SIP Messages and Methods Overview

This chapter provides a brief overview of SIP messages and methods. This chapter contains information about the following:

SIP is a new protocol developed by the Internet Engineering Task Force (IETF) Multiparty Multimedia Session Control (MMUSIC) Working Group as an alternative to the ITU-T H.323 specification. SIP is defined by RFC 2543 and is used for multimedia call session setup and control over IP networks.

Format

All SIP messages are either requests from a server or client or responses to a request. The messages are formatted according to RFC 822, "Standard for the format of ARPA internet text messages." For all messages, the general format is:

A start line
One or more header fields
An empty line
A message body (optional)

Requests

INVITE—Indicates a user or service is being invited to participate in a call session.
ACK—Confirms that the client has received a final response to an INVITE request.
BYE—Terminates a call and can be sent by either the caller or the callee.
CANCEL—Cancels any pending searches but does not terminate a call that has already been accepted.
OPTIONS—Queries the capabilities of servers.
REGISTER—Registers the address listed in the To header field with a SIP server.
PRACK—Insures reliability of provisional 1xx responses if a UAS supports reliability of provisional responses.
UPDATE—Updates offer for not-yet-established sessions. It can also be used to update session parameters before a call is active.
REFER—The mechanism to initiate a session transfer. It indicates the recipient (identified by the Request-URI) should contact a third party using the contact information provided in the request.
SUBSCRIBE—Subscribes to be notified of an event occurrence.
NOTIFY—Notifies that an event has occurred.
MESSAGE—The means of carrying data for instant messages. The gateway does not support this method.

Responses

The following types of responses are used by SIP and generated by the Cisco SIP Proxy Server:

SIP 1xx—Informational Responses
SIP 2xx—Successful Responses
SIP 3xx—Redirection Responses
SIP 4xx—Client Failure Responses
SIP 5xx—Server Failure Responses
SIP 6xx—Global Failure Responses

The Registration Process

A registration occurs when a client needs to inform a proxy or redirect server of its location. During this process, the client sends a REGISTER request to the proxy or redirect server and includes the address (or addresses) at which it can be reached.

The Invitation Process

An invitation occurs when one SIP end point (user A) "invites" another SIP endpoint (user B) to join in a call. During this process, user A sends an INVITE message requesting that user B join a particular conference or establish a two-party conversation. If user B wants to join the call, it sends an affirmative response (SIP 2xx). Otherwise, it sends a failure response (SIP 4xx). Upon receiving the response, user A acknowledges the response with an ACK message. User A always send an ACK message to 200 or 4xx. If user A wants to disconnect the call, it sends the BYE message.

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