Re: IID length text [was Re: Review of draft-ietf-6man-rfc4291bis-06]

Fernando Gont <fgont@si6networks.com> Mon, 16 January 2017 20:28 UTC

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Subject: Re: IID length text [was Re: Review of draft-ietf-6man-rfc4291bis-06]
To: Brian E Carpenter <brian.e.carpenter@gmail.com>, 6man <ipv6@ietf.org>
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From: Fernando Gont <fgont@si6networks.com>
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On 01/14/2017 04:49 PM, Brian E Carpenter wrote:
> A modest suggestion:
> 
> OLD
>    For all unicast addresses, except those that start with the binary
>    value 000, Interface IDs are required to be 64 bits long.  Background
>    on the 64 bit boundary in IPv6 addresses can be found in [RFC7421].
> 
> NEW
>    IPv6 routing is based on prefixes of any valid length up to 128 [BCP198].
>    For example, [RFC6164] standardises 127 bit  prefixes on point-to-point
>    links. However, consistent use of Stateless Address Autoconfiguration
>    (SLAAC)[RFC4862] requires that all interfaces on a link use the same length
>    of Interface ID. In practice, this means that to guarantee interoperability
>    of SLAAC, a fixed length of Interface ID is necessary.

fixed as "all nodes on the link use the same IID length" or as in "a
hardcoded IID length, as the current 64 value"?

If the former, I agree. If the later, I don't.  The 64-bit length seems
to have a lot to do with embedding MAC addresses (IIRC, the IID length
was something like 48, but then changed to 64 in response to EUI-64).
Certainly, if you generate IIDs by embedding some number which has
constraints (as a MAC address), you need a fixed length. OTOH, if you
select the IIDs from a stream of bits (as RFC7217 does), then you don't
care about the length (other than "as many bits as necessary such that
the host density is low, and hence collisions of IIDs are reduced).

Thanks!

Cheers,
-- 
Fernando Gont
SI6 Networks
e-mail: fgont@si6networks.com
PGP Fingerprint: 6666 31C6 D484 63B2 8FB1 E3C4 AE25 0D55 1D4E 7492