What are SIP methods - requests and responses?

SIP uses Methods / Requests and corresponding Responses to communicate and establish a call session.

SIP Methods Diagram

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

Further reading