An Introduction to SIP

1. An Introduction to SIP Moshe Sambol Services Research Lab November 18, 1998

2. The Session Initiation Protocol • Communications protocol under development by the IETF’s MMUSIC working group. Last call for comments is over, SIP is about to become a standard. • Text-based IP application-layer protocol (derived from HTTP) for Internet multi-party multi-media conferencing establishment and management. GTE Laboratories SRL Presentation

3. SIP Basics • Separation of call data and connection control • as opposed to PSTN: DTMF / pulses and on-hook/off-hook signals carried in band with call. • Carries service and participant information • provides natural support for advanced telephony services. GTE Laboratories SRL Presentation

4. SIP Basics continued • Simpler language than H.323 supports flexibility, expandability, greater expression • Media and/or participants can be added to or removed from existing conferences • Supports personal mobility, degrees of security... GTE Laboratories SRL Presentation

5. The SIP Call Model • Session: A set of multimedia senders and receivers and the data which flows between them. • Conference: A multimedia session identified by a session description. Composed of one or more calls. • Call: All participants in a conference invited by a single source. • Call Leg: All communication between one caller and callee within a call. • Transaction: All messages within a call leg from a first request until a final* response. GTE Laboratories SRL Presentation

6. Actors in a SIP Conference • User Agent • Client application which initiates and accepts invitations; interacts with media applications. • SIP Proxy Server • Intermediary which relays requests after possible lookup and/or translation. • SIP Redirect Server • Maps a request to a set of alternate addresses and returns those to the client without initiating requests of its own. GTE Laboratories SRL Presentation

7. SIP Actors, continued • Location Service • Not defined by SIP but used by its servers to locate other SIP servers. • No Gatekeeper • Gatekeeper traditionally provides admission control, address translation. These functions provided by other SIP actors. GTE Laboratories SRL Presentation

8. SIP Operation in Proxy Mode All illustrations courtesy of H.Schulzrinne GTE Laboratories SRL Presentation

9. SIP Operation in Redirect Mode GTE Laboratories SRL Presentation

10. SIP Requests • REGISTER • Notify server of user location and availability • OPTIONS • Determine user capabilities • INVITE CANCEL • Call setup  Transaction termination • BYE • Call termination / transfer GTE Laboratories SRL Presentation

11. SIP Responses • SIP Provides a rich set of response codes which make it well suited to support advanced telephony services: • 1xx: Provisional responses (ringing, queued, etc.) • 2xx: Final success responses • 3xx: Redirect responses (moved perm/temp, mult. choices, etc.) • 4xx: Client-Error (formatting, authorization, timeout, loop, etc.) • 5xx: Server-Error • 6xx: Global-Failure (busy, decline, does not exist, etc.) GTE Laboratories SRL Presentation

12. An Example SIP Message INVITE sip:watson@boston.bell-tel.com SIP/2.0 Via: SIP/2.0/UDP kton.bell-tel.com From: A. Bell <sip:a.g.bell@bell-tel.com> To: T. Watson <sip:watson@bell-tel.com> Call-ID: 43243250128u50@kton.bell-tel.com Cseq: 1 INVITE Subject: Mr. Watson, come here. Content-Type: application/sdp Content-Length: … v=0 o=bell 53655765 2353687637 IN IP4 128.3.4.5 m=audio 3456 RTP/AVP 0 3 4 5 GTE Laboratories SRL Presentation

13. … Sample Response SIP/2.0 180 Ringing Via: SIP/2.0/UDP kton.bell-tel.com From: A. Bell <sip:a.g.bell@bell-tel.com> To: T. Watson <sip:watson@bell.tel.com> Call-ID: 43243250128u50@kton.bell-tel.com Cseq: 1 INVITE Content-Length: 0 ----------------------------------------------------------------------------- SIP/2.0 200 OK Via: SIP/2.0/UDP kton.bell-tel.com From: A. Bell <sip:a.g.bell@bell-tel.com> To: T. Watson <sip:watson@bell.tel.com> Call-ID: 43243250128u50@kton.bell-tel.com Cseq: 1 INVITE Contact: sip:watson@boston.bell-tel.com Content-Type: … m=audio 5004 RTP/AVP 0 3 … GTE Laboratories SRL Presentation

14. Sample Services • Multi-party call transfers • generic addressing  directed pick-up • multiple pick-up  hold music control • third party call control  camp-on • click to dial back  availability notification • SIP Supports • Multicast  UDP + TCP • Privacy (authentication, encryption, path hiding) GTE Laboratories SRL Presentation

15. Transfer Example: Telemarketing GTE Laboratories SRL Presentation

16. State of SIP • SIP implementations are being worked on at: • ObjectSoftware  Nokia Research • Lucent  Mediatrix • ISI  HP Labs • GMD Fokus  Ericsson • Dynamicsoft  others? • My own graduate work was on... GTE Laboratories SRL Presentation

17. Open Issues • Billing • Mappings to telephony APIs • Interfacing to external media applications • Location services Topics for Future Presentations • SIP vs. H.323  SDP • Security  Richer examples! GTE Laboratories SRL Presentation

18. Additional Info At: • Professor Henning Schulzrinne: • http://www.cs.columbia.edu/~hgs/sip • http://www.cs.columbia.edu/~hgs/internet/sdp.html • Working group mailing list: • confctrl@isi.edu • Struggling ex-graduate student: • msambol@gte.com GTE Laboratories SRL Presentation