Autonomni system

[Martin Mares]

Hello, world!

> parkrat jsem v ruznych konferencich zahledl neco o Autonomnim Systemu a
> nikdo zatim nevysvetlil k co to je a k cemu to je. Najde se nekdo tady?
> Vetsinou se to objevuje v souvislosti se chtenim pripojeni do dvou siti
> zaroven prez modem a vetsinou s poznamkou, ze je to drahe, slozite,
> rozlozite ...

Definice AS (viz RFC 1722):

   The classic definition of an Autonomous System is a set of routers
   under a single technical administration, using an interior gateway
   protocol and common metrics to route packets within the AS and using
   an exterior gateway protocol to route packets to other AS's. Since
   this classic definition was developed, it has become common for a
   single AS to use several interior gateway protocols and sometimes
   several sets of metrics within an AS. The use of the term Autonomous
   System here stresses the fact that, even when multiple IGPs and
   metrics are used, the administration of an AS appears to other AS's
   to have a single coherent interior routing plan and presents a
   consistent picture of which destinations are reachable through it.

Z pohledu exterior-gateway protokolu (tedy tech, kterymi je routovana
"paterni cast" Internetu; typicky BGP) je cely Internet slozen z navzajem
propojenych AS, ktere si mezi sebou vymenuji informace o dostupnosti
jednotlivych siti spolu s tim, jakou cestou (i.e., pres jakou posloupnost
cisel AS) je dana sit pripojena. Kazdy AS ma svoji vlastni jednoznacnou
routovaci politiku (tedy kudy posila ktery packet ven) => potrebujete-li
provozovat BGP, abyste zjistil, kterym smerem (pres ktery AS) je vyhodnejsi
dany packet poslat (na rozdil od klasickeho reseni pres default route
do jednoho AS), musite mit vlastni AS.

   Cisla AS prideluje IANA, pripadne lokalni registracni urad typu RIPE.
Typicky se prideluji providerum a specialnim sitovym uzlum (jako je napr.
cesky NIX). Tez nektere organizace s komplikovanou strukturou interni
site (typicky vice vnejsich pripojeni v ruznych castech sveta apod.)
maji vlastni AS. Tech cisel je ovsem docela malo (65536), takze je
prideluji pomerne neradi.

   Summary: Pro beznou organizaci je daleko vyhodnejsi pripojit se
k providerovi, ktery ma slusnou jak lokalni, tak svetovou konektivitu.
Mimo jine take proto, ze zvladnout zpracovavat kompletni routovaci
tabulky do celeho sveta vyzaduje dosti vykonny router s alespon 32MB
RAM (pro pripad dvou az trech vnejsich pripojeni).

								Martin