Communicating over a Network
Have you ever wanted to send a simple string of text over your local LAN? Or maybe you have wanted to send and receive pictures and files with a friend from the other side of the world. Whatever your needs, the Winsock control can help out.
In this article you will learn the basics
of the Winsock control, and how you can use it to transfer data and
information over a network and the Internet.
The Winsock control does not provide us
with many properties, but it gives us many events and methods
to play with. The most important properties are:
RemoteHost - The IP/Machine name of the
RemotePort - The remote port number that
the remote machine is listening
LocalPort - The local port number
to listen to
As you can see, these three properties
are quite important. When connecting, you will need to set the
RemoteHost and RemotePort, but when you are listening you will only
need to set the local port property.
Like the Internet Transfer Control,
the Winsock control provides us with some constants. These include
ErrorConstants, StateConstants and ProtocolConstants. To see a
list of them, click View, Object Browser (or Press F2). From the
drop down combo, select MSWinsockLib. You can now see the
enumerations of constants. For this article, we will mainly be
using the default TCP (Transmission Control Protocol).
Connecting to a computer is not as
easy as just calling the Connect method and specifying a remote
IP and port. You need to do a fair amount of 'handshaking', which
is making sure that the other computer is still there and
wanting to connect.
First, before starting a connection, it's always
good practice to close any current operations before you
start to ensure that the Winsock control is available:
See the use of the While Wend loop?
That is to make sure that you don't start trying to do something
while the winsock control is still finishing an operation.
When connecting, you will also need to check for errors:
If Err Then
Msgbox Err.Number, Err.Description
Err = 0
As done before, you need to make sure
that you don't start doing tasks before the control is ready:
Most of the code you have seen now
forms the bulk of a Connect method. You will now learn about the Listen method. The Listen method is what the receiving computer must do to hear Connect methods.
As before, we need to check for a
If Winsock1.State Then
Otherwise, listening is quite
simple. Just use the Listen method of the Winsock control to
Winsock1.LocalPort = 1008
You may have noticed the use of a
port number above. This is possible because when creating comms apps you can use
just about any port number. Below is a list of ports to avoid
(or use for that matter) if you are creating a program because
if you try and listen to the same port as a mail server, then
one of the two fail.
- SMTP: 25
- POP: 110
- FTP: 21
- HTTP: 80
- NNTP: 119
- Telnet: 23
- Gopher: 70
- IRC: 6667
When it comes to sending text,
things become very messy. Before starting, you need to
sort out how your applications are going to communicate. For instance, assume you
need to let the other end know what you are sending (text,
file, etc). The best way to do this is to create a module that
contains a load of constants that you can use to tell your
program what's going on:
Public Const DATATEXT As Byte = 0
This way, every time you want to
send a piece of text you send the DATATEXT constant first,
then the size of it, and finally the actual content. Using
this standard of operation, you can make your apps work
together very nicely. At each point along the way you need
to do two things: check for errors and wait until the
current operation has been completed.
The best way to handle errors
is to use events.
In Visual Basic, you can
create your own events. An example event might be when you
click on a button:
Private Sub Command1_Click()
You call or 'raise' events
using the RaiseEvent method. This method accepts one
parameter, which is one of the events declared. You
declare an event like this:
Public Event TextSent(TextContent As String)
The event procedure for this
event will look like this:
Private Sub MyModule_TextSent(TextContent As String)
The last thing you will discover in this article are operation time outs. When using the
Winsock control, or in fact any Internet comms
control, you need to be able to detect a time out. This
occurs, for instance, when an application you are
talking to stops responding while you are communicating
with it. If you didn't watch for a time out, then you
program would just sit around waiting for the other
program to respond. The easiest way to watch for these
is by using a timer control.
By setting the Interval
property to a reasonable length, for instance 30000, then you
can detect time outs. Again, use events to tell the user
what's going on. You need to do this in the timers Timer
Private Sub Timer1_Timer()
To finish, here
are some quick guidelines for specifying addresses:
- Where possible use an IP
address (127.0.0.1) instead of computer names because
the Winsock control has to resolve a host name to an IP
address - this takes some time.
- If you connect to the net
over a dial up connection, you can still get this to
work. When you have logged on, use the Winsock's LocalIP
method to return to current IP address. This way you can
send it to your friend and he/she can connect to you.