Understanding TCP over TCP: Effects of TCP Tunneling on End-to-End Throughput and Latency

TCP tunnel is a technology that aggregates and transfers packets sent between end hosts as a single or multiple TCP connections. By using a TCP tunnel, the fairness among aggregate flows can be improved and several protocols can be transparently transmitted through an Internet firewall. However, since most applications running on end hosts generally use TCP, two TCP congestion controls (i.e., end-to-end TCP and tunnel TCP) operate simultaneously and might interfere each other. Under certain conditions, it has been known that using a TCP tunnel severely degrades the end-to-end TCP performance. On the contrary, under other conditions, it has been known that using a TCP tunnel significantly improves the end-to-end TCP performance. However, it is still an open issue — how, when, and why is TCP tunnel malicious for end-to-end TCP performance? In this paper, we therefore investigate effect of TCP tunnel on the end-to-end TCP performance using simulation experiments. Specifically, we quantitatively reveal effects of several factors on performances of end-to-end TCP (e.g., the link propagation delay, usage of TCP SACK option, TCP socket buffer size, and the buffer size of the ingress router) .