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An Architecture for Use of PCE and the PCE Communication Protocol (PCEP) in a Network with Central Control

 


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Internet Engineering Task Force (IETF)                    A. Farrel, Ed.
Request for Comments: 8283                              Juniper Networks
Category: Informational                                     Q. Zhao, Ed.
ISSN: 2070-1721                                                    R. Li
                                                     Huawei Technologies
                                                                 C. Zhou
                                                           Cisco Systems
                                                           December 2017


An Architecture for Use of PCE and the PCE Communication Protocol (PCEP)
                   in a Network with Central Control

Abstract

   The Path Computation Element (PCE) is a core component of Software-
   Defined Networking (SDN) systems.  It can compute optimal paths for
   traffic across a network and can also update the paths to reflect
   changes in the network or traffic demands.

   PCE was developed to derive paths for MPLS Label Switched Paths
   (LSPs), which are supplied to the head end of the LSP using the Path
   Computation Element Communication Protocol (PCEP).

   SDN has a broader applicability than signaled MPLS traffic-engineered
   (TE) networks, and the PCE may be used to determine paths in a range
   of use cases including static LSPs, segment routing, Service Function
   Chaining (SFC), and most forms of a routed or switched network.  It
   is, therefore, reasonable to consider PCEP as a control protocol for
   use in these environments to allow the PCE to be fully enabled as a
   central controller.

   This document briefly introduces the architecture for PCE as a
   central controller, examines the motivations and applicability for
   PCEP as a control protocol in this environment, and introduces the
   implications for the protocol.  A PCE-based central controller can
   simplify the processing of a distributed control plane by blending it
   with elements of SDN and without necessarily completely replacing it.

   This document does not describe use cases in detail and does not
   define protocol extensions: that work is left for other documents.

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Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Not all documents
   approved by the IESG are a candidate for any level of Internet
   Standard; see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8283.

Copyright Notice

   Copyright (c) 2017 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.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Architecture  . . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Resilience and Scaling  . . . . . . . . . . . . . . . . .   8
       2.1.1.  Partitioned Network . . . . . . . . . . . . . . . . .   9
       2.1.2.  Multiple Parallel Controllers . . . . . . . . . . . .  10
       2.1.3.  Hierarchical Controllers  . . . . . . . . . . . . . .  12
   3.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .  13
     3.1.  Technology-Oriented Applicability . . . . . . . . . . . .  14
       3.1.1.  Applicability to Control-Plane Operated Networks  . .  14
       3.1.2.  Static LSPs in MPLS . . . . . . . . . . . . . . . . .  14
       3.1.3.  MPLS Multicast  . . . . . . . . . . . . . . . . . . .  15
       3.1.4.  Transport SDN . . . . . . . . . . . . . . . . . . . .  15
       3.1.5.  Segment Routing . . . . . . . . . . . . . . . . . . .  15
       3.1.6.  Service Function Chaining . . . . . . . . . . . . . .  16
     3.2.  High-Level Applicability  . . . . . . . . . . . . . . . .  16
       3.2.1.  Traffic Engineering . . . . . . . . . . . . . . . . .  16
       3.2.2.  Traffic Classification  . . . . . . . . . . . . . . .  17
       3.2.3.  Service Delivery  . . . . . . . . . . . . . . . . . .  17
   4.  Protocol Implications / Guidance for Solution Developers  . .  18
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  19
   6.  Manageability Considerations  . . . . . . . . . . . . . . . .  19
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  20
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  20
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  20
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  21
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  23
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  24
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

1.  Introduction

   The Path Computation Element (PCE) [RFC4655] was developed to offload
   path computation function from routers in an MPLS traffic-engineered
   network.  Since then, the role and function of the PCE has grown to
   cover a number of other uses (such as GMPLS [RFC7025]) and to allow
   delegated control [RFC8231] and PCE-initiated use of network
   resources [RFC8281].

   According to [RFC7399], Software-Defined Networking (SDN) refers to a
   separation between the control elements and the forwarding components
   so that software running in a centralized system, called a
   controller, can act to program the devices in the network to behave
   in specific ways.  A required element in an SDN architecture is a
   component that plans how the network resources will be used and how
   the devices will be programmed.  It is possible to view this
   component as performing specific computations to place traffic flows

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   within the network given knowledge of the availability of network
   resources, how other forwarding devices are programmed, and the way
   that other flows are routed.  This is the function and purpose of a
   PCE, and the way that a PCE integrates into a wider network control
   system (including an SDN system) is presented in [RFC7491].

   In early PCE implementations, where the PCE was used to derive paths
   for MPLS Label Switched Paths (LSPs), paths were requested by network
   elements (known as Path Computation Clients (PCCs)), and the results
   of the path computations were supplied to network elements using the
   Path Computation Element Communication Protocol (PCEP) [RFC5440].
   This protocol was later extended to allow a PCE to send unsolicited
   requests to the network for LSP establishment [RFC8281].

   SDN has a far broader applicability than just signaled MPLS or GMPLS
   traffic-engineered networks.  The PCE component in an SDN system may
   be used to determine paths in a wide range of use cases including
   static LSPs, segment routing [SR-ARCH], SFC [RFC7665], and indeed any
   form of routed or switched network.  It is, therefore, reasonable to
   consider PCEP as a general southbound control protocol (i.e., a
   control protocol for communicating from the central controller to
   network elements) for use in these environments to allow the PCE to
   be fully enabled as a central controller.

   This document introduces the architecture for PCE as a central
   controller as an extension of the architecture described in [RFC4655]
   and assumes the continued use of PCEP as the protocol used between
   PCE and PCC.  This document also examines the motivations and
   applicability for PCEP as a Southbound Interface (SBI) and introduces
   the implications for the protocol used in this way.  A PCE-based
   central controller can simplify the processing of a distributed
   control plane by blending it with elements of SDN and without
   necessarily completely replacing it.

   This document does not describe use cases in detail and does not
   define protocol extensions: that work is left for other documents.

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2.  Architecture

   The architecture for the use of PCE within centralized control of a
   network is based on the understanding that a PCE can determine how
   connections should be placed and how resources should be used within
   the network, and that the PCE can then cause those connections to be
   established.  Figure 1 shows how this control relationship works in a
   network with an active control plane.  This is a familiar view for
   those who have read and understood [RFC4655] and [RFC8281].

   In this mode of operation, the central controller is asked to create
   connectivity by a network orchestrator, a service manager, an
   Operations Support System (OSS), a Network Management Station (NMS),
   or some other application.  The PCE-based controller computes paths
   with awareness of the network topology, the available resources, and
   the other services supported in the network.  This information is
   held in the Traffic Engineering Database (TED) and other databases
   available to the PCE.  Then the PCE sends a request using PCEP to one
   of the Network Elements (NEs), and that NE uses a control plane to
   establish the requested connections and reserve the network
   resources.

   Note that other databases (such as an LSP Database (LSP-DB)) might
   also be used, but for simplicity of illustration, just the TED is
   shown.

              --------------------------------------------
             | Orchestrator / Service Manager / OSS / NMS |
              --------------------------------------------
                      ^
                      |
                      v
                  ------------
                 |            |     -----
                 | PCE-Based  |<---| TED |
                 | Controller |     -----
                 |            |
                  ------------
                    ^
                PCEP|
                    v
                   ----             ----       ----       ----
                  | NE |<--------->| NE |<--->| NE |<--->| NE |
                   ----  Signaling  ----       ----       ----
                         Protocol

          Figure 1: Architecture for the Central Controller with
                              a Control Plane

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   Although the architecture shown in Figure 1 represents a form of SDN,
   one objective of SDN in some environments is to remove the dependency
   on a control plane.  A transition architecture toward this goal is
   presented in [RFC7491] and is shown in Figure 2.  In this case,
   services are still requested in the same way, and the PCE-based
   controller still requests use of the network using PCEP.  The main
   difference is that the consumer of the PCEP messages is a network
   controller that provisions the resources and instructs the data plane
   using an SBI that provides an interface to each NE.

                --------------------------------------------
               | Orchestrator / Service Manager / OSS / NMS |
                --------------------------------------------
                                   ^
                                   |
                                   v
                              ------------
                             |            |     -----
                             | PCE-Based  |<---| TED |
                             | Controller |     -----
                             |            |
                              ------------
                                   ^
                                   | PCEP
                                   v
                              ------------
                             |  Network   |
                             | Controller |
                             /------------\
                        SBI /   ^       ^  \
                           /    |       |   \
                          /     v       v    \
                     ----/    ----     ----   \----
                    | NE |   | NE |   | NE |  | NE |
                     ----     ----     ----    ----

           Figure 2: Architecture Including a Network Controller

   The approach in Figure 2 delivers the SDN functionality but is overly
   complicated and insufficiently flexible.

   o  The complication is created by the use of two controllers in a
      hierarchical organization and the resultant use of two protocols
      in a southbound direction.

   o  The lack of flexibility arises from the assumed or required lack
      of a control plane.

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   This document describes an architecture that reduces the number of
   components and is flexible to a number of deployment models and use
   cases.  In this hybrid approach (shown in Figure 3), the network
   controller is PCE enabled and can also speak PCEP as the SBI (i.e.,
   it can communicate with each node along the path using PCEP).  That
   means that the controller can communicate with a conventional
   control-plane-enabled NE using PCEP and can also use the same
   protocol to program individual NEs.  In this way, the PCE-based
   controller can control a wider range of networks and deliver many
   different functions as described in Section 3.

   There will be a trade-off in different application scenarios.  In
   some cases, the use of a control plane will simplify deployment (for
   example, by distributing recovery actions), and in other cases, a
   control plane may add operational complexity.

   PCEP is essentially already capable of acting as an SBI and only
   small, use-case-specific modifications to the protocol are needed to
   support this architecture.  The implications for the protocol are
   discussed further in Section 4.

                  --------------------------------------------
                 | Orchestrator / Service Manager / OSS / NMS |
                  --------------------------------------------
                                      ^
                                      |
                                      v
                                ------------
                               |            |     -----
                               | PCE-Based  |<---| TED |
                               | Controller |     -----
                               |            |
                               /------------\
                         PCEP /   ^       ^  \
                             /    |       |   \
                            /     v       v    \
                           /    ----     ----   \
                          /    | NE |   | NE |   \
                     ----/      ----     ----     \----
                    | NE |                        | NE |
                     ----                          ----
                       ^        ----     ----      ^
                       :......>| NE |...| NE |<....:
             Signaling Protocol ----     ----

          Figure 3: Architecture for Node-by-Node Central Control

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2.1.  Resilience and Scaling

   Systems with central controllers are vulnerable to two problems:
   failure of the controller or overload of the controller.  These
   concerns are not unique to the use of a PCE-based controller, but
   they need to be addressed in this document before the PCE-based
   controller architecture can be considered for use in all but the
   smallest networks.

   There are three architectural mechanisms that can be applied to
   address these issues.  The mechanisms are described separately for
   clarity, but a deployment may use any combination of the approaches.

   For simplicity of illustration, these three approaches are shown in
   the sections that follow without a control plane.  However, the
   general, hybrid approach of Figure 3 is applicable in each case.

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2.1.1.  Partitioned Network

   The first and simplest approach to handling controller overload or
   scalability is to use multiple controllers, each responsible for a
   part of the network.  We can call the resultant areas of control
   "domains" [RFC4655].

   This approach is shown in Figure 4.  It can clearly address some of
   the scaling and overload concerns since each controller now only has
   responsibility for a subset of the network elements.  But this comes
   at a cost because end-to-end connections require coordination between
   the controllers.  Furthermore, this technique does not remove the
   concern about a single point-of-failure even if it does reduce the
   impact on the network of the failure of a single controller.

   Note that PCEP is designed to work as a PCE-to-PCE protocol as well
   as a PCE-to-PCC protocol, so it should be possible to use it to
   coordinate between PCE-based controllers in this model.

                    --------------------------------------------
                   | Orchestrator / Service Manager / OSS / NMS |
                    --------------------------------------------
                                ^                 ^
                                |                 |
                                v                 v
                        ------------  Coordi-   ------------
             -----     |            |  nation  |            |     -----
            | TED |--->| PCE-Based  |<-------->| PCE-Based  |<---| TED |
             -----     | Controller |          | Controller |     -----
                       |            |    ::    |            |
                       /------------     ::     ------------\
                      /    ^       ^     ::    ^        ^    \
                     /     |       |     ::    |        |     \
                    |      |       |     ::    |        |      |
                    v      v       v     ::    v        v      v
                  ----    ----    ----   ::   ----    ----    ----
                 | NE |  | NE |  | NE |  ::  | NE |  | NE |  | NE |
                  ----    ----    ----   ::   ----    ----    ----
                                         ::
                                Domain 1 :: Domain 2
                                         ::

          Figure 4: Multiple Controllers on a Partitioned Network

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2.1.2.  Multiple Parallel Controllers

   Multiple controllers may be deployed where each controller is capable
   of controlling all of the network elements.  Thus, the failure of any
   one controller will not leave the network unmanageable and, in normal
   circumstances, the load can be distributed across the controllers.

   Multiple parallel controllers may be deployed as shown in Figure 5.
   Each controller is capable of controlling all of the network
   elements; thus, the failure of any one controller will not leave the
   network unmanageable, and in normal circumstances, the load can be
   distributed across the controllers.  In this model, the orchestrator
   (or any requester) must select a controller to consume its request.

                         --------------------------------------------
                        | Orchestrator / Service Manager / OSS / NMS |
                         --------------------------------------------
                                ^                            ^
                                |    ___________________     |
                                |   |  Synchronization  |    |
                                v   v                   v    v
                          ------------                 ------------
                         |            |     -----     |            |
                         | PCE-Based  |<---| TED |--->| PCE-Based  |
                         | Controller |     -----     | Controller |
                         |            |__  ...........|            |
                          ------------\  \_:__        :------------
                                ^  ^   \___:  \  .....:  ^   ^
                                |  |  .....:\  \_:___  ..:   :
                                |  |__:___   \___:_  \_:___  :
                                | ....:   | .....: | ..:   | :
                                | :       | :      | :     | :
                                v v       v v      v v     v v
                               ----      ----     ----     ----
                              | NE |    | NE |   | NE |   | NE |
                               ----      ----     ----     ----

                 Figure 5: Multiple Redundant Controllers

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   An alternate approach is to present the controllers as a "cluster"
   that represents itself externally as a single controller as in
   Figure 3 but that is actually comprised of multiple controllers.  The
   size of the cluster may be varied according to the load in the manner
   of Network Functions Virtualization (NFV), and the cluster is
   responsible for sharing load among the members of the cluster.  This
   approach is shown in Figure 6.

                       --------------------------------------------
                      | Orchestrator / Service Manager / OSS / NMS |
                       --------------------------------------------
                                             ^
                                             |
                   --------------------------+-------------------------
                  | Controller ______________|_____________            |
                  | Cluster   |                            |           |
                  |           |    ___________________     |           |
                  |           |   |  Synchronization  |    |           |
                  |           v   v                   v    v           |
                  |     ------------      -----      ------------      |
                  |    | PCE-Based  |<---| TED |--->| PCE-Based  |     |
                  |    | Controller |     -----     | Controller |     |
                  |    | Instance   |               | Instance   |     |
                  |     ------------                 ------------      |
                  |           ^                            ^           |
                  |           |____________________________|           |
                  |                          |                         |
                   --------------------------+-------------------------
                                _____________|_____________
                               |         |        |        |
                               v         v        v        v
                             ----      ----     ----     ----
                            | NE |    | NE |   | NE |   | NE |
                             ----      ----     ----     ----

           Figure 6: Multiple Controllers Presented as a Cluster

   To achieve full redundancy and to be able to continue to provide full
   function in the event of a controller failure, the controllers must
   synchronize with each other.  This is nominally a simple task if
   there are just two controllers but can actually be quite complex if
   state changes in the network are not to be lost.  Furthermore, if
   there are more than two controllers, the synchronization between
   controllers can become a hard problem.

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   Synchronization issues are often off-loaded as "database
   synchronization" problems, because distributed database packages have
   already had to address these challenges, or by using a shared
   database.  In networking, the problem may also be addressed by
   collecting the state from the network (effectively using the network
   as a database) using normal routing protocols such as OSPF, IS-IS,
   and BGP.  It should be noted that addressing the synchronization
   problem through a shared database may be hiding the issues of
   congestion and of a single point of failure: while the controllers
   may have been made resilient by allowing redundancy, the shared
   database is still a problem, so the whole system is still vulnerable.

2.1.3.  Hierarchical Controllers

   Figure 7 shows an approach with hierarchical controllers.  This
   approach was developed for PCEs in [RFC6805] and appears in various
   SDN architectures where a "parent PCE", an "orchestrator", or a
   "super controller" takes responsibility for a high-level view of the
   network before distributing tasks to lower-level PCEs or controllers.

   On its own, this approach does little to protect against the failure
   of a controller, but it can make significant improvements in loading
   and scaling of the individual controllers.  It also offers a good way
   to support end-to-end connectivity across multiple administrative or
   technology-specific domains.

   Note that this model can be arbitrarily recursive with a PCE-based
   controller being the child of one parent PCE-based controller while
   acting as the parent of another set of PCE-based controllers.

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                      --------------------------------------------
                     | Orchestrator / Service Manager / OSS / NMS |
                      --------------------------------------------
                                           ^
                                           |
                                           v
                                      ------------
                                     |   Parent   |     -----
                                     | PCE-Based  |<---| TED |
                                     | Controller |     -----
                                     |            |
                                      ------------
                                       ^        ^
                                       |        |
                                       v   ::   v
                             ------------  ::  ------------
                  -----     |            | :: |            |     -----
                 | TED |--->| PCE-Based  | :: | PCE-Based  |<---| TED |
                  -----     | Controller | :: | Controller |     -----
                           /|            | :: |            |\
                          /  ------------  ::  ------------  \
                         /   ^       ^     ::    ^        ^   \
                        /    |       |     ::    |        |    \
                       /     |       |     ::    |        |     \
                      |      |       |     ::    |        |      |
                      v      v       v     ::    v        v      v
                    ----    ----    ----   ::   ----    ----    ----
                   | NE |  | NE |  | NE |  ::  | NE |  | NE |  | NE |
                    ----    ----    ----   ::   ----    ----    ----
                                           ::
                                  Domain 1 :: Domain 2
                                           ::

                    Figure 7: Hierarchical Controllers

3.  Applicability

   This section gives a very high-level introduction to the
   applicability of a PCE-based centralized controller.  There is no
   attempt to explain each use case in detail, and the inclusion of a
   use case is not intended to suggest that deploying a PCE-based
   controller is a mandatory or recommended approach.  The sections
   below are provided as a stimulus to the discussion of the
   applicability of a PCE-based controller, and it is expected that
   separate documents will be written to develop the use cases in which
   there is interest for implementation and deployment.  As described in

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   Section 4, specific enhancements to PCEP may be needed for some of
   these use cases, and it is expected that the documents that develop
   each use case will also address any extensions to PCEP.

   The rest of this section is divided into two sub-sections.  The first
   approaches the question of applicability from a consideration of the
   network technology.  The second looks at the high-level functions
   that can be delivered by using a PCE-based controller.

   As previously mentioned, this section is intended to just make
   suggestions.  Thus, the material supplied is very brief.  The
   omission of a use case is in no way meant to imply some limit on the
   applicability of PCE-based control.

3.1.  Technology-Oriented Applicability

   This section provides a list of use cases based on network
   technology.

3.1.1.  Applicability to Control-Plane Operated Networks

   This mode of operation is the common approach for an active, stateful
   PCE to control a traffic-engineered MPLS or GMPLS network [RFC8231].
   Note that the PCE-based controller determines what LSPs are needed
   and where to place them.  PCEP is used to instruct the head end of
   each LSP, and the head end signals in the control plane to set up the
   LSP.

   In this mode of operation, the PCE may construct its TED in a number
   of ways as described in [RFC4655], including (but not limited to)
   participating in the IGP or receiving information from a network
   element via BGP-LS [RFC7752].

3.1.2.  Static LSPs in MPLS

   Static LSPs are provisioned without the use of a control plane.  This
   means that they are established using a management plane or "manual"
   configuration.

   Static LSPs can be provisioned as explicit label instructions at each
   hop on the end-to-end path LSP.  Each router along the path must be
   told what label-forwarding instructions to program and what resources
   to reserve.  The PCE-based controller keeps a view of the network and
   determines the paths of the end-to-end LSPs just as it does for the
   use case described in Section 3.1.1, but the controller uses PCEP to
   communicate with each router along the path of the end-to-end LSP.
   In this case, the PCE-based controller will take responsibility for
   managing some part of the MPLS label space for each of the routers

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   that it controls, and it may taker wider responsibility for
   partitioning the label space for each router and allocating different
   parts for different uses, communicating the ranges to the router
   using PCEP.

3.1.3.  MPLS Multicast

   Multicast LSPs may be provisioned with a control plane or as static
   LSPs.  No extra considerations apply above those described in
   Sections 3.1.1 and 3.1.2 except, of course, to note that the PCE must
   also include the instructions about where the LSP branches, i.e.,
   where packets must be copied.

3.1.4.  Transport SDN

   Transport SDN (T-SDN) is the application of SDN techniques to
   transport networks.  In this respect, a transport network is a
   network built from any technology below the IP layer and designed to
   carry traffic transparently in a connection-oriented way.  Thus, an
   MPLS traffic-engineered network is a transport network, although it
   is more common to consider technologies such as Time Division
   Multiplexing (TDM) and Optical Transport Networks (OTNs) to be
   transport networks.

   Transport networks may be operated with or without a control plane
   and may have point-to-point or point-to-multipoint connections.
   Thus, all of the considerations in Sections 3.1.1, 3.1.2, and 3.1.3
   apply so that the normal PCEP message allows a PCE-based central
   controller to provision a transport network.  It is usually the case
   that additional technology-specific parameters are needed to
   configure the NEs or LSPs in transport networks, such as optical
   characteristic.  Such parameters will need to be carried in the PCEP
   messages: new protocol extensions may be needed, as described, for
   example, in [PCEP-WSON-RWA].

3.1.5.  Segment Routing

   Segment routing is described in [SR-ARCH].  It relies on a series of
   forwarding instructions being placed in the header of a packet.  At
   each hop in the network, a router looks at the first instruction and
   may: continue to forward the packet unchanged; strip the top
   instruction and forward the packet; or strip the top instruction,
   insert some additional instructions, and forward the packet.

   The segment routing architecture supports operations that can be used
   to steer packet flows in a network, thus providing a form of traffic
   engineering.  A PCE-based controller can be responsible for computing
   the paths for packet flows in a segment routing network, configuring

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   the forwarding actions on the routers, and telling the edge routers
   what instructions to attach to packets as they enter the network.
   These last two operations can be achieved using PCEP, and the
   PCE-based controller will assume responsibility for managing the
   space of labels or path identifiers used to determine how packets are
   forwarded.

3.1.6.  Service Function Chaining

   SFC is described in [RFC7665].  It is the process of directing
   traffic in a network such that it passes through specific hardware
   devices or virtual machines (known as service function nodes) that
   can perform particular desired functions on the traffic.  The set of
   functions to be performed and the order in which they are to be
   performed is known as a service function chain.  The chain is
   enhanced with the locations at which the service functions are to be
   performed to derive a Service Function Path (SFP).  Each packet is
   marked as belonging to a specific SFP, and that marking lets each
   successive service function node know which functions to perform and
   to which service function node to send the packet next.

   To operate an SFC network, the service function nodes must be
   configured to understand the packet markings, and the edge nodes must
   be told how to mark packets entering the network.  Additionally, it
   may be necessary to establish tunnels between service function nodes
   to carry the traffic.

   Planning an SFC network requires load balancing between service
   function nodes and traffic engineering across the network that
   connects them.  These are operations that can be performed by a
   PCE-based controller, and that controller can use PCEP to program the
   network and install the service function chains and any required
   tunnels.

3.2.  High-Level Applicability

   This section provides a list of the high-level functions that can be
   delivered by using a PCE-based controller.

3.2.1.  Traffic Engineering

   According to [RFC2702], TE is concerned with performance optimization
   of operational networks.  In general, it encompasses the application
   of technology and scientific principles to the measurement, modeling,
   characterization, control of Internet traffic, and application of
   such knowledge and techniques to achieve specific performance
   objectives.

Top      ToC       Page 17 
   From a practical point of view, this involves having an understanding
   of the topology of the network, the characteristics of the nodes and
   links in the network, and the traffic demands and flows across the
   network.  It also requires that actions can be taken to ensure that
   traffic follows specific paths through the network.

   PCE was specifically developed to address TE in an MPLS network, so a
   PCE-based controller is well suited to analyze TE problems and supply
   answers that can be installed in the network using PCEP.  PCEP can be
   responsible for initiating paths across the network through a control
   plane or for installing state in the network node by node such as in
   a segment-routed network (see Section 3.1.5) or by configuring IGP
   metrics.

3.2.2.  Traffic Classification

   Traffic classification is an important part of traffic engineering.
   It is the process of looking at a packet to determine how it should
   be treated as it is forwarded through the network.  It applies in
   many scenarios including MPLS traffic engineering (where it
   determines what traffic is forwarded onto which LSPs); segment
   routing (where it is used to select which set of forwarding
   instructions to add to a packet); and SFC (where it indicates along
   which service function path a packet should be forwarded).  In
   conjunction with traffic engineering, traffic classification is an
   important enabler for load balancing.

   Traffic classification is closely linked to the computational
   elements of planning for the network functions just listed because it
   determines how traffic load is balanced and distributed through the
   network.  Therefore, selecting what traffic classification should be
   performed by a router is an important part of the work done by a
   PCE-based controller.

   Instructions can be passed from the controller to the routers using
   PCEP.  These instructions tell the routers how to map traffic to
   paths or connections.

3.2.3.  Service Delivery

   Various network services may be offered over a network.  These
   include protection services (including end-to-end protection
   [RFC4427], restoration after failure, and fast reroute [RFC4090]);
   Virtual Private Network (VPN) services (such as Layer 3 VPNs
   [RFC4364] or Ethernet VPNs [RFC7432]); or Pseudowires [RFC3985].

Top      ToC       Page 18 
   Delivering services over a network in an optimal way requires
   coordination in the way that network resources are allocated to
   support the services.  A PCE-based central controller can consider
   the whole network and all components of a service at once when
   planning how to deliver the service.  It can then use PCEP to manage
   the network resources and to install the necessary associations
   between those resources.

4.  Protocol Implications / Guidance for Solution Developers

   PCEP is a push-pull protocol that is designed to move requests and
   responses between a server (the PCE) and clients (the PCCs, i.e., the
   network elements).  In particular, it has a message (the LSP Initiate
   Request (PCInitiate); see [RFC8281]) that can be sent by the PCE to
   install state or cause actions at the PCC and a response message
   (Path Computation State Report (PCRpt)) that is used to confirm the
   request.

   As such, there is an expectation that only relatively minor changes
   to PCEP are required to support the concept of a PCE-based
   controller.  The only work expected to be needed is extensions to
   existing PCEP messages to carry additional or specific information
   elements for the individual use cases, which maintain backward
   compatibility and do not impact existing PCEP deployments.  [RFC5440]
   already describes how legacy implementations handle unknown protocol
   extensions and how to use the PCEP Open message to indicate support
   for PCEP features.  Where possible, consistent with the general
   principles of how protocols are extended, any additions to the
   protocol should be made in a generic way such that they are open to
   use in a range of applications.

   It is anticipated that new documents (such as [PCEP-CONTROLLER]) will
   be produced for each use case dependent on support and demand.  Such
   documents will explain the use case and define the necessary protocol
   extensions.

   Protocol extensions could have impact on existing PCEP deployments
   and the interoperability between different implementations.  It is
   anticipated that changes of the PCEP protocol or addition of
   information elements could require additional testing to ensure
   interoperability between different PCEP implementations.

   It is reasonable to expect that implementations are able to select a
   subset or profile of the protocol extensions and PCEP features that
   are relevant for the application scenario in which they will be
   deployed.  Identification of these profiles should form part of the
   protocol itself so that interoperability can be easily determined and
   testing can be limited to the specific profiles.

Top      ToC       Page 19 
   Note that protocol mechanisms to handle synchronization of state in
   parallel PCE-based controllers will also be required if parallel
   controllers are used as described in Section 2.1.2.  In [RFC8231],
   there is a discussion of mechanisms to achieve PCE state
   synchronization.

5.  Security Considerations

   Security considerations for a PCE-based controller are little
   different from those for any other PCE system.  That is, the
   operation relies heavily on the use and security of PCEP, so
   consideration should be given to the security features discussed in
   [RFC5440] and the additional mechanisms described in [RFC8253].

   It should be observed that the trust model of a network that operates
   without a control plane is different from one with a control plane.
   The conventional "chain of trust" used with a control plane is
   replaced by individual trust relationships between the controller and
   each individual NE.  This model may be considerably easier to manage,
   so it is more likely to be operated with a high level of security.

   However, an architecture with a central controller has a central
   point of failure, and this is also a security weakness since the
   network can be vulnerable to denial-of-service attacks on the
   controller.  Similarly, the central controller provides a focus for
   interception and modification of messages sent to individual NEs.  In
   short, while the interactions with a PCE-based controller are not
   substantially different to those in any other SDN architecture, the
   security implications of SDN have not been fully discussed or
   described.  Therefore, protocol and applicability work-around
   solutions for this architecture must take proper account of these
   concerns.

   It is expected that each new document that is produced for a specific
   use case will also include considerations of the security impacts of
   the use of a PCE-based central controller on the network type and
   services being managed.

6.  Manageability Considerations

   The architecture described in this document is a management
   architecture: the PCE-based controller is a management component that
   controls the network through a southbound control protocol (PCEP).

   An implementation of a PCE-based controller will require access to
   information about the state of the network, its nodes, and its links.
   Some of this will be the TED as is normal for a PCE and can be
   collected using the mechanisms already in place (such as listening to

Top      ToC       Page 20 
   the IGPs, using BGP-LS [RFC7752], or northbound export of
   YANG-encoded data [YANG-TE] from the network elements to the
   controller).  More information may be collected in the LSP database
   for stateful PCEs as described in [RFC7399] and [RFC8231].
   Additional information may be needed for other specific use cases and
   will need to be collected and passed to the controller.  This may
   require protocol extensions for the mechanisms listed in this
   paragraph.

   The use of different PCEP options and protocol extensions may have an
   impact on interoperability, which is a management issue.  As noted in
   Section 4, protocol extensions should be done in a way that makes it
   possible to identify profiles of PCEP to aid interoperability, and
   this will aid deployment and manageability.

   [RFC5440] contains a substantive Manageability Considerations section
   that examines how a PCE-based system and a PCE-enabled system may be
   managed.  A MIB module for PCEP was published as [RFC7420], and a
   YANG module for PCEP has also been proposed [YANG-PCEP].

7.  IANA Considerations

   This document does not require any IANA actions.

8.  References

8.1.  Normative References

   [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,
              <https://www.rfc-editor.org/info/rfc5440>.

   [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>.

Top      ToC       Page 21 
8.2.  Informative References

   [PCECC]    Zhao, Q., Li, Z., Khasanov, B., Ke, Z., Fang, L., Zhou,
              C., Communications, T., Rachitskiy, A., and A. Gulida,
              "The Use Cases for Using PCE as the Central
              Controller(PCECC) of LSPs", Work in Progress,
              draft-zhao-teas-pcecc-use-cases-02, October 2016.

   [PCEP-CONTROLLER]
              Zhao, Q., Li, Z., Dhody, D., Karunanithi, S., Farrel, A.,
              and C. Zhou, "PCEP Procedures and Protocol Extensions for
              Using PCE as a Central Controller (PCECC) of LSPs", Work
              in Progress, draft-zhao-pce-pcep-extension-for-pce-
              controller-06, October 2017.

   [PCEP-WSON-RWA]
              Lee, Y. and R. Casellas, "PCEP Extension for WSON Routing
              and Wavelength Assignment", Work in Progress,
              draft-ietf-pce-wson-rwa-ext-07, November 2017.

   [RFC2702]  Awduche, D., Malcolm, J., Agogbua, J., O'Dell, M., and J.
              McManus, "Requirements for Traffic Engineering Over MPLS",
              RFC 2702, DOI 10.17487/RFC2702, September 1999,
              <https://www.rfc-editor.org/info/rfc2702>.

   [RFC3985]  Bryant, S., Ed. and P. Pate, Ed., "Pseudo Wire Emulation
              Edge-to-Edge (PWE3) Architecture", RFC 3985,
              DOI 10.17487/RFC3985, March 2005,
              <https://www.rfc-editor.org/info/rfc3985>.

   [RFC4090]  Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast
              Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090,
              DOI 10.17487/RFC4090, May 2005,
              <https://www.rfc-editor.org/info/rfc4090>.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, DOI 10.17487/RFC4364, February
              2006, <https://www.rfc-editor.org/info/rfc4364>.

   [RFC4427]  Mannie, E., Ed. and D. Papadimitriou, Ed., "Recovery
              (Protection and Restoration) Terminology for Generalized
              Multi-Protocol Label Switching (GMPLS)", RFC 4427,
              DOI 10.17487/RFC4427, March 2006,
              <https://www.rfc-editor.org/info/rfc4427>.

Top      ToC       Page 22 
   [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,
              <https://www.rfc-editor.org/info/rfc6805>.

   [RFC7025]  Otani, T., Ogaki, K., Caviglia, D., Zhang, F., and C.
              Margaria, "Requirements for GMPLS Applications of PCE",
              RFC 7025, DOI 10.17487/RFC7025, September 2013,
              <https://www.rfc-editor.org/info/rfc7025>.

   [RFC7399]  Farrel, A. and D. King, "Unanswered Questions in the Path
              Computation Element Architecture", RFC 7399,
              DOI 10.17487/RFC7399, October 2014,
              <https://www.rfc-editor.org/info/rfc7399>.

   [RFC7420]  Koushik, A., Stephan, E., Zhao, Q., King, D., and J.
              Hardwick, "Path Computation Element Communication Protocol
              (PCEP) Management Information Base (MIB) Module",
              RFC 7420, DOI 10.17487/RFC7420, December 2014,
              <https://www.rfc-editor.org/info/rfc7420>.

   [RFC7432]  Sajassi, A., Ed., Aggarwal, R., Bitar, N., Isaac, A.,
              Uttaro, J., Drake, J., and W. Henderickx, "BGP MPLS-Based
              Ethernet VPN", RFC 7432, DOI 10.17487/RFC7432, February
              2015, <https://www.rfc-editor.org/info/rfc7432>.

   [RFC7491]  King, D. and A. Farrel, "A PCE-Based Architecture for
              Application-Based Network Operations", RFC 7491,
              DOI 10.17487/RFC7491, March 2015,
              <https://www.rfc-editor.org/info/rfc7491>.

   [RFC7665]  Halpern, J., Ed. and C. Pignataro, Ed., "Service Function
              Chaining (SFC) Architecture", RFC 7665,
              DOI 10.17487/RFC7665, October 2015,
              <https://www.rfc-editor.org/info/rfc7665>.

   [RFC7752]  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and
              S. Ray, "North-Bound Distribution of Link-State and
              Traffic Engineering (TE) Information Using BGP", RFC 7752,
              DOI 10.17487/RFC7752, March 2016,
              <https://www.rfc-editor.org/info/rfc7752>.

   [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>.

Top      ToC       Page 23 
   [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>.

   [SR-ARCH]  Filsfils, C., Previdi, S., Ginsberg, L., Decraene, B.,
              Litkowski, S., and R. Shakir, "Segment Routing
              Architecture", Work in Progress, draft-ietf-spring-
              segment-routing-13, October 2017.

   [YANG-PCEP]
              Dhody, D., Hardwick, J., Beeram, V., and j.
              jefftant@gmail.com, "A YANG Data Model for Path
              Computation Element Communications Protocol (PCEP)", Work
              in Progress, draft-ietf-pce-pcep-yang-05, June 2017.

   [YANG-TE]  Liu, X., Bryskin, I., Beeram, V., Saad, T., Shah, H., and
              O. Dios, "YANG Data Model for Traffic Engineering (TE)
              Topologies", Work in Progress, draft-ietf-teas-yang-te-
              topo-13, October 2017.

Acknowledgments

   The ideas in this document owe a lot to the work started by the
   authors of [PCECC] and [PCEP-CONTROLLER].  The authors of this
   document fully acknowledge the prior work and thank those involved
   for opening the discussion.  The individuals concerned are: King Ke,
   Luyuan Fang, Chao Zhou, Boris Zhang, and Zhenbin Li.

   This document has benefited from the discussions within a small ad
   hoc design team; the members of which are listed as document
   contributors.

   Thanks to Michael Scharf and Andy Malis for a lively discussion of
   this document.

   Thanks to Phil Bedard, Aijun Wang, and Elwyn Davies for last call
   comments on this document.

   Spencer Dawkins, Adam Roach, and Ben Campbell provided helpful
   comments during IESG review.

Top      ToC       Page 24 
Contributors

   The following people contributed to discussions that led to the
   development of this document:

      Cyril Margaria
      Email: cmargaria@juniper.net

      Sudhir Cheruathur
      Email: scheruathur@juniper.net

      Dhruv Dhody
      Email: dhruv.dhody@huawei.com

      Daniel King
      Email: daniel@olddog.co.uk

      Iftekhar Hussain
      Email: IHussain@infinera.com

      Anurag Sharma
      Email: AnSharma@infinera.com

      Eric Wu
      Email: eric.wu@huawei.com

Top      ToC       Page 25 
Authors' Addresses

   Adrian Farrel (editor)
   Juniper Networks

   Email: afarrel@juniper.net


   Quintin Zhao (editor)
   Huawei Technologies
   125 Nagog Technology Park
   Acton, MA  01719
   United States of America

   Email: quintin.zhao@huawei.com


   Robin Li
   Huawei Technologies
   Huawei Bld., No.156 Beiqing Road
   Beijing  100095
   China

   Email: lizhenbin@huawei.com


   Chao Zhou
   Cisco Systems

   Email: chao.zhou@cisco.com