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Re: [DNSOP] Clarification question: compression pointers always to names earlier in the packet?

Ted Lemon <mellon@fugue.com> Wed, 24 October 2018 20:27 UTC

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References: <BC2CDF40-4FF0-4111-88B7-04969491D2E0@dukhovni.org> <EC514300-F235-41DB-A413-2F9F8F8B04C8@sinodun.com> <f3c89916-6622-c153-f662-9ac42b61d81c@time-travellers.org>
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From: Ted Lemon <mellon@fugue.com>
Date: Wed, 24 Oct 2018 16:26:37 -0400
Message-ID: <CAPt1N1nq2-9NxoGSg3M+XHehAeT47A5T1w54vvmG=83j-hRoUw@mail.gmail.com>
To: Shane Kerr <shane@time-travellers.org>
Cc: dnsop WG <dnsop@ietf.org>
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Subject: Re: [DNSOP] Clarification question: compression pointers always to names earlier in the packet?
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The good news is that in a typical DNS message, N is pretty small.
 Although I suppose in an internet-facing name server, pretty small is
still pretty big...

On Wed, Oct 24, 2018 at 4:25 PM Shane Kerr <shane@time-travellers.org>
wrote:

> John,
>
> On 24/10/2018 15.38, John Dickinson wrote:
> > On 24 Oct 2018, at 10:01, Viktor Dukhovni wrote:
> >
> >> My reading of RFC 1035 is that DNS name "compression"
> >> via "pointers" is restricted to name strictly earlier
> >> in the DNS message:
> >>
> >>    4.1.4. Message compression
> >>
> >>    In order to reduce the size of messages, the domain system utilizes a
> >>    compression scheme which eliminates the repetition of domain names
> >> in a
> >>    message.  In this scheme, an entire domain name or a list of labels
> at
> >>    the end of a domain name is replaced with a pointer to a prior
> >> occurance
> >>
> >> ---------------
> >>    of the same name.
> >>
> >
> > Not strictly to do with loops but we noticed that not all nameservers
> > use the same compression algorithm. See section 9.1 and appendix B of
> > https://datatracker.ietf.org/doc/draft-ietf-dnsop-dns-capture-format
>
> It's not too surprising. I am pretty sure that producing optimal name
> compression (by size) is the same as one of packing problems, which are
> either NP-complete or NP-hard depending on the particulars:
>
> https://en.wikipedia.org/wiki/Packing_problems
>
> I think that this is only true if you allow re-ordering of RRset as well
> as RR within a set, but still.
>
> If you don't allow re-ordering then I think the overall complexity of
> compressing a message is the same as sorting, O(NlgN), which while a lot
> better is still not great, because you need to search through all prior
> labels to find the longest match when compressing each name.
>
> I think it's due to these fundamentally expensive costs that different
> name servers use the various heuristics you mention in the C-DNS draft
> to try to get a good balance of speed & size.
>
> Cheers,
>
> --
> Shane
>
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