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.
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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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Internet Draft SCTP Checksum Change January 2002
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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Internet Draft SCTP Checksum Change January 2002
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);
}
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