Interface parameters

We need some text around the interface parameters that defines the
behavior when an unknown interface parameter is seen.  E.g., some
vendors have implemented FCS retention, and there is apparently no way
to turn it off.  Any code that pre-dates FCS retention may or may not
work with this up-level code because:
  - there are mandatory interface parameters
  - there are optional interface parameters
  - there is no way to tell if, for a PW, an interface parameter is
optional or mandatory
  - any new interface parameter may also be mandatory or optional

So it is fine for the FCS retention draft to say that it is backwards
compatible with any pre-FCS retention implementation.  However, that is
true only if the behavior of the older implementations is to ignore any
unknown interface parameters.  This is what draft-ietf-pwe3-control
says:

  5.4. Interface Parameters Field

     This field specifies interface specific parameters. When
applicable,
                                                         ^^^^^^^^^^^^^^^
     it MUST be used to validate that the PEs, and the ingress and
egress
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
     ports at the edges of the circuit, have the necessary capabilities
to
     interoperate with each other. The field structure is defined as
     follows: