  The interference channel models a wireless network where several
source-destination pairs compete for the same resources. When nodes transmit
simultaneously the destinations experience interference. This paper considers a
4-node network, where two nodes are sources and the other two are destinations.
All nodes are full-duplex and cooperate to mitigate interference. A sum-rate
outer bound is derived, which is shown to unify a number of previously derived
outer bounds for special cases of cooperation. The approach is shown to extend
to cooperative interference networks with more than two source-destination
pairs and for any partial sum-rate. How the derived bound relates to similar
bounds for channel models including cognitive nodes, i.e., nodes that have
non-causal knowledge of the messages of some other node, is also discussed.
Finally, the bound is evaluated for the Gaussian noise channel and used to
compare different modes of cooperation.
