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

  • From: John Dupuy
  • Date: Mon Mar 28 16:59:58 2005

I'll be brief, but I do want to perhaps word Alex's definition in a different way that might be more useful.

Even "tier 1" providers regularly trade transit. They must since no single network is connected to all the other ones. Not even close. Even UUNet (ASN 701), arguably the most-connected network on the planet, only connects to a fraction of the possible peerings.

The true definition is more vague: if a peering or transit circuit between A or B is taken down, who will be hurt the most: A or B? If it predominantly B, and much less A, then A is "more Tier 1" and B is of a "lesser Tier". If they are equally hurt, they the are of equal status. Essentially, "Tier 1" is whatever the other "Tier 1" providers believe at the moment is "Tier 1". It is self-referential and not distinct at all.

This is, frustratingly, a very non-technical definition. But it seems to map with what I've actually seen the industry do.


At 09:17 AM 3/28/2005, Stephen J. Wilcox wrote:

On Mon, 28 Mar 2005, Randy Bush wrote:

> > Firstly, peering isn't binary. Is peering vs transit a distinction based on
> > routes taken / accepted & readvertised, or on cost? Does "paid for peering"
> > count as peering or transit? If you pay by volume? If you pay for "more than
> > your fair share" of the interconnect pipes? (if the latter, I am guessing
> > there are actually no Tier 1s as everyone reckons they pay for more than
> > their fair share...).
> pay?  did i say pay?  i discussed announcement and receipt of prefixes.  this
> was not an accident.  it is measurable.

i also avoided money.. i dont think its that relevant, everyone is paying for
peering or transit in one form or another, i dont think any peering is free
(free != settlement free)

> > Secondly, it doesn't cover scenarios that have have happened in the past.
> > For instance, the route swap. EG Imagine networks X1, X2, X3, X4 are "Tier
> > 1" as Randy describes them. Network Y peers with all the above except X1.
> > Network Z peers with all the above except X2. Y & Z peer. To avoid Y or Z
> > needing to take transit, Y sends Z X2's routes (and sends Z's routes to X2
> > routes marked "no export" to X2's peers), and Z sends Y X1's routes (and
> > sends Y's routes to X1 marked "no export" to X1's peers). Perhaps they do
> > this for free. Perhaps they charge eachother for it and settle up at the end
> > of each month. Perhaps it's one company that's just bought another.

"transit (n). The act of passing over, across, or through; passage."

whether it is a settlement arrangement or a mutual swap, they do NOT have
peering, they ARE transitting and by our definition are not transit-free (and
hence not tier1)

however alex, you do highlight an excellent point - things are not as simple as
'tier1, tier2', there are complicated routing and financial arrangements in
operation, which brings me back to my earlier point: does it matter what a
network is paying for some connectivity providing they deliver to you the
connectivity you need at the quality you desire?


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