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Internet Engineering Task Force (IETF)                         S. BryantRequest for Comments: 8372                                        HuaweiCategory: Informational                                     C. PignataroISSN: 2070-1721                                                    Cisco                                                                 M. Chen                                                                   Z. Li                                                                  Huawei                                                               G. Mirsky                                                               ZTE Corp.                                                                May 2018MPLS Flow Identification ConsiderationsAbstract   This document discusses aspects to consider when developing a   solution for MPLS flow identification.  The key application that   needs this solution is in-band performance monitoring of MPLS flows   when MPLS is used to encapsulate user data packets.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Not all documents   approved by the IESG are candidates for any level of Internet   Standard; seeSection 2 of RFC 7841.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttps://www.rfc-editor.org/info/rfc8372.Bryant, et al.                Informational                     [Page 1]

RFC 8372                MPLS Flow Identification                May 2018Copyright Notice   Copyright (c) 2018 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (https://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Table of Contents1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .22.  Loss Measurement Considerations . . . . . . . . . . . . . . .33.  Delay Measurement Considerations  . . . . . . . . . . . . . .44.  Units of Identification . . . . . . . . . . . . . . . . . . .45.  Types of LSP  . . . . . . . . . . . . . . . . . . . . . . . .66.  Network Scope . . . . . . . . . . . . . . . . . . . . . . . .77.  Backwards Compatibility . . . . . . . . . . . . . . . . . . .78.  Data Plane  . . . . . . . . . . . . . . . . . . . . . . . . .79.  Control Plane . . . . . . . . . . . . . . . . . . . . . . . .910. Privacy Considerations  . . . . . . . . . . . . . . . . . . .911. Security Considerations . . . . . . . . . . . . . . . . . . .912. IANA Considerations . . . . . . . . . . . . . . . . . . . . .913. Informative References  . . . . . . . . . . . . . . . . . . .10   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .11   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .111.  Introduction   This document discusses the aspects that need to be considered when   developing a solution for MPLS flow identification.  The key   application that needs this is in-band performance monitoring of MPLS   flows when MPLS is used to encapsulate user data packets.   There is a need to identify flows in MPLS networks for various   applications such as determining packet loss and packet delay   measurement.  A method of loss and delay measurement in MPLS networks   was defined in [RFC6374].  When used to measure packet loss,   [RFC6374] depends on the use of injected Operations, Administration,   and Maintenance (OAM) packets to designate the beginning and the end   of the packet group over which packet loss is being measured.  If the   misordering of packets from one group relative to the following groupBryant, et al.                Informational                     [Page 2]

RFC 8372                MPLS Flow Identification                May 2018   or the misordering of any of the packets being counted relative to   the Loss Measurement packet [RFC6374] occurs, then an error will   occur in the packet loss measurement.   In addition, [RFC6374] did not support different granularities of   flow or address a number of multipoint cases in which two or more   ingress Label Switching Routers (LSRs) could send packets to one or   more destinations.   Due to the very low loss rate in normal operation, improvements in   link and transmission technologies have made it more difficult to   assess packet loss using active performance measurement methods with   synthetic traffic.  That, together with more demanding service-level   requirements, means that network operators now need to be able to   measure the loss of the actual user data traffic using passive   performance measurement methods.  Any technique deployed needs to be   transparent to the end user, and it needs to be assumed that they   will not take any active part in the measurement process.  Indeed, it   is important that any flow identification technique be invisible to   them and that no remnant of the measurement process leaks into their   network.   Additionally, when there are multiple traffic sources, such as in   multipoint-to-point and multipoint-to-multipoint network   environments, there needs to be a method whereby the sink can   distinguish between packets from the various sources; that is to say,   a multipoint measurement model needs to be developed.2.  Loss Measurement Considerations   Modern networks, if not oversubscribed, generally drop relatively few   packets; thus, packet loss measurement is highly sensitive to the   common demarcation of the exact set of packets to be measured for   loss.  Without some form of coloring or batch marking such as that   proposed in [RFC8321], it may not be possible to achieve the required   accuracy in the loss measurement of customer data traffic.  Thus,   when accurate measurement of packet loss is required, it may be   economically advantageous, or even be a technical requirement, to   include some form of marking in the packets to assign each packet to   a particular counter for loss measurement purposes.   When this level of accuracy is required and the traffic between a   source-destination pair is subject to Equal-Cost Multipath (ECMP), a   demarcation mechanism is needed to group the packets into batches.   Once a batch is correlated at both ingress and egress, the packet   accounting mechanism is then able to operate on the batch of packets   that can be accounted for at both the packet ingress and the packetBryant, et al.                Informational                     [Page 3]

RFC 8372                MPLS Flow Identification                May 2018   egress.  Errors in the accounting are particularly acute in Label   Switched Paths (LSPs) subjected to ECMP because the network transit   time will be different for the various ECMP paths since:   1.  the packets may traverse different sets of LSRs;   2.  the packets may depart from different interfaces on different       line cards on LSRs; and   3.  the packets may arrive at different interfaces on different line       cards on LSRs.   A consideration with this solution on modifying the identity label   (the MPLS label ordinarily used to identify the LSP, Virtual Private   Network, Pseudowire, etc.) to indicate the batch is the impact that   this has on the path chosen by the ECMP mechanism.  When the member   of the ECMP path set is chosen by deep packet inspection, a change of   batch represented by a change of identity label will have no impact   on the ECMP path.  If the path member is chosen by reference to an   entropy label [RFC6790], then changing the batch identifier will not   result in a change to the chosen ECMP path.  ECMP is so pervasive in   multipoint-to-(multi)point networks that some method of avoiding   accounting errors introduced by ECMP needs to be supported.3.  Delay Measurement Considerations   Most of the existing delay measurement methods are active methods   that depend on the extra injected test packet to evaluate the delay   of a path.  With the active measurement method, the rate, numbers,   and interval between the injected packets may affect the accuracy of   the results.  Due to ECMP (or link aggregation techniques), injected   test packets may traverse different links from the ones used by the   data traffic.  Thus, measuring the delay of the real traffic is   required.   For combined loss and delay measurements, both the loss and the delay   considerations apply.4.  Units of Identification   The most basic unit of identification is the identity of the node   that processed the packet on its entry to the MPLS network.  However,   the required unit of identification may vary depending on the use   case for accounting, performance measurement, or other types of   packet observations.  In particular, note that there may be a need to   impose identity at several different layers of the MPLS label stack.Bryant, et al.                Informational                     [Page 4]

RFC 8372                MPLS Flow Identification                May 2018   This document considers the identification of the following traffic   components:   o  Per source LSR: everything from one source is aggregated   o  Per group of LSPs chosen by an ingress LSR: an ingress LSP      aggregates a group of LSPs (e.g., all the LSPs of a tunnel)   o  Per LSP: the basic form   o  Per flow [RFC6790] within an LSP: a fine-grained method   Note that a fine-grained identity resolution is needed when there is   a need to perform these operations on a flow not readily identified   by some other element in the label stack.  Such a fine-grained   resolution may be possible by deep packet inspection.  However, this   may not always be possible, or it may be desired to minimize   processing costs by doing this only on entry to the network.  Adding   a suitable identifier to the packet for reference by other network   elements minimizes the processing needed by other network elements.   An example of such a fine-grained case might be traffic belonging to   a certain service or from a specific source, particularly if matters   related to service level agreement or application performance were   being investigated.   We can thus characterize the identification requirement in the   following broad terms:   o  There needs to be some way for an egress LSR to identify the      ingress LSR with an appropriate degree of scope.  This concept is      discussed further inSection 6.   o  There needs to be a way to identify a specific LSP at the egress      node.  This allows for the case of instrumenting multiple LSPs      operating between the same pair of nodes.  In such cases, the      identity of the ingress LSR is insufficient.   o  In order to conserve resources such as labels, counters, and/or      compute cycles, it may be desirable to identify an LSP group so      that an operation can be performed on the group as an aggregate.   o  There needs to be a way to identify a flow within an LSP.  This is      necessary when investigating a specific flow that has been      aggregated into an LSP.   The unit of identification and the method of determining which   packets constitute a flow will be specific to the application or use   case and are out of scope of this document.Bryant, et al.                Informational                     [Page 5]

RFC 8372                MPLS Flow Identification                May 20185.  Types of LSP   We need to consider a number of types of LSP.  The two simplest types   to monitor are point-to-point LSPs and point-to-multipoint LSPs.  The   ingress LSR for a point-to-point LSP, such as those created using the   Resource Reservation Protocol - Traffic Engineering (RSVP-TE)   [RFC5420] signaling protocol or those that conform to the MPLS   Transport Profile (MPLS-TP) [RFC5654], may be identified by   inspection of the top label in the stack because, at any provider-   edge (PE) or provider (P) router on the path, the top label is unique   to the ingress-egress pair at every hop at a given layer in the LSP   hierarchy.  Provided that Penultimate Hop Popping (PHP) is disabled,   the identity of the ingress LSR of a point-to-point LSP is available   at the egress LSR; thus, determining the identity of the ingress LSR   must be regarded as a solved problem.  Note, however, that the   identity of a flow cannot to be determined without further   information being carried in the packet or gleaned from some aspect   of the packet payload.   In the case of a point-to-multipoint LSP, and in the absence of PHP,   the identity of the ingress LSR may also be inferred from the top   label.  However, it may not possible to adequately identify the flow   from the top label alone; thus, further information may need to be   carried in the packet or gleaned from some aspect of the packet   payload.  In designing any solution, it is desirable that a common   flow identification solution be used for both point-to-point and   point-to-multipoint LSP types.  Similarly, it is desirable that a   common method of LSP group identification be used.  In the above   cases, a context label [RFC5331] needs to be used to provide the   required identity information.  This is a widely supported MPLS   feature.   A more interesting case is the case of a multipoint-to-point LSP.  In   this case, the same label is normally used by multiple ingress or   upstream LSRs; hence, source identification is not possible by   inspection of the top label by the egress LSRs.  It is therefore   necessary for a packet to be able to explicitly convey any of the   identity types described inSection 4.   Similarly, in the case of a multipoint-to-multipoint LSP, the same   label is normally used by multiple ingress or upstream LSRs; hence,   source identification is not possible by inspection of the top label   by egress LSRs.  The various identity types described inSection 4   are again needed.  Note, however, that the scope of the identity may   be constrained to be unique within the set of multipoint-to-   multipoint LSPs terminating on any common node.Bryant, et al.                Informational                     [Page 6]

RFC 8372                MPLS Flow Identification                May 20186.  Network Scope   The scope of identification can be constrained to the set of flows   that are uniquely identifiable at an ingress LSR or some aggregation   thereof.  There is no need for an ingress LSR to seek assistance from   outside the MPLS protocol domain.   In any solution that constrains itself to carrying the required   identity in the MPLS label stack rather than in some different   associated data structure, constraints on the choice of label and   label stack size imply that the scope of identity resides within that   MPLS domain.  For similar reasons, the identity scope of a component   of an LSP is constrained to the scope of that LSP.7.  Backwards Compatibility   In any network, it is unlikely that all LSRs will have the same   capability to support the methods of identification discussed in this   document.  It is therefore an important constraint on any flow   identity solution that it is backwards compatible with deployed MPLS   equipment to the extent that deploying the new feature will not   disable anything that currently works on the legacy equipment.   This is particularly the case when the deployment is incremental or   the feature is not required for all LSRs or all LSPs.  Thus, the flow   identification design must support the coexistence of LSRs that can   identify the traffic components described inSection 4 and those that   cannot.  In addition, the identification of the traffic components   described inSection 4 must be an optional feature that is disabled   by default.  As a design simplification, a solution may require that   all egress LSRs of a point-to-multipoint or a multipoint-to-   multipoint LSP support the identification type in use so that a   single packet can be correctly processed by all egress devices.  The   corollary of this last point is that either all egress LSRs are   enabled to support the required identity type or none of them are.8.  Data Plane   There is a huge installed base of MPLS equipment; typically, this   type of equipment remains in service for an extended period of time,   and in many cases, hardware constraints mean that it is not possible   to upgrade its data-plane functionality.  Changes to the MPLS data   plane are therefore expensive to implement, add complexity to the   network, and may significantly impact the deployability of a solution   that requires such changes.  For these reasons, MPLS users have set a   very high bar to changes to the MPLS data plane, and only a very   small number have been adopted.  Hence, it is important that the   method of identification must minimize changes to the MPLS dataBryant, et al.                Informational                     [Page 7]

RFC 8372                MPLS Flow Identification                May 2018   plane.  Ideally, method(s) of identification that require no changes   to the MPLS data plane should be given preferential consideration.   If a method of identification that makes a change to the data plane   is chosen, it will need to have a significant advantage over any   method that makes no change, and the advantage of the approach will   need to be carefully evaluated and documented.  If a change to the   MPLS data plane proves necessary, it should be (a) as small a change   as possible and (b) a general-purpose method, so as to maximize its   use for future applications.  It is imperative that, as far as can be   foreseen, any necessary change made to the MPLS data plane does not   impose any foreseeable future limitation on the MPLS data plane.   Stack size is an issue with many MPLS implementations both as a   result of hardware limitations and due to the impact on networks and   applications in which a large number of small payloads need to be   transported.  In particular, one MPLS payload may be carried inside   another.  For example, one LSP may be carried over another LSP, or a   Pseudowire (PW) or similar multiplexing construct may be carried over   an LSP, and identification may be required at both layers.  Of   particular concern is the implementation of low-cost edge LSRs that,   for cost reasons, have a significant limit on the number of Label   Stack Entries (LSEs) that they can impose or dispose.  Therefore, any   method of identity must not consume an excessive number of unique   labels and must not result in an excessive increase in the size of   the label stack.   The design of the MPLS data plane provides two types of special-   purpose labels: the original 16 reserved labels and the much larger   set of special-purpose labels defined in [RFC7274].  The original   reserved labels need one LSE, and the newer special-purpose labels   [RFC7274] need two LSEs.  Given the tiny number of original reserved   labels, it is core to the MPLS design philosophy that this scarce   resource is only used when it is absolutely necessary.  Using a   special-purpose label to encode flow identity requires two label   stack entries, one for the reserved label and one for the flow   identity.  Use of extended special-purpose labels [RFC7274] requires   a total of three label stack entries to encode the flow identity.   The larger set of [RFC7274] labels requires two label stack entries   for the special-purpose label itself; hence, a total of three label   stack entries is needed to encode the flow identity.   The use of special-purpose labels [RFC7274] as part of a method to   encode the identity information therefore has a number of undesirable   implications for the data plane.  Thus, while a solution may use   special-purpose labels, methods that do not require special-purpose   labels need to be carefully considered.Bryant, et al.                Informational                     [Page 8]

RFC 8372                MPLS Flow Identification                May 20189.  Control Plane   Any flow identity design should both seek to minimize the complexity   of the control plane and minimize the amount of label coordination   needed amongst LSRs.10.  Privacy Considerations   The inclusion of originating and/or flow information in a packet   provides more identity information and hence potentially degrades the   privacy of the communication.   Recent IETF concerns on pervasive monitoring [RFC7258] have resulted   in a preference for a solution that does not degrade the privacy of   user traffic below that of an MPLS network not implementing the flow   identification feature.  The choice of using MPLS technology for this   OAM solution has a privacy advantage, as the choice of the label   identifying a flow is limited to the scope of the MPLS domain and   does not have any dependency on the identification of the user data.   This minimizes the observability of the flow characteristics.11.  Security Considerations   Any flow identification solution must not degrade the security of the   MPLS network below that of an equivalent network not deploying the   specified identity solution.  In order to preserve present   assumptions about MPLS privacy properties, propagation of   identification information outside the MPLS network imposing it must   be disabled by default.  Any solution should provide for the   restriction of the identity information to those components of the   network that need to know it.  It is thus desirable to limit the   knowledge of the identify of an endpoint to only those LSRs that need   to participate in traffic flow.  The choice of using MPLS technology   for this OAM solution, with MPLS encapsulation of user traffic,   provides for a key advantage over other data-plane solutions, as it   provides for a controlled access and trusted domain within a service   provider's network.   For a more comprehensive discussion of MPLS security and attack   mitigation techniques, please see "Security Framework for MPLS and   GMPLS Networks" [RFC5920].12.  IANA Considerations   This document has no IANA considerations.Bryant, et al.                Informational                     [Page 9]

RFC 8372                MPLS Flow Identification                May 201813.  Informative References   [RFC5331]  Aggarwal, R., Rekhter, Y., and E. Rosen, "MPLS Upstream              Label Assignment and Context-Specific Label Space",RFC 5331, DOI 10.17487/RFC5331, August 2008,              <https://www.rfc-editor.org/info/rfc5331>.   [RFC5420]  Farrel, A., Ed., Papadimitriou, D., Vasseur, JP., and A.              Ayyangarps, "Encoding of Attributes for MPLS LSP              Establishment Using Resource Reservation Protocol Traffic              Engineering (RSVP-TE)",RFC 5420, DOI 10.17487/RFC5420,              February 2009, <https://www.rfc-editor.org/info/rfc5420>.   [RFC5654]  Niven-Jenkins, B., Ed., Brungard, D., Ed., Betts, M., Ed.,              Sprecher, N., and S. Ueno, "Requirements of an MPLS              Transport Profile",RFC 5654, DOI 10.17487/RFC5654,              September 2009, <https://www.rfc-editor.org/info/rfc5654>.   [RFC5920]  Fang, L., Ed., "Security Framework for MPLS and GMPLS              Networks",RFC 5920, DOI 10.17487/RFC5920, July 2010,              <https://www.rfc-editor.org/info/rfc5920>.   [RFC6374]  Frost, D. and S. Bryant, "Packet Loss and Delay              Measurement for MPLS Networks",RFC 6374,              DOI 10.17487/RFC6374, September 2011,              <https://www.rfc-editor.org/info/rfc6374>.   [RFC6790]  Kompella, K., Drake, J., Amante, S., Henderickx, W., and              L. Yong, "The Use of Entropy Labels in MPLS Forwarding",RFC 6790, DOI 10.17487/RFC6790, November 2012,              <https://www.rfc-editor.org/info/rfc6790>.   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an              Attack",BCP 188,RFC 7258, DOI 10.17487/RFC7258, May              2014, <https://www.rfc-editor.org/info/rfc7258>.   [RFC7274]  Kompella, K., Andersson, L., and A. Farrel, "Allocating              and Retiring Special-Purpose MPLS Labels",RFC 7274,              DOI 10.17487/RFC7274, June 2014,              <https://www.rfc-editor.org/info/rfc7274>.   [RFC8321]  Fioccola, G., Ed., Capello, A., Cociglio, M., Castaldelli,              L., Chen, M., Zheng, L., Mirsky, G., and T. Mizrahi,              "Alternate-Marking Method for Passive and Hybrid              Performance Monitoring",RFC 8321, DOI 10.17487/RFC8321,              January 2018, <https://www.rfc-editor.org/info/rfc8321>.Bryant, et al.                Informational                    [Page 10]

RFC 8372                MPLS Flow Identification                May 2018Acknowledgments   The authors thank Nobo Akiya, Nagendra Kumar Nainar, George Swallow,   and Deborah Brungard for their comments.Authors' Addresses   Stewart Bryant   Huawei   Email: stewart.bryant@gmail.com   Carlos Pignataro   Cisco Systems, Inc.   Email: cpignata@cisco.com   Mach(Guoyi) Chen   Huawei   Email: mach.chen@huawei.com   Zhenbin Li   Huawei   Email: lizhenbin@huawei.com   Gregory Mirsky   ZTE Corp.   Email: gregimirsky@gmail.comBryant, et al.                Informational                    [Page 11]