Rockwell Automation 1768-EWEB EtherNet/IP Web Server Module User Manual - Manual (Page 145)

Publication ENET-UM527E-EN-P - October 2006
Socket Interface 145
MSG Destination Element
Not used; the MSG instruction does not return any data.
Considerations
In some cases, the OpenConnection service can return before the
timeout period without creating a TCP connection. For example, if the
destination device is running, but is not listening for connections on
the specified port number, OpenConnection returns with an error
before the timeout period.
For UDP, if you use OpenConnection, you do not have to specify the
IP address and port number each time you send data. If you do not
specify an IP address and port number, you can only receive data
from the previously specified IP address and port number until you
call OpenConnection to specify a different IP address and port
number.
For UDP, if you do not use OpenConnection, you must specify the
destination address each time you call the Write service to send data.
When you call the Read service, in addition to the data, you receive
the address of the sender. You can then use the address of the sender
to send a response via the Write service.
If you call OpenConnection on a UDP socket with an AddrLen of 0,
this removes the association with the destination address.