  WebRTC enables browsers to exchange data directly but the number of possible
concurrent connections to a single source is limited. We overcome the
limitation by organizing participants in a fat-tree overlay: when the maximum
number of connections of a tree node is reached, the new participants connect
to the node's children. Our design quickly scales when a large number of
participants join in a short amount of time, by relying on a novel scheme that
only requires local information to route connection messages: the destination
is derived from the hash value of the combined identifiers of the message's
source and of the node that is holding the message. The scheme provides
deterministic routing of a sequence of connection messages from a single source
and probabilistic balancing of newer connections among the leaves. We show that
this design puts at least 83% of nodes at the same depth as a deterministic
algorithm, can connect a thousand browser windows in 21-55 seconds in a local
network, and can be deployed for volunteer computing to tap into 320 cores in
less than 30 seconds on a local network to increase the total throughput on the
Collatz application by two orders of magnitude compared to a single core.
