Digisol DG-FS4526E, Management Manual

Page: 254 / 1162

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HAPTER
12 | Quality of Service
Creating QoS Policies
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Policing is based on a token bucket, where bucket depth (that is, the
maximum burst before the bucket overflows) is specified by the “burst”
field (BC), and the average rate tokens are removed from the bucket is
specified by the “rate” option (CIR). Action may be taken for traffic
conforming to the maximum throughput, or exceeding the maximum
throughput.
srTCM Police Meter – Defines an enforcer for classified traffic based on a
single rate three color meter scheme defined in RFC 2697. This metering
policy monitors a traffic stream and processes its packets according to the
committed information rate (CIR, or maximum throughput), committed
burst size (BC, or burst rate), and excess burst size (BE). Action may taken
for traffic conforming to the maximum throughput, exceeding the
maximum throughput, or exceeding the excess burst size.
The PHB label is composed of five bits, three bits for per-hop behavior,
and two bits for the color scheme used to control queue congestion. In
addition to the actions defined by this command to transmit, remark
the DSCP service value, or drop a packet, the switch will also mark the
two color bits used to set the drop precedence of a packet for Random
Early Detection. A packet is marked green if it doesn't exceed the
committed information rate and committed burst size, yellow if it does
exceed the committed information rate and committed burst size, but
not the excess burst size, and red otherwise.
The meter operates in one of two modes. In the color-blind mode, the
meter assumes that the packet stream is uncolored. In color-aware
mode the meter assumes that some preceding entity has pre-colored
the incoming packet stream so that each packet is either green, yellow,
or red. The marker (re)colors an IP packet according to the results of
the meter. The color is coded in the DS field [RFC 2474] of the packet.
The behavior of the meter is specified in terms of its mode and two
token buckets, C and E, which both share the common rate CIR. The
maximum size of the token bucket C is BC and the maximum size of the
token bucket E is BE.
The token buckets C and E are initially full, that is, the token count
Tc(0) = BC and the token count Te(0) = BE. Thereafter, the token
counts Tc and Te are updated CIR times per second as follows:
If Tc is less than BC, Tc is incremented by one, else
if Te is less then BE, Te is incremented by one, else
neither Tc nor Te is incremented.
When a packet of size B bytes arrives at time t, the following happens if
srTCM is configured to operate in Color-Blind mode:
If Tc(t)-B  0, the packet is green and Tc is decremented by B down
to the minimum value of 0, else