Default selection is the loop-back interface. Users who require
interoperation between WSJT-X instances cooperating applications
running on different hosts should select a suitable network interface
and carefully choose a multicast group address, and TTL, that has
minimal scope covering the necessary network(s). Using 224.0.0.1 is a
reasonable strategy if all hosts are on the same
subnet. Administratively scoped multicast group addresses like those
within 239.255.0.0/16 can cover larger boundaries, but care must be
taken if the local subnet has access to a multicast enabled router.
The IPv4 broadcast address (255.255.255.255) may be used as an
alternative to multicast UDP, but note that WSJT-X will only send
broadcast UDP datagrams on the loop-back interface, so all recipient
applications must be running on the same host system.
The reference UDP Message protocol applications are being extended to
be configurable with a list of interfaces to join a multicast group
address on. By default they will only join on the loop-back interface,
which is also recommended for any applications designed to take part
in the WSJT-X UDP Message Protocol. This allows full user control of
the scope of multicast group membership with a very conservative
default mode that will work with all interoperating applications
running on the same host system.
