Aboutsip - intro to transactions and dialogs
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The document introduces the Session Initiation Protocol (SIP), focusing on the concepts of transactions and dialogs. It details the request/response model, emphasizing the independent nature of transactions and the role of unique identifiers in managing them. Additionally, it explains that dialogs establish a peer-to-peer relationship between SIP endpoints, containing routing information and consisting of sequences of transactions.
Aboutsip - intro to transactions and dialogs
- 1. About SIP An introduction to Transactions & Dialogs Jonas Borjesson @borjessonjonas
- 2. NOTE ● This version has been adapted to be viewed without transitions. ● Go to aboutsip.com to download the original version. ● Also be sure to check out vimeo.com/aboutsip for any recorded presentations. ● Follow @borjessonjonas to receive updates. @borjessonjonas
- 3. re ca SIP Messages p ● SIP – Request/Response model ● UAC sends the request, UAS responds ● Two types of responses ● Provisional (1xx) ● Final (2xx - 6xx) @borjessonjonas
- 4. Transactions ● SIP is a transactional protocol. ● Every request & response goes within a transaction. ● Transactions are independent of each other. ● SIP transaction: ● 1 request INVITE ● 0..* provisional responses 100 Trying 180 Ringing ● 0..* final responses 200 OK @borjessonjonas
- 5. Client Transactions ● Responsible for: ● Receiving request from the TU it and... ● Deliver requests reliably to the server transaction ● Processing responses: – Filter out retransmissions – Filter out disallowed responses – Pass response to the TU ● Generating ACK for non 2xx final responses to INVITE transactions @borjessonjonas
- 6. Server Transactions ● Responsible for: ● Receive requests and pass them up the TU ● Filter out any retransmissions ● Accepts responses from the TU and sends them. ● Absorbing the ACK request for non 2xx final responses on invite transactions @borjessonjonas
- 7. Transaction Relationships request request request server transaction server transaction server transaction client transaction client transaction client transaction response response response UAC Outbound Inbound UAS Proxy Proxy @borjessonjonas
- 8. Transaction Identifier ● Each transaction is uniquely identified by: ● the branch-id on the Via-header plus ● the Cseq header ● 3261 branch-id starts with “z9hG4bK” INVITE sip:bob@aboutsip.com SIP/2.0 3 Via: SIP/2.0/UDP <ip_3>;branch=z9hG4bK-lkjsalkfjoijlkjlkj 2 Via: SIP/2.0/UDP <ip_2>;branch=z9hG4bK-8jijlk-asfk-iji0kj 1 Via: SIP/2.0/TCP <ip_1>;branch=z9hG4bK-llkowe-lkjko39d CSeq: 1 INVITE ... @borjessonjonas
- 9. Transaction Lifecycle ● Slightly different depending on: ● Server or client transaction ● Invite or non-invite transaction ● Reliable vs. non-reliable transport (e.g. udp vs tcp) ● To summarize them all* ● Starts when request is sent or received ● Final responses takes it to completed state ● Timer fires and moves it to terminated state. *You can't really group them together like this. Please view this information as a very generic summary of the life cycle of a SIP transaction. In reality, it is much more complicated. @borjessonjonas
- 10. Dialogs ? ! @borjessonjonas
- 11. Dialogs ● Dialogs are extremely important within SIP ● A dialog: ● represent a p2p relation between two SIP endpoints. ● exists for some time ● contains important routing information ● facilitates proper sequencing of messages ● consists of a sequence of transactions @borjessonjonas
- 12. Dialog establishing methods ● Not all methods establish a dialog! ● The ones that do are: ● INVITE (RFC 3261) – For establishing a dialog ● SUBSCRIBE (RFC 3265) – Creates a subscription. Used e.g. in presence scenarios ● REFER (RFC 3515) – Also a subscription but only for the refer event-package. Used e.g. for call transfer @borjessonjonas
- 13. INVITE Scenario INVITE sip:bob@aboutsip.com 100 Trying transaction 180 Ringing 200 OK ACK transaction dialog BYE transaction 200 OK @borjessonjonas
- 14. Dialog Id ● A dialog is uniquely identified by: ● The Call-ID header ● The remote-tag ● The local-tag ● My remote tag is your local tag and vice versa ● Therefore, the dialog id is different for both ends (a very common thing SIP stacks mess up) SIP/2.0 200 OK To: <sip:bob@aboutsip.com>;tag=e103a059-a9ca-4bc2-96d1-779636810bfe From: <sip:alice@aboutsip.com>;tag=f7389c89-ee9f-4802-af3e-636ce53883cb Call-ID: 9d8eccee-02f2-4952-a1bf-01fe1bae45d6 CSeq: 2 INVITE Contact: <sip:127.0.0.1:1557;transport=TCP> @borjessonjonas
- 15. Establishing a dialog ● UAC ● UAS ● establishes the dialog ● creates the initial through a 2xx final request. response ● Fills in “half” of the ● fills in the other “half” of dialog-id. the dialog-id. INVITE sip:bob@aboutsip.com From: alice@aboutsip.com ;tag=kjlkjoilkjlkjasdflkj To: sip:bob@aboutsip.com Call-ID: lkjasdlfkjasldkfjla 200 OK From: alice@aboutsip.com;tag=kjlkjoilkjlkjasdflkj To: sip:bob@aboutsip.com;tag=abckjo219898df Call-ID: lkjasdlfkjasldkfjla @borjessonjonas
- 16. Subsequent Requests ● Requests that goes within an established dialog are called subsequent requests ● Subsequent requests follow the established route INVITE BYE @borjessonjonas
- 17. To Complicate things... ● There is such a thing as early-dialogs. ● One INVITE request can be forked and create multiple dialogs. e bil mo @ bob E V IT IN INVITE bob INV ITE bob @w or k @borjessonjonas
- 18. Tearing down a dialog ● Depends on the method ● BYE request for INVITE dialogs ● Un-subscribe request for SUBSCRIBE dialogs ● REFER dialogs typically die when the reference to which the subscription is referring to goes away. ● Of course, lots of corner cases that can lead to the destruction of a dialog... @borjessonjonas
- 19. Summary @borjessonjonas
- 20. Transactions ● Consists of 1 request and 0..* responses ● Two types of transactions: ● Invite transactions ● Non-Invite transactions ● The branch-id on the Via uniquely identifies the transaction (plus the Cseq) @borjessonjonas
- 21. Dialogs ● Consists of a sequence of transactions ● Not all methods establish dialogs. ● Represent a p2p relation between two SIP endpoints. ● Contains routing information. ● The dialog-id is different for the two parties. @borjessonjonas
- 22. More presentations and material at aboutsip.com Thanks! @borjessonjonas