Explicit Congestion Marking in MPLS
RFC 5129

Network Working Group                                           B. Davie
Request for Comments: 5129                           Cisco Systems, Inc.
Category: Standards Track                                     B. Briscoe
                                                                  J. Tay
                                                             BT Research
                                                            January 2008

                  Explicit Congestion Marking in MPLS

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.

Abstract

   RFC 3270 defines how to support the Diffserv architecture in MPLS
   networks, including how to encode Diffserv Code Points (DSCPs) in an
   MPLS header.  DSCPs may be encoded in the EXP field, while other uses
   of that field are not precluded.  RFC 3270 makes no statement about
   how Explicit Congestion Notification (ECN) marking might be encoded
   in the MPLS header.  This document defines how an operator might
   define some of the EXP codepoints for explicit congestion
   notification, without precluding other uses.

Davie, et al.               Standards Track                     [Page 1]
RFC 5129                      ECN for MPLS                  January 2008

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Background . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Intent . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Use of MPLS EXP Field for ECN  . . . . . . . . . . . . . . . .  5
   3.  Per-Domain ECT Checking  . . . . . . . . . . . . . . . . . . .  7
   4.  ECN-Enabled MPLS Domain  . . . . . . . . . . . . . . . . . . .  8
     4.1.  Pushing (Adding) One or More Labels to an IP Packet  . . .  8
     4.2.  Pushing One or More Labels onto an MPLS Labeled Packet . .  8
     4.3.  Congestion Experienced in an Interior MPLS Node  . . . . .  8
     4.4.  Crossing a Diffserv Domain Boundary  . . . . . . . . . . .  8
     4.5.  Popping an MPLS Label (Not the End of the Stack) . . . . .  9
     4.6.  Popping the Last MPLS Label in the Stack . . . . . . . . .  9
     4.7.  Diffserv Tunneling Models  . . . . . . . . . . . . . . . . 10
   5.  ECN-Disabled MPLS Domain . . . . . . . . . . . . . . . . . . . 10
   6.  The Use of More Codepoints with E-LSPs and L-LSPs  . . . . . . 10
   7.  Relationship to Tunnel Behavior in RFC 3168  . . . . . . . . . 11
   8.  Deployment Considerations  . . . . . . . . . . . . . . . . . . 11
     8.1.  Marking Non-ECN-Capable Packets  . . . . . . . . . . . . . 11
     8.2.  Non-ECN-Capable Routers in an MPLS Domain  . . . . . . . . 12
   9.  Example Uses . . . . . . . . . . . . . . . . . . . . . . . . . 13
     9.1.  RFC 3168-Style ECN . . . . . . . . . . . . . . . . . . . . 13
     9.2.  ECN Co-Existence with Diffserv E-LSPs  . . . . . . . . . . 13
     9.3.  Congestion-Feedback-Based Traffic Engineering  . . . . . . 14
     9.4.  PCN Flow Admission Control and Flow Termination  . . . . . 14
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   11. Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 15
   Appendix A.   Extension to Pre-Congestion Notification . . . . . . 16
     A.1. Label Push onto IP Packet . . . . . . . . . . . . . . . . . 16
     A.2. Pushing Additional MPLS Labels  . . . . . . . . . . . . . . 16
     A.3. Admission Control or Flow Termination Marking Inside
          MPLS Domain . . . . . . . . . . . . . . . . . . . . . . . . 17
     A.4. Popping an MPLS Label (Not End of Stack)  . . . . . . . . . 17
     A.5. Popping the Last MPLS Label to Expose IP Header . . . . . . 17
   Normative References . . . . . . . . . . . . . . . . . . . . . . . 18
   Informative References . . . . . . . . . . . . . . . . . . . . . . 18

Davie, et al.               Standards Track                     [Page 2]
RFC 5129                      ECN for MPLS                  January 2008

1.  Introduction

1.1.  Background

   [RFC3168] defines Explicit Congestion Notification (ECN) for IP.  The
   primary purpose of ECN is to allow congestion to be signalled without
   dropping packets.

   [RFC3270] defines how to support the Diffserv architecture in MPLS
   networks, including how to encode Diffserv Code Points (DSCPs) in an
   MPLS header.  DSCPs may be encoded in the EXP field, while other uses