FD.io is a very fast userspace networking library, which allows to create programs for packet processing. While DPDK allows fast read and write access to the NICs, FD.io is foccussed on processing the packets. Possible use cases are a packet forwarder, implementing a NAT, or a VPN.

More details also in this APNIC blogpost: https://blog.apnic.net/2020/04/17/kernel-bypass-networking-with-fd-io-and-vpp/

FD.io: The World's Secure Networking Data Plane