diff --git a/lib/ex_webrtc/peer_connection.ex b/lib/ex_webrtc/peer_connection.ex
index 9b11507c..b75cd48b 100644
--- a/lib/ex_webrtc/peer_connection.ex
+++ b/lib/ex_webrtc/peer_connection.ex
@@ -383,7 +383,7 @@ defmodule ExWebRTC.PeerConnection do
     demuxer = %{
       state.demuxer
       | extensions: SDPUtils.get_extensions(sdp),
-        payload_types: SDPUtils.get_payload_types(sdp)
+        pt_to_mid: SDPUtils.get_payload_types(sdp)
     }
 
     dtls =
diff --git a/lib/ex_webrtc/peer_connection/demuxer.ex b/lib/ex_webrtc/peer_connection/demuxer.ex
index 249886ea..3951e5c3 100644
--- a/lib/ex_webrtc/peer_connection/demuxer.ex
+++ b/lib/ex_webrtc/peer_connection/demuxer.ex
@@ -5,7 +5,7 @@ defmodule ExWebRTC.PeerConnection.Demuxer do
   alias ExRTP.Packet.Extension
   alias ExRTP.Packet.Extension.SourceDescription
 
-  defstruct ssrcs: %{}, extensions: %{}, payload_types: %{}
+  defstruct ssrc_to_mid: %{}, extensions: %{}, pt_to_mid: %{}
 
   def process_data(demuxer, data) do
     with {:ok, %Packet{} = packet} <- decode(data),
@@ -15,14 +15,12 @@ defmodule ExWebRTC.PeerConnection.Demuxer do
   end
 
   # RFC 8843, 9.2
-  defp match_to_mid(demuxer, packet) do
-    with demuxer <- update_mapping(demuxer, packet),
-         :error <- match_by_extension(demuxer, packet),
-         :error <- match_by_payload_type(demuxer, packet) do
-      {:error, :unmatched_stream}
-    else
-      {:ok, mid} -> {:ok, demuxer, mid}
-      {:ok, _demuxer, _mid} = res -> res
+  defp match_to_mid(demuxer, %Packet{ssrc: ssrc} = packet) do
+    demuxer = update_mapping(demuxer, packet)
+
+    case Map.get(demuxer.ssrc_to_mid, ssrc) do
+      {last_mid, _last_sn} -> {:ok, demuxer, last_mid}
+      nil -> match_by_payload_type(demuxer, packet)
     end
   end
 
@@ -40,29 +38,22 @@ defmodule ExWebRTC.PeerConnection.Demuxer do
         end
       end)
 
-    case Map.get(demuxer.ssrcs, ssrc) do
+    case Map.get(demuxer.ssrc_to_mid, ssrc) do
       {_last_mid, last_sn} when mid != nil and sn > last_sn ->
-        put_in(demuxer.ssrcs[ssrc], {mid, sn})
+        put_in(demuxer.ssrc_to_mid[ssrc], {mid, sn})
 
       nil when mid != nil ->
-        put_in(demuxer.ssrcs[ssrc], {mid, sn})
+        put_in(demuxer.ssrc_to_mid[ssrc], {mid, sn})
 
       _other ->
         demuxer
     end
   end
 
-  defp match_by_extension(demuxer, %Packet{ssrc: ssrc}) do
-    case Map.get(demuxer.ssrcs, ssrc) do
-      {last_mid, _last_sn} -> {:ok, last_mid}
-      nil -> :error
-    end
-  end
-
-  defp match_by_payload_type(demuxer, %Packet{ssrc: ssrc, payload_type: pt}) do
-    case Map.get(demuxer.payload_types, pt) do
+  defp match_by_payload_type(demuxer, %Packet{ssrc: ssrc, payload_type: pt, sequence_number: sn}) do
+    case Map.get(demuxer.pt_to_mid, pt) do
       nil -> :error
-      mid -> {:ok, put_in(demuxer.ssrcs[ssrc], mid), mid}
+      mid -> {:ok, put_in(demuxer.ssrc_to_mid[ssrc], {mid, sn}), mid}
     end
   end
 
diff --git a/test/configuration_test.exs b/test/configuration_test.exs
index e53c15b2..e335fc2e 100644
--- a/test/configuration_test.exs
+++ b/test/configuration_test.exs
@@ -1,5 +1,5 @@
 defmodule ExWebRTC.PeerConnection.ConfigurationTest do
-  use ExUnit.Case
+  use ExUnit.Case, async: true
 
   alias ExWebRTC.{PeerConnection, RTPCodecParameters, RTPTransceiver, SessionDescription}
 
diff --git a/test/peer_connection/demuxer_test.exs b/test/peer_connection/demuxer_test.exs
new file mode 100644
index 00000000..ab2a56b0
--- /dev/null
+++ b/test/peer_connection/demuxer_test.exs
@@ -0,0 +1,76 @@
+defmodule ExWebRTC.PeerConnection.DemuxerTest do
+  use ExUnit.Case, async: true
+
+  alias ExRTP.Packet
+  alias ExRTP.Packet.Extension
+  alias ExWebRTC.PeerConnection.Demuxer
+
+  @mid "1"
+
+  @sequence_number 500
+  @payload_type 111
+  @ssrc 333_333
+  @raw_packet %Packet{
+    payload_type: @payload_type,
+    sequence_number: @sequence_number,
+    timestamp: 0,
+    ssrc: @ssrc,
+    payload: <<>>
+  }
+
+  @packet Packet.encode(@raw_packet)
+  @packet_mid Packet.encode(
+                Packet.set_extension(@raw_packet, :two_byte, [%Extension{id: 15, data: @mid}])
+              )
+  @demuxer %Demuxer{extensions: %{15 => {Extension.SourceDescription, :mid}}}
+
+  test "ssrc already mapped and, without extension" do
+    seq_num = 1
+    demuxer = %Demuxer{@demuxer | ssrc_to_mid: %{@ssrc => {@mid, seq_num}}}
+
+    assert {:ok, new_demuxer, @mid, _packet} = Demuxer.process_data(demuxer, @packet)
+    assert new_demuxer == %Demuxer{demuxer | ssrc_to_mid: %{@ssrc => {@mid, seq_num}}}
+  end
+
+  test "ssrc already mapped, with extension with the same mid and bigger sequence number" do
+    demuxer = %Demuxer{@demuxer | ssrc_to_mid: %{@ssrc => {@mid, 1}}}
+
+    assert {:ok, new_demuxer, @mid, _packet} = Demuxer.process_data(demuxer, @packet_mid)
+    assert new_demuxer == %Demuxer{demuxer | ssrc_to_mid: %{@ssrc => {@mid, @sequence_number}}}
+  end
+
+  test "ssrc already mapped, with extension with new mid and smaller sequence number" do
+    seq_num = 600
+    mid = "2"
+    demuxer = %Demuxer{@demuxer | ssrc_to_mid: %{@ssrc => {mid, seq_num}}}
+
+    assert {:ok, new_demuxer, ^mid, _packet} = Demuxer.process_data(demuxer, @packet_mid)
+    assert new_demuxer == %Demuxer{demuxer | ssrc_to_mid: %{@ssrc => {mid, seq_num}}}
+  end
+
+  test "ssrc already mapped, with extension with new mid and bigger sequence number" do
+    seq_num = 1
+    mid = "2"
+    demuxer = %Demuxer{@demuxer | ssrc_to_mid: %{@ssrc => {mid, seq_num}}}
+
+    assert {:ok, new_demuxer, @mid, _packet} = Demuxer.process_data(demuxer, @packet_mid)
+    assert new_demuxer == %Demuxer{demuxer | ssrc_to_mid: %{@ssrc => {@mid, @sequence_number}}}
+  end
+
+  test "ssrc not mapped, with extension" do
+    assert {:ok, new_demuxer, @mid, _packet} = Demuxer.process_data(@demuxer, @packet_mid)
+    assert new_demuxer == %Demuxer{@demuxer | ssrc_to_mid: %{@ssrc => {@mid, @sequence_number}}}
+  end
+
+  test "ssrc not mapped, without extension, with unique payload type" do
+    mid = "2"
+    demuxer = %Demuxer{@demuxer | pt_to_mid: %{@payload_type => mid}}
+
+    assert {:ok, new_demuxer, ^mid, _packet} = Demuxer.process_data(demuxer, @packet)
+    assert new_demuxer == %Demuxer{demuxer | ssrc_to_mid: %{@ssrc => {mid, @sequence_number}}}
+  end
+
+  test "unmatchable ssrc" do
+    assert :error = Demuxer.process_data(@demuxer, @packet)
+  end
+end