Definition
TCP is one of the two main transport protocols of the internet. It provides reliable, ordered, error-checked delivery of a stream of bytes between two endpoints. If a packet is lost, TCP detects this and retransmits it; if packets arrive out of order, TCP reassembles them in the correct sequence before handing the data to the application. The cost of these guarantees is overhead and latency.
TCP and VPNs
Some VPN protocols can run over either TCP or UDP. UDP is the default and preferred mode because it is faster and avoids a known performance pathology — TCP-over-TCP — where two layers of retransmission compound on a lossy link. TCP mode is useful when UDP is blocked (some restrictive networks block all UDP except DNS) or when you want to wrap the VPN in an obfuscation layer that requires reliability. SSL VPNs and OpenVPN over TCP 443 are common examples.
TCP-over-TCP Problem
When a VPN runs TCP inside a TCP tunnel and the network drops a packet, both the inner TCP and the outer TCP try to retransmit, often with conflicting timing. This causes throughput collapse and high latency. The well-known fix is to use UDP as the outer transport, even if the inner traffic is TCP. UDP-based VPN protocols (WireGuard, OpenVPN/UDP, IKEv2) all avoid the problem; TCP-based VPNs are tolerated for compatibility but not recommended for performance.