[DNSOP] valid value range for SOA REFRESH/RETRY/EXPIRE

[神明達哉 <[e-mail]>]

I have a question for which I believe there's an answer already that I
couldn't find: what's the valid range for SOA REFRESH/RETRY/EXPIRE
values?

RFC1035 says:

  REFRESH         A 32 bit time interval ...
  RETRY           A 32 bit time interval ...
  EXPIRE          A 32 bit time value ...

and since it explicitly uses "unsigned" for SERIAL and MINIMUM, e.g:

  SERIAL          The unsigned 32 bit version number of the original copy

one obvious interpretation is that REFRESH/RETRY/EXPIRE are signed 32
bit integers.  And, since negative values for these don't make much
sense, we might even interpret it similar to RR TTLs as clarified in
RFC2181, i.e., 0 <= REFRESH/RETRY/EXPIRE <= 2^31-1.

Is this correct?  Implementations seem to vary on this point, btw.
>From my quick experiment with some code reading,
- BIND 9 accepts any unsigned 32-bit values
- Same for Knot
- NSD treats them like TTL (values >= 2^31 are reduced to the "default
  TTL" value)
(I've only checked at the primary side; I didn't do any test how the
secondary side of the implementation uses these values when they are
very large).

Such huge values for these parameters don't make sense in practice
anyway, so this is probably a pedantic question.  But if anyone knows
an authoritative reference that can answer it I'd appreciate it very
much.

Thanks,

--
JINMEI, Tatuya