usually VPNs in linux push IP (TUN) or ethernet (TAP) frames between the nodes so you really need to be using UDP or else you are going to have problems with running TCP over TCP and the congestion algorithms conflicting with each other. openvpn which supports TCP refers to this problem as TCP meltdown and advise using UDP where possible (https://openvpn.net/faq/what-is-tcp-meltdown/) VPNs that use TCP as the transport layer could try and special case TCP handling and treat them as a flow and just transport the TCP data streams instead of the IP packets but you would still be left with issues when you are transferring UDP across TCP which is not ideal.
Thanks! I don’t have a lot of experience with VPNs but for sure UDP is much better suited for packets which is the abstraction layer that VPNs operate on.
> VPNs that use TCP as the transport layer could try and special case TCP handling and treat them as a flow and just transport the TCP data streams instead
Yes! An interesting observation is that TCP composes really well, ie relaying works excellent. However, for VPNs it’d be nesting TCP which melts down quickly.
