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6. Compressed Header Formats

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

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

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

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

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

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

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

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

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

  b) COMPRESSED_TCP_NODELTA header format

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

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

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

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

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