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Building Telephony Systems with OpenSER by Flavio E. Goncalves

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Routing Basics

It is not easy to figure out how to route SIP packets. We will present in this section some of the basic concepts to route SIP packets over the proxy server. The first important concept is the one involving transactions and dialogs.

Transactions and Dialogs

A transaction starts with a request and finishes, usually, with a response code. The branch parameter in the VIA header field identifies a transaction. A dialog may start with an INVITE transaction and finish with a BYE transaction. A dialog is identified by the combination of the FROM, TO, and CALL-ID header fields. Not all SIP methods start a dialog, the REGISTER and MESSAGE methods do not.

Initial and Sequential Requests

It is important to understand the difference between initial ...

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