FAQs

SIP Methods / Requests and Responses

SIP uses methods / requests and corresponding responses to communicate and establish a call session.

SIP Requests

There are fourteen SIP request methods of which the first six are the most basic request / method types:

• INVITE = Establishes a session.
• ACK = Confirms an INVITE request.
• BYE = Ends a session.
• CANCEL = Cancels establishing of a session.
• REGISTER = Communicates user location (host name, IP).
• OPTIONS = Communicates information about the capabilities of the calling and receiving SIP phones.
• PRACK = Provisional acknowledgement.
• SUBSCRIBE = Subscribes for notification from the notifier.
• NOTIFY = Notifies the subscriber of a new event.
• PUBLISH = Publishes an event to the server.
• INFO = Sends mid session information.
• REFER = Asks the recipient to issue call transfer.
• MESSAGE = Transports instant messages.
• UPDATE = Modifies the state of a session.

SIP Responses

SIP requests are answered with SIP responses, of which there are six classes:

1xx = Informational responses, such as 180 (ringing).
2xx = Success responses.
3xx = Redirection responses.
4XX = Request failures.
5xx = Server errors.
6xx = Global failures.