How VPNs Actually Work: From Tunneling Protocols to the Hidden Latency Costs
In 2019, a network engineer at a major financial institution noticed something odd. Their newly deployed VPN, configured with OpenVPN over TCP, was causing a 40% drop in throughput for database replication traffic. The latency between their New York and London data centers had jumped from 75ms to over 200ms. After weeks of troubleshooting, they discovered the culprit wasn’t bandwidth or hardware—it was TCP-over-TCP meltdown, a fundamental interaction between the VPN protocol and the underlying transport layer. ...