1. Camarillo / Schulzrinne / Kantola November 26th, 2001 SIP over SCTP performance analysis Gonzalo.Camarillo@ericsson.com Schulzrinne@cs.columbia.edu Raimo.Kantola@hut.fi

2. Camarillo / Schulzrinne / Kantola November 26th, 2001 Outline Problem statement: signalling transport Introduction to SCTP SCTP performance analysis SCTP vs UDP SCTP vs TCP

3. Camarillo / Schulzrinne / Kantola November 26th, 2001 Signalling Transport (SIGTRAN) Providers want to transport telephony signalling over IP IP trunking Let us try to use existing transport protocols PSTNIP PSTN SG SG: Signalling Gateway

4. Camarillo / Schulzrinne / Kantola November 26th, 2001 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

5. Camarillo / Schulzrinne / Kantola November 26th, 2001 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

6. Camarillo / Schulzrinne / Kantola November 26th, 2001 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

7. Camarillo / Schulzrinne / Kantola November 26th, 2001 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) SCTPTCP

8. Camarillo / Schulzrinne / Kantola November 26th, 2001 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

9. Camarillo / Schulzrinne / Kantola November 26th, 2001 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) TSN:Transmission Sequence Number STREAM 0

10. Camarillo / Schulzrinne / Kantola November 26th, 2001 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

11. Camarillo / Schulzrinne / Kantola November 26th, 2001 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

12. Camarillo / Schulzrinne / Kantola November 26th, 2001 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

13. Camarillo / Schulzrinne / Kantola November 26th, 2001 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)

14. Camarillo / Schulzrinne / Kantola November 26th, 2001 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 TSN = 1 10 TSN = 2 00 TSN = 3 01 SCTP DATA chunk

15. Camarillo / Schulzrinne / Kantola November 26th, 2001 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

16. Camarillo / Schulzrinne / Kantola November 26th, 2001 Head Of the Line (HOL) blocking Comparison between ordered SCTP (which behaves like TCP SACK) and unordered SCTP Time a packet is generated (ms) Time a packet is received (ms)

17. Camarillo / Schulzrinne / Kantola November 26th, 2001 Measuring HOL: buffer limited router 15 ms

18. Camarillo / Schulzrinne / Kantola November 26th, 2001 Measuring HOL: induced packet loss 15 ms

19. Camarillo / Schulzrinne / Kantola November 26th, 2001 Measuring HOL: competing traffic 15 ms

20. Camarillo / Schulzrinne / Kantola November 26th, 2001 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