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List:       centos
Subject:    Re: [CentOS] IP6 Anyone?
From:       Cameron Kerr <cameron () humbledown ! org>
Date:       2011-02-28 18:06:33
Message-ID: E59CD06B-2266-410F-A850-7EEF1EE3E2CD () humbledown ! org
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On 28/02/2011, at 10:30 AM, Les Mikesell wrote:

> On 2/27/11 12:50 PM, John R Pierce wrote:
> > On 02/27/11 9:16 AM, Les Mikesell wrote:
> > > Is there any difference in efficiency in how well the NIC hardware filters the
> > > assigned addresses?

Yes, as mentioned below, the NIC is given a list of Ethernet MAC addresses. Some \
[old, cheap] cards had a very small filter list [occassionally just a single entry], \
so the NIC would have to pass up pretty much any multicast so the next-layer (IP) \
could decide if it was wanted or not. This shouldn't be an issue today, unless you're \
using very old or very cheap cards.

> > NIC's work in MAC addresses, not IP.
> 
> Sort-of.  Most NICs know enough about IPv4 multicast to at least help filter 
> unwanted addresses without bothering the CPU.

Not quite. NICs have a list of Ethernet multicast MAC addresses that it can be \
assigned to pass on up the stack. These Ethernet multicast MAC addresses are \
determined from the IPv6 or IPv4 multicast address, but this is a kernel/driver \
issue, the NIC still does't have to know anything about IP.

Although, there certainly are NICs which know about higher-level protocols \
(TCP-offload engine), but that's a separate issue.

Ethernet _switches_, on the other hand, really need to know enough about IPv4/6 \
multicast in order to figure out which switch ports it should forward multicast \
traffic to (otherwise it floods, and we end up with the same traffic flow as \
broadcast). In IPv4, the switch needs to implement what is commonly called IGMP \
snooping. IGMP being the IPv4 protocol client devices use for subscribing to \
multicast groups. In IPv6, IGMP is instead part of ICMPv6 called Multicast Listener \
Discovery (typically version 2), and the switch feature to look for is called MLD[v2] \
snooping.

Hope this helps to clear up any misunderstanding.
Cameron
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