I have put a patch to the mue(4) driver on ftp.n.o that adds support for checksum offloading. It seems to be working fine but maybe could use some more testing before I commit it. <https://ftp.NetBSD.org/pub/NetBSD/misc/rjs/if_mue.patch.gz> Copied to port-arm as the RPi B3+ uses this driver. Robert Swindells