[babel] Dummy source address [was: I-D Action: draft-ietf-babel-v4viav6-01.txt]

Juliusz Chroboczek <jch@irif.fr> Tue, 13 April 2021 22:50 UTC

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Date: Wed, 14 Apr 2021 00:49:51 +0200
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From: Juliusz Chroboczek <jch@irif.fr>
To: Donald Eastlake <d3e3e3@gmail.com>
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Subject: [babel] Dummy source address [was: I-D Action: draft-ietf-babel-v4viav6-01.txt]
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> a separate Babel dummy address

I've been thinking about this all evening, and I think I now see what
makes me uncomfortable.  (Dinner helped.)

By the time the IPv4 module has decided it needs to send an ICMPv4 packet,
it does not necessarily know whether the issue it's reporting is due to
Babel or to some routing protocol.  In fact, there might not even be
a well defined responsible protocol -- if a router running both Babel and
BGP finds out that it has no route to a given destination, is the ICMPv4
packet associated with Babel or BGP?

Thus, any implementable procedure for sending ICMPv4 must be independent
of any given routing protocol -- so it's not reasonable to have a Babel
dummy address, distinct from a hypothetical BGP dummy address or an OSPF
dummy address.  If we define a dummy address, that address must be common
across all routing protocols.

Not sure how to proceed:

 1. reuse the 4rd dummy address, as David suggested, repurposing it
    to be a generic dummy address?
 2. write a new RFC defining a cross-protocol dummy address?
 3. define a cross-protocol dummy address in this draft?
 4. remain vague, as in the current text?
 5. some other solution I'm missing?

I'm not volunteering for (2), although I could perhaps be a co-author.
(3) seems like exceeding the scope of this document.  (1) and (4) would be
fine with me.

-- Juliusz