<s>
Network	B-Protocol
congestion	I-Protocol
in	O
data	B-Architecture
networking	I-Architecture
and	O
queueing	O
theory	O
is	O
the	O
reduced	O
quality	O
of	O
service	O
that	O
occurs	O
when	O
a	O
network	O
node	O
or	O
link	O
is	O
carrying	O
more	O
data	O
than	O
it	O
can	O
handle	O
.	O
</s>
<s>
Typical	O
effects	O
include	O
queueing	B-Protocol
delay	I-Protocol
,	O
packet	B-Protocol
loss	I-Protocol
or	O
the	O
blocking	O
of	O
new	O
connections	O
.	O
</s>
<s>
Network	O
protocols	O
that	O
use	O
aggressive	O
retransmissions	B-Protocol
to	O
compensate	O
for	O
packet	B-Protocol
loss	I-Protocol
due	O
to	O
congestion	O
can	O
increase	O
congestion	O
,	O
even	O
after	O
the	O
initial	O
load	O
has	O
been	O
reduced	O
to	O
a	O
level	O
that	O
would	O
not	O
normally	O
have	O
induced	O
network	B-Protocol
congestion	I-Protocol
.	O
</s>
<s>
These	O
include	O
:	O
exponential	B-Algorithm
backoff	I-Algorithm
in	O
protocols	O
such	O
as	O
CSMA/CA	O
in	O
802.11	O
and	O
the	O
similar	O
CSMA/CD	O
in	O
the	O
original	O
Ethernet	O
,	O
window	O
reduction	O
in	O
TCP	B-Protocol
,	O
and	O
fair	B-Algorithm
queueing	I-Algorithm
in	O
devices	O
such	O
as	O
routers	B-Protocol
and	O
network	B-Protocol
switches	I-Protocol
.	O
</s>
<s>
Other	O
techniques	O
that	O
address	O
congestion	O
include	O
priority	O
schemes	O
which	O
transmit	O
some	O
packets	B-Protocol
with	O
higher	O
priority	O
ahead	O
of	O
others	O
and	O
the	O
explicit	O
allocation	O
of	O
network	O
resources	O
to	O
specific	O
flows	O
through	O
the	O
use	O
of	O
admission	O
control	O
.	O
</s>
<s>
Network	O
resources	O
are	O
limited	O
,	O
including	O
router	B-Protocol
processing	O
time	O
and	O
link	O
throughput	O
.	O
</s>
<s>
Resource	B-General_Concept
contention	I-General_Concept
may	O
occur	O
on	O
networks	O
in	O
several	O
common	O
circumstances	O
.	O
</s>
<s>
A	O
wireless	B-Architecture
LAN	I-Architecture
is	O
easily	O
filled	O
by	O
a	O
single	O
personal	O
computer	O
.	O
</s>
<s>
Even	O
on	O
fast	O
computer	B-Architecture
networks	I-Architecture
,	O
the	O
backbone	B-General_Concept
can	O
easily	O
be	O
congested	O
by	O
a	O
few	O
servers	O
and	O
client	O
PCs	O
.	O
</s>
<s>
Denial-of-service	O
attacks	O
by	O
botnets	B-Architecture
are	O
capable	O
of	O
filling	O
even	O
the	O
largest	O
Internet	B-General_Concept
backbone	I-General_Concept
network	O
links	O
,	O
generating	O
large-scale	O
network	B-Protocol
congestion	I-Protocol
.	O
</s>
<s>
Connection	O
points	O
between	O
a	O
local	B-General_Concept
area	I-General_Concept
network	I-General_Concept
and	O
a	O
wide	O
area	O
network	O
are	O
common	O
choke	O
points	O
.	O
</s>
<s>
When	O
a	O
network	O
is	O
in	O
this	O
condition	O
,	O
it	O
settles	O
into	O
a	O
stable	O
state	O
where	O
traffic	O
demand	O
is	O
high	O
but	O
little	O
useful	O
throughput	O
is	O
available	O
,	O
during	O
which	O
packet	B-Protocol
delay	I-Protocol
and	O
loss	O
occur	O
and	O
quality	O
of	O
service	O
is	O
extremely	O
poor	O
.	O
</s>
<s>
It	O
was	O
first	O
observed	O
on	O
the	O
early	O
Internet	O
in	O
October	O
1986	O
,	O
when	O
the	O
NSFNET	O
phase-I	O
backbone	B-General_Concept
dropped	O
three	O
orders	O
of	O
magnitude	O
from	O
its	O
capacity	O
of	O
32kbit/s	O
to	O
40bit/s	O
,	O
which	O
continued	O
until	O
end	O
nodes	O
started	O
implementing	O
Van	O
Jacobson	O
and	O
Sally	O
Floyd	O
's	O
congestion	O
control	O
between	O
1987	O
and	O
1988	O
.	O
</s>
<s>
When	O
more	O
packets	B-Protocol
were	O
sent	O
than	O
could	O
be	O
handled	O
by	O
intermediate	O
routers	B-Protocol
,	O
the	O
intermediate	O
routers	B-Protocol
discarded	O
many	O
packets	B-Protocol
,	O
expecting	O
the	O
end	O
points	O
of	O
the	O
network	O
to	O
retransmit	O
the	O
information	O
.	O
</s>
<s>
However	O
,	O
early	O
TCP	B-Protocol
implementations	O
had	O
poor	O
retransmission	B-Protocol
behavior	O
.	O
</s>
<s>
When	O
this	O
packet	B-Protocol
loss	I-Protocol
occurred	O
,	O
the	O
endpoints	O
sent	O
extra	O
packets	B-Protocol
that	O
repeated	O
the	O
information	O
lost	O
,	O
doubling	O
the	O
incoming	O
rate	O
.	O
</s>
<s>
This	O
is	O
typically	O
accomplished	O
by	O
reducing	O
the	O
rate	O
of	O
packets	B-Protocol
.	O
</s>
<s>
Examples	O
of	O
optimal	O
rate	O
allocation	O
are	O
max-min	B-Algorithm
fair	I-Algorithm
allocation	I-Algorithm
and	O
Kelly	O
's	O
suggestion	O
of	O
proportionally	B-Algorithm
fair	I-Algorithm
allocation	O
,	O
although	O
many	O
others	O
are	O
possible	O
.	O
</s>
<s>
The	O
Lagrange	B-Algorithm
dual	I-Algorithm
of	O
this	O
problem	O
decouples	O
so	O
that	O
each	O
flow	O
sets	O
its	O
own	O
rate	O
,	O
based	O
only	O
on	O
a	O
price	O
signaled	O
by	O
the	O
network	O
.	O
</s>
<s>
Many	O
current	O
congestion	O
control	O
algorithms	O
can	O
be	O
modelled	O
in	O
this	O
framework	O
,	O
with	O
being	O
either	O
the	O
loss	O
probability	O
or	O
the	O
queueing	B-Protocol
delay	I-Protocol
at	O
link	O
.	O
</s>
<s>
By	O
incremental	O
deployability	O
:	O
Only	O
sender	O
needs	O
modification	O
;	O
sender	O
and	O
receiver	O
need	O
modification	O
;	O
only	O
router	B-Protocol
needs	O
modification	O
;	O
sender	O
,	O
receiver	O
and	O
routers	B-Protocol
need	O
modification	O
.	O
</s>
<s>
Mechanisms	O
have	O
been	O
invented	O
to	O
prevent	O
network	B-Protocol
congestion	I-Protocol
or	O
to	O
deal	O
with	O
a	O
network	O
collapse	O
:	O
</s>
<s>
Other	O
strategies	O
such	O
as	O
slow	O
start	O
ensure	O
that	O
new	O
connections	O
do	O
n't	O
overwhelm	O
the	O
router	B-Protocol
before	O
congestion	O
detection	O
initiates	O
.	O
</s>
<s>
Common	O
router	B-Protocol
congestion	O
avoidance	O
mechanisms	O
include	O
fair	B-Algorithm
queuing	I-Algorithm
and	O
other	O
scheduling	O
algorithms	O
,	O
and	O
random	B-Algorithm
early	I-Algorithm
detection	I-Algorithm
(	O
RED	O
)	O
where	O
packets	B-Protocol
are	O
randomly	O
dropped	O
as	O
congestion	O
is	O
detected	O
.	O
</s>
<s>
Some	O
end-to-end	O
protocols	O
are	O
designed	O
to	O
behave	O
well	O
under	O
congested	O
conditions	O
;	O
TCP	B-Protocol
is	O
a	O
well	O
known	O
example	O
.	O
</s>
<s>
The	O
first	O
TCP	B-Protocol
implementations	O
to	O
handle	O
congestion	O
were	O
described	O
in	O
1984	O
,	O
but	O
Van	O
Jacobson	O
's	O
inclusion	O
of	O
an	O
open	O
source	O
solution	O
in	O
the	O
Berkeley	O
Standard	O
Distribution	O
UNIX	O
(	O
"	O
BSD	B-Operating_System
"	O
)	O
in	O
1988	O
first	O
provided	O
good	O
behavior	O
.	O
</s>
<s>
UDP	B-Protocol
does	O
not	O
control	O
congestion	O
.	O
</s>
<s>
Protocols	O
built	O
atop	O
UDP	B-Protocol
must	O
handle	O
congestion	O
independently	O
.	O
</s>
<s>
Real-time	O
streaming	O
protocols	O
,	O
including	O
many	O
Voice	B-Application
over	I-Application
IP	I-Application
protocols	O
,	O
have	O
this	O
property	O
.	O
</s>
<s>
Thus	O
,	O
special	O
measures	O
,	O
such	O
as	O
quality	O
of	O
service	O
,	O
must	O
be	O
taken	O
to	O
keep	O
packets	B-Protocol
from	O
being	O
dropped	O
in	O
the	O
presence	O
of	O
congestion	O
.	O
</s>
<s>
Connection-oriented	B-Protocol
protocols	I-Protocol
,	O
such	O
as	O
the	O
widely	O
used	O
TCP	B-Protocol
protocol	I-Protocol
watch	O
for	O
packet	B-Protocol
loss	I-Protocol
,	O
or	O
queuing	B-Protocol
delay	I-Protocol
to	O
adjust	O
their	O
transmission	O
rate	O
.	O
</s>
<s>
Various	O
network	B-Protocol
congestion	I-Protocol
avoidance	O
processes	O
support	O
different	O
trade-offs	O
.	O
</s>
<s>
The	O
TCP	B-Protocol
congestion	O
avoidance	O
algorithm	O
is	O
the	O
primary	O
basis	O
for	O
congestion	O
control	O
on	O
the	O
Internet	O
.	O
</s>
<s>
Problems	O
occur	O
when	O
concurrent	O
TCP	B-Protocol
flows	O
experience	O
tail-drops	B-Protocol
,	O
especially	O
when	O
bufferbloat	O
is	O
present	O
.	O
</s>
<s>
This	O
delayed	O
packet	B-Protocol
loss	I-Protocol
interferes	O
with	O
TCP	B-Protocol
's	O
automatic	O
congestion	O
avoidance	O
.	O
</s>
<s>
All	O
flows	O
that	O
experience	O
this	O
packet	B-Protocol
loss	I-Protocol
begin	O
a	O
TCP	B-Protocol
retrain	O
at	O
the	O
same	O
moment	O
–	O
this	O
is	O
called	O
TCP	B-Protocol
global	I-Protocol
synchronization	I-Protocol
.	O
</s>
<s>
Active	B-Algorithm
queue	I-Algorithm
management	I-Algorithm
(	O
AQM	O
)	O
is	O
the	O
reordering	O
or	O
dropping	O
of	O
network	B-Protocol
packets	I-Protocol
inside	O
a	O
transmit	O
buffer	O
that	O
is	O
associated	O
with	O
a	O
network	B-Protocol
interface	I-Protocol
controller	I-Protocol
(	O
NIC	O
)	O
.	O
</s>
<s>
This	O
task	O
is	O
performed	O
by	O
the	O
network	B-Application
scheduler	I-Application
.	O
</s>
<s>
One	O
solution	O
is	O
to	O
use	O
random	B-Algorithm
early	I-Algorithm
detection	I-Algorithm
(	O
RED	O
)	O
on	O
the	O
network	B-Device
equipment	I-Device
's	O
egress	O
queue	O
.	O
</s>
<s>
On	O
networking	B-Device
hardware	I-Device
ports	O
with	O
more	O
than	O
one	O
egress	O
queue	O
,	O
weighted	O
random	B-Algorithm
early	I-Algorithm
detection	I-Algorithm
(	O
WRED	O
)	O
can	O
be	O
used	O
.	O
</s>
<s>
RED	O
indirectly	O
signals	O
TCP	B-Protocol
sender	O
and	O
receiver	O
by	O
dropping	O
some	O
packets	B-Protocol
,	O
e.g.	O
</s>
<s>
50%	O
)	O
and	O
deletes	O
linearly	O
or	O
cubically	O
more	O
packets	B-Protocol
,	O
up	O
to	O
e.g.	O
</s>
<s>
The	O
robust	O
random	B-Algorithm
early	I-Algorithm
detection	I-Algorithm
(	O
RRED	O
)	O
algorithm	O
was	O
proposed	O
to	O
improve	O
the	O
TCP	B-Protocol
throughput	O
against	O
denial-of-service	O
(	O
DoS	O
)	O
attacks	O
,	O
particularly	O
low-rate	O
denial-of-service	O
(	O
LDoS	O
)	O
attacks	O
.	O
</s>
<s>
Experiments	O
confirmed	O
that	O
RED-like	O
algorithms	O
were	O
vulnerable	O
under	O
LDoS	O
attacks	O
due	O
to	O
the	O
oscillating	O
TCP	B-Protocol
queue	O
size	O
caused	O
by	O
the	O
attacks	O
.	O
</s>
<s>
Some	O
network	B-Device
equipment	I-Device
is	O
equipped	O
with	O
ports	O
that	O
can	O
follow	O
and	O
measure	O
each	O
flow	O
and	O
are	O
thereby	O
able	O
to	O
signal	O
a	O
too	O
big	O
bandwidth	O
flow	O
according	O
to	O
some	O
quality	O
of	O
service	O
policy	O
.	O
</s>
<s>
This	O
is	O
better	O
than	O
the	O
indirect	O
congestion	O
notification	O
signaled	O
by	O
packet	B-Protocol
loss	I-Protocol
by	O
the	O
RED/WRED	O
algorithms	O
,	O
but	O
it	O
requires	O
support	O
by	O
both	O
hosts	O
.	O
</s>
<s>
When	O
a	O
router	B-Protocol
receives	O
a	O
packet	B-Protocol
marked	O
as	O
ECN-capable	O
and	O
the	O
router	B-Protocol
anticipates	O
congestion	O
,	O
it	O
sets	O
the	O
ECN	O
flag	O
,	O
notifying	O
the	O
sender	O
of	O
congestion	O
.	O
</s>
<s>
The	O
sender	O
should	O
respond	O
by	O
decreasing	O
its	O
transmission	O
bandwidth	O
,	O
e.g.	O
,	O
by	O
decreasing	O
its	O
sending	O
rate	O
by	O
reducing	O
the	O
TCP	B-Protocol
window	I-Protocol
size	O
or	O
by	O
other	O
means	O
.	O
</s>
<s>
Effective	O
congestion	O
notifications	O
can	O
be	O
propagated	O
to	O
transport	O
layer	O
protocols	O
,	O
such	O
as	O
TCP	B-Protocol
and	O
UDP	B-Protocol
,	O
for	O
the	O
appropriate	O
adjustments	O
.	O
</s>
<s>
WiFi	O
,	O
3G	B-Architecture
and	O
other	O
networks	O
with	O
a	O
radio	O
layer	O
are	O
susceptible	O
to	O
data	O
loss	O
due	O
to	O
interference	O
and	O
may	O
experience	O
poor	O
throughput	O
in	O
some	O
cases	O
.	O
</s>
<s>
The	O
TCP	B-Protocol
connections	O
running	O
over	O
a	O
radio-based	O
physical	B-Application
layer	I-Application
see	O
the	O
data	O
loss	O
and	O
tend	O
to	O
erroneously	O
believe	O
that	O
congestion	O
is	O
occurring	O
.	O
</s>
<s>
Older	O
web	B-Application
browsers	I-Application
created	O
many	O
short-lived	O
connections	O
and	O
opened	O
and	O
closed	O
the	O
connection	O
for	O
each	O
file	O
.	O
</s>
<s>
To	O
avoid	O
this	O
problem	O
,	O
modern	O
browsers	B-Application
either	O
open	O
multiple	O
connections	O
simultaneously	O
or	O
reuse	B-Protocol
one	I-Protocol
connection	I-Protocol
for	O
all	O
files	O
requested	O
from	O
a	O
particular	O
server	O
.	O
</s>
<s>
Examples	O
include	O
Contention-Free	O
Transmission	O
Opportunities	O
(	O
CFTXOPs	O
)	O
in	O
the	O
ITU-T	B-General_Concept
G.hn	I-General_Concept
standard	O
for	O
home	O
networking	O
over	O
legacy	O
wiring	O
,	O
Resource	B-Protocol
Reservation	I-Protocol
Protocol	I-Protocol
for	O
IP	O
networks	O
and	O
Stream	O
Reservation	O
Protocol	O
for	O
Ethernet	O
.	O
</s>
