Web Management Guide-R04

Chapter 10 | Quality of Service
Creating QoS Policies
– 276 –
(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
incremented by one PIR times per second up to BP and the token count Tc is
incremented by one CIR times per second up to BC.
When a packet of size B bytes arrives at time t, the following happens if trTCM is
configured to operate in Color-Blind mode:
■
If Tp(t)-B < 0, the packet is red, else
■
if Tc(t)-B < 0, the packet is yellow and Tp is decremented by B, else
■
the packet is green and both Tp and Tc are decremented by B.
When a packet of size B bytes arrives at time t, the following happens if trTCM is
configured to operate in Color-Aware mode:
■
If the packet has been precolored as red or if Tp(t)-B < 0, the packet is red,
else
■
if the packet has been precolored as yellow or if Tc(t)-B < 0, the packet is
yellow and Tp is decremented by B, else
■
the packet is green and both Tp and Tc are decremented by B.
◆ The trTCM can be used to mark a IP packet stream in a service, where different,
decreasing levels of assurances (either absolute or relative) are given to packets