Avoid "MAY ... only if"

Combining the RFC2119 term "MAY" with a negation or restrictions runs counter to what MAY wants to express.

Clients MAY do A only if B

sounds as if

• the client may choose not to do A if B (which is not always implied)
• there is no rule for the client if not B.

Instead, write

Clients MUST NOT do A if (not B)