Multipath TCP : controlling congestion
Since the publication of Van Jacobson’s seminal paper on Congestion Avoidance and Control in 1988 [Jac88], congestion control has been one of the most active topics in transport protocol research. This was one of the key scientific challenges for the design of a multipath transport protocol.
As explained in RFC 6182, one of the goals of Multipath TCP was to solve the congestion problem while remaining fair with regular TCP traffic. As Multipath TCP uses multiple parallel TCP connections that are called subflows, a naive implementation of Multipath TCP that would use a standard TCP congestion control scheme for each subflow would be unfair against single TCP flows that would compete for the same resources.
Several researchers had addressed problems that are similar to multipath congestion control earlier but from a mathematical viewpoint and without considering an implementation in a real protocol [KV05], [KMassoulieT07]. An interesting survey article was published later [KMassoulieT11].
The first pratical multipath congestion control scheme was proposed in the paper Design, Implementation and Evaluation of Congestion Control for Multipath TCP [WRGH11]. This is an evolution of the classical TCP congestion control scheme RFC 5681. Compared to the single flow TCP congestion control, the multipath congestion control scheme slows down the increase of the congestion window while still halving it when congestion occurs. A summary of this multipath congestion control algorithm can be found below (source [WRGH11]).
The IETF has adopted this multipath congestion control algorithm as the default one for Multipath TCP RFC 6356. It was initially designed based on simulations with htsim and a preliminary userspace implementation of MPTCP. It has later been added to the Multipath TCP implementation in the Linux kernel.
Since the publication of [WRGH11], several other congestion control algorithms have been proposed and imlemented. These include OLIA [KGP+12], BALIA [PWHL16] and a multipath adaptation of TCP Vegas [CXF12]. The authors of these three algorithms released their implementation in Multipath TCP implementation in the Linux kernel. OLIA [KGP+12] is often preferred by Multipath TCP users.
Each of the above articles discussed the merits of the proposed congestion control scheme and compares it by simulations or measurements with other congestion control schemes. It is likely that other multipath congestion control algorithms will be proposed by the research community. As the IETF is adopting the CUBIC congestion control algorithm for single path TCP RFC 8312, a multipath variant would cleary be a useful contribution. It would also be interesting to develop a multipath variant of BBR [CCG+16].
A recent survey [KL18] summarises the multipath congestion control schemes implemented in the Linux kernel in a nice table shown below.
References
| [CXF12] | Yu Cao, Mingwei Xu, and Xiaoming Fu. Delay-based congestion control for multipath tcp. In Network Protocols (ICNP), 2012 20th IEEE International Conference on, 1–10. IEEE, 2012. |
| [CCG+16] | Neal Cardwell, Yuchung Cheng, C Stephen Gunn, Soheil Hassas Yeganeh, and Van Jacobson. Bbr: congestion-based congestion control. Queue, 14(5):50, 2016. |
| [Jac88] | V. Jacobson. Congestion avoidance and control. ACM SIGCOMM Computer Communication Review, 18(4):314–329, 1988. |
| [KV05] | Frank Kelly and Thomas Voice. Stability of end-to-end algorithms for joint routing and rate control. ACM SIGCOMM Computer Communication Review, 35(2):5–12, 2005. |
| [KMassoulieT07] | Peter Key, Laurent Massoulié, and Don Towsley. Path selection and multipath congestion control. In INFOCOM 2007. 26th IEEE International Conference on Computer Communications. IEEE, 143–151. IEEE, 2007. |
| [KMassoulieT11] | Peter Key, Laurent Massoulié, and Don Towsley. Path selection and multipath congestion control. Communications of the ACM, 54(1):109–116, 2011. |
| [KGP+12] | (1, 2) R. Khalili, N. Gast, M. Popovic, U. Upadhyay, and J.-Y. Le Boudec. MPTCP is not Pareto-Optimal: Performance Issues and a Possible Solution. In Proceedings of the 8th International Conference on Emerging Networking Experiments and Technologies (CoNEXT). 2012. |
| [KL18] | Bruno Yuji Lino Kimura and Antonio Alfredo Frederico Loureiro. Mptcp linux kernel congestion controls. Technical Report arXiv:1812.03210, Arxiv, Dec. 2018. URL: https://arxiv.org/abs/1812.03210. |
| [PWHL16] | Qiuyu Peng, Anwar Walid, Jaehyun Hwang, and Steven H Low. Multipath tcp: analysis, design, and implementation. IEEE/ACM Transactions on Networking (ToN), 24(1):596–609, 2016. |
| [WRGH11] | (1, 2, 3) D. Wischik, C. Raiciu, A. Greenhalgh, and M. Handley. Design, Implementation and Evaluation of Congestion Control for Multipath TCP. In Proceedings of the 8th USENIX Symposium on Networked Systems Design and Implementation (NSDI). 2011. |