Digisol DG-FS4526E, Management Manual

Page: 952 / 1162

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37 | Quality of Service Commands
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The srTCM as defined in RFC 2697 meters a traffic stream and
processes its packets according to three traffic parameters –
Committed Information Rate (CIR), Committed Burst Size (BC), and
Excess Burst Size (BE).
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 green if it doesn't exceed the CIR and BC, yellow if it
does exceed the CIR and BC, but not the BE, and red otherwise.
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.