Information Technology Reference
In-Depth Information
CANCEL:
Terminates a
pending
request (a request for which a final response has not yet been received).
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When a caller's UA (UAC) wants to hang up when there has not yet been a final response to an INVITE, the session will still
be pending, so a CANCEL is sent. In this case, the CANCEL constitutes a request to stop ringing.
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REGISTER:
This message is used to register contact information.
When a user wants to initiate a session with another user, SIP must locate the host at which the destination user is currently
reachable. The location of the host is often determined by a proxy or redirect server because these servers will frequently
receive requests that they are then responsible for forwarding to the destination user's host.
To locate the destination user host, the server consults a location service, the purpose of which is to provide address bindings.
These address bindings consist of URI to user agent at which a user is currently located.
Address bindings can be created using a process called
registration
. Registration entails sending REGISTER messages to an
element called a registrar server. As previously mentioned, a registrar server is a front end for the location service that both
reads and writes mappings based on REGISTER messages.
Cisco SIP phones send their MAC addresses and register their lines with the primary CUCM using REGISTER messages.
Cisco SIP phones also use REGISTER messages (with expires=0) as a keepalive with a secondary CUCM.
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OPTIONS:
A UA can query another UA or SIP server about its capabilities using an OPTIONS request. In this way, a client
can find out such capabilities as supported methods, content types, codecs, and so on before, for example, sending an INVITE
specifying required options.
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INFO:
This method is defined in RFC 2976 and can be used to carry session-related control information, such as ISUP or
ISDN signaling information.
There are six different ranges of response:
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1XX:
Responses in this range are provisional or informational.
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2XX:
These indicate success.
