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Understanding SIP: Key Concepts & Implementation

Explore Session Initiation Protocol (SIP) modes, headers, SDP, NAT, and more. Learn about SIP entities, requests, and design guidelines. Enhance your knowledge of SIP for multimedia systems. 8 Relevant

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Understanding SIP: Key Concepts & Implementation

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  1. CSc 461/561Multimedia Systems Part C: 2. SIP

  2. Summary • Why SIP? • What is SIP? • SIP proxy mode • SIP redirect mode • SIP header • SDP • SIP and NAT • More from Henning’s Tutorial

  3. Why SIP (1) What we have so far … • Network: IP and IP multicast • move packets from one host to other hosts • Transport: RTP and RTCP • end2end media transport and control feedback • Application: what we can do now • voice over IP (VoIP): yes! • IP telephony: not yet • what’s missing?: signaling! (e.g., how to setup calls)

  4. Why SIP (2): What we need: signalling support • E.g., in telephone networks • in addition to speech path: voice • signaling path: out-of-band, packetized SS7 • E.g., in cellular systems (+mobility support) • handoff between base stations • roaming across service providers • On the Internet • SIP: session initiation protocol

  5. 2. SIP: quick fact (1) • SIP is not limited to IP telephony • SIP is the Internet’s signaling protocol • SIP offers • setup calls (or sessions) • make changes to ongoing calls • terminate calls, and more (e.g., presence) • SIP does not offer • media transport, QoS support, server control,etc

  6. 2. SIP design guidelines (2) • Client-server model • request-reply transaction • HTTP+MIME-like format • Common headers in plain text • request/response line (e.g., INVITE a@b.com SIP/2.0) • message headers (identification, routing, etc) • message body • e.g., session description (SDP)

  7. 2. SIP requests (3) • REGISTER: register user agents • INVITE: initiate calls • ACK: confirm responses • BYE: terminate or transfer calls • Other methods: • CANCEL, OPTIONS, INFO, COMET, PRACK, SUBSCRIBE, NOTIFY, REFER • SIP response: HTTP-like (e.g., SIP/2.0 200 OK)

  8. 2. SIP entities (4) • User agent (UA) • UA client (UAC) and UA server (UAS) • Proxy server • relay calls; chaining; forking • Redirect server • redirect calls • Registrar server • UA registration (UA whereabouts)

  9. REGISTER sip:b.com SIP/2.0From: sip:a@b.comTo: sip:a@b.comContact: <sip:1.2.3.4> Expires: 3600 a@1.2.3.4 SIP/2.0 200 OK 2. SIP: register (5) • Create UA name/location binding • name (URI): e.g., sip:a@b.com • location: sip@1.2.3.4 • valid for a specific time period locationserver sip:a@b.com b.comregistrar sip:1.2.3.4

  10. INVITE sip:a@1.2.3.4 SIP/2.0 From: sip:x@y.comTo: sip:a@b.comCall-ID: 5678@y.com INVITE sip:a@b.com SIP/2.0 From: sip:x@y.comTo: sip:a@b.comCall-ID: 5678@y.com ?a SIP/2.0 200 OK From: sip:x@y.comTo: sip:a@b.comCall-ID: 5678@y.com ACK sip:a@b.com SIP/2.0 media media BYE OK 3. SIP: proxy mode (1) locationserver b.comproxy

  11. a@west ?a@west ?a a@1.2.3.4 INVITEa@1.2.3.4 INVITEa@west.b.com INVITE a@b.com INVITE a@5.6.7.8 CANCEL INVITE a@1.2.3.4 OK 3. Proxy: chaining and forking (2) west.b.com b.com

  12. a@home.com ?a INVITE sip:a@b.com SIP/2.0 SIP/2.0 302 Moved temporarilyContact: a@home.com INVITE sip:a@home.com SIP/2.0 SIP/2.0 200 OK ACK sip:a@home.com SIP/2.0 media media 4. SIP: redirect mode (1) locationserver b.comredirect

  13. 5. SIP headers • From/To: caller/callee URI • Via: proxy routing • request/reply: use the same route • may be different from the media route • Call-ID: identification • unique at caller • Content-Type/Length: payload info • e.g., Content-Type: application/sdp

  14. 6. SDP • SDP: session description protocol • media type, network/transport parameters • e.g., media: media, port, protocol, format_list • m=audio 2000/2 RTP/AVP 0 98 • a=rtpmap:0 PCMU/8000 • connection: net_type, add_type, address • c=IN IP4 1.2.3.4/127/3 • Ref: http://www.ietf.org/rfc/rfc2327.txt

  15. 7. SIP and NAT • NAT: network address translation • was to deal with IPv4 address shortage • now pervasive in all networking scenarios • “directional” connectivity • Communications in SIP • UA<=>server • UA<=>UA • Problem: when UAC and UAS behind NAT

  16. 8. More Content More teaching content from Henning Schulzrinne’s tutorial: • http://www.cs.columbia.edu/~coms6181/slides/11/sip_long.pdf

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