RE: [PCN] PCN & tunnelling

<> Wed, 24 October 2007 19:06 UTC

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Subject: RE: [PCN] PCN & tunnelling
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I don't know.


For inspiration, I looked at rfc2983, diffserv & tunnels. S3.2 of this
talks about partially capable DS configs - only tunnel ingress is
DS-capable but not the egress. It says that 

If tunnel decapsulation processing discards
   the outer header's DSCP value without changing the inner header's
   DSCP value, the DS-capable tunnel ingress node is obligated to set
   the inner header's DSCP to a value compatible with the network at the
   tunnel egress.  The value 0 [is a good suggestion]


this approach is along the same lines to the one in the original email
below. Unfortunately 2983 doesn't say how the tunnel ingress node knows
about the characteristics of the network at the tunnel egress. Did the
DS people have anything in mind that might apply in the PCN case?




-----Original Message-----
From: Anna Charny (acharny) [] 
Sent: 24 October 2007 14:30
To: Eardley,PL,Philip,CXR9 R;
Subject: RE: [PCN] PCN & tunnelling


Hi Phil,


Question: what would be the mechanism for knowing whether the egress of
the tunnel is in the PCN domain or not?  Are we assuming that a node
somehow advertises its PCN capability?  I do not think we ever
explicitly assumed that?





	From: [] 
	Sent: Wednesday, October 24, 2007 6:24 AM
	Subject: [PCN] PCN & tunnelling



	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.)






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