2.5 Stream Control Transmission
Protocol (SCTP)
SCTP provides services similar to those offered
by UDP and TCP. SCTP is described in RFC 2960 [Stewart et al.
2000], and updated by RFC 3309 [Stone, Stewart, and Otis 2002]. An
introduction to SCTP is available in RFC 3286 [Ong and Yoakum
2002]. SCTP provides associations
between clients and servers. SCTP also provides applications with
reliability, sequencing, flow control, and full-duplex data
transfer, like TCP. The word "association" is used in SCTP instead of
"connection" to avoid the connotation that a connection involves
communication between only two IP addresses. An association refers
to a communication between two systems, which may involve more than
two addresses due to multihoming.
Unlike TCP, SCTP is message-oriented. It provides sequenced
delivery of individual records. Like UDP, the length of a record
written by the sender is passed to the receiving application.
SCTP can provide multiple streams between
connection endpoints, each with its own reliable sequenced delivery
of messages. A lost message in one of these streams does not block
delivery of messages in any of the other streams. This approach is
in contrast to TCP, where a loss at any point in the single stream
of bytes blocks delivery of all future data on the connection until
the loss is repaired.
SCTP also provides a multihoming feature, which
allows a single SCTP endpoint to support multiple IP addresses.
This feature can provide increased robustness against network
failure. An endpoint can have multiple redundant network
connections, where each of these networks has a different
connection to the Internet infrastructure. SCTP can work around a
failure of one network or path across the Internet by switching to
another address already associated with the SCTP association.
Similar robustness can be obtained from TCP with
help from routing protocols. For example, BGP connections within a
domain (iBGP) often use addresses that are assigned to a virtual
interface within the router as the endpoints of the TCP connection.
The domain's routing protocol ensures that if there is a route
between two routers, it can be used, which would not be possible if
the addresses used belonged to an interface that went down, for
example. SCTP's multihoming feature allows hosts to multihome, not just routers, and
allows this multihoming to occur across different service
providers, which the routing-based TCP method cannot allow.
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