> > Doing that the "normal way" (value modulo some prime) seems like it
> would be
> > a much too expensive operation to contemplate. Doing it by
> calculating some
> > kind of CRC of the 272 bits, then using however many bits of that
> would be
> > better, but still not blindingly cheap.
>
> Associative memory is your friend :-)
In fact, one of the points that were made clear in the debate is that
the router manufacturers will never "just trust the host", i.e. just
trust that the "flow label" is sufficiently random and sufficiently
unique that they could use it without looking at the addresses. Then,
even if the flow was unique, we should note that even 20 bits would be
two small to be practical: the birthday paradox would kick in as soon as
the router processes 1,024 flows, which is not a very large number; you
would need some kind of tie-breaking logic, which means that you would
have at a minimum to process the source address. So, it is much safer to
assume that the primary classification will be based on the address
pair, and that the label is just a differentiator for packets that carry
the same address pair and require specific treatment; Mike points out
one possible solution; there are certainly others; hashing 256 bits of
source-destination should not require an inordinate number of gates in a
VLSI.
-- Christian Huitema
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