Big Temporary Networks

Masataka Ohta mohta at necom830.hpcl.titech.ac.jp
Tue Sep 18 12:16:32 UTC 2012


William Herrin wrote:

>> OTOH, IPv6 requires many multicast received by STAs: RA and NS
>> for DAD, for example.
>>
>> Worse, minimum intervals of ND messages are often very large,
>> which means a lot of delay occurs when a message is lost.
> 
> Hi Masataka,
> 
> Where do things go wrong?

>> OTOH, IPv6 requires many multicast received by STAs: RA and NS
>> for DAD, for example.

> Wifi station to station communications comprises
> a relatively minor portion of wifi's use so we don't burn a lot of
> worry on them in the general analysis.

>> OTOH, IPv6 requires many multicast received by STAs: RA and NS
>> for DAD, for example.

> 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.

and a comment in rtadvd on the solicited advertisement:

        /*
         * unicast advertisements
         * XXX commented out.  reason: though spec does not forbit it,
unicast
         * advert does not really help

> In the reverse direction,

Poor SLAAC with a lot of configured states is unnecessarily a lot
more complex than simply bidirectional ARP, because it must
involve all the distributed states of all the hosts on the link.

> 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.

						Masataka Ohta



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