[IPv6]Re: Using different router addresses (draft-link-6man-gulla) to solve the router fail-over scenario in (draft-gont-v6ops-multi-ipv6)

[Fernando Gont <[e-mail]>]

Hi, David,

On 23/3/25 09:36, David 'equinox' Lamparter wrote:
> (inline below)
> 
> On Fri, Mar 21, 2025 at 03:20:15PM -0300, Fernando Gont wrote: […]
>> 1. You have ROUTER_A advertising PREFIX_A, and ROUTER_B
>> advertising PREFIX_B.
>> 
>> 2. You have a destination "HOST_X" that you want to connect to.
> 
> X > 3. Under normal circumnstances, X >     NH(SRC_A, HOST_X) =
> ROUTER_A X >     NH(SRC_B, HOST_X) = ROUTER_B
> 
> 3. NH(*, HOST_X) = { ROUTER_A, ROUTER_B }
> 
> "*" is used to express "unspecified/not-chosen-yet source address". 
> Without 8028 or dst-src-routing, only NH(HOST_X) exists and is the
> same.

In this example, we are assuming that RFC8028 is in place.
There's not such a thing such as NH(*, HOST_X), because by the time you
have to make a decision to route a packet... er... you have a packet
already (i.e., it does have all the IPv6 fields completed).

And RFC8028 tells you to only use next-hops that advertised the prefix
you are sourcing from.



> This is the root of miscommunication here.  6724 rule 5.5 requires
> you to choose a nexthop before choosing the source.

At this point, we are doing Destination Address selection. So there's no
Rule 5.5 here.

Besides, how could a protocol spec about address selection possibly rule
routing?

The key part here is that Rule 5.5 does not belong in RFC6724. -- It
is already part of RFC8028. And if RFC6724 required RFC8024 (as it 
should), then we wouldn't need Rule 5.5.




>> 4. Now ROUTER_B becomes unreachable
> 
> Now this changes:  NH(*, HOST_X) = { ROUTER_A]
>
> 
>> 5. You resolve www.example.com, and it resolves to HOST_X
>> 
>> 6. You know run Dest address selection. Since you have a single 
>> destination addresses, there are not really any addresses to
>> "sort"
>> 
>> 7. Dest address selection is done at this point.
> 
> 7.5. You select a nexthop for HOST_X.  Due to the change in 4.:
> 
> NH(*, HOST_X) = { ROUTER_A }
> 
> Due to 4861 6.3.6, ROUTER_B is rejected.

How you possibly select a next hop here if what you are doing is simply
sorting the list of IPv6 addresses that you return via e.g. getaddrinfo()?

Thanks,
-- 
Fernando Gont
e-mail: [e-mail]
PGP Fingerprint: 7F7F 686D 8AC9 3319 EEAD C1C8 D1D5 4B94 E301 6F01