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Always include track id in MSID/SSRC attributes #221

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Merged
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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85 changes: 48 additions & 37 deletions lib/ex_webrtc/rtp_transceiver.ex
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Original file line number Diff line number Diff line change
Expand Up @@ -558,10 +558,8 @@ defmodule ExWebRTC.RTPTransceiver do
else: transceiver.sender.codecs

get_sender_attrs(
transceiver.sender.track,
codecs,
transceiver.sender.ssrc,
transceiver.sender.rtx_ssrc
transceiver.sender,
codecs
)
else
[]
Expand All @@ -577,75 +575,76 @@ defmodule ExWebRTC.RTPTransceiver do
end

@doc false
defp get_sender_attrs(track, codecs, ssrc, rtx_ssrc) do
# Don't include track id. See RFC 8829 sec. 5.2.1
defp get_sender_attrs(sender, codecs) 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
%MediaStreamTrack{streams: streams} when streams != [] ->
Enum.map(streams, &ExSDP.Attribute.MSID.new(&1, nil))
case sender.track do
%MediaStreamTrack{streams: streams, id: id} when streams != [] ->
Enum.map(streams, &ExSDP.Attribute.MSID.new(&1, id))

_other ->
%MediaStreamTrack{id: id} ->
# In theory, we should do this "for each MediaStream that was associated with the transceiver",
# but web browsers (chrome, ff) include MSID even when there aren't any MediaStreams
[ExSDP.Attribute.MSID.new("-", nil)]
[ExSDP.Attribute.MSID.new("-", id)]

nil ->
[ExSDP.Attribute.MSID.new("-", sender.id)]
end

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

msid_attrs ++ ssrc_attrs
end

defp get_ssrc_attrs(codecs, ssrc, rtx_ssrc, track) do
defp get_ssrc_attrs(sender, codecs) 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(sender, codec, rtx_codec)
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(_sender, false, _rtx_codec) do
[]
end

# we have a codec but not rtx codec
defp do_get_ssrc_attrs(_codec, false, ssrc, _rtx_ssrc, track) do
streams = (track && track.streams) || []

case streams do
[] ->
[%ExSDP.Attribute.SSRC{id: ssrc, attribute: "msid", value: "-"}]

streams ->
defp do_get_ssrc_attrs(%{ssrc: ssrc} = sender, _codec, false) 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}
%ExSDP.Attribute.SSRC{id: ssrc, attribute: "msid", value: "#{stream} #{id}"}
end)

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

nil ->
# 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
streams = (track && track.streams) || []

defp do_get_ssrc_attrs(%{ssrc: ssrc, rtx_ssrc: rtx_ssrc} = sender, _codec, _rtx_codec) do
fid = %ExSDP.Attribute.SSRCGroup{semantics: "FID", ssrcs: [ssrc, rtx_ssrc]}

ssrc_attrs =
case streams do
[] ->
[
%ExSDP.Attribute.SSRC{id: ssrc, attribute: "msid", value: "-"},
%ExSDP.Attribute.SSRC{id: rtx_ssrc, attribute: "msid", value: "-"}
]

streams ->
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} ->
ssrc_attr = %ExSDP.Attribute.SSRC{id: ssrc, attribute: "msid", value: stream}
ssrc_value = "#{stream} #{id}"

ssrc_attr = %ExSDP.Attribute.SSRC{id: ssrc, attribute: "msid", value: ssrc_value}
ssrc_attrs = [ssrc_attr | ssrc_attrs]

rtx_ssrc_attr = %ExSDP.Attribute.SSRC{
id: rtx_ssrc,
attribute: "msid",
value: stream
value: ssrc_value
}

rtx_ssrc_attrs = [rtx_ssrc_attr | rtx_ssrc_attrs]
Expand All @@ -654,6 +653,18 @@ defmodule ExWebRTC.RTPTransceiver do
end)

Enum.reverse(ssrc_attrs) ++ Enum.reverse(rtx_ssrc_attrs)

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

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

[fid | ssrc_attrs]
Expand Down
60 changes: 43 additions & 17 deletions test/ex_webrtc/rtp_transceiver_test.exs
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Original file line number Diff line number Diff line change
Expand Up @@ -74,13 +74,14 @@ defmodule ExWebRTC.RTPTransceiverTest do
)

mline = RTPTransceiver.to_offer_mline(tr, @opts)
ssrc_value = ssrc_msid_value(@stream_id, @track.id)

assert [%ExSDP.Attribute.MSID{id: @stream_id}] =
ExSDP.get_attributes(mline, ExSDP.Attribute.MSID)

assert [] = ExSDP.get_attributes(mline, ExSDP.Attribute.SSRCGroup)

assert [%ExSDP.Attribute.SSRC{id: @ssrc, attribute: "msid", value: @stream_id}] =
assert [%ExSDP.Attribute.SSRC{id: @ssrc, attribute: "msid", value: ^ssrc_value}] =
ExSDP.get_attributes(mline, ExSDP.Attribute.SSRC)
end

Expand All @@ -93,16 +94,17 @@ defmodule ExWebRTC.RTPTransceiverTest do
)

mline = RTPTransceiver.to_offer_mline(tr, @opts)
ssrc_value = ssrc_msid_value(@stream_id, @track.id)

assert [%ExSDP.Attribute.MSID{id: @stream_id, app_data: nil}] =
assert [%ExSDP.Attribute.MSID{id: @stream_id, app_data: @track.id}] ==
ExSDP.get_attributes(mline, ExSDP.Attribute.MSID)

assert [%ExSDP.Attribute.SSRCGroup{semantics: "FID", ssrcs: [@ssrc, @rtx_ssrc]}] =
ExSDP.get_attributes(mline, ExSDP.Attribute.SSRCGroup)

assert [
%ExSDP.Attribute.SSRC{id: @ssrc, attribute: "msid", value: @stream_id},
%ExSDP.Attribute.SSRC{id: @rtx_ssrc, attribute: "msid", value: @stream_id}
%ExSDP.Attribute.SSRC{id: @ssrc, attribute: "msid", value: ^ssrc_value},
%ExSDP.Attribute.SSRC{id: @rtx_ssrc, attribute: "msid", value: ^ssrc_value}
] = ExSDP.get_attributes(mline, ExSDP.Attribute.SSRC)
end

Expand All @@ -118,16 +120,16 @@ defmodule ExWebRTC.RTPTransceiverTest do

mline = RTPTransceiver.to_offer_mline(tr, @opts)

assert [%ExSDP.Attribute.MSID{id: "-", app_data: nil}] =
assert [%ExSDP.Attribute.MSID{id: "-", app_data: track.id}] ==
ExSDP.get_attributes(mline, ExSDP.Attribute.MSID)

assert [%ExSDP.Attribute.SSRCGroup{semantics: "FID", ssrcs: [@ssrc, @rtx_ssrc]}] =
ExSDP.get_attributes(mline, ExSDP.Attribute.SSRCGroup)

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

test "with multiple media streams" do
Expand All @@ -145,20 +147,23 @@ defmodule ExWebRTC.RTPTransceiverTest do

mline = RTPTransceiver.to_offer_mline(tr, @opts)

ssrc1_value = ssrc_msid_value(s1_id, track.id)
ssrc2_value = ssrc_msid_value(s2_id, track.id)

assert [
%ExSDP.Attribute.MSID{id: ^s1_id, app_data: nil},
%ExSDP.Attribute.MSID{id: ^s2_id, app_data: nil}
] = ExSDP.get_attributes(mline, ExSDP.Attribute.MSID)
%ExSDP.Attribute.MSID{id: s1_id, app_data: track.id},
%ExSDP.Attribute.MSID{id: s2_id, app_data: track.id}
] == ExSDP.get_attributes(mline, ExSDP.Attribute.MSID)

assert [%ExSDP.Attribute.SSRCGroup{semantics: "FID", ssrcs: [@ssrc, @rtx_ssrc]}] =
ExSDP.get_attributes(mline, ExSDP.Attribute.SSRCGroup)

assert [
%ExSDP.Attribute.SSRC{id: @ssrc, attribute: "msid", value: ^s1_id},
%ExSDP.Attribute.SSRC{id: @ssrc, attribute: "msid", value: ^s2_id},
%ExSDP.Attribute.SSRC{id: @rtx_ssrc, attribute: "msid", value: ^s1_id},
%ExSDP.Attribute.SSRC{id: @rtx_ssrc, attribute: "msid", value: ^s2_id}
] = ExSDP.get_attributes(mline, ExSDP.Attribute.SSRC)
%ExSDP.Attribute.SSRC{id: @ssrc, attribute: "msid", value: ssrc1_value},
%ExSDP.Attribute.SSRC{id: @ssrc, attribute: "msid", value: ssrc2_value},
%ExSDP.Attribute.SSRC{id: @rtx_ssrc, attribute: "msid", value: ssrc1_value},
%ExSDP.Attribute.SSRC{id: @rtx_ssrc, attribute: "msid", value: ssrc2_value}
] == ExSDP.get_attributes(mline, ExSDP.Attribute.SSRC)
end

test "without codecs" do
Expand All @@ -174,12 +179,31 @@ defmodule ExWebRTC.RTPTransceiverTest do

mline = RTPTransceiver.to_offer_mline(tr, @opts)

assert [%ExSDP.Attribute.MSID{id: @stream_id, app_data: nil}] =
assert [%ExSDP.Attribute.MSID{id: @stream_id, app_data: @track.id}] ==
ExSDP.get_attributes(mline, ExSDP.Attribute.MSID)

assert [] = ExSDP.get_attributes(mline, ExSDP.Attribute.SSRCGroup)
assert [] = ExSDP.get_attributes(mline, ExSDP.Attribute.SSRC)
end

test "without track" do
tr =
RTPTransceiver.new(:video, nil, @config,
ssrc: @ssrc,
rtx_ssrc: @rtx_ssrc,
direction: :sendrecv
)

mline = RTPTransceiver.to_offer_mline(tr, @opts)

assert [%ExSDP.Attribute.MSID{id: "-", app_data: tr.sender.id}] ==
ExSDP.get_attributes(mline, ExSDP.Attribute.MSID)

assert [
%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

defp test_sender_attrs_present(tr) do
Expand All @@ -201,4 +225,6 @@ defmodule ExWebRTC.RTPTransceiverTest do
assert [] == ExSDP.get_attributes(mline, ExSDP.Attribute.SSRCGroup)
assert [] == ExSDP.get_attributes(mline, ExSDP.Attribute.SSRC)
end

defp ssrc_msid_value(stream, app_data), do: "#{stream} #{app_data}"
end
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