UDP Message Passing

[AdamOhio76]

So, as always, I'll preface this email by saying this is for a class assignment. I'm looking for debugging assistance.

I have two files running on two machines. The sending program is taking 100 byte chunks of a file, adding a header file, adding a crc-16 tail and sending the message to the remote server. The remote server is validating the code, ensuring it's not duplicated and sending an ack back to the local machine.

My problem is this...I'm not seeing the output I'd expect when the file is longer than 1 frame. (>100 bytes). The program works perfectly when the file is only 1 frame long.

File: send.c

#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <netdb.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define BUFMAX 128

//Command Arguments:
// [0] - sendf
// [1] - destination machine
// [2] - destination port number
// [3] - file to be sent

int main(int argc, char* argv[]){
  int sk;                       // Socket
  int n;                        // # of char read to buffer
  int i;                        // Looping Index
  int fin;                      // Final Frame 1=true 0=false
  int seq;                      // Frame Seq No
  int ack;                      // Frame Ack
  unsigned char buf[BUFMAX];    // Send Buffer
  unsigned char res[BUFMAX];    // Response Buffer
  unsigned short reply;         // CRC Reply

  struct sockaddr_in remote;
  struct hostent *hp;

  // Define and open the input file for transfer
  FILE *fp;
  fp = fopen(argv[3],"r");

  // Create the socket
  sk = socket(AF_INET, SOCK_DGRAM, 0);
  remote.sin_family = AF_INET;
  hp = gethostbyname(argv[1]);

  // If the host cannot be found terminate program
  if(hp==NULL){
    printf("Can't find hostname. %s\n", argv[1]);
    exit(1);
  }

  bcopy(hp->h_addr, &remote.sin_addr.s_addr, hp->h_length);
  remote.sin_port = ntohs(atoi(argv[2]));

  seq = 1; // set the initial seq

  fd_set rfds;
  struct timeval tv;
  int retval;

  FD_ZERO(&rfds);
  FD_SET(sk, &rfds);

  // Read 100 bytes at a time from the file to the buffer
  while((n=fread(&buf[1], 1, 100, fp)>0)){
    printf("Preparing to send a new frame!\n");
    printf("%d bytes read to buffer.\n", strlen(buf));
    for(i=1; i<strlen(buf); i++){ printf("%c", buf[i]); }
    printf("\n");

    // Prepare header byte
    // fin = 1 << 5;
    // seq = 1 << 2;
    // ack = 1;

    if(strlen(buf)<100){                // Is this the final frame?
      printf("Found the final frame! %d\n", strlen(buf));
      fin = 1 << 5;     // Set the 6th bit to 1
    } else {
      fin = 0;
    }

    seq = (seq == 0) ? 1 : 0;           // Need to alternate seq no
    seq = seq << 2;

    ack = 0;                    // Not needed for the sendf function
    buf[0] = fin | seq | ack;   // Bitwise or to set the header
    printf("Preparing Header: %x\n", buf[0]);

    // Generate CRC Code //
    reply = crc(&buf, strlen(buf));
    buf[strlen(buf)] = reply >> 8 & 0xff;
    buf[strlen(buf)] = reply & 0xff;
    printf("Did I get here?\n");
    for(i=0; i<strlen(buf); i++){ printf("buf[%i] = %02x\n", i, buf[i]); }

    // Send to the remote machine //
    while(1){
      dlsendto(sk, buf, strlen(buf)+1, 0, &remote,sizeof(remote),0);

      // Stop and Wait for a response from the remote machine //
      tv.tv_sec = 10;
      tv.tv_usec = 0;
      if(select(32, &rfds, NULL, NULL, &tv)==0){
        printf("Timeout - resending the frame!!\n");
      }
      else {
        read(sk,res,BUFMAX);
        reply = crc(res, strlen(res));

        if(crc(res, strlen(res))==0){           // Is CRC Correct?
          if(((res[0] << 5) & 0x80) == seq){    // Is Seq Correct?
            if(((res[0] << 7) & 0x80)){ // Is Ack?
                printf("Ack Received\n");
                //buf[1] = '\0';
                break;
            }
            else{
                printf("Not an Ack! Resend Frame!\n");
            }
          }
          else {
            printf("Duplicate Ack Received! Resend Frame!\n");
          }
        }
        else {
          printf("Ack Frame Damaged! Resend Frame!\n");
        }
      }
    } //printf("Exited to here\n");
  }
  close(sk);
}

The output I'm getting looks like this....

Preparing to send a new frame!
101 bytes read to buffer.
total 4945
-rwxr-xr-x 1 root root  57548 2005-01-29 12:20 afio
-rwxr-xr-x 1 root root   3200 2006-05Preparing Header: 0
Did I get here?
Ack Received
Preparing to send a new frame!
0 bytes read to buffer.
Found the final frame! 0
Preparing Header: 24
Did I get here?
buf[0] = 24
... <intentionally removed for space>
buf[102] = c1
Duplicate Ack Received! Resend Frame!
Timeout - resending the frame!!
Timeout - resending the frame!!

Notice that the first read appears to work fine. One the second read, it thinks I'm not reading anything into the buffer so it turns the final frame bit on.

For reference, my header should be:
8 - 0
7 - 0
6 - final frame? 1 = yes; 0 = no;
5 - 0
4 - 0
3 - sequence - alternates between 1 and 0
2 - 0
1 - ack? 1 = yes; 0 = no;

Hoping some fresh eyes might see what I'm doing wrong. If you need it, I can also post the receive program.

[CommonTater]

Because you are getting "Timeout -- Resending the frame", it would seem your sender is working more or less as it should...

I'm betting the problem is on the receiving end...

[AdamOhio76]

Because you are getting "Timeout -- Resending the frame", it would seem your sender is working more or less as it should...

I'm betting the problem is on the receiving end...

That was my original thought and I haven't ruled out a problem on the ack side.

That being said, I did find that my strlen(buf) was causing one problem. When the header was 00000000 it was reading as the end-of-string. I've fixed that so now I can read what's in the buffer.

My problem is actually somewhere in the Ack process on the sender side. It is hanging up on the ack because it thinks it's a duplicate frame.

I added more code to validate what comes across back from the receiver. My if statements are generating false negatives.

if(((res[0] << 5) & 0x80) == seq){    // Is Seq Correct?

I thought this would generate a 1 or 0. It's generating 128 or 0.