<s>
Path	B-Protocol
MTU	I-Protocol
Discovery	I-Protocol
(	O
PMTUD	B-Protocol
)	O
is	O
a	O
standardized	O
technique	O
in	O
computer	B-Architecture
networking	I-Architecture
for	O
determining	O
the	O
maximum	B-Protocol
transmission	I-Protocol
unit	I-Protocol
(	O
MTU	B-Protocol
)	O
size	O
on	O
the	O
network	O
path	O
between	O
two	O
Internet	O
Protocol	O
(	O
IP	O
)	O
hosts	O
,	O
usually	O
with	O
the	O
goal	O
of	O
avoiding	O
IP	B-Protocol
fragmentation	I-Protocol
.	O
</s>
<s>
PMTUD	B-Protocol
was	O
originally	O
intended	O
for	O
routers	O
in	O
Internet	B-Protocol
Protocol	I-Protocol
Version	I-Protocol
4	I-Protocol
(	O
IPv4	B-Protocol
)	O
.	O
</s>
<s>
In	O
IPv6	B-Protocol
,	O
this	O
function	O
has	O
been	O
explicitly	O
delegated	O
to	O
the	O
end	O
points	O
of	O
a	O
communications	O
session	O
.	O
</s>
<s>
As	O
an	O
extension	O
to	O
the	O
standard	O
path	B-Protocol
MTU	I-Protocol
discovery	I-Protocol
,	O
a	O
technique	O
called	O
Packetization	O
Layer	O
Path	B-Protocol
MTU	I-Protocol
Discovery	I-Protocol
works	O
without	O
support	O
from	O
ICMP	B-Protocol
.	O
</s>
<s>
For	O
IPv4	B-Protocol
packets	O
,	O
Path	B-Protocol
MTU	I-Protocol
Discovery	I-Protocol
works	O
by	O
setting	O
the	O
Do	O
n't	O
Fragment	O
(	O
DF	O
)	O
flag	O
bit	O
in	O
the	O
IP	O
headers	O
of	O
outgoing	O
packets	O
.	O
</s>
<s>
Then	O
,	O
any	O
device	O
along	O
the	O
path	O
whose	O
MTU	B-Protocol
is	O
smaller	O
than	O
the	O
packet	O
will	O
drop	O
it	O
,	O
and	O
send	O
back	O
an	O
Internet	B-Protocol
Control	I-Protocol
Message	I-Protocol
Protocol	I-Protocol
(	O
ICMP	B-Protocol
)	O
Fragmentation	O
Needed	O
(	O
Type	O
3	O
,	O
Code	O
4	O
)	O
message	O
containing	O
its	O
MTU	B-Protocol
,	O
allowing	O
the	O
source	O
host	O
to	O
reduce	O
its	O
Path	B-Protocol
MTU	I-Protocol
appropriately	O
.	O
</s>
<s>
The	O
process	O
is	O
repeated	O
until	O
the	O
MTU	B-Protocol
is	O
small	O
enough	O
to	O
traverse	O
the	O
entire	O
path	O
without	O
fragmentation	O
.	O
</s>
<s>
As	O
IPv6	B-Protocol
routers	O
do	O
not	O
fragment	O
packets	O
,	O
there	O
is	O
no	O
Do	O
n't	O
Fragment	O
option	O
in	O
the	O
IPv6	B-Protocol
header	O
.	O
</s>
<s>
For	O
IPv6	B-Protocol
,	O
Path	B-Protocol
MTU	I-Protocol
Discovery	I-Protocol
works	O
by	O
initially	O
assuming	O
the	O
path	B-Protocol
MTU	I-Protocol
is	O
the	O
same	O
as	O
the	O
MTU	B-Protocol
on	O
the	O
link	B-Protocol
layer	I-Protocol
interface	O
where	O
the	O
traffic	O
originates	O
.	O
</s>
<s>
Then	O
,	O
similar	O
to	O
IPv4	B-Protocol
,	O
any	O
device	O
along	O
the	O
path	O
whose	O
MTU	B-Protocol
is	O
smaller	O
than	O
the	O
packet	O
will	O
drop	O
the	O
packet	O
and	O
send	O
back	O
an	O
ICMPv6	B-Protocol
Packet	O
Too	O
Big	O
(	O
Type	O
2	O
)	O
message	O
containing	O
its	O
MTU	B-Protocol
,	O
allowing	O
the	O
source	O
host	O
to	O
reduce	O
its	O
Path	B-Protocol
MTU	I-Protocol
appropriately	O
.	O
</s>
<s>
The	O
process	O
is	O
repeated	O
until	O
the	O
MTU	B-Protocol
is	O
small	O
enough	O
to	O
traverse	O
the	O
entire	O
path	O
without	O
fragmentation	O
.	O
</s>
<s>
If	O
the	O
Path	B-Protocol
MTU	I-Protocol
changes	O
after	O
the	O
connection	O
is	O
set	O
up	O
and	O
is	O
lower	O
than	O
the	O
previously	O
determined	O
Path	B-Protocol
MTU	I-Protocol
,	O
the	O
first	O
large	O
packet	O
will	O
cause	O
an	O
ICMP	B-Protocol
error	O
and	O
the	O
new	O
,	O
lower	O
Path	B-Protocol
MTU	I-Protocol
will	O
be	O
found	O
.	O
</s>
<s>
If	O
the	O
path	O
changes	O
and	O
the	O
new	O
path	B-Protocol
MTU	I-Protocol
is	O
larger	O
,	O
the	O
source	O
will	O
not	O
learn	O
about	O
the	O
increase	O
because	O
routers	O
do	O
not	O
store	O
state	O
.	O
</s>
<s>
Many	O
network	O
security	O
devices	O
block	O
all	O
ICMP	B-Protocol
messages	O
for	O
perceived	O
security	O
benefits	O
,	O
including	O
the	O
errors	O
that	O
are	O
necessary	O
for	O
the	O
proper	O
operation	O
of	O
PMTUD	B-Protocol
.	O
</s>
<s>
This	O
state	O
is	O
referred	O
to	O
as	O
a	O
black	B-Protocol
hole	I-Protocol
connection	I-Protocol
.	O
</s>
<s>
Some	O
implementations	O
of	O
PMTUD	B-Protocol
attempt	O
to	O
prevent	O
this	O
problem	O
by	O
inferring	O
that	O
large	O
payload	O
packets	O
have	O
been	O
dropped	O
due	O
to	O
MTU	B-Protocol
rather	O
than	O
because	O
of	O
link	O
congestion	O
.	O
</s>
<s>
However	O
,	O
in	O
order	O
for	O
the	O
Transmission	B-Protocol
Control	I-Protocol
Protocol	I-Protocol
(	O
TCP	O
)	O
to	O
operate	O
most	O
efficiently	O
,	O
ICMP	B-Protocol
Unreachable	O
messages	O
(	O
type	O
3	O
)	O
should	O
be	O
permitted	O
.	O
</s>
<s>
A	O
robust	O
method	O
for	O
PMTUD	B-Protocol
that	O
relies	O
on	O
TCP	O
or	O
another	O
protocol	O
to	O
probe	O
the	O
path	O
with	O
progressively	O
larger	O
packets	O
has	O
been	O
standardized	O
in	O
RFC	O
4821	O
.	O
</s>
<s>
A	O
workaround	O
used	O
by	O
some	O
routers	O
is	O
to	O
change	O
the	O
maximum	B-Protocol
segment	I-Protocol
size	I-Protocol
(	O
MSS	O
)	O
of	O
all	O
TCP	O
connections	O
passing	O
through	O
links	O
with	O
MTU	B-Protocol
lower	O
than	O
the	O
Ethernet	O
default	O
of	O
1500	O
.	O
</s>
