RE: nsec++

[Roy Arends <roy@logmess.com>]

On Thu, 4 Dec 2003, Andrew Draper wrote:

> Hi,
>
> While not wishing to reopen (too much) the debate about how to encode
> this efficiently, I have a suggestion which might make the encoding of
> sparse NSEC records a lot smaller, while being only slightly harder to
> understand and I think no harder to implement.
>
> If the types encoded are sparse then the bitmaps will tend to have a lot
> of zeros at the start of them.  For example the RDATA for:
>
> example.com. IN NSEC host.example.com. 385
>
> will be encoded as, I think:
>
>       04 'h' 'o' 's' 't'  06 'e' 'x' 'a' 'm' 'p' 'l' 'e'  03 'c' 'o' 'm'  00
>       01 11  00 00 00 00  00 00 00 00  00 00 00 00  00 00 00 00  02
>       (all numbers are in hex)
>
> which has a lot of superfluous zeros.
>
> There are three bits spare in the "bitmap length" byte (if you store bitmap_length-1 instead of just bitmap_length) so it would be much more efficient, for sparse bitmaps, if the block start was allowed to be on any boundary of 32.
>
> The encoding of the same RR would then be:
>
>       04 'h' 'o' 's' 't'  06 'e' 'x' 'a' 'm' 'p' 'l' 'e'  03 'c' 'o' 'm'  00
>       0C 00  02
>
>
> If you like this then suggested text is below (I wrote this yesterday so some of the alternative suggested descriptions of bitmap might be better than this one):
>
>
>    The type space is represented using one or more blocks.  Each block
>    starts at a location in type space which is a multiple of 32 and
>    describes upto 256 active types.  Each block consists of a 13 bit
>    start type number, a 5 bit length field (from 1 .. 32 octets) and
>    a bitmap (covering upto 256 type codes) in network bit order
>    (similar to NXT).

One of the design requirements, and therefor one of the reasons some
proposals did not make it is that the bit map must be able to respresent
every type in one bitmap.

Your encoding gives us a worst case length of 69632 which is >64K

Roy



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