rfc2550.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

Network Working Group                                       S. Glassman
Request for Comments: 2550                                   M. Manasse
Category: Stinkards Track                                      J. Mogul
                                            Compaq Computer Corporation
                                                           1 April 1999


                            Y10K and Beyond

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 (1999).  All Rights Reserved.

Abstract

   As we approach the end of the millennium, much attention has been
   paid to the so-called "Y2K" problem.  Nearly everyone now regrets the
   short-sightedness of the programmers of yore who wrote programs
   designed to fail in the year 2000.  Unfortunately, the current fixes
   for Y2K lead inevitably to a crisis in the year 10,000 when the
   programs are again designed to fail.

   This specification provides a solution to the "Y10K" problem which
   has also been called the "YAK" problem (hex) and the "YXK" problem
   (Roman numerals).

1. Introduction, Discussion, and Related Work

   Many programs and standards contain, manipulate and maintain dates.
   Comparing and sorting dates is a common activity.  Many different
   formats and standards for dates have been developed and all have been
   found wanting.

   Early date formats reserved only two digits to represent the year
   portion of a date.  Programs that use this format make mistakes when
   dealing with dates after the year 2000.  This is the so-called Y2K
   problem.








Glassman, et. al.            Informational                      [Page 1]

RFC 2550                    Y10K and Beyond                 1 April 1999


   The most common fix for the Y2K problem has been to switch to 4-digit
   years.  This fix covers roughly the next 8,000 years (until the year
   9999) by which time, everyone seems convinced that all current
   programs will have been retired.  This is exactly the faulty logic
   and lazy programming practice that led to the current Y2K problem!
   Programmers and designers always assume that their code will
   eventually disappear, but history suggests that code and programs are
   often used well past their intended circumstances.

   The 4-digit year leads directly to programs that will fail in the
   year 10,000.  This proposal addresses the Y10K problem in a general
   way that covers the full range of date and time format issues.

1.1 Current approaches

   A large number of approaches exist for formatting dates and times.
   All of them have limitations.  The 2-digit year runs into trouble
   next year.  The 4-digit year hits the wall in the year 10,000.  A
   16-bit year runs out in the year 65,536.  A 32-bit counter for the
   number of seconds since 1970 [UNIX] wraps in 2038.  A 32-bit counter
   for the number of milli-seconds since booting crashes a Windows (TM)
   PC in 49.7 days [Microsoft].

   In this specification, we focus on the Y10K problems since they are
   most common and a large number of existing standards and protocols
   are susceptible to them (section 7).  These standards, and new
   proposals on their way, will lead to a serious world-wide problem
   unless efforts are made now to correct the computing, government, and
   business communities.

   Already, a small cottage industry is popping up to deal with the Y10K
   problem [YUCK].  We encourage these efforts and, in the coming years,
   this effort can only grow in size and importance.

1.2 A Fixed Format Y10K Fix

   At the time of this writing, only one proposal [Wilborne] directly
   deals with the Y10K problem.  In that proposal, dates are represented
   as decimal numbers with the dates compared numerically.  The proposed
   format is simply YYYYYMMDD - i.e. 5-digit years.

   To allow numerical comparison of dates, this representation requires
   a completely fixed representation for the date.  There can be no
   optional fields, the date resolution is limited to the granularity of
   one day, and this solution fails in the year 100,000 (Y100K).






Glassman, et. al.            Informational                      [Page 2]

RFC 2550                    Y10K and Beyond                 1 April 1999


1.2.2 Limitations of Numerical Comparison

   While sufficient for the specific Y10K problem, this solution is
   limited.  Even if extended for 6-digit years, it fails on 32-bit
   systems (and future 32-bit system emulators) after the date
   represented by the number 2147481231 (December 31, 214748) leading to
   a Y214749 problem.  Similarly, 64-bit and 128-bit systems also will
   fail, although somewhat later (after December 31, 922,337,203,685,477
   and December 31, 17,014,118,346,046,923,173,168,730,371,588,410
   respectively).

1.2.3 Granularity Issues

   The granularity problems of a fixed format date can be improved by
   extending the date format to include greater precision in the date.
   However, since numerical comparison of dates requires a fixed
   representation date, an extended format can not provide sufficient
   resolution for all foreseeable needs.

   For instance, if the precision were extended to the femto-second
   range the date format would become YYYYYMMDDHHMMSSmmmuuunnnpppfff
   (year, month, day, hour, minute, second, milli-second, micro-second,
   nano-second, pico-second, and femto-second).  The additional 21
   digits of this format limit the set of representable dates.  Compared
   to 1.2.2, the 32-bit and 64-bit forms of the date are instantly
   exceeded, while the 128-bit version would be viable - expiring on
   December 31, 17,014,118,346,046.

1.2.3.1 Extrapolation of Future Granularity Issues

   However, a simple extrapolation of Moore's law shows that even
   femto-second resolution will soon be inadequate.  Projecting current
   CPU clock speeds forward, a femto-second clock speed will be achieved
   in only 30 years.  And, by the year 10,000 the projected clock speed
   of the Intel Pentium MMDCLXVI (TM) will be approximately 10 ** (-
   1609) seconds.

   This discussion clearly shows that any fixed-format, integer
   representation of a date is likely to be insufficiently precise for
   future uses.

1.2.3.2 Floating Point Is No Solution

   The temptation to use floating point numbers to represent dates
   should be avoided.  Like the longer fixed-format, integer
   representations of the date, floating point representations merely
   delay the inevitable time when their range is exceeded.  In addition,




Glassman, et. al.            Informational                      [Page 3]

RFC 2550                    Y10K and Beyond                 1 April 1999


   the well known problems of real numbers - rounding, de-normalization,
   non-uniform distribution, etc. - just add to the problems of dealing
   with dates.

2 Structure of Y10K Solution

   Any Y10K solution should have the following characteristics.

2.1 Compatibility

   The format must be compatible with existing 4-digit date formats.
   Y2K compliant programs and standards must continue to work with Y10K
   dates before the year 10,000.  Y10K compliant programs can gradually
   be developed over time and coexist with non-Y10K compliant programs.

2.2 Simplicity and Efficiency

   Y10K dates must allow dates after 10,000 to be easily identified.
   Within a program, there must be a simple procedure for recognizing
   the Y10K dates and distinguishing them from legacy dates.

2.3 Lexical Sorting

   Y10K dates must be sortable lexically based on their ASCII
   representation.  The dates must not require specialized libraries or
   procedures.

2.4 Future Extensibility

   Y10K dates must support arbitrary precision dates, and should support
   dates extending arbitrarily far into the future and past.  Y10K dates
   from different eras and with different precisions must be directly
   comparable and sortable.

2.4.1 Environmental Considerations

   The known universe has a finite past and future.  The current age of
   the universe is estimated in [Zebu] as between 10 ** 10 and 2 * 10 **
   10 years.  The death of the universe is estimated in [Nigel] to occur
   in 10 ** 11 - years and in [Drake] as occurring either in 10 ** 12
   years for a closed universe (the big crunch) or 10 ** 14 years for an
   open universe (the heat death of the universe).

   In any case, the prevailing belief is that the life of the universe
   (and thus the range of possible dates) is finite.






Glassman, et. al.            Informational                      [Page 4]

RFC 2550                    Y10K and Beyond                 1 April 1999


2.4.2 Transcending Environmental Considerations

   However, we might get lucky.  So, Y10K dates are able to represent
   any possible time without any limits to their range either in the
   past or future.

   Y10K compliant programs MAY choose to limit the range of dates they
   support to those consistent with the expected life of the universe.
   Y10K compliant systems MUST accept Y10K dates from 10 ** 12 years in
   the past to 10 ** 20 years into the future.  Y10K compliant systems
   SHOULD accept dates for at least 10 ** 29 years in the past and
   future.

3 Syntax Overview

   The syntax of Y10K dates consists of simple, printable ASCII
   characters.  The syntax and the characters are chosen to support a
   simple lexical sort order for dates represented in Y10K format.  All
   Y10K dates MUST conform to these rules.

   Every Y10K date MUST begin with a Y10K year.  Following the year,
   there MAY be an arbitrary sequence of digits.  The digits are
   interpreted as MMDDHHMMSSmmmuuunnnpppfff...  (month, day, hour,
   minute, second, milli-second, micro-second, nano-second pico-second,
   femto-second, etc. - moving left to right in the date, digits always
   decrease in significance).

   All dates and times MUST be relative to International Atomic Time
   (TAI) [NRAO].

   When comparing dates, a date precedes every other date for which it
   is a prefix.  So, the date "19990401000000" precedes the date
   "19990401000000000".  In particular, dates with the format YYYYMMDD
   are interpreted to represent the exact instant that the day begins
   and precede any other date contained in that day.

3.1 Years 1 - 9999

   The current 4-digit year syntax covers all years from 1000 - 9999.
   These years are represented as 4 decimal digits.  Leading 0's MUST be
   added to the years before 1000 to bring the year to 4 digits.  Files
   containing legacy pre-Y1K [Mike] dates will have to be converted.

3.2 Years 10,000 through 99,999

   Four digits are not sufficient to represent dates beyond the year
   9999.  So, all years from 10,000 - 99,999 are represented by 5 digits
   preceded by the letter 'A'.  So, 10,000 becomes "A10000" and 99,999



Glassman, et. al.            Informational                      [Page 5]

RFC 2550                    Y10K and Beyond                 1 April 1999


   becomes "A99999".  Since 'A' follows '9' in the ASCII ordering, all
   dates with 5-digit years will follow all dates with 4-digit years
   (for example, "A10000" will sort after "9999").  This gives us the
   sort and comparison behaviour we want.

3.3 Years 100,000 up to 10 ** 30

   By a simple generalization of 3.2, 6-digit years are preceded by the
   letter 'B', 7-digit years by 'C', etc.  Using just the 26 upper-case
   ASCII characters, we can cover all years up to 10**30 (the last year
   representable is "Z999999999999999999999999999999").  Again, since
   the ASCII characters are sorted alphabetically, all dates sort
   appropriately.

3.4 Years 10 ** 30 and beyond (Y10**30)

   As discussed in 2.4.1, the end of the universe is predicted to occur
   well before the year 10 ** 30.  However, if there is one single
   lesson to be learned from the current Y2K problems, it is that
   specifications and conventions have a way of out living their
   expected environment.  Therefore we feel it is imperative to
   completely solve the date representation problem once and for all.

3.4.1 Naive Approach for Y10**30 Problem

   The naive solution is to prepend a single '^' (caret) - caret sorts
   after all letters in the ASCII order) before all years from 10 ** 30
   to 10 ** 56.  Thus the year "Z999999999999999999999999999999" is
   followed by the year "^A1000000000000000000000000000000".  Similarly,
   all years from 10 ** 56 to 10 ** 82 get one more caret.  So, the year
   "^Z99999999999999999999999999999999999999999999999999999999" is
   followed by the year
   "^^A100000000000000000000000000000000000000000000000000000000".  This
   scheme can be extended indefinitely by prepending one addition caret
   for each additional factor of 10 ** 26 in the range of the year.

   In this approach, the number of digits in a date that are used to
   represent the year is simply:

      26 * <number of '^'> + ASCII(<prefix letter>) - ASCII('A') + 5

   Note: this algorithm is provided for informational purposes only and
   to show the path leading to the true solution.  Y10K dates MUST NOT
   use this format.  They MUST use the format in the next section.







Glassman, et. al.            Informational                      [Page 6]

RFC 2550                    Y10K and Beyond                 1 April 1999


3.4.2 Space Efficient Approach for Y10**30 Problem

   The solution in 3.4.1 is not a space efficient format for giving the
   number of digits in the year.  The length of the prefix grows
   linearly in the length of the year (which, itself, grows
   logarithmically over time).  Therefore, Y10K format dates use an
   improved, more compact encoding of the number of digits in the year.

3.4.2.1 Years 10 ** 30 to 10 ** 56

   As in 3.4.1, a single '^' and letter precede the year.

3.4.2.2 Years 10 ** 56 to 10 ** 732

   The year is preceded by two carets ("^^") and two letters.  The
   letters create a two digit, base 26 number which is the number of
   digits in the year minus 57.  So, the year
   "^Z99999999999999999999999999999999999999999999999999999999" is
   followed by the year
   "^^AA100000000000000000000000000000000000000000000000000000000".  The
   last representable year with two carets is the year (10 ** 732) - 1
   and is "^^ZZ999..999" (i.e. two carets and two Z's, followed by 732
   consecutive 9's).

   The formula for the number of digits in the year is, based on the two
   digit prefix is:

    26 * (ASCII(<prefix letter1>) - ASCII('A')) +
          ASCII(<prefix letter2>) - ASCII('A') + 57

3.4.2.3 Years 10 ** 732 to 10 ** 18308

   The next block of years has the number of digits given by three
   carets ("^^^") followed by three letters forming a three-digit, base
   26 number.  The number of digits in the year is given by the formula:

    676 * (ASCII(<prefix letter1>) - ASCII('A')) +
     26 * (ASCII(<prefix letter2>) - ASCII('A')) +
           ASCII(<prefix letter3>) - ASCII('A') + 733

3.4.2.4 General Format for Y10K Dates

   In general, if there is at least one letter in a Y10K year, the
   number of the digits in the year portion of the date is given by the
   formula:

       base26(fib(n) letters) + y10k(n)




Glassman, et. al.            Informational                      [Page 7]