Quality of Service Overview
February 22, 2008
Page 4 of 29
Figure 1 Assigning and Marking Traffic with a Priority
The
ICMP
protocol,
used
for
error
messaging,
has
a
low
bandwidth
requirement,
with
a
high
tolerance
for
delay
and
jitter,
and
is
appropriate
for
a
low
priority
setting.
HTTP
and
FTP
protocols,
used
respectively
for
browser
‐
generated
and
file
transfer
traffic,
have
a
medium
to
high
bandwidth
requirement,
with
a
medium
to
high
tolerance
for
delay
and
jitter,
and
are
appropriate
for
a
medium
priority
level.
Voice
(VoIP),
used
for
voice
calls,
has
a
low
bandwidth
requirement,
but
is
very
sensitive
to
delay
and
jitter
and
is
appropriate
for
a
high
priority
level.
See
RFC
1349
for
further
details
on
ToS.
See
RFCs
2474
and
2475
for
further
details
on
DSCP.
Preferential Queue Treatment for Packet Forwarding
There
are
three
types
of
preferential
queue
treatments
for
packet
forwarding:
strict,
weighted
fair,
and
hybrid.
Strict Queuing
With
Strict
Priority
Queuing,
a
higher
priority
queue
must
be
empty
before
a
lower
priority
queue
can
transmit
any
packets.
Strict
queuing
is
depicted
in
Figure 2
.
Inbound
packets
enter
on
the
upper
left
and
proceed
to
the
appropriate
queue,
based
upon
the
TxQ
configuration
in
the
CoS.
Outbound
packets
exit
the
queues
on
the
lower
right.
At
this
time
only
queue
3
packets
are
forwarded.
This
will
be
true
until
queue
3
is
completely
empty.
Queue
2
packets
will
then
be
forwarded.
Queue
1
packets
will
only
forward
if
both
queue
2
and
queue
3
are
empty.
Queue
0
packets
will
only
forward
if
all
other
queues
are
empty.
Strict
queuing
assures
that
the
highest
priority
queue
with
any
packets
in
it
will
get
100
percent
of
the
bandwidth
available.
This
is
particularly
useful
for
one
or
more
priority
levels
with
low
bandwidth
and
low
tolerance
for
delay.
The
problem
with
strict
queuing
is
that
should
the
higher
level
queues
never
fully
empty,
lower
level
queues
can
be
starved
of
bandwidth.
