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 <[email protected]> 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 > > _______________________________________________ > DNSOP mailing list > [email protected] > https://www.ietf.org/mailman/listinfo/dnsop >
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