<s>
On	O
an	O
Ethernet	O
connection	O
,	O
a	O
duplex	B-Protocol
mismatch	I-Protocol
is	O
a	O
condition	O
where	O
two	O
connected	O
devices	O
operate	O
in	O
different	O
duplex	O
modes	O
,	O
that	O
is	O
,	O
one	O
operates	O
in	O
half	O
duplex	O
while	O
the	O
other	O
one	O
operates	O
in	O
full	O
duplex	O
.	O
</s>
<s>
The	O
effect	O
of	O
a	O
duplex	B-Protocol
mismatch	I-Protocol
is	O
a	O
link	O
that	O
operates	O
inefficiently	O
.	O
</s>
<s>
Duplex	B-Protocol
mismatch	I-Protocol
may	O
be	O
caused	O
by	O
manually	O
setting	O
two	O
connected	O
network	O
interfaces	O
at	O
different	O
duplex	O
modes	O
or	O
by	O
connecting	O
a	O
device	O
that	O
performs	O
autonegotiation	B-Protocol
to	O
one	O
that	O
is	O
manually	O
set	O
to	O
a	O
full	O
duplex	O
mode	O
.	O
</s>
<s>
When	O
a	O
device	O
set	O
to	O
autonegotiation	B-Protocol
is	O
connected	O
to	O
a	O
device	O
that	O
is	O
not	O
using	O
autonegotiation	B-Protocol
,	O
the	O
autonegotiation	B-Protocol
process	O
fails	O
.	O
</s>
<s>
For	O
backward	O
compatibility	O
with	O
Ethernet	B-Protocol
hubs	I-Protocol
,	O
the	O
standard	O
requires	O
the	O
autonegotiating	O
device	O
to	O
use	O
half	O
duplex	O
in	O
these	O
conditions	O
.	O
</s>
<s>
Therefore	O
,	O
the	O
autonegotiating	O
end	O
of	O
the	O
connection	O
uses	O
half	O
duplex	O
while	O
the	O
non-negotiating	O
peer	O
is	O
locked	O
at	O
full	O
duplex	O
,	O
and	O
this	O
is	O
a	O
duplex	B-Protocol
mismatch	I-Protocol
.	O
</s>
<s>
The	O
Ethernet	O
standards	O
and	O
major	O
Ethernet	O
equipment	O
manufacturers	O
recommend	O
enabling	O
autonegotiation	B-Protocol
.	O
</s>
<s>
Nevertheless	O
,	O
network	O
equipment	O
allows	O
autonegotiation	B-Protocol
to	O
be	O
disabled	O
and	O
on	O
some	O
networks	O
,	O
autonegotiation	B-Protocol
is	O
disabled	O
on	O
all	O
ports	O
and	O
a	O
fixed	O
modality	O
of	O
100	O
Mbit/s	O
and	O
full	O
duplex	O
is	O
used	O
.	O
</s>
<s>
That	O
was	O
often	O
done	O
by	O
network	O
administrators	O
intentionally	O
upon	O
the	O
introduction	O
of	O
autonegotiation	B-Protocol
,	O
because	O
of	O
interoperability	O
issues	O
with	O
the	O
initial	O
autonegotiation	B-Protocol
specification	O
.	O
</s>
<s>
Since	O
autonegotiation	B-Protocol
is	O
generally	O
the	O
manufacturer	O
’s	O
default	O
setting	O
it	O
is	O
almost	O
certain	O
that	O
,	O
in	O
an	O
environment	O
where	O
the	O
policy	O
is	O
to	O
have	O
fixed	O
port	O
settings	O
,	O
someone	O
will	O
sooner	O
or	O
later	O
leave	O
a	O
port	O
set	O
to	O
use	O
autonegotiation	B-Protocol
by	O
mistake	O
.	O
</s>
<s>
Communication	O
is	O
possible	O
over	O
a	O
connection	O
in	O
spite	O
of	O
a	O
duplex	B-Protocol
mismatch	I-Protocol
.	O
</s>
<s>
As	O
a	O
result	O
,	O
a	O
simple	O
ping	B-Protocol
command	I-Protocol
fails	O
to	O
detect	O
a	O
duplex	B-Protocol
mismatch	I-Protocol
because	O
single	O
packets	O
and	O
their	O
resulting	O
acknowledgments	O
at	O
1-second	O
intervals	O
do	O
not	O
cause	O
any	O
problem	O
on	O
the	O
network	O
.	O
</s>
<s>
A	O
duplex	B-Protocol
mismatch	I-Protocol
causes	O
problems	O
when	O
both	O
ends	O
of	O
the	O
connection	O
attempt	O
to	O
transfer	O
data	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
Indeed	O
,	O
when	O
a	O
large	O
data	O
transfer	O
is	O
sent	O
over	O
a	O
TCP	B-Protocol
,	O
data	O
is	O
sent	O
in	O
multiple	O
packets	O
,	O
some	O
of	O
which	O
will	O
trigger	O
an	O
acknowledgment	O
packet	O
back	O
to	O
the	O
sender	O
.	O
</s>
<s>
The	O
lost	O
packets	O
force	O
the	O
TCP	B-Protocol
protocol	I-Protocol
to	O
perform	O
error	O
recovery	O
,	O
but	O
the	O
initial	O
(	O
streamlined	O
)	O
recovery	O
attempts	O
fail	O
because	O
the	O
retransmitted	O
packets	O
are	O
lost	O
in	O
exactly	O
the	O
same	O
way	O
as	O
the	O
original	O
packets	O
.	O
</s>
<s>
Eventually	O
,	O
the	O
TCP	B-Protocol
transmission	O
window	O
becomes	O
full	O
and	O
the	O
TCP	B-Protocol
protocol	I-Protocol
refuses	O
to	O
transmit	O
any	O
further	O
data	O
until	O
the	O
previously-transmitted	O
data	O
is	O
acknowledged	O
.	O
</s>
<s>
This	O
,	O
in	O
turn	O
,	O
will	O
quiesce	B-Device
the	O
new	O
traffic	O
over	O
the	O
connection	O
,	O
leaving	O
only	O
the	O
retransmissions	O
and	O
acknowledgments	O
.	O
</s>
<s>
This	O
will	O
restart	O
the	O
TCP	B-Protocol
traffic	O
,	O
which	O
in	O
turn	O
immediately	O
causes	O
lost	O
packets	O
as	O
streaming	O
resumes	O
.	O
</s>
<s>
The	O
end	O
result	O
is	O
a	O
connection	O
that	O
is	O
working	O
but	O
performs	O
extremely	O
poorly	O
because	O
of	O
the	O
duplex	B-Protocol
mismatch	I-Protocol
.	O
</s>
<s>
Symptoms	O
of	O
a	O
duplex	B-Protocol
mismatch	I-Protocol
are	O
connections	O
that	O
seem	O
to	O
work	O
fine	O
with	O
a	O
ping	B-Protocol
command	I-Protocol
,	O
but	O
"	O
lock	O
up	O
"	O
easily	O
with	O
very	O
low	O
throughput	O
on	O
data	O
transfers	O
;	O
the	O
effective	O
data	O
transfer	O
rate	O
is	O
likely	O
to	O
be	O
asymmetrical	O
,	O
performing	O
much	O
worse	O
in	O
the	O
half-duplex	O
to	O
full-duplex	O
direction	O
than	O
the	O
other	O
.	O
</s>
<s>
However	O
,	O
in	O
a	O
duplex	B-Protocol
mismatch	I-Protocol
the	O
collisions	O
seen	O
on	O
the	O
half-duplex	O
side	O
of	O
the	O
link	O
are	O
often	O
late	O
collisions	O
.	O
</s>
<s>
The	O
full-duplex	O
side	O
usually	O
will	O
register	O
frame	B-Protocol
check	I-Protocol
sequence	I-Protocol
errors	O
,	O
or	O
runt	O
frames	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
setting	O
one	O
side	O
to	O
automatic	O
(	O
either	O
speed	O
or	O
duplex	O
or	O
both	O
)	O
and	O
setting	O
the	O
other	O
to	O
be	O
fixed	O
(	O
either	O
speed	O
or	O
duplex	O
or	O
both	O
)	O
will	O
likely	O
result	O
in	O
either	O
a	O
speed	O
mismatch	O
,	O
a	O
duplex	B-Protocol
mismatch	I-Protocol
or	O
both	O
.	O
</s>
<s>
A	O
duplex	B-Protocol
mismatch	I-Protocol
can	O
be	O
fixed	O
by	O
either	O
enabling	O
autonegotiation	B-Protocol
(	O
if	O
available	O
and	O
working	O
)	O
on	O
both	O
ends	O
or	O
by	O
forcing	O
the	O
same	O
settings	O
on	O
both	O
ends	O
(	O
availability	O
of	O
a	O
configuration	O
interface	O
permitting	O
)	O
.	O
</s>
<s>
If	O
there	O
is	O
no	O
option	O
but	O
to	O
have	O
a	O
locked	O
setting	O
on	O
one	O
end	O
and	O
autonegotiation	B-Protocol
the	O
other	O
(	O
for	O
example	O
,	O
an	O
old	O
device	O
with	O
broken	O
autonegotiation	B-Protocol
connected	O
to	O
an	O
unmanaged	O
switch	O
)	O
half	O
duplex	O
must	O
be	O
used	O
.	O
</s>
<s>
All	O
modern	O
LAN	O
equipment	O
comes	O
with	O
autonegotiation	B-Protocol
enabled	O
and	O
the	O
various	O
compatibility	O
issues	O
have	O
been	O
resolved	O
.	O
</s>
<s>
The	O
best	O
way	O
to	O
avoid	O
duplex	B-Protocol
mismatches	I-Protocol
is	O
to	O
use	O
autonegotiation	B-Protocol
and	O
to	O
replace	O
any	O
legacy	O
equipment	O
that	O
does	O
not	O
use	O
autonegotiation	B-Protocol
or	O
does	O
not	O
autonegotiate	B-Protocol
correctly	O
.	O
</s>
