On 2/1/13 7:56 PM, Fernando Gont wrote:
On 02/01/2013 09:41 PM, Joel M. Halpern wrote:
With regard to section 3, I would ask the question in the reverse.
(which may actually be the same quewstion Fernando is asking.)
If we assume that there is value in being able to perform the diagnostic
operation described, then it seems that one needs to be able to tell
when it can be applied.
Currently, that is u==1 in the IID field.
The text says that u==1 must be used n the IID field for IIDs derived
from EUI-64.
But it the goes on to say that u=1 can be used by other addresses.
If u=1 can be used by addresses not derived from EUI-64, then the
diagnostic operation can not be applied without a-priori knowledge.
And in the presence of such knowledge, the u bit does not seem to help.
The obvious answer is that to enable the diagnostic, and because there
is no need to make a gratuitous change, we stick with u=1 being used
always and only for addresses derived from EUI-64.
And to state the obvious, that implies that u==g==1 means we are looking
at an identifier derived from a hardware-based identifier (e.g., IEEE).
But if we enforce u=1 only for IIDS based on EUI-64, there might be
other IIDs generated with some other mechanism, that lead to IIDs with
such bit set to 1.
Unless u=1 is enforced for all identifiers, chances are that eventually
some other technology will generate an IID with u==1, and one would
mistakenly assume that such IID was based on IEEE identifiers...
(yes, in order for such "collisions" to occur, u would have to be 1, and
the other technology would also have to produce the 0xfffe we currently
insert to generate the Modified EUI64 identifiers... but "s* happens",
they say...)
The assumption that 0xFFFE will be in the identifier is incorrect. That
pattern is only inserted when the 64-bit IID is generated from a 48-bit
IEEE identifier. If I use a 64-bit IEEE identifier, the u bit will be
set to 1, but the 0xFFFE pattern will (most likely) not be there.
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