<s>
thumb|400px|An	O
example	O
of	O
the	O
fragmentation	O
of	O
a	O
protocol	B-Protocol
data	I-Protocol
unit	I-Protocol
in	O
a	O
given	O
layer	O
into	O
smaller	O
fragments	O
.	O
</s>
<s>
IP	B-Protocol
fragmentation	I-Protocol
is	O
an	O
Internet	B-Protocol
Protocol	I-Protocol
(	O
IP	O
)	O
process	O
that	O
breaks	O
packets	B-Protocol
into	O
smaller	O
pieces	O
(	O
fragments	O
)	O
,	O
so	O
that	O
the	O
resulting	O
pieces	O
can	O
pass	O
through	O
a	O
link	O
with	O
a	O
smaller	O
maximum	B-Protocol
transmission	I-Protocol
unit	I-Protocol
(	O
MTU	B-Protocol
)	O
than	O
the	O
original	O
packet	B-Protocol
size	O
.	O
</s>
<s>
The	O
fragments	O
are	O
reassembled	O
by	O
the	O
receiving	O
host	B-Device
.	O
</s>
<s>
The	O
details	O
of	O
the	O
fragmentation	O
mechanism	O
,	O
as	O
well	O
as	O
the	O
overall	O
architectural	O
approach	O
to	O
fragmentation	O
,	O
are	O
different	O
between	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
.	O
</s>
<s>
describes	O
the	O
procedure	O
for	O
IP	B-Protocol
fragmentation	I-Protocol
,	O
and	O
transmission	O
and	O
reassembly	O
of	O
IP	O
packets	B-Protocol
.	O
</s>
<s>
The	O
Identification	O
field	O
along	O
with	O
the	O
foreign	O
and	O
local	O
internet	O
address	O
and	O
the	O
protocol	O
ID	O
,	O
and	O
Fragment	O
offset	O
field	O
along	O
with	O
Do	O
n't	O
Fragment	O
and	O
More	O
Fragments	O
flags	O
in	O
the	O
IP	B-Protocol
header	I-Protocol
are	O
used	O
for	O
fragmentation	O
and	O
reassembly	O
of	O
IP	O
packets	B-Protocol
.	O
</s>
<s>
If	O
a	O
receiving	O
host	B-Device
receives	O
a	O
fragmented	O
IP	O
packet	B-Protocol
,	O
it	O
has	O
to	O
reassemble	O
the	O
packet	B-Protocol
and	O
pass	O
it	O
to	O
the	O
higher	O
protocol	O
layer	O
.	O
</s>
<s>
Reassembly	O
is	O
intended	O
to	O
happen	O
in	O
the	O
receiving	O
host	B-Device
but	O
in	O
practice	O
it	O
may	O
be	O
done	O
by	O
an	O
intermediate	O
router	B-Protocol
,	O
for	O
example	O
,	O
network	B-Protocol
address	I-Protocol
translation	I-Protocol
(	O
NAT	O
)	O
may	O
need	O
to	O
reassemble	O
fragments	O
in	O
order	O
to	O
translate	O
data	O
streams	O
.	O
</s>
<s>
Under	O
IPv4	B-Protocol
,	O
a	O
router	B-Protocol
that	O
receives	O
a	O
network	B-Protocol
packet	I-Protocol
larger	O
than	O
the	O
next	O
hop	O
's	O
MTU	B-Protocol
has	O
two	O
options	O
:	O
drop	O
the	O
packet	B-Protocol
if	O
the	O
Do	O
n't	O
Fragment	O
(	O
DF	O
)	O
flag	O
bit	O
is	O
set	O
in	O
the	O
packet	B-Protocol
's	O
header	O
and	O
send	O
an	O
Internet	B-Protocol
Control	I-Protocol
Message	I-Protocol
Protocol	I-Protocol
(	O
ICMP	O
)	O
message	O
which	O
indicates	O
the	O
condition	O
Fragmentation	O
Needed	O
(	O
Type	O
3	O
,	O
Code	O
4	O
)	O
,	O
or	O
fragment	O
the	O
packet	B-Protocol
and	O
send	O
it	O
over	O
the	O
link	O
with	O
a	O
smaller	O
MTU	B-Protocol
.	O
</s>
<s>
Although	O
originators	O
may	O
produce	O
fragmented	O
packets	B-Protocol
,	O
IPv6	B-Protocol
routers	B-Protocol
do	O
not	O
have	O
the	O
option	O
to	O
fragment	O
further	O
.	O
</s>
<s>
Instead	O
,	O
network	O
equipment	O
is	O
required	O
to	O
deliver	O
any	O
IPv6	B-Protocol
packets	B-Protocol
or	O
packet	B-Protocol
fragments	O
smaller	O
than	O
or	O
equal	O
to	O
1280	O
bytes	O
and	O
IPv6	B-Protocol
hosts	O
are	O
required	O
to	O
determine	O
the	O
optimal	O
MTU	B-Protocol
through	O
Path	B-Protocol
MTU	I-Protocol
Discovery	I-Protocol
before	O
sending	O
packets	B-Protocol
.	O
</s>
<s>
Though	O
the	O
header	O
formats	O
are	O
different	O
for	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
,	O
analogous	O
fields	O
are	O
used	O
for	O
fragmentation	O
,	O
so	O
the	O
same	O
algorithm	O
can	O
be	O
reused	O
for	O
IPv4	B-Protocol
and	O
IPv6	B-Protocol
fragmentation	O
and	O
reassembly	O
.	O
</s>
<s>
In	O
IPv4	B-Protocol
,	O
hosts	O
must	O
make	O
a	O
best-effort	O
attempt	O
to	O
reassemble	O
fragmented	O
IP	O
packets	B-Protocol
with	O
a	O
total	O
reassembled	O
size	O
of	O
up	O
to	O
576	O
bytes	O
.	O
</s>
<s>
They	O
may	O
also	O
attempt	O
to	O
reassemble	O
fragmented	O
IP	O
packets	B-Protocol
larger	O
than	O
576	O
bytes	O
,	O
but	O
they	O
are	O
also	O
permitted	O
to	O
silently	O
discard	O
such	O
larger	O
packets	B-Protocol
.	O
</s>
<s>
Applications	O
are	O
recommended	O
to	O
refrain	O
from	O
sending	O
packets	B-Protocol
larger	O
than	O
576	O
bytes	O
unless	O
they	O
have	O
prior	O
knowledge	O
that	O
the	O
remote	O
host	B-Device
is	O
capable	O
of	O
accepting	O
or	O
reassembling	O
them	O
.	O
</s>
<s>
In	O
IPv6	B-Protocol
,	O
hosts	O
must	O
make	O
a	O
best-effort	O
attempt	O
to	O
reassemble	O
fragmented	O
packets	B-Protocol
with	O
a	O
total	O
reassembled	O
size	O
of	O
up	O
to	O
1500	O
bytes	O
,	O
larger	O
than	O
IPv6	B-Protocol
's	O
minimum	O
MTU	B-Protocol
of	O
1280	O
bytes	O
.	O
</s>
<s>
Fragmented	O
packets	B-Protocol
with	O
a	O
total	O
reassembled	O
size	O
larger	O
than	O
1500	O
bytes	O
may	O
optionally	O
be	O
silently	O
discarded	O
.	O
</s>
<s>
Applications	O
relying	O
upon	O
IPv6	B-Protocol
fragmentation	O
to	O
overcome	O
a	O
path	B-Protocol
MTU	I-Protocol
limitation	O
must	O
explicitly	O
fragment	O
the	O
packet	B-Protocol
at	O
the	O
point	O
of	O
origin	O
;	O
however	O
,	O
they	O
should	O
not	O
attempt	O
to	O
send	O
fragmented	O
packets	B-Protocol
with	O
a	O
total	O
size	O
larger	O
than	O
1500	O
bytes	O
unless	O
they	O
know	O
in	O
advance	O
that	O
the	O
remote	O
host	B-Device
is	O
capable	O
of	O
reassembly	O
.	O
</s>
<s>
When	O
a	O
network	O
has	O
multiple	O
parallel	O
paths	O
,	O
technologies	O
like	O
LAG	B-Architecture
and	O
CEF	B-Protocol
split	O
traffic	O
across	O
the	O
paths	O
according	O
to	O
a	O
hash	B-Error_Name
algorithm	I-Error_Name
.	O
</s>
<s>
One	O
goal	O
of	O
the	O
algorithm	O
is	O
to	O
ensure	O
all	O
packets	B-Protocol
of	O
the	O
same	O
flow	B-Operating_System
are	O
sent	O
out	O
the	O
same	O
path	O
to	O
minimize	O
unnecessary	O
packet	B-Protocol
reordering	I-Protocol
.	O
</s>
<s>
IP	B-Protocol
fragmentation	I-Protocol
can	O
cause	O
excessive	O
retransmissions	O
when	O
fragments	O
encounter	O
packet	B-Protocol
loss	I-Protocol
and	O
reliable	O
protocols	O
such	O
as	O
TCP	O
must	O
retransmit	O
all	O
of	O
the	O
fragments	O
in	O
order	O
to	O
recover	O
from	O
the	O
loss	O
of	O
a	O
single	O
fragment	O
.	O
</s>
<s>
Thus	O
,	O
senders	O
typically	O
use	O
two	O
approaches	O
to	O
decide	O
the	O
size	O
of	O
IP	O
packets	B-Protocol
to	O
send	O
over	O
the	O
network	O
.	O
</s>
<s>
The	O
first	O
is	O
for	O
the	O
sending	O
host	B-Device
to	O
send	O
an	O
IP	O
packet	B-Protocol
of	O
size	O
equal	O
to	O
the	O
MTU	B-Protocol
of	O
the	O
first	O
hop	O
of	O
the	O
source-destination	O
pair	O
.	O
</s>
<s>
The	O
second	O
is	O
to	O
run	O
the	O
Path	B-Protocol
MTU	I-Protocol
Discovery	I-Protocol
algorithm	O
to	O
determine	O
the	O
path	B-Protocol
MTU	I-Protocol
between	O
two	O
IP	B-Device
hosts	I-Device
so	O
that	O
IP	B-Protocol
fragmentation	I-Protocol
can	O
be	O
avoided	O
.	O
</s>
<s>
,	O
IP	B-Protocol
fragmentation	I-Protocol
is	O
considered	O
fragile	O
and	O
often	O
undesired	O
due	O
to	O
its	O
security	O
impact	O
.	O
</s>
