SPX Fields
Connection control: flags
- bit #0: SPX-2 extended header (ignored by SPX)
- 1: reserved (Xmit as 0, ignore on receipt)
- 2: ignored by SPX, means “negotiate size” for SPX-2
- 3: ignored by SPX, indicates this is SPX-2
- 4: end of msg (user bit)
- 5: ignored by SPX, “attention” in SPX-2
- 6: send ack after this pkt (always 1 for SPX)
- 7: System packet (does not consume seq #)