Big Temporary Networks

Masataka Ohta mohta at
Wed Sep 19 07:26:28 UTC 2012

William Herrin wrote:

>> Unicast since its responding to a solicitation?
>> RFC4861 states:
>>     A router MAY choose to unicast the
>>     response directly to the soliciting host's address (if the
>>     solicitation's source address is not the unspecified address), but
>>     the usual case is to multicast the response to the all-nodes group.
> Ah, okay. So the IPv6 router usually responds to router discovery with

Don't ignore how is the implementations in the real world:

: and a comment in rtadvd on the solicited advertisement:
:         /*
:          * unicast advertisements
:          * XXX commented out.  reason: though spec does not forbit it,
:          * advert does not really help

> But correct me if I'm wrong: the router advertisement daemon could be
> altered to reply with unicast without changing the standard, right?

See above.

> What do the radvd and rtadvd developers say about this when confronted
> with the 802.11 multicast problem?

I reported the problem to IPv6 (or IPng?) WG more than 10 years
ago (before rtadvd was developed) and Christian Huitema
acknowledged that the problem does exist.

Since then, nothing happened.

> Are there any Internet drafts
> active in the IETF to replace that "MAY" with a "SHOULD," noting that
> replying with multicast can defeat layer 2 error recovery needed for
> the successful use of some layer 1 media?

Didn't you say "without changing the standard"?

>>> What did I
>>> miss? Where does IPv6 take the bad turn that IPv4 avoided?

You still miss DAD.

DupAddrDetectTransmits should be 3, 5 or maybe 10 (depending on
level of congestion), which means even more time is wasted.

Worse, increasing DupAddrDetectTransmits increases level of
congestion, which means congestion collapse occurs with
use case senario of IEEE802.11ai.

> I have no interest in defending IPv6. We're network operators here.
> You just told us (and offered convincing reasoning) that when
> selecting a router vendor for use with an IPv6 wifi network, one of
> our evaluation check boxes should should be, "Responds to ICMPv6
> router solicitation with a unicast message? Yes or Fail." And when we
> provide the list of deficiencies to our vendor and wave the wad of
> cash around, one of them should be, "Responds to ICMPv6 router
> solicitations with a multicast packet - unreliable in a wifi
> environment."

> That's strikes me as something valuable to know. Far more valuable
> than, "Dood, IPv6 has problems on wifi networks."

The only thing operators have to know about IPv6 is that IPv6, as
is currently specified, is not operational.

Then, let IETF bother.

						Masataka Ohta

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