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Performance-Based Path Selection for Explicitly Routed Label Switched Paths (LSPs) Using TE Metric Extensions
RFC 7823

Document Type RFC - Informational (May 2016)
Authors Alia Atlas , John Drake , Spencer Giacalone , Stefano Previdi
Last updated 2016-05-09
RFC stream Internet Engineering Task Force (IETF)
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IESG Responsible AD Deborah Brungard
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RFC 7823
Internet Engineering Task Force (IETF)                          A. Atlas
Request for Comments: 7823                                      J. Drake
Category: Informational                                 Juniper Networks
ISSN: 2070-1721                                             S. Giacalone
                                                              S. Previdi
                                                           Cisco Systems
                                                                May 2016

                  Performance-Based Path Selection for
Explicitly Routed Label Switched Paths (LSPs) Using TE Metric Extensions


   In certain networks, it is critical to consider network performance
   criteria when selecting the path for an explicitly routed RSVP-TE
   Label Switched Path (LSP).  Such performance criteria can include
   latency, jitter, and loss or other indications such as the
   conformance to link performance objectives and non-RSVP TE traffic
   load.  This specification describes how a path computation function
   may use network performance data, such as is advertised via the OSPF
   and IS-IS TE metric extensions (defined outside the scope of this
   document) to perform such path selections.

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 a candidate for any level of Internet
   Standard; see Section 2 of RFC 5741.

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

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Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( 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
     1.1.  Basic Requirements  . . . . . . . . . . . . . . . . . . .   4
     1.2.  Oscillation and Stability Considerations  . . . . . . . .   4
   2.  Using Performance Data Constraints  . . . . . . . . . . . . .   5
     2.1.  End-to-End Constraints  . . . . . . . . . . . . . . . . .   5
     2.2.  Link Constraints  . . . . . . . . . . . . . . . . . . . .   6
     2.3.  Links out of Compliance with Link Performance Objectives    6
       2.3.1.  Use of Anomalous Links for New Paths  . . . . . . . .   7
       2.3.2.  Links Entering the Anomalous State  . . . . . . . . .   7
       2.3.3.  Links Leaving the Anomalous State . . . . . . . . . .   8
   3.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   4.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Normative References  . . . . . . . . . . . . . . . . . .   8
     4.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .   9
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10

1.  Introduction

   In certain networks, such as financial information networks, network
   performance information is becoming as critical to data-path
   selection as other existing metrics.  Network performance information
   can be obtained via either the TE Metric Extensions in OSPF [RFC7471]
   or IS-IS [RFC7810] or via a management system.  As with other TE
   information flooded via OSPF or IS-IS, the TE metric extensions have
   a flooding scope limited to the local area or level.  This document
   describes how a path computation function, whether in an ingress LSR
   or a PCE [RFC4655], can use that information for path selection for
   explicitly routed LSPs.  The selected path may be signaled via RSVP-
   TE [RFC3209] [RFC3473] or simply used by the ingress with segment

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   routing [SEG-ROUTE-MPLS] to properly forward the packet.  Methods of
   optimizing path selection for multiple parameters are generally
   computationally complex.  However, there are good heuristics for the
   delay-constrained lowest-cost (DCLC) computation problem
   [k-Paths_DCLC] that can be applied to consider both path cost and a
   maximum delay bound.  Some of the network performance information can
   also be used to prune links from a topology before computing the

   The path selection mechanisms described in this document apply to
   paths that are fully computed by the head-end of the LSP and then
   signaled in an Explicit Route Object (ERO) where every sub-object is
   strict.  This allows the head-end to consider IGP-distributed
   performance data without requiring the ability to signal the
   performance constraints in an object of the RSVP Path message.

   When considering performance-based data, it is obvious that there are
   additional contributors to latency beyond just the links.  Clearly
   end-to-end latency is a combination of router latency (e.g., latency
   from traversing a router without queueing delay), queuing latency,
   physical link latency, and other factors.  While traversing a router
   can cause delay, that router latency can be included in the
   advertised link delay.  As described in [RFC7471] and [RFC7810],
   queuing delay must not be included in the measurements advertised by
   OSPF or IS-IS.

   Queuing latency is specifically excluded to insure freedom from
   oscillations and stability issues that have plagued prior attempts to
   use delay as a routing metric.  If application traffic follows a path
   based upon latency constraints, the same traffic might be in an
   Expedited Forwarding Per-Hop Behavior (PHB) [RFC3246] with minimal
   queuing delay or another PHB with potentially very substantial per-
   hop queuing delay.  Only traffic that experiences relatively low
   congestion, such as Expedited Forwarding traffic, will experience
   delays very close to the sum of the reported link delays.

   This document does not specify how a router determines what values to
   advertise by the IGP; it does assume that the constraints specified
   in [RFC7471] and [RFC7810] are followed.  Additionally, the end-to-
   end performance that is computed for an LSP path should be built from
   the individual link data.  Any end-to-end characterization used to
   determine an LSP's performance compliance should be fully reflected
   in the Traffic Engineering Database so that a path calculation can
   also determine whether a path under consideration would be in

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1.1.  Basic Requirements

   The following are the requirements considered for a path computation
   function that uses network performance criteria.

   1.  Select a TE tunnel's path based upon a combination of existing
       constraints as well as on link-latency, packet loss, jitter,
       conformance with link performance objectives, and bandwidth
       consumed by non-RSVP-TE traffic.

   2.  Ability to define different end-to-end performance requirements
       for each TE tunnel regardless of common use of resources.

   3.  Ability to periodically verify with the TE Link State Database
       (LSDB) that a TE tunnel's current LSP complies with its
       configured end-to-end performance requirements.

   4.  Ability to move tunnels, using make-before-break, based upon
       computed end-to-end performance complying with constraints.

   5.  Ability to move tunnels away from any link that is violating an
       underlying link performance objective.

   6.  Ability to optionally avoid setting up tunnels using any link
       that is violating a link performance objective, regardless of
       whether end-to-end performance would still meet requirements.

   7.  Ability to revert back, using make-before-break, to the best path
       after a configurable period.

1.2.  Oscillation and Stability Considerations

   Past attempts to use unbounded delay or loss as a metric suffered
   from severe oscillations.  The use of performance based data must be
   such that undamped oscillations are not possible and stability cannot
   be impacted.

   The use of timers is often cited as a cure.  Oscillation that is
   damped by timers is known as "slosh".  If advertisement timers are
   very short relative to the jitter applied to RSVP-TE Constrained
   Shortest Path First (CSPF) timers, then a partial oscillation occurs.
   If RSVP-TE CSPF timers are short relative to advertisement timers,
   full oscillation (all traffic moving back and forth) can occur.  Even
   a partial oscillation causes unnecessary reordering that is
   considered at least minimally disruptive.

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   Delay variation or jitter is affected by even small traffic levels.
   At even tiny traffic levels, the probability of a queue occupancy of
   one can produce a measured jitter proportional to or equal to the
   packet serialization delay.  Very low levels of traffic can increase
   the probability of queue occupancies of two or three packets enough
   to further increase the measured jitter.  Because jitter measurement
   is extremely sensitive to very low traffic levels, any use of jitter
   is likely to oscillate.  However, there may be uses of a jitter
   measurement in path computation that can be considered free of

   Delay measurements that are not sensitive to traffic loads may be
   safely used in path computation.  Delay measurements made at the link
   layer or measurements made at a queuing priority higher than any
   significant traffic (such as Differentiated Services Code Point
   (DSCP) CS7 or CS6 [RFC4594], but not CS2 if traffic levels at CS3 and
   higher or Expedited Forwarding and Assured Forwarding can affect the
   measurement).  Making delay measurements at the same priority as the
   traffic on affected paths is likely to cause oscillations.

2.  Using Performance Data Constraints

2.1.  End-to-End Constraints

   The per-link performance data available in the IGP [RFC7471]
   [RFC7810] includes: unidirectional link delay, unidirectional delay
   variation, and link loss.  Each (or all) of these parameters can be
   used to create the path-level link-based parameter.

   It is possible to compute a CSPF where the link latency values are
   used instead of TE metrics; this results in ignoring the TE metrics
   and causing LSPs to prefer the lowest-latency paths.  In practical
   scenarios, latency constraints are typically a bound constraint
   rather than a minimization objective.  An end-to-end latency upper
   bound merely requires that the path computed be no more than that
   bound and does not require that it be the minimum latency path.  The
   latter is exactly the DCLC problem to which good heuristics have been
   proposed in the literature (e.g., [k-Paths_DCLC]).

   An end-to-end bound on delay variation can be used similarly as a
   constraint in the path computation on what links to explore where the
   path's delay variation is the sum of the used links' delay

   For link loss, the path loss is not the sum of the used links'
   losses.  Instead, the path loss fraction is 1 - (1 - loss_L1)*
   (1 - loss_L2)*...*(1 - loss_Ln), where the links along the path are
   L1 to Ln with loss_Li in fractions.  This computation is discussed in

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   more detail in Sections 5.1.4 and 5.1.5 in [RFC6049].  The end-to-end
   link loss bound, computed in this fashion, can also be used as a
   constraint in the path computation.

   The heuristic algorithms for DCLC only address one constraint bound
   but having a CSPF that limits the paths explored (i.e., based on hop
   count) can be combined [hop-count_DCLC].

2.2.  Link Constraints

   In addition to selecting paths that conform to a bound on performance
   data, it is also useful to avoid using links that do not meet a
   necessary constraint.  Naturally, if such a parameter were a known
   fixed value, then resource attribute flags could be used to express
   this behavior.  However, when the parameter associated with a link
   may vary dynamically, there is not currently a configuration-time
   mechanism to enforce such behavior.  An example of this is described
   in Section 2.3, where links may move in and out of conformance for
   link performance objectives with regards to latency, delay variation,
   and link loss.

   When doing path selection for TE tunnels, it has not been possible to
   know how much actual bandwidth is available that includes the
   bandwidth used by non-RSVP-TE traffic.  In [RFC7471] and [RFC7810],
   the Unidirectional Available Bandwidth is advertised as is the
   Residual Bandwidth.  When computing the path for a TE tunnel, only
   links with at least a minimum amount of Unidirectional Available
   Bandwidth might be permitted.

   Similarly, only links whose loss is under a configurable value might
   be acceptable.  For these constraints, each link can be tested
   against the constraint and only explored in the path computation if
   the link passes.  In essence, a link that fails the constraint test
   is treated as if it contained a resource attribute in the exclude-any

2.3.  Links out of Compliance with Link Performance Objectives

   Link conformance to a link performance objective can change as a
   result of rerouting at lower layers.  This could be due to optical
   regrooming or simply rerouting of an FA-LSP.  When this occurs, there
   are two questions to be asked:

   a.  Should the link be trusted and used for the setup of new LSPs?

   b.  Should LSPs using this link automatically be moved to a secondary

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2.3.1.  Use of Anomalous Links for New Paths

   If the answer to (a) is no for link latency performance objectives,
   then any link that has the Anomalous bit set in the Unidirectional
   Link Delay sub-TLV [RFC7471] [RFC7810] should be removed from the
   topology before a path calculation is used to compute a new path.  In
   essence, the link should be treated exactly as if it fails the
   exclude-any resource attributes filter [RFC3209].

   Similarly, if the answer to (a) is no for link loss performance
   objectives, then any link that has the Anomalous bit set in the Link
   Loss sub-TLV should be treated as if it fails the exclude-any
   resource attributes filter.

2.3.2.  Links Entering the Anomalous State

   When the Anomalous bit transitions from clear to set, this indicates
   that the associated link has entered the Anomalous state with respect
   to the associated parameter; similarly, a transition from set to
   clear indicates that the Anomalous state has been exited for that
   link and associated parameter.

   When a link enters the Anomalous state with respect to a parameter,
   this is an indication that LSPs using that link might also no longer
   be in compliance with their performance bounds.  It can also be
   considered an indication that something is changing that link and so
   it might no longer be trustworthy to carry performance-critical
   traffic.  Naturally, which performance criteria are important for a
   particular LSP is dependent upon the LSP's configuration; thus, the
   compliance of a link with respect to a particular link performance
   objective is indicated per performance criterion.

   At the ingress of a TE tunnel, a TE tunnel may be configured to be
   sensitive to the Anomalous state of links in reference to latency,
   delay variation, and/or loss.  Additionally, such a TE tunnel may be
   configured to either verify continued compliance, to switch
   immediately to a standby LSP, or to move to a different path.

   When a sub-TLV is received with the Anomalous bit set when previously
   it was clear, the list of interested TE tunnels must be scanned.
   Each such TE tunnel should have its continued compliance verified, be
   switched to a hot standby, or do a make-before-break to a secondary

   It is not sufficient to just look at the Anomalous bit in order to
   determine when TE tunnels must have their compliance verified.  When
   changing to set, the Anomalous bit merely provides a hint that

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   interested TE tunnels should have their continued compliance

2.3.3.  Links Leaving the Anomalous State

   When a link leaves the Anomalous state with respect to a parameter,
   this can serve as an indication that those TE tunnels, whose LSPs
   were changed due to administrative policy when the link entered the
   Anomalous state, may want to reoptimize to a better path.  The hint
   provided by the Anomalous state change may help optimize when to
   recompute for a better path.

3.  Security Considerations

   This document is not currently believed to introduce new security

4.  References

4.1.  Normative References

   [RFC3209]  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
              and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,

   [RFC7471]  Giacalone, S., Ward, D., Drake, J., Atlas, A., and S.
              Previdi, "OSPF Traffic Engineering (TE) Metric
              Extensions", RFC 7471, DOI 10.17487/RFC7471, March 2015,

   [RFC7810]  Previdi, S., Ed., Giacalone, S., Ward, D., Drake, J., and
              Q. Wu, "IS-IS Traffic Engineering (TE) Metric Extensions",
              RFC 7810, DOI 10.17487/7810, May 2016,

4.2.  Informative References

              Agrawal, H., Grah, M., and M. Gregory, "Optimization of
              QoS Routing", 6th IEEE/AACIS International Conference on
              Computer and Information Science,
              DOI 10.1109/ICIS.2007.144, July 2007,

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RFC 7823        Path Selection with TE Metric Extensions        May 2016

              Jia, Z. and P. Varaiya, "Heuristic methods for delay
              constrained least cost routing using k-shortest-paths",
              IEEE Transactions on Automatic Control, vol. 51, no. 4,
              April 2006, <>.

   [RFC3246]  Davie, B., Charny, A., Bennet, J., Benson, K., Le Boudec,
              J., Courtney, W., Davari, S., Firoiu, V., and D.
              Stiliadis, "An Expedited Forwarding PHB (Per-Hop
              Behavior)", RFC 3246, DOI 10.17487/RFC3246, March 2002,

   [RFC3473]  Berger, L., Ed., "Generalized Multi-Protocol Label
              Switching (GMPLS) Signaling Resource ReserVation Protocol-
              Traffic Engineering (RSVP-TE) Extensions", RFC 3473,
              DOI 10.17487/RFC3473, January 2003,

   [RFC4594]  Babiarz, J., Chan, K., and F. Baker, "Configuration
              Guidelines for DiffServ Service Classes", RFC 4594,
              DOI 10.17487/RFC4594, August 2006,

   [RFC4655]  Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
              Element (PCE)-Based Architecture", RFC 4655,
              DOI 10.17487/RFC4655, August 2006,

   [RFC6049]  Morton, A. and E. Stephan, "Spatial Composition of
              Metrics", RFC 6049, DOI 10.17487/RFC6049, January 2011,

              Filsfils, C., Ed., Previdi, S., Ed., Bashandy, A.,
              Decraene, B., Litkowski, S., Horneffer, M., Shakir, R.,
              Tantsura, J., and E. Crabbe, "Segment Routing with MPLS
              data plane", Work in Progress, draft-ietf-spring-segment-
              routing-mpls-04, March 2016.


   The authors would like to thank Curtis Villamizar for his extensive
   detailed comments and suggested text in Sections 1 and 1.2.  The
   authors would like to thank Dhruv Dhody for his useful comments and
   his care and persistence in making sure that these important
   corrections weren't missed.  The authors would also like to thank
   Xiaohu Xu and Sriganesh Kini for their reviews.

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   Dave Ward and Clarence Filsfils contributed to this document.

Authors' Addresses

   Alia Atlas
   Juniper Networks
   10 Technology Park Drive
   Westford, MA  01886
   United States


   John Drake
   Juniper Networks
   1194 N. Mathilda Ave.
   Sunnyvale, CA  94089
   United States


   Spencer Giacalone


   Stefano Previdi
   Cisco Systems
   Via Del Serafico 200
   Rome  00142


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