Re: IPv6 fc00::/7 — Unique local addresses

Owen DeLong owen at delong.com
Sat Oct 23 14:45:23 UTC 2010


On Oct 23, 2010, at 7:26 AM, Mark Smith wrote:

> On Fri, 22 Oct 2010 15:42:41 -0700
> Owen DeLong <owen at delong.com> wrote:
> 
>>>>> 
>>>> Actually, it's not pointless at all. The RA system assumes that all routers
>>>> capable of announcing RAs are default routers and that virtually all routers
>>>> are created equal (yes, you have high/medium/low, but, really, since you
>>>> have to use high for everything in any reasonable deployment...)
>>>> 
>>> 
>>> No it doesn't. You can set the router lifetime to zero, which indicates
>>> to the end-node that the RA isn't announcing a default router. In this
>>> case, it may be announcing M/O bit, prefix or other parameters.
>>> 
>> DHCPv6 can selectively give different information to different hosts
>> on the same wire segment.
>> 
>> RA cannot.
>> 
> 
> That was not the assertion you made.
> 
> You said that 
> 
> "The RA system assumes that all routers
>>>> capable of announcing RAs are default routers"
> 
> and I said, no, that is not the case if you set the RA lifetime to
> zero. To cite explicitly, RFC4861 says,
> 
Right... I oversimplified the point I was attempting to make and you
called me on it... Move on.

>      Router Lifetime
>                     16-bit unsigned integer.  The lifetime associated
>                     with the default router in units of seconds.  The
>                     field can contain values up to 65535 and receivers
>                     should handle any value, while the sending rules in
>                     Section 6 limit the lifetime to 9000 seconds.  A
>                     Lifetime of 0 indicates that the router is not a
>                     default router and SHOULD NOT appear on the default
> 
> 
> 
> Narten, et al.              Standards Track                    [Page 20]
> ^L
> RFC 4861               Neighbor Discovery in IPv6         September 2007
> 
> 
>                     router list.  The Router Lifetime applies only to
>                     the router's usefulness as a default router; it
>                     does not apply to information contained in other
>                     message fields or options.  Options that need time
>                     limits for their information include their own
>                     lifetime fields.
> 
> 
> I was not making any statements about whether DHCPv6 could be
> selective about providing certain options to selected end-nodes.
> 
> You might think I'm being overlay pedantic, however changing the
> question to then disagree with answer that doesn't agree with yours is
> being disingenuous. 
> 
>>>> There are real environments where it's desirable to have a way to tell
>>>> different clients on a network to use different default gateways or
>>>> default gateway sets.
>>>> 
> 
> I wouldn't necessarily disagree, although in my experience they're
> really quite rare, to the point where segmenting them into a separate
> subnet, via e.g. a different VLAN, becomes a somewhat better and easier
> option.
> 
While I would agree with you operationally, sometimes they involve
software that discovers other devices by broadcast and does not
permit other mechanisms.

I've seen environments where they're able to deal with this in IPv4
because of this flexibility in DHCPv4 and would be limited to static
addressing in IPv6 because it lacks this ability.

Owen





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