Subject: [Anima-signaling] Fwd: [homenet] Multicast in 802.11

From: Brian E Carpenter <[email-masked]>
Date: Fri, 07 Aug 2015 18:46:42 +1200

This is something we should bear in mind for GRASP discovery:
UDP multicast can be highly unreliable.

   Brian

-------- Forwarded Message --------
Subject: [homenet] Multicast in 802.11 [was: Despair]
Date: Fri, 07 Aug 2015 02:42:53 +0200
From: Juliusz Chroboczek <[email-masked]>
To: Eric Gray <[email-masked]>
CC: Homenet <[email-masked]>

> I wasn't aware of the treatment of multicast packets as less than best
> effort in wireless transmission.  That is not exactly intuitive, given
> that radio is inherently broadcast.

Yes, that's counter-intuitive, but actually quite natural.

802.11 uses two different MAC sublayers: for unicast, it uses ARQ
(typically 8-persistent) and varies the PHY rate (depending on
e.g. pre-ARQ packet loss), while for multicast, it uses no ARQ and a fixed
PHY rate.

The effect is that unicast uses a higher data rate and virtually zero
packet loss, while multicast uses a low data rate and suffers from
significant packet loss (20% - 40% is not unusual for full-size frames,
and I've seen 80% on a link that was quite usable, post-ARQ, for reading
mail over IMAP).  This is good enough for IPv6 (both ND and RA use little
throughput and deal gracefully with reasonable packet loss), but makes
multicast useless for some other applications.

-- Juliusz

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