Send empty packet before initial delay.

udpbench.1

@@ -85,8 +85,9 @@ With dynamic
 .Ic ssh
 the option is passed to the remote process.
 .It Fl d Ar delay
-Sleep some seconds before sending packets.
+Sleep some seconds before sending packets with payload.
 This is useful to wait for setup of multiple processes via ssh.
+An initial empty packet is sent before delay to trigger setup.
 Default is 0 seconds which disables the feature.
 .It Fl H
 Use IPv6 hop-by-hop option by adding a router alert when sending.

udpbench.c

@@ -304,8 +304,11 @@ udp_connect_send(void)
 			    bitrate, etherlen);
 	} else
 		sendrate = packetrate;
-	if (delay)
+	if (delay) {
+		if (send(udp_socket, "", 0, 0) == -1)
+			err(1, "send initial");
 		sleep(delay);
+	}
 	if (timeout > 0)
 		alarm(timeout);
 	udp_send(udp_socket, udp_family, sendrate);