Appendix C. Delay Pools

Delay pools are Squid’s answer to rate limiting and traffic shaping. They work by limiting the rate at which Squid returns data for cache misses. Cache hits are sent as quickly as possible, under the assumption that local bandwidth is plentiful.

Delay pools were written by David Luyer while at the University of Western Australia. The feature was designed for a LAN environment in which different groups of users (for example, students, instructors, and staff) are on different subnets. You’ll see some evidence of this in the following descriptions.


The delay pools are, essentially “bandwidth buckets.” A response is delayed until some amount of bandwidth is available from an appropriate bucket. The buckets don’t actually store bandwidth (e.g., 100 Kbit/s), but rather some amount of traffic (e.g., 384 KB). Squid adds some amount of traffic to the buckets each second. Cache clients take some amount of traffic out when they receive data from an upstream source (origin server or neighbor).

The size of a bucket determines how much burst bandwidth is available to a client. If a bucket starts out full, a client can take as much traffic as it needs until the bucket becomes empty. The client then receives traffic allotments at the fill rate.

The mapping between Squid clients and actual buckets is a bit complicated. Squid uses three different constructs to do it: access rules, delay pool classes, and types of buckets. First, Squid checks a client request against the ...

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