Re: Forwarding Packets With Link Local Destination Addresses

神明達哉 <jinmei@wide.ad.jp> Fri, 08 January 2021 00:56 UTC

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Date: Thu, 7 Jan 2021 16:56:45 -0800
Message-ID: <CAJE_bqfSkvpT0PfbGxPmJ450+_DWH_66O9h=pbRkn36mB27sBA@mail.gmail.com>
Subject: Re: Forwarding Packets With Link Local Destination Addresses
To: Fernando Gont <fgont@si6networks.com>
Cc: Ron Bonica <rbonica=40juniper.net@dmarc.ietf.org>, "6man@ietf.org" <6man@ietf.org>
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At Thu, 7 Jan 2021 18:40:18 -0300,
Fernando Gont <fgont@si6networks.com> wrote:

> > According to RFC 4291, “routers must not forward any packets with
> > Link-Local source or destination addresses to other links”.
> >
> > I interpret this statement to include packets that contain routing
> > headers. For example, it forbids an SRv6 packet whose final segment has
> > a locator that begins with FE80.
> >
> > Does everyone share this interpretation? If so, do RFC 4291 or RFC 8200
> > make this sufficiently clear?
>
> Let me ask a different question:
> Why should this be any different for a routing header? The specs still
> apply.

The RFC 4291 spec still applies, but if it were only with RFC 4291, we
could allow the following:

- A node sends a packet with a routing header that would route the
  packet to a global address (D1) and then to a link-local (final)
  destination (D2).
- The packet leaves the source link and is delivered to the interface
  corresponding to the global address D1.
- The receiving node now sets D2 as the destination address of the
  IPv6 header and forwards the packet to the link where the packet is
  delivered to D2 (so it does not forward the packet to "other link").

RFC 4007 makes it clearer that this scenario is forbidden.

> I'd say that the case of link-locals is probably even "worse", because
> it would mean a link-local address was included in a routing header, to
> be processed by some router elsewhere in the networks. Something that
> doesn't seem to make any sense to me.

Actually, I can imagine a source routing usage where all intermediate
and final destination addresses are link-local, and the packet is
forwarded through nodes within the same link.  It may still be an
imaginary scenario, but it still makes some sense to me (I don't
intend to champion that usage though).

--
JINMEI, Tatuya