Last updated: 30/12/04

In this C exercise you will write an ECHO server implementing the ECHO  protocol  following all the steps of the ECHO RFC.

Content:

• Introduction
• Synopsis
• Basic requirements of the server design
• Details
• Testing Your Program
• Relevant/interesting Man Pages
• Error Handling
• General
• Policies, Tips and Gradings

Introduction:

The program must be written in C.

Synopsis:

Basic requirements of the server design:

• The server handles all connections in a single-threaded process. It creates neither new processes nor new threads.
  Whenever a client connects to the server, a new socket is opened to communicate with the client. All these sockets are managed by a single process.
  The server will handle concurrency using UNIX I/O multiplexing, provided by system call select(2).
   
• The server must provide for a design that guarantees:
  • fare share of the server resources among all active clients (for example, the situation when the server gets stuck waiting for data from one of the clients, while other clients are waiting for the service, is unacceptable).
  • a misbehaving client cannot disrupt service to other clients. In particular, the server must keep running properly no matter what (malicious) clients do.

Details:

When designing the server, think about the following points:

• You may refuse or fail to serve new clients only when the system resources are exhausted (e.g. too many sockets are already open).
• After the server has entered the main loop (waiting for connection request, accepting connection and serving it) successfully, it must keep running properly forever, no matter what clients do.
• Since the server serves all the clients in a single process, this process becomes a single point of failure: if the process crashes, ALL clients are cut off the services. Thus, one must beware that one misbehaving client may disrupt the service for all other clients.
• If the server uses blocking I/O, and a client just doesn't read what the server sends it, the server sooner or later will get stuck in write(2) to this client, and all other clients will be stuck as well, which is violation of our basic requirements to the server.
  Therefore, our server must use non blocking I/O for all its communications with clients.
• Since TCP is a byte-stream protocol, read()/write() may input or output fewer bytes than requested, but this is not an error.
  If you are interested in exactly the requested amount of data to be input/output, all that is required is for the caller to invoke read() or write() function again, to input or output remaining bytes.
• Dispose immediately of unnecessary resources (memory, open files/sockets etc) to avoid the server running out of the system resources.
• You cannot assume that on return select() modifies the last argument to hold unslept time.
• Use sigaction() function instead of signal() when handling signals.

Testing Your Program:

• First, run a single client on the same machine where the server runs.
• Run server and a client on different machines.
• Run a set of clients simultaneously.

Relevant/interesting Man Pages:

• The Sockets API system calls
  • socket(2), bind(2), listen(2), accept(2), connect(2), read(2), write(2), send(2), recv(2), shutdown(2), close(2)
• Auxiliary functions
  • gethostbyname(3), getprotobyname(3)
• Device/Descriptor Manipulation
  • ioctl(2), fcntl(2), getsockopt(2), setsockopt(2)
• Signal handling
  • kill(2), alarm(3), ualarm(3), raise(3), sigaction(2), sigprocmask(2), sigpending(2), sigsuspend(2)
• Synchronous I/O multiplexing
  • select(2)
• Time handling functions
  • time(2), gettimeofday(2)

Error Handling:

• In all cases of errors, the program must print an informative error message to stderr (use perror(3) when needed).
• Check thoroughly the command line arguments.
• Check EVERY system call return value.
• In case of an unrecoverable error (e.g., if 'server' can't open a server socket), the program must exit with status 1.
• After the server have entered the main loop (waiting for connection request, accepting connection and serving it) successfully, it must keep running properly forever, no matter what clients do.
• If a client's request cannot be fulfilled, and the connection still exists, 'server' closes the connection without response.

General:

• Write this ex in C.
• The server should be implemented in a single file named echoserver.c. Don't submit any other source/header files.
• Do not submit any client you write.
• Submit your source echoserver.c, a README, and a Makefile that prepares one target: echoserver.

Policies, Tips and Gradings:

1. The grading of this exercise will be as follows:

ü      85%: Correctness.

ü      10%: Code structure.

ü      5%: Submission.