Re: [babel] Martin Duke's Discuss on draft-ietf-babel-source-specific-07: (with DISCUSS)

Martin Duke <> Wed, 21 April 2021 15:18 UTC

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From: Martin Duke <>
Date: Wed, 21 Apr 2021 08:18:20 -0700
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To: Juliusz Chroboczek <>
Cc: The IESG <>,,,, Donald Eastlake <>
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Subject: Re: [babel] Martin Duke's Discuss on draft-ietf-babel-source-specific-07: (with DISCUSS)
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Sorry, I overlooked these replies in my email. I will remove my DISCUSS.

On Thu, Nov 5, 2020 at 8:25 AM Juliusz Chroboczek <> wrote:

> I've done some minor reordering to your comments.  Hopefully this doesn't
> change their meaning.
> > I am concerned about the congestion implications of this architecture.
> If there
> > are P prefixes in the network, the number of TLVs exchanged potentially
> goes
> > from P to P^2.
> Source-specific routes don't arise ex nihilo: they are injected into the
> routing domain through redistribution, just like ordinary, destination-only
> routes.  There is no way for Babel to exchange P² prefixes unless the
> network administrator has explicitly selected P² prefixes for
> redistribution.
> > But are there any safeguards in the protocol against this happening
> There are no safeguards in the protocol against a network administrator
> redistributing unreasonable numbers of source-specific routes into a Babel
> domain, just like there are no safeguards in the protocol against
> redistributing the full BGP routing table.
> > Or ought there to be some operational guidance to be somewhat selective
> > about source prefixes?
> Source-specific routing is a mechanism, which, as you rightly note, has
> a worst-case quadratic cost.  We currently know of three applications for
> source-specific routing, outlined in [SS-ROUTING] Sections II.B and II.C,
> and none of the three known applications lead to the worst-case quadratic
> explosion that you fear.  Interestingly, two of these applications have
> the potential to reduce the amount of routing data propagated globally in
> the Internet, at a moderate cost in the amount of data propagated locally.
> This seems to me like an excellent tradeoff.
> As source-specific routing becomes widely available and well-known, people
> will hopefully find new and exciting applications for this mechanism.
> Each of these new applications will need to be studied carefully, and its
> complexity understood.  It is not the role of the mechanism, however, to
> prohibit future applications just because they might turn out to be too
> costly.
> -- Juliusz