  The goal of this paper is to increase our understanding of the fundamental
performance limits of mobile and Delay Tolerant Networks (DTNs), where
end-to-end multi-hop paths may not exist and communication routes may only be
available through time and mobility. We use analytical tools to derive generic
theoretical upper bounds for the information propagation speed in large scale
mobile and intermittently connected networks. In other words, we upper-bound
the optimal performance, in terms of delay, that can be achieved using any
routing algorithm. We then show how our analysis can be applied to specific
mobility and graph models to obtain specific analytical estimates. In
particular, in two-dimensional networks, when nodes move at a maximum speed $v$
and their density $\nu$ is small (the network is sparse and surely
disconnected), we prove that the information propagation speed is upper bounded
by ($1+O(\nu^2))v$ in the random way-point model, while it is upper bounded by
$O(\sqrt{\nu v} v)$ for other mobility models (random walk, Brownian motion).
We also present simulations that confirm the validity of the bounds in these
scenarios. Finally, we generalize our results to one-dimensional and
three-dimensional networks.
