Terms - 3Com MSR 50 Series Configuration Manual

2368 C 137: SIP O
HAPTER VERVIEW

Terms

Multimedia session
According to RFC2327, "A multimedia session is a set of multimedia senders and
receivers and the data streams flowing from senders to receivers. A multimedia
conference is an example of a multimedia session."
A session is identified by a set of username, session ID, network type, address
type, and address elements in the origin field.
User agent
A user agent (UA), or a SIP endpoint, is a SIP-supported multimedia session
endpoint, which can be a phone, a gateway, or a router. Usually, a SIP-supported
router serves as a SIP UA.
There are two types of UAs: user agent clients (UAC) and user agent server (UASs).
Generally, a SIP endpoint can not only originate calls as a UAC but also receive
calls as a UAS.
A UAC is a device that creates a new session request. It can be a calling SIP
endpoint or a proxy server forwarding a request to a called endpoint for example.
A UAS is a device that generates response to a SIP request. It can be a called SIP
endpoint or a proxy server receiving a request from a calling endpoint for example.
Proxy server
A proxy server is a device that forwards session requests to a called UA on behalf
of a calling UA (a SIP endpoint) and responds to the calling UA on behalf of the
called UA.
When the proxy server receives a request from a calling UA, it first requests its
registrar for information on callee location and call policies of the calling UA and
the called UA. If the location information of the called UA is available and the
calling UA is allowed to make the call, the proxy server then forwards the request
to the called UA.
Most SIP sessions involve at least one proxy server.
Redirect server
A redirect server is a device that directs a calling UA to contact an alternate
location for calling the called UA.
When receiving a request from a calling UA, the redirect server searches for the
location information of the called UA and returns information on a location. This
location can be that of the called UA or another proxy server, to which the UA can
initiate the session request again. The subsequent procedure is the same as that
for calling a called UA directly or for calling a proxy server.
Location server
A location server is a device that provides UA information to proxy and redirect
servers; it retains UA information received by a registrar. Normally, location server
and registrar are co-located.