Timer in Selective Repeat ARQ - c

Question

My professor has given me an assignment to implement the Selective Repeat ARQ algorithm in C for packet transaction between sender and receiver.
There is a timer associated with each packet to be sent at the sender which is triggered ON when that packet is sent, according to which it is decided which packet duplicate is needed to be sent.
But I don't know how to set the timer of each packet.
Please suggest some method for it.
Thanks is advance!!

Answer 1

Keep a data structure (e.g. a priority queue or ordered map or some such) that contains each packet you're planning to (re)send, along with the time at which you're intending to (re)send it. Ideally this data structure will be such that it is efficient to determine the smallest timestamp currently in the data structure, but if the number of scheduled packets will be relatively small, a simpler unordered data structure like a linked list could work too.
On each iteration of your event loop, determine the smallest timestamp value in the data structure. Subtract the current time from that timestamp value to get a delay time (in milliseconds or microseconds or similar).
If you're using select() or similar, you can pass that delay time as your timeout argument. If you're doing something simpler without multiplexing, you might be able to get away with passing the delay time to usleep() or similar instead.
After select() (or usleep()) returns, check the current time again. If the current time is now greater than or equal to your target time, you can send the packet with the smallest timestamp, and then remove it from your data structure. (If you think you might want to resend it again later, you can re-insert it into the data structure with a new/updated timestamp value)

Answer 2

You can also use Threads for that purpose, which is quite easy and needs fewer lines of codes.
You just need to create and define this function:
unsigned long CALLBACK packetTimer(void *pn){
//This is our Packet Timer
//It's going to run on a Tread
//If ack[thisPacketNumber] is not true
//We gonna check will check for packet time
//If it's reached to its limit we gonna send this packet again
//and reset the timer
//If ack[] becomes true
//break the while loop and this will end this tread
int pno = (int)pn;
std::clock_t start;
double duration;
start = std::clock();
while(1){
if(!ack[pno]){
duration = ( std::clock() - start ) / (double) CLOCKS_PER_SEC;
if(duration > 0.5){
//This tells that we haven't received our ACk yet for this packet
//So send it again
printf("SendBuffer for Packet %d: %s", pno, packets[pno]->data);
//Resending packet again
send_unreliably(s,packets[pno]->data,(result->ai_addr));
//Reseting the timer
start = std::clock();
}
}else{break;}
}
}
And inside your while loop, where you send and receive packets to receiver, you simply define:
unsigned long tid;//This should be outside the while loop,
//Ideally in the beginning of main function
CreateThread(NULL,0,packetTimer,(void *)packetNumber,0,&tid);
This implementaion is for windows, for UNIX we need to use pthread()
This is it.
And don't forgot to add required header files like:
#include <stdlib.h>
#include <cstdio>
#include <ctime>