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Re: Level 3's side of the story

  • From: JC Dill
  • Date: Sat Oct 08 10:25:38 2005

Eric Louie wrote:
DISCLAIMER:  From one of the clueless
As a disclaimer, I will point out that there are some in this debate who consider me clueless as well. However, I don't believe that any of the following is in error.

During this entire debaucle, I never saw any mention of:
I've mentioned most of it in prior posts but perhaps it hasn't been explained well enough for you to fully grasp it. I hope this helps.

1) Cogent sending "transit" traffic to Level3, which leads me to
Cogent was sending peering traffic, not transit traffic.

believe that all the traffic from Cogent through the peering points was actually *destined* for Level3 customers.
Yes, that's what peering is. Peering gets you the other ISP's customers but NOT all the rest of the internet that is reachable thru that ISP. Transit gets you all the internet that is reachable thru that ISP.

Does the routing support this idea?

Is it safe to assume the opposite, also... that only traffic destined for Cogent customers came through the Level3 peering points?

And that Level3 had one and only one path to Cogent (no one else providing transit for them to Cogent AS'es?)

2) Level3 making any contingency for their own customers to reach Cogent networks (any announcements to their own customers)
L3 made no contingency plans for their customers to reach Cogent (and visa versa).

For the L3 customers that were multi-homed, they had other paths to Cogent thru their other service providers. Those that were not multi-homed had no path to Cogent when L3 cut the peering, because as a "tier 1" (the widely accepted definition of "tier-1" is that the network only has SFI (Settlement Free Interchange i.e. peering) links to other networks and thus no transit links) none of the other networks L3 connects to will carry L3's traffic to a third network (another peer).

Cogent was a "tier 1" until prior de-peering incidents left them unable to reach other networks. They solved this by buying filtered transit thru Verio to reach the networks they couldn't reach via peering.

L3 was hoping to force Cogent to increase that transit to include the traffic destined for L3's customers, thus raising Cogent's transport costs at no additional (transport) cost to L3.

3) Possible traffic issues. Was Cogent guilty of not transporting the Level3-bound packets within the Cogent network to the closest point-of-entry peer to the host in the Level3 network, therefore "costing" Level3 transit of their own packets?
Possible, in fact probable. Most ISPs hand off traffic to peers under a "hot potato" policy, they hand it off at the closest point where they connect. If the traffic is equal in both directions then this works. If the traffic is not equal, then this lowers the cost of the network that has high outbound traffic, as the other network bears the brunt of the total cost for transporting the combined traffic between their respective customers.

In other words is it also a traffic engineering issue?
Or rather, that it could be mostly solved with a traffic engineering fix called "cold potato" routing, where the sending network carries the traffic as far as they can before handing it off to the recipient network.

Consider a simple hypothetical closest-exited network setup (hot potato routing) between 2 peers:

ISP Eyeballs: Router-E1----2,000 Mile Link----Router-E2----Customer
| |
| |
peering peering
| |
| |
ISP Content: Server--Router-C1---2,000 Mile Link-----Router-C2

When the customer on ISP E (Eyeballs) requests content (web page, music file, etc.) from the server on ISP C (Content), packets travel like this:


When the server returns traffic to the customer, traffic goes like this:


The problem is the customer->server direction would typically be a 500 byte request and 64 byte ACK packets, where as the server->customer data includes many 1500 byte data packets. So, ISP Eyeballs may carry 2Mbs of data over its 2,000 mile link, where as ISP Content will only carry 128Kbs over its 2,000 mile link.

Even though both companies met in the middle, ISP Content shifted some of its costs to ISP Eyeballs.

Back when most ISPs had the same types of traffic (even mixes of content and eyeballs), they had even ratios which equalized this effect, as it was happening the same amount in both directions. But as some ISPs started specializing in one type of content or the other, uneven flows were produced. Some bean-counter felt that these uneven flows meant that the network that was sending more traffic should now pay for transit, even though this traffic was traffic that their own customers were requesting and paying them to transmit!

There are ways to deal with it though, like cold potato routing.

Are some of the business issues solvable by proper engineering and filtering (or statistics-jockeying)?
Yes, see above.

jc  (no coffee yet, errors possible)