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Admission Control over DiffServ with Pre-Congestion Notification: Framework and Challenges

This document discusses the implementation of Controlled Load (CL) services in DiffServ networks using Pre-Congestion Notification (PCN). The aim is to achieve admission control and flow pre-emption without maintaining flow states or signaling in the backbone. Key concepts include earlier packet marking by PCN routers, feedback mechanisms, and deployment models. The paper also reviews ongoing work, challenges in PCN deployment, and solutions for improving congestion management, including algorithm enhancements and addressing known limitations.

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Admission Control over DiffServ with Pre-Congestion Notification: Framework and Challenges

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  1. Admission Control over DiffServ using Pre-Congestion Notification draft-briscoe-tsvwg-cl-phb-02.pdf draft-briscoe-tsvwg-cl-architecture-03.txt Philip Eardley,Bob Briscoe, Dave Songhurst - BT Francois Le Faucheur, Anna Charny, Vassilis Liatsos – Cisco Kwok-Ho Chan, Joe Babiarz, Stephen Dudley – Nortel Georgios Karagiannis - University of Twente / Ericsson Attila Bader, Lars Westberg - Ericsson IETF-66 tsvwg 10th July 2006

  2. Summary • Aim: • End-to-end Controlled Load (CL) service without flow state or signalling in the core / backbone • Solution: • Pre-Congestion Notification (PCN) builds on the concepts of ECN, RFC 3168, “The addition of Explicit Congestion Notification to IP”. • PCN-router marks packet “earlier” than ECN-router (bulk marking, not per flow) • admission marking • pre-emption marking • Feedback of these markings used in a particular network framework to achieve flow admission control and flow pre-emption • Applied to real-time flows (such as voice, video and multimedia streaming) in DiffServ networks. • History • Drafts from BT, Cisco, Nortel & Ericsson - working together intensively

  3. Drafts – ‘Pre-Congestion Notification marking’ draft-briscoe-tsvwg-cl-phb-02.pdf ‘An edge-to-edge Deployment Model for Pre-Congestion Notification: Admission Control over a DiffServ Region’ draft-briscoe-tsvwg-cl-architecture-03 • further deployment models using PCN (future work) • end-to-end / Open • others? Controlled environment • Signalling extensions • RSVP, draft-lefaucheur-rsvp-ecn-01 • NSIS (future work) • Border anti-cheating (related work) • draft-briscoe-tsvwg-re-ecn-border-cheat-01 • extending CL-region across operators • PCN over MPLS (related work) • MPLS & ECN/PCN, draft-davie-ecn-mpls-00

  4. Recent work On-going • Understanding behaviour of PCN better • Sensitivity to measurement accuracy, reaction timing & choice of parameter settings • Further simulations • Supplementing those reported last time • On-going & will be reported at the next ietf • Investigating algorithm variants • we believe the current algorithms are reasonably good • On-going work to assess if alternatives are better • eg would like same algorithm to work in other deployment models • Understanding & addressing known “corner cases” • New Section 5 indraft-briscoe-tsvwg-cl-architecture-03: Limitations of PCN & potential solutions • Otherwise drafts have (just) had clarifications & corrections, eg due to comments received (thanks) On-going Report now

  5. ECMP • PCN mechanism works well when all packets between a pair of ingress and egress gateways follow the same path • With ECMP (Equal Cost Multipath Routing) congestion-level-estimate is averaged over several paths • Therefore if paths have different pre-congestion level, the reaction is not quite accurate. • Similarly for Sustainable-aggregate-rate • Can avoid problem by tunnelling from ingress to egress (so get a path-specific Congestion-level-estimate) • Investigating other possible alternatives: • Eg (for flow pre-emption), only select from sub-set of flows that have actually had a Pre-emption Marked packet (or Router Marked packet)

  6. Sub-optimalities • Global fairness: • ‘Beat down’ effect: if more than one router congested, some aggregates may pre-empt more flows than they need to • Common problem for congestion control algorithms eg TCP, XCP • Could just accept it • Bi-directional sessions: • flow pre-emption for Aggregate A=>B independent of B=>A. • But if pre-empt flow in only one direction of a voice call, session is still torn down • End result: too many flows are pre-empted • Could just live with these sub-optimalities? • Investigating a number of possible enhancements

  7. Next steps & future work • Progress PCN marking draft • Standards track • get feedback / support • Progress deployment model draft • Informational track • CL-region: Edge-to-edge / Controlled • Do limited work on this doc • Signalling extensions for NSIS • To be discussed at NSIS meeting • Please say if you’d like to work on this • Other deployment model • End-to-end / Open • Ie fewer flows per link, and may be some non-PCN-routers • Not working on at the moment, but trying to ensure that marking (algorithms & encoding) wouldn’t preclude the End-to-end/Open deployment model

  8. Comments please!

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