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Michael Welzl University of Oslo

The Benefits to Applications of using Explicit Congestion Notification (ECN) draft-welzl-ecn-benefits-00. Michael Welzl University of Oslo. Gorry Fairhurst University of Aberdeen. 89th IETF Meeting London, UK 4 March 2014. What are we doing?. Point of draft: document gains of ECN

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Michael Welzl University of Oslo

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  1. The Benefits to Applications of using Explicit Congestion Notification (ECN)draft-welzl-ecn-benefits-00 Michael WelzlUniversity of Oslo Gorry FairhurstUniversity of Aberdeen 89th IETF Meeting London, UK4 March 2014

  2. What are we doing? • Point of draft: • document gains of ECN • includes less obvious gains • Out of scope: • To recommend a specific behavior

  3. ECN pro’s seem obvious • “The CE codepoint of an ECN-Capable packet SHOULD only be set if the router would otherwise have dropped the packet as an indication of congestion to the end nodes.” [RFC3168] • The receiver gets packets instead of loosing them. • Benefits shown in [RFC 2884], are not always biggest gain • In light to moderately loaded networks, number of dropped packets dropped due to congestion is small [RFC 3649] • …but that’s only a part of the story (Note: some but not all of ECN’s benefits need a different configuration than RFC 3168’s “SHOULD” above)

  4. Benefit from avoiding congestion loss • Reduced Head-of-Line Blocking • Relevant for transports providing in-order delivery • TCP/SCTP: Reduced Probability of RTO Expiry • RTO collapses cwnd, with significant bad impact; several mechanisms try to prevent this • Some applications do not retransmit lost packets • Typically VoIP, interactive video, real-time data • Need to apply loss-hiding mechanisms, with immediate effect on user-perceived quality

  5. Benefits that require special configuration • If ECN is configured such that routers mark packets at a lower level of congestion before they would otherwise drop packets from queue overflow: • Can avoid capacity overshoot; relevant e.g. in Slow Start • Can make congestion visible; relevant in ConEx • If a special configuration and reaction are used: • E.g. DCTCP has shown benefits when: • packets are marked earlier than they would otherwise be dropped • an instantaneous (= not averaged) queue is used for this decision (can be achieved with a special configuration of RED) • Receiver precisely feeds back number of ECN marks received in an RTT

  6. Conclusion • Motivates people configuring host stacks and network devices to enable ECN. • Application developers should where possible use transports that enable ECN. • Once enabled, the benefits of ECN are provided by the transport layer and the application does not need to be rewritten to gain these benefits.

  7. Next Steps • Is a document of this type helpful? • Are there other benefits to list? • Do we need to explain (potential) problems?

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