[relay] reduce QUIC initial packet size to 1280 (IPv6 min MTU) (#5374)

* [relay] reduce QUIC initial packet size to 1280 (IPv6 min MTU)

* adjust QUIC initial packet size to 1232 based on RFC 9000 §14

shared/relay/client/dialer/quic/quic.go

@@ -12,6 +12,7 @@ import (
 	log "github.com/sirupsen/logrus"
 
 	nbnet "github.com/netbirdio/netbird/client/net"
+	nbRelay "github.com/netbirdio/netbird/shared/relay"
 	quictls "github.com/netbirdio/netbird/shared/relay/tls"
 )
 
@@ -42,7 +43,7 @@ func (d Dialer) Dial(ctx context.Context, address string) (net.Conn, error) {
 		KeepAlivePeriod:   30 * time.Second,
 		MaxIdleTimeout:    4 * time.Minute,
 		EnableDatagrams:   true,
-		InitialPacketSize: 1452,
+		InitialPacketSize: nbRelay.QUICInitialPacketSize,
 	}
 
 	udpConn, err := nbnet.ListenUDP("udp", &net.UDPAddr{IP: net.IPv4zero, Port: 0})

shared/relay/constants.go

@@ -3,4 +3,9 @@ package relay
 const (
 	// WebSocketURLPath is the path for the websocket relay connection
 	WebSocketURLPath = "/relay"
+
+	// QUICInitialPacketSize is the conservative initial QUIC packet size (bytes)
+	// for unknown-path PMTU, per RFC 9000 §14: 1280 (IPv6 min MTU) − 40 (IPv6
+	// header) − 8 (UDP header) = 1232. DPLPMTUD may probe larger sizes later.
+	QUICInitialPacketSize = 1232
 )