Re: IID length text [was Re: Review of draft-ietf-6man-rfc4291bis-06]

神明達哉 <jinmei@wide.ad.jp> Wed, 18 January 2017 19:29 UTC

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Date: Wed, 18 Jan 2017 11:29:35 -0800
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Subject: Re: IID length text [was Re: Review of draft-ietf-6man-rfc4291bis-06]
To: Fernando Gont <fgont@si6networks.com>
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At Tue, 17 Jan 2017 23:04:08 -0300,
Fernando Gont <fgont@si6networks.com> wrote:

> Has anyone tred what happens if a prefix from::/3 is advertised for slaac?

I didn't, but I'm quite sure about BSD variants (perhaps including
iOS) that they don't check the prefix (as long as they are global) for
the purpose of SLAAC.  This is based on the interpretation of RFC4862
that for the purpose of SLAAC the IID length is solely a parameter of
the underlying link:

   interface identifier -  a link-dependent identifier for an interface
      that is (at least) unique per link [RFC4291].  Stateless address
      autoconfiguration combines an interface identifier with a prefix
      to form an address.  From address autoconfiguration's perspective,
      an interface identifier is a bit string of known length.  The
      exact length of an interface identifier and the way it is created
      is defined in a separate link-type specific document that covers
      issues related to the transmission of IP over a particular link
      type (e.g., [RFC2464]).  Note that the address architecture
      [RFC4291] also defines the length of the interface identifiers for
      some set of addresses, but the two sets of definitions must be
      consistent.  In many cases, the identifier will be derived from
      the interface's link-layer address.

So, for example, if the advertised prefix length on an Ethernet link
is not 64, that prefix is simply ignored for SLAAC according to
Section 5.5.3 d) of RFC4862:

      If the sum of the prefix length and interface identifier length
      does not equal 128 bits, the Prefix Information option MUST be
      ignored.

That's the same regardless of what prefix it is, whether it's in or
outside of ::/3.

--
JINMEI, Tatuya