One Simple Queue for the Whole Network with PCQ

What it is

PCQ lets one simple queue share bandwidth fairly among all hosts of a network.

Writing one queue per host does not scale. With PCQ, which stands for per connection queuing, you create a queue type once, and the router splits the traffic into sub-streams by address. Each sub-stream is a host or connection group that gets its own share.

The result is that every host gets an equal part of the link, and no single host can take it all.

How it works

You define PCQ queue types for each direction, and then attach them to one simple queue.

The PCQ queue type

A queue type is created in the queue type menu with kind=pcq.

/queue type add name=pcq-down kind=pcq pcq-classifier=dst-address pcq-rate=0
/queue type add name=pcq-up kind=pcq pcq-classifier=src-address pcq-rate=0

The classifier decides how traffic is split into sub-streams. For download, traffic goes to the LAN hosts, so the destination address identifies the host. For upload, traffic comes from the hosts, so the source address identifies it.

Other classifiers exist, such as ports, but the address is the usual choice for a simple fair share.

Using the queue types

One simple queue ties both queue types to the whole network.

/queue simple add name=all-lan target=192.168.88.0/24 max-limit=20M/20M queue=pcq-up/pcq-down

The queue setting takes two values, in the same upload/download order as the limits. The first is the queue type for upload and the second is the one for download.

The max-limit is the total the whole network may use. PCQ then divides this total among the active hosts.

If one host is active, it can use the whole limit. If ten hosts are active, each gets about a tenth. When a host goes quiet, its share returns to the others.

The pcq-rate setting

pcq-rate sets a fixed rate limit for each sub-stream.

With pcq-rate=0, hosts share the total equally, and a single host can use all of it when the others are idle. With a rate such as 2M, each host is held to that rate, even when the link has room.

/queue type add name=pcq-down-2m kind=pcq pcq-classifier=dst-address pcq-rate=2M

A fixed rate is useful for plans where every customer gets the same speed. Equal sharing is better when you want to use the whole link at all times.

The pcq-limit setting

pcq-limit is the size of the queue of one sub-stream. It sets how much data can wait for each host before the router starts to drop packets.

/queue type add name=pcq-down kind=pcq pcq-classifier=dst-address pcq-rate=0 pcq-limit=50KiB

A larger queue absorbs bursts and avoids dropping packets, but it adds delay. A smaller queue keeps delay low but drops sooner. A related setting, pcq-total-limit, caps the combined size of all sub-streams.

If hosts see long delays under load, a smaller pcq-limit can help. If downloads are slow because of drops, a larger one may help.

PCQ compared with one queue per host

Use PCQ for a large or changing network. Use separate simple queues when particular hosts need different limits.

You can combine them. Place the special host queues above the PCQ queue, so that they match first, and let PCQ handle everyone else.

Checking the queue

/queue simple print stats
/queue type print

The statistics show how much traffic the queue handles and whether packets are being dropped. If the counters stay at zero, the target may not match, or FastTrack may be taking the traffic first.