NFD

Currently, when UdpFactory creates a UdpChannel, the latter is put into listening mode unconditionally. This is different from e.g. TcpChannel, where listening or non-listening is decided by a configuration file setting (face_system.tcp.listen).

This task implements a similar knob (face_system.udp.listen) that applies to UdpChannel. The use case is allowing "outgoing-only" channels, in which UDP faces can only be created explicitly through management requests, and incoming datagrams from unknown peers are ignored.