User Agent Client (Uac) - Yealink SIP-T2xP Administration Manual

Administrator's Guide for SIP-T2xP IP Phones
SIP provides the capabilities to:
Determine the location of the target end point -- SIP supports address resolution,

name mapping, and call redirection.
Determine the media capabilities of the target end point -- Via Session Description

Protocol (SDP), SIP determines the ―lowest level‖ of common services between the
end points. Conferences are established using only the media capabilities that can
be supported by all end points.
Determine the availability of the target end point -- A call cannot be completed

because the target end point is unavailable, SIP determines whether the called
party is already on the phone or did not answer in the allotted number of rings. It
then returns a message indicating why the target end point was unavailable.
Establish a session between the origin and target end point -- The call can be

completed, SIP establishes a session between the end points. SIP also supports
mid-call changes, such as the addition of another end point to the conference or
the changing of a media characteristic or codec.
Handle the transfer and termination of calls -- SIP supports the transfer of calls from

one end point to another. During a call transfer, SIP simply establishes a session
between the transferee and a new end point (specified by the transferring party)
and terminates the session between the transferee and the transferring party. At
the end of a call, SIP terminates the sessions between all parties.
SIP is a peer-to-peer protocol. The peers in a session are called User Agents (UAs). A
user agent can function as one of the following roles:
User Agent Client (UAC) -- A client application that initiates the SIP request.

User Agent Server (UAS) -- A server application that contacts the user when a SIP

request is received and that returns a response on behalf of the user.

User Agent Client (UAC)

The UAC is an application that initiates up to six feasible SIP requests to the UAS. The six
requests issued by the UAC are: INVITE, ACK, OPTIONS, BYE, CANCEL and REGISTER.
When the SIP session is being initiated by the UAC SIP component, the UAC determines
the information essential for the request, which is the protocol, the port and the IP
address of the UAS to which the request is being sent. This information can be dynamic
and this will make it challenging to put through a firewall. For this reason it may be
recommended to open the specific application type on the firewall. The UAC is also
capable of using the information in the request URI to establish the course of the SIP
request to its destination, as the request URI always specifies the host which is essential.
The port and protocol are not always specified by the request URI. Thus if the request
does not specify a port or protocol, a default port or protocol is contacted. Using this
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