Re: [LLMNR] Use of IPv6 link-local multicast addresses [Potential new issue]

[Bill Sommerfeld <sommerfeld@orchard.arlington.ma.us>]

> It is unlikely (correct me if I'm wrong) that a NIC will support this
> many multicast filters in hardware.

I just took a look at drivers for a few commonly used fast ethernet
chipsets, and found the following limits:

	- 80 literal multicast addresses
	- 64-entry hash.
	- 128-entry hash.
	- 256-entry hash.
	- 512-entry hash.

The "N-entry hash" variety maintain a N-bit vector, and run the
recieved multicast address through a hash function with N possible
outputs; if the corresponding bit in the vector is set, the packet is
received, so you get more graceful degradation as the number of groups
increases at the cost of slightly more false positives received.

One 802.11 card I looked at has a lower limits (16 literal addresses).
Another 802.11 card seems to have no multicast filtering at all!
(though that may be an inadequate driver rather than a deficiency in
the hardware/firmware).

							- Bill

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