BigISP<-->SmallISP peerings

all the Real Big ISPs use a technique of routing called 'closest exit'.

They do, and the effect is that they then wash their hands of the
congestion problems that occur.

Congestion where? Closest Exit routing has the effect of getting the packets
onto the destination's providers backbone as soon as possible; where the hops
from backbone to backbone take place is all that it changes, not the number
of hops there are. Which means that with the current dialup customer's typical
small-outbound-big-inbound traffic patters, it is the dialup customer's provider
(or their provider's provider perhaps) whos backbone is the most congested.

Furthest exit would give a provider
the most control over quality of service but would be expensive*.
* - but many might be willing to pay for it

I disagree; if everyone were to do 'furthest exit' (in which you transit
outgoing packets across your backbone to the point closest to the
destination and then pass them off to the destination's provider's
backbone) , routing would still exhibit the asymmetry (and therefore
would have the exact same amount of 'control over QoS) it does today. In
fact, I believe that the loading would be the same as we have now, but
with the labels 'Inbound Load' and 'Outbound Load' reversed. And it's
only expensive because everyone is doing closest exit, and so the 'odd
man out' trying to do 'closest-exit-to-destination' loses because he
then bears the cost of transporting not only his incoming load across
his backbone, but his outgoing load as well, when everyone else is
mostly bearing the cost of just their incoming load. So in the current
internet, yes, if an ISP was to do 'shortest external path' routing,
their backbone would carry both their inbound and outbound traffic
loads, and hence it its possible that they would have more control over
the quality of service, as you pointed out. But let me point out that
that situation only lasts as long as everyone else is doing closest
exit routing; when they switch to 'shortest external path' then you no
longer carry the brunt of the load of your inbound traffic across your

Best exit (perhaps closest unless there is congestion, then furthest)
doesn't seem possible with current routing technology.

What *is* best exit? It's unclear to me which one is better; they're flip
sides of the same coin.


With all the recent talk about BGP, etc., I thought I'd see if anybody
knows the reasoning behind a particular short-coming of BGP that I've
noticed and found particularly bothersome...

We peer, using BGP, with several "backbone" provider networks for transit
purposes. Some of these links are "faster" than others (e.g. T-3 vs.
multiple T-1 and single T-1) for various reasons. If our router sees
a route to a particular destination via a "high-speed" link and a "low-
speed" link that has the _same_ number of AS "hops", it picks the link
with the "lowest" IP address! (At least that's what I'm told and what
I observe...)

Why doesn't BGP pick the link with the highest bandwidth, or, better
yet, pick the link with the highest bandwidth AND least congestion to
label as the "best" available route? The needed information is avail-
able in the router (and if it was somebody doing BGP from a host that
was separate from the box with the interfaces, well, then too bad I
guess) and can't be _that_ hard to incorporate can it?

I'll get off my soapbox now...


Can't you adjust your metrics/weights to prefer the low speed links less?


Well, sure, but why should I _have_ to? I thought we, in part, pay
the big bucks for routers that are supposed to figure some of this
stuff out on their own without having to "band-aid" things with AS
path manipulations, etc.