Constrained-path LSP in which the path is
obtained by querying an external source, such as a PCE server.
In the case that an LSP is defined to be externally queried, it may
also have associated explicit definitions (which are provided to the
external source to aid computation); and the path that is returned by
the external source is not required to provide a wholly resolved
path back to the originating system - that is to say, some local
computation may also be required