CS 471G: Networking and Distributed Operating Systems Spring 2009
Programming Assignment 2
Assigned: March 27, 2009 Due: April 13 (progress report) and April 20, 2009
In this programming assignment, you will implement a reliable data transfer protocol using emulab.
You can modify the UDP echo server (UDPEchoServer.c) and UDP echo client (UDPEchoClient-Timeout.c)
codes from the “TCP/IP Sockets in C” book. You should not use TCP at all. You can use the UDP
echo client as the sender and the UDP echo server as the receiver. If you run the UDP codes on a pair of
machines in the multilab, most likely you will not see any loss. Therefore, we need an environment that
we can control the loss rate and create loss scenarios to test your codes. Fortunately, UK Laboratory for
Advanced Networking has an emulab facility that meets these requirements.
The protocol you will implement in this assignment is go-back-N.
Sender or Client
You can modify the UDP client code to send packets to the receiver, which is a modified version of UDP
Echo server. The sender will send data in a buffer using UDP. Its interface should be:
myclient server_IP server_port chunk_size window_size
where server_IP is the IP address (starting with “192.”) of the server, server_port is the port number the
server is listening to, chunk_size is the default value for the length of data in each packet and it should be
less than 1024, window_size is the maximal number of packets the sender can send before receiving an ack.
To avoid the burden of reading data from a file, you can declare a variable called buffer of type character
array and initialize it with a constant. Here is an example:
char buffer[4096]=‘‘Here is the message. Put at least two thousand characters here!!!’’;
The message sent to the receiver is in the following format: type (4 bytes, int), sequence number (4 bytes,
int), length (4 bytes, int) and data (the size of the data is specified by datalen). The field type should be
1 to indicate it is a data message. You can define the following structure for the messages you will send.
struct {
int type;
int sno;
int datalen;
char data[1024];
}
However, the size of message you will send should be 4+4+4+chunk size.
In GBN, assume “base” is the smallest sequence number within the window. All packets up to “base”-1
have been acknowledged by the receiver. The number of packets the sender can send is window_size. The
packets in the window are from “base” to “base”+“window size”-1. After sending them, it will set a timer
(3 seconds). Then it has to wait for the ack from the receiver or a timeout.
1. If it is a duplicate ack, just ignore it.
2. If it is a new ack, clear the timer, update “base”, and send new packets allowed by the “window size”;
if there are packets sent out but not acknowledged yet, the sender should restart the timer.
3. If it is a timeout, it will resend all packets (retransmission) from “base” to whatever packet allowed
by the “window size” and set a timer. After 10 successive retransmissions, it will give up and declare
failure.
After receiving the ack for the last message, the sender will send a special tear-down message with
type=4 (other fields irrelevant). This message possibly needs to be retransmitted for at most 10 times
unless it receives an ack message of type=8 from the receiver. Then the sender can exit.
