Question

implementing time-out in stop and wait protocol

Asked by: shwaqar82

Hi Experts
i need some modification in already made that works fine with no error:
I have the following code for sender.c
#include "assign1.h"
#include "sender.h"

extern int sTOr[2];
extern int rTOs[2];

FILE *fp;
static FRAME sendFrame;

void sender(char filename[]) {
        short blockNum = 0;
        char reply[5];

        sigignore(SIGINT);

        sendFrame.SOH = 00000001;
        sendFrame.blockNum=0;

        if ((fp = fopen(filename, "r")) == NULL) {
                perror("receiver(): fopen: ");
                exit(errno);
        }
        while ((readFile(sendFrame.msg)) > 0) {
                ++sendFrame.blockNum;
                sendFrame.blockNumComp = ~sendFrame.blockNum;
                sendFrame.checksum = makeChecksum(sendFrame.msg);

                printf("\nSENDER: blockNum = %d blockNumComp = %d checksum = %d\n", sendFrame.blockNum,
                        sendFrame.blockNumComp, sendFrame.checksum);
                write(sTOr[1], &sendFrame, sizeof(FRAME));

                while(1) {
                        read(rTOs[0], reply, sizeof("ACK"));
                        printf("%s was received\n", reply);

                        if (!strcmp(reply, "NAK")) {
                                printf("SENDER2: blockNum = %d blockNumComp = %d checksum = %d\n", sendFrame.blockNum,
                                        sendFrame.blockNumComp, sendFrame.checksum);
                                write(sTOr[1], &sendFrame, sizeof(FRAME));
                        }
                        else if (!strcmp(reply, "ACK"))
                                break;
                        else
                                exit(12);
                }
        }
}
int readFile(char *str) {
        int x;
        for(x = 0; x < 128; x++)
                str[x] = '\0';
        return fread(str, sizeof(char), 128, fp);
}
and the following code for receiver.c
#include "assign1.h"
#include "receiver.h"

FRAME rFrame;

void receiver() {

        sigset(SIGINT, makeError);

        while (1) {
                if (read(STDIN_FILENO, &rFrame, sizeof(FRAME)) > 0) {
                        sleep(1);

                        fprintf(stderr, "RECEIVER: blockNum = %d blockNumComp = %d checksum = %d\n", rFrame.blockNum,
                                rFrame.blockNumComp, rFrame.checksum);
                        if (!checkSOH(rFrame.SOH) ||
                            !checkBlockNum(rFrame.blockNum, rFrame.blockNumComp) ||
                            !checkChecksum(rFrame.msg, rFrame.checksum))
                                write(STDOUT_FILENO, "NAK", sizeof("NAK"));
                        else
                                write(STDOUT_FILENO, "ACK", sizeof("ACK"));
                }
        }
}

int checkSOH(char SOH){
        return (SOH == 00000001);
}

int checkBlockNum(const short blockNum, const short blockNumComp) {
        short newComp = ~blockNum;

        if (newComp == blockNumComp)
                return 1; // No error

        return 0; // Error
}

int checkChecksum(char *str, const short checksum) {
        if (makeChecksum(str) == checksum)
                return 1; // No error

        return 0; // Error
}

void makeError() {
        int n = rand()%3;
        switch (n) {
                case 0:
                        rFrame.blockNum = 0;
                        fputs("\nCHANGED BLOCK NUMBER\n", stderr);
                        break;
                case 1:
                        rFrame.checksum = 0;
                        fputs("\nCHANGED CHECKSUM\n", stderr);
                        break;
                case 2:
                        rFrame.blockNumComp = 10;
                        fputs("\nCHANGED BLOCK NUMBER COMPLIEMENT\n", stderr);
                        break;
        }
}

The program simulates different errors that the protocol checks for.I need to modify the program so that it implements "time out".
Theory:
Sender: sends a frame and calls a SIGALRM function and sets the timer for say, 2 seconds. If the sender gets a response within two seconds from the receiver (either "ACK" or "NAK"), turn off the alarm and transmit the next frame. If no response is received, time out function kicks in and retransmits the frame.

Receiver: When it sees an error in transmission, do not send back anything to the sender and this causes the to time out and retransmit.
Im new to C language please help me solve my problem

regards
shaukat

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Asked On
2007-09-20 at 07:17:40ID22841407
Tags

wait

,

stop

,

protocol

,

c

Topics

Networking Protocols

,

C Programming Language

,

Network Analysis Software

Participating Experts
1
Points
500
Comments
10

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Answers

 

by: itsmeandnobodyelsePosted on 2007-09-20 at 08:38:10ID: 19929155

>>>> extern int sTOr[2];
>>>> extern int rTOs[2];
Are these sockets? If yes, you could realize a timeout by 'select' call which timeouts when there is no input on one or more sockets within a given time period.

Regards, Alex

 

by: shwaqar82Posted on 2007-09-20 at 09:16:18ID: 19929472

i think yes these are the sockets im not sure about it ...but lets consider both assumption 1- if these are socket then how to implement it 2- if not then how to implement it

 

by: itsmeandnobodyelsePosted on 2007-09-20 at 10:37:55ID: 19930027

>>>> 1- if these are socket then how to implement it

               while(1) {
      // Create the timeout struct
      timeval tval;
      fd_set set;
                           int rc;
 
      FD_ZERO (&set);
      FD_SET ((SOCKET)rTOs[0], &set);

      // Call select
      tval.tv_sec  = 2;
      tval.tv_usec = 0;
      rc = SOCK_SELECT (rTOs[0]+ 1, &set, 0, 0, &tval);
      if (rc < 0) {
          // We had an error - go into error state
          // m_state = SOCK_ERROR;
          // return -1;
      } else if (rc == 0) {
          // We had a timeout
          // return -2;
      }
                         
                        read(rTOs[0], reply, sizeof("ACK"));
                        printf("%s was received\n", reply);

So, the read only would be made if there is something to receive,i. e. it is not blocking.

2. if not then how to implement it
Hmmm, most probably you would need some kind of a timer. I think there is someone in the pthread library but actualy a read on a file never would block but return with EOF if come to end. So, it is a good chance that it is a socket (or at least a pipe)(though I don't know much of pipes either).

Regards, Alex

 

by: itsmeandnobodyelsePosted on 2007-09-20 at 10:43:37ID: 19930077

Note, the FD_SET and FD_ZERO you should find in sys/select.h. The SOCK_SELECT can be replaced by select (or bsdselect) and the SOCKET_ERROR is -1.

 

by: shwaqar82Posted on 2007-09-20 at 10:52:33ID: 19930155

hmm lets talk little more about the code in detail.
where exactly im going to write this code.... i mean where do i append this code in my already existing code..........?
i think using timer is a good option as my prof. discuss the time out issue with timer. i saw only one thread with stop n wait protocol in previous thread liberary
Any suggesstion........?
thanks for your great help

 

by: shwaqar82Posted on 2007-09-20 at 20:11:29ID: 19933139

forget about my previous question ...just help me fix this error. Im using the following code:

#include "assign1.h"
#include "sender.h"

extern int sTOr[2];
extern int rTOs[2];

FILE *fp;
static FRAME sendFrame;

void sender(char filename[]) {
        short blockNum = 0;
        char reply[5];

        sigignore(SIGINT);

        sendFrame.SOH = 00000001;
        sendFrame.blockNum=0;

        if ((fp = fopen(filename, "r")) == NULL) {
                perror("receiver(): fopen: ");
                exit(errno);
        }
        while ((readFile(sendFrame.msg)) > 0) {
                ++sendFrame.blockNum;
                sendFrame.blockNumComp = ~sendFrame.blockNum;
                sendFrame.checksum = makeChecksum(sendFrame.msg);

                printf("\nSENDER: blockNum = %d blockNumComp = %d checksum = %d\n", sendFrame.blockNum,
                        sendFrame.blockNumComp, sendFrame.checksum);
                write(sTOr[1], &sendFrame, sizeof(FRAME));

                while(1) {
                        read(rTOs[0], reply, sizeof("ACK"));
                        printf("%s was received\n", reply);

                        if (!strcmp(reply, "NAK")) {
                                printf("SENDER2: blockNum = %d blockNumComp = %d checksum = %d\n", sendFrame.blockNum,
                                        sendFrame.blockNumComp, sendFrame.checksum);
                                write(sTOr[1], &sendFrame, sizeof(FRAME));
                        }
                        else if (!strcmp(reply, "ACK"))
                                break;
                        else
                                 signal(SIGALRM, sender());
                                alarm(2);
                }
        }
}
int readFile(char *str) {
        int x;
        for(x = 0; x < 128; x++)
                str[x] = '\0';
        return fread(str, sizeof(char), 128, fp);
}
 
and it gives me the following error:
ender.c: In function `sender':
sender.c:46: too few arguments to function `sender'
sender.c:46: invalid use of void expression

and this error is on line : signal(SIGALRM, sender());

please help me fix this function call

best regards
shaukat

 

by: itsmeandnobodyelsePosted on 2007-09-21 at 01:10:56ID: 19933957

>>>> signal(SIGALRM, sender());
turn it to

    signal(SIGALRM, sender);

The 'signal' expects a function pointer. If you add parantheses () you are 'calling' sender function which was refused because sender needs a char* argument.

 

by: itsmeandnobodyelsePosted on 2007-09-21 at 01:13:55ID: 19933970

>>>> where exactly im going to write this code....
>>>> i mean where do i append this code in my
>>>> already existing code..........?

The code should be inserted between the 'while' and above the 'read'.
 
              while(1) {
                        // here put the additional code I posted
                        ...

                        read(rTOs[0], reply, sizeof("ACK"));
                        printf("%s was received\n", reply);

 

by: shwaqar82Posted on 2007-09-21 at 05:49:32ID: 19935065

by replacing the code from :
signal(SIGALRM, sender());
to
signal(SIGALRM, sender);

it gives me the following error:
sender.c: In function `sender':
sender.c:46: warning: passing arg 2 of `signal' from incompatible pointer type

 

by: itsmeandnobodyelsePosted on 2007-09-22 at 01:35:19ID: 19940986

>>>> sender.c:46: warning: passing arg 2 of `signal' from incompatible pointer type
the signal function expects a function pointer of a function which prototype is 'void signalfunc(int)'  while the sender has 'void sender(char*)'.  Actually I don't know why you try to install the sender function as a signal handler. A signal handler was called in case of a signal event, e. g. when CTRL+c was pressed and the integer argument indicates the kind of event. The sender function is supposed to send a text message to a client or a server what hardly is the task of a signal handler.

Regards, Alex

20120131-EE-VQP-002

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