stopwait.java

本程序是用java编写的一些关于arq协议实现的小程序

import datalink.*;

public class Stopwait
    extends Protocol
{
    Packet buffer;      // to store outstanding packet in case of time out
    int frameToSend;    // sequence number to sue for current packet
    int frameExpected;  // sequence number expected
    double timer;       // time out value

    boolean waiting;    // indicates that a packet is waiting to be sent

    public Stopwait( int windowsize, double timer)
    {
	super( windowsize, timer);
	frameToSend = 0;
	frameExpected = 0;
	waiting = false;
	this.timer = timer;
    }

    public void FrameArrival( Object frame)
    {
	DLL_Frame f = (DLL_Frame) frame;
	/* a frame has arrived from the physical layer */
	/* check that it is the one that is expected */
	if (f.sequence == frameExpected)
	    {
		sendPacket(f.info);	// valid frame
		                       // is returned to network layer */
		frameExpected = 1-frameExpected; // flip bit
	    }

	if( f.acknowledgement == frameToSend) /* acknowledgment has arrived */
	    {
		stopTimer(0);            /* cancel timer */
		enableNetworkLayer();   /* allow new packets */
		frameToSend = 1-frameToSend;
	    }
    }

    public void PacketArrival( Packet p)
    {
	DLL_Frame f;
	buffer = p;
	if ( isChannelIdle() )
	    {
		f = new DLL_Frame( frameToSend, 1-frameExpected, buffer);
		sendFrame( f);        // send it to physical layer
		startTimer(0, timer); // start timer
		waiting = false;
	    }
	else
	    waiting = true;
	disableNetworkLayer();
    }

    public void TimeOut( int code)
    {
	DLL_Frame f;
	if ( isChannelIdle())
	    {
		/* a frame has not been ACKed in time, */
		/* so re-send the outstanding frame */
		f = new DLL_Frame( frameToSend, 1-frameExpected, buffer);
		sendFrame( f);
		startTimer(0, timer);
	    }
	else
	    waiting = true;
    }

    public void CheckSumError()
    {  // ignore check sum errors and allow frames to time out
    }

    public void ChannelIdle()
    {
	DLL_Frame f;
	if ( waiting )
	    {
		f = new DLL_Frame( frameToSend, 1-frameExpected, buffer);
		sendFrame( f);
		startTimer(0, timer);
		waiting = false;
	    }
    }
}

class DLL_Frame {
    Packet info;
    int sequence;
    int acknowledgement;

    DLL_Frame ( int s, int a, Packet p)
    {
	info = p;
	sequence = s;
	acknowledgement = a;
    }
}