ax25.c

/*
 * ax25.c
 *
 *  Created on: Apr 20, 2012
 *      Author: alpsayin
 */

#include <inttypes.h>
#include <stdint.h>
#include <stdio.h>
#include <string.h>

#include "ax25.h"

static uint8_t ax25_local_callsign[7]="SA0BXI\x0a";
static const uint8_t ax25_broadcast_address[7] = "\0\0\0WIDE";

#define INITFCS      0xffff  /* Initial FCS value */

static uint16_t ax25_fcstab[256] = {
   0x0000, 0x1189, 0x2312, 0x329b, 0x4624, 0x57ad, 0x6536, 0x74bf,
   0x8c48, 0x9dc1, 0xaf5a, 0xbed3, 0xca6c, 0xdbe5, 0xe97e, 0xf8f7,
   0x1081, 0x0108, 0x3393, 0x221a, 0x56a5, 0x472c, 0x75b7, 0x643e,
   0x9cc9, 0x8d40, 0xbfdb, 0xae52, 0xdaed, 0xcb64, 0xf9ff, 0xe876,
   0x2102, 0x308b, 0x0210, 0x1399, 0x6726, 0x76af, 0x4434, 0x55bd,
   0xad4a, 0xbcc3, 0x8e58, 0x9fd1, 0xeb6e, 0xfae7, 0xc87c, 0xd9f5,
   0x3183, 0x200a, 0x1291, 0x0318, 0x77a7, 0x662e, 0x54b5, 0x453c,
   0xbdcb, 0xac42, 0x9ed9, 0x8f50, 0xfbef, 0xea66, 0xd8fd, 0xc974,
   0x4204, 0x538d, 0x6116, 0x709f, 0x0420, 0x15a9, 0x2732, 0x36bb,
   0xce4c, 0xdfc5, 0xed5e, 0xfcd7, 0x8868, 0x99e1, 0xab7a, 0xbaf3,
   0x5285, 0x430c, 0x7197, 0x601e, 0x14a1, 0x0528, 0x37b3, 0x263a,
   0xdecd, 0xcf44, 0xfddf, 0xec56, 0x98e9, 0x8960, 0xbbfb, 0xaa72,
   0x6306, 0x728f, 0x4014, 0x519d, 0x2522, 0x34ab, 0x0630, 0x17b9,
   0xef4e, 0xfec7, 0xcc5c, 0xddd5, 0xa96a, 0xb8e3, 0x8a78, 0x9bf1,
   0x7387, 0x620e, 0x5095, 0x411c, 0x35a3, 0x242a, 0x16b1, 0x0738,
   0xffcf, 0xee46, 0xdcdd, 0xcd54, 0xb9eb, 0xa862, 0x9af9, 0x8b70,
   0x8408, 0x9581, 0xa71a, 0xb693, 0xc22c, 0xd3a5, 0xe13e, 0xf0b7,
   0x0840, 0x19c9, 0x2b52, 0x3adb, 0x4e64, 0x5fed, 0x6d76, 0x7cff,
   0x9489, 0x8500, 0xb79b, 0xa612, 0xd2ad, 0xc324, 0xf1bf, 0xe036,
   0x18c1, 0x0948, 0x3bd3, 0x2a5a, 0x5ee5, 0x4f6c, 0x7df7, 0x6c7e,
   0xa50a, 0xb483, 0x8618, 0x9791, 0xe32e, 0xf2a7, 0xc03c, 0xd1b5,
   0x2942, 0x38cb, 0x0a50, 0x1bd9, 0x6f66, 0x7eef, 0x4c74, 0x5dfd,
   0xb58b, 0xa402, 0x9699, 0x8710, 0xf3af, 0xe226, 0xd0bd, 0xc134,
   0x39c3, 0x284a, 0x1ad1, 0x0b58, 0x7fe7, 0x6e6e, 0x5cf5, 0x4d7c,
   0xc60c, 0xd785, 0xe51e, 0xf497, 0x8028, 0x91a1, 0xa33a, 0xb2b3,
   0x4a44, 0x5bcd, 0x6956, 0x78df, 0x0c60, 0x1de9, 0x2f72, 0x3efb,
   0xd68d, 0xc704, 0xf59f, 0xe416, 0x90a9, 0x8120, 0xb3bb, 0xa232,
   0x5ac5, 0x4b4c, 0x79d7, 0x685e, 0x1ce1, 0x0d68, 0x3ff3, 0x2e7a,
   0xe70e, 0xf687, 0xc41c, 0xd595, 0xa12a, 0xb0a3, 0x8238, 0x93b1,
   0x6b46, 0x7acf, 0x4854, 0x59dd, 0x2d62, 0x3ceb, 0x0e70, 0x1ff9,
   0xf78f, 0xe606, 0xd49d, 0xc514, 0xb1ab, 0xa022, 0x92b9, 0x8330,
   0x7bc7, 0x6a4e, 0x58d5, 0x495c, 0x3de3, 0x2c6a, 0x1ef1, 0x0f78
};

static uint16_t ax25_fcs(uint16_t fcs, uint8_t* cp, uint16_t len)
{
	while (len--)
	{
    	fcs = (fcs >> 8) ^ ax25_fcstab[(fcs ^ *cp++) & 0xff];
	}
	return fcs;
}

static uint16_t ax25_compute_crc(uint8_t* buf, uint16_t len)
{
	uint16_t fcs = INITFCS;
	fcs = ax25_fcs(fcs, buf, len);
	fcs ^= 0xffff;
	return fcs;
}

void ax25_initialize_network(uint8_t* myCallsign)
{
    uint32_t i;
    for(i=0; i<7; i++)
    	ax25_local_callsign[i] = myCallsign[i];
}
uint8_t* ax25_get_local_callsign(uint8_t* callsign_out)
{
    uint8_t i;

    if(callsign_out!=NULL)
    {
        for(i=0; i<7; i++)
        	callsign_out[i] = ax25_local_callsign[i];
    }
    return ax25_local_callsign;
}

uint8_t* ax25_get_broadcast_callsign(uint8_t* callsign_out)
{
    uint8_t i;

    if(callsign_out!=NULL)
    {
        for(i=0; i<7; i++)
        	callsign_out[i] = ax25_broadcast_address[i];
    }
    return ax25_broadcast_address;
}

uint32_t ax25_create_ui_packet(uint8_t* src_in, uint8_t* dst_in, uint8_t* payload_in, uint16_t payload_length, uint8_t* packet_out)
{
	uint16_t crc=0;
	uint16_t len=0;
	uint16_t ax25_len=0;

    //check for input errors
    if(payload_length > AX25_MAX_PAYLOAD_LENGTH || packet_out==NULL)
        return 0;

    //destination address
    memcpy(packet_out+AX25_DESTINATION_OFFSET, dst_in, AX25_DESTINATION_LENGTH);
    len+=AX25_DESTINATION_LENGTH;

    //source address
    memcpy(packet_out+AX25_SOURCE_OFFSET, src_in, AX25_SOURCE_LENGTH);
    len+=AX25_SOURCE_LENGTH;

    //control field
    packet_out[AX25_CONTROL_OFFSET]=AX25_CONTROL_UI_FINAL;
    len+=AX25_CONTROL_LENGTH;

    //PID
    packet_out[AX25_PID_OFFSET]=AX25_PID_NO_PROTOCOL;
    len+=AX25_PID_LENGTH;

    //payload
    memcpy(packet_out+AX25_PAYLOAD_OFFSET, payload_in, payload_length);
    len+=payload_length;

    //fcs (crc16)
    crc=ax25_compute_crc(packet_out, len);

    packet_out[len]=crc>>8 & 0xFF;
    packet_out[len+1]=crc & 0xFF;
    len+=AX25_FCS_LENGTH;

    return len;
}

uint8_t ax25_check_destination(uint8_t* my_dst, uint8_t* packet_dst_out, uint8_t* packet_in)
{
    uint8_t result;
    //check for address match
    result=strncmp(my_dst, packet_in+AX25_DESTINATION_OFFSET, AX25_DESTINATION_LENGTH);
    if(result)
    {
        result=strncmp(ax25_broadcast_address, packet_in+AX25_DESTINATION_OFFSET, AX25_DESTINATION_LENGTH);
    }

    //copy the destination address in the packet
    if(packet_dst_out!=NULL)
    	memcpy(packet_dst_out, packet_in+AX25_DESTINATION_OFFSET, AX25_DESTINATION_LENGTH);
    return result;
}

uint16_t ax25_open_ui_packet(uint8_t* src_out, uint8_t* dst_out, uint8_t* payload_out, uint8_t* packet_in, uint16_t packet_length)
{
    uint8_t control=0, pid=0;
    uint16_t ax25_len=0, len=0;
    uint16_t crc=0, packet_crc=0;
    //copy destination address
    if(dst_out!=NULL)
    	memcpy(dst_out, packet_in+AX25_DESTINATION_OFFSET, AX25_DESTINATION_LENGTH);

    //copy source address
    if(src_out!=NULL)
    	memcpy(src_out, packet_in+AX25_SOURCE_OFFSET, AX25_SOURCE_LENGTH);

    //copy ax25 control field
    control=packet_in[AX25_CONTROL_OFFSET];

    //copy ax25 pid
    pid=packet_in[AX25_PID_OFFSET];

    //calculate payload length
    ax25_len=packet_length;
    len=ax25_len-AX25_DESTINATION_LENGTH-AX25_SOURCE_LENGTH-AX25_CONTROL_LENGTH-AX25_PID_LENGTH-AX25_FCS_LENGTH;

    //copy the crc came with packet
    packet_crc=packet_in[AX25_FCS_OFFSET(len)] & 0xFF;
    packet_crc=packet_crc<<8;
    packet_crc|=packet_in[AX25_FCS_OFFSET(len)+1] & 0xFF;

    //calculate fcs (crc32)
    crc=ax25_compute_crc(packet_in, ax25_len-AX25_FCS_LENGTH);

    //check for match
    if(packet_crc != crc)
        return 0;

    if(payload_out!=NULL)
    {
        memcpy(payload_out, packet_in+AX25_PAYLOAD_OFFSET, len);
        payload_out[len]=0;
    }

    return len;
}