
Quality of Service (QoS)
©2008 Allied Telesis Inc. All rights reserved.
39.16
AlliedWare Plus
TM
Operating System Software Reference C613-50003-00 REV E
Software Version 5.2.1
Single-rate three-color policing
This policing method is based that defined in RFC 2697. The principle of single-rate three-color
policing is shown in
Figure 39-7 on page 39.16
. For a given class-map, a meter monitors both
the token count in the buckets, and the input data flow.
Figure 39-7: Single-Rate Three-Color Policing
Each byte entering the meter is paired with a token in each bucket, and a token is removed as
each byte is accepted. If the input data rate is the same as the CIR then the data passes through
the port at the same rate as the bucket fills. Hence the bucket level remains constant. In this
model the data buffer is represented by two data buckets. You can specify the CIR using the
police single-rate exceed-action
command.
Initially both buckets have their full token count. A surge of date exceeding the CIR will begin
to empty the bucket. As the data and tokens are paired, data bytes that match tokens below
the CBS level are marked green, those that are between CBS and EBS will be marked yellow,
and those that are above EBS are marked red.
Note that although the data is metered per byte, the color marking process is applied per
packet. This means that if there were only sufficient tokens available to match part of a packet,
then the whole packet would be marked red. Then, depending on the
exceed action
parameter of the
police single-rate exceed-action
command, the whole packet will be either
dropped or forwarded. In either situation, the red marked packet will leave the bucket counts
unchanged.
M
e
t
e
r
Te = Tokens in
Excess Bucket
Tc = Tokens
in Committed
Bucket
SR-TC_bucket_meter
CBS
(minburstsize)
EBS
(maxburstsize)
Data packets (at data rate > CIR)
packet size < Tc
packet size > Tc < Te
Packets marked after metering
Used green and yellow
tokens are replaced at the
Committed Information
Rate (CIR)
green
green
green, yellow or red
yellow
red (no token top-up)
Bucket
Excess
Committed
Bucket
yellow
red
yellow
red
Single-Rate Three-Color Policing (color aware mode)
green
yellow
red
red