Chapter 10 Quality of Service (QoS)
VMG1312-B Series User’s Guide
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The srTCM evaluat es incom ing packet s and m arks them with one of t hree colors which refer t o
packet loss pr iority levels. High packet loss pr iority level is referred to as red, m edium is referred to
as yellow and low is referred t o as green.
The srTCM is based on the t oken bucket filter and has t wo token buckets ( CBS and EBS). Tokens
are generat ed and added int o the bucket at a constant rat e, called Com m itt ed I nform ation Rat e
( CI R). When t he first bucket ( CBS) is full, new t okens overflow into the second bucket ( EBS) .
All packets are evaluat ed against the CBS. I f a packet does not exceed t he CBS it is m arked green.
Ot herwise it is evaluated against the EBS. I f it is below t he EBS then it is m arked yellow. I f it
exceeds the EBS then it is m arked red.
The following shows how tokens wor k wit h incom ing packet s in srTCM:
• A packet arrives. The packet is m arked green and can be t ransm it ted if t he num ber of t okens in
the CBS bucket is equal to or greater t han the size of the packet ( in bytes).
• After a packet is transm it t ed, a num ber of tokens corresponding t o t he packet size is rem oved
from the CBS bucket .
• I f t here are not enough tokens in the CBS bucket, the Device checks the EBS bucket. The packet
is m arked yellow if there are sufficient tokens in the EBS bucket . Otherwise, the packet is m arked
red. No tokens are rem oved if t he packet is dr opped.
Two Rate Three Color Marker
The Two Rat e Three Color Marker ( trTCM, defined in RFC 2698) is a t y pe of t raffic policing t hat
identifies packet s by com paring t hem t o two user- defined rat es: t he Com m itt ed I nfor m ation Rate
( CI R) and t he Peak I nform ation Rat e ( PI R). The CI R specifies the average rate at which packet s are
adm itt ed to the network. The PI R is great er than or equal t o the CI R. CI R and PI R values are based
on the guarant eed and m axim um bandwidth respect ively as negotiat ed betw een a service provider
and client .
The t rTCM evaluat es incom ing packets and m arks them with one of t hree colors which refer to
packet loss pr iority levels. High packet loss pr iority level is referred to as red, m edium is referred to
as yellow and low is referred t o as green.
The t rTCM is based on the t oken bucket filt er and has t wo token buckets ( Com m it t ed Burst Size
( CBS) and Peak Burst Size ( PBS) ) . Tokens are generated and added into the t wo buckets at the CI R
and PI R respect ively.
All packet s are evaluat ed against t he PI R. I f a packet exceeds t he PI R it is m arked red. Ot herwise it
is evaluated against t he CI R. I f it exceeds t he CIR t hen it is m arked yellow. Finally, if it is below the
CIR t hen it is m arked green.
The following shows how tokens wor k wit h incom ing packet s in t rTCM:
• A packet ar r ives. I f t he num ber of t oken s in t he PBS bucket is less t han the size of the packet (in
byt es) , the packet is m arked red and m ay be dropped regardless of the CBS bucket . No t okens
are rem oved if the packet is dropped.
• I f t he PBS bucket has enough t okens, the Device checks the CBS bucket . The packet is m arked
green and can be transm itt ed if t he num ber of tokens in t he CBS bucket is equal to or greater
than the size of the packet (in bytes) . Otherwise, t he packet is m arked yellow.
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