Re: [PCN] PCN & tunnelling

Hi Phil,

philip.eardley@bt.com wrote:
>
> Hi,
>
> I was chatting with Bob about section 5.8 tunnelling. We’ve realised 
> there’s the following nasty case which we hadn’t thought about. The 
> scenario is when a tunnel starts inside a PCN-domain and finishes 
> outside it.
>
> First we follow what happens when the current text is followed. 
> Imagine that the pkt is PCN-marked by some PCN-node before the tunnel 
> start node. On encapsulation the PCN-mark is copied onto the outer 
> header. Hence the PCN-egress-node ‘sees’ the PCN-mark as normal – this 
> is ok. The PCN-egress-node clears the marking in the (outer) header 
> and forwards the pkt into the next domain. The pkt is decapsulated at 
> the tunnel end point. But the decapsulated pkt is already PCN-marked. 
> Potential problems: [1] if it’s decapsulated in a non-PCN-domain, then 
> the pkt may confuse nodes in this domain [depending on what encoding 
> is used for a PCN-mark] ; [2] if it’s decapsulated in a PCN-domain, 
> then the pkt is PCN-marked [which might lead this PCN-domain to 
> terminate or block a flow unnecessarily].
>
> The problem arises because the PCN-egress-node clears PCN-marking on 
> the outer header but not on the inner header. (This is a new problem 
> compared with ECN & tunnelling.)
>
> Possible solution: if the pkt is PCN-marked, then the tunnel start 
> node checks whether the tunnel egress is inside or outside the 
> PCN-domain - if it’s outside, then it clears the PCN-marking on the 
> inner header (effectively it does this on behalf of the 
> PCN-egress-node). (Also, the PCN-mark is copied onto the outer header, 
> as the current text says.)
>
> Thoughts?
>

At first sight I find this solution quite simple and good.

There are many other open issues regarding how PCN should behave in the 
case of tunneling. In general, I think multi-domain issues are not yet 
well understood and tunneling can create very nasty side effects.
http://www1.ietf.org/mail-archive/web/pcn/current/msg00687.html
I find it is important to record these caveats linked with tunneling and 
interdomain aspects, but I don't think that PCN must be bullet-proof for 
everything, at least these problems should not prevent simple solutions 
in the near future.

Regards,

Michael

> Thanks,
>
> Phil/
>
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-- 
Dr. Michael Menth, Assistant Professor
University of Wuerzburg, Institute of Computer Science
Am Hubland, D-97074 Wuerzburg, Germany, room B206
phone: (+49)-931/888-6644, fax: (+49)-931/888-6632
mailto:menth@informatik.uni-wuerzburg.de
http://www3.informatik.uni-wuerzburg.de/research/ngn

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