





Internet Engineering Task Force (IETF)                         A. Farrel
Request for Comments: 7399                              Juniper Networks
Category: Informational                                          D. King
ISSN: 2070-1721                                       Old Dog Consulting
                                                            October 2014


   Unanswered Questions in the Path Computation Element Architecture

Abstract

   The Path Computation Element (PCE) architecture is set out in RFC
   4655.  The architecture is extended for multi-layer networking with
   the introduction of the Virtual Network Topology Manager (VNTM) in
   RFC 5623 and generalized to Hierarchical PCE (H-PCE) in RFC 6805.

   These three architectural views of PCE deliberately leave some key
   questions unanswered, especially with respect to the interactions
   between architectural components.  This document draws out those
   questions and discusses them in an architectural context with
   reference to other architectural components, existing protocols, and
   recent IETF efforts.

   This document does not update the architecture documents and does not
   define how protocols or components must be used.  It does, however,
   suggest how the architectural components might be combined to provide
   advanced PCE function.

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 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7399.








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Copyright Notice

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   described in the Simplified BSD License.





































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Table of Contents

   1. Introduction ....................................................3
      1.1. Terminology ................................................4
   2. What Is Topology Information? ...................................4
   3. How Is Topology Information Gathered? ...........................5
   4. How Do I Find My PCE? ...........................................6
   5. How Do I Select between PCEs? ...................................7
   6. How Do Redundant PCEs Synchronize TEDs? .........................8
   7. Where Is the Destination? .......................................9
   8. Who Runs or Owns a Parent PCE? .................................10
   9. How Do I Find My Parent PCE? ...................................11
   10. How Do I Find My Child PCEs? ..................................11
   11. How Is the Parent PCE Domain Topology Built? ..................12
   12. Does H-PCE Solve the Internet? ................................12
   13. What are Sticky Resources? ....................................13
   14. What Is a Stateful PCE for? ...................................14
   15. How Is the LSP-DB Built? ......................................14
   16. How Do Redundant Stateful PCEs Synchronize State? .............15
   17. What Is an Active PCE? What Is a Passive PCE? .................16
   18. What is LSP Delegation? .......................................17
   19. Is an Active PCE with LSP Delegation Just a Fancy NMS? ........18
   20. Comparison of Stateless and Stateful PCE ......................18
   21. How Does a PCE Work with a Virtual Network Topology? ..........19
   22. How Does PCE Communicate with VNTM ............................21
   23. How Does Service Scheduling and Calendering Work? .............21
   24. Where Does Policy Fit In? .....................................22
   25. Does PCE Play a Role in SDN? ..................................23
   26. Security Considerations .......................................23
   27. References ....................................................25
      27.1. Normative References .....................................25
      27.2. Informative References ...................................25
   Acknowledgements ..................................................29
   Authors' Addresses ................................................29

1.  Introduction

   Over the years since the architecture for the Path Computation
   Element (PCE) was documented in [RFC4655], many new people have
   become involved in the work of the PCE working group and wish to use
   or understand the PCE architecture.  These people often missed out on
   early discussions within the working group and are unfamiliar with
   questions that were raised during the development of the
   documentation.







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   Furthermore, the base architecture has been extended to handle other
   situations and requirements: the architecture was extended for multi-
   layer networking with the introduction of the Virtual Network
   Topology Manager (VNTM) [RFC5623] and was generalized to include
   Hierarchical PCE (H-PCE) [RFC6805].

   These three architectural views of PCE deliberately leave some key
   questions unanswered, especially with respect to the interactions
   between architectural components.  This document draws out those
   questions and discusses them in an architectural context with
   reference to other architectural components, existing protocols, and
   recent IETF efforts.

   This document does not update the architecture documents and does not
   define how protocols or components must be used.  It does, however,
   suggest how the architectural components might be combined to provide
   advanced PCE function.

1.1.  Terminology

   Readers are assumed to be thoroughly familiar with terminology
   defined in [RFC4655], [RFC4726], [RFC5440], [RFC5623], and [RFC6805].
   More information about terms related to stateful PCE can be found in
   [STATEFUL-PCE].

   Throughout this document, the term "area" is used to refer equally to
   an OSPF area and an IS-IS level.  It is assumed that the reader is
   able to map the small differences between these two use cases.

2.  What Is Topology Information?

   [RFC4655] specifies that a PCE performs path computations based on a
   view of the available network resources and network topology.  This
   information is collected into a Traffic Engineering Database (TED).

   However, [RFC4655] does not provide a detailed description of what
   information is present in the TED.  It simply says that the TED
   "contains the topology and resource information of the domain."  The
   precise information that needs to be held in a TED depends on the
   type of network and nature of the computation that has to be
   performed.  As a basic minimum, the TED must contain the nodes and
   links that form the domain, and it must identify the connectivity in
   the domain.

   For most traffic-engineering needs (for example, MPLS Traffic
   Engineering - MPLS-TE), the TED would additionally contain a basic
   metric for each link and knowledge of the available (unallocated)
   resources on each link.



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   More advanced use cases might require that the TED contain additional
   data that represents qualitative information such as:

      - link delay
      - link jitter
      - node throughput capabilities
      - optical impairments
      - switching capabilities
      - limited node cross-connect capabilities

   Additionally, an important information element for computing paths,
   especially for protected services, is the Shared Risk Group (SRG).
   This is an indication of resources in the TED that have a common risk
   of failure.  That is, they have a shared risk of failure from a
   single event.

   In short, the TED needs to contain as much information as is needed
   to satisfy the path computation requests subject to the objective
   functions (OFs).  This, in itself, may not be a trivial issue in some
   network technologies.  For example, in some optical networks, the
   path computation for a new Label Switched Path (LSP) may need to
   consider the impact that turning up a new laser would have on the
   optical signals already being carried by fibers.  It may be possible
   to abstract this information as parameters of the optical links and
   nodes in the TED, but it may be easier to capture this information
   through a database of existing LSPs (see Sections 14 and 15).

3.  How Is Topology Information Gathered?

   Clearly, the information in the TED discussed in Section 2 needs to
   be gathered and maintained somehow.  [RFC4655] simply says "The TED
   may be fed by Interior Gateway Protocol (IGP) extensions or
   potentially by other means."  In this context, "fed" means built and
   maintained.

   Thus, one way that the PCE may construct its TED is by participating
   in the IGP running in the network.  In an MPLS-TE network, this would
   depend on OSPF TE [RFC3630] and IS-IS TE [RFC5305].  In a GMPLS
   network, it would utilize the GMPLS extensions to OSPF and IS-IS,
   [RFC4203] and [RFC5307].

   However, participating in an IGP, even as a passive receiver of IGP
   information, can place a significant load on the PCE.  The IGP can be
   quite "chatty" when there are frequent updates to the use of the
   network, meaning that the PCE must dedicate significant processing to
   parsing protocol messages and updating the TED.  Furthermore, to be
   truly useful, a PCE implementation would need to support OSPF and IS-
   IS.



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   An alternative feed from the network to the PCE's TED is offered by
   BGP-LS [LS-DISTRIB].  This approach offers the alternative of
   leveraging an in-network BGP speaker (such as an Autonomous System
   Border Router or a Route Reflector) that already has to participate
   in the IGP and that is specifically designed to apply filters to IGP
   advertisements.  In this usage, the BGP speaker filters and
   aggregates topology information according to configured policy before
   advertising it "north-bound" to the PCE to update the TED.  The PCE
   implementation has to support just a simplified subset of BGP rather
   than two full IGPs.

   But BGP might not be convenient in all networks (for example, where
   BGP is not run, such as in an optical network or a BGP-free core).
   Furthermore, not all relevant information is made available through
   standard TE extensions to the IGPs.  In these cases, the TED must be
   built or supplemented from other sources such as the Network
   Management System (NMS), inventory management systems, and directly
   configured data.

   It has also been proposed that the PCE Communication Protocol (PCEP)
   [RFC5440] could be extended to serve as an information collection
   protocol to supply information from network devices to a PCE.  The
   logic is that the network devices may already speak PCEP; so, the
   protocol could easily be used to report details about the resources
   and state in the network, including the LSP state discussed in
   Sections 14 and 15.

   Note that a PCE that is responsible for more than one domain must, of
   course, collect TE information from each domain to build its TED or
   TEDs.

4.  How Do I Find My PCE?

   A Path Computation Client (PCC) needs to know the identity/location
   of a PCE in order to be able to make computation requests.  This is
   because PCEP is a transaction-based protocol carried over TCP, and
   the architectural decision made in Section 6.4 of RFC 4655 required
   targeted PCC-PCE communications.

   As described in [RFC4655], a PCC could be configured with the
   knowledge of the IP address of its PCE.  This is a relatively
   lightweight option considering all of the other configuration that a
   router may require, but it is open to configuration errors, and does
   not meet the need for minimal-configuration operation.  Furthermore,
   configuration communication with multiple PCEs could become onerous,
   while handling changes in PCE identities and coping with failure
   events would be an issue for a configured system.




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   [RFC4655] offers the possibility for PCEs to advertise themselves in
   the IGP, and this requirement is developed in [RFC4674] and made
   possible in OSPF and IS-IS through [RFC5088] and [RFC5089].  In
   general, these mechanisms should be sufficient for PCCs in a network
   where an IGP is used and where the PCE participates in the IGP.

   Note, however, that not all PCEs will participate in the IGP (see
   Section 3).  In these cases, assuming configuration is not
   appropriate as a discovery mechanism, some other server
   announcement/discovery function may be needed, such as DNS [RFC4848]
   as used for discovery of the Local Location Information Server (LIS)
   [RFC5986] and in the Application-Layer Traffic Optimization (ALTO)
   discovery function [ALTO-SERVER-DISC].

5.  How Do I Select between PCEs?

   When more than one PCE is discovered or configured, a PCC will need
   to select which PCE to use.  It may make this decision on any
   arbitrary algorithm (for example, first-listed, or round robin), but
   it may also be the case that different PCEs have different
   capabilities and path computation scope; in which case, the PCC will
   want to select the PCE most likely to be able to satisfy any one
   request.  The first requirement, of course, is that the PCE can
   compute paths for the relevant domain.

   PCE advertisement in OSPF or IS-IS per [RFC5088] and [RFC5089] allows
   a PCE to announce its capabilities as required in [RFC4657].  A PCC
   can select between PCEs based on the capabilities that they have
   announced.  However, these capabilities are expressed as flags in the
   PCE advertisement so only the core capabilities are presented, and
   there is not scope for including detailed information (such as
   support for specific objective functions) in the advertisement.

   Additional and more complex PCE capabilities, including the
   capability to perform point-to-multipoint (P2MP) path computations
   [RFC6006], may be announced by the PCE as optional PCEP type-length-
   value (TLV) Type Indicators in the Open message described in
   [RFC5440].  This mechanism is not limited to just a set of flags, and
   detailed capability information may be presented in sub-TLVs.

   Note that this exchange of PCE capabilities is in the form of an
   announcement, not a negotiation.  That is, a PCC that wants specific
   function from a PCE must examine the advertised capabilities and
   select which PCE to use for a specific request.  There is no scope
   for a PCC to request a PCE to support features or functions that it
   does not offer or announce.





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   A PCC may also vary which PCE it uses according to congestion
   information reported by the PCEs using the Notification Object and
   Notification Type [RFC5440].  In a heavily overloaded PCE system,
   note that reports from one PCE that it is overloaded may simply
   result in all PCCs switching to another PCE, which will, itself,
   immediately become overloaded.  Thus, PCCs should exercise a certain
   amount of discretion and queueing theory before selecting a PCE
   purely based on reported load.

   Note that a PCC could send all requests to all PCEs that it knows
   about.  It can then select between the results, perhaps choosing the
   first result it receives, but this approach is very likely to
   overload all the PCEs in the network considering that one of the
   reasons for multiple PCEs is to share the load.

6.  How Do Redundant PCEs Synchronize TEDs?

   A network may have more than one PCE, as discussed in the previous
   sections.  These PCEs may provide redundancy for load-sharing,
   resilience, or partitioning of computation features.

   In order to achieve some consistency between the results of different
   PCEs, it is desirable that they operate on the same TE information.

   The TED reflects the actual state of the network and is not a
   resource reservation or booking scheme.  Therefore, a PCE-based
   system does not prevent competition for network resources during the
   provisioning phase, although a process of "sticky resources" that are
   temporarily reduced in the TED after a computation may be applied
   purely as a local implementation feature.

   One option for ensuring that multiple PCEs use the same TE
   information is simply to have the PCEs driven from the same TED.
   This could be achieved in implementations by utilizing a shared
   database, but it is unlikely to be efficient.

   More likely is that each PCE is responsible for building its own TED
   independently, using the techniques described in Section 3.  If the
   PCEs participate in the IGP, it is likely that they will attach at
   different points in the network; so, there may be minor and temporary
   inconsistencies between their TEDs caused by IGP convergence issues.
   If the PCEs gather TE information via BGP-LS [LS-DISTRIB] from
   different sources, the same inconsistencies may arise.  However, if
   the PCEs attach to the same BGP speaker, it may be possible to
   achieve consistency between TEDs modulo the BGP-LS process itself.






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   A final option is to provide an explicit synchronization process
   between the TED of a "master" PCE and the TEDs of other PCEs.  Such a
   process could be achieved using BGP-LS or a database synchronization
   protocol (which would allow check-pointing and sequential updates).
   This approach is fraught with issues around selection of the master
   PCE and handling failures.  It is, in fact, a mirrored database
   scenario: a problem that is well known and the subject of plenty of
   work.

   Noting that the provisioning protocols such as RSVP-TE [RFC3209]
   already handle contention for resources, that the differences between
   TEDs are likely to be relatively small with moderate arrival rates
   for new services, and that contention in all but the most busy
   networks is relatively unlikely, there may be no value in any attempt
   to synchronize TEDs between PCEs.

   However, see Section 16 for a discussion of synchronizing other state
   between redundant PCEs.

7.  Where Is the Destination?

   Path computation provides an end-to-end path between a source and a
   destination.  If the destination lies in the source domain, then its
   location will be known to the PCE and there are no issues to be
   solved.  However, in a multi-domain system a path must be found to a
   remote domain that contains the destination, and that can only be
   achieved by knowledge of the location of the destination or at least
   knowing the next domain in the path toward the domain that contains
   the destination.

   The simplest solution here is achieved when a PCE has visibility into
   multiple domains.  Such may be the case in a multi-area network where
   the PCE is aware of the contents of all of the IGP areas.  This
   approach is only likely to be appropriate where the number of nodes
   is manageable, and it is unlikely to extend over administrative
   boundaries.

   The per-domain path computation method for establishing inter-domain
   traffic engineering LSPs [RFC5152] simply requires a PCE to compute a
   path to the next domain toward the destination.  As the LSP setup
   (through signaling) progresses domain by domain, the Label Switching
   Router (LSR) at the entry point to each domain requests its local PCE
   to compute the next segment of the path, that is from that LSR to the
   next domain in the sequence toward the destination.  Thus, it is not
   necessary for any PCE (except the last) to know in which domain the
   destination exists.  But, in this approach, each PCE must somehow
   determine the next domain toward the destination, and it is not
   obvious how this is achieved.



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   [RFC5152] suggests that, in an IP/MPLS network, it is good enough to
   leverage the IP reachability information distributed by BGP and
   assume that TE reachability can follow the same Autonomous System
   (AS) path.  This approach might not guarantee the optimal TE path
   and, of course, might result in no path being found in degenerate
   cases.  Furthermore, in many network technologies (such as optical
   networks operated by GMPLS) there may be limited or no end-to-end IP
   connectivity.

   The Backward Recursive PCE-based Computation (BRPC) procedure
   [RFC5441] is able to achieve a more optimal end-to-end path than the
   per-domain method, but depends on the knowledge of both the domain in
   which the destination is located and the sequence of domains toward
   the destination.  This information is described in [RFC5441] as being
   known a priori.  Clearly, however, information is not always known a
   priori, and it may be hard for the PCE that serves the source PCC to
   discover the necessary details.  While there are several approaches
   to solving the question of establishing the domain sequence (for
   example, BRPC trial and error or H-PCE [RFC6805]), none of them
   addresses the issue of determining where the destination lies.

   One argument that is often made is that an end-to-end connection
   expressed as an LSP is a feature of a service agreement between
   source and destination.  If that is the case, it is argued, it stands
   to reason that the location of the destination must be known to the
   source node in the same way that the source has determined the IP
   address of the destination.  Presumably, this would be through a
   commercial process or an administrative protocol.

   [RFC4974] introduced the concept of Calls and Connections for LSPs.
   A Call does not provide the actual connectivity for transmitting user
   traffic, but builds a relationship that will allow subsequent
   Connections to be made.  A Call might be considered an agreement to
   support an end-to-end LSP that is made between the endpoint nodes.
   Call messages are sent and routed as normal IP messages, so the
   sender does not need to know the location of the destination.

   Furthermore, Call requests are responded, and the Call Response can
   carry information (such as the identity of the domain containing the
   destination) for use by Call initiator.  Thus, the use of GMPLS Calls
   might provide a mechanism to discover destination's location.

8.  Who Runs or Owns a Parent PCE?

   A parent PCE [RFC6805] is responsible for selecting inter-domain path
   by coordinating with child PCEs and maintaining a domain topology
   map.




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   In the case of multi-domains (e.g., IGP areas or multiple ASes)
   within a single service provider network, the management
   responsibility for the parent PCE would most likely be handled by the
   service provider.

   In the case of multiple ASes within different service provider
   networks, it may be necessary for a third party to manage the parent
   PCEs according to commercial and policy agreements from each of the
   participating service providers.  Note that the H-PCE architecture
   does not require disclosure of internals of a child domain to the
   parent PCE.  Thus, there is ample scope for a parent PCE to be run by
   one of the connected service providers or by a third party without
   compromising commercial issues.  In fact, each service provider could
   run its own parent PCE while allowing its child PCEs to be contacted
   by outsider parent PCEs according to configured policy and security.

9.  How Do I Find My Parent PCE?

   [RFC6805] specifies that a child PCE must be configured with the
   address of its parent PCE in order for it to interact with its parent
   PCE.  There is no scope for parent PCEs to advertise their presence;
   however, there is potential for directory systems (such as DNS
   [RFC4848] as used in the ALTO discovery function [ALTO-SERVER-DISC])
   to be used as described in Section 4.

   According to [RFC6805], note that the child PCE must also be
   authorized to peer with the parent PCE.  This is discussed from the
   viewpoint of the parent PCE in Section 10.  The child PCE may need to
   participate in a key distribution protocol in order to properly
   authenticate its identity to the parent PCE.

10.  How Do I Find My Child PCEs?

   Within the hierarchical PCE framework [RFC6805], the parent PCE must
   only accept path computation requests from authorized child PCEs.  If
   a parent PCE receives a request from an unauthorized child PCE, the
   request should be dropped.

   This requires a parent PCE to be configured with the identities and
   security credentials of all of its child PCEs, or there must be some
   form of shared secret that allows an unknown child PCE to be
   authorized by the parent PCE.









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11.  How Is the Parent PCE Domain Topology Built?

   The parent PCE maintains a domain topology map of the child domains
   and their interconnectivity.  This map does not include any
   visibility into the child domains.  Where inter-domain connectivity
   is provided by TE links, the capabilities of those links may also be
   known to the parent PCE.

   The parent PCE maintains a TED for the parent domain in the same way
   that any PCE does.  The nodes in the parent domain will be
   abstractions of the child domains (connected by real or virtual TE
   links), but the parent domain may also include real nodes and links.

   The mechanism for building the parent TED is likely to rely heavily
   on administrative configuration and commercial issues because the
   network was probably partitioned into domains specifically to address
   these issues.  However, note that in some configurations (for
   example, collections of small optical domains) a separate instance of
   a routing protocol (probably an IGP) may be run within the parent
   domain to advertise the domain interconnectivity.  Additionally,
   since inter-domain TE links can be advertised by the IGPs operating
   in the child domains, this information could be exported to the
   parent PCE either by the child PCEs or using a north-bound export
   mechanism such as BGP-LS [LS-DISTRIB].

12.  Does H-PCE Solve the Internet?

   The model described in [RFC6805] introduced a hierarchical
   relationship between domains.  It is applicable to environments with
   small groups of domains where visibility from the ingress LSRs is
   limited.  Applying the hierarchical PCE model to large groups of
   domains such as the Internet is not considered feasible or desirable.

   This does open up a harder question: how many domains can be handled
   by an H-PCE system?  In other words: what is a small group of
   domains?  The answer is not clear and might be "I know it when I see
   it."  At the moment, a rough guide might be around 20 domains as a
   maximum.

   An associated question would be: how many hierarchy levels can be
   handled by H-PCE?  Architecturally, the answer is that there is no
   limit, but it is hard to construct practical examples where more than
   two or possibly three levels are needed.








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13.  What are Sticky Resources?

   When a PCE computes a path, it has a reasonable idea that an LSP will
   be set up and that resources will be allocated within the network.
   If the arrival rate of computation requests is faster than the LSP
   setup rate combined with the IGP convergence time, it is quite
   possible that the PCE will perform its next computation before the
   TED has been updated to reflect the setup of the previous LSP.  This
   can result in LSP setup failures if there is contention for
   resources.  The likelihood of this problem is particularly high
   during recovery from network failures when a large number of LSPs
   might need new paths.

   A PCE may choose to make a provisional assignment of the resources
   that would be needed for an LSP and to reduce the available resources
   in its TED so that the problem is mitigated.  Such resources are
   informally known as "sticky resources".

   Note that using sticky resources introduces a number of other
   problems that can make managing the TED difficult.  For example:

   -  When the TED is updated as a result of new information from the
      IGP, how does the PCE know whether the reduction in available
      resources is due to the successful setup of the LSP for which it
      is holding sticky resources or due to some other network event
      (such as the setup of another LSP)?  This problem may be
      particularly evident if there are multiple PCEs that do not
      synchronize their sticky resources or if not all LSPs utilize PCE
      computation.

   -  When LSP setup fails, how are the sticky resources released?
      Since the PCE doesn't know about the failure of the LSP setup, it
      needs some other mechanism to release them.

   -  What happens if a path computation was made only to investigate
      the potential for an LSP but not to actually set one up?

   -  What if the path used by the LSP does not match that provided by
      the PCE (for example, because the control plane routes around some
      problem)?

   Some of these issues can be mitigated by using a Stateful PCE (see
   Section 14) or by timers.








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14.  What Is a Stateful PCE for?

   A Stateless PCE can perform path computations that take into account
   the existence of other LSPs if the paths of those LSPs are supplied
   on the computation request.  This function can be particularly useful
   when arranging protection paths so that a working and protection LSP
   do not share any links or nodes.  It can also be used when a group of
   LSPs are to be reoptimized at the same time in the process known as
   Global Concurrent Optimization (GCO) [RFC5557].

   However, this mechanism can be quite a burden on the protocol
   messages, especially when large numbers of LSP paths need to be
   reported.

   A Stateful PCE [STATEFUL-PCE] maintains a database of LSPs (the LSP-
   DB) that are active in the network, i.e., have been provisioned such
   that they use network resources although they might or might not be
   carrying traffic.  This database allows a PCC to refer to an LSP
   using only its identifier -- all other details can be retrieved by
   the PCE from the LSP-DB.

   A Stateful PCE can use the LSP-DB for many other functions, such as
   balancing the distribution of LSPs in the network.  Furthermore, the
   PCE can correlate LSPs with network resource availability placing new
   LSPs more cleverly.

   A Stateful PCE that is also an Active PCE (see Section 17) can
   respond to changes in network resource availability and predicted
   demands to reroute LSPs that it knows about.

   Section 20 offers a brief comparison of the different modes of PCE
   with reference to stateful and stateless PCE.

15.  How Is the LSP-DB Built?

   The LSP-DB contains information about the LSPs that are active in the
   network, as mentioned in Section 14.  This state information can be
   constructed by the PCE from information it receives from a number of
   sources including from provisioning tools and from the network, but
   no matter how the information is gleaned, a Stateful PCE needs to
   synchronize its LSP-DB with the state in the network.  Just as
   described in Section 13, the PCE cannot rely on knowledge about
   previous computations it has made, but it must find out the actual
   LSPs in the network.







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   A simple solution is for all ingress LSRs to report all LSPs to the
   PCE as they are set up, modified, or torn down.  Since PCEP already
   has the facility to fully describe LSP routes and resources in the
   protocol messages, this is not a difficult problem, and the LSP State
   Report (PCRpt) message has been defined for this purpose
   [STATEFUL-PCE].

   The situation can be more complex, however, if there are ingress LSRs
   that do not support PCEP, support PCEP but not the PCRpt, or that are
   unaware of the requirement to report LSPs to the PCE.  This might
   happen if the LSRs are able to compute paths themselves or if they
   receive LSP setup instructions with pre-computed paths from an NMS.

   An alternative approach is to note that any LSR on the path of an LSP
   can probably see the whole path (through the Record Route object in
   RSVP-TE signaling [RFC3209]) and knows the bandwidth reserved for the
   LSP.  Thus, any LSR could report the LSP to the PCE, noting that it
   will not hurt (beyond additional message processing and potential
   overload of the PCE or the network) for the LSP to be reported
   multiple times because it is clearly identified.  In fact, this would
   also provide a cross-check mechanism.

   Nevertheless, it is possible that some LSPs will traverse only LSRs
   that are not aware of the PCE's need to learn LSP state and build an
   LSP-DB.  In these cases, the stateful PCE must either only have
   limited knowledge of the LSPs in the network or must learn about LSPs
   through some other mechanism (such as reading the MPLS and GMPLS MIB
   modules [RFC3812] [RFC4802]).

   Ultimately, there may be no substitute for all LSRs being aware of
   Stateful PCEs and able to respond to requests for reports on all LSPs
   that they know about.  This will allow a Stateful PCE to build its
   LSP-DB from scratch (which it may need to do at start of day) and to
   verify its LSP-DB against the network (which may be important if the
   PCE has suffered some form of outage).

16.  How Do Redundant Stateful PCEs Synchronize State?

   It is important that two PCEs operating in a network have similar
   views of the available resources.  That is, they should have the same
   or substantially similar TEDs.  This is easy to achieve either by
   building the TEDs from the network in the same way or by one PCE
   synchronizing its TED to the other PCE using a TED export protocol
   such as BGP-LS [LS-DISTRIB] or the Network Configuration Protocol
   (NETCONF) [RFC6241] (see Section 6).






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   Synchronizing the LSP-DB can be a more complicated issue.  As
   described in Section 15, building the LSP-DB can be an involved
   process, so it would be best to not have multiple PCEs each trying to
   build an LSP-DB from the network.  However, it is still important
   that where multiple PCEs operate in the network (either as
   distributed PCEs or with one acting as a backup for the other), their
   LSP-DBs are kept synchronized.

   Thus, there is likely to be a need for a protocol mechanism for one
   PCE to update its LSP-DB with that of another PCE.  This is no
   different from any other database-synchronization problem and could
   use existing mechanisms or a new protocol.  Note, however, that in
   the case of distributed PCEs that are also Active PCEs (see Section
   17), each PCE will be creating entries in its own LSP-DB; so, the
   synchronization of databases must be incremental and bidirectional,
   not just simply a database dump.

   It may be helpful to clarify the word "redundant" in the context of
   this question.  One interpretation is that a redundant PCE exists
   solely as a backup such that it only performs a function in the
   network in the event of a failure of the primary PCE.  This seems
   like a waste of expensive resources, and it would make more sense for
   the redundant PCE to take its share of computation load all the time.
   However, that scenario of two (or more) active PCEs creates exactly
   the state synchronization issue described above.

   Various deployment options have been suggested where one PCE serves a
   set of PCCs as the primary computation server, and only addresses
   requests from other PCCs in the event of the failure of some other
   PCE; however, this mode of operation still raises questions about the
   need for synchronized state even in non-failure scenarios if the LSPs
   that will be computed by the different PCEs may traverse the same
   network resources.

17.  What Is an Active PCE? What Is a Passive PCE?

   A Passive PCE is one that only responds to path computation requests.
   It takes no autonomous actions.  A Passive PCE may be stateless or
   stateful.

   An Active PCE is one that issues provisioning "recommendations" to
   the network.  These recommendations may be new routes for existing
   LSPs or routes for new LSPs (that is, an Active PCE may recommend the
   instantiation of new LSPs).  An Active PCE may be stateless or
   stateful, but in order for it to reroute existing LSPs effectively,
   it is likely to hold state for at least those LSPs that it will
   reroute.




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   Many people consider that the PCE, itself, cannot be Active.  That
   is, they hold that the PCE's function is purely to compute paths.  In
   that worldview, the "Active PCE" is actually the combination of a
   normal, passive PCE and an additional architectural component
   responsible for issuing commands or recommendations to the network.

   In some configurations, the VNTM discussed in Sections 21 and 22
   provides this additional component.

   Section 20 offers a brief comparison of the different modes of PCE
   with reference to passive and active PCE.

18.  What is LSP Delegation?

   LSP delegation [STATEFUL-PCE] is the process where a PCC (usually an
   ingress LSR) passes responsibility for triggering updates to the
   attributes of an LSP (such as bandwidth or path) to the PCE.  In this
   case, the PCE would need to be both Stateful and Active.

   LSP delegation allows an LSP to be set up under the control of the
   ingress LSR potentially using the services of a PCE.  Once the LSP
   has been set up, the LSR (a PCC) tells the PCE about the LSP by
   providing details of the path and resources used.  It delegates
   responsibility for the LSP to the PCE so that the PCE can make
   adjustments to the LSP as dictated by changes to the TED and the
   policies in force at the PCE.  The PCE makes the adjustments by
   sending a new path to the LSR with the instruction/recommendation
   that the LSP be re-signaled.

   There may be some debate over whether the PCE "owns" the LSP after
   delegation.  That is, if the PCE supplies a new path, is the ingress
   LSR required to act or can it take the information "under
   advisement"?  It may be too soon to answer this question
   definitively; however, there is certainly an expectation that the LSR
   will act on the advice it receives.  A comparison may be drawn with a
   visit to the doctor: the doctor has an expectation that the patient
   will take the medicine, but the patient has free will.

   It is important, however, to distinguish between an LSP established
   within the network and subsequently delegated to a PCE and an LSP
   that was established as the result of an Active PCE's recommendation
   for LSP instantiation.

   Section 20 offers a brief comparison of the different modes of PCE
   with reference to LSP delegation.






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19.  Is an Active PCE with LSP Delegation Just a Fancy NMS?

   In many ways the answer here is "yes".  But the PCE architecture
   forms part of a new way of looking at network operation and
   management.  In this new view, the network operation is more dynamic
   and under the control of software applications without direct
   intervention from operators.  This is not to say that the operator
   has no say in how their network runs, but it does mean that the
   operator sets policies (see Section 24) and that new components (such
   as an Active PCE) are responsible for acting on those policies to
   dynamically control the network.

   There is a subtle distinction between an NMS and an Active PCE with
   LSP delegation.  An NMS is in control of the LSPs in the network and
   can command that they are set up, modified, or torn down.  An Active
   PCE can only make suggestions about LSPs that have been delegated to
   the PCE by a PCC, or make recommendations for the instantiation of
   new LSPs.

   For more details, see the discussion of an architecture for
   Application-Based Network Operation (ABNO) in [NET-OPS]

20.  Comparison of Stateless and Stateful PCE

   Table 1 shows a comparison of stateless and stateful PCEs to show how
   they how might be instantiated as passive or active PCEs with or
   without control of LSPs.  The terms used relate to the concepts
   introduced in the previous sections.  The entries in the table refer
   to the notes that follow.






















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                           | Stateless |  Stateful |
   ------------------------+-----------+-----------+
   Passive                 |     1     |     2     |
   Active delegated LSPs   |     3     |     4     |
   Active suggest new LSPs |     5     |     6     |
   Active instantiate LSPs |     7     |     7     |

   Notes:
   1. Passive is the normal mode for a stateless PCE.
   2. A passive mode stateful PCE may have value for more complex
      environments and for computing protected services.
   3. Delegation of LSPs to a stateless PCE is relatively pointless,
      but could add value at moment of delegation.
   4. This is the normal mode for a stateful PCE.
   5. There is only marginal potential for a stateless PCE to
      recommend new LSPs because without a view of existing LSPs, the
      PCE cannot determine when new ones might be needed.
   6. This mode has potential for recommending the instantiation of
      new LSPs.
   7. These modes are out of scope for PCE as currently described.
      That is, the PCE can recommend instantiation, but cannot
      actually instantiate the LSPs.

              Table 1 : Comparing Stateless and Stateful PCE

21.  How Does a PCE Work with a Virtual Network Topology?

   A Virtual Network Topology (VNT) is described in [RFC4397] as a set
   of Hierarchical LSPs that is created (or could be created) in a
   particular network layer to provide network flexibility (data links)
   in other layers.  Thus, the TE topology of a network can be
   constructed from TE links that are simply data links, from TE links
   that are supported by LSPs in another layer of the network, or from
   TE links that could be supported by LSPs ("potential LSPs") that
   would be set up on demand in another network layer.  This third type
   of TE link is known as a Virtual TE Link in [RFC5212].

   [RFC5212] also gives a more detailed explanation of a VNT, and it
   should be noted that the network topology in a packet network could
   be supported by LSPs in a number of different lower-layer networks.
   For example, the TE links in the packet network could be achieved by
   connections (LSPs) in underlying Synchronous Optical Network or
   Synchronous Digital Hierarchy (SONET/SDH) and photonic networks.
   Furthermore, because of the hierarchical nature of MPLS, the TE links
   in a packet network may be achieved by setting up packet LSPs in the
   same packet network.





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   A PCE obviously works with the TED that contains information about
   the TE links in the network.  Those links may be already established
   or may be virtual TE links.  In a simple TED, there is no distinction
   between the types of TE link; however, there may be advantages to
   selecting TE links that are based on real data links over those based
   on dynamic LSPs in lower layers because the data links may be more
   stable.  Conversely, the TE links based on dynamic LSPs may be able
   to be repaired dynamically giving better resilience.  Similarly, a
   PCE may prefer to select a TE link that is supported by a data link
   or existing LSP in preference to using a virtual TE link because the
   latter may need to be set up (taking time) and the setup could
   potentially fail.  Thus, a PCE might want to employ additional
   metrics or indicators to help it view the TED and select the right
   path for LSPs.

   If a PCE uses a virtual TE link, then some action will be needed to
   establish the LSP that supports that link.  Some models (such as that
   in [RFC5212]) trigger the setup of the lower-layer LSPs on-demand
   during the signaling of the upper-layer LSP (i.e., when the upper
   layer comes to use the virtual TE link, the upper-layer signaling is
   paused and the lower-layer LSP is established).  Another view,
   described in [RFC5623], is that when the PCE computes a path that
   will use a virtual TE link, it should trigger the setup of the lower-
   layer LSP to properly create the TE link so that the path it returns
   will be sure to be viable.  This latter mode of operation can be
   extended to allow the PCE to spot the need for additional TE links
   and to trigger LSPs in lower layers in order to create those links.

   Of course, such "interference" in a lower-layer network by a PCE
   responsible for a higher-layer network depends heavily on policy.  In
   order to make a clean architectural separation and to facilitate
   proper policy control, [RFC5623] introduces the Virtual Network
   Topology Manager (VNTM) as a functional element that manages and
   controls the VNT.  [RFC5623] notes that the PCE and VNT Manager are
   distinct functional elements that may or may not be collocated.
   indeed, it should be noted that there will be a PCE for the upper
   layer, and a PCE for each lower layer, and a VNTM responsible for
   coordinating between the PCEs and for triggering LSP setup in the
   lower layers.  Therefore, the combination of all of the PCEs and the
   VNTM produces functionally similar to an Active, multi-layer PCE.

   See [TE-INFO] for additional discussion of the construction of
   networks using virtual and potential links.








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22.  How Does PCE Communicate with VNTM

   The VNTM described in Section 21 and [RFC5623] has several interfaces
   (see also [NET-OPS]).

   -  In order to make decisions on whether to create new TE links, the
      VNTM needs to learn from the upper-layer PCE about resource
      shortages and the need for additional TE links.  It can then make
      policy-based decisions to determine whether to create new TE links
      and how to support them through existing or new LSPs.

   -  The VNTM will need to coordinate with the PCEs in the lower
      layers, but this is simply a normal use of PCEP.

   -  The VNTM will need to issue provisioning requests/commands (via
      the Provisioning Manager described in [NET-OPS]) to the lower-
      layer networks to cause LSPs to be set up to act as TE links in
      the higher layer network.  A number of potential protocols exist
      for this function as described in [NET-OPS], but it should be
      noted that it makes a lot of sense for this interface to be the
      same as that used by an Active PCE when providing paths to the
      network.

23.  How Does Service Scheduling and Calendering Work?

   LSP scheduling or calendaring is a process where LSPs are planned
   ahead of time, and they are only set up when needed.  The challenge
   here is to ensure that the resources needed by an LSP and that were
   available when the LSP's path was computed are still available when
   the LSP needs to be set up.  This needs to be achieved using a
   mechanism that allows those resources to be used in the meantime.

   Previous discussion of this topic has suggested that LSPs should be
   pre-signaled so that each LSR along the path could make a "temporal
   reservation" of resources.  But this approach can become very
   complicated requiring each network node to store multi-dimensional
   state.

   Conversely, a centralized database of resources and LSPs (such as the
   database maintained by a Stateful PCE) can be enhanced with a time-
   based booking system.  If the PCE is also Active, then when the time
   comes for the LSP to be set up (or later, when it is to be torn
   down), the PCE can issue recommendations to the network.

   In a busy network (and why would one bother with a scheduling service
   in a network that is not busy?), it should be noted that the
   computation algorithm can be quite complex.  It may also be necessary
   to reposition existing or planned LSPs as new bookings arrive.



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   Furthermore, the booking database that contains both the scheduled
   LSPs and their impact on the network resources can become quite
   large.  A very important factor in the size of the active database
   (depending on implementation) may be the timeslices that are
   available in the calendering process.

24.  Where Does Policy Fit In?

   Policy is critical to the operation of a network.  In a PCE context,
   it provides control and management of how a PCE selects network
   resources for use by different PCEs.

   [RFC5394] introduced the concept of PCE-based policy-enabled path
   computation.  It is based on the Policy Core Information Model (PCIM)
   [RFC3060] as extended by [RFC3460], and provides a framework for
   supporting path computation policy.

   Policy enters into all aspects of the use of a PCE starting from the
   very decision to use a PCE to off-load computation function from the
   LSRs.

   -  Each PCC must select which computations will be delegated to a
      PCE.

   -  Each PCC must select which PCEs it will use.

   -  Each PCE must determine which PCCs are allowed to use its services
      and for what computations.

   -  The PCE must determine how to collect the information in its TED,
      who to trust for that information, and how to refresh/update the
      information.

   -  Each PCE must determine which objective functions and which
      algorithms to apply.

   -  Inter-domain (and particularly H-PCE) computations will need to be
      sensitive to commercial and reliability information about domains
      and their interactions.

   -  Stateful PCEs must determine what state to hold, when to refresh
      it, and which network elements to trust for the supply of the
      state information.

   -  An Active PCE must have a policy relationship with its LSRs to
      determine which LSPs can be modified or triggered, and what LSP
      delegation is supported.




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   -  Multi-layer interactions (especially those using virtual or
      dynamic TE links) must provide policy control to stop server layer
      LSPs (which are fat and expensive by definition) from being set up
      on a whim to address micro-flows or speculative computations in
      higher layers.

   -  A PCE may supply, along with a computed path, policy information
      that should be signaled during LSP setup for use by the LSRs along
      the path.

   It may be seen, therefore, that a PCE is substantially a policy
   engine that computes paths.  It should also be noted that the work of
   the PCE can be substantially controlled by configured policy in a way
   that will reduce the options available to the PCC, but also
   significantly reduce the need for the use of optional parameters in
   the PCEP messages.

25.  Does PCE Play a Role in SDN?

   Software-Defined Networking (SDN) is the latest shiny thing in
   networking.  It 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 flows 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, it may be concluded, is the same function that a PCE
   might offer in a network operated using a dynamic control plane.
   Thus, a PCE could form part of the infrastructure for an SDN.

   A view of how PCE integrates into a wider network control system
   including SDN is presented in [NET-OPS].

26.  Security Considerations

   The use of a PCE-based architecture and subsequent impact on network
   security must, itself, be considered in the context of existing
   routing and signaling protocols and techniques.  The nature of multi-
   domain network scenarios and establishment of relationships between
   PCCs and PCEs may increase the vulnerability of the network to
   security attacks.  However, this informational document does not
   define any new protocol elements or mechanism.  As such, it does not
   introduce any new security issues and security is deemed to be a



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   "previously answered question" even if the answers previously
   supplied are not perfect.  Previous PCE RFCs have given some
   attention to security concerns in the use of PCE (RFC 4655), PCE
   discovery (RFC 4674, RFC 5088, and RFC 5089), and PCEP (RFC 4657 and
   RFC 5440).

   It is worth noting that PCEP operates over TCP.  An analysis of the
   security issues for routing protocols that use TCP (including PCEP)
   is provided in [RFC6952], while [PCE-PCEPS] discusses an experimental
   approach to provide secure transport for PCEP.

   A number of the questions raised and answered in this document should
   be given consideration in the light of security requirements.  Some
   of these are called out explicitly (Sections 8 and 10), but attention
   should also be paid to security in all aspects of the use of PCE.
   For example:

   -  Topology and other information about the network needs to be kept
      private and protected from modification or forgery.  That means
      that access to the TED, LSP-DB, etc., needs to be secured and that
      mechanisms used to gather topology and other information (Sections
      2, 11, 14, and 15) need to include security.

   -  PCE discovery (Sections 4, 5, 9, and 10) needs to protect against
      impersonation or misconfiguration so that PCCs know that they are
      getting correct paths and so that PCEs know that they are only
      serving legitimate computation requests.

   -  Synchronization of information and state between PCEs (Sections 6
      and 16) is subject to the same security requirements in that the
      information exchanged is sensitive and needs to be protected
      against interception and modification.

   -  PCE computes paths for components that may provision the network.
      Those component are responsible for the security of the
      provisioning mechanisms, however, if PCE operates as a
      provisioning protocol (Sections 17, 18, 19, and 25).

   -  A PCE may also need to interface with other network components
      (Sections 19, 21, 22, and 25).  Those communications, if external
      to an implementation, also need to be secure.










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27.  References

27.1.  Normative References

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

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

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

   [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, November 2012,
                  <http://www.rfc-editor.org/info/rfc6805>.

27.2.  Informative References

   [ALTO-SERVER-DISC]
                  Kiesel, S., Stiemerling, M., Schwan, N., Scharf, M.,
                  and H. Song, "ALTO Server Discovery", Work in
                  Progress, draft-ietf-alto-server-discovery-10,
                  September 2013.

   [LS-DISTRIB]   Gredler, H., Medved, J., Previdi, S., Farrel, A., and
                  S. Ray, "North-Bound Distribution of Link-State and TE
                  Information using BGP", Work in Progress,
                  draft-ietf-idr-ls-distribution-06, September 2014.

   [NET-OPS]      King, D., and A. Farrel, "A PCE-based Architecture for
                  Application-based Network Operations", Work in
                  Progress, draft-farrkingel-pce-abno-architecture-13,
                  October 2014.

   [PCE-PCEPS]    Lopez, D., Gonzalez de Dios, O., Wu, Q., and D. Dhody,
                  "Secure Transport for PCEP", Work in Progress,
                  draft-ietf-pce-pceps-02, October 2014.





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   [RFC3060]      Moore, B., Ellesson, E., Strassner, J., and A.
                  Westerinen, "Policy Core Information Model -- Version
                  1 Specification", RFC 3060, February 2001,
                  <http://www.rfc-editor.org/info/rfc3060>.

   [RFC3209]      Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan,
                  V., and G. Swallow, "RSVP-TE: Extensions to RSVP for
                  LSP Tunnels", RFC 3209, December 2001,
                  <http://www.rfc-editor.org/info/rfc3209>.

   [RFC3460]      Moore, B., Ed., "Policy Core Information Model (PCIM)
                  Extensions", RFC 3460, January 2003
                  <http://www.rfc-editor.org/info/rfc3460>.

   [RFC3630]      Katz, D., Kompella, K., and D. Yeung, "Traffic
                  Engineering (TE) Extensions to OSPF Version 2", RFC
                  3630, September 2003,
                  <http://www.rfc-editor.org/info/rfc3630>.

   [RFC3812]      Srinivasan, C., Viswanathan, A., and T. Nadeau,
                  "Multiprotocol Label Switching (MPLS) Traffic
                  Engineering (TE) Management Information Base (MIB)",
                  RFC 3812, June 2004,
                  <http://www.rfc-editor.org/info/rfc3812>.

   [RFC4203]      Kompella, K., Ed., and Y. Rekhter, Ed., "OSPF
                  Extensions in Support of Generalized Multi-Protocol
                  Label Switching (GMPLS)", RFC 4203, October 2005,
                  <http://www.rfc-editor.org/info/rfc4203>.

   [RFC4397]      Bryskin, I. and A. Farrel, "A Lexicography for the
                  Interpretation of Generalized Multiprotocol Label
                  Switching (GMPLS) Terminology within the Context of
                  the ITU-T's Automatically Switched Optical Network
                  (ASON) Architecture", RFC 4397, February 2006,
                  <http://www.rfc-editor.org/info/rfc4397>.

   [RFC4657]      Ash, J., Ed., and J. Le Roux, Ed., "Path Computation
                  Element (PCE) Communication Protocol Generic
                  Requirements", RFC 4657, September 2006,
                  <http://www.rfc-editor.org/info/rfc4657>.

   [RFC4674]      Le Roux, J., Ed., "Requirements for Path Computation
                  Element (PCE) Discovery", RFC 4674, October 2006,
                  <http://www.rfc-editor.org/info/rfc4674>.






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

   [RFC4802]      Nadeau, T., Ed., and A. Farrel, Ed., "Generalized
                  Multiprotocol Label Switching (GMPLS) Traffic
                  Engineering Management Information Base", RFC 4802,
                  February 2007,
                  <http://www.rfc-editor.org/info/rfc4802>.

   [RFC4848]      Daigle, L., "Domain-Based Application Service Location
                  Using URIs and the Dynamic Delegation Discovery
                  Service (DDDS)", RFC 4848, April 2007,
                  <http://www.rfc-editor.org/info/rfc4848>.

   [RFC4974]      Papadimitriou, D. and A. Farrel, "Generalized MPLS
                  (GMPLS) RSVP-TE Signaling Extensions in Support of
                  Calls", RFC 4974, August 2007,
                  <http://www.rfc-editor.org/info/rfc4974>.

   [RFC5088]      Le Roux, JL., Ed., Vasseur, JP., Ed., Ikejiri, Y., and
                  R. Zhang, "OSPF Protocol Extensions for Path
                  Computation Element (PCE) Discovery", RFC 5088,
                  January 2008,
                  <http://www.rfc-editor.org/info/rfc5088>.

   [RFC5089]      Le Roux, JL., Ed., Vasseur, JP., Ed., Ikejiri, Y., and
                  R. Zhang, "IS-IS Protocol Extensions for Path
                  Computation Element (PCE) Discovery", RFC 5089,
                  January 2008,
                  <http://www.rfc-editor.org/info/rfc5089>.

   [RFC5152]      Vasseur, JP., Ed., Ayyangar, A., Ed., and R. Zhang, "A
                  Per-Domain Path Computation Method for Establishing
                  Inter-Domain Traffic Engineering (TE) Label Switched
                  Paths (LSPs)", RFC 5152, February 2008,
                  <http://www.rfc-editor.org/info/rfc5152>.

   [RFC5212]      Shiomoto, K., Papadimitriou, D., Le Roux, JL.,
                  Vigoureux, M., and D. Brungard, "Requirements for
                  GMPLS-Based Multi-Region and Multi-Layer Networks
                  (MRN/MLN)", RFC 5212, July 2008,
                  <http://www.rfc-editor.org/info/rfc5212>.

   [RFC5305]      Li, T. and H. Smit, "IS-IS Extensions for Traffic
                  Engineering", RFC 5305, October 2008,
                  <http://www.rfc-editor.org/info/rfc5305>.



Farrel & King                 Informational                    [Page 27]

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   [RFC5307]      Kompella, K., Ed., and Y. Rekhter, Ed., "IS-IS
                  Extensions in Support of Generalized Multi-Protocol
                  Label Switching (GMPLS)", RFC 5307, October 2008,
                  <http://www.rfc-editor.org/info/rfc5307>.

   [RFC5394]      Bryskin, I., Papadimitriou, D., Berger, L., and J.
                  Ash, "Policy-Enabled Path Computation Framework", RFC
                  5394, December 2008,
                  <http://www.rfc-editor.org/info/rfc5394>.

   [RFC5441]      Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le
                  Roux, "A Backward-Recursive PCE-Based Computation
                  (BRPC) Procedure to Compute Shortest Constrained
                  Inter-Domain Traffic Engineering Label Switched
                  Paths", RFC 5441, April 2009,
                  <http://www.rfc-editor.org/info/rfc5441>.

   [RFC5557]      Lee, Y., Le Roux, JL., King, D., and E. Oki, "Path
                  Computation Element Communication Protocol (PCEP)
                  Requirements and Protocol Extensions in Support of
                  Global Concurrent Optimization", RFC 5557, July 2009,
                  <http://www.rfc-editor.org/info/rfc5557>.

   [RFC5986]      Thomson, M. and J. Winterbottom, "Discovering the
                  Local Location Information Server (LIS)", RFC 5986,
                  September 2010,
                  <http://www.rfc-editor.org/info/rfc5986>.

   [RFC6006]      Zhao, Q., Ed., King, D., Ed., Verhaeghe, F., Takeda,
                  T., Ali, Z., and J. Meuric, "Extensions to the Path
                  Computation Element Communication Protocol (PCEP) for
                  Point-to-Multipoint Traffic Engineering Label Switched
                  Paths", RFC 6006, September 2010,
                  <http://www.rfc-editor.org/info/rfc6006>.

   [RFC6241]      Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J.,
                  Ed., and A. Bierman, Ed., "Network Configuration
                  Protocol (NETCONF)", RFC 6241, June 2011,
                  <http://www.rfc-editor.org/info/rfc6241>.

   [RFC6952]      Jethanandani, M., Patel, K., and L. Zheng, "Analysis
                  of BGP, LDP, PCEP, and MSDP Issues According to the
                  Keying and Authentication for Routing Protocols (KARP)
                  Design Guide", RFC 6952, May 2013,
                  <http://www.rfc-editor.org/info/rfc6952>.






Farrel & King                 Informational                    [Page 28]

RFC 7399              Questions in PCE Architecture         October 2014


   [STATEFUL-PCE] Crabbe, E., Minei, I., Medved, J., and R. Varga, "PCEP
                  Extensions for Stateful PCE", Work in Progress,
                  draft-ietf-pce-stateful-pce-10, October 2014.

   [TE-INFO]      Farrel, A., Ed., Drake, J., Bitar, N., Swallow, G.,
                  Ceccarelli, D, and X. Zhang, "Problem Statement and
                  Architecture for Information Exchange Between
                  Interconnected Traffic Engineered Networks", Work in
                  Progress, draft-farrel-interconnected-te-info-
                  exchange-07, September 2014.

Acknowledgements

   Thanks for constructive comments go to Fatai Zhang, Oscar Gonzalez de
   Dios, Xian Zhang, Cyril Margaria, Denis Ovsienko, Ina Minei, Dhruv
   Dhody, and Qin Wu.

   This work was supported in part by the FP-7 IDEALIST project under
   grant agreement number 317999.

   This work received funding from the European Union's Seventh
   Framework Programme for research, technological development and
   demonstration through the PACE project under grant agreement no.
   619712.

Authors' Addresses

   Adrian Farrel
   Juniper Networks
   EMail: adrian@olddog.co.uk


   Daniel King
   Old Dog Consulting
   EMail: daniel@olddog.co.uk
















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