The SIP device would then have to determine how to process the NOTIFY
and what action should be taken based on its own capabilities and applications.
Options The OPTIONS method is used to query application servers about their capabilities.
This allows the network to ensure that a particular server is able to support
requested services prior to establishing a session with that server.
It can also be used to query another user device about its capabilities prior to sending
an INVITE to the device. This way the requesting device can determine if the endpoint
can even support the type of session it is about to establish without sending the actual
request.
The response to the OPTION method contains the ACCEPT header, containing the
methods and capabilities supported by the responding entity. For example, if the device
supports the SDP application, then the response would contain the ACCEPT header
with application/SDP as its parameter.
Register The REGISTER method is usually the first method to be initiated by a device
right after the device is turned on. The purpose of the REGISTER is to notify the
network of the device??™s location (IP address). This is so the network knows how to route
messages to the device, so callers can find the subscriber wherever they are located.
This is one of the unique features of an IP network: mobility. Certainly this functionality
can be provided through the Internet in general, but simply providing a location
for a subscriber is not a secure means of providing a service.
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