Sending UDP Broadcasts

How to Send UDP Broadcasts

UDP datagrams can be sent as broadcasts. A broadcast, instead of specifying a particular network host as a destination, targets a group of hosts on the network.

A tip note icon. The sock. object supports link-level broadcasts. Such broadcast packets have their destination MAC address set to 255.255.255.255.255.255. Link-level broadcasts are received by all network hosts connected to the current network segment. Link-level broadcasts cannot penetrate routers, bridges, etc.

To make the socket send its outgoing UDP datagrams as broadcasts, set the sock.targetbcast property to 1 — YES:

Tibbo BASIC

							...
							sock.targetbcast= YES
							sock.setdata("ABC") 'this is explained in 'Working With Buffers' section
							sock.send 'this is explained in 'Working With Buffers' section
							sock.connect 'broadcast UDP datagram with string 'ABC' will be sent out at this point
							...
						

There is one difference to grasp regarding the socket that is sending its outgoing packets as broadcasts: No incoming UDP packet that would have normally be interpreted as a reconnect will cause the socket to "switchover" to the source IP address of the packet.

Let's evaluate two examples.

Example 1: Regular UDP Communications

Here is a sample setup for the UDP socket:

Tibbo BASIC

							 
							sock.protocol= PL_SOCK_PROTOCOL_UDP
							sock.inconmode= PL_SOCK_INCONMODE_ANY_IP_ANY_PORT
							sock.reconmode= PL_SOCK_RECONMODE_3
							sock.targetip= "192.168.100.40"
							sock.targetport= 1000
							sock.localportlist= "2000"
							sock.setdata("ABC")
							sock.send
							sock.connect
							 
						

And here is a hypothetical sequence of events:

Incoming/outgoing datagram	Comment
Datagram with contents "ABC" sent to 192.168.100.40:1000 as soon as the sock.connect method is invoked.	We now have the UDP "connection" with 192.168.100.40:1000.
Incoming UDP datagram from 192.168.100.41:20.	This will be taken as a reconnect — the socket is now engaged in a connection with 192.168.100.41:20.
Socket sends out another datagram — this time to 192.168.100.41:20!	Complete switchover happened — the socket is now transmitting data to the IP address and port of the sender of the previous incoming datagram.

Example 2: Regular UDP Communications

Here is another setup for the UDP socket:

Tibbo BASIC

							sock.protocol= PL_SOCK_PROTOCOL_UDP
							sock.inconmode= PL_SOCK_INCONMODE_ANY_IP_ANY_PORT
							sock.reconmode= PL_SOCK_RECONMODE_3
							sock.targetbcast= YES
							sock.targetport=1000
							sock.localportlist= "2000"
							sock.setdata("ABC")
							sock.send
							sock.connect
						

The sequence of events:

Incoming/outgoing datagram	Comment
Datagram with contents "ABC" sent as broadcast to all stations on the network segment.	We now have a UDP "connection" with ... err ... everybody on the segment.
Incoming UDP datagram from 192.168.100.41:20.	This is still a reconnect, but the socket won't switch over to the IP address of the sender. Only port switchover will take place!
Socket sends out another datagram — still as a broadcast but this time to port 20.	Port switchover happened because it is allowed by sock.reconmode.