CS 3700 - Networks and Distributed Systems

Project 4: Simple Transport Protocol

This project is due at 11:59pm on Friday, March 19, 2020.

Description

You will design a simple transport protocol that provides reliable datagram service. Your protocol will be responsible for ensuring data is delivered in order, without duplicates, missing data, or errors. Since the local area networks at Northeastern are far too reliable to be interesting, we will provide you with scripts to set up a vagrant image to emulate an unreliable network.

For the assignment, you will write code that will transfer a file reliably between two nodes (a sender and a receiver). You do NOT have to implement connection open/close etc. You may assume that the receiver is run first and will wait indefinitely, and the sender can just send the data to the receiver.

Requirements

You have to design your own packet format and use UDP as a carrier to transmit packets. Your packet might include fields for packet type, acknowledgment number, advertised window, data, etc. This part of the assignment is entirely up to you. Your code must meet the following specifications:

You may implement any reliability algorithm(s) you choose. However, your implementation must meet certain minimums of performance. Desirable properties in any reliability algorithm include (but are not limited to): Being said, correctness matters most: your receiver must output exactly the input to your sender. We will test your code and measure these two performance metrics; better performance will result in higher credit. Remember that network-facing code should be written defensively. Your code should check the integrity of every packet received. We will test your code by reordering packets, delaying packets, and dropping packets. You should handle these errors gracefully, recover, and not crash.

Your Programs

For this project, you will submit two programs: a sending program 3700send that accepts data on STDIN and sends it across the network, and a receiving program 3700recv that receives data and prints it to STDOUT in-order. You must use UDP. You may not use any transport protocol libraries in your project (TCP, QUIC, etc).

Language

You can write your code in whatever language you choose, as long as your code compiles and runs on unmodified Khoury College Linux machines on the command line. Do not use libraries that are not installed by default on the Khoury College Linux machines, or that are disallowed for this project. You may use IDEs (e.g. Eclipse) during development, but do not turn in your IDE project without a Makefile. Make sure you code has no dependencies on your IDE.

Testing Environment

The goal of this project is to get you to develop a TCP-like, reliable transport protocol. As such, we need a way for you to test your code on an unreliable network, i.e., one that is slow, drops packets, reorders and duplicates packets, etc. Unfortunately, real networks are far too reliable for this purpose. As such, you will develop and test your programs inside a virtual machine (VM) that is pre-configured with tools for creating lossy, slow, unreliable network links.

You will need to download, install, and setup two tools to gain access to the VM for this project: VirtualBox and Vagrant. The former is a tool that can run VMs; the latter is a tool for automating the deployment and configuration of VMs. Both tools are available for Windows, OSX, and major Linux distributions. Note: Vagrant is compatible with other VM tools besides VirtualBox, but we will only be providing support for VirtualBox in this class. You may use other VM tools at your own peril.

Installation

Download and install Oracle VirtualBox v6.1.18 from here. Download and install Vagrant v2.2.14 from here.
After you have installed VirtualBox and Vagrant, you are ready to setup the VM that you will use for this project. Open a terminal and create a folder that will house (1) the VM, (2) the Vagrant configuration files, and (3) synchronized files from within the Vagrant VM (more on this in a little bit). Create this folder in a place you will remember, like your home directory or desktop. Once this directory is created, cd into it and run the following command: 

$ vagrant init cs3700/transport-environment --box-version 0.0.1

This will create a Vagrant configuration file in the directory that describes the VM. You do not need to understand or modify this configuration file (unless you have problems with your Vagrant VM, see the notes on issues at the end of the webpage).

Managing and Accessing the VM

From within the directory where the Vagrant configuration is stored, run the following command to boot up the Vagrant VM: 

$ vagrant up

This command can take a while, as it downloads the image for the VM, unpacks it, and executes it. Once the command is done you won't see any changes to your computer, but the Vagrant VM is running in the background. To access the Vagrant VM, use the following command to ssh into it: 

$ vagrant ssh

If you are asked for password, use "vagrant". At this point, you should have sshed into the Vagrant VM, and you are ready to begin working on the project.
To temporarily pause the Vagrant VM, open a terminal, cd into the directory where the Vagrant configuration is stored, and use the following command: 

$ vagrant halt

This command is data preserving: the next time you run vagrant up the VM will resume in the same state it was previously in. If you created files in the VM, they will still be present.
To permanently destroy the Vagrant VM and all data within it, use the following command: 

$ vagrant destroy

Getting Files Into and Out of the Vagrant VM

The Vagrant VM is a copy of CentOS Linux, and it has its own file system and virtual hard drive that is totally distinct from the file system in your host OS. However, there is a convenient way to get files into and out of the Vagrant VM: by default, any files placed in the /vagrant directory inside the VM will automatically be synchronized to the directory where the Vagrant configuration is stored in the host OS.

PUT ALL YOUR SOURCE CODE IN THE /vagrant FOLDER

We highly recommend that students place all of their source code for this project in the the /vagrant directory so that it will be synchronized and backed-up to their host OS. If the Vagrant VM crashes, is destroyed, or the host OS is rebooted without halting the VM first, all data inside the VM will be lost. We cannot help you recover source code in these cases.

Starter Code

Very basic starter code in C and Python 2 for the project is available in the /transport-starter directory in the Vagrant VM. You may use this code as a basis for your project, or you may work from scratch. Provided is a simple implementation that sends one packet at a time; it does not handle any packet retransmissions, delayed packets, or duplicated packets. It will only work if the network is perfectly reliable. Moreover, if the latency is significant, the implementation will use very little of the available bandwidth. To get started, you should copy down this directory into your own local directory. You can compile the code by running make. You can also delete any compiled code and object files by running make clean.

Program Specification

The command line syntax for your sending is given below. The syntax for launching your sending program must be: 

./3700send <recv_host>:<recv_port>

The sender must open a UDP socket to the given IP address on the given port.
The data that the sender must transmit to the receiver must be supplied in STDIN. The sender must read in the data from STDIN and transmit it to the receiver via the UDP socket.
Your sender may print out any debug information that you wish, but it must do so to STDERR. The syntax for launching your receiving program must be: 

./3700recv <recv_port>

On startup, the receiver must bind to the specified UDP port and wait for datagrams from the sender. Similar to 3700send, you may add your own output messages to your receiver but they must be printed to STDERR.
The receiver program must print out the data that it receives from the sender to STDOUT. The data that it prints must be identical to the data that was supplied to the sender via STDIN. In other words, data cannot be missing, reordered, or contain any bit-level errors.

Testing Your Code

In order for you to test your code over an unreliable network, we are providing scripts to setup a Vagrant image that will emulate a network that drops, reorders, duplicates, and delays your packets. These scripts are tested to work on the Vagrant image, and are known NOT TO WORK on Khoury machines managed by Systems. You will need to use the loopback interface in order to leverage the emulated network. In other words, you might run something like ./3700recv 3992 in one terminal and then run ./3700send 127.0.0.1:3992 in another terminal. Note the use of the loopback IP address 127.0.0.1 .

You may configure the emulated network conditions by calling the following program: 

netsim [--bandwidth <bw-in-mbps>] 
     [--latency <latency-in-ms>] [--delay <percent>] 
     [--drop <percent>] 
     [--reorder <percent>] [--duplicate <percent>]

Once you call this program, it will configure the emulator to delay/drop/reorder/duplicate all UDP and ICMP packets sent by or to you at the specified rate. For example, if you called 

netsim --bandwidth 0.5 --latency 100 --delay 20 --drop 40

the simulator will configure a network with 500 Kb/s bandwidth and a latency of 100 ms, and will randomly delay 20% of your packets and drop 40%. In order to reset it so that none of your packets are disturbed, you can simply call netsim with no arguments. Note that the simulator is stateful, meaning your settings will persist across multiple sessions.

Helper Script

In order to make testing your code easier, we have included a script in the Vagrant VM that will launch your receiver, launch your sender, feed the sender input, read the output from the receiver, compare the two, and print out statistics about the transfer. This script is included in the Vagrant VM, and you can run it by executing 

$ nettest
This script also takes a couple of arguments to determine what it should do: 
$ nettest [--live] [--size (small|medium|large|huge)] [--timeout TIMEOUT]
The output of this script include some statistics about the transfer: 

$ nettest --size large
  Data match: Yes
  Msg Size: 10000 B
  Time elapsed: 174.982 ms
  Packets sent: 23
  Data on Wire: 13761 B

where Data match indicates whether the data was transferred correctly.

Testing Script

Additionally, we have included a testing script that runs your code under a variety of network conditions and checks your code's compatibility with the grading script. If your code fails in the test script we provide, you can be assured that it will fare poorly when run under the grading script. To run the test script, simply type 

$ ./testall
This test your programs on a number of inputs. If any errors are detected, the test will print out the expected and actual output. Note that you must run the testall command from within the directory that contains your 3700send and 3700recv programs.

Performance Testing

10% of your grade on this project will come from performance. This includes the time it takes for sender to fully transmit files to the receiver and the number of bytes sent during the transmission. Your project will be graded against a series of benchmarks that we have established.
To help you know how you're doing, the testing script will run a series of performance tests at the end. For example, you might see something like the following: 

Performance tests
    huge 5 Mb/s, 10 ms, 0% drop, 0% duplicate 0% delay        [DATAOK]
    0.401 sec elapsed, 976KB sent
    Rate: 19Mb/s                                              [ OKAY ]

The first line presents the parameters of the current test. [DATAOK] indicates that the file was delivered successfully, without any errors. [DATAERR] would indicate that the receiver printed out a file containing errors. The following lines present the performance characteristics of the file transfer, including how long it took, how much data was sent, and the overall transmission rate. [OKAY] indicates that performance in this case was acceptable, and would receive full credit. Alternatively, you might see output like this: 

    Rate: 1Mb/s                                               [PERF50]

[PERFXX] indicates that the performance was at XX% of our target benchmark. In this example, performance hit 50% of the desired benchmark. In these cases, credit would be awarded proportionately to the achieved performance level. [FAIL] indicates that performance was insufficient, and would not receive any credit for performance in this test.

Submitting Your Project

Before turning in your project, you and your partner(s) must register your group. To register yourself in a group, execute the following script: 

$ /course/cs3700sp21/bin/register project4 [team name]
This will either report back success or will give you an error message. If you have trouble registering, please contact the course staff. You and your partner(s) must all run this script with the same [team name]. This is how we know you are part of the same group. Note that this command must be run on a Khoury Linux machine, not within a Vagrant VM.

To turn-in your project, you should submit your (thoroughly documented) code along with two other files: Your README.md, Makefile, source code, etc. should all be placed in a directory. You submit your project by running the turn-in script on a Khoury machine as follows: 

$ /course/cs3700sp21/bin/turnin project4 [project directory]

Alternatively, you may turn-in from within the Vagrant VM using the following command: 

$ remotesubmit project4 [project directory]

In both cases, [project directory] is the name of the directory with your submission. The script will print out every file that you are submitting, so make sure that it prints out all of the files you wish to submit! The turn-in script will not accept submissions that are missing a README.md or a Makefile. Only one group member needs to submit your project. Your group may submit as many times as you wish; only the last submission will be graded, and the time of the last submission will determine whether your assignment is late.

Double Checking Your Submission

To try and make sure that your submission is (1) complete and (2) will work with our grading scripts, we provide a simple script that checks the formatting of your submission. This script is available on the Khoury College Linux machines and can be executed using the following command: 

/course/cs3700sp21/code/project4/project4_format_check.py [path to your project directory]
This script will attempt to make sure that the correct files (e.g. README.md and Makefile) are available in the given directory, that your Makefile will run without errors (or is empty), and that after running the Makefile two programs named 3700send and 3700recv exist in the directory. The script will also try to determine if your files use Windows-style line endings (\r\n) as opposed to Unix-style line endings (\n). If your files are Windows-encoded, you must convert them to Unix-encoding using the dos2unix utility before turning in.

Grading

This project is worth 15% of your final grade. The grading in this project will consist of:

Points will be taken off for submissions that send datagrams with greater than 1500 bytes of data.

By definition, you are going to be graded on how gracefully you handle errors. Particularly, your code must never print out incorrect data. Your code will definitely see corrupted packets, delays, duplicated packets, and so forth. You should always assume that everyone is trying to break your program. To paraphrase John F. Woods, "Always code as if the [the remote machine you're communicating with] will be a violent psychopath who knows where you live."

You can see your grades for this course at any time by using the gradesheet program that is available on the Khoury College machines. 
$ /course/cs3700sp21/bin/gradesheet