The Addition of Explicit Congestion Notification (ECN) to IP
RFC 3168

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Network Working Group                                    K. Ramakrishnan
Request for Comments: 3168                            TeraOptic Networks
Updates: 2474, 2401, 793                                        S. Floyd
Obsoletes: 2481                                                    ACIRI
Category: Standards Track                                       D. Black
                                                                     EMC
                                                          September 2001

      The Addition of Explicit Congestion Notification (ECN) to IP

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2001).  All Rights Reserved.

Abstract

   This memo specifies the incorporation of ECN (Explicit Congestion
   Notification) to TCP and IP, including ECN's use of two bits in the
   IP header.

Table of Contents

   1.  Introduction..................................................  3
   2.  Conventions and Acronyms......................................  5
   3.  Assumptions and General Principles............................  5
   4.  Active Queue Management (AQM).................................  6
   5.  Explicit Congestion Notification in IP........................  6
   5.1.  ECN as an Indication of Persistent Congestion............... 10
   5.2.  Dropped or Corrupted Packets................................ 11
   5.3.  Fragmentation............................................... 11
   6.  Support from the Transport Protocol........................... 12
   6.1.  TCP......................................................... 13
   6.1.1  TCP Initialization......................................... 14
   6.1.1.1.  Middlebox Issues........................................ 16
   6.1.1.2.  Robust TCP Initialization with an Echoed Reserved Field. 17
   6.1.2.  The TCP Sender............................................ 18
   6.1.3.  The TCP Receiver.......................................... 19
   6.1.4.  Congestion on the ACK-path................................ 20
   6.1.5.  Retransmitted TCP packets................................. 20

Ramakrishnan, et al.        Standards Track                     [Page 1]
RFC 3168               The Addition of ECN to IP          September 2001

   6.1.6.  TCP Window Probes......................................... 22
   7.  Non-compliance by the End Nodes............................... 22
   8.  Non-compliance in the Network................................. 24
   8.1.  Complications Introduced by Split Paths..................... 25
   9.  Encapsulated Packets.......................................... 25
   9.1.  IP packets encapsulated in IP............................... 25
   9.1.1.  The Limited-functionality and Full-functionality Options.. 27
   9.1.2.  Changes to the ECN Field within an IP Tunnel.............. 28
   9.2.  IPsec Tunnels............................................... 29
   9.2.1.  Negotiation between Tunnel Endpoints...................... 31
   9.2.1.1.  ECN Tunnel Security Association Database Field.......... 32
   9.2.1.2.  ECN Tunnel Security Association Attribute............... 32
   9.2.1.3.  Changes to IPsec Tunnel Header Processing............... 33
   9.2.2.  Changes to the ECN Field within an IPsec Tunnel........... 35
   9.2.3.  Comments for IPsec Support................................ 35
   9.3.  IP packets encapsulated in non-IP Packet Headers............ 36
   10.  Issues Raised by Monitoring and Policing Devices............. 36
   11.  Evaluations of ECN........................................... 37
   11.1.  Related Work Evaluating ECN................................ 37
   11.2.  A Discussion of the ECN nonce.............................. 37
   11.2.1.  The Incremental Deployment of ECT(1) in Routers.......... 38
   12.  Summary of changes required in IP and TCP.................... 38
   13.  Conclusions.................................................. 40
   14.  Acknowledgements............................................. 41
   15.  References................................................... 41
   16.  Security Considerations...................................... 45
   17.  IPv4 Header Checksum Recalculation........................... 45
   18.  Possible Changes to the ECN Field in the Network............. 45
   18.1.  Possible Changes to the IP Header.......................... 46
   18.1.1.  Erasing the Congestion Indication........................ 46
   18.1.2.  Falsely Reporting Congestion............................. 47
   18.1.3.  Disabling ECN-Capability................................. 47
   18.1.4.  Falsely Indicating ECN-Capability........................ 47