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Versions: 00 01 02 03 04 05 06 07 rfc3309                               
    Network Working Group                                         R. Stewart
    Request for Comments: 2960                                      C. Sharp
    Category: Internet Draft                                   Cisco Systems
                                                                    J. Stone
                                                                    Stanford
                                                                     D. Otis
                                                                    SANlight

                                                            January 05, 2002

                             SCTP Checksum Change
                        draft-ietf-tsvwg-sctpcsum-01.txt

    Status of this Memo

    This document is an internet-draft and is in full conformance with all
    provisions of Section 10 of RFC2026.

    Internet-Drafts are working documents of the Internet Engineering Task
    Force (IETF), its areas, and its working groups.  Note that other groups
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    Drafts are draft documents valid for a maximum of six months and may be
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    inappropriate to use Internet- Drafts as reference material or to cite
    them other than as "work in progress."
    The list of current Internet-Drafts can be accessed at
    http://www.ietf.org/ietf/1id-abstracts.txt
    The list of Internet-Draft Shadow Directories can be accessed at
    http://www.ietf.org/shadow.html.

    Abstract

    SCTP [RFC2960] currently uses an Adler-32 checksum.  For small packets,
    this provides weak protection against the detection of errors.  This
    document changes that checksum and updates SCTP to use a 32 bit CRC
    checksum.

    Table of Contents

    1 Introduction ................................................ 1
    2 Checksum Procedures ......................................... 2
    3 Acknowledgments ............................................. 4
    4 Authors' Addresses .......................................... 4
    5 References .................................................. 5
    6 Appendix .................................................... 5


    1 Introduction

    A fundamental weakness has been detected in SCTP's current Adler-32
    checksum algorithm [STONE].  One requirement of an effective checksum is
    that it evenly and smoothly spreads its input packets over the available
    check bits.

    From an email from Jonathan Stone, who analyzed the Adler-32 as part

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    of his doctoral thesis:

    "Briefly, the problem is that, for very short packets, Adler32 is
    guaranteed to give poor coverage of the available bits.  Don't take my
    word for it, ask Mark Adler. :-).

    Adler-32 uses two 16-bit counters, s1 and s2.  s1 is the sum of the
    input, taken as 8-bit bytes.  s2 is a running sum of each value of s1.
    Both s1 and s2 are computed mod-65521 (the largest prime less than 2^16).
    Consider a packet of 128 bytes.  The *most* that each byte can be is 255.
    There are only 128 bytes of input, so the greatest value which the s1
    accumulator can have is 255 * 128 = 32640.  So for 128-byte packets, s1
    never_ wraps.  That is critical.  Why?

    The key is to consider the distribution of the s1 values, over some
    distribution of the values of the individual input bytes in each packet.
    Because s1 never wraps, s1 is simply the sum of the individual input
    bytes. (even Doug's trick of adding 0x5555 doesn't help here, and an even
    larger value doesn't really help: we can get at most one mod-565521
    reduction).

    Given the further assumption that the input bytes are drawn independently
    from some distribution (they probably aren't: for file system data, it's
    even worse than that!), the Central Limit Theorem tells us that that s1
    will tend to have a normal distribution.  That's bad: it tells us that
    the value of s1 will have hot-spots at around 128 times the mean of the
    input distribution: around 16k, assuming a uniform distribution.  That's
    bad. We want the accumulator to wrap as many times as possible, so that
    the resulting sum has as close to a uniform distribution as possible. (I
    call this "fairness").

    So, for short packets, the Adler-32 s1 sum is guaranteed to be unfair.
    Why is that bad?  It's bad because the space of valid packets-- input
    data, plus checksum values -- is also small.  If all packets have
    checksum values very close to 32640, then the likelihood of even a
    'small' error leaving a damaged packet with a valid checksum is higher
    than if all checksum values are equally likely."

    Due to this inherent weakness, exacerbated by the fact that SCTP will
    first be used as a signaling transport protocol where signaling messages
    are usually less than 128 bytes, a new checksum algorithm is specified by
    this document, replacing the current Adler-32 algorithm with CRC-32c.

    1.1 Conventions

    The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,SHOULD
    NOT, RECOMMENDED, NOT RECOMMENDED, MAY, and OPTIONAL, when they appear in
    this document, are to be interpreted as described in [RFC2119].

    2 Checksum Procedures

    The procedures described in section 2.1 of this document MUST be
    followed, replacing the current checksum defined in [RFC2960].
    Furthermore any references within [RFC2960] to Adler-32 MUST be treated

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    as a reference to CRC-32c.  Section 2.1 of this document describes the
    new calculation and verification procedures that MUST be followed.

    2.1 Checksum Calculation

    When sending an SCTP packet, the endpoint MUST include in the checksum
    field the CRC-32c value calculated on the packet, as described below.

    After the packet is constructed (containing the SCTP common header and
    one or more control or DATA chunks), the transmitter MUST do the
    following:

    1) Fill in the proper Verification Tag in the SCTP common header and
       initialize the checksum field to 0's.

    2) Calculate the CRC-32c of the whole packet, including the SCTP common
       header and all the chunks.

    3) Put the resultant value into the checksum field in the common header,
       and leave the rest of the bits unchanged.

    When an SCTP packet is received, the receiver MUST first perform the
    following:

    1) Store the received CRC-32c value,

    2) Replace the 32 bits of the checksum field in the received SCTP packet
       with all '0's and calculate a CRC-32c value of the whole received
       packet.  And,

    3) Verify that the calculated CRC-32c value is the same as the received
       CRC-32c value.  If not, the receiver MUST treat the packet as an
       invalid SCTP packet.

    The default procedure for handling invalid SCTP packets is to silently
    discard them.

    The 32 bit CRC is calculated as described for CRC-32c and uses the
    polynomial code 0x11EDC6F41 (Castagnoli93) or x^32+x^28+x^27+x^26+x^25
    +x^23+x^22+x^20+x^19+x^18+x^14+x^13+x^11+x^10+x^9+x^8+x^6+x^0 with
    (reflected) placement.  With most serial media, the bits within each byte
    are shifted out least significant bit first whereas CRC is calculated
    from most significant to least.  To accommodate the serial bit order, a
    reflected table is used.  Reflected means bit 31 becomes bit 0, bit 30
    becomes bit 1, etc.  This reflected technique also reduces the number of
    instructions needed for each lookup.

    It becomes a minor problem dealing with this unusual reflected value in
    that both bit and byte order is reversed from that of the CPU.  As the
    bits within each byte are to remain reflected as that is how they are
    sent out, then ideally only the byte order is adjusted to provide most to
    least serial presentation.  To utilize existing byte placement routines
    defined for various architectures however, the CRC-32c value will be
    placed as reflected in network order.  This incorrect byte order

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    placement with respect to the serial sequence eliminates new byte order
    placement definitions.

    To improve leading zero detection, the CRC value is initialized to all
    one's prior to the packet calculation but is not inverted before being
    placed.  Placement in the SCTP common header and jumbo frames cause
    variances from the Ethernet CRC algorithm.  The [Castagnoli93] polynomial
    offers error detection enhancements for jumbo frames at the expense of
    gates.  The software table implementations for any 32 bit polynomial has
    the same overhead however.

    3 Acknowledgments

    The authors would like to thank the following people that have
    provided comments and input on the checksum issue:

    Ran Atkinson, Stephen Bailey, David Black, Scott Bradner, Mikael
    Degermark, Laurent Glaude, Klaus Gradischnig, Alf Heidermark, Jacob
    Heitz, Gareth Kiely, David Lehmann, Allision Mankin, Lyndon Ong, Craig
    Partridge, Vern Paxson, Kacheong Poon, Michael Ramalho, David Reed, Ian
    Rytina, Hanns Juergen Schwarzbauer, Bill Sommerfeld, Michael Tuxen, Jim
    Williams, Jim Wendt, Michael Welzl, Jonathan Wood, Lloyd Wood, Qiaobing
    Xie, La Monte Yarroll, Dafna Sheinwald, and Julian Satran, Pat Thaler,
    Vince Cavanna, Matt Wakeley.


    4 Authors' Addresses

    Randall R. Stewart
    24 Burning Bush Trail.
    Crystal Lake, IL 60012
    USA

    EMail: rrs@cisco.com


    Chip Sharp
    Cisco Systems Inc.
    7025 Kit Creek Road
    Research Triangle Park, NC  27709
    USA

    EMail: chsharp@cisco.com

    Jonathan Stone
    Room 446, Mail code 9040
    Gates building 4A
    Stanford, Ca 94305

    EMail: jonathan@dsg.stanford.edu


    Douglas Otis
    800 E. Middlefield

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    Mountain View, CA 94043
    USA

    Email dotis@sanlight.net


    5 References

    [Castagnoli93] Guy Castagnoli, Stefan Braeuer and Martin Herrman
    "Optimization of Cyclic Redundancy-Check Codes with 24 and 32 Parity
    Bits", IEEE Transactions on Communications, Vol. 41,  No. 6, June 1993

    5.1 Informative References

    [STONE]  Jonathan Stone "Checksums in the Internet", Doctoral
             dissertation - August 2001


    6 Appendix

    Example code using 256 word lookup table.

    /* Example of the crc table file */
    #ifndef __crc32cr_table_h__
    #define __crc32cr_table_h__

    #define CRC32C_POLY 0x1EDC6F41
    #define CRC32C(c,d) (c=(c>>8)^crc_c[(c^(d))&0xFF])
    /* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */
    /* Copyright 2001, D. Otis.  Use this program, code or tables    */
    /* extracted from it, as desired without restriction.            */
    /*                                                               */
    /* 32 Bit Reflected CRC table generation for SCTP.               */
    /* To accommodate serial byte data being shifted out least       */
    /* significant bit first, the table's 32 bit words are reflected */
    /* which flips both byte and bit MS and LS positions.  The CRC   */
    /* is calculated MS bits first from the perspective of the serial*/
    /* stream.  The x^32 term is implied and the x^0 term may also   */
    /* be shown as +1.  The polynomial code used is 0x1EDC6F41.      */
    /* Castagnoli93                                                  */
    /* x^32+x^28+x^27+x^26+x^25+x^23+x^22+x^20+x^19+x^18+x^14+x^13+  */
    /* x^11+x^10+x^9+x^8+x^6+x^0                                     */
    /* Guy Castagnoli Stefan Braeuer and Martin Herrman              */
    /* "Optimization of Cyclic Redundancy-Check Codes                */
    /* with 24 and 32 Parity Bits",                                  */
    /* IEEE Transactions on Communications, Vol.41, No.6, June 1993  */
    /* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */

    unsigned long  crc_c[256] =
    {
    0x00000000L, 0xF26B8303L, 0xE13B70F7L, 0x1350F3F4L,
    0xC79A971FL, 0x35F1141CL, 0x26A1E7E8L, 0xD4CA64EBL,
    0x8AD958CFL, 0x78B2DBCCL, 0x6BE22838L, 0x9989AB3BL,
    0x4D43CFD0L, 0xBF284CD3L, 0xAC78BF27L, 0x5E133C24L,

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    0x105EC76FL, 0xE235446CL, 0xF165B798L, 0x030E349BL,
    0xD7C45070L, 0x25AFD373L, 0x36FF2087L, 0xC494A384L,
    0x9A879FA0L, 0x68EC1CA3L, 0x7BBCEF57L, 0x89D76C54L,
    0x5D1D08BFL, 0xAF768BBCL, 0xBC267848L, 0x4E4DFB4BL,
    0x20BD8EDEL, 0xD2D60DDDL, 0xC186FE29L, 0x33ED7D2AL,
    0xE72719C1L, 0x154C9AC2L, 0x061C6936L, 0xF477EA35L,
    0xAA64D611L, 0x580F5512L, 0x4B5FA6E6L, 0xB93425E5L,
    0x6DFE410EL, 0x9F95C20DL, 0x8CC531F9L, 0x7EAEB2FAL,
    0x30E349B1L, 0xC288CAB2L, 0xD1D83946L, 0x23B3BA45L,
    0xF779DEAEL, 0x05125DADL, 0x1642AE59L, 0xE4292D5AL,
    0xBA3A117EL, 0x4851927DL, 0x5B016189L, 0xA96AE28AL,
    0x7DA08661L, 0x8FCB0562L, 0x9C9BF696L, 0x6EF07595L,
    0x417B1DBCL, 0xB3109EBFL, 0xA0406D4BL, 0x522BEE48L,
    0x86E18AA3L, 0x748A09A0L, 0x67DAFA54L, 0x95B17957L,
    0xCBA24573L, 0x39C9C670L, 0x2A993584L, 0xD8F2B687L,
    0x0C38D26CL, 0xFE53516FL, 0xED03A29BL, 0x1F682198L,
    0x5125DAD3L, 0xA34E59D0L, 0xB01EAA24L, 0x42752927L,
    0x96BF4DCCL, 0x64D4CECFL, 0x77843D3BL, 0x85EFBE38L,
    0xDBFC821CL, 0x2997011FL, 0x3AC7F2EBL, 0xC8AC71E8L,
    0x1C661503L, 0xEE0D9600L, 0xFD5D65F4L, 0x0F36E6F7L,
    0x61C69362L, 0x93AD1061L, 0x80FDE395L, 0x72966096L,
    0xA65C047DL, 0x5437877EL, 0x4767748AL, 0xB50CF789L,
    0xEB1FCBADL, 0x197448AEL, 0x0A24BB5AL, 0xF84F3859L,
    0x2C855CB2L, 0xDEEEDFB1L, 0xCDBE2C45L, 0x3FD5AF46L,
    0x7198540DL, 0x83F3D70EL, 0x90A324FAL, 0x62C8A7F9L,
    0xB602C312L, 0x44694011L, 0x5739B3E5L, 0xA55230E6L,
    0xFB410CC2L, 0x092A8FC1L, 0x1A7A7C35L, 0xE811FF36L,
    0x3CDB9BDDL, 0xCEB018DEL, 0xDDE0EB2AL, 0x2F8B6829L,
    0x82F63B78L, 0x709DB87BL, 0x63CD4B8FL, 0x91A6C88CL,
    0x456CAC67L, 0xB7072F64L, 0xA457DC90L, 0x563C5F93L,
    0x082F63B7L, 0xFA44E0B4L, 0xE9141340L, 0x1B7F9043L,
    0xCFB5F4A8L, 0x3DDE77ABL, 0x2E8E845FL, 0xDCE5075CL,
    0x92A8FC17L, 0x60C37F14L, 0x73938CE0L, 0x81F80FE3L,
    0x55326B08L, 0xA759E80BL, 0xB4091BFFL, 0x466298FCL,
    0x1871A4D8L, 0xEA1A27DBL, 0xF94AD42FL, 0x0B21572CL,
    0xDFEB33C7L, 0x2D80B0C4L, 0x3ED04330L, 0xCCBBC033L,
    0xA24BB5A6L, 0x502036A5L, 0x4370C551L, 0xB11B4652L,
    0x65D122B9L, 0x97BAA1BAL, 0x84EA524EL, 0x7681D14DL,
    0x2892ED69L, 0xDAF96E6AL, 0xC9A99D9EL, 0x3BC21E9DL,
    0xEF087A76L, 0x1D63F975L, 0x0E330A81L, 0xFC588982L,
    0xB21572C9L, 0x407EF1CAL, 0x532E023EL, 0xA145813DL,
    0x758FE5D6L, 0x87E466D5L, 0x94B49521L, 0x66DF1622L,
    0x38CC2A06L, 0xCAA7A905L, 0xD9F75AF1L, 0x2B9CD9F2L,
    0xFF56BD19L, 0x0D3D3E1AL, 0x1E6DCDEEL, 0xEC064EEDL,
    0xC38D26C4L, 0x31E6A5C7L, 0x22B65633L, 0xD0DDD530L,
    0x0417B1DBL, 0xF67C32D8L, 0xE52CC12CL, 0x1747422FL,
    0x49547E0BL, 0xBB3FFD08L, 0xA86F0EFCL, 0x5A048DFFL,
    0x8ECEE914L, 0x7CA56A17L, 0x6FF599E3L, 0x9D9E1AE0L,
    0xD3D3E1ABL, 0x21B862A8L, 0x32E8915CL, 0xC083125FL,
    0x144976B4L, 0xE622F5B7L, 0xF5720643L, 0x07198540L,
    0x590AB964L, 0xAB613A67L, 0xB831C993L, 0x4A5A4A90L,
    0x9E902E7BL, 0x6CFBAD78L, 0x7FAB5E8CL, 0x8DC0DD8FL,
    0xE330A81AL, 0x115B2B19L, 0x020BD8EDL, 0xF0605BEEL,
    0x24AA3F05L, 0xD6C1BC06L, 0xC5914FF2L, 0x37FACCF1L,

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    0x69E9F0D5L, 0x9B8273D6L, 0x88D28022L, 0x7AB90321L,
    0xAE7367CAL, 0x5C18E4C9L, 0x4F48173DL, 0xBD23943EL,
    0xF36E6F75L, 0x0105EC76L, 0x12551F82L, 0xE03E9C81L,
    0x34F4F86AL, 0xC69F7B69L, 0xD5CF889DL, 0x27A40B9EL,
    0x79B737BAL, 0x8BDCB4B9L, 0x988C474DL, 0x6AE7C44EL,
    0xBE2DA0A5L, 0x4C4623A6L, 0x5F16D052L, 0xAD7D5351L,
    };

    #endif

    /* Example of table build routine */

    #include <stdio.h>
    #include <stdlib.h>

    #define OUTPUT_FILE   "crc32cr.h"
    #define CRC32C_POLY    0x1EDC6F41L
    #define CRC_TYPE  "\
    /* Castagnoli93                                                  */\n\
    /* x^32+x^28+x^27+x^26+x^25+x^23+x^22+x^20+x^19+x^18+x^14+x^13+  */\n\
    /* x^11+x^10+x^9+x^8+x^6+x^0                                     */\n\
    /* Guy Castagnoli Stefan Braeuer and Martin Herrman              */\n\
    /* \"Optimization of Cyclic Redundancy-Check Codes                */\n\
    /* with 24 and 32 Parity Bits\",                                  */\n\
    /* IEEE Transactions on Communications, Vol.41, No.6, June 1993  */\n"

    FILE *tf;

    unsigned long
      reflect_32 (unsigned long b)
    {
      int i;
      unsigned long rw = 0L;

      for (i = 0; i < 32; i++)
        {
          if (b & 1)
            rw |= 1 << (31 - i);
          b >>= 1;
        }
      return (rw);
    }

    unsigned long
      build_crc_table (int index)
    {
      int i;
      unsigned long rb;

      rb = reflect_32 (index);

      for (i = 0; i < 8; i++)
        {
          if (rb & 0x80000000L)

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           rb = (rb << 1) ^ CRC32C_POLY;
          else
           rb <<= 1;
        }
      return (reflect_32 (rb));
    }

    main ()
    {
      int i;

      printf ("\nGenerating CRC-32c table file <%s>\n", OUTPUT_FILE);
      if ((tf = fopen (OUTPUT_FILE, "w")) == NULL)
        {
          printf ("Unable to open %s\n", OUTPUT_FILE);
          exit (1);
        }
      fprintf (tf, "#ifndef __crc32cr_table_h__\n");
      fprintf (tf, "#define __crc32cr_table_h__\n\n");
      fprintf (tf, "#define CRC32C_POLY 0x%08lX\n", CRC32C_POLY);
      fprintf (tf, "#define CRC32C(c,d) (c=(c>>8)^crc_c[(c^(d))&0xFF])\n");
      fprintf (tf, "\
    /* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */\n\
    /* Copyright 2001, D. Otis.  Use this program, code or tables    */\n\
    /* extracted from it, as desired without restriction.            */\n\
    /*                                                               */\n\
    /* 32 Bit Reflected CRC table generation for SCTP.               */\n\
    /* To accommodate serial byte data being shifted out least       */\n\
    /* significant bit first, the table's 32 bit words are reflected */\n\
    /* which flips both byte and bit MS and LS positions.  The CRC   */\n\
    /* is calculated MS bits first from the perspective of the serial*/\n\
    /* stream.  The x^32 term is implied and the x^0 term may also   */\n\
    /* be shown as +1.  The polynomial code used is 0x%08lX.      */\n%s\
    /* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * */\n"
                   , CRC32C_POLY, CRC_TYPE);

      fprintf (tf, "\nunsigned long  crc_c[256] =\n{\n");
      for (i = 0; i < 256; i++)
        {
          fprintf (tf, "0x%08lXL, ", build_crc_table (i));
          if ((i & 3) == 3)
            fprintf (tf, "\n");
        }

      fprintf (tf, "};\n\n#endif\n");

      if (fclose (tf) != 0)
        printf ("Unable to close <%s>." OUTPUT_FILE);
      else
        printf ("\nThe CRC-32c table has been written to <%s>.\n",
          OUTPUT_FILE);
    }



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    /* Example of crc insertion */

    #include "crc32cr.h"


    int
    insert_crc32(unsigned char *buffer, unsigned int length)
    {
    SCTP_message *message;
    unsigned int i;
    unsigned long crc32 = ~0L;

    /* check packet length */
    if (length > NMAX  || length < NMIN)
      return -1;

    message = (SCTP_message *) buffer;
    message->common_header.checksum = 0L;

    for (i = 0; i < length; i++)
    {
      CRC32C(crc32, buffer[i]);
    }

    /* and insert it into the message */
    message->common_header.checksum = htonl(crc32);
        return 1;
    }

    /* Example of crc validation */
    /* Test of 32 zeros should yield 0x756EC955 placed in network order */
    /* 13 zeros followed by byte values of 1 - 0x1f should yield
    /* 0x5b988D47 */

    int
    validate_crc32(unsigned char *buffer, unsigned int length)
    {
    SCTP_message *message;
    unsigned int i;
    unsigned long original_crc32;
    unsigned long crc32 = ~0L;

    /* check packet length */
    if (length > NMAX  || length < NMIN)
      return -1;

    /* save and zero checksum */
    message = (SCTP_message *) buffer;
    original_crc32 = ntohl(message->common_header.checksum);
    message->common_header.checksum = 0L;

    for (i = 0; i < length; i++)
    {

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      CRC32C(crc32, buffer[i]);
    }

      return ((original_crc32 == crc32)? 1 : -1);
    }

    Full Copyright Statement

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