direwolf/dedupe.c

//
//    This file is part of Dire Wolf, an amateur radio packet TNC.
//
//    Copyright (C) 2011, 2013  John Langner, WB2OSZ
//
//    This program is free software: you can redistribute it and/or modify
//    it under the terms of the GNU General Public License as published by
//    the Free Software Foundation, either version 2 of the License, or
//    (at your option) any later version.
//
//    This program is distributed in the hope that it will be useful,
//    but WITHOUT ANY WARRANTY; without even the implied warranty of
//    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
//    GNU General Public License for more details.
//
//    You should have received a copy of the GNU General Public License
//    along with this program.  If not, see <http://www.gnu.org/licenses/>.
//


/*------------------------------------------------------------------
 *
 * Name:	dedupe.c
 *
 * Purpose:	Avoid transmitting duplicate packets which are too
 *		close together.
 *
 *
 * Description:	We want to avoid digipeating duplicate packets to
 *		to help reduce radio channel congestion with 
 *		redundant information.
 *		Duplicate packets can occur in several ways:
 *
 *		(1) A digipeated packet can loop between 2 or more
 *			digipeaters.  For example:
 *
 *			W1ABC>APRS,WIDE3-3
 *			W1ABC>APRS,mycall*,WIDE3-2
 *			W1ABC>APRS,mycall,RPT1*,WIDE3-1
 *			W1ABC>APRS,mycall,RPT1,mycall*
 *
 *		(2) We could hear our own original transmission
 *			repeated by someone else.  Example:
 *
 *			mycall>APRS,WIDE3-3
 *			mycall>APRS,RPT1*,WIDE3-2
 *			mycall>APRS,RPT1*,mycall*,WIDE3-1
 *
 *		(3) We could hear the same packet from multiple
 *			digipeaters (with or without the original).
 *
 *			W1ABC>APRS,WIDE3-2
 *			W1ABC>APRS,RPT1*,WIDE3-2
 *			W1ABC>APRS,RPT2*,WIDE3-2
 *			W1ABC>APRS,RPT3*,WIDE3-2
 *
 *		(4) Someone could be sending the same thing over and 
 *			over with very little delay in between.
 *
 *			W1ABC>APRS,WIDE3-3
 *			W1ABC>APRS,WIDE3-3
 *			W1ABC>APRS,WIDE3-3
 *
 *		We can catch the first two by looking for 'mycall' in
 *		the source or digipeater fields.
 *
 *		The other two cases require us to keep a record of what
 *		we transmitted recently and test for duplicates that 
 *		should be dropped.
 *		
 *		Once we have the solution to catch cases (3) and (4)
 *		there is no reason for the special case of looking for
 *		mycall.  The same technique catches all four situations.
 *
 *		For detecting duplicates, we need to look
 *			+ source station
 *			+ destination 
 *			+ information field
 *		but NOT the changing list of digipeaters.
 *
 *		Typically, only a checksum is kept to reduce memory 
 *		requirements and amount of compution for comparisons.
 *		There is a very very small probability that two unrelated 
 *		packets will result in the same checksum, and the
 *		undesired dropping of the packet.
 *
 * References:	Original APRS specification:
 *
 *			TBD...
 *
 *		"The New n-N Paradigm"
 *
 *			http://www.aprs.org/fix14439.html
 *		
 *------------------------------------------------------------------*/

#define DEDUPE_C


#include <stdlib.h>
#include <string.h>
#include <assert.h>
#include <stdio.h>
#include <time.h>


#include "ax25_pad.h"
#include "dedupe.h"
#include "fcs_calc.h"
#include "textcolor.h"


/*------------------------------------------------------------------------------
 *
 * Name:	dedupe_init
 * 
 * Purpose:	Initialize the duplicate detection subsystem.
 *
 * Input:	ttl	- Number of seconds to retain information
 *			  about recent transmissions.
 *	
 *		
 * Returns:	None
 *
 * Description:	This should be called at application startup.
 *
 *		
 *------------------------------------------------------------------------------*/

static int history_time = 30;		/* Number of seconds to keep information */
					/* about recent transmissions. */

#define HISTORY_MAX 25			/* Maximum number of transmission */
					/* records to keep.  If we run out of */
					/* room the oldest ones are overwritten */
					/* before they expire. */

static int insert_next;			/* Index, in array below, where next */
					/* item should be stored. */
					
static struct {

	time_t time_stamp;		/* When the packet was transmitted. */

	unsigned short checksum;	/* Some sort of checksum for the */
					/* source, destination, and information. */
					/* is is not used anywhere else. */

	short xmit_channel;		/* Radio channel number. */

} history[HISTORY_MAX];


void dedupe_init (int ttl)
{
	history_time = ttl;
	insert_next = 0;
	memset (history, 0, sizeof(history));
}


/*------------------------------------------------------------------------------
 *
 * Name:	dedupe_remember
 * 
 * Purpose:	Save information about a packet being transmitted so we
 *		can detect, and avoid, duplicates later.
 *
 * Input:	pp	- Pointer to packet object.
 *		
 *		chan	- Radio channel for transmission.
 *		
 * Returns:	None
 *
 * Rambling:	At one time, my thinking is that we want to keep track of
 *		ALL transmitted packets regardless of origin or type.
 *
 *			+ my beacons
 *			+ anything from a connected application 
 *			+ anything digipeated
 *
 *		The easiest way to catch all cases is to call dedup_remember()
 *		from inside tq_append().  
 *
 *		But I don't think that is the right approach.
 *		When acting as a KISS TNC, we should just shovel everything
 *		through and not question what the application is doing.
 *		If the connected application has a digipeating function,
 *		it's responsible for those decisions.
 *
 *		My current thinking is that dedupe_remember() should be 
 *		called BEFORE tq_append() in the digipeater case.
 *
 *		We should also capture our own beacon transmissions.
 *		
 *------------------------------------------------------------------------------*/

void dedupe_remember (packet_t pp, int chan)
{
	history[insert_next].time_stamp = time(NULL);
	history[insert_next].checksum = ax25_dedupe_crc(pp);
	history[insert_next].xmit_channel = chan;

	insert_next++;
	if (insert_next >= HISTORY_MAX) {
	  insert_next = 0;
	}
}


/*------------------------------------------------------------------------------
 *
 * Name:	dedupe_check
 * 
 * Purpose:	Check whether this is a duplicate of another sent recently.
 *
 * Input:	pp	- Pointer to packet object.
 *		
 *		chan	- Radio channel for transmission.
 *		
 * Returns:	True if it is a duplicate.
 *
 *		
 *------------------------------------------------------------------------------*/

int dedupe_check (packet_t pp, int chan)
{
	unsigned short crc = ax25_dedupe_crc(pp);
	time_t now = time(NULL);
	int j;

	for (j=0; j<HISTORY_MAX; j++) {
	  if (history[j].time_stamp >= now - history_time &&
	      history[j].checksum == crc && 
	      history[j].xmit_channel == chan) {
	    return 1;
	  }
	}
	return 0;
}


/* end dedupe.c */