Source Coding and Fountain Codes meet Multipath TCP
Multipath TCP continues to attract the interest of networking researchers. In less than one week, two preprints of papers that apply networking coding techniques to improve the performance Multipath TCP have been published.
In the first paper [FMTCP] , Yong Cui and his colleagues propose to apply Fountain Codes, as defined in RFC 6380 to Multipath TCP. This is an interesting approach that encodes the data and adds redundancy to the data before transmission. Thanks to this coding scheme, FMTCP does not need to retransmit all segments when losses occur. Of course, the tradeoff is that FMTCP generates more data than Multipath TCP. The paper evaluates the performance of FMTCP by comparing it with Multipath TCP by relying on ns-2 simulations. The simulations show that on lossy links FMTCP performs better than Multipath TCP. This is not surprising given that Multipath TCP’s congestion control scheme tries to move traffic away from lossy paths by reducing its transmission rate on those paths. FMTCP on the other hand can easily recover from a limited number of losses by relying on its coding scheme. It would be interesting to complement these simulations by :
- evaluating the performance of several competing FMTCP flows. This would show the impact of the losses on the congestion control scheme that is used by FMTCP (the paper is not very clear about the congestion control scheme that it used and unfortunately, the source code of the simulator used is not referenced in the paper)
- evaluating the support of interactive applications. Such applications are more likely to benefit from FMTCP than bulk transfer, but this might have an impact on the coding scheme that will not have to deal anymore with fixed size segments
Another related paper is SC-MPTCP [SCMPTCP] . SC-MPTCP relies on source coding to encode the segments before being transmitted. It also compares SC-MPTCP with regular Multipath TCP, but explicitly mentions the utilisation of the coupled congestion control scheme and relies on ns-3 simulations. The simulations analyze the impact of losses on the performance of both SC-MPTCP and regular Multipath TCP. In addition, it also proposes a scheduling technique to select on which subflows segments have to be sent to minimize reordering on the receiver and thus prevent head-of-line blocking when the receive buffer is limited. This technique is applicable to both regular Multipath TCP and SC-MPTCP. The paper then evaluates two interesting multihoming scenarios and shows the benefits of SC-MPTCP compared to Multipath TCP.
These two papers show that by adding redundancy to the data segments, it is possible to improve the performance of Multipath TCP in lossy environments. These results were based on simulations and there are two remaining questions that need to be answered :
- How can these techniques be efficiently implemented in TCP stacks, either kernel-based on by modifying user-level TCP stacks that start to (re)appear ? How do applications interact with such modified stacks ?
- What happens when several sources using source coding or fountain codes compete for the same bottleneck link where packets are discarded due to congestion ? Is the loss pattern as favorable for the source coding and fountain codes are random losses that mainly correspond to wireless links ?
Bibliography
| [FMTCP] | Yong Cui, Lian Wang, Xin Wang,Hongyi Wang, and Yining Wang, FMTCP: A Fountain Code-Based Multipath Transmission Control Protocol, to appear in IEEE/ACM Transactions on Networking, http://dx.doi.org/10.1109/TNET.2014.2300140 |
| [SCMPTCP] | Ming Li, Andrey Lukyanenko, Sasu Tarkoma, Yong Cui, Antti Yla-Jaaski, Tolerating path heterogeneity in multipath TCP with bounded receive buffers, Computer Networks, Available online 6 February 2014, ISSN 1389-1286, http://dx.doi.org/10.1016/j.comnet.2014.01.011. |