diff --git a/Program.cs b/Program.cs
index 30ce55c..87c935a 100644
--- a/Program.cs
+++ b/Program.cs
@@ -1,5 +1,5 @@
 using System.IO;
-using System;
+using System;
 using System.Net.Sockets;
 using System.Collections.Generic;
 using System.Net;
@@ -475,7 +475,6 @@ static P2pEndPoint GetExternalEndPoint(Socket socket)
             return null;
         }
 
-
         static int SleepTime(DateTime now)
         {
             List<int> seconds = new List<int>() {10, 20, 30, 40, 50, 60};
diff --git a/Receiver.cs b/Receiver.cs
index 7a1ef20..f49cc76 100644
--- a/Receiver.cs
+++ b/Receiver.cs
@@ -1,15 +1,13 @@
 using System;
-using System.Collections.Generic;
 using System.IO;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
 using UdtSharp;
 
 namespace p2pcopy
 {
     static class Receiver
     {
+        const Roles myRole = Roles.Receiver;
+
         static internal void Run(UdtSocket conn)
         {
             int ini = Environment.TickCount;
@@ -18,6 +16,17 @@ static internal void Run(UdtSocket conn)
             using (var writer = new BinaryWriter(netStream))
             using (var reader = new BinaryReader(netStream))
             {
+                // transmit your role and check if connected peer has the opposite role
+                writer.Write(myRole.ToString());
+                string role = reader.ReadString();
+
+                if (role == myRole.ToString())
+                {
+                    Console.Error.WriteLine("Peers can't have the same role.");
+                    Environment.Exit(1);
+                    return;
+                }
+
                 string fileName = reader.ReadString();
                 long size = reader.ReadInt64();
 
diff --git a/Roles.cs b/Roles.cs
new file mode 100644
index 0000000..f7ab3a7
--- /dev/null
+++ b/Roles.cs
@@ -0,0 +1,8 @@
+namespace p2pcopy
+{
+    enum Roles
+    {
+        Receiver,
+        Sender,
+    }
+}
diff --git a/Sender.cs b/Sender.cs
index d052f3b..3f6b5da 100644
--- a/Sender.cs
+++ b/Sender.cs
@@ -6,6 +6,8 @@ namespace p2pcopy
 {
     static class Sender
     {
+        const Roles myRole = Roles.Sender;
+
         static internal void Run(UdtSocket conn, string file, bool bVerbose)
         {
             int ini = Environment.TickCount;
@@ -15,6 +17,17 @@ static internal void Run(UdtSocket conn, string file, bool bVerbose)
             using (var reader = new BinaryReader(netStream))
             using (var fileReader = new FileStream(file, FileMode.Open, FileAccess.Read))
             {
+                // transmit your role and check if connected peer has the opposite role
+                writer.Write(myRole.ToString());
+                string role = reader.ReadString();
+
+                if (role == myRole.ToString())
+                {
+                    Console.Error.WriteLine("Peers can't have the same role.");
+                    Environment.Exit(1);
+                    return;
+                }
+
                 long fileSize = new FileInfo(file).Length;
 
                 writer.Write(Path.GetFileName(file));
diff --git a/p2pcopy.csproj b/p2pcopy.csproj
index fc14775..6d062e4 100644
--- a/p2pcopy.csproj
+++ b/p2pcopy.csproj
@@ -45,6 +45,7 @@
     <Compile Include="Program.cs" />
     <Compile Include="Properties\AssemblyInfo.cs" />
     <Compile Include="Receiver.cs" />
+    <Compile Include="Roles.cs" />
     <Compile Include="Sender.cs" />
     <Compile Include="SizeConverter.cs" />
     <Compile Include="StunClient.cs" />
@@ -64,4 +65,4 @@
   <Target Name="AfterBuild">
   </Target>
   -->
-</Project>
+</Project>
\ No newline at end of file