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

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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