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Tunnelling of Explicit Congestion Notification
RFC 6040

Internet Engineering Task Force (IETF)                        B. Briscoe
Request for Comments: 6040                                            BT
Updates: 3168, 4301, 4774                                  November 2010
Category: Standards Track
ISSN: 2070-1721

             Tunnelling of Explicit Congestion Notification

Abstract

   This document redefines how the explicit congestion notification
   (ECN) field of the IP header should be constructed on entry to and
   exit from any IP-in-IP tunnel.  On encapsulation, it updates RFC 3168
   to bring all IP-in-IP tunnels (v4 or v6) into line with RFC 4301
   IPsec ECN processing.  On decapsulation, it updates both RFC 3168 and
   RFC 4301 to add new behaviours for previously unused combinations of
   inner and outer headers.  The new rules ensure the ECN field is
   correctly propagated across a tunnel whether it is used to signal one
   or two severity levels of congestion; whereas before, only one
   severity level was supported.  Tunnel endpoints can be updated in any
   order without affecting pre-existing uses of the ECN field, thus
   ensuring backward compatibility.  Nonetheless, operators wanting to
   support two severity levels (e.g., for pre-congestion notification --
   PCN) can require compliance with this new specification.  A thorough
   analysis of the reasoning for these changes and the implications is
   included.  In the unlikely event that the new rules do not meet a
   specific need, RFC 4774 gives guidance on designing alternate ECN
   semantics, and this document extends that to include tunnelling
   issues.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in 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/rfc6040.

Briscoe                      Standards Track                    [Page 1]
RFC 6040                     ECN Tunnelling                November 2010

Copyright Notice

   Copyright (c) 2010 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
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Briscoe                      Standards Track                    [Page 2]
RFC 6040                     ECN Tunnelling                November 2010

Table of Contents

   1. Introduction ....................................................4
      1.1. Scope ......................................................5
   2. Terminology .....................................................6
   3. Summary of Pre-Existing RFCs ....................................7
      3.1. Encapsulation at Tunnel Ingress ............................7
      3.2. Decapsulation at Tunnel Egress .............................8
   4. New ECN Tunnelling Rules ........................................9
      4.1. Default Tunnel Ingress Behaviour ..........................10
      4.2. Default Tunnel Egress Behaviour ...........................10
      4.3. Encapsulation Modes .......................................12
      4.4. Single Mode of Decapsulation ..............................14
   5. Updates to Earlier RFCs ........................................15
      5.1. Changes to RFC 4301 ECN Processing ........................15
      5.2. Changes to RFC 3168 ECN Processing ........................16
      5.3. Motivation for Changes ....................................17
           5.3.1. Motivation for Changing Encapsulation ..............17
           5.3.2. Motivation for Changing Decapsulation ..............18
   6. Backward Compatibility .........................................21
      6.1. Non-Issues Updating Decapsulation .........................21
      6.2. Non-Update of RFC 4301 IPsec Encapsulation ................21
      6.3. Update to RFC 3168 Encapsulation ..........................22
   7. Design Principles for Alternate ECN Tunnelling Semantics .......22

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