Re: [v6ops] [IPv6] draft-ietf-6man-rfc6724-update might break current ULA+IPv4 hostnames

[Brian E Carpenter <[e-mail]>]

On 13-Nov-23 08:48, David Farmer wrote:
> Ole,
> 
> On Sun, Nov 12, 2023 at 8:37 AM Ole Trøan <otroan=[e-mail] <mailto:[e-mail]>> wrote:
> 
>     Matk,
> 
>      > On 12 Nov 2023, at 12:19, Mark Smith <[e-mail] <mailto:[e-mail]>> wrote:
>      >
>      > Normally, Host A's ULA address should have a path through the local
>      > network to Host B's ULA address and vice versa, so Host A and B's ULA
>      > addresses should be preferred over Host A and Host B's GUA addresses.
> 
>     Come to think about it. Wouldn’t RFC6724 also fail in the MPMH case?
> 
> 
> Yes, it will fail in some cases but succeed in others.

That's because the model is fundamentally broken - I mean the model of
picking a set of destination addresses without considering source addresses.
We *cannot* fix all cases without changing that model - to a large
extent Happy Eyeballs is an ambulance at the bottom of the cliff.

If we want to fix this problem properly, let's just advance rfc6724-update
ASAP as a mitigation, and then tackle the fundamental problem caused by
getaddrinfo().

     Brian
  
> 
>     Source host A with GUA1+GUA2 (from ISPA AND ISPB)
>     And destination host B with GUA3.
> 
>     6724 will return the source address with the longest matching prefix to GUA3. So only one of these:
> 
>     {GUA1, GUA3}
>     {GUA2, GUA3}
> 
>     The SA determines exit path, so depending on which ISP is down. Communication will fail.
>     Redundancy is the whole point of multi-homing…
> 
> 
> If there is a soft failure of the provider of the selected SA, there will be a failure. If there is a hard enough failure to cause the withdrawal of one of the RAs, then redundancy will work.
> 
>     The other case I am concerned about is the one where within a single network multiple routers make up their own ULA and use that to assign addresses to directly connected hosts but do not participate in routing. We then have multiple disjointed ULA domains within the network. With RFC7078 and SNAC routers I am not sure if we can avoid that. This would possibly have been cleaner with site-locals.
> 
> 
> In many cases you refer to, the disjoint ULA domains are likely to be ULA-only domains. If there are ULA-only domains, there will only be ULA destinations to try, and they will be reachable or not. When we mix ULA, GUA, and IPv4, things get really complicated. ULA-only domains should be relatively straightforward.
> Thanks
> 
> -- 
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> David Farmer Email:[e-mail] <mailto:[e-mail]>
> Networking & Telecommunication Services
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