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

Errata ID: 1470
Status: Verified
Type: Technical
Publication Format(s) : TEXT
Reported By: Iñaki Baz Castillo
Date Reported: 2008-07-14
Verifier Name: Robert Sparks
Date Verified: 2010-04-03

Section 17.2.2 says:

If the TU passes a final response (status codes 200-699) to the 
server while in the "Proceeding" state, the transaction MUST enter 
the "Completed" state...

It should say:

If the TU passes a final response (status codes 200-699) to the 
server while in the "Trying" or "Proceeding" state, the transaction 
MUST enter the "Completed" state...

Notes:

"17.2.2 Non-INVITE Server Transaction" doesn't consider the case in which the transaction state is "Trying" and the transaction receives from the TU a final response.
It's totally possible that TU sends a final response without sending before a provisional response. Note that this case is perfectly valid in the diagram of page 139.

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