<s>
Head-of-line	B-Protocol
blocking	I-Protocol
(	O
HOL	B-Protocol
blocking	I-Protocol
)	O
in	O
computer	B-Architecture
networking	I-Architecture
is	O
a	O
performance-limiting	O
phenomenon	O
that	O
occurs	O
when	O
a	O
line	O
of	O
packets	B-Protocol
is	O
held	O
up	O
in	O
a	O
queue	B-Application
by	O
a	O
first	O
packet	B-Protocol
.	O
</s>
<s>
Examples	O
include	O
input	O
buffered	B-General_Concept
network	B-Protocol
switches	I-Protocol
,	O
out-of-order	B-Protocol
delivery	I-Protocol
and	O
multiple	O
requests	O
in	O
HTTP	B-Protocol
pipelining	I-Protocol
.	O
</s>
<s>
A	O
switch	O
may	O
be	O
composed	O
of	O
buffered	B-General_Concept
input	O
ports	O
,	O
a	O
switch	O
fabric	O
and	O
buffered	B-General_Concept
output	O
ports	O
.	O
</s>
<s>
If	O
first-in	B-Operating_System
first-out	I-Operating_System
(	O
FIFO	B-Operating_System
)	O
input	B-General_Concept
buffers	I-General_Concept
are	O
used	O
,	O
only	O
the	O
oldest	O
packet	B-Protocol
is	O
available	O
for	O
forwarding	O
.	O
</s>
<s>
If	O
the	O
oldest	O
packet	B-Protocol
cannot	O
be	O
transmitted	O
due	O
to	O
its	O
target	O
output	O
being	O
busy	O
,	O
then	O
more	O
recent	O
arrivals	O
cannot	O
be	O
forwarded	O
.	O
</s>
<s>
The	O
output	O
may	O
be	O
busy	O
if	O
there	O
is	O
output	O
contention	B-General_Concept
.	O
</s>
<s>
Without	O
HOL	B-Protocol
blocking	I-Protocol
,	O
the	O
new	O
arrivals	O
could	O
potentially	O
be	O
forwarded	O
around	O
the	O
stuck	O
oldest	O
packet	B-Protocol
to	O
their	O
respective	O
destinations	O
.	O
</s>
<s>
HOL	B-Protocol
blocking	I-Protocol
can	O
produce	O
performance-degrading	O
effects	O
in	O
input-buffered	O
systems	O
.	O
</s>
<s>
For	O
FIFO	B-Operating_System
input	B-General_Concept
buffers	I-General_Concept
,	O
a	O
simple	O
model	O
of	O
fixed-sized	O
cells	O
to	O
uniformly	O
distributed	O
destinations	O
,	O
causes	O
the	O
throughput	O
to	O
be	O
limited	O
to	O
58.6	O
%	O
of	O
the	O
total	O
as	O
the	O
number	O
of	O
links	O
becomes	O
large	O
.	O
</s>
<s>
One	O
way	O
to	O
overcome	O
this	O
limitation	O
is	O
by	O
using	O
virtual	O
output	O
queues	B-Application
.	O
</s>
<s>
Only	O
switches	O
with	O
input	O
buffering	O
can	O
suffer	O
HOL	B-Protocol
blocking	I-Protocol
.	O
</s>
<s>
With	O
sufficient	O
internal	O
bandwidth	O
,	O
input	O
buffering	O
is	O
unnecessary	O
;	O
all	O
buffering	O
is	O
handled	O
at	O
outputs	O
and	O
HOL	B-Protocol
blocking	I-Protocol
is	O
avoided	O
.	O
</s>
<s>
This	O
no-input-buffering	O
architecture	O
is	O
common	O
in	O
small	O
to	O
medium-sized	O
ethernet	B-Protocol
switches	I-Protocol
.	O
</s>
<s>
Out-of-order	B-Protocol
delivery	I-Protocol
occurs	O
when	O
sequenced	O
packets	B-Protocol
arrive	O
out	O
of	O
order	O
.	O
</s>
<s>
This	O
may	O
happen	O
due	O
to	O
different	O
paths	O
taken	O
by	O
the	O
packets	B-Protocol
or	O
from	O
packets	B-Protocol
being	O
dropped	O
and	O
resent	O
.	O
</s>
<s>
HOL	B-Protocol
blocking	I-Protocol
can	O
significantly	O
increase	O
packet	B-Protocol
reordering	I-Protocol
.	O
</s>
<s>
While	O
atomic	B-Operating_System
broadcast	I-Operating_System
algorithms	O
solve	O
the	O
single	B-Architecture
point	I-Architecture
of	I-Architecture
failure	I-Architecture
problem	O
of	O
centralized	O
servers	O
,	O
those	O
algorithms	O
introduce	O
a	O
head-of-line	B-Protocol
blocking	I-Protocol
problem	O
.	O
</s>
<s>
The	O
Bimodal	O
Multicast	O
algorithm	O
,	O
a	O
randomized	B-General_Concept
algorithm	I-General_Concept
that	O
uses	O
a	O
gossip	B-Operating_System
protocol	I-Operating_System
,	O
avoids	O
head-of-line	B-Protocol
blocking	I-Protocol
by	O
allowing	O
some	O
messages	O
to	O
be	O
received	O
out-of-order	O
.	O
</s>
<s>
One	O
form	O
of	O
HOL	B-Protocol
blocking	I-Protocol
in	O
HTTP/1.1	O
is	O
when	O
the	O
number	O
of	O
allowed	O
parallel	O
requests	O
in	O
the	O
browser	O
is	O
used	O
up	O
,	O
and	O
subsequent	O
requests	O
need	O
to	O
wait	O
for	O
the	O
former	O
ones	O
to	O
complete	O
.	O
</s>
<s>
HTTP/2	B-Protocol
addresses	O
this	O
issue	O
through	O
request	O
multiplexing	O
,	O
which	O
eliminates	O
HOL	B-Protocol
blocking	I-Protocol
at	O
the	O
application	O
layer	O
,	O
but	O
HOL	O
still	O
exists	O
at	O
the	O
transport	O
(	O
TCP	O
)	O
layer	O
.	O
</s>
<s>
Head-of-line	B-Protocol
blocking	I-Protocol
can	O
occur	O
in	O
reliable	O
byte	O
streams	O
:	O
if	O
packets	B-Protocol
are	O
reordered	O
or	O
lost	B-Protocol
and	O
need	O
to	O
be	O
retransmitted	O
(	O
and	O
thus	O
arrive	O
out-of-order	O
)	O
,	O
data	O
from	O
sequentially	O
later	O
parts	O
of	O
the	O
stream	O
may	O
be	O
received	O
before	O
sequentially	O
earlier	O
parts	O
of	O
the	O
stream	O
;	O
however	O
,	O
the	O
later	O
data	O
cannot	O
typically	O
be	O
used	O
until	O
the	O
earlier	O
data	O
has	O
been	O
received	O
,	O
incurring	O
network	B-Protocol
latency	I-Protocol
.	O
</s>
<s>
If	O
multiple	O
independent	O
higher-level	O
messages	O
are	O
encapsulated	B-Protocol
and	O
multiplexed	B-Protocol
onto	O
a	O
single	O
reliable	O
byte	O
stream	O
,	O
then	O
head-of-line	B-Protocol
blocking	I-Protocol
can	O
cause	O
processing	O
of	O
a	O
fully-received	O
message	O
that	O
was	O
sent	O
later	O
to	O
wait	O
for	O
delivery	O
of	O
a	O
message	O
that	O
was	O
sent	O
earlier	O
.	O
</s>
<s>
This	O
affects	O
,	O
for	O
example	O
,	O
HTTP/2	B-Protocol
,	O
which	O
frames	B-Protocol
multiple	O
requestresponse	O
pairs	O
onto	O
a	O
single	O
stream	O
;	O
HTTP/3	B-Protocol
,	O
which	O
has	O
an	O
application-layer	B-Protocol
framing	I-Protocol
design	O
and	O
uses	O
datagram	B-Protocol
rather	O
than	O
stream	O
transport	O
,	O
avoids	O
this	O
problem	O
.	O
</s>
<s>
The	O
latency	O
degradation	O
from	O
head-of-line	B-Protocol
blocking	I-Protocol
depends	O
on	O
the	O
underlying	O
packet	B-Protocol
loss	I-Protocol
rate	O
and	O
round-trip	O
time	O
,	O
with	O
higher	O
losses	O
producing	O
worse	O
latency	O
.	O
</s>
<s>
Without	O
changing	O
the	O
stream	O
abstraction	O
,	O
reducing	O
packet	B-Protocol
loss	I-Protocol
can	O
reduce	O
the	O
harm	O
from	O
head-of-line	B-Protocol
blocking	I-Protocol
;	O
an	O
alternative	O
is	O
to	O
implement	O
the	O
reliable	O
byte	O
stream	O
using	O
forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
to	O
send	O
redundant	O
data	O
so	O
that	O
a	O
certain	O
amount	O
of	O
loss	O
can	O
be	O
tolerated	O
without	O
incurring	O
retransmissions	O
.	O
</s>
