the Internet Backbone

Howard C. Berkowitz hcb at
Sat Apr 6 15:54:18 UTC 1996

I frequently teach routing and operations, and find this discussion of
terms to be coming up with a useful consensus to answer some very
frequently asked questions.  I'd like to see the results of this discussion
spread more widely than the NANOG list.

It strikes me that the discussion is the kernel of an informational RFC
complementary to Dave Crocker's RFC1775, "To be "on" the Internet."  Here,
perhaps we are defining what it means to be "in" the Internet.  RFC1775
emphasized the user perspective; it would seem that a different document
emphasizing the operator perspective also would be useful.

Michael Dillon and others have put forth some good definitions.  Would any
of you active contributors be interested in trying an RFC on this?  I'll
step up to editing it if it would be useful.

My comments on Michael's last posting:

HCB>  Another FAQ is the meaning of "multihoming."  There seem to be
HCB> two cases in common usage:  IP-level multiple paths to a single
HCB> AS, and BGP peerings to >1 AS.

If we think like an onion (shades of TinyBASIC!) then the core of the
Internet are these providers who supply transit over their own national
and international backbones and who do not need to buy transit from other
providers. The providers who form the Internet core are sometimes called
NSP's (Network Service Providers) and sometimes called Tier 1 providers

HCB>  I'd add that a Tier 1 provider/NSP has at least some routers that
HCB>  are default-free.

> That's pretty common usage.  Sounds much better than an-as-pee.
> The term for second-tier is "regional provider" and third tier is
> usually local providers.

The next layer of the onion is the Tier 2 providers sometimes referred to
as regional providers although they may actually serve overlapping
geographical regions. These providers do not provide transit but they do
supply other providers in a lower tier.

HCB>  I'm not sure about the part that a Tier 2 provider never provides
HCB>  transit.

This brings us to the Tier 3 providers commonly known as ISP's (Internet
Service Providers. These organizations may connect to Tier 2 providers
or Tier 1 providers but their distinguishing characteristic is that they
do no normally supply organizations who resell Internet access.

Tier 4 networks belong to those organizations who provide Internet access
for their own members or employees. These could be corporations, schools,
or universities who operate both internal networks and provide dialup
services that are not available to the general public. Sometimes a Tier 4
network provides access to other organizations such as a company which
supplies its subcontractors with their Internet connectivity.

HCB>  A Tier 4 might be the first level that provides proxy aggregation,
HCB>  if it supports external organizations such as subcontractors.
HCB>  Proxy aggregation is more likely to be at Tier 3 or higher.

Tier 5 is the end user. They may have a single PC that dials up to the
Internet or they may be sitting in front of a workstation on a corporate LAN.

Unlike an onion skin, these layers are not precise and there is some overlap
especially in Tier 2. Until recently most ISP's connected directly to
Tier 1 providers and although there are some providers who are starting
to specialize in Tier 2 services it will remain common for both Tier 1
and 3 organizations to be in that market.

This seems to explain the relationships in a way that I think the average
person or journalist could understand and still form concepts fairly
close to the reality of today's global Internet.

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