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Session Initiation Protocol (SIP)

This comprehensive guide provides an overview of the Session Initiation Protocol (SIP), a core protocol for establishing sessions on the internet. Learn about SIP addressing, mobility, building blocks, operation modes, architecture, and more. Discover how SIP enables voice-over-IP calls, multimedia conferences, event notifications, text messaging, and signaling transport. Dive into the SIP syntax, methods, and basic user location mechanisms. Explore SIP's role in peer-to-peer communication, VoIP to PSTN connections, and proxy operations.

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Session Initiation Protocol (SIP)

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  1. Session Initiation Protocol (SIP) Onno W. Purbo Onno@indo.net.id

  2. Referensi • Henning Schulzrinne, Columbia University, New York, May 2001 • Dr. Andreas Steffen, Komunikationsysteme

  3. SIP in short .. • Dr. Andreas Steffen, Komunikationsysteme

  4. SIP addressing ..

  5. SIP processes

  6. SIP format

  7. Overview • Protocol architecture • Typical component architecture • Addressing and locating SIP entities. • Protocol operation and extensions.

  8. Introduction .. • SIP = core protocol for establishing sessions in the internet. • Transports session description information from initiator (caller) to callees. • Allows to exchange parameters in mid-session. • Terminate session.

  9. VoIP protocol architecture

  10. Media Protocol Stack

  11. SIP Protocol Use

  12. SIP Applications .. • Setting up voice-over-IP calls. • Setting up multimedia conferences. • Event notification (subscribe / notify)  IM and presence. • Text and general messaging. • Signaling transport.

  13. SIP addressing • SIP uses e-mail style addressing to identify users.

  14. Personal Mobility

  15. SIP addressing .. • Typically, same as user’s e-mail address: • djancuk@surabaya.com • 02114204701@gateway-r-us.com • Written as URL, e.g, • Sip:djancuk@surabaya.com • Can add parameters, such as, type (user=“phone”) or transport protocol.

  16. Tel URLs (RFC 2806) • Also can use tel URL for telephone numbers, e.g., • Tel:+0811797677 • Fax:+022.253.4677 • Either global (tel:+001..) or local (tel:0w00222534677; phone-context=+0222534677) number. • Allow post-dialing digits; postd=pp32. • Also modem:+52990000; type=v32b?7e1; type=v110.

  17. SIP building blocks

  18. Back-to-Back UA (B2BUA) • Two (or more) user agents, where incoming calls trigger outgoing calls to somebody else. • Also, “third-party call control” (later) • Useful for services and anonymity

  19. Back-to-Back UA (B2BUA)

  20. Maintaining state in SIP enitities. • Stateless: • Each request and response handled independently. • (Transaction) Stateful: • Remember a whole request/response transaction. • Call stateful: • Remember a call from beginning to end.

  21. SIP archirecture • Peer-to-peer • Outbound proxy • VoIP to PSTN • PSTN to VoIP

  22. peer-to-peer

  23. outbound proxy

  24. VoIP to PSTN

  25. PSTN to VoIP

  26. SIP operation in proxy mode

  27. SIP operation in redirect mode • 302: redirection for a single call • 301: permanently

  28. Locating SIP users

  29. Registrar and location servers

  30. Basic user location mechanism • Host (SIP URL)  hostname of proxy • DNS: hostname of proxy SIP server(s) • If SIP UAS: alter user; done. • If SIP proxy/redirect server: map URLn  URLn+1, using any information in the request • Go to step 1 One minor exception ..

  31. Basic SIP “routing” mechanism • Will fill in details later • Route using request URIs. • All but first request in call typically bypass proxies and go direct UAC-UAS. • However, can use “record-routing” to force certain proxies to be visited all the time. • Responses always traverse the same route as requests

  32. Outbound proxies • Normally, proxy servers one or more domains. • Outbound proxies are used for all outbound requests from within a domain. • Typically, for managing corporate firewalls and policy enforcement. • May also provide dial plans or route tel/fax URLs. • Other uses: lawyer client billing, ..

  33. Locating users: DNS SRV .. • e-mail: DNS MX record mapping domain to mail server(s). • SIP: use newer record for general purpose mapping, SRV (RFC 2782) • Mapping from service and transport protocol to one or more servers, including protocols.

  34. Locating users: DNS SRV .. • Allow priority (for back-up) and weight (for load balancing).

  35. Using DNS SRV for scalable load balacing ..

  36. Aside: SIP scaling .. • HTTP request director  SIP client-based • HTTP randomized DNS (short TTL!)  SRV weight and priorities. • Can’t just distribute request randomly, since backend (registration) synchronization is needed.

  37. Aside: SIP scaling .. • Registration scaling: request/second * 3600; e.g. 100 request/second  360.000 user/server. • Major bottleneck are logging and database updates. • Generally, higher registration than INVITE rates.

  38. SIP operation ..

  39. SIP requests and responses • Text, not binary, format. • Look very similar to HTTP/1.1 • Requests and responses are similar except for first line. • Requests and responses can contain message bodies; typically session descriptions, but also ASCII or HTML.

  40. SIP syntax

  41. SIP syntax • Field names and some tokens, e.g. media type, are case-insensitive. • Everything else is case-sensitive. • White space doesn’t matter except in the first line. • Lines can be folded. • Multi-valued header fields can be combined as a comma-lists.

  42. SIP methods

  43. SIP invitation and media negotiation

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