I use SLiRP over SSH as a poor man's VPN. It's ancient. The bulk of the code is a userspace TCP stack that performs NAT. I can use any SSH host as full VPN w

I use SLiRP over SSH as a poor man's VPN. It's ancient. The bulk of the code is a userspace TCP stack that performs NAT. I can use any SSH host as full VPN with only basic user privileges. I spent quite a bit of time poking around the code to try to improve performance. Increasing the receive window was sufficient to achieve >1Mb/s over a high latency link, but I can't go much higher without <100ms latency. It turns out that SLiRP never implemented window scaling, it was unnecessary for the links back then. The code has been reused for virtualization applications like VirtualBox, etc; but window scaling was never needed for that since client-host latency is basically zero there. Digging through old network code gives an appreciation for how far we've come.