Layer 4 Transport

A single 16-bit packet of data is formed as follows

(msb)
bit-15 Unused
bit-14 Unused (possibly parity)
bit-13 section
bit-12 section (00 - invalid, 01 - header, 10 - payload, 11 - trailer)
bit-11 sequence
bit-10 sequence
bit-9  sequence
bit-8  sequence Sequence is a 4 bit cyclic counter. The first packet in the header, payload, or trailer has a sequence of 0000.
bit-7  payload
bit-6  payload
bit-5  payload
bit-4  payload
bit-3  payload
bit-2  payload
bit-1  payload
bit-0  payload
(lsb)

Since the maximum length of a trailer is 4 packets, there are 12 out-of-band signal packets available for assignment.

0011 0000 xxxx xxxx - Not an out-of-band signal. This is the first packet of the trailer.
0011 0001 xxxx xxxx - Not an out-of-band signal. This is the first packet of the trailer.
0011 0010 xxxx xxxx - Not an out-of-band signal. This is the first packet of the trailer.
0011 0011 xxxx xxxx - Not an out-of-band signal. This is the first packet of the trailer.
0011 0100 xxxx xxxx - Unassigned.
0011 0101 xxxx xxxx - Unassigned.
0011 0110 xxxx xxxx - Unassigned.
0011 0111 xxxx xxxx - Unassigned.
0011 1000 xxxx xxxx - Unassigned.
0011 1001 xxxx xxxx - Unassigned.
0011 1010 xxxx xxxx - Unassigned.
0011 1011 xxxx xxxx - Unassigned.
0011 1100 xxxx xxxx - Unassigned.
0011 1101 xxxx xxxx - Unassigned.
0011 1110 xxxx xxxx - Unassigned.
0011 1111 xxxx xxxx - Abort. The current message MUST be aborted and MUST be ignored.