Subject: [v6ops] multiple A and/or AAAA in CDNs - draft-palet-v6ops-464xlat-opt-cdn-caches

From: JORDI PALET MARTINEZ <[email-masked]>
Date: Fri, 29 Mar 2019 10:08:49 +0100

Question for CDN folks.

When you provide multiple A or AAAA records, from the view of a "given" ISP, all them work the same from the perspective of the host?

Otherwise, I don't see how, if a host uses a different DNS resolver (i.e., 1.1.1.1, 8.8.8.8, etc., from the ISP one) you avoid breaking "something".

If there is no "difference" in what A or AAAA is used, then we don't need to consider which one to pick for the EAMT entry (same chances than when the host is doing the choice).

Regards,
Jordi

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From: Erik Nygren <[email-masked]>
Date: Fri, 29 Mar 2019 17:13:47 -0400

On Fri, Mar 29, 2019 at 5:09 AM JORDI PALET MARTINEZ <jordi.palet=
[email-masked]> wrote:

> Question for CDN folks.
>
> When you provide multiple A or AAAA records, from the view of a "given"
> ISP, all them work the same from the perspective of the host?

> If there is no "difference" in what A or AAAA is used, then we don't need
> to consider which one to pick for the EAMT entry (same chances than when
> the host is doing the choice).

>From a given ISP or resolver, multiple A and AAAA records in the respective
rrsets generally is just intended for load balancing and to allow client
fail-over.  What generally matters is that they should preferably be
round-robined or permuted by clients on-average.

For the draft-palet-v6ops-464xlat-opt-cdn-caches use-case, of you see
multiple A and multiple AAAA responses, I'd suggest doing something like
randomizing or permuting their order and then pairing them up.  For example
if you have RRSETs of:

192.0.2.1
192.0.2.2
192.0.2.3
192.0.2.4

2001:db8::4:a
2001:db8::4:b
2001:db8::4:c

Then you might want to end up with a pairing like:

192.0.2.1 ==> 2001:db8::4:c
192.0.2.2 ==> 2001:db8::4:a
192.0.2.3 ==> 2001:db8::4:a
192.0.2.4 ==> 2001:db8::4:b

Don't assume both are the same length.  This preserves the ability of the
client to do fail-over and also doesn't get in the way of round-robin style
behaviors.

Otherwise, I don't see how, if a host uses a different DNS resolver (i.e.,
> 1.1.1.1, 8.8.8.8, etc., from the ISP one) you avoid breaking "something".
>

This seems to be a different topic?  But here it depends on the CDN and
their architecture.  Many CDNs do "DNS-based mapping" meaning that which
resolver you use determines which cluster you get directed to.  This makes
is strongly preferential to use the network-provided resolver as it is
generally configured to send you to a local CDN cluster.  If you use a DNS
resolver that is off-net, you may sometimes be sent to a CDN cluster
off-net.  One way that some public resolvers deal with this is by
implementing ECS (RFC 7871) whereby they send a subnet to the authority
when resolving.

     Erik