Re: IPv6 Routing & ND vs. Addressing, (Was: Re: <draft-ietf-6man-rfc4291bis-09.txt>)

[David Farmer <[e-mail]>]

On Mon, Jul 10, 2017 at 7:21 PM, Manfredi, Albert E <
[e-mail]> wrote:

> -----Original Message-----
> From: ipv6 [mailto:[e-mail]] On Behalf Of Brian E Carpenter
>
> > Agreed. On the other hand, maybe rough consensus could be
> > achieved with s/required/recommended/, even if some objections
> > remain.
>
> The problem I have is mainly that in the ebb and flow of these continuing
> discussions, one sees the "IIDs must be 64 bits wide" creeping back in,
> again and again. Where instead, the rough consensus seems to have become,
> must be 64-bits wide unless the addresses are statically configured or are
> provided by DHCPv6. In the latter cases, 64-bit IIDs might still be
> "recommended," but in fact, the on-link prefix length can be from zero to
> 128 bits, and the IID must therefore be 128 - prefix length.
>
> Why not just state those two exceptions outright? Citing RFCs that
> mandated 64-bit IIDs doesn't hold much weight, if the rationale for doing
> so has been deprecated. Or they may be cited, but with that proviso. Yes,
> LLAs and ULAs still require 64-bit IIDs, as does SLAAC (to try to thwart a
> "race to the bottom").
>
> I too was impressed with the thoroughness of David Farmer's thesis, but as
> far as I can tell, it doesn't change the above.
>

IPv6 as currently defined does actually require IIDs to be 64 bits, if this
wasn't the case then you could use subnets of any length without any
special requirements or considerations.  RFC6164 is quite clear that there
are /127 prefixes that should not be used and there are other requirements
like disabling Subnet-Router anycast. However, as IPv6 unicast routing is
based on on-link prefixes of any length, up to 128 bits, limited IPv6
functionality can be provided using any length IID.  In the case of
point-to-point links, the limitations are not a big deal, however it would
be a stretch to that the limitations are not an issue is all cases.

So, how about something like this;

For the full functioning of all IPv6 capabilities, when assigning unicast
addresses, except those that start with the binary value 000, Interface
Identifiers are required to be 64 bits long. However, as IPv6 unicast
routing and on-link determination are separate from address assignment and
will operate with Interface Identifiers of any length. It is therefore
possible, with several limitations, to use Interface Identifiers of other
lengths in limited scenarios, these include; 128 bit prefixes, such as for
router loopback addresses, point-to-point router-links [RFC6164], and links
where all node are manually configured.

-- 
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Networking & Telecommunication Services
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