



MPLS Working Group                                             S. Bryant
Internet-Draft                                Futurewei Technologies Inc
Intended status: Standards Track                                 M. Chen
Expires: April 5, 2021                                            Huawei
                                                              G. Swallow
                                               Southend Technical Center
                                                            S. Sivabalan
                                                       Ciena Corporation
                                                               G. Mirsky
                                                               ZTE Corp.
                                                        October 02, 2020


                    Synonymous Flow Label Framework
                    draft-ietf-mpls-sfl-framework-11

Abstract

   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
   which mimic the behaviour 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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
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   This Internet-Draft will expire on April 5, 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 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 Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   2
   3.  Synonymous Flow Labels  . . . . . . . . . . . . . . . . . . .   3
   4.  User Service Traffic in the Data Plane  . . . . . . . . . . .   4
     4.1.  Application Label Present . . . . . . . . . . . . . . . .   4
       4.1.1.  Setting TTL and the Traffic Class Bits  . . . . . . .   5
     4.2.  Single Label Stack  . . . . . . . . . . . . . . . . . . .   5
       4.2.1.  Setting TTL and the Traffic Class Bits  . . . . . . .   6
     4.3.  Aggregation of SFL Actions  . . . . . . . . . . . . . . .   6
   5.  Equal Cost Multipath Considerations . . . . . . . . . . . . .   7
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .   8
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   9.  Contributing Authors  . . . . . . . . . . . . . . . . . . . .   8
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     10.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

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 (SFL) in which labels which mimic the behaviour 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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP




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   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
   behaviour 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
   the measurement of 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 identification
   of the packet source.  In, for example, a Performance Monitoring (PM)
   application, the agreed action could be the recording of 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
   [I-D.bryant-mpls-sfl-control].

   Now consider an MPLS application that is multi-point 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 sub-flow 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 an 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



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   introduce network-wide forwarding state.  This would not be
   acceptable for scaling reasons.  We therefore 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
   batch.

    +-----------------+          +-----------------+
    |      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






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   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 Label Stack Entry (LSE) MAY be set these 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




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   2.  Where the LSP label is penultimate hop popped

   If the LSP label is present, it is processed exactly as it would
   normally processed and then it is popped.  This reveals the SFL,
   which, in the case of [RFC6374] measurements, 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 normal.

   If the LSP label is not present due to PHP action in the upstream
   LSR, two almost equivalent processing actions can take place.  Either
   the SFL can be treated as an LSP label that was not PHPed and the
   additional associated SFL action is taken when the label is
   processed.  Alternatively, it can be treated as an MPLS Explicit NULL
   with associated SFL actions.  From the perspective of the measurement
   system described in this document the behaviour of the two approaches
   is indistinguishable and 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 becomes 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 signalling,
   management and configuration tools would be applicable.














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                                 +-----------------+
                                 |      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 Multi-path (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 [RFC6790] Entropy Labels 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].



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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 inspection of 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 DPI technique is used, their relavent 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 draft makes no IANA requests.

9.  Contributing Authors

   Zhenbin Li
   Huawei
   Email: lizhenbin@huawei.com

10.  References

10.1.  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,
              <https://www.rfc-editor.org/info/rfc2119>.







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   [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,
              <https://www.rfc-editor.org/info/rfc3032>.

   [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,
              <https://www.rfc-editor.org/info/rfc6790>.

   [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>.

10.2.  Informative References

   [I-D.bryant-mpls-sfl-control]
              Bryant, S., Swallow, G., and S. Sivabalan, "A Simple
              Control Protocol for MPLS SFLs", draft-bryant-mpls-sfl-
              control-08 (work in progress), June 2020.

   [RFC3985]  Bryant, S., Ed. and P. Pate, Ed., "Pseudo Wire Emulation
              Edge-to-Edge (PWE3) Architecture", RFC 3985,
              DOI 10.17487/RFC3985, March 2005,
              <https://www.rfc-editor.org/info/rfc3985>.

   [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>.

   [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,
              <https://www.rfc-editor.org/info/rfc7011>.

   [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>.






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   [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,
              <https://www.rfc-editor.org/info/rfc8372>.

Authors' Addresses

   Stewart Bryant
   Futurewei Technologies Inc

   Email: sb@stewartbryant.com


   Mach Chen
   Huawei

   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










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