refactor(ish) of special frame processing in udp frame handler

- pull all special frame handling in one place
- generalise the condition for detecting special frames
- add some explanation

router/ethernet_decoder.go

@@ -75,7 +75,7 @@ func (dec *EthernetDecoder) DropFrame() bool {
 	return bytes.Equal(stpMACPrefix, dec.eth.DstMAC[:len(stpMACPrefix)])
 }
 
-func (dec *EthernetDecoder) IsPMTUVerify() bool {
-	return bytes.Equal(zeroMAC, dec.eth.SrcMAC) &&
-		bytes.Equal(zeroMAC, dec.eth.DstMAC)
+func (dec *EthernetDecoder) IsSpecial() bool {
+	return dec.eth.Length == 0 && dec.eth.EthernetType == layers.EthernetTypeLLC &&
+		bytes.Equal(zeroMAC, dec.eth.SrcMAC) && bytes.Equal(zeroMAC, dec.eth.DstMAC)
 }

router/router.go

@@ -247,13 +247,38 @@ func (router *Router) handleUDPPacketFunc(dec *EthernetDecoder, po PacketSink) F
 
 		dec.DecodeLayers(frame)
 		decodedLen := len(dec.decoded)
+
+		// Handle special frames produced internally (rather than
+		// captured/forwarded) by the remote router.
+		//
+		// We really shouldn't be decoding these above, since they are
+		// not genuine Ethernet frames. However, it is actually more
+		// efficient to do so, as we want to optimise for the common
+		// (i.e. non-special) frames. These always need decoding, and
+		// detecting special frames is cheaper post decoding than pre.
+		if decodedLen == 0 || (decodedLen == 1 && dec.IsSpecial()) {
+			if srcPeer != relayConn.Remote() || dstPeer != router.Ourself.Peer {
+				// A special frame not originating from the remote, or
+				// not for us? How odd; let's just drop it.
+				return nil
+			}
+			if frameLen == 0 {
+				relayConn.ReceivedHeartbeat(sender)
+			} else if frameLen == FragTestSize && bytes.Equal(frame, FragTest) {
+				relayConn.SendTCP(ProtocolFragmentationReceivedByte)
+			} else if frameLen == PMTUDiscoverySize && bytes.Equal(frame, PMTUDiscovery) {
+			} else if decodedLen == 1 {
+				frameLenBytes := []byte{0, 0}
+				binary.BigEndian.PutUint16(frameLenBytes, uint16(frameLen-EthernetOverhead))
+				relayConn.SendTCP(Concat(ProtocolPMTUVerifiedByte, frameLenBytes))
+			}
+			return nil
+		}
+
 		df := decodedLen == 2 && (dec.ip.Flags&layers.IPv4DontFragment != 0)
 
 		if dstPeer != router.Ourself.Peer {
 			// it's not for us, we're just relaying it
-			if decodedLen == 0 {
-				return nil
-			}
 			if df {
 				router.LogFrame("Relaying DF", frame, &dec.eth)
 			} else {
@@ -262,29 +287,6 @@ func (router *Router) handleUDPPacketFunc(dec *EthernetDecoder, po PacketSink) F
 			return checkFrameTooBig(router.Ourself.Relay(srcPeer, dstPeer, df, frame, dec), srcPeer)
 		}
 
-		if relayConn.Remote() == srcPeer {
-			if frameLen == 0 {
-				relayConn.ReceivedHeartbeat(sender)
-				return nil
-			} else if frameLen == FragTestSize && bytes.Equal(frame, FragTest) {
-				relayConn.SendTCP(ProtocolFragmentationReceivedByte)
-				return nil
-			} else if frameLen == PMTUDiscoverySize && bytes.Equal(frame, PMTUDiscovery) {
-				return nil
-			}
-		}
-
-		if decodedLen == 0 {
-			return nil
-		}
-
-		if dec.IsPMTUVerify() && relayConn.Remote() == srcPeer {
-			frameLenBytes := []byte{0, 0}
-			binary.BigEndian.PutUint16(frameLenBytes, uint16(frameLen-EthernetOverhead))
-			relayConn.SendTCP(Concat(ProtocolPMTUVerifiedByte, frameLenBytes))
-			return nil
-		}
-
 		srcMac := dec.eth.SrcMAC
 		dstMac := dec.eth.DstMAC