[6lo] IID length in draft-ietf-6lo-btle-06.txt - why64?

[Alexandru Petrescu <[e-mail]>]

Hello,

I wonder why is the length of the IID in draft-ietf-6lo-btle-06.txt set 
to 64 since the BTLE MAC address is just 48bit?

Is there a problem if we set the IID length to 48?

Because if we do, then we may have an advantage in using longer than 64 
prefix lengths in the BTLE subnet - e.g. /65.  This is helpful in 
networks at the very edge of the Internet - typically the prefix 
arriving from the Internet to the 6LBR is is of length 64.  With such a 
prefix arriving to the 6LBR it is practically impossible to create 
another prefix of length also 64 beyond 6LBR, in the BTLE subnet.  Hence 
we'd need a prefix length of /65 in the BTLE subnet.  But a /65 in BTLE 
is impossible if the IID is of length 64.

Let me illustrate this by way of comparison:

                          6LN
                           \              ____________
            /65 impossible  \       /64  /            \
                    6LN ---- 6LBR ----- |  Internet   |
                            /            \____________/
                           /
                          6LN

             - with IID of length 64 -

--------------------------------------------------------------

                          6LN
                           \              ____________
              /65 possible  \       /64  /            \
                    6LN ---- 6LBR ----- |  Internet   |
                            /            \____________/
                           /
                          6LN

             - with IID of length 48 -

Alex
PS:  fyi a soon-to-be-RFC on the topic draft-ietf-6man-why64-08
      "Analysis of the 64-bit Boundary in IPv6 Addressing"