MikroTik RouterOS v2.9 Reference Manual page 441

The whole SFQ queue can contain 128 packets and there are 1024 subqueues available for these
packets.
Use SFQ for congested links to ensure that some connections do not starve
PCQ
To solve some SFQ imperfectness, Per Connection Queuing (PCQ) was created. It is the only
classless queuing type that can do limitation. It is an improved version of SFQ without its stohastic
nature. PCQ also creates subqueues, regarding the pcq-classifier parameter. Each subqueue has a
data rate limit of pcq-rate and size of pcq-limit packets. The total size of a PCQ queue cannot be
greater than pcq-total-limit packets.
The following example demonstrates the usage of PCQ with packets, classified by their source
address.
If you classify the packets by src-address then all packets with different source IP addresses will be
grouped into different subqueues. Now you can do the limitation or equalization for each subqueue
with the pcq-rate parameter. Perhaps, the most significant part is to decide to which interface
should we attach this queue. If we will attach it to the Local interface, all traffic from the Public
interface will be grouped by src-address (probably it's not what we want), but if we attach it to the
Public interface, all traffic from our clients will be grouped by src-address - so we can easily limit
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