[nfsv4] Sending a stronger message in rpc-tls

[David Noveck <[e-mail]>]

One possibility to consider is replacing the penultimate paragraph of the
Introduction by the following two paragraphs.

The current document assumes policies in line with [RFC7435]
in order to enable RPC-on-TLS to be deployed opportunistically in
environments that contain RPC implementations that do not support
TLS. Specifications for RPC-based upper-layer protocols will often
choose to require stricter policies in order to guarantee that encryption
or host authentication is in use on every connection.

Imposition of such stricter policies is of particular importance with
regard to
protocols for which the within-protocol security infrastrucure is weak,
allowing common deployments without encryption of request and response
data or providing attackers the opportunity to easily obtain server
execution
of substantively unauthenticated requests, with authentication presumptively

provided by the clients (e.g. using AUTH_SYS), which themselves have not
been authenticated.