Add Generic NACK feedback packet type

lib/ex_rtcp/packet.ex

@@ -17,6 +17,7 @@ defmodule ExRTCP.Packet do
           | __MODULE__.ReceiverReport.t()
           | __MODULE__.SourceDescription.t()
           | __MODULE__.Goodbye.t()
+          | __MODULE__.TransportFeedback.NACK.t()
           | __MODULE__.TransportFeedback.CC.t()
 
   @typedoc """
@@ -87,6 +88,7 @@ defmodule ExRTCP.Packet do
   defp get_type_module(201, _count), do: {:ok, __MODULE__.ReceiverReport}
   defp get_type_module(202, _count), do: {:ok, __MODULE__.SourceDescription}
   defp get_type_module(203, _count), do: {:ok, __MODULE__.Goodbye}
+  defp get_type_module(205, 1), do: {:ok, __MODULE__.TransportFeedback.NACK}
   defp get_type_module(205, 15), do: {:ok, __MODULE__.TransportFeedback.CC}
   defp get_type_module(_type, _count), do: {:error, :unknown_type}
 end

lib/ex_rtcp/packet/transport_feedback/nack.ex

@@ -0,0 +1,83 @@
+defmodule ExRTCP.Packet.TransportFeedback.NACK do
+  @moduledoc """
+  Transport layer feedback message with generic NACKs
+  packet type (`RFC 4585`, sec. 6.2.1).
+  """
+
+  alias ExRTCP.Packet
+
+  @behaviour ExRTCP.PacketTranscoder
+
+  @packet_type 205
+  @feedback_type 1
+
+  @typedoc """
+  Generic NACK message, as described in `RFC 4585`, sec. 6.2.1.
+  `blp` must be a 16-bit binary.
+  """
+  @type generic_nack() :: %{
+          pid: Packet.uint16(),
+          blp: binary()
+        }
+
+  @typedoc """
+  Struct representing Transport layer NACK feedback RTCP message.
+  """
+  @type t() :: %__MODULE__{
+          sender_ssrc: Packet.uint32(),
+          media_ssrc: Packet.uint32(),
+          nacks: [generic_nack()]
+        }
+
+  @enforce_keys [:sender_ssrc, :media_ssrc, :nacks]
+  defstruct @enforce_keys
+
+  @impl true
+  def encode(packet) do
+    %__MODULE__{
+      sender_ssrc: sender_ssrc,
+      media_ssrc: media_ssrc,
+      nacks: nacks
+    } = packet
+
+    encoded_nacks = encode_nacks(nacks)
+    encoded = <<sender_ssrc::32, media_ssrc::32, encoded_nacks::binary>>
+
+    {encoded, @feedback_type, @packet_type}
+  end
+
+  defp encode_nacks(nacks, acc \\ <<>>)
+  defp encode_nacks([], acc), do: acc
+
+  defp encode_nacks([nack | nacks], acc) do
+    %{pid: pid, blp: blp} = nack
+    encoded = <<pid::16, blp::binary-size(2)>>
+
+    encode_nacks(nacks, acc <> encoded)
+  end
+
+  @impl true
+  def decode(<<sender_ssrc::32, media_ssrc::32, rest::binary>>, _count) do
+    with {:ok, nacks} <- decode_nacks(rest) do
+      packet = %__MODULE__{
+        sender_ssrc: sender_ssrc,
+        media_ssrc: media_ssrc,
+        nacks: nacks
+      }
+
+      {:ok, packet}
+    end
+  end
+
+  def decode(_raw, _count), do: {:error, :invalid_packet}
+
+  defp decode_nacks(raw, acc \\ [])
+  defp decode_nacks(<<>>, acc), do: {:ok, Enum.reverse(acc)}
+
+  defp decode_nacks(<<pid::16, blp::binary-size(2), rest::binary>>, acc) do
+    nack = %{pid: pid, blp: blp}
+    decode_nacks(rest, [nack | acc])
+  end
+
+  defp decode_nacks(_raw, _acc), do: {:error, :invalid_packet}
+end

test/ex_rtcp/packet/transport_feedback/nack_test.exs

@@ -0,0 +1,60 @@
+defmodule ExRTCP.Packet.TransportFeedback.NACKTest do
+  use ExUnit.Case, async: true
+
+  alias ExRTCP.Packet.TransportFeedback.NACK
+
+  @sender_ssrc 123_321
+  @media_ssrc 112_231
+
+  test "encode/1" do
+    pid_1 = 35_042
+    blp_1 = <<5, 1>>
+    pid_2 = 21_042
+    blp_2 = <<14, 81>>
+
+    packet = %NACK{
+      sender_ssrc: @sender_ssrc,
+      media_ssrc: @media_ssrc,
+      nacks: [%{pid: pid_1, blp: blp_1}, %{pid: pid_2, blp: blp_2}]
+    }
+
+    assert {encoded, 1, 205} = NACK.encode(packet)
+
+    assert <<
+             @sender_ssrc::32,
+             @media_ssrc::32,
+             pid_1::16,
+             blp_1::binary-size(2),
+             pid_2::16,
+             blp_2::binary-size(2)
+           >> == encoded
+  end
+
+  describe "decode/2" do
+    test "valid packet" do
+      pid_1 = 35_043
+      blp_1 = <<2, 9>>
+      nack_1 = <<pid_1::16, blp_1::binary>>
+      pid_2 = 53_221
+      blp_2 = <<4, 5>>
+      nack_2 = <<pid_2::16, blp_2::binary>>
+
+      raw_packet = <<@sender_ssrc::32, @media_ssrc::32, nack_1::binary, nack_2::binary>>
+
+      assert {:ok, packet} = NACK.decode(raw_packet, 15)
+
+      assert %NACK{
+               sender_ssrc: @sender_ssrc,
+               media_ssrc: @media_ssrc,
+               nacks: [%{pid: ^pid_1, blp: ^blp_1}, %{pid: ^pid_2, blp: ^blp_2}]
+             } = packet
+    end
+
+    test "invalid packet" do
+      # blp of first nack is cut after 8 bits
+      raw_packet = <<@sender_ssrc::32, @media_ssrc::32, 53::16, 0>>
+
+      assert {:error, :invalid_packet} = NACK.decode(raw_packet, 15)
+    end
+  end
+end