  We study a multi-source Gaussian relay network consisting of $K$
source--destination pairs having $K$ unicast sessions. We assume $M$ layers of
relays between the sources and the destinations. We find achievable degrees of
freedom of the network. Our schemes are based on interference alignment at the
transmitters and symbol extension and opportunistic interference cancellation
at the relays. For $K$-$L$-$K$ networks, i.e., 2-hop network with $L$ relays,
we show $\min\{K,K/2+L/(2(K-1))\}$ degrees of freedom are achievable. For
$K$-hop networks with $K$ relays in each layer, we show the full $K$ degrees of
freedom are achievable provided that $K$ is even and the channel distribution
satisfies a certain symmetry.
