[OpenSIPS-Users] RTP proxy RE-INVITE with late SDP (ACK cannot have SDP body)

David Villasmil david.villasmil.work at gmail.com
Wed Sep 7 07:45:07 UTC 2022

Hello Callum,

Can you share how you are doing late negotiation?


David Villasmil
email: david.villasmil.work at gmail.com
phone: +34669448337

On Wed, May 15, 2019 at 6:37 PM Callum Guy <callum.guy at x-on.co.uk> wrote:

> Hi All,
> I am working on a problem where for a few destinations my OpenSIPs is
> receiving RE-INVITE messages with late SDP. This is causing a breakdown in
> the rtpproxy engagement and causing the audio to fail mid call.
> The OpenSIPs deployment is acting as a SIP proxy which traverses NAT and
> rtpproxy is used in bridging mode. I am using rtpproxy_engage to tie the
> integration to the dialog session and this is for all other purposes
> working as expected.
> My failure scenario is when the remote system sends a RE-INVITE message
> which includes no SDP. This passes through to my FreeSWITCH server which
> responds with a 200 including SDP. This message is processed fine and
> interacts with rtpproxy as expected and provides the remote with the
> correct public IP and port for RTP (the same as returned during call
> setup). In response the remote system returns an ACK with SDP which
> triggers an OpenSIPs error message (below) which results in the remotes
> public IP being passed through in SDP which causes the FreeSWITCH to start
> sending RTP direct resulting in one way audio as the media server is not
> publicly accessible.
> *ERROR:rtpproxy:engage_force_rtpproxy: not a late negotiation - ACK cannot
> have SDP body*
> As I understand it the FreeSWITCH behaviour is OK, although I am not clear
> why it feels the need to resend the SDP. All I want to happen in this
> scenario is for rtpproxy module to re-write the SDP in the way it has for
> all previous messages. I am very interested to hear if there is any reason
> for rtpproxy to disallow late negotiation in this scenario, if anyone can
> point to a relevant RFC that would be interesting!
> Is there any way around this other than some sort of manual SDP re-write
> (not helpful to me as I am using a pool of rtpproxy instances)? Might I
> have more luck with offer/answer or indeed rtpengine?
> I've illustrated the scenario better on the following link (sngrep paste):
> https://gist.githubusercontent.com/spacetourist/ef0478c0bf4e2d736f9b5663042087dd/raw/6f0a984a1a2838e7e2c4539f059fd68935a3b0b1/gistfile1.txt
> Thanks, looking forward to any advice!
> Best regards,
> Callum
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