T1 vs. T2 [WAS: Apology: [Tier-2 reachability and multihoming]]

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1

My apologies to UUNet/MCI, I'm not trying to pick on you, but you are useful to the discussion.

But by the technical description of a "transit free zone", then 701 is not tier one, since I have encountered scenarios where many AS are transversed between 701 and other networks, not just a peer of a peer. Unless, by "transit free zone" you mean "transit trading" where large providers permit each other to transit for free. (Which gets back to my 'who hurts more' discussion.)

I'm willing to be wrong. If any of the large providers on the list will say that their network does not transit beyond the customer of a peer; and they still maintain full connectivity, I will gladly be corrected.

John

2

But by the technical description of a "transit free
zone", then 701 is not
tier one, since I have encountered scenarios where
many AS are transversed
between 701 and other networks, not just a peer of a
peer. Unless, by
"transit free zone" you mean "transit trading" where
large providers permit
each other to transit for free. (Which gets back to
my 'who hurts more'
discussion.)

<oversimplification>

Transit = being someone's customer

Peering = permitting your customers to go to your
peer's customers or the peer's network, but not the
peer's peers, without exchange of money.

Any other relationship != peering for my purposes
(although lots of subtly different relationships
exist, the largest networks tend to take a view which
is not too dissimilar to the one shown above)

</oversimplification>

Are you implying that 701 is paying someone to carry
their prefixes? While I'm not the peering coordinator
for 701, I would find that improbable. I would expect
that money would flow the other direction (and thus
701 would become a more valuable peer for other
networks).

I'm willing to be wrong. If any of the large
providers on the list will say
that their network does not transit beyond the
customer of a peer; and they
still maintain full connectivity, I will gladly be
corrected.

oodles and oodles of people can say this (and already
have). A paying customer of mine can readvertise
(with a non-munged AS_PATH) any of my prefixes which
they want, and thus provide transit for other people
to reach me. That does not change the fact that I'm
not paying for transit.

So in short, I would say that T1 vs T2 etc is a
"follow the money":

T1 => doesn't pay anyone else to carry their prefixes,
and runs a default-free network.

T2 => pays one or more T1 providers to carry their
prefixes, may or may not run a default-free network.

T3 => leaf node, pays one or more T1/T2 providers to
carry their traffic, probably uses default route.

YMMV, blah blah blah

David Barak
Need Geek Rock? Try The Franchise:
http://www.listentothefranchise.com

3

I guess I'm looking at this too much from the point of view of a BGP Admin.

Yes, if you are looking at this from the point of view of payment, then the top ISPs do not pay each other.

I was looking at it from a route announcement point of view. Transit is where AS A advertises full routes to AS B. Thus, AS B is getting transit from A. Peering is where A & B only advertise their network and, possibly, the networks that stub or purchase transit from them.

It is my understanding that the top ISPs "trade transit". They provide full routes to each other without payment, regardless of how or where the route was learned from. They are willing to pass some traffic without compensation because it makes for better connectivity. From an announcement POV they are not peering.

I am still curious: do any of the larger ISPs on this list want to confirm/deny the previous paragraph?

I think we are getting into "defining terms" territory. So, I will bow out of the discussion.

John

4

I was looking at it from a route announcement point of view. Transit is where
AS A advertises full routes to AS B. Thus, AS B is getting transit from A.
Peering is where A & B only advertise their network and, possibly, the
networks that stub or purchase transit from them.

no, they MUST send their customer nets else their customers will not have
global reachability

It is my understanding that the top ISPs "trade transit". They provide full
routes to each other without payment, regardless of how or where the route
was learned from. They are willing to pass some traffic without
compensation because it makes for better connectivity. From an announcement
POV they are not peering.

ahhh. no, they send peering only between each other (approx 50000 routes for
each of the biggest providers - level3, sprint, uunet, at&t)

Steve

5

I would be AMAZINGLY interested if anyone confirms the above paragraph.

AFAIK, 701/1239/209/etc. do not give full tables to _anyone_ unless they are paid.

Someone care to correct me?

6

ISPs formerly known as tier1s in general peer with each other, not trade transit.
If one of the peers started sending us full routes, that would quickly result in a
NOC to NOC chat about route leaks.

If they exchanged full routes, wouldn't that be mutual transit, not peering?

This isn't meant to imply that networks don't play kinky games with each other
at various times that can confuse outside observers, but peering is peering
and transit is transit, most of the time.

-dorian

7

Settlement free transit? Sounds like the wave of the future to me. Oh wait
it's only March 29th, we're still 3 days away. :slight_smile:

Alas, as anyone who has ever watched Internap when they go flappy flappy
can attest, BGP does not handle an excessive number of transit paths very
well. I'd really hate to picture the size of the boom that would happen if
people WERE to exchange transit paths with each other on anything other
than a rare and isolated basis.

8

Alas, as anyone who has ever watched Internap when they go flappy flappy
can attest, BGP does not handle an excessive number of transit paths

very

well. I'd really hate to picture the size of the boom that would happen

if

people WERE to exchange transit paths with each other on anything other
than a rare and isolated basis.

True. And I fully support the common practice of heavy filtering on both
ends of most BGP sessions to prevent route leakage. Nothing upsets an
upstream more than announcing a major network via a smaller connection.

Perhaps things have changed a lot in the last six years, which is the last
time I got much face-to-face time with other BGP admins. Back then it
seemed that the larger networks horse-traded transit pretty regularly. I
do not know if was partly automated or case-by-case for each route. (And I
suspect it was not always with corporate knowledge.) Especially since some
networks (foreign government networks, etc.) were not as "flexible" as one
would hope about peering.

Again, I'd be interested in hearing from one of the bigger ones on this:
UUNet, AT&T, Sprint, Level3, QWest.... If you can't say anything, I
understand.

John

9

In a message written on Tue, Mar 29, 2005 at 02:27:56PM -0600, John Dupuy wrote:

I was looking at it from a route announcement point of view. Transit is
where AS A advertises full routes to AS B. Thus, AS B is getting transit
from A. Peering is where A & B only advertise their network and, possibly,
the networks that stub or purchase transit from them.

This is oversimplistic. Transit does not have to be full routes.
Don't confuse the business case with the technical configuration.
That is, all combinations of:

{paid,settlement free}-{customer routes only, full routes, no routes,
you leak mine, I leak yours}

exist. Some are more common than others. Sometimes multiple
combinations exist between the same two parties.

It is my understanding that the top ISPs "trade transit". They provide full
routes to each other without payment, regardless of how or where the route
was learned from. They are willing to pass some traffic without
compensation because it makes for better connectivity. From an announcement
POV they are not peering.

The top of the food chain is a full mesh of customer routes only.
I have never seen anyone at the top of the food chain trade full
routing tables, something that would likely be obvious from time
to time in various outage scenarios. There is no business case to
provide free transit on that level. It would be too easily abused.

That's not limited to "top" ISP's either. Full tables are not done
on a peering level, ever. If anything wonky is being done it's
done with selective leaking of routes in one or both directions,
never ever ever with a full table.

10

jdupuy-list@socket.net writes:

Again, I'd be interested in hearing from one of the bigger ones on this:
UUNet, AT&T, Sprint, Level3, QWest.... If you can't say anything, I
understand.

You don't need them to say anything - just look at what they are
advertising. Are they advertising each other's routes? If not, then
they aren't given each other transit.

11

[ snip ]

I'd really hate to picture the size of the boom that would happen if
people WERE to exchange transit paths with each other on anything other
than a rare and isolated basis.

Yup... Already happening a lot in IPv6 today, mostly from legacy 6bone
operators who still refuse to clean up. Worse, such mutual full swapping
/ settlement-free transit exchange on large part is done over tunnels...
(oh snap...)

I can already go on and name at least five ASNs already that are doing this
on large scale but I think I'll refrain from doing so on a public mailing
list :smiley:

-J