Multiple variable-length sub-structures in ExternalStructure ?

Does anyone know of a neat way to describe an ExternalStructure which has more
than one variable-length sub-structure ?

(This is actually for parsing a network protocol, so if there's no easy way to
handle it, I'll just drop the idea of overlaying an ExternalStructure on the
received bytes, and parse the data field-by-field.)

TIA.

    -- chris