Priority Based Subflow Selection in MPTCP: A Case Study

Traditionally routing is an IP layer functionality. Multipath TCP creates multiple flows within a single TCP connection, and it has the luxury of selecting a path based on some criteria. This essentially pushes some routing functionality (not hop by hop, but end to end) to the TCP layer. Now TCP has to do flow control, etc., with "routing." However, TCP may not be able to discover all the metrics required for the right path selection. There are metrics available only to the user (not even to the application). A case in point is the monetary cost of different paths based on the different ISPs. The user may have a policy to select the paths based on these metrics. In this work, we used MPTCP over two paths with different costs from the user; A path over an ad-hoc WiFi network and another path through a fixed infrastructure based network. The user policy dictates that the path over the ad-hoc network is preferred. We devised a mechanism to pass this preference/policy as a hint to the kernel's TCP/IP stack. We show that the user can influence the kernel on the path selection with a minimal or non-negligible cost. We argue that the proposed research has a lots of security implications.