PCE Working Group                                               D. Dhody
Internet-Draft                                                  X. Zhang
Intended status: Informational                       Huawei Technologies
Expires: January 5, 2015                                    July 4, 2014


  Stateful Path Computation Element (PCE) Inter-domain Considerations
              draft-dhody-pce-stateful-pce-interdomain-00

Abstract

   A stateful Path Computation Element (PCE) maintains information about
   Label Switched Path (LSP) characteristics and resource usage within a
   network in order to provide traffic engineering path calculations for
   its associated Path Computation Clients (PCCs).  Furthermore, PCEs
   are used to compute shortest constrained Traffic Engineering Label
   Switched Paths (TE LSPs) in Multiprotocol Label Switching (MPLS) and
   Generalized MPLS (GMPLS) networks across multiple domains.

   This document describes general considerations for the deployment of
   stateful PCE(s) in inter-domain scenarios including inter-area and
   inter-AS.

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
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   This Internet-Draft will expire on January 5, 2015.

Copyright Notice

   Copyright (c) 2014 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
   (http://trustee.ietf.org/license-info) in effect on the date of



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   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.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  LSP State Synchronization . . . . . . . . . . . . . . . .   4
   3.  Stateful PCE Deployments  . . . . . . . . . . . . . . . . . .   4
     3.1.  Single Stateful PCE, Multiple Domains . . . . . . . . . .   4
     3.2.  Multiple Stateful PCE, Multiple Domains . . . . . . . . .   5
       3.2.1.  Per Domain Path Computation . . . . . . . . . . . . .   6
       3.2.2.  Backward-Recursive PCE-based Computation  . . . . . .   7
       3.2.3.  Hierarchical PCE  . . . . . . . . . . . . . . . . . .   7
   4.  Other Considerations  . . . . . . . . . . . . . . . . . . . .   8
     4.1.  Delegation  . . . . . . . . . . . . . . . . . . . . . . .   8
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  Manageability Considerations  . . . . . . . . . . . . . . . .   9
     6.1.  Control of Function and Policy  . . . . . . . . . . . . .   9
     6.2.  Information and Data Models . . . . . . . . . . . . . . .   9
     6.3.  Liveness Detection and Monitoring . . . . . . . . . . . .   9
     6.4.  Verify Correct Operations . . . . . . . . . . . . . . . .   9
     6.5.  Requirements On Other Protocols . . . . . . . . . . . . .   9
     6.6.  Impact On Network Operations  . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
   8.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  10
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Appendix A.  Contributor Addresses  . . . . . . . . . . . . . . .  12

1.  Introduction

   The Path Computation Element communication Protocol (PCEP) provides
   mechanisms for Path Computation Elements (PCEs) to perform path
   computations in response to Path Computation Clients' (PCCs)
   requests.

   [I-D.ietf-pce-stateful-pce-app] describes general considerations for
   a stateful PCE deployment and examines its applicability and
   benefits, as well as its challenges and limitations through a number
   of use cases.  [I-D.ietf-pce-stateful-pce] describes a set of
   extensions to PCEP to provide stateful control.  A stateful PCE has
   access to not only the information carried by the network's Interior



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   Gateway Protocol (IGP), but also the set of active paths and their
   reserved resources for its computations.  The additional state allows
   the PCE to compute constrained paths while considering individual
   LSPs and their interactions.

   The ability to compute shortest constrained TE LSPs in Multiprotocol
   Label Switching (MPLS) and Generalized MPLS (GMPLS) networks across
   multiple domains has been identified as a key motivation for PCE
   development.  In this context, a domain is a collection of network
   elements within a common sphere of address management or path
   computational responsibility such as an Interior Gateway Protocol
   (IGP) area or an Autonomous Systems (AS).

   This document presents general considerations for stateful PCE(s)
   deployment in multi-domain scenarios.

2.  Overview

   A stateful PCE maintains two sets of information for use in path
   computation.  The first is the Traffic Engineering Database (TED)
   which includes the topology and resource state in the network.  The
   second is the LSP State Database (LSP-DB), in which a PCE stores
   attributes of all active LSPs in the network, such as their paths
   through the network, bandwidth/resource usage, switching types and
   LSP constraints.  This state information allows the PCE to compute
   constrained paths while considering individual LSPs and their inter-
   dependency.  [I-D.ietf-pce-stateful-pce] applies equally to MPLS-TE
   and GMPLS LSPs and distinguishes between an active and a passive
   stateful PCE.  A passive stateful PCE uses LSP state information to
   optimize path computations but does not actively update LSP state.
   In contrast, an active stateful PCE may issue recommendations to the
   network.  For example, an active stateful PCE may update LSP
   parameters for those LSPs that have been delegated, by its PCCs, the
   control over to the PCE.

   The capability to compute the routes of end-to-end inter-domain MPLS-
   TE LSPs is expressed as requirements in [RFC4105] and [RFC4216] and
   may be realized by PCE(s).  PCEs may use one of the following
   mechanisms to compute end-to-end paths:

   o  a per-domain path computation technique [RFC5152];

   o  a Backward-Recursive PCE-based Computation (BRPC) mechanism
      [RFC5441];

   o  a Hierarchical PCE mechanism [RFC6805];





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   This document examines the stateful PCE inter-domain considerations
   for all of these mechanisms.

2.1.  LSP State Synchronization

   The population of the LSP-DB using information received from PCCs
   (ingress LSR) is supported by the stateful PCE extensions defined in
   [I-D.ietf-pce-stateful-pce] , i.e., via LSP state report messages.

   The inter-domain LSP state is synchronised to the ingress-PCE from
   the ingress LSR (PCC), but this PCC cannot synchronise to other PCEs
   (in transit or egress domains), thus other mechanism must be
   investigated for this purpose.

3.  Stateful PCE Deployments

   There are multiple models to perform PCE-based inter-domain path
   computation:

      o A single PCE;

      o Multiple PCE;

          *  without inter-PCE communication;

          *  with inter-PCE communication;

   This section describe stateful PCE considerations for each of these
   deployment models.

3.1.  Single Stateful PCE, Multiple Domains

   In this model, inter-domain path computation is performed by a single
   stateful PCE that has topology visibility into all domains.  The
   inter-domain LSP state is synchronised to this PCE from the ingress
   LSR (PCC) itself.  This PCC may also choose to delegate control over
   this LSP to the PCE.  Thus this model is similar to a single domain
   in all aspects.

   Following figure show an example of inter-area case comprising of
   Area 0,1 and 2.  A single stateful PCE is deployed for all areas.










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                                  *******
                                  * PCE *
                                  *******

                         !                      !
                         !                      !
         A----B----C----ABR1----D----E----F----ABR2----G----H----I
         |    |    |     |      |    |    |     |      |    |    |
         |    |    |     |      |    |    |     |      |    |    |
         J----K----L----ABR3----M----N----O----ABR4----P----Q----R
                         !                      !
              Area 1     !         Area 0       !      Area 2

   In this model PCE has visibility into the topology of all domains as
   well as the state of all active LSPs including inter-domain LSPs.
   This model is thus well suited to take advantage of all stateful PCE
   capabilities.

   It should be noted that a single PCE may not be possible because of
   administrative and confidentiality concerns.

3.2.  Multiple Stateful PCE, Multiple Domains

   In this model, there is at least one PCE per domain, and each PCE has
   topology visibility restricted to its own domain.  The inter-domain
   LSP state is synchronised to the ingress-PCE from the ingress LSR
   (PCC), but this PCC cannot synchronise to other PCEs (in transit or
   egress domains).  This PCC may also choose to delegate control over
   this LSP to the Ingress-PCE, which may issue inter-domain path
   computation or re-optimization request to other PCEs.  An inter-
   domain LSP that originates in the domain, is synchronised to the PCE
   in that domain.  But a mechanism is needed to synchronize state of
   inter-domain LSP that do not originate in the domain.  In other
   words, inter-domain LSP state should also be synchronised to transit
   and egress PCEs.

   Following figure show an example of inter-AS case comprising of AS
   100 and AS 200.  A stateful PCE is deployed per AS.













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                  ********                      ********
                  * PCE1 *                      * PCE2 *
                  ********                      ********

                              !          !
                              !          !
              A----B----C----ASBR1------ASBR2----D----E----F
              |    |    |     |          |       |    |    |
              |    |    |     |          |       |    |    |
              G----H----I----ASBR3------ASBR4----J----K----L
                              !          !
                   AS100      !          !       AS200

   In order to conceal the information, a PCE may use path-key based
   confidentiality mechanisms as per [RFC5520].

   This section further describes considerations with respect to each of
   the inter-domain path computation techniques.

3.2.1.  Per Domain Path Computation

   The per domain path computation technique [RFC5152] is based on
   Multiple PCE Path Computation without Inter-PCE Communication Model
   as described in [RFC4655].  It defines a method where the path is
   computed during the signaling process (on a per-domain basis).  The
   entry Boundary Node (BN) of each domain is responsible for performing
   the path computation for the section of the LSP that crosses the
   domain, or for requesting that a PCE for that domain computes that
   piece of the path.

   The ingress LSR would synchronise the the state to the ingress PCE,
   further the entry boundary nodes should synchronize the state of
   inter-domain LSP to transit and egress PCEs.  Note that the BN on the
   path of an LSP can probably see the path (through the Record Route
   object in RSVP-TE signaling [RFC3209]) and knows the bandwidth
   reserved for the LSP.  Thus each entry BN along the path could be
   made responsible to synchronise the LSP state to the transit/egress
   PCE(s).

   Since the stateful PCE(s) do not communicate during this inter-domain
   path computation technique and each entry BN would perform path
   computation via Path Computation Request (PCReq) and Reply (PCRep)
   messages, a passive stateful PCE is well suited for this case.

   In case of delegation to the ingress PCE (active stateful PCE), it
   would be capable of loose path computation only and make updates to
   the ingress LSR with this limited visibility.  The entry BN would
   perform path computation via Path Computation Request and Reply



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   messages (and thus rely on the passive stateful mode).  Thus the
   inter-domain LSP is delegated only to the ingress PCE.

3.2.2.  Backward-Recursive PCE-based Computation

   The BRPC [RFC5441] technique is based on Multiple PCE Path
   Computation with Inter-PCE Communication Model as described in
   [RFC4655].  It involves cooperation and communication between PCEs in
   order to compute an optimal end-to-end path across multiple domains.
   The sequence of domains to be traversed may be known before the path
   computation, but it can also be used when the domain path is unknown
   and determined during path computation.

   As described in Section 3.2.1, the entry boundary nodes may
   synchronize the state of inter-domain LSPs to transit and egress
   PCEs.  An alternative approach may be for each PCE to synchronise the
   state along the path across domains, i.e., each PCE would report the
   state to the next PCE(s) in the adjacent domain along the domain
   sequence of the inter-domain path.  A mechanism similar to LSP-DB
   backup [I-D.palle-pce-stateful-pce-lspdb-sync] may be utilized for
   this purpose.

   Some path segment in the end to end path may also be hidden via path-
   key as per [RFC5520] during state synchronization.

   In case of passive path computation request to the ingress PCE from
   the ingress LSR the BRPC path computation procedure is applied to
   compute end-to-end path by using PCReq and PCRep messages among
   stateful PCE(s) in passive mode.

   In case of delegation to the ingress PCE (active stateful PCE), the
   ingress PCE may trigger the end-to-end path computation via the same
   BRPC procedure using the path computation request and reply messages
   among stateful PCE(s) in passive mode.  For re-optimization or update
   the ingress PCE still rely on the same BRPC procedure triggered by
   the ingress PCE.  Ultimately the inter-domain LSP is delegated to the
   ingress PCE and only the ingress PCE can issue updates to the inter-
   domain LSP.  It may trigger E2E path re-optimization with help of
   transit/egress PCE using the BRPC procedure.

3.2.3.  Hierarchical PCE

   In H-PCE [RFC6805] architecture, the parent PCE is used to compute a
   multi-domain path based on the domain connectivity information.  The
   parent PCE may be requested to provide a end-to-end path or only the
   sequence of domains.





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   As described in Section 3.2.1 and Section 3.2.2, the entry boundary
   nodes may synchronize the state of inter-domain LSP to transit and
   egress child PCEs.  If the parent PCE provides the sequence of
   domains and BRPC procedure is used to get the E2E path, each PCE may
   be responsible to synchronise the state along the path across domains
   similar to Section 3.2.2.  An alternative approach may be for ingress
   PCE to synchronise LSP state with the Parent PCE and it may further
   synchronise the state to the child PCE(s) along the path across
   domains, i.e. parent PCE would report the state to the child PCE(s)
   along the domain sequence.

   Some path segment in the end to end path may also be hidden via path-
   key as per [RFC5520] during state synchronization.

   In case of passive path computation request to the ingress PCE from
   the ingress LSR, the H-PCE path computation procedure is applied to
   compute sequence of domains or end-to-end path by using PCReq and
   PCRep messages among stateful PCE(s) in passive mode.

   In case of delegation to the ingress PCE (active stateful PCE), the
   ingress PCE may trigger the H-PCE path computation via the same
   procedure using the PCReq and PCRep messages among stateful PCE(s) in
   passive mode.  For re-optimization or update the ingress PCE still
   rely on the same H-PCE procedure triggered by the ingress PCE.
   Ultimately the inter-domain LSP is delegated to the ingress PCE and
   only the ingress PCE can issue updates to the inter-domain LSP.  It
   may trigger E2E path re-optimization with help of parent and child
   PCEs using the H-PCE procedure.

4.  Other Considerations

4.1.  Delegation

   As noted in this document, the inter-domain LSP is delegated to the
   ingress PCE and only the ingress PCE can issue updates to the inter-
   domain LSP.  The ingress PCE is responsible to trigger E2E path re-
   optimization.

   Thus the ingress PCE can recommend updation for all aspects of the
   inter-domain LSP including the segment of path in another domain
   (which it may have computed with the help of other cooperating PCEs).
   These interaction between PCEs for the inter-domain path computation
   are done using PCReq/PCRep messages (i.e., in a passive mode).

   The transit/egress PCE cannot update any attribute of the inter-
   domain LSP on its own as it may not have any interaction with the
   ingress LSR.  A mechanism may be developed for transit/egress PCE to
   inform the ingress PCE to trigger E2E re-optimization and choose to



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   update the inter-domain LSP based on the result.  Also the ingress
   PCE may use combination of local information and events along with
   some external mechanism (management / monitoring interface) to
   trigger E2E path re-optimization.

   Though Ingress PCE can recommend update for path segments in other
   domains, the entry boundary node of that domain can apply policy
   control during signalling as explained in [RFC4105] and [RFC4216].

5.  Security Considerations

   The security considerations are as per [RFC5440] and
   [I-D.ietf-pce-stateful-pce].  Any multi-domain operation necessarily
   involves the exchange of information across domain boundaries.  This
   may represent a significant security and confidentiality risk
   especially when the domains are controlled by different commercial
   entities.  PCEP allows individual PCEs to maintain confidentiality of
   their domain path information by using path-keys [RFC5520].

6.  Manageability Considerations

6.1.  Control of Function and Policy

   Mechanisms defined in this document do not imply any new control of
   function and policy requirements.

6.2.  Information and Data Models

   [I-D.ietf-pce-pcep-mib] describes the PCEP MIB, there are no new MIB
   Objects for this document.

6.3.  Liveness Detection and Monitoring

   Mechanisms defined in this document do not imply any new liveness
   detection and monitoring requirements in addition to those already
   listed in [RFC5440].

6.4.  Verify Correct Operations

   Mechanisms defined in this document do not imply any new operation
   verification requirements in addition to those already listed in
   [RFC5440].

6.5.  Requirements On Other Protocols

   Mechanisms defined in this document do not imply any new requirements
   on other protocols.




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6.6.  Impact On Network Operations

   Mechanisms defined in this document do not have any impact on network
   operations in addition to those already listed in [RFC5440].

7.  IANA Considerations

   This is an informational document and has no IANA considerations.

8.  Acknowledgments

   TBD.

9.  References

9.1.  Normative References

   [RFC5440]  Vasseur, JP. and JL. Le Roux, "Path Computation Element
              (PCE) Communication Protocol (PCEP)", RFC 5440, March
              2009.

   [I-D.ietf-pce-stateful-pce]
              Crabbe, E., Minei, I., Medved, J., and R. Varga, "PCEP
              Extensions for Stateful PCE", draft-ietf-pce-stateful-
              pce-09 (work in progress), June 2014.

9.2.  Informative 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, December 2001.

   [RFC4105]  Le Roux, J., Vasseur, J., and J. Boyle, "Requirements for
              Inter-Area MPLS Traffic Engineering", RFC 4105, June 2005.

   [RFC4216]  Zhang, R. and J. Vasseur, "MPLS Inter-Autonomous System
              (AS) Traffic Engineering (TE) Requirements", RFC 4216,
              November 2005.

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

   [RFC5152]  Vasseur, JP., Ayyangar, A., and R. Zhang, "A Per-Domain
              Path Computation Method for Establishing Inter-Domain
              Traffic Engineering (TE) Label Switched Paths (LSPs)", RFC
              5152, February 2008.





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   [RFC5441]  Vasseur, JP., Zhang, R., Bitar, N., and JL. Le Roux, "A
              Backward-Recursive PCE-Based Computation (BRPC) Procedure
              to Compute Shortest Constrained Inter-Domain Traffic
              Engineering Label Switched Paths", RFC 5441, April 2009.

   [RFC5520]  Bradford, R., Vasseur, JP., and A. Farrel, "Preserving
              Topology Confidentiality in Inter-Domain Path Computation
              Using a Path-Key-Based Mechanism", RFC 5520, April 2009.

   [RFC6805]  King, D. and A. Farrel, "The Application of the Path
              Computation Element Architecture to the Determination of a
              Sequence of Domains in MPLS and GMPLS", RFC 6805, November
              2012.

   [I-D.ietf-pce-stateful-pce-app]
              Zhang, X. and I. Minei, "Applicability of a Stateful Path
              Computation Element (PCE)", draft-ietf-pce-stateful-pce-
              app-02 (work in progress), June 2014.

   [I-D.ietf-pce-pcep-mib]
              Koushik, K., Emile, S., Zhao, Q., King, D., and J.
              Hardwick, "Path Computation Element Protocol (PCEP)
              Management Information Base", draft-ietf-pce-pcep-mib-08
              (work in progress), April 2014.

   [I-D.palle-pce-stateful-pce-lspdb-sync]
              Palle, U., Dhody, D., and X. Zhang, "LSP-DB
              Synchronization between Stateful PCEs", draft-palle-pce-
              stateful-pce-lspdb-sync-02 (work in progress), January
              2014.





















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Appendix A.  Contributor Addresses

   Udayasree Palle
   Huawei Technologies
   Leela Palace
   Bangalore, Karnataka  560008
   INDIA

   EMail: udayasree.palle@huawei.com


   Avantika
   Huawei Technologies
   Leela Palace
   Bangalore, Karnataka  560008
   INDIA

   EMail: avantika.sushilkumar@huawei.com



Authors' Addresses

   Dhruv Dhody
   Huawei Technologies
   Leela Palace
   Bangalore, Karnataka  560008
   INDIA

   EMail: dhruv.ietf@gmail.com


   Xian Zhang
   Huawei Technologies
   Bantian, Longgang District
   Shenzhen  518129
   P.R.China

   EMail: zhang.xian@huawei.com












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