rfc1570.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

      specific information to make use of the Callback.  The
      codification of the range of allowed usage of this field is
      outside the scope of this specification.



2.4.  Compound-Frames

   Description

      This Configuration Option provides a method for an implementation
      to send multiple PPP encapsulated packets within the same frame.
      This option might be used for many diverse purposes, such as
      savings on toll charges.




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      Only those PPP Protocols which have determinate lengths or
      integral length fields may be aggregated into a compound frame.

      When Compound-Frames is successfully negotiated, the sender MAY
      add additional packets to the same frame.  Each packet is
      immediately followed by another Protocol field, with its attendant
      datagram.

      When padding is added to the end of the Information field, the
      procedure described in Self-Describing-Padding is used.
      Therefore, this option MUST be negotiated together with the Self-
      Describing-Padding option.

      If the FCS-Alternatives option has been negotiated, self
      describing padding MUST always be added.  That is, the final
      packet MUST be followed by a series of octets, the first of which
      contains the value one (1).

      On receipt, the first Protocol field is examined, and the packet
      is processed as usual.  For those datagrams which have a
      determinate length, the remainder of the frame is returned to the
      demultiplexor.  Each succeeding Protocol field is processed as a
      separate packet.  This processing is complete when a packet is
      processed which does not have a determinate length, when the
      remainder of the frame is empty, or when the Protocol field is
      determined to have a value of one (1).

      The PPP Protocol value of one (1) is reserved as the Padding
      Protocol.  Any following octets are removed as padding.

   A summary of the Compound-Frames Option format is shown below.  The
   fields are transmitted from left to right.

    0                   1
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |    Length     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Type

      15

   Length

      2




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2.4.1.  LCP considerations

   During initial negotiation, the Compound-Frames option can be used to
   minimize the negotiation latency, by reducing the number of frames
   exchanged.

   The first LCP Configure-Request packet is sent as usual in a single
   frame, including the Self-Describing-Padding and Compound-Frames
   options.

   The peer SHOULD respond with a Configure-Ack, followed in a compound
   frame by its LCP Configure-Request, and any NCP Configure-Requests
   desired.

   Upon receipt, the local implementation SHOULD process the Configure-
   Ack as usual.  Since the peer has agreed to send compound frames, the
   implementation MUST examine the remainder of the frame for additional
   packets.  If the peer also specified the Self-Describing-Padding and
   Compound-Frames options in its Configure-Request, the local
   implementation SHOULD retain its Configure-Ack, and further NCP
   configuration packets SHOULD be added to the return frame.

   Together with the peer's final return frame, the minimum number of
   frames to complete configuration is 4.



























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A.  Fast Frame Check Sequence (FCS) Implementation

A.1.  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.

   /*
    * 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,
      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,



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      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 */

   /*
    * 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--)



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           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");
   }


Security Considerations

   Security issues are briefly discussed in sections concerning the
   Callback Configuration Option.


References

   [1]   Simpson, W., Editor, "The Point-to-Point Protocol (PPP)", RFC
         1548, Daydreamer, December 1993.

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

   [3]   Simpson, W., Editor, "PPP in HDLC Framing", RFC 1549, 
         Daydreamer, December 1993.






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RFC 1570                   PPP LCP extensions               January 1994


Acknowledgments

   The Identification feature was suggested by Bob Sutterfield (Morning
   Star Technologies).

   The Time-Remaining feature was suggested by Brad Parker (FCR).

   Some of the original text for FCS-Alternatives was provided by Arthur
   Harvey (then of DEC).  The Null FCS was requested by Peter Honeyman
   (UMich).  The 32-bit FCS example code was provided by Karl Fox
   (Morning Star Technologies).

   Self-Describing-Padding was suggested and named by Fred Baker (ACC).

   Compound-Frames was suggested by Keith Sklower (Berkeley).

   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

      EMail: [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

      EMail: [e-mail]
             [e-mail]







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