User guide

QoS implementation for DIFFSERV QOS
7-7
iMG/RG Software Reference Manual (Quality of Service)
7.1.3.3.4 trTCM (two rate three color marker) meter profile
Another three-level meter profile named as trTCM (two rate three color marker) meter profile consists of four
parameters:
• Committed information rate (CIR)
• Committed burst size (CBS)
• Peak Information rate (PIR)
• Peak burst size (PBS)
If the packet stream's average rate is within CIR and the burst size is within CBS then the packet is marked
Green. If the packet stream's average rate is within PIR and the burst size is within PBS then the packet is
marked Yellow. Otherwise the packet is marked Red.
7.1.3.3.5 Metering packets
The Meter device will take one of the following actions for each packet which has been metered:
• Drop (default action for Red packets): Drop the packet buffer.
• Set DSCP value in IP header: Set the DSCP value in the DS field in the header of the IP packet. The Meter
will overwrite the DSCP value the packet had before metering. The checksum for the IP packet is also
updated following an update to the DSCP value.
• Pass (Default Action for green and yellow packets): Does not set any DSCP value in the IP header. The
packet has the same DSCP value it had before metering.
The above actions can be configured by the user.
For every metering result, the Meter will also set a drop priority value in the buffer.
This drop priority value is used by the Algorithmic dropper (if configured) that is used by the Scheduler in order
to select a packet to drop if the system detects congestion in queues.
The following drop priority values can be set in the buffer:
• If the packet is completely in profile (Green) then a low drop priority value '0' is set in the buffer.
• If the packet if partially in profile (Yellow) then a medium drop priority value '1' is set in the buffer.
• If the packet is out of profile (Red) then a high drop priority value '2' is set in the buffer.
The following diagram illustrates the complete metering process: