Subject: Re: Macs which may not like IP Multicast in the background

From: "Jeffrey S. Curtis" <[email-masked]>
Date: Tue, 16 Apr 1996 23:18:07 -0500

Hugh LaMaster writes:
}I have been advised recently of a problem on one of our IP multicast
}enabled subnets wherein a Mac Centris seems to slow down drastically 
}when IP multicast traffic, e.g. mbone, is going on on the same ethernet.
}I am wondering if this is a common problem, if there is an FAQ or
}other resource which describes this problem, and, if there is a 
}cure other than divorcing the Mac and the multicast traffic.

Macs use AppleTalk.  AppleTalk uses various multicast groups for things
like routing, zone updates, or address resolution.  Therefore, the Mac
will be involved in listening to certain multicast groups on your LAN.

My guess is that the Mac in question has a NIC with a very small number
of hash buckets, such that when the Mac turns on buckets X, Y, and Z to
listen to AppleTalk multicasts, certain IP Multicast groups from the
Mbone happen to map into either X, Y, or Z as well with that NIC's
particular hash function, and therefore some of the Mbone multicast
groups are having their traffic streams "leak" past the NIC's hardware
filter into the CPU on the machine.  I put "leak" in quotes because
this is normal behavior, and therefore the use of the word "leak" may
be misleading because "leak" implies incorrect behavior, but it is
only incorrect behavior from a logical standpoint rather than an
implementation standpoint.

Hope this helps,

Jeff
-- 
Jeffrey S. Curtis                      | Internetwork Manager
Argonne National Laboratory            | Email: [email-masked]
9700 South Cass Avenue, ECT-221        | Voice: 708/252-1789
Argonne, IL 60439                      | Fax:   708/252-9689

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From: [email-masked] (Joe Pallas)
Date: Thu, 18 Apr 1996 10:24:11 -0700

Macs running OpenTransport (including all the PCI machines) are
multicast-aware, and Macs running MacTCP 2.x should properly ignore
multicast traffic.  I haven't noticed any performance problems on my
machines, which are on an Mbone-connected net.  If you can conduct a
controlled experiment that shows multicast traffic affects the performance
of your machine, I expect the OpenTransport development team would be very
interested.

joe