Network Working Group George Swallow Internet Draft Cisco Systems, Inc. Category: Standards Track Expiration Date: August 2007 Stewart Bryant Cisco Systems, Inc. Loa Andersson Acreo
September 2005February 2007 Avoiding Equal Cost Multipath Treatment in MPLS Networks draft-ietf-mpls-ecmp-bcp-02.txtdraft-ietf-mpls-ecmp-bcp-03.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." The list of current Internet-Drafts can be accessed at http://www.ietf.org/1id-abstracts.html The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html Abstract This document describes the Equal Cost Multipath (ECMP) behavior of currently deployed MPLS networks andnetworks. This document makes best practice recommendations for anyone defining an application to run over an MPLS network andthat wishes to avoid such treatment.the reordering that can result from transmission of different packets from the same flow over multiple different equal cost paths. Contents 1 Introduction .............................................. 3 1.1 Terminology ............................................... 3 2 Current ECMP Practices .................................... 3 3 Recommendations for Avoiding ECMP Treatment ............... 5 4 Security Considerations ................................... 6 5 References ................................................ 6 5.1 Normative References ...................................... 6 5.2 Informative References .................................... 7 6 Authors' Addresses ........................................ 68 1. Introduction This document describes the Equal Cost Multipath (ECMP) behavior of currently deployed MPLS networks andnetworks. We discuss cases where multiple packets from the same top-level LSP might be transmitted over differ- ent equal cost paths, resulting in possible mis-ordering of packets which are part of the same top-level LSP. This document also makes best practice recommenda- tionsrecommendations for anyone defining an application to run over an MPLS network andthat wishes to avoid such treatment.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.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 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 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 inis 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 labels. 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 inferredexisting 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 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 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 which 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 com- putation 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 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 per- formed. For example if an ethernet frame immediately follows the label 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, but Label ECMP may be per- formed. If it is 0x4, then the constant values of the MAC addresses overlay the fields that would behave been occupied by the source and destina- tiondestination addresses of an IP header. As a result the ECMP algo- rithm would be feed a constant value and thus 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 a 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 for- mat of the payload 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 | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ . . . . . . . . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Label | Exp |0| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Application Label | Exp |1| TTL | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1st Nbl| | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ In order to avoid IP ECMP treatment it is necessary that an 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, 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. For an example of how ECMP is avoided in Pseudowires, see [RFC4385]. 4. Security Considerations This memo documents current practices. As such it creates no newdiscusses the conditions under which MPLS traffic associ- ated with a single top-level LSP either does or does not have the possibility of being split between multiple paths, implying the pos- sibility 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 considerations.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, a hacker 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 of the scope of this document, but are discussed in other MPLS specifica- tions such as RFCs 3031, 3036, 3107, 3209, 3478, 3479, 4206, 4220, 4221, 4378, AND 4379. 5. References 5.1. Normative References [RFC3031] Rosen, E. et al., "Multiprotocol Label Switching Architecture", RFC 3031, January 2001. 5.2. Informative References [RFC3036] Andersson, L., et. al., "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., et. al., "RSVP-TE: Extensions to RSVP for LSP Tunnels", RFC 3209, December 2001. [RFC3478] Leelanivas, M., et. al., "Graceful Restart Mechanism for Label Distribution Protocol", RFC 3478, February 2003. [RFC3479] Farrel, A., "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., et. al., "Traffic Engineering Link Management Information Base", RFC 4220, November 2005. [RFC4221] Nadeau, T., et. al., "Multiprotocol Label Switching (MPLS) Management Overview", RFC 4221, November 2005. [RFC4378] Allan, D. and T. Nadeau, "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., et. al., "Pseudowire Emulation Edge-to-Edge (PWE3) Control Word for Use over an MPLS PSN", RFC 4385, February 2006. 6. Authors' Addresses Loa Andersson Acreo Email: email@example.com Stewart Bryant Cisco Systems 250, Longwater, Green Park, Reading, RG2 6GB, UK Email: firstname.lastname@example.org George Swallow Cisco Systems, Inc. 1414 Massachusetts Ave Boxborough, MA 01719 Email: email@example.com Copyright Notice Copyright (C) The Internet Society (2005). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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