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JUNOSe 11.1.x Policy Management Configuration Guide
To configure a single-rate hard limit, set the committed rate and burst rate to the
desired values, the committed action to transmit, the conformed action to drop, and
the exceeded action to drop. The peak rate must be set to zero.
NOTE: You can also achieve the characteristics of the single-rate hard limit by
configuring a one-rate rate-limit profile with the extended burst rate set to zero.
Related Topics
Configuring a TCP-Friendly One-Rate Rate-Limit Profile
You can configure a committed rate, committed burst, and excess burst for the token
bucket. For example, to configure a rate-limit process with hard tail dropping of
packets when tokens are unavailable, set the committed rate and committed burst
to a nonzero value, and set the excess burst to zero. Setting the excess burst to a
nonzero value causes the router to drop packets in a more friendly way.
The configuration values for the preceding attributes determine the degree of
friendliness of the rate-limit process. Instead of tail dropping packets that arrive
outside the committed and burst rate envelope, the TCP-friendly bucket enables
more tokens to be borrowed, up to a limit determined by the excess burst size. The
next packet that borrows tokens in excess of the excess burst size is deemed excessive
and is dropped if the exceeded action is set to drop.
The rate-limit algorithm is designed to avoid consecutive packet drops in the initial
stages of congestion when the packet flow rate exceeds the committed rate of the
token bucket. The intention is that just a few packet drops are sufficient for TCP's
congestion control algorithm to drastically scale back its sending rate. Eventually,
the packet flow rate falls below the committed rate, which enables the token bucket
to replenish faster because of the reduced load.
If the packet flow rate exceeds the committed rate for an extended period of time,
the rate-limit algorithm tends toward hard tail dropping. In a properly configured
scenario, the rate limiter is consistently driven to borrow tokens because of TCP's
aggressive nature, but it replenishes the tokens as TCP backs off, resulting in a
delivered rate that is very close to the rate configured in the rate-limit profile.
The recommended burst sizes for TCP-friendly behavior are:
For example, if the committed rate is 1,000,000 bps, the recommended burst sizes
are as follows:
84
Configuring a TCP-Friendly One-Rate Rate-Limit Profile
Rate Limits for Interfaces Overview on page 62
Monitoring Policy Management Overview on page 181
Committed burst 0.2 to 2.0 seconds of the committed rate
Excess burst 1.0 to 2.0 seconds of the committed rate, plus the committed
burst
Committed burst is 1,000,000 x 1.0 x 1/8 = 125,000 bytes
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