SCTP

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SCTP Stream Control Transmission Protocol
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What is SCTP?

• A (relatively) new general purpose transport
  protocol
• Main motivation TCP and UDP are inadequate for
  telephony signaling transport
• Provides a reliable message-oriented transport
  service
• a number of functions beneficial for telephony
  signaling

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TCP Limitations

• Enforces total ordering of data
• May cause head-of-line blocking
• Is byte-oriented, i.e. does not support framing
  of message boundaries
• Has no support for multi-homing
• Difficult to build link or path-level redundancy
• Is vulnerable to denial of service attacks
• Security is often a top priority for phone appl.

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UDP Limitations

• Provides unreliable transport service
• Packets can be lost, duplicated or arrive
  out-of-order
• Has no congestion control mechanism
• Possible for the application to build its own
  mechanism for the above, but

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Comparing SCTP to TCP - similarities

• Both are connection-oriented, i.e. exchange
  messages at startup and closing down
• Both provides reliability through
  retransmissions, using either timeouts or fast
  retransmit
• Both provides for orderly delivery of data (but
  SCTP also allows for no or partial ordering)
• Both use the same congestion control mechanism
• Slow start, congestion avoidance (AIMD)

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Comparing SCTP to TCP - differences

• Startup procedure
• Better protection against SYN flooding
• Message abstraction
• Easier buffering and framing for receiving
  application
• Multi-streaming
• Protection against head-of-line blocking
• Multi-homing
• Better robustness in the presence of network
  failure

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Startup
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Four-way handshake

• Cookie mechanism to guard against SYN flooding
• On receiving INIT chunk, server does not allocate
  memory for TCB
• Instead sends cookie in INIT ACK for
  authentication
• Data can be included in 3rd or 4th message

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Multi-streaming

• Streams allows multiple ordered flows of messages
  within a single association.
• Messages are sent in their respective streams and
  if a message in one stream is lost, it will not
  hold up delivery of a message in the other
  streams
• Prevents head of line blocking

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Multi-streaming (contd)

• Both ordered and un-ordered data can be sent
  within a stream.
• For un-ordered data, delivery to the upper layer
  is immediate upon receipt.
• For ordered data, delivery may be delayed due to
  reassembly from network reordering.
• A stream is uni-directional
• SCTP makes NO correlation between an inbound and
  outbound stream

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Multi-homing

• When a peer is multi-homed, a primary
  destination address is selected by the SCTP
  endpoint.
• By default, all data is sent to this primary
  address.
• When the primary address fails, the sender
  selects an alternate primary address until it is
  restored or the user changes the primary address.

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Implementations

• FreeBSD version 1.4
• Linux 2.4/2.6
• Sun Solaris 10
• .