© Copyright Lenovo 2018
Chapter 12: Quality of Service
223
Using DSCP Values to Provide QoS
The
six
most
significant
bits
in
the
TOS
byte
of
the
IP
header
are
defined
as
DiffServ
Code
Points
(DSCP).
Packets
are
marked
with
a
certain
value
depending
on
the
type
of
treatment
the
packet
must
receive
in
the
network
device.
DSCP
is
a
measure
of
the
Quality
of
Service
(QoS)
level
of
the
packet.
Differentiated Services Concepts
To
differentiate
between
traffic
flows,
packets
can
be
classified
by
their
DSCP
value.
The
Differentiated
Services
(DS)
field
in
the
IP
header
is
an
octet,
and
the
first
six
bits,
called
the
DS
Code
Point
(DSCP),
can
provide
QoS
functions.
Each
packet
carries
its
own
QoS
state
in
the
DSCP.
There
are
64
possible
DSCP
values
(0
‐
63).
Figure 23.
Layer
3
IPv4
Packet
The
NE2552E
can
perform
the
following
actions
to
the
DSCP:
Read
the
DSCP
value
of
ingress
packets
Re
‐
mark
the
DSCP
value
to
a
new
value
Map
the
DSCP
value
to
an
802.1p
priority
Once
the
DSCP
value
is
marked,
the
NE2552E
can
use
it
to
direct
traffic
prioritization.
7 6 5 4 3 2 1 0
Differentiated Services Code Point (DSCP)
unused
Version
Length
ToS
Length
ID
Offset
TTL
Proto
FCS
DIP
SIP
Data
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