Internet Transfer Control

Most of you might have worked with Internet Transfer Control which is very handy control when it comes to Internet Programming but there is another control which even more robust and helps programmers creating more flexible applications. Winsock control comes with VB6 and is used to create applications that access the low-level functions of the Transmission Control Protocol/Internet Protocol (TCP/IP).

TCP/IP is a specification that defines a series of protocols used to standardize how computers exchange information with each other. TCP/IP provides communication across interconnected networks that use diverse hardware architectures and various operating systems. The protocols in TCP/IP are arranged in a series of layers known as a protocol stack. Each layer has its own functionality.

Winsock is a standard that is maintained by Microsoft. This standard is basically a set of routines that describe communications to and from the TCP/IP stack. These routines reside in a dynamic link library that runs under Windows. The winsock DLL is interfaced with TCP/IP and from there through the Internet.

In this article, I am going to show how to use the winsock in a client server environment, we will create two separate applications, one of which will be a server and the other will be a client. Both client and server will interact with each other to exchange data. Client will send a request to the server and the server which will be connected to a database will retrieve the information requested by the client from the database and will return the requested information back to the client. You will a database with this article, the database contains the item numbers and their prices. In real life situations, database might be located on a machine different from the one that hosts the client application.

I think it would be better to talk about the ports before we proceed any further. A port is a special memory location that exists when two computers are in communication via TCP/IP. Applications use a port number as an identifier to other computers, both the sending and receiving computers use this port to exchange data.

To make the job of communication easier, some port numbers have been standardized. These standard port numbers have no inherent value other than that users have agreed to use them with certain applications. Table below lists a number of popular and publicly accepted port numbers and their corresponding applications.

Service	Port
HTTP	80
FTP	20,21
Gopher	70
SMTP	25
POP3	110
Telnet	23
Finger	79
Local loops/callbacks	0

Using the Winsock Control

Winsock is above the TCP/IP protocol stack in the ISO/OSI model. TCP/IP is an industry standard communication protocol that defines methods for packaging data into packets for transmission between computing devices on a heterogeneous network. TCP/IP is the standard for data transmission over networks, including the Internet. TCP establishes a connection for data transmission and IP defines the method for sending data packets.

The Microsoft Winsock control makes using the TCP/IP a breeze. Microsoft has wrapped up the Winsock and INetAPI API calls into a nice neat package that you can easily incorporate into your Visual Basic applications.

Winsock Operating Modes

The Transport layer (also known as the Host-to-Host Transport layer) is responsible for providing the Application layer with session and datagram communication services. The core protocols of the Transport layer are TCP and User Datagram Protocol (UDP). The Winsock control supports the following two operating modes:

• sckTCPProtocol
• sckUDPProtocol

Winsock Properties

Winsock enables you to create clients and servers using the same control. This dual functionality enables you to specify through property setting the type of application you will be building. The Winsock control uses a number of the same properties, whether you are creating client or a server, thereby all but eliminating the learning curve needed to create applications. Some of the important properties of the control are as following:

BytesReceived Property

This property returns the number of bytes currently in the receive buffer. This is a read-only property and is unavailable at design time. The value returned is a long integer.

LocalHostName Property

The LocalHostName property returns the name of the local host system. This is read-only property and is unavailable at the design time. The value returned is a string.

LocalIP Property

The LocalIP property returns the local host system IP address in the form of a string, such as 11.0.0.127. This property is read-only and is unavailable at design time.

LocalPort Property

This property returns or sets the local port number. This can be both read from and written to and is available at both design time and runtime. The value returned is a long integer.

Protocol Property

Returns or sets the protocol, either TCP or UDP, used by the Winsock control.

RemoteHost Property

The RemoteHost property returns or sets the remote host. This can be both read from and written to and is available both in design time and runtime. The value returned is a string and can be specified either as an IP address or as a DNS name.

RemotePort Property

This property returns or sets the remote port number.

State Property

This returns the state of the control as expressed by an enumerated list. This is read-only property and is unavailable at design time.

Some of the important methods of Winsock control are as following: