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: 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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