Add ssrc attributes to the SDP

lib/ex_webrtc/peer_connection.ex

@@ -870,18 +870,19 @@ defmodule ExWebRTC.PeerConnection do
   @impl true
   def handle_call({:add_track, %MediaStreamTrack{kind: kind} = track}, _from, state) do
     # we ignore the condition that sender has never been used to send
-    {ssrc, rtx_ssrc} = generate_ssrcs(state)
 
     {transceivers, sender} =
       state.transceivers
       |> Enum.with_index()
       |> Enum.find(fn {tr, _idx} -> RTPTransceiver.can_add_track?(tr, kind) end)
       |> case do
         {tr, idx} ->
-          tr = RTPTransceiver.add_track(tr, track, ssrc, rtx_ssrc)
+          tr = RTPTransceiver.add_track(tr, track)
           {List.replace_at(state.transceivers, idx, tr), tr.sender}
 
         nil ->
+          {ssrc, rtx_ssrc} = generate_ssrcs(state)
+
           options = [
             direction: :sendrecv,
             added_by_add_track: true,
@@ -910,8 +911,7 @@ defmodule ExWebRTC.PeerConnection do
         {:reply, {:error, :invalid_track_type}, state}
 
       {tr, idx} when tr.direction in [:sendrecv, :sendonly] ->
-        {ssrc, rtx_ssrc} = generate_ssrcs(state)
-        tr = RTPTransceiver.replace_track(tr, track, ssrc, rtx_ssrc)
+        tr = RTPTransceiver.replace_track(tr, track)
         transceivers = List.replace_at(state.transceivers, idx, tr)
         state = %{state | transceivers: transceivers}
         {:reply, :ok, state}
@@ -1649,7 +1649,13 @@ defmodule ExWebRTC.PeerConnection do
       transceivers =
         sdp.media
         |> Enum.reject(&SDPUtils.data_channel?/1)
-        |> process_mlines_remote(state.transceivers, type, state.config, state.owner)
+        |> process_mlines_remote(
+          state.transceivers,
+          type,
+          Map.keys(state.demuxer.ssrc_to_mid),
+          state.config,
+          state.owner
+        )
 
       # infer our role from the remote role
       dtls_role = if dtls_role in [:actpass, :passive], do: :active, else: :passive
@@ -1803,7 +1809,7 @@ defmodule ExWebRTC.PeerConnection do
     end
   end
 
-  # See W3C WebRTC 4.4.1.5-4.7.10.1
+  # See W3C WebRTC 4.4.1.4-4.7.10.1
   defp process_mlines_local(_mlines, transceivers, :offer, _owner), do: transceivers
 
   defp process_mlines_local([], transceivers, :answer, _owner), do: transceivers
@@ -1834,26 +1840,36 @@ defmodule ExWebRTC.PeerConnection do
     process_mlines_local(mlines, transceivers, :answer, owner)
   end
 
-  # See W3C WebRTC 4.4.1.5-4.7.10.2
-  defp process_mlines_remote(mlines, transceivers, sdp_type, config, owner) do
+  # See W3C WebRTC 4.4.1.4-4.7.10.2
+  defp process_mlines_remote(mlines, transceivers, sdp_type, demuxer_ssrcs, config, owner) do
     mlines_idx = Enum.with_index(mlines)
-    do_process_mlines_remote(mlines_idx, transceivers, sdp_type, config, owner)
+    do_process_mlines_remote(mlines_idx, transceivers, sdp_type, demuxer_ssrcs, config, owner)
   end
 
-  defp do_process_mlines_remote([], transceivers, _sdp_type, _config, _owner), do: transceivers
+  defp do_process_mlines_remote([], transceivers, _sdp_type, _demuxer_ssrcs, _config, _owner),
+    do: transceivers
 
-  defp do_process_mlines_remote([{mline, idx} | mlines], transceivers, sdp_type, config, owner) do
+  defp do_process_mlines_remote(
+         [{mline, idx} | mlines],
+         transceivers,
+         sdp_type,
+         demuxer_ssrcs,
+         config,
+         owner
+       ) do
     direction =
       if SDPUtils.rejected?(mline),
         do: :inactive,
         else: SDPUtils.get_media_direction(mline) |> reverse_direction()
 
+    {ssrc, rtx_ssrc} = generate_ssrcs(transceivers, demuxer_ssrcs)
+
     # Note: in theory we should update transceiver codecs
     # after processing remote track but this shouldn't have any impact
     {idx, tr} =
       case find_transceiver_from_remote(transceivers, mline) do
         {idx, tr} -> {idx, RTPTransceiver.update(tr, mline, config)}
-        nil -> {nil, RTPTransceiver.from_mline(mline, idx, config)}
+        nil -> {nil, RTPTransceiver.from_mline(mline, idx, ssrc, rtx_ssrc, config)}
       end
 
     tr = process_remote_track(tr, direction, owner)
@@ -1867,11 +1883,11 @@ defmodule ExWebRTC.PeerConnection do
     case idx do
       nil ->
         transceivers = transceivers ++ [tr]
-        do_process_mlines_remote(mlines, transceivers, sdp_type, config, owner)
+        do_process_mlines_remote(mlines, transceivers, sdp_type, demuxer_ssrcs, config, owner)
 
       idx ->
         transceivers = List.replace_at(transceivers, idx, tr)
-        do_process_mlines_remote(mlines, transceivers, sdp_type, config, owner)
+        do_process_mlines_remote(mlines, transceivers, sdp_type, demuxer_ssrcs, config, owner)
     end
   end
 
@@ -2177,15 +2193,21 @@ defmodule ExWebRTC.PeerConnection do
   end
 
   defp generate_ssrcs(state) do
-    rtp_sender_ssrcs = Enum.map(state.transceivers, & &1.sender.ssrc)
-    ssrcs = MapSet.new(Map.keys(state.demuxer.ssrc_to_mid) ++ rtp_sender_ssrcs)
-    ssrc = do_generate_ssrc(ssrcs, 200)
-    rtx_ssrc = do_generate_ssrc(MapSet.put(ssrcs, ssrc), 200)
+    generate_ssrcs(state.transceivers, Map.keys(state.demuxer.ssrc_to_mid))
+  end
+
+  defp generate_ssrcs(transceivers, demuxer_ssrcs) do
+    rtp_sender_ssrcs = Enum.map(transceivers, & &1.sender.ssrc)
+    ssrcs = MapSet.new(demuxer_ssrcs ++ rtp_sender_ssrcs)
+    ssrc = do_generate_ssrc(ssrcs)
+    rtx_ssrc = do_generate_ssrc(MapSet.put(ssrcs, ssrc))
     {ssrc, rtx_ssrc}
   end
 
   # this is practically impossible so it's easier to raise
   # than to propagate the error up to the user
+  defp do_generate_ssrc(ssrcs, max_attempts \\ 200)
+
   defp do_generate_ssrc(_ssrcs, 0), do: raise("Couldn't find free SSRC")
 
   defp do_generate_ssrc(ssrcs, max_attempts) do

lib/ex_webrtc/rtp_sender.ex

@@ -21,8 +21,11 @@ defmodule ExWebRTC.RTPSender do
           mid: String.t() | nil,
           pt: non_neg_integer() | nil,
           rtx_pt: non_neg_integer() | nil,
-          ssrc: non_neg_integer() | nil,
-          rtx_ssrc: non_neg_integer() | nil,
+          # ssrc and rtx_ssrc are always present, even if there is no track,
+          # or transceiver direction is recvonly.
+          # We preallocate them so they can be included in SDP when needed.
+          ssrc: non_neg_integer(),
+          rtx_ssrc: non_neg_integer(),
           packets_sent: non_neg_integer(),
           bytes_sent: non_neg_integer(),
           retransmitted_packets_sent: non_neg_integer(),
@@ -68,8 +71,8 @@ defmodule ExWebRTC.RTPSender do
           RTPCodecParameters.t() | nil,
           [Extmap.t()],
           String.t() | nil,
-          non_neg_integer() | nil,
-          non_neg_integer() | nil,
+          non_neg_integer(),
+          non_neg_integer(),
           [atom()]
         ) :: sender()
   def new(track, codec, rtx_codec, rtp_hdr_exts, mid, ssrc, rtx_ssrc, features) do
@@ -131,6 +134,83 @@ defmodule ExWebRTC.RTPSender do
     }
   end
 
+  @spec get_mline_attrs(sender()) :: [ExSDP.Attribute.t()]
+  def get_mline_attrs(sender) do
+    # Don't include track id. See RFC 8829 sec. 5.2.1
+    msid_attrs =
+      case sender.track do
+        %MediaStreamTrack{streams: streams} when streams != [] ->
+          Enum.map(streams, &ExSDP.Attribute.MSID.new(&1, nil))
+
+        _other ->
+          # 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)]
+      end
+
+    ssrc_attrs =
+      get_ssrc_attrs(sender.pt, sender.rtx_pt, sender.ssrc, sender.rtx_ssrc, sender.track)
+
+    msid_attrs ++ ssrc_attrs
+  end
+
+  # we didn't manage to negotiate any codec
+  defp get_ssrc_attrs(nil, _rtx_pt, _ssrc, _rtx_ssrc, _track) do
+    []
+  end
+
+  # we have a codec but not rtx
+  defp get_ssrc_attrs(_pt, nil, ssrc, _rtx_ssrc, track) do
+    streams = (track && track.streams) || []
+
+    case streams do
+      [] ->
+        [%ExSDP.Attribute.SSRC{id: ssrc, attribute: "msid", value: "-"}]
+
+      streams ->
+        Enum.map(streams, fn stream ->
+          %ExSDP.Attribute.SSRC{id: ssrc, attribute: "msid", value: stream}
+        end)
+    end
+  end
+
+  # we have both codec and rtx
+  defp get_ssrc_attrs(_pt, _rtx_pt, ssrc, rtx_ssrc, track) do
+    streams = (track && track.streams) || []
+
+    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 ->
+          {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_attrs = [ssrc_attr | ssrc_attrs]
+
+              rtx_ssrc_attr = %ExSDP.Attribute.SSRC{
+                id: rtx_ssrc,
+                attribute: "msid",
+                value: stream
+              }
+
+              rtx_ssrc_attrs = [rtx_ssrc_attr | rtx_ssrc_attrs]
+
+              {ssrc_attrs, rtx_ssrc_attrs}
+            end)
+
+          Enum.reverse(ssrc_attrs) ++ Enum.reverse(rtx_ssrc_attrs)
+      end
+
+    [fid | ssrc_attrs]
+  end
+
   @doc false
   @spec send_packet(sender(), ExRTP.Packet.t(), boolean()) :: {binary(), sender()}
   def send_packet(sender, packet, rtx?) do

lib/ex_webrtc/rtp_sender/nack_responder.ex

@@ -28,8 +28,12 @@ defmodule ExWebRTC.RTPSender.NACKResponder do
 
     {packets, seq_no} =
       seq_nos
-      |> Enum.map(fn seq_no -> {seq_no, Map.get(responder.packets, rem(seq_no, @max_packets))} end)
-      |> Enum.filter(fn {seq_no, packet} -> packet != nil and packet.sequence_number == seq_no end)
+      |> Enum.map(fn seq_no ->
+        {seq_no, Map.get(responder.packets, rem(seq_no, @max_packets))}
+      end)
+      |> Enum.filter(fn {seq_no, packet} ->
+        packet != nil and packet.sequence_number == seq_no
+      end)
       # ssrc will be assigned by the sender
       |> Enum.map_reduce(responder.seq_no, fn {seq_no, packet}, rtx_seq_no ->
         rtx_packet = %Packet{

lib/ex_webrtc/rtp_transceiver.ex

@@ -174,8 +174,14 @@ defmodule ExWebRTC.RTPTransceiver do
   end
 
   @doc false
-  @spec from_mline(ExSDP.Media.t(), non_neg_integer(), Configuration.t()) :: transceiver()
-  def from_mline(mline, mline_idx, config) do
+  @spec from_mline(
+          ExSDP.Media.t(),
+          non_neg_integer(),
+          non_neg_integer(),
+          non_neg_integer(),
+          Configuration.t()
+        ) :: transceiver()
+  def from_mline(mline, mline_idx, ssrc, rtx_ssrc, config) do
     header_extensions = Configuration.intersect_extensions(config, mline)
     codecs = Configuration.intersect_codecs(config, mline)
 
@@ -205,7 +211,16 @@ defmodule ExWebRTC.RTPTransceiver do
     receiver = RTPReceiver.new(track, codec, header_extensions, config.features)
 
     sender =
-      RTPSender.new(nil, codec, codec_rtx, header_extensions, mid, nil, nil, config.features)
+      RTPSender.new(
+        nil,
+        codec,
+        codec_rtx,
+        header_extensions,
+        mid,
+        ssrc,
+        rtx_ssrc,
+        config.features
+      )
 
     %{
       id: id,
@@ -281,10 +296,9 @@ defmodule ExWebRTC.RTPTransceiver do
   end
 
   @doc false
-  @spec add_track(transceiver(), MediaStreamTrack.t(), non_neg_integer(), non_neg_integer()) ::
-          transceiver()
-  def add_track(transceiver, track, ssrc, rtx_ssrc) do
-    sender = %{transceiver.sender | track: track, ssrc: ssrc, rtx_ssrc: rtx_ssrc}
+  @spec add_track(transceiver(), MediaStreamTrack.t()) :: transceiver()
+  def add_track(transceiver, track) do
+    sender = %{transceiver.sender | track: track}
 
     direction =
       case transceiver.direction do
@@ -297,11 +311,9 @@ defmodule ExWebRTC.RTPTransceiver do
   end
 
   @doc false
-  @spec replace_track(transceiver(), MediaStreamTrack.t(), non_neg_integer(), non_neg_integer()) ::
-          transceiver()
-  def replace_track(transceiver, track, ssrc, rtx_ssrc) do
-    ssrc = transceiver.sender.ssrc || ssrc
-    sender = %{transceiver.sender | track: track, ssrc: ssrc, rtx_ssrc: rtx_ssrc}
+  @spec replace_track(transceiver(), MediaStreamTrack.t()) :: transceiver()
+  def replace_track(transceiver, track) do
+    sender = %{transceiver.sender | track: track}
     %{transceiver | sender: sender}
   end
 
@@ -526,39 +538,37 @@ defmodule ExWebRTC.RTPTransceiver do
         [rtp_mapping, codec.sdp_fmtp_line, codec.rtcp_fbs]
       end)
 
-    msids =
-      case transceiver.sender.track do
-        nil ->
-          []
-
-        %MediaStreamTrack{id: id, streams: streams} ->
-          case Enum.map(streams, &ExSDP.Attribute.MSID.new(&1, id)) do
-            [] -> [ExSDP.Attribute.MSID.new("-", id)]
-            other -> other
-          end
-      end
+    direction = Keyword.get(opts, :direction, transceiver.direction)
 
     attributes =
       if(Keyword.get(opts, :rtcp, false), do: [{"rtcp", "9 IN IP4 0.0.0.0"}], else: []) ++
         Keyword.get(opts, :simulcast, []) ++
         [
-          Keyword.get(opts, :direction, transceiver.direction),
+          direction,
           {:mid, transceiver.mid},
           {:ice_ufrag, Keyword.fetch!(opts, :ice_ufrag)},
           {:ice_pwd, Keyword.fetch!(opts, :ice_pwd)},
           {:ice_options, Keyword.fetch!(opts, :ice_options)},
           {:fingerprint, Keyword.fetch!(opts, :fingerprint)},
           {:setup, Keyword.fetch!(opts, :setup)},
           :rtcp_mux
-        ] ++ transceiver.header_extensions ++ msids
+        ] ++ transceiver.header_extensions
+
+    # add sender attrs only if we send
+    sender_attrs =
+      if direction in [:sendonly, :sendrecv] do
+        RTPSender.get_mline_attrs(transceiver.sender)
+      else
+        []
+      end
 
     %ExSDP.Media{
       ExSDP.Media.new(transceiver.kind, 9, "UDP/TLS/RTP/SAVPF", pt)
       | # mline must be followed by a cline, which must contain
         # the default value "IN IP4 0.0.0.0" (as there are no candidates yet)
         connection_data: [%ExSDP.ConnectionData{address: {0, 0, 0, 0}}]
     }
-    |> ExSDP.add_attributes(attributes ++ media_formats)
+    |> ExSDP.add_attributes(attributes ++ media_formats ++ sender_attrs)
   end
 
   # RFC 3264 (6.1) + RFC 8829 (5.3.1)