Retransmissions and Destination Status

When transmitting data on a multihomed association, the initial transmission is made to the primary address on the primary path. If the initial transmission times out, then the first retransmission is made to an alternate destination in a round-robin, consecutive fashion. The SCTP layer will continue to send the initial transmission of new data arriving for transmission from upper layers on the primary path.

If a unihomed SCTP endpoint is not in contact after RTIMES errors, the end point address is marked as unreachable. For multihomed associations, if an endpoint’s address is not in contact after RTIMES/2 errors, the address is marked as unreachable.

An error is a failure to Selectively Acknowledge (SACK) a transmitted packet or acknowledge a heartbeat within a Retransmission Time Out (RTO). Alternate paths exchange heartbeats as a means of confirming connectivity, and failure to acknowledge heartbeats would cause an alternate destination to be marked as unreachable.