Point 2 point IPs between ASes

joel jaeggli joelja at bogus.com
Thu Jun 29 02:10:02 CST 2017

On 6/28/17 15:44, William Herrin wrote:
> On Wed, Jun 28, 2017 at 5:09 PM, Thomas Bellman <bellman at nsc.liu.se> wrote:
>> On 2017-06-28 17:03, William Herrin wrote:
>>> The common recommendations for IPv6 point to point interface numbering
>> are:
>>> /64
>>> /124
>>> /126
>>> /127
>> I thought the only allowed subnet prefix lengths for IPv6 were /64 and
>> /127.  RFC 4291 states:
>>    For all unicast addresses, except those that start with the binary
>>    value 000, Interface IDs are required to be 64 bits long and to be
>>    constructed in Modified EUI-64 format.
>> (and addresses starting with 000 are only used for special things,
>> like the localhost address ::1).  And then RFC 6164 adds /127 to
>> the allowed prefix lengths.
>> I know that many devices allow you to configure any subnet size,
>> but is there any RFC allowing you to use e.g. /124 or /126?
> Hi Thomas,
> AFAICT, the IETF has not caught up with operations practice... 
there's a certain amount of style drift, I think the rfc series actually
captures quite a bit of it.
> and
> operations practice itself is still in flux. I do see some discussion of
> longer-than-/64 prefixes in RFC 7421.
I'm not so sure about that, While operators have a variety of
preferences some of which I fix quixotic; which were formed as much as 2
decades ago. it's been about 6 years since we had a standards track
consensus describing the rational for numbering point-to-point links out
of /127s (6164). Which is long enough for text books to have been
updated, silicon implemntations of tcams to use exact match instead of
longest match lookups for your connected  neighbor on a /127 and so on.
likewise mitigations for ND exhaustion attacks exist even if they are
not universally implemented or perfect so some if not all the motivation
for short prefixes has been ameliorated. one can argue that concern in
rfc3627 (subnet router anycast) is entirely irrelevant for point to
point links (the rfc is now historic for that reason) which was the
major motivation for /126 vs /127 14 years ago.

in other news isps that apparently haven't run out of ipv4 addresses are
still assigning me /30 point-to-point links.
> The difference between theory and practice? In theory, there is no
> difference.
> IPv6 overall is designed to support CIDR addressing at any netmask. Correct
> implementations may not assume that any given interface will host a /64.
> Some specific protocols (like SLAAC) intentionally do not work if the
> interface ID is not exactly 64 bits. Others become more difficult than
> necessary if the prefix is not on a nibble boundary (the /CIDR number is
> not evenly divisible by 4).
> In the mean time, the options that have come out of OPERATIONS activity for
> point to point connections have converged on the above 4.
> Regards,
> Bill Herrin

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