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13. Demultiplexing

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

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

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

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

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

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

  0110     regular IPv6 header

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

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

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

       0        COMPRESSED_TCP
       1        COMPRESSED_TCP_NODELTA
       2        COMPRESSED_NON_TCP
       3        CONTEXT_STATE

The currently assigned CONTEXT_STATE type values are

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