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

Stephen J. Wilcox steve at telecomplete.co.uk
Tue Mar 29 01:23:06 UTC 2005

On Mon, 28 Mar 2005, John Dupuy wrote:

> 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.

701 is not the most connected, it has only customers and a restrictive set of 

you dont need to peer with all networks tho, if all networks are buying from 701 
or one of its peers then it will get those routes via peering not transit or 
transit trades... you seem to be forgetting what peering is.

and if you peer with all networks in the 'transit free zone' then you too become 
transit free also.

> 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.

i believe the distinction exists as shown above ie transit free.. as to why this 
might be considered a goal i'm not sure, its not obvious that transit free is 
cheaper than buying transit!

this thing about 'who hurts most' is an entirely different topic and has nothing 
to do with who is in the transit free zone. altho destructive depeering does 
seem to be common practice within that zone :)

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

thats because non-technical definitions mean anyone can call themselves anything 
they like.. wiltel recently spammed me to buy their 'tier1 transit'.. presumably 
they are tier1 within their own definition of tier1.

if you want to be technical tho, and aiui we are a technical forum, then tier1 
means transit free.

i reaffirm my earlier point - but why care, isnt it about cost and reliability, 
and as peering and transit are about the same cost who cares who you dont peer 


> John
> 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?
>       Steve

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