Avoiding Equal Cost Multipath Treatment in MPLS Networks

Network Working Group                                         G. Swallow
Request for Comments: 4928                                     S. Bryant
BCP: 128                                             Cisco Systems, Inc.
Category: Best Current Practice                             L. Andersson
                                                                Acreo AB
                                                               June 2007


        Avoiding Equal Cost Multipath Treatment in MPLS Networks

Status of This Memo

   This document specifies an Internet Best Current Practices for the
   Internet Community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This document describes the Equal Cost Multipath (ECMP) behavior of
   currently deployed MPLS networks.  This document makes best practice
   recommendations for anyone defining an application to run over an
   MPLS network that wishes to avoid the reordering that can result from
   transmission of different packets from the same flow over multiple
   different equal cost paths.  These recommendations rely on inspection
   of the IP version number field in packets.  Despite the heuristic
   nature of the recommendations, they provide a relatively safe way to
   operate MPLS networks, even if future allocations of IP version
   numbers were made for some purpose.

Table of Contents

   1. Introduction ....................................................2
      1.1. Terminology ................................................2
   2. Current ECMP Practices ..........................................2
   3. Recommendations for Avoiding ECMP Treatment .....................4
   4. Security Considerations .........................................5
   5. IANA Considerations .............................................5
   6. References ......................................................6
      6.1. Normative References .......................................6
      6.2. Informative References .....................................6

1.  Introduction

   This document describes the Equal Cost Multipath (ECMP) behavior of
   currently deployed MPLS networks.  We discuss cases where multiple
   packets from the same top-level LSP might be transmitted over
   different equal cost paths, resulting in possible mis-ordering of
   packets that are part of the same top-level LSP.  This document also
   makes best practice recommendations for anyone defining an
   application to run over an MPLS network that wishes to avoid the
   resulting potential for mis-ordered packets.  While disabling ECMP
   behavior is an option open to most operators, few (if any) have
   chosen to do so, and the application designer does not have control
   over the behavior of the networks that the application may run over.
   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 ECMP Practices

   The MPLS label stack and Forwarding Equivalence Classes are defined
   in [RFC3031].  The MPLS label stack does not carry a Protocol
   Identifier.  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 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 is based mainly or exclusively on
   the IP source and destination addresses.  The reason for splitting
   aggregated flows in this manner is to minimize the re-ordering of
   packets belonging to individual flows contained within the aggregated
   flow.  Within this document, we use the term IP ECMP for this type of
   forwarding algorithm.

   For packets that contain both a label stack and an encapsulated IPv4
   (or IPv6) packet, current implementations in some cases may hash on
   any combination of labels and IPv4 (or IPv6) source and destination
   addresses.

   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 the payload.  By
   inspecting the first nibble beyond the label stack, existing
   equipment infers 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
   ECMP.

   A consequence of this is that any application that defines an FEC
   that 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
   flows.

   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 that correctly identify a payload as
   not being IP most often will load-share traffic across multiple
   equal-cost paths based on the label stack.  Any reserved label, no
   matter where it is located in the stack, may be included in the
   computation for load balancing.  Modification 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, being mistaken for IPv4 may not be
   detrimental.  The trivial case being 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 would be
   performed.  For example, if an Ethernet frame immediately follows the
   label and the LSR does ECMP on IPv4, but 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, but
   Label ECMP may be performed.  If it is 0x4, then the constant values
   of the MAC addresses overlay the fields that would have been occupied
   by the source and destination addresses of an IP header.  In this
   case, the input to the ECMP algorithm would be a constant value and
   thus the algorithm would always return the same result.

3.  Recommendations for Avoiding ECMP Treatment

   We will use the term "Application Label" to refer to a label that has
   been allocated with an FEC Type that is defined (or simply used) by
   an application.  Such labels necessarily appear at the bottom of the
   label stack, that is, below labels associated with transporting the
   packet across an MPLS network.  The FEC Type of the Application label
   defines the payload that follows.  Anyone defining an application to
   be transported over MPLS is free to define new FEC Types and the
   format of the payload that 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     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   .                                       .     . .               .
   .                                       .     . .               .
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Label                  | Exp |0|       TTL     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Application Label            | Exp |1|       TTL     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1st Nbl|                                                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   In order to avoid IP ECMP treatment, it is necessary that an
   application 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, the chosen format must disallow the
   values 0x4 and 0x6 in the first nibble of their payload.

   It is REQUIRED, however, that applications depend upon in-order
   packet delivery restrict the first nibble values to 0x0 and 0x1.
   This will ensure that their traffic flows will not be affected if
   some future routing equipment does similar snooping on some future
   version(s) of IP.

   This behavior implies that if in the future an IP version is defined
   with a version number of 0x0 or 0x1, then equipment complying with
   this BCP would be unable to look past one or more MPLS headers, and
   loadsplit traffic from a single LSP across multiple paths based on a
   hash of specific fields in the IPv0 or IPv1 headers.  That is, IP
   traffic employing these version numbers would be safe from
   disturbances caused by inappropriate loadsplitting, but would also
   not be able to get the performance benefits.

   For an example of how ECMP is avoided in Pseudowires, see [RFC4385].

4.  Security Considerations

   This memo discusses the conditions under which MPLS traffic
   associated with a single top-level LSP either does or does not have
   the possibility of being split between multiple paths, implying the
   possibility of mis-ordering between packets belonging to the same
   top-level LSP.  From a security point of view, the worse that could
   result from a security breach of the mechanisms described here would
   be mis-ordering of packets, and possible corresponding loss of
   throughput (for example, TCP connections may in some cases reduce the
   window size in response to mis-ordered packets).  However, in order
   to create even this limited result, an attacker would need to either
   change the configuration or implementation of a router, or change the
   bits on the wire as transmitted in a packet.

   Other security issues in the deployment of MPLS are outside the scope
   of this document, but are discussed in other MPLS specifications,
   such as [RFC3031], [RFC3036], [RFC3107], [RFC3209], [RFC3478],
   [RFC3479], [RFC4206], [RFC4220], [RFC4221], [RFC4378], AND [RFC4379].

5.  IANA Considerations

   IANA has marked the value 0x1 in the IP protocol version number space
   as "Reserved" and placed a reference to this document to both values
   0x0 and 0x1.

   Note that this document does not in any way change the policies
   regarding the allocation of version numbers, including the possible
   use of the reserved numbers for some future purpose.

6.  References

6.1.  Normative References

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031, January 2001.

6.2.  Informative References

   [RFC3036]  Andersson, L., Doolan, P., Feldman, N., Fredette, A., and
              B. Thomas, "LDP Specification", RFC 3036, January 2001.

   [RFC3107]  Rekhter, Y. and E. Rosen, "Carrying Label Information in
              BGP-4", RFC 3107, May 2001.

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, December 2001.

   [RFC3478]  Leelanivas, M., Rekhter, Y., and R. Aggarwal, "Graceful
              Restart Mechanism for Label Distribution Protocol", RFC
              3478, February 2003.

   [RFC3479]  Farrel, A., Ed., "Fault Tolerance for the Label
              Distribution Protocol (LDP)", RFC 3479, February 2003.

   [RFC4206]  Kompella, K. and Y. Rekhter, "Label Switched Paths (LSP)
              Hierarchy with Generalized Multi-Protocol Label Switching
              (GMPLS) Traffic Engineering (TE)", RFC 4206, October 2005.

   [RFC4220]  Dubuc, M., Nadeau, T., and J. Lang, "Traffic Engineering
              Link Management Information Base", RFC 4220, November
              2005.

   [RFC4221]  Nadeau, T., Srinivasan, C., and A. Farrel, "Multiprotocol
              Label Switching (MPLS) Management Overview", RFC 4221,
              November 2005.

   [RFC4378]  Allan, D., Ed., and T. Nadeau, Ed., "A Framework for
              Multi-Protocol Label Switching (MPLS) Operations and
              Management (OAM)", RFC 4378, February 2006.

   [RFC4379]  Kompella, K. and G. Swallow, "Detecting Multi-Protocol
              Label Switched (MPLS) Data Plane Failures", RFC 4379,
              February 2006.

   [RFC4385]  Bryant, S., Swallow, G., Martini, L., and D. McPherson,
              "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for
              Use over an MPLS PSN", RFC 4385, February 2006.

Authors' Addresses

   Loa Andersson
   Acreo AB
   Electrum 236
   SE-146 40 Kista
   Sweden

   EMail:  loa@pi.se


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

   EMail: stbryant@cisco.com


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

   EMail:  swallow@cisco.com

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