Re: [spfbis] RFC7208 dual-cidr-length clarification

[David Bürgin <[e-mail]>]

Jan Schaumann:
> Reading RFC7208 leaves me wondering whether the
> correct syntax when specifying both an ip4-cidr-length
> and an ip6-cidr-length requires two slashes to
> separate them:
> 
> a                = "a"      [ ":" domain-spec ] [ dual-cidr-length ]
> ip4-cidr-length  = "/" ("0" / %x31-39 0*1DIGIT) ; value range 0-32
> ip6-cidr-length  = "/" ("0" / %x31-39 0*2DIGIT) ; value range 0-128
> dual-cidr-length = [ ip4-cidr-length ] [ "/" ip6-cidr-length ]
> 
> This reads to me that the correct syntax would need to
> be:
> 
> a:example.com/24//64
>              +--++--+
>              || |||
>              || ||+---> "64" (ip6-cidr-length value range 0-128)
>              || ||
>              || |+----> "/" from 'ip6-cidr-length' definition
>              || |
>              || +-----> "/" from 'dual-cidr-length' definition
>              ||
>              |+-------> "24" (ip4-cidr-length value range 0-32)
>              |
>              +--------> "/" from 'ip4-cidr-length' definition
> 
> I suspect that the "/" in 'dual-cidr-length' is
> intended to allow an empty ip4-cidr-length, so that
> it'd be possible to say
> 
> a:example.com//64
> 
> to convey that no ip4-cidr-length is specified (and
> would then be equivalent to "a:example.com/32/64"),
> but I think as written above, specifying both an
> ip4-cidr-length and an ip6-cidr-length would require
> dual-slashes as shown above, rather than the more
> intuitive
> 
> a:example.com/24/64
> 
> Perhaps the intended grammar was
> 
> dual-cidr-length = ( "/" / [ ip4-cidr-length ] ) [ ip6-cidr-length ]
> 
> ?

Interesting. I interpreted the syntax to allow all of

mx
mx/24
mx//64
mx/24//64

for IPv4 or IPv6 CIDR prefix length, both, or neither. Right?