RE: OMNI Interface - formal issues

["Jaron, Zdenek" <[e-mail]>]

Dear Fred
Thank you for your reply.

> The OMNI spec is an "IPv6-over-foo" document which specifies the
> operation of IPv6 over specific link layers. That includes the formation and
> use of IPv6 link-local addresses. So, since only nodes that implement the
> OMNI spec will ever see these link-local addresses it should be OK to define
> the link local addresses as being relative to fe80::/10 as permitted by RFC4291
> while overriding the default zero settings of the 54 intermediate bits. So,
> since this is wholly contained within the OMNI spec I don't think we need to
> formally update RFC4291.
I still believe that any "IPv6-over-foo" must conform to the IPv6 Addressing
Architecture [RFC4291]. This document says that all addresses from FE80::/10
are interpreted as link-local (see Section 2.4.), but only addresses from
range FE80::/64 may be actually used (see Section 2.5.6.: the 54 zeros are
not "default", they are mandatory). In my understanding, this makes the rest
of FE80::/10 reserved for future updates of [RFC4291]. Although I agree
that chances to collision seem to be low, I think that using the whole
FE80::/10 is otherwise equivalent to using any other currently
unassigned/reserved IP address range.


> Good point, Zdenek - I honestly never dug deeply into RFC4193 and thought
> that all of fc::/7 was our own playground for free expression. However, I
> notice that RFC4193 says that bit 7 is "Set to 1 if the prefix is locally assigned
> and Set to 0 may be defined in the future". The behavior you are describing
> above is therefore with respect to "fd::/8", so what if we were to change the
> OMNI spec to define "fc::/8" as permitted by RFC4193? That way, we would
> be free to define the setting of fc80::/10 in the way we currently do in the
> OMNI spec. However, I think you are correct we would still need to say
> "updates RFC4193. What do you think?
FC00::/8 is currently reserved and cannot be used at all without update
to [RFC4193]. The "locally assigned" range FD00::/8 is limited to have
bits 8-47 randomly generated (see [RFC4193], Section 3.2.), so currently it
could only be used as a source of (any number of) unrelated /48 prefixes.

Personally, I would reconsider the need to introduce "OMNI ULAs". They seem
to be used only for segment routing (which could probably use some
deployment-specific addresses) and fragmentation (which, if I understand
it correctly, could use link-local addresses).

Best regards
Zdenek Jaron