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

[David Farmer <[e-mail]>]

Geoff,

I thought of another issue: Changing the loopback address to a loopback
prefix will require a few changes to the Default IPv6 Address Selection
policy table in RFC 6724. Here is the updated table from
draft-ietf-6man-rfc6724-update.

Prefix        Precedence Label
::1/128               50     0
$known_local/4x       45    14 (**)
::/0                  40     1
fc00::/7              30    13 (*)
::ffff:0:0/96         20     4 (*)
2002::/16              5     2 (*)
2001::/32              5     5
::/96                  1     3
fec0::/10              1    11

3ffe::/16              1    12


(*) value(s) changed in update
(**) $known_local = the ULA Known-Local IPv6 prefix(es), with lengths

between /40 and /48 (if any) with precedence and labels per the rules

in Sec 5.3


If we change ::1/128 to a prefix, it will include ::/128, the unspecified
address, which currently falls under ::/0 in the policy table. Will this
create any side effects? I don't think so, but I'm not completely sure. If
necessary, we could add ::/128 to Label 1 with a Precedence of 40, but that
seems unnecessary; again, I'm not completely sure.

Also, note that ::/96 is already in the policy table with a Precedence of 1
since the IPv4-Compatible IPv6 Addresses are deprecated.

Thanks.

On Tue, Nov 25, 2025 at 7:29 PM David Farmer <[e-mail]> wrote:

> Geoff,
>
> To clarify, I don't object to 4B loopback addresses, or possibly even
> more. My concern was corner cases regarding the reuse of IPv4-Compatible
> IPv6 Addresses. Even if something was still using them, it seems unlikely
> that ::0/104 or the equivalent of 0.0.0.0/8 would cause any problems.
> However, I accept your argument that it's been long enough. Other than
> ::ffff:0:0/96 being used by IPv4-Mapped IPv6 Address, I would not object to
> using ::0/48 or even ::0/32. Nevertheless, you make a good point about
> Mark's proposed use case.
>
> Thanks.
>
> On Tue, Nov 25, 2025 at 6:49 PM 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'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.
>>
>> 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.
>>
>> regards,
>>
>>   Geoff
>>
>>
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>
>
> --
> ===============================================
> David Farmer               Email:[e-mail]
> Networking & Telecommunication Services
> Office of Information Technology
> University of Minnesota
> 2218 University Ave SE        Phone: [phone]
> Minneapolis, MN 55414-3029   Cell: 612-812-9952
> ===============================================
>


-- 
===============================================
David Farmer               Email:[e-mail]
Networking & Telecommunication Services
Office of Information Technology
University of Minnesota
2218 University Ave SE        Phone: [phone]
Minneapolis, MN 55414-3029   Cell: 612-812-9952
===============================================