This is an old version of this course, from Fall 2015. A newer version is available here.

Lab Assignment

This is a team assignment; each team should be two members unless a different size is approved by the instructor.

Introduction

The goal of this lab is to write a short Python program, to respond to HTTP requests and return web resources, acting as an HTTP server.

Procedure

1. Download the skeleton Python template: httpserver.py
2. Edit the header of the file to include your team members' names and usernames. (Note: the header format has changed from prior labs.)
3. Complete the handle_request method to parse a request and respond by returning the designated resource. You will want to add other helper methods, but do not change the any other code provided in the template.
   • Note that this method will be invoked on a separate thread for each request received. This means that there may be multiple copies of this method running simultaneously, if the web client opens more than one connection at a time (e.g., to download resources that are referenced in a main HTML file).
   • For this reason, you should not rely on any global variables, but instead pass data as arguments to related functions. Each thread will have its own execution stack.
4. Add comments at the end of your Python file, with the following information:
   • A description of the functionality you implemented and the results of your testing.
   • Comments on your experience in completing the lab, including any problems you encountered. Briefly explain what you learned.
   • Questions and suggestions.

You may not use a prebuilt library like Lib/BaseHTTPServer; the point of this lab is for you to understand the low-level implementation of the HTTP protocol.

You may and should use the utility functions that are included near the end of the skeleton template file. Read the description for each function and ask the instructor if you have questions about them.

If this base functionality turns out to be too easy, you may experiment with adding additional functions, but be sure the basic requirements are still met.

Plan as a team and divide up the primary responsibility for parts of the program in an equitable way.

Assignment details

• Your server is only expected to handle "file" resources, so that you can service a client request by returning the contents of a file associated with the resource identifier.
  • You must be able to serve at least the following resources. Download each of them from the table below:

    Relative URL	File path, relative to the directory with your Python server code file	File to serve (right-click to download)
    / (default)	./index.html	index.html
    index.html	./index.html	index.html
    sebern1.jpg	./sebern1.jpg	sebern1.jpg
    style.css	./style.css	style.css
    e-sebern2.gif	./e-sebern2.gif	e-sebern2.gif

• You must parse the request Request-Line and all request header lines, storing the key/value pairs in a Python dictionary. (See slides on dictionaries in Python)
  • Unless you implement additional functionality, it is unlikely that you will need to make use of any of the request headers, but you should store and print them after the entire request is received, so you can verify that you are handling the request correctly.
• You must return an appropriate response Status-Line and header lines to the requesting client.
• Use a Python dictionary to store the response header lines, and then send them all at once at an appropriate time.
• The response header lines must include:
  • A Date header (in proper RFC format) indicating the time that the request was satisfied.
  • A Connection header indicating that a persistent connection will NOT be used.
  • A Content-Type header with an appropriate MIME type (you should use the provided function to get the MIME type).
  • A Content-Length header to specify the size of the resource being returned (if there is one to return).
    • You are not required to use chunked encoding for any file type.
    • Optionally, you may use chunked encoding for text/html resources. If you do so, you must include the appropriate Transfer-Encoding header instead of Content-Length, and format the resource data appropriately in the response.

Hints and Notes

• As in the HTTP client lab, you will have to both send and receive on the TCP connection to the HTTP client. On the receiving side, since we will only be handling GET requests with no entity bodies, there will likely be only one kind of data that needs to be processed: "Textual" data, organized as a sequence of characters followed by the CR/LF pair. Data in this category includes:
  • The HTTP Request-Line.
  • Request header lines.
  • "Blank" lines (e.g., to terminate a header). You should probably have a "read line" function from the HTTP client lab, which you can likely use here.
• Remember that when you read from the network stream with a function like recv, or from a file with a function like read, you can only control the maximum number of bytes that will be returned. You will always get at least one byte, unless there is no more data (in the case of a file or a socket that has been closed), but there is no way to predict absolutely in advance how many bytes will be available when you make the recv or read call.
  • At times, you may get fewer than the number of bytes needed (e.g., in a block of "binary byte" data). If this happens, you will have to make another recv call to get additional data.
• When serving resource data from a file, open the file in binary ('rb') mode to avoid problems with line-ending modification on Windows.
• Getting the proper HTTP "Date" value can be a little tricky. You can try something like this:

  timestamp = datetime.datetime.utcnow()
  timestring = timestamp.strftime('%a, %d %b %Y %H:%M:%S GMT')
   				#Sun, 06 Nov 1994 08:49:37 GMT
  

When you have questions you can't resolve, consult the instructor as soon as possible, in person or by email.

Submission (Due Friday, Week 6, 11PM)

One team member should submit your Python file by uploading it to the upload page.

Please submit your file below. Please make sure your usernames are separated by hyphens and in alphabetical order.

(Acknowledgements: The original version of this lab written by Dr. Sebern.)