TCP Server

Table of Contents

First, solve the problem. Then, write the code.

John Johnson

GitHub Classroom

Use the GitHub Classroom link posted in the Slack channel for the lab to accept the assignment.

Overview

In this lab, you will be building a TCP server that implements your favorite protocol (the same one as the last three labs). The individual parts of the lab are outlined below. This lab (and all future labs) must be done in Python, but many of the algorithms that you have developed and refined in the previous labs can be reused in this lab.

Protocol

This lab will be using the same version of the protocol you developed in TCP Client v3 (binary header and pipelining).

The request protocol will be formatted as follows:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|  Action |                    Message Length                   |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             Data                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Action: 5 bits
Message Length: 27 bits
Data: variable

Once your server has received a request, it will parse it, transform the message, and send back a response. In processing a request, if an unexpected action is encountered or a request is malformed, then the message “error” should be returned. Your response must be formatted as:

 0                   1                   2                   3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                         Message Length                        |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|                             Data                              |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Message Length: 32 bits
Data: variable

To simplify this lab, a few adjustments will be made:

  • You only need to support one client at a time.

  • Typically for a server, you provide the IP address you would like the server to bind to (e.g., 127.0.0.1, 192.0.2.33), or 0.0.0.0 to bind to all interfaces. For this lab, you can bind to all interfaces. In Python this is done by passing in an empty string "" as the binding IP address.

Command-line Interface (CLI)

The CLI will have no arguments and three options: port, verbose flag, and help flag. The port option changes the port that your server binds to. Your program will generally print nothing to stdout. The only exception is if the --help option is set. Your program must print to stderr any errors while running the server and log messages, if the verbose flag is set.

Your server will be designed to block forever. Once it has handled one client, it will wait for another client to connect. As a result, you must be able to properly handle an interrupt signal (SIGINT). A process is usually sent this signal by typing ctrl-c in a terminal window. Your server must catch this signal and properly shutdown the server.

Here is a video of the program running:

Logging

Like the previous labs, I strongly encourage you to use logging to help debug your program and understand its flow. Python provides a powerful logging library. Spend some time learning it and it will pay off later.

Objectives

  • Learn to build a server.

  • Become familiar with Python network programming.

Requirements

  • You must use Python 3.8+.

  • You can not use any third party Python libraries for this lab. If you have to pip install or clone any repos, in order to import a library, stop. The only exception is the formatter, Black, which you have to pip install. However, you do not use it in your code.

  • For all socket related tasks, you must only use the low-level socket interface that Python provides. No high-level server socket interfaces are allowed.

  • The name of your program must be named tcp_server.py.

  • tcp_server.py accepts no arguments, and three options, as outlined above.

Usage: tcp_server.py [--help] [-v] [-p PORT]

Options:
  --help
  -v, --verbose
  --port PORT, -p PORT

  • The default port must be 8083.

  • You must set the SO_REUSEADDR option on the server socket.

  • Your server must handle any request size.

  • Your server does not need to support IPv6 or concurrent clients.

  • Return “error” (with the appropriate header) if an error occurs when processing a request, such as an invalid action.

  • If a client does not send enough data (e.g., the length is 10 but they only send 4 bytes), your server is allowed to block waiting for the remaining bytes of data.

  • Properly shutdown server when an interrupt signal (ctrl-c) is sent to your process. If your server is in the middle of sending/receiving to a client, you can let that finish before stopping the server.

  • As per the coding standard, you must use the Black formatter.

Testing

Netcat is going to be your best friend for this lab. This will allow you to connect directly to your server and test out different input. You can also use the client that you created in lab 3.

Submission

To submit your code, push it to your Github repository. Tag the commit you want to be graded with a tag named final.

Resources