Mysterious missing DHCPv6 feature, was Re: How does one obtain an IPv6 DNS server when VPNing to an ASA?

[Paul Timmins]

Doug Barton wrote:
> On 05/27/10 08:56, Gert Doering wrote:
>> Hi,
>>
>> On Thu, May 27, 2010 at 04:46:33PM +0100, Sam Wilson wrote:
>>> On 27 May 2010, at 16:38, Gert Doering wrote:
>>>
>>>> But thanks for agreeing that it's hard to find a scenario that backs
>>>> the claim "with RA+DHCP, I need to touch many more places than with
>>>> DHCP-only".
>>>
>>> 100% more places.  Anyone care to define whether that's "many" or not?
>>
>> Huh?  You need to touch the router, and the server.  In both cases.
>>
>> Sounds like "same number of places" to me.
>
> Sure, in your contrived example the number of things to configure is 
> equal. I can contrive equally effective examples of stuff that doesn't 
> require the people doing the configuration to touch the router.
Give an example where you could change the subnet assigned on a segment 
without touching the router on it. (And only the subnet, since that's 
all RA can set anyway)

> You still haven't answered the question, "Why should RA be _required_ 
> in IPv6 when DHCP is perfectly capable of doing everything that RA can 
> do and more?"

Because RA is way easier to develop embedded devices for than DHCP. 
Getting an interrupt and checking the buffer, finding an RA and then do 
the address set and change routine, and going back to normal work is way 
easier than maintaining a state machine that has to keep track of 
address expiry, renewal, and even the request/handle response/ack can be 
complex if you're doing everything in 8k/16k of space.

I'm talking telemetry units that either respond to polls or blatt data 
out on to the wire as multicast as soon as it's collected here, not 
smart fridges that have enough ram to drive a LCD display, and thus 
could maybe fit a DHCP implementation.

The nice thing is that if you use stateless DHCP, smart devices will 
work okay, and so will really dumb devices scarcely bigger than the 
RJ-45 jack they plug into.

-Paul