diff --git a/components/net/sf/briar/transport/ConnectionWindowImpl.java b/components/net/sf/briar/transport/ConnectionWindowImpl.java
index 25910776f..fe0c1735a 100644
--- a/components/net/sf/briar/transport/ConnectionWindowImpl.java
+++ b/components/net/sf/briar/transport/ConnectionWindowImpl.java
@@ -31,6 +31,7 @@ class ConnectionWindowImpl implements ConnectionWindow {
 	public void setSeen(long connectionNumber) {
 		int offset = getOffset(connectionNumber);
 		int mask = 0x80000000 >>> offset;
+		if((bitmap & mask) != 0) throw new IllegalArgumentException();
 		bitmap |= mask;
 		// If the new connection number is above the centre, slide the window
 		if(connectionNumber >= centre) {
diff --git a/components/net/sf/briar/transport/TransportModule.java b/components/net/sf/briar/transport/TransportModule.java
index a2a6807cb..e1dd260a0 100644
--- a/components/net/sf/briar/transport/TransportModule.java
+++ b/components/net/sf/briar/transport/TransportModule.java
@@ -1,7 +1,6 @@
 package net.sf.briar.transport;
 
 import net.sf.briar.api.transport.ConnectionWindowFactory;
-import net.sf.briar.api.transport.PacketWriter;
 
 import com.google.inject.AbstractModule;
 
diff --git a/test/build.xml b/test/build.xml
index a7b466eef..f034b76f4 100644
--- a/test/build.xml
+++ b/test/build.xml
@@ -32,6 +32,7 @@
 			<test name='net.sf.briar.serial.ReaderImplTest'/>
 			<test name='net.sf.briar.serial.WriterImplTest'/>
 			<test name='net.sf.briar.setup.SetupWorkerTest'/>
+			<test name='net.sf.briar.transport.ConnectionWindowImplTest'/>
 			<test name='net.sf.briar.transport.PacketEncrypterImplTest'/>
 			<test name='net.sf.briar.transport.PacketWriterImplTest'/>
 			<test name='net.sf.briar.util.FileUtilsTest'/>
diff --git a/test/net/sf/briar/transport/ConnectionWindowImplTest.java b/test/net/sf/briar/transport/ConnectionWindowImplTest.java
new file mode 100644
index 000000000..32a32ff6a
--- /dev/null
+++ b/test/net/sf/briar/transport/ConnectionWindowImplTest.java
@@ -0,0 +1,82 @@
+package net.sf.briar.transport;
+
+import junit.framework.TestCase;
+
+import org.junit.Test;
+
+public class ConnectionWindowImplTest extends TestCase {
+
+	@Test
+	public void testWindowSliding() {
+		ConnectionWindowImpl w = new ConnectionWindowImpl(0L, 0);
+		for(int i = 0; i < 100; i++) {
+			assertFalse(w.isSeen(i));
+			w.setSeen(i);
+			assertTrue(w.isSeen(i));
+		}
+	}
+
+	@Test
+	public void testWindowJumping() {
+		ConnectionWindowImpl w = new ConnectionWindowImpl(0L, 0);
+		for(int i = 0; i < 100; i += 13) {
+			assertFalse(w.isSeen(i));
+			w.setSeen(i);
+			assertTrue(w.isSeen(i));
+		}
+	}
+
+	@Test
+	public void testWindowUpperLimit() {
+		ConnectionWindowImpl w = new ConnectionWindowImpl(0L, 0);
+		// Centre is 0, highest value in window is 15
+		w.setSeen(15);
+		// Centre is 16, highest value in window is 31
+		w.setSeen(31);
+		try {
+			// Centre is 32, highest value in window is 47
+			w.setSeen(48);
+			fail();
+		} catch(IllegalArgumentException expected) {}
+	}
+
+	@Test
+	public void testWindowLowerLimit() {
+		ConnectionWindowImpl w = new ConnectionWindowImpl(0L, 0);
+		// Centre is 0, negative values should never be allowed
+		try {
+			w.setSeen(-1);
+			fail();
+		} catch(IllegalArgumentException expected) {}
+		// Slide the window
+		w.setSeen(15);
+		// Centre is 16, lowest value in window is 0
+		w.setSeen(0);
+		// Slide the window
+		w.setSeen(16);
+		// Centre is 17, lowest value in window is 1
+		w.setSeen(1);
+		try {
+			w.setSeen(0);
+			fail();
+		} catch(IllegalArgumentException expected) {}
+		// Slide the window
+		w.setSeen(25);
+		// Centre is 26, lowest value in window is 10
+		w.setSeen(10);
+		try {
+			w.setSeen(9);
+			fail();
+		} catch(IllegalArgumentException expected) {}
+	}
+
+	@Test
+	public void testCannotSetSameValueTwice() {
+		ConnectionWindowImpl w = new ConnectionWindowImpl(0L, 0);
+		w.setSeen(15);
+		try {
+			w.setSeen(15);
+			fail();
+		} catch(IllegalArgumentException expected) {}
+	}
+}