Re: [RRG] How to Incrementally Deploy APT

[Scott Brim <swb@employees.org>]

On 2/21/08 10:52 PM, Dan Jen allegedly wrote:
> The only new task for an edge network is to provide
> traffic engineering information to its providers.

How is this done?

> Second,
> inside an APT island, the APT networks exchange their mapping
> information with each other. This allows their default mappers to
> maintain mapping information for the entire island.  Let's call this
> table the Island Mapping Table.

I'm a little concerned about three things.

 - Scaling.  If I understand correctly, we will have inter-domain
   routing carrying all of the fine-grain intra-domain changes, e.g.
   ETR up/down.  In the past we have been concerned about fault
   isolation, which is one reason we still have strong domain
   boundaries: so that errors in one domain do not "poison" a
   neighboring domain.

 - Cooperation.  This seems to require providers to work together for
   the common good.  That's not something we can easily guarantee
   :-).  Do you have business motivators that would encourage them to
   share ops data?

 - Policy.  I believe it's possible to say that traffic to one
   sub-prefix of a site should come in to a site one way, and traffic
   to another sub-prefix should come in a different way.  Is is
   possible, using APT, to say that traffic *from* some remote site
   should come in to my site one way, while traffic from some other
   remote site should come in via some other set of CEs?

> As APT islands grow larger and merge with each
> other, this BGP table can be gradually reduced and eventually
> contain only transit prefixes.  Smaller BGP table provides an
> incentive for transit networks to deploy APT.

First, smaller BGP tables are an incentive only if the total difficulty 
of running both (small) BGP and the APT protocol(s) is less than the 
difficulty of running BGP right now.  I suspect that it will be years 
before everyone converts to a new routing architecture, therefore we 
can't talk about "transition" so much as "interworking" and 
"coexistence".  How much incremental difference do you believe there
is between an APT entry and a BGP entry?  What percent of a site's
routing needs to be converted to APT in order for BGP+APT to be less
complex to operate than BGP alone?

Getting a smaller BGP table is certainly an incentive for a domain to
get its *neighbors* to convert. :-)

swb


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