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Skriven 2010-03-13 10:34:42 av Michiel van der Vlist (2:280/5555)
Ärende: FSP-1029
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Hello All,
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FTSC FIDONET TECHNICAL STANDARDS COMMITTEE
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Publication: FSP-1029
Revision: 2 (draft 3)
Title: Serial number allocation in Mesage IDs.
Author(s): Michiel van der Vlist
Date: 25 February 2010
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Contents: 0. Status of this document.
1. Introduction.
2. The problem.
3. The solution.
4. The standard.
4.1 The serial number file.
4.2 Name and path of the serial number file.
4.3 Issuing numbers using the serial number file.
4.4 Upon successful open and lock.
4.5 Upon failure to open the serial number file.
4.6 Upon failure to obtain a lock.
4.7 When the serial number file is corrupted.
4.8 A note on "increasing by at least one".
5. References.
6. Contact Info.
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0. Status of this document
--------------------------
This document is a Fidonet Standards Proposal (FSP).
This document proposes a Fidonet standard for the Fidonet
community.
This document is released to the public domain, and may be used,
copied or modified for any purpose whatever.
1. Introduction
---------------
FTS-0009 mandates that serial numbers used in FTN Message IDs be
unique on a given system within a period of three years. FTS-0009
does not mandate any particular way to achieve this goal. It does
not even offer suggestions, It leaves it entirely to the
implementation.
This has led to software developers using a variety of methods,
using the system clock is the most popular, but random number
generators and checksums over the message header and/or body are
also used.
2. The Problem
--------------
The problem is that none of the above mentioned methods is really
satisfactory as it can not guarantee that no duplicate message IDs
will be issued. Issuing duplicate message ID's is undesirable as
echomail processors and netmail trackers may discard or quarantine
messages as dupes.
Using the system clock was good enough when computers were slow and
single tasking. With computers getting faster, only issuing a new
serial number every second however is too coarse. Fast systems can
create tens if not hundreds of messages a second. Using a finer
scale, i.e. hundreds of a second or even thousands only shifts the
problem to the future. Some day we shall see systems that can
spew out more than one message in a millisecond.
Multitasking systems have worsened the problem. Not only can one
application give out the same number twice, Another application
running in another task may give out that same number as well!
Using random number generators and/or checksums is simply wrong.
There is just no guarantee no duplicates will occur. It is roulette,
Some day the same number will come up again.
3. The Solution
---------------
The obvious solution is to use some sort of "sequencer" that
is shared by all applications running on a given system that
issue message ID's. The idea is simple, it is the same as
drawing a "ticket" from the ticket dispenser in the Post Office.
No two tickets in the machine have the same number and only
one customer can draw a ticket at the time. If the machine is busy
dispensing a ticket to a customer, the next customer has to wait.
This way it is ensured that no two customers get the same number.
There are many ways to implement the equivalent of such a system
for FTN serial numbers, but it will only work properly if all
applications use the same method. Hence the need for a standard.
4. The standard
---------------
4.1 The serial number file.
The serial number file is a plain text file who's first
line is exactly eight characters long. It holds an eight
digit hexadicimal number, representing a 32 bit unsigned
integer. Leading zeros may not be omitted.
It is recommended that applications write this number in
lower case. When reading it must be treated as case
insensitive.
The eight character line may optionally be terminated by a
line terminator. The file may contain additional lines
containing additional information, thus making it possible
to use this file for supplementary future standards. Appli-
cations issuing serial numbers according to this proposed
standard shall only consider the first eight bytes of the
serial number file.
4.2 Name, extension and path of the serial number file.
The default name of the serial number file shall be MSGIDNR.NXT.
It is recommended that the name be configurable.
The path pointing to the file MUST be configurable. For ease
of configuration it is strongly recommended that the
implementation supports reading the path from the environment
variable SERNRPATH.
4.3 Issuing serial numbers using the serial number file.
An application needing to issue a serial number for a message
ID shall attempt to open the serial number file and lock the
file. Or alternatively, if the OS permits partial locking, lock
part of it with a minimum of the first 8 bytes.
4.4 Upon successful open and lock.
On a successful lock it shall read the 32 bit hexadecimal
number from the file and use that as the next serial number
to be issued.
After having issued this number it shall increase the serial
number by at least one and use that for the next serial
number. When the programme is done issuing serial numbers,
it shall write the last issued serial number increased by at
least one to the serial number file, release the lock and
close the file.
Alternatively if more than one serial number is to be issued
in one run, the programme may write the next to issue serial
number to the file, unlock and close it; and open, lock and
read it again between issuing serial numbers so that other
applications can obtain a serial number in the meantime.
4.5 Upon failure to open the serial number file.
Upon failure to open the file because it does not exist, the
application shall attempt to create it. What method is used
to "seed" the serial number generator is up to the implemen-
tation. The system clock might be a good suggestion, but this
document shall not attempt to mandate any particular method.
4.6 Upon failure to obtain a lock.
Failure to obtain a lock normally means another application
has locked the file. The logical course of action would be to
wait for the lock to clear and retry. Limiting the time to a
maximum before aborting and/or supplying a user abort option
are at the discretion of the implementation.
Another reason for failure to obtain a lock can be that the
application is running on a single tasking system that does
not support file sharing and locking. (F.e. plain DOS, SHARE
not loaded.) If the application is aware of being run in a
single tasking environment, either by autodetect or by
a configuration switch, it can of course skip the locking
and just read and update the sequence number file.
4.7 When the serial number file is corrupted.
When the serial number file contains characters other than
0-9, A-Z or a-z in the first eight positions, this is an error
condition. One possible course of action would be to re-
initialize the serial number generator. Another would be to
abort or prompt the sysop for intervention.
All methods have pros and cons, so this proposal does not
impose any specific course of action to resolve this error and
leaves it to the implementation.
4.8 A note on "increasing by at least one".
Simply starting at zero and increasing the serial number by
one for each number issued would be perfectly valid and
satisfy FTS-0009. That is, provided all applications running
on a given system were to use this method, use the same serial
number file and the serial number file is never lost.
In practise these ideal condition may not be met so using a
more sophisticated method of increasing the serial number than
just increasing it by one may be preferred. Using the system
clock to prime the number generator in case the serial number
file is absent and to use the larger value of the system clock
and the last number incremented by one for the next number may
give reasonable insurance against the loss of the serial
number file.
5. References
-------------
[FTS-0009] A standard for unique message identifiers and reply chain
linkage.
Jim Nutt, 17 Dec 1991.
[FSC-0083] A proposed standard for message IDs on FTN systems.
Jonathan de Boyne Pollard. 17 June 1995.
6. Contact Data
---------------
Michiel van der Vlist
Fidonet: 2:280/5555
E-mail: pa0mmv at vrza dot org
E-mail: administrator at ftsc dot org
History
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Rev.1, 20040206: Initial Release.
Principal author Michiel van der Vlist.
Rev.2, 20100225 Par. 4.1: 1st paragraph changed for more clearity.
Made some small textual changes.
Added section about corrupted sequence number file
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Cheers, Michiel
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