release/src/router/pptp-client/ppp_fcs.c

   1 /* Fast Frame Check Sequence (FCS) Implementation, for HDLC-like framing of
   2  * PPP.  Adapted by C. Scott Ananian <cananian@alumni.princeton.edu>
   3  * from RFC1662:
   4  *
   5  * C.2.  16-bit FCS Computation Method
   6  *
   7  *  The following code provides a table lookup computation for
   8  *  calculating the Frame Check Sequence as data arrives at the
   9  *  interface.  This implementation is based on [7], [8], and [9].
  10  *
  11  *  [7]   Perez, "Byte-wise CRC Calculations", IEEE Micro, June 1983.
  12  *
  13  *  [8]   Morse, G., "Calculating CRC's by Bits and Bytes", Byte,
  14  *        September 1986.
  15  *
  16  *  [9]   LeVan, J., "A Fast CRC", Byte, November 1987.
  17  *
  18  * $Id: ppp_fcs.c,v 1.2 2003/06/17 17:25:47 reink Exp $
  19  */
  20
  21 #include <sys/types.h>
  22 #include <assert.h>
  23 #include "ppp_fcs.h"
  24 #define ASSERT(x) assert(x)
  25
  26 /*
  27  * u16 represents an unsigned 16-bit number.  Adjust the typedef for
  28  * your hardware.
  29  */
  30 typedef u_int16_t u16;
  31
  32 /*
  33  * FCS lookup table as calculated by the table generator.
  34  */
  35 static u16 fcstab[256] = {
  36    0x0000, 0x1189, 0x2312, 0x329b, 0x4624, 0x57ad, 0x6536, 0x74bf,
  37    0x8c48, 0x9dc1, 0xaf5a, 0xbed3, 0xca6c, 0xdbe5, 0xe97e, 0xf8f7,
  38    0x1081, 0x0108, 0x3393, 0x221a, 0x56a5, 0x472c, 0x75b7, 0x643e,
  39    0x9cc9, 0x8d40, 0xbfdb, 0xae52, 0xdaed, 0xcb64, 0xf9ff, 0xe876,
  40    0x2102, 0x308b, 0x0210, 0x1399, 0x6726, 0x76af, 0x4434, 0x55bd,
  41    0xad4a, 0xbcc3, 0x8e58, 0x9fd1, 0xeb6e, 0xfae7, 0xc87c, 0xd9f5,
  42    0x3183, 0x200a, 0x1291, 0x0318, 0x77a7, 0x662e, 0x54b5, 0x453c,
  43    0xbdcb, 0xac42, 0x9ed9, 0x8f50, 0xfbef, 0xea66, 0xd8fd, 0xc974,
  44    0x4204, 0x538d, 0x6116, 0x709f, 0x0420, 0x15a9, 0x2732, 0x36bb,
  45    0xce4c, 0xdfc5, 0xed5e, 0xfcd7, 0x8868, 0x99e1, 0xab7a, 0xbaf3,
  46    0x5285, 0x430c, 0x7197, 0x601e, 0x14a1, 0x0528, 0x37b3, 0x263a,
  47    0xdecd, 0xcf44, 0xfddf, 0xec56, 0x98e9, 0x8960, 0xbbfb, 0xaa72,
  48    0x6306, 0x728f, 0x4014, 0x519d, 0x2522, 0x34ab, 0x0630, 0x17b9,
  49    0xef4e, 0xfec7, 0xcc5c, 0xddd5, 0xa96a, 0xb8e3, 0x8a78, 0x9bf1,
  50    0x7387, 0x620e, 0x5095, 0x411c, 0x35a3, 0x242a, 0x16b1, 0x0738,
  51    0xffcf, 0xee46, 0xdcdd, 0xcd54, 0xb9eb, 0xa862, 0x9af9, 0x8b70,
  52    0x8408, 0x9581, 0xa71a, 0xb693, 0xc22c, 0xd3a5, 0xe13e, 0xf0b7,
  53    0x0840, 0x19c9, 0x2b52, 0x3adb, 0x4e64, 0x5fed, 0x6d76, 0x7cff,
  54    0x9489, 0x8500, 0xb79b, 0xa612, 0xd2ad, 0xc324, 0xf1bf, 0xe036,
  55    0x18c1, 0x0948, 0x3bd3, 0x2a5a, 0x5ee5, 0x4f6c, 0x7df7, 0x6c7e,
  56    0xa50a, 0xb483, 0x8618, 0x9791, 0xe32e, 0xf2a7, 0xc03c, 0xd1b5,
  57    0x2942, 0x38cb, 0x0a50, 0x1bd9, 0x6f66, 0x7eef, 0x4c74, 0x5dfd,
  58    0xb58b, 0xa402, 0x9699, 0x8710, 0xf3af, 0xe226, 0xd0bd, 0xc134,
  59    0x39c3, 0x284a, 0x1ad1, 0x0b58, 0x7fe7, 0x6e6e, 0x5cf5, 0x4d7c,
  60    0xc60c, 0xd785, 0xe51e, 0xf497, 0x8028, 0x91a1, 0xa33a, 0xb2b3,
  61    0x4a44, 0x5bcd, 0x6956, 0x78df, 0x0c60, 0x1de9, 0x2f72, 0x3efb,
  62    0xd68d, 0xc704, 0xf59f, 0xe416, 0x90a9, 0x8120, 0xb3bb, 0xa232,
  63    0x5ac5, 0x4b4c, 0x79d7, 0x685e, 0x1ce1, 0x0d68, 0x3ff3, 0x2e7a,
  64    0xe70e, 0xf687, 0xc41c, 0xd595, 0xa12a, 0xb0a3, 0x8238, 0x93b1,
  65    0x6b46, 0x7acf, 0x4854, 0x59dd, 0x2d62, 0x3ceb, 0x0e70, 0x1ff9,
  66    0xf78f, 0xe606, 0xd49d, 0xc514, 0xb1ab, 0xa022, 0x92b9, 0x8330,
  67    0x7bc7, 0x6a4e, 0x58d5, 0x495c, 0x3de3, 0x2c6a, 0x1ef1, 0x0f78
  68 };
  69
  70 #ifndef PPPINITFCS16
  71 #define PPPINITFCS16    0xffff  /* Initial FCS value */
  72 #endif
  73 #ifndef PPPGOODFCS16
  74 #define PPPGOODFCS16    0xf0b8  /* Good final FCS value */
  75 #endif
  76
  77 /*
  78  * Calculate a new fcs given the current fcs and the new data.
  79  */
  80 u16 pppfcs16(u16 fcs, void *_cp, int len)
  81 {
  82     register unsigned char *cp = (unsigned char *)_cp;
  83     /* don't worry about the efficiency of these asserts here.  gcc will
  84      * recognise that the asserted expressions are constant and remove them.
  85      * Whether they are usefull is another question.
  86      */
  87     ASSERT(sizeof (u16) == 2);
  88     ASSERT(((u16) -1) > 0);
  89     while (len--)
  90         fcs = (fcs >> 8) ^ fcstab[(fcs ^ *cp++) & 0xff];
  91     return (fcs);
  92 }
  93
  94 #if 0
  95 /*
  96  * How to use the fcs
  97  */
  98 tryfcs16(cp, len)
  99     register unsigned char *cp;
 100     register int len;
 101 {
 102     u16 trialfcs;
 103
 104     /* add on output */
 105     trialfcs = pppfcs16( PPPINITFCS16, cp, len );
 106     trialfcs ^= 0xffff;                 /* complement */
 107     cp[len] = (trialfcs & 0x00ff);      /* least significant byte first */
 108     cp[len+1] = ((trialfcs >> 8) & 0x00ff);
 109
 110     /* check on input */
 111     trialfcs = pppfcs16( PPPINITFCS16, cp, len + 2 );
 112     if ( trialfcs == PPPGOODFCS16 )
 113         printf("Good FCS\n");
 114 }
 115
 116 #endif