In SIP , you will find 2 types of SIP Messages, there are:
- Request: A message sent by a client to a server that is used to invoke certains operations or functions.
- Response : A message sent by a server to a client that indicates the status of the request received from the client.
Here are now the request messages which invoke function on the server ( note that these functions are also called methods)
- Invite : When UAC want to initiate a session, it sends an Invite request to a server. When the request is received by the UAS, it process it and sends the appropriate response message.
- Ack : This message is sent in reply to a final response message from a server.
- Bye : Used to terminate a session
- Cancel : a Cancel request is used to terminate a pending request ( a request for which a final response has not yet been received)
- Register : This message is used to register contact information.
- Options : An UA can query another UA or SIP Proxy server about its capabilities using Options request. In this way, a client can find out capabilities such as supported methods, content types, codecs, and so on.
- Info : This is used to carry session-related control information such a ISUP, ISDN signaling information.
Now let’s see the 6 availables types of responses:
- 1XX : Responses in this range are provisional or informational
- 2XX : Responses indicate a success
- 3XX : Responses indicate a redirection
- 4XX : Responses indicate a client error
- 5XX : Responses indicate a server error
- 6xx : Responses indicate a global failure