[OpenSIPS-Users] Source port changing during a transaction

Bogdan-Andrei Iancu bogdan at opensips.org
Tue Jun 26 10:20:50 EDT 2018

Hi John,

According to RFC3261, a re-INVITE or UPDATE can change only the remote 
contacts (the end point contacts), but not the Record-Route hops.

In order to properly reflect the change of the port at network level, of 
course, the SIP contact URI has to be changes (via re-INVITE or UPDATE).

You say that OpenSIPS with dialog + TH does not obey this ? (it might be 
true :D)

Best regards,

Bogdan-Andrei Iancu

OpenSIPS Founder and Developer
OpenSIPS Summit 2018

On 06/25/2018 08:08 PM, John Quick wrote:
> 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
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