A Graph Minor Perspective to Multicast Network Coding

Network coding encourages information coding across a communication network. While the necessity, benefit and complexity of network coding are sensitive to the underlying graph structure of a network, existing theory on network coding often treats the network topology as a black box, focusing on algebraic or information theoretic aspects of the problem. This paper aims at an in-depth examination of the relation between algebraic coding and network topologies. We mathematically establish a series of results along the direction of: if network coding is necessary/beneficial, or if a particular finite field is required for coding, then the network must have a corresponding hidden structure embedded in its underlying topology, and such embedding is computationally efficient to verify. Specifically, we first formulate a meta-conjecture, the NC-minor conjecture, that articulates such a connection between graph theory and network coding, in the language of graph minors. We next prove that the NC-minor conjecture for multicasting two information flows is almost equivalent to the Hadwiger conjecture, which connects graph minors with graph coloring. Such equivalence implies the existence of K₄, K₅, K₆, and K_O(q/log q) minors, for networks that require F₃, F₄, F₅, and F_q to multicast two flows, respectively. We finally prove that, for the general case of multicasting arbitrary number of flows, network coding can make a difference from routing only if the network contains a K₄ minor, and this minor containment result is tight. Practical implications of the above results are discussed.