No one behind the wheel at WorldCom

Tue Jul 16 06:44:29 UTC 2002

msa at samurai.sfo.dead-dog.com (Majdi S. Abbas) writes:

 >	Actually, I think you'll find that bad data is only a small part
 > of the problem; even with good data, there isn't enough support from
 > various router vendors to make it worthwhile; it's effectively impossible
 > to prefix filter a large peer due to router software restrictions.  We
 > need support for very large (256k+ to be safe) prefix filters, and the
 > routing process performance to actually handle a prefix list this large,
 > and not just one list, but many.
 >
 > 	IRR support for automagically building these prefix lists would
 > be a real plus too.  Building and then pushing out filters on another
 > machine can be quite time consuming, especially for a large network.
 >

 From a point of view of routing software the major challenge of
handling a 256k prefix list is not actually applying it to the
received prefixes. The most popular BGP implementations all, to my
knowledge, have prefix filtering algorithms that are O(log2(N)) and
which probably scale ok... while it would be not very hard to make
this a O(4) algorithm that is probably not the issue.

Implementations do always have to do a O(log2(N)) lookup on the
routing table with a received prefix, and to afaik that is not
a performance problem for anyone.

What all implementations that i'm familiar with do have a problem with
is to actually accept the configuration of 256k lines of text to use
as a filter. Configuration parsing is typically not designed for such
numbers... it tends to work with major vendors albeith a bit slowly.

If the above disagrees with your experience please let me know.

Assuming that the bottleneck is in fact being able to parse
configuration, it begs the question what to do about it...

I'm sure all vendors will be able to, given enought incentive,
optimize their text parsing code in order to do this faster... but it
begs the question, would you actually fix anything by doing that ?

My inclination would be to think that you would just tend to move the
bottleneck to the backend systems managing the configuration of such
lists, if it isn't there already, presently.

Of course i'm completly ignorant of the backends that most of you use
to manage your systems and the above is just uneducated guessing,
although i would apreciate further education.

I would be inclined to agree with your statement that the major blame
should lie on "router vendors" if you see your router vendor as
someone that sells you the network elements + the NMS to manage it.

But in my guestimate the focal point of our search for a culprit
should be the NMS or the NMS -> router management mechanism. Idealy
the latter should be more computer friendly than text parsing.

Just an attempt to equally and democratically distribute blame around :-)

regards,
   Pedro.