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

[Mark Smith <[e-mail]>]

Hi Geoff, all,

On Wed, 26 Nov 2025 at 11:48, Geoff Huston <[e-mail]> wrote:
>
> Hi Mark,
>
> I hear you, but I also observe that a prior effort to expand this designation
> in 2013 did not get beyoind a draft. This incarnation of the proposal is deliberately
> more modest (even then I see David Farmer saying (paraphrased) "whoa! Way
> too much!" So we have one comment saying "too big" and another saying "too
> small"!
>

I would argue for what is a convenient larger loopback prefix size to
suit all reasonable and conceivable use cases, not just what is
minimal and necessary. That's the freedom we have in IPv6 that we
haven't had in IPv4 since RFC791 when classes were introduced (vs
RFC760s 8 bit network numbers and 24 bit host numbers).

>
> I'm inclined to say that the draft will keep this proposal  at a /96 for now but
> doubtless Warren and I will keep a close eye on any other comments
> that have a view about the appropriate size for a loopback prefix.
>

I think using a /96 for a loopback interface would be shoehorning IPv4
addressing into IPv6 rather than following IPv6's addresses
conventions. At a minimum I think it should be a /64 to support 64 bit
IIDs on the loopback interface that can be automatically generated by
the host at boot time via RFC7217 and/or RFC8981 IIDs.

> But I do note:  "Those loopback interface addresses were all announced into
> the routing protocol" might bne interpreted as a bit contrary to RFC 4291
> which says quite  definitively "keep loopback em to yourself" So I am not
> exactly on board with a rationale that relates to a use scenario in routing
> protocol that "exports" these addresses out to neighbours, if I interpret your
> description correctly.

I probably confused things. I should have said "loopback *interface*"
addresses. They weren't from 127/8, they were standard unicast IPv4
addresses. Using loopback virtual interfaces and announcing those /32s
into the routing protocol was for the purposes of making them
reachable via any of the router's physical interfaces for BGP sessions
etc.

I was using that example to demonstrate the value of supporting
multiple loopback interfaces on a node/host, and in conjunction with
supporting IPv6 addressing conventions, that the larger loopback
prefix should be large enough to provide multiple /64s at a minimum. A
/48 would suffice, however a /32 is cheap enough so that's why I
bumped the size up from a /48 to a /32 in my larger loopback ID.

Regards,
Mark.



>
> regards,
>
>   Geoff
>
>