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When two devices communicate with one another, they exchange a set of messages (referred to as transactions). For example, a cell phone originating a call to another cell phone would send an INVITE to the other phone requesting a connection be made. The recipient of the INVITE will determine whether to accept or reject the transaction. If the cell phone chooses to accept the invitation to a session, then it will send a response to the request and exchange other messages, entering into a dialog with the other device. The dialog then becomes a logical connection between the communicating entities for the purpose of exchanging SIP messages regarding a session. Each user agent establishes its own dialog with the user agent client (the requestor). The dialog is then used by the user agent to maintain the status of the dialog and associated sessions. The session is not the same as a dialog, since a session can involve multiple user agents communicating with one user agent client. In other words, the dialog is established between each entity involved in the same session as depicted in Figure 2-1. The illustration shows the user agent client with a dialog established with multiple user agent servers, exchanging session control information for the same session (as would be the case in a conference call). As can be seen in the illustration, the user agent client can now delineate transactions between each of the entities participating in the same session. This allows the UAC to correlate responses from each individual UAS and treat each one independently even though they are all participating in the same session. Each dialog requires a dialog ID, which is derived from the SIP headers. When the UAC sends a request for a session, it will expect a response. In that response will be required headers (as we will discuss in a moment). The response must contain the TO, FROM, and CALL-ID headers. The TO and the FROM headers will include the TAG parameter. The TAG parameter is used by each user agent for correlating requests with responses, but it is also used for calculating the dialog ID.
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User agent server (UAS)
User agent client (UAC)
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User agent server (UAS)
User agent server (UAS) Figure 2.1 The difference between a dialog and a session
The dialog ID then becomes unique for the UAC and each of the UASs. In other words, the UAC creates its own dialog ID, while each of the UASs will create its own unique dialog ID. This is not communicated to other entities, since the use of the dialog ID is local (used by the user agent). Note that a dialog ID cannot be determined until the UAC receives a 2xx response back from each of the UASs. This is because the TO header does not actually contain the TAG parameter until the response is generated by the UAS. Remember that the TAG is used by the UAS for correlation of responses (more on that when we discuss the various message headers and their parameters). So in summary, a UAC will generate a SIP message and send it to one or more UASs. Each UAS will then answer the UAC by sending a response message. The response message contains at a minimum the TO, FROM, and CALL-ID headers, which are used in creating the dialog ID. Once all of these are done, the dialog is considered established, as depicted in the call flow shown in Figure 2-2. Figure 2-2 also shows a handshake. Once the dialog has been established and the acknowledgment is received, the handshake sequence between two entities has been completed. This marks the beginning of a session where bearer traffic is exchanged. A session cannot begin without completion of a handshake between the two entities. Think of the handshake as an agreement between all involved user agents to engage in the transfer of bearer traffic. To establish a session, a sequence of messages must be exchanged between all involved parties. This sequence of messages consists of an offer and an answer. The offer is established by the user agent client wishing to begin a session. It must contain all of the data necessary to establish the session and transfer bearer traffic.
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