The SNDCP-XID (or layer-3 xid) is used to exchange layer-3 parameters
such as compression. The encoder encodes a bytestream that is then
sent as regular XID field from LLC.
We will need the SNDCP-XID to negotiate the parameters for our
upcomming GPRS data and header compression features
Change-Id: If2d63fe2550864cafef3156b1dc0629037c49c1e