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To eliminate the traditional
connotation that a “connection” is between a single
source and destination address,
SCTP uses the term association to define the protocol state installed on two
peer SCTP hosts exchanging messages. An SCTP association can employ multiple
addresses at each end. SCTP supports some features inherited from TCP and
others that provide additional functionality: Message boundary reservation: SCTP
reserves applications’ message-framing boundaries by placing messages inside
one or more SCTP data structures, called chunks. Multiple messages can be
bundled into a single chunk, or a large message can be spread across multiple
chunks.
■ No “head-of-line” blocking. :
SCTP eliminates the head-of-line blocking delay that can occur when a TCP
receiver is forced to resequence packets that arrive out of order because of
network reordering or packet loss.
■ Multiple delivery modes. : SCTP
supports several modes of delivery including strict order-oftransmission (like
TCP), partially ordered (per stream), and unordered delivery (like UDP).
■ Multihoming support. : SCTP
sends packets to one destination IP address, but can reroute messages to an
alternate if the current IP address becomes unreachable.
■ TCP-friendly congestion control
: SCTP employs the standard techniques pioneered in TCP for congestion
control,6 including slow-start, congestion avoidance, and fast retransmit. SCTP
applications can thus receive their share of network resources when coexisting
with TCP applications.
■ Selective acknowledgments.:
SCTP employs a selective acknowledgment scheme, derived from TCP, for packet
loss recovery.7 The SCTP receiver provides feedback to the sender about which
messages to retransmit when any are lost.
■ User data fragmentation:. SCTP
will fragment messages to conform to the maximum transmit unit (MTU) size along
a particular routed path between communicating hosts. This function is
described in RFC 1191 and is optionally
employed by TCP/IP to avoid the
performance degradation that results when IP routers have to perform
fragmentation.8
■ Heartbeat keep-alive mechanism:
SCTP sends heartbeat control packets to idle
destination addresses that are
part of the association. The protocol declares the IP
address to be down once it
reaches the threshold of unreturned heartbeat acknowledgments.
■ DOS protection: To mitigate the
impact of TCP SYN flooding attacks on a target
host, SCTP employs a security “cookie”
mechanism during association initialization.
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