RE: [PCN] PCN & tunnelling

Phil,

I guess this is a general DiffServ issue too. In case [2] you assume the
same DSCP to be used by the tunnel end point PCN domain as at the tunnel
entry. This assumption may not hold, if the set DSCP is used for another
service class or unused at the end point. 

I had a brief glance on RFC2983, "Differentiated Services and Tunnels".
Downpropagation of outer IP header DSCP/PCN settings may be a fairly
good recommendation.

Regards,

Rudiger

-----Original Message-----
From: philip.eardley@bt.com [mailto:philip.eardley@bt.com]
Sent: Wednesday, October 24, 2007 12:24 PM
To: pcn@ietf.org
Subject: [PCN] PCN & tunnelling

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?

Thanks,

Phil/  