RFC 3261, section 6
A SIP transaction occurs between a client and a server and comprises all messages from the first request sent from the client to the server up to a final (non-1xx) response sent from the server to the client. If the request is and the final response is a non-2xx, the transaction also includes an to the response. The ACK for a 2xx response to an INVITE request is a separate transaction.
Further down the road, the spec says that this ACK for a non-2xx response bypasses the transaction layer and thus is not a transaction.
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