Description
Assignment Objective The goal of this assignment is to implement the Go-‐Back-‐N protocol, which could be used to transfer a text file from one host to another across an unreliable network. The protocol should be able to handle network errors such as packet loss and duplicate packets. For simplicity, your protocol is unidirectional, i.e., data will flow in one direction (from the sender to the receiver) and the acknowledgements (ACKs) in the opposite direction. To implement this protocol, you will write two programs: a sender and a receiver, with the specifications given below. You will test your implementation using an emulated network link (which will be provided to you) as shown in the diagram below:
When the sender needs to send packets to the receiver, it sends them to the network emulator instead of sending them directly to the receiver. The network emulator then forwards the received packets to the receiver. However, it may randomly discard and/or delay received packets. The same scenario happens when the receiver sends ACKs to the sender. Note: The assignment description (data structure and program names) assumes an implementation in Java. 2 Packet format All packets exchanged between the sender and the receiver should have the following structure (consult packet.java provided with the assignment): public class packet { … … … private int type; // 0: ACK, 1: Data, 2: EOT private int seqnum; // Modulo 32 private int length; // Length of the String variable ‘data’ private String data; // String with Max Length 500 … … … }
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The type field indicates the type of the packet. It is set to 0 if it is an ACK, 1 if it is a data packet, 2 if it is an end-‐of-‐transmission (EOT) packet (see the definition and use of an end-‐of-‐transmission packet below). For data packets, seqnum is the modulo 32 sequence number of the packet. The sequence number of the first packet should be zero. For ACK packets, seqnum is the sequence number of the packet being acknowledged. The length field specifies the number of characters carried in the data field. It should be in the range of 0 to 500. For ACK packets, length should be set to zero. 3 Sender Program (sender) You should implement a sender program, named sender, on a UNIX system. Its command line input includes the following: 3
3 Receiver Program (receiver) You should implement the receiver program, named as receiver, on a UNIX system. Its command line input includes the following: 4
6 Hints • Use the packet class given in packet.java containing necessary declarations, definitions, and helper methods. • All the packets must be sent and received as byte arrays instead of as Java packet objects. Since the network emulator is written in C/C++, it cannot read Java objects. Necessary code to convert the packet class into byte array and vice versa is provided in the packet.java file. • You must run the programs in the CS Undergrad Environment in order to allow nEmulator to work. • Experiment with network delay values and sender time-‐out to understand the performance of the protocol. • Run nEmulator, receiver, and sender on three different machines in this order to obtain meaningful results. 6.1 Example Execution 1. On the host host1: nEmulator 9991 host2 9994 9993 host3 9992 1 0.2 0 2. On the host host2: java receiver host1 9993 9994
