Zach, I may miss something big, but here goes my comment anyways...
Zach Shelby a écrit :
Alexandru Petrescu wrote:
Zach Shelby a écrit :
[...]
Multicast with link-local scope, is simply a link-layer broadcast and
is not retransmitted.
I am not sure what you mean (in general? 802.15.4 in particular?) but
multicast with link-local scope maps into link-layer multicast, and is
not simply link-layer broadcast. Some link-layers do support
link-layer multicast and it is all an advantage to ND.
In general, for the types of link-layer this WG deals with. Please see
RFC4944, RFC4919, IEEE 802.15.4. Link-local scope multicast is mapped to
a broadcast (0xffff)... see page 4 of RFC4944. Other wireless
link-layers are similar.
Unless I miss something big I'm afraid there may be an error both in
what is said above, and in rfc4944.
rfc4944 doesn't seem to say a link-local scope multicast address is
mapped into 0xffff, but probably into something like 0x8001.
BEsides, it doesn't seem to say what IP address corresponds to the 16bit
MAC broadcast address 0xffff. If the rule used for mapping ff02::1 into
0x8001 were used to map 0xffff into an IP address then that would be
ff02::9f, which is different than the all-nodes link-scoped multicast
address ff02::1.
In short, by rfc4944:
ff02::1 maps into 0x8001
0xffff maps into ff02::9f
HEre's the relevant rfc4944 text:
rfc4944 says "An IPv6 packet with a multicast destination address (DST),
consisting of the sixteen octets DST[1] through DST[16], is
transmitted to the following 802.15.4 16-bit multicast address:
100 dst[15] dst [16]"
rfc4944 later says " This document creates a new IANA registry
for the 16-bit short address fields as used in 6LoWPAN packets.
[...]
This registry MUST include the addresses 0xffff (16-bit broadcast
address accepted by all devices currently listening to the channel)
and 0xfffe as defined in [ieee802.15.4]."
Additionally, when it talks use of link-layer broadcast to immitate
link-layer multicast, it doesn't say which IP link-scoped multicast
address is mapped into 0xffff.
Hence, IPv6 level multicast packets MUST be carried as link-layer
broadcast frames in IEEE 802.15.4 networks. [...]
2. A short destination address is included in the frame, and it MUST
match the broadcast address (0xffff).
With real link-layer multicast there are several multicast addresses,
like 33:33::1, 33:33::2, etc. Each corresponds to IP link-scope
multicast address which is ff02::1, ff02::2, etc.
For example NS is sent to 33:33::1/ff02::1
whereas RS is sent to 33:33::2/ff02::2
Now, with the above rfc4944 step 2, are both RS and NS sent to 0xffff?
Or only RS? rfc4944 is silent about this, yet of paramount importance
to implementer.
Alex
PS: there is more error in rfc4944 saying that "IPv6 level multicast
packets MUST be carried as link layer broadcast frames" - because only
the link-local scoped IP multicast packets should, and not the
globally-scoped multicast packets.
PPS: there is even more error in rfc4944 requiring every byte in the IP
multicast address (bytes 1 to 16) to correspond to "100 DST[15] DST[16]"
because globally multicast packets (i.e. group ff0e::1) I doubt should
be multicast at link-layer, they should be unicasted to the router.
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