Re: Generic anycast addresses...

[Ted Lemon <[e-mail]>]

On May 30, 2019, at 5:05 PM, Mark Smith <[e-mail]> wrote:
> Adding, even if both upsream/parent NSPs are providing DNS resolver service with the same NSP scoped anycast address (because it will be a generic NSP scoped DNS resolver address), the downstream customers' routing will pick only one of them to use.

This might be dysfunctional.  Sometimes if you query the DNS server for one upstream and then use the answer on the other upstream, the results will not be the same, and might be quite suboptimal.   That said, this isn’t likely a problem with the real point you are making, so I only mention it for completeness.

> Similar to multicast, what I've proposed supports embedding a unicast /64 prefix. The all-zeros /64 is used as "unspecified".
> 
> So in this scenario, both NSPs would advertising the same generic NSP scoped anycast DNS resolver address with an all zeros /64 part - and likely such a well-known anycast address that it could be a factory default for devices. They may also each advertise an NSP scoped anycast DNS address which embeds one of their GUA /64s.
> 
> So a customer could leave the choice of which NSP's DNS resolver they use entirely up to their routing system by using the generic DNS anycast address, which would be the default, or they could prefer only one of their NSP's DNS anycast resolver services by configuring their hosts to use that NSP's specific anycast DNS address that includes the NSP's chosen GUA /64.
> 
> They could go further and configure the second NSP's specific NSP scope anycast DNS resolver address as a second DNS entry on a host to get NSP independent DNS redundancy, if the first and preferred NSP's DNS anycast resolution service isn't considered reliable enough. 

This isn’t actually the use case that I’m talking about—I’m a bit puzzled.  Here you are configuring the anycast address just like a regular IP address.  The use case I need is for a well-defined anycast address that does not need to be discovered.   So while what you are doing may be useful, it doesn’t have anything to do with my use case.   The reason I was pointing out that your scoping model didn’t work is that I didn’t understand that you intended to use different anycast addresses for different ISPs.   For that use case, there’s no need for a special scoped anycast address—each ISP can simply allocate an IP address or IP addresses and configure their routing to treat them as anycast addresses.   There are problems with this, but these problems are already discussed e.g. in RFC 7094.