MPLS                                                           S. Bryant
Internet-Draft                                              C. Pignataro
Intended status: Informational                             Cisco Systems
Expires: April 27, 2015                                 October 24, 2014

                        MPLS Flow Identification


   This memo discusses the desired capabilities for MPLS flow
   identification.  The key application that needs this is in-band
   performance monitoring of user data packets.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on April 27, 2015.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Loss Measurement Considerations . . . . . . . . . . . . . . .   3
   3.  Units of identification . . . . . . . . . . . . . . . . . . .   3
   4.  Types of LSP  . . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Network Scope . . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Backwards Compatibility . . . . . . . . . . . . . . . . . . .   6
   7.  Dataplane . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   8.  Control Plane . . . . . . . . . . . . . . . . . . . . . . . .   7
   9.  Manageability Considerations  . . . . . . . . . . . . . . . .   8
   10. Privacy Considerations  . . . . . . . . . . . . . . . . . . .   8
   11. Security Considerations . . . . . . . . . . . . . . . . . . .   8
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   8
   14. References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     14.1.  Normative References . . . . . . . . . . . . . . . . . .   8
     14.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

1.  Introduction

   This memo discusses the desired capabilities for MPLS flow
   identification.  The key application that needs this is in-band
   performance monitoring of user data packets.

   There is a need to identify flows in MPLS networks for applications
   such as packet loss and packet delay measurement.  A method of loss
   and delay measurement in MPLS networks was defined in [RFC6374].
   However this work needs to be extended to deal with different
   granularities of flow and to address a number of the multi-point
   cases in which a number of ingress LSRs could send to one or more

   Improvements in link and transmission technologies mean that it is
   difficult to assess a loss using synthetic traffic due to the very
   low loss rate in normal operation.  That together with more demanding
   service level requirements mean that network operators need to be
   able to measure the loss of the actual user data traffic.  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 identification of measurement process leak into their network.

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2.  Loss Measurement Considerations

   Modern networks normally drop very few packets, thus packet loss
   measurement are highly sensitive to counter errors.  Without some
   form of coloring or batch marking such as that proposed in
   [I-D.tempia-opsawg-p3m] it may not be possible to achieve the
   required accuracy in the loss measurement of customer data traffic.
   Where accuracy better than the data link loss performance of a modern
   optical network is required it may be economically advantage to
   include temporal marking.

   Where this level of accuracy is required and the traffic between a
   source-destination pair is subject to ECMP a demarcation mechanism is
   needed to group the packets into batches.  The packet accounting
   mechanism is then able to operate on a batch of packets which can be
   accounted for at both the packet ingress and the packet egress.
   Errors in the accounting are particularly acute in LSPs subjected to
   ECMP because the network transit time will be different for the
   various ECMP paths since:

   a.  The packets may traverse different sets of LSRs.

   b.  The packets may depart from different interfaces on different
       line cards on LSRs

   c.  The packets may arrive at different interfaces on different line
       cards on LSRs.

   A consideration in modifying the identity label 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 colour represented by a change of identity
   label will have no impact on the ECMP path.  Where the path member is
   chosen by reference to an entropy label [RFC6790] then provided that
   the entropy label is higher in the stack than the label that is
   changing colour again there will be no change to the chosen ECMP
   path.  ECMP is so pervasive in multi-point to (multi-) point networks
   that some method of avoiding accounting errors introduced by ECMP
   needs to be supported.

3.  Units of identification

   The most basic unit of identification is the identity of the node
   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 mat be a need to impose
   identify at several different layers of the MPLS label stack.

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   This document considers follow unit of identifications:

   o  Per source LSR - everything from one source is aggregated.

   o  Per group of LSPs chosen by an ingress LSR - an ingress LSP
      aggregates group of LSPs (ex: all LSPs of a tunnel).

   o  Per LSP - the basic form.

   o  Per flow [RFC6790] within an LSP - fine graining method.

   Note that a finer 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 fine grained
   resolution may be possible by deep packet inspection, but this may
   not always be possible, or it may be desired to minimise processing
   costs by doing only in entry to the network, and adding a suitable
   identifier to the packet for reference by other network elements.  An
   example of such a fine grained case might be traffic from a specific
   application, or from a specific application 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 in Section 5.

   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
      operate 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 a 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 method of determining which packets constitute a flow is outside
   the scope of this memo.

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4.  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 multi-point LSPs.
   The ingress LSR for a point to point LSP, such as those created using
   the RSVP-TE signalling protocol, or those that conform to the MPLS-TP
   may be identified by inspection of the top label in the stack, since
   at any PE or P router on the path this is unique to the ingress-
   egress pair at every hop at a given layer in the LSP hierarchy.
   Provided that penultimate hop popping is disabled, the identity of
   the ingress LSR of a point to point LSP is available at the egress
   LSR and 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.

   In the case of a point to multi-point LSP the identity of the ingress
   LSR may also be inferred from the top label.  However it is not
   possible to identify a flow from the top label, nor is it possible to
   directly identify the ingress LSR since there may be many point to
   multi-point LSP originating at that LSR.  In designing any solution
   it is desirable that a common flow identity solution be used for both
   point to point and point to multi-point LSP types.  Similarly it is
   desirable that a common method of LSP group identification be used.

   In the above cases, an explicit non-null label is needed to provide
   context at the egress LSR.  This is widely supported MPLS feature.

   A more interesting case, and the core purpose of this memo, is the
   case of a multi-point to point LSP.  In this case the same label is
   normally used by multiple ingress or upstream LSRs and hence source
   identification is not possible by inspection of the top label by
   egress LSRs.  It is therefore necessary for a packet to be able to
   explicitly convey any of the identity types described in Section 3.

   Similarly, in the case of a multi-point to multi-point LSP the same
   label is normally used by multiple ingress or upstream LSRs and hence
   source identification is not possible by inspection of the top label
   by egress LSRs.  The various types of identity described in Section 3
   are again needed.  Note however, that the scope of the identity may
   be constrained to be unique within the set of multi-point to multi-
   point LSPs terminating on any common node.

   Any method of identity must not consume an excessive number of unique
   labels, nor result in an excessive increase in the size of the label
   stack (Section 7).

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5.  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 question of an ingress LSR seeking assistance
   from outside the MPLS 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 label stack size imply
   that the scope of identity reside within that MPLS domain.  For
   similar reasons the identity scope of a component of an LSP should be
   constrained to the scope of that LSP.

6.  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
   memo.  It is therefore an important constraint on any 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 a legacy equipment.

   This is particularly the case when the deployment is incremental or
   when the feature is not required for all LSRs or all LSPs.  Thus in
   broad the flow identification design must support the co-existence of
   LSRs that can and cannot identify the traffic components described in
   . (Section 3).  In addition the identification of the traffic
   components described in Section 3 needs to 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 multi-
   point to multipoint LSP to 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.

7.  Dataplane

   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 dataplane 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, the MPLS designers 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

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   method of identification must minimize changes to the MPLS data
   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 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 is necessary to
   the MPLS data plane proves necessary, it should be (a) be as small a
   change as possible and (b) be a general purpose method so as to
   maximise 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 where 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
   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 LSEs that they can impose
   or dispose.

   The MPLS data plane design provides only a tiny number of reserved
   labels, it is therefore core to the MPLS design philosophy that this
   scarce resource is only used when it is absolutely necessary.  Using
   a single LSE reserved or special purpose label to encode flow
   identity thus requires two stack entries.  A larger special purpose
   labels space is available [RFC7274] but this requires two labels
   stack entries for the reserved label itself and hence a total of
   three label stack entries to encode the flow identity.

   The use of special purpose labels (SPL) [RFC7274]as part of a method
   to encode the identity information therefore has a number of
   undesirable implications for the data plane and hence whilst a
   solution may use SPL(s), methods that do not require SPLs need to be
   carefully considered.

8.  Control Plane

   Any flow identity design should both seek to minimise the complexity
   of the control plane and should minimise the amount of label co-
   ordination needed amongst LSRs.

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9.  Manageability Considerations

   This will be provided in a future version of this document.

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 would lead it to prefer a solution that does not degrade
   the privacy of user traffic below that of an MPLS network not
   implementing the flow identification feature.  The minimizing the
   scope of the identity indication can be useful in minimizing the
   observability of the flow characteristics.

11.  Security Considerations

   Any solution to the flow identification needs must not degrade the
   security of the MPLS network below that of an equivalent network not
   deploying the specified identity solution.  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.

12.  IANA Considerations

   EDITOR'S NOTE: This section may be removed on publication

   This memo has no IANA considerations.

13.  Acknowledgements

   The authors thank Nobo Akiya (, Nagendra Kumar Nainar
   ( and George Swallow ( for their

14.  References

14.1.  Normative References

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

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14.2.  Informative References

              Capello, A., Cociglio, M., Castaldelli, L., and A. Bonda,
              "A packet based method for passive performance
              monitoring", draft-tempia-opsawg-p3m-04 (work in
              progress), February 2014.

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

   [RFC6790]  Kompella, K., Drake, J., Amante, S., Henderickx, W., and
              L. Yong, "The Use of Entropy Labels in MPLS Forwarding",
              RFC 6790, November 2012.

   [RFC7274]  Kompella, K., Andersson, L., and A. Farrel, "Allocating
              and Retiring Special-Purpose MPLS Labels", RFC 7274, June

Authors' Addresses

   Stewart Bryant
   Cisco Systems


   Carlos Pignataro
   Cisco Systems


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