On Fri, 12 Mar 2010, Tony Li wrote:
Are you referring to Geoff's recent APRICOT talk on "BGP in 2009"?
Yes.
If so, that would seem to indicate convergence times for single
prefixes are relatively stable and seem to be proportional to AS
path length.
Maybe. Though I also got the following from his presentation:
- the diameter of the internet is fairly stable (not unexpected,
given we know the internet topology tends to be scale-free in
distribution)
- despite the continued growth in the number of the ASes, the
convergence time has been fairly stable too
This latter point is very interesting as, if I'm understanding
Geoff's results and the previous academic work on BGP convergence
correctly (Labovitz, Griffin), it means that the common-case
convergence of BGP+internet topology scales a lot more nicely than
the (worst-case) scaling predicted by academic work.
I could be completely misunderstanding stuff maybe, and I guess we'd
also need more data beyond Geoff's too, however Geoff's presentation
seemed to say "the BGP sky isn't falling anywhere like we thought it
should" and based on his charts I'd have to agree.
It is not terribly surprising that these are constant, as a
significant fraction of changes are due to tail circuit changes
where only a small number of prefixes are actually going to shift
paths.
Since BGP convergence at a single node is going to take
time that is linear in the number of affected prefixes, and only a
few prefixes are changing, there would seem to be nearly a constant
amount of work to do.
Well, the per-prefix work ought to be increasing as the number of
prefixes advertised were increasing, surely? Unless somehow the
growth in new prefixes is not distributed evenly - i.e. all the new
prefixes are in the core, while the failures are mostly at the edges
(tail circuits).
However, I wonder if Geoff is measuring per-UPDATE alone and not
breaking out UPDATE activity out by the number of NLRIs? That would
be consistent with what you say.
Whatever, his findings are that there isn't any evidence of a BGP
meltdown, surely? To the contrary, internet routing is doing
surprisingly well.
That same talk reinforces the fact that we're still on a quadratic
growth curve, for both v4 and v6. ;-(
I can't remember where I saw it (another one of Geoff's lovely
graph-filled presentations maybe?? :) ) but didn't someone do a
comparison of growth in memory/CPU resources versus growth in DFZ
prefixes? Iirc they found RAM/compute resources were increasing at a
sufficient rate to keep ahead of DFZ growth.
I.e. the exponential Moore's law growth in transistors-per-device
beats the quadratic DFZ growth.
The growth in prefixes is simply to be expected, obviously - to some
extent its natural and good.
What I didn't see in there was any discussion of the convergence
time when there is a point failure in the network, such as when a
router reboots. These are the cases where the BGP convergence time
is going to be linear in the number of prefixes.
I think what Geoff was suggesting is that it's linear in the diameter
of the internet, which is a lot better than the "linear in the size
of the internet topology" O(|V|+|E|) worst-scaling from the
literature - particularly when the internet topology grows in such a
way as to keep diameter-growth very limited.
Or, put another way, Geoff's work suggests that routers in the
middle of the net aren't rebooting frequently. Whew. ;-)
Maybe :)
FWIW, I'm not claiming that BGP scalability should not be improved.
However I don't see evidence for any terrible scaling problem that
must urgently be solved. Without evidence of such a problem, I would
argue we should be careful about picking too extreme solutions
(extreme in the disruptive sense).
I.e., in trying to find a way to insulate the core of the internet
from growth at the edges we should be careful that we do not rush
towards a solution that is less good than the null-solution (e.g.
because it has a mapping system that adds latency and/or
instability).
It's possible that the current architecture of the internet is
holding back growth that should otherwise be there. I guess we could
test for that by comparing growth today to growth in earlier days. If
it's significantly slowed down then that must be the case. I havn't
checked, but my vague memory of the address-space reports is that
this is not the case and that the internet is growing normally (how
this will change as/when v4 runs out and v6 is the only space that
allows growth will be interesting to see).
In short, I havn't seen compelling evidence to suggest that the
current routing architecture can not cope with the historical and
current modes of growth.
Anyway, I could be completely under- and/or mis- informed, in which
case apologies in advance :).
regards,
--
Paul Jakma p...@jakma.org Key ID: 64A2FF6A
Fortune:
According to the latest official figures, 43% of all statistics are
totally worthless.
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