Big Temporary Networks

William Herrin bill at
Tue Sep 18 20:24:22 UTC 2012

On Tue, Sep 18, 2012 at 8:16 AM, Masataka Ohta
<mohta at> wrote:
> William Herrin wrote:
>> In IPv6, the station sends an ICMPv6 router solicitation instead of an
>> ARP for the default gateway. This is a multicast message but since
>> it's from the station to the AP it's subject to layer 2 error recovery
>> by the 802.11 protocol. The default gateway sends back a router
>> advertisement (unicast since its responding to a solicitation)
> 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
multicast where arp would have responded with unicast. This multicast
message is not subject to 802.11's layer 2 error recovery so as
previously discussed it has a high probability of being lost during
some relatively ordinary wifi usage scenarios.

But correct me if I'm wrong: the router advertisement daemon could be
altered to reply with unicast without changing the standard, right?
What do the radvd and rtadvd developers say about this when confronted
with the 802.11 multicast problem? 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?

>> What did I
>> miss? Where does IPv6 take the bad turn that IPv4 avoided?
> If you still want to defend IPv6, you must say multicast RA and
> DAD are unnecessary features of IPv6, which means the current
> IPv6 is broken.

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

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

So, let's keep going. IPv6 falls down compared to IPv4 on wifi
networks when it responds to a router solicitation with a multicast
(instead of unicast) router advertisement. Where else does it fall
down compared to the equivalent behavior in an IPv4 wifi network?

Bill Herrin

William D. Herrin ................ herrin at  bill at
3005 Crane Dr. ...................... Web: <>
Falls Church, VA 22042-3004

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