 



PCE Working Group                                               D. Dhody
Internet-Draft                                       Huawei Technologies
Intended status: Informational                                    Y. Lee
Expires: April 23, 2020                                             SKKU
                                                           D. Ceccarelli
                                                                Ericsson
                                                                 J. Shin
                                                              SK Telecom
                                                                 D. King
                                                    Lancaster University
                                                        October 21, 2019


          Hierarchical Stateful Path Computation Element (PCE)
                    draft-ietf-pce-stateful-hpce-15

Abstract

   A Stateful Path Computation Element (PCE) maintains information on
   the current network state received from the Path Computation Clients
   (PCCs), including: computed Label Switched Path (LSPs), reserved
   resources within the network, and pending path computation requests.
   This information may then be considered when computing the path for a
   new traffic-engineered LSP or for any associated/dependent LSPs. The
   Path computation response from a PCE is helpful for the PCC to
   gracefully establish the computed LSP.

   The Hierarchical Path Computation Element (H-PCE) architecture
   provides an architecture to allow the optimum sequence of
   inter-connected domains to be selected, and network policy to be
   applied if applicable, via the use of a hierarchical relationship
   between PCEs.

   Combining the capabilities of Stateful PCE and the Hierarchical PCE
   would be advantageous. This document describes general considerations
   and use cases for the deployment of Stateful, and not Stateless, PCEs
   using the Hierarchical PCE architecture.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.
   Internet-Drafts are draft documents valid for a maximum of six months
 


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   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 23, 2020.


Copyright Notice

   Copyright (c) 2019 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
   (https://trustee.ietf.org/license-info) 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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Background . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Use cases and Applicability of Hierarchical Stateful PCE .  4
       1.2.1.  Applicability to ACTN  . . . . . . . . . . . . . . . .  5
       1.2.2.  End-to-End Contiguous LSP  . . . . . . . . . . . . . .  5
       1.2.3.  Applicability of a Stateful P-PCE  . . . . . . . . . .  6
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Requirement Language . . . . . . . . . . . . . . . . . . .  7
   3.  Hierarchical Stateful PCE  . . . . . . . . . . . . . . . . . .  7
     3.1.  Passive Operations . . . . . . . . . . . . . . . . . . . .  9
     3.2.  Active Operations  . . . . . . . . . . . . . . . . . . . . 11
     3.3.  PCE Initiation of LSPs . . . . . . . . . . . . . . . . . . 12
       3.3.1.  Per-Domain Stitched LSP  . . . . . . . . . . . . . . . 13
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 15
   5.  Manageability Considerations . . . . . . . . . . . . . . . . . 16
     5.1.  Control of Function and Policy . . . . . . . . . . . . . . 16
     5.2.  Information and Data Models  . . . . . . . . . . . . . . . 16
     5.3.  Liveness Detection and Monitoring  . . . . . . . . . . . . 16
     5.4.  Verify Correct Operations  . . . . . . . . . . . . . . . . 17
     5.5.  Requirements On Other Protocols  . . . . . . . . . . . . . 17
     5.6.  Impact On Network Operations . . . . . . . . . . . . . . . 17
     5.7.  Error Handling between PCEs  . . . . . . . . . . . . . . . 17
 


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   6.  Other Considerations . . . . . . . . . . . . . . . . . . . . . 17
     6.1.  Applicability to Inter-Layer Traffic Engineering . . . . . 18
     6.2.  Scalability Considerations . . . . . . . . . . . . . . . . 18
     6.3.  Confidentiality  . . . . . . . . . . . . . . . . . . . . . 19
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 19
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 19
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 19
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 19
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 20
   Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23



1.  Introduction

1.1.  Background 

   The Path Computation Element communication Protocol (PCEP) [RFC5440]
   provides mechanisms for Path Computation Elements (PCEs) to perform
   path computations in response to Path Computation Clients' (PCCs)
   requests.

   A stateful PCE is capable of considering, for the purposes of path
   computation, not only the network state in terms of links and nodes
   (referred to as the Traffic Engineering Database or TED) but also the
   status of active services (previously computed paths, and currently
   reserved resources, stored in the Label Switched Paths Database
   (LSP-DB).

   [RFC8051] 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.

   [RFC8231] 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 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.  [RFC8281] describes the setup, maintenance and
   teardown of PCE-initiated LSPs under the stateful PCE model.

   [RFC8231] also describes the active stateful PCE. The active PCE
   functionality allows a PCE to reroute an existing LSP or make changes
   to the attributes of an existing LSP, or delegate control of specific
   LSPs to a new PCE.

 


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   The ability to compute constrained paths for 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.  [RFC6805] describes a Hierarchical PCE (H-PCE)
   architecture which can be used for computing end-to-end paths for
   inter-domain MPLS Traffic Engineering (TE) and GMPLS Label Switched
   Paths (LSPs).  Within the Hierarchical PCE (H-PCE) architecture
   [RFC6805], the Parent PCE (P-PCE) is used to compute a multi-domain
   path based on the domain connectivity information.  A Child PCE
   (C-PCE) may be responsible for a single domain or multiple domains.
   The C-PCE is used to compute the intra-domain path based on its
   domain topology information.

   This document presents general considerations for stateful PCEs, and
   not Stateless PCEs, in the hierarchical PCE architecture.  It focuses
   on the behavior changes and additions to the existing stateful PCE
   mechanisms (including PCE-initiated LSP setup and active stateful PCE
   usage) in the context of networks using the H-PCE architecture.

   In this document, Sections 3.1 and 3.2 focus on end to end (E2E)
   inter-domain TE LSP. Section 3.3.1 describes the operations for
   stitching Per-Domain LSPs.

1.2.  Use cases and Applicability of Hierarchical Stateful PCE

   As per [RFC6805], in the hierarchical PCE architecture, a P-PCE
   maintains a domain topology map that contains the child domains and
   their interconnections.  Usually, the P-PCE has no information about
   the content of the child domains. But if the PCE is applied to the
   Abstraction and Control of TE Networks (ACTN) [RFC8453] as described
   in [RFC8637], the Provisioning Network Controller (PNC) can provide
   an abstract topology to the Multi-Domain Service Coordinator (MDSC).
   Thus the P-PCE in MDSC could be aware of topology information in much
   more detail than just the domain topology.

   In a PCEP session between a PCC (Ingress) and a C-PCE, the C-PCE acts
   as per the stateful PCE operations described in [RFC8231] and
   [RFC8281]. The same C-PCE behaves as a PCC on the PCEP session
   towards the P-PCE. The P-PCE is stateful in nature and thus maintains
   the state of the inter-domain LSPs that are reported to it. The
   inter-domain LSP could also be delegated by the C-PCE to the P-PCE,
   so that the P-PCE could update the inter-domain path. The trigger for
   this update could be the LSP state change reported for this LSP or
   any other LSP. It could also be a change in topology at the P-PCE,
   such as inter-domain link status change. In case of use of stateful
   H-PCE in ACTN, a change in abstract topology learned by the P-PCE
   could also trigger the update. Some other external factors (such as a
   measurement probe) could also be a trigger at the P-PCE. Any such
 


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   update would require an inter-domain path recomputation as described
   in [RFC6805].


   The end-to-end inter-domain path computation and setup is described
   in [RFC6805]. Additionally, a per-domain stitched LSP model is also
   applicable in a P-PCE initiation model. Section 3.1, Section 3.2, and
   Section 3.3 describe the end-to-end Contiguous LSP setup, whereas
   Section 3.3.1 describes the per-domain stitching.

1.2.1.  Applicability to ACTN

   [RFC8453] describes a framework for the Abstraction and Control of TE
   Networks (ACTN), where each Provisioning Network Controller (PNC) is
   equivalent to a C-PCE, and the P-PCE is the Multi-Domain Service
   Coordinator (MDSC).  The Per-Domain stitched LSP as per the
   Hierarchical PCE architecture described in Section 3.3.1 and Section
   4.1 is well suited for ACTN deployments.

   [RFC8637] examines the applicability of PCE to the ACTN framework. To
   support the function of multi-domain coordination via hierarchy, the
   hierarchy of stateful PCEs plays a crucial role.

   In the ACTN framework, a Customer Network Controller (CNC) can
   request the MDSC to check whether there is a possibility to meet
   Virtual Network (VN) requirements before requesting that the VN be
   provisioned.  The H-PCE architecture as described in [RFC6805] can
   support this function using PCReq and PCRep messages between the
   P-PCE and C-PCEs. When the CNC requests VN provisioning, the MDSC
   decomposes this request into multiple inter-domain LSP provisioning
   requests, which might be further decomposed to per-domain path
   segments. This is described in Section 3.3.1. The MDSC uses the LSP
   Initiate Request (PCInitiate) message from the P-PCE towards the
   C-PCE, and the C-PCE reports the state back to the P-PCE via a Path
   Computation State Report (PCRpt) message. The P-PCE could make
   changes to the LSP via the use of a Path Computation Update Request
   (PCUpd) message.

   In this case, the P-PCE (as MDSC) interacts with multiple C-PCEs (as
   PNCs) along the inter-domain path of the LSP.

1.2.2.  End-to-End Contiguous LSP

   Different signaling options for inter-domain RSVP-TE are identified
   in [RFC4726].  Contiguous LSPs are achieved using the procedures of
   [RFC3209] and [RFC3473] to create a single end-to-end LSP that spans
   all domains. [RFC6805] describes the technique to establish the
   optimum path when the sequence of domains is not known in advance. 
 


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   It shows how the PCE architecture can be extended to allow the
   optimum sequence of domains to be selected, and the optimum
   end-to-end path to be derived.

   A stateful P-PCE has to be aware of the inter-domain LSPs for it to
   consider them during path computation. For instance when a domain
   diverse path is required from another LSP, the P-PCE needs to be
   aware of the LSP. This is the Passive Stateful P-PCE as described in
   Section 3.1. Additionally, the inter-domain LSP could be delegated to
   the P-PCE, so that P-PCE could trigger an update via a PCUpd message.
   The update could be triggered on receipt of the PCRpt message that
   indicates a status change of this LSP or some other LSP. The other
   LSP could be an associated LSP (such as protection
   [I-D.ietf-pce-stateful-path-protection]) or an unrelated LSP whose
   resource change leads to re-optimization at the P-PCE. This is the
   Active Stateful Operation as described in Section 3.2. Further, the
   P-PCE could be instructed to create an inter-domain LSP on its own
   using the PCInitiate message for an E2E contiguous LSP. The P-PCE
   would send the PCInitiate message to the Ingress domain C-PCE, which
   would further instruct the Ingress PCC.

   In this document, for the Contiguous LSP, the above interactions are
   only between the ingress domain C-PCE and the P-PCE. The use of
   stateful operations for an inter-domain LSP between the
   transit/egress domain C-PCEs and the P-PCE is out of scope of this
   document.

1.2.3.  Applicability of a Stateful P-PCE

   [RFC8051] 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. These
   are also applicable to the stateful P-PCE when used for the inter-
   domain LSP path computation and setup. It should be noted that though
   the stateful P-PCE has limited direct visibility inside the child
   domain, it could still trigger re-optimization with the help of child
   PCEs based on LSP state changes, abstract topology changes, or some
   other external factors.

   The C-PCE would delegate control of the inter-domain LSP to the P-PCE
   so that the P-PCE can make changes to it. Note that, if the C-PCE
   becomes aware of a topology change that is hidden from the P-PCE, it
   could take back the delegation from the P-PCE to act on it itself.
   Similarly, a P-PCE could also request delegation if it needs to make
   a change to the LSP (refer to [I-D.ietf-pce-lsp-control-request]).


2.  Terminology
 


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   The terminology is as per [RFC4655], [RFC5440], [RFC6805], [RFC8051],
   [RFC8231], and [RFC8281]. 

   Some key terms are listed below for easy reference.  

      ACTN: Abstraction and Control of Traffic Engineering Networks

      CNC: Customer Network Controller 

      C-PCE: Child Path Computation Element  

      H-PCE: Hierarchical Path Computation Element 

      IGP: Interior Gateway Protocol

      LSP: Label Switched Path

      LSP-DB: Label Switched Path Database

      LSR: Label Switching Router

      MDSC: Multi-Domain Service Coordinator

      PCC: Path Computation Client

      PCE: Path Computation Element 

      PCEP: Path Computation Element communication Protocol 

      PNC: Provisioning Network Controller

      P-PCE: Parent Path Computation Element  

      TED: Traffic Engineering Database 

      VN: Virtual Network

2.1.  Requirement Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Hierarchical Stateful PCE

   As described in [RFC6805], in the hierarchical PCE architecture a
 


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   P-PCE maintains a domain topology map that contains the child domains
   (seen as vertices in the topology) and their interconnections (links
   in the topology).  Usually, the P-PCE has no information about the
   content of the child domains.  Each child domain has at least one PCE
   capable of computing paths across the domain.  These PCEs are known
   as Child PCEs (C-PCEs) [RFC6805] and have a direct relationship with
   the P-PCE.  The P-PCE builds the domain topology map either via
   direct configuration or from learned information received from each
   C-PCE. The network policy could be be applied while building the
   domain topology map. This has been described in detail in [RFC6805]. 

   Note that, in the scope of this document, both the C-PCEs and the P-
   PCE are stateful in nature.

   [RFC8231] specifies new functions to support a stateful PCE.  It also
   specifies that a function can be initiated either from a PCC towards
   a PCE (C-E) or from a PCE towards a PCC (E-C).

   This document extends these functions to support H-PCE Architecture
   from a C-PCE towards P-PCE (EC-EP) or from a P-PCE towards C-PCE
   (EP-EC).  All PCE types herein (EC-EP and EP-EC) are assumed to be
   "Stateful PCE".

   A number of interactions are expected in the Hierarchical Stateful
   PCE architecture. These include:

   LSP State Report (EC-EP):  a child stateful PCE sends an LSP state
      report to a Parent Stateful PCE to indicate the state of a LSP.

   LSP State Synchronization (EC-EP):  after the session between the
      Child and Parent stateful PCEs is initialized, the P-PCE must
      learn the state of C-PCE's TE LSPs.

   LSP Control Delegation (EC-EP,EP-EC):  a C-PCE grants to the P-PCE
      the right to update LSP attributes on one or more LSPs; the C-PCE
      may withdraw the delegation or the P-PCE may give up the
      delegation at any time.

   LSP Update Request (EP-EC):  a stateful P-PCE requests modification
      of attributes on a C-PCE's TE LSP.

   PCE LSP Initiation Request (EP-EC):  a stateful P-PCE requests C-PCE
      to initiate a TE LSP.


   Note that this hierarchy is recursive, so a Label Switching Router
   (LSR), as a PCC, could delegate control to a PCE, which may in turn
   delegate to its parent, which may further delegate to its parent (if
 


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   it exists). Similarly, update operations can also be applied
   recursively.

   [I-D.ietf-pce-hierarchy-extensions] defines the H-PCE Capability TLV
   that is used in the Open message to advertise the H-PCE capability.
   [RFC8231] defines the Stateful PCE Capability TLV used in the Open
   message to indicate stateful support. To indicates the support for
   stateful H-PCE operations described in this document, a PCEP speaker
   MUST include both TLVs in an Open message. It is RECOMMENDED that any
   implementation that supports stateful operations [RFC8231] and H-PCE
   [I-D.ietf-pce-hierarchy-extensions] would also implements the
   stateful H-PCE operations as described in this document. 

   Further consideration may be made for optional procedures for
   stateful communication coordination between PCEs, including
   procedures to minimize computational loops. The procedures described
   in [I-D.litkowski-pce-state-sync] facilitate stateful communication
   between PCEs for various use cases. The procedures and extensions as
   described in Section 3 of [I-D.litkowski-pce-state-sync] are also
   applicable to Child and Parent PCE communication. The
   SPEAKER-IDENTITY-TLV (defined in [RFC8232]) is included in the LSP
   object to identify the Ingress (PCC). The PLSP-ID (PCEP-specific
   identifier for the LSP, as per [RFC8231]) used in the forwarded PCRpt
   by the C-PCE to P-PCE is same as the original one used by the PCC.

3.1.  Passive Operations

   Procedures as described in [RFC6805] are applied, where the ingress
   PCC triggers a path computation request for the destination towards
   the C-PCE in the domain where the LSP originates. The C-PCE further
   forwards the request to the P-PCE. The P-PCE selects a set of
   candidate domain paths based on the domain topology and the state of
   the inter-domain links.  It then sends computation requests to the
   C-PCEs responsible for each of the domains on the candidate domain
   paths.  Each C-PCE computes a set of candidate path segments across
   its domain and sends the results to the P-PCE. The P-PCE uses this
   information to select path segments and concatenate them to derive
   the optimal end-to-end inter-domain path.  The end-to-end path is
   then sent to the C-PCE that received the initial path request, and
   this C-PCE passes the path on to the PCC that issued the original
   request.

   As per [RFC8231], PCC sends an LSP State Report carried on a PCRpt
   message to the C-PCE, indicating the LSP's status.  The C-PCE may
   further propagate the State Report to the P-PCE.  A local policy at
   C-PCE may dictate which LSPs are reported to the P-PCE.  The PCRpt
   message is sent from C-PCE to P-PCE.

 


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   State synchronization mechanisms as described in [RFC8231] and
   [RFC8232] are applicable to a PCEP session between C-PCE and P-PCE as
   well.

   We use the hierarchical domain topology example from [RFC6805] as the
   reference topology for the entirety of this document.  It is shown in
   Figure 1.

      -----------------------------------------------------------------
     |   Domain 5                                                      |
     |                              -----                              |
     |                             |PCE 5|                             |
     |                              -----                              |
     |                                                                 |
     |    ----------------     ----------------     ----------------   |
     |   | Domain 1       |   | Domain 2       |   | Domain 3       |  |
     |   |                |   |                |   |                |  |
     |   |        -----   |   |        -----   |   |        -----   |  |
     |   |       |PCE 1|  |   |       |PCE 2|  |   |       |PCE 3|  |  |
     |   |        -----   |   |        -----   |   |        -----   |  |
     |   |                |   |                |   |                |  |
     |   |            ----|   |----        ----|   |----            |  |
     |   |           |BN11+---+BN21|      |BN23+---+BN31|           |  |
     |   |   -        ----|   |----        ----|   |----        -   |  |
     |   |  |S|           |   |                |   |           |D|  |  |
     |   |   -        ----|   |----        ----|   |----        -   |  |
     |   |           |BN12+---+BN22|      |BN24+---+BN32|           |  |
     |   |            ----|   |----        ----|   |----            |  |
     |   |                |   |                |   |                |  |
     |   |         ----   |   |                |   |   ----         |  |
     |   |        |BN13|  |   |                |   |  |BN33|        |  |
     |    -----------+----     ----------------     ----+-----------   |
     |                \                                /               |
     |                 \       ----------------       /                |
     |                  \     |                |     /                 |
     |                   \    |----        ----|    /                  |
     |                    ----+BN41|      |BN42+----                   |
     |                        |----        ----|                       |
     |                        |                |                       |
     |                        |        -----   |                       |
     |                        |       |PCE 4|  |                       |
     |                        |        -----   |                       |
     |                        |                |                       |
     |                        | Domain 4       |                       |
     |                         ----------------                        |
     |                                                                 |
      -----------------------------------------------------------------

 


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              Figure 1: Hierarchical Domain Topology Example


   Steps 1 to 11 are exactly as described in section 4.6.2 of [RFC6805]
   (Hierarchical PCE End-to-End Path Computation Procedure), the
   following additional steps are added for stateful PCE, to be executed
   at the end:

   (A)  The Ingress LSR initiates the setup of the LSP as per the path
        and reports to the PCE1 the LSP status ("GOING-UP").

   (B)  The PCE1 further reports the status of the LSP to the P-PCE
        (PCE5).

   (C)  The Ingress LSR notifies the LSP state to PCE1 when the state is
        "UP".

   (D)  The PCE1 further reports the status of the LSP to the P-PCE
        (PCE5).

   The Ingress LSR could trigger path re-optimization by sending the
   path computation request as described in [RFC6805], at this time it
   can include the LSP object in the PCReq message as described in
   [RFC8231].

3.2.  Active Operations

   [RFC8231] describes the case of active stateful PCE. The active PCE
   functionality uses two specific PCEP messages:

   o Update Request (PCUpd)

   o State Report (PCRpt)

   The first is sent by the PCE to a PCC for modifying LSP attributes.
   The PCC sends back a PCRpt to acknowledge the requested operation or
   report any change in LSP's state.

   As per [RFC8051], Delegation is an operation to grant a PCE temporary
   rights to modify a subset of LSP parameters on one or more PCC's
   LSPs.  The C-PCE may further choose to delegate to its P-PCE based on
   a local policy.  The PCRpt message with the "D" (delegate) flag is
   sent from C-PCE to P-PCE.

   To update an LSP, a PCE sends an LSP Update Request to the PCC using
   a PCUpd message.  For an LSP delegated to a P-PCE via the C-PCE; the
   P-PCE can use the same PCUpd message to request a change to the C-PCE
   (the Ingress domain PCE).  The C-PCE further propagates the update
 


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   request to the PCC.

   The P-PCE uses the same mechanism described in Section 3.1 to compute
   the end to end path using PCReq and PCRep messages.

   For active operations, the following steps are required when
   delegating the LSP, again using the reference architecture described
   in Figure 1 (Hierarchical Domain Topology Example).

   (A)  The Ingress LSR delegates the LSP to the PCE1 via PCRpt message
        with D flag set.

   (B)  The PCE1 further delegates the LSP to the P-PCE (PCE5).

   (C)  Steps 4 to 10 of section 4.6.2 of [RFC6805] are executed at
        P-PCE (PCE5) to determine the end to end path.

   (D)  The P-PCE (PCE5) sends the update request to the C-PCE (PCE1)
        via PCUpd message.

   (E)  The PCE1 further updates the LSP to the Ingress LSR (PCC).

   (F)  The Ingress LSR initiates the setup of the LSP as per the path
        and reports to the PCE1 the LSP status ("GOING-UP").

   (G)  The PCE1 further reports the status of the LSP to the P-PCE
        (PCE5).

   (H)  The Ingress LSR notifies the LSP state to PCE1 when the state is
        "UP".

   (I)  The PCE1 further reports the status of the LSP to the P-PCE
        (PCE5).

3.3.  PCE Initiation of LSPs

   [RFC8281] describes the setup, maintenance and teardown of PCE-
   initiated LSPs under the stateful PCE model, without the need for
   local configuration on the PCC, thus allowing for a dynamic network
   that is centrally controlled and deployed.  To instantiate or delete
   an LSP, the PCE sends the Path Computation LSP Initiate Request
   (PCInitiate) message to the PCC.  In case of an inter-domain LSP in
   Hierarchical PCE architecture, the initiation operations can be
   carried out at the P-PCE.  In which case after the P-PCE finishes the
   E2E path computation, it can send the PCInitiate message to the C-PCE
   (the Ingress domain PCE), the C-PCE further propagates the initiate
   request to the PCC.

 


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   The following steps are performed for PCE-initiated operations, again
   using the reference architecture described in Figure 1 (Hierarchical
   Domain Topology Example):


   (A)  The P-PCE (PCE5) is requested to initiate a LSP. Steps 4 to 10
        of section 4.6.2 of [RFC6805] are executed to determine the end
        to end path.

   (B)  The P-PCE (PCE5) sends the initiate request to the child PCE
        (PCE1) via PCInitiate message.

   (C)  The PCE1 further propagates the initiate message to the Ingress
        LSR (PCC).

   (D)  The Ingress LSR initiates the setup of the LSP as per the path
        and reports to the PCE1 the LSP status ("GOING-UP").

   (E)  The PCE1 further reports the status of the LSP to the P-PCE
        (PCE5).

   (F)  The Ingress LSR notifies the LSP state to PCE1 when the state is
        "UP".

   (G)  The PCE1 further reports the status of the LSP to the P-PCE
        (PCE5).

   The Ingress LSR (PCC) would generate the PLSP-ID for the LSP and
   inform the C-PCE, which is propagated to the P-PCE.

3.3.1.  Per-Domain Stitched LSP

   The Hierarchical PCE architecture as per [RFC6805] is primarily used
   for E2E LSP.  With PCE-Initiated capability, another mode of
   operation is possible, where multiple intra-domain LSPs are initiated
   in each domain, which are further stitched to form an E2E LSP.  The
   P-PCE sends PCInitiate message to each C-PCE separately to initiate
   individual LSP segments along the domain path.  These individual Per-
   Domain LSP are stitched together by some mechanism, which is out of
   scope of this document (Refer [I-D.dugeon-pce-stateful-
   interdomain]).

   The following steps are performed for the Per-Domain stitched LSP
   operation, again using the reference architecture described in Figure
   1 (Hierarchical Domain Topology Example):

   (A)  The P-PCE (PCE5) is requested to initiate a LSP. Steps 4 to 10
        of section 4.6.2 of [RFC6805] are executed to determine the end
 


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        to end path, which are broken into per-domain LSPs say -

        o  S-BN41

        o  BN41-BN33

        o  BN33-D

        It should be noted that the P-PCE may use other mechanisms to
        determine the suitable per-domain LSPs (apart from [RFC6805]).

   For LSP (BN33-D)

   (B)  The P-PCE (PCE5) sends the initiate request to the child PCE
        (PCE3) via PCInitiate message for LSP (BN33-D).

   (C)  The PCE3 further propagates the initiate message to BN33.

   (D)  BN33 initiates the setup of the LSP as per the path and reports
        to the PCE3 the LSP status ("GOING-UP").

   (E)  The PCE3 further reports the status of the LSP to the P-PCE
        (PCE5).

   (F)  The node BN33 notifies the LSP state to PCE3 when the state is
        "UP".

   (G)  The PCE3 further reports the status of the LSP to the P-PCE
        (PCE5).

   For LSP (BN41-BN33)

   (H)  The P-PCE (PCE5) sends the initiate request to the child PCE
        (PCE4) via PCInitiate message for LSP (BN41-BN33).

   (I)  The PCE4 further propagates the initiate message to BN41.

   (J)  BN41 initiates the setup of the LSP as per the path and reports
        to the PCE4 the LSP status ("GOING-UP").

   (K)  The PCE4 further reports the status of the LSP to the P-PCE
        (PCE5).

   (L)  The node BN41 notifies the LSP state to PCE4 when the state is
        "UP".

   (M)  The PCE4 further reports the status of the LSP to the P-PCE
        (PCE5).
 


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   For LSP (S-BN41)

   (N)  The P-PCE (PCE5) sends the initiate request to the child PCE
        (PCE1) via PCInitiate message for LSP (S-BN41).

   (O)  The PCE1 further propagates the initiate message to node S.

   (P)  S initiates the setup of the LSP as per the path and reports to
        the PCE1 the LSP status ("GOING-UP").

   (Q)  The PCE1 further reports the status of the LSP to the P-PCE
        (PCE5).

   (R)  The node S notifies the LSP state to PCE1 when the state is
        "UP".

   (S)  The PCE1 further reports the status of the LSP to the P-PCE
        (PCE5).

   Additionally:

   (T)  Once P-PCE receives report of each per-domain LSP, it should use
        suitable stitching mechanism, which is out of scope of this
        document. In this step, P-PCE (PCE5) could also initiate an E2E
        LSP (S-D) by sending the PCInitiate message to Ingress C-PCE
        (PCE1).

   Note that each per-domain LSP can be set up in parallel. Further, it
   is also possible to stitch the per-domain LSP at the same time as the
   per-domain LSPs are initiated. This option is defined in
   [I-D.dugeon-pce-stateful-interdomain].


4.  Security Considerations

   The security considerations listed in [RFC8231],[RFC6805] and
   [RFC5440] apply to this document as well. As per [RFC6805], it is
   expected that the parent PCE will require all child PCEs to use full
   security (i.e. the highest security mechanism available for PCEP)
   when communicating with the parent.

   Any multi-domain operation necessarily involves the exchange of
   information across domain boundaries.  This is bound to represent a
   significant security and confidentiality risk especially when the
   child domains are controlled by different commercial concerns.  PCEP
   allows individual PCEs to maintain confidentiality of their domain
   path information using path-keys [RFC5520], and the hierarchical PCE
   architecture is specifically designed to enable as much isolation of
 


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   domain topology and capabilities information as is possible. The LSP
   state in the PCRpt message must continue to maintain the internal
   domain confidentiality when required.

   The security consideration for PCE-Initiated LSP as per [RFC8281] is
   also applicable from P-PCE to C-PCE. 

   Further, section 6.3 describes the use of path-key [RFC5520] for
   confidentiality between C-PCE and P-PCE. 

   Thus it is RECOMMENDED to secure the PCEP session (between the P-PCE
   and the C-PCE) using Transport Layer Security (TLS) [RFC8446] (per
   the recommendations and best current practices in BCP 195 [RFC7525])
   and/or TCP Authentication Option (TCP-AO) [RFC5925]. The guidance for
   implementing PCEP with TLS can be found in [RFC8253].

   In case of TLS, due care needs to be taken while exposing the
   parameters of the X.509 certificate, such as subjectAltName:otherName
   which is set to Speaker Entity Identifier [RFC8232] as per [RFC8253],
   to ensure uniqueness and avoid any mismatch.

5.  Manageability Considerations

   All manageability requirements and considerations listed in
   [RFC5440], [RFC6805], [RFC8231], and [RFC8281] apply to Stateful H-
   PCE defined in this document.  In addition, requirements and
   considerations listed in this section apply.

5.1.  Control of Function and Policy

   Support of the hierarchical procedure will be controlled by the
   management organization responsible for each child PCE. The parent
   PCE must only accept path computation requests from authorized child
   PCEs.  If a parent PCE receives a report from an unauthorized child
   PCE, the report should be dropped. All mechanisms as described in
   [RFC8231] and [RFC8281] continue to apply.

5.2.  Information and Data Models

   An implementation should allow the operator to view the stateful and
   H-PCE capabilities advertised by each peer. The "ietf-pcep" PCEP YANG
   module is specified in [I-D.ietf-pce-pcep-yang]. This YANG module
   will be required to be augmented to also include details for stateful
   H-PCE deployment and operation. The exact model and attributes are
   out of scope for this document.

5.3.  Liveness Detection and Monitoring

 


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

5.4.  Verify Correct Operations

   Mechanisms defined in this document do not imply any new operation
   verification requirements in addition to those already listed in
   [RFC5440] and [RFC8231].

5.5.  Requirements On Other Protocols

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

5.6.  Impact On Network Operations

   Mechanisms defined in [RFC5440] and [RFC8231] also apply to PCEP
   extensions defined in this document.

   The stateful H-PCE technique brings the applicability of stateful PCE
   as described in [RFC8051], for the LSP traversing multiple domains.

   As described in Section 3, a PCEP speaker includes both the H-PCE
   Capability TLV [I-D.ietf-pce-hierarchy-extensions] and Stateful PCE
   Capability TLV [RFC8231] to indicate support for Stateful H-PCE. Note
   that there is a possibility of a PCEP speaker that does not support
   the Stateful H-PCE feature but does provide support for Stateful PCE
   [RFC8231] and H-PCE [I-D.ietf-pce-hierarchy-extensions] features.
   This PCEP speaker will also include both the TLVs and in this case a
   PCEP peer could falsely assume that the stateful H-PCE feature is
   also supported. On further PCEP message exchange, the stateful
   messages will not get further propagated (as described in this
   document) and a stateful H-PCE based 'parent' control of the LSP will
   not happen. A PCEP peer should be prepared for this eventuality as a
   part of normal procedures.


5.7.  Error Handling between PCEs

   Apart from the basic error handling described in this document, an
   implementation could also use the enhanced error and notification
   mechanism for stateful H-PCE operations as per [I-D.ietf-pce-
   enhanced-errors]. Enhanced features such as error behavior
   propagation, notification and error criticality level, are further
   defined in [I-D.ietf-pce-enhanced-errors].

6.  Other Considerations
 


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6.1.  Applicability to Inter-Layer Traffic Engineering

   [RFC5623] describes a framework for applying the PCE-based
   architecture to inter-layer (G)MPLS traffic engineering.  The H-PCE
   Stateful architecture with stateful P-PCE coordinating with the
   stateful C-PCEs of higher and lower layer is shown in the figure
   below.


                                                 +----------+
                                                 | Parent   |
                                                /| PCE      |
                                               / +----------+
                                              /     /   Stateful
                                             /     /    P-PCE
                                            /     /
                                           /     /
                          Stateful+-----+ /     /
                          C-PCE   | PCE |/     /
                          Hi      | Hi  |     /
                                  +-----+    /
          +---+    +---+                    /     +---+    +---+
         + LSR +--+ LSR +........................+ LSR +--+ LSR +
         + H1  +  + H2  +                 /      + H3  +  + H4  +
          +---+    +---+\         +-----+/       /+---+    +---+
                         \        | PCE |       /
                          \       | Lo  |      /
                Stateful   \      +-----+     /
                C-PCE       \                /
                Lo           \+---+    +---+/
                             + LSR +--+ LSR +
                             + L1  +  + L2  +
                              +---+    +---+

                 Figure 2: Sample Inter-Layer Topology

   All procedures described in Section 3 are applicable to inter-layer
   (and therefore separate domains) path setup as well.

6.2.  Scalability Considerations

   It should be noted that if all the C-PCEs would report all the LSPs
   in their domain, it could lead to scalability issues for the P-PCE.
   Thus it is recommended to only report the LSPs which are involved in
   H-PCE, i.e. the LSPs which are either delegated to the P-PCE or
   initiated by the P-PCE. Scalability considerations for PCEP as per
   [RFC8231] continue to apply for the PCEP session between child and
   parent PCE.
 


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6.3.  Confidentiality

   As described in section 4.2 of [RFC6805], information about the
   content of child domains is not shared for both scaling and
   confidentiality reasons. The child PCE could also conceal the path
   information during path computation. A C-PCE may replace a path
   segment with a path-key [RFC5520], effectively hiding the content of
   a segment of a path.

7.  IANA Considerations

   There are no IANA considerations.

8.  Acknowledgments

   Thanks to Manuela Scarella, Haomian Zheng, Sergio Marmo, Stefano
   Parodi, Giacomo Agostini, Jeff Tantsura, Rajan Rao, Adrian Farrel and
   Haomian Zheng, for their reviews and suggestions.

   Thanks to Tal Mazrahi for the RTGDIR review, Paul Kyzivat for the
   GENART review, and Stephen Farrell for SECDIR review.

   Thanks to Barry Leiba, Martin Vigoureux, Benjamin Kaduk, and Roman
   Danyliw for IESG review. 

9.  References

9.1.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, DOI
              10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC4655]  Farrel, A., Vasseur, J., and J. Ash, "A Path Computation
              Element (PCE)-Based Architecture", RFC 4655, DOI
              10.17487/RFC4655, August 2006,
              <https://www.rfc-editor.org/info/rfc4655>.

   [RFC5440]  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation
              Element (PCE) Communication Protocol (PCEP)", RFC 5440,
              DOI 10.17487/RFC5440, March 2009,
              <http://www.rfc-editor.org/info/rfc5440>.

   [RFC5520]  Bradford, R., Ed., Vasseur, JP., and A. Farrel,
              "Preserving Topology Confidentiality in Inter-Domain Path
              Computation Using a Path-Key-Based Mechanism", RFC 5520,
              DOI 10.17487/RFC5520, April 2009,
 


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              <https://www.rfc-editor.org/info/rfc5520>.

   [RFC5925]  Touch, J., Mankin, A., and R. Bonica, "The TCP
              Authentication Option", RFC 5925, DOI 10.17487/RFC5925,
              June 2010, <https://www.rfc-editor.org/info/rfc5925>.

   [RFC6805]  King, D., Ed. and A. Farrel, Ed., "The Application of the
              Path Computation Element Architecture to the Determination
              of a Sequence of Domains in MPLS and GMPLS", RFC 6805, DOI
              10.17487/RFC6805, November 2012,
              <http://www.rfc-editor.org/info/rfc6805>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <https://www.rfc-editor.org/info/rfc7525>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8231]  Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for Stateful PCE", RFC 8231,
              DOI 10.17487/RFC8231, September 2017,
              <https://www.rfc-editor.org/info/rfc8231>.

   [RFC8253]  Lopez, D., Gonzalez de Dios, O., Wu, Q., and D. Dhody,
              "PCEPS: Usage of TLS to Provide a Secure Transport for the
              Path Computation Element Communication Protocol (PCEP)",
              RFC 8253, DOI 10.17487/RFC8253, October 2017,
              <https://www.rfc-editor.org/info/rfc8253>.

   [RFC8281]  Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "Path
              Computation Element Communication Protocol (PCEP)
              Extensions for PCE-Initiated LSP Setup in a Stateful PCE
              Model", RFC 8281, DOI 10.17487/RFC8281, December 2017,
              <https://www.rfc-editor.org/info/rfc8281>.

   [RFC8446]   Rescorla, E., "The Transport Layer Security (TLS)
              Protocol Version 1.3", RFC 8446, DOI 10.17487/RFC8446,
              August 2018, <https://www.rfc-editor.org/info/rfc8446>.

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
 


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              Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001,
              <https://www.rfc-editor.org/info/rfc3209>.

   [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,
              <https://www.rfc-editor.org/info/rfc3473>.

   [RFC4726]  Farrel, A., Vasseur, J., and A. Ayyangar, "A Framework for
              Inter-Domain Multiprotocol Label Switching Traffic
              Engineering", RFC 4726, DOI 10.17487/RFC4726, November
              2006, <https://www.rfc-editor.org/info/rfc4726>.

   [RFC5623]  Oki, E., Takeda, T., Le Roux, JL., and A. Farrel,
              "Framework for PCE-Based Inter-Layer MPLS and GMPLS
              Traffic Engineering", RFC 5623, DOI 10.17487/RFC5623,
              September 2009,
              <https://www.rfc-editor.org/info/rfc5623>.

   [RFC8051]  Zhang, X., Ed. and I. Minei, Ed., "Applicability of a
              Stateful Path Computation Element (PCE)", RFC 8051,
              DOI 10.17487/RFC8051, January 2017,
              <https://www.rfc-editor.org/info/rfc8051>.

   [RFC8232]  Crabbe, E., Minei, I., Medved, J., Varga, R., Zhang, X.,
              and D. Dhody, "Optimizations of Label Switched Path State
              Synchronization Procedures for a Stateful PCE", RFC 8232,
              DOI 10.17487/RFC8232, September 2017,
              <https://www.rfc-editor.org/info/rfc8232>.

   [RFC8453]  Ceccarelli, D., Ed. and Y. Lee, Ed., "Framework for
              Abstraction and Control of TE Networks (ACTN)", RFC 8453,
              DOI 10.17487/RFC8453, August 2018,
              <https://www.rfc-editor.org/info/rfc8453>.

   [RFC8637]  Dhody, D., Lee, Y., and D. Ceccarelli, "Applicability of
              the Path Computation Element (PCE) to the Abstraction and
              Control of TE Networks (ACTN)", RFC 8637, DOI
              10.17487/RFC8637, July 2019,
              <https://www.rfc-editor.org/info/rfc8637>.

   [I-D.litkowski-pce-state-sync]
              Litkowski, S., Sivabalan, S., and D. Dhody, "Inter
              Stateful Path Computation Element communication
              procedures", draft-litkowski-pce-state-sync-06 (work in
              progress), July 2019.

 


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   [I-D.ietf-pce-hierarchy-extensions]
              Zhang, F., Zhao, Q., Dios, O., Casellas, R., and D. King,
              "Extensions to Path Computation Element Communication
              Protocol (PCEP) for Hierarchical Path Computation Elements
              (PCE)", draft-ietf-pce-hierarchy-extensions-11 (work in
              progress), June 2019.

   [I-D.ietf-pce-pcep-yang] 
              Dhody, D., Hardwick, J., Beeram, V., and J. Tantsura, "A 
              YANG Data Model for Path Computation Element 
              Communications Protocol (PCEP)", 
              draft-ietf-pce-pcep-yang-12 (work in progress), July 2019.

   [I-D.dugeon-pce-stateful-interdomain]
              Dugeon, O., Meuric, J., Lee, Y., Dhody, D., and D.
              Ceccarelli, "PCEP Extension for Stateful Inter-Domain
              Tunnels", draft-dugeon-pce-stateful-interdomain-02 (work
              in progress), March 2019.

   [I-D.ietf-pce-lsp-control-request]
              Raghuram, A., Goddard, A., Yadlapalli, C., Karthik, J.,
              Sivabalan, S., Parker, J., and M. Negi, "Ability for a
              stateful Path Computation Element (PCE) to request and
              obtain control of a LSP", draft-ietf-pce-lsp-control-
              request-11 (work in progress), October 2019.

   [I-D.ietf-pce-enhanced-errors]
              Pouyllau, et al., "Extensions to PCEP for Enhanced Errors"
              , draft-ietf-pce-enhanced-errors-06 (work in progress),
              August 2019.

   [I-D.ietf-pce-stateful-path-protection]
              Ananthakrishnan, H., Sivabalan, S., Barth, C., Minei, I.,
              and M. Negi, "PCEP Extensions for Associating Working and
              Protection LSPs with Stateful PCE", draft-ietf-pce-
              stateful-path-protection-11 (work in progress), September 
              2019.

Contributors

   Avantika
   ECI Telecom
   India

   EMail: avantika.srm@gmail.com

   Xian Zhang
   Huawei Technologies
 


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   Bantian, Longgang District
   Shenzhen, Guangdong  518129
   P.R.China

   EMail: zhang.xian@huawei.com

   Udayasree Palle

   EMail: udayasreereddy@gmail.com

   Oscar Gonzalez de Dios
   Telefonica I+D
   Don Ramon de la Cruz 82-84
   Madrid,   28045
   Spain
   Phone: +34913128832

   EMail: oscar.gonzalezdedios@telefonica.com

Authors' Addresses

   Dhruv Dhody
   Huawei Technologies
   Divyashree Techno Park, Whitefield
   Bangalore, Karnataka  560066
   India

   EMail: dhruv.ietf@gmail.com

   Young Lee
   SKKU

   EMail: younglee.tx@gmail.com

   Daniele Ceccarelli
   Ericsson
   Torshamnsgatan,48
   Stockholm
   Sweden

   EMail: daniele.ceccarelli@ericsson.com

   Jongyoon Shin
   SK Telecom
   6 Hwangsaeul-ro, 258 beon-gil, Bundang-gu, Seongnam-si,
   Gyeonggi-do  463-784
   Republic of Korea

 


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   EMail: jongyoon.shin@sk.com

   Daniel King
   Lancaster University
   UK

   EMail: d.king@lancaster.ac.uk












































Dhody, et al.              Expires April 2020                  [Page 24]
