Subject: Re: [rrg] RG futures

From: [email-masked] (Noel Chiappa)
Date: Sat, 8 May 2010 17:49:42 -0400 (EDT)

> From: Paul Jakma <[email-masked]>

    > Note that you can organise routing around AS numbers and so reduce
    > routing state requirements perhaps without having to change IP
    > forwarding. I.e. you can compute paths on an AS-based topology, and
    > then compute IP routes based on which ASes terminate which prefixes. As
    > the association of IP prefix:AS is a lot more static than the topology,
    > you could reduce the amount of information that had to be passed around.

Not a bad idea, but... First, as some people have pointed out, there are
things like 'traffic engineering being done with injection of more-specifics'
to deal with. Second, fundamental characteristics of the current routing
architecture mean that deploying something like this is going to be a really
hard.

That's because the current path selection is, for better or worse, a giant
distributed computation, in which intermediate results are passed from node
to node. I.e. nobody computes the entire path; a each node does a bit, and
relies on the rest of the network to do the rest. That 'pass intermediate
results around' means that you can't really 'evolve' it to use a different
kind of data in any simple way, e.g. by a process which involves some nodes
using one type of data (addresses), and others using a different type (AS
numbers).

Even assuming you have a contiguous area of the networking using one type, and
the rest using the other, at the border between the two you have to
'translate' from one kind of data to the other. And if you have 'islands' of
stuff it gets really horrible: you have to 'translate' from one kind of data
to the other going in, and then as you exit the island, you have to translate
back.

	Noel

---

From: Paul Jakma <[email-masked]>
Date: Sat, 8 May 2010 23:35:51 +0100 (BST)

On Sat, 8 May 2010, Noel Chiappa wrote:

> Not a bad idea, but... First, as some people have pointed out, 
> there are things like 'traffic engineering being done with 
> injection of more-specifics' to deal with.

More specifics, in the sense of discontinuous ASes, are easy to deal 
with. You allow for ASes to have sub-entitities, e.g. AS X, X:1, X:2, 
etc.. where the :Y part can be defined at will by the AS X 
administrator. This is of course only worth doing if these 
"topologically continuous" entities tend to advertise more than 1 
prefix.

My memory of ISO CLNx and IS-IS is very vague, but I think it's got 
some kind of similar mechanism.

I think that's one of the easiest problems though, if trying to go 
down this path.

regards,
-- 
Paul Jakma	[email-masked]	Key ID: 64A2FF6A
Fortune:
The people rule.