Overview¶
Fast Path QoS - Exception Rate Limitation is a feature of Fast Path QoS that limits the rate of packets sent as exceptions from the fast path to Linux.
Features¶
Exception Rate Limitation to limit the rate of packets sent as exceptions from the fast path to Linux
When the MCORE_TC_ERL option is enabled, the fast path limits the rate of exceptions sent to the slow path. The rate can be configured by “fp-cli tc-erl-add” command.
A priority is associated with each exception: low, medium and high. The pass or drop action depends on this priority and the packet color, as marked by the QoS marker:
If exception priority is low, green packets pass and yellow or red packets are dropped,
If exception priority is medium, green and yellow packets pass, and red packets are dropped,
If exception priority is high, packets pass,
If exception priority is unknown, packets are dropped.
Whenever a packet is found to be an exception, a class value (8 bits) is associated with it:
The two most significant bits define the priority of the exception (3 for high priority, 2 for medium, 1 for low, and 0 for unknown),
The six last significant bits define the type of the exception (ARP is required; IKE negotiation is needed, etc.).
The default setting is low priority for all exceptions type, so that a simple token bucket is used to let packets go through or to drop them.