app-protocol
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... experimental-service / iana-registered-service
app-protocol = experimental-protocol / iana-registered-protocol
...
... DIGIT / SYM
; The app-service and app-protocol tags are limited to 32
; characters and must start ...
... service, or an app-service with one or more app-protocol
specifications separated by the ":" symbol.
...
... The protocol identifiers valid for the "app-protocol" production are
standard, registered protocols; see section 7 for instructions on
registering new application protocol ...