Brocade Virtual ADX Server Load Balancing Guide (Supporting ADX v03.1.00) - Manual (Page 342)

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SIP overview
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The 200 OK message is routed back through the SIP proxy server to the User1's SIP phone, which
then stops the ringback tone and indicates that the call has been answered. Finally, User1's SIP
phone sends an acknowledgement (ACK) message to User2's SIP phone to confirm the reception of
the final response (200 OK). This ACK is sent directly from User1's SIP phone to User2's SIP phone,
bypassing the SIP proxy server. This occurs because the endpoints have now learned each other's
IP addresses from the Contact header fields through the INVITE/200 OK exchange, which was not
known when the initial INVITE was sent. This completes the INVITE/200/ACK three-way handshake
used to establish SIP sessions.
The media exchange between User1 and User2 now begins using the format that they have agreed
upon through SDP. In general, the end-to-end media packets take a different path from the SIP
signaling messages. At the end of the call, User2 disconnects (hangs up) the phone and generates
a BYE message. This BYE is routed directly to User1's SIP phone, again bypassing the SIP proxy.
User1 confirms the session termination with a 200 OK response.
SIP client registration
Registration is another common SIP operation. Registration is the means through which the SIP
domain's registrar server learns the current location of SIP clients (UACs). Upon initialization, and at
periodic intervals, the SIP clients send REGISTER messages to the domain's SIP registrar server.
The REGISTER messages associate an individual SIP URI (sip:[email protected]) with the IP
address of the machine which the user is currently logged on. The registrar server writes this
association to a database, called the location service, where it can be used by the SIP proxy server
of the domain. Often, a registrar server and the location service for a domain are co-located with
the proxy server for that domain.
SIP terminology
This section describes terms and concepts that you might find useful when configuring SIP SLB.
• Request-URI: Every SIP user has a URI. One SIP user calls another by setting the SIP URI of the
latter in the request message, also called the request-URI, which appears before all message
headers.
• UAC: A User Agent Client (UAC) is a logical entity that creates a new request. The role of UAC
lasts only for the duration of the transaction.
• UAS: A User Agent Server (UAS) is a logical entity that generates a response to a SIP request.
The response accepts, rejects, or redirects the request.
• Proxy server: A proxy server is 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 is primarily a router,
which means its job is to ensure that a request is sent to another entity nearer to the targeted
user. A proxy interprets and, if necessary, rewrites specific parts of a request message before
forwarding it.
• Redirect server: A redirect server is a UAS that generates 3xx responses to requests it receives,
directing the client to contact an alternate set of URIs.
• Registrar server: A registrar server accepts REGISTER requests and places the information it
receives in those requests into the location service for the domain it handles.