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Because SIP is a text-based
protocol like HTTP, it is used to send information between clients and servers,
and User Agent clients and User Agent servers, as a series of requests and
responses. When requests are made, there are a number of possible signaling
commands that might be used:
REGISTER Used when a user agent
first goes online and registers their SIP address and IP address with a
Registrar server.
■ INVITE Used to invite another
User agent to communicate, and then establish a
SIP session between them.
■ ACK Used to accept a session
and confirm reliable message exchanges.
■OPTIONS Used to obtain information
on the capabilities of another user agent,
so that a session can be
established between them. When this information is provided a session isn’t
automatically created as a result.
■ SUBSCRIBE Used to request
updated presence information on another user agent’s status.This is used to
acquire updated information on whether a User agent
is online, busy, offline, and so
on.
■NOTIFY Used to send updated
information on a User agent’s current status.This
sends presence information on
whether a User agent is online, busy, offline, and so
on.
■ CANCEL Used to cancel a pending
request without terminating the session.
■ BYE Used to terminate the
session. Either the user agent who initiated the session, or the one being
called can use the BYE command at any time to terminate the session.
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