CLI Reference Guide-R04

Chapter 24 | Quality of Service Commands
– 656 –
◆ 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
■
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.
Example
This example creates a policy called “rd-policy,” uses the class command to specify
the previously defined “rd-class,” uses the set phb command to classify the service
that incoming packets will receive, and then uses the police srtcm-color-blind
command to limit the average bandwidth to 100,000 Kbps, the committed burst
rate to 4000 bytes, the excess burst rate to 6000 bytes, to remark any packets
exceeding the committed burst size, and to drop any packets exceeding the excess
burst size.