Edge-Core ECS4110-28T, Руководство по управлению, Страница: 293 / 1514

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HAPTER
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| Quality of Service
Creating QoS Policies
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When a packet of size B bytes arrives at time t, the following happens if
srTCM is configured to operate in Color-Aware mode:
■
If the packet has been precolored as green and Tc(t)-B  0, the
packet is green and Tc is decremented by B down to the minimum
value of 0, else
■
If the packet has been precolored as yellow or green and if
Te(t)-B  0, the packets is yellow and Te is decremented by B down
to the minimum value of 0, else
■
the packet is red and neither Tc nor Te is decremented.
The metering policy guarantees a deterministic behavior where the
volume of green packets is never smaller than what has been
determined by the CIR and BC, that is, tokens of a given color are
always spent on packets of that color. Refer to RFC 2697 for more
information on other aspects of srTCM.
trTCM Police Meter – Defines an enforcer for classified traffic based on a
two rate three color meter scheme defined in RFC 2698. This metering
policy monitors a traffic stream and processes its packets according to the
committed information rate (CIR, or maximum throughput), peak
information rate (PIR), and their associated burst sizes – committed burst
size (BC, or burst rate), and peak burst size (BP). Action may taken for
traffic conforming to the maximum throughput, exceeding the maximum
throughput, or exceeding the peak 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 red if it exceeds the PIR. Otherwise
it is marked either yellow or green depending on whether it exceeds or
doesn't exceed the CIR.
The trTCM is useful for ingress policing of a service, where a peak rate
needs to be enforced separately from a committed rate.
◆ 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, P and C, which are based on the rates PIR and CIR,
respectively. The maximum size of the token bucket P is BP and the
maximum size of the token bucket C is BC.
The token buckets P and C are initially (at time 0) full, that is, the token
count Tp(0) = BP and the token count Tc(0) = BC. Thereafter, the token