Multipath TCP is an extension of TCP that will soon be standardized by IETF. It is a succesful attempt to resolve major TCP shortcomings emerged from the change in the way we use our devices to communicate. There’s particularly the change in the way our new devices like iPhones and laptops are talking across network. All the devices like the networks are becoming multipath. Networks redundancy and devices multiple 3G and wireless connections made that possible.
Almost all today’s web applications are using TCP to communicate. This is due to TCP virtue of reliable packet delivery and ability to adapt to variable network throughput conditions. Multipath TCP is created so that it is backwards compatible with standard TCP. In that way it’s possible for today’s applications to use Multipath TCP without any changes. They think that they are using normal TCP.
We know that TCP is single path. It means that there can be only one path between two devices that have TCP session open. That path is sealed as a communication session defined by source and destination IP address of communicating end devices. If some device wants to switch the communication from 3G to wireless as it happens on smartphones when they come in range of known WiFi connection, TCP session is disconnected and new one is created over WiFi. Using multiple paths/subsessions inside one TCP communication MPTCP will enable that new WiFi connection makes new subsession inside established MPTCP connection without braking TCP that’s already in place across 3G. Basically more different paths that are available will be represented by more subsessions inside one MPTCP connection. Device connected to 3G will expand the connection to WiFi and then will use algorithm to decide if it will use 3G and WiFi in the same time or it will stop using 3G and put all the traffic to cheaper and faster WiFi.
TCP single path property is TCP’s fundamental problem
In datacenter environment there is a tricky situation where two servers are talking to each other using TCP to communicate and that TCP session is created across random path between servers and switches in the datacenter. If there are more paths of course. If there are (and there are!) another two servers talking in the same time, it will possibly happen that this second TCP session will be established using partially the same path as the first TCP session. In that situation there will be a collision that will reduce the throughput for both sessions. There is actually no way to control this phenomenon in TCP world. As in our datacenter example the same thing works for every multipath environment so it it true for example for the Internet.
Answer is MPTCP!
Multipath TCP – MPTCP is better as TCP in that enables the use of multiple paths inside a single transport connection. It meets the goal to work well at any place where “normal” TCP would work.