New subject: [EXTERNAL] Per-branch SDP modification (strip video) in parallel forking

9 Apr 2026


      Hi All,
I am using Kamailio with rtpengine and have a parallel forking scenario (single INVITE -> multiple branches via append_branch()).
Incoming INVITE has SDP with video
Goal:
- For Main branch : keep audio + video
- For Forked branch: strip video (audio only)
Problem:
SDP manipulation seems to be applied globally to the message, not per branch.
What I tried:
1. Using textops:
   sdp_remove_media("video");
   msg_apply_changes();
2. Trying to do it per branch:
I attempted to move this logic into branch_route, but msg_apply_changes() is not allowed there, so SDP changes are not applied.
Documentation stated that it can be used from from REQUEST_ROUTE or REPLY_ROUTE.
3. Using rtpengine:
According to rtpengine NG protocol, "offer" supports:
  "sdp-media-remove": ["video"]
However, I cannot find a way to pass this parameter via Kamailio's rtpengine module.
Tried to add flag sdp-media-remove=video to the rtpengine_manage() but it doesn't work.
Environment:
- Kamailio version: 6.0.6
- rtpengine version: 13.5.1.6
Question:
What is the correct way to modify SDP per branch in a parallel forking scenario in Kamailio?
My test routing config:
```
request_route {
if (is_method("CANCEL")) {
    if (t_check_trans()) {
      route(RELAY);
    }
    exit;
  }
if (!is_method("ACK")) {
    if(t_precheck_trans()) {
      t_check_trans();
      exit;
    }
    t_check_trans();
  }
# remove unused codecs and media
 # if (has_body("application/sdp")) {
 #   sdp_remove_media("video");
 #   msg_apply_changes();
 # }
if (is_method("INVITE") && !has_totag()) {
    dlg_manage();
  }
remove_hf("Route");
  if (is_method("INVITE|SUBSCRIBE")) {
    record_route();
  }
if ($rU==$null) {
    sl_send_reply("484","Address Incomplete");
    exit;
  }
route(LOCATION);
return;
}
route[RELAY] {
if (is_method("INVITE|BYE|SUBSCRIBE|UPDATE")) {
    if(!t_is_set("branch_route")) t_on_branch("MANAGE_BRANCH");
  }
  if (is_method("INVITE|SUBSCRIBE|UPDATE")) {
    if(!t_is_set("onreply_route")) t_on_reply("MANAGE_REPLY");
  }
  if (is_method("INVITE")) {
    if(!t_is_set("failure_route")) t_on_failure("MANAGE_FAILURE");
  }
if (!t_relay()) {
    sl_reply_error();
  }
  exit;
}
route[LOCATION] {
route(ADD_FORK);
  route(RELAY);
}
route[ADD_FORK] {
  $avp(extNum) = "abcd@192.168.106.34";
  append_branch("sip:" + $(avp(extNum)));
}
branch_route[MANAGE_BRANCH] {
route(NATMANAGE);
}
onreply_route[MANAGE_REPLY] {
  # remove video media
  # if (has_body("application/sdp")) {
  #   sdp_remove_media("video");
  #   msg_apply_changes();
  # }
if(status=~"[12][0-9][0-9]") {
    route(NATMANAGE);
  }
}
failure_route[MANAGE_FAILURE] {
  route(NATMANAGE);
if (t_is_canceled()) exit;
}
route[NATMANAGE] {
if (is_request()) {
    if (has_body("application/sdp")) {
      xlog("L_INFO","Negotiate RTP offer for branch $T_branch_idx\n");
      if ($ru =~ "^sip:1[0-9]{4}@v") {
        xlog("L_INFO","Main leg\n");
        rtpengine_manage("replace-origin RTP/AVP external internal");
      } else if ($ru =~ "192.168.106.34") {
        xlog("L_INFO","Forked leg\n");
      rtpengine_manage("replace-origin RTP/AVP codec-mask=telephone-event transcode=PCMA always-transcode external internal via-branch=1");
      //rtpengine_manage("label=$fu--->$ru sdp-media-remove=video replace-origin RTP/AVP codec-mask=telephone-event transcode=PCMA always-transcode ptime=20 external internal via-branch=1"); # only with patched module
      }
    }
  }
if (is_reply()) {
    if (has_body("application/sdp") && (status=="200")) {
      xlog("L_INFO","Negotiate RTP answer for branch $T_branch_idx\n");
    }
  }
return;
}
```