3.2.2 Timers

Retransmission Timer: This timer is initialized or reset whenever a Callback Request message is sent to the caller. This is a periodic timer with the default value of a 2-second interval; that is, unless the timer is stopped, it will expire periodically as specified by the timer interval. This timer value SHOULD be chosen appropriately, taking the link speed into consideration as well <6>. This timer has to be stopped when the Callback Response message is received by the answerer.