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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