Big Temporary Networks

William Herrin bill at herrin.us
Tue Sep 18 04:11:35 UTC 2012


On Sun, Sep 16, 2012 at 7:42 PM, Masataka Ohta
<mohta at necom830.hpcl.titech.ac.jp> wrote:
> ARP and DHCP usually work.
>
> For an unusual case of ARP for other STAs, collisions do
> increase initial latencies, but as refreshes are attempted
> several times, there will be no latter latencies.
>
> 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?

As I understand it from your description, we're mostly talking about
data between a wifi station and remote servers somewhere off the wired
side of the network. 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.

In the wifi to remote wired case, all IPv4 traveling the wifi network
is subject to layer-2 error recovery except for the ARP packet from
the default gateway to the station, requesting the station's MAC
address. This works out OK because the default gateway is somewhat
noisy about resending that arp request until it gets a response from
the station and then it caches the response for a long time.


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) with
prefix info, its MAC, its IP address, etc. Unicast = layer 2 error
recovery. It then configures its address and sends packets to the
default gateway.

In the reverse direction, the gateway sends a neighbor solicitation
via multicast looking for the MAC association with the station's IP.
Like the ARP broadcast this is not subject to layer-2 error recovery.
When the station eventually receives one of the repeated
solicitations, it responds with a neighbor advertisement to the
default gateway (station to AP, error recovered) which the default
gateway caches for a while.


In terms of the number and nature of packets sent without wifi's layer
2 error recovery, they look the same, at least in theory. What did I
miss? Where does IPv6 take the bad turn that IPv4 avoided?

Thanks,
Bill Herrin


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



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