<s>
A	O
Tier	B-Architecture
1	I-Architecture
network	I-Architecture
is	O
an	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
network	O
that	O
can	O
reach	O
every	O
other	O
network	O
on	O
the	O
Internet	O
solely	O
via	O
settlement-free	B-Protocol
interconnection	I-Protocol
(	O
also	O
known	O
as	O
settlement-free	B-Protocol
peering	I-Protocol
)	O
.	O
</s>
<s>
Tier	B-Architecture
1	I-Architecture
networks	I-Architecture
can	O
exchange	O
traffic	O
with	O
other	O
Tier	B-Architecture
1	I-Architecture
networks	I-Architecture
without	O
paying	O
any	O
fees	O
for	O
the	O
exchange	O
of	O
traffic	O
in	O
either	O
direction	O
.	O
</s>
<s>
In	O
contrast	O
,	O
some	O
Tier	B-Architecture
2	I-Architecture
networks	I-Architecture
and	O
all	O
Tier	B-Architecture
3	I-Architecture
networks	I-Architecture
must	O
pay	O
to	O
transmit	O
traffic	O
on	O
other	O
networks	O
.	O
</s>
<s>
The	O
most	O
common	O
and	O
well-accepted	O
definition	O
of	O
a	O
Tier	B-Architecture
1	I-Architecture
network	I-Architecture
is	O
a	O
network	O
that	O
can	O
reach	O
every	O
other	O
network	O
on	O
the	O
Internet	O
without	O
purchasing	O
IP	B-Application
transit	I-Application
or	O
paying	O
for	O
peering	B-Protocol
.	O
</s>
<s>
By	O
this	O
definition	O
,	O
a	O
Tier	B-Architecture
1	I-Architecture
network	I-Architecture
must	O
be	O
a	O
transit-free	B-Application
network	I-Application
(	O
purchases	O
no	O
transit	B-Application
)	O
that	O
peers	O
for	O
free	O
with	O
every	O
other	O
Tier	B-Architecture
1	I-Architecture
network	I-Architecture
and	O
can	O
reach	O
all	O
major	O
networks	O
on	O
the	O
Internet	O
.	O
</s>
<s>
Not	O
all	O
transit-free	B-Application
networks	I-Application
are	O
Tier	B-Architecture
1	I-Architecture
networks	I-Architecture
,	O
as	O
it	O
is	O
possible	O
to	O
become	O
transit-free	O
by	O
paying	O
for	O
peering	B-Protocol
,	O
and	O
it	O
is	O
also	O
possible	O
to	O
be	O
transit-free	O
without	O
being	O
able	O
to	O
reach	O
all	O
major	O
networks	O
on	O
the	O
Internet	O
.	O
</s>
<s>
The	O
most	O
widely	O
quoted	O
source	O
for	O
identifying	O
Tier	B-Architecture
1	I-Architecture
networks	I-Architecture
is	O
published	O
by	O
Renesys	O
Corporation	O
,	O
but	O
the	O
base	O
information	O
to	O
prove	O
the	O
claim	O
is	O
publicly	O
accessible	O
from	O
many	O
locations	O
,	O
such	O
as	O
the	O
RIPE	O
RIS	O
database	O
,	O
the	O
Oregon	O
Route	B-Protocol
Views	I-Protocol
servers	O
,	O
Packet	O
Clearing	O
House	O
,	O
and	O
others	O
.	O
</s>
<s>
It	O
can	O
be	O
difficult	O
to	O
determine	O
whether	O
a	O
network	O
is	O
paying	O
for	O
peering	B-Protocol
or	O
transit	B-Application
,	O
as	O
these	O
business	O
agreements	O
are	O
rarely	O
public	O
information	O
,	O
or	O
are	O
covered	O
under	O
a	O
non-disclosure	O
agreement	O
.	O
</s>
<s>
The	O
Internet	O
peering	B-Protocol
community	O
is	O
roughly	O
the	O
set	O
of	O
peering	B-Protocol
coordinators	O
present	O
at	O
the	O
Internet	B-Protocol
exchange	I-Protocol
points	I-Protocol
on	O
more	O
than	O
one	O
continent	O
.	O
</s>
<s>
The	O
subset	O
representing	O
Tier	B-Architecture
1	I-Architecture
networks	I-Architecture
is	O
collectively	O
understood	O
in	O
a	O
loose	O
sense	O
,	O
but	O
not	O
published	O
as	O
such	O
.	O
</s>
<s>
Common	O
definitions	O
of	O
Tier	B-Architecture
2	I-Architecture
and	O
Tier	B-Architecture
3	I-Architecture
networks	I-Architecture
:	O
</s>
<s>
Tier	B-Architecture
2	I-Architecture
network	I-Architecture
:	O
A	O
network	O
that	O
peers	O
for	O
free	O
with	O
some	O
networks	O
,	O
but	O
still	O
purchases	O
IP	B-Application
transit	I-Application
or	O
pays	O
for	O
peering	B-Protocol
to	O
reach	O
at	O
least	O
some	O
portion	O
of	O
the	O
Internet	O
.	O
</s>
<s>
Tier	B-Architecture
3	I-Architecture
network	I-Architecture
:	O
A	O
network	O
that	O
solely	O
purchases	O
transit/peering	O
from	O
other	O
networks	O
to	O
participate	O
in	O
the	O
Internet	O
.	O
</s>
<s>
The	O
original	O
Internet	B-General_Concept
backbone	I-General_Concept
was	O
the	O
ARPANET	O
when	O
it	O
provided	O
the	O
routing	O
between	O
most	O
participating	O
networks	O
.	O
</s>
<s>
The	O
development	O
of	O
the	O
British	O
JANET	O
(	O
1984	O
)	O
and	O
U.S.	O
NSFNET	O
(	O
1985	O
)	O
infrastructure	O
programs	O
to	O
serve	O
their	O
nations	O
 '	O
higher	O
education	O
communities	O
,	O
regardless	O
of	O
discipline	O
,	O
resulted	O
in	O
1989	O
with	O
the	O
NSFNet	O
backbone	B-General_Concept
.	O
</s>
<s>
The	O
Internet	O
could	O
be	O
defined	O
as	O
the	O
collection	O
of	O
all	O
networks	O
connected	O
and	O
able	O
to	O
interchange	O
Internet	B-Protocol
Protocol	I-Protocol
datagrams	O
with	O
this	O
backbone	B-General_Concept
.	O
</s>
<s>
When	O
the	O
Internet	O
was	O
opened	O
to	O
the	O
commercial	O
markets	O
,	O
multiple	O
for-profit	O
Internet	B-General_Concept
backbone	I-General_Concept
and	O
access	O
providers	O
emerged	O
.	O
</s>
<s>
The	O
network	O
routing	O
architecture	O
then	O
became	O
decentralized	O
and	O
attained	O
a	O
need	O
for	O
exterior	O
routing	O
protocols	O
,	O
in	O
particular	O
the	O
Border	B-Protocol
Gateway	I-Protocol
Protocol	I-Protocol
emerged	O
.	O
</s>
<s>
New	O
Tier	B-Architecture
1	I-Architecture
ISPs	I-Architecture
and	O
their	O
peering	B-Protocol
agreements	O
supplanted	O
the	O
government-sponsored	O
NSFNet	O
,	O
a	O
program	O
that	O
was	O
officially	O
terminated	O
on	O
April	O
30	O
,	O
1995	O
.	O
</s>
<s>
A	O
bilateral	O
private	O
peering	B-Protocol
agreement	O
typically	O
involves	O
a	O
direct	O
physical	O
link	O
between	O
two	O
partners	O
.	O
</s>
<s>
A	O
Tier	B-Architecture
1	I-Architecture
network	I-Architecture
may	O
have	O
various	O
such	O
links	O
to	O
other	O
Tier	B-Architecture
1	I-Architecture
networks	I-Architecture
.	O
</s>
<s>
Peering	B-Protocol
is	O
founded	O
on	O
the	O
principle	O
of	O
equality	O
of	O
traffic	O
between	O
the	O
partners	O
and	O
as	O
such	O
disagreements	O
may	O
arise	O
between	O
partners	O
in	O
which	O
usually	O
one	O
of	O
the	O
partners	O
unilaterally	O
disconnects	O
the	O
link	O
in	O
order	O
to	O
force	O
the	O
other	O
into	O
a	O
payment	O
scheme	O
.	O
</s>
<s>
Such	O
disruptive	O
de-peering	O
has	O
happened	O
several	O
times	O
during	O
the	O
first	O
decade	O
of	O
the	O
21st	O
century	O
.	O
</s>
<s>
In	O
the	O
worst	O
case	O
,	O
single-homed	B-Protocol
customers	O
of	O
each	O
network	O
will	O
not	O
be	O
able	O
to	O
reach	O
the	O
other	O
network	O
at	O
all	O
.	O
</s>
<s>
The	O
de-peering	O
party	O
then	O
hopes	O
that	O
the	O
other	O
network	O
's	O
customers	O
will	O
be	O
hurt	O
more	O
by	O
the	O
decision	O
than	O
its	O
own	O
customers	O
which	O
may	O
eventually	O
conclude	O
the	O
negotiations	O
in	O
its	O
favor	O
.	O
</s>
<s>
The	O
disputes	O
referenced	O
have	O
also	O
typically	O
involved	O
transit-free	O
peering	B-Protocol
in	O
which	O
one	O
player	O
only	O
exchanged	O
data	O
with	O
the	O
other	O
that	O
involved	O
each	O
other	O
's	O
networks	O
—	O
there	O
was	O
no	O
data	O
transiting	O
through	O
the	O
other	O
's	O
network	O
destined	O
for	O
other	O
parts	O
of	O
the	O
Internet	O
.	O
</s>
<s>
By	O
the	O
strict	O
definition	O
of	O
peering	B-Protocol
and	O
the	O
strict	O
definition	O
of	O
a	O
Tier	B-Architecture
1	I-Architecture
network	I-Architecture
,	O
a	O
Tier	B-Architecture
1	I-Architecture
network	I-Architecture
only	O
peers	O
with	O
other	O
Tier	B-Architecture
1	I-Architecture
networks	I-Architecture
and	O
has	O
no	O
transit	B-Application
routes	O
going	O
anywhere	O
.	O
</s>
<s>
More	O
practically	O
speaking	O
,	O
Tier	B-Architecture
1	I-Architecture
networks	I-Architecture
serve	O
as	O
transit	B-Application
networks	O
for	O
lower	O
tier	O
networks	O
and	O
only	O
peer	O
with	O
other	O
Tier	B-Architecture
1	I-Architecture
networks	I-Architecture
that	O
offer	O
the	O
same	O
services	O
on	O
an	O
adequate	O
scale	O
—	O
effectively	O
being	O
"	O
peers	O
"	O
in	O
the	O
truest	O
sense	O
of	O
the	O
word	O
.	O
</s>
<s>
More	O
appropriately	O
then	O
,	O
peering	B-Protocol
means	O
the	O
exchange	O
of	O
an	O
equitable	O
and	O
fair	O
amount	O
of	O
data-miles	O
between	O
two	O
networks	O
,	O
agreements	O
of	O
which	O
do	O
not	O
preclude	O
any	O
pay-for-transit	O
contracts	O
to	O
exist	O
between	O
the	O
very	O
same	O
parties	O
.	O
</s>
<s>
On	O
the	O
subject	O
of	O
routing	O
,	O
settlement-free	B-Protocol
peering	I-Protocol
involves	O
conditions	O
disallowing	O
the	O
abuse	O
of	O
the	O
other	O
's	O
network	O
by	O
sending	O
it	O
traffic	O
not	O
destined	O
for	O
that	O
network	O
(	O
i.e.	O
</s>
<s>
intended	O
for	O
transit	B-Application
)	O
.	O
</s>
<s>
Transit	B-Application
agreements	O
however	O
would	O
typically	O
cater	O
for	O
just	O
such	O
outbound	O
packets	O
.	O
</s>
<s>
Tier	O
1	O
providers	O
are	O
more	O
central	O
to	O
the	O
Internet	B-General_Concept
backbone	I-General_Concept
and	O
would	O
only	O
purchase	O
transit	B-Application
from	O
other	O
Tier	O
1	O
providers	O
,	O
while	O
selling	O
transit	B-Application
to	O
providers	O
of	O
all	O
tiers	O
.	O
</s>
<s>
Given	O
their	O
huge	O
networks	O
,	O
Tier	O
1	O
providers	O
do	O
not	O
participate	O
in	O
public	O
Internet	B-Protocol
Exchanges	I-Protocol
but	O
rather	O
sell	O
transit	B-Application
services	O
to	O
such	O
participants	O
.	O
</s>
<s>
In	O
the	O
most	O
logical	O
definition	O
,	O
a	O
Tier	O
1	O
provider	O
will	O
never	O
pay	O
for	O
transit	B-Application
because	O
the	O
set	O
of	O
all	O
Tier	O
1	O
providers	O
sells	O
transit	B-Application
to	O
all	O
of	O
the	O
lower	O
tier	O
providers	O
everywhere	O
,	O
and	O
becauseAs	O
such	O
,	O
by	O
the	O
peering	B-Protocol
agreement	O
,	O
all	O
the	O
customers	O
of	O
any	O
Tier	O
1	O
provider	O
already	O
have	O
access	O
to	O
all	O
the	O
customers	O
of	O
all	O
the	O
other	O
Tier	O
1	O
providers	O
without	O
the	O
Tier	O
1	O
provider	O
itself	O
having	O
to	O
pay	O
transit	B-Application
costs	O
to	O
the	O
other	O
networks	O
.	O
</s>
<s>
Effectively	O
,	O
the	O
actual	O
transit	B-Application
costs	O
incurred	O
by	O
provider	O
A	O
on	O
behalf	O
of	O
provider	O
B	O
are	O
logically	O
identical	O
to	O
the	O
transit	B-Application
costs	O
incurred	O
by	O
provider	O
B	O
on	O
behalf	O
of	O
provider	O
A	O
—	O
hence	O
there	O
not	O
being	O
any	O
payment	O
required	O
.	O
</s>
<s>
These	O
networks	O
are	O
universally	O
recognized	O
as	O
Tier	B-Architecture
1	I-Architecture
networks	I-Architecture
,	O
because	O
they	O
can	O
reach	O
the	O
entire	O
internet	O
(	O
IPv4	O
and	O
IPv6	O
)	O
via	O
settlement-free	B-Protocol
peering	I-Protocol
.	O
</s>
<s>
A	O
common	O
point	O
of	O
contention	O
regarding	O
Tier	B-Architecture
1	I-Architecture
networks	I-Architecture
is	O
the	O
concept	O
of	O
a	O
regional	O
Tier	B-Architecture
1	I-Architecture
network	I-Architecture
.	O
</s>
<s>
A	O
regional	O
Tier	B-Architecture
1	I-Architecture
network	I-Architecture
is	O
a	O
network	O
which	O
is	O
not	O
transit	B-Application
free	O
globally	O
,	O
but	O
which	O
maintains	O
many	O
of	O
the	O
classic	O
behaviors	O
and	O
motivations	O
of	O
a	O
Tier	B-Architecture
1	I-Architecture
network	I-Architecture
within	O
a	O
specific	O
region	O
.	O
</s>
<s>
Within	O
their	O
specific	O
countries	O
or	O
regions	O
of	O
origin	O
,	O
these	O
networks	O
maintain	O
peering	B-Protocol
policies	O
which	O
mimic	O
those	O
of	O
Tier	B-Architecture
1	I-Architecture
networks	I-Architecture
(	O
such	O
as	O
lack	O
of	O
openness	O
to	O
new	O
peering	B-Protocol
relationships	O
and	O
having	O
existing	O
peering	B-Protocol
with	O
every	O
other	O
major	O
network	O
in	O
that	O
region	O
)	O
.	O
</s>
<s>
However	O
,	O
this	O
network	O
may	O
then	O
extend	O
to	O
another	O
country	O
,	O
region	O
,	O
or	O
continent	O
outside	O
of	O
its	O
core	O
region	O
of	O
operations	O
,	O
where	O
it	O
may	O
purchase	O
transit	B-Application
or	O
peer	O
openly	O
like	O
a	O
Tier	B-Architecture
2	I-Architecture
network	I-Architecture
.	O
</s>
<s>
Less	O
extreme	O
examples	O
of	O
much	O
less	O
restrictive	O
peering	B-Protocol
requirements	O
being	O
set	O
for	O
regions	O
in	O
which	O
a	O
network	O
peers	O
,	O
but	O
does	O
not	O
sell	O
services	O
or	O
have	O
a	O
significant	O
market	O
share	O
,	O
are	O
relatively	O
common	O
among	O
many	O
networks	O
,	O
not	O
just	O
regional	O
Tier	B-Architecture
1	I-Architecture
networks	I-Architecture
.	O
</s>
<s>
While	O
the	O
classification	O
regional	O
Tier	O
1	O
holds	O
some	O
merit	O
for	O
understanding	O
the	O
peering	B-Protocol
motivations	O
of	O
such	O
a	O
network	O
within	O
different	O
regions	O
,	O
these	O
networks	O
do	O
not	O
meet	O
the	O
requirements	O
of	O
a	O
true	O
global	O
Tier	O
1	O
because	O
they	O
are	O
not	O
transit	B-Application
free	O
globally	O
.	O
</s>
<s>
This	O
is	O
a	O
list	O
of	O
networks	O
that	O
are	O
often	O
considered	O
and	O
close	O
to	O
the	O
status	O
of	O
Tier	O
1	O
,	O
because	O
they	O
can	O
reach	O
the	O
majority	O
(	O
50%+	O
)	O
of	O
the	O
internet	O
via	O
settlement	O
free	O
peering	B-Protocol
with	O
their	O
global	O
rings	O
.	O
</s>
<s>
Therefore	O
,	O
they	O
are	O
technically	O
Tier	B-Architecture
2	I-Architecture
,	O
though	O
practically	O
something	O
in	O
between	O
.	O
</s>
<s>
480952Purchases	O
transit	B-Application
from	O
Level	O
3/AS3356	O
,	O
Cogent/AS174.Singtel747315Purchases	O
transit	B-Application
from	O
Arelion/AS1299	O
,	O
Zayo/AS6461	O
,	O
Tata	O
Communications/AS6453.Cogent	O
Communications	O
(	O
formerly	O
PSINet	O
)	O
1743No	O
IPv6	O
routes	O
to	O
Hurricane	O
Electric/AS6939	O
.	O
</s>
<s>
''	O
Comcast792219Network	O
limited	O
to	O
the	O
US	O
;	O
Purchases	O
transit	B-Application
from	O
Tata/AS6453	O
,	O
otherwise	O
full	O
reach	O
via	O
peering	B-Protocol
Hurricane	O
Electric69396IPv4	O
:	O
Purchases	O
transit	B-Application
from	O
Arelion/AS1299	O
to	O
reach	O
NTT/AS2914	O
,	O
Cogent/AS174	O
,	O
and	O
Tata/AS6453IPv6	O
:	O
Lack	O
of	O
peering	B-Protocol
with	O
Cogent/AS174.RETN900212Purchases	O
transit	B-Application
from	O
Level	O
3/AS3356Vodafone	O
Carrier	O
Services( formerly	O
Cable	O
&	O
Wireless	O
)	O
1273	O
11	O
Purchases	O
transit	B-Application
from	O
Arelion/AS1299	O
to	O
reach	O
AT&	O
T/AS7018.Verizon	O
Enterprise	O
Solutions(formerly XO Communications )	O
282883IPv6	O
:	O
Purchases	O
transit	B-Application
from	O
T-Mobile	O
US/AS1239	O
to	O
reach	O
Vodafone	O
(	O
CW	O
)	O
/AS1273	O
and	O
Telecom	O
Italia	O
Sparkle	O
(	O
Seabone	O
)	O
/AS6763.Telstra463714Purchases	O
transit	B-Application
from	O
Level	O
3/AS3356	O
,	O
Arelion/AS1299	O
,	O
Zayo/AS6461	O
.	O
</s>
