rfc3385.txt

一个学习iSCSI协议的文档

Network Working Group                                       D. Sheinwald
Request for Comments: 3385                                     J. Satran
Category: Informational                                              IBM
                                                               P. Thaler
                                                              V. Cavanna
                                                                 Agilent
                                                          September 2002


       Internet Protocol Small Computer System Interface (iSCSI)
         Cyclic Redundancy Check (CRC)/Checksum Considerations

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

Abstract

   In this memo, we attempt to give some estimates for the probability
   of undetected errors to facilitate the selection of an error
   detection code for the Internet Protocol Small Computer System
   Interface (iSCSI).

   We will also attempt to compare Cyclic Redundancy Checks (CRCs) with
   other checksum forms (e.g., Fletcher, Adler, weighted checksums), as
   permitted by available data.

1. Introduction

   Cyclic Redundancy Check (CRC) codes [Peterson] are shortened cyclic
   codes used for error detection.  A number of CRC codes have been
   adopted in standards: ATM, IEC, IEEE, CCITT, IBM-SDLC, and more
   [Baicheva].  The most important expectation from this kind of code is
   a very low probability for undetected errors.  The probability of
   undetected errors in such codes has been, and still is, subject to
   extensive studies that have included both analytical models and
   simulations.  Those codes have been used extensively in
   communications and magnetic recording as they demonstrate good "burst
   error" detection capabilities, but are also good at detecting several
   independent bit errors.  Hardware implementations are very simple and
   well known; their simplicity has made them popular with hardware




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   developers for many years.  However, algorithms and software for
   effective implementations of CRC are now also widely available
   [Williams].

   The probability of undetected errors depends on the polynomial
   selected to generate the code, the error distribution (error model),
   and the data length.

2. Error Models and Goals

   We will analyze the code behavior under two conditions:

      - noisy channel - burst errors with an average length of n bits
      - low noise channel - independent single bit errors

   Burst errors are the prevalent natural phenomenon on communication
   lines and recording media.  The numbers quoted for them revolve
   around the BER (bit error rate).  However, those numbers are
   frequently nothing more than a reflection of the Burst Error Rate
   multiplied by the average burst length.  In field engineering tests,
   three numbers are usually quoted together -- BER, error-free-seconds
   and severely-error-seconds; this illustrates our point.

   Even beyond communication and recording media, the effects of errors
   will be bursty.  An example of this is a memory error that will
   affect more than a single bit and the total effect will not be very
   different from the communication error, or software errors that occur
   while manipulating packets will have a burst effect.  Software errors
   also result in burst errors.  In addition, serial internal
   interconnects will make this type of error the most common within
   machines as well.

   We also analyze the effects of single independent bit errors, since
   these may be caused by certain defects.

   On burst, we assume an average burst error duration of bd, which at a
   given transmission rate s, will result in an average burst of a =
   bd*s bits.  (E.g., an average burst duration of 3 ns at 1Gbs gives an
   average burst of 3 bits.)

   For the burst error rate, we will take 10^-10.  The numbers quoted
   for BER on wired communication channels are between 10^-10 to 10^-12
   and we consider the BER as burst-error-rate*average-burst-length.
   Nevertheless, please keep in mind that if the channel includes
   wireless links, the error rates may be substantially higher.

   For independent single bit errors, we assume a 10^-11 error rate.




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   Because the error detection mechanisms will have to transport large
   amounts of data (petabytes=10^16 bits) without errors, we will target
   very low probabilities for undetected errors for all block lengths
   (at 10Gb/s that much data can be sent in less than 2 weeks on a
   single link).

   Alternatively, as iSCSI has to perform efficiently, we will require
   that the error detection capability of a selected protection
   mechanism be very good, at least up to block lengths of 8k bytes
   (64kbits).

   The error detection capability should keep the probability of
   undetected errors at values that would be "next-to-impossible".  We
   recognize, however, that such attributes are hard to quantify and we
   resorted to physics.  The value 10^23 is the Avogadro number while
   10^45 is the number of atoms in the known Universe (or it was many
   years ago when we read about it) and those are the bounds of
   incertitude we could live with.  (10^-23 at worst and 10^-45 if we
   can afford it.)  For 8k blocks, the per/bit equivalent would be
   (10^-28 to 10^-50).

3. Background and Literature Survey

   Each codeword of a binary (n,k) CRC code C consists of n = k+r bits.
   The block of r parity bits is computed from the block of k
   information bits.  The code has a degree r generator polynomial g(x).

   The code is linear in the sense that the bitwise addition of any two
   codewords yields a codeword.

   For the minimal m such that g(x) divides (x^m)-1, either n=m, and the
   code C comprises the set D of all the multiplications of g(x) modulo
   (x^m)-1, or n<m, and C is obtained from D by shortening each word in
   the latter in m-n specific positions.  (This also reduces the number
   of words since all zero words are then discarded and duplicates are
   not maintained.)

   Error detection at the receiving end is made by computing the parity
   bits from the received information block, and comparing them with the
   received parity bits.

   An undetected error occurs when the received word c' is a codeword,
   but is different from the c that is transmitted.

   This is only possible when the error pattern e=c'-c is a codeword by
   itself (because of the linearity of the code).  The performance of a
   CRC code is measured by the probability Pud of undetected channel
   errors.



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   Let Ai denote the number of codewords of weight i, (i.e., with i 1-
   bits).  For a binary symmetric channel (BSC), with sporadic,
   independent bit error ratio of probability 0<=epsilon<=0.5, the
   probability of undetected errors for the code C is thus given by:

Pud(C,epsilon) = Sigma[for i=d to n] (Ai*(epsilon^i)*(1-epsilon)^(n-i))

   where d is the distance of the code:  the minimal weight difference
   between two codewords in C which, by the linearity of the code, is
   also the minimal weight of any codeword in the code.  Pud can also be
   expressed by the weight distribution of the dual code:  the set of
   words each of which is orthogonal (bitwise AND yields an even number
   of 1-bits) to every word of C.  The fact that Pud can be computed
   using the dual code is extremely important; while the number of
   codewords in the code is 2^k, the number of codewords in the dual
   code is 2^r.  k is in the orders of thousands, and r in the order of
   16 or 24 or 32.  If we use Bi to denote the number of codewords in
   the dual code which are of weight i, then ([LinCostello]):

Pud (C,epsilon) = 2^-r Sigma [for i=0 to n] Bi*(1-2*epsilon)^i -
(1-epsilon)^n

   Wolf [Wolf94o] introduced an efficient algorithm for enumerating all
   the codewords of a code and finding their weight distribution.

   Wolf [Wolf82] found that, counter to what was assumed, (1) there
   exist codes for which Pud(C,epsilon)>Pud(C,0.5) for some epsilon
   not=0.5 and (2) Pud is not always increasing for 0<=epsilon<=0.5.
   The value of what was assumed to be the worst Pud is Pud(C,0.5)=(2^-
   r) - (2^-n).  This stems from the fact that with epsilon=0.5, all 2^n
   received words are equally likely and out of them 2^(n-r)-1 will be
   accepted as codewords of no errors, although they are different from
   the codeword transmitted.  Previously Pud had been assumed to equal
   [2^(n-r)-1]/(2^n-1) or the ratio of the number of non-zero multiples
   of the polynomial of degree less than n (each such multiple is
   undetected) and the number of possible error polynomials.  With
   either formula Pud approaches 1/2^r as n approaches infinity, but
   Wolf's formula is more accurate.

   Wolf [Wolf94j] investigated the CCITT code of r=16 parity bits.  This
   code is a member of the family of (shortened codes of) BCH codes of
   length 2^(r-1) -1 (r=16 in the CCITT 16-bit case) generated by a
   polynomial of the form g(x) =(x+1)p(x) with p(x) being a primitive
   polynomial of degree r-1 (=15 in this case).  These codes have a BCH
   design distance of 4.  That is, the minimal distance between any two
   codewords in the code is at least 4 bits (which is earned by the fact





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   that the sequence of powers of alpha, the root of p(x), which are
   roots of g(x), includes three consecutive powers -- alpha^0, alpha^1,
   alpha^2).  Hence, every 3 single bit errors are detectable.

   Wolf found that different shortened versions of a given code, of the
   same codeword length, perform the same (independent of which specific
   indexes are omitted from the original code).  He also found that for
   the unshortened codes, all primitive polynomials yield codes of the
   same performance.  But for the shortened versions, the choice of the
   primitive polynomial does make a difference.  Wolf [Wolf94j] found a
   primitive polynomial which (when multiplied by x+1) yields a
   generating polynomial that outperforms the CCITT one by an order of
   magnitude.  For 32-bit redundancy bits, he found an example of two
   polynomials that differ in their probability of undetected burst of
   length 33 by 4 orders of magnitude.

   It so happens, that for some shortened codes, the minimum distance,
   or the distribution of the weights, is better than for others derived
   from different unshortened codes.

   Baicheva, et. al. [Baicheva] made a comprehensive comparison of
   different generating polynomials of degree 16 of the form g(x) =
   (x+1)p(x), and of other forms.  They computed their Pud for code
   lengths up to 1024 bits.  They measured their "goodness"  -- if
   Pud(C,epsilon)  <= Pud(C,0.5) and being "well-behaved" -- if
   Pud(C,epsilon) increases with epsilon in the range (0,0.5).  The
   paper gives a comprehensive table that lists which of the polynomials
   is good and which is well-behaved for different length ranges.

   For a single burst error, Wolf [Wolf94J] suggested the model of (b:p)
   burst -- the errors only occur within a span of b bits, and within
   that span, the errors occur randomly, with a bit error probability 0
   <= p <= 1.

   For p=0.5, which used to be considered the worst case, it is well
   known [Wolf94J] that the probability of undetected one burst error of
   length b <= r is 0, of length b=r+1 is 2^-(r-1), and of b > r+1, is
   2^-r, independently of the choice of the primitive polynomial.

   With Wolf's definition, where p can be different from 0.5, indeed it
   was found that for a given b there are values of p, different from
   0.5 which maximize the probability of undetected (b:p) burst error.









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   Wolf proved that for a given code, for all b in the range r < b < n,
   the conditional probability of undetected error for the (n, n-r)
   code, given that a (b:p) burst occurred, is equal to the probability
   of undetected errors for the same code (the same generating
   polynomial), shortened to block length b, when this shortened code is
   used with a binary symmetric channel with channel (sporadic,
   independent) bit error probability p.

   For the IEEE-802.3 used CRC32, Fujiwara et al. [Fujiwara89] measured
   the weights of all words of all shortened versions of the IEEE 802.3
   code of 32 check bits.  This code is generated by a primitive
   polynomial of degree 32:

   g(x) = x^32 + x^26 + x^23 + x^22 + x^16 + x^12 + x^11 + x^10 + x^8 +
   x^7 + x^5 + x^4 + x^2 + x + 1 and hence the designed distance of it
   is only 3.  This distance holds for codes as long as 2^32-1.
   However, the frame format of the MAC (Media Access Control) of the
   data link layer in IEEE 802.3, as well as that of the data link layer
   for the Ethernet (1980) forbid lengths exceeding 12,144 bits.  Thus,
   only such bounded lengths are investigated in [Fujiwara89].  For
   shortened versions, the minimum distance was found to be 4 for
   lengths 4096 to 12,144; 5 for lengths 512 to 2048; and even 15 for
   lengths 33 through 42.  A chart of results of calculations of Pud is
   presented in [Fujiwara89] from which we can see that for codes of
   length 12,144 and BSC of epsilon = 10^-5 - 10^-4,
   Pud(12,144,epsilon)= 10^-14 - 10^-13 and for epsilon = 10^-4 - 10^-3,
   Pud(512,epsilon) = 10^-15, Pud(1024,epsilon) = 10^-14,
   Pud(2048,epsilon) = 10^-13, Pud(4096,epsilon) = 10^-12 - 10^-11, and
   Pud(8192,epsilon) = 10^-10 which is rather close to 2^-32.

   Castagnoli, et. al. [Castagnoli93] extended Fujiwara's technique for
   efficiently calculating the minimum distance through the weight
   distribution of the dual code and explored a large number of CRC
   codes with 24 and 32 redundancy bit.  They explored several codes
   built as a multiplication of several lower degree irreducible
   polynomials.

   In the popular class of (x+1)*deg31-irreducible-polynomial they