Network Working Group                                     George Swallow
Internet Draft                                       Cisco Systems, Inc.
Category: Standards Track
Expiration Date: March 2005 January 2006
                                                          Stewart Bryant
                                                     Cisco Systems, Inc.

                                                           Loa Andersson

                                                          September 2004

                                                               July 2005

        Avoiding Equal Cost Multipath Treatment in MPLS Networks



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Copyright Notice

   Copyright (C) The Internet Society (2004).


      This document describes the Equal Cost Multipath (ECMP) behavior
      of currently deployed MPLS networks and makes best practice
      recommendations for anyone defining an application to run over an
      MPLS network and wishes to avoid such treatment.


 1      Introduction  ...........................................  ..............................................   3
 1.1    Terminology  ...............................................   3
 2      Current EMCP Practices  .................................  ....................................   3
 3      Recommendations for Avoiding ECMP Treatment  ............   4  ...............   5
 4      Security Considerations  ................................   5  ...................................   6
 5      References  .............................................   5  ................................................   6
 5.1    Normative References  ...................................   5  ......................................   6
 6      Authors' Addresses  .....................................   6
    7      Full Copyright and Intellectual Property Statements  ....  ........................................   6

1. Introduction

   This document describes the Equal Cost Multipath (ECMP) behavior of
   currently deployed MPLS networks and makes best practice
   recommendations recommenda-
   tions for anyone defining an application to run over an MPLS network
   and wishes to avoid such treatment.  While turning disabling ECMP
   off behavior is
   an option open to most operators, few (if any) have chosen to do so.
   Thus ECMP behavior is a reality that must be reckoned with.

1.1. Terminology

   ECMP        Equal Cost Multipath

   FEC         Forwarding Equivalence Class

   IP ECMP     A forwarding behavior in which the selection of the
               next-hop between equal cost routes is based on the
               header(s) of an IP packet

   Label ECMP  A forwarding behavior in which the selection of the
               next-hop between equal cost routes is based on the
               label stack of an MPLS packet

   LSP         Label Switched Path

   LSR         Label Switching Router

2. Current EMCP Practices

   The MPLS label stack and Fowarding Forwarding Equivalence Classes are defined
   in [RFC3031].  The MPLS label stack does not carry a Protocol
   Identifier. Identi-
   fier.  Instead the payload of an MPLS packet is identified by the
   Forwarding Equivalence Class (FEC) of the bottom most label.  Thus it
   is not possible to know the payload type if one does not know the
   label binding for the bottom most label.  Since an LSR which is
   processing pro-
   cessing a label stack need only know the binding for the label(s) it
   must process, it is very often the case that LSRs along an LSP are
   unable to determine the payload type of the carried contents.

   As a means of potentially reducing delay and congestion, IP networks
   have taken advantage of multiple paths through a network by splitting
   traffic flows across those paths.  The general name for this practice
   is Equal Cost Multipath or ECMP.  In general this is done by hashing
   on various fields on the IP or contained headers.  In practice,
   within a network core, the hashing in based mainly or exclusively on
   the IP source and destination addresses.  The reason for splitting
   aggregated flows in this manner is to minimize the
   mis-ordering re-ordering of flows between
   packets belonging to individual IP hosts flows contained with in within the aggregated
   flow.  Within this document we use the term IP ECMP for this type of
   forwarding algorithm.

   In the early days of MPLS, the payload was almost exclusively IP.
   Even today the overwhelming majority of carried traffic remains IP.
   Providers of MPLS equipment sought to continue this IP ECMP behavior.
   As shown above, it is not possible to know whether the payload of an
   MPLS packet is IP at every place where IP ECMP needs to be performed.
   Thus vendors have taken the liberty of guessing what the payload is.
   By inspecting the first nibble beyond the label stack, it can be
   inferred that a packet is not IPv4 or IPv6 if the value of the nibble
   (where the IP version number would be found) is not 0x4 or 0x6
   respectively.  Most deployed LSRs will treat a packet whose first
   nibble is equal to 0x4 as if the payload were IPv4 for purposes of IP

   A consequence of this is that any application which defines a FEC
   which does not take measures to prevent the values 0x4 and 0x6 from
   occurring in the first nibble of the payload may be subject to IP
   ECMP and thus having their flows take multiple paths and arriving
   with considerable jitter and possibly out of order.  While none of
   this is in violation of the basic service offering of IP, it is
   detrimental to the performance of various classes of applications.
   It also complicates the measurement, monitoring and tracing of those

   New MPLS payload types are emerging such as those specified by the
   IETF PWE3 and AVT working groups. These payloads are not IP and, if
   specified without constraint might be mistaken for IP.

   It must also be noted that LSRs which correctly identify a payload as
   not being IP, may still need to load-share this traffic across multi-
   ple equal-cost paths.  In this case a LABEL ECMP algorithm is
   employed, where a hash is computed on all or part(s) of the label
   stack.  Any reserved label, no matter where it is located in the
   stack, may be included in the computation for load balancing.  Modi-
   fication of the label stack between packets of a single flow could
   result in re-ordering that flow.  That is, were an explicit null or a
   router-alert label to be added to a packet, that packet could take a
   different path through the network.

   Note that for some applications applications, being mistaken for IPv4 may not be
   detrimental.  The trivial case where the payload behind the top label
   is a packet belonging to an MPLS IPv4 VPN.  Here the real payload is
   IP and most (if not all) deployed equipment will locate the end of
   the label stack and correctly perform IP ECMP.

   A less obvious case is when the packets of a given flow happen to
   have constant values in the fields upon which IP ECMP will would be performed.
   Consider an MPLS PSN that only does ECMP on IPv4 (i.e. not on IPv6).
   If per-
   formed.  For example if an ethernet frame immediately follows the
   label stack, and the LSR does not do ECMP on IPv6, then either the first
   nibble will be 0x4 or it will be something else.  If the nibble is
   not 0x4 then no IP ECMP is performed. performed, but Label ECMP may be per-
   formed.  If it is 0x4, that is
   it is mistaken for IPv4, then the constant values of the MAC addresses
   overlay the fields that would be occupied by the source and
   destination destina-
   tion addresses of an IP header.  Thus all packets of the flow
   receive the same ECMP treatment.

3. Recommendations for Avoiding ECMP Treatment

   The field in the figure below tagged "Application Label" is a label
   of the FEC Type used/defined by the application.  It is the bottom
   most label in the label stack.  As such its FEC Type defines the
   payload pay-
   load which follows.  Anyone defining an application to be transported
   over MPLS is free to define new FEC Types and the format of the payload pay-
   load which will be carried.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   |                Label                  | Exp |0|       TTL     | MPLS
   .                                       .     . .               .
   .                                       .     . .               .
   |                Label                  | Exp |0|       TTL     | Label
   |          Application Label            | Exp |1|       TTL     | Stack
   |1st Nbl|                                                       |

              Figure 1: Label Stack and one Word of Payload

   In order to avoid IP ECMP treatment it is necessary that an application applica-
   tion take precautions to not be mistaken as IP by deployed equipment
   that snoops on the presumed location of the IP Version field.  Thus,
   at a
   minimum that minimum, the chosen format must disallow the values 0x4 and 0x6
   in the first nibble of their payload.

   It is strongly recommended, however, that applications restrict the
   first nibble values to 0x0 and 0x1.  This will ensure that that their
   traffic flows will not be affected if some future routing equipment
   does similar snooping on some future version of IP.

4. Security Considerations

   This memo documents current practices.  As such it creates no new
   security considerations.

5. References

5.1. Normative References

   [RFC3031]  Rosen, E. et al., "Multiprotocol Label Swithing Switching
              Architecture", January 2001.

6. Authors' Addresses

      Loa Andersson


      Stewart Bryant
      Cisco Systems
      250, Longwater,
      Green Park,
      Reading, RG2 6GB, UK


      George Swallow
      Cisco Systems, Inc.
      1414 Massachusetts Ave
      Boxborough, MA 01719


7. Full

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   Copyright (C) The Internet Society (2004). (2005).  This document is subject
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   January 2006

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