[OpenSIPS-Users] Source port changing during a transaction

[John Quick]

Is it permissible according to the relevant RFC's for the address and/or
port where responses are sent to change during a transaction?
[I assume it must be okay within a dialogue, such as if a re-INVITE is
sent?].

This is why I am asking:
I have been checking through a pcap capture for a UAC device that is sending
INVITE requests to OpenSIPS.
The topmost Via has the rport parameter present - this tells OpenSIPS to
respond to the source port of the request.
There are some configuration issues which mean the response fails to reach
the UAC.
So the UAC sends the same INVITE request again, but this time it sends from
a different port.
OpenSIPS continues to send responses to the source port of the first INVITE
and ignores the source port of the second INVITE.

UAC port 59500  ----  INVITE ------->   OpenSIPS Proxy
UAC port 59500  <--- 100 Trying ---   OpenSIPS Proxy
UAC port 5062    ----  INVITE ------->   OpenSIPS Proxy
UAC port 59500  <--- 100 Trying ---   OpenSIPS Proxy

The CSeq and Call-ID on both INVITE requests are identical - only the source
port changed. Is this why the second INVITE has no impact on where the
responses are sent?
Or is there something special about the first request in a transaction that
fixes the destination address/port for all subsequent responses?
The OpenSIPS Proxy in my pcap example uses topology hiding which could, I
suppose, be relevant. It is OpenSIPS v 1.9.

Thanks.

John Quick
Smartvox Limited
Web: www.smartvox.co.uk