Subject: RE: WLAN (was Re: IPv6 Host Configuration of Recursive DNS Server)

From: "Christian Huitema" <[email-masked]>
Date: Tue, 8 Jun 2004 09:06:44 -0700

> >>I know ND is wrong. That is, I know it is wrong to have generic
> >>link protocols ignoring link specific properties and has been
> >>pondering on how such protocols suffer.
> 
> > I think ND is not wrong.
> 
> ND is wrong, because it was designed to be applicable to all the
> link types.

The reliance of ND on multicast is indeed questionable. Some networks
support multicast poorly. As Masataka points out in his message,
multicast does not play well with CSMA-CA networks, where transmission
reliability relies on frame-level ACK. We could also mention the growing
use of directive antennas or phased arrays antennas, which improve
unicast transmission but don't apply to multicast. I would expect most
wireless networks to be "somewhat multicast capable", but the multicast
operation will not be reliable.

The only standard solution so far is to treat these networks as NBMA,
i.e. on the router for ND. This is not satisfactory, because it places a
lot of burden on the router and also because it leaves out ad hoc
networks. 

-- Christian Huitema

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From: Erik Nordmark <[email-masked]>
Date: Tue, 8 Jun 2004 11:31:31 -0700 (PDT)

> lot of burden on the router and also because it leaves out ad hoc

There is a middle ground since ND can be configured to have the routers
not advertise any on-link prefixes (while still advertising prefixes
for stateless addrconf or not; that's orthogonal).

In this mode the hosts would send all packets to a default router
and the router has the option to redirect the host to the on-link link-layer 
address. Thus the only nodes multicasting NS messages would be the routers.

I don't know how much this would improve the situation, and whether it
would also be benefitial if the routers broadcast the NS messages.

  Erik

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From: Masataka Ohta <[email-masked]>
Date: Wed, 09 Jun 2004 15:31:53 +0900

Erik Nordmark;

> address. Thus the only nodes multicasting NS messages would be the routers.

NS with link multicast over congested WLAN is a lot less reliable
than unicast one and has little chance to be received by its target.

						Masataka Ohta

PS

I'm not a member of ipv6 ML.

---

From: Masataka Ohta <[email-masked]>
Date: Wed, 09 Jun 2004 15:15:14 +0900

Christian Huitema;

> operation will not be reliable.

As base stations with sector or active antennas are expected to
know the locations of mobile hosts and transmit to all the
related sectors or undirectionaly, I think lack of ACK is the
only problem.
 
> The only standard solution so far is to treat these networks as NBMA,

The irony is that radio network is *THE* broadcast network.

PS

I'm not a member of ipv6 ML. That is, CC to me, if you want your mail
read by me.