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Re: [Idr] Fwd: New Version Notification for draft-scudder-idr-optional-transitive-00

On Mar 3, 2009, at 3:12 AM, Danny McPherson wrote:

I'm not sure I like the recommended behavior in S 2
where we're treating malformed updates "as though all
contained prefixes had been withdrawn".  I'd strongly
prefer that in ALL cases the update be discarded.
Making assumptions and providing rules for how to
selectively handle mal-formed updates seems much like
something you yourself would caution us against.  I'd
much prefer a 'discard and log' requirement.
The example below should refresh your memory of why "discard" doesn't fly. "Treat as withdraw" is basically the closest *safe* version of "discard". 

--John


From: "John G. Scudder" <jgs at juniper.net>
Date: December 15, 2008 2:34:34 PM GMT-05:00
To: Danny McPherson <danny at tcb.net>
Cc: Jeffrey Haas <jhaas at pfrc.org>, cayle.spandon at gmail.com, Enke Chen <enkechen at cisco.com>, quaizar.vohra at gmail.com, skh at nexthop.com, Inter-Domain Routing List <idr at ietf.org>, Yakov Rekhter <yakov at juniper.net >, Tony Li <tony.li at tony.li>, Quaizar Vohra <qv at juniper.net> 
Subject: Re: [Idr] RFC-4893 handling malformed AS4_PATH attributes

On Dec 15, 2008, at 2:21 PM, Danny McPherson wrote:

After thinking about this for a few minutes, I can't currently
come up with any configuration where a forwarding loop would occur
as a result of just dropping the update (because the "translation"
occurs on ingress to an AS), so discarding the update is likely
to be a cleaner action.



AS1---AS2
  \   /
   \ /
   AS3

- AS1 prefers to reach AS3 directly, and advertises its route to AS2.
- AS2 prefers to reach AS3 directly, and advertises its route to AS1.
- Connections AS3-AS1 and AS3-AS2 fail simultaneously.
- AS1 switches to prefer AS2's route, and sends an update message which 
  includes a withdraw of its previous announcement.  The withdraw is
bundled with some advertisements. It includes a bad attribute. As a 
  result, AS2 ignores the message.
- AS2 switches to prefer AS1's route, and sends an update message which 
  includes a withdraw of its previous announcement.  The withdraw is
bundled with some advertisements. It includes a bad attribute. As a 
  result, AS1 ignores the message.
End result is that AS1 forwards traffic for AS3 towards AS2, and AS2 forwards traffic for AS3 towards AS1. This is a permanent (until corrected) forwarding loop.
I'm sure a less-contrived example can also be constructed, but this one should be sufficient to prove the point. 

--John