4.1. Configuration Option Format

A summary of the IP-Compression-Protocol Configuration Option format to negotiate Van Jacobson TCP/IP header compression is shown below. The fields are transmitted from left to right.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |     IP-Compression-Protocol   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Max-Slot-Id  | Comp-Slot-Id  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Type

2

Length

6

IP-Compression-Protocol

002d (hex) for Van Jacobson Compressed TCP/IP headers.

Max-Slot-Id

The Max-Slot-Id field is one octet and indicates the maximum slot identifier. This is one less than the actual number of slots; the slot identifier has values from zero to Max-Slot-Id.

Note: There may be implementations that have problems with only one slot (Max-Slot-Id = 0). See the discussion in reference [3]. The example implementation in [3] will only work with 3 through 254 slots.

Comp-Slot-Id

The Comp-Slot-Id field is one octet and indicates whether the slot identifier field may be compressed.

         0  The slot identifier must not be compressed.  All compressed
            TCP packets must set the C bit in every change mask, and
            must include the slot identifier.

         1  The slot identifer may be compressed.

The slot identifier must not be compressed if there is no ability for the PPP link level to indicate an error in reception to the decompression module. Synchronization after errors depends on receiving a packet with the slot identifier. See the discussion in reference [3].