Function effective_plan 

pub fn effective_plan(
    policy: &PathPolicy,
    a: &PathOffer,
    b: &PathOffer,
    pair: PairFeasibility,
) -> EffectivePlan

The plan for a pair: each enabled rule of policy, in order up to and including the server relay, kept only when neither peer refuses it and the pair can use it. Direct needs NATs that admit a hole punch; PeerRelay needs a forwarder; Turn needs both peers to hold a TURN config.