Assignment 2, COSC 4377, Fall 2000

UNIVERSITY of HOUSTON
Department of Computer Science

COSC 4377 - Introduction to Computer Networks
Section 07925
Fall 2000

Assignment 2 - Socket Programming

Due on Monday, October 16, 2000

This programming assignment is designed to let you familiar with BSD sockets , client-server application, and persistent connection. This assignment weights 200 points.

The usual note:

At the beginning of each file, you should include "COSC 4377, Fall 2000, Assignment #2", your NAME, and your E-MAIL address in the comment.
Structural programming and good programming style are necessary.
Comments / in-line documents for functions and variables are a must.
You can discuss with your classmate, but no collaboration / copy. And your programs will be checked by MOSS (Measure Of Software Similarity) to find any similarity. Any student found guilty of academic dishonesty will receive severe punishment.
You are responsible for ensuring the code you write for the assignment is not readable by others.

Latest Hint:

Please see FAQ.

Overview:

You will write two C/C++ programs using BSD sockets (using AF_INET and SOCK_STREAM) to implement a client and a server which follow a protocol (called Oracle) to exchange message and perform some functions on server. There will be only one client and one server for this assignment.

After connection between client and server is established, client sends a "Hello" to server.

Server responses with an "OK" and a random session id (client will use this session id to communicate with server).

Then client sends the user name, and server responses with "HI username".

Client sends a question to server and server responses with an answer. The sample questions are "server name", "date/time", "user name", "add 1 2", "subtract 4 2", etc.

At the end of message exchange, client sends "Bye" to server and server responses with "Bye username". And then connection is closed.

If client sends another "Hello", server issues another random session ID. And continue as (3).

Server keeps track of client's hostname, current user's name and session ID. If client's session ID doesn't match current session ID at server, server sends an error message and terminates the connection.

Each message includes a unique ID (4 bytes, first 2 letters of your last name + first letter of your first name + last digit of your SSN). Client and server has a copy of this unique ID. If server receives a message with different unique ID, server responses a error message and closes the connection.

If any command received by server is out-of-order (for example, server receives "ASK" command as the first massage from client), server responses an error message and terminate the connection.

Oracle Protocol

Message format

Each message has 42 bytes which are consisted of 4 parts.

Unique ID (4 bytes, characters): Unique ID is used as an identification between client and server, so client communicates with intended server. Unique ID is constructed by first two letters of your last name + first letter of your first name + last digit of your SSN.

Session ID (2 bytes, an unsigned integer): Session ID is randomly generated (greater than 0) and assigned by server. When client connects to server with a "HELO" command, server will return a session ID. Session ID "0" is used only for client sending "HELO" command.

Code (2 bytes, an unsigned integer): The code is used to identify the actions between client and server.

Message (34 bytes, characters): The message part contains the actions or information to be exchanged. All commands are in upper case and words in the messages are separated by space.

Messages exchanged between client and server

The messages exchanged between client and server are listed (but not limited to) in the following table. You can add extra error messages to cover out-of-order commands.

Sent from

Code

Message

Explanation

Client

100

HELO from clienthost

send client's hostname (clienthost) to server

110

USER username

send client's current user name (username)

300

ASK SRVR

request server's hostname

ASK SNID

request current session ID

ASK CLNT
request client's hostname

ASK USER request current user's name

ASK DATE

request date and time on the server

ASK ADD numbr1 numbr2

request the result of (numbr1 + numbr2)

ASK SUB numbr1 numbr2

request the result of (numbr1 - numbr2)

500
BYE
notify the end of connection

Server

200
OK SNID
reply with session ID (SNID)

210
HI username
acknowledge current user (username)

400
ANSW result
reply with the result requested by ASK command

600
BYE usrname from clnthost
acknowledge the termination to current user (usrname) at client (clnthost)

800
ERR unknown command
server receives an unknown command

810
ERR incorrect UNIQ ID
server receives mismatched unique ID

820
ERR incorrect SNID
server receives mismatched session ID

...
...... ......

The state diagram of messages exchanged between client and server is described in the following feature.

There were some confusions regarding the state diagram above. So I drew another state diagram to make the transition between different states easier to understand. Start, C2, C3, and End are on the client; S1, S2, S3, and S4 are on the server. "Error" state indicates server receives a message containing error(s) and labels on the edges reaching "Error" state indicate the messages sent to client.

Requirements:

The client and server program will use command line parameters to input server name (servername) and port number (port_no). The commands would look like this. [Hint: you can use getopt().]
        client -h servername -p port_no
    or  client servername port_no
        server -p port_no
    or  server port_no
Users (you or TA) will type a message from client program (with code, and message) and send it to server. Your program should provide a debugging mode, so TA can type "unique ID" and "session ID" (plus code and message) to test your error detecting mechanism. You can add an option in the command line parameter, like "client -d -h servername -p port_no" or "client servername port_no y" to activate the debugging mode.

You should print out entire message block of all messages exchanged with proper prefixes to identify the message is sent or received (or from client or server). The "session ID" and "Code" should be converted into integral numbers. [Hint: there are lots of string related functions which will significantly shorten the time you spend on this assignment.]

You can assume that integral numbers used in "ADD" and "SUB" commands are positive. Also, when checking for unknown command, you can check if you can identify the code and the first word in the message (except "ASK" command which you have to check next word). [Hint: you can use strtok() to retrieve tokens.]

If you plan to use Java (instead of C/C++), you should get instructor's approval and make a arrangement with TA (on which machine to run your program and maybe a domestication.)

Discussion Questions:

When client receives message from server, client finds the unique ID from server is different from client's (somehow another server hijacks the connection and uses different unique ID.) Should client close the connection right away, or sent a message to server before close the connection? Which way is better? Why?

Why does this protocol provide persistent connections?

When server receives an out-of-order command, it would be better to tolerate the error and continue the execution (instead of terminate the connection). Give an example (or sequence) that server receives an out-of-order command and continues execution and still preserve the authentication sequence ("HELO" and then "USER"). You may add extra message to achieve fault-tolerance.

Submit your assignment: (updated)

You should turn in all necessary programs (client, server, include files, makefile, readme, etc.)

Your document should include in-line document / comments in your programs, instruction to run your programs, special instruction for compilation, sample outputs (at least one with error and one without), answers to discussion questions, and comments.

Here is how to submit your programs and documents (must run on pegasus or SunOS machines in the CS Dept).

Your programs, readme, makefile (due midnight, Monday, Oct. 16, 2000): run the following command
~xjiang/submit assignment# file1 file2 ...
where assignment# is which assignment you want to submit (in this case it's "2"), and file1, file2, ... are the files you want to submit. Here is an example for submitting 4 files for Assignment #2.
~xjiang/submit 2 Makefile server.c client.c readme.txt

Discussion questions and comments: Please turn-in hardcopy in the Monday's class .

If you have trouble submitting your programs, or you can't get your CS account in time, you can email your programs to TA, Mark, or Dr. Subhlok.

Reference:

A simple Socket example - a simple but sufficient example for this assignment. You can use this example as a base to implement your programs. I have added some debugging messages and you can download here (client.c, server.c, and readdir.c).

BSD Sockets: A Quick And Dirty Primer - a brief introduction to Sockets.

BSD Sockets - a more detail introduction to Sockets with examples.

Davin's collection of UNIX programming links - an excellent resource for UNIX programming (including Sockets).

A great UNIX network programming reference book, "UNIX Network Programming," by W. Richard Stevens, Prentice Hall, 1990. (ANDERSON/RESERVES QA76.76.O63 S755 1990). Sockets programming covered in 84 pages.

Another book about client/server programming is "Power Programming with RPC," by John Bloomer, O'Reilly, 1991. (ANDERSON/RESERVES QA76.8.U65B4 1991 3-WK LOAN). You can find some sockets programming example from this book (actually the example in the first link is from this book.) Socket programming covered in 25 pages.

If you want to know how sockets is implemented, you should check out "TCP/IP Illustrated, Vol. 2 - The Implementation," by Gary R. Wright and W. Richard Stevens, Addison-Wesley, 1995. Sockets covered in 124 pages.

The slide I used in the class is from "TCP/IP Protocol Suite," by Behrouz A. Forouzan, McGraw-Hill, 2000. This is a good reference book for TCP/IP protocols, but it's quite expensive. This book gives explanations on data structures and functions. The examples provided are not compilable. Sockets covered in 34 pages. You can get all the overhead slides of this book from Kevin Long's web page.

Last modified: October 12, 2000.
tihuang at cs . uh . edu

Sent from	Code	Message	Explanation
Client	100	HELO from clienthost	send client's hostname (clienthost) to server
	110	USER username	send client's current user name (username)
	300	ASK SRVR	request server's hostname
		ASK SNID	request current session ID
		ASK CLNT	request client's hostname
		ASK USER	request current user's name
		ASK DATE	request date and time on the server
		ASK ADD numbr1 numbr2	request the result of (numbr1 + numbr2)
		ASK SUB numbr1 numbr2	request the result of (numbr1 - numbr2)
	500	BYE	notify the end of connection
Server	200	OK SNID	reply with session ID (SNID)
	210	HI username	acknowledge current user (username)
	400	ANSW result	reply with the result requested by ASK command
	600	BYE usrname from clnthost	acknowledge the termination to current user (usrname) at client (clnthost)
	800	ERR unknown command	server receives an unknown command
	810	ERR incorrect UNIQ ID	server receives mismatched unique ID
	820	ERR incorrect SNID	server receives mismatched session ID
	...	......	......

UNIVERSITY of HOUSTON Department of Computer Science

COSC 4377 - Introduction to Computer Networks Section 07925 Fall 2000