Stop me if I've misunderstood...

[Mark Andrews]

In message <CAD7705D4A93814F97D3EF00790AF0B315F59BF2 at tk5ex14mbxc105.redmond.corp.microsoft.com>, Shawn Steele writes:
> > So it's
> > not quite right that microsoft.com and MICROSOFT.COM "don't compare
> > equal in binary form".
> 
> They differ in the 0x20 bits.  Therefore a bit-by-bit (eg: binary)
> comparison has them differ.
>
> > The protocol is quite specific that they _do_ in fact compare equal,
> > even though they happen to have different 0x20 bits on the octets.
> 
> That's not quite true.  0x30-0x39 are permitted, 0x10-0x19 are
> not.  ASCII ignore-case is "mapping", it's just a very simple and
> easy to implement form.  (And doesn't DNS recommend lowercase now
> anyway?)

	No.  It's just the cannonical form when you want to compute
	crypto hashes of records that contain domain names in records
	that at are case insensitive.  If we thought it would treated
	as a new presentation standard we would have choosen
	alternating upper lower case just to make the point that
	it isn't a presentation standard.

> You can probably cheat by throwing out < 0x30 which takes
> care of 0x10-0x19 anyway, but it's still mapping.  If you ignore
> the 0x20 by OR 0x20, then your mapping one way (lower case).  If
> it's by AND 0xdf (or whatever) then its the other way (upper case).
> You're still mapping, it just doesn't look like it :)
> 
> > As a matter of actual implementation, however, it's worth noting that
> > in just about every real implementation in the wild, the server uses
> > the form of the name _as supplied by the user_.
> 
> Yes, the mapping happens at a different layer.  That doesn't make comparison
> impossible, we just need consistent rules.
> 
> > This is because most name servers use the compression trick
> 
> Sorry, I don't know what the compression trick is :)
> 
> -Shawn
> 
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-- 
Mark Andrews, ISC
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: marka at isc.org