Session Initiation Protocol - SIP Messages

SIP Messages

SIP is a text-based protocol with syntax similar to that of HTTP. There are two different types of SIP messages: requests and responses. The first line of a request has a method, defining the nature of the request, and a Request-URI, indicating where the request should be sent. The first line of a response has a response code.

For SIP requests, RFC 3261 defines the following methods:

• REGISTER: Used by a UA to indicate its current IP address and the URLs for which it would like to receive calls.
• INVITE: Used to establish a media session between user agents.
• ACK: Confirms reliable message exchanges.
• CANCEL: Terminates a pending request.
• BYE: Terminates a session between two users in a conference.
• OPTIONS: Requests information about the capabilities of a caller, without setting up a call.

A new method has been introduced in SIP in RFC 3262:

• PRACK (Provisional Response Acknowledgement): PRACK improves network reliability by adding an acknowledgement system to the provisional Responses (1xx). PRACK is sent in response to provisional response (1xx).

The SIP response types defined in RFC 3261 fall in one of the following categories:

• Provisional (1xx): Request received and being processed.
• Success (2xx): The action was successfully received, understood, and accepted.
• Redirection (3xx): Further action needs to be taken (typically by sender) to complete the request.
• Client Error (4xx): The request contains bad syntax or cannot be fulfilled at the server.
• Server Error (5xx): The server failed to fulfill an apparently valid request.
• Global Failure (6xx): The request cannot be fulfilled at any server.