Re: [babel] Alvaro Retana's Discuss on draft-ietf-babel-source-specific-07: (with DISCUSS and COMMENT)

Alvaro Retana <aretana.ietf@gmail.com> Wed, 04 November 2020 17:23 UTC

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From: Alvaro Retana <aretana.ietf@gmail.com>
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Subject: Re: [babel] Alvaro Retana's Discuss on draft-ietf-babel-source-specific-07: (with DISCUSS and COMMENT)
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On November 3, 2020 at 4:45:44 PM, Toke Høiland-Jørgensen wrote:

Toke:

Hi!


> Alvaro Retana via Datatracker writes:
>
...
> > Still, it shows how easy it is to define a policy that may not result
> > in the expected behavior because of the destination-first operation.
> > Please add some guidance on the advertisements and how they may
> > interact in the routing table.
> >
> > Note that what I'm asking for may just be a good example of what to do/
> > not to do. rfc8678/§5 presents an example of how the forwarding table is
> > constructed based on a set of routes -- something like that would be
> > great!
>
> Since your concern seems to be related to a particular example of what
> the source-specific extension can be used for, mentioned in the
> introduction, I'm not sure I see how it would improve the document to
> write a whole bunch of text addressing this particular operational
> concern? I mean, the section you're quoting the example from (section 4)
> literally says that the choice of dest first / source first is arbitrary
> and all that is required is that it is consistent, before then going
> ahead and spelling out the choice.

The choice may have been arbitrary, but §4 explicitly requires
destination-first: "A Babel implementation MUST choose routing table
entries by using the so-called destination-first ordering..."

Note that I am not questioning the choice, I'm just asking for more details.



> One small change in the introduction that may make this clearer, was to
> amend this sentence in the third paragraph of the introduction:
>
> "When using
> this technique in a network connected to multiple providers, every
> host is assigned multiple addresses, one per provider."
>
> to read something like:
>
> "When using
> this technique in a network connected to multiple providers, every
> host is assigned multiple addresses, one per provider, and each edge
> router exports a default route with a source prefix corresponding to
> the prefix delegated from its upstream."

That is not a bad change.  However, my concern is not just about the
multihomed case, but about the general handling of advertisements into
the routing table.



> > (b) How is the source address selected by the host?
>
> [...]
>
> > Even though this document is about Babel and not the host, I would
> > like to see a (short) discussion about how the host is expected to
> > behave (or what it needs to consider) in light of the
> > destination-first operation.
>
> Juliusz can correct me if I'm wrong here, but I believe this is left out
> because it really doesn't matter. The host can pick source addresses
> however it likes, and packets will make their way to the right upstream
> (in the multi-homing scenario).

If it doesn't matter, I would like to understand why.

The multihomed case with 2 default routes is straight forward -- but
how would it work in a more complex case (§4.4/rfc8678, for example)
where a specific source address is expected?  Or is this use case not
supported?


Thanks!

Alvaro.