Description
Objective
1. To build a Stop and Wait reliable protocol on top of UDP to provide a reliable transport service while
considering loss
TFv3 – Stop and Wait for an Unreliable Channel, with Loss
In Lab 5, you have developed the TFv2 which implements a reliable file transfer using Stop and Wait protocol
rdt2.2. This version of file transfer is then TFv3.
TFv3 implements basically the protocol rdt3.0 presented in the text book. It consists of a client and a server.
Communication is unidirectional, i.e., data flows from the client to the server.
The server starts first and waits for messages. The client starts the communication. Messages have sequence or
ack number 0 or 1 (start with zero). Before sending each message, a checksum is calculated and added to the
header. After sending each message, the client starts a timer. Use select for that. If select returns zero, there is
no data, and the client needs to retransmit, restart the timer, and call select again. If select returns 1, there is
data, so the client calls recvfrom to receive the ACK and then processes it. If ACK is not corrupted and the ack
number is right, the client can now send one more message.
The server, after receiving a message, checks its checksum. If the message is correct and the seq number is
right, the server sends an ACK message (according to the seq number) to the client, and the data is ready to be
written in the file. Otherwise the server repeats the last ACK message and waits to receive a message again. The
server does not change much from the previous lab.
To verify your protocol, use the result of a random function to decide to send or skip a message, to decide to send
or skip an ACK message (only change to the server), and to decide whether to send the right checksum or just
zero. This will fake the error and loss effect.
SELECT
This is an example on how to use select (check the man page for includes):
// local variables needed
struct timeval tv; // timer
int rv; // select returned value
// set it up, in the beginning of the function
fd_set readfds;
fcntl (sock, F_SETFL, O_NONBLOCK);
…
// start before calling select
FD_ZERO (&readfds);
FD_SET (sock, &readfds);
// set the timer
tv.tv_sec = 10;
tv.tv_usec = 0;
// call select
rv = select (sock + 1, &readfds, NULL, NULL, &tv); // sock is the socket you are using
if (rv == 0)
{
// timeout, no data
}
COEN 146 – Lab assignment 6 2/2
else if (rv == 1)
{
// there is data to be received
}
Important note
The server closes the file and terminates execution after the message with zero bytes arrives. If the ack sent for
that last message does not make it to the client, the client will keep resending it forever to a non-responding
server. To avoid that, the client will start a counter after sending a message with zero bytes and will only resend
that last message 3 times. After 3 times, it will return to the main function.
Requirements to complete the lab
Show the TA correct execution of the programs you wrote and upload source code to Camino.
Be sure to retain copies (machine and/or printed) of your source code. You will want these for study purposes and
to resolve any grading questions (should they arise)
Please start each program with a descriptive block that includes minimally the following information:
/*
* Name:
* Date:
* Title: Lab6 – ….
* Description: This program …
*/
Rdt3.0 sender – FSM
