When a large number of clients register with a SIP registrar server
at approximately the same time, the server may become overloaded.
Near-simultaneous floods of SIP SUBSCRIBE and PUBLISH requests may
have similar effects. Such request avalanches can occur, for
example, after a power failure and recovery in a metropolitan area.
This document describes how to avoid such overload situations. Under
this mechanism, a server estimates an avalanche restart backoff
interval during its normal operation and conveys this interval to its
clients through a new Restart-Timer header in normal response
messages. Once an avalanche restart actually occurs, the clients
perform backoff based on the previously received Restart-Timer header
value before sending out the first request attempt. Thus, the
mechanism spreads all the initial client requests and prevents them
from overloading the server.