[Cbor] Supporting IPv6 Link-Local with scope (was Re: Éric Vyncke's Discuss on draft-ietf-cbor-network-addresses-09: (with DISCUSS and COMMENT))

Michael Richardson <mcr+ietf@sandelman.ca> Tue, 05 October 2021 18:55 UTC

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From: Michael Richardson <mcr+ietf@sandelman.ca>
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Captured as issue:

Éric Vyncke via Datatracker <noreply@ietf.org> wrote:
    > == DISCUSS ==

    > Generic comment how are link-local address (LLA) with scope encoded ? I would
    > expect CBOR to work also on LLA only networks... At the bare minimum, please
    > state that link-local addresses cannot be encoded with their scope, hence, they
    > cannot represent an interface.

    > -- Section 3.1.3 --
    > How can 2 valid link-local addresses (fe80::1%eth0, fe80::1%eth1) can be
    > represented in order to identity two interfaces ?

There are three kinds of things encoded:
      a) addresses.
      b) prefixes
      c) interface definitions

For (b) and (c), we could easily entertain (and did we discuss this in the
thread that was CC'ed to 6man?) adding a third element to the array to store
the interface ID.
For (a), I'm not sure what we can do to add the interface ID, but see below.

That's kinda the easy part.
The hard part is deciding how to encode the scope.
The simplest is as an integer, being the ifindex.

CBOR makes that easy and efficient, and many systems don't have more than 24
interfaces.  However, on systems where interfaces come/go a lot, the ifindex
often increments anyway.  Using the ifindex is probably clearer on most any
system than a string which can change, but it does change from one boot to
While the ifindex is system specific and has no outside meaning, the purposes
where I imagine this being used would be in some system specific container.
(My use case, which drove me to do this, actually probably needed scope-id)

One way to do (a) could be to append to the IPv6 string.
Another way would be not to bother, to always use the interface definition
when a IPv6-LL is needed.  Whether the length is 0, 128, or the actual
interface prefix (probably 64) is something we could specify.

Michael Richardson <mcr+IETF@sandelman.ca>   . o O ( IPv6 IøT consulting )
           Sandelman Software Works Inc, Ottawa and Worldwide