Two questions about the name server address sorting functionality

Kevin Darcy kcd at
Mon Oct 14 21:08:03 UTC 2002

"Johan Larsson (EAB)" wrote:

> Hi Mark!
> Do you know when BIND 9.3 will be available?
> I also would like to know if the following features are going to be supported in BIND 9.3:
> True Round Robin (not only random-cyclic as in BIND 9.1.3)?

Do you *really* want *true* round-robin? With *true* round-robin, whenever you have one
address fail, the address which is rotationally "after" that address, gets hammered by all of
the clients which fail over, e.g. in a ABC/BCA/CAB sequence, "B" gets hammered with as much
as 2x normal traffic if "A" fails.

AFAIK, the optimal sort order is "permuted", i.e. all possible permutations presented with
equal frequency (e.g. ABC/BCA/CAB/ACB/BAC/CBA, for a 3-valued RRset), so that the traffic is
as equally spread as possible, even in the face of failures (in the same failure scenario,
"B" only gets max 1.5x traffic, and, unlike "true" round-robin, this ratio decreases the
larger the RRset gets). Unfortunately, "permuted" sort order requires storing a significant
amount of state about each multi-valued RRset (the number of permutations goes up factorially
with the number of records in the RRset), so it tends to be rather resource-intensive.

Still, if someone is coding "true" round-robin anyway, perhaps they might give some
consideration to adding "permuted" order as well...

- Kevin

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