Fixed TCP checksums for odd length headers

Author: Noah Potash

Diff for: src/LibPacketGremlin/LibPacketGremlin.csproj

@@ -14,9 +14,9 @@
     <PackageProjectUrl>https://github.com/SapientGuardian/LibPacketGremlin</PackageProjectUrl>
     <PackageLicenseUrl>http://opensource.org/licenses/MIT</PackageLicenseUrl>
     <PackageTargetFallback>$(PackageTargetFallback);dnxcore50</PackageTargetFallback>
-    <Version>1.1.0</Version>
-    <AssemblyVersion>0.1.1.0</AssemblyVersion>
-    <FileVersion>0.1.1.0</FileVersion>
+    <Version>1.1.1</Version>
+    <AssemblyVersion>0.1.1.1</AssemblyVersion>
+    <FileVersion>0.1.1.1</FileVersion>
   </PropertyGroup>
 
   <ItemGroup>

Diff for: src/LibPacketGremlin/Packets/TCP.cs

@@ -330,12 +330,20 @@ internal static ushort ComputeChecksum(byte[] header, int start, int length)
         {
             ushort word16;
             long sum = 0;
-            for (var i = start; i < length + start; i += 2)
+
+            int i;
+            for (i = start; i < length + start - 1; i += 2)
             {
                 word16 = (ushort)(((header[i] << 8) & 0xFF00) + (header[i + 1] & 0xFF));
                 sum += word16;
             }
 
+            if (length % 2 != 0)
+            {
+                word16 = (ushort)((header[i] << 8) & 0xFF00);
+                sum += word16;
+            }
+
             while (sum >> 16 != 0)
             {
                 sum = (sum & 0xFFFF) + (sum >> 16);

Diff for: test/LibPacketGremlinTests/Packets/TCPTests.cs

@@ -43,7 +43,7 @@ public void ParsesBasicFields()
             packet.Fin.Should().BeFalse();
             packet.WindowSize.Should().Be(64078);
             packet.Checksum.Should().Be(0x1b5e);
-            packet.UrgentPointer.Should().Be(0);            
+            packet.UrgentPointer.Should().Be(0);
         }
 
         [Fact]
@@ -57,10 +57,10 @@ public void SerializesCorrectly()
                 , 0xcb, 0xde, 0xa7, 0xcf, 0xeb, 0x46, 0xd4, 0x88, 0x48, 0x48, 0x91, 0xda
                 , 0xb9, 0xd8, 0xd7, 0x52, 0xef, 0x36, 0x2a}, out packet).Should().BeTrue();
 
-            
+
             using (var ms = new MemoryStream())
             {
-                packet.WriteToStream(ms);                
+                packet.WriteToStream(ms);
                 ms.ToArray()
                     .SequenceEqual(
                         new byte[]
@@ -76,22 +76,21 @@ public void SerializesCorrectly()
         }
 
         [Fact]
-        public void CorrectsFields()
-        {
-            // This test is poor. It seems the original test for CorrectFields didn't set an IPv4 parent, and thus was basically worthless.
-
-            TCP packet;
-            TCPFactory.Instance.TryParse(new byte[]{0x04, 0x0a
-                , 0x14, 0x46, 0x45, 0x0e, 0x6e, 0xc8, 0xfb, 0xcd, 0x4f, 0xcc, 0x50, 0x18
-                , 0xfa, 0x4e, 0x1b, 0x5e, 0x00, 0x00, 0x17, 0x03, 0x01, 0x00, 0x20, 0x0f
-                , 0x52, 0xba, 0x8d, 0xda, 0x74, 0x2a, 0xd7, 0x3d, 0x59, 0x2f, 0x22, 0x5e
-                , 0xcb, 0xde, 0xa7, 0xcf, 0xeb, 0x46, 0xd4, 0x88, 0x48, 0x48, 0x91, 0xda
-                , 0xb9, 0xd8, 0xd7, 0x52, 0xef, 0x36, 0x2a}, out packet).Should().BeTrue();
-
-            var correctChecksum = packet.Checksum;            
-            packet.CorrectFields();
-            packet.Checksum.Should().Be(correctChecksum);
-            ((int)packet.DataOffset).Should().Be(5);
+        public void CalculatesChecksum()
+        {            
+            IPv4 packet;
+            IPv4Factory.Instance.TryParse(new byte[]{ 0x45, 0x00,0x00, 0x29,0x10, 0xe9,
+0x40, 0x00, 0x80, 0x06, 0x00, 0x00, 0x7f, 0x00,
+0x00, 0x01, 0x7f, 0x00, 0x00, 0x01, 0xd4, 0x68,
+0xd4, 0x69, 0xb6, 0x08, 0xff, 0x50, 0x01, 0x1e,
+0x37, 0xd9, 0x50, 0x18, 0x01, 0x00, 0xf8, 0xa5,
+0x00, 0x00, 0x21 }, out packet).Should().BeTrue();
+
+            var e2 = EthernetIIFactory.Instance.Default(packet);
+            var tcp = packet.Layer<TCP>();
+            var correctChecksum = tcp.Checksum;
+            e2.CorrectFields();
+            tcp.Checksum.Should().Be(correctChecksum);
         }
 
         [Fact]