This is the ST-2 version 3 flowspec.
The FlowSpec is used to convey stream service requirements
end-to-end. We expect that other versions of FlowSpec will
be needed in the future, which may or may not be subsets or
supersets of the version described here. PBytes will allow
new constraints to be added to the end without having to
simultaneously update all implementations in the field.
Implementations are expected to be able to process in a
graceful manner a Version 4 (or higher) structure that has
more elements than shown here.