CS 60 Computer Networks


IM Application: Your First Network App!

Ready to design, implement and test your first network application? In this lab, we will implement a simple Instant messaging and presence protocol in C. This will require socket programming in C.

Read the socket programming notes. You have coded a client but now you will need to know how to do the server side too. There are examples in the socket programming notes that you can cut-paste and run to get some experience.

Note that there will be an x-hour by the TA this week on concurrency, threads, synchronization in C, etc, associated with completing this assignment. The synchronisation issues need some thought.

The description below in on the verbose side, we apologies in advance.

OK, off we go.


Getting Started

The design of your IM system is an important aspect of this assignment. We provide a functional description of an IM system that your design should conform to. However, we will leave the precise details of the design up to you (maybe you want to make it state driven, using an Finite State Machine design, or use other techniques, maybe you want to add bells and whistles beyond what we outline all up to you). But please think about the design before diving into coding it will help. Feel free to look at some of the IM references presented at the end of this assignment, on the web, existing specs ( MSNP always cite sources (no use of any existing code base of course but you can look at other open source code for sure, for inspiration, ideas, etc.). Be a detective. If you use any ideas, techniques, please cite sources, again.

When you do begin to implement your IM system we recommend you start with a basic set of features and proceed incrementally start simply and keep it simple. Perhaps start with just two clients exchanging messages and build up from there. Test each feature as you proceed, build confidence, and keep working towards the design specification that you developed. This assignment may require some aspects of the C language with which you maybe unfamiliar (for instance dealing with concurrency/threads). You will find the text book provides some useful examples and there are some excellent tutorials provided by Sun that we list in the references section at the end of this assignment.

Blueprint for a Simple IM System

Many of you are familiar with IM from a user’s perspective but may not have given much consideration to its operation. The precise operation of an IM system will vary between different systems (e.g., AIM, MSN, iChat) but the essential functions remain the same. For the purposes of this assignment we require your simple IM system to incorporate the following functionality:

1) IM clients are able to join and leave the IM network. Every client has a user name and at any one particular time no two clients connected to the IM network can have the same user name. So some name checking is required in your code.

2) When IM clients join the network other connected clients are made aware of this change within the network the presence component. Similarly, the reverse should be true; that is, when clients leave the network other connected clients should be informed. So you need notification and state management a consistent view.

3) Connected IM clients are able to send messages to any other connected IM client. Messages are addressed to particular IM clients using their user name.

Given that constructing an elaborate IM system would be a semester long project we have limited the scope of this assignment in comparison to a full blown IM system such as supported by MSNP. It is perfectly acceptable to adopt the following simplifications:

1) A command line user interface is acceptable, only add a GUI if you would like to again keep it simple. We looked at the command line interface for MSNP in class. It is fairly complex because it supports many functions, e.g., file sharing. Your clear text interface would be quite simple in comparison to that.

2) You are not required to maintain buddy lists specific to each IM client. It is fine to assume everyone in the IM network is part of a single universal buddy list. For instance all IM clients should be notified when any IM client joins or leaves the network. Furthermore, all IM clients should be able to exchange messages with any other IM client.

3) Your design only needs to support a single IM server (phew ;-). Although, this is not a scalable approach it simplifies the task of building your IM system. You may assume all clients are aware of this single well-known IM server. So your server will sit on a known host and have a known port. Clients simply connect using sockets. By only having a single server you do not need to be concerned with maintaining consistent state between multiple servers, so essential in a real system.

However, feel free to provide any additional functionality if you wish, it will be rewarded as extra-credit. But completing the base line system with no extras would be outstanding. See how things go.

The objective of this assignment is for you to design and implement an IM system that conforms to the description outlined above. It is a fairly high level description, so the details are up to you. Supporting these features will require you to define an application level protocol that controls how participants in the system communicate. We suggest you adopt the client/server approach when designing your IM system let us leave P2P aside for the moment.

The server should perform operations, such as:

(a) monitor clients that join and leave the IM network;

(b) maintain a reasonably consistent view of clients currently connected to the network and provide this state information to all the currently connected clients; and

(c) facilitate the exchange of messages between clients.

The clients should perform operations, such as:

(a) display to “the user” the currently connected IM clients; and

(b) allow “the use” to send and receive messages to and from any of these clients, respectively.

IM Design Spec

Here is what we would like to see other than running code and a working simple IM system. We would like you to provide as part of the submission a short (read: we do not expect you to write a book the goal is to write good, working code) specification. For example, take a look at the original Gnutella specification that we will talk about in class - this is not an IM system but it gives a good example of how an application level protocol (albeit for a P2P system) can be specified. It is good that you start to look at specs and read them - try and puzzle them out for yourself. We would expect a spec of 3 or 4 pages max that describes your protocol.

The spec should include:

1) A high level description of the types of participants within your system (i.e., IM clients, the IM server, and perhaps others). Describe what are the functions that each of these participants perform and how these combine to produce the required system behavior.

2) Define the protocols that occur between these participants. You should provide illustrative examples of the protocols in operation (such as seen in the textbook; see Figure 3.16 - note this is for the transport layer but is shows examples of sender and receiver interaction). You may find an FSM diagram (e.g., Figure 3.10 in the textbook) useful in representing the state and operation of your system.

3) Provide a comment on reasons behind any important design decisions so we can best understand your thinking. You could even add a short section on ideas you jettisoned because of x, y, z reasons. What brought you to your final design? Could be you ran out of time and hacked it! Well, that would not be a great design methodology but industry sometimes operates like that when developing prototype ideas to test the waters against crushing deadlines.

After your design is complete and you start the implementation of your design we suggest you first consult the example client/server application provided in the textbook (page 159). The textbook example provides a description (and source code) of two Java applications using socket programming. The client application is able to send strings to the server. These strings are echoed back to the client from the server but are altered so that the strings now only use upper case letters. It would be quite simple to alter this example and allow the two end hosts to exchange messages directly between each other. Doing this would be a good initial step when approaching this assignment. However, you must write this assignment in C so you should only use the Java code as a reference. Note that lab 2 will come in handy for this lab because in lab 2 you more or less implemented the client part of this lab. You will have to tweak it to conform to your protocol but the fundamental building blocks should already be there. Thus the majority of your time will more than likely be spent on writing the server portion of this lab.

During the design process you will determine that your server needs to be “multi-threaded” so that it can concurrently service more than one IM client at a time multithreading might be a new concept to you; it allows a software process to deal with concurrent events and operations. If you lack experience in threaded programming you will find the POSIX Threads Programming tutorials on the subject an excellent resource (this reference is provided at the end of this assignment). Your IM system requires threads to concurrently access state that is shared between all thread instances. To ensure the correct operation of your server you will be required to perform some form of synchronization between these threads when they access shared data they need mutual exclusion. The pthreads library provides support for threading and the necessary synchronization primitives. So this assignment brings in a lot of concepts that are important, one being shared state and the issue of concurrent access. You will come across these ideas in the software industry as designers and programmers.


Make sure you test your code against the requirements discussed above and that it works. Please submit the source code - please add comments in your code and employ good programming practices. We will not grade you on style in this lab but it is good to get into good habits early in your careers. Please also provide a README file that contains any information necessary to run your application. The source code submitted must be entirely self-contained. You should also provide in the README any information that explains how to setup your IM network; for example, provide a step-by-step set of instructions regarding how to use your system to demonstrate it supports the list of functions discussed.

That is a lot of information but not too much to limit any innovations you would like to add, if you wish. The assignment will take some effort so plan your time carefully.

Extra hacker credit: Allow authentication through Dartmouth’s DND. Allow users to maintain a buddy list of users.

Good luck.

Here are some useful links for IM systems and pthreads.


Instant Messenger Systems

Yahoo Instant Messenger Protocol

MSNP (Early Internet Draft)


POSIX Threads

POSIX Threads Programming

Thread-Specific Data and Signal Handling in Multi-Threaded Applications

Signal Handling for Threads

Good luck!

Tip: Make sure you always logout when you are done and see the prompt to login again before you leave the terminal.