Multipath TCP : beyond grandmother’s TCP

TCP, the Transmission Control Protocol, is the default transport protocol in the TCP/IP protocol suite. TCP is essentially based on research carried out during the 1970s that resulted in the publication of RFC 793. Since then, TCP has slowly evolved. A major step in TCP’s history is the congestion control scheme proposed and implemented by Van Jacobson [2]. RFC 1323 proposed a few years later extended TCP to support larger windows, a key extension for today’s high speed networks. Various changes have been included in TCP over the years and RFC 4614 provides pointers to many of the TCP standardisation documents.

In the late 1990s and early 2000s, the IETF developed the Stream Control Transmission Protocol (SCTP) RFC 4960. Compared to TCP, SCTP is a more advanced transport protocol that provides a richer API to the application. SCTP was also designed with multihoming and mind and can support hosts with multiple interfaces, something that TCP cannot easily do. Unfortunately, as of this writing, SCTP has not yet been widely deployed despite being implemented in major operating systems. The key difficulties in deploying SCTP appear to be :

  • the lack of SCTP support in middleboxes such as NAT, firewalls, ...
  • the need to update applications to support SCTP, although this is changing with RFC 6458

SCTP seems to be stuck in a classical chicken and egg problem. As there are not enough SCTP applications, middleboxes vendor do not support it and application developers do not use SCTP since middleboxes do not support. Multipath TCP is a major extension to TCP whose specification is currently being finalised by the MPTCP working group of the IETF. Multipath TCP allows a TCP connection to be spread over several interfaces during the lifetime of the connection. Multipath TCP has several possible use cases :

  • datacenters where Multipath TCP allows to better spread the load among all available paths [4]
  • smartphones where Multipath TCP allows to use both WiFi and 3G a the same time [3]

The design of Multipath TCP [1] has been more complicated than expected due to the difficulty of supporting various types of middleboxes [5], but the protocol is now ready and you can even try our implementation in the Linux kernel from http://www.multipath-tcp.org

[1]Alan Ford, Costin Raiciu, Mark Handley, and Olivier Bonaventure. Tcp extensions for multipath operation with multiple addresses. Internet draft, draft-ietf-mptcp-multiaddressed-07, March 2012.
[2]V. Jacobson. Congestion avoidance and control. In Symposium proceedings on Communications architectures and protocols, SIGCOMM ‘88, 314–329. New York, NY, USA, 1988. ACM.
[3]Christoph Paasch, Gregory Detal, Fabien Duchene, Costin Raiciu, and Olivier Bonaventure. Exploring mobile/wifi handover with multipath tcp. In ACM SIGCOMM workshop on Cellular Networks (Cellnet12). 2012.
[4]Costin Raiciu, Sebastien Barre, Christopher Pluntke, Adam Greenhalgh, Damon Wischik, and Mark Handley. Improving datacenter performance and robustness with multipath tcp. In Proceedings of the ACM SIGCOMM 2011 conference, SIGCOMM ‘11, 266–277. New York, NY, USA, 2011. ACM.
[5]Costin Raiciu, Christoph Paasch, Sebastien Barre, Alan Ford, Michio Honda, Fabien Duchene, Olivier Bonaventure, and Mark Handley. How hard can it be? designing and implementing a deployable multipath tcp. In USENIX Symposium of Networked Systems Design and Implementation (NSDI‘12), San Jose (CA). 2012.