A.3   Lab: Implementing a reliable transport protocol

Overview

In this laboratory programming assignment, you will be writing the sending and receiving transport-level code for implementing a simple reliable data transfer protocol. There are two versions of this lab, the Alternating-Bit-Protocol version and the Go-Back-N version. This lab should be FUN since your implementation will differ very little from what would be required in a real-world situation.

Since you probably don't have standalone machines (with an OS that you can modify), your code will have to execute in a simulated hardware/software environment. However, the programming interface provided to your routines, i.e., the code that would call your entities from above and from below is very close to what is done in an actual UNIX environment. (Indeed, the software interfaces described in this programming assignment are much more realistic that the infinite loop senders and receivers that many texts describe). Stopping/starting of timers are also simulated, and timer interrupts will cause your timer handling routine to be activated.

The routines you will write

The procedures you will write are for the sending entity (A) and the receiving entity (B). Only unidirectional transfer of data (from A to B) is required. Of course, the B side will have to send packets to A to acknowledge (positively or negatively) receipt of data. Your routines are to be implemented in the form of the procedures described below. These procedures will be called by (and will call) procedures that I have written which emulate a network environment. The overall structure of the environment is shown in Figure Lab.3-1:

structure of the emulated environment
Figure Lab.3-1: structure of the emulated environment

The unit of data passed between the upper layers and your protocols is a message, which is declared as:

struct msg {
    char data[20];
    };

This declaration, and all other data structure and emulator routines, as well as stub routines (i.e., those you are to complete) are in the file, prog2.c, described later. Your sending entity will thus receive data in 20-byte chunks from layer5; your receiving entity should deliver 20-byte chunks of correctly received data to layer5 at the receiving side.

The unit of data passed between your routines and the network layer is the packet, which is declared as:

struct pkt {
   int seqnum;
   int acknum;
   int checksum;
   char payload[20];
    };

Your routines will fill in the payload field from the message data passed down from layer5. The other packet fields will be used by your protocols to insure reliable delivery, as we've seen in class.

The routines you will write are detailed below. As noted above, such procedures in real-life would be part of the operating system, and would be called by other procedures in the operating system.

Software Interfaces

The procedures described above are the ones that you will write. I have written the following routines which can be called by your routines:

The simulated network environment

A call to procedure tolayer3() sends packets into the medium (i.e., into the network layer). Your procedures A_input() and B_input() are called when a packet is to be delivered from the medium to your protocol layer.

The medium is capable of corrupting and losing packets. It will not reorder packets. When you compile your procedures and my procedures together and run the resulting program, you will be asked to specify values regarding the simulated network environment:

The Alternating-Bit-Protocol Version of this lab.

You are to write the procedures, A_output(), A_input(), A_timerinterrupt(), A_init(), B_input(), and B_init() which together will implement a stop-and-wait (i.e., the alternating bit protocol, which we referred to as rdt3.0 in the text) unidirectional transfer of data from the A-side to the B-side. Your protocol should use both ACK and NACK messages.

You should choose a very large value for the average time between messages from sender's layer5, so that your sender is never called while it still has an outstanding, unacknowledged message it is trying to send to the receiver. I'd suggest you choose a value of 1000. You should also perform a check in your sender to make sure that when A_output() is called, there is no message currently in transit. If there is, you can simply ignore (drop) the data being passed to the A_output() routine.

You should put your procedures in a file called prog2.c. You will need the initial version of this file, containing the emulation routines we have writen for you, and the stubs for your procedures. You can obtain this program from http://gaia.cs.umass.edu/kurose/transport/prog2.c.

This lab can be completed on any machine supporting C. It makes no use of UNIX features. (You can simply copy the prog2.c file to whatever machine and OS you choose).

We recommend that you should hand in a code listing, a design document, and sample output. For your sample output, your procedures might print out a message whenever an event occurs at your sender or receiver (a message/packet arrival, or a timer interrupt) as well as any action taken in response. You might want to hand in output for a run up to the point (approximately) when 10 messages have been ACK'ed correctly at the receiver, a loss probability of 0.1, and a corruption probability of 0.3, and a trace level of 2. You might want to annotate your printout with a colored pen showing how your protocol correctly recovered from packet loss and corruption.

Make sure you read the ``helpful hints'' for this lab following the description of the Go_Back-N version of this lab.

The Go-Back-N version of this lab.

You are to write the procedures, A_output(), A_input(), A_timerinterrupt(), A_init(),B_input(), and B_init() which together will implement a Go-Back-N unidirectional transfer of data from the A-side to the B-side, with a window size of 8. Your protocol should use both ACK and NACK messages. Consult the alternating-bit-protocol version of this lab above for information about how to obtain the network emulator.

We would STRONGLY recommend that you first implement the easier lab (Alternating Bit) and then extend your code to implement the harder lab (Go-Back-N). Believe me – it will not be time wasted! However, some new considerations for your Go-Back-N code (which do not apply to the Alternating Bit protocol) are:

Consult the Alternating-bit-protocol version of this lab above for a general description of what you might want to hand in. You might want to hand in output for a run that was long enough so that at least 20 messages were successfully transfered from sender to receiver (i.e., the sender receives ACK for these messages) transfers, a loss probability of 0.2, and a corruption probability of 0.2, and a trace level of 2, and a mean time between arrivals of 10. You might want to annotate parts of your printout with a colored pen showing how your protocol correctly recovered from packet loss and corruption.

For extra credit, you can implement bidirectional transfer of messages. In this case, entities A and B operate as both a sender and receiver. You may also piggyback acknowledgments on data packets (or you can choose not to do so). To get my emulator to deliver messages from layer 5 to your B_output() routine, you will need to change the declared value of BIDIRECTIONAL from 0 to 1.

Helpful Hints and the like

Q&A

When we've taught this lab in our introductory neworking course, students have posed versious questions. If you are interested in looking at the questions we've received (and answers), check out http://gaia.cs.umass.edu/kurose/transport/programming_assignment_QA.htm