RFC 3261, "SIP: Session Initiation Protocol", June 2002

Note: This RFC has been updated by RFC 3265, RFC 3853, RFC 4320, RFC 4916, RFC 5393, RFC 5621, RFC 5626, RFC 5630, RFC 5922, RFC 5954, RFC 6026, RFC 6141, RFC 6665, RFC 6878, RFC 7462, RFC 7463, RFC 8217, RFC 8591, RFC 8760, RFC 8898, RFC 8996

Errata ID: 5653
Status: Reported
Type: Technical
Publication Format(s) : TEXT
Reported By: Vimal Chandra Tewari
Date Reported: 2019-03-12

Section 17.1.1.2 says:

Notes:

When a INVITE client transaction transitions to Proceeding state (upon receiving a provisional response), the request retransmission stops (for an unreliable transport) i.e. Timer A is stopped.
However in Proceeding state, Timer B, i.e. Transaction Timeout Timer can still fire if no final response is received in stipulated time in which case the TU should be informed and the transaction should transition to Terminated state.
"Figure 5: INVITE client transaction" in RFC 3261 does not show the Timer B expiry event in Proceeding state. This means that we are saying there is a guarantee that a final response will always be received in Proceeding state which may not always be the case.
In my opinion, the Proceeding state in Figure 5 should be updated to include a Timer B event.

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