Why Peering Ratios?


Argument #2 - “OK, but there is massive asymmetry here. Look at how many bit miles I have to carry your content, while you have only to deliver the traffic across the exchange point.”

Counter Argument #1 - This is a valid observation, and perhaps a good argument for requiring a peer to spread the peering load across multiple geographically distributed interconnect regions.

This is not however an argument for using Peering traffic ratios to restrict Peering. As demonstrated earlier, Peering traffic ratios are irrelevant; eventually this traffic traverses your network to get to your customers. The asymmetry is merely a side affect of the type of traffic requested by or served on behalf of your customers.

One discussion with a large backbone provider highlighted the folly. This Peering Coordinator was proud that the peering ratio required a peer to “adjust” traffic to maintain an “appropriate” traffic ratio. How did the peer accomplish this shift? They redirected some portion of traffic to a transit provider who in turn forwarded that traffic directly back to the large backbone provider, onto the same router as the peer! There was no change in backbone load, simply a shift of traffic from one interface card to the other. The ISP mandating peering traffic ratios realized no cost savings, enhanced revenue, or performance benefits from this mandate. In fact, traffic now takes a more circuitous path to reach their customers yielding slightly worse performance.

Counter-Counter Argument – Multiple interconnect points may make the problem worse. With hot potato routing the large volume of content enters my backbone sooner!  On the other end of the spectrum, “cold-potato” routing that some have advocated to distribute the load more evenly across the interconnect points simply does not scale. Cold Potato routing using Multi-Exit Discriminators (MEDs) requires breaking up aggregate routing blocks into more granular chunks that can be routed to a specific interconnect point. This breaks the scaling characteristics of CIDR, and can become an operational nightmare as load-balancing and fail-over scenarios are encountered. So, unless there is a way to effectively spread the traffic across the interconnect points without breaking things, I will have to carry the traffic further; more costs to me.

Counter-Counter-Counter Argument – But you will have to pick up that traffic somewhere, either via a transit link that costs you money or a peering link. I can get you that same traffic for free more directly so with lower latency.  No matter what, you have to carry that traffic to your customers.

Counter-Argument #2 – The asymmetry is simply a short-term artifact of the currently popular HTTP web traffic. For the longer term, this will change. For example, there may be more peer-to-peer traffic, or other types of traffic that will swap the traffic asymmetry. Any policy based on traffic ratios is short-sighted.

Argument #3The_Folly_of_Peering_Ratios_3.html