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Assignment 2

CS 436

Reliable file transfer protocol

Work on this assignment is to be completed individually or in groups of 2

Assignment Objective

The goal of this assignment is to implement a reliable file transfer protocol, which could be used to reliably transfer a text file from one host to another over UDP. The protocol should be able to handle network errors. 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.

Packet Format

All packets exchanged between the sender and the receiver should have the following structure:

integer type; // 0: ACK, 1: Data, 2: EOT integer seqnum; // sequence number of the packet

integer length; // Length of the String variable ‘data’

String data; // String with Max Length 500

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 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.

Sender Program (sender)

You should implement a sender program, named sender. Its command line input includes the following:

<host address of the receiver>, <UDP port number used by the receiver to receive data from the sender>, <UDP port number used by the sender to send data and receive ACKs from the receiver>, <timeout interval in units of millisecond>, and <name of the file to be transferred> in the given order.

Upon execution, the sender program should be able to read data from a text file (10KB to 15KB) and send the data packet by packet to the receiver. After all packets are sent, the sender will start a countdown timer set to the inputted <timeout interval in units of millisecond>. When the sender receives an acknowledgement packet with seqnum n, the ACK means that the packet with sequence number n

has been correctly received at the receiver. When a timeout occurs, the sender retransmits all non- acknowledged packets and restarts the timer. These steps are repeated until there are no outstanding packets. After all content of the file has been transmitted successfully to the receiver (and corresponding ACKs have been received), the sender should send an EOT packet to the receiver. The EOT packet is in the same format as a regular data packet, except that its type field is set to 2 and its length is set to zero. The sender can close its connection and exit only after it has received ACKs for all data packets it has sent and received an EOT from the receiver. To keep the project simple, you can assume that the end-of- transmission packet never gets lost in the network.

Output

For both testing and grading purposes, your sender program should be able to generate two log files, named as seqnum.log, and ack.log. Whenever a packet is sent, its sequence number should be recorded in seqnum.log. The file ack.log should record the sequence numbers of all the ACK packets that the sender receives during the entire period of transmission. The format for these two log files is one number per line. You must follow this format to avoid losing marks.

Receiver Program (receiver)

You should implement the receiver program, named as receiver, on a UNIX system. Its command line input includes the following: <UDP port number used by the receiver to receive data from the sender>, <drop probability> and <name of the file into which the received data is written> in the given order.

When receiving packets sent by the sender via the network emulator, it should execute the following:

• if the packet is not EOT, drop the packet with the inputted probability
• if the packet is not an EOT and was not dropped, acknowledge the packet; the seqnum of the ACK packet should be the same as the seqnum of the received packet;
• if the packet is a duplicate (i.e. if it was previously received, not dropped and acknowledged), it should be discarded;
• if the packet is out-of-order, it should be buffered. Received data should be reordered before it is saved in the file.

After the receiver has received all data packets and an EOT from the sender, it should send an EOT packet then exit. Make sure that the file is saved before exiting!

Output

The receiver program is also required to generate 3 log files, named as arrival.log, and drop.log. The file arrival.log should record the sequence numbers of all the data packets that the receiver receives during the entire period of transmission. drop.log should record the sequence numbers of all the data packets dropped by the receiver. The format for the log files is one number per line. You must follow the format to avoid losing marks.