RFC 8957

Internet Engineering Task Force (IETF)                         S. Bryant
Request for Comments: 8957                   Futurewei Technologies Inc.
Category: Standards Track                                        M. Chen
ISSN: 2070-1721                                                   Huawei
                                                              G. Swallow
                                               Southend Technical Center
                                                            S. Sivabalan
                                                       Ciena Corporation
                                                               G. Mirsky
                                                               ZTE Corp.
                                                            January 2021

                    Synonymous Flow Label Framework


   RFC 8372 ("MPLS Flow Identification Considerations") describes the
   requirement for introducing flow identities within the MPLS
   architecture.  This document describes a method of accomplishing this
   by using a technique called "Synonymous Flow Labels" in which labels
   that mimic the behavior of other labels provide the identification
   service.  These identifiers can be used to trigger per-flow
   operations on the packet at the receiving label switching router.

Status of This Memo

   This is an Internet Standards Track document.

   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).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction
   2.  Requirements Language
   3.  Synonymous Flow Labels
   4.  User Service Traffic in the Data Plane
     4.1.  Application Label Present
       4.1.1.  Setting TTL and the Traffic Class Bits
     4.2.  Single-Label Stack
       4.2.1.  Setting TTL and the Traffic Class Bits
     4.3.  Aggregation of SFL Actions
   5.  Equal-Cost Multipath Considerations
   6.  Privacy Considerations
   7.  Security Considerations
   8.  IANA Considerations
   9.  References
     9.1.  Normative References
     9.2.  Informative References

   Authors' Addresses

1.  Introduction

   [RFC8372] ("MPLS Flow Identification Considerations") describes the
   requirement for introducing flow identities within the MPLS
   architecture.  This document describes a method of providing the
   required identification by using a technique called "Synonymous Flow
   Labels (SFLs)" in which labels that mimic the behavior of other MPLS
   labels provide the identification service.  These identifiers can be
   used to trigger per-flow operations on the packet at the receiving
   label switching router.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Synonymous Flow Labels

   An SFL is defined to be a label that causes exactly the same behavior
   at the egress Label Edge Router (LER) as the label it replaces,
   except that it also causes one or more additional actions that have
   been previously agreed between the peer LERs to be executed on the
   packet.  There are many possible additional actions, such as
   measuring the number of received packets in a flow, triggering an IP
   Flow Information Export (IPFIX) [RFC7011] capture, triggering other
   types of deep packet inspection, or identifying the packet source.
   For example, in a Performance Monitoring (PM) application, the agreed
   action could be recording the receipt of the packet by incrementing a
   packet counter.  This is a natural action in many MPLS
   implementations, and where supported, this permits the implementation
   of high-quality packet loss measurement without any change to the
   packet-forwarding system.

   To illustrate the use of this technology, we start by considering the
   case where there is an "application" label in the MPLS label stack.
   As a first example, let us consider a pseudowire (PW) [RFC3985] on
   which it is desired to make packet loss measurements.  Two labels,
   synonymous with the PW labels, are obtained from the egress
   terminating provider edge (T-PE).  By alternating between these SFLs
   and using them in place of the PW label, the PW packets may be
   batched for counting without any impact on the PW forwarding behavior
   [RFC8321] (note that strictly only one SFL is needed in this
   application, but that is an optimization that is a matter for the
   implementor).  The method of obtaining these additional labels is
   outside the scope of this text; however, one control protocol that
   provides a method of obtaining SFLs is described in

   Next, consider an MPLS application that is multipoint to point, such
   as a VPN.  Here, it is necessary to identify a packet batch from a
   specific source.  This is achieved by making the SFLs source
   specific, so that batches from one source are marked differently from
   batches from another source.  The sources all operate independently
   and asynchronously from each other, independently coordinating with
   the destination.  Each ingress LER is thus able to establish its own
   SFL to identify the subflow and thus enable PM per flow.

   Finally, we need to consider the case where there is no MPLS
   application label such as occurs when sending IP over a Label
   Switched Path (LSP), i.e., there is a single label in the MPLS label
   stack.  In this case, introducing an SFL that was synonymous with the
   LSP label would introduce network-wide forwarding state.  This would
   not be acceptable for scaling reasons.  Therefore, we have no choice
   but to introduce an additional label.  Where penultimate hop popping
   (PHP) is in use, the semantics of this additional label can be
   similar to the LSP label.  Where PHP is not in use, the semantics are
   similar to an MPLS Explicit NULL [RFC3032].  In both of these cases,
   the label has the additional semantics of the SFL.

   Note that to achieve the goals set out above, SFLs need to be
   allocated from the platform label table.

4.  User Service Traffic in the Data Plane

   As noted in Section 3, it is necessary to consider two cases:

   1.  Application label is present

   2.  Single-label stack

4.1.  Application Label Present

   Figure 1 shows the case in which both an LSP label and an application
   label are present in the MPLS label stack.  Traffic with no SFL
   function present runs over the "normal" stack, and SFL-enabled flows
   run over the SFL stack with the SFL used to indicate the packet

    +-----------------+          +-----------------+
    |      LSP        |          |      LSP        |
    |     Label       |          |     Label       |
    |  (May be PHPed) |          |  (May be PHPed) |
    +-----------------+          +-----------------+
    |                 |          |                 |
    |  Application    |          | Synonymous Flow |
    |     Label       |          |     Label       |
    +-----------------+ <= BoS   +-----------------+ <= Bottom of Stack
    |                 |          |                 |
    |   Payload       |          |   Payload       |
    |                 |          |                 |
    +-----------------+          +-----------------+

   "Normal" Label Stack         Label Stack with SFL

     Figure 1: Use of Synonymous Labels in a Two-Label MPLS Label Stack

   At the egress LER, the LSP label is popped (if present).  Then, the
   SFL is processed executing both the synonymous function and the
   corresponding application function.

4.1.1.  Setting TTL and the Traffic Class Bits

   The TTL and the Traffic Class bits [RFC5462] in the SFL label stack
   entry (LSE) would normally be set to the same value as would have
   been set in the label that the SFL is synonymous with.  However, it
   is recognized that, if there is an application need, these fields in
   the SFL LSE MAY be set to some other value.  An example would be
   where it was desired to cause the SFL to trigger an action in the TTL
   expiry exception path as part of the label action.

4.2.  Single-Label Stack

   Figure 2 shows the case in which only an LSP label is present in the
   MPLS label stack.  Traffic with no SFL function present runs over the
   "normal" stack, and SFL-enabled flows run over the SFL stack with the
   SFL used to indicate the packet batch.  However, in this case, it is
   necessary for the ingress Label Edge Router (LER) to first push the
   SFL and then to push the LSP label.

                                 |      LSP        |
                                 |     Label       |
                                 |  (May be PHPed) |
    +-----------------+          +-----------------+
    |      LSP        |          |                 | <= Synonymous with
    |     Label       |          | Synonymous Flow |    Explicit NULL
    |  (May be PHPed) |          |     Label       |
    +-----------------+ <= BoS   +-----------------+ <= Bottom of Stack
    |                 |          |                 |
    |   Payload       |          |   Payload       |
    |                 |          |                 |
    +-----------------+          +-----------------+

   "Normal" Label Stack         Label Stack with SFL

   Figure 2: Use of Synonymous Labels in a Single-Label MPLS Label Stack

   At the receiving Label Switching Router (LSR), it is necessary to
   consider two cases:

   1.  Where the LSP label is still present

   2.  Where the LSP label is penultimate hop popped

   If the LSP label is present, it is processed exactly as it would
   normally be processed, and then it is popped.  This reveals the SFL,
   which, in the case of the measurements defined in [RFC6374], is
   simply counted and then discarded.  In this respect, the processing
   of the SFL is synonymous with an MPLS Explicit NULL.  As the SFL is
   the bottom of stack, the IP packet that follows is processed as

   If the LSP label is not present due to PHP action in the upstream
   LSR, two almost equivalent processing actions can take place.  The
   SFL can be treated either 1) as an LSP label that was not PHPed and
   the additional associated SFL action is taken when the label is
   processed or 2) as an MPLS Explicit NULL with associated SFL actions.
   From the perspective of the measurement system described in this
   document, the behavior of the two approaches is indistinguishable;
   thus, either may be implemented.

4.2.1.  Setting TTL and the Traffic Class Bits

   The TTL and the Traffic Class considerations described in
   Section 4.1.1 apply.

4.3.  Aggregation of SFL Actions

   There are cases where it is desirable to aggregate an SFL action
   against a number of labels, for example, where it is desirable to
   have one counter record the number of packets received over a group
   of application labels or where the number of labels used by a single
   application is large and the resultant increase in the number of
   allocated labels needed to support the SFL actions may become too
   large to be viable.  In these circumstances, it would be necessary to
   introduce an additional label in the stack to act as an aggregate
   instruction.  This is not strictly a synonymous action in that the
   SFL is not replacing an existing label but is somewhat similar to the
   single-label case shown in Section 4.2, and the same signaling,
   management, and configuration tools would be applicable.

                                 |      LSP        |
                                 |     Label       |
                                 |  (May be PHPed) |
    +-----------------+          +-----------------+
    |      LSP        |          |                 |
    |     Label       |          |   Aggregate     |
    |  (May be PHPed) |          |      SFL        |
    +-----------------+          +-----------------+
    |                 |          |                 |
    |  Application    |          |  Application    |
    |     Label       |          |     Label       |
    +-----------------+ <=BoS    +-----------------+ <= Bottom of Stack
    |                 |          |                 |
    |   Payload       |          |   Payload       |
    |                 |          |                 |
    +-----------------+          +-----------------+

   "Normal" Label Stack         Label Stack with SFL

                      Figure 3: Aggregate SFL Actions

   The aggregate SFL is shown in the label stack depicted in Figure 3 as
   preceding the application label; however, the choice of position
   before or after the application label will be application specific.
   In the case described in Section 4.1, by definition, the SFL has the
   full application context.  In this case, the positioning will depend
   on whether the SFL action needs the full context of the application
   to perform its action and whether the complexity of the application
   will be increased by finding an SFL following the application label.

5.  Equal-Cost Multipath Considerations

   The introduction of an SFL to an existing flow may cause that flow to
   take a different path through the network under conditions of Equal-
   Cost Multipath (ECMP).  This, in turn, may invalidate certain uses of
   the SFL, such as performance measurement applications.  Where this is
   a problem, there are two solutions worthy of consideration:

   1.  The operator MAY elect to always run with the SFL in place in the
       MPLS label stack.

   2.  The operator can elect to use entropy labels [RFC6790] in a
       network that fully supports this type of ECMP.  If this approach
       is adopted, the intervening MPLS network MUST NOT load balance on
       any packet field other than the entropy label.  Note that this is
       stricter than the text in Section 4.3 of [RFC6790].

6.  Privacy Considerations

   IETF concerns on pervasive monitoring are described in [RFC7258].
   The inclusion of originating and/or flow information in a packet
   provides more identity information and hence potentially degrades the
   privacy of the communication to an attacker in a position to observe
   the added identifier.  Whilst the inclusion of the additional
   granularity does allow greater insight into the flow characteristics,
   it does not specifically identify which node originated the packet
   unless the attacker can inspect the network at the point of ingress
   or inspect the control protocol packets.  This privacy threat may be
   mitigated by encrypting the control protocol packets by regularly
   changing the synonymous labels or by concurrently using a number of
   such labels, including the use of a combination of those methods.
   Minimizing the scope of the identity indication can be useful in
   minimizing the observability of the flow characteristics.  Whenever
   IPFIX or other deep packet inspection (DPI) technique is used, their
   relevant privacy considerations apply.

7.  Security Considerations

   There are no new security issues associated with the MPLS data plane.
   Any control protocol used to request SFLs will need to ensure the
   legitimacy of the request, i.e., that the requesting node is
   authorized to make that SFL request by the network operator.

8.  IANA Considerations

   This document has no IANA actions.

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC3032]  Rosen, E., Tappan, D., Fedorkow, G., Rekhter, Y.,
              Farinacci, D., Li, T., and A. Conta, "MPLS Label Stack
              Encoding", RFC 3032, DOI 10.17487/RFC3032, January 2001,

   [RFC5462]  Andersson, L. and R. Asati, "Multiprotocol Label Switching
              (MPLS) Label Stack Entry: "EXP" Field Renamed to "Traffic
              Class" Field", RFC 5462, DOI 10.17487/RFC5462, February
              2009, <https://www.rfc-editor.org/info/rfc5462>.

   [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,

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

9.2.  Informative References

              Bryant, S., Swallow, G., and S. Sivabalan, "A Simple
              Control Protocol for MPLS SFLs", Work in Progress,
              Internet-Draft, draft-bryant-mpls-sfl-control-09, 7
              December 2020, <https://tools.ietf.org/html/draft-bryant-

   [RFC3985]  Bryant, S., Ed. and P. Pate, Ed., "Pseudo Wire Emulation
              Edge-to-Edge (PWE3) Architecture", RFC 3985,
              DOI 10.17487/RFC3985, March 2005,

   [RFC6374]  Frost, D. and S. Bryant, "Packet Loss and Delay
              Measurement for MPLS Networks", RFC 6374,
              DOI 10.17487/RFC6374, September 2011,

   [RFC7011]  Claise, B., Ed., Trammell, B., Ed., and P. Aitken,
              "Specification of the IP Flow Information Export (IPFIX)
              Protocol for the Exchange of Flow Information", STD 77,
              RFC 7011, DOI 10.17487/RFC7011, September 2013,

   [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>.

   [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>.

   [RFC8372]  Bryant, S., Pignataro, C., Chen, M., Li, Z., and G.
              Mirsky, "MPLS Flow Identification Considerations",
              RFC 8372, DOI 10.17487/RFC8372, May 2018,


   Zhenbin Li

   Email: lizhenbin@huawei.com

Authors' Addresses

   Stewart Bryant
   Futurewei Technologies Inc.

   Email: sb@stewartbryant.com

   Mach(Guoyi) Chen

   Email: mach.chen@huawei.com

   George Swallow
   Southend Technical Center

   Email: swallow.ietf@gmail.com

   Siva Sivabalan
   Ciena Corporation

   Email: ssivabal@ciena.com

   Gregory Mirsky
   ZTE Corp.

   Email: gregimirsky@gmail.com