I'm not aware of any formal spec, but Michael N6MEF's suggestion pretty much covers it. The only difference I've come across between implementations is that some require a specific source port as well as the defined destination port.
FYI John, in case you aren't aware:
JNOS used to require that the source port for incoming packets be specified when setting up the AXUDP interface in JNOS. Of course, that's very unusual for IP protocols and also easily broken by firewalls. In JNOS 2.0j.5 (June 2014), Maiko added the option to specify "-1" for the incoming source port, meaning "don't care". This is what I use with all AXUDP connections, including to BPQ systems. http://www.langelaar.net/projects/jnos2/documents/changes.txt
There seem to be two commonly used ports (93 and 10093).though others are also used. Using low numbered ports can be a problem with some operating systems (requiring root/admin rights) so I prefer 10093
Agreed.
Michael N6MEF