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Assignment 3: Reliable Transport

Due: November 16th, 2018 at 11:59 PM


In this project, you will build a simple reliable transport protocol, WTP, on top of UDP. Your WTP implementation must provide in­order, reliable delivery of UDP datagrams in the presence of events like packet loss, delay, corruption, duplication, and re­ordering.

There are a variety of ways to ensure a message is reliably delivered from a sender to a receiver. You are to implement a sender (wSender) and a receiver (wReceiver) that follows the following WTP specification.

WTP Specification

WTP sends data in the format of a header, followed by a chunk of data.

WTP has four header types: START, END, DATA, and ACK, all following the same format:

struct PacketHeader {
	unsigned int type;     // 0: START; 1: END; 2: DATA; 3: ACK
	unsigned int seqNum;   // Described below
	unsigned int length;   // Length of data; 0 for ACK, START and END packets
	unsigned int checksum; // 32-bit CRC

To initiate a connection, wSender starts with a START message along with a random seqNum value, and wait for an ACK for this START message. After sending the START message, additional packets in the same connection are sent using the DATA message type, adjusting seqNum appropriately. After everything has been transferred, the connection should be terminated with wSender sending an END message, and waiting for the corresponding ACK for this message.

The ACK seqNum values for START and END messages should both be set to whatever the seqNum values are that were sent by wSender.

wSender will use 0 as the initial sequence number for data packets in that connection. Furthremore, it sends back cumulative ACK packets (described in more details below).

Packet Size

An important limitation is the maximum size of your packets. The UDP protocol has an 8 byte header, and the IP protocol underneath it has a header of 20 bytes. Because we will be using Ethernet networks, which have a maximum frame size of 1500 bytes, this leaves 1472 bytes for your entire packet structure (including both the header and the chunk of data).

Overall, this assignment has the following components:

Learning Outcomes

After completing this programming assignment, students should be able to:

  • Explain the mechanisms required to reliably transfer data
  • Describe how different sliding window protocols work


  • TBA

Part 1: Implement wSender

wSender should read an input file and transmit it to a specified receiver using UDP sockets following the WTP protocol. It should split the input file into appropriately sized chunks of data, and append a checksum to each packet. seqNum should increment by one for each additional packet in a connection. Please use the 32-bit CRC header we provide here, in order to add a checksum to your packet.

You will implement reliable transport using a sliding window mechanism. The size of the window (window-size) will be specified in the command line. wSender must accept cumulative ACK packets from wReceiver.

After transferring the entire file, you should send an END message to mark the end of connection.

wSender must ensure reliable data transfer under the following network conditions:

  • Loss of arbitrary levels;
  • Re­ordering of ACK messages;
  • Duplication of any amount for any packet;
  • Delay in the arrivals of ACKs.

To handle cases where ACK packets are lost, you should implement a 500 milliseconds retransmission timer to automatically retransmit packets that were never acknowledged. Whenever the window moves forward (i.e., some ACK(s) are received and some new packets are sent out), you reset the timer. If after 500ms the window still has not advanced, you retransmit all packets in the window because they are all never acknowledged.

Running wSender

wSender should be invoked as follows:

./wSender <input_file> <window_size> <log> <receiver_IP> <receiver_port>

  • input_file Path to the file that has to be transferred. It can be a text as well as a binary file (e.g., image or video).
  • window_size Maximum number of outstanding packets.
  • log The file path to which you should log the messages as described below.
  • receiver_IP The IP address of the host that wReceiver is running on.
  • receiver_port The port number on which wReceiver is listening.

###Logging wSender should create a log of its activity. After sending or receiving each packet, it should append the following line to the log (i.e., everything except the data of the packet structure described earlier):

<type> <seqNum> <length> <checksum>

Part 2: Implement wReceiver

wReceiver needs to handle only one wSender at a time and should ignore START messages while in the middle of an existing connection. It must receive and store the file sent by the sender on disk completely and correctly; i.e., if it received a video file, we should be able to play it!. The stored file should be named FILE-i, where i=1 for the file from the first connection, i=2 for the second, and so on.

wReceiver should also calculate the checksum value for the data in each packet it receives using the header mentioned in part 1. If the calculated checksum value does not match the checksum provided in the header, it should drop the packet (i.e. not send an ACK back to the sender).

For each packet received, it sends a cumulative ACK with the seqNum it expects to receive next. If it expects a packet of sequence number N, the following two scenarios may occur:

  1. If it receives a packet with seqNum not equal to N, it will send back an ACK with seqNum=N.
  2. If it receives a packet with seqNum=N, it will check for the highest sequence number (say M) of the in­order packets it has already received and send ACK with seqNum=M+1.

If the next expected seqNum is N, wReceiver will drop all packets with seqNum greater than or equal to N + window_size to maintain a window_size window.

wReceiver should also log every single packet it sends and receives using the same format as the wSender log.

Put the programs written in parts 1 and 2 of this assignment into a folder called WTP-base.

Running wReceiver

wReceiver should be invoked as follows: ./wReceiver <port_num> <log> <window_size> <file_dir>

  • port_num The port number on which wReceiver is listening for data.
  • log The file path to which you should log the messages as described above.
  • window_size Maximum number of outstanding packets.
  • file_dir Path to the directory that holds the output file, i.e., where FILE-i files are stored. Assume the directory already exists.

For example: ./wReceiver 5001 log.txt 2 /tmp

Part 3: Optimizations

For this part of the assignment, you will be making a few modifications to the programs written in the previous two sections. Consider how the programs written in the previous sections would behave for the following case where there is a window of size 3:

In this case wReceiver would send back two ACKs both with the sequence number set to 0 (as this is the next packet it is expecting). This will result in a timeout in wSender and a retransmission of packets 0, 1 and 2. However, since wReceiver has already received and buffered packets 1 and 2. Thus, there is an unnecessary retransmission of these packets.

In order to account for situations like this, you will be modifying your wReceiver and wSender accordingly (save these different versions of the program in a folder called WTP-opt):

  • wReceiver will not send cumulative ACKs anymore; instead, it will send back an ACK with seqNum set to whatever it was in the data packet (i.e., if a sender sends a data packet with seqNum set to 2, wReceiver will also send back an ACK with seqNum set to 2). It should still drop all packets with seqNum greater than or equal to N + window_size, where N is the next expected seqNum.
  • wSender must maintain information about all the ACKs it has received in its current window and maintain an individual timer for each packet. So, for example, packet 0 having a timeout would not necessarily result in a retransmission of packets 1 and 2.

For a more concrete example, here is how your improved wSender and wReceiver should behave for the case described at the beginning of this section:

wReceiver individually ACKs both packet 1 and 2.

wSender receives these ACKs and denotes in its buffer that packets 1 and 2 have been received. Then, the it waits for the 500 ms timeout and only retransmits packet 0 again.

The command line parameters passed to these new wSender and wReceiver are the same as the previous two sections.

Important Notes

  • Please closely follow updates on Piazza. All further clarifications will be posted on Piazza via pinned Instructor Notes. We recommend you follow these notes to receive updates in time.
  • It is up to you how you choose to read from and write to files, but you may find the and std::ofstream.write() functions particularly helpful.
  • You MUST NOT use TCP sockets.
  • You should still transmit all fields of the PacketHeader in the correct Byte Order, as you have done in Assignment 2 Part 2.
  • We are NOT providing any test cases. Instead, we will be using an automatic testing platform as discussed below.

Automatic Testing Platform

We will be using an automatic testing platform similar to the autograder of EECS482:

  • We pull the code from your GitHub repos once a day (twice a day near the deadline) at fixed time (TBD).
  • We then test your code, and send email feedbacks to your UMich mailbox at fixed time (TBD).
  • We will start this process about halfway of this assignment.
  • In total you will receive about 10 chances of testing. The final grade of your code is ONLY determined by the final testing.

Note that:

  • Your code will only be tested once/twice per day. Please do not rely on the platform for debugging.
  • The testing involves random packet drop/loss/duplication/corruption. If your code contains some bug which affects the handling of dropped packets, you may observe passing the packet drop test cases occasionally.

Submission Instructions

You must submit:

  • The source code for wSender and wReceiver from parts 1 and 2: all source files should be in a folder called WTP-base
  • A Makefile directly under WTP-base directory to compile WTP-base source code with one single make command, and to clean up with make clean
  • The source code for wSender and wReceiver from part 3: all source files should be in a folder called WTP-opt
  • A Makefile directly under WTP-opt directory to compile WTP-opt source code with one single make command, and to clean up with make clean
  • Your WTP-base and WTP-opt directory should be directly under your assignment 3 group folder (i.e., <repo>/WTP-base and <repo>/WTP-opt).


This programming assignment is based on UC Berkeley's Project 2 from EE 122: Introduction to Communication Networks.