Hello,

Hi Henning and Daniel,

Thank you very much for the replies. We moved the following configuration to reply_route{}, and now it inserts "a=sendonly" immediately before the "a=sendrecv" rather than appending it to the SDP as before. This configuration does use RTPengine as the call is from a WebRTC client. Have you any further idea why it's not actually replacing the "a=sendonly"?

                                replace_body_all( "a=sendrecv", "a=sendonly" );
                                msg_apply_changes();

Daniel, from reading the sdpops documentation it's not clear how to iterate through an SDP. Could you please give us a pointer on that?

the functions to remove/insert/append should show how to use the iterator, e.g.,:

- https://www.kamailio.org/docs/modules/stable/modules/sdpops.html#sdpops.f.sdp_iterator_rm

For example, to replace for all media sessions:

    sdp_iterator_start("s1");
    while(sdp_iterator_next("s1")) {
        if($sdpitval(s1) =~ "^a=sendrecv") {
            sdp_iterator_rm("s1");
            sdp_iterator_append("s1", "a=sendonly\r\n");
        }
    }
    sdp_iterator_end("s1");

Cheers,
Daniel

Thanks again.

On Thu, 18 Dec 2025 at 00:23, Daniel-Constantin Mierla <miconda@gmail.com> wrote:

Hello,

On 17.12.25 02:08, David Cunningham via sr-users wrote:

Hello,

We need to replace a=sendrecv with a=sendonly in the 200 OK reply to an INVITE. In theory it should be something straightforward, like in the onreply_route using:

replace_body_all( "a=sendrecv", "a=sendonly" );

or:

subst_body( '/^a=sendrecv/a=sendonly/' )

Our first problem is that these both are appending a=sendonly to the end of the SDP, not replacing values within it. Does anyone know why this might happen?

it could be because of conflicting operations with rtpengine (or rtpproxy). You could try the replacement in the core reply_route() and then use msg_apply_changes.

Our second problem is that we really only want to do this for video media, and not touch the audio media. Are there any text tools that allow you to work with specific media within the SDP?

The sdpops has an sdp iterator that could help: once you match the video stream, look for the sendrecv attrbute and replace the line.

Cheers,
Daniel

-- 
Daniel-Constantin Mierla (@ asipto.com)
twitter.com/miconda -- linkedin.com/in/miconda
Kamailio Consultancy, Training and Development Services -- asipto.com
Kamailio World Conference, May 7-8, 2026 - Berlin, Germany -- kamailioworld.com

--

David Cunningham, Voisonics Limited
http://voisonics.com/
USA: +1 213 221 1092
New Zealand: +64 (0)28 2558 3782

-- 
Daniel-Constantin Mierla (@ asipto.com)
twitter.com/miconda -- linkedin.com/in/miconda
Kamailio Consultancy, Training and Development Services -- asipto.com
Kamailio World Conference, May 7-8, 2026 - Berlin, Germany -- kamailioworld.com