As mentioned earlier, SIP phones can be connected together in pairs with an Ethernet cable, or they can be nodes on a LAN or a WAN that supports SIP. The SIP-based network can also work with translators to talk to VoIP phones and networks that work with other protocols such as the Media Gateway Control Protocol (MGCP) and H.323 (the International Telecommunications Union (ITU) VoIP standard). Finally, since most people and organizations are connected to the PSTN, VoIP networks normally contain gateways to translate the protocol messages into Integrated Services Digital Network (ISDN) or Signaling System 7 (SS7) signals for transmission to the phone company’s central office.
Many request and response messages are identified in the RFC. This section introduces you to some of the more common messages and shows how they are used by the network components.
The following are the most common request messages described in the standard and the only ones that are used in our example call flows:
Indicates that the user or service is being invited to participate in a session.
Confirms that the client has received a final response to an INVITE request.
Indicates that the user wishes to terminate the session.
Cancels a previous request. This can be confused with BYE; see "BYE and CANCEL: Similar but Not Interchangeable" later in this chapter.
Registers the address listed in the To header field with a SIP server. ...