Add/refactor set_local_description and create_answer

lib/ex_webrtc/peer_connection.ex

@@ -12,8 +12,8 @@ defmodule ExWebRTC.PeerConnection do
     IceCandidate,
     MediaStreamTrack,
     RTPTransceiver,
-    SessionDescription,
     SDPUtils,
+    SessionDescription,
     Utils
   }
 
@@ -192,26 +192,31 @@ defmodule ExWebRTC.PeerConnection do
   @impl true
   def handle_call({:create_answer, _options}, _from, state)
       when state.signaling_state in [:have_remote_offer, :have_local_pranswer] do
+    {:offer, remote_offer} = state.pending_remote_desc
+
     {:ok, ice_ufrag, ice_pwd} = ICEAgent.get_local_credentials(state.ice_agent)
     {:ok, dtls_fingerprint} = ExDTLS.get_cert_fingerprint(state.dtls_client)
 
-    answer = %ExSDP{ExSDP.new() | timing: %ExSDP.Timing{start_time: 0, stop_time: 0}}
+    answer =
+      %ExSDP{ExSDP.new() | timing: %ExSDP.Timing{start_time: 0, stop_time: 0}}
+      # we only support trickle ICE, so non-trickle offers should be rejected earlier
+      |> ExSDP.add_attribute({:ice_options, "trickle"})
 
     config =
       [
         ice_ufrag: ice_ufrag,
         ice_pwd: ice_pwd,
         ice_options: "trickle",
         fingerprint: {:sha256, Utils.hex_dump(dtls_fingerprint)},
-        # TODO offer will always contain actpass
-        # and answer should contain active
-        # see RFC 8829 sec. 5.3.1
-        setup: :passive
+        setup: :active
       ]
 
+    # TODO: rejected media sections
     mlines =
-      Enum.map(state.transceivers, fn transceiver ->
-        RTPTransceiver.to_mline(transceiver, config)
+      Enum.map(remote_offer.media, fn mline ->
+        {:mid, mid} = ExSDP.Media.get_attribute(mline, :mid)
+        {_ix, transceiver} = RTPTransceiver.find_by_mid(state.transceivers, mid)
+        SDPUtils.get_answer_mline(mline, transceiver, config)
       end)
 
     mids =
@@ -381,7 +386,7 @@ defmodule ExWebRTC.PeerConnection do
         %{dtls_buffered_packets: packets} = state
       ) do
     # we got DTLS packets from the other side but
-    # we haven't established ICE connection yet so 
+    # we haven't established ICE connection yet so
     # packets to retransmit have to be the same as dtls_buffered_packets
     {:noreply, state}
   end
@@ -392,17 +397,33 @@ defmodule ExWebRTC.PeerConnection do
     {:noreply, state}
   end
 
-  defp apply_local_description(_type, _sdp, state) do
-    # TODO: implement
-    {:ok, state}
+  defp apply_local_description(type, sdp, state) do
+    new_transceivers = update_local_transceivers(type, state.transceivers, sdp)
+    state = set_description(:local, type, sdp, state)
+
+    {:ok, %{state | transceivers: new_transceivers}}
+  end
+
+  defp update_local_transceivers(:offer, transceivers, sdp) do
+    sdp.media
+    |> Enum.zip(transceivers)
+    |> Enum.map(fn {mline, transceiver} ->
+      {:mid, mid} = ExSDP.Media.get_attribute(mline, :mid)
+      # TODO: check if mid from mline == mid from transceiver
+      %{transceiver | mid: mid}
+    end)
   end
 
-  defp apply_remote_description(_type, sdp, state) do
+  defp update_local_transceivers(:answer, transceivers, _sdp) do
+    transceivers
+  end
+
+  defp apply_remote_description(type, sdp, state) do
     # TODO apply steps listed in RFC 8829 5.10
     with :ok <- SDPUtils.ensure_mid(sdp),
          :ok <- SDPUtils.ensure_bundle(sdp),
          {:ok, {ice_ufrag, ice_pwd}} <- SDPUtils.get_ice_credentials(sdp),
-         {:ok, new_transceivers} <- update_transceivers(state.transceivers, sdp) do
+         {:ok, new_transceivers} <- update_remote_transceivers(state.transceivers, sdp) do
       :ok = ICEAgent.set_remote_credentials(state.ice_agent, ice_ufrag, ice_pwd)
       :ok = ICEAgent.gather_candidates(state.ice_agent)
 
@@ -419,13 +440,15 @@ defmodule ExWebRTC.PeerConnection do
         notify(state.owner, {:track, track})
       end
 
-      {:ok, %{state | current_remote_desc: sdp, transceivers: new_transceivers}}
+      state = set_description(:remote, type, sdp, state)
+
+      {:ok, %{state | transceivers: new_transceivers}}
     else
       error -> error
     end
   end
 
-  defp update_transceivers(transceivers, sdp) do
+  defp update_remote_transceivers(transceivers, sdp) do
     Enum.reduce_while(sdp.media, {:ok, transceivers}, fn mline, {:ok, transceivers} ->
       case ExSDP.Media.get_attribute(mline, :mid) do
         {:mid, mid} ->
@@ -504,5 +527,40 @@ defmodule ExWebRTC.PeerConnection do
   defp maybe_next_state(:have_remote_pranswer, :remote, :answer), do: {:ok, :stable}
   defp maybe_next_state(:have_remote_pranswer, _, _), do: {:error, :invalid_transition}
 
+  defp set_description(:local, :answer, sdp, state) do
+    # NOTICE: internaly, we don't create SessionDescription
+    # as it would require serialization of sdp
+    %{
+      state
+      | current_local_desc: {:answer, sdp},
+        current_remote_desc: state.pending_remote_desc,
+        pending_local_desc: nil,
+        pending_remote_desc: nil,
+        # W3C 4.4.1.5 (.4.7.5.2) suggests setting these to "", not nil
+        last_offer: nil,
+        last_answer: nil
+    }
+  end
+
+  defp set_description(:local, other, sdp, state) when other in [:offer, :pranswer] do
+    %{state | pending_local_desc: {other, sdp}}
+  end
+
+  defp set_description(:remote, :answer, sdp, state) do
+    %{
+      state
+      | current_remote_desc: {:answer, sdp},
+        current_local_desc: state.pending_local_desc,
+        pending_remote_desc: nil,
+        pending_local_desc: nil,
+        last_offer: nil,
+        last_answer: nil
+    }
+  end
+
+  defp set_description(:remote, other, sdp, state) when other in [:offer, :pranswer] do
+    %{state | pending_remote_desc: {other, sdp}}
+  end
+
   defp notify(pid, msg), do: send(pid, {:ex_webrtc, self(), msg})
 end

lib/ex_webrtc/sdp_utils.ex

@@ -1,6 +1,51 @@
 defmodule ExWebRTC.SDPUtils do
   @moduledoc false
 
+  alias ExWebRTC.RTPTransceiver
+
+  @spec get_answer_mline(ExSDP.Media.t(), RTPTransceiver.t(), Keyword.t()) :: ExSDP.Media.t()
+  def get_answer_mline(mline, transceiver, config) do
+    # TODO: we need to filter the media formats according to our capabilities
+    media_formats =
+      Enum.filter(mline.attributes, fn
+        %ExSDP.Attribute.RTPMapping{} -> true
+        %ExSDP.Attribute.FMTP{} -> true
+        _other -> false
+      end)
+
+    payload_types =
+      Enum.flat_map(media_formats, fn
+        %ExSDP.Attribute.RTPMapping{payload_type: pt} -> [pt]
+        _other -> []
+      end)
+
+    offered_direction = ExSDP.Media.get_attribute(mline, :direction)
+    direction = get_direction(offered_direction, transceiver.direction)
+
+    attributes =
+      [
+        direction,
+        {:mid, transceiver.mid},
+        {:ice_ufrag, Keyword.fetch!(config, :ice_ufrag)},
+        {:ice_pwd, Keyword.fetch!(config, :ice_pwd)},
+        {:ice_options, Keyword.fetch!(config, :ice_options)},
+        {:fingerprint, Keyword.fetch!(config, :fingerprint)},
+        {:setup, Keyword.fetch!(config, :setup)},
+        # TODO: probably should fail if the offer doesn't contain rtcp-mux
+        # as we don't support non-muxed streams
+        :rtcp_mux
+      ]
+
+    # TODO: validation of some the stuff in remote SDP
+    %ExSDP.Media{
+      ExSDP.Media.new(mline.type, 9, mline.protocol, payload_types)
+      | # 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.Media.add_attributes(attributes ++ media_formats)
+  end
+
   @spec get_media_direction(ExSDP.Media.t()) ::
           :sendrecv | :sendonly | :recvonly | :inactive | nil
   def get_media_direction(media) do
@@ -145,4 +190,14 @@
       _ -> {:error, :conflicting_ice_credentials}
     end
   end
+
+  # RFC 3264 (6.1) + RFC 8829 (5.3.1)
+  # AFAIK one of the cases should always match
+  # bc we won't assign/create an inactive transceiver to i.e. sendonly mline
+  # also neither of the arguments should ever be :stopped
+  defp get_direction(_, :inactive), do: :inactive
+  defp get_direction(:sendonly, t) when t in [:sendrecv, :recvonly], do: :recvonly
+  defp get_direction(:recvonly, t) when t in [:sendrecv, :sendonly], do: :sendonly
+  defp get_direction(o, other) when o in [:sendrecv, nil], do: other
+  defp get_direction(:inactive, _), do: :inactive
 end

lib/ex_webrtc/session_description.ex

@@ -16,7 +16,7 @@
   defstruct @enforce_keys
 
   @spec from_json(%{String.t() => String.t()}) :: {:ok, t()} | :error
-  def from_init(%{"type" => type})
+  def from_json(%{"type" => type})
       when type not in ["answer", "offer", "pranswer", "rollback"],
       do: :error
 

test/peer_connection_test.exs

@@ -102,6 +102,20 @@ defmodule ExWebRTC.PeerConnectionTest do
     refute_receive {:ex_webrtc, ^pc, {:track, %MediaStreamTrack{}}}
   end
 
+  test "offer/answer exchange" do
+    {:ok, pc1} = PeerConnection.start_link()
+    {:ok, _} = PeerConnection.add_transceiver(pc1, :audio)
+    {:ok, offer} = PeerConnection.create_offer(pc1)
+    :ok = PeerConnection.set_local_description(pc1, offer)
+
+    {:ok, pc2} = PeerConnection.start_link()
+    :ok = PeerConnection.set_remote_description(pc2, offer)
+    {:ok, answer} = PeerConnection.create_answer(pc2)
+    :ok = PeerConnection.set_local_description(pc2, answer)
+
+    :ok = PeerConnection.set_remote_description(pc1, answer)
+  end
+
   describe "set_remote_description/2" do
     test "MID" do
       {:ok, pc} = PeerConnection.start_link()