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Versions: 00 01 02 RFC 5468

Networking Working Group                                     S. Dasgupta
Internet-Draft                                            J. de Oliveira
Intended status: Informational                         Drexel University
Expires: November 25, 2007                                   JP. Vasseur
                                                           Cisco Systems
                                                            May 24, 2007

  Performance Analysis of Inter-Domain Path Computation Methodologies

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   This document presents a performance comparison between the per-
   domain path computation method and the Path Computation Element (PCE)
   Architecture based Backward Recursive Path Computation (BRPC)
   procedure.  Metrics to capture the significant performance aspects
   are identified and detailed simulations are carried out on realistic
   scenarios.  A performance analysis for each of the path computation

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   methods is then undertaken.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

Table of Contents

   1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Evaluation Metrics . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Simulation Setup . . . . . . . . . . . . . . . . . . . . . . .  5
   5.  Results and Analysis . . . . . . . . . . . . . . . . . . . . .  7
     5.1.  Path Cost  . . . . . . . . . . . . . . . . . . . . . . . .  7
     5.2.  Crankback/Setup Delay  . . . . . . . . . . . . . . . . . .  8
     5.3.  Signaling Failures . . . . . . . . . . . . . . . . . . . .  8
     5.4.  Failed TE-LSPs/Bandwidth on link failures  . . . . . . . .  9
     5.5.  TE LSP/Bandwidth setup capacity  . . . . . . . . . . . . .  9
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
   Intellectual Property and Copyright Statements . . . . . . . . . . 12

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1.  Terminology

   Terminology used in this document

   TE LSP: Traffic Engineered Label Switched Path.

   CSPF: Constraint Shortest Path First.

   PCE: Path Computation Element.

   BRPC: Backward Recursive PCE based Computation.

   AS: Autonomous System.

   ABR: Routers used to connect two IGP areas (areas in OSPF or levels
   in IS-IS).

   ASBR: Routers used to connect together ASes of a different or the
   same Service Provider via one or more Inter-AS links.

   Border LSR: A border LSR is either an ABR in the context of inter-
   area TE or an ASBR in the context of inter-AS TE.

   VSPT: Virtual Shortest Path Tree.

   LSA: Link State Advertisement.

   LSR: Label Switching Router.

   IGP: Interior Gateway Protocol.

   TED: Traffic Engineering Database.

2.  Introduction

   The IETF has specified two approaches for the computation of inter-
   domain (Generalized) Multi-Protocol Label Switching (MPLS) Traffic
   Engineering (TE) Label Switched Paths (LSP): the per-domain path
   computation approach defined in
   [I-D.ietf-ccamp-inter-domain-pd-path-comp] and the PCE based approach
   specified in [RFC4655].  More specifically we study the PCE based path
   computation model that makes use of the BRPC method outlined
   in[I-D.ietf-pce-brpc].  In the rest of this document, we will call PD
   and PCE the per-domain path computation approach and the PCE path
   computation approach respectively.

   In the per-domain path computation approach, each path segment within

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   a domain is computed during the signaling process by each entry node
   of the domain up to the next hop exit node of that same domain.

   By contrast the PCE-based approach and in particular the BRPC method
   defined in [I-D.ietf-pce-brpc] relies the collaboration between a set
   of PCEs to find to shortest inter-domain path after the computation
   of which the corresponding TE LSP is signaled: path computation is
   undertaken using multiple PCEs in a backward recursive fashion from
   the destination domain to the source domain.  The notion of a Virutal
   Shortest Path Tree (VSPT) is introduced.  Each link of a VSPT
   represents the shortest path satisfying the set of required
   constraints between the border nodes of a domain and the destination
   LSR.  The VSPT of each domain is returned by the corresponding PCE to
   create a new VSPT by PCEs present in other domains.
   [I-D.ietf-pce-brpc] discusses the BRPC procedure in complete detail.

   This document presents some simulation results and analysis to
   compare the performance of the above two inter-domain path
   computation approaches.  Two realistic topologies with accompanying
   traffic matrices are used to undertake the simulations.

   Note that although the simulations results discussed in this document
   have used inter-area networks, they also apply to Inter-AS cases.

   Disclaimer: although simulations have been made on different and
   realistic topologies showing consistent results, the metrics shown
   below may vary with the network topology.

3.  Evaluation Metrics

   This section discusses the metrics that are used to quantify and
   compare the performance of the two approaches.

   o  Path Cost.  The maximum and average path costs are observed for
      each TE LSP.  The distributions for the maximum and average path
      costs are then compared for the two path computation approaches.

   o  Signaling Failures. signaling failures may occur in various
      circumstances.  With the PD or PCE approach, the signaling of a TE
      LSP my fail because of lack of synchronization between the Traffic
      Engineering Database (TED) used by the computing entity and the
      actual link reservation state.  With the PD approach, another
      situation leading to a signaling failure is when the selected exit
      domain router does not have any path obeying the set of
      constraints toward a downstream exit node or the TE LSP
      destination.  The signaling failure metric captures the total
      number of signaling failures that occur during initial setup and

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      reroute (on link failure) of a TE LSP.  The distribution of the
      number of signaling failures encountered for all TE LSPs is

   o  Crankback Signaling.  In this document we made the assumption that
      in the case of PD, when an entry border node fails to find a route
      in the corresponding domain, crankback [I-D.ietf-ccamp-crankback]
      signaling was used.  A crankback signaling message propagates to
      the entry border node of the domain and a new exit border node is
      chosen.  After this, path computation takes place to find a path
      segment to a new entry border node of the next domain.  This
      causes a additional delay in setup time.  This metric captures the
      distribution of the number of crankback signals and the
      corresponding delay in setup time for a TE LSP when using PD.  The
      total delay arising from the crankback signaling is proportional
      to the costs of the links over which the signal travels, i.e., the
      path which is setup from the entry border node of a domain to its
      exit border node (the assumption was made that link metrics
      reflect propagation delays).  Similar to above metrics, the
      distribution of total crankback signaling and corresponding
      proportional delay across all TE LSPs is compared.

   o  TE LSPs/Bandwidth Setup Capacity.  Due to the different path
      computation techniques, there is a significant difference in the
      amount of TE LSPs/bandwidth that can be setup.  This metric
      captures the difference in the number of TE LSPs and corresponding
      bandwidth that can be setup using the two path computation
      techniques.  The traffic matrix is continuously scaled and stopped
      when the first TE LSP cannot be setup for both the methods.  The
      difference in the scaling factor gives the extra bandwidth that
      can be setup using the corresponding path computation technique.

   o  Failed TE LSPs/Bandwidth on link failure.  Link failures are
      induced in the network during the course of the simulations
      conducted.  This metric captures the number of TE LSPs and the
      corresponding bandwidth that failed to find a route when one or
      more links lying on its path failed.

4.  Simulation Setup

   A very detailed simulator has been developed to replicate a real life
   network scenario accurately.  Following is the set of entities used
   in the simulation with a brief description of their behavior.

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   |   Domain   |  # of |  # of |  OC48  |  OC192 |  [0,20) | [20,100] |
   |    Name    | nodes | links |  links |  links |   Mbps  |   Mbps   |
   |     D1     |   17  |   24  |   18   |    6   |   125   |    368   |
   |     D2     |   14  |   17  |   12   |    5   |    76   |    186   |
   |     D3     |   19  |   26  |   20   |    6   |    14   |    20    |
   |     D4     |   9   |   12  |    9   |    3   |    7    |    18    |
   |  MESH-CORE |   83  |  167  |   132  |   35   |    0    |     0    |
   | (backbone) |       |       |        |        |         |          |
   |  SYM-CORE  |   29  |  377  |   26   |   11   |    0    |     0    |
   | (backbone) |       |       |        |        |         |          |

           Table 1.  Domain Details and TE LSP Size Distribution

   o  Topology Description.  To obtain meaningful results applicable to
      present day Service Provider topologies, simulations have been run
      on two representative topologies.  They consists of a large
      backbone area to which four smaller areas are connected.  For the
      first topology named MESH-CORE, a densely connected backbone was
      obtained from RocketFuel [ROCKETFUEL].  The second topology has a
      symmetrical backbone and is called SYM-CORE.  The four connected
      smaller areas are obtained from [DEF-DES].  Details of the
      topologies are shown in Table 1 along with their layout in Figure
      1.  All TE LSPs setup on this network have their source and
      destinations in different areas and all of them need to traverse
      the backbone network.  Table 1 also shows the number of TE LSPs
      that have their sources in the corresponding areas along with
      their size distribution.

   o  Node behavior.  Every node in the topology represents a router
      that maintains states for all the TE LSPs passing through it.
      Each node in a domain is a source for TE LSPs to all the other
      nodes in the other domains.  As in a real life scenario, where
      routers boot up at random points in time, the nodes in the
      topologies also start sending traffic on the TE LSPs originating
      from them at a random start time (to take into account the
      different boot-up times).  All nodes are up within an hour of the
      start of simulation.  All nodes maintain a TED that is updated
      using LSA updates as outlined in [RFC3630].  The flooding scope of
      the Traffic Engineering IGP updates are restricted only to the
      domain in which they originate in compliance with [RFC3630] and

   o  TE LSP Setup.  When a node boots up, it sets up all TE LSPs that
      originate from it in descending order of size.  The network is
      dimensioned such that all TE LSPs can find a path.  Once setup,

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      all TE LSPs stay in the network for the complete duration of the
      simulation unless they fail due to a link failure.  Eventhough the
      TE LSPs are setup in descending order of size from a head-end
      router, from the network perspective, TE LSPs are setup in random
      fashion as the routers bootup at random times.

   o  Inducing Failures.  For thorough performance analysis and
      comparison, link failures are induced in all the areas.  Each link
      in a domain can fail independently with a mean failure time of 24
      hours and be restored with a mean restore time of 15 minutes.
      Both inter-failure and inter-restore times are uniformly
      distributed.  No attempt to re-optimize the path of a TE LSP is
      made when a link is restored.  The links that join two domains
      never fail.  This step has been taken to concentrate only on how
      link failures within domains affect the performance.

5.  Results and Analysis

   Simulations were carried out on the two topologies previously
   described.  The results are presented and discussed in this section.
   All figures are from the PDF version of this document.  In the
   figures, `PD-Setup' and `PCE-Setup' represent results corresponding
   to the initial setting up of TE LSPs on an empty network using the
   per-domain and the PCE approach, respectively.  Similarly, `PD-
   Failure' and `PCE-Failure' denote the results under the link failure
   scenario.  A period of one week was simulated and results were
   collected after the transient period.  Figure 2 and Figure 3
   illustrate the behavior of the metrics for topologies MESH-CORE and
   SYM-CORE, respectively.

5.1.  Path Cost

   Figures 2a and 3a show the distribution of the average path cost of
   the TE LSPs for MESH-CORE and SYM-CORE, respectively.  During initial
   setup, roughly 40% of TE LSPs for MESH-CORE and 70% of TE LSPs for
   SYM-CORE have path costs greater with PD (PD-Setup) than with PCE
   approach (PCE-Setup).  This is due to the ability of the BRPC
   procedure to select the inter-domain shortest constrained paths that
   satisfy the constraints.  Since the per-domain approach to path
   computation is undertaken in stages where every entry border router
   to a domain computes the path in the corresponding domain, the most
   optimal (shortest constrained inter-domain) route is not always
   found.  When failures start to take place in the network, TE LSPs are
   rerouted over different paths resulting in path costs that are
   different from the initial costs.  PD-Failure and PCE-Failure in
   Figures 2a and 3a show the distribution of the average path costs
   that the TE LSPs have over the duration of the simulation with link

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   failures occurring.  Similarly, the average path costs with the PD
   approach are much higher than the PCE approach when link failures
   occur.  Figures 2b and 3b show similar trends and present the maximum
   path costs for a TE LSP for the two topologies, respectively.  It can
   be seen that with per-domain path computation, the maximum path costs
   are larger for 30% and 100% of the TE LSPs for MESH-CORE and SYM-
   CORE, respectively.

5.2.  Crankback/Setup Delay

   Due to crankbacks that take place in the per-domain approach of path
   computation, TE LSP setup time is significantly increased.  This
   could lead to QoS requirements not being met, especially during
   failures when rerouting needs to be quick in order to keep traffic
   disruption to a minimum (for example in the absence of local repair
   mechanisms such as defined in [RFC4090]).  Since crankbacks do not
   take place during path computation with a PCE, setup delays are
   significantly reduced.  Figures 2c and 3c show the distributions of
   the number of crankbacks that took place during the setup of the
   corresponding TE LSPs for MESH-CORE and SYM-CORE, respectively.  It
   can be seen that all crankbacks occurred when failures were taking
   place in the networks.  Figures 2d and 3d illustrate the
   'proportional' setup delays experienced by the TE LSPs due to
   crankbacks for the two topologies.  It can be observed that for a
   large proportion of the TE LSPs, the setup delays arising out of
   crankbacks is very large possibly proving to be very detrimental to
   QoS requirements.  The large delays arise out of the crankback
   signaling that needs to propagate back and forth from the exit border
   router of a domain to its entry border router.  More crankbacks occur
   for SYM-CORE as compared to MESH-CORE as it is a very `restricted'
   and `constrained' network in terms of connectivity.  This causes a
   lack of routes and often several cycles of crankback signaling are
   required to find a constrained path.

5.3.  Signaling Failures

   As discussed in the previous sections, signaling failures occur
   either due to an outdated TED or when a path cannot be found from the
   selected entry border router.  Figures 2e and 3e shows the
   distribution of the total number of signaling failures experienced by
   the TE LSPs during setup.  About 38% and 55% of TE LSPs for MESH-CORE
   and SYM-CORE, respectively, experience a signaling failures with per-
   domain path computation when link failures take place in the network.
   In contrast, only about 3% of the TE LSPs experience signaling
   failures with the PCE method.  It should be noted that the signaling
   failures experienced with the PCE correspond only to the TEDs being
   out of date.

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5.4.  Failed TE-LSPs/Bandwidth on link failures

   Figures 2f and 3f show the number of TE LSPs and the associated
   required bandwidth that fail to find a route when link failures are
   taking place in the topologies.  For MESH-CORE, with the per-domain
   approach, 395 TE LSPs failed to find a path corresponding to 1612
   Mbps of bandwidth.  For PCE, this number is lesser at 374
   corresponding to 1546 Mbps of bandwidth.  For SYM-CORE, with the per-
   domain approach, 434 TE LSPs fail to find a route corresponding to
   1893 Mbps of bandwidth.  With the PCE approach, only 192 TE LSPs fail
   to find a route, corresponding to 895 Mbps of bandwidth.  It is
   clearly visible that the PCE allows more TE LSPs to find a route thus
   leading to better performance during link failures.

5.5.  TE LSP/Bandwidth setup capacity

   Since PCE and the per-domain path computation approach differ in how
   path computation takes place, more bandwidth can be setup with PCE.
   This is primarily due to the way in which BRPC functions.  To observe
   the extra bandwidth that can fit into the network, the traffic matrix
   was scaled.  Scaling was stopped when the first TE LSP failed to
   setup with PCE.  This metric, like all the others discussed above, is
   topology dependent (therefore the choice of two topologies for this
   study).  This metric highlights the ability of PCE to fit more
   bandwidth in the network.  For MESH-CORE, on scaling, 1556 Mbps more
   could be setup with PCE.  In comparison, for SYM-CORE this value is
   986 Mbps.  The amount of extra bandwidth that can be setup on SYM-
   CORE is lesser due to its restricted nature and limited capacity.

6.  IANA Considerations

   This document makes no request to IANA for action.

7.  Security Considerations

   This document does not raise any security issue.

8.  References

8.1.  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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8.2.  Informative References

   [DEF-DES]  J. Guichard, F. Le Faucheur, and J.-P. Vasseur, "Definitve
              MPLS Network Designs", Cisco Press, 2005.

              Farrel, A., "Crankback Signaling Extensions for MPLS and
              GMPLS RSVP-TE", draft-ietf-ccamp-crankback-06 (work in
              progress), January 2007.

              Vasseur, J., "A Per-domain path computation method for
              establishing Inter-domain Traffic  Engineering (TE) Label
              Switched Paths (LSPs)",
              draft-ietf-ccamp-inter-domain-pd-path-comp-04 (work in
              progress), February 2007.

              Ayyangar, A., "Inter domain Multiprotocol Label Switching
              (MPLS) and Generalized MPLS  (GMPLS) Traffic Engineering -
              RSVP-TE extensions",
              draft-ietf-ccamp-inter-domain-rsvp-te-05 (work in
              progress), March 2007.

              Ayyangar, A., "Label Switched Path Stitching with
              Generalized Multiprotocol Label Switching  Traffic
              Engineering (GMPLS TE)", draft-ietf-ccamp-lsp-stitching-05
              (work in progress), March 2007.

              Vasseur, J., "A Backward Recursive PCE-based Computation
              (BRPC) procedure to compute  shortest inter-domain Traffic
              Engineering Label Switched Paths", draft-ietf-pce-brpc-04
              (work in progress), March 2007.

   [RFC3630]  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering
              (TE) Extensions to OSPF Version 2", RFC 3630,
              September 2003.

   [RFC3784]  Smit, H. and T. Li, "Intermediate System to Intermediate
              System (IS-IS) Extensions for Traffic Engineering (TE)",
              RFC 3784, June 2004.

   [RFC4090]  Pan, P., Swallow, G., and A. Atlas, "Fast Reroute
              Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
              May 2005.

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

              N. Spring, R. Mahajan, and D. Wehterall, "Measuring ISP
              Topologies with Rocketfuel", Proceedings of  ACM SIGCOMM,

Authors' Addresses

   Sukrit Dasgupta
   Drexel University
   Dept of ECE, 3141 Chestnut Street
   Philadelphia, PA  19104

   Phone: 215-895-1862
   Email: sukrit@ece.drexel.edu
   URI:   pages.drexel.edu/~sd88

   Jaudelice C. de Oliveira
   Drexel University
   Dept. of ECE, 3141 Chestnut Street
   Philadelphia, PA  19104

   Phone: 215-895-2248
   Email: jau@ece.drexel.edu
   URI:   www.ece.drexel.edu/faculty/deoliveira

   JP Vasseur
   Cisco Systems
   1414 Massachussetts Avenue
   Boxborough, MA  01719

   Email: jpv@cisco.com

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