diff --git a/lib/openflow/group_mod.ex b/lib/openflow/group_mod.ex
index 4cff99d..bb3c217 100644
--- a/lib/openflow/group_mod.ex
+++ b/lib/openflow/group_mod.ex
@@ -19,7 +19,7 @@ defmodule Openflow.GroupMod do
   @type t :: %GroupMod{
           version: 4,
           datapath_id: String.t() | nil,
-          aux_id: 0..0xF | nil,
+          aux_id: 0..0xFF | nil,
           xid: 0..0xFFFFFFFF,
           command: command(),
           type: type(),
diff --git a/lib/openflow/multipart/group/reply.ex b/lib/openflow/multipart/group/reply.ex
index 5e4b86f..499973d 100644
--- a/lib/openflow/multipart/group/reply.ex
+++ b/lib/openflow/multipart/group/reply.ex
@@ -11,14 +11,21 @@ defmodule Openflow.Multipart.Group.Reply do
 
   alias __MODULE__
 
+  @type t :: %Reply{
+          version: 4,
+          xid: 0..0xFFFFFFFF,
+          datapath_id: String.t() | nil,
+          aux_id: 0..0xFF | nil,
+          flags: [:more],
+          groups: [Openflow.Multipart.GroupStats.t()]
+        }
+
+  @spec ofp_type() :: 18
   def ofp_type, do: 18
 
-  def new(groups \\ []) do
-    %Reply{groups: groups}
-  end
-
+  @spec read(binary()) :: t()
   def read(<<groups_bin::bytes>>) do
-    groups = Openflow.Multipart.Group.read(groups_bin)
+    groups = Openflow.Multipart.GroupStats.read(groups_bin)
     %Reply{groups: groups}
   end
 
@@ -36,7 +43,7 @@ defmodule Openflow.Multipart.Group.Reply do
   end
 end
 
-defmodule Openflow.Multipart.Group do
+defmodule Openflow.Multipart.GroupStats do
   defstruct(
     group_id: 0,
     ref_count: 0,
@@ -51,6 +58,15 @@ defmodule Openflow.Multipart.Group do
 
   alias __MODULE__
 
+  @type t :: %GroupStats{
+          group_id: 0..0xFFFFFFFF,
+          ref_count: 0..0xFFFFFFFF,
+          packet_count: 0..0xFFFFFFFFFFFFFFFF,
+          byte_count: 0..0xFFFFFFFFFFFFFFFF,
+          duration_sec: 0..0xFFFFFFFF,
+          duration_nsec: 0..0xFFFFFFFF
+        }
+
   def read(binary) do
     do_read([], binary)
   end
@@ -72,11 +88,10 @@ defmodule Openflow.Multipart.Group do
     <<bucket_stats_bin::size(bucket_stats_size)-bytes, _rest::bytes>> = tail
 
     bucket_stats =
-      for <<packet_count::64, byte_count::64 <- bucket_stats_bin>> do
-        %{packet_count: packet_count, byte_count: byte_count}
-      end
+      for <<packet_count::64, byte_count::64 <- bucket_stats_bin>>,
+        do: %{packet_count: packet_count, byte_count: byte_count}
 
-    %Group{
+    %GroupStats{
       group_id: group_id,
       ref_count: ref_count,
       packet_count: packet_count,
diff --git a/lib/openflow/multipart/group/request.ex b/lib/openflow/multipart/group/request.ex
index 30103d8..523467f 100644
--- a/lib/openflow/multipart/group/request.ex
+++ b/lib/openflow/multipart/group/request.ex
@@ -4,34 +4,43 @@ defmodule Openflow.Multipart.Group.Request do
     xid: 0,
     # virtual field
     datapath_id: nil,
+    aux_id: nil,
     flags: [],
     group_id: :all
   )
 
   alias __MODULE__
 
+  @type t :: %Request{
+          version: 4,
+          xid: 0..0xFFFFFFFF,
+          datapath_id: String.t() | nil,
+          aux_id: 0..0xFF | nil,
+          flags: [],
+          group_id: Openflow.GroupMod.id()
+        }
+
+  @spec ofp_type() :: 18
   def ofp_type, do: 18
 
-  def new(options) when is_list(options) do
+  @spec new(xid: 0..0xFFFFFFFF, group_id: Openflow.GroupMod.id()) :: t()
+  def new(options \\ []) do
     %Request{
       xid: options[:xid] || 0,
       group_id: options[:group_id] || :all
     }
   end
 
-  def new(group_id) when is_integer(group_id) or is_atom(group_id) do
-    %Request{group_id: group_id}
-  end
-
+  @spec read(<<_::32, _::_*8>>) :: t()
   def read(<<group_id_int::32, _::size(4)-unit(8)>>) do
     group_id = Openflow.Utils.get_enum(group_id_int, :group_id)
     %Request{group_id: group_id}
   end
 
+  @spec to_binary(t()) :: binary()
   def to_binary(%Request{group_id: group_id} = msg) do
     group_id_int = Openflow.Utils.get_enum(group_id, :group_id)
-    body_bin = <<group_id_int::32, 0::size(4)-unit(8)>>
     header_bin = Openflow.Multipart.Request.header(msg)
-    <<header_bin::bytes, body_bin::bytes>>
+    <<header_bin::bytes, group_id_int::32, 0::size(4)-unit(8)>>
   end
 end
diff --git a/test/lib/openflow/ofp_group_stats_test.exs b/test/lib/openflow/ofp_group_stats_test.exs
new file mode 100644
index 0000000..a02496f
--- /dev/null
+++ b/test/lib/openflow/ofp_group_stats_test.exs
@@ -0,0 +1,47 @@
+defmodule OfpGroupStatsTest do
+  use ExUnit.Case
+
+  describe "Openflow.Multipart.Group.Request" do
+    test "with default values" do
+      group_stats =
+        %Openflow.Multipart.Group.Request{}
+        |> Map.to_list()
+        |> Openflow.Multipart.Group.Request.new()
+        |> Openflow.to_binary()
+        |> Openflow.read()
+        |> Kernel.elem(1)
+
+      assert group_stats.xid == 0
+      assert group_stats.group_id == :all
+    end
+  end
+
+  describe "Openflow.Multipart.Group.Reply" do
+    test "with test packet_data" do
+      group_stats =
+        "test/packet_data/4-58-ofp_group_stats_reply.packet"
+        |> File.read!()
+        |> Openflow.read()
+        |> Kernel.elem(1)
+
+      %Openflow.Multipart.Group.Reply{
+        aux_id: nil,
+        datapath_id: nil,
+        flags: [],
+        groups: [
+          %Openflow.Multipart.GroupStats{
+            bucket_stats: [%{byte_count: 2345, packet_count: 234}],
+            byte_count: 12345,
+            duration_nsec: 609_036_000,
+            duration_sec: 9,
+            group_id: 1,
+            packet_count: 123,
+            ref_count: 2
+          }
+        ],
+        version: 4,
+        xid: 0
+      } = group_stats
+    end
+  end
+end