Simplifying Multipath TCP configuration with Babel

Multipath TCP builds its sub-flows based on pairs of the client’s and server’s IP addresses. When a host is connected to two different providers, it should have one IP address associated to each provider, which allows Multipath TCP to effectively use these two paths simultaneously. However, with a single default route, packets will follow the same path, independently of their source address, which prevents Multipath TCP from working properly.

The Multipath TCP website provides a recipe for configuring multiple routes manually on a single host directly connected to multiple providers using policy routing. Even for such simple topology, the procedure consists of 7 commands. Analogous configuration for a more complex network, while possible, is exceedingly tedious, error-prone and fragile.

Consider for example the following topology, where routers A and B are connected to distinct providers, and the host H is connected to router C. Routers A and B have default routes for packets sourced in and respectively.    --- A ----- B ---
from       \     /      from
                        \   /
                         \ /

A manual configuration of this network will require an intervention on at least routers A, B and C. If for some reason the topology changes, a link fails or a router crashes, the network will fail to work correctly until an explicit intervention of the administrator.

Routing configuration of complex networks should not be done manually. That is the role of a dynamic routing protocol.

The source-specific version of the Babel routing protocol RFC 6126 will dynamically configure a network to route packets based both on both their source and destination addresses. Babel will learn routes installed in the kernel of the gateway routers (A and B), announce them through the network, and install them in the routing tables of the routers. Only the edge gateways need manual configuration to teach Babel which source prefix to apply to the default route they provide. A contrario, internal routers (C) will not need any manual configuration. In our example, we need to add the following directives to the Babel configuration file (/etc/babeld.conf) of the two gateways:


On router A:

redistribute ip eq 0 src-prefix
redistribute ip eq 0 proto 3 src-prefix

On router B:

redistribute ip eq 0 src-prefix
redistribute ip eq 0 proto 3 src-prefix

The second line specifies proto 3 because Babel does not redistribute proto boot routes by default.

The case of IPv6 is similar. In fact, it may be simpler, because Babel recognises kernel-side source-specific routes: in the best case, no additional configuration may be needed in IPv6.

The source-specific version of Babel works on Linux, and is available in OpenWRT as the babels package. You can also retrieve its source code from:

git clone

Additional information about the Babel protocol and its operation with Multipath TCP may be found in the technical report entitled Source-specific routing written by Matthieu Boutier and Juliusz Chroboczek