RE: Provider Independent addressing usage

[Pekka Savola <pekkas@netcore.fi>]

On Fri, 13 Jul 2001, Tony Hain wrote:
> > Problems with the PI approach:
> >
> >  1) people don't seem to want too specific routes in the DFZ.
> > With current policies, I'd estimate 100-200 routes is the
> > maximum. (though the technical constraints are a couple of
> > orders of magnitude larger). Specifying these with prefix length
> > may give the wrong impression, as the space would be rather
> > densely populated (minus the oceans, sahara etc.)
>
> I agree that removing specifics from the DFZ is the goal, but punching holes
> in provider aggregates is the current reality and that mechanism is directly
> contrary to the goal. I don't understand your point about prefix length.
> What I was trying to say in the draft was that each of the regions outside
> of the current scope would be represented by single prefix. The population
> density of these remote regions has no impact on routing in the local
> region.

Punching holes is the current reality with _IPv4_.  In IPv6 world (at
least 6bone), too specific routes are filtered by almost everybody; so
this would not work.

> >  2) because of non-specific advertisements, operators would be
> > forced to carry a lot of traffic not their own, or throw it
> > out.  Neither is acceptable, economically or from user's
> > point-of-view.
>
> Simply FUD. There may not be a customer relationship between an intermediate
> operator and the destination, but the traffic was originated via a customer
> relationship with the source, and there is some operator with a relationship
> to the destination. Inter-provider agreements are what they are, and they
> have little relationship to end customers. When a site is connected to
> multiple operators each of them become identifiable entities to the
> intermediate operator, so there is a potential for customer relationships
> between providers. If end-sites are directly connecting to exchanges, the
> exchange becomes the logical customer of the intermediate operators and
> passes through the costs to the connecting customer.

FUD how?  I don't see flaw in my point _if_ there are no significant
exchanges in the regions in question.  If there are a lot of exchanges,
ie. "neutral ground", this is not a problem.

Consider:

Assume source is in e.g. Europe and the destination in someone from a
different provider in a backwater city in the U.S.  Assume there is no
regional exchange close by.

There are a few issues here:
1) which out-of-the-country provider is selected
2) which intercontinental carrier is selected
3) where the intercontinental carrier drops the traffic
4) where someone else picks it up

1) no problem and no matter; the customer has default routes and direct
relation with his ISP's.

2) all EU-US carriers probably want to advertise reachability to all US
destinations, I'd suspect.  So, one of these is picked.

3) The carrier does not know what to actually do about the packet yet, but
let's assume it just pushes it toward a PoP nearest to the region.

If there is no IX in the region, the carrier can either dump the packet
(if it doesn't know a more specific route), or throw it at some operator
operating in the region.

4) Some operator operating in the region _might_ get a packet, but it
can't know yet whether the destination in the area is reachable or not.
So it could take the packet, muck around with it, perhaps route it closer
to the destination (geographically) and "throw the hot potato to the next
guy".

As said, these problems are "fixed" by:
 1) lots and lots of IX's (up to the level where having full /48's is
acceptable), or
 2) full /48 routes in all ISP's, carriers etc. which makes the entire
routing table size consideration moot

> > 3) the only ones that could really gain from advertisements
> > are regional/area/etc. IX's that have a very high level of
> > regional ISP penetration; e.g. if address belongs to the
> > region, it'd be reachable through the members of an IX with
> > over 99.9% probability.
>
> The PI addresses that actually have an attached site will always be
> reachable through some set of the ISP's in the region. It appears that you
> are concerned that some of the ISP's in the region are not participants in
> an exchange. This is not required as long as one of the providers that are
> participating is willing to tie it all together. The only mechanism to
> enforce this is for the inter-region providers to refuse more specifics, or
> charge exorbitant fees.

What I'm worried about is situation where SiteA is customer of ISPA, and
ISPA does not participate in exchange in SiteA's region, but does
participate in some other exchanges.  Assume SiteB is in the same region
as SiteA, using ISPB which does participate in the regional exchange.  Now
the exchange advertises the reachability of regional prefix, but SiteA
will not be reachable.  ISPB and ISPA might not be willing to "tie it
together" if there is no customer relationship between them (ie, not ISP -
Carrier).

I think one of the goals is that inter-region providers, or "upper layers"
in general, don't have to know all the specific routes.

> > 4) ISP's operating at the IX would have to advertise, among
> > themselves, full /48 routes.  In some regions, where there
> > are dozens of millions of Internet users, this is probably
> > not an acceptable solution either.  So the problem goes back
> > to 2), but in smaller scale where there are often _no_ IX's
> > to exchange the traffic.
>
> Lack of an exchange in the current topology is not relevant to what might be
> necessary to scale millions of multi-homed sites in a specific scope. Look
> back a few years and you would find only a few exchanges globally.
> Additional exchanges were built as the engineering trade-offs showed value
> in establishing an exchange at a particular place. The design of this PI
> mechanism allows exchanges to be created at the scopes that make sense for
> local engineering reasons, without impact about on any other scopes.

Perhaps the practical requirement for IX's should be noted more clearly in
the draft.

> > So, it's often the case, e.g. in Europe, that a country has
> > about one IX. The PI solution would work if the prefix(es)
> > of the region belonging to that country were advertised to
> > by that IX, and _everyone's_ (in that country) /48 PI prefix
> > be advertised within that IX (whether it's directly associated,
> > or a smaller client ISP of the peering bigger ISP).
>
> Forget country boundaries, and see the previous note about when and where
> exchanges get built. Certainly the scopes relevant to an existing IX should
> have their prefixes exchanged by the participants in the IX, or a route
> server at the IX coordinated the actual prefix/provider relationships.

Countries often have an IX, and as international connections often require
paying for carrier service or getting possibly rather expensive
connections, it's often that regional boundaries are actually
de-facto country boundaries.

Bigger IX's (probably most of the ones we regard as real exchanges now)
are most often "international".

> > If this was not done, some regional ISP's would have to capture
> > and carry some traffic they have no idea if they can deliver or > dumped.
> Or, the traffic could be sent to some smaller-than-
> > regional IX's which by definition didn't exist.
>
> I don't understand. If there is a site at the defined prefix there will be
> an explicit entry known to the providers for that scope. If there is no site
> there may be connection attempts, but that is no different from unassigned
> provider-based prefix addresses.

See the concern above on ISPA offering service in an area, but not
necessarily peering, or participating in an exchange (if one exists) in
that particular area (but perhaps knowing ISPB from some other areas).

> > Take the netcraft web server count by domain for example
> > [http://www.netcraft.com/Survey/Reports/0106/bydomain/];
> > I think it's fair to assume that each /48 site would have at
> > least one if not more web servers.  United Kingdom appears to
> > have about 2.8 million, so say 3 million /48's.  Even if there
> > were 10 IX's in the UK (distributed nicely by geographical
> > areas, no less), this would still be a whole lot of specific
> > /48 routes between participating ISP's.
>
> Yes; if all sites were multi-homed and the providers chose to implement it
> that way. When they become concerned about scale, history shows they will
> implement more IX's to break up the space.

If PI addressing were to be adopted at large, there would have to be more
IX's.  Someone has to administer, and pay for them.  This might become an
important point to consider.

-- 
Pekka Savola                 "Tell me of difficulties surmounted,
Netcore Oy                   not those you stumble over and fall"
Systems. Networks. Security.  -- Robert Jordan: A Crown of Swords