CS 5523 Operating Systems
Laboratory 2 - Spring 2001
A UDP Port Server

Objectives:

• Use UDP protocol in a communication protocol.
• Explore the use of time-outs in managing UDP communication.
• Understand request-reply protocols.
• Learn more about services and ports.

Overview:

In this laboratory you will write a server that uses UDP to provide information about the services that are available on the host on which it is running. Prior to doing the lab, print out and read through the man page for getservbyname. Also get a copy of the include file netdb.h. Your server will call the getservbyname as described in Stevens I Section 9.9.

Requirements:

• Your source code must be clean and clear. (It will be read and you will be graded on its quality as well as whether the code works.)
• This project must be done in C.
• You must hand in all source for each part separately.
• You must hand in the output files for each part separately.
• You must hand in copies of the lint output for each part separately. It should not contain an indication of any errors.
• You must hand in a word-processed report of 3 to 5 pages summarizing the structure of your program, known bugs, how you tested the program, and answers to the questions posed in the assignment. Your report should also an introductory paragraph and a diagram of the architectural model implemented in this assignment.

Part I: UDP warmup

The following code consists of a server that waits to receive a UDP message and echoes its contents to standard output. The client sends a UDP message to the server consisting of a line that the user types in from the keyboard. Download the following test programs, compile them and run them. Make sure that you understand them.

• clientudp.c
• serverudp.c
• makefile

Modify the programs so that the server replies with a copy of the message sent from the client. The client should echo the message to standard input.

Modify the programs so that the client uses a timeout value (as specified as a command line argument) to determine how long to wait for a response. Experiment with different values of the time out to determine reasonable values. How does the needed timeout value depend on time of day?

Part II: Accessing host services

In this part of the assignment you will design and write routines to access the information about services available on a host. The client sends a UDP request containing:

• A random sequence number (an integer in network byte order)
• A protocol name (a null-terminated string)
• A name of the service (a null-terminated string)

• The same random sequence number as in the request.
• An integer port number (in network byte order).
• A set of null-terminated strings giving alias of the service.

    #define NAMESIZE 256
    struct service {
        int sequence;
        int port;
        char names[NAMESIZE];
    } hostsev; 

Rewrite the client so that it prompts the user for host information, protocol and service name. The client chooses a sequence number at random, marshals the request and sends it to the server.

The timeout value and UDP port number of the server should be command line arguments. The client should wait until it receives a response from the server or times out before prompting the user for another request. If the sequence number of a received response does not match the sequence number of the most recent request, the client should print the response noting the mismatch and resume waiting for the server to respond. As part of your testing, set the timeout in the client very short and insert a delay in the server between the receipt of the request and the response. This will cause a previous packet to be received on the next request. During some of your tests you should run several servers on different machines and have a client access different ones in turn. You should also test your client against at least one other class member's server and your server against at least one other class member's client.