Re: [RRG] Not moving the problem to the global mapping system

["William Herrin" <bill@herrin.us>]

On Thu, Apr 10, 2008 at 5:49 PM, Brian E Carpenter
<brian.e.carpenter@gmail.com> wrote:
>  I would suggest turning this into a concrete goal, such as:
>
>  <strawman>
>  The update rate in the mapping system should be at least
>  two orders of magnitude less than the update rate in
>  the BGP4 system, at any point in time.
>  </strawman>

Brian,

Depending on the character of the map system, this is either
absolutely critical or blithering nonsense.

Before pushing similar concrete goals, I'd ask you glance through TRRP
for an example of a system in which constraints on the system-wide map
update rate appear unwarranted.

TRRP isn't a push-based system. That difference may prove significant
when considering permissible map update rates and that significance
shouldn't be overlooked by the specified goals.


Anyway, while I think the idea of a map system that doesn't convey
reachability information is interesting at a conceptual level, I don't
see how you can implement it without making the system at least
hybrid-pull.

Just because a particular service provider (BGP AS) can usually reach
a particular endpoint doesn't mean they always can. That holds true
regardless of whether the service provider itself is reachable. And as
we see every time Cogent gets in a spat with someone, just because you
can reach either of two ISPs doesn't necessarily mean they can reach
each other.

If you don't convey reachability information in the map, then the ITR
will have to ask its selected ETR whether a particular endpoint is
currently reachable and it will have to keep verifying that
reachability throughout the session. If you're going to do all that,
why not use a pull system from the beginning and gain all of its
advantages?

Remember, you can't rely on the ETR to tell the ITR that a destination
is no longer reachable. There are common, trivial cases in which that
will fail as discussed here:
http://bill.herrin.us/network/statechange.html

Regards,
Bill Herrin


-- 
William D. Herrin ................ herrin@dirtside.com bill@herrin.us
3005 Crane Dr. ...................... Web: <http://bill.herrin.us/>
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