Project 2 - Implementation of a Reliable Transport Protocol

Part A Due - Thursday, November 6, 2003

Part B Due - Tuesday, November 11, 2003

Overview

This project is a slightly modified version of Programming Assignment 3 from the textbook. The goal of this assignment is to give you a better understanding of reliable transport protocols by having you actually program one. You are going to write the code for a Go-Back-N protocol (GBN) as described in section 3.4.3 of your textbook. As the original assignment specification describes, you must first implement an Alternating-Bit protocol and then extend it to support the GBN. The original assignment specification can be found at: http://media.pearsoncmg.com/aw/aw_kurose_network_2/labs/lab3.html

Rather than actually modifying the OS on a given machine, your code will run in a simulated environment. The goal of the simulator (written in C) is to mimic an actual Unix environment. You will be provided with a number of routines which you can call, and a number of routines that will simulate sending and receiving of messages and call your routines when appropriate.

The GBN Protocol

In the GBN protocol, the sender has to process three different types of events:

1. Receive data from the application layer (layer 5)
2. Receive an ACK/NAK from the network layer (layer 3)
3. Timer Expiration

1. Receive packet from the network layer (layer 3)

The simulator will generate these types of events, and it is your job to implement the code necessary to process the event. The general structure of the simulator is available here.

Your Routines

You are given two structures which define the data units passed to (and/or from) your routines:

struct msg {
      char data[20] ;
      };

struct pkt {
       int seqnum;
       int acknum;
       int checksum;

       char payload[20];
        };

You will implement the following routines:

• SenderSendMsg(message) - You will need to take the structure of type msg, create a packet, and send it to layer 3. Keep in mind that you will also need to manage the "sliding window" and buffer messages as necessary.
• SenderProcessAck(packet) - You will need to take the structure of type pkt, determine whether or not it is corrupted, and process the ACK/NAK.
• SenderTimerExpiration() - You will need to resend outstanding data.
• SenderInit() - This routine will be called one time at the beginning of the run. Use it to do any initialization.
• RcvrProcessPkt(packet) - You will need to take the structure of type pkt, determine whether or not it is corrupted, determine whether is the the "correct" packet, and process it accordingly.
• RcvrInit() - This routine will be called one time at the beginning of the run. Use it to do any initialization.

Software Interfaces

You may (and should) call the following routines:

• starttimer(SenderOrRcvr,increment) - Called by students routine to start a timer. In your code, the sender will be the only entity calling the starttimer routine. Call the routine as follows: starttimer(Sender, RTT);
• stoptimer(SenderOrRcvr) - Called by students routine to cancel a previously-started timer. In your code, the sender will be the only entity calling the stoptimer routine. Call the routine as follows: stoptimer(Sender);
• tolayer3(SenderOrRcvr,packet) - Called by students routine to send a message from one entity to the other. If the sender is sending a packet to the receiver, call the routine as follows: tolayer3(Sender, mypacket); If the receiver is sending an ACK/NAK to the sender, call the routine as follows: tolayer3(Rcvr, mypacket);
• tolayer5(SenderOrRcvr,datasent) - Called by students routine to deliver a message to the application. In this assignment, the receiver will be the only entity calling this routine. Call the routine as follows: tolayer5(Rcvr, mypacket.payload);

I have also provided two additional helper functions which you can use to calculate and check the packet checksum. Those functions and the correct method of calling them can be found in the simulator code skeleton: proj2.c.

Running the Code

Once you have completed your portion of the code and run the simulator, you will be prompted for a number of paramenters. Following is a typical sample run:

----- Network Simulator --------

Enter the number of messages to simulate: 5
Enter  packet loss probability [enter 0.0 for no loss]:.2
Enter packet corruption probability [0.0 for no corruption]:.2
Enter average time between messages from sender's layer5 [ > 0.0]:20
Enter TRACE:0

The parameters are as follows:

• Number of messages to simulate - The number of messages passed from layer 5 to your SenderSendMsg routine. This number should always be greater than 1.
• Packet loss probability - This should be between 0 and 1. For example, 0.2 means that 2 in 10 packets are lost (a 20% probability).
• Packet corruption probability - This should be between 0 and 1. Note that the contents of the payload, sequence, ack or checksum fields can be corrupted.
• Time between messages - This value should be greater than 0 and indicates the amount of time between messages passed down from layer 5. When you are debugging, it is easiest to use large values (like 1000).
• Trace - This can be 0, 1, 2, or 3. It indicates how much information is printed throughout the run of the simulation. Higher values mean that more debugging information is printed.

Getting Started

For this assignment, you may work in teams of two. Download the simulator and skeleton for your code here: proj2.c. You can edit the file with your favorite editor, and compile with your favorite compiler. You may discuss high-level concepts with the other teams, but you are not to copy or even look at another team's code.

There is a bit of an issue with the random number generator. You may need to change the value of the variable mmm in the function jimsrand. If you get an error message asking you to look at the function, you can let me know and we will figure out the solution for the environment you are running in.

What to Turn In

Send me your completed code via email by class time on the due dates. Part A must be turned in by Thursday, November 6 and Part B must be turned in by Tuesday, November 11. Make sure you include all files required to compile and run your program and any special instructions with respect to compilation, etc. I will compile it and run it under a number of scenarios. You do not need to turn in the output of your test cases. However, make sure you test your code for all possible cases.

Misc

You are not asked to implement the three way handshake for this assignment. You can assume that connection setup has already taken place and you can hard code an initial sequence number.

You may use both ACK and NAK messages if you like, or just ACK messages (as described in the text). It is up to you.