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

[Geoff Huston <[e-mail]>]

Hi David,

Thanks indeed for repairing an obvious gap in my memory here! I'll remove my comment in the next rev of the draft.

Geoff


> On 26 Nov 2025, at 6:12 am, David Farmer <[e-mail]> wrote:
> 
> Regarding Geoff's comment at the end of Section 4;
> 
> ((Geoff: this last sentence of the paragraph on the unspecified address, lifted from RFC4291, strikes me as incorrect!  If a packet is sent out an interface with a source address of all zeros then nobody can respond and the packet has no purpose.  I'm inclined to drop this sentence from the proposed update, but as I don't think I was paying attention to IPv6 when this text was originally incorporated into the IPv6 Address Architecture so I have no ide what scenario was being referenced in this case.))
> 
> Duplicate Address Detection (DAD), as defined in RFC 4862 Section 5.4 and DHCPv6 defined in RFC 8415, uses the unspecified address for a host that doesn't know its address yet; it is not used to return a DAD or DHCPv6 result. A multicast address is the return address; the all-hosts multicast address is the return address used.
>  
> Thanks
> 
> On Tue, Nov 25, 2025 at 11:39 AM Warren Kumari <[e-mail] <mailto:[e-mail]>> wrote:
>> Dear 6MAN and V6OPS,
>> 
>> Geoff Huston and I have just submitted draft-kumari-ipv6-loopback - "The IPv6 Loopback Address Prefix" <https://datatracker.ietf.org/doc/draft-kumari-ipv6-loopback/>
>> 
>> We believe that it is within the 6MAN charter ("The 6man working group is responsible for the maintenance, upkeep, and advancement of the IPv6 protocol specifications and addressing architecture."), but I have CCed V6OPS as well, as it is clearly operational as well.
>> 
>> Abstract: 
>> "This document updates the IP Version 6 Address Architecture to define the IPv6 address prefix ::/32 as the Loopback address prefix."
>> 
>> Basically, this document expands the single loopback address ::1/128 into a prefix.
>> 
>> Yes, we are aware that there have been some previous discussions[0] on the need (or lack thereof!) of a loopback prefix in IPv6, but we believe that they are worth revisiting. 
>> 
>> There are a number of situations in which having more than a single address is helpful; an obvious example of this is Dockers/k8s use of 127.0.0.11 for the DNS resolver, SPAM RBL use of the last octet on 127.0.0.x to encode the type of SPAM. It is also relatively common it use this for inter-service communication in container environments. 
>> 
>> It is also a common practice to bind different services to different addresses in the IPv4 loopback space to allow for scaling (avoiding the "Port already in use" issue), testing, etc.  Yes, these can be somewhat emulated with ULAs and / or additional interfaces and scopes, but they are all more complicated, and much more likely to result in leakage or collision. 
>> 
>> Another, more recent example is the ICANN Public Comment on "Name Collision IPv6 Research Study" and proposed use of ::ffff:7f00:3535 [1] - if there was a loopback prefix this would have been a better option[2]
>> 
>> 
>> We expect a fairly robust discussion :-), 
>> W
>> 
>> [0]: I know I've seen them, but I quick search of my mail was unable to find these — the authors are more than happy to link to previous documents, etc. 
>> 
>> [1]: See long threads on 6MAN https://mailarchive.ietf.org/arch/msg/ipv6/-HrYFMwHhsUWYxSXsFIkLpF_Qgk/ and V6OPS.
>> 
>> [2]: Solving the technical concerns, but not necessarily the policy ones.
>> 
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> 
> 
> 
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