There is a test scenario in ETSI (ETSI TS 102 027-2 V4.1.1 (2006-07)) as
Status: PICS: A.77/3.2
Ref: RFC 3261  annex A and section 188.8.131.52.
Purpose: Ensure that the IUT, when a BYE client transaction is in the
Proceeding state, repeats its BYE request after timer E set in Trying state to
T1 value expires.
I tried this scenario in the following steps (Using version 1.2.148):
1. Make a call from caller to callee via sip proxy.
2. Caller sends BYE to proxy, and proxy forwards it to the callee.
3. Callee responses 100 Trying to proxy, and sends no final response. At this
moment, sip proxy's BYE client transaction goes to the Proceeding state.
Sip proxy should retransmit BYE request in 500ms (T1) for the first time, then
retransmit it for the second time in 4s(T2).
Sip proxy retransmits BYE request in 4s (T2) for the first time.
When proxy sends the first BYE request, the client transaction is in the Trying
state, Timer E is set to T1. When 100 Trying response comes, the client
transaction goes to the Proceeding state, the Timer E will be reset to T2.
According the RFC3261, the Timer E should be reset to T2 when Timer E times
Reference: RFC3261 section 184.108.40.206
"If Timer E fires while in the "Proceeding" state, the request MUST be passed
to the transport layer for retransmission, and Timer E MUST be reset with a
value of T2 seconds."