Re: SLAAC, Static & DHCPv6 day 1 interoperability issue

[Brian E Carpenter <[e-mail]>]

Gyan,

I don't think you were around for the original discussions, so there is an aspect that is missing from your logic below.

The inclusion of a separate interface identifier field in IP addresses was an entirely intentional feature of IPng. If all we had wanted to do was IPv4 with bigger addresses, that's what we would have done and the address length would have undoubtedly been 64 bits. In fact there were various proposals to do exactly that, with a variety of associated transition and coexistence mechanisms.

But the rough consensus was to do more than that, and to allow *extra* space in the address for an interface identifier that was not part of the subnetting mechanism. Originally it was going to be 48 bits, so the longest subnet prefix would have been 80; on second thoughts it was set to 64, which gave *exactly* the same extension to the subnettable space as we would have got from IPv4 with bigger addresses.

That isn't inconsistent with what we now call BCP198, which says that on links where an interface identifier & SLAAC isn't needed, subnetting can extend out to /127.

All that was despite the fact that we hadn't even realised the potential privacy benefits of a host-defined interface identifier at the time; that is much more recent.

As far "day 1" goes, please remember that DHCPv6 is a retro-fit:

RFC1971 IPv6 Stateless Address Autoconfiguration. August 1996
RFC3315 Dynamic Host Configuration Protocol for IPv6 (DHCPv6). July 2003.

(In fairness, draft-ietf-addrconf-ipv6-auto-00 was dated January 1995 and draft-ietf-dhc-dhcpv6-00 was dated February 1995, but it advanced very slowly compared to SLAAC.)

Regards
   Brian Carpenter

On 08-Nov-20 19:34, Gyan Mishra wrote:
> 
> A day 1 issue that has existed with SLAAC and another critical operational reason as to why longer prefix should be supported by SLAAC.
> 
> Since SLAAC only supports /64 plen due to the fixed 64 bit IID, that issue ends up shackling IPv6 from using longer  prefix length when other devices using static or DHCPV6 are configured on the same subnet.  
> 
> The major issue that exists is that their maybe devices that do not support DHCPV6 or static and have to use SLAAC and you have a subnet configured on the router with longer then /64 prefix length like a /96 for example.  
> 
> So now with those devices running SLAAC you end up with interoperability issue with remaining devices on the subnet that support longer prefix length via static or DHCPV6.
> 
> The workaround has always been to not use longer prefix lengths for DHCPV6 and static and only use the /64 prefix length as that is only supported by SLAAC.
> 
> So that hinders any and  all operators both enterprises and service providers from using longer prefix lengths, so you are now back to being stuck with the 64 bit IID boundary for all host subnets till the end of time using any of the three IPv6 addressing schemes.
> 
> Gyan
> 
> 
> 
> 
> 
> -- 
> 
> <http://www.verizon.com/>
> 
> *Gyan Mishra*
> 
> /Network Solutions A//rchitect /
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