<s>
An	O
IPv6	B-Protocol
packet	I-Protocol
is	O
the	O
smallest	O
message	O
entity	O
exchanged	O
using	O
Internet	B-Protocol
Protocol	I-Protocol
version	I-Protocol
6	I-Protocol
(	O
IPv6	B-Protocol
)	O
.	O
</s>
<s>
Packets	B-Protocol
consist	O
of	O
control	O
information	O
for	O
addressing	O
and	O
routing	O
and	O
a	O
payload	O
of	O
user	O
data	O
.	O
</s>
<s>
The	O
control	O
information	O
in	O
IPv6	B-Protocol
packets	I-Protocol
is	O
subdivided	O
into	O
a	O
mandatory	O
fixed	O
header	B-General_Concept
and	O
optional	O
extension	O
headers	B-General_Concept
.	O
</s>
<s>
The	O
payload	O
of	O
an	O
IPv6	B-Protocol
packet	I-Protocol
is	O
typically	O
a	O
datagram	B-Protocol
or	O
segment	O
of	O
the	O
higher-level	O
transport	B-Architecture
layer	I-Architecture
protocol	O
,	O
but	O
may	O
be	O
data	O
for	O
an	O
internet	B-Protocol
layer	I-Protocol
(	O
e.g.	O
,	O
ICMPv6	B-Protocol
)	O
or	O
link	B-Protocol
layer	I-Protocol
(	O
e.g.	O
,	O
OSPF	B-Protocol
)	O
instead	O
.	O
</s>
<s>
IPv6	B-Protocol
packets	I-Protocol
are	O
typically	O
transmitted	O
over	O
the	O
link	B-Protocol
layer	I-Protocol
(	O
i.e.	O
,	O
over	O
Ethernet	O
or	O
Wi-Fi	O
)	O
,	O
which	O
encapsulates	O
each	O
packet	B-Protocol
in	O
a	O
frame	B-Protocol
.	O
</s>
<s>
Packets	B-Protocol
may	O
also	O
be	O
transported	O
over	O
a	O
higher-layer	O
tunneling	B-Protocol
protocol	I-Protocol
,	O
such	O
as	O
IPv4	B-Protocol
when	O
using	O
6to4	B-Protocol
or	O
Teredo	B-Protocol
transition	O
technologies	O
.	O
</s>
<s>
In	O
contrast	O
to	O
IPv4	B-Protocol
,	O
routers	B-Protocol
do	O
not	O
fragment	O
IPv6	B-Protocol
packets	I-Protocol
larger	O
than	O
the	O
maximum	B-Protocol
transmission	I-Protocol
unit	I-Protocol
(	O
MTU	B-Protocol
)	O
,	O
it	O
is	O
the	O
sole	O
responsibility	O
of	O
the	O
originating	O
node	O
.	O
</s>
<s>
A	O
minimum	O
MTU	B-Protocol
of	O
1,280	O
octets	O
is	O
mandated	O
by	O
IPv6	B-Protocol
,	O
but	O
hosts	B-Device
are	O
"	O
strongly	O
recommended	O
"	O
to	O
use	O
Path	B-Protocol
MTU	I-Protocol
Discovery	I-Protocol
to	O
take	O
advantage	O
of	O
MTUs	B-Protocol
greater	O
than	O
the	O
minimum	O
.	O
</s>
<s>
Since	O
July	O
2017	O
,	O
the	O
Internet	O
Assigned	O
Numbers	O
Authority	O
(	O
IANA	O
)	O
has	O
been	O
responsible	O
for	O
registering	O
all	O
IPv6	B-Protocol
parameters	O
that	O
are	O
used	O
in	O
IPv6	B-Protocol
packet	I-Protocol
headers	B-General_Concept
.	O
</s>
<s>
The	O
fixed	O
header	B-General_Concept
starts	O
an	O
IPv6	B-Protocol
packet	I-Protocol
and	O
has	O
a	O
size	O
of	O
40	O
octets	O
(	O
320	O
bits	O
)	O
.	O
</s>
<s>
The	O
six	O
most-significant	O
bits	O
hold	O
the	O
differentiated	O
services	O
field	O
(	O
DS	O
field	O
)	O
,	O
which	O
is	O
used	O
to	O
classify	O
packets	B-Protocol
.	O
</s>
<s>
A	O
high-entropy	O
identifier	O
of	O
a	O
flow	O
of	O
packets	B-Protocol
between	O
a	O
source	O
and	O
destination	O
.	O
</s>
<s>
A	O
flow	O
is	O
a	O
group	O
of	O
packets	B-Protocol
,	O
e.g.	O
,	O
a	O
TCP	B-Protocol
session	O
or	O
a	O
media	O
stream	O
.	O
</s>
<s>
The	O
special	O
flow	O
label	O
0	O
means	O
the	O
packet	B-Protocol
does	O
not	O
belong	O
to	O
any	O
flow	O
(	O
using	O
this	O
scheme	O
)	O
.	O
</s>
<s>
An	O
older	O
scheme	O
identifies	O
flow	O
by	O
source	O
address	O
and	O
port	O
,	O
destination	O
address	O
and	O
port	O
,	O
protocol	O
(	O
value	O
of	O
the	O
last	O
Next	O
Header	B-General_Concept
field	O
)	O
.	O
</s>
<s>
It	O
has	O
further	O
been	O
suggested	O
that	O
the	O
flow	O
label	O
be	O
used	O
to	O
help	O
detect	O
spoofed	O
packets	B-Protocol
.	O
</s>
<s>
The	O
size	O
of	O
the	O
payload	O
in	O
octets	O
,	O
including	O
any	O
extension	O
headers	B-General_Concept
.	O
</s>
<s>
The	O
length	O
is	O
set	O
to	O
zero	O
when	O
a	O
Hop-by-Hop	O
extension	O
header	B-General_Concept
carries	O
a	O
Jumbo	B-Protocol
Payload	I-Protocol
option	O
.	O
</s>
<s>
Specifies	O
the	O
type	B-Protocol
of	O
the	O
next	O
header	B-General_Concept
.	O
</s>
<s>
This	O
field	O
usually	O
specifies	O
the	O
transport	B-Architecture
layer	I-Architecture
protocol	O
used	O
by	O
a	O
packet	B-Protocol
's	O
payload	O
.	O
</s>
<s>
When	O
extension	O
headers	B-General_Concept
are	O
present	O
in	O
the	O
packet	B-Protocol
this	O
field	O
indicates	O
which	O
extension	O
header	B-General_Concept
follows	O
.	O
</s>
<s>
The	O
values	O
are	O
shared	O
with	O
those	O
used	O
for	O
the	O
IPv4	B-Protocol
protocol	O
field	O
,	O
as	O
both	O
fields	O
have	O
the	O
same	O
function	O
(	O
see	O
List	B-Protocol
of	I-Protocol
IP	I-Protocol
protocol	I-Protocol
numbers	I-Protocol
)	O
.	O
</s>
<s>
Replaces	O
the	O
time	B-Protocol
to	I-Protocol
live	I-Protocol
field	O
in	O
IPv4	B-Protocol
.	O
</s>
<s>
This	O
value	O
is	O
decremented	O
by	O
one	O
at	O
each	O
forwarding	O
node	O
and	O
the	O
packet	B-Protocol
is	O
discarded	O
if	O
it	O
becomes	O
0	O
.	O
</s>
<s>
However	O
,	O
the	O
destination	O
node	O
should	O
process	O
the	O
packet	B-Protocol
normally	O
even	O
if	O
received	O
with	O
a	O
hop	B-Protocol
limit	I-Protocol
of	O
0	O
.	O
</s>
<s>
The	O
unicast	O
IPv6	B-Protocol
address	I-Protocol
of	O
the	O
sending	O
node	O
.	O
</s>
<s>
The	O
IPv6	B-Protocol
unicast	O
or	O
multicast	O
address	O
of	O
the	O
destination	O
node(s )	O
.	O
</s>
<s>
In	O
order	O
to	O
increase	O
performance	O
,	O
and	O
since	O
current	O
link	B-Protocol
layer	I-Protocol
technology	O
and	O
transport	B-Architecture
layer	I-Architecture
protocols	O
are	O
assumed	O
to	O
provide	O
sufficient	O
error	O
detection	O
,	O
the	O
header	B-General_Concept
has	O
no	O
checksum	B-Algorithm
to	O
protect	O
it	O
.	O
</s>
<s>
Extension	O
headers	B-General_Concept
carry	O
optional	O
internet	B-Protocol
layer	I-Protocol
information	O
and	O
are	O
placed	O
between	O
the	O
fixed	O
header	B-General_Concept
and	O
the	O
upper-layer	O
protocol	O
header	B-General_Concept
.	O
</s>
<s>
Extension	O
headers	B-General_Concept
form	O
a	O
chain	O
,	O
using	O
the	O
Next	O
Header	B-General_Concept
fields	I-General_Concept
.	O
</s>
<s>
The	O
Next	O
Header	B-General_Concept
field	O
in	O
the	O
fixed	O
header	B-General_Concept
indicates	O
the	O
type	B-Protocol
of	O
the	O
first	O
extension	O
header	B-General_Concept
;	O
the	O
Next	O
Header	B-General_Concept
field	O
of	O
the	O
last	O
extension	O
header	B-General_Concept
indicates	O
the	O
type	B-Protocol
of	O
the	O
upper-layer	O
protocol	O
header	B-General_Concept
in	O
the	O
payload	O
of	O
the	O
packet	B-Protocol
.	O
</s>
<s>
All	O
extension	O
headers	B-General_Concept
are	O
a	O
multiple	O
of	O
8	O
octets	O
in	O
size	O
;	O
some	O
extension	O
headers	B-General_Concept
require	O
internal	O
padding	O
to	O
meet	O
this	O
requirement	O
.	O
</s>
<s>
There	O
are	O
several	O
extension	O
headers	B-General_Concept
defined	O
,	O
and	O
new	O
extension	O
headers	B-General_Concept
may	O
be	O
defined	O
in	O
the	O
future	O
.	O
</s>
<s>
Most	O
extension	O
headers	B-General_Concept
are	O
examined	O
and	O
processed	O
at	O
the	O
packet	B-Protocol
's	O
destination	O
.	O
</s>
<s>
All	O
extension	O
headers	B-General_Concept
are	O
optional	O
and	O
should	O
appear	O
at	O
most	O
once	O
,	O
except	O
for	O
the	O
Destination	O
Options	O
header	B-General_Concept
extension	O
,	O
which	O
may	O
appear	O
twice	O
.	O
</s>
<s>
If	O
a	O
node	O
does	O
not	O
recognize	O
a	O
specific	O
extension	O
header	B-General_Concept
,	O
it	O
should	O
discard	O
the	O
packet	B-Protocol
and	O
send	O
a	O
Parameter	O
Problem	O
message	O
(	O
ICMPv6	B-Protocol
type	B-Protocol
4	O
,	O
code	O
1	O
)	O
.	O
</s>
<s>
The	O
defined	O
extension	O
headers	B-General_Concept
below	O
are	O
listed	O
in	O
the	O
preferred	O
order	O
for	O
the	O
case	O
where	O
there	O
is	O
more	O
than	O
one	O
extension	O
header	B-General_Concept
following	O
the	O
fixed	O
header	B-General_Concept
.	O
</s>
<s>
Value	O
59	O
(	O
No	O
Next	O
Header	B-General_Concept
)	O
in	O
the	O
Next	O
Header	B-General_Concept
field	O
indicates	O
that	O
there	O
is	O
no	O
next	O
header	B-General_Concept
whatsoever	O
following	O
this	O
one	O
,	O
not	O
even	O
a	O
header	B-General_Concept
of	O
an	O
upper-layer	O
protocol	O
.	O
</s>
<s>
It	O
means	O
that	O
,	O
from	O
the	O
header	B-General_Concept
's	O
point	O
of	O
view	O
,	O
the	O
IPv6	B-Protocol
packet	I-Protocol
ends	O
right	O
after	O
it	O
:	O
the	O
payload	O
should	O
be	O
empty	O
.	O
</s>
<s>
There	O
could	O
,	O
however	O
,	O
still	O
be	O
data	O
in	O
the	O
payload	O
if	O
the	O
payload	O
length	O
in	O
the	O
first	O
header	B-General_Concept
of	O
the	O
packet	B-Protocol
is	O
greater	O
than	O
the	O
length	O
of	O
all	O
extension	O
headers	B-General_Concept
in	O
the	O
packet	B-Protocol
.	O
</s>
<s>
This	O
data	O
should	O
be	O
ignored	O
by	O
hosts	B-Device
,	O
but	O
passed	O
unaltered	O
by	O
routers	B-Protocol
.	O
</s>
<s>
The	O
Hop-by-Hop	O
Options	O
extension	O
header	B-General_Concept
may	O
be	O
examined	O
and	O
altered	O
by	O
all	O
nodes	O
on	O
the	O
packet	B-Protocol
's	O
path	O
,	O
including	O
sending	O
and	O
receiving	O
nodes	O
.	O
</s>
<s>
The	O
Destination	O
Options	O
extension	O
header	B-General_Concept
needs	O
to	O
be	O
examined	O
by	O
the	O
destination	O
node(s )	O
only	O
.	O
</s>
<s>
The	O
extension	O
headers	B-General_Concept
are	O
both	O
at	O
least	O
8	O
octets	O
in	O
size	O
;	O
if	O
more	O
options	O
are	O
present	O
than	O
will	O
fit	O
in	O
that	O
space	O
,	O
blocks	O
of	O
8	O
octets	O
,	O
containing	O
options	O
and	O
padding	O
,	O
are	O
added	O
to	O
the	O
header	B-General_Concept
repeatedly	O
until	O
all	O
options	O
are	O
represented	O
.	O
</s>
<s>
Next	O
Header	B-General_Concept
(	O
8	O
bits	O
)	O
Specifies	O
the	O
type	B-Protocol
of	O
the	O
next	O
header	B-General_Concept
.	O
</s>
<s>
Header	B-General_Concept
extension	O
length	O
(	O
8	O
bits	O
)	O
Length	O
of	O
this	O
header	B-General_Concept
in	O
8-octet	O
units	O
,	O
not	O
including	O
the	O
first	O
8	O
octets	O
.	O
</s>
<s>
Options	O
and	O
padding	O
(	O
variable	O
)	O
Contains	O
one	O
or	O
more	O
options	O
,	O
and	O
optional	O
padding	O
fields	O
to	O
align	O
options	O
and	O
to	O
make	O
the	O
total	O
header	B-General_Concept
length	O
a	O
multiple	O
of	O
8	O
octets	O
.	O
</s>
<s>
Options	O
are	O
TLV-coded	O
.	O
</s>
<s>
The	O
Routing	O
extension	O
header	B-General_Concept
is	O
used	O
to	O
direct	O
a	O
packet	B-Protocol
to	O
one	O
or	O
more	O
intermediate	O
nodes	O
before	O
being	O
sent	O
to	O
its	O
destination	O
.	O
</s>
<s>
The	O
header	B-General_Concept
is	O
at	O
least	O
8	O
octets	O
in	O
size	O
;	O
if	O
more	O
Type-specific	O
Data	O
is	O
needed	O
than	O
will	O
fit	O
in	O
4	O
octets	O
,	O
blocks	O
of	O
8	O
octets	O
are	O
added	O
to	O
the	O
header	B-General_Concept
repeatedly	O
,	O
until	O
all	O
Type-specific	O
Data	O
is	O
placed	O
.	O
</s>
<s>
|	O
colspan	O
=	O
"	O
32	O
"	O
rowspan	O
=	O
"	O
2	O
"	O
|Optional	O
:	O
more	O
type-specific	O
data	O
...	O
</s>
<s>
Next	O
header	B-General_Concept
(	O
8	O
bits	O
)	O
Indicates	O
the	O
type	B-Protocol
of	O
the	O
next	O
header	B-General_Concept
.	O
</s>
<s>
Header	B-General_Concept
extension	O
length	O
(	O
8	O
bits	O
)	O
The	O
length	O
of	O
this	O
header	B-General_Concept
,	O
in	O
multiples	O
of	O
8	O
octets	O
,	O
not	O
including	O
the	O
first	O
8	O
octets	O
.	O
</s>
<s>
Routing	O
type	B-Protocol
(	O
8	O
bits	O
)	O
A	O
value	O
between	O
0	O
and	O
255	O
,	O
as	O
assigned	O
by	O
IANA	O
.	O
</s>
<s>
|	O
Due	O
to	O
the	O
fact	O
that	O
with	O
Routing	O
Header	B-General_Concept
type	B-Protocol
0	O
a	O
simple	O
but	O
effective	O
denial-of-service	O
attack	O
could	O
be	O
launched	O
,	O
this	O
header	B-General_Concept
was	O
deprecated	O
in	O
2007	O
and	O
host	O
and	O
routers	B-Protocol
are	O
required	O
to	O
ignore	O
these	O
headers	B-General_Concept
.	O
</s>
<s>
|	O
A	O
limited	O
version	O
of	O
type	B-Protocol
0	O
and	O
is	O
used	O
for	O
Mobile	B-Protocol
IPv6	I-Protocol
,	O
where	O
it	O
can	O
hold	O
the	O
home	O
address	O
of	O
the	O
mobile	O
node	O
.	O
</s>
<s>
|	O
RPL	O
Source	O
Route	O
Header	B-General_Concept
for	O
low-power	O
and	O
lossy	O
networks	O
.	O
</s>
<s>
|	O
Segment	O
Routing	O
Header	B-General_Concept
(	O
SRH	O
)	O
.	O
</s>
<s>
Segments	O
Left	O
(	O
8	O
bits	O
)	O
Number	O
of	O
nodes	O
this	O
packet	B-Protocol
still	O
has	O
to	O
visit	O
before	O
reaching	O
its	O
final	O
destination	O
.	O
</s>
<s>
Type-specific	O
Data	O
(	O
variable	O
)	O
Data	O
that	O
belongs	O
to	O
this	O
type	B-Protocol
of	O
routing	O
header	B-General_Concept
.	O
</s>
<s>
In	O
order	O
to	O
send	O
a	O
packet	B-Protocol
that	O
is	O
larger	O
than	O
the	O
path	B-Protocol
MTU	I-Protocol
,	O
the	O
sending	O
node	O
splits	O
the	O
packet	B-Protocol
into	O
fragments	O
.	O
</s>
<s>
The	O
Fragment	O
extension	O
header	B-General_Concept
carries	O
the	O
information	O
necessary	O
to	O
reassemble	O
the	O
original	O
(	O
unfragmented	O
)	O
packet	B-Protocol
.	O
</s>
<s>
Next	O
header	B-General_Concept
(	O
8	O
bits	O
)	O
Identifies	O
the	O
type	B-Protocol
of	O
the	O
next	O
header	B-General_Concept
.	O
</s>
<s>
Fragment	O
offset	O
(	O
13	O
bits	O
)	O
Offset	O
,	O
in	O
8-octet	O
units	O
,	O
relative	O
to	O
the	O
start	O
of	O
the	O
fragmentable	O
part	O
of	O
the	O
original	O
packet	B-Protocol
.	O
</s>
<s>
Identification	O
(	O
32	O
bits	O
)	O
Packet	B-Protocol
identification	O
value	O
,	O
generated	O
by	O
the	O
source	O
node	O
.	O
</s>
<s>
Needed	O
for	O
reassembly	O
of	O
the	O
original	O
packet	B-Protocol
.	O
</s>
<s>
The	O
Authentication	O
Header	B-General_Concept
and	O
the	O
Encapsulating	O
Security	O
Payload	O
are	O
part	O
of	O
IPsec	B-Protocol
and	O
are	O
used	O
identically	O
in	O
IPv6	B-Protocol
and	O
in	O
IPv4	B-Protocol
.	O
</s>
<s>
The	O
fixed	O
and	O
optional	O
IPv6	B-Protocol
headers	B-General_Concept
are	O
followed	O
by	O
the	O
upper-layer	O
payload	O
,	O
the	O
data	O
provided	O
by	O
the	O
transport	B-Architecture
layer	I-Architecture
,	O
for	O
example	O
a	O
TCP	B-Protocol
segment	O
or	O
a	O
UDP	B-Protocol
datagram	B-Protocol
.	O
</s>
<s>
The	O
Next	O
Header	B-General_Concept
field	O
of	O
the	O
last	O
IPv6	B-Protocol
header	B-General_Concept
indicates	O
what	O
type	B-Protocol
of	O
payload	O
is	O
contained	O
in	O
this	O
packet	B-Protocol
.	O
</s>
<s>
The	O
payload	O
length	O
field	O
of	O
IPv6	B-Protocol
(	O
and	O
IPv4	B-Protocol
)	O
has	O
a	O
size	O
of	O
16bits	O
,	O
capable	O
of	O
specifying	O
a	O
maximum	O
length	O
of	O
octets	O
for	O
the	O
payload	O
.	O
</s>
<s>
In	O
practice	O
,	O
hosts	B-Device
determine	O
the	O
maximum	O
usable	O
payload	O
length	O
using	O
Path	B-Protocol
MTU	I-Protocol
Discovery	I-Protocol
(	O
yielding	O
the	O
minimum	O
MTU	B-Protocol
along	O
the	O
path	O
from	O
sender	O
to	O
receiver	O
)	O
,	O
to	O
avoid	O
having	O
to	O
fragment	O
packets	B-Protocol
.	O
</s>
<s>
Most	O
link-layer	B-Protocol
protocols	O
have	O
MTUs	B-Protocol
considerably	O
smaller	O
than	O
octets	O
.	O
</s>
<s>
An	O
optional	O
feature	O
of	O
IPv6	B-Protocol
,	O
the	O
jumbo	B-Protocol
payload	I-Protocol
option	O
in	O
a	O
Hop-By-Hop	O
Options	O
extension	O
header	B-General_Concept
,	O
allows	O
the	O
exchange	O
of	O
packets	B-Protocol
with	O
payloads	O
of	O
up	O
to	O
one	O
octet	O
less	O
than	O
4GB	O
(	O
232−1	O
=	O
octets	O
)	O
,	O
by	O
making	O
use	O
of	O
a	O
32-bit	O
length	O
field	O
.	O
</s>
<s>
Packets	B-Protocol
with	O
such	O
payloads	O
are	O
called	O
jumbograms	B-Protocol
.	O
</s>
<s>
Since	O
both	O
TCP	B-Protocol
and	O
UDP	B-Protocol
include	O
fields	O
limited	O
to	O
16bits	O
(	O
length	O
,	O
urgent	O
data	O
pointer	O
)	O
,	O
support	O
for	O
IPv6	B-Protocol
jumbograms	B-Protocol
requires	O
modifications	O
to	O
the	O
transport	B-Architecture
layer	I-Architecture
protocol	O
implementation	O
.	O
</s>
<s>
Jumbograms	B-Protocol
are	O
only	O
relevant	O
for	O
links	O
that	O
have	O
a	O
MTU	B-Protocol
larger	O
than	O
octets	O
(	O
more	O
than	O
octets	O
for	O
the	O
payload	O
,	O
plus	O
40	O
octets	O
for	O
the	O
fixed	O
header	B-General_Concept
,	O
plus	O
8	O
octets	O
for	O
the	O
Hop-by-Hop	O
extension	O
header	B-General_Concept
)	O
.	O
</s>
<s>
Only	O
a	O
few	O
link-layer	B-Protocol
protocols	O
can	O
process	O
packets	B-Protocol
larger	O
than	O
octets	O
.	O
</s>
<s>
Unlike	O
in	O
IPv4	B-Protocol
,	O
IPv6	B-Protocol
routers	B-Protocol
never	O
fragment	O
IPv6	B-Protocol
packets	I-Protocol
.	O
</s>
<s>
Packets	B-Protocol
exceeding	O
the	O
size	O
of	O
the	O
maximum	B-Protocol
transmission	I-Protocol
unit	I-Protocol
(	O
MTU	B-Protocol
)	O
of	O
the	O
destination	O
link	O
are	O
dropped	O
and	O
this	O
condition	O
is	O
signaled	O
by	O
a	O
Packet	B-Protocol
too	O
big	O
ICMPv6	B-Protocol
message	O
to	O
the	O
originating	O
node	O
,	O
similarly	O
to	O
the	O
IPv4	B-Protocol
method	O
when	O
the	O
Do	O
n't	O
Fragment	O
bit	O
is	O
set	O
.	O
</s>
<s>
End	O
nodes	O
in	O
IPv6	B-Protocol
are	O
expected	O
to	O
perform	O
Path	B-Protocol
MTU	I-Protocol
Discovery	I-Protocol
to	O
determine	O
the	O
maximum	O
size	O
of	O
packets	B-Protocol
to	O
send	O
,	O
and	O
the	O
upper-layer	O
protocol	O
is	O
expected	O
to	O
limit	O
the	O
payload	O
size	O
.	O
</s>
<s>
If	O
the	O
upper-layer	O
protocol	O
is	O
unable	O
to	O
do	O
so	O
,	O
the	O
sending	O
host	O
may	O
use	O
the	O
Fragment	O
extension	O
header	B-General_Concept
instead	O
.	O
</s>
<s>
Any	O
data	O
link	B-Protocol
layer	I-Protocol
conveying	O
IPv6	B-Protocol
data	O
must	O
be	O
capable	O
of	O
transmitting	O
an	O
IP	O
packet	B-Protocol
containing	O
up	O
to	O
1,280	O
bytes	O
,	O
thus	O
the	O
sending	O
endpoint	O
may	O
limit	O
its	O
packets	B-Protocol
to	O
1,280	O
bytes	O
and	O
avoid	O
any	O
need	O
for	O
fragmentation	O
or	O
Path	B-Protocol
MTU	I-Protocol
Discovery	I-Protocol
.	O
</s>
<s>
A	O
packet	B-Protocol
containing	O
the	O
first	O
fragment	O
of	O
an	O
original	O
(	O
larger	O
)	O
packet	B-Protocol
consists	O
of	O
five	O
parts	O
:	O
the	O
per-fragment	O
headers	B-General_Concept
(	O
the	O
crucial	O
original	O
headers	B-General_Concept
that	O
are	O
repeatedly	O
used	O
in	O
each	O
fragment	O
)	O
,	O
followed	O
by	O
the	O
Fragment	O
extension	O
header	B-General_Concept
containing	O
a	O
zero	O
Offset	O
,	O
then	O
all	O
the	O
remaining	O
original	O
extension	O
headers	B-General_Concept
,	O
then	O
the	O
original	O
upper-layer	O
header	B-General_Concept
(	O
alternatively	O
the	O
ESP	O
header	B-General_Concept
)	O
,	O
and	O
a	O
piece	O
of	O
the	O
original	O
payload	O
.	O
</s>
<s>
Each	O
subsequent	O
packet	B-Protocol
consists	O
of	O
three	O
parts	O
:	O
the	O
per-fragment	O
headers	B-General_Concept
,	O
followed	O
by	O
the	O
Fragment	O
extension	O
header	B-General_Concept
,	O
and	O
by	O
a	O
part	O
of	O
the	O
original	O
payload	O
as	O
identified	O
by	O
a	O
Fragment	O
Offset	O
.	O
</s>
<s>
The	O
per-fragment	O
headers	B-General_Concept
are	O
determined	O
based	O
on	O
whether	O
the	O
original	O
contains	O
Routing	O
or	O
Hop-by-Hop	O
extension	O
header	B-General_Concept
.	O
</s>
<s>
If	O
neither	O
exists	O
,	O
the	O
per-fragment	O
part	O
is	O
just	O
the	O
fixed	O
header	B-General_Concept
.	O
</s>
<s>
If	O
the	O
Routing	O
extension	O
header	B-General_Concept
exists	O
,	O
the	O
per-fragment	O
headers	B-General_Concept
include	O
the	O
fixed	O
header	B-General_Concept
and	O
all	O
the	O
extension	O
headers	B-General_Concept
up	O
to	O
and	O
including	O
the	O
Routing	O
one	O
.	O
</s>
<s>
If	O
the	O
Hop-by-Hop	O
extension	O
header	B-General_Concept
exists	O
,	O
the	O
per-fragment	O
headers	B-General_Concept
consist	O
of	O
only	O
the	O
fixed	O
header	B-General_Concept
and	O
the	O
Hop-by-Hop	O
extension	O
header	B-General_Concept
.	O
</s>
<s>
In	O
any	O
case	O
,	O
the	O
last	O
header	B-General_Concept
of	O
the	O
per-fragment	O
part	O
has	O
its	O
Next	O
Header	B-General_Concept
value	O
set	O
to	O
to	O
indicate	O
that	O
a	O
Fragment	O
extension	O
header	B-General_Concept
follows	O
.	O
</s>
<s>
Each	O
Fragment	O
extension	O
header	B-General_Concept
has	O
its	O
M	O
flag	O
set	O
to	O
(	O
indicating	O
more	O
fragments	O
follow	O
)	O
,	O
except	O
the	O
last	O
,	O
whose	O
flag	O
is	O
set	O
to	O
.	O
</s>
<s>
The	O
per-fragment	O
headers	B-General_Concept
were	O
historically	O
called	O
the	O
"	O
unfragmentable	O
part	O
"	O
,	O
referring	O
to	O
pre-2014	O
possibility	O
of	O
fragmenting	O
the	O
rest	O
of	O
the	O
header	B-General_Concept
.	O
</s>
<s>
Now	O
no	O
headers	B-General_Concept
are	O
actually	O
fragmentable	O
.	O
</s>
<s>
The	O
original	O
packet	B-Protocol
is	O
reassembled	O
by	O
the	O
receiving	O
node	O
by	O
collecting	O
all	O
fragments	O
and	O
placing	O
each	O
fragment	O
at	O
its	O
indicated	O
offset	O
and	O
discarding	O
the	O
Fragment	O
extension	O
headers	B-General_Concept
of	O
the	O
packets	B-Protocol
that	O
carried	O
them	O
.	O
</s>
<s>
Packets	B-Protocol
containing	O
fragments	O
need	O
not	O
arrive	O
in	O
sequence	O
;	O
they	O
will	O
be	O
rearranged	O
by	O
the	O
receiving	O
node	O
.	O
</s>
<s>
If	O
not	O
all	O
fragments	O
are	O
received	O
within	O
60	O
seconds	O
after	O
receiving	O
the	O
first	O
packet	B-Protocol
with	O
a	O
fragment	O
,	O
reassembly	O
of	O
the	O
original	O
packet	B-Protocol
is	O
abandoned	O
and	O
all	O
fragments	O
are	O
discarded	O
.	O
</s>
<s>
If	O
the	O
first	O
fragment	O
was	O
received	O
(	O
which	O
contains	O
the	O
fixed	O
header	B-General_Concept
)	O
and	O
one	O
or	O
more	O
others	O
are	O
missing	O
,	O
a	O
Time	O
Exceeded	O
message	O
(	O
ICMPv6	B-Protocol
type	B-Protocol
3	O
,	O
code	O
1	O
)	O
is	O
returned	O
to	O
the	O
node	O
originating	O
the	O
fragmented	O
packet	B-Protocol
.	O
</s>
<s>
When	O
reassembling	O
node	O
detects	O
a	O
fragment	O
that	O
overlaps	O
with	O
another	O
fragment	O
,	O
the	O
reassembly	O
of	O
the	O
original	O
packet	B-Protocol
is	O
aborted	O
and	O
all	O
fragments	O
are	O
dropped	O
.	O
</s>
<s>
Receiving	O
hosts	B-Device
must	O
make	O
a	O
best-effort	O
attempt	O
to	O
reassemble	O
fragmented	O
IP	O
datagrams	B-Protocol
that	O
,	O
after	O
reassembly	O
,	O
contain	O
up	O
to	O
1500	O
bytes	O
.	O
</s>
<s>
Hosts	B-Device
are	O
permitted	O
to	O
make	O
an	O
attempt	O
to	O
reassemble	O
fragmented	O
datagrams	B-Protocol
larger	O
than	O
1,500	O
bytes	O
,	O
but	O
they	O
are	O
also	O
permitted	O
to	O
silently	O
discard	O
any	O
datagram	B-Protocol
after	O
it	O
becomes	O
apparent	O
that	O
the	O
reassembled	O
packet	B-Protocol
would	O
be	O
larger	O
than	O
1,500	O
bytes	O
.	O
</s>
<s>
Therefore	O
,	O
senders	O
should	O
avoid	O
sending	O
fragmented	O
IP	O
datagrams	B-Protocol
with	O
a	O
total	O
reassembled	O
size	O
larger	O
than	O
1,500	O
bytes	O
,	O
unless	O
they	O
have	O
knowledge	O
that	O
the	O
receiver	O
is	O
capable	O
of	O
reassembling	O
such	O
large	O
datagrams	B-Protocol
.	O
</s>
<s>
As	O
a	O
result	O
,	O
in	O
2014	O
the	O
earlier	O
allowance	O
for	O
overflowing	O
the	O
IPv6	B-Protocol
header	B-General_Concept
chain	O
beyond	O
the	O
first	O
fragment	O
became	O
forbidden	O
in	O
order	O
to	O
avoid	O
some	O
very	O
pathological	O
fragmentation	O
cases	O
.	O
</s>
<s>
Additionally	O
,	O
as	O
a	O
result	O
of	O
research	O
on	O
the	O
evasion	O
of	O
Router	B-Protocol
Advertisement	O
Guard	O
,	O
the	O
use	O
of	O
fragmentation	O
with	O
Neighbor	B-Protocol
Discovery	I-Protocol
is	O
deprecated	O
,	O
and	O
the	O
use	O
of	O
fragmentation	O
with	O
Secure	B-Protocol
Neighbor	I-Protocol
Discovery	I-Protocol
(	O
SEND	O
)	O
is	O
discouraged	O
.	O
</s>
