I've followed the docs, extracted vmlinuz and initrd from the casper dir of the Ubuntu ISO and just placed them at the base tftpboot dir. Things hum along, the ISO downloads to the client, after it enters BusyBox and fires up initramfs, it completes loading the ISO, but then stops at "Unable to find a live file system on the network". The thing is, I'm able to navigate and find the ISO on the client, and I'm even able to mount it and see that all of the contents are there.