SIP Transactions and Dialogs

A transaction is a SIP message exchange between two user agents (UA) that starts with a request and ends with its final response (it can also contain zero or more provisional responses in between). For example, during the termination of a SIP session, one user releases the call by sending a BYE request, and the other party replies with a 200 OK response. This message exchange is called a transaction. Transactions are actually part of a dialog.

A dialog is a complete exchange of SIP messages between two user agents. A dialog is a peer-to-peer SIP relationship between two UAs that persists for some time. A dialog is established by SIP messages, such as a 2xx response to an INVITE request. A dialog is identified by a call identifier, a local tag, and a remote tag.

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