Simpler flowlabel parsing; avoid using 0 flowlabel.

src/yggdrasil/session.go

@@ -425,28 +425,28 @@ func (sinfo *sessionInfo) doSend(bs []byte) {
 	if !sinfo.init {
 		return
 	} // To prevent using empty session keys
+
+	var coords []byte
+	// Read IPv6 flowlabel field (20 bits).
+	// XXX(cathugger): is len(bs) validated there?
+	flowlabel := uint(bs[1]&0x0f)<<16 | uint(bs[2])<<8 | uint(bs[3])
+	if flowlabel != 0 {
 		// Now we append something to the coords
 		// Specifically, we append a 0, and then arbitrary data
 		// The 0 ensures that the destination node switch forwards to the self peer (router)
 		// The rest is ignored, but it's still part as the coords, so it affects switch queues
 		// This helps separate traffic streams (coords, flowlabel) to be queued independently
-	var coords []byte
-	addUint64 := func(bs []byte) {
-		// Converts bytes to a uint64
-		// Converts that back to variable length bytes
-		// Appends it to coords
-		var u uint64
-		for _, b := range bs {
-			u <<= 8
-			u |= uint64(b)
-		}
-		coords = append(coords, wire_encode_uint64(u)...)
-	}
+
 		coords = append(coords, sinfo.coords...)                          // Start with the real coords
 		coords = append(coords, 0)                                        // Then target the local switchport
-	flowlabel := append([]byte(nil), bs[1:4]...)
-	flowlabel[0] &= 0x0f
-	addUint64(flowlabel)
+		coords = append(coords, wire_encode_uint64(uint64(flowlabel))...) // Then variable-length encoded flowlabel
+	} else {
+		// 0 value means that flowlabels aren't being generated by OS.
+		// To save bytes, we're not including it, therefore we won't need self-port override either.
+		// So just use sinfo.coords directly to avoid golang GC allocations.
+		// XXX: investigate where flowlabels aren't included, and attempt to look into TCP/UDP/SCTP/DCCP headers' sport/dport fields?
+		coords = sinfo.coords
+	}
 	payload, nonce := boxSeal(&sinfo.sharedSesKey, bs, &sinfo.myNonce)
 	defer util_putBytes(payload)
 	p := wire_trafficPacket{