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Re: a draft about on-link and submit prefixes

otroan@employees.org Tue, 14 March 2017 08:04 UTC

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Subject: Re: a draft about on-link and submit prefixes
Date: Tue, 14 Mar 2017 09:04:33 +0100
In-Reply-To: <20170314.080739.74664517.sthaug@nethelp.no>
To: sthaug@nethelp.no
References: <CAJE_bqdd9OXOi+SZ8_OfGWXxEoKSfoR6=Lp3-_=vEaWbjx4udw@mail.gmail.com> <CAKD1Yr3ncJkNwZgpWpr049K497iLAQ3dCzJ6dCHM1VsrC8UHog@mail.gmail.com> <20170314.080739.74664517.sthaug@nethelp.no>
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>> 
>> If possible, I would suggest making this document even more general. It's
>> mostly geared to how implementations treat PIOs, and refers to RFC 4861 and
>> RFC 4862, but really I think the clarifications it makes have much broader
>> impact. As I see it, the core issues here are:
>> 
>>   - IPv6 addresses don't specify any on-link information
>>   - For most (but not all) unicast addresses, subnet prefixes are 64 bits
>>   per RFC 4291.
>>   - A given subnet prefix can be spread across multiple links, and a given
>>   link can support multiple subnet prefixes.
>> 
>> This is not immediately obvious to people who are familiar with the IPv4
>> subnet model, because it is quite different.
> 
> I agree that a generalization is a good idea. In particular, it would
> be good to have (in one place) a definition of what an IPv6 subnet is,
> especially if a subnet can be spread across multiple links (which is
> *not* obvious to me from reading RFC 4861 and RFC 4862).

Indeed. I think we can generalize that statement and say it is not obvious in general.

There were proposals around multi-link subnets:
https://tools.ietf.org/html/draft-thaler-ipngwg-multilink-subnets-02

Which pretty quickly decayed into implementing a spanning tree in ND and flooding DAD messages.

The remnants of that you can find in proxy RA and DAD proxy work.

I would suggest that redefining existing terms to be able to shoe-horn in a 64 bit boundary isn't helpful.

Best regards,
Ole

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