The “Experimental” status of Multipath TCP

Multipath TCP is defined in RFC 6824 and I recently heard feedback from someone working for industry who mentioned that Multipath TCP should not be considered for deployment given its Experimental status. I was surprised by this comment and I think that it would be useful to clarify some facts about the maturity of Multipath TCP.

First, from a administrative viewpoint, the Experimental status of Multipath TCP was decided at the creation of the IETF MPTCP working group. At that time, it was unclear whether it would be even possible to specify a protocol like Multipath TCP and the IESG wanted to encourage experiments with the new protocol. By selecting this option , the IESG prepared a future standardisation of the protocol and this is happening right now with the definition of a standards-track version of Multipath TCP in RFC6824bis . According to the milestones of the IETF MPTCP working group, this revision should be ready in 2017.

Second, from a technical viewpoint, the maturity of a protocol cannot be inferred from the status of its specification. The best way to measure this maturity is to observe the interoperable implementations and the deployment of the protocol. From these two viewpoints, Multipath TCP is a clear success. There are endhost implementations on Linux, FreeBSD, Apple iOS, MacOS and Oracle Solaris. Multipath TCP is also supported on various middleboxes including Citrix Netscaler, F5 BIG-IP LTM and Ericsson.

From a deployment viewpoint, Multipath TCP is also a huge success. Hundreds of millions of users of Apple devices (iPhone, iPad, laptops) use Multipath TCP every time they use the Siri voice recognition application. In Korea, a dozen of models of high-end smartphones from Samsung and LG include a port of the reference implementation of Multipath TCP in the Linux kernel and use SOCKS proxies to bond WiFi and fast LTE. Several network operators provide those proxies as a commercial service. Other companies such as Swisscom or OVH also rely on SOCKS proxies to bond different types of links together. Another emerging use case are hybrid access networks. In various countries, network operators are require to provide fast broadband services, even in rural areas where deploying fiber is too expensive. Many of these operators want to combine their xDSL and LTE networks in order to improve the bandwidth to their customers. Tessares has already deployed a pilot hybrid access network solution that leverages Multipath TCP in Belgium.