tell if a peer supports DATAGRAM frames in the ConnectionState

interface.go

@@ -264,7 +264,8 @@ type Config struct {
 
 // ConnectionState records basic details about a QUIC connection
 type ConnectionState struct {
-	TLS handshake.ConnectionState
+	TLS               handshake.ConnectionState
+	SupportsDatagrams bool
 }
 
 // A Listener for incoming QUIC connections

session.go

@@ -632,9 +632,14 @@ func (s *session) Context() context.Context {
 	return s.ctx
 }
 
+func (s *session) supportsDatagrams() bool {
+	return s.peerParams.MaxDatagramFrameSize != protocol.InvalidByteCount
+}
+
 func (s *session) ConnectionState() ConnectionState {
 	return ConnectionState{
-		TLS: s.cryptoStreamHandler.ConnectionState(),
+		TLS:               s.cryptoStreamHandler.ConnectionState(),
+		SupportsDatagrams: s.supportsDatagrams(),
 	}
 }