Problem on round robin DNS

[Mark.Andrews at nominum.com]

> Barry Margolin wrote:
> 
> > In article <20000523125125.A88713 at lucifer.bart.nl>,
> > Jeroen Ruigrok van der Werven  <asmodai at bart.nl> wrote:
> > >-On [20000523 06:46], Ching, Calvin KW (Calvin.KW.Ching at cwhkt.com) wrote:
> > >>A round robin DNS is configured to return two IP addresses of a domain na
> me,
> > >>e.g. www.abc.com. It is supposed that it can return the IP in strictly ro
> und
> > >>robin fashion like the following:
> > >
> > >[snip]
> > >
> > >>However, we found that it is not strictly following this sequence. It
> > >>sometimes returns like this:
> >
> > Are you sure you're the only one querying the server?  If someone else is
> > also looking up the same name while you are, they'll interrupt your
> > sequence.
> >
> > >For all I know, its totally random what the server returns as the first
> > >address.
> >
> > By default it's strict round-robin, although in BIND 8 you can configure
> > other orders, including random.
> 
> Nope. What ISC calls "cyclic" isn't strict round-robin. The "cyclic" algorith
> m as
> implemented in 8.2.2-p5 consists of picking the first address at random and t
> hen
> observing proper rotational order relative to that (see src/bin/named/ns_resp
> .c
> lines 3107 through 3111). So for an RRset of only two addresses, there is
> currently no difference between so-called "cyclic" and "random". Older versio
> ns
> used to implement *real* round-robin because they did the sorting via databas
> e
> manipulations, thus keeping state. But now the sorting is done statelessly
> on-the-fly, so true round-robin is not possible.
> 
> Looks like BIND 9 operates on the same principle. Sigh...
> 
> 
> - Kevin
> 
> 
> 
	Cyclic is statistically identical to round-robin.

	For load balancing "random" generally gives better results
	especially when one of the servers has failed.  Cyclic (and
	round-robin) overload the server after the failed server.
	Random does not have this problem.

	Mark
--
Mark Andrews, Nominum Inc.
1 Seymour St., Dundas Valley, NSW 2117, Australia
PHONE: +61 2 9871 4742                 INTERNET: Mark.Andrews at nominum.com