Re: IPv6 address assignment for strictly point-to-point links and Device Loopbacks

Ole Trøan <otroan@employees.org> Wed, 26 September 2012 08:36 UTC

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Subject: Re: IPv6 address assignment for strictly point-to-point links and Device Loopbacks
From: =?iso-8859-1?Q?Ole_Tr=F8an?= <otroan@employees.org>
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Date: Wed, 26 Sep 2012 10:36:11 +0200
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To: Usman Latif <osmankh@yahoo.com>
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> Also its a good idea to encompass recommendations for p2p and loopbacks (and for that matter any assignment where prefix length is going beyond /64 into smaller subnets) into one standard track because the cautions and potential overlap issues that may exist for a /127 would pretty much be similar for /128 or any prefix that goes into the lower-64 bit territory...

> One major concern I have with using /127 and /128 on p2p and loopbacks respectively is that we need to be careful that there are existing (ISATAP etc) and potentially future implementations that would use/reserve bits in the lower 64 bits -so unless we set aside bit boundaries in the lower 64 bits, we are likely to overlap with these... which is why if we use /127 with a whole /64 reserved for the p2p subnet, then it should be okay but if /127 or /128s are numbered from the same /64 consecutively, then obviously its likely that the reserved bits used by other implementations would overlap. When this happens, any scenario where the router (which has this overlap) is SRC/DST of packets would be confused whether to interpret those lower-64 bits as simple global unicast prefix or try to treat the lower-64 bits in a different way (according to the protocol implementation which is using that bit pattern).

a few general points:
 - the 64 bit boundary is a "policy". implementations should handle any prefix length.
   implementations should, and as far as I know do, consider all bits in the address as transparent.
 - a loopback interface using a /128 cannot by definition overlap with anything else. 
 - for a point to point link the operator should know if there applications at either of two end points requiring the use of
   reserved addresses, if so the link must use a /64 otherwise /127 is fine.

these are operational choices, and I don't see a need to change IPv6 addressing architecture or protocol specifications.

cheers,
Ole