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[psg.com #250] Reception of prefix option with prefix length > 64

This issue was discussed in some detail.

I've done the following:

- Clarified the prefix length field in 4.6.2
- Added clarifications to the second last paragraph in 

Text in 4.6.2:

Prefix Length  8-bit unsigned integer. The number of leading bits
                     in the Prefix that are valid.  The value ranges
                     from 0 to 128. The prefix length field provides
                     necessary information for on-link determination
                     (when combined with other flags in the prefix
                     option).  It also assists with address
                     autoconfiguration as specified in [ADDRCONF], for
                     which there may be more restrictions on the prefix

Text in 6.3.4: 

   Stateless address autoconfiguration [ADDRCONF] may in some
   circumstances increase the Valid Lifetime of a prefix or ignore it
   completely in order to prevent a particular denial of service attack.
   However, since the effect of the same denial of service targeted at
   the on-link prefix list is not catastrophic (hosts would send packets
   to a default router and receive a redirect rather than sending
   packets directly to a neighbor) the Neighbor Discovery protocol does
   not impose such a check on the prefix lifetime values. Similarly,
   [ADDRCONF] may impose certain restrictions on the prefix length for
   address configuration purposes. Therefore, the prefix might be
   rejected by [ADDRCONF] implementation in the host. However, the
   prefix length is still valid for on-link determination when combined     
   with other flags in the prefix option.

Currently there is no text that limits the prefix length
to 64 if the A flag is set (as recommended by the IAB). 
I'd like to hear from the WG if this should be added.


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