Web Management Guide-R01

Chapter 10 | Quality of Service
Creating QoS Policies
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else the packet is red and neither Tc nor Te is decremented.
When a packet of size B bytes arrives at time t, the following happens if srTCM is
configured to operate in Color-Aware mode:
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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
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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
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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. 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 count Tp is