Re: [DNSOP] Clarification question: compression pointers always to names earlier in the packet?
Shane Kerr <shane@time-travellers.org> Wed, 24 October 2018 20:25 UTC
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From: Shane Kerr <shane@time-travellers.org>
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Date: Wed, 24 Oct 2018 22:25:12 +0200
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Subject: Re: [DNSOP] Clarification question: compression pointers always to names earlier in the packet?
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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
- [DNSOP] Clarification question: compression point… Viktor Dukhovni
- Re: [DNSOP] Clarification question: compression p… bert hubert
- Re: [DNSOP] Clarification question: compression p… Shane Kerr
- Re: [DNSOP] Clarification question: compression p… Joe Abley
- Re: [DNSOP] Clarification question: compression p… Martin Hoffmann
- Re: [DNSOP] Clarification question: compression p… Tony Finch
- Re: [DNSOP] Clarification question: compression p… Martin Hoffmann
- Re: [DNSOP] Clarification question: compression p… John Dickinson
- Re: [DNSOP] Clarification question: compression p… Ray Bellis
- Re: [DNSOP] Clarification question: compression p… Tony Finch
- Re: [DNSOP] Clarification question: compression p… Viktor Dukhovni
- Re: [DNSOP] Clarification question: compression p… Tony Finch
- Re: [DNSOP] Clarification question: compression p… Ted Lemon
- Re: [DNSOP] Clarification question: compression p… Mark Andrews
- Re: [DNSOP] Clarification question: compression p… Shane Kerr
- Re: [DNSOP] Clarification question: compression p… Ted Lemon
- Re: [DNSOP] Clarification question: compression p… Mukund Sivaraman
- Re: [DNSOP] Clarification question: compression p… George Michaelson