pfifo_fast is the default linux qdisc. Shadow's C interface kinda sorta emulates it, but in Rust we'll want to change the design to push packets into the network queue instead of sockets. The network queue can then prioritize based on the TOS bits set in each packet's header.

pfifo_fast: https://tldp.org/HOWTO/Traffic-Control-HOWTO/classless-qdiscs.html
priomap priorities used in pfifo_fast: https://lartc.org/howto/lartc.qdisc.classless.html

Some discussion about how we might implement this is in #3480 (comment)

We'll have to rework the interface design so that sockets push packets directly to the interface when packets are ready, and the interface imposes the necessary queue size limits for each socket.