Hierarchical Rate Limits - Juniper E320 Configuration Manual

Rate limiters are implemented using a dual token bucket scheme: a token bucket
for conformed (yellow) packets and a token bucket for committed (green) packets.
One token is synonymous with one byte. The capacity of the buckets is the
maximum number of tokens that can be placed in each bucket.
You configure the bucket capacity with the peak burst parameter or the committed
burst parameter. The burst parameters are in bytes (not bytes per second), which is
the number of tokens in a full bucket. When a packet passes through a rate limiter,
its size is compared to the contents of both buckets, the packet is categorized, and
the rate-limiter action is taken on the packet.
Peak rate and committed rate determine the fill rate of their respective buckets. If
you set the committed rate to 128,000 bps, tokens are added to the committed
(green) bucket at a rate of 128,000 bps (16 K bytes per second), regardless of the
traffic. If no traffic passes through the rate limiter, the bucket continues to fill until it
reaches the committed burst setting.
Traffic passes through the rate limiter causing a draining of tokens. The drain rate is
dependent on how large the packets are and how much time elapses between
packets. At any given instant the level of tokens in each bucket is a function of the
fill rate, size of packets, and elapsed time between packets.
When packets are received on an interface with a rate limiter applied, the level of
tokens in each bucket dynamically changes in both of the following ways:
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Hierarchical Rate Limits

In another type of rate limiting, rate-limit hierarchies enable lower priority traffic to
access unused bandwidth allocated for real-time traffic, such as voice or video,
during times when no real-time traffic is flowing. IP subscribers receive multiple
services, such as Web, video, and file transfer, that have a maximum bandwidth. A
rate-limit hierarchy can apply a common rate limit to several classified flows,
enabling them to share bandwidth according to the preferences set in the
hierarchical rate limits.
You can also use rate-limit hierarchies in a layer 2 (ATM) access network for DSL
where many routing gateways lead into one Broadband Access Server. The
Broadband Access Server uses rate-limit hierarchies to allocate shareable bandwidth
to each routing gateway, which enables unused bandwidth from one routing
gateway to be used by others. The hierarchy in the rate limit represents the
hierarchy in the access network.
Rate-limit hierarchies enable you to share unused bandwidth dynamically, taking
unused preferred bandwidth. They also enable real-time traffic to use all guaranteed
bandwidth at any time without violating the configured limit on the total interface
bandwidth. While preferred traffic fluctuates, the interface rate limit adjusts,
dropping non-preferred packets to keep the total flow through the interface under a
configured maximum rate, because preferred packets cannot be dropped by the
shared rate limits, only by their individual rate limits.
Tokens are added every 100-ms sample period
Tokens are removed based on the size and rate of incoming packets
Chapter 3: Creating Rate-Limit Profiles
Hierarchical Rate Limits
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