Dual Token Bucket Algorithms

Token Bucket Concepts

When you apply traffic policing to the input or output traffic at an interface, the rate limits and actions specified in the policer configuration are used to enforce a limit on the average throughput rate at the interface while also allowing bursts of traffic up to a maximum number of bytes based on the overall traffic load. Junos OS policers measure traffic-flow conformance to a policing rate limit by using a token bucket algorithm. An algorithm based on a single token bucket allows burst of traffic for short periods, whereas an algorithm based dual token buckets allows more sustained bursts of traffic.

Guaranteed Bandwidth for Three-Color Marking

A committed information rate (CIR) defines the guaranteed bandwidth for traffic arriving at or departing from the interface under normal line conditions. A flow of traffic at an average rate that conforms to the CIR is categorized green, and packets in a green flow are implicitly marked with low packet loss priority (PLP) and then passed through the interface. During periods of relatively low traffic (traffic that arrives at or departs from the interface at average rates below the CIR), any unused bandwidth capacity accumulates in the first token bucket, but only up to a configured number of bytes. If any unused bandwidth capacity overflows the first bucket, the excess accumulates in a second token bucket.

The committed burst size (CBS) defines the maximum number of bytes for which unused amounts of the guaranteed bandwidth can be accumulated in the first token bucket. A burst of traffic at an average rate that exceeds the CIR is also categorized as green provided that sufficient unused bandwidth capacity is available in the first token bucket.

Nonconformance Measurement for Single-Rate Three-Color Marking

Single-rate three-color policer configurations specify a second burst size—the excess burst size (EBS)—that defines the maximum number of bytes for which the second token bucket can accumulate unused bandwidth that overflows from the first bucket.

A traffic flow is categorized yellow if its average rate exceeds the CIR and the available bandwidth capacity accumulated in the first bucket if sufficient unused bandwidth capacity is available in the second token bucket. Packets in a yellow flow are implicitly marked with medium-high PLP and then passed through the interface.

A traffic flow is categorized red its average rate exceeds the CIR and the available bandwidth capacity accumulated in the second bucket. Packets in a red flow are implicitly marked with high PLP and then either passed through the interface or optionally discarded.

Nonconformance Measurement for Two-Rate Three-Color Marking

Two-rate three-color policer configurations include a second rate limit—the peak-information-rate (PIR)—that you set to the expected average data rate for traffic arriving at or departing from the interface under peak conditions.

Two-rate three-color policer configurations also include a second burst size—the peak burst size (PBS)—that defines the maximum number of bytes for which the second token bucket can accumulate unused peak bandwidth capacity. During periods of relatively little peak traffic (traffic that arrives at or departs from the interface at average rates that exceed the PIR), any unused peak bandwidth capacity accumulates in the second token bucket, but only up to the maximum number of bytes specified by the PBS.

A traffic flow is categorized yellow if it exceeds the CIR and the available committed bandwidth capacity accumulated in the first token bucket but conforms to the PIR. Packets in a yellow flow are implicitly marked with medium-high PLP and then passed through the interface.

A traffic flow is categorized red if it exceeds the PIR and the available peak bandwidth capacity accumulated in the second token bucket. Packets in a red flow are implicitly marked with high PLP and then either passed through the interface or optionally discarded.