MPLS Working Group                                              A. Atlas
Internet-Draft                                                  J. Drake
Intended status: Informational                          Juniper Networks
Expires: August 10, 2013                                    S. Giacalone
                                                         Thomson Reuters
                                                                 D. Ward
                                                              S. Previdi
                                                             C. Filsfils
                                                           Cisco Systems
                                                        February 6, 2013

      Performance-based Path Selection for Explicitly Routed LSPs


   In certain networks, it is critical to consider network performance
   criteria when selecting the path for an explicitly routed RSVP-TE
   LSP.  Such performance criteria can include latency, jitter, and loss
   or other indications such as the conformance to link SLAs and non-
   RSVP TE traffic load.  This specification uses IGP extension data
   (which is defined outside the scope of this document) to perform such
   path selections.

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 August 10, 2013.

Copyright Notice

   Copyright (c) 2013 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

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   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
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   described in the Simplified BSD License.

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.  The ability to distribute
   network performance information in OSPF
   [I-D.ietf-ospf-te-metric-extensions] and in ISIS
   [I-D.previdi-isis-te-metric-extensions] is being defined (outside the
   scope of this document).  This document describes how to use that
   information for path selection for explicitly routed LSPs signaled
   via RSVP-TE [RFC3209].  The method suggested is not optimal for both
   minimizing path cost and additional constraints, such as latency;
   optimal solutions are computationally complex.

   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 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 beyond just the links.  Clearly end-to-end
   latency is a combination of router latency, queuing latency, physical
   link latency and other factors.  However, if application traffic
   requires paths to be selected based upon latency constraints, the
   same traffic might be in an Expedited Forwarding Per-Hop-
   Behavior[RFC3246] with minimal queuing delay or another PHB with
   known maximal per-hop queuing delay.  While traversing a router can
   cause delay, that can be included in the advertised link delay.

   This document does not specify how a router determines what values to
   advertise by the IGP.  However, 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 CSPF calculation can also determine
   whether a path under consideration would be in compliance.

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1.1.  Basic Requirements

   The following are the requirements that motivate this solution.

   1.  Select a TE tunnel's path based upon a combination of existing
       constraints as well as on link-latency, packet loss, jitter, link
       SLA conformance, 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 that a TE tunnel's current LSP
       complies with its configured end-to-end perforance requirements.

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

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

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

   7.  Ability to revert back to the best path after a configurable

2.  Using Performance Data Constraints

2.1.  End-to-End Constraints

   The per-link performance data available in the IGP
   [I-D.previdi-isis-te-metric-extensions] 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

   While it has been 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.
   Instead of this approach to minimize path latency, an end-to-end
   latency bound merely requires that the path computed be no more than
   that bound without being the minimum.  This bound can be used as a
   constraint in CSPF to prevent exploring links that would create a
   path over the end-to-end latency bound.

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   This is illustrated as follows.  Let the LSP have an end-to-end
   latency bound of 20ms.  Assume that the path to node X has been
   minimized and its latency is 12ms.  When X's links are to be
   explored, the link X<->Y has a link latency of 5ms and the link X<->Z
   has a link latency of 9ms.  The path via X to Y along link X<->Y
   would have a path latency of 12ms + 5ms = 17ms < 20ms; therefore, the
   link X<->Y can be explored.  In contrast, reaching Z via link X<->Z
   would result in a path latency of 12ms + 9ms = 21ms > 20ms; therefore
   the link X<->Z would not be explored in the CSPF.

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

   For link loss, the path loss is not the sum of the used links'
   losses.  Instead, the path loss percentage is (100 - loss_L1)*(100 -
   loss_L2)*...*(100 - loss_Ln), where the links along the path are L1
   to Ln.  The end-to-end link loss bound, computed in this fashion, can
   also be used as a constraint in the CSPF on what links to explore.

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 SLA-conformance
   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 inludes the
   bandwidth used by non-RSVP-TE traffic.  In
   [I-D.previdi-isis-te-metric-extensions], 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 configurable
   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 CSPF 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 filter.

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2.3.  Links out of SLA

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

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

   b.  Should LSPs using this link be immediately verified for continued
       compliance to their end-to-end constraints?

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

2.3.1.  Use of Anomalous Links for New Paths

   If the answer to (a) is no for latency SLAs, then any link which has
   the Anomalous bit set in the Unidirectional Link Delay sub-
   [I-D.previdi-isis-te-metric-extensions] should be removed from the
   topology before a CSPF 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 SLAs, then any
   link which has the Anomalous bit set in the Link Los sub-TLV should
   be treated as if it fails the exclude-any resource attributes filter.
   If the answer to (a) is no for jitter SLAs, then any link that has
   the Anomalous bit set in the Unidirectional Delay Variation sub-
   TLV[I-D.previdi-isis-te-metric-extensions] should be treated as if it
   fails the exclude-any resource attributes filter.

2.3.2.  Links entering the Anomalous State

   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 and thus the
   SLA compliance of a link 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

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   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 either have its continued compliance
   verified, be switched to a hot standby, or do a make-before-break to
   a secondary path.

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 when the link entered the Anomalous state, may want to
   reoptimize to a better path.

3.  IANA Considerations

   This document includes no request to IANA.

4.  Security Considerations

   This document is not currently believed to introduce new security

5.  References

5.1.  Normative References

              Giacalone, S., Ward, D., Drake, J., Atlas, A., and S.
              Previdi, "OSPF Traffic Engineering (TE) Metric
              Extensions", draft-ietf-ospf-te-metric-extensions-02 (work
              in progress), December 2012.

              Previdi, S., Giacalone, S., Ward, D., Drake, J., Atlas,
              A., and C. Filsfils, "IS-IS Traffic Engineering (TE)
              Metric Extensions",
              draft-previdi-isis-te-metric-extensions-02 (work in
              progress), October 2012.

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

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5.2.  Informative References

   [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, March 2002.

   [RFC5420]  Farrel, A., Papadimitriou, D., Vasseur, JP., and A.
              Ayyangarps, "Encoding of Attributes for MPLS LSP
              Establishment Using Resource Reservation Protocol Traffic
              Engineering (RSVP-TE)", RFC 5420, February 2009.

Authors' Addresses

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


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


   Spencer Giacalone
   Thomson Reuters
   195 Broadway
   New York, NY  10007


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   Dave Ward
   Cisco Systems
   170 West Tasman Dr.
   San Jose, CA  95134


   Stefano Previdi
   Cisco Systems
   Via Del Serafico 200
   Rome  00142


   Clarence Filsfils
   Cisco Systems


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