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Network Working Group                                         G. SwallowRequest for Comments: 4928                                     S. BryantBCP: 128                                             Cisco Systems, Inc.Category: Best Current Practice                             L. Andersson                                                                Acreo AB                                                               June 2007Avoiding Equal Cost Multipath Treatment in MPLS NetworksStatus 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 Contents1. Introduction ....................................................21.1. Terminology ................................................22. Current ECMP Practices ..........................................23. Recommendations for Avoiding ECMP Treatment .....................44. Security Considerations .........................................55. IANA Considerations .............................................56. References ......................................................66.1. Normative References .......................................66.2. Informative References .....................................6Swallow, et al.          Best Current Practice                  [Page 1]

RFC 4928        Avoiding ECMP Treatment in MPLS Networks       June 20071.  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 Router2.  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 splittingSwallow, et al.          Best Current Practice                  [Page 2]

RFC 4928        Avoiding ECMP Treatment in MPLS Networks       June 2007   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 thatSwallow, et al.          Best Current Practice                  [Page 3]

RFC 4928        Avoiding ECMP Treatment in MPLS Networks       June 2007   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|                                                       |   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+Swallow, et al.          Best Current Practice                  [Page 4]

RFC 4928        Avoiding ECMP Treatment in MPLS Networks       June 2007   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.Swallow, et al.          Best Current Practice                  [Page 5]

RFC 4928        Avoiding ECMP Treatment in MPLS Networks       June 2007   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.  References6.1.  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",RFC3478, 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.Swallow, et al.          Best Current Practice                  [Page 6]

RFC 4928        Avoiding ECMP Treatment in MPLS Networks       June 2007   [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.comSwallow, et al.          Best Current Practice                  [Page 7]

RFC 4928        Avoiding ECMP Treatment in MPLS Networks       June 2007Full Copyright Statement   Copyright (C) The IETF Trust (2007).   This document is subject to the rights, licenses and restrictions   contained inBCP 78, and except as set forth therein, the authors   retain all their rights.   This document and the information contained herein are provided on an   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Swallow, et al.          Best Current Practice                  [Page 8]