Sip Messages Structure

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)

Each line must end with a carriage return-line feed (CRLF).
Requests

SIP uses six types (methods) of 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.

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. If user A no longer wants to establish this conference, it sends a BYE message instead of an ACK message.

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.

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