Subject: [PCN] Signalling protocols for CL and SM modes

From: Tom Taylor <[email-masked]>
Date: Fri, 24 Jun 2011 13:26:02 -0400

Rereading the signalling requirements document, I note that we actually 
need two protocols:

1) A non-reliable protocol carrying reports from the PCN-egress-node to 
the decision point, whether centralized or collocated with the 
PCN-ingress-node.

2) A reliable request-response protocol between a centralized decision 
point and a PCN-ingress-node.

1) Protocol between the PCN-egress-node and the decision point
    -----------------------------------------------------------

This protocol would be based on UDP, subject to security analysis. 
(Alternatives are UDP/IPSec or DTLS).

For the first protocol, the question arises: do we need back-off in the 
face of congestion? I argue not, since the very purpose of the protocol 
is to carry information that may help to mitigate that congestion. (I 
say "may" because the report path doesn't necessarily coincide with the 
data path of the received PCN packets.)

I await comments on that topic, but I suggest that the protocol itself 
would consist of two message types. The first is REPORT, and would be 
the only one used in normal operation. REPORT passes from the 
PCN-egress-node to the decision point.

The second message is REQUEST-REPORT. It is sent under the following 
circumstances from the decision point to the PCN-egress-node:

  -- possibly as part of a start-up sequence. Assuming that the decision 
point knows the address of the PCN-egress-node, this message could push 
the address of the decision point to the PCN-egress-node, avoiding the 
need to configure it there.

  -- possibly as part of a recovery attempt if the failure timer 
T-rcvFail expires;

  -- possibly as part of a manual debugging procedure.

The response of the PCN-egress-node to a REQUEST-REPORT message would be 
to immediately send a REPORT message with contents based on the latest 
measurement interval.

2) Protocol between the decision point and the PCN-ingress-node
    ------------------------------------------------------------

This one needs a bit of discussion to decide whether we actually want a 
stand-alone protocol consisting of REQUEST and RESPONSE messages running 
over TCP. The decision point already needs a protocol to install policy 
on the PCN-ingress-node. If this protocol is COPS, the PIB could easily 
be extended to request the estimated PCN-sent-rate from the 
PCN-ingress-node. In general we should think about protocol integration.

Comments?

Tom