rfc1662.txt

pptp第二层隧道模块

    * FCS lookup table as calculated by the table generator.
    */
   static u16 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,



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      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
   };

   #define PPPINITFCS16    0xffff  /* Initial FCS value */
   #define PPPGOODFCS16    0xf0b8  /* Good final FCS value */

   /*
    * Calculate a new fcs given the current fcs and the new data.
    */
   u16 pppfcs16(fcs, cp, len)
       register u16 fcs;
       register unsigned char *cp;
       register int len;
   {
       ASSERT(sizeof (u16) == 2);
       ASSERT(((u16) -1) > 0);
       while (len--)
           fcs = (fcs >> 8) ^ fcstab[(fcs ^ *cp++) & 0xff];

       return (fcs);
   }

   /*
    * How to use the fcs
    */
   tryfcs16(cp, len)
       register unsigned char *cp;
       register int len;
   {
       u16 trialfcs;

       /* add on output */
       trialfcs = pppfcs16( PPPINITFCS16, cp, len );
       trialfcs ^= 0xffff;                 /* complement */
       cp[len] = (trialfcs & 0x00ff);      /* least significant byte first */
       cp[len+1] = ((trialfcs >> 8) & 0x00ff);




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       /* check on input */
       trialfcs = pppfcs16( PPPINITFCS16, cp, len + 2 );
       if ( trialfcs == PPPGOODFCS16 )
           printf("Good FCS\n");
   }



C.3.  32-bit FCS Computation Method

   The following code provides a table lookup computation for
   calculating the 32-bit Frame Check Sequence as data arrives at the
   interface.

   /*
    * The FCS-32 generator polynomial: x**0 + x**1 + x**2 + x**4 + x**5
    *                      + x**7 + x**8 + x**10 + x**11 + x**12 + x**16
    *                      + x**22 + x**23 + x**26 + x**32.
    */

   /*
    * u32 represents an unsigned 32-bit number.  Adjust the typedef for
    * your hardware.
    */
   typedef unsigned long u32;

   static u32 fcstab_32[256] =
      {
      0x00000000, 0x77073096, 0xee0e612c, 0x990951ba,
      0x076dc419, 0x706af48f, 0xe963a535, 0x9e6495a3,
      0x0edb8832, 0x79dcb8a4, 0xe0d5e91e, 0x97d2d988,
      0x09b64c2b, 0x7eb17cbd, 0xe7b82d07, 0x90bf1d91,
      0x1db71064, 0x6ab020f2, 0xf3b97148, 0x84be41de,
      0x1adad47d, 0x6ddde4eb, 0xf4d4b551, 0x83d385c7,
      0x136c9856, 0x646ba8c0, 0xfd62f97a, 0x8a65c9ec,
      0x14015c4f, 0x63066cd9, 0xfa0f3d63, 0x8d080df5,
      0x3b6e20c8, 0x4c69105e, 0xd56041e4, 0xa2677172,
      0x3c03e4d1, 0x4b04d447, 0xd20d85fd, 0xa50ab56b,
      0x35b5a8fa, 0x42b2986c, 0xdbbbc9d6, 0xacbcf940,
      0x32d86ce3, 0x45df5c75, 0xdcd60dcf, 0xabd13d59,
      0x26d930ac, 0x51de003a, 0xc8d75180, 0xbfd06116,
      0x21b4f4b5, 0x56b3c423, 0xcfba9599, 0xb8bda50f,
      0x2802b89e, 0x5f058808, 0xc60cd9b2, 0xb10be924,
      0x2f6f7c87, 0x58684c11, 0xc1611dab, 0xb6662d3d,
      0x76dc4190, 0x01db7106, 0x98d220bc, 0xefd5102a,
      0x71b18589, 0x06b6b51f, 0x9fbfe4a5, 0xe8b8d433,
      0x7807c9a2, 0x0f00f934, 0x9609a88e, 0xe10e9818,
      0x7f6a0dbb, 0x086d3d2d, 0x91646c97, 0xe6635c01,



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      0x6b6b51f4, 0x1c6c6162, 0x856530d8, 0xf262004e,
      0x6c0695ed, 0x1b01a57b, 0x8208f4c1, 0xf50fc457,
      0x65b0d9c6, 0x12b7e950, 0x8bbeb8ea, 0xfcb9887c,
      0x62dd1ddf, 0x15da2d49, 0x8cd37cf3, 0xfbd44c65,
      0x4db26158, 0x3ab551ce, 0xa3bc0074, 0xd4bb30e2,
      0x4adfa541, 0x3dd895d7, 0xa4d1c46d, 0xd3d6f4fb,
      0x4369e96a, 0x346ed9fc, 0xad678846, 0xda60b8d0,
      0x44042d73, 0x33031de5, 0xaa0a4c5f, 0xdd0d7cc9,
      0x5005713c, 0x270241aa, 0xbe0b1010, 0xc90c2086,
      0x5768b525, 0x206f85b3, 0xb966d409, 0xce61e49f,
      0x5edef90e, 0x29d9c998, 0xb0d09822, 0xc7d7a8b4,
      0x59b33d17, 0x2eb40d81, 0xb7bd5c3b, 0xc0ba6cad,
      0xedb88320, 0x9abfb3b6, 0x03b6e20c, 0x74b1d29a,
      0xead54739, 0x9dd277af, 0x04db2615, 0x73dc1683,
      0xe3630b12, 0x94643b84, 0x0d6d6a3e, 0x7a6a5aa8,
      0xe40ecf0b, 0x9309ff9d, 0x0a00ae27, 0x7d079eb1,
      0xf00f9344, 0x8708a3d2, 0x1e01f268, 0x6906c2fe,
      0xf762575d, 0x806567cb, 0x196c3671, 0x6e6b06e7,
      0xfed41b76, 0x89d32be0, 0x10da7a5a, 0x67dd4acc,
      0xf9b9df6f, 0x8ebeeff9, 0x17b7be43, 0x60b08ed5,
      0xd6d6a3e8, 0xa1d1937e, 0x38d8c2c4, 0x4fdff252,
      0xd1bb67f1, 0xa6bc5767, 0x3fb506dd, 0x48b2364b,
      0xd80d2bda, 0xaf0a1b4c, 0x36034af6, 0x41047a60,
      0xdf60efc3, 0xa867df55, 0x316e8eef, 0x4669be79,
      0xcb61b38c, 0xbc66831a, 0x256fd2a0, 0x5268e236,
      0xcc0c7795, 0xbb0b4703, 0x220216b9, 0x5505262f,
      0xc5ba3bbe, 0xb2bd0b28, 0x2bb45a92, 0x5cb36a04,
      0xc2d7ffa7, 0xb5d0cf31, 0x2cd99e8b, 0x5bdeae1d,
      0x9b64c2b0, 0xec63f226, 0x756aa39c, 0x026d930a,
      0x9c0906a9, 0xeb0e363f, 0x72076785, 0x05005713,
      0x95bf4a82, 0xe2b87a14, 0x7bb12bae, 0x0cb61b38,
      0x92d28e9b, 0xe5d5be0d, 0x7cdcefb7, 0x0bdbdf21,
      0x86d3d2d4, 0xf1d4e242, 0x68ddb3f8, 0x1fda836e,
      0x81be16cd, 0xf6b9265b, 0x6fb077e1, 0x18b74777,
      0x88085ae6, 0xff0f6a70, 0x66063bca, 0x11010b5c,
      0x8f659eff, 0xf862ae69, 0x616bffd3, 0x166ccf45,
      0xa00ae278, 0xd70dd2ee, 0x4e048354, 0x3903b3c2,
      0xa7672661, 0xd06016f7, 0x4969474d, 0x3e6e77db,
      0xaed16a4a, 0xd9d65adc, 0x40df0b66, 0x37d83bf0,
      0xa9bcae53, 0xdebb9ec5, 0x47b2cf7f, 0x30b5ffe9,
      0xbdbdf21c, 0xcabac28a, 0x53b39330, 0x24b4a3a6,
      0xbad03605, 0xcdd70693, 0x54de5729, 0x23d967bf,
      0xb3667a2e, 0xc4614ab8, 0x5d681b02, 0x2a6f2b94,
      0xb40bbe37, 0xc30c8ea1, 0x5a05df1b, 0x2d02ef8d
      };

   #define PPPINITFCS32  0xffffffff   /* Initial FCS value */
   #define PPPGOODFCS32  0xdebb20e3   /* Good final FCS value */



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   /*
    * Calculate a new FCS given the current FCS and the new data.
    */
   u32 pppfcs32(fcs, cp, len)
       register u32 fcs;
       register unsigned char *cp;
       register int len;
       {
       ASSERT(sizeof (u32) == 4);
       ASSERT(((u32) -1) > 0);
       while (len--)
           fcs = (((fcs) >> 8) ^ fcstab_32[((fcs) ^ (*cp++)) & 0xff]);

       return (fcs);
       }

   /*
    * How to use the fcs
    */
   tryfcs32(cp, len)
       register unsigned char *cp;
       register int len;
   {
       u32 trialfcs;

       /* add on output */
       trialfcs = pppfcs32( PPPINITFCS32, cp, len );
       trialfcs ^= 0xffffffff;             /* complement */
       cp[len] = (trialfcs & 0x00ff);      /* least significant byte first */
       cp[len+1] = ((trialfcs >>= 8) & 0x00ff);
       cp[len+2] = ((trialfcs >>= 8) & 0x00ff);
       cp[len+3] = ((trialfcs >> 8) & 0x00ff);

       /* check on input */
       trialfcs = pppfcs32( PPPINITFCS32, cp, len + 4 );
       if ( trialfcs == PPPGOODFCS32 )
           printf("Good FCS\n");
   }













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Security Considerations

   As noted in the Physical Layer Requirements section, the link layer
   might not be informed when the connected state of the physical layer
   has changed.  This results in possible security lapses due to over-
   reliance on the integrity and security of switching systems and
   administrations.  An insertion attack might be undetected.  An
   attacker which is able to spoof the same calling identity might be
   able to avoid link authentication.



References

   [1]   Simpson, W., Editor, "The Point-to-Point Protocol (PPP)",
         STD 50, RFC 1661, Daydreamer, July 1994.

   [2]   ISO/IEC 3309:1991(E), "Information Technology -
         Telecommunications and information exchange between systems -
         High-level data link control (HDLC) procedures - Frame
         structure", International Organization For Standardization,
         Fourth edition 1991-06-01.

   [3]   ISO/IEC 3309:1991/Amd.2:1992(E), "Information Technology -
         Telecommunications and information exchange between systems -
         High-level data link control (HDLC) procedures - Frame
         structure - Amendment 2: Extended transparency options for
         start/stop transmission", International Organization For
         Standardization, 1992-01-15.

   [4]   ISO/IEC 4335:1991(E), "Information Technology -
         Telecommunications and information exchange between systems -
         High-level data link control (HDLC) procedures - Elements of
         procedures", International Organization For Standardization,
         Fourth edition 1991-09-15.

   [5]   Simpson, W., Editor, "PPP LCP Extensions", RFC 1570,
         Daydreamer, January 1994.

   [6]   ANSI X3.4-1977, "American National Standard Code for
         Information Interchange", American National Standards
         Institute, 1977.

   [7]   Perez, "Byte-wise CRC Calculations", IEEE Micro, June 1983.

   [8]   Morse, G., "Calculating CRC's by Bits and Bytes", Byte,
         September 1986.




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   [9]   LeVan, J., "A Fast CRC", Byte, November 1987.

   [10]  Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC
         1340, USC/Information Sciences Institute, July 1992.



Acknowledgements

   This document is the product of the Point-to-Point Protocol Working
   Group of the Internet Engineering Task Force (IETF).  Comments should
   be submitted to the [e-mail] mailing list.

   This specification is based on previous RFCs, where many
   contributions have been acknowleged.

   The 32-bit FCS example code was provided by Karl Fox (Morning Star
   Technologies).

   Special thanks to Morning Star Technologies for providing computing
   resources and network access support for writing this specification.



Chair's Address

   The working group can be contacted via the current chair:

      Fred Baker
      Advanced Computer Communications
      315 Bollay Drive
      Santa Barbara, California  93117

      [e-mail]


Editor's Address

   Questions about this memo can also be directed to:

      William Allen Simpson
      Daydreamer
      Computer Systems Consulting Services
      1384 Fontaine
      Madison Heights, Michigan  48071

      [e-mail]
          [e-mail]


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