Problem Statement and Requirements for Increased Accuracy in Explicit Congestion Notification (ECN) Feedback
RFC 7560

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Internet Engineering Task Force (IETF)                M. Kuehlewind, Ed.
Request for Comments: 7560                                    ETH Zurich
Category: Informational                                 R. Scheffenegger
ISSN: 2070-1721                                             NetApp, Inc.
                                                              B. Briscoe
                                                                      BT
                                                             August 2015

       Problem Statement and Requirements for Increased Accuracy
           in Explicit Congestion Notification (ECN) Feedback

Abstract

   Explicit Congestion Notification (ECN) is a mechanism where network
   nodes can mark IP packets, instead of dropping them, to indicate
   congestion to the endpoints.  An ECN-capable receiver will feed this
   information back to the sender.  ECN is specified for TCP in such a
   way that it can only feed back one congestion signal per Round-Trip
   Time (RTT).  In contrast, ECN for other transport protocols, such as
   RTP/UDP and SCTP, is specified with more accurate ECN feedback.
   Recent new TCP mechanisms (like Congestion Exposure (ConEx) or Data
   Center TCP (DCTCP)) need more accurate ECN feedback in the case where
   more than one marking is received in one RTT.  This document
   specifies requirements for an update to the TCP protocol to provide
   more accurate ECN feedback.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7560.

Kuehlewind, et al.            Informational                     [Page 1]
RFC 7560       Requirements for More Accurate ECN Feedback   August 2015

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Recap of Classic ECN and ECN Nonce in IP/TCP  . . . . . . . .   5
   3.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   4.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Design Approaches . . . . . . . . . . . . . . . . . . . . . .  11
     5.1.  Redefinition of ECN/NS Header Bits  . . . . . . . . . . .  11
     5.2.  Using Other Header Bits . . . . . . . . . . . . . . . . .  13
     5.3.  Using a TCP Option  . . . . . . . . . . . . . . . . . . .  13
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Appendix A.  Ambiguity of the More Accurate ECN Feedback in DCTCP  16
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  17

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RFC 7560       Requirements for More Accurate ECN Feedback   August 2015

1.  Introduction

   Explicit Congestion Notification (ECN) [RFC3168] is a mechanism where
   network nodes can mark IP packets instead of dropping them to
   indicate congestion to the endpoints.  An ECN-capable receiver will
   feed this information back to the sender.  ECN is specified for TCP
   in such a way that only one feedback signal can be transmitted per
   Round-Trip Time (RTT).  This is sufficient for preexisting TCP
   congestion control mechanisms that perform only one reduction in
   sending rate per RTT, independent of the number of ECN congestion
   marks.  But recently proposed or deployed mechanisms like Congestion
   Exposure (ConEx) [RFC6789] or Data Center TCP (DCTCP) [DCTCP] need