plumb ECN through via relays

inside.go

@@ -520,21 +520,16 @@ func (f *Interface) prepareSendVia(via *HostInfo,
 // ad is the plaintext data to authenticate, but not encrypt
 // nb is a buffer used to store the nonce value, re-used for performance reasons.
 // out is a buffer used to store the result of the Encrypt operation
+// outerECN is the 2-bit codepoint to stamp on the carrier datagram (0 for control traffic).
 // q indicates which writer to use to send the packet.
-func (f *Interface) SendVia(via *HostInfo,
-	relay *Relay,
-	ad,
-	nb,
-	out []byte,
-	nocopy bool,
-) {
+func (f *Interface) SendVia(via *HostInfo, relay *Relay, ad, nb, out []byte, nocopy bool, outerECN byte, q int) {
 	toSend, err := f.prepareSendVia(via, relay, ad, nb, out, nocopy)
 	if err != nil {
 		// already logged by prepareSendVia
 		return
 	}
 
-	err = f.writers[0].WriteTo(toSend, via.GetRemote())
+	err = f.writers[q].WriteTo(toSend, via.GetRemote(), outerECN)
 	if err != nil {
 		via.logger(f.l).Info("Failed to WriteTo in sendVia", "error", err)
 	}
@@ -595,8 +590,15 @@ func (f *Interface) sendNoMetrics(t header.MessageType, st header.MessageSubType
 		return
 	}
 
+	// Data packets copy the inner packet's ECN codepoint onto the outer
+	// carrier per RFC 6040; control traffic stays Not-ECT.
+	var outerECN byte
+	if t == header.Message && f.ecnEnabled.Load() {
+		outerECN = innerECN(p)
+	}
+
 	if remote.IsValid() {
-		err = f.writers[q].WriteTo(out, remote)
+		err = f.writers[q].WriteTo(out, remote, outerECN)
 		if err != nil {
 			hostinfo.logger(f.l).Error("Failed to write outgoing packet",
 				"error", err,
@@ -604,7 +606,7 @@ func (f *Interface) sendNoMetrics(t header.MessageType, st header.MessageSubType
 			)
 		}
 	} else if hr := hostinfo.GetRemote(); hr.IsValid() {
-		err = f.writers[q].WriteTo(out, hr)
+		err = f.writers[q].WriteTo(out, hr, outerECN)
 		if err != nil {
 			hostinfo.logger(f.l).Error("Failed to write outgoing packet",
 				"error", err,
@@ -623,7 +625,7 @@ func (f *Interface) sendNoMetrics(t header.MessageType, st header.MessageSubType
 				)
 				continue
 			}
-			f.SendVia(relayHostInfo, relay, out, nb, fullOut[:header.Len+len(out)], true)
+			f.SendVia(relayHostInfo, relay, out, nb, fullOut[:header.Len+len(out)], true, outerECN, q)
 			break
 		}
 	}