MPTCP is not Pareto-Optimal: Performance Issues and a Possible Solution

MPTCP has been proposed recently as a mechanism for sup- porting transparently multiple connections to the applica- tion layer. It is under discussion at the IETF. We show, however, that the current MPTCP suffers from two prob- lems: (P1) Upgrading some TCP users to MPTCP can re- duce the throughput of others without any benefit to the upgraded users, which is a symptom of not being Pareto- optimal; and (P2) MPTCP users could be excessively ag- gressive towards TCP users. We attribute these problems to the linked-increases algorithm (LIA) of MPTCP and, more specifically, to an excessive amount of traffic transmitted over congested paths. The design of LIA forces a tradeoff between optimal re- source pooling and responsiveness. We revisit the problem and show that it is possible to provide these two proper- ties simultaneously. We implement the resulting algorithm, called opportunistic linked increases algorithm (OLIA), in the Linux kernel, and we study its performance over our testbed, by simulations and by theoretical analysis. We prove that OLIA is Pareto-optimal and satisfies the design goals of MPTCP. Hence it can avoid the problems P1 and P2. Our measurements and simulations indicate that MPTCP with OLIA is as responsive and non-flappy as MPTCP with LIA, and that it solves problems P1 and P2.