<s>
A	O
content	B-Operating_System
delivery	I-Operating_System
network	I-Operating_System
,	O
or	O
content	B-Operating_System
distribution	I-Operating_System
network	I-Operating_System
(	O
CDN	O
)	O
,	O
is	O
a	O
geographically	O
distributed	O
network	O
of	O
proxy	B-Application
servers	I-Application
and	O
their	O
data	B-Operating_System
centers	I-Operating_System
.	O
</s>
<s>
CDNs	O
are	O
a	O
layer	B-Architecture
in	O
the	O
internet	O
ecosystem	O
.	O
</s>
<s>
In	O
turn	O
,	O
a	O
CDN	O
pays	O
Internet	O
service	O
providers	O
(	O
ISPs	O
)	O
,	O
carriers	O
,	O
and	O
network	O
operators	O
for	O
hosting	O
its	O
servers	O
in	O
their	O
data	B-Operating_System
centers	I-Operating_System
.	O
</s>
<s>
CDN	O
is	O
an	O
umbrella	O
term	O
spanning	O
different	O
types	O
of	O
content	O
delivery	O
services	O
:	O
video	B-Application
streaming	I-Application
,	O
software	O
downloads	O
,	O
web	O
and	O
mobile	O
content	O
acceleration	O
,	O
licensed/managed	O
CDN	O
,	O
transparent	O
caching	O
,	O
and	O
services	O
to	O
measure	O
CDN	O
performance	O
,	O
load	B-Application
balancing	I-Application
,	O
Multi	O
CDN	O
switching	O
and	O
analytics	O
and	O
cloud	O
intelligence	O
.	O
</s>
<s>
CDN	O
vendors	O
may	O
cross	O
over	O
into	O
other	O
industries	O
like	O
security	O
,	O
DDoS	O
protection	O
and	O
web	B-Protocol
application	I-Protocol
firewalls	I-Protocol
(	O
WAF	O
)	O
,	O
and	O
WAN	O
optimization	O
.	O
</s>
<s>
Marking	O
the	O
25th	O
anniversary	O
of	O
the	O
birth	O
of	O
CDNs	O
,	O
the	O
history	O
of	O
the	O
transformational	O
role	O
that	O
CDNs	O
played	O
in	O
creating	O
the	O
first	O
edge	O
networks	O
that	O
soon	O
led	O
to	O
edge	B-Device
computing	I-Device
,	O
edge	B-Protocol
security	I-Protocol
services	I-Protocol
,	O
edge	B-Device
clouds	I-Device
,	O
and	O
other	O
edge	O
technologies	O
was	O
told	O
in	O
an	O
of	O
ACM	O
SIGCOMM	O
's	O
.	O
</s>
<s>
CDN	O
nodes	O
are	O
usually	O
deployed	O
in	O
multiple	O
locations	O
,	O
often	O
over	O
multiple	O
Internet	B-General_Concept
backbones	I-General_Concept
.	O
</s>
<s>
This	O
may	O
be	O
measured	O
by	O
choosing	O
locations	O
that	O
are	O
the	O
fewest	O
hops	B-Protocol
,	O
the	O
lowest	O
number	O
of	O
network	O
seconds	O
away	O
from	O
the	O
requesting	O
client	O
,	O
or	O
the	O
highest	O
availability	O
in	O
terms	O
of	O
server	O
performance	O
(	O
both	O
current	O
and	O
historical	O
)	O
,	O
to	O
optimize	O
delivery	O
across	O
local	O
networks	O
.	O
</s>
<s>
CDN	O
providers	O
profit	O
either	O
from	O
direct	O
fees	O
paid	O
by	O
content	O
providers	O
using	O
their	O
network	O
,	O
or	O
profit	O
from	O
the	O
user	O
analytics	O
and	O
tracking	O
data	O
collected	O
as	O
their	O
scripts	O
are	O
being	O
loaded	O
onto	O
customers	O
 '	O
websites	O
inside	O
their	O
browser	B-Protocol
origin	I-Protocol
.	O
</s>
<s>
For	O
example	O
,	O
in	O
2021	O
a	O
German	O
court	O
forbade	O
the	O
use	O
of	O
a	O
CDN	O
on	O
a	O
university	O
website	O
,	O
because	O
this	O
caused	O
the	O
transmission	O
of	O
the	O
user	O
's	O
IP	B-Protocol
address	I-Protocol
to	O
the	O
CDN	O
,	O
which	O
violated	O
the	O
GDPR	O
.	O
</s>
<s>
CDNs	O
serving	O
JavaScript	B-Language
have	O
also	O
been	O
targeted	O
as	O
a	O
way	O
to	O
inject	O
malicious	O
content	O
into	O
pages	O
using	O
them	O
.	O
</s>
<s>
The	O
Internet	O
was	O
designed	O
according	O
to	O
the	O
end-to-end	B-Application
principle	I-Application
.	O
</s>
<s>
This	O
principle	O
keeps	O
the	O
core	B-General_Concept
network	I-General_Concept
relatively	O
simple	O
and	O
moves	O
the	O
intelligence	O
as	O
much	O
as	O
possible	O
to	O
the	O
network	O
end-points	O
:	O
the	O
hosts	O
and	O
clients	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
the	O
core	B-General_Concept
network	I-General_Concept
is	O
specialized	O
,	O
simplified	O
,	O
and	O
optimized	O
to	O
only	O
forward	O
data	O
packets	O
.	O
</s>
<s>
Content	B-Operating_System
Delivery	I-Operating_System
Networks	I-Operating_System
augment	O
the	O
end-to-end	O
transport	O
network	O
by	O
distributing	O
on	O
it	O
a	O
variety	O
of	O
intelligent	O
applications	O
employing	O
techniques	O
designed	O
to	O
optimize	O
content	O
delivery	O
.	O
</s>
<s>
The	O
resulting	O
tightly	O
integrated	O
overlay	O
uses	O
web	B-Protocol
caching	I-Protocol
,	O
server-load	O
balancing	O
,	O
request	O
routing	O
,	O
and	O
content	O
services	O
.	O
</s>
<s>
Web	B-Protocol
caches	I-Protocol
store	O
popular	O
content	O
on	O
servers	O
that	O
have	O
the	O
greatest	O
demand	O
for	O
the	O
content	O
requested	O
.	O
</s>
<s>
Web	B-Protocol
caches	I-Protocol
are	O
populated	O
based	O
on	O
requests	O
from	O
users	O
(	O
pull	O
caching	O
)	O
or	O
based	O
on	O
preloaded	O
content	O
disseminated	O
from	O
content	O
servers	O
(	O
push	O
caching	O
)	O
.	O
</s>
<s>
Server-load	O
balancing	O
uses	O
one	O
or	O
more	O
techniques	O
including	O
service-based	O
(	O
global	O
load	B-Application
balancing	I-Application
)	O
or	O
hardware-based	O
(	O
i.e.	O
</s>
<s>
layer	B-Application
4	I-Application
–	I-Application
7	I-Application
switches	I-Application
,	O
also	O
known	O
as	O
a	O
web	O
switch	O
,	O
content	O
switch	O
,	O
or	O
multilayer	B-Application
switch	I-Application
)	O
to	O
share	O
traffic	O
among	O
a	O
number	O
of	O
servers	O
or	O
web	B-Protocol
caches	I-Protocol
.	O
</s>
<s>
Here	O
the	O
switch	O
is	O
assigned	O
a	O
single	O
virtual	O
IP	B-Protocol
address	I-Protocol
.	O
</s>
<s>
Traffic	O
arriving	O
at	O
the	O
switch	O
is	O
then	O
directed	O
to	O
one	O
of	O
the	O
real	O
web	B-Application
servers	I-Application
attached	O
to	O
the	O
switch	O
.	O
</s>
<s>
This	O
has	O
the	O
advantage	O
of	O
balancing	O
load	O
,	O
increasing	O
total	O
capacity	O
,	O
improving	O
scalability	O
,	O
and	O
providing	O
increased	O
reliability	O
by	O
redistributing	O
the	O
load	O
of	O
a	O
failed	O
web	B-Application
server	I-Application
and	O
providing	O
server	O
health	O
checks	O
.	O
</s>
<s>
A	O
content	O
cluster	O
or	O
service	O
node	O
can	O
be	O
formed	O
using	O
a	O
layer	B-Architecture
4	O
–	O
7	O
switch	O
to	O
balance	O
load	O
across	O
a	O
number	O
of	O
servers	O
or	O
a	O
number	O
of	O
web	B-Protocol
caches	I-Protocol
within	O
the	O
network	O
.	O
</s>
<s>
These	O
include	O
Global	B-Application
Server	I-Application
Load	I-Application
Balancing	I-Application
,	O
DNS-based	O
request	O
routing	O
,	O
Dynamic	O
metafile	O
generation	O
,	O
HTML	O
rewriting	O
,	O
and	O
anycasting	O
.	O
</s>
<s>
CDNs	O
use	O
a	O
variety	O
of	O
methods	O
of	O
content	O
delivery	O
including	O
,	O
but	O
not	O
limited	O
to	O
,	O
manual	O
asset	O
copying	O
,	O
active	O
web	B-Protocol
caches	I-Protocol
,	O
and	O
global	O
hardware	O
load	B-Application
balancers	I-Application
.	O
</s>
<s>
The	O
Internet	B-Protocol
Content	I-Protocol
Adaptation	I-Protocol
Protocol	I-Protocol
(	O
ICAP	O
)	O
was	O
developed	O
in	O
the	O
late	O
1990s	O
to	O
provide	O
an	O
open	O
standard	O
for	O
connecting	O
application	O
servers	O
.	O
</s>
<s>
Edge	B-Language
Side	I-Language
Includes	I-Language
or	O
ESI	O
is	O
a	O
small	O
markup	O
language	O
for	O
edge-level	O
dynamic	O
web	O
content	O
assembly	O
.	O
</s>
<s>
This	O
means	O
that	O
,	O
unlike	O
client	B-Operating_System
–	I-Operating_System
server	I-Operating_System
systems	O
,	O
the	O
content-centric	O
networks	O
can	O
actually	O
perform	O
better	O
as	O
more	O
users	O
begin	O
to	O
access	O
the	O
content	O
(	O
especially	O
with	O
protocols	O
such	O
as	O
Bittorrent	B-Protocol
that	O
require	O
users	O
to	O
share	O
)	O
.	O
</s>
<s>
These	O
PoPs	O
can	O
be	O
caching	O
servers	O
,	O
reverse	B-General_Concept
proxies	I-General_Concept
or	O
application	O
delivery	O
controllers	O
.	O
</s>
<s>
Large	O
content	B-Operating_System
distribution	I-Operating_System
networks	I-Operating_System
may	O
even	O
build	O
and	O
set	O
up	O
their	O
own	O
private	O
network	O
to	O
distribute	O
copies	O
of	O
content	O
across	O
cache	O
locations	O
.	O
</s>
<s>
The	O
rapid	O
growth	O
of	O
streaming	B-Application
video	I-Application
traffic	O
uses	O
large	O
capital	O
expenditures	O
by	O
broadband	O
providers	O
in	O
order	O
to	O
meet	O
this	O
demand	O
and	O
retain	O
subscribers	O
by	O
delivering	O
a	O
sufficiently	O
good	O
quality	O
of	O
experience	O
.	O
</s>
<s>
To	O
address	O
this	O
,	O
telecommunications	O
service	O
providers	O
(	O
TSPs	O
)	O
have	O
begun	O
to	O
launch	O
their	O
own	O
content	B-Operating_System
delivery	I-Operating_System
networks	I-Operating_System
as	O
a	O
means	O
to	O
lessen	O
the	O
demands	O
on	O
the	O
network	B-General_Concept
backbone	I-General_Concept
and	O
reduce	O
infrastructure	O
investments	O
.	O
</s>
<s>
They	O
own	O
the	O
last	B-Protocol
mile	I-Protocol
and	O
can	O
deliver	O
content	O
closer	O
to	O
the	O
end-user	O
because	O
it	O
can	O
be	O
cached	O
deep	O
in	O
their	O
networks	O
.	O
</s>
<s>
This	O
deep	O
caching	O
minimizes	O
the	O
distance	B-Protocol
that	O
video	O
data	O
travels	O
over	O
the	O
general	O
Internet	O
and	O
delivers	O
it	O
more	O
quickly	O
and	O
reliably	O
.	O
</s>
<s>
In	O
June	O
2011	O
,	O
StreamingMedia.com	O
reported	O
that	O
a	O
group	O
of	O
TSPs	O
had	O
founded	O
an	O
Operator	O
Carrier	O
Exchange	O
(	O
OCX	O
)	O
to	O
interconnect	O
their	O
networks	O
and	O
compete	O
more	O
directly	O
against	O
large	O
traditional	O
CDNs	O
like	O
Akamai	B-Application
and	O
Limelight	O
Networks	O
,	O
which	O
have	O
extensive	O
PoPs	O
worldwide	O
.	O
</s>
<s>
The	O
Open	O
Caching	O
specification	O
by	O
Streaming	O
Media	O
Alliance	O
defines	O
a	O
set	O
of	O
APIs	B-Application
that	O
allows	O
a	O
Content	O
Provider	O
to	O
deliver	O
its	O
content	O
using	O
several	O
CDNs	O
in	O
a	O
consistent	O
way	O
,	O
seeing	O
each	O
CDN	O
provider	O
the	O
same	O
way	O
through	O
these	O
APIs	B-Application
.	O
</s>
<s>
Indeed	O
,	O
a	O
recent	O
study	O
showed	O
that	O
in	O
many	O
countries	O
where	O
public	O
DNS	O
resolvers	O
are	O
in	O
popular	O
use	O
,	O
the	O
median	O
distance	B-Protocol
between	O
the	O
clients	O
and	O
their	O
recursive	O
DNS	O
resolvers	O
can	O
be	O
as	O
high	O
as	O
a	O
thousand	O
miles	O
.	O
</s>
<s>
In	O
August	O
2011	O
,	O
a	O
global	O
consortium	O
of	O
leading	O
Internet	O
service	O
providers	O
led	O
by	O
Google	B-Application
announced	O
their	O
official	O
implementation	O
of	O
the	O
edns-client-subnet	O
IETF	O
Internet	O
Draft	O
,	O
which	O
is	O
intended	O
to	O
accurately	O
localize	O
DNS	O
resolution	O
responses	O
.	O
</s>
<s>
The	O
initiative	O
involves	O
a	O
limited	O
number	O
of	O
leading	O
DNS	O
service	O
providers	O
,	O
such	O
as	O
Google	B-Protocol
Public	I-Protocol
DNS	I-Protocol
,	O
and	O
CDN	O
service	O
providers	O
as	O
well	O
.	O
</s>
<s>
With	O
the	O
edns-client-subnet	O
EDNS0	B-Protocol
option	I-Protocol
,	O
CDNs	O
can	O
now	O
utilize	O
the	O
IP	B-Protocol
address	I-Protocol
of	O
the	O
requesting	O
client	O
's	O
subnet	O
when	O
resolving	O
DNS	O
requests	O
.	O
</s>
<s>
However	O
,	O
the	O
use	O
of	O
EDNS0	B-Protocol
also	O
has	O
drawbacks	O
as	O
it	O
decreases	O
the	O
effectiveness	O
of	O
caching	O
resolutions	O
at	O
the	O
recursive	O
resolvers	O
,	O
increases	O
the	O
total	O
DNS	O
resolution	O
traffic	O
,	O
and	O
raises	O
a	O
privacy	O
concern	O
of	O
exposing	O
the	O
client	O
's	O
subnet	O
.	O
</s>
<s>
In	O
2017	O
,	O
Addy	O
Osmany	O
of	O
Google	B-Application
started	O
referring	O
to	O
software	O
solutions	O
that	O
could	O
integrate	O
naturally	O
with	O
the	O
Responsive	B-General_Concept
Web	I-General_Concept
Design	I-General_Concept
paradigm	O
(	O
with	O
particular	O
reference	O
to	O
the	O
<picture>	O
element	O
)	O
as	O
Image	O
CDNs	O
.	O
</s>
<s>
The	O
expression	O
referred	O
to	O
the	O
ability	O
of	O
a	O
web	O
architecture	O
to	O
serve	O
multiple	O
versions	O
of	O
the	O
same	O
image	O
through	O
HTTP	B-Protocol
,	O
depending	O
on	O
the	O
properties	O
of	O
the	O
browser	O
requesting	O
it	O
,	O
as	O
determined	O
by	O
either	O
the	O
browser	O
or	O
the	O
server-side	O
logic	O
.	O
</s>
<s>
The	O
purpose	O
of	O
Image	O
CDNs	O
was	O
,	O
in	O
Google	B-Application
's	I-Application
vision	O
,	O
to	O
serve	O
high-quality	O
images	O
(	O
or	O
,	O
better	O
,	O
images	O
perceived	O
as	O
high-quality	O
by	O
the	O
human	O
eye	O
)	O
while	O
preserving	O
download	O
speed	O
,	O
thus	O
contributing	O
to	O
a	O
great	O
User	B-Application
experience	I-Application
(	O
UX	O
)	O
.	O
</s>
<s>
Arguably	O
,	O
the	O
Image	O
CDN	O
term	O
was	O
originally	O
a	O
misnomer	O
,	O
as	O
neither	O
Cloudinary	O
nor	O
Imgix	O
(	O
the	O
examples	O
quoted	O
by	O
Google	B-Application
in	O
the	O
2017	O
guide	O
by	O
Addy	O
Osmany	O
)	O
were	O
,	O
at	O
the	O
time	O
,	O
a	O
CDN	O
in	O
the	O
classical	O
sense	O
of	O
the	O
term	O
.	O
</s>
<s>
Many	O
of	O
these	O
solutions	O
were	O
built	O
on	O
top	O
of	O
traditional	O
CDNs	O
,	O
such	O
as	O
Akamai	B-Application
,	O
CloudFront	O
,	O
Fastly	B-General_Concept
,	O
Edgecast	B-Application
and	O
Cloudflare	B-Application
.	O
</s>
<s>
At	O
the	O
same	O
time	O
,	O
other	O
solutions	O
that	O
already	O
provided	O
an	O
image	O
multi-serving	O
service	O
joined	O
the	O
Image	O
CDN	O
definition	O
by	O
either	O
offering	O
CDN	O
functionality	O
natively	O
(	O
ImageEngine	O
)	O
or	O
integrating	O
with	O
one	O
of	O
the	O
existing	O
CDNs	O
(	O
Cloudinary/Akamai	O
,	O
Imgix/Fastly	O
)	O
.	O
</s>
<s>
A	O
Content	B-Operating_System
Delivery	I-Operating_System
Network	I-Operating_System
(	O
CDN	O
)	O
for	O
the	O
fast	O
serving	O
of	O
images	O
.	O
</s>
<s>
the	O
ability	O
to	O
determine	O
the	O
properties	O
of	O
the	O
requesting	O
browser	O
and/or	O
device	O
through	O
analysis	O
of	O
the	O
User-Agent	B-Protocol
string	O
,	O
HTTP	B-Protocol
Accept	O
headers	O
,	O
Client-Hints	O
or	O
JavaScript	B-Language
.	O
</s>
<s>
BitTorrent	B-Protocol
,	O
Inc	O
.	O
</s>
