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More Accurate ECN Feedback in TCP
draft-kuehlewind-tcpm-accurate-ecn-03

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Last updated: 2014-07-03
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Transport Area Working Group                                  B. Briscoe
Internet-Draft                                                        BT
Intended status: Experimental                           R. Scheffenegger
Expires: January 3, 2015                                    NetApp, Inc.
                                                           M. Kuehlewind
                                                 University of Stuttgart
                                                           July 02, 2014

                   More Accurate ECN Feedback in TCP
                 draft-kuehlewind-tcpm-accurate-ecn-03

Abstract

   Explicit Congestion Notification (ECN) is a mechanism where network
   nodes can mark IP packets instead of dropping them to indicate
   incipient congestion to the end-points.  Receivers with an ECN-
   capable transport protocol feed back this information 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).  Recently, new TCP
   mechanisms like Congestion Exposure (ConEx) or Data Center TCP
   (DCTCP) need more accurate ECN feedback information whenever more
   than one marking is received in one RTT.  This document specifies an
   experimental scheme to provide more than one feedback signal per RTT
   in the TCP header.  Given TCP header space is scarce, it overloads
   the three existing ECN-related flags in the TCP header.  Also, to
   improve robustness it uses 15 more bits if available.  For initial
   experiments it places these in a TCP option.  However, if the Urgent
   flag is cleared, zero header overhead could be achieved by reusing
   the Urgent Pointer opportunistically.  Therefore this document
   reserves space in the Urgent Pointer to be used if the protocol
   progresses to the standards track.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

Briscoe, et al.          Expires January 3, 2015                [Page 1]
Internet-Draft          Accurate TCP-ECN Feedback              July 2014

   This Internet-Draft will expire on January 3, 2015.

Copyright Notice

   Copyright (c) 2014 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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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Document Roadmap  . . . . . . . . . . . . . . . . . . . .   4
     1.2.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Experiment Goals  . . . . . . . . . . . . . . . . . . . .   5
     1.4.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   6
     1.5.  Recap of Existing ECN feedback in IP/TCP  . . . . . . . .   6
   2.  AccECN Protocol Overview  . . . . . . . . . . . . . . . . . .   8
     2.1.  Essential and Supplementary Parts . . . . . . . . . . . .   8
     2.2.  Capability Negotiation  . . . . . . . . . . . . . . . . .   9
     2.3.  Two Complementary Feedback Methods  . . . . . . . . . . .  10
     2.4.  Resilience Against ACK Loss . . . . . . . . . . . . . . .  11
     2.5.  Order of Arrival of IP-ECN Markings . . . . . . . . . . .  11
   3.  AccECN Protocol Specification . . . . . . . . . . . . . . . .  12
     3.1.  Negotiation during the TCP handshake  . . . . . . . . . .  12
     3.2.  Essential AccECN Feedback . . . . . . . . . . . . . . . .  15
       3.2.1.  The ACE Field . . . . . . . . . . . . . . . . . . . .  15
       3.2.2.  Safety against Ambiguity of the ACE Field . . . . . .  17
       3.2.3.  ACE Counter Selection . . . . . . . . . . . . . . . .  17
     3.3.  The Supplementary AccECN Field (SupAccECN)  . . . . . . .  18
       3.3.1.  Placement of the SupAccECN Field  . . . . . . . . . .  19

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