SIP INVITE

> In what amount of time does "100 Trying message" have to be
> sent to asterisk?  I see asterisk retransmitting the INVITE
> message multiple times before receiving the "100 Trying message".

The INVITEs are retransmitted based on a timer T1, which starts at a default
of 500 ms and then exponentially backoffs and caps at 64*T1. The first
INVITE retransmission is supposed to happen in 500 ms. However, Asterisk has
a minor bug in this place. Asterisk sends the first INVITE retransmission
after 1 second instead of 500 ms.

This means Asterisk will wait for a second for a response such as 100 Trying
before it will start retransmitting the INVITE. Asterisk will retransmit the
INVITE after 1, 1, 2, 4, 8, 16, 32 seconds (ideally, this should be 500ms,
1, 2, 4, 8, 16, 32) from the start if it doesn't see a response.