[IPv6]Re: [v6ops] Re: New draft: "The IPv6 Loopback Address Prefix"

[Mark Smith <[e-mail]>]

Hi,

On Wed, 26 Nov 2025 at 18:26, Gert Doering <[e-mail]> wrote:
>
> Hi,
>
> On Wed, Nov 26, 2025 at 11:03:20AM +1100, Mark Smith wrote:
> > On Wed, 26 Nov 2025 at 10:19, Geoff Huston <[e-mail]> wrote:
> > > thanks Mark - except that you should  substitute the /32 to a /96 as I got confused between my left and my right!
> > >
> >
> > One of my use cases, which I think I hinted to but didn't specifically
> > state, was multiple loopback interfaces on a node.
> >
> > On Cisco routers you can create multiple loopback interfaces. I've
> > done that so that I could have say a management loopback interface and
> > address, a BGP session loopback interface and address and then other
> > loopback interfaces and addresses for other functions such as an
> > L2TPv2 end point, RADIUS service client address etc.. Those loopback
> > interface addresses were all announced into the routing protocol (or
> > not depending on function).
>
> This is a very different thing from what the draft proposes to establish.
>
> You are basically deliberately choosing and assigning routable addresses
> to "non physical" device interfaces, for externally-reachable functions
> (and this is a very reasonable thing to do, so "all the router people"
> do it, and "all the anycast service people" do it as well...).
>

Yes. I mentioned it to demonstrate that I think a larger loopback
prefix should be big enough to support multiple loopback interfaces
within a host.

> The draft at hand proposes to extend the single-ip loopback interface
> to a prefix, with well-known IPv6 addresses "these are available on every
> single node out there, so software can use it without being told what
> addresses are available".  Not manually administered, not routed.
>

I wrote my own version of a larger IPv6 loopback prefix draft 12 years
ago, and which also covered DNS and Default Address Selection and
automatic address configuration including on a host that has multiple
loopback interfaces.

"A Larger Loopback Prefix for IPv6"
https://datatracker.ietf.org/doc/html/draft-smith-v6ops-larger-ipv6-loopback-prefix-04

Regarding forwarding of 127/8 loopback addresses. MPLS does that per
RFC4379. I documented the equivalent function for IPv6 loopback
address forwarding.

My draft was also cited in RFC7439, "Gap Analysis for Operating
IPv6-Only MPLS Networks"

Regards,
Mark.

> On the draft itself, *shrug*.  I dislike all the ways modern linux systems
> use 127.0.0.<random> for magic things, but I don't have to use it, so I
> could care less.
>
> Gert Doering
>         -- NetMaster
> --
> have you enabled IPv6 on something today...?
>
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