Another type of attack on Multipath TCP ?

In a recent paper presented at Hotnets, M. Zubair Shafiq and his colleagues discuss a new type of “attack” on Multipath TCP.

When the paper was announced on the Multipath TCP mailing list, I was somewhat concerned by the title. However, after having read it in details, I do not consider the inference “attack” discussed in this paper as a concern. The paper explains that thanks to Multipath TCP, it is possible for an operator to infer about the performance of “another operator” by observing the Multipath TCP packets that pass through its own network. The “attack” is discussed in the paper and some measurements are carried out in the lab to show that it is possible to infer some characteristics about the performance of the other network.

After having read the paper, I don’t think that the problem is severe and should be classified as an “attack”. First, if I want to test the performance of TCP in my competitor’s network, I can easily subscribe to this network, in particular for wireless networks that would likely benefit from Multipath TCP. There are even public measurements facilities that collect measurement data, see SamKnows, the FCC measurement app, speedtest or MLab.

More fundamentally, if an operator observes one subflow of a Multipath TCP connection, it cannot easily determine how many subflows are used in this Multipath TCP connection and what are the endpoints of these subflows. Without this information, it becomes more difficult to infer TCP performance in another specific network.

The technique proposed in the paper mainly considers the measurement throughput on each subflow as a time series whose evolution needs to be predicted. A passive measurement device could get more accurate predictions by looking at the packets that are exchanged, in particular the DATA level sequence number and acknowledgements. There is plenty of room to improve the inference technique described in this paper. Once Multipath TCP gets widely deployed and used for many applications, it might be possible to extend the technique to learn more about the performance of TCP in the global Internet.