...
Customers may want to increase or decrease the SIP request timers depending on what they are seeking to accomplish. This document will explain how each of the SIP timers behave on the SIP page within the Dialogic® Media Gateways. Exercise caution when adjusting these timers. Undesired outcomes, ranging from lengthy SIP retransmits to an increase in traffic across the network, may result. The following timers are specified by the standard RFC3261.
T1 Time (ms)
The T1 timer, which is defined in milliseconds, specifies the amount of round trip time (RTT), that the client will attempt to send a SIP Request and expect a resonse. By default, the T1 timer is set to 500ms. T1 timeout is defined as: T1 (500ms)*64. So the amount of time the client would attempt to send the initial INVITE request would be 32 seconds. If this timer is decreased to 200ms, the T1 timeout would be decreased to approximately 12.8 seconds. Once this initial timeout occurs, without a received response, the timeout is doubled (2*T1). If no response is received before the new timeout, the request is retransmitted and doubled to (4*T1). Request transmit retries would look like this: 500ms, 1000ms, 2000ms, 4000ms, 8000ms, and so on, to a maximum amount of time defined by T2. The T1 values allowed by Dialogic® Media Gateways are from 100 – 60000 milliseconds.
...