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 Introducti
on 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 Mes
sage 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-b
ased congestion control