The CANCEL method is used to terminate PENDING searches or call attempts. It can be generated by either user agents or proxy servers provided that a 1XX response containing a tag has been received, but no final response has been received. The branch ID for a CANCEL MATCHES the INVITE that it is canceling. A CANCEL only has meaning for an INVITE since only an INVITE may take several seconds (or minutes) to complete. All other SIP requests complete immediately. A user agent CONFIRMS the cancellation with a 200 OK response to the CANCEL and replies to the INVITE with a 487 Request Terminated response.

The CANCEL method is used to terminate pending searches or call attempts. It can be generated by either user agents or proxy servers provided that a 1xx response containing a tag has been received, but no final response has been received. The branch ID for a CANCEL matches the INVITE that it is canceling. A CANCEL only has meaning for an INVITE since only an INVITE may take several seconds (or minutes) to complete. All other SIP requests complete immediately. A user agent confirms the cancellation with a 200 OK response to the CANCEL and replies to the INVITE with a 487 Request Terminated response.