Re: Multicast MAC and Unicast IP Address

In article <4305c5d5$0$11080$e4fe514c@xxxxxxxxxxxxxx>,
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
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