Network Working Group                                         R. Stewart
    Category: Internet Draft                                   Cisco Systems
                                                                    J. Stone
                                                                     D. Otis

                                                               March 1, 2002

                             SCTP Checksum Change

    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
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    SCTP [RFC2960] currently uses an Adler-32 checksum.  For small packets
    Adler-32 provides weak 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 Security Considerations...................................... 5
    4 IANA Considerations.......................................... 5
    5 Acknowledgments ............................................. 5
    6 Authors' Addresses .......................................... 6
    7 References .................................................. 7
    8 Appendix .................................................... 8

    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-65521

    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

    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

    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

    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.

    We define a 'reflected value' as one that is the opposite of the
    normal bit order of the machine.  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
    The CRC is computed using a procedure similar to  ETHERNET CRC [ITU32],
    modified to reflect transport level usage.

    CRC computation uses polynomial division. A message bit-string M
    is transformed to a polynomial, M(X), and the CRC is calculated
    from M(X) using polynomial arithmetic [Peterson 72].
    When CRCs are used at the link layer, the polynomial is derived from
    on-the-wire bit ordering: the first bit  `on the wire' is
    the high-order coefficient. Since SCTP is a transport-level protocol,
    it  cannot know the actual serial-media bit ordering. Moreover,
    different links in the path between SCTP endpoints may use
    different link-level bit orders)

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    A  convention must therefore be established for mapping SCTP transport
    messages to polynomials for purposes of CRC computation.
    The bit-ordering for mapping SCTP messages to  polynomials is
    that bytes are taken most-significant first; but within each byte,
    bits are taken least-significant first.  The first byte of the
    message provides the eight highest coefficients.
    Within each  byte, the least-significant SCTP bit gives the
    most significant polynomial coefficient within that byte, and
    the most-significant SCTP bit is the most significant polynomial
    coefficient in that byte.   (This bit ordering is sometimes
    called `mirrored' or `reflected'   [Williams93].)  CRC polynomials
    are to be transformed back into SCTP transport-level byte values
    using a consistent mapping.

    The SCTP transport-level CRC value should be calculated as follows:
           - CRC input data are assumed to a byte stream numbered from 0
             to N-1.
           - the transport-level byte-stream is mapped to a polynomial value.
             An N-byte PDU with bytes 0 to N-1, is  considered as
             coefficients of a polynomial M(x) of order 8N-1, with
             bit 0 of byte j being coefficient x^(8j-1), bit 7 of  byte
             0 being coefficient x(8j^-8).
           - the CRC remainder register is initialized with all
             1s (equivalent to complementing the first 32 bits of the message)
           - the polynomial is multiplied by x^32 and divided by G(x),
             the generator polynomial, producing a remainder R(x) of degree
             less than or equal to 31.
           - the coefficients of R(x) are considered a 32 bit sequence.
           - the bit sequence is complemented. The resulting is the CRC
           - The CRC polynomial is mapped back into SCTP transport-level
             bytes.   Coefficient of x^31 gives the value of bit 0 of
             SCTP byte 0, the coefficient of x^24 gives the value of
             bit 7 of byte 0.  the coefficient of x^7 gives bit 0 of
             bit 0 and the coefficient of x^0 0 gives bit 7 of byte 3.
             The resulting four-byte transport-level sequence is the
             32-bit SCTP checksum value.

    When an SCTP packet is transmitted, the sender MUST perform this
    checksum procedure, using the preceding CRC computation:

    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 32-bit SCTP checksum 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

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    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 the SCTP CRC-32c checksum 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.

    If SCTP could follow link level CRC use, the CRC would be computed
    over the link-level bit-stream. The first bit on the link
    mapping to the highest-order coefficient, and so on down to the
    last link-level bit as the lowest-order coefficient. The CRC value
    would be transmitted immediately after the input message as a link-level
    `trailer'. The resulting link-level bit-stream would be
    (M(X)x) * x^32) + (M(X)*x^32))/ G(x), which is divisible by G(X).
    There would thus be a constant CRC remainder for `good' packets.
    However, given that implementations of RFC2960 have already
    proliferated, the IETF discussions considered that the benefit of
    a `trailer' CRC did not outweigh the cost of making a very large
    change in the protocol processing.   Further, packets accepted by
    the SCTP `header' CRC are in one-to-one correspondence with
    packets accepted by a modified procedure using  a  `trailer'
    CRC value, and where the SCTP common checksum header is set to zero
    on transmission and is received as zero.

    There may be a computational advantage in validating the Association
    against the Verification Tag prior to performing a checksum as
    invalid tags will result in the same action as a bad checksum in
    most cases. The exceptions for this technique would be INIT and some
    SHUTDOWN-COMPLETE exchanges as well as a stale COOKIE-ECHO.  These
    special case exchanges must represent small packets and will
    minimize the effect of the checksum calculation.

    3 Security Considerations

    In general, the security considerations of RFC2960 apply to
    the protocol with the new checksum as well.

    4 IANA Considerations

    There are no IANA considerations required in this document.

    5 Acknowledgments

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

    Mark Adler, Ran Atkinson, Stephen Bailey, David Black, Scott
    Bradner, Mikael Degermark, Laurent Glaude, Klaus Gradischnig, Alf

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    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,
    Chip Sharp, Bill Sommerfeld, Michael Tuexen, Jim Williams, Jim Wendt,
    Michael Welzl, Jonathan Wood, Lloyd Wood, Qiaobing Xie, La Monte

    Special thanks to Dafna Scheinwald, Julian Satran  Pat Thaler, Matt
    Wakeley, and  Vince Cavanna, for  selection criteria of polynomials and
    examination of CRC polynomials, particularly CRC-32c [Castagnoli93].

    Special thanks to Mr. Ross Williams and his document [Williams93].
    This non-formal perspective on software aspects of CRCs furthered
    understanding of authors previously unfamiliar with CRC computation.
    More formal treatments of [Blahut 94] or [Peterson 72], was
    also  essential.

    6 Authors' Addresses

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


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


    Douglas Otis
    800 E. Middlefield
    Mountain View, CA 94043


    7 References

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

    [McKee75]  H. McKee, "Improved {CRC} techniques detects erroneous
               leading and trailing 0's in transmitted data blocks",
               Computer Design Volume 14 Number 10 Pages 102-4,106,
               October 1975

    [RFC2026]  Bradner, S., "The Internet Standards Process -- Revision

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               3", BCP 9, RFC 2026, October 1996.

    [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
               Requirement Levels", BCP 14, RFC 2119, March 1997.

    [RFC2960] R. R. Stewart, Q. Xie, K. Morneault, C. Sharp,
        H. J. Schwarzbauer, T. Taylor, I. Rytina, M. Kalla, L. Zhang,
        and, V. Paxson, "Stream Control Transmission Protocol," RFC
        2960, October 2000.

    [ITU32] ITU-T Recommendation V.42, "Error-correcting
            procedures for DCEs using asynchronous-to-synchronous
            conversion", section, October 1996.

    7.1 Informative References

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

                 ALGORITHMS" - Internet publication, August 1993,

    [Blahut 1994], R.E. Blahut, Theory and Practice of Error Control
    Codes, Addison-Wesley, 1994.

    [Easics 2001].  Online tools
    for synthesis of CRC Verilog and VHDL.

    [Feldmeier 95], David C. Feldmeier, Fast software implementation of
    error detection codes, IEEE Transactions on Networking, vol 3 no 6,
    pp 640-651, December, 1995.

    [Glaise 1997] R.  J. Glaise, A two-step computation of cyclic
    redundancy code CRC-32 for ATM networks, IBM Journal of Research and
    Development} vol 41 no 6, 1997.  URL=

    [Prange 1957], E. Prange, Cyclic Error-Correcting codes in two
    symbols, Technical report AFCRC-TN-57-103, Air Force Cambridge
    Research Center, Cambridge, Mass. 1957.

    [Peterson 1972], W. W. Peterson and E.J Weldon, Error Correcting
    Codes, 2nd. edition, MIT Press, Cambridge, Massachusetts.

    [Shie2001] Ming-Der Shieh et. al, A Systematic Approach for Parallel
    CRC Computations. Journal of Information Science and Engineering,
    Vol.17 No.3, pp.445-461

    [Sprachman2001] Michael Sprachman, Automatic Generation of Parallel
    CRC Circuits, IEEE Design & Test May-June 2001

    8 Appendix

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    This appendix is for information only and is NOT part of the

    The anticipated deployment of SCTP ranges over several orders of
    magnitude of link speed: from cellular-power telephony devices at
    tens of kilobits, to local links at tens of gigabits.  Implementors
    of SCTP should consider their link speed and choose, from the wide
    range of CRC implementations, one which matches their own design
    point for size, cost, and throughput. Many techniques for computing
    CRCs are known. This Appendix surveys just a few, to give a feel for
    the range of techniques available.

    CRCs are derived from early work by Prange in the 1950s [Prange 57].
    The theory underlying CRCs and choice of generator polynomial can be
    introduced by either via the theory of Galois fields [Blahut 94]
    or as ideals of an algebra over cyclic codes [cite Peterson 72].

    One of the simplest techniques is direct bit-serial hardware
    implementations, using the generator polynomial as the taps of a
    linear feedback shift register (LSFR).  LSFR computation follows
    directly from the mathematics, and is generally attributed to Prange.
    Tools exist which, a CRC generator polynomial, will produce
    synthesizable Verilog code for  CRC hardware [Easics 2001].

    Since LSFRs do not scale well in speed, a variety of other
    techniques have been explored.  One technique exploits the fact that
    the divisor of the polynomial long-division, G, is known in
    advance. It is thus possible to pre-compute lookup tables giving the
    polynomial remainder of multiple input bits --- typically 2, 4, or 8
    bits of input at a time.  This technique can be used either in
    software or in hardware.  Software to compute lookup tables yielding
    2, 4, or 8 bits of result is freely available. [Williams93]

    For multi-gigabit links, the above techniques may still not be fast
    enough. One technique for computing CRCS at OC-48 rates is
    `two-stage' CRC computation [Glaise 1997]. Here, some multiple
    of G(x), G(x)H(x), is chosen so as to minimize the number of nonzero
    coefficients, or weight, of the product G(x)H(x).  The low weight of
    the product polynomial makes it susceptible to efficient hardware
    divide-by-constant implementations. This first stage gives M(x) /
    (G(x)H(x)) as its result.  The second stage then divides the result
    of the first stage by H(x), yielding (M(x) / (G(x)H(x))) / H(x).  If
    H(x) is also relatively prime to G(x), this gives M(x)/G(x).
    Further developments on this approach can be found in [Shie2001] and

    The literature also includes a variety of software CRC
    implementations.  One approach is to use carefully-tuned assembly
    code for direct polynomial division. [Feldmeier 95] reports that for
    low-weight polynomials, tuned polynomial arithmetic gives higher
    throughput than table-lookup algorithms.  Even within table-lookup
    algorithms, the size of the table can be tuned, either for total
    cache footprint, or (for space-restricted environments) to minimize

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    total size.

    Implementors should keep in mind the bit ordering described in
    Section 2: the ordering of bits within bytes for computing CRCs in
    SCTP is the least significant bit of each byte is the
    most-significant polynomial coefficient(and vice-versa). This
    `reflected' SCTP CRC bit ordering matches on-the-wire bit order for
    Ethernet and other serial media, but is the reverse of traditional
    Internet bit ordering.

    One technique to accommodate this bit-reversal can be explained as
    follows: sketch out a hardware implementation assuming the bits are
    in CRC bit order; then perform a left-to-right inversion (mirror
    image) on the entire algorithm.  (We defer for a moment the issue of
    byte order within words.) Then compute that "mirror image" in
    software.  The CRC from the ``mirror image'' algorithm will be the
    bit-reversal of a correct hardware implementation.  When the
    link-level media sends each byte, the byte is sent in the reverse of
    the host CPU bit-order.  Serialization of each byte of the
    ``reflected'' CRC value re-reverses the bit order, so in the end,
    each byte will be transmitted on-the-wire in the specified bit

    The following non-normative sample code is taken from an open-source
    CRC generator [Williams93] using the ``mirroring'' technique
    and yielding a lookup table for SCTP CRC32-c with 256 entries, each
    32 bits wide.  While neither especially slow nor especially fast, as
    software table-lookup CRCs go, it has the advantage of working on
    both big-endian and little-endian CPUs, using the same (host-order)
    lookup tables, and using only the pre-defined ntohl() and htonl()
    operations.  The code is somewhat modified from [Williams93], to
    ensure portability between big-endian and little-endian
    architectures.  (Note that if the byte endian-ness of the target
    architecture is known to be little-endian the final bit-reversal and
    byte-reversal steps can be folded into a single operation.)

    /* Note Definition for Ross Williams table generator would */
    /* be: TB_WIDTH=4, TB_POLLY=0x1EDC6F41, TB_REVER=TRUE        */
    /* For Mr. Williams direct calculation code use the settings */
    /* cm_width=32, cm_poly=0x1EDC6F41, cm_init=0xFFFFFFFF,      */
    /* cm_refin=TRUE, cm_refot=TRUE, cm_xorort=0x00000000        */

    /* 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])

    unsigned long  crc_c[256] =
    0x00000000L, 0xF26B8303L, 0xE13B70F7L, 0x1350F3F4L,

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    0xC79A971FL, 0x35F1141CL, 0x26A1E7E8L, 0xD4CA64EBL,
    0x8AD958CFL, 0x78B2DBCCL, 0x6BE22838L, 0x9989AB3BL,
    0x4D43CFD0L, 0xBF284CD3L, 0xAC78BF27L, 0x5E133C24L,
    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,

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    0x9E902E7BL, 0x6CFBAD78L, 0x7FAB5E8CL, 0x8DC0DD8FL,
    0xE330A81AL, 0x115B2B19L, 0x020BD8EDL, 0xF0605BEEL,
    0x24AA3F05L, 0xD6C1BC06L, 0xC5914FF2L, 0x37FACCF1L,
    0x69E9F0D5L, 0x9B8273D6L, 0x88D28022L, 0x7AB90321L,
    0xAE7367CAL, 0x5C18E4C9L, 0x4F48173DL, 0xBD23943EL,
    0xF36E6F75L, 0x0105EC76L, 0x12551F82L, 0xE03E9C81L,
    0x34F4F86AL, 0xC69F7B69L, 0xD5CF889DL, 0x27A40B9EL,
    0x79B737BAL, 0x8BDCB4B9L, 0x988C474DL, 0x6AE7C44EL,
    0xBE2DA0A5L, 0x4C4623A6L, 0x5F16D052L, 0xAD7D5351L,


    /* Example of table build routine */

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

    #define OUTPUT_FILE   "crc32cr.h"
    #define CRC32C_POLY    0x1EDC6F41L
    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)
           rb = (rb << 1) ^ CRC32C_POLY;
           rb <<= 1;
      return (reflect_32 (rb));

    main ()

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      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, "\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);
        printf ("\nThe CRC-32c table has been written to <%s>.\n",

    /* Example of crc insertion */

    #include "crc32cr.h"

    unsigned long
    generate_crc32c(unsigned char *buffer, unsigned int length)
      unsigned int i;
      unsigned long crc32 = ~0L;
      unsigned long result;
      unsigned char byte0,byte1,byte2,byte3;

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

      /*  result  now holds the negated polynomial remainder;
       *  since the table and algorithm is "reflected" [williams95].
       *  That is,  result has the same value as if we mapped the message
       *  to a polyomial, computed the host-bit-order polynomial
       *  remainder, performed final negation, then did an end-for-end
       *  bit-reversal.
       *  Note that a 32-bit bit-reversal is identical to four inplace
       *  8-bit reversals followed by an end-for-end byteswap.

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       *  In other words, the bytes of each bit are in the right order,
       *  but the bytes have been byteswapped.  So we now do an explicit
       *  byteswap.  On a little-endian machine, this byteswap and
       *  the final ntohl cancel out and could be elided.
      byte0 = result & 0xff;
      byte1 = (result>>8) & 0xff;
      byte2 = (result>>16) & 0xff;
      byte3 = (result>>24) & 0xff;

      crc32 = ((byte0 << 24) |
               (byte1 << 16) |
               (byte2 << 8)  |
      return ( crc32 );

    insert_crc32(unsigned char *buffer, unsigned int length)
      SCTP_message *message;
      unsigned long crc32;
      message = (SCTP_message *) buffer;
      message->common_header.checksum = 0L;
      crc32 = generate_crc32c(buffer,length);
      /* 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 */

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

      /* save and zero checksum */
      message = (SCTP_message *) buffer;
      original_crc32 = ntohl(message->common_header.checksum);
      message->common_header.checksum = 0L;
      crc32 = generate_crc32c(buffer,length);
      return ((original_crc32 == crc32)? 1 : -1);

    Full Copyright Statement

    Copyright (C) The Internet Society (2001).  All Rights Reserved.

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    This document and translations of it may be copied and furnished to
    others, and derivative works that comment on or otherwise explain it or
    assist in its implementation may be prepared, copied, published and
    distributed, in whole or in part, without restriction of any kind,
    provided that the above copyright notice and this paragraph are included
    on all such copies and derivative works.  However, this document itself
    may not be modified in any way, such as by removing the copyright notice
    or references to the Internet Society or other Internet organizations,
    except as needed for the purpose of developing Internet standards in
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    Standards process must be followed, or as required to translate it into
    languages other than English.

    The limited permissions granted above are perpetual and will not be
    revoked by the Internet Society or its successors or assigns.

    This document and the information contained herein is provided on an "AS

    Funding for the RFC Editor function is currently provided by the
    Internet Society.

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