add a quic.Config option to disable Path MTU discovery

config.go

@@ -106,6 +106,7 @@ func populateConfig(config *Config) *Config {
 		StatelessResetKey:                     config.StatelessResetKey,
 		TokenStore:                            config.TokenStore,
 		EnableDatagrams:                       config.EnableDatagrams,
+		DisablePathMTUDiscovery:               config.DisablePathMTUDiscovery,
 		Tracer:                                config.Tracer,
 	}
 }

config_test.go

@@ -71,6 +71,8 @@ var _ = Describe("Config", func() {
 				f.Set(reflect.ValueOf(true))
 			case "EnableDatagrams":
 				f.Set(reflect.ValueOf(true))
+			case "DisablePathMTUDiscovery":
+				f.Set(reflect.ValueOf(true))
 			case "Tracer":
 				f.Set(reflect.ValueOf(mocklogging.NewMockTracer(mockCtrl)))
 			default:
@@ -144,6 +146,7 @@ var _ = Describe("Config", func() {
 			Expect(c.MaxReceiveConnectionFlowControlWindow).To(BeEquivalentTo(protocol.DefaultMaxReceiveConnectionFlowControlWindow))
 			Expect(c.MaxIncomingStreams).To(BeEquivalentTo(protocol.DefaultMaxIncomingStreams))
 			Expect(c.MaxIncomingUniStreams).To(BeEquivalentTo(protocol.DefaultMaxIncomingUniStreams))
+			Expect(c.DisablePathMTUDiscovery).To(BeFalse())
 		})
 
 		It("populates empty fields with default values, for the server", func() {

interface.go

@@ -266,6 +266,9 @@ type Config struct {
 	StatelessResetKey []byte
 	// KeepAlive defines whether this peer will periodically send a packet to keep the connection alive.
 	KeepAlive bool
+	// DisablePathMTUDiscovery disables Path MTU Discovery (RFC 8899).
+	// Packets will then be at most 1252 (IPv4) / 1232 (IPv6) bytes in size.
+	DisablePathMTUDiscovery bool
 	// See https://datatracker.ietf.org/doc/draft-ietf-quic-datagram/.
 	// Datagrams will only be available when both peers enable datagram support.
 	EnableDatagrams bool