Archived from groups: comp.dcom.lans.ethernet (
More info?)
Rich Seifert wrote:
> In article <4305c5d5$0$11080$e4fe514c@news.xs4all.nl>,
> Geert Jan de Groot <> wrote:
>
>
>>>Is it allowed to have a multicast MAC address linked to a unicast IP
>>>address? If not what standard specifies this?
>>>I know that this mechanism is used for clustering but that certain vendors
>>>do not like it very much. I am interested in knowing what the rules are
>>>because up to now everybody just told me that it is a strange way to use
>>>multicast but some also said that it is illegal. Without proving it.
>>
>>RFC1122, page 67:
>> When a host sends a datagram to a link-layer broadcast address,
>> the IP destination address MUST be a legal IP broadcast or IP
>> multicast address.
>>
>> A host SHOULD silently discard a datagram that is received via
>> a link-layer broadcast (see Section 2.4) but does not specify
>> an IP multicast or broadcast destination address.
>>
>>I can think of several stacks (including ones on which I've worked myself),
>>that would fail in the scenario you describe: they flag the packet
>>as broadcast (multicast = limited broadcast) and hence,
>>for instance TCP SYN packets are dropped on the floor in this case.
>>
>
>
>
> The RFC discusses *broadcasts*; it is moot on the issue of *multicasts*.
> A multicast is NOT a broadcast; the standards treat them as distinct
> entities. I believe that the use of a multicast MAC address to send a
> unicast IP packet to a group of devices (e.g., a cluster) is a perfectly
> correct use of link-layer multicast, assuming that the cluster software
> knows how to deal with the fact that multiple devices are receiving the
> same IP datagram.
>
> The reason for the wording of RFC1122 becomes clearer when one considers
> that some link-layer technologies provide broadcast capability, but not
> Ethernet-style multicast (e.g., ARCnet). The RFC simply says that
> link-layer broadcast can be used to send IP broadcast or IP multicasts,
> but not IP unicasts. The RFC says nothing about how one would use an
> Ethernet-style multicast mechanism.
>
>
> --
> Rich Seifert Networks and Communications Consulting
> 21885 Bear Creek Way
> (408) 395-5700 Los Gatos, CA 95033
> (408) 228-0803 FAX
>
> Send replies to: usenet at richseifert dot com
Hi Rich,
as I am no implementor or LAN HW I ask myself the question:
How if not by looking at the multicast bit does a switch decide to
multicast/broadcast a frame? I doubt that many implementors make a
distinction between broadcast and multicast by looking at more bits than
that one.
But you are right, it's not specified clear enough.
Thanks for your suggestion.
Mat
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