ISUP / SIP mapping

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SIP over SCTP performance analysis
Gonzalo.Camarillo_at_ericsson.com Schulzrinne_at_cs.colu
mbia.edu Raimo.Kantola_at_hut.fi
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Outline

• Problem statement signalling transport
• Introduction to SCTP
• SCTP performance analysis
• SCTP vs UDP
• SCTP vs TCP

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Signalling Transport (SIGTRAN)

• Providers want to transport telephony signalling
  over IP
• IP trunking
• Let us try to use existing transport protocols

PSTN
IP
PSTN
SG
SG
SG Signalling Gateway
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Traditional transports UDP and TCP

• UDP (User Datagram Protocol)
• Unreliable transfer of datagrams
• Demultiplexing based on port numbers
• Checksum
• Example RTP
• TCP (Transmission Control Protocol)
• Reliable transfer of streams of bytes
• Congestion control network friendly
• Example HTTP, FTP, Telnet

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TCP limitations

• Carry every SS7 call on top of a TCP session
• Every SS7 call is affected by the TCP three-way
  handshake
• Multiple SS7 calls on top of a single TCP session
• TCP provides a stream service Head of the line
  blocking problem

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UDP limitations

• Use UDP and application level retransmissions
• No flow control Congestion
• Large amount of state information in the
  application
• In a slightly different context, this solution
  was adopted by SIP

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Stream Control Transmission Protocol (SCTP)

• SIGTRAN is chartered to develop a transport
  protocol that fulfils signalling transport
  requirements
• October 2000 RFC 2960 Stream Control
  Transmission Protocol
• A connection in SCTP terminology is an
  association
• Four-way handshake (avoids DoS attacks)

SCTP
TCP
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Stream Control Transmission Protocol (SCTP)

• SCTP allows multihoming
• A receiver can be reachable at multiple IP
  addresses
• Robustness rather than load balancing
• SCTP is a message-based protocol
• Easier parsing
• No need of application specific boundaries

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Stream Control Transmission Protocol (SCTP)

• SCTP allows multiple streams within an
  association
• Flow control performed on association basis (TSN)
• Delivery performed on stream basis (no head of
  the line blocking)

STREAM 0
TSNTransmission Sequence Number
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Stream Control Transmission Protocol (SCTP)

• SCTP uses TCP SACK congestion and flow control
  mechanisms (per association)
• Slow start, congestion avoidance, fast
  retransmit, fast recovery
• SCTP provides two services (per stream)
• Ordered message delivery
• Unordered message delivery
• It also provides unordered messages within an
  ordered stream

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SIP over SCTP

• Two ways of transporting SIP over SCTP
• Send all SIP requests and responses over a single
  unordered SCTP stream.
• Send requests and responses belonging to the same
  SIP transaction over the same SCTP stream.
• Stream ID can be used as a lightweight
  transaction identifier instead of the Call-ID,
  From, To, Via and Cseq header fields
• Server side Incoming ACKs (non-2xx) and CANCELs
• Client side Incoming responses

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Simulations SCTP performance analysis
We implemented SCTP in the network simulator
(ns) We only analyzed the SIP hop-by-hop
handshake (INVITE-100 Trying)
15 ms
15 ms
15 ms
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SCTP vs. UDP
Fast retransmit detects losses much faster than
UDP-based timeouts UDP lacks congestion control
Delay (ms)
Time a packet is generated (ms)
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SCTP transport layer fragmentation

• Every fragment a different TSN
• Begin and End bits are used for reassembling
• Once an SCTP packet is sent it cannot be
  re-fragmented again
• Sudden changes in the path MTU trigger IP
  fragmentation
• Problems with NATs and firewalls

SCTP DATA chunk
TSN 1 10
TSN 2 00
TSN 3 01
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SCTP and TCP window-based congestion control

• Congestion window (cwnd) limits the data rate at
  the sender
• Slow start exponential growth of cwnd
• Congestion avoidance linear growth of cwnd
• When the data rate at the sender is limited by
  the application rather than by cwnd, the
  congestion window grows dramatically
• Bursts of traffic produce heavy congestion

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Head Of the Line (HOL) blocking
Comparison between ordered SCTP (which behaves
like TCP SACK) and unordered SCTP
Time a packet is received (ms)
Time a packet is generated (ms)
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Measuring HOL buffer limited router
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Measuring HOL induced packet loss
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Measuring HOL competing traffic
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Conclusions

• UDP is not a suitable protocol for proxy to proxy
  communications
• SCTP offers some advantages over TCP
• Protection against DoS attacks
• Multihoming
• Message based
• Lightweight transaction identifiers
• HOL avoidance
• Supposedly the biggest advantage of SCTP
• Only significant gain when anyway the delay is
  unacceptable
• SCTP has some limitations
• Transport layer fragmentation
• Window-based congestion control