[AusNOG] IPv6 Reachable and Retrans Times

[Mark Smith]

On Thu, 10 Jun 2010 18:59:41 +0930
Andrew Cox <andrew at accessplus.com.au> wrote:

> Hey All,
> 
> Can anyone here give me some suggestions or 'best practice' advice for 
> setting IPv6 ND Reachable Time and Retrans Timer *1
> I've been playing around with it on one of our test networks and it 
> seems that some vendors have this set by default *2 while others leave 
> it disabled/blank.
> 
> *1 http://tools.ietf.org/html/rfc2461#section-4.2
> *2 
> http://www.cisco.com/en/US/products/ps5989/products_configuration_guide_chapter09186a0080664d94.html#wp1216529 
> 

http://tools.ietf.org/html/rfc4861#section-10 (Updated RFC2461) shows
the default values should be :


REACHABLE_TIME               30,000 milliseconds

RETRANS_TIMER                 1,000 milliseconds


If a router is setting the values to 0, it means that the router isn't
specifying a value, and that the hosts should either use the above
defaults, or use values that a different router is supplying if they
are non-zero.

At face value, hard setting this parameters in the RAs to the
defaults would seem to be a way of ensuring all hosts used the
same/correct values. However, if the hosts aren't following the RFC
defaults when the values are set to 0/unspecified, how could you be
confident that the hosts are going to correctly follow the parameters in
the RA instead? If you do need to get a broken host working (e.g. on
that is literally taking 0 to mean zero milliseconds), then specifying
it would be an acceptable work around. However I'd also try to report a
bug to the OS vendor if possible, because one day in the future,
somebody might change out the router, notice that you've set the values
to the defaults anyway, not realise that is to overcome a broken host,
and therefore not update the new router with those explicit values.

If I have to specify a value, i.e. if the router forces me to specify
one, then I think I'd leave it at 0/unspecified, and only override that
if the 30 seconde/1 seconds RFC defaults weren't appropriate. (I'd
probably not override Cisco specific setting it thought, so that if I
had to lodge a fault with them I can tell them I'm using their defaults
- usually that makes having them accept there is a fault quicker)



Regards,
Mark.