原创 A reliable communication protocol

In
computer communication, there are many cases in which a protocol needs to cope
with packet loss. In order to provide a reliable channel, lost packets must be
retransmitted. This example shows a simple communication protocol that uses
(the absence of) acknowledgment packets to detect a lost packet. If a lost
packet is detected, the packet is retransmitted.

The
example shows two protothreads, one for the sender and one for the receiver.

＃i nclude "pt.h"

PT_THREAD(sender(struct
pt *pt))

{

  PT_BEGIN(pt); 

  do {

    send_packet();

    /* Wait until an ackowledgement has been
received, or until the timer expires. If the timer expires, we should send the
packet again. */

    timer_set(&timer, TIMEOUT);

    PT_WAIT_UNTIL(pt, acknowledgment_received()
|| timer_expired(&timer));

  } while(timer_expired(&timer)); 

  PT_END(pt);

}

PT_THREAD(receiver(struct
pt *pt))

{

  PT_BEGIN(pt);

  /* Wait until a packet has been received, and
send an acknowledgment. */

  PT_WAIT_UNTIL(pt, packet_received());

  send_acknowledgement(); 

  PT_END(pt);