Lab Project 6: Stop & Wait Protocol

COEN 146L – Computer Networks
Lab Project 6: Stop & Wait Protocol (reliable data transmission over UDP)
• Use C
This project consists of building and Stop and Wait reliable protocol on top of UDP. Messages are sent
one at a time, and each message needs to be acknowledged when received, before a new message can
be sent.
The program consists of a client and a server. The client transmits a file to the 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 number 0 or 1 (start with zero).
Before sending each message, a 1-byte checksum is calculated and added to the header. After sending
each message, the client waits for a corresponding ACK. When it arrives, if it is not the corresponding
ACK or if the checksum does not match, the message is sent again. If it is the corresponding ACK, the
client changes state and returns to the application (main() function here), which can now send one more
message. This means that the client blocks on writes until an ACK is received.
Use select() function for timed wait.
• 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 sequence
number is right, the server sends an ACK message (according to the sequence 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 protocol should deal properly with duplicate data messages and duplicate ACK messages.
Each message contains the header and the application data. No reordering is necessary, since the client
is sending the exact message given by the application, one by one.
The checksum must be calculated for messages from the server and client. To calculate the checksum,
calculate the XOR of all the bytes (header + data, or just header if there is no data) when member cksum
field (see below) is zero.
To verify your protocol, use the result of a random function to decide whether to send the right checksum
or just zero. This will fake the error effect. Follow a similar approach to fake packet loss.
Packet format is defined as follows:
typedef struct
{
 int seq_ack;
 int len;
 int checksum;
} HEADER;
COEN 146L – Computer Networks
Santa Clara University | Instructor: Behnam Dezfouli
typedef struct
{
 HEADER header;
 char data[SIZE];
} PACKET;
SENDER
• Member seq_ack is used as sequence number, and the data is in member data.
• Each packet may have 100 or less bytes of data.
• After transmitting the file, a packet with no data (len = 0) is sent to notify the receiver that the
file is complete.
RECEIVER
• Member seq_ack is used as ACK, and data is empty (len = 0).
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 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.
Using select() function
This is an example on how to use select (check the man page for includes):
// local variables needed
// 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;
struct timeval tv;
// timer
int rv;
// select returned value
// call select
rv = select (sock + 1, &readfds, NULL, NULL, &tv);
if (rv == 0)
{
 // timeout, no data
}
else if (rv == 1)
{
 // there is data to be received
}
COEN 146L – Computer Networks
Santa Clara University | Instructor: Behnam Dezfouli
Deliverables:
• Demo your project to the TA in the lab
• Submit your code to Camino