<s>
The	O
Subnetwork	B-Protocol
Access	I-Protocol
Protocol	I-Protocol
(	O
SNAP	O
)	O
is	O
a	O
mechanism	O
for	O
multiplexing	B-Architecture
,	O
on	O
networks	O
using	O
IEEE	O
802.2	O
LLC	O
,	O
more	O
protocols	O
than	O
can	O
be	O
distinguished	O
by	O
the	O
8-bit	O
802.2	O
Service	B-Protocol
Access	I-Protocol
Point	I-Protocol
(	O
SAP	O
)	O
fields	O
.	O
</s>
<s>
SNAP	O
supports	O
identifying	O
protocols	O
by	O
EtherType	B-Protocol
field	O
values	O
;	O
it	O
also	O
supports	O
vendor-private	O
protocol	O
identifier	O
spaces	O
.	O
</s>
<s>
It	O
is	O
used	O
with	O
IEEE	O
802.3	O
,	O
IEEE	B-Protocol
802.4	I-Protocol
,	O
IEEE	B-Device
802.5	I-Device
,	O
IEEE	O
802.11	O
and	O
other	O
IEEE	O
802	O
physical	O
network	O
layers	O
,	O
as	O
well	O
as	O
with	O
non-IEEE	O
802	O
physical	O
network	O
layers	O
such	O
as	O
FDDI	B-Protocol
that	O
use	O
802.2	O
LLC	O
.	O
</s>
<s>
The	O
SNAP	O
and	O
LSAP	O
fields	O
are	O
added	B-Protocol
to	O
the	O
packets	B-Protocol
at	O
the	O
transmitting	O
node	O
in	O
order	O
to	O
allow	O
the	O
receiving	O
node	O
to	O
pass	B-Architecture
each	O
received	O
frame	B-Protocol
to	O
an	O
appropriate	O
device	B-Application
driver	I-Application
which	O
understands	O
given	O
protocol	O
.	O
</s>
<s>
The	O
OSI	O
model	O
uses	O
a	O
Service	B-Protocol
Access	I-Protocol
Point	I-Protocol
(	O
SAP	O
)	O
to	O
define	O
the	O
communication	O
between	O
layers	O
(	O
like	O
Network	O
,	O
Transport	O
,	O
Session	O
,	O
and	O
the	O
other	O
layers	O
of	O
the	O
Seven-Layered	O
Model	O
)	O
,	O
that	O
is	O
to	O
identify	O
which	O
protocol	O
should	O
process	O
an	O
incoming	O
message	O
.	O
</s>
<s>
The	O
most	O
common	O
reference	O
to	O
SAP	O
,	O
including	O
a	O
Source	O
Service	B-Protocol
Access	I-Protocol
Point	I-Protocol
(	O
SSAP	O
)	O
and	O
a	O
Destination	O
Service	B-Protocol
Access	I-Protocol
Point	I-Protocol
(	O
DSAP	O
)	O
refers	O
to	O
the	O
boundary	O
between	O
the	O
Data	O
Link	O
Layer	O
and	O
the	O
Network	O
Layer	O
.	O
</s>
<s>
Link	O
Service	B-Protocol
Access	I-Protocol
Point	I-Protocol
(	O
LSAP	O
)	O
includes	O
both	O
Destination	O
Service	B-Protocol
Access	I-Protocol
Point	I-Protocol
(	O
DSAP	O
)	O
and	O
Source	O
Service	B-Protocol
Access	I-Protocol
Point	I-Protocol
(	O
SSAP	O
)	O
.	O
</s>
<s>
Thus	O
,	O
the	O
Subnetwork	B-Protocol
Access	I-Protocol
Protocol	I-Protocol
(	O
SNAP	O
)	O
is	O
a	O
mechanism	O
for	O
multiplexing	B-Architecture
,	O
on	O
networks	O
using	O
IEEE	O
802.2	O
LLC	O
,	O
more	O
protocols	O
than	O
can	O
be	O
distinguished	O
by	O
the	O
8-bit	O
802.2	O
Service	B-Protocol
Access	I-Protocol
Point	I-Protocol
(	O
SAP	O
)	O
fields	O
.	O
</s>
<s>
It	O
is	O
used	O
with	O
IEEE	O
802.3	O
,	O
IEEE	B-Protocol
802.4	I-Protocol
,	O
IEEE	B-Device
802.5	I-Device
,	O
IEEE	O
802.11	O
and	O
other	O
IEEE	O
802	O
physical	O
network	O
layers	O
,	O
as	O
well	O
as	O
with	O
non-IEEE	O
802	O
physical	O
network	O
layers	O
such	O
as	O
FDDI	B-Protocol
that	O
use	O
802.2	O
LLC	O
.	O
</s>
<s>
The	O
5-octet	O
SNAP	B-Protocol
header	I-Protocol
follows	O
the	O
802.2	O
LLC	O
header	O
if	O
the	O
destination	O
SAP	O
(	O
DSAP	O
)	O
and	O
the	O
source	O
SAP	O
(	O
SSAP	O
)	O
contain	O
hexadecimal	O
values	O
of	O
AA	O
or	O
AB	O
:	O
</s>
<s>
The	O
SNAP	B-Protocol
header	I-Protocol
consists	O
of	O
a	O
3-octet	O
IEEE	O
organizationally	O
unique	O
identifier	O
(	O
OUI	O
)	O
followed	O
by	O
a	O
2-octet	O
protocol	O
ID	O
.	O
</s>
<s>
If	O
the	O
OUI	O
is	O
zero	O
,	O
the	O
protocol	O
ID	O
is	O
the	O
registered	O
EtherType	B-Protocol
value	O
for	O
the	O
protocol	O
running	O
on	O
top	O
of	O
SNAP	O
.	O
</s>
<s>
SNAP	O
is	O
usually	O
used	O
with	O
Unnumbered	O
Information	O
802.2	O
protocol	B-Protocol
data	I-Protocol
units	I-Protocol
(	O
PDUs	O
)	O
,	O
with	O
a	O
control	O
field	O
value	O
of	O
3	O
,	O
and	O
the	O
LSAP	O
values	O
are	O
usually	O
hexadecimal	O
AA	O
,	O
so	O
the	O
802.2	O
LLC	O
header	O
for	O
a	O
SNAP	O
packet	B-Protocol
is	O
usually	O
AA	O
AA	O
03	O
;	O
however	O
,	O
SNAP	O
can	O
be	O
used	O
with	O
other	O
PDU	O
types	O
as	O
well	O
.	O
</s>
<s>
On	O
Ethernet	O
,	O
the	O
8	O
octets	O
occupied	O
by	O
the	O
LLC	O
and	O
SNAP	B-Protocol
headers	I-Protocol
reduce	O
the	O
size	O
of	O
the	O
available	O
payload	O
for	O
protocols	O
such	O
as	O
the	O
Internet	B-Protocol
Protocol	I-Protocol
to	O
1492	O
bytes	O
,	O
compared	O
to	O
the	O
use	O
of	O
the	O
Ethernet	B-Protocol
II	I-Protocol
framing	I-Protocol
;	O
therefore	O
,	O
for	O
protocols	O
that	O
have	O
EtherType	B-Protocol
values	O
,	O
packets	B-Protocol
are	O
usually	O
transmitted	O
with	O
Ethernet	O
II	O
headers	O
rather	O
than	O
with	O
LLC	O
and	O
SNAP	B-Protocol
headers	I-Protocol
.	O
</s>
<s>
On	O
other	O
network	O
types	O
,	O
the	O
LLC	O
and	O
SNAP	B-Protocol
headers	I-Protocol
are	O
required	O
in	O
order	O
to	O
multiplex	O
different	O
protocols	O
on	O
the	O
link	O
layer	O
,	O
as	O
the	O
MAC	O
layer	O
does	O
n't	O
itself	O
have	O
an	O
EtherType	B-Protocol
field	O
,	O
so	O
there	O
's	O
no	O
alternative	O
framing	O
that	O
would	O
have	O
a	O
larger	O
available	O
payload	O
.	O
</s>
<s>
The	O
hexadecimal	O
AA	O
and	O
AB	O
values	O
were	O
reserved	O
,	O
and	O
an	O
additional	O
header	O
—	O
the	O
SNAP	B-Protocol
header	I-Protocol
—	O
was	O
developed	O
;	O
it	O
can	O
support	O
all	O
EtherType	B-Protocol
values	O
and	O
multiple	O
spaces	O
of	O
private	O
protocol	O
values	O
.	O
</s>
<s>
As	O
per	O
IETF	O
RFC	O
1042	O
,	O
IP	O
datagrams	O
and	O
ARP	O
datagrams	O
are	O
transmitted	O
over	O
IEEE	O
802	O
networks	O
using	O
LLC	O
and	O
SNAP	B-Protocol
headers	I-Protocol
,	O
except	O
on	O
Ethernet/IEEE	O
802.3	O
,	O
where	O
they	O
are	O
transmitted	O
with	O
Ethernet	O
II	O
headers	O
,	O
as	O
per	O
RFC	O
894	O
.	O
</s>
