Sip Components - Yealink CP969 Administrator's Manual

SIP establishes a session between endpoints. SIP also supports mid-call changes, such as
the addition of another endpoint to the conference or the change of a media characteristic
or codec.
Handle the transfer and termination of calls -- SIP supports the transfer of calls from one
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endpoint to another. During a call transfer, SIP simply establishes a session between the
transferee and a new endpoint (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 Components

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 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 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. It may be
preferential to use this method when not using an application layer firewall. Application layer
firewalls like to know what applications are flowing through which ports and it is possible to use
content types of other applications other than the one you are trying to let through what has
been denied.
User Agent Server (UAS)
UAS is a server that hosts the application responsible for receiving the SIP requests from a UAC,
and on reception it returns a response to the request back to the UAC. The UAS may issue
multiple responses to the UAC, not necessarily a single response. Communication between UAC
and UAS is client/server and peer-to–peer.
Typically, a SIP endpoint is capable of functioning as both a UAC and a UAS, but it functions only
as one or the other per transaction. Whether the endpoint functions as a UAC or a UAS depends
on the UA that initiates the request.
Introduction
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