[ietf-dkim] NEW ISSUE: SSP-02: Policy Scope

[Douglas Otis]

The prior syntax did not seem to permit enough flexibility.

s= Policy Scope (plain-text; OPTIONAL; default is "*").  A colon-
   separated list of policy scopes specify which protocols to which
   this record applies.  Verifiers for a given service type MUST
   ignore this record if the appropriate type is not listed.
   Currently defined protocol types are as follows:

       *  matches all unlisted service types
       !  disavows protocol use
       -  excludes from policy

       SMTP	RFC2821
       NNTP	RFC3977
       MSRP	RFC4975
       UUCP	RFC976

   This tag is intended to constrain the use of policy for various
   transport protocols that may implement, should DKIM be defined by
   other protocols in the future. This tag can also disavow use
   of specific protocols to repudiate references to this domain.

   As example, "s=SMTP:-UUCP:!*" would mean this domain only uses
   SMTP and UUCP to exchange messages, but that this policy does not
   apply to UUCP.

ABNF:

   policy-s-tag  = %x73 [FWS] "=" [FWS][exclude|disavow] policy-s-tag- 
type
                   0*( [FWS] ":" [FWS] policy-s-tag-type )
   disavow = "!"
   exclude = "-"
   policy-s-tag-type   = "SMTP" /
                         "NNTP" /
                         "MSRP" /
                          UUCP /
                         "*" /
                         x-policy-s-tag-type
   x-policy-s-tag-type = hyphenated-word   ; for future extension