19.10 Java4660: sockets  (Page 2/8)

Data flow

Socket programming makes it possible for you to cause data to flow in a full-duplex mode between a client and a server . This data flow can be viewed in almost exactly the same way that we view data flow to and from adisk: as a stream of bytes.

As with most stream data processing, the system is responsible for moving the bytes from the source to the destination. It is the responsibility of theprogrammer to assign meaning to those bytes.

An application protocol

Assigning meaning takes on a special significance for socket programming. In particular, as mentioned above, it is the responsibility of the programmer toimplement a mutually acceptable communication protocol, at the application level, to cause the data to flow in an orderly manner. Some of the bytes areused to implement the protocol, and some of the bytes are used to transfer data.

An application protocol is a set of rules by which the programs in the two computers can carry on a conversation and transfer data in the process.

The HTTP protocol

For example, we will write a very abbreviated form of the HTTP protocol to download a web page from a server and to display it as raw text.

This program will involve adherence to a fairly simple protocol. (At least the part that we implement will be simple.)

The daytime protocol

We will also write a program that obtains the date and time from the same or another computer. In this case, the protocol is about as simple as it canpossibly be. The client will simply make the connection and listen for a string containing the date and time. In this case, the client isn't even required tomake a request.

The echo protocol

We will write another program that sends a line of text to a computer and receives an echo of that text. This protocol is only slightly more complicatedthan the daytime protocol in that it is necessary to sent text to the other computer in order to elicit a response.

The application protocol is the hard part

It is easy to use sockets to write code that will cause a stream of bytes to flow in both directions between a client and a server . This is no more difficult than causing a stream of bytes to flow in both directions betweenmemory and a file on a disk.

Getting the bytes to flow is the easy part. Beyond that, you must do all of the programming to implement an application protocol that is understood by boththe client and the server . Often that is the more difficult part.

Discussion and sample code

Simple TCP/IP services

This discussion applies to the Windows operating system only. If you are using a different operating system you will need to do the necessary research totranslate this information for use with your operating system.

See Windows 7 Simple TCP/IP Services - What and How? for more detailed information on this topic.

A simple server

Windows and some other operating systems make it possible to cause a computer to act as a simple server to deliver the following services:

• Echo on port 7
• Daytime on port 13
• Quote of the Day on port 17
• Character generator on port 19
• Discard on port 9