handshake: Add ix_psk2_mlkem1024, IX with an ephemeral ML-KEM-1024 exchange in the psk2 slot

header/header.go

@@ -67,8 +67,19 @@ const (
 const (
 	HandshakeIXPSK0 MessageSubType = 0
 	HandshakeXXPSK0 MessageSubType = 1
+
+	// HandshakeIXPSK2MLKEM1024 is IX with an ephemeral ML-KEM-1024 exchange mixed
+	// in through the psk2 slot. Its messages exceed the MTU and travel as
+	// fragments (see handshake/fragment).
+	HandshakeIXPSK2MLKEM1024 MessageSubType = 2
 )
 
+// IsFragmented reports whether handshake messages of this subtype travel as
+// fragments.
+func IsFragmented(st MessageSubType) bool {
+	return st == HandshakeIXPSK2MLKEM1024
+}
+
 var ErrHeaderTooShort = errors.New("header is too short")
 
 var subTypeTestMap = map[MessageSubType]string{
@@ -88,7 +99,8 @@ var subTypeMap = map[MessageType]*map[MessageSubType]string{
 	Test:        &subTypeTestMap,
 	CloseTunnel: &subTypeNoneMap,
 	Handshake: {
-		HandshakeIXPSK0: "ix_psk0",
+		HandshakeIXPSK0:          "ix_psk0",
+		HandshakeIXPSK2MLKEM1024: "ix_psk2_mlkem1024",
 	},
 	Control: &subTypeNoneMap,
 }
@@ -199,7 +211,7 @@ func IsValidSubType(t MessageType, s MessageSubType) bool {
 	case Message:
 		return s == MessageNone || s == MessageRelay
 	case Handshake:
-		return s == HandshakeIXPSK0
+		return s == HandshakeIXPSK0 || s == HandshakeIXPSK2MLKEM1024
 	case Test:
 		return s == TestReply || s == TestRequest
 	case Control, CloseTunnel, RecvError, LightHouse: