Which servers use Multipath TCP ?

Since the publication of RFC 6824, Multipath TCP has been deployed for several use cases described in RFC 8041 and [BS16]. However, there has been no public study of the deployment of Multipath TCP on servers.

A first study appeared in [MHFB15]. They used zmap to send SYN packets with the MP_CAPABLE option and checked whether they replied with the MP_CAPABLE option. They used 452,008 unique IP addresses corresponding to a bit less than 2 million domains. Their results are summarised in the table below (source [MHFB15]).


Unfortunately, there was a flaw in the methodology. Checking the presence of the MP_CAPABLE option in the SYN+ACKis not sufficient as some middleboxes simply echo in the SYN+ACK any option contained in the SYN even if they do not understand it. The dataset was later corrected and updated. Any zmap scan must check that a returned TCP option in a SYN+ACK is not simply echoed by a middlebox. For Multipath TCP, this means that the returned MP_CAPABLE option must differ from the option sent in the SYN.

Since the publication of this paper, no other study using zmap has been published. Several research groups use zmap to carry out measurement studies. One of them is the netray Internet Observatory at RWTH Aachen University. They have used zmap to study the deployment of HTTP/2, QUIC or the initial TCP congestion window. I recently discussed with Jan Ruth and Oliver Hohlfeld who had some interesting data. In August 2015, a scan of the entire IPv4 addressing space revealed 921 Multipath TCP servers, with about one sixth of them hosted by Apple. At the same time, about 24.000 addresses returned in the SYN+ACK an MP_CAPABLE option that was exactly the same as the one sent in the SYN packet.

They proposed to launch another zmap scan last week over the entire IPv4 addressing space on port 443. 88484 addresses replied with an MP_CAPABLE option. Out of these, 77384 simply returned exactly the same MP_CAPABLE option as the one included in the SYN packet. There are thus more middleboxes that echo unsupported TCP options than in August 2015. The interesting point of their study is that 11k addresses replied with a valid MP_CAPABLE option. This indicates an important growth in the deployment of Multipath TCP on servers. This analysis ignores the utilisation of Multipath TCP on clients such as iPhones and other smartphones, but also in Hybrid Access Networks since none of those devices respond to SYN packets.

The exact usage of these servers is not known. If you manage one of them, we’d be interested in knowing more about your use case to document it in revisions of RFC 8041. If it receives a lot of traffic, we’d also be interested in analysing packet traces in more details to extend the measurements published in [CBHB16a][CBHB16b][HTSB15][TCH+16].


[BS16]Olivier Bonaventure and SungHoon Seo. Multipath tcp deployments. IETF Journal, 12(2):24–27, 2016. URL: https://www.ietfjournal.org/multipath-tcp-deployments/.
[CBHB16a]Quentin De Coninck, Matthieu Baerts, Benjamin Hesmans, and Olivier Bonaventure. A first analysis of multipath tcp on smartphones. In 17th International Passive and Active Measurements Conference, volume 17. Springer, March-April 2016. URL: https://inl.info.ucl.ac.be/publications/first-analysis-multipath-tcp-smartphones.html.
[CBHB16b]Quentin De Coninck, Matthieu Baerts, Benjamin Hesmans, and Olivier Bonaventure. Observing real smartphone applications over multipath tcp. IEEE Communications Magazine, Network Testing Series, March 2016. URL: https://inl.info.ucl.ac.be/publications/observing-real-smartphone-applications-over-multipath-tcp.html.
[HTSB15]Benjamin Hesmans, Viet-Hoang Tran, Ramin Sadre, and Olivier Bonaventure. A first look at real multipath tcp traffic. In Traffic Monitoring and Analysis. 2015. URL: https://inl.info.ucl.ac.be/publications/first-look-real-multipath-tcp-traffic.html.
[MHFB15](1, 2) Olivier Mehani, Ralph Holz, Simone Ferlin, and Roksana Boreli. An early look at multipath tcp deployment in the wild. In Proceedings of the 6th International Workshop on Hot Topics in Planet-Scale Measurement, HotPlanet ‘15, 7–12. New York, NY, USA, 2015. ACM. URL: http://doi.acm.org/10.1145/2798087.2798088, doi:10.1145/2798087.2798088.
[TCH+16]Viet-Hoang Tran, Quentin De Coninck, Benjamin Hesmans, Ramin Sadre, and Olivier Bonaventure. Observing real multipath tcp traffic. Computer Communications, 2016. URL: https://inl.info.ucl.ac.be/publications/observing-real-multipath-tcp-traffic.html, doi:10.1016/j.comcom.2016.01.014.