conn.go, handler.go: Added SystemUptime tracking and method to Conn.

conn.go

@@ -37,6 +37,10 @@ type Conn struct {
 	// connection. The rtt is measured in nanoseconds.
 	rtt atomic.Int64
 
+	// systemStart is the timestamp indicating when the remote system was started,
+	// collected via timestamps.
+	systemStart time.Time
+
 	closing atomic.Int64
 
 	ctx        context.Context
@@ -690,6 +694,11 @@ func (conn *Conn) writeTo(p []byte, raddr net.Addr) error {
 	return nil
 }
 
+// SystemUptime returns the uptime of the senders system or client.
+func (conn *Conn) SystemUptime() time.Duration {
+	return time.Since(conn.systemStart)
+}
+
 // startTime is the time the system or client was started.
 var startTime = time.Now()
 

handler.go

@@ -195,6 +195,7 @@ func (h listenerConnectionHandler) handleConnectionRequest(conn *Conn, b []byte)
 	if err := pk.UnmarshalBinary(b); err != nil {
 		return fmt.Errorf("read CONNECTION_REQUEST: %w", err)
 	}
+	conn.systemStart = time.Now().Add(-time.Millisecond * time.Duration(pk.RequestTime))
 	return conn.send(&message.ConnectionRequestAccepted{ClientAddress: resolve(conn.raddr), PingTime: pk.RequestTime, PongTime: timestamp()})
 }