ACK
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... SIP request was delivered to.
Because the ACK request for 2xx responses to INVITE constitutes a
different transaction ...
... to the same server that received the original request (indeed, if
that server did not record-route, it will not get the ACK).
We define TARGET ...
... retransmissions of the request
for the transaction, as well as ACK for a non-2xx final response, and
CANCEL requests for that transaction, MUST go to the same server.
...