1. PCN WG (Pre-Congestion Notification) – a brief status update Philip Eardley, BT TSVAREA, IETF-73 Minneapolis 18 Nov 08 http://tools.ietf.org/wg/pcn/

2. Main message The key docs are at critical stage… –Architecture (Info): about to do IETF Last Call –Baseline encoding (Std): WG Last Call very soon –Marking behaviour (Std): WG Last Call very soon –Feedback welcome asap … so you may want to think about how to implement and/or use it, or how it might impact on what you’re doing

3. What is Pre-Congestion Notification? Objective: –Protect the quality of service of established inelastic flows within a single DiffServ domain, –by using flow admission and termination based on pre-congestion information Flow Admission (for “normal” situations) Flow Termination (for “unusual” situations, eg failures) Motivation: –Admission control: there are a range of existing CAC approaches with a range of scalability-accuracy tradeoffs classic trade-off between flow admission ctrl (smarts) vs. generous sizing (capacity) PCN offers a scalable light-weight solution with strong QoS assurances –Flow termination: typical current approach is just to wait for users to hang up PCN rapidly removes some flows in order to preserve QoS of the rest

4. BTS MSC PCN V V PCN-boundary- node PCN-interior-node V BTS MSC PCN-boundary- node PCN: Example scenario PCN-boundary-node = VoIP Trunk GW VoIP Trunk GW handles high number of calls All per-flow state restricted to VoIP GWs

5. BTS MSC PCN V V PCN-boundary- node PCN-interior-node V BTS MSC PCN-boundary- node PCN: Example scenario monitor PCN-marks (for a pair of boundary nodes) and hence make decisions about whether to admit new flows (or terminate existing flows) measure current load of PCN-traffic selectively mark packets if congestion appears imminent (hence pre-congestion) DiffServ forwarding typically EF PHB

6. PCN Concept 2 levels of pre-congestion notification encoded into pkt headers or 1 level if short of PCN codepoints

7. Encoding (STD) Baseline encoding –Uses 1 DSCP Allows re-use of other DSCPs –Switches on PCN behaviour using ECN field –Compatible with existing tunnels (RFC 3168 & 4301) –Gives 1 level of PCN-marking DSCP00100111 DSCP1Not-PCNNM (not marked) EXPPM (PCN- marked) (EXPTL) Extension encodings –Extra encoding states so can distinguish different sorts of PCN- marking

8. Next steps Complete current WG docs to RFC –Architecture; baseline encoding; marking behaviour Progress one or more boundary node behaviours (EXPTL) –ie methods to do flow admission & termination based on the PCN-marked pkts –Several on the table Progress one or more Encoding extensions (EXPTL) –Several on the table Signalling reqts for PCN info (eg ingress informed about measurements made by egress)

9. Spare slides

10. PCN motivation flow admission ctrl (smarts) vs. generous sizing (capacity) classic trade-off with diseconomy of scale either way the more hops away from admission control smarts the more generous sizing is needed for the voice/video class edge & interior flow admission control over Diffserv edge flow admission control over Diffserv Backbone provider Customer N/wk AccessBackhaul Core Customer router Metro Node MSAN CustomerAccess Provider Customer N/wk AccessBackhaul Customer router Metro Node MSAN CustomerAccess Provider €€€ € € € €€ €€€ €€ generous sizing PCN with Diffserv

11. Marking behaviour PCN-interior-node Is it a PCN-pkt? (optional) If rate of PCN-pkts > PCN-threshold-rate then mark all pkts Write encoding in pkt If rate of PCN-pkts > PCN-excess-rate then mark pkts above this rate