Switching to VoIP

This is the Title of the Book, eMatter Edition

Copyright © 2007 O’Reilly & Associates, Inc. All rights reserved.

SIP

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In this example, though, the user, [e-mail], does exist and is able to respond

to the proxied INVITE method. His endpoint’s responses are proxied back to the

caller, and ultimately, the 200 OK response is sent, indicating the call is clear to

proceed. One of the most important pieces of this startup signaling process occurs

during the INVITE methods and 200 responses: SDP capabilities negotiation.

SIP redirect

When a SIP server responds to a calling endpoint’s INVITE method with a 3xx

response, that SIP server is redirecting the calling endpoint to a different SIP server.

The calling endpoint should then contact that server with an INVITE method for fur-

ther assistance in connecting the media stream. This feature is not implemented on

all systems that support SIP. In fact, where complex, signaling-neutral dial-plan pro-

gramming is available (like in Asterisk), SIP redirection isn’t always necessary. The

use of SIP redirects is more common in large, SIP-only networks with multiple serv-

ers, such as those that span the Internet.

Session Description Protocol

SDP is the de facto session capabilities protocol of SIP, similar to the H.245 protocol

in H.323. It is defined in RFC 2327. When a call is placed from one SIP endpoint to

another, an SDP capabilities construct is