Subject: Re: [arch-d] It's the routing, people!

From: "Fergie" <[email-masked]>
Date: Sun, 12 Nov 2006 18:27:46 GMT

-- Geoff Huston <[email-masked]> wrote:

At 01:40 PM 12/11/2006, Schliesser, Benson wrote:

>>
>>I assume Geoff can correct me on this, but my gut response to the 
>>15k number is that some visibility is probably missing. I.e. Those 
>>ASs may be multihomed, and the analysis tools just don't see the 
>>secondary path(s) to their prefixes.
>
>It depends on the nature of the multi-homed configuration. If the AS 
>is multi-homed for transit then the alternative transit providers get 
>exposed on withdrawl, in general.
>
>If the multi-homing is actually multi-peering, then you just can't 
>see this from a remote location - BGP won't expose this.
>

I know I'm probably going to hate myself for saying this, but
perhaps it is time for us to revisit the value of "selective"
methods for proxy aggregation? It's a sensitive topic, for sure,
which is why I highly emphasize the word: "selective"...

- ferg

--
"Fergie", a.k.a. Paul Ferguson
 Engineering Architecture for the Internet
 fergdawg(at)netzero.net
 ferg's tech blog: http://fergdawg.blogspot.com/

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---

From: Geoff Huston <[email-masked]>
Date: Mon, 13 Nov 2006 09:57:20 +1100

>which is why I highly emphasize the word: "selective"...

I hate my life too, so lets go there..

Its true that our various methods for attempting to limit the 
propagation of localised information have been an abject failure and 
around 50% of local information (more specifics) appears in the 
global table. Its not that this information should go away - in local 
contexts it serves a valid purpose and has a valid reason for 
existence. But thats no reason why this should flood globally.

I always wondered how you gain the knowledge that by absorbing the 
more specifics and not re-advertising them you are not just allowing 
the more specifics to take longer paths and still propagate anyway. 
But if I'm obsessing over the wrong thing then that's nothing new!

  Geoff

---

From: "Schliesser, Benson" <[email-masked]>
Date: Mon, 13 Nov 2006 19:21:43 -0600

> which is why I highly emphasize the word: "selective"...

The basic issue with proxy aggregation is that it breaks TE. (In turn,
possibly breaking connectivity.)  If we had some way of doing proxy
aggregation while maintaining TE functionality then I suppose we'd have
something useful, but as far as I can imagine that would require
awareness/information beyond the borders of the aggregating AS's
perspective.

Thinking about this inspires me to propose a requirement. It's possible
that everybody agrees and I'm simply stating the obvious, but let me be
explicit regardless: The routing system must support arbitrary
connectivity between transit networks.

For instance, one could envision a (poor) solution to the aggregation
problem that involves tunneling between the "primary" AS that assigns PA
space to the end network and each other AS it multi-homes to. This would
allow the global routing system to forward packets to the primary
network, which would be responsible for forwarding over tunnels to the
secondary networks during link failure etc. This constrains more
specific prefixes to just those networks that are directly supporting
the end customer. However this defeats one of the most important reasons
that a network multi-homes--survival during failure of the primary
provider's network. Thus each AS needs the ability to provide
connectivity to the end network independently of any other AS (to the
degree allowed by their peering/connectivity, of course).

To extend on this, one could possibly imagine yet other schemes that
allow for aggressive aggregation in the global system but which create a
reliance on the "Tier 1" crowd for routing and connectivity. (I.e.,
similar to the "tunnel" model above, but with a group of "core" networks
that everybody relies upon instead of a single "primary" network.) This
might have better survivability/resiliency properties, but it also
institutes an economic hierarchy that would ultimately be stifling and
dangerous (socially, economically, politically, in terms of long-term
security and stability of the network, etc).

The way to avoid these issues is to allow networks to connect directly
to each other, and have any connectivity graph / topology they might
desire. This is the tussle counter-balance...

Note that I'm talking specifically about transit networks here, which
excludes end networks (leaf networks) that don't want to provide
connectivity but merely want to receive connectivity services. (and
possibly don't need an ASN)

Is this an obvious requirement? Does anybody disagree? I have to admit,
it certainly makes things more difficult in any case. (Is it even
possible theoretically? I'm only slightly aware of Dima's/Claffy's
recent work, so I don't quite know its properties versus my above
proposed requirement. Perhaps somebody with a topology/graph theory
background can comment?)

Cheers,
-Benson

---

From: "Tony Li" <[email-masked]>
Date: Mon, 13 Nov 2006 21:51:04 -0800

> awareness/information beyond the borders of the aggregating AS's

The whole point of doing proxy aggregation is to create an aggregate
that is sufficiently remote from the originating AS so that TE is not
affected.  An aggregate should be injected at the same point that the
longer prefixes are dropped, so connectivity should not suffer.

A perfect example: why should Geoff, on his Wee Island Down Under (tm),
really need to carry all of the /24s for Europe?  Once all of the
components of an aggregate share the same next hop, then they can all be
replaced by the aggregate.

Tony

---

From: Jefsey_Morfin <[email-masked]>
Date: Tue, 14 Nov 2006 10:19:26 +0100

No.
The user should in a way keep control of the path of his data.
For obvious security reasons.
jfc

---

From: "Schliesser, Benson" <[email-masked]>
Date: Tue, 14 Nov 2006 08:26:39 -0600

> longer prefixes are dropped, so connectivity should not suffer.

In an ideal and well-engineered environment this is true. But as a
general rule I'm not so sure. For instance, when provider A announces
their PA space and provider B announces a proxy aggregate for the same
space, which path does network C follow? Assume that C is connected to
both A and B. Even if B maintains the more specific prefixes/paths so
that it can forward traffic to the end-network, network C might still
forward to provider A. If the connection between the end-network and
provider A is down, then the only way traffic arrives at the destination
is if A and B are exchanging more specific prefixes across their
peering. Proxy aggregation in this case is nice to ensure that
downstream "default-free" networks which filter on prefix length can
still reach all endpoints, but it doesn't help the "core" networks whom
still have to carry all routes.

Or did you have something else in mind that I'm not thinking about? If
we can find a way to work-around/fix what I'm talking about then maybe
we have a useful tool, so please point out if I've missed something.

Cheers,
-Benson

---

From: "Tony Li" <[email-masked]>
Date: Tue, 14 Nov 2006 09:00:55 -0800

> space, which path does network C follow? 

If A announces longer prefixes that C is hearing, C has no choice, he
must follow the longer prefixes.  If A is announcing an aggregate, then
B has no business announcing an aggregate too.

This is the problem: for proxy aggregation to be effective, all
providers at a certain radius must collaborate to proxy aggregate.
Otherwise, longer prefixes will pull traffic in non-intuitive
directions.

> Proxy aggregation in this case is nice to ensure that

In the general case, it can help contain some of the longer prefixes
used for TE.

Tony