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SIP – Session Initiation Protocol

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SIP protocol structure through an example

This example illustrates, as a slide show, the structure of the SIP protocol, as outlined in RFC 3261 – chapter 5:
Legend for SIP Transport LayerThe lowest layer is the Transport layer. It defines how a Client sends requests and receives responses and how a Server receives requests and sends responses over the network. All SIP elements contain a transport layer.
Legend for SIP Transaction LayerThe second layer is the Transaction layer. A Transaction is a request sent by a Client transaction (using the transport layer) to a Server transaction, along with all responses to that request sent from the server transaction back to the client. Any task that a user agent client (UAC) accomplishes takes place using a series of transactions. Stateless proxies do not contain a transaction layer.
Legend for SIP Transaction-User LayerThe layer above the transaction layer is called the Transaction User (TU). Each of the SIP entities, except the stateless proxy, is a transaction user.
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Sequence chart for SIP protocol structure through an example

 Each number-box gives access to the detailed slideHereTopNext
SIP Session Set-Up Sequence ChartSIP protocol structure through an example: 01. SIP INVITE request from Alice's UA to Proxy1SIP protocol structure through an example: 02. 407 Proxy Authentication Required, from Proxy1 to UACSIP protocol structure through an example: 03. ACK from UAC to Proxy1SIP protocol structure through an example: 04a. new INVITE with credentials, from UAC to Proxy1SIP protocol structure through an example: 05a. 100 Trying, from Proxy1 to UACSIP protocol structure through an example: 04b. INVITE from Proxy1 to Proxy2SIP protocol structure through an example: 04c. INVITE from Proxy2 to Proxy3SIP protocol structure through an example: 05b. 100 Trying, from Proxy3 to Proxy2SIP protocol structure through an example: 04d. INVITE from Proxy3 to UASSIP protocol structure through an example: 06a. 180 Ringing, from UAS to Proxy3SIP protocol structure through an example: 06b. 180 Ringing, from Proxy3 to Proxy2SIP protocol structure through an example: 06c. 180 Ringing, from Proxy2 to Proxy1SIP protocol structure through an example: 06c. 180 Ringing, from Proxy1 to UACSIP protocol structure through an example: 07a. 200 OK, from UAS to Proxy3SIP protocol structure through an example: 07b. 200 OK, from Proxy3 to Proxy2SIP protocol structure through an example: 07c. 200 OK, from Proxy2 to Proxy1SIP protocol structure through an example: 07d. 200 OK, from Proxy1 to UACSIP protocol structure through an example: 08a. ACK from UAC to Proxy1SIP protocol structure through an example: 08b. ACK from Proxy1 to Proxy3SIP protocol structure through an example: 08c. ACK from Proxy3 to UASSIP protocol structure through an example: 09. Media Session between Alice-UA and Bob-UASIP protocol structure through an example: 10a. BYE, from UAS to Proxy3SIP protocol structure through an example: 10b. BYE, from Proxy3 to Proxy1SIP protocol structure through an example: 10c. BYE, from Proxy1 to UACSIP protocol structure through an example: 11a. 200 OK, from UAC to Proxy1SIP protocol structure through an example: 11b. 200 OK, from Proxy1 to Proxy3SIP protocol structure through an example: 11c. 200 OK, from Proxy3 to UAS
In this example, the rejection of the first INVITE request, followed by a valid INVITE request, enables the analysis of the processing of the ACK for these two situations. It is assumed that both Proxy 1 and Proxy 3 stateful proxy servers are in the final signalling path because they requested it in the INVITE requests they routed on.