Re: what problem is solved by proscribing non-64 bit prefixes?

[Alexandru Petrescu <alexandru.petrescu at gmail.com>]

Pekka, I also think having that 64bit boundary helped in designing CGAs. 
 They're more secure when we know IIDs must be 64bit in length.

Pekka Savola wrote:
> On Tue, 30 Sep 2008, Brian Dickson wrote:
> > Dunn, Jeffrey H. wrote:
> >
> > My basic question is: What basic engineering problem is solved by
> > proscribing non-64 bit prefixes?
>
> If the non-64 prefixes had not been proscribed, we wouldn't have been 
> able to use the existing engineering method to develop CGA/SEND 
> specifications.

If CGA addresses are a departure allowing the IID on Ethernet be 
something else than an Ethernet-derived IID (e.g. not always have fffe 
somewhere in the middle) then they (CGA IIDs) could also settle for 
less, like being 63 in length, instead of 64.  This would allow for 
further subnetting the typical /64 into two /65s, with only minimal 1bit 
decrease in security.

It may be that CGA addresses and longer-than-64bit SLAAC are not 
incompatible.

Just some thoughts.

Alex
SLAAC: IPv6 StateLess Address AutoConf

>  These are being leveraged in SHIM6 and potentially in 
> other applications as well. Likely we couldn't even solve these 
> engineering problems (at least without major drawbacks/other 
> assumptions) if we couldn't have made assumptions about the widely-used 
> prefix length.
>
> That said, I personally use non-64 bit prefixes on point-to-point links 
> between routers, but I have done so willingly and knowing that if the 
> IETF develops anything fancy new stuff, I might not be able to use it or 
> I might need to renumber.
>
> > When managing such a scheme alongside an IPv6 prefix which needs to be 
> > assigned to the same set of servers, which are all dual-stack, the 
> > *number* of prefixes, their *relative* numbering, and the host 
> > *addresses* within the prefixes, it is quickly apparent that use of 
> > only /64 prefixes makes for a management nightmare, particularly if 
> > renumbering of prefixes and/or servers occurs, e.g. re-balancing the 
> > VLSM arrangement itself in IPv4-land.
>
> I don't understand why *relative* numbering (i.e: overlapping subnet 
> masks) is important in IPv6, and I'm not sure if I see the case even for 
> v4.  Could you enlighten me?
>
> I assume you refer to a scenario where on the same broadcast domain 
> there are hosts which are configured with say A.B.C.0/24 length, and 
> some others are configured with, say, A.B.C.D/28 prefix length.


______________________________________________________________________
This email has been scanned by the MessageLabs Email Security System.
For more information please visit http://www.messagelabs.com/email 
______________________________________________________________________
--------------------------------------------------------------------
IETF IPv6 working group mailing list
ipv6 at ietf.org
Administrative Requests: https://www.ietf.org/mailman/listinfo/ipv6
--------------------------------------------------------------------