Subject: Re: Why, technically, MIP and IPv6 can't be deployed

From: Greg Daley <[email-masked]>
Date: Wed, 10 Nov 2004 03:51:26 +1100

Hi Francis and Ohta-san.

----- Original Message -----
From: Francis Dupont <[email-masked]>
Date: Wednesday, November 10, 2004 2:15 am
Subject: Re: Why, technically, MIP and IPv6 can't be deployed

> In your previous mail you wrote:
> 
>   > Could you describe why exactly IPv6 can't run on the (layer 
> 2?) WLAN
>   > infrastructure?
>   
>   That ND extensively, without any valid reason to do so, use
>   multicast, which is not acknowledged at WLAN L2, means IPv6
>   or its ND is unreliable over congested WLAN. If multicast
>   ND packet is lost by congestion, it is not retransmitted by L2.
>   
> => Masataka san, your argument is right (I saw 40% lost rate
> on multicast over IEEE 802.11b) but it applies to IPv4 (ARP
> uses broadcast) too...

I understand this is the case, but we can still modify the hosts
to do something about it.

Multicast or Broadcast are necessary if we want generic 
address discovery in IPv4/6 networks. Obviously we'll have problems
with MAC level acknowledgements for messages received by multiple
hosts.

RFC 2461 (as described by a recent individual submission of mine)
allow Neighbour cache entries to be created with reliable 
802.11 MAC transport (unicast toDS=0,toDS=1, multicast toDS=1)
where there is only one MAC recipient in the contention domain.

This may be done to preemptively place neighbour cache entries
into first hop routers.

Since Ohta-san mentioned MIPv6 here, the router becomes the most
relevant peer for neighbour discovery.
 
The mechanism doesn't require protocol modification, and is
available today (If it's practically helpful, and implemented).
It may need some further documentation though.

Greg Daley

draft-daley-ipv6-preempt-nd-00.txt

This has been implemented in <40 lines of C in the Linux Kernel.
Results, while early are fairly promising.

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---

From: Masataka Ohta <[email-masked]>
Date: Wed, 10 Nov 2004 09:34:29 +0900

Greg Daley wrote:

> address discovery in IPv4/6 networks.

True.

 > Obviously we'll have problems

Not necessarily. Discovery by beacons repeatedly broadcast works
just fine, if they are sent frequently enough.

> where there is only one MAC recipient in the contention domain.

I'm afraid your mistake is that you are still insisting on a
generic adaptation approach of ND.

The point is that ND approach is invalidated by the world most
popular (maybe even more than Ether) link technology.

To let IP work over various link technologies, the adaptation
mechanisms MUST be different link technology by link technology.

ND assumes not only that broadcast/multicast is reliable but
also that we can wait several minuites before using terminals,
both of which was valid 20 years ago with immobile workstations
connected to Ether. And, there are other hidden, now invalid,
assumptions.

 > relevant peer for neighbour discovery.

Such approach works if terminals moves very slowly, which
is unlikely the case with MIPv6.

						Masataka Ohta