p4.c

在linux下模拟不同数据链路层协议的工作状况

/* Protocol 4 (sliding window) is bidirectional and is more robust than
 * protocol 3.
 *
 * To compile: cc -o protocol4 p4.c simulator.o
 * To run: protocol4 events timeout  pct_loss  pct_cksum  debug_flags
 *
 * Written by Andrew S. Tanenbaum
 * Revised by Shivakant Mishra
 */


#define MAX_SEQ 1
typedef enum {frame_arrival, cksum_err, timeout} event_type;
#include "protocol.h"

void protocol4 (void)
{
  seq_nr next_frame_to_send;	/* 0 or 1 only */
  seq_nr frame_expected;	/* 0 or 1 only */
  frame r, s;	/* scratch variables */
  packet buffer;	/* current packet being sent */
  event_type event;

  next_frame_to_send = 0;	/* next frame on the outbound stream */
  frame_expected = 0;	/* number of frame arriving frame expected */
  from_network_layer(&buffer);	/* fetch a packet from the network layer */
  init_frame(&s);
  s.kind = data;
  s.info = buffer;	/* prepare to send the initial frame */
  s.seq = next_frame_to_send;	/* insert sequence number into frame */
  s.ack = 1 - frame_expected;	/* piggybacked ack */
  to_physical_layer(&s);	/* transmit the frame */
  start_timer(s.seq);	/* start the timer running */

  while (true) {
        wait_for_event(&event);	/* could be: frame_arrival, cksum_err, timeout */
        if ((event != frame_arrival) && (event != cksum_err) &&
            (event != timeout)) printf("\n SOMETHING WEIRD\n");
        if (event == frame_arrival) { /* a frame has arrived undamaged. */
                from_physical_layer(&r);	/* go get it */

                if (r.seq == frame_expected) {
                        /* Handle inbound frame stream. */
                        to_network_layer(&r.info);	/* pass packet to network layer */
                        inc(frame_expected);	/* invert sequence number expected next */
                }

                if (r.ack == next_frame_to_send) { /* handle outbound frame stream. */
                        from_network_layer(&buffer);	/* fetch new packet from network layer */
                        inc(next_frame_to_send);	/* invert sender's sequence number */
                }
        }
        init_frame(&s);
        s.kind = data; 
        s.info = buffer;	/* construct outbound frame */
        s.seq = next_frame_to_send;	/* insert sequence number into it */
        s.ack = 1 - frame_expected;	/* seq number of last received frame */
        to_physical_layer(&s);	/* transmit a frame */
        start_timer(s.seq);	/* start the timer running */
  }
}

main (int argc, char *argv[])
{
  int timeout_interval, pkt_loss, garbled, debug_flags;
  long event;

  if (!parse_first_five_parameters(argc, argv, &event, &timeout_interval,
                                   &pkt_loss, &garbled, &debug_flags))  {
    printf ("Usage: p4 events timeout loss cksum debug\n");
    exit(1);
  }

  printf("\n\n Simulating Protocol 4\n");
  start_simulator(protocol4, protocol4, event, timeout_interval, pkt_loss, garbled, debug_flags);
}