Subject: Re: [dnsop] ipv6-dns-configuration-02 comments

From: Iljitsch van Beijnum <[email-masked]>
Date: Tue, 12 Oct 2004 14:03:49 +0200

On 1-aug-04, at 16:47, Pekka Savola wrote:

> if folks thought multicast on WLAN had such
> big problems, they might have devised a different mechanism for it -- 
> for
> example, if pure broadcast does not suffer from this issue, certain
> multicast addresses, which are expected to have a larger number of
> recipients, could be mapped to the broadcast address instead.

Sorry to jump back more than two months in the past, but I think this 
is an important issue and it doesn't look like it has been resolved in 
later discussions.

I've been using wireless at home (where I do most of my work) for more 
than a year now, using Apple equipment, and I haven't experienced any 
problems with ND. Apple also extensively uses multicasts for service 
discovery through multicast DNS / zeroconf, I don't remember seeing 
problem with that either.

I've searched through "802.11 Wireless Networks: The Definitive Guide" 
and although it doesn't say so specifically, it's pretty clear that 
broadcasts and multicasts are treated the same in 802.11. This means no 
acknowledgments and no fragmentation. Which in turn means that 
broad/multicast packets are delivered less reliably than unicast 
packets. (So this goes for IPv4 ARP too.)

However, the difference isn't as big as it may seem at first glance. 
With unicast, transmissions initially happen at a high speed, and if 
that doesn't work, they're resent at a lower speed. Since non-unicast 
packets must be received by all stations, they are generally 
transmitted at 1 or 2 Mbps, which conforms to the original 802.11 
(pre-b) standard. Because of this, the chance of a non-multicast packet 
actually making it through to a given host are quite good, given even 
low but reasonable radio coverage.

.
dnsop resources:_____________________________________________________
web user interface: http://darkwing.uoregon.edu/~llynch/dnsop.html
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From: Masataka Ohta <[email-masked]>
Date: Wed, 13 Oct 2004 15:40:18 +0900

Pekka Savola;

As has been discussed extensively enough in this and
IPv6 ML, multicast and unicast packets of 802.11 are
not acknowledged. They can not be.

Though first half of non-STA multicast, which is unicast, is
acknowledged, it merely means unicast is acknowledged.

							Masataka Ohta

> On Tue, 12 Oct 2004, Iljitsch van Beijnum wrote:
> 
> Good.  Maybe Masataka can given an exact reference to the spec (and
> section or page) which says that multicast packets are acknowledged at
> the link layer.
> 

.

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From: Pekka Savola <[email-masked]>
Date: Wed, 13 Oct 2004 08:49:23 +0300 (EEST)

-- 
Pekka Savola                 "You each name yourselves king, yet the
Netcore Oy                    kingdom bleeds."
Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings

.