Pekka, hi,
Pekka Savola wrote:
On Thu, 2 Oct 2008, Dunn, Jeffrey H. wrote:
While I agree with your assertions concerning flexibility and
robustness, I do not agree that 2^64 is the minimum number of nodes
that should be supported on a link. Consider current IPv4 deployments.
I doubt anyone have configured a single router interface with any
prefix shorter than /8 (24 bits for hosts). As a result, it would seem
to me to be a reasonable and conservative to assume that one might want
to use an IPv6 /96 (32 bits for hosts). Regardless of that, I see no
reason to constraint folks from using other numbers of bits, both more
and less than 64. The bottom line is that I do not understand why
folks feel the need to constrain network architects and administrators
to use only 64 bit prefixes.
Back then, folks felt that mapping an Ethernet MAC address (one-to-one)
was a good idea. That requires 48 bits. Initially, the interface
identifier part was 48 bits (RFC1884), but that was thought inadequate
and it was expanded to 64, probably because other media needed more than
48 and bits were there. I don't think this design decision is worth
revisiting.
I'm aware of several IEEE link layers and none uses 64bit addresses.
IEEE tries to have them all 48bit. Even non-IEEE (like USB) tries to be
48bit.
Alex
But no one will force you to use this numbering in your own network if
you don't want to. It just might be that some protocols don't work
quite as well.
You're not reserving a block of 2^12 IPv4 prefixes (or addresses in
degenerate case) for each VLAN-capable router interface today, so I
fail to see how this argument would apply to IPv6.
That is because you did not read the initial paragraph, which points
out that SLAAC requires a one-to-one mapping of prefixes to links,
e.g., Ethernet broadcast domains. In addition, OSPFv3 also forbids
prefixes from spanning interfaces. As a result, 2^13 prefixes could be
required if one wanted to be sure that all 2^13 VLAN identifiers were
available to a given physical interface. This fits in with your
statement about the ability to design new protocols and mechanisms to
leverage address bits.
I read it, and read it again. Again, I ask, how do you solve this
problem with IPv4? I'm not seeing it. IPv4 has (AFAIK) the same
constraints except that IPv6 has link-local addresses and you actually
don't need any prefix at all unless you attach hosts to the link.
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