ASN draft

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Yakov asked for my concerns.... here goes:


                                                            January 1995

           Guidelines for creation, selection, and registration
                       of an Autonomous System (AS)


   The definition of AS has been unclear and ambiguous for some
   time.  [BGP-4] states:


        The classic definition of an Autonomous System is a set of
        routers under a single technical administration, using an inte-
        rior gateway protocol and common metrics to route packets within
        the AS, and using an exterior gateway protocol to route packets
        to other ASes.  Since this classic definition was developed, it
        has become common for a single AS to use several interior gate-
        way 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 ASes to have a single
        coherent interior routing plan and presents a consistent picture
        of what networks are reachable through it.

   To rephrase succinctly:

        An AS is a connected group of IP networks run by one or more
        network operators which has a SINGLE and CLEARLY DEFINED routing
        policy.

this rephrasing (and the rest of this document) discount the concept
of "a single technical administration" as a basic criteria for ASN
assignment and focus on the Exterior routing aspects. 
Witness the immediatly following paragraph....

   Routing policy here is defined as how routing decisions are made in
   the Internet today.  It is the exchange of routing information
   between ASes that is subject to routing policies. 

.... Skipping  a few pages

   Policies are not configured for each prefix separately but for groups
   of prefixes.  These groups of prefixes are ASes.

Blanket Assumption here.   Even given the previously rephrased definition,
I can have a perfectly leagl entry like this:  ASN:2886 146.1.1.1/32
One prefix (host route too!), one ASN.

   An AS has a globally unique number (sometimes referred to as an ASN,
   or Autonomous System Number) associated with it; this number is used
   in both the exchange of exterior routing information (between neigh-
   boring ASes), and as an identifier of the AS itself.

So ASN's have two uses here... Interesting.

   In routing terms, an AS will normally use one or more interior gate-
   way protocols (IGPs) when exchanging reachability information within
   its own AS. See ``IGP Issues''.

And one assumes (I know, a bad thing to do) that the BGP definition  is
put into play here. But wait, can't an AS have different policies internal
to itself?  You bet.  But as they are under a common administrative bound,
that is up to them how things work inside the AS.  It's only when they
cross the trust/administrative bound that things get dicey.


4. Common errors in allocating ASes

   The term AS is often confused or even misused as a convenient way of
   grouping together a set of prefixes which belong under the same
   administrative umbrella, even if within that group of prefixes there
   are various different routing policies. Without exception, an AS must
   have only one routing policy.

This seems to fly in the face of the BGP definition (and the one to which
I subscribe). ASNs have more uses than just routing. They are the way to
define administrative/political/trust bounds.

   of having an AS is to be avoided, as is the worst-case scenario of
   one AS per classful network (the IDEAL situation is to have one pre-

Nope, Worst Case is one ASN per prefix. (can you say host route? sure you can)
This is also the best case, especially with a small magnitude mask. To use
the previous example:  ASN 2886 146.1.1/8..               



   *    Exchange of external routing information

        An AS must be used for exchanging external routing information
        with other ASes through an exterior routing protocol. The
        current recommended exterior routing protocol is BGP, the Border
        Gateway Protocol. However, the exchange of external routing
        information alone does not constitute the need for an AS. See
        ``Sample Cases'' below.

Uh no, an ASN must be used, not an AS.

   *    Single-homed site, single prefix

        A separate AS is not needed; the prefix should be placed in an
        AS of the provider. The site's prefix has exactly the same rout-
        ing policy as the other customers of the site's service pro-
        vider, and there is no need to make any distinction in routing
        information.

Hogwash. A seperate AS is needed and to demarc the administrative split
an ASN should be assigned.  Does it need to be used for routing?  Probably.
It could be argued that from the -exterior- they both share the same 
policy.  I debate that statement at the peering point between the providers.
They do not share the same trust model and the normal model is to demarc 
each as an AS. One assumes that an AS can get an ASN as demand dictates.
The exception cited below is a lot more common than the authors think.

        In some situations, a single site, or piece of a site, may find
        it necessary to have a policy different from that of its pro-
        vider, or the rest of the site. In such an instance, a separate
        AS must be created for the affected prefixes. This situation is
        rare and should almost never happen. Very few stub sites require
        different routing policies than their parents. Because the AS is
        the unit of policy, however, this sometimes occurs.

   *    Topolgy

        Routing policy decisions such as geography, AUP (Acceptable Use
        Policy) compliance and network topology can influence decisions
        of AS creation. However, all too often these are done without
        consideration of whether or not an AS is needed in terms of
        adding additional information for routing policy decisions by
        the rest of the Internet. Careful consideration should be taken
        when basing AS creation on these type of criteria.

One more case of excessive emphasis on routing as the only basis for
AS creation and ASN assignment.  

   *    History

        AS number application forms have historically made no reference
        to routing policy. All too often ASes have been created purely

I wonder why this was/is?  Could it be that AS'es and ASN's do more
than just twist routing?  Could be.... :)

	because it was seen as ``part of the process'' of connecting to
        the Internet. The document should act as reference to future
        application forms to show clearly when an AS is needed.

......

   A prefix can and must belong to only one AS. This is a direct conse-
How about:
	A prefix can and must have a single orign AS.

   With the introduction of aggregation it should be noted that an AS
                                                                a prefix 

   can occasionally be represented as residing in more than one AS, how-
   ever, this is very much the exception rather than the rule. This hap-



-- 
--bill