Stream Control Transmission Protocol (SCTP)

SCTP provides
for the reliable and fast delivery of signaling messages. It is reliable
because it includes mechanisms for the detection and recovery of lost or corrupted
messages. It is faster than TCP, however, because it avoids head-ofline
blocking, which can occur with TCP, and it also has more efficient
retransmission mechanisms than TCP.

Head-of-line blocking is avoided in SCTP through the use of streams. A
stream is a logical channel between SCTP endpoints. It may also be thought
of as a sequence of user messages between two SCTP users. When an association
is established between endpoints, part of the establishment of the association
involves each endpoint specifying how many inbound streams and
how many outbound streams are to be supported. If we think of a given association
as a one-way highway between endpoints, then the individual streams
are analogous to the individual traffic lanes on that highway. The advantage
with the stream concept is that resources (or queues) are allocated individually
to each stream, rather than to the complete set of packets that might
pass between two endpoints. Consequently, a message from one stream does
not have to wait in a queue behind a message from another stream.
Retransmission in SCTP is based on the fact that SCTP packets carrying
user data (known as chunks) include a transmission sequence number
(TSN). The receiver of the chuncks checks to make sure that all chunks
have been received by ensuring that no gap exists in the TSNs. If a gap is
found, then SCTP enables the receiver to specify which TSNs are missing
and it is only those TSNs that need to be retransmitted, which is more efficient
than TCP.
Consider, for example, the situation depicted in Figure 8-17. Chunks
with TSNs 1 to 4 have been received correctly, the chunk with TSN 5 is
missing, the chunk with TSN 9 is missing, and the chunks with TSNs 8 and
11 have been received twice. If TCP were to deal with this situation, then all
chunks from 5 onwards would be retransmitted. SCTP, however, has the
means for the receiver to clearly specify to the sender what is missing and
what is duplicated so that the minimum retransmission takes place.
Not only does SCTP support fast transmission and efficient retransmission,
it also supports congestion avoidance and it supports networklevel
redundancy. Congestion avoidance is achieved through the use of a
parameter in SCTP messages called the Advertised Receiver Credit Window.
This parameter indicates to the far end how much buffer space the
receiver has for the receipt of new messages. This helps to avoid flooding a
receiver with more messages than it can handle. Redundancy is achieved
through the fact that a given endpoint can be logically distributed across
multiple platforms with multiple IP addresses. If a given platform fails,
then another platform can take over. SCTP includes messages for monitoring
the reachability of a given endpoint and failover messages for one
endpoint to indicate to another that a different IP address should be used
for future messages. 367