CS475: Computer Networks - TFTP Protocol (100 Points)

Assignment Goals

The goals of this assignment are:
  1. To implement a protocol according to its RFC

The Assignment

A tftp Server

You should construct a file transfer server that follows the TFTP protocol (RFC 1350). Part one of this assignment involves constructing the server, while part two involves constructing the client.

Your programs should adhere to the published specifications so that they can inter-operate with existing clients and servers. The protocol follows this state machine [1]:

TFTP State Machine

You could test your server with the tftp client available under most Unix (and Unix-like) operating systems (e.g. Solaris, Linux, OpenBSD, etc.). Here is an article describing how to install and configure tftp on Ubuntu. However, if you do, note that some extensions to tftp are implemented by modern clients, rendering this spec non-backwards compatible. You’ll need to implement slightly different message formats to do so (look at the tftp OACK extensions). You might, instead, copy your server implementation and write a small client for testing instead, which is also acceptable.

Your server should handle multiple clients at the same time. You can accomplish this by putting your server code into a threaded routine, and starting the thread with the connection socket as soon as a connection is made by the primary server socket in main().

The Protocol State Machine

Before you begin writing code, read the TFTP protocol RFC and sketch a state machine that indicates:

  1. What action should be taken when each message is received
  2. What response should be sent when each message is received
  3. What message should be expected next

For example, upon receiving an RRQ, you should read the filename from the message and, if it exists, respond with a DATA packet containing the first 512 bytes of the file. If the file does not exist (or you can’t read it), you should respond with an ERROR packet and quit.

Implementing the State Machine

This state machine can be implemented using a do loop and an if statement that checks the opcode of each message. Depending on the opcode received, you’ll know how to read the rest of the message, what to do, how to respond, and what to expect next. Initially, you should expect to receive an RRQ or a WRQ message to kick things off - similarly, this is what you would send first (and you’ll know what to expect in reply right away thanks to the state machine you’ve just made!).

Bit Packing with Java

You can use the DataInputStream and DataOutputStream to send individual bytes, short integers, integers, etc., as part of your message format. For example, to read a String, you can call readChar() in a loop until the character returned is equal to the null terminator \0. You can append each character to a StringBuilder, and call its toString() method to obtain the resulting String.

Network (Big-Endian) Byte Order

When you write a byte[] array, you will want to be careful to put the array into network order before sending it with the DataOutputStream.write(byte[], 0, n) method. Similarly, you’ll want to re-order the bytes when you receive them from network byte order into your local computer architecture byte ordering. Java facilitates this byte swapping with the java.nio.ByteBuffer library, which you can use for this purpose. For example:

import java.nio.ByteBuffer;

// this operation is known as ntohl in C
int networkOrderToInt(byte[] data) {
    ByteBuffer buf = ByteBuffer.wrap(data);
    return buf.getInt(); // can be getShort, getLong, ...
}

// this operation is known as htonl in C
byte[] intToNetworkOrder(int data) {
    ByteBufer buf = ByteBuffer.allocate(4); // sizeof(int) is 4
    buf.putInt(data);
    return buf.array();
}

Notes

The tftp service is using port 69 which requires superuser (administrator) access rights. Since you want to be able to run your server on machines where you may not have superuser privileges, you should include the option to run the server from another (non-privileged) port.

  1. Image created by staskobzar under the GPLv3 license. ↩

Submission

In your submission, please include answers to any questions asked on the assignment page in your README file. If you wrote code as part of this assignment, please describe your design, approach, and implementation in your README file as well. Finally, include answers to the following questions:
  • Describe what you did, how you did it, what challenges you encountered, and how you solved them.
  • Please answer any questions found throughout the narrative of this assignment.
  • If collaboration with a buddy was permitted, did you work with a buddy on this assignment? If so, who? If not, do you certify that this submission represents your own original work?
  • Please identify any and all portions of your submission that were not originally written by you (for example, code originally written by your buddy, or anything taken or adapted from a non-classroom resource). It is always OK to use your textbook and instructor notes; however, you are certifying that any portions not designated as coming from an outside person or source are your own original work.
  • Approximately how many hours it took you to finish this assignment (I will not judge you for this at all...I am simply using it to gauge if the assignments are too easy or hard)?
  • Your overall impression of the assignment. Did you love it, hate it, or were you neutral? One word answers are fine, but if you have any suggestions for the future let me know.
  • Using the grading specifications on this page, discuss briefly the grade you would give yourself and why. Discuss each item in the grading specification.
  • Any other concerns that you have. For instance, if you have a bug that you were unable to solve but you made progress, write that here. The more you articulate the problem the more partial credit you will receive (it is fine to leave this blank).

Assignment Rubric

Description Pre-Emerging (< 50%) Beginning (50%) Progressing (85%) Proficient (100%)
Algorithm Implementation (60%) The algorithm fails on the test inputs due to major issues, or the program fails to compile and/or run The algorithm fails on the test inputs due to one or more minor issues The algorithm is implemented to solve the problem correctly according to given test inputs, but would fail if executed in a general case due to a minor issue or omission in the algorithm design or implementation A reasonable algorithm is implemented to solve the problem which correctly solves the problem according to the given test inputs, and would be reasonably expected to solve the problem in the general case
Code Quality and Documentation (30%) Code commenting and structure are absent, or code structure departs significantly from best practice, and/or the code departs significantly from the style guide Code commenting and structure is limited in ways that reduce the readability of the program, and/or there are minor departures from the style guide Code documentation is present that re-states the explicit code definitions, and/or code is written that mostly adheres to the style guide Code is documented at non-trivial points in a manner that enhances the readability of the program, and code is written according to the style guide
Writeup and Submission (10%) An incomplete submission is provided The program is submitted, but not according to the directions in one or more ways (for example, because it is lacking a readme writeup) The program is submitted according to the directions with a minor omission or correction needed, and with at least superficial responses to the bolded questions throughout The program is submitted according to the directions, including a readme writeup describing the solution, and thoughtful answers to the bolded questions throughout

Please refer to the Style Guide for code quality examples and guidelines.