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Re: [homenet] draft-boutier-homenet-source-specific-routing-00

Teco Boot <teco@inf-net.nl> Wed, 10 July 2013 19:44 UTC

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From: Teco Boot <teco@inf-net.nl>
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Date: Wed, 10 Jul 2013 21:44:09 +0200
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To: Juliusz Chroboczek <jch@pps.univ-paris-diderot.fr>
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Cc: Henning Rogge <hrogge@googlemail.com>, "homenet@ietf.org" <homenet@ietf.org>
Subject: Re: [homenet] draft-boutier-homenet-source-specific-routing-00
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Op 10 jul. 2013, om 18:19 heeft Juliusz Chroboczek <jch@pps.univ-paris-diderot.fr> het volgende geschreven:

>> In my example I assumed that every gateway announces a
>> non-source-specific route too, so there should be no special code
>> necessary for the "normal" routers.
> 
> Unless I'm missing something, that's not as simple as you make it.
> Consider this:
> 
>  -- A -- B --
> 
> Suppose that A announces (::/0, alpha), and B announces (::/0, beta)
> (two source-specific default routes).  Suppose further that, as you
> suggest, both A and B announce default routes (::/0, ::/0).
> 
> What happens when A receives a packet that's not sourced in either
> alpha or beta?  It will route it towards B (which, you will recall, is
> announcing a non-specific default route).  Now B will send the packet
> back to A, and you're in trouble.
No. A knows B sends out a specific default route. A shall not send packets to B where B doesn't know what to do with it. This is delegated ingress filtering.
In a homenet, "normal" routers know how to handle multi-homing. 

Teco

> 
> So you cannot just announce a default route, you must redistribute
> a blackhole route.  But then, you might end up overriding a bona fide
> default route with your blackhole.  Grr.
> 
> I'm pretty sure various workarounds can be devised (such as using
> a very high metric for the blackhole route, or adding a flag "this
> route must be ignored by source-specific routers"), but I'd obviously
> prefer avoiding such hacks.
> 
> -- Juliusz
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