Objective

The purpose of this assignment is to implement a simple Web service over the UDP transport service. You will write the Web client and server programs that will communicate over the College of Engineering LAN. You will also learn other important functions in computer networks: (1) implementation of segmentation and re-assembly of long messages, (2) detecting errors in the received packets, and (3) emulation of packet errors generation and detection.

Overview

In this project you will implement a simple web client application, a simple web server application, the segmentation and re-assembly function, an error detection function and a gremlin function (that can corrupt packets with a specified probability). The overview of these software components is show in Figure 1 below.

Figure 1. Overview of the Software Components

The Web client initiates the communication by sending an HTTP request to the Web server. This outgoing HTTP request is not processed by the segmentation and reassembly, error detection or the Gremlin function. The HTTP request is sent through the transport UDP datagram socket to the Web server.

At the Web server host, the HTTP request is also not processed by those functions at the Web server host. The Web server will then process the request, e.g. GET command, by reading the file requested by the Web client. Since the requested file may be large, the server application will use the segmentation function to partition the file into smaller segments that will fit into a packet of size allowable by the network. Each segment is then placed into a 256-byte packet that is allowed by the network. The packet must contain a header that contains information for error detection and other protocol information. You may design your own header fields that are of reasonable sizes. Another field that must be in the header is a sequence number. The packet is then passed to the error detection function which, at the server (sending process), will compute the checksum and place the checksum in the header. The packet is finally sent via the UDP socket to the Web client.

When the packet is received by the Web client host UDP socket, the packet will be processed by the Gremlin function which may randomly cause errors in some packet. This will emulate errors that may be generated by the network links and routers. The packet is then processed by the error detection function that will detect possibility of error based on the checksum. The packet is then processed by the segmentation and reassembly function that re-assembles all the segments of the file from the packets received into the original file. The file is then displayed by the Web client application using a display software or browser.

Descriptions

This project introduces you to network programming in the College of Engineering Unix computing environment (Linux workstations) which you will use for your programming assignments.

Use the UDP java client and server programs presented in class. You need to modify the programs to ensure that they run correctly in your environments. For example, you must change the server IP address to the address of the machine in which you will be running the server program. Also make sure that the correct ports are used.

Now, modify the programs to implement a simple Web service as follows:

1. Modify the client program so that it will send a HTTP request to a simple Web server to retrieve a data file. The HTTP request message is supposed to be of the form:

GET TestFile.html HTTP/1.0

The server will send HTTP response messages in 256-byte packets until the end of the file (see below). The client will then receive each 256-byte packet in a loop and writes them into a file sequentially. When it reads a 1-byte message with a NULL character that indicates the end of the file, it will then close the file. Add print statements in the client program to indicate that it is sending and receiving the packets correctly, i.e. print the messages that it sends and receives.

2. Modify the server program so that it responds to clients HTTP requests. The server constructs HTTP response messages by putting header lines before the object itself that is to be sent. The 4 header lines are supposed to be of the form:

HTTP/1.0 200 Document Followsr

Content-Type: text/plainr

Content-Length: xxxr

r

Data

(note: r is a carriage return,
is a line feed, xxx is the number of bytes in the HTML file being sent and data is the requested HTML file)

The server then reads the requested HTML file (an ASCII file, must be at least 10 Kbytes), put them in abuffer and sends the content of the buffer to the Web client who made the request. The HTTP responsemessages are sent in 256-byte packets until the end of the file. At the end of the file, it transmits 1 byte(NULL character) that indicates the end of the file. It will then close the file.

Add print statements in the server program to indicate that it is receiving and sending the packets correctly, i.e.print the messages that it receives and sends.

Gremlin Function

Your program must allow the probability of damaged packets to be input as an argument when the program is executed. This packet damage probability is passed to your Gremlin function. You will implement a gremlin function to simulate two possible scenarios in the transmission line: transmission error that cause packet corruption and correct delivery. When the receiving process receive each packet, it first pass the packet to a gremlin function which will randomly determine whether to change (corrupt) some of the bits or pass the packet as it is to the receiving function. The gremlin function uses a randomnumber generator to determine whether to damage a packet or pass the packet as it is to the receiving function.

If it decides to damage a packet, it will decide on how many and which byte to change. The probability that a given packet will be damaged, P(d), is entered as an argument at runtime. If the probability of damaging a packet is .3, then three out of every ten packets will be damaged. If the packet is to be damaged, the probability of changing one byte is .5, the probability of changing two bytes is .3, and the probability of changing 3 bytes is .2. Every byte in the packet is equally likely to be damaged.

The packet is then passed from the gremlin function to the error detection function that will check for errors in the packet.

Error Detection Function

The sending process, e.g. the Web Server, will compute the checksum for the packet that is to be sent. The checksum is calculated by simply summing all the bytes in the packet. The checksum is then inserted into the checksum header field of the packet.

The receiving process, e.g. the Web client, will then use the same algorithm for computing the checksum that the sending process used. It will calculate the checksum by summing all the bytes in the received packet. It then compares the computed checksum with the checksum received in the packet. If the two checksums match, then it assumes that there is no error, otherwise there is at least an error in the packet.

When the receiving process detects an error in a packet, it will print out a message indicating the packets sequence number and that there is an error in the packet.

In this project, you must not try to correct for errors in the packet using any network protocol or scheme; you are required only to indicate that a packet is in error.

Testing

Run the modified UDP Java client and UDP Java server programs, with the segmentation and re-assembly, error detection and gremlin functions, on different tux Linux computers and capture the execution trace of the programs. In Linux, use the script command to capture the trace of the execution of the UDP Java client and UDP Java server programs.

Print the content of the input file read by the server program and the output file received by the client program.