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W O R K S H O P

It's Time To Take a Look At SIP

April 17, 2003
By Peter Morrissey

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Anatomy of SIP

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	In this article
	Introduction
	At Your Service
	Anatomy of SIP

The Header

• To: Contains a display name and URI (Uniform Resource Identifier) of the destination UA (user agent) or endpoint.

• From: Contains a display name and URI of the UA that's sending the request.

• Contact: Information needed so the destination UA can contact the requesting UA once a call is set up.

• Via: Contains the version of SIP and the transport protocol to be used. Also includes an IP address to ensure the sender receives responses to its requests. Each proxy server in a call puts its IP address in the via field so responses get routed through it.

Commands

• INVITE: Main command used by SIP to make a call to another endpoint.

• REGISTER: Used by a SIP endpoint to inform a register server of its current location.

Servers

• Proxy Server: Makes requests and sets up connections on behalf of a UA.

• Redirect Server: Provides alternative location information to a UA in response to a request, but doesn't participate in the connection setup.

• Registration Server: Used by a SIP device, such as a phone, to register its current location.

• Location Server: Stores location information in a database. Usually runs on the same physical server as the registration server.