Avaya 9608 Administrator's Manual page 148

System Failover and Survivability
When registration is successful, the phone sets the SIPPROXYSRVR_IN_USE parameter to
the IP address of this (Active) Controller. The phone also performs the other registration tasks.
If the value of the SIPREGPROXYPOLICY parameter is "simultaneous" and a user is logged-in,
the phone attempts and maintains active SIP registrations with all Available Controller(s).
If the value of the FAILBACK_POLICY parameter is "automatic", the phone'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.
The phone initiates a search for a new Active Controller whenever one of the following triggers
is encountered:
Fast Response Timer Expiry,
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TCP keep-alive failure (or other socket error),
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The phone receives an administrative failback trigger,
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An incoming INVITE is received from a non-Active controller,
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A re-registration with the Active Controller times out, or
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Whenever one of these triggers is encountered and a user is logged in, the deskphone initiates
parallel REGISTER transactions with every controller in its configured list, including the
currently active controller.
Simultaneous Registration (R6.0 and later SIP software Releases)
For SIP software Release 6.0+, deskphone behavior for simultaneous registration is based on
one of four triggers:
Trigger 1: The TCP socket closes.
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Trigger 2: A TCP Keep-alive timeout occurs.
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Trigger 3: The deskphone receives an administrative failback trigger from a Configured
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Controller.
Trigger 4: "Fast Response Timer"
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Simultaneous registration functions in response to a trigger are:
1. Controller search (maintenance check) - The deskphone tries to establish a connection (if
needed) and then register (or refresh registration) with each of the controllers. If it gets a
successful response to the REGISTER, it marks the controller as "available." If the
deskphone cannot establish the connection or if it does not receive a successful response,
it marks the controller as "unavailable".
2. Controller Subscription Refresh - The deskphone sends a refresh SUBSCRIBE to the
current controller for all the subscriptions that it has. If it gets any failure response other than
"489 Bad Event" for any of the refresh SUBSCRIBE messages, it removes that subscription
and re-establishes a subscription for that event package.
3. Controller Failover - The deskphone removes all the existing subscriptions and establishes
subscriptions with the highest priority controller that is available.
148 Deskphone SIP 9608, 9611G, 9621G, 9641G Administrator Guide Release 6.0.1