LevelOne GTL-2691 - Manual (Page 1180)

C HAPTER 41 | Quality of Service Commands
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Rate (CIR) and Peak Information Rate (PIR), and their associated burst
sizes - Committed Burst Size (BC) and Peak Burst Size (BP).
◆ 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. 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 pre-colored as red or if Tp(t)-B < 0, the
packet is red, else
■
if the packet has been pre-colored 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 which are green, yellow, or red. Refer to RFC 2698
for more information on other aspects of trTCM.