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SIP - The Basics


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If you don't know what SIP is, what it is used for or why you should even care then this is the section for you. This presentation will go over the very basics of SIP and assumes no previous knowledge of SIP or really any other network experience either. Topics it will touch upon is:

- What SIP is all about
- SIP and sessions management
- Basic call flow
- Brief discussion of SIP messages
- SIP and audio

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SIP - The Basics

  1. 1. About SIP SIPThe Basics Jonas Borjesson @borjessonjonas
  2. 2. NOTE● This version has been adapted to be viewed without transitions.● Go to to download the original version.● Also be sure to check out for any recorded presentations. @borjessonjonas
  3. 3. This is SIP! @borjessonjonas
  4. 4. SIP is about communication! Gaming Voice Video Chat Presence Whatever @borjessonjonas
  5. 5. SIP is about sessions● A session represents a p2p relationship between two SIP endpoints.● SIP helps you: ● Setup sessions. ● Negotiate what you want to do with the session ● Tear down sessions● Session can be anything ● Voice/video/gaming/your own whatever ● SIP only helps you establish and manage the session, you decide what to do! @borjessonjonas
  6. 6. SIP is about finding your friends ● Before you can “call” your friends (establish a session), you must find them!● No magic though... you need to tell someone where you are... a t o g et W en t coffee /Bob @borjessonjonas
  7. 7. No Magic Hey, I m comin g ove r, nee d a coff ee too . ..? Hey, have you seen Bob?! He is over at Cafe Capuchino ! Tell my friends that I am online at Cafe Capuchino @borjessonjonas
  8. 8. Basic User Scenario ProxyINVITE INVITE sip:bob@ 180 Ringing 180 Ringing 200 OK 200 OK ACK ACK audio @borjessonjonas
  9. 9. SIP Components● User Agents (UA) ● User Agent Client (UAC) ● User Agent Server (UAS)● Servers ● Proxy – forwards the request to the next hop ● Registrar – accepts registrar requests ● Redirect Server – finds alternative locations ● Location Service – stores bindings. (note, all SIP components are logical components and a such one server can act as all of the roles) @borjessonjonas
  10. 10. SIP Messages● Request/Response model ● UAC sends the request, UAS responds● Requests starts with a request line ● INVITE SIP/2.0● Methods ● INVITE, ACK, BYE, CANCEL, REGISTER, OPTIONS ● And many more... @borjessonjonas
  11. 11. SIP Messages● SIP responses starts with a status line ● SIP/2.0 200 OK● Response Codes: ● 1xx – Provisional. INVITE ● 2xx – Success. 180 Ringing ● 3xx – Redirection. 500 Server Error ● 4xx – Client Error. ● 5xx – Server Error. ● 6xx - Global Failures. @borjessonjonas
  12. 12. INVITE RequestINVITE SIP/2.0To: <>From: <>;tag=987lkajsdf89auCall-ID: lkjasdf90989lkjCSeq: 1 INVITEContent-Type: application/sdpContent-Length: 450Record-Route: <sip:;transport=tcp;lr>Via: SIP/2.0/TCP;branch=xxx;rport...Contact: <sip:;transport=TCP> @borjessonjonas
  13. 13. 200 OK ResponseSIP/2.0 200 OKTo: <>;tag=89uasdkfjoiuFrom: <>;tag=987lkajsdf89auCall-ID: lkjasdf90989lkjCSeq: 1 INVITEVia: SIP/2.0/TCP;branch=xxx;rport...Contact: <sip:;transport=TCP>Record-Route: <sip:;transport=tcp;lr>Content-Type: application/sdpContent-Length: 451 @borjessonjonas
  14. 14. SIP Headers● Headers carries important information about e.g. routing or request and responses.● Similar in syntax and meaning to HTTP● The more important headers: ● To & From ● Via ● Contact ● Call-ID ● Route & Record-Route ● CSeq @borjessonjonas
  15. 15. Addressing● SIP-address is used to locate and communicate with other users. ● ●● Each user typically have an Address of Record (AOR) through which that user can be contacted. @borjessonjonas
  16. 16. What about the actual Audio● SIP doesnt care about audio● SIP only cares about managing sessions*● Allows any type of sessions to be established, such as an audio session.● SDP + RTP = gets audio going ● SDP – describes e.g. an audio session ● RTP carries the actual audio @borjessonjonas
  17. 17. Summary● SIP != VoIP● SIP can do VoIP but is so much more● SIP actually doesnt care about audio at all● SIP helps you route messages through the network.● SIP helps to locate your friends. @borjessonjonas
  18. 18. More presentations and material at Thanks! @borjessonjonas