SIP-TIMERS “Timing is everything”
There are several aspects of SIP that ensure that messages are delivered on time. Three basic SIP timer parameters – T1, Timer B, and Timer F. T1 is the estimated round trip time of an IP packet. By default, T1 is set to 500 milliseconds which for many networks can be a little on the high side. However, the SIP standard allows you to ignore the default and set it to something that better matches your network characteristics. The second SIP timer parameter is Timer B. Timer B is the maximum amount of time that a sender will wait for an INVITE message to be acknowledged — i.e. a SIP response message is received. Timer F is the maximum amount of time that a sender will wait for a non-INVITE message to be acknowledged. SIP messages such as REFER, INFO, MESSAGE, BYE, and CANCEL fall into this category. The way that Timer B and Timer F function is pretty straightforward. Both timers default to 64 times T1. If you take the T1 default of 500ms, Timer B and Time