More Accurate ECN Feedback in TCP
draft-ietf-tcpm-accurate-ecn-11

Document Type Active Internet-Draft (tcpm WG)
Last updated 2020-05-04 (latest revision 2020-03-05)
Replaces draft-kuehlewind-tcpm-accurate-ecn
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Intended RFC status Proposed Standard
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TCP Maintenance & Minor Extensions (tcpm)                     B. Briscoe
Internet-Draft                                               Independent
Updates: 3168 (if approved)                                M. Kuehlewind
Intended status: Standards Track                                Ericsson
Expires: September 6, 2020                              R. Scheffenegger
                                                                  NetApp
                                                           March 5, 2020

                   More Accurate ECN Feedback in TCP
                    draft-ietf-tcpm-accurate-ecn-11

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).  Recent new TCP
   mechanisms like Congestion Exposure (ConEx), Data Center TCP (DCTCP)
   or Low Latency Low Loss Scalable Throughput (L4S) need more accurate
   ECN feedback information whenever more than one marking is received
   in one RTT.  This document specifies a scheme to provide more than
   one feedback signal per RTT in the TCP header.  Given TCP header
   space is scarce, it allocates a reserved header bit, that was
   previously used for the ECN-Nonce which has now been declared
   historic.  It also overloads the two existing ECN flags in the TCP
   header.  The resulting extra space is exploited to feed back the IP-
   ECN field received during the 3-way handshake as well.  Supplementary
   feedback information can optionally be provided in a new TCP option,
   which is never used on the TCP SYN.

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
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   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

Briscoe, et al.         Expires September 6, 2020               [Page 1]
Internet-Draft          Accurate TCP-ECN Feedback             March 2020

   This Internet-Draft will expire on September 6, 2020.

Copyright Notice

   Copyright (c) 2020 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Document Roadmap  . . . . . . . . . . . . . . . . . . . .   5
     1.2.  Goals . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
     1.4.  Recap of Existing ECN feedback in IP/TCP  . . . . . . . .   6
   2.  AccECN Protocol Overview and Rationale  . . . . . . . . . . .   8
     2.1.  Capability Negotiation  . . . . . . . . . . . . . . . . .   9
     2.2.  Feedback Mechanism  . . . . . . . . . . . . . . . . . . .   9
     2.3.  Delayed ACKs and Resilience Against ACK Loss  . . . . . .   9
     2.4.  Feedback Metrics  . . . . . . . . . . . . . . . . . . . .  10
     2.5.  Generic (Dumb) Reflector  . . . . . . . . . . . . . . . .  11
   3.  AccECN Protocol Specification . . . . . . . . . . . . . . . .  12
     3.1.  Negotiating to use AccECN . . . . . . . . . . . . . . . .  12
       3.1.1.  Negotiation during the TCP handshake  . . . . . . . .  12
       3.1.2.  Backward Compatibility  . . . . . . . . . . . . . . .  13
       3.1.3.  Forward Compatibility . . . . . . . . . . . . . . . .  15
       3.1.4.  Retransmission of the SYN . . . . . . . . . . . . . .  15
       3.1.5.  Implications of AccECN Mode . . . . . . . . . . . . .  16
     3.2.  AccECN Feedback . . . . . . . . . . . . . . . . . . . . .  18
       3.2.1.  Initialization of Feedback Counters . . . . . . . . .  18
       3.2.2.  The ACE Field . . . . . . . . . . . . . . . . . . . .  19
       3.2.3.  The AccECN Option . . . . . . . . . . . . . . . . . .  26
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