Brocade 6910 Ethernet Access Switch Configuration Guide (Supporting R2.2.0.0) - Manual (Page 888)

832 Brocade 6910 Ethernet Access Switch Configuration Guide
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Creating QoS Policies
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
• 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.
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.