TCP Tunnels: Avoiding Congestion Collapse

This paper examines the attributes of TCP tunnels which are TCP circuits that carry IP packets and benefit from the congestion control mechanism of TCP/IP. The deployment of TCP tunnels reduces the many flows situation on the Internet to that of a few flows. TCP tunnels eliminate unnecessary packet loss in the core routers of the congested backbones which waste precious bandwidth leading to congestion collapse due to unresponsive UDP flows. We also highlight that the use of TCP tunnels can, in principle, help prevent certain forms of congestion collapse described by Floyd & Fall [FF98]. Using a testbed of ten Intel PCs running the Linux operating system and traffic generators simulating user applications, we explore: the benefits which TCP tunnels confer upon its payload of user IP traffic; the impact on the congestion within network backbones; and the protection that tunnels offer with respect to the various competing classes of traffic in terms of bandwidth allocation and reduced retransmissions. The deployment of TCP tunnels on the Internet and the issues involved are also discussed and we conclude that with the recent RFC2309 recommendation of using Random Early Drop (RED) as the default packetdrop policy in Internet routers, coupled with the implementation of a pure tunnel environment on backbone networks makes the deployment of TCP tunnels a feasible endeavour worthy of further investigation.