SCTP Programmer's Guide

• Restrain from sending a FORWARD TSN chunk at any time during the lifetime of
an association.
• Check with the upper layer if it has requested a notification on whether the peer
endpoint supports the Forward-TSN-Supported parameter.
Error Handling in SCTP
The network traffic in the Internet is unpredictable. Sudden network failures and traffic
surges can occur, which result in non-reachability of an endpoint. Such a network is
error prone and a sending application must be cautious while transmitting or
retransmitting data, because the receiving endpoint may be unavailable to receive data.
The unavailability of the endpoint is caused either by a path failure or an endpoint
failure.
SCTP offers appropriate error handling methods, to overcome this problem. Before
transmitting data, SCTP sends chunks of information to verify whether a destination
is active. Even before using a different path to reach a destination or closing an
association, SCTP ensures that the destination address is not reachable or inactive.
SCTP uses the following error handling methods:
• Retransmission of DATA chunks
• HEARTBEATs to identify path failures
• HEARTBEATs to identify endpoint failures
This section addresses the following topics:
• “Retransmission of DATA Chunks” (page 37)
• “HEARTBEATs to Identify Path Failures” (page 38)
• “HEARTBEATs to Identify Endpoint Failure” (page 38)
Retransmission of DATA Chunks
SCTP uses DATA chunks to exchange information between two addresses. Upon
receiving a DATA chunk, the receiving address sends an acknowledgement to the
sending address. If the receiving address does not receive the DATA chunk properly,
it sends a SACK packet that triggers the sending address to retransmit the DATA chunk.
The sending address also retransmits the DATA chunk when the retransmission timer
times out.
SCTP limits the rate of retransmission of DATA chunks, to reduce chances of congestion.
It modifies the retransmission timeout (RTO) value, based on the estimates of the round
trip delay and reduces the transmission rate exponentially when the message loss
increases.
In an active SCTP association with constant DATA transmission, SACKs are more likely
to cause retransmission than the retransmission timeout. To reduce unnecessary
retransmission of data, SCTP uses the four SACK rule, so that SCTP retransmits a DATA
chunk only after receiving the fourth SACK, which indicates a missing DATA chunk.
Error Handling in SCTP 37