Administrator Guides EN Owner's manual

Abstract (from RFC3261):
13-3 41-001391-00 Rev 03 – 04.2012
Outbound Proxy:
A proxy that receives requests from a client, even though it may not be the server resolved by the Request-URI. Typ-
ically, a UA is manually configured with an outbound proxy, or can learn about one through auto-configuration
protocols.
Proxy, Proxy Server:
An intermediary entity that acts as both a server and a client for the purpose of making requests on behalf of other
clients. A proxy server primarily plays the role of routing, which means its job is to ensure that a request is sent to
another entity "closer" to the targeted user. Proxies are also useful for enforcing policy (for example, making sure a
user is allowed to make a call). A proxy interprets, and, if necessary, rewrites specific parts of a request message
before forwarding it.
Redirect Server:
A redirect server is a user agent server that generates 3xx responses to requests it receives, directing the client to
contact an alternate set of URIs.
Registrar:
A registrar is a server that accepts REGISTER requests and places the information it receives in those requests into
the location service for the domain it handles.
Server:
A server is a network element that receives requests in order to service them and sends back responses to those
requests. Examples of servers are proxies, user agent servers, redirect servers, and registrars.
User Agent Client (UAC):
A user agent client is a logical entity that creates a new request, and then uses the client transaction state machin-
ery to send it. The role of UAC lasts only for the duration of that transaction. In other words, if a piece of software ini-
tiates a request, it acts as a UAC for the duration of that transaction. If it receives a request later, it assumes the role
of a user agent server for the processing of that transaction.
User Agent Server (UAS):
A user agent server is a logical entity that generates a response to a SIP request. The response accepts, rejects, or
redirects the request. This role lasts only for the duration of that transaction. In other words, if a piece of software
responds to a request, it acts as a UAS for the duration of that transaction. If it generates a request later, it assumes
the role of a user agent client for the processing of that transaction.
User Agent (UA):
A logical entity that can act as both a user agent client and user agent server.
The role of UAC and UAS as well as proxy and redirect servers are defined on a transaction-by-transaction basis. For
example, the user agent initiating a call acts as a UAC when sending the initial INVITE request and as a UAS when
receiving a BYE request from the callee. Similarly, the same software can act as a proxy server for one request and as
a redirect server for the next request. Proxy, location, and registrar servers defined above are logical entities; imple-
mentations MAY combine them into a single application.