SCTP Programmer's Guide

The communication failure detection and protection capabilities of reliable SCTP data
traffic are also applicable to partially reliable data traffic. PR-SCTP enables an endpoint
to detect a failure destination address quickly and to failover to an alternate destination
address. It also notifies when the destination address becomes unreachable.
The chunk bundling capability in SCTP enables reliable and unreliable messages to be
multiplexed over a single PR-SCTP association. Multiplexing enables a single protocol
(that is SCTP) to be used to transmit different types of messages, instead of using
separate protocols.
SCTP includes the following parameter and chunk to support the partially reliable data
transmission service:
The Forward-TSN-Supported
parameter
This is an optional parameter in the INIT and
INIT ACK chunks. When an association is
initialized, the SCTP sender must include this
parameter in the INIT or INIT ACK chunk to
inform its peer that it supports partially reliable
data service.
The Forward Cumulative TSN
(FORWARD TSN) chunk
The receiver sends this chunk to a sender to
inform its support for PR-SCTP. An SCTP sender
uses this chunk to inform the receiver to move
its cumulative received TSN forward, because
the missing TSNs are associated with data chunks
that must not be transmitted or retransmitted by
the sender.
The timed-reliability service is an example of a partially reliable service that SCTP
provides to the upper layer using PR-SCTP. This service enables the service user to
indicate a limit on the duration of time that the sender must try to transmit or retransmit
the message.
If an SCTP endpoint supports the FORWARD TSN chunk, it can include the
Forward-TSN-supported parameter in the INIT chunk to indicate support for FORWARD
TSN chunk to its peer. If an endpoint chooses not to include the Forward-TSN-Supported
parameter, it cannot send or process a FORWARD TSN chunk anytime during the lifetime
of an association. Instead, it must pretend as if it does not support the FORWARD TSN
chunk and return an error to the peer upon the receipt of any FORWARD TSN chunk.
When a receiver of an INIT or INIT ACK chunk detects a Forward-TSN-Supported
parameter and does not support the Forward-TSN chunk type, the receiver may
optionally respond with the Unsupported Parameters parameter, as defined in
Section 3.3.3 of RFC 2960.
A receiver can perform the following tasks if it receives an INIT chunk that does not
contain the Forward-TSN-Supported parameter:
• Include the Forward-TSN-Supported parameter in INIT-ACK.
• Record the information that the peer does not support the FORWARD TSN chunk.
36 Introduction