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Packet Loss Priority

Loss priority settings help determine which packets are dropped from the network during periods of congestion. The software supports multiple packet loss priority (PLP) designations: low and high. (In addition, medium-low and medium-high PLPs are supported when you configure tricolor marking, as discussed in Configuring Tricolor Marking.) You can set PLP by configuring a behavior aggregate or multifield classifier, as discussed in Classifying Packets by Behavior Aggregate and Classifying Packets Based on Various Packet Header Fields.

A drop-profile map examines the loss priority setting of an outgoing packet: high, medium-high, medium-low, low, or any. In addition, some Layer 4 protocol information is examined to determine if the packet is associated with the Transmission Control Protocol (TCP), non-TCP, or any form of traffic.

Obviously, low, medium-low, medium-high, and high are relative terms, which by themselves have no meaning. Drop profiles define the meanings of the loss priorities. In the following example, the low-drop drop profile defines the meaning of low PLP as a 10 percent drop probability when the fill level is 75 percent, and a 40 percent drop probability when the fill level is 95 percent. The high-drop drop profile defines the meaning of high PLP as a 50 percent drop probability when the fill level is 25 percent, and a 90 percent drop probability when the fill level is 50 percent.

In this example, the scheduler includes two drop-profile maps, which specify that packets are evaluated by the low-drop drop profile if they have a low loss priority and are from any protocol. Packets are evaluated by the high-drop drop profile if they have a high loss priority and are from any protocol.



[edit class-of-service]
drop-profiles {


low-drop {


interpolate {
drop-probability [ 10 40];
fill-level [ 75 95];
}


}




high-drop {


interpolate {
drop-probability [ 50 90];
fill-level [ 25 50];
}


}


}
schedulers {


best-effort {
drop-profile-map loss-priority low protocol any drop-profile
low-drop;
drop-profile-map loss-priority high protocol any drop-profile
high-drop;
}


}

For more information, see Configuring Schedulers.

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