DEFAULT);
  }

  DEFAULT {
    ENFORCE(outer_ip_flag == 0);



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    tos_tc          =:= static;
    dest_addr       =:= static;
    ttl_hopl        =:= static;
    src_addr        =:= static;
    df              =:= static;
    flow_label      =:= static;
    next_header     =:= static;
    src_port        =:= static;
    dst_port        =:= static;
    pad_bit         =:= static;
    extension       =:= static;
    cc              =:= static;
    // When marker not present in packets, it is assumed 0
    marker          =:= uncompressed_value(1, 0);
    payload_type    =:= static;
    sequence_number =:= static;
    timestamp       =:= static;
    ssrc            =:= static;
    csrc_list       =:= static;
    ts_stride       =:= static;
    time_stride     =:= static;
    ts_scaled       =:= static;
    ts_offset       =:= static;
    reorder_ratio   =:= static;
    ip_id_behavior_innermost =:= static;
  }

  // Replacement for UOR-2-ext3
  COMPRESSED co_common {
    ENFORCE(outer_ip_flag == outer_ip_indicator.CVALUE);
    discriminator        =:= '11111010'                    [ 8 ];
    marker               =:= irregular(1)                  [ 1 ];
    header_crc   =:= crc7(THIS.UVALUE, THIS.ULENGTH)       [ 7 ];
    flags1_indicator     =:= irregular(1)                  [ 1 ];
    flags2_indicator     =:= irregular(1)                  [ 1 ];
    tsc_indicator        =:= irregular(1)                  [ 1 ];
    tss_indicator        =:= irregular(1)                  [ 1 ];
    ip_id_indicator      =:= irregular(1)                  [ 1 ];
    control_crc3         =:= control_crc3_encoding         [ 3 ];

    outer_ip_indicator : ttl_hopl_indicator :
      tos_tc_indicator : df : ip_id_behavior_innermost : reorder_ratio
      =:= profile_1_7_flags1_enc(flags1_indicator.CVALUE,
        ip_version.UVALUE)                                 [ 0, 8 ];
    list_indicator : pt_indicator : tis_indicator : pad_bit :
      extension =:= profile_1_flags2_enc(
        flags2_indicator.CVALUE)                           [ 0, 8 ];
    tos_tc =:= static_or_irreg(tos_tc_indicator.CVALUE, 8) [ 0, 8 ];



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    ttl_hopl =:= static_or_irreg(ttl_hopl_indicator.CVALUE,
      ttl_hopl.ULENGTH)                                    [ 0, 8 ];
    payload_type =:= pt_irr_or_static(pt_indicator)        [ 0, 8 ];
    sequence_number =:=
      sdvl_sn_lsb(sequence_number.ULENGTH)                [ VARIABLE ];
    ip_id =:= ip_id_sequential_variable(
      ip_id_behavior_innermost.UVALUE,
      ip_id_indicator.CVALUE) [ 0, 8, 16 ];
    ts_scaled =:= variable_scaled_timestamp(tss_indicator.CVALUE,
      tsc_indicator.CVALUE, ts_stride.UVALUE,
      time_stride.UVALUE)                                 [ VARIABLE ];
    timestamp =:= variable_unscaled_timestamp(tss_indicator.CVALUE,
      tsc_indicator.CVALUE)                               [ VARIABLE ];
    ts_stride =:= sdvl_or_static(tss_indicator.CVALUE)    [ VARIABLE ];
    time_stride =:= sdvl_or_static(tis_indicator.CVALUE)  [ VARIABLE ];
    csrc_list =:= csrc_list_presence(list_indicator.CVALUE,
      cc.UVALUE)                                          [ VARIABLE ];
  }

  // UO-0
  COMPRESSED pt_0_crc3 {
    discriminator =:= '0'                             [ 1 ];
    msn           =:= msn_lsb(4)                      [ 4 ];
    header_crc    =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
    timestamp     =:= inferred_scaled_field           [ 0 ];
    ip_id         =:= inferred_sequential_ip_id       [ 0 ];
  }

  // New format, Type 0 with strong CRC and more SN bits
  COMPRESSED pt_0_crc7 {
    discriminator =:= '1000'                          [ 4 ];
    msn           =:= msn_lsb(5)                      [ 5 ];
    header_crc    =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
    timestamp     =:= inferred_scaled_field           [ 0 ];
    ip_id         =:= inferred_sequential_ip_id       [ 0 ];
  }

  // UO-1 replacement
  COMPRESSED pt_1_rnd {
    ENFORCE(ts_stride.UVALUE != 0);
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_RANDOM) ||
            (ip_id_behavior_innermost.UVALUE == IP_ID_BEHAVIOR_ZERO));
    discriminator =:= '101'                                [ 3 ];
    marker        =:= irregular(1)                         [ 1 ];
    msn           =:= msn_lsb(4)                           [ 4 ];
    ts_scaled     =:= scaled_ts_lsb(time_stride.UVALUE, 5) [ 5 ];
    header_crc    =:= crc3(THIS.UVALUE, THIS.ULENGTH)      [ 3 ];



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  }

  // UO-1-ID replacement
  COMPRESSED pt_1_seq_id {
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
    discriminator =:= '1001'                                [ 4 ];
    ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 4) [ 4 ];
    msn           =:= msn_lsb(5)                            [ 5 ];
    header_crc    =:= crc3(THIS.UVALUE, THIS.ULENGTH)       [ 3 ];
    timestamp     =:= inferred_scaled_field                 [ 0 ];
  }

  // UO-1-TS replacement
  COMPRESSED pt_1_seq_ts {
    ENFORCE(ts_stride.UVALUE != 0);
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
    discriminator =:= '101'                                [ 3 ];
    marker        =:= irregular(1)                         [ 1 ];
    msn           =:= msn_lsb(4)                           [ 4 ];
    ts_scaled     =:= scaled_ts_lsb(time_stride.UVALUE, 5) [ 5 ];
    header_crc    =:= crc3(THIS.UVALUE, THIS.ULENGTH)      [ 3 ];
    ip_id         =:= inferred_sequential_ip_id       [ 0 ];
  }

  // UOR-2 replacement
  COMPRESSED pt_2_rnd {
    ENFORCE(ts_stride.UVALUE != 0);
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_RANDOM) ||
            (ip_id_behavior_innermost.UVALUE == IP_ID_BEHAVIOR_ZERO));
    discriminator =:= '110'                                [ 3 ];
    msn           =:= msn_lsb(7)                           [ 7 ];
    ts_scaled     =:= scaled_ts_lsb(time_stride.UVALUE, 6) [ 6 ];
    marker        =:= irregular(1)                         [ 1 ];
    header_crc    =:= crc7(THIS.UVALUE, THIS.ULENGTH)      [ 7 ];
  }

  // UOR-2-ID replacement
  COMPRESSED pt_2_seq_id {
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.UVALUE ==



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RFC 5225                    ROHCv2 Profiles                   April 2008


             IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
    discriminator =:= '11000'                               [ 5 ];
    msn           =:= msn_lsb(7)                            [ 7 ];
    ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 5) [ 5 ];
    header_crc    =:= crc7(THIS.UVALUE, THIS.ULENGTH)       [ 7 ];
    timestamp     =:= inferred_scaled_field                 [ 0 ];
  }

  // UOR-2-ID-ext1 replacement (both TS and IP-ID)
  COMPRESSED pt_2_seq_both {
    ENFORCE(ts_stride.UVALUE != 0);
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
    discriminator =:= '11001'                               [ 5 ];
    msn           =:= msn_lsb(7)                            [ 7 ];
    ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 5) [ 5 ];
    header_crc    =:= crc7(THIS.UVALUE, THIS.ULENGTH)       [ 7 ];
    ts_scaled     =:= scaled_ts_lsb(time_stride.UVALUE, 7)  [ 7 ];
    marker        =:= irregular(1)                          [ 1 ];
  }

  // UOR-2-TS replacement
  COMPRESSED pt_2_seq_ts {
    ENFORCE(ts_stride.UVALUE != 0);
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
    discriminator =:= '1101'                               [ 4 ];
    msn           =:= msn_lsb(7)                           [ 7 ];
    ts_scaled     =:= scaled_ts_lsb(time_stride.UVALUE, 5) [ 5 ];
    marker        =:= irregular(1)                         [ 1 ];
    header_crc    =:= crc7(THIS.UVALUE, THIS.ULENGTH)      [ 7 ];
    ip_id         =:= inferred_sequential_ip_id            [ 0 ];
  }
}

////////////////////////////////////////////
// UDP profile
////////////////////////////////////////////

udp_baseheader(profile_value, outer_ip_flag, ip_id_behavior_value,
               reorder_ratio_value)
{
  UNCOMPRESSED v4 {
    outer_headers  =:= baseheader_outer_headers        [ VARIABLE ];



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    ip_version     =:= uncompressed_value(4, 4)        [  4 ];
    header_length  =:= uncompressed_value(4, 5)        [  4 ];
    tos_tc                                             [  8 ];
    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 ];
    next_header                                        [  8 ];
    ip_checksum =:= inferred_ip_v4_header_checksum     [ 16 ];
    src_addr                                           [ 32 ];
    dest_addr                                          [ 32 ];
    extension_headers =:= baseheader_extension_headers [ VARIABLE ];
    src_port                                           [ 16 ];
    dst_port                                           [ 16 ];
    udp_length     =:= inferred_udp_length             [ 16 ];
    udp_checksum                                       [ 16 ];
  }

  UNCOMPRESSED v6 {
    ENFORCE(ip_id_behavior.UVALUE == IP_ID_BEHAVIOR_RANDOM);
    outer_headers  =:= baseheader_outer_headers        [ VARIABLE ];
    ip_version     =:= uncompressed_value(4, 6)        [  4 ];
    tos_tc                                             [  8 ];
    flow_label                                         [ 20 ];
    payload_length =:= inferred_ip_v6_length           [ 16 ];
    next_header                                        [  8 ];
    ttl_hopl                                           [  8 ];
    src_addr                                           [ 128 ];
    dest_addr                                          [ 128 ];
    extension_headers =:= baseheader_extension_headers [ VARIABLE ];
    src_port                                           [ 16 ];
    dst_port                                           [ 16 ];
    udp_length     =:= inferred_udp_length             [ 16 ];
    udp_checksum                                       [ 16 ];
    df    =:= uncompressed_value(0,0)                  [  0 ];
    ip_id =:= uncompressed_value(0,0)                  [  0 ];
  }

  CONTROL {
    ENFORCE(profile_value == PROFILE_UDP_0102);
    ENFORCE(profile == profile_value);
    ENFORCE(reorder_ratio.UVALUE == reorder_ratio_value);
    ENFORCE(ip_id_behavior_innermost.UVALUE == ip_id_behavior_value);
  }




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RFC 5225                    ROHCv2 Profiles                   April 2008


  DEFAULT {
    ENFORCE(outer_ip_flag == 0);
    tos_tc         =:= static;
    dest_addr      =:= static;
    ip_version     =:= static;
    ttl_hopl       =:= static;
    src_addr       =:= static;
    df             =:= static;
    flow_label     =:= static;
    next_header    =:= static;
    src_port       =:= static;
    dst_port       =:= static;
    reorder_ratio  =:= static;
    ip_id_behavior_innermost =:= static;
  }

  // Replacement for UOR-2-ext3
  COMPRESSED co_common {
    ENFORCE(outer_ip_flag == outer_ip_indicator.CVALUE);
    discriminator        =:= '11111010'                    [ 8 ];
    ip_id_indicator      =:= irregular(1)                  [ 1 ];
    header_crc   =:= crc7(THIS.UVALUE, THIS.ULENGTH)       [ 7 ];
    flags_indicator      =:= irregular(1)                  [ 1 ];
    ttl_hopl_indicator   =:= irregular(1)                  [ 1 ];
    tos_tc_indicator     =:= irregular(1)                  [ 1 ];
    reorder_ratio        =:= irregular(2)                  [ 2 ];
    control_crc3         =:= control_crc3_encoding         [ 3 ];
    outer_ip_indicator : df : ip_id_behavior_innermost =:=
      profile_2_3_4_flags_enc(
      flags_indicator.CVALUE, ip_version.UVALUE)           [ 0, 8 ];
    tos_tc =:= static_or_irreg(tos_tc_indicator.CVALUE, 8) [ 0, 8 ];
    ttl_hopl =:= static_or_irreg(ttl_hopl_indicator.CVALUE,
      ttl_hopl.ULENGTH)                                    [ 0, 8 ];
    msn                  =:= msn_lsb(8)                    [ 8 ];
    ip_id =:= ip_id_sequential_variable(ip_id_behavior_innermost.UVALUE,
      ip_id_indicator.CVALUE)                          [ 0, 8, 16 ];
  }

  // UO-0
  COMPRESSED pt_0_crc3 {
    discriminator =:= '0'                             [ 1 ];
    msn           =:= msn_lsb(4)                      [ 4 ];
    header_crc    =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
    ip_id         =:= inferred_sequential_ip_id       [ 0 ];
  }

  // New format, Type 0 with strong CRC and more SN bits
  COMPRESSED pt_0_crc7 {



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    discriminator =:= '100'                           [ 3 ];
    msn           =:= msn_lsb(6)                      [ 6 ];
    header_crc    =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
    ip_id         =:= inferred_sequential_ip_id       [ 0 ];
  }

  // UO-1-ID replacement (PT-1 only used for sequential)
  COMPRESSED pt_1_seq_id {
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
    discriminator =:= '101'                                 [ 3 ];
    header_crc    =:= crc3(THIS.UVALUE, THIS.ULENGTH)       [ 3 ];
    msn           =:= msn_lsb(6)                            [ 6 ];
    ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 4) [ 4 ];
  }

  // UOR-2-ID replacement
  COMPRESSED pt_2_seq_id {
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
    discriminator =:= '110'                                 [ 3 ];
    ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 6) [ 6 ];
    header_crc    =:= crc7(THIS.UVALUE, THIS.ULENGTH)       [ 7 ];
    msn           =:= msn_lsb(8)                            [ 8 ];
  }
}

////////////////////////////////////////////
// ESP profile
////////////////////////////////////////////

esp_baseheader(profile_value, outer_ip_flag, ip_id_behavior_value,
               reorder_ratio_value)
{
  UNCOMPRESSED v4 {
    ENFORCE(msn.UVALUE == sequence_number.UVALUE % 65536);
    outer_headers  =:= baseheader_outer_headers        [ VARIABLE ];
    ip_version     =:= uncompressed_value(4, 4)        [  4 ];
    header_length  =:= uncompressed_value(4, 5)        [  4 ];
    tos_tc                                             [  8 ];
    length         =:= inferred_ip_v4_length           [ 16 ];
    ip_id                                              [ 16 ];
    rf             =:= uncompressed_value(1, 0)        [  1 ];
    df                                                 [  1 ];



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    mf             =:= uncompressed_value(1, 0)        [  1 ];
    frag_offset    =:= uncompressed_value(13, 0)       [ 13 ];
    ttl_hopl                                           [  8 ];
    next_header                                        [  8 ];
    ip_checksum =:= inferred_ip_v4_header_checksum     [ 16 ];
    src_addr                                           [ 32 ];
    dest_addr                                          [ 32 ];
    extension_headers =:= baseheader_extension_headers [ VARIABLE ];
    spi                                                [ 32 ];
    sequence_number                                    [ 32 ];
  }

  UNCOMPRESSED v6 {
    ENFORCE(msn.UVALUE == (sequence_number.UVALUE % 65536));
    ENFORCE(ip_id_behavior_innermost.UVALUE == IP_ID_BEHAVIOR_RANDOM);
    outer_headers  =:= baseheader_outer_headers        [ VARIABLE ];
    ip_version     =:= uncompressed_value(4, 6)        [   4 ];
    tos_tc                                             [   8 ];
    flow_label                                         [  20 ];
    payload_length =:= inferred_ip_v6_length           [  16 ];
    next_header                                        [   8 ];
    ttl_hopl                                           [   8 ];
    src_addr                                           [ 128 ];
    dest_addr                                          [ 128 ];
    extension_headers =:= baseheader_extension_headers [ VARIABLE ];
    spi                                                [  32 ];
    sequence_number                                    [  32 ];
    df    =:= uncompressed_value(0,0)                  [   0 ];
    ip_id =:= uncompressed_value(0,0)                  [   0 ];
  }

  CONTROL {
    ENFORCE(profile_value == PROFILE_ESP_0103);
    ENFORCE(profile == profile_value);
    ENFORCE(ip_id_behavior_innermost.UVALUE == ip_id_behavior_value);
    ENFORCE(reorder_ratio.UVALUE == reorder_ratio_value);
  }

  DEFAULT {
    ENFORCE(outer_ip_flag == 0);
    tos_tc          =:= static;
    dest_addr       =:= static;
    ttl_hopl        =:= static;
    src_addr        =:= static;
    df              =:= static;
    flow_label      =:= static;
    next_header     =:= static;
    spi             =:= static;



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    sequence_number =:= static;
    reorder_ratio   =:= static;
    ip_id_behavior_innermost =:= static;
  }

  // Replacement for UOR-2-ext3
  COMPRESSED co_common {
    ENFORCE(outer_ip_flag == outer_ip_indicator.CVALUE);
    discriminator        =:= '11111010'                    [ 8 ];
    ip_id_indicator      =:= irregular(1)                  [ 1 ];
    header_crc   =:= crc7(THIS.UVALUE, THIS.ULENGTH)       [ 7 ];
    flags_indicator      =:= irregular(1)                  [ 1 ];
    ttl_hopl_indicator   =:= irregular(1)                  [ 1 ];
    tos_tc_indicator     =:= irregular(1)                  [ 1 ];
    reorder_ratio        =:= irregular(2)                  [ 2 ];
    control_crc3         =:= control_crc3_encoding         [ 3 ];

    outer_ip_indicator : df : ip_id_behavior_innermost =:=
      profile_2_3_4_flags_enc(
      flags_indicator.CVALUE, ip_version.UVALUE)           [ 0, 8 ];
    tos_tc =:= static_or_irreg(tos_tc_indicator.CVALUE, 8) [ 0, 8 ];
    ttl_hopl =:= static_or_irreg(ttl_hopl_indicator.CVALUE,
      ttl_hopl.ULENGTH)                                    [ 0, 8 ];
    sequence_number =:=
      sdvl_sn_lsb(sequence_number.ULENGTH)             [ VARIABLE ];
    ip_id =:= ip_id_sequential_variable(ip_id_behavior_innermost.UVALUE,
      ip_id_indicator.CVALUE)                          [ 0, 8, 16 ];
  }

  // Sequence number sent instead of MSN due to field length
  // UO-0
  COMPRESSED pt_0_crc3 {
    discriminator   =:= '0'                             [ 1 ];
    sequence_number =:= msn_lsb(4)                      [ 4 ];
    header_crc      =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
    ip_id           =:= inferred_sequential_ip_id       [ 0 ];
  }

  // New format, Type 0 with strong CRC and more SN bits
  COMPRESSED pt_0_crc7 {
    discriminator   =:= '100'                           [ 3 ];
    sequence_number =:= msn_lsb(6)                      [ 6 ];
    header_crc      =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
    ip_id           =:= inferred_sequential_ip_id       [ 0 ];
  }

  // UO-1-ID replacement (PT-1 only used for sequential)
  COMPRESSED pt_1_seq_id {



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    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
    discriminator   =:= '101'                               [ 3 ];
    header_crc      =:= crc3(THIS.UVALUE, THIS.ULENGTH)     [ 3 ];
    sequence_number =:= msn_lsb(6)                          [ 6 ];
    ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 4) [ 4 ];
  }

  // UOR-2-ID replacement
  COMPRESSED pt_2_seq_id {
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
    discriminator   =:= '110'                               [ 3 ];
    ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 6) [ 6 ];
    header_crc      =:= crc7(THIS.UVALUE, THIS.ULENGTH)     [ 7 ];
    sequence_number =:= msn_lsb(8)                          [ 8 ];
  }
}

////////////////////////////////////////////
// IP-only profile
////////////////////////////////////////////

iponly_baseheader(profile_value, outer_ip_flag, ip_id_behavior_value,
                  reorder_ratio_value)
{
  UNCOMPRESSED v4 {
    outer_headers  =:= baseheader_outer_headers        [ VARIABLE ];
    ip_version     =:= uncompressed_value(4, 4)        [  4 ];
    header_length  =:= uncompressed_value(4, 5)        [  4 ];
    tos_tc                                             [  8 ];
    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 ];
    next_header                                        [  8 ];
    ip_checksum =:= inferred_ip_v4_header_checksum     [ 16 ];
    src_addr                                           [ 32 ];
    dest_addr                                          [ 32 ];
    extension_headers =:= baseheader_extension_headers [ VARIABLE ];
  }



Pelletier & Sandlund        Standards Track                    [Page 89]

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  UNCOMPRESSED v6 {
    ENFORCE(ip_id_behavior_innermost.UVALUE == IP_ID_BEHAVIOR_RANDOM);
    outer_headers     =:= baseheader_outer_headers     [ VARIABLE ];
    ip_version        =:= uncompressed_value(4, 6)     [   4 ];
    tos_tc                                             [   8 ];
    flow_label                                         [  20 ];
    payload_length    =:= inferred_ip_v6_length        [  16 ];
    next_header                                        [   8 ];
    ttl_hopl                                           [   8 ];
    src_addr                                           [ 128 ];
    dest_addr                                          [ 128 ];
    extension_headers =:= baseheader_extension_headers [ VARIABLE ];
    df    =:= uncompressed_value(0,0)                  [   0 ];
    ip_id =:= uncompressed_value(0,0)                  [   0 ];
  }

  CONTROL {
    ENFORCE(profile_value == PROFILE_IP_0104);
    ENFORCE(profile == profile_value);
    ENFORCE(reorder_ratio.UVALUE == reorder_ratio_value);
    ENFORCE(ip_id_behavior_innermost.UVALUE == ip_id_behavior_value);
  }

  DEFAULT {
    ENFORCE(outer_ip_flag == 0);
    tos_tc         =:= static;
    dest_addr      =:= static;
    ttl_hopl       =:= static;
    src_addr       =:= static;
    df             =:= static;
    flow_label     =:= static;
    next_header    =:= static;
    reorder_ratio  =:= static;
    ip_id_behavior_innermost =:= static;
  }

  // Replacement for UOR-2-ext3
  COMPRESSED co_common {
    ENFORCE(outer_ip_flag == outer_ip_indicator.CVALUE);
    discriminator        =:= '11111010'                    [ 8 ];
    ip_id_indicator      =:= irregular(1)                  [ 1 ];
    header_crc   =:= crc7(THIS.UVALUE, THIS.ULENGTH)       [ 7 ];
    flags_indicator      =:= irregular(1)                  [ 1 ];
    ttl_hopl_indicator   =:= irregular(1)                  [ 1 ];
    tos_tc_indicator     =:= irregular(1)                  [ 1 ];
    reorder_ratio        =:= irregular(2)                  [ 2 ];
    control_crc3         =:= control_crc3_encoding         [ 3 ];
    outer_ip_indicator : df : ip_id_behavior_innermost =:=



Pelletier & Sandlund        Standards Track                    [Page 90]

RFC 5225                    ROHCv2 Profiles                   April 2008


      profile_2_3_4_flags_enc(
      flags_indicator.CVALUE, ip_version.UVALUE)           [ 0, 8 ];
    tos_tc =:= static_or_irreg(tos_tc_indicator.CVALUE, 8) [ 0, 8 ];
    ttl_hopl =:= static_or_irreg(ttl_hopl_indicator.CVALUE,
      ttl_hopl.ULENGTH)                                    [ 0, 8 ];
    msn                  =:= msn_lsb(8)                    [ 8 ];
    ip_id =:= ip_id_sequential_variable(ip_id_behavior_innermost.UVALUE,
      ip_id_indicator.CVALUE)                          [ 0, 8, 16 ];
  }

  // UO-0
  COMPRESSED pt_0_crc3 {
    discriminator =:= '0'                             [ 1 ];
    msn           =:= msn_lsb(4)                      [ 4 ];
    header_crc    =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
    ip_id         =:= inferred_sequential_ip_id       [ 0 ];
  }

  // New format, Type 0 with strong CRC and more SN bits
  COMPRESSED pt_0_crc7 {
    discriminator =:= '100'                           [ 3 ];
    msn           =:= msn_lsb(6)                      [ 6 ];
    header_crc    =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
    ip_id         =:= inferred_sequential_ip_id       [ 0 ];
  }

  // UO-1-ID replacement (PT-1 only used for sequential)
  COMPRESSED pt_1_seq_id {
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
    discriminator =:= '101'                                 [ 3 ];
    header_crc    =:= crc3(THIS.UVALUE, THIS.ULENGTH)       [ 3 ];
    msn           =:= msn_lsb(6)                            [ 6 ];
    ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 4) [ 4 ];
  }

  // UOR-2-ID replacement
  COMPRESSED pt_2_seq_id {
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
    discriminator =:= '110'                                 [ 3 ];
    ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 6) [ 6 ];
    header_crc    =:= crc7(THIS.UVALUE, THIS.ULENGTH)       [ 7 ];
    msn           =:= msn_lsb(8)                            [ 8 ];



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  }
}

////////////////////////////////////////////
// UDP-lite/RTP profile
////////////////////////////////////////////

udplite_rtp_baseheader(profile_value, ts_stride_value,
                       time_stride_value, outer_ip_flag,
                       ip_id_behavior_value, reorder_ratio_value,
                       coverage_behavior_value)
{
  UNCOMPRESSED v4 {
    ENFORCE(msn.UVALUE == sequence_number.UVALUE);
    outer_headers  =:= baseheader_outer_headers        [ VARIABLE ];
    ip_version     =:= uncompressed_value(4, 4)        [  4 ];
    header_length  =:= uncompressed_value(4, 5)        [  4 ];
    tos_tc                                             [  8 ];
    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 ];
    next_header                                        [  8 ];
    ip_checksum =:= inferred_ip_v4_header_checksum     [ 16 ];
    src_addr                                           [ 32 ];
    dest_addr                                          [ 32 ];
    extension_headers =:= baseheader_extension_headers [ VARIABLE ];
    src_port                                           [ 16 ];
    dst_port                                           [ 16 ];
    checksum_coverage                                  [ 16 ];
    udp_checksum                                       [ 16 ];
    rtp_version    =:= uncompressed_value(2, 2)        [  2 ];
    pad_bit                                            [  1 ];
    extension                                          [  1 ];
    cc                                                 [  4 ];
    marker                                             [  1 ];
    payload_type                                       [  7 ];
    sequence_number                                    [ 16 ];
    timestamp                                          [ 32 ];
    ssrc                                               [ 32 ];
    csrc_list                                          [ VARIABLE ];
  }

  UNCOMPRESSED v6 {
    ENFORCE(ip_id_behavior_innermost.UVALUE == IP_ID_BEHAVIOR_RANDOM);



Pelletier & Sandlund        Standards Track                    [Page 92]

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    outer_headers  =:= baseheader_outer_headers        [ VARIABLE ];
    ip_version     =:= uncompressed_value(4, 6)        [   4 ];
    tos_tc                                             [   8 ];
    flow_label                                         [  20 ];
    payload_length =:= inferred_ip_v6_length           [  16 ];
    next_header                                        [   8 ];
    ttl_hopl                                           [   8 ];
    src_addr                                           [ 128 ];
    dest_addr                                          [ 128 ];
    extension_headers =:= baseheader_extension_headers [ VARIABLE ];
    src_port                                           [  16 ];
    dst_port                                           [  16 ];
    checksum_coverage                                  [  16 ];
    udp_checksum                                       [  16 ];
    rtp_version =:= uncompressed_value(2, 2)           [   2 ];
    pad_bit                                            [   1 ];
    extension                                          [   1 ];
    cc                                                 [   4 ];
    marker                                             [   1 ];
    payload_type                                       [   7 ];
    sequence_number                                    [  16 ];
    timestamp                                          [  32 ];
    ssrc                                               [  32 ];
    csrc_list                                          [ VARIABLE ];
    df    =:= uncompressed_value(0,0)                  [   0 ];
    ip_id =:= uncompressed_value(0,0)                  [   0 ];
  }

  CONTROL {
    ENFORCE(profile_value == PROFILE_RTP_0107);
    ENFORCE(profile == profile_value);
    ENFORCE(time_stride.UVALUE == time_stride_value);
    ENFORCE(ts_stride.UVALUE == ts_stride_value);
    ENFORCE(coverage_behavior.UVALUE == coverage_behavior_value);
    ENFORCE(reorder_ratio.UVALUE == reorder_ratio_value);
    ENFORCE(ip_id_behavior_innermost.UVALUE == ip_id_behavior_value);
    dummy_field =:= field_scaling(ts_stride.UVALUE,
      ts_scaled.UVALUE, timestamp.UVALUE, ts_offset.UVALUE) [ 0 ];
  }

  INITIAL {
    ts_stride     =:= uncompressed_value(32, TS_STRIDE_DEFAULT);
    time_stride   =:= uncompressed_value(32, TIME_STRIDE_DEFAULT);
  }

  DEFAULT {
    ENFORCE(outer_ip_flag == 0);
    tos_tc            =:= static;



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    dest_addr         =:= static;
    ttl_hopl          =:= static;
    src_addr          =:= static;
    df                =:= static;
    flow_label        =:= static;
    next_header       =:= static;
    src_port          =:= static;
    dst_port          =:= static;
    pad_bit           =:= static;
    extension         =:= static;
    cc                =:= static;
    // When marker not present in packets, it is assumed 0
    marker            =:= uncompressed_value(1, 0);
    payload_type      =:= static;
    sequence_number   =:= static;
    timestamp         =:= static;
    ssrc              =:= static;
    csrc_list         =:= static;
    ts_stride         =:= static;
    time_stride       =:= static;
    ts_scaled         =:= static;
    ts_offset         =:= static;
    reorder_ratio     =:= static;
    ip_id_behavior_innermost =:= static;
  }

  // Replacement for UOR-2-ext3
  COMPRESSED co_common {
    ENFORCE(outer_ip_flag == outer_ip_indicator.CVALUE);
    discriminator        =:= '11111010'                    [ 8 ];
    marker               =:= irregular(1)                  [ 1 ];
    header_crc   =:= crc7(THIS.UVALUE, THIS.ULENGTH)       [ 7 ];
    flags1_indicator     =:= irregular(1)                  [ 1 ];
    flags2_indicator     =:= irregular(1)                  [ 1 ];
    tsc_indicator        =:= irregular(1)                  [ 1 ];
    tss_indicator        =:= irregular(1)                  [ 1 ];
    ip_id_indicator      =:= irregular(1)                  [ 1 ];
    control_crc3         =:= control_crc3_encoding         [ 3 ];

    outer_ip_indicator : ttl_hopl_indicator :
      tos_tc_indicator : df : ip_id_behavior_innermost : reorder_ratio
      =:= profile_1_7_flags1_enc(flags1_indicator.CVALUE,
        ip_version.UVALUE)                                 [ 0, 8 ];
    list_indicator : pt_indicator : tis_indicator : pad_bit :
      extension : coverage_behavior =:=
      profile_7_flags2_enc(flags2_indicator.CVALUE)        [ 0, 8 ];
    tos_tc =:= static_or_irreg(tos_tc_indicator.CVALUE, 8) [ 0, 8 ];
    ttl_hopl =:=



Pelletier & Sandlund        Standards Track                    [Page 94]

RFC 5225                    ROHCv2 Profiles                   April 2008


      static_or_irreg(ttl_hopl_indicator.CVALUE, 8)        [ 0, 8 ];
    payload_type =:= pt_irr_or_static(pt_indicator.CVALUE) [ 0, 8 ];
    sequence_number =:=
      sdvl_sn_lsb(sequence_number.ULENGTH)               [ VARIABLE ];
    ip_id =:= ip_id_sequential_variable(ip_id_behavior_innermost.UVALUE,
      ip_id_indicator.CVALUE)                            [ 0, 8, 16 ];
    ts_scaled =:= variable_scaled_timestamp(tss_indicator.CVALUE,
      tsc_indicator.CVALUE, ts_stride.UVALUE,
      time_stride.UVALUE)                                [ VARIABLE ];
    timestamp =:= variable_unscaled_timestamp(tss_indicator.CVALUE,
      tsc_indicator.CVALUE)                              [ VARIABLE ];
    ts_stride =:= sdvl_or_static(tss_indicator.CVALUE)   [ VARIABLE ];
    time_stride =:= sdvl_or_static(tis_indicator.CVALUE) [ VARIABLE ];
    csrc_list            =:=
        csrc_list_presence(list_indicator.CVALUE,
          cc.UVALUE)                                     [ VARIABLE ];
  }

  // UO-0
  COMPRESSED pt_0_crc3 {
    discriminator =:= '0'                             [ 1 ];
    msn           =:= msn_lsb(4)                      [ 4 ];
    header_crc    =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
    timestamp     =:= inferred_scaled_field           [ 0 ];
    ip_id         =:= inferred_sequential_ip_id       [ 0 ];
  }

  // New format, Type 0 with strong CRC and more SN bits
  COMPRESSED pt_0_crc7 {
    discriminator =:= '1000'                          [ 4 ];
    msn           =:= msn_lsb(5)                      [ 5 ];
    header_crc    =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
    timestamp     =:= inferred_scaled_field           [ 0 ];
    ip_id         =:= inferred_sequential_ip_id       [ 0 ];
  }

  // UO-1 replacement
  COMPRESSED pt_1_rnd {
    ENFORCE(ts_stride.UVALUE != 0);
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_RANDOM) ||
            (ip_id_behavior_innermost.UVALUE == IP_ID_BEHAVIOR_ZERO));
    discriminator =:= '101'                                [ 3 ];
    marker        =:= irregular(1)                         [ 1 ];
    msn           =:= msn_lsb(4)                           [ 4 ];
    ts_scaled     =:= scaled_ts_lsb(time_stride.UVALUE, 5) [ 5 ];
    header_crc    =:= crc3(THIS.UVALUE, THIS.ULENGTH)      [ 3 ];
  }



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  // UO-1-ID replacement
  COMPRESSED pt_1_seq_id {
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
    discriminator =:= '1001'                                [ 4 ];
    ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 4) [ 4 ];
    msn           =:= msn_lsb(5)                            [ 5 ];
    header_crc    =:= crc3(THIS.UVALUE, THIS.ULENGTH)       [ 3 ];
    timestamp     =:= inferred_scaled_field                 [ 0 ];
  }

  // UO-1-TS replacement
  COMPRESSED pt_1_seq_ts {
    ENFORCE(ts_stride.UVALUE != 0);
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
    discriminator =:= '101'                                [ 3 ];
    marker        =:= irregular(1)                         [ 1 ];
    msn           =:= msn_lsb(4)                           [ 4 ];
    ts_scaled     =:= scaled_ts_lsb(time_stride.UVALUE, 5) [ 5 ];
    header_crc    =:= crc3(THIS.UVALUE, THIS.ULENGTH)      [ 3 ];
    ip_id         =:= inferred_sequential_ip_id            [ 0 ];
  }

  // UOR-2 replacement
  COMPRESSED pt_2_rnd {
    ENFORCE(ts_stride.UVALUE != 0);
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_RANDOM) ||
            (ip_id_behavior_innermost.UVALUE == IP_ID_BEHAVIOR_ZERO));
    discriminator =:= '110'                                [ 3 ];
    msn           =:= msn_lsb(7)                           [ 7 ];
    ts_scaled     =:= scaled_ts_lsb(time_stride.UVALUE, 6) [ 6 ];
    marker        =:= irregular(1)                         [ 1 ];
    header_crc    =:= crc7(THIS.UVALUE, THIS.ULENGTH)      [ 7 ];
  }

  // UOR-2-ID replacement
  COMPRESSED pt_2_seq_id {
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
    discriminator =:= '11000'                               [ 5 ];



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    msn           =:= msn_lsb(7)                            [ 7 ];
    ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 5) [ 5 ];
    header_crc    =:= crc7(THIS.UVALUE, THIS.ULENGTH)       [ 7 ];
    timestamp     =:= inferred_scaled_field                 [ 0 ];
  }

  // UOR-2-ID-ext1 replacement (both TS and IP-ID)
  COMPRESSED pt_2_seq_both {
    ENFORCE(ts_stride.UVALUE != 0);
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
    discriminator =:= '11001'                               [ 5 ];
    msn           =:= msn_lsb(7)                            [ 7 ];
    ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 5) [ 5 ];
    header_crc    =:= crc7(THIS.UVALUE, THIS.ULENGTH)       [ 7 ];
    ts_scaled     =:= scaled_ts_lsb(time_stride.UVALUE, 7)  [ 7 ];
    marker        =:= irregular(1)                          [ 1 ];
  }

  // UOR-2-TS replacement
  COMPRESSED pt_2_seq_ts {
    ENFORCE(ts_stride.UVALUE != 0);
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
    discriminator =:= '1101'                               [ 4 ];
    msn           =:= msn_lsb(7)                           [ 7 ];
    ts_scaled     =:= scaled_ts_lsb(time_stride.UVALUE, 5) [ 5 ];
    marker        =:= irregular(1)                         [ 1 ];
    header_crc    =:= crc7(THIS.UVALUE, THIS.ULENGTH)      [ 7 ];
    ip_id         =:= inferred_sequential_ip_id            [ 0 ];
  }
}

////////////////////////////////////////////
// UDP-lite profile
////////////////////////////////////////////

udplite_baseheader(profile_value, outer_ip_flag, ip_id_behavior_value,
                   reorder_ratio_value, coverage_behavior_value)
{
  UNCOMPRESSED v4 {
    outer_headers  =:= baseheader_outer_headers        [ VARIABLE ];
    ip_version     =:= uncompressed_value(4, 4)        [  4 ];
    header_length  =:= uncompressed_value(4, 5)        [  4 ];



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    tos_tc                                             [  8 ];
    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 ];
    next_header                                        [  8 ];
    ip_checksum =:= inferred_ip_v4_header_checksum     [ 16 ];
    src_addr                                           [ 32 ];
    dest_addr                                          [ 32 ];
    extension_headers =:= baseheader_extension_headers [ VARIABLE ];
    src_port                                           [ 16 ];
    dst_port                                           [ 16 ];
    checksum_coverage                                  [ 16 ];
    udp_checksum                                       [ 16 ];
  }

  UNCOMPRESSED v6 {
    ENFORCE(ip_id_behavior_innermost.UVALUE == IP_ID_BEHAVIOR_RANDOM);
    outer_headers  =:= baseheader_outer_headers        [ VARIABLE ];
    ip_version     =:= uncompressed_value(4, 6)        [   4 ];
    tos_tc                                             [   8 ];
    flow_label                                         [  20 ];
    payload_length =:= inferred_ip_v6_length           [  16 ];
    next_header                                        [   8 ];
    ttl_hopl                                           [   8 ];
    src_addr                                           [ 128 ];
    dest_addr                                          [ 128 ];
    extension_headers =:= baseheader_extension_headers [ VARIABLE ];
    src_port                                           [  16 ];
    dst_port                                           [  16 ];
    checksum_coverage                                  [  16 ];
    udp_checksum                                       [  16 ];
    df    =:= uncompressed_value(0,0)                  [   0 ];
    ip_id =:= uncompressed_value(0,0)                  [   0 ];
  }

  CONTROL {
    ENFORCE(profile_value == PROFILE_UDPLITE_0108);
    ENFORCE(profile == profile_value);
    ENFORCE(coverage_behavior.UVALUE == coverage_behavior_value);
    ENFORCE(reorder_ratio.UVALUE == reorder_ratio_value);
    ENFORCE(ip_id_behavior_innermost.UVALUE == ip_id_behavior_value);
  }

  DEFAULT {



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    ENFORCE(outer_ip_flag == 0);
    tos_tc            =:= static;
    dest_addr         =:= static;
    ttl_hopl          =:= static;
    src_addr          =:= static;
    df                =:= static;
    flow_label        =:= static;
    next_header       =:= static;
    src_port          =:= static;
    dst_port          =:= static;
    reorder_ratio     =:= static;
    ip_id_behavior_innermost =:= static;
  }

  // Replacement for UOR-2-ext3
  COMPRESSED co_common {
    ENFORCE(outer_ip_flag == outer_ip_indicator.CVALUE);
    discriminator        =:= '11111010'                    [ 8 ];
    ip_id_indicator      =:= irregular(1)                  [ 1 ];
    header_crc   =:= crc7(THIS.UVALUE, THIS.ULENGTH)       [ 7 ];
    flags_indicator      =:= irregular(1)                  [ 1 ];
    ttl_hopl_indicator   =:= irregular(1)                  [ 1 ];
    tos_tc_indicator     =:= irregular(1)                  [ 1 ];
    reorder_ratio        =:= irregular(2)                  [ 2 ];
    control_crc3         =:= control_crc3_encoding         [ 3 ];
    outer_ip_indicator : df : ip_id_behavior_innermost :
      coverage_behavior  =:=
      profile_8_flags_enc(flags_indicator.CVALUE,
      ip_version.UVALUE)                                   [ 0, 8 ];
    tos_tc =:= static_or_irreg(tos_tc_indicator.CVALUE, 8) [ 0, 8 ];
    ttl_hopl =:= static_or_irreg(ttl_hopl_indicator.CVALUE,
      ttl_hopl.ULENGTH)                                    [ 0, 8 ];
    msn                  =:= msn_lsb(8)                    [ 8 ];
    ip_id =:= ip_id_sequential_variable(ip_id_behavior_innermost.UVALUE,
      ip_id_indicator.CVALUE)                          [ 0, 8, 16 ];
  }

  // UO-0
  COMPRESSED pt_0_crc3 {
    discriminator =:= '0'                             [ 1 ];
    msn           =:= msn_lsb(4)                      [ 4 ];
    header_crc    =:= crc3(THIS.UVALUE, THIS.ULENGTH) [ 3 ];
    ip_id         =:= inferred_sequential_ip_id       [ 0 ];
  }

  // New format, Type 0 with strong CRC and more SN bits
  COMPRESSED pt_0_crc7 {
    discriminator =:= '100'                           [ 3 ];



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    msn           =:= msn_lsb(6)                      [ 6 ];
    header_crc    =:= crc7(THIS.UVALUE, THIS.ULENGTH) [ 7 ];
    ip_id         =:= inferred_sequential_ip_id       [ 0 ];
  }

  // UO-1-ID replacement (PT-1 only used for sequential)
  COMPRESSED pt_1_seq_id {
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
    discriminator =:= '101'                                 [ 3 ];
    header_crc    =:= crc3(THIS.UVALUE, THIS.ULENGTH)       [ 3 ];
    msn           =:= msn_lsb(6)                            [ 6 ];
    ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 4) [ 4 ];
  }

  // UOR-2-ID replacement
  COMPRESSED pt_2_seq_id {
    ENFORCE((ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL) ||
            (ip_id_behavior_innermost.UVALUE ==
             IP_ID_BEHAVIOR_SEQUENTIAL_SWAPPED));
    discriminator =:= '110'                                 [ 3 ];
    ip_id =:= ip_id_lsb(ip_id_behavior_innermost.UVALUE, 6) [ 6 ];
    header_crc    =:= crc7(THIS.UVALUE, THIS.ULENGTH)       [ 7 ];
    msn           =:= msn_lsb(8)                            [ 8 ];
  }
}

6.9.  Feedback Formats and Options

6.9.1.  Feedback Formats

   This section describes the feedback format for ROHCv2 profiles, using
   the formats described in Section 5.2.3 of [RFC4995].

   The Acknowledgment Number field of the feedback formats contains the
   least significant bits of the MSN (see Section 6.3.1) that
   corresponds to the reference header that is being acknowledged.  A
   reference header is a header that has been successfully CRC-8
   validated or CRC verified.  If there is no reference header
   available, the feedback MUST carry an ACKNUMBER-NOT-VALID option.
   FEEDBACK-1







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        0   1   2   3   4   5   6   7
      +---+---+---+---+---+---+---+---+
      |     Acknowledgment Number     |
      +---+---+---+---+---+---+---+---+

      Acknowledgment Number: The eight least significant bits of the
      MSN.

   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| Acknowledgment Number |
      +---+---+---+---+---+---+---+---+
      |     Acknowledgment Number     |
      +---+---+---+---+---+---+---+---+
      |              CRC              |
      +---+---+---+---+---+---+---+---+
      /       Feedback options        /
      +---+---+---+---+---+---+---+---+

      Acktype:

         0 = ACK

         1 = NACK

         2 = STATIC-NACK

         3 is reserved (MUST NOT be used for parsability)

      Acknowledgment Number: The least significant bits of the MSN.

      CRC: 8-bit CRC computed over the entire feedback payload including
      any CID fields but excluding the feedback type, the 'Size' field,
      and the 'Code' octet, using the polynomial defined in Section
      5.3.1.1 of [RFC4995].  If the CID is given with an Add-CID octet,
      the Add-CID octet immediately precedes the FEEDBACK-1 or
      FEEDBACK-2 format.  For purposes of computing the CRC, the CRC
      field is zero.

      Feedback options: A variable number of feedback options, see
      Section 6.9.2.  Options may appear in any order.





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   A FEEDBACK-2 of type NACK or STATIC-NACK is always implicitly an
   acknowledgment for a successfully decompressed packet, which
   corresponds to a packet whose LSBs match the Acknowledgment Number of
   the feedback element, unless the ACKNUMBER-NOT-VALID option (see
   Section 6.9.2.2) 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
   comparing the result with the CRC carried in the feedback format.  If
   the two are not identical, the feedback element MUST be discarded.

6.9.2.  Feedback Options

   A 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 Len (octets)
      +---+---+---+---+---+---+---+---+

      Opt Type: Unsigned integer that represents the type of the
      feedback option.  Section 6.9.2.1 through Section 6.9.2.4
      describes the ROHCv2 feedback options.

      Opt Len: Unsigned integer that represents the length of the Option
      Data field, in octets.

      Option Data: Feedback type specific data.  Present if the value of
      the Opt Len field is set to a non-zero value.

6.9.2.1.  The REJECT Option

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

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




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

6.9.2.2.  The ACKNUMBER-NOT-VALID Option

   The ACKNUMBER-NOT-VALID option indicates that the Acknowledgment
   Number field of the feedback is not valid.

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

   A compressor MUST NOT use the Acknowledgment Number of the feedback
   to find the corresponding sent header when this option is present.
   When this option is used, the Acknowledgment Number field of the
   FEEDBACK-2 format is set to zero.  Consequently, a NACK or a STATIC-
   NACK feedback type sent with the ACKNUMBER-NOT-VALID option is
   equivalent to a STATIC-NACK with respect to the type of context
   repair requested by the decompressor.

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

6.9.2.3.  The CONTEXT_MEMORY Option

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

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

   When receiving a CONTEXT_MEMORY option, the compressor SHOULD take
   actions to compress the packet flow in a way that requires less
   decompressor memory resources or stop compressing the packet flow.
   The CONTEXT_MEMORY option MUST NOT appear more than once in the
   FEEDBACK-2 format; otherwise, the compressor MUST discard the entire
   feedback element.

6.9.2.4.  The CLOCK_RESOLUTION Option

   The CLOCK_RESOLUTION option informs the compressor of the clock
   resolution of the decompressor.  It also informs whether or not the
   decompressor supports timer-based compression of the RTP TS timestamp



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   (see Section 6.6.9).  The CLOCK_RESOLUTION option is applicable per
   channel, i.e., it applies to any context associated with a profile
   for which the option is relevant between a compressor and
   decompressor pair.

        0   1   2   3   4   5   6   7
      +---+---+---+---+---+---+---+---+
      | Opt Type = 10 |  Opt Len = 1  |
      +---+---+---+---+---+---+---+---+
      |     Clock resolution (ms)     |
      +---+---+---+---+---+---+---+---+

      Clock resolution: Unsigned integer that represents the clock
      resolution of the decompressor expressed in milliseconds.

   The smallest clock resolution that can be indicated is 1 millisecond.
   The value zero has a special meaning: it indicates that the
   decompressor cannot do timer-based compression of the RTP Timestamp.
   The CLOCK_RESOLUTION option MUST NOT appear more than once in the
   FEEDBACK-2 format; otherwise, the compressor MUST discard the entire
   feedback element.

6.9.2.5.  Unknown Option Types

   If an option type other than those defined in this document is
   encountered, the compressor MUST discard the entire feedback element.

7.  Security Considerations

   Impairments such as bit errors on the received compressed headers,
   missing packets, and reordering between packets could cause the
   header decompressor to reconstitute erroneous packets, i.e., packets
   that do not match the original packet, but still have a valid IP, UDP
   (or UDP-Lite), and RTP headers, and possibly also valid UDP (or UDP-
   Lite) checksums.

   The header compression profiles defined herein use an internal
   checksum for verification of reconstructed headers.  This reduces the
   probability that a header decompressor delivers erroneous packets to
   upper layers without the error being noticed.  In particular, the
   probability that consecutive erroneous packets are not detected by
   the internal checksum is close to zero.

   This small but non-zero probability remains unchanged when integrity
   protection is applied after compression and verified before
   decompression, in the case where an attacker could discard or reorder
   packets between the compression endpoints.




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   The impairments mentioned above could be caused by a malfunctioning
   or malicious header compressor.  Such corruption may be detected with
   end-to-end integrity mechanisms that will not be affected by the
   compression.  Moreover, the internal checksum can also be useful in
   the case of malfunctioning compressors.

   Denial-of-service attacks are possible if an intruder can introduce
   (for example) bogus IR 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.

8.  IANA Considerations

   The following ROHC profile identifiers have been assigned by the IANA
   for the profiles defined in this document:

     Identifier        Profile
     ----------        -------
     0x0101            ROHCv2 RTP
     0x0102            ROHCv2 UDP
     0x0103            ROHCv2 ESP
     0x0104            ROHCv2 IP
     0x0107            ROHCv2 RTP/UDP-Lite
     0x0108            ROHCv2 UDP-Lite

9.  Acknowledgements

   The authors would like to thank Mark West, Robert Finking, Haipeng
   Jin, and Rohit Kapoor for serving as committed document reviewers,
   and also for constructive discussions during the development of this
   document.  Thanks to Carl Knutsson for his extensive contribution to
   this specification, as well as to Jani Juvan and Anders Edqvist for
   useful comments and feedback.  Thanks also to Elwyn Davies for his
   review as the General Area Review Team (Gen-ART) reviewer, and to
   Stephen Kent for his review on behalf of the IETF security
   directorate, during IETF last-call.  Finally, thanks to the many
   people who have contributed to previous ROHC specifications and
   supported this effort.











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10.  References

10.1.  Normative References

   [RFC0768]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              August 1980.

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

   [RFC3550]  Schulzrinne, H., Casner, S., Frederick, R., and V.
              Jacobson, "RTP: A Transport Protocol for Real-Time
              Applications", STD 64, RFC 3550, July 2003.

   [RFC3828]  Larzon, L-A., Degermark, M., Pink, S., Jonsson, L-E., and
              G. Fairhurst, "The Lightweight User Datagram Protocol
              (UDP-Lite)", RFC 3828, July 2004.

   [RFC4019]  Pelletier, G., "RObust Header Compression (ROHC): Profiles
              for User Datagram Protocol (UDP) Lite", RFC 4019,
              April 2005.

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

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

   [RFC4995]  Jonsson, L-E., Pelletier, G., and K. Sandlund, "The RObust
              Header Compression (ROHC) Framework", RFC 4995, July 2007.





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

10.2.  Informative References

   [RFC2675]  Borman, D., Deering, S., and R. Hinden, "IPv6 Jumbograms",
              RFC 2675, August 1999.

   [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.

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

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





























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Appendix A.  Detailed Classification of Header Fields

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

   In this appendix, all fields in the headers compressible by these
   profiles are classified and analyzed.  The analysis is based on
   behavior for the types of traffic that are expected to be the most
   frequently compressed (e.g., RTP field behavior is based on voice
   and/or video traffic behavior).

   Fields are classified as belonging to one of the following classes:

   INFERRED - These fields contain values that can be inferred from
   other values, for example the size of the frame carrying the packet,
   and thus do not have to be included in compressed packets.

   STATIC - These fields are expected to be constant throughout the
   lifetime of the flow; in general, it is sufficient to design a
   compressed format so that these fields are only updated by IR
   packets.

   STATIC-DEF - These fields are expected to be constant throughout the
   lifetime of the flow and their values can be used to define a flow.
   They are only sent in IR packets.

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

   SEMISTATIC - These fields are unchanged most of the time.  However,
   occasionally the value changes but will revert to its original value.
   For ROHCv2, the values of such fields do not need to be possible to
   change with the smallest compressed packet formats, but should be
   possible to change via slightly larger compressed packets.

   RARELY CHANGING (RACH) - These are fields that change their values
   occasionally and then keep their new values.  For ROHCv2, the values
   of such fields do not need to be possible to change with the smallest
   compressed packet formats, but should be possible to change via
   slightly larger compressed packets.

   IRREGULAR - These are the fields for which no useful change pattern
   can be identified and should be transmitted uncompressed in all
   compressed packets.



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   PATTERN - These are fields that change between each packet, but
   change in a predictable pattern.

A.1.  IPv4 Header Fields

   +------------------------+----------------+
   | Field                  | Class          |
   +------------------------+----------------+
   | Version                | STATIC-KNOWN   |
   | Header Length          | STATIC-KNOWN   |
   | Type Of Service        | RACH           |
   | Packet Length          | INFERRED       |
   | Identification         |                |
   |             Sequential | PATTERN        |
   |             Seq. swap  | PATTERN        |
   |             Random     | IRREGULAR      |
   |             Zero       | STATIC         |
   | Reserved flag          | STATIC-KNOWN   |
   | Don't Fragment flag    | RACH           |
   | More Fragments flag    | STATIC-KNOWN   |
   | Fragment Offset        | STATIC-KNOWN   |
   | Time To Live           | RACH           |
   | Protocol               | STATIC-DEF     |
   | Header Checksum        | INFERRED       |
   | Source Address         | STATIC-DEF     |
   | Destination Address    | STATIC-DEF     |
   +------------------------+----------------+

   Version

      The version field states which IP version is used and is set to
      the value four.

   Header Length

      As long as no options are present in the IP header, the header
      length is constant with the value five.  If there are options, the
      field could be RACH or STATIC-DEF, but only option-less headers
      are compressed by ROHCv2 profiles.  The field is therefore
      classified as STATIC-KNOWN.

   Type Of Service

      For the type of flows compressed by the ROHCv2 profiles, the DSCP
      (Differentiated Services Code Point) and ECN (Explicit Congestion
      Notification) fields are expected to change relatively seldom.





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   Packet Length

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

   IPv4 Identification

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

      Sequential

         In this behavior, the IP-ID is expected to increment by one for
         most packets, but may increment by a value larger than one,
         depending on the behavior of the transmitting IPv4 stack.

      Sequential Swapped

         When using this behavior, the IP-ID behaves as in the
         Sequential behavior, but the two bytes of IP-ID are byte-
         swapped.  Therefore, the IP-ID can be swapped before
         compression to make it behave exactly as the Sequential
         behavior.

      Random

         Some IP stacks assign IP-ID values using a pseudo-random number
         generator.  There is thus no correlation between the ID values
         of subsequent datagrams, and therefore there is no way to
         predict the IP-ID value for the next datagram.  For header
         compression purposes, this means that the IP-ID field needs to
         be sent uncompressed with each datagram, resulting in two extra
         octets of header.

      Zero

         This behavior, although not a legal implementation of IPv4, is
         sometimes seen in existing IPv4 stacks.  When this behavior is
         used, all IP packets have the IP-ID value set to zero.





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   Flags

      The Reserved flag must be set to zero and is therefore classified
      as STATIC-KNOWN.  The Don't Fragment (DF) flag changes rarely and
      is therefore classified as RACH.  Finally, the More Fragments (MF)
      flag is expected to be zero because IP fragments will not be
      compressed by ROHC and is therefore classified as STATIC-KNOWN.

   Fragment Offset

      Under the assumption that no fragmentation occurs, the fragment
      offset is always zero and is therefore classified as STATIC-KNOWN.

   Time To Live

      The Time To Live field is expected to be constant during the
      lifetime of a flow or to alternate between a limited number of
      values due to route changes.

   Protocol

      This field will have the same value in all packets of a flow and
      is therefore classified as STATIC-DEF.

   Header Checksum

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

   Source and Destination addresses

      These fields are part of the definition of a flow and must thus be
      constant for all packets in the flow.















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A.2.  IPv6 Header Fields

   +----------------------+----------------+
   | Field                | Class          |
   +----------------------+----------------+
   | Version              | STATIC-KNOWN   |
   | Traffic Class        | RACH           |
   | Flow Label           | STATIC-DEF     |
   | Payload Length       | INFERRED       |
   | Next Header          | STATIC-DEF     |
   | Hop Limit            | RACH           |
   | Source Address       | STATIC-DEF     |
   | Destination Address  | STATIC-DEF     |
   +----------------------+----------------+

   Version

      The version field states which IP version is used and is set to
      the value six.

   Traffic Class

      For the type of flows compressed by the ROHCv2 profiles, the DSCP
      and ECN fields are expected to change relatively seldom.

   Flow Label

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

   Payload Length

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

   Next Header

      This field will have the same value in all packets of a flow and
      is therefore classified as STATIC-DEF.








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   Hop Limit

      The Hop Limit field is expected to be constant during the lifetime
      of a flow or to alternate between a limited number of values due
      to route changes.

   Source and Destination addresses

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

A.3.  UDP Header Fields

   +------------------+-------------+
   | Field            | Class       |
   +------------------+-------------+
   | Source Port      | STATIC-DEF  |
   | Destination Port | STATIC-DEF  |
   | Length           | INFERRED    |
   | Checksum         |             |
   |         Disabled | STATIC      |
   |         Enabled  | IRREGULAR   |
   +------------------+-------------+

   Source and Destination ports

      These fields are part of the definition of a flow and must thus be
      constant for all packets in the flow.

   Length

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

   Checksum

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










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A.4.  UDP-Lite Header Fields

   +--------------------+-------------+
   | Field              | Class       |
   +--------------------+-------------+
   | Source Port        | STATIC-DEF  |
   | Destination Port   | STATIC-DEF  |
   | Checksum Coverage  |             |
   |        Zero        | STATIC-DEF  |
   |        Constant    | INFERRED    |
   |        Variable    | IRREGULAR   |
   | Checksum           | IRREGULAR   |
   +--------------------+-------------+

   Source and Destination Port

      These fields are part of the definition of a flow and must thus be
      constant for all packets in the flow.

   Checksum Coverage

      The Checksum Coverage field may behave in different ways: it may
      have a value of zero, it may be equal to the datagram length, or
      it may have any value between eight octets and the length of the
      datagram.  From a compression perspective, this field is expected
      to either be entirely predictable (for the cases where it follows
      the same behavior as the UDP Length field or where it takes on a
      constant value) or to change randomly for each packet (making the
      value unpredictable from a header-compression perspective).  For
      all cases, the behavior itself is not expected to change for this
      field during the lifetime of a packet flow, or to change
      relatively seldom.

   Checksum

      The information used for the calculation of the UDP-Lite checksum
      is governed by the value of the checksum coverage and minimally
      includes the UDP-Lite header.  The checksum is a changing field
      that must always be sent as-is.












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A.5.  RTP Header Fields

   +----------------+----------------+
   | Field          | Class          |
   +----------------+----------------+
   | Version        | STATIC-KNOWN   |
   | Padding        | RACH           |
   | Extension      | RACH           |
   | CSRC Counter   | RACH           |
   | Marker         | SEMISTATIC     |
   | Payload Type   | RACH           |
   | Sequence Number| PATTERN        |
   | Timestamp      | PATTERN        |
   | SSRC           | STATIC-DEF     |
   | CSRC           | RACH           |
   +----------------+----------------+

   Version

      This field is expected to have the value two and the field is
      therefore classified as STATIC-KNOWN.

   Padding

      The use of this field is application-dependent, but when payload
      padding is used, it is likely to be present in most or all
      packets.  The field is classified as RACH to allow for the case
      where the value of this field changes.

   Extension

      If RTP extensions are used by the application, these extensions
      are often present in all packets, although the use of extensions
      is infrequent.  To allow efficient compression of a flow using
      extensions in only a few packets, this field is classified as
      RACH.

   CSRC Count

      This field indicates the number of CSRC items present in the CSRC
      list.  This number is expected to be mostly constant on a packet-
      to-packet basis and when it changes, change by small amounts.  As
      long as no RTP mixer is used, the value of this field will be
      zero.







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   Marker

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

   Payload Type

      Applications could adapt to congestion by changing payload type
      and/or frame sizes, but that is not expected to happen frequently,
      so this field is classified as RACH.

   RTP Sequence Number

      The RTP Sequence Number will be incremented by one for each packet
      sent.

   Timestamp

      In the audio case:

         As long as there are no pauses in the audio stream, the RTP
         Timestamp will be incremented by a constant value, which
         corresponds to the number of samples in the speech frame.  It
         will thus mostly follow the RTP Sequence Number.  When there
         has been a silent period and a new talkspurt begins, the
         timestamp will jump in proportion to the length of the silent
         period.  However, the increment will probably be within a
         relatively limited range.

      In the video case:

         Between two consecutive packets, the timestamp will either be
         unchanged or increase by a multiple of a fixed value
         corresponding to the picture clock frequency.  The timestamp
         can also decrease by a multiple of the fixed value for certain
         coding schemes.  The change in timestamp value, expressed as a
         multiple of the picture clock frequency, is in most cases
         within a limited range.

   SSRC

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





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   Contributing Sources (CSRC)

      The participants in a session, who are identified by the CSRC
      fields, are usually expected to be unchanged on a packet-to-packet
      basis, but will infrequently change by a few additions and/or
      removals.

A.6.  ESP Header Fields

   +------------------+-------------+
   | Field            | Class       |
   +------------------+-------------+
   | SPI              | STATIC-DEF  |
   | Sequence Number  | PATTERN     |
   +------------------+-------------+

   SPI

      This field is used to identify a distinct flow between two IPsec
      peers and it changes rarely; therefore, it is classified as
      STATIC-DEF.

   ESP Sequence Number

      The ESP Sequence Number will be incremented by one for each packet
      sent.

A.7.  IPv6 Extension Header Fields

   +-----------------------+---------------+
   | Field                 | Class         |
   +-----------------------+---------------+
   | Next Header           | STATIC-DEF    |
   | Ext Hdr Len           |               |
   |      Routing          | STATIC-DEF    |
   |      Hop-by-hop       | STATIC        |
   |      Destination      | STATIC        |
   | Options               |               |
   |      Routing          | STATIC-DEF    |
   |      Hop-by-hop       | RACH          |
   |      Destination      | RACH          |
   +-----------------------+---------------+

   Next Header

      This field will have the same value in all packets of a flow and
      is therefore classified as STATIC-DEF.




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   Ext Hdr Len

      For the Routing header, it is expected that the length will remain
      constant for the duration of the flow, and that a change in the
      length should be classified as a new flow by the ROHC compressor.
      For Hop-by-hop and Destination options headers, the length is
      expected to remain static, but can be updated by an IR packet.

   Options

      For the Routing header, it is expected that the option content
      will remain constant for the duration of the flow, and that a
      change in the routing information should be classified as a new
      flow by the ROHC compressor.  For Hop-by-hop and Destination
      options headers, the options are expected to remain static, but
      can be updated by an IR packet.

A.8.  GRE Header Fields

   +--------------------+---------------+
   | Field              | Class         |
   +--------------------+---------------+
   | C flag             | STATIC        |
   | K flag             | STATIC        |
   | S flag             | STATIC        |
   | R flag             | STATIC-KNOWN  |
   | Reserved0, Version | STATIC-KNOWN  |
   | Protocol           | STATIC-DEF    |
   | Checksum           | IRREGULAR     |
   | Reserved           | STATIC-KNOWN  |
   | Sequence Number    | PATTERN       |
   | Key                | STATIC-DEF    |
   +--------------------+---------------+

   Flags

      The four flag bits are not expected to change for the duration of
      the flow, and the R flag is expected to always be set to zero.

   Reserved0, Version

      Both of these fields are expected to be set to zero for the
      duration of any flow.

   Protocol

      This field will have the same value in all packets of a flow and
      is therefore classified as STATIC-DEF.



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   Checksum

      When the checksum field is present, it is expected to behave
      unpredictably.

   Reserved

      When present, this field is expected to be set to zero.

   Sequence Number

      When present, the Sequence Number increases by one for each
      packet.

   Key

      When present, the Key field is used to define the flow and does
      not change.

A.9.  MINE Header Fields

   +---------------------+----------------+
   | Field               | Class          |
   +---------------------+----------------+
   | Protocol            | STATIC-DEF     |
   | S bit               | STATIC-DEF     |
   | Reserved            | STATIC-KNOWN   |
   | Checksum            | INFERRED       |
   | Source Address      | STATIC-DEF     |
   | Destination Address | STATIC-DEF     |
   +---------------------+----------------+

   Protocol

      This field will have the same value in all packets of a flow and
      is therefore classified as STATIC-DEF.

   S bit

      The S bit is not expected to change during a flow.

   Reserved

      The reserved field is expected to be set to zero.







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   Checksum

      The header checksum protects individual routing hops from
      processing a corrupted header.  Since all fields of this header
      are compressed away, there is no need to include this checksum in
      compressed packets and it can be regenerated at the decompressor
      side.

   Source and Destination Addresses

      These fields can be used to define the flow and are not expected
      to change.

A.10.  AH Header Fields

   +---------------------+----------------+
   | Field               | Class          |
   +---------------------+----------------+
   | Next Header         | STATIC-DEF     |
   | Payload Length      | STATIC         |
   | Reserved            | STATIC-KNOWN   |
   | SPI                 | STATIC-DEF     |
   | Sequence Number     | PATTERN        |
   | ICV                 | IRREGULAR      |
   +---------------------+----------------+

   Next Header

      This field will have the same value in all packets of a flow and
      is therefore classified as STATIC-DEF.

   Payload Length

      It is expected that the length of the header is constant for the
      duration of the flow.

   Reserved

      The value of this field will be set to zero.

   SPI

      This field is used to identify a specific flow and only changes
      when the sequence number wraps around, and is therefore classified
      as STATIC-DEF.






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   Sequence Number

      The Sequence Number will be incremented by one for each packet
      sent.

   ICV

      The ICV is expected to behave unpredictably and is therefore
      classified as IRREGULAR.

Appendix B.  Compressor Implementation Guidelines

   This section describes some guiding principles for implementing a
   ROHCv2 compressor with focus on how to efficiently select appropriate
   packet formats.  The text in this appendix should be considered
   guidelines; it does not define any normative requirement on how
   ROHCv2 profiles are implemented.

B.1.  Reference Management

   The compressor usually maintains a sliding window of reference
   headers, which contains as many references as needed for the
   optimistic approach.  Each reference contains a description of which
   changes occurred in the flow between two consecutive headers in the
   flow, and a new reference is inserted into the window each time a
   packet is compressed by this context.  A reference may for example be
   implemented as a stored copy of the uncompressed header being
   represented.  When the compressor is confident that a specific
   reference is no longer used by the decompressor (for example by using
   the optimistic approach or feedback received), the reference is
   removed from the sliding window.

B.2.  Window-based LSB Encoding (W-LSB)

   Section 5.1.1 describes how the optimistic approach impacts the
   packet format selection for the compressor.  Exactly how the
   compressor selects a packet format is up to the implementation to
   decide, but the following is an example of how this process can be
   performed for lsb-encoded fields through the use of Window-based LSB
   encoding (W-LSB).

   With W-LSB encoding, the compressor uses a number of references (a
   window) from its context.  What references to use is determined by
   its optimistic approach.  The compressor extracts the value of the
   field to be W-LSB encoded from each reference in the window, and
   finds the maximum and minimum values.  Once it determines these
   values, the compressor uses the assumption that the decompressor has
   a value for this field within the range given by these boundaries



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   (inclusively) as its reference.  The compressor can then select a
   number of LSBs from the value to be compressed, so that the LSBs can
   be decompressed regardless of whether the decompressor uses the
   minimum value, the maximum value or any other value in the range of
   possible references.

B.3.  W-LSB Encoding and Timer-based Compression

   Section 6.6.9 defines decompressor behavior for timer-based RTP
   timestamp compression.  This section gives guidelines on how the
   compressor should determine the number of LSB bits it should send for
   the timestamp field.  When using timer-based compression, this number
   depends on the sum of the jitter before the compressor and the jitter
   between the compressor and decompressor.

   The jitter before the compressor can be estimated using the following
   computation:

       Max_Jitter_BC =
            max {|(T_n - T_j) - ((a_n - a_j) / time_stride)|,
               for all headers j in the sliding window}

   where (T_n - T_j) is the difference in the timestamp between the
   currently compressed header and a reference header and (a_n - a_j) is
   the difference in arrival time between those same two headers.

   In addition to this, the compressor needs to estimate an upper bound
   for the jitter between the compressor and decompressor
   (Max_Jitter_CD).  This information may for example come from lower
   layers.

   A compressor implementation can determine whether the difference in
   clock resolution between the compressor and decompressor induces an
   error when performing integer arithmetics; it can then treat this
   error as additional jitter.

   After obtaining estimates for the jitters, the number of bits needed
   to transmit is obtained using the following calculation:

       ceiling(log2(2 * (Max_Jitter_BC + Max_Jitter_CD + 2) + 1))

   This number is then used to select a packet format that contains at
   least this many scaled timestamp bits.








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Authors' Addresses

   Ghyslain Pelletier
   Ericsson
   Box 920
   Lulea  SE-971 28
   Sweden

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


   Kristofer Sandlund
   Ericsson
   Box 920
   Lulea  SE-971 28
   Sweden

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































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Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

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   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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   The IETF takes no position regarding the validity or scope of any
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   this document or the extent to which any license under such rights
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   Copies of IPR disclosures made to the IETF Secretariat and any
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   The IETF invites any interested party to bring to its attention any
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   ietf-ipr@ietf.org.












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