<s>
Packet	B-Protocol
loss	I-Protocol
occurs	O
when	O
one	O
or	O
more	O
packets	B-Protocol
of	O
data	O
travelling	O
across	O
a	O
computer	B-Architecture
network	I-Architecture
fail	O
to	O
reach	O
their	O
destination	O
.	O
</s>
<s>
Packet	B-Protocol
loss	I-Protocol
is	O
either	O
caused	O
by	O
errors	O
in	O
data	O
transmission	O
,	O
typically	O
across	O
wireless	B-General_Concept
networks	I-General_Concept
,	O
or	O
network	B-Protocol
congestion	I-Protocol
.	O
</s>
<s>
Packet	B-Protocol
loss	I-Protocol
is	O
measured	O
as	O
a	O
percentage	O
of	O
packets	B-Protocol
lost	O
with	O
respect	O
to	O
packets	B-Protocol
sent	O
.	O
</s>
<s>
The	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
detects	O
packet	B-Protocol
loss	I-Protocol
and	O
performs	O
retransmissions	O
to	O
ensure	O
reliable	B-Protocol
messaging	I-Protocol
.	O
</s>
<s>
Packet	B-Protocol
loss	I-Protocol
in	O
a	O
TCP	O
connection	O
is	O
also	O
used	O
to	O
avoid	O
congestion	O
and	O
thus	O
produces	O
an	O
intentionally	O
reduced	O
throughput	O
for	O
the	O
connection	O
.	O
</s>
<s>
In	O
real-time	O
applications	O
like	O
streaming	O
media	O
or	O
online	B-Application
games	I-Application
,	O
packet	B-Protocol
loss	I-Protocol
can	O
affect	O
a	O
user	O
's	O
quality	O
of	O
experience	O
(	O
QoE	O
)	O
.	O
</s>
<s>
The	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
is	O
designed	O
according	O
to	O
the	O
end-to-end	B-Application
principle	I-Application
as	O
a	O
best-effort	O
delivery	O
service	O
,	O
with	O
the	O
intention	O
of	O
keeping	O
the	O
logic	O
routers	B-Protocol
must	O
implement	O
,	O
as	O
simple	O
as	O
possible	O
.	O
</s>
<s>
If	O
the	O
network	O
made	O
reliable	B-Protocol
delivery	I-Protocol
guarantees	O
on	O
its	O
own	O
,	O
that	O
would	O
require	O
store	B-Protocol
and	I-Protocol
forward	I-Protocol
infrastructure	O
,	O
where	O
each	O
router	B-Protocol
devotes	O
a	O
significant	O
amount	O
of	O
storage	O
space	O
to	O
packets	B-Protocol
while	O
it	O
waits	O
to	O
verify	O
that	O
the	O
next	O
node	O
properly	O
received	O
them	O
.	O
</s>
<s>
A	O
reliable	O
network	O
would	O
not	O
be	O
able	O
to	O
maintain	O
its	O
delivery	O
guarantees	O
in	O
the	O
event	O
of	O
a	O
router	B-Protocol
failure	O
.	O
</s>
<s>
For	O
example	O
,	O
with	O
live	O
streaming	O
media	O
,	O
it	O
is	O
more	O
important	O
to	O
deliver	O
recent	O
packets	B-Protocol
quickly	O
than	O
to	O
ensure	O
that	O
stale	O
packets	B-Protocol
are	O
eventually	O
delivered	O
.	O
</s>
<s>
An	O
application	O
or	O
user	O
may	O
also	O
decide	O
to	O
retry	O
an	O
operation	O
that	O
is	O
taking	O
a	O
long	O
time	O
,	O
in	O
which	O
case	O
another	O
set	O
of	O
packets	B-Protocol
will	O
be	O
added	O
to	O
the	O
burden	O
of	O
delivering	O
the	O
original	O
set	O
.	O
</s>
<s>
To	O
avoid	O
all	O
of	O
these	O
problems	O
,	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
allows	O
for	O
routers	B-Protocol
to	O
simply	O
drop	O
packets	B-Protocol
if	O
the	O
router	B-Protocol
or	O
a	O
network	O
segment	O
is	O
too	O
busy	O
to	O
deliver	O
the	O
data	O
in	O
a	O
timely	O
fashion	O
.	O
</s>
<s>
Dropping	O
of	O
packets	B-Protocol
acts	O
as	O
an	O
implicit	O
signal	O
that	O
the	O
network	O
is	O
congested	O
,	O
and	O
may	O
cause	O
senders	O
to	O
reduce	O
the	O
amount	O
of	O
bandwidth	O
consumed	O
,	O
or	O
attempt	O
to	O
find	O
another	O
path	O
.	O
</s>
<s>
For	O
example	O
,	O
using	O
perceived	O
packet	B-Protocol
loss	I-Protocol
as	O
feedback	O
to	O
discover	O
congestion	O
,	O
the	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
is	O
designed	O
so	O
that	O
excessive	O
packet	B-Protocol
loss	I-Protocol
will	O
cause	O
the	O
sender	O
to	O
throttle	O
back	O
and	O
stop	O
flooding	O
the	O
bottleneck	O
point	O
with	O
data	O
.	O
</s>
<s>
Packets	B-Protocol
may	O
also	O
be	O
dropped	O
if	O
the	O
IPv4	B-Protocol
header	I-Protocol
checksum	I-Protocol
or	O
the	O
Ethernet	O
frame	B-Protocol
check	I-Protocol
sequence	I-Protocol
indicates	O
the	O
packet	B-Protocol
has	O
been	O
corrupted	O
.	O
</s>
<s>
Packet	B-Protocol
loss	I-Protocol
can	O
also	O
be	O
caused	O
by	O
a	O
packet	B-Protocol
drop	I-Protocol
attack	I-Protocol
.	O
</s>
<s>
Wireless	B-General_Concept
networks	I-General_Concept
are	O
susceptible	O
to	O
a	O
number	O
of	O
factors	O
that	O
can	O
corrupt	O
or	O
lose	O
packets	B-Protocol
in	O
transit	O
,	O
such	O
as	O
radio	O
frequency	O
interference	O
(	O
RFI	O
)	O
,	O
radio	O
signals	O
that	O
are	O
too	O
weak	O
due	O
to	O
distance	O
or	O
multi-path	O
fading	O
,	O
faulty	O
networking	O
hardware	O
,	O
or	O
faulty	O
network	O
drivers	O
.	O
</s>
<s>
Wi-Fi	O
is	O
inherently	B-Protocol
unreliable	I-Protocol
and	O
even	O
when	O
two	O
identical	O
Wi-Fi	O
receivers	O
are	O
placed	O
within	O
close	O
proximity	O
of	O
each	O
other	O
,	O
they	O
do	O
not	O
exhibit	O
similar	O
patterns	O
of	O
packet	B-Protocol
loss	I-Protocol
,	O
as	O
one	O
might	O
expect	O
.	O
</s>
<s>
Cellular	O
networks	O
can	O
experience	O
packet	B-Protocol
loss	I-Protocol
caused	O
by	O
,	O
"	O
high	O
bit	O
error	O
rate	O
(	O
BER	O
)	O
,	O
unstable	O
channel	O
characteristics	O
,	O
and	O
user	O
mobility.	O
"	O
</s>
<s>
TCP	O
's	O
intentional	O
throttling	O
behavior	O
prevents	O
wireless	B-General_Concept
networks	I-General_Concept
from	O
performing	O
near	O
their	O
theoretical	O
potential	O
transfer	O
rates	O
because	O
unmodified	O
TCP	O
treats	O
all	O
dropped	O
packets	B-Protocol
as	O
if	O
they	O
were	O
caused	O
by	O
network	B-Protocol
congestion	I-Protocol
,	O
and	O
so	O
may	O
throttle	O
wireless	B-General_Concept
networks	I-General_Concept
even	O
when	O
they	O
are	O
n't	O
actually	O
congested	O
.	O
</s>
<s>
Network	B-Protocol
congestion	I-Protocol
is	O
a	O
cause	O
of	O
packet	B-Protocol
loss	I-Protocol
that	O
can	O
affect	O
all	O
types	O
of	O
networks	O
.	O
</s>
<s>
When	O
content	O
arrives	O
for	O
a	O
sustained	O
period	O
at	O
a	O
given	O
router	B-Protocol
or	O
network	O
segment	O
at	O
a	O
rate	O
greater	O
than	O
it	O
is	O
possible	O
to	O
send	O
through	O
,	O
there	O
is	O
no	O
other	O
option	O
than	O
to	O
drop	O
packets	B-Protocol
.	O
</s>
<s>
If	O
a	O
single	O
router	B-Protocol
or	O
link	O
is	O
constraining	O
the	O
capacity	O
of	O
the	O
complete	O
travel	O
path	O
or	O
of	O
network	O
travel	O
in	O
general	O
,	O
it	O
is	O
known	O
as	O
a	O
bottleneck	O
.	O
</s>
<s>
In	O
some	O
cases	O
,	O
packets	B-Protocol
are	O
intentionally	O
dropped	O
by	O
routing	O
routines	O
,	O
or	O
through	O
network	O
dissuasion	O
technique	O
for	O
operational	O
management	O
purposes	O
.	O
</s>
<s>
Packet	B-Protocol
loss	I-Protocol
directly	O
reduces	O
throughput	O
for	O
a	O
given	O
sender	O
as	O
some	O
sent	O
data	O
is	O
never	O
received	O
and	O
ca	O
n't	O
be	O
counted	O
as	O
throughput	O
.	O
</s>
<s>
Packet	B-Protocol
loss	I-Protocol
indirectly	O
reduces	O
throughput	O
as	O
some	O
transport	O
layer	O
protocols	O
interpret	O
loss	O
as	O
an	O
indication	O
of	O
congestion	O
and	O
adjust	O
their	O
transmission	O
rate	O
to	O
avoid	O
congestive	O
collapse	O
.	O
</s>
<s>
When	O
reliable	B-Protocol
delivery	I-Protocol
is	O
necessary	O
,	O
packet	B-Protocol
loss	I-Protocol
increases	O
latency	O
due	O
to	O
additional	O
time	O
needed	O
for	O
retransmission	O
.	O
</s>
<s>
Assuming	O
no	O
retransmission	O
,	O
packets	B-Protocol
experiencing	O
the	O
worst	O
delays	O
might	O
be	O
preferentially	O
dropped	O
(	O
depending	O
on	O
the	O
queuing	B-Application
discipline	I-Application
used	O
)	O
,	O
resulting	O
in	O
lower	O
latency	O
overall	O
.	O
</s>
<s>
Packet	B-Protocol
loss	I-Protocol
may	O
be	O
measured	O
as	O
frame	B-Protocol
loss	I-Protocol
rate	I-Protocol
defined	O
as	O
the	O
percentage	O
of	O
frames	O
that	O
should	O
have	O
been	O
forwarded	O
by	O
a	O
network	O
but	O
were	O
not	O
.	O
</s>
<s>
Packet	B-Protocol
loss	I-Protocol
is	O
closely	O
associated	O
with	O
quality	O
of	O
service	O
considerations	O
.	O
</s>
<s>
The	O
amount	O
of	O
packet	B-Protocol
loss	I-Protocol
that	O
is	O
acceptable	O
depends	O
on	O
the	O
type	O
of	O
data	O
being	O
sent	O
.	O
</s>
<s>
For	O
example	O
,	O
for	O
voice	B-Application
over	I-Application
IP	I-Application
traffic	O
,	O
one	O
commentator	O
reckoned	O
that	O
"[m]issing	O
one	O
or	O
two	O
packets	B-Protocol
every	O
now	O
and	O
then	O
will	O
not	O
affect	O
the	O
quality	O
of	O
the	O
conversation	O
.	O
</s>
<s>
Losses	O
between	O
5%	O
and	O
10%	O
of	O
the	O
total	O
packet	B-Protocol
stream	O
will	O
affect	O
the	O
quality	O
significantly.	O
"	O
</s>
<s>
Another	O
described	O
less	O
than	O
1%	O
packet	B-Protocol
loss	I-Protocol
as	O
"	O
good	O
"	O
for	O
streaming	O
audio	O
or	O
video	O
,	O
and	O
1	O
–	O
2.5	O
%	O
as	O
"	O
acceptable	O
"	O
.	O
</s>
<s>
Packet	B-Protocol
loss	I-Protocol
is	O
detected	O
by	O
reliable	O
protocols	O
such	O
as	O
TCP	O
.	O
</s>
<s>
Reliable	O
protocols	O
react	O
to	O
packet	B-Protocol
loss	I-Protocol
automatically	O
,	O
so	O
when	O
a	O
person	O
such	O
as	O
a	O
network	O
administrator	O
needs	O
to	O
detect	O
and	O
diagnose	O
packet	B-Protocol
loss	I-Protocol
,	O
they	O
typically	O
use	O
status	O
information	O
from	O
network	O
equipment	O
or	O
purpose-built	O
tools	O
.	O
</s>
<s>
The	O
Internet	B-Protocol
Control	I-Protocol
Message	I-Protocol
Protocol	I-Protocol
provides	O
an	O
echo	O
functionality	O
,	O
where	O
a	O
special	O
packet	B-Protocol
is	O
transmitted	O
that	O
always	O
produces	O
a	O
reply	O
.	O
</s>
<s>
Tools	O
such	O
as	O
ping	B-Protocol
,	O
traceroute	B-Protocol
,	O
MTR	B-Protocol
and	O
PathPing	B-Protocol
use	O
this	O
protocol	O
to	O
provide	O
a	O
visual	O
representation	O
of	O
the	O
path	O
packets	B-Protocol
are	O
taking	O
,	O
and	O
to	O
measure	O
packet	B-Protocol
loss	I-Protocol
at	O
each	O
hop	B-Protocol
.	O
</s>
<s>
Many	O
routers	B-Protocol
have	O
status	O
pages	O
or	O
logs	O
,	O
where	O
the	O
owner	O
can	O
find	O
the	O
number	O
or	O
percentage	O
of	O
packets	B-Protocol
dropped	O
over	O
a	O
particular	O
period	O
.	O
</s>
<s>
Per	O
the	O
end-to-end	B-Application
principle	I-Application
,	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
leaves	O
responsibility	O
for	O
packet	B-Protocol
recovery	O
through	O
the	O
retransmission	O
of	O
dropped	O
packets	B-Protocol
to	O
the	O
endpoints	O
-	O
the	O
computers	O
sending	O
and	O
receiving	O
the	O
data	O
.	O
</s>
<s>
Network	O
transport	O
protocols	O
such	O
as	O
TCP	O
provide	O
endpoints	O
with	O
an	O
easy	O
way	O
to	O
ensure	O
reliable	B-Protocol
delivery	I-Protocol
of	O
packets	B-Protocol
so	O
that	O
individual	O
applications	O
do	O
n't	O
need	O
to	O
implement	O
the	O
logic	O
for	O
this	O
themselves	O
.	O
</s>
<s>
In	O
the	O
event	O
of	O
packet	B-Protocol
loss	I-Protocol
,	O
the	O
receiver	O
asks	O
for	O
retransmission	O
or	O
the	O
sender	O
automatically	O
resends	O
any	O
segments	O
that	O
have	O
not	O
been	O
acknowledged	O
.	O
</s>
<s>
Although	O
TCP	O
can	O
recover	O
from	O
packet	B-Protocol
loss	I-Protocol
,	O
retransmitting	O
missing	O
packets	B-Protocol
reduces	O
the	O
throughput	O
of	O
the	O
connection	O
as	O
receivers	O
wait	O
for	O
retransmissions	O
and	O
additional	O
bandwidth	O
is	O
consumed	O
by	O
them	O
.	O
</s>
<s>
In	O
certain	O
variants	O
of	O
TCP	O
,	O
if	O
a	O
transmitted	O
packet	B-Protocol
is	O
lost	O
,	O
it	O
will	O
be	O
re-sent	O
along	O
with	O
every	O
packet	B-Protocol
that	O
had	O
already	O
been	O
sent	O
after	O
it	O
.	O
</s>
<s>
Protocols	O
such	O
as	O
User	B-Protocol
Datagram	I-Protocol
Protocol	I-Protocol
(	O
UDP	O
)	O
provide	O
no	O
recovery	O
for	O
lost	B-Protocol
packets	I-Protocol
.	O
</s>
<s>
Applications	O
that	O
use	O
UDP	O
are	O
expected	O
to	O
implement	O
their	O
own	O
mechanisms	O
for	O
handling	O
packet	B-Protocol
loss	I-Protocol
,	O
if	O
needed	O
.	O
</s>
<s>
There	O
are	O
many	O
queuing	B-Application
disciplines	I-Application
used	O
for	O
determining	O
which	O
packets	B-Protocol
to	O
drop	O
.	O
</s>
<s>
Most	O
basic	O
networking	O
equipment	O
will	O
use	O
FIFO	B-Operating_System
queuing	I-Operating_System
for	O
packets	B-Protocol
waiting	O
to	O
go	O
through	O
the	O
bottleneck	O
and	O
they	O
will	O
drop	O
the	O
packet	B-Protocol
if	O
the	O
queue	O
is	O
full	O
at	O
the	O
time	O
the	O
packet	B-Protocol
is	O
received	O
.	O
</s>
<s>
This	O
type	O
of	O
packet	B-Protocol
dropping	I-Protocol
is	O
called	O
tail	B-Protocol
drop	I-Protocol
.	O
</s>
<s>
Other	O
full	O
queue	O
mechanisms	O
include	O
random	B-Algorithm
early	I-Algorithm
drop	I-Algorithm
or	O
weighted	O
random	B-Algorithm
early	I-Algorithm
drop	I-Algorithm
.	O
</s>
<s>
Dropping	O
packets	B-Protocol
is	O
undesirable	O
as	O
the	O
packet	B-Protocol
is	O
either	O
lost	O
or	O
must	O
be	O
retransmitted	O
and	O
this	O
can	O
impact	O
real-time	O
throughput	O
;	O
however	O
,	O
increasing	O
the	O
buffer	O
size	O
can	O
lead	O
to	O
bufferbloat	O
which	O
has	O
its	O
own	O
impact	O
on	O
latency	O
and	O
jitter	O
during	O
congestion	O
.	O
</s>
<s>
In	O
cases	O
where	O
quality	O
of	O
service	O
is	O
rate	O
limiting	O
a	O
connection	O
,	O
e.g.	O
,	O
using	O
a	O
leaky	B-Algorithm
bucket	I-Algorithm
algorithm	I-Algorithm
,	O
packets	B-Protocol
may	O
be	O
intentionally	O
dropped	O
in	O
order	O
to	O
slow	O
down	O
specific	O
services	O
to	O
ensure	O
available	O
bandwidth	O
for	O
other	O
services	O
marked	O
with	O
higher	O
importance	O
.	O
</s>
<s>
For	O
this	O
reason	O
,	O
packet	B-Protocol
loss	I-Protocol
is	O
not	O
necessarily	O
an	O
indication	O
of	O
poor	O
connection	O
reliability	O
or	O
signs	O
of	O
a	O
bandwidth	O
bottleneck	O
.	O
</s>
