OpenVPN implements its own reliability layer on top of UDP (it's worthy to note that it's used for TCP too). We should follow the reference implementation as closely as possible.

This basically boils down to keep track of acknowledged packets on a fixed-size structure, and retry sending if acks have not been received after a given period. Retries follow an exponential back-off, and incoming packets with ids beyond a certain acknowledgment window will be dropped since sequentiality cannot be guaranteed by the data structure.

edit: for some reason I had originally written "sentimentality" in the last sentence.

Relevant pointers:

• https://build.openvpn.net/doxygen/group__reliable.html#details
• https://github.com/roburio/openvpn/blob/main/src/engine.ml
• https://github.com/glacjay/govpn/blob/master/reliable_udp.go very elegant implementation, imho.