Re: [dhcwg] dhc WG review of draft-ietf-geopriv-lis-discovery-05

["Thomson, Martin" <[e-mail]>]

My earlier mail was bounced (forgot to subscribe).  Apologies to the moderators if this is a double-up.

> -----Original Message-----
> From: Thomson, Martin
> Sent: Wednesday, 28 January 2009 9:06 AM
> To: 'Ralph Droms'; David W. Hankins
> Cc: DHC WG; Richard Barnes; Winterbottom, James
> Subject: RE: [dhcwg] dhc WG review of draft-ietf-geopriv-lis-discovery-
> 05
> 
> Hi Ralph, David,
> 
> Thanks both for your feedback.
> 
> Obviously, I missed a critical piece of text on cardinality.  The
> intent is to convey one URI with one or more fingerprints.  The URI is
> the point of the exercise, and the fingerprints describe a property of
> that URI.  A fingerprint without a URI would be pointless, so I wanted
> a format that would inherently enforce the restriction.  What Ralph
> proposes is almost identical to what is in the current draft with the
> limitation of one (or no) fingerprints.  It would also be possible to
> overload the meaning of hash-type-len so that a zero length means that
> there are no more fingerprints, which would allow for multiple
> fingerprints.
> 
> Allowing for multiple fingerprints ensures that new hash functions can
> be invented and actually used without breaking backward compatibility.
> That's the idea at least--I'm sure that our security expert friends
> would be happy to tell me otherwise if this were a bad or unworkable
> idea.
> 
> For David's benefit, concatenation would only be useful in providing
> for a longer URI.  I'll take the suggestion on restricting the total
> option length to 253 and I'll be clearer on the lack of the same
> restriction in v6.  As an aside, 225 happens to be the length to the
> URI after a SHA-1 fingerprint is included...I think :)  253 - f-
> code(1)x2 - hashlen(1) - 'sha-1'(5) -fprint-len(1) - fprint(20) =
> 224... oh well.
> 
> Cheers,
> Martin
> 
> > -----Original Message-----
> > From: Ralph Droms [mailto:[e-mail]]
> > Sent: Wednesday, 28 January 2009 7:06 AM
> > To: David W. Hankins
> > Cc: Ralph Droms; DHC WG; Richard Barnes; Winterbottom, James;
> Thomson,
> > Martin
> > Subject: Re: [dhcwg] dhc WG review of draft-ietf-geopriv-lis-
> discovery-
> > 05
> >
> > David - if, indeed, the idea is for one URI, with an optional
> > fingerprint, would the following format be sensible and simpler to
> > parse?
> >
> >      0                   1                   2                   3
> >      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
> >     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
> >     |    LIS_URI    |    Length     | Hash-Type-Len | Hash-Type   ...
> >     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
> >     |                           Hash-Type (cont.)                 ...
> >     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
> >     |  Fprint-Len   |           Fingerprint-Value                 ...
> >     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
> >     |                           URI                               ...
> >     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
> >     |                           URI (cont.)                       ...
> >     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
> >
> > where Hash-Type-Len and Fprint-Len are both either zero (no
> > fingerprint) or greater than 0 (fingerprint included).
> >
> > - Ralph
> >
> > On Jan 27, 2009, at 2:39 PM 1/27/09, David W. Hankins wrote:
> >
> > > On Tue, Jan 27, 2009 at 06:41:06AM -0500, Ralph Droms wrote:
> > >> Can there be more than one fingerprint and more than one URI
> > >> carried in the
> > >> LIS-URI option?  I don't think the draft explicitly restricts the
> > >> option.
> > >> Because the URI does not include a length field, it would seem to
> > >> limit the
> > >> option to one URI, and the fingerprint MUST come before the URI.
> I
> > >> think
> > >> the text should be explicit on this point.
> > >>
> > >> If I understand this restriction correctly, it may be possible to
> > >> simplify
> > >> the option syntax by assuming the order and number of fields in
> the
> > >> option.
> > >> I'll defer to dhc WG members for opinions on whether there might
> be
> > a
> > >> modification to the syntax that would be more in line with
> existing
> > >> options
> > >> and implementations.
> > >
> > > I presumed the intent was to provide only one URI.
> > >
> > > I think that if you squint and turn your head sideways, the 'f-
> code,
> > > f-length' looks like a suboption (except that the URI's f-code
> > implies
> > > no length octet, evidently a format size optimization).
> > >
> > > I want to ask if the idea is to intentionally make a distinction
> > > between 'f-code's and suboptions, such as in order to support
> > multiple
> > > instances of one f-code (and not provoke option concatenation as
> > would
> > > happen with multiple sub-option codes of the same value).
> > >
> > > If they can be suboptions, then why not multiple regular options?
> > > I think the answer is that the certificate is useless without the
> > URI,
> > > and vice versa, so if a DHCP server omitted one due to running out
> of
> > > space it is better to omit both.  Containering them together solves
> > > this neatly.
> > >
> > >> The DHCPv6 option doesn't have the 253 byte length restriction.
> It
> > >> would
> > >> be good to mention that difference between the two option
> versions.
> > >
> > > I was curious what you meant (253, not 255?), so went back to re-
> read
> > > the draft.  I don't see the number 253 mentioned, nor 255!
> > >
> > > It /does/ mention '225'.  This appears to be a typo?
> > >
> > > My only suggestion for the mention of the option size is to refer
> > > specifically to the 'length field value', and not to terms like
> 'URI
> > > length' or 'option length', which can be misinterpreted from
> > different
> > > starting positions.
> > >
> > > Otherwise I like this approach of referring to the extension of
> > option
> > > space as a DHCPv4 protocol feature, and not trying to reinvent its
> > > specification.
> > >
> > > --
> > > David W. Hankins	"If you don't do it right the first time,
> > > Software Engineer		     you'll just have to do it again."
> > > Internet Systems Consortium, Inc.		-- Jack T. Hankins
> > > _______________________________________________
> > > dhcwg mailing list
> > > [e-mail]
> > > https://www.ietf.org/mailman/listinfo/dhcwg
> >

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