[okl]

tc is the way to go if the options are sufficient. You can bridge two interfaces (also virtual ones) in a VM and set qdiscs. Also works asynchronously, e.g., different rules for up/downlink. Instead of a VM setup one could use a cheap linux/openWrt router or an SBC-type board.

[touisteur]

Tc is always a pain to search for, I add 'netem' as a recommendation for keywords and also the IFB trick to put a qdisc on an input stream.