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Always include track id in MSID/SSRC attributes #221

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Merged
Karolk99 merged 6 commits into from
Jun 26, 2025
Merged

Always include track id in MSID/SSRC attributes #221

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e47e165
Add track to MSID
Karolk99 Jun 23, 2025
b22d21c
Rename ssrc_value to ssrc_msid_value
Karolk99 Jun 25, 2025
1f6ec0c
Use sender_id as default msid app_data
Karolk99 Jun 25, 2025
f4a4ab4
Requested changes
Karolk99 Jun 26, 2025
00ac212
Add MSID event if there is no track
Karolk99 Jun 26, 2025
a62d350
Remove tool versions
Karolk99 Jun 26, 2025
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30 changes: 16 additions & 14 deletions lib/ex_webrtc/rtp_transceiver.ex
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Original file line number Diff line number Diff line change
Expand Up @@ -558,7 +558,7 @@ defmodule ExWebRTC.RTPTransceiver do
else: transceiver.sender.codecs

get_sender_attrs(
transceiver.sender.track,
transceiver.sender,
codecs,
transceiver.sender.ssrc,
transceiver.sender.rtx_ssrc
Expand All @@ -577,11 +577,11 @@ defmodule ExWebRTC.RTPTransceiver do
end

@doc false
defp get_sender_attrs(track, codecs, ssrc, rtx_ssrc) do
defp get_sender_attrs(sender, codecs, ssrc, rtx_ssrc) do
# According to RFC 8829 sec. 5.2.1, track IDs should not be included.
# However, most browsers support track IDs in MSID. We will follow this practice.
msid_attrs =
case track do
case sender.track do
%MediaStreamTrack{streams: streams, id: id} when streams != [] ->
Enum.map(streams, &ExSDP.Attribute.MSID.new(&1, id))

Expand All @@ -594,26 +594,26 @@ defmodule ExWebRTC.RTPTransceiver do
[]
end

ssrc_attrs = get_ssrc_attrs(codecs, ssrc, rtx_ssrc, track)
ssrc_attrs = get_ssrc_attrs(codecs, ssrc, rtx_ssrc, sender)

msid_attrs ++ ssrc_attrs
end

defp get_ssrc_attrs(codecs, ssrc, rtx_ssrc, track) do
defp get_ssrc_attrs(codecs, ssrc, rtx_ssrc, sender) do
codec = Enum.any?(codecs, fn codec -> not String.ends_with?(codec.mime_type, "/rtx") end)
rtx_codec = Enum.any?(codecs, fn codec -> String.ends_with?(codec.mime_type, "/rtx") end)

do_get_ssrc_attrs(codec, rtx_codec, ssrc, rtx_ssrc, track)
do_get_ssrc_attrs(codec, rtx_codec, ssrc, rtx_ssrc, sender)
end

# we didn't manage to negotiate any codec
defp do_get_ssrc_attrs(false, _rtx_codec, _ssrc, _rtx_ssrc, _track) do
defp do_get_ssrc_attrs(false, _rtx_codec, _ssrc, _rtx_ssrc, _sender) do
[]
end

# we have a codec but not rtx codec
defp do_get_ssrc_attrs(_codec, false, ssrc, _rtx_ssrc, track) do
case track do
defp do_get_ssrc_attrs(_codec, false, ssrc, _rtx_ssrc, sender) do
case sender.track do
%MediaStreamTrack{streams: streams, id: id} when streams != [] ->
Enum.map(streams, fn stream ->
%ExSDP.Attribute.SSRC{id: ssrc, attribute: "msid", value: "#{stream} #{id}"}
Expand All @@ -623,16 +623,18 @@ defmodule ExWebRTC.RTPTransceiver do
[%ExSDP.Attribute.SSRC{id: ssrc, attribute: "msid", value: "- #{id}"}]

nil ->
[%ExSDP.Attribute.SSRC{id: ssrc, attribute: "msid", value: "- -"}]
# If the track_id is missing, we will default to using the sender_id, following Chromium's approach:
# See: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/pc/sdp_offer_answer.cc;l=739;drc=b8b5768e5d0d1c2a84fe4896eae884d97fd1131e;bpv=1;bpt=1
[%ExSDP.Attribute.SSRC{id: ssrc, attribute: "msid", value: "- #{sender.id}"}]
end
end

# we have both codec and rtx codec
defp do_get_ssrc_attrs(_codec, _rtx_codec, ssrc, rtx_ssrc, track) do
defp do_get_ssrc_attrs(_codec, _rtx_codec, ssrc, rtx_ssrc, sender) do
fid = %ExSDP.Attribute.SSRCGroup{semantics: "FID", ssrcs: [ssrc, rtx_ssrc]}

ssrc_attrs =
case track do
case sender.track do
%MediaStreamTrack{streams: streams, id: id} when streams != [] ->
{ssrc_attrs, rtx_ssrc_attrs} =
Enum.reduce(streams, {[], []}, fn stream, {ssrc_attrs, rtx_ssrc_attrs} ->
Expand Down Expand Up @@ -662,8 +664,8 @@ defmodule ExWebRTC.RTPTransceiver do

nil ->
[
%ExSDP.Attribute.SSRC{id: ssrc, attribute: "msid", value: "- -"},
%ExSDP.Attribute.SSRC{id: rtx_ssrc, attribute: "msid", value: "- -"}
%ExSDP.Attribute.SSRC{id: ssrc, attribute: "msid", value: "- #{sender.id}"},
%ExSDP.Attribute.SSRC{id: rtx_ssrc, attribute: "msid", value: "- #{sender.id}"}
]
end

Expand Down
6 changes: 3 additions & 3 deletions test/ex_webrtc/rtp_transceiver_test.exs
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Original file line number Diff line number Diff line change
Expand Up @@ -199,9 +199,9 @@ defmodule ExWebRTC.RTPTransceiverTest do
assert [] = ExSDP.get_attributes(mline, ExSDP.Attribute.MSID)

assert [
%ExSDP.Attribute.SSRC{id: @ssrc, attribute: "msid", value: "- -"},
%ExSDP.Attribute.SSRC{id: @rtx_ssrc, attribute: "msid", value: "- -"}
] = ExSDP.get_attributes(mline, ExSDP.Attribute.SSRC)
%ExSDP.Attribute.SSRC{id: @ssrc, attribute: "msid", value: "- #{tr.sender.id}"},
%ExSDP.Attribute.SSRC{id: @rtx_ssrc, attribute: "msid", value: "- #{tr.sender.id}"}
] == ExSDP.get_attributes(mline, ExSDP.Attribute.SSRC)
end
end

Expand Down
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