Avaya 9608 Administration page 159

The only exception to the above rule is if the deskphone receives a 301 response to a
REGISTER request which contains a list of controllers. In this case, the deskphone discards the
aggregated controller list and use ONLY the controllers received in the 301 response.
2. Determine which monitored controllers are available
Using the Monitored Controllers list, the deskphone performs DNS queries to resolve
hostnames and the signaling protocol (TLS, TCP, UDP in that order when no DNS NAPTR or
SIP URI parameter is located). To determine which of the Monitored Controllers is actually
available to provide service, the deskphone performs a maintenance activity for each Monitored
Controller. The deskphone starts the controller search timer and sends a SIP REGISTER
(adding bindings) message to each controller, which may necessitate establishing a TLS or
TCP connection to the controller.
The controller is considered available once a 200 OK response is received in response to the
REGISTER request. If all the Monitored Controllers are available before the end of the
CONTROLLER_SEARCH_INTERVAL, the deskphone continues with selecting the Active
Controller. If at least one Monitored Controller is available at the end of the
CONTROLLER_SEARCH_INTERVAL, the deskphone continues determining which controllers
are available.
If a failure response to the REGISTER request is received, the controller is considered
unavailable and depending on the failure code, either retries the query, provides the requested
credentials, abandons the query, or stops monitoring this specific controller entirely.
If no response to the REGISTER request is received within the timeout period, the deskphone
retries the monitoring attempt using the RECOVERYREGISTERWAIT parameter value as a
guideline.
3. Select the active controller
If the value of the SIPREGPROXYPOLICY parameter is "alternate" and a user is logged in, the
deskphone must attempt and maintain a single active SIP registration with the highest priority
available controller; the number of available controllers that the deskphone simultaneously
registers with is the value of the SIMULTANEOUS_REGISTRATIONS parameter. Any additional
controllers are treated as alternate registrations. The deskphone attempts to register using the
username and password provided during the login process. It also uses the SIPDOMAIN
parameter. The deskphone uses a SIP URI unless SRTP is enabled where a SIPS URI is used.
When registration is successful, the deskphone sets the SIPPROXYSRVR_IN_USE parameter
to the IP address of this (Active) controller. The deskphone also performs the other registration
tasks.
If the value of the SIPREGPROXYPOLICY parameter is "simultaneous" and a user is logged-in,
the deskphone attempts and maintains active SIP registrations with all available controllers.
If the value of the FAILBACK_POLICY parameter is "automatic", the deskphone's active
controller will always be the highest priority available controller. If the value of the
FAILBACK_POLICY parameter is "admin", then a controller lower down the priority list may be
active.
Controller Determination and Survivability Activity
Issue 3 January 2014 155