PCE Working Group                                                D. King
Internet Draft                                        Old Dog Consulting
Intended status: Informational                                  H. Zheng
Expires: January 9, 2020                             Huawei Technologies
                                                            July 8, 2019
                                         
     Applicability of the Path Computation Element to Inter-Area and    
            Inter-AS MPLS and GMPLS Traffic Engineering            

           draft-ietf-pce-inter-area-as-applicability-08


   Abstract 

   The Path Computation Element (PCE) may be used for computing services
   that traverse multi-area and multi-AS Multiprotocol Label Switching  
   (MPLS) and Generalized MPLS (GMPLS) Traffic Engineered (TE) networks.

   This document examines the applicability of the PCE architecture,   
   protocols, and protocol extensions for computing multi-area and  
   multi-AS paths in MPLS and GMPLS networks.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on January 9, 2020.


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   to this document.  Code Components extracted from this document must


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   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.




   1. Introduction.................................................3
      1.1. Domains.................................................4
      1.2. Path Computation........................................4
        1.2.1 PCE-based Path Computation Procedure.................5
      1.3. Traffic Engineering Aggregation and Abstraction.........6
      1.4. Traffic Engineered Label Switched Paths.................6
      1.5. Inter-area and Inter-AS Capable PCE Discovery...........6
      1.6. Objective Functions.....................................6
   2. Terminology..................................................7
   3. Issues and Considerations....................................7
      3.1 Multi-homing.............................................7
      3.2 Destination Location.....................................8
      3.3 Domain Confidentiality ..................................8
   4. Domain Topologies............................................8
      4.1 Selecting Domain Paths...................................8
      4.2 Domain Sizes.............................................9
      4.3 Domain Diversity.........................................9
      4.4 Synchronized Path Computations...........................9
      4.5 Domain Inclusion or Exclusion............................9
   5. Applicability of the PCE to Inter-area Traffic Engineering...10
      5.1. Inter-area Routing......................................11
      5.1.1. Area Inclusion and Exclusion..........................11
      5.1.2. Strict Explicit Path and Loose Path...................11
      5.1.3. Inter-Area Diverse Path Computation...................11
   6. Applicability of the PCE to Inter-AS Traffic Engineering.....12
      6.1. Inter-AS Routing........................................12
      6.1.1. AS Inclusion and Exclusion............................12
      6.2. Inter-AS Bandwidth Guarantees...........................12
      6.3. Inter-AS Recovery.......................................13
      6.4. Inter-AS PCE Peering Policies...........................13
   7. Multi-Domain PCE Deployment..................................13
      7.1 Traffic Engineering Database.............................13
      7.1.1. Applicability of BGP-LS to PCE........................14
      7.2 Pre-Planning and Management-Based Solutions..............14
   8. Domain Confidentiality.......................................15
      8.1 Loose Hops...............................................15
      8.2 Confidential Path Segments and Path Keys.................15
   9. Point-to-Multipoint..........................................16
   10. Optical Domains.............................................16
      10.1 Abstraction and Control of TE Networks (ACTN)...........17
   11. Policy......................................................17


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   12. Manageability Considerations................................18
     12.1 Control of Function and Policy...........................18
     12.2 Information and Data Models..............................18
     12.3 Liveness Detection and Monitoring........................19
     12.4 Verifying Correct Operation..............................19
     12.5 Impact on Network Operation..............................19
   13. Security Considerations.....................................19
     13.1 Multi-domain Security....................................19
   14. IANA Considerations.........................................20
   15. Acknowledgements............................................20
   16. References..................................................20
     16.1. Normative References....................................20
     16.2. Informative References..................................21
   17. Contributors................................................24
   18. Author's Addresses..........................................25


1. Introduction

   Computing paths across large multi-domain environments may 
   require special computational components and cooperation between
   entities in different domains capable of complex path computation. 
   
   Issues that may exist when routing in multi-domain networks include:
   
   o Often there is a lack of full topology and TE information across 
     domains;
   o No single node has the full visibility to determine an optimal or 
     even feasible end-to-end path across domains;
   o How to evaluate and select the exit point and next domain boundary 
     from a domain?
   o How might the ingress node determine which domains should be used 
     for the end-to-end path?
   
   Often information exchange across multiple domains is limited due to 
   the lack of trust relationship, security issues, or scalability 
   issues even if there is a trust relationship between domains.
   
   The Path Computation Element (PCE) [RFC4655] provides an architecture
   and a set of functional components to address the problem space, and 
   issues highlighted above.

   A PCE may be used to compute end-to-end paths across multi-domain 
   environments using a per-domain path computation technique [RFC5152]. 
   The so called backward recursive path computation (BRPC) mechanism 
   [RFC5441] defines a PCE-based path computation procedure to compute
   inter-domain constrained Multiprotocol Label Switching (MPLS) and 
   Generalized MPLS (GMPLS) Traffic Engineered (TE) networks. However, 


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   both per-domain and BRPC techniques assume that the sequence of 
   domains to be crossed from source to destination is known, either 
   fixed by the network operator or obtained by other means. 

   In more advanced deployments (including multi-area and multi-
   Autonomous System (multi-AS) environments) the sequence of domains 
   may not be known in advance and the choice of domains in the end-to-
   end domain sequence might be critical to the determination of an 
   optimal end-to-end path. In this case the use of the Hierarchical PCE 
   [RFC6805] architecture and mechanisms may be used to discover the 
   intra-area path and select the optimal end-to-end domain sequence. 

   This document describes the processes and procedures available when 
   using the PCE architecture and protocols, for computing inter-area 
   and inter-AS MPLS and GMPLS Traffic Engineered paths.
   
   This document scope does not include discussion on stateful PCE, 
   active PCE, remotely initiated PCE, or PCE as a central controller 
   (PCECC) deployment scenarios. 

1.1 Domains

   Generally, a domain can be defined as a separate administrative, 
   geographic, or switching environment within the network. A domain 
   may be further defined as a zone of routing or computational ability.
   Under these definitions a domain might be categorized as an 
   Autonomous System (AS) or an Interior Gateway Protocol (IGP) area 
   (as per [RFC4726] and [RFC4655]). 

   For the purposes of this document, a domain is considered to be a
   collection of network elements within an area or AS that has a 
   common sphere of address management or path computational 
   responsibility. Wholly or partially overlapping domains are not 
   within the scope of this document.

   In the context of GMPLS, a particularly important example of a domain
   is the Automatically Switched Optical Network (ASON) subnetwork
   [G-8080]. In this case, computation of an end-to-end path requires
   the selection of nodes and links within a parent domain where some
   nodes may, in fact, be subnetworks. Furthermore, a domain might be an
   ASON routing area [G-7715]. A PCE may perform the path computation
   function of an ASON routing controller as described in [G-7715-2].

   It is assumed that the PCE architecture is not applied to a large 
   group of domains, such as the Internet.

1.2 Path Computation



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   For the purpose of this document, it is assumed that the 
   path computation is the sole responsibility of the PCE as per the
   architecture defined in [RFC4655]. When a path is required the Path
   Computation Client (PCC) will send a request to the PCE. The PCE 
   will apply the required constraints and compute a path and return a 
   response to the PCC. In the context of this document it may be
   necessary for the PCE to co-operate with other PCEs in adjacent
   domains (as per BRPC [RFC5441]) or cooperate with a Parent PCE 
   (as per [RFC6805]).

   It is entirely feasible that an operator could compute a path across
   multiple domains without the use of a PCE if the relevant domain 
   information is available to the network planner or network management
   platform. The definition of what relevant information is required to
   perform this network planning operation and how that information is
   discovered and applied is outside the scope of this document.
   
1.2.1 PCE-based Path Computation Procedure

   As highlighted, the PCE is an entity capable of computing an 
   inter-domain TE path upon receiving a request from a PCC. There could 
   be a single PCE per domain, or single PCE responsible for all 
   domains. A PCE may or may not reside on the same node as the 
   requesting PCC. A path may be computed by either a single PCE node 
   or a set of distributed PCE nodes that collaborate during path 
   computation. 
   
   [RFC4655] defines that a PCC should send a path computation request
   to a particular PCE, using [RFC5440] (PCC-to-PCE communication). 
   This negates the need to broadcast a request to all the PCEs. Each 
   PCC can maintain information about the computation capabilities
   of the PCEs, it is aware of. The PCC-PCE capability awareness can be
   configured using static configurations or by automatic and dynamic 
   PCE discovery procedures.
      
   If a network path is required, the PCC will send a path computation 
   request to the PCE. A PCE may then compute the end-to-end path
   if it is aware of the topology and TE information required to 
   compute the entire path. If the PCE is unable to compute the 
   entire path, the PCE architecture provides co-operative PCE 
   mechanisms for the resolution of path computation requests when an
   individual PCE does not have sufficient TE visibility.
   
   End-to-end path segments may be kept confidential through the 
   application of path keys, to protect partial or full path 
   information. A path key that is a token that replaces a path segment
   in an explicit route. The path key mechanism is described in 
   [RFC5520]


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1.3 Traffic Engineering Aggregation and Abstraction

   Networks are often constructed from multiple areas or ASes that are
   interconnected via multiple interconnect points. To maintain 
   network confidentiality and scalability TE properties of each area
   and AS are not generally advertized outside each specific area or AS. 

   TE aggregation or abstraction provide mechanism to hide information
   but may cause failed path setups or the selection of suboptimal 
   end-to-end paths [RFC4726]. The aggregation process may also have
   significant scaling issues for networks with many possible routes
   and multiple TE metrics. Flooding TE information breaks
   confidentiality and does not scale in the routing protocol.

   The PCE architecture and associated mechanisms provide a solution
   to avoid the use of TE aggregation and abstraction. 
   
1.4 Traffic Engineered Label Switched Paths

   This document highlights the PCE techniques and mechanisms that exist
   for establishing TE packet and optical LSPs across multiple areas 
   (inter-area TE LSP) and ASes (inter-AS TE LSP). In this context and
   within the remainder of this document, we consider all LSPs to be
   constraint-based and traffic engineered. 

   Three signaling options are defined for setting up an inter-area or 
   inter-AS LSP [RFC4726]:

    o Contiguous LSP
    o Stitched LSP 
    o Nested LSP 

   All three signaling methods are applicable to the architectures and
   procedures discussed in this document.  
   
1.5 Inter-area and Inter-AS Capable PCE Discovery

   When using a PCE-based approach for inter-area and inter-AS path
   computation, a PCE in one area or AS may need to learn information
   related to inter-AS capable PCEs located in other ASes. The PCE
   discovery mechanism defined in [RFC5088] and [RFC5089] facilitates
   the discovery of PCEs, and disclosure of information related to
   inter-area and inter-AS capable PCEs.

1.6 Objective Functions

   An Objective Function (OF) [RFC5541], or set of OFs, specifies the 
   intentions of the path computation and so defines the "optimality", 


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   in the context of the computation request. 
   
   An OF specifies the desired outcome of a computation. An OF does not
   describe or specify the algorithm to use. Also, an implementation 
   may apply any algorithm, or set of algorithms, to achieve the result
   indicated by the OF. A number of general OFs are specified in 
   [RFC5541]. 

   Various OFs may be included in the PCE computation request to 
   satisfy the policies encoded or configured at the PCC, and a PCE 
   may be subject to policy in determining whether it meets the OFs 
   included in the computation request or applies its own OFs.

   During inter-domain path computation, the selection of a domain 
   sequence, the computation of each (per-domain) path fragment, and 
   the determination of the end-to-end path may each be subject to 
   different OFs and policy.   


2. Terminology

   This document also uses the terminology defined in [RFC4655] and 
   [RFC5440]. Additional terminology is defined below:

   ABR: IGP Area Border Router, a router that is attached to more than
   one IGP area.

   ASBR: Autonomous System Border Router, a router used to connect
   together ASes of a different or the same Service Provider via one or
   more inter-AS links.
   
   Inter-area TE LSP: A TE LSP whose path transits through two or more
   IGP areas.

   Inter-AS MPLS TE LSP: A TE LSP whose path transits through two or
   more ASes or sub-ASes (BGP confederations
   
   SRLG: Shared Risk Link Group.
   
   TED: Traffic Engineering Database, which contains the topology and
   resource information of the domain.  The TED may be fed by Interior
   Gateway Protocol (IGP) extensions or potentially by other means.


3. Issues and Considerations

3.1 Multi-homing



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   Networks constructed from multi-areas or multi-AS environments
   may have multiple interconnect points (multi-homing). End-to-end path
   computations may need to use different interconnect points to avoid
   a single point failure disrupting both primary and backup services.
   
3.2 Destination Location

   The PCC asking for an inter-domain path computation is typically
   aware of the identity of the destination node. If the PCC is aware 
   of the destination domain, it may supply the destination domain 
   information as part of the path computation request. However, if the
   PCC does not know the destination domain this information must be 
   determined by another method.

3.3 Domain Confidentiality 

   Where the end-to-end path crosses multiple domains, it may be 
   possible that each domain (AS or area) are administered by separate
   Service Providers, it would break confidentiality rules for a PCE
   to supply a path segment to a PCE in another domain, thus disclosing
   AS-internal topology information.
   
   If confidentiality is required between domains (ASes and areas) 
   belonging to different Service Providers, then cooperating PCEs 
   cannot exchange path segments or else the receiving PCE or PCC will 
   be able to see the individual hops through another domain.
   
   This topic is discussed further in Section 8 of this document. 


4. Domain Topologies

   Constraint-based inter-domain path computation is a fundamental 
   requirement for operating traffic engineered MPLS [RFC3209] and 
   GMPLS [RFC3473] networks, in inter-area and inter-AS (multi-domain) 
   environments. Path computation across multi-domain networks is 
   complex and requires computational co-operational entities like the 
   PCE. 
   
4.1 Selecting Domain Paths
 
   Where the sequence of domains is known a priori, various techniques
   can be employed to derive an optimal multi-domain path. If the 
   domains are connected to a simple path with no branches and single
   links between all domains, or if the preferred points of 
   interconnection is also known, the Per-Domain Path Computation 
   [RFC5152] technique may be used. Where there are multiple connections
   between domains and there is no preference for the choice of points 


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   of interconnection, BRPC [RFC5441] can be used to derive an optimal
   path.
 
   When the sequence of domains is not known in advance, or the 
   end-to-end path will have to navigate a mesh of small domains 
   (especially typical in optical networks), the optimum path may be 
   derived through the application of a Hierarchical PCE [RFC6805].   

4.2 Domain Sizes

   Very frequently network domains are composed of dozens or hundreds of
   network elements. These network elements are usually interconnected 
   in a partial-mesh fashion, to provide survivability against dual 
   failures, and to benefit from the traffic engineering capabilities 
   from MPLS and GMPLS protocols. Network operator feedback in the 
   development of the document highlighted that node degree (the number 
   of neighbors per node) typically ranges from 3 to 10 (4-5 is quite 
   common).

4.3 Domain Diversity

   Domain and path diversity may also be required when computing 
   end-to-end paths. Domain diversity should facilitate the selection
   of paths that share ingress and egress domains, but do not share 
   transit domains. Therefore, there must be a method allowing the 
   inclusion or exclusion of specific domains when computing end-to-end
   paths. 
   
4.4 Synchronized Path Computations

   In some scenarios, it would be beneficial for the operator to rely on
   the capability of the PCE to perform synchronized path computation.
   
   Synchronized path computations, known as Synchronization VECtors 
   (SVECs) are used for dependent path computations. SVECs are 
   defined in [RFC5440] and [RFC6007] provides an overview for the 
   use of the PCE SVEC list for synchronized path computations when 
   computing dependent requests.
   
   In H-PCE deployments, a child PCE will be able to request both 
   dependent and synchronized domain diverse end to end paths from its 
   parent PCE. 

4.5 Domain Inclusion or Exclusion

   A domain sequence is an ordered sequence of domains traversed to 
   reach the destination domain.  A domain sequence may be supplied 
   during path computation to guide the PCEs or derived via the use of 


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   Hierarchical PCE (H-PCE).    

   During multi-domain path computation, a PCC may request 
   specific domains to be included or excluded in the domain sequence  
   using the Include Route Object (IRO) [RFC5440] and Exclude Route 
   Object (XRO) [RFC5521]. The use of Autonomous Number (AS) as an 
   abstract node representing a domain is defined in [RFC3209]. 
   [RFC7897] specifies new sub-objects to include or exclude domains 
   such as an IGP area or a 4-Byte AS number.
   
   An operator may also need to avoid a path that uses specified nodes 
   for administrative reasons, or if a specific connectivity 
   service required to have a 1+1 protection capability, two    
   completely disjoint paths must be established. A mechanism known as
   Shared Risk Link Group (SRLG) information may be used to ensure 
   path diversity.


5. Applicability of the PCE to Inter-area Traffic Engineering

   As networks increase in size and complexity, it may be required to 
   introduce scaling methods to reduce the amount of information 
   flooded within the network and make the network more manageable. An 
   IGP hierarchy is designed to improve IGP scalability by dividing the
   IGP domain into areas and limiting the flooding scope of topology
   information to within area boundaries. This restricts visibility of
   the area to routers in a single area. If a router needs to compute 
   the route to a destination located in another area, a method would 
   be required to compute a path across area boundaries. 
  
   In order to support multiple vendors in a network, in cases where
   data or control plane technologies cannot interoperate, it is useful 
   to divide the network into vendor domains. Each vendor domain is
   an IGP area, and the flooding scope of the topology (as well as any
   other relevant information) is limited to the area boundaries. 

   Per-domain path computation [RFC5152] exists to provide a method of 
   inter-area path computation. The per-domain solution is based on 
   loose hop routing with an Explicit Route Object (ERO) expansion on
   each Area Border Router (ABR).  This allows an LSP to be established
   using a constrained path, however at least two issues exist:

   o This method does not guarantee an optimal constrained path.

   o The method may require several crankback signaling messages, as per
     [RFC4920], increasing signaling traffic and delaying the LSP setup.

   The PCE-based architecture [RFC4655] is designed to solve inter-area


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   path computation problems. The issue of limited topology visibility 
   is resolved by introducing path computation entities that are able to
   cooperate in order to establish LSPs with source and destinations 
   located in different areas. 

5.1. Inter-area Routing

   An inter-area TE-LSP is an LSP that transits through at least two
   IGP areas. In a multi-area network, topology visibility remains
   local to a given area for scaling and privacy purposes, a node 
   in one area will not be able to compute an end-to-end path across 
   multiple areas without the use of a PCE.
   
5.1.1. Area Inclusion and Exclusion

   The BRPC method [RFC5441] of path computation provides a more optimal
   method to specify inclusion or exclusion of an ABR. Using the BRPC 
   procedure an end-to-end path is recursively computed in reverse from
   the destination domain, towards the source domain. Using this method, 
   an operator might decide if an area must be included or excluded from 
   the inter-area path computation.  
     
5.1.2. Strict Explicit Path and Loose Path

   A strict explicit Path is defined as a set of strict hops, while a
   loose path is defined as a set of at least one loose hop and zero or
   more strict hops.  It may be useful to indicate, during the
   path computation request, if a strict explicit path is required or 
   not. An inter-area path may be strictly explicit or loose (e.g., a
   list of ABRs as loose hops). 
      
   A PCC request to a PCE does allow the indication of whether a strict 
   explicit path across specific areas ([RFC7897]) is required or 
   desired, or if the path request is loose.
   
5.1.3. Inter-Area Diverse Path Computation

   It may be necessary to compute a path that is partially or entirely
   diverse, from a previously computed path, to avoid fate sharing of
   a primary service with a corresponding backup service. There are 
   various levels of diversity in the context of an inter-area network:
   
   o Per-area diversity (intra-area path segments are link, node or
     SRLG disjoint.
         
   o Inter-area diversity (end-to-end inter-area paths are link,
     node or SRLG disjoint).



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   Note that two paths may be disjoint in the backbone area but non-
   disjoint in peripheral areas. Also, two paths may be node disjoint
   within areas but may share ABRs, in which case path segments within
   an area is node disjoint, but end-to-end paths are not node-disjoint.
   Per-Domain [RFC5152], BRPC [RFC5441] and H-PCE [RFC6805] mechanisms 
   all support the capability to compute diverse paths across multi-area 
   topologies.


6. Applicability of the PCE to Inter-AS Traffic Engineering

   As discussed in section 4 (Applicability of the PCE to Inter-area
   Traffic Engineering) it is necessary to divide the network into
   smaller administrative domains, or ASes. If an LSR within an AS needs
   to compute a path across an AS boundary, it must also use an inter-AS
   computation technique. [RFC5152] defines mechanisms for the 
   computation of inter-domain TE LSPs using network elements along the
   signaling paths to compute per-domain constrained path segments.

   The PCE was designed to be capable of computing MPLS and GMPLS paths
   across AS boundaries. This section outlines the features of a 
   PCE-enabled solution for computing inter-AS paths.  

6.1 Inter-AS Routing

6.1.1. AS Inclusion and Exclusion

   [RFC5441] allows the specifying of inclusion or exclusion of an AS 
   or an ASBR. Using this method, an operator might decide if an AS 
   must be include or exclude from the inter-AS path computation. 
   Exclusion and/or inclusion could also be specified at any step in 
   the LSP path computation process by a PCE (within the BRPC 
   algorithm) but the best practice would be to specify them at the 
   edge. In opposition to the strict and loose path, AS inclusion or 
   exclusion doesn't impose topology disclosure as ASes are public 
   entity as well as their interconnection.

6.2 Inter-AS Bandwidth Guarantees

   Many operators with multi-AS domains will have deployed MPLS-TE 
   DiffServ either across their entire network or at the domain edges 
   on CE-PE links. In situations where strict QOS bounds are required, 
   admission control inside the network may also be required.

   When the propagation delay can be bounded, the performance targets,
   such as maximum one-way transit delay may be guaranteed by providing
   bandwidth guarantees along the DiffServ-enabled path, these 
   requirements are described in [RFC4216].


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   One typical example of the requirements in [RFC4216] is to provide 
   bandwidth guarantees over an end-to-end path for VoIP traffic 
   classified as EF (Expedited Forwarding) class in a DiffServ-enabled 
   network. In the case where the EF path is extended across multiple 
   ASes, inter-AS bandwidth guarantee would be required.

   Another case for inter-AS bandwidth guarantee is the requirement for
   guaranteeing a certain amount of transit bandwidth across one or
   multiple ASes.

6.3 Inter-AS Recovery

   During a path computation process, a PCC request may contain the
   requirement to compute a backup LSP for protecting the primary LSP, 
   1+1 protection. A single LSP or multiple backup LSPs may also be 
   used for a group of primary LSPs, this is typically known as m:n 
   protection. 
      
   Other inter-AS recovery mechanisms include [RFC4090] which adds fast 
   re-route (FRR) protection to an LSP. So, the PCE could be used to 
   trigger computation of backup tunnels in order to protect Inter-AS 
   connectivity. 
   
   Inter-AS recovery clearly requires backup LSPs for service 
   protection but it would also be advisable to have multiple PCEs 
   deployed for path computation redundancy, especially for service 
   restoration in the event of catastrophic network failure.

6.4 Inter-AS PCE Peering Policies

   Like BGP peering policies, inter-AS PCE peering policies is a
   requirement for operator. In inter-AS BRPC process, PCE must 
   cooperate in order to compute the end-to-end LSP. So, the AS path 
   must not only follow technical constraints, e.g. bandwidth 
   availability, but also policies defined by the operator.

   Typically PCE interconnections at an AS level must follow agreed 
   contract obligations, also known as peering agreements. The PCE 
   peering policies are the result of the contract negotiation and 
   govern the relation between the different PCE.


7. Multi-domain PCE Deployment Options

7.1 Traffic Engineering Database and Synchronization

   An optimal path computation requires knowledge of the available 
   network resources, including nodes and links, constraints, 


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   link connectivity, available bandwidth, and link costs.  The PCE 
   operates on a view of the network topology as presented by a 
   TED.  As discussed in [RFC4655] the TED used by a PCE may be learnt 
   by the relevant IGP extensions. 

   Thus, the PCE may operate its TED is by participating 
   in the IGP running in the network.  In an MPLS-TE network, this 
   would require OSPF-TE [RFC3630] or ISIS-TE [RFC5305].  In a GMPLS 
   network it would utilize the GMPLS extensions to OSPF and IS-IS 
   defined in [RFC4203] and [RFC5307]. Inter-as connectivity 
   information may be populated via [RFC5316] and [RFC5392].
 
   An alternative method to provide network topology and resource 
   information is offered by [RFC7752], which is described in the 
   following section. 

7.1.1 Applicability of BGP-LS to PCE

   The concept of exchange of TE information between Autonomous Systems
   (ASes) is discussed in [RFC7752].  The information exchanged in this
   way could be the full TE information from the AS, an aggregation of
   that information, or a representation of the potential connectivity
   across the AS.  Furthermore, that information could be updated
   frequently (for example, for every new LSP that is set up across the
   AS) or only at threshold-crossing events.

   In an H-PCE deployment, the parent PCE will require the inter-domain 
   topology and link status between child domains. This information may 
   be learnt by a BGP-LS speaker and provided to the parent PCE, 
   furthermore link-state performance including delay, available 
   bandwidth and utilized bandwidth may also be provided to the parent 
   PCE for optimal path link selection.  

7.2 Pre-Planning and Management-Based Solutions
 
   Offline path computation is performed ahead of time, before the LSP
   setup is requested.  That means that it is requested by, or performed
   as part of, an Operation Support System (OSS) management application.
   This model can be seen in Section 5.5 of [RFC4655].

   The offline model is particularly appropriate to long-lived LSPs
   (such as those present in a transport network) or for planned
   responses to network failures.  In these scenarios, more planning is
   normally a feature of LSP provisioning.

   The management system may also use a PCE and BRPC to pre-plan an AS 
   sequence, and the source domain PCE and per-domain path    
   computation to be used when the actual end-to-end path is 


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   required. This model may also be used where the operator 
   wishes to retain full manual control of the placement of LSPs, 
   using the PCE only as a computation tool to assist the operator, 
   not as part of an automated network.
   
   In environments where operators peer with each other to provide end-
   to-end paths, the operator responsible for each domain must agree 
   to what extent paths must be pre-planned or manually controlled.


8. Domain Confidentiality

   This section discusses the techniques that co-operating PCEs
   can use to compute inter-domain paths without each domain
   disclosing sensitive internal topology information (such as
   explicit nodes or links within the domain) to the other domains.

   Confidentiality typically applies to inter-provider (inter-AS) PCE
   communication. Where the TE LSP crosses multiple domains (ASes or 
   areas), the path may be computed by multiple PCEs that cooperate 
   together. With each local PCE responsible for computing a segment 
   of the path. 
   
   In situations where ASes are administered by separate Service 
   Providers, it would break confidentiality rules for a PCE to supply
   a path segment details to a PCE responsible another domain, thus 
   disclosing AS-internal or area topology information.
   
   
8.1 Loose Hops

   A method for preserving the confidentiality of the path segment is 
   for the PCE to return a path containing a loose hop in place of the
   segment that must be kept confidential.  The concept of loose and
   strict hops for the route of a TE LSP is described in [RFC3209]. 
   
   [RFC5440] supports the use of paths with loose hops, and it is a
   local policy decision at a PCE whether it returns a full explicit
   path with strict hops or uses loose hops.  A path computation 
   request may require an explicit path with strict hops, or may allow 
   loose hops as detailed in [RFC5440].

8.2 Confidential Path Segments and Path Keys

   [RFC5520] defines the concept and mechanism of Path-Key. A Path-Key 
   is a token that replaces the path segment information in an explicit
   route. The Path-Key allows the explicit route information to be 
   encoded and in the PCEP ([RFC5440]) messages exchanged between the 


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   PCE and PCC.
   
   This Path-Key technique allows explicit route information to be used
   for end-to-end path computation, without disclosing internal topology
   information between domains.   


9. Point-to-Multipoint

   For inter-domain point-to-multipoint application scenarios using 
   MPLS-TE LSPs, the complexity of domain sequences, domain policies, 
   choice and number of domain interconnects is magnified compared to 
   point-to-point path computations. As the size of the network 
   grows, the number of leaves and branches increase, further 
   increasing the complexity of the overall path computation problem. 
   A solution for managing point-to-multipoint path computations may 
   be achieved using the PCE inter-domain point-to-multipoint path 
   computation [RFC7334] procedure.


10. Optical Domains

   The International Telecommunications Union (ITU) defines the ASON
   architecture in [G-8080]. [G-7715] defines the routing architecture
   for ASON and introduces a hierarchical architecture. In this
   architecture, the Routing Areas (RAs) have a hierarchical
   relationship between different routing levels, which means a parent
   (or higher level) RA can contain multiple child RAs. The
   interconnectivity of the lower RAs is visible to the higher-level RA.

   In the ASON framework, a path computation request is termed a Route
   Query. This query is executed before signaling is used to establish
   an LSP termed a Switched Connection (SC) or a Soft Permanent
   Connection (SPC). [G-7715-2] defines the requirements and
   architecture for the functions performed by Routing Controllers (RC)
   during the operation of remote route queries - an RC is synonymous
   with a PCE. 
    
   In the ASON routing environment, an RC responsible for an RA may
   communicate with its neighbor RC to request the computation of an
   end-to-end path across several RAs. The path computation components
   and sequences are defined as follows:
   
   o Remote route query. An operation where a routing controller 
     communicates with another routing controller, which does not have
     the same set of layer resources, in order to compute a routing 
     path in a collaborative manner.



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   o Route query requester. The connection controller or RC that sends a
     route query message to a routing controller requesting for one or
     more routing paths that satisfy a set of routing constraints.
   
   o Route query responder. An RC that performs path computation upon
     reception of a route query message from a routing controller or
     connection controller, sending a response back at the end of 
     computation.

   When computing an end-to-end connection, the route may be computed by
   a single RC or multiple RCs in a collaborative manner and the two
   scenarios can be considered a centralized remote route query model
   and distributed remote route query model. RCs in an ASON environment
   can also use the hierarchical PCE [RFC6805] model to match fully the 
   ASON hierarchical routing model. 
   
10.1 Abstraction and Control of TE Networks (ACTN)

   Where a single operator operates multiple TE domains (including 
   optical environments) then Abstraction and Control of TE Networks 
   (ACTN) framework [RFC8453] may be used to create an abstracted 
   (virtualized network) view of underlay interconnected domains. This 
   underlay connectivity then be exposed to higher-layer control 
   entities and applications.
   
   ACTN describes the method and procedure for coordinating the 
   underlay per-domain Physical Network Controllers (PNCs), which may 
   be PCEs, via a hierarchical model to facilitate setup of 
   end-to-end connections across inter-connected TE domains.


11. Policy

   Policy is important in the deployment of new services and the 
   operation of the network. [RFC5394] provides a framework for PCE-
   based policy-enabled path computation. This framework is based on 
   the Policy Core Information Model (PCIM) as defined in [RFC3060] and 
   further extended by [RFC3460]. 
   
   When using a PCE to compute inter-domain paths, policy may be 
   invoked by specifying: 

   o Each PCC must select which computations will be requested to a PCE;

   o Each PCC must select which PCEs it will use;

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


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   o The PCE must determine how to collect the information in its TED,
     whom to trust for that information, and how to refresh/update the
     information;

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


12. Manageability Considerations

   General PCE management considerations are discussed in [RFC4655].    
   In the case of multi-domains within a single service provider 
   network, the management responsibility for each PCE would most 
   likely be handled by the same service provider.  In the case of 
   multiple ASes within different service provider networks, it will 
   likely be necessary for each PCE to be configured and managed   
   separately by each participating service provider, with policy 
   being implemented based on a previously agreed set of principles. 

12.1 Control of Function and Policy

   As per PCEP [RFC5440] implementation allow the user to configure
   a number of PCEP session parameters. These are detailed in section 
   8.1 of [RFC5440]. 
   
   In H-PCE deployments the administrative entity responsible for the 
   management of the parent PCEs for multi-areas would typically be a 
   single service provider. In the multiple ASes (managed by different 
   service providers), it may be necessary for a third party to manage 
   the parent PCE.

12.2  Information and Data Models

   A PCEP MIB module is defined in [RFC7420] that describes managed
   objects for modeling of PCEP communication including:

   o  PCEP client configuration and status,

   o  PCEP peer configuration and information,

   o  PCEP session configuration and information,

   o  Notifications to indicate PCEP session changes.
   
   A YANG module for PCEP has also been proposed [PCEP-YANG].
   
   An H-PCE MIB module, or YANG data model, will be required to
   report parent PCE and child PCE information, including:


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   o  parent PCE configuration and status,

   o  child PCE configuration and information,

   o  notifications to indicate session changes between parent PCEs and
      child PCEs, and

   o  notification of parent PCE TED updates and changes. 

12.3 Liveness Detection and Monitoring

   PCEP includes a keepalive mechanism to check the liveliness of a PCEP
   peer and a notification procedure allowing a PCE to advertise its
   overloaded state to a PCC. In a multi-domain environment [RFC5886] 
   provides the procedures necessary to monitor the liveliness and 
   performances of a given PCE chain.

12.4 Verifying Correct Operation

   It is important to verify the correct operation of PCEP, [RFC5440] 
   specifies the monitoring of key parameters. These parameters are 
   detailed in [RFC5520]. 

12.5 Impact on Network Operation

   [RFC5440] states that in order to avoid any unacceptable impact on 
   network operations, a PCEP implementation should allow a limit to be 
   placed on the number of sessions that can be set up on a PCEP 
   speaker, it may also be practical to place a limit on the rate 
   of messages sent by a PCC and received my the PCE. 


13. Security Considerations

   PCEP Security considerations are discussed in [RFC5440] and [RFC6952] 
   Potential vulnerabilities include spoofing, snooping, falsification 
   and using PCEP as a mechanism for denial of service attacks. 
    
   As PCEP operates over TCP, it may make use of TCP security 
   encryption mechanisms, such as Transport Layer Security (TLS) and TCP 
   Authentication Option (TCP-AO). Usage of these security mechanisms 
   for PCEP is described in [RFC8253], and recommendations and best 
   current practices in [RFC7525].

13.1 Multi-domain Security 

   Any multi-domain operation necessarily involves the exchange of 
   information across domain boundaries.  This does represent 


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   significant security and confidentiality risk. 
   
   It is expected that PCEP is used between PCCs and PCEs belonging to 
   the same administrative authority, and using one of the 
   aforementioned encryption mechanisms. Furthermore,  PCEP allows 
   individual PCEs to maintain confidentiality of their domain path 
   information using path-keys.


14. IANA Considerations 

   This document makes no requests for IANA action.


15. Acknowledgements

   The author would like to thank Adrian Farrel for his review, and 
   Meral Shirazipour and Francisco Javier Jimenex Chico for their 
   comments.


16. References 

16.1. Normative References 

     [RFC3209] Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V.,
               and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP
               Tunnels", RFC 3209, December 2001.

     [RFC3473] Berger, L., Ed., "Generalized Multi-Protocol Label
               Switching (GMPLS) Signaling Resource ReserVation
               Protocol-Traffic Engineering (RSVP-TE) Extensions", RFC
               3473, January 2003.

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

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

     [RFC4726] Farrel, A., Vasseur, J., and A. Ayyangar, "A Framework
               for Inter-Domain Multiprotocol Label Switching Traffic
               Engineering", RFC 4726, November 2006.
               
     [RFC5152] Vasseur, JP., Ayyangar, A., and R. Zhang, "A Per-Domain
               Path Computation Method for Establishing Inter-Domain
               Traffic Engineering (TE) Label Switched Paths (LSPs)",


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               RFC 5152, February 2008.

     [RFC5440] Ayyangar, A., Farrel, A., Oki, E., Atlas, A., Dolganow,
               A., Ikejiri, Y., Kumaki, K., Vasseur, J., and J. Roux,
               "Path Computation Element (PCE) Communication Protocol
               (PCEP)", RFC 5440, March 2009.

     [RFC5441] Vasseur, J.P., Ed., "A Backward Recursive PCE-based
               Computation (BRPC) procedure to compute shortest inter-
               domain Traffic Engineering Label Switched Paths", 
               RFC5441, April 2009.

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

     [RFC5541] Le Roux, J., Vasseur, J., Lee, Y., "Encoding
               of Objective Functions in the Path Computation Element
               Communication Protocol (PCEP)", RFC5541, December 2008.

     [RFC6805] King, D. and A. Farrel, "The Application of the Path
               Computation Element Architecture to the Determination
               of a Sequence of Domains in MPLS & GMPLS", RFC6805, July 
               2010.

16.2. Informative References 

     [RFC3060] Moore, B., Ellesson, E., Strassner, J., and A.
               Westerinen, "Policy Core Information Model -- Version 1
               Specification", RFC 3060, February 2001.

     [RFC3460] Moore, B., Ed., "Policy Core Information Model (PCIM)
               Extensions", RFC 3460, January 2003.

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

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

     [RFC4203] Kompella, K., Ed., and Y. Rekhter, Ed., "OSPF
               Extensions in Support of Generalized Multi-
               Protocol Label Switching (GMPLS)", RFC
               4203, October 2005.



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     [RFC4920] Farrel, A., Ed., Satyanarayana, A., Iwata, A., Fujita,
                N., and G. Ash, "Crankback Signaling Extensions for MPLS
                and GMPLS RSVP-TE", RFC 4920, July 2007.

     [RFC5088] Le Roux, JL., Vasseur, JP., Ikejiri, Y., and R. Zhang,
               "OSPF Protocol Extensions for Path Computation Element
               (PCE) Discovery", RFC 5088, January 2008.

     [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.

     [RFC5305] Li, T. and H. Smit, "IS-IS Extensions for Traffic
               Engineering", RFC 5305, October 2008.
                  
     [RFC5307] Kompella, K., Ed., and Y. Rekhter, Ed., "IS-IS
               Extensions in Support of Generalized Multi-Protocol
               Label Switching (GMPLS)", RFC 5307,
               October 2008.   

     [RFC5316] Chen, M., Zhang, R., and X. Duan, "ISIS Extensions in
               Support of Inter-Autonomous System (AS) MPLS and GMPLS
               Traffic Engineering", December 2008. 

     [RFC5392] Chen, M., Zhang, R., and X. Duan, "OSPF Extensions in
               Support of Inter-Autonomous System (AS) MPLS and GMPLS
               Traffic Engineering", RFC 5392, January 2009.

     [RFC5394] Bryskin, I., Papadimitriou, D., Berger, L., and J. Ash,
               "Policy-Enabled Path Computation Framework", RFC 5394,
               December 2008.

     [RFC5521] Oki, E., Takeda, T., and A. Farrel, "Extensions to the 
               Path Computation Element Communication Protocol (PCEP) 
               for Route Exclusions", RFC 5521, April 2009.

     [RFC5886] Vasseur, JP., Le Roux, JL., and Y. Ikejiri, "A Set of 
               Monitoring Tools for Path ComputationElement (PCE)-Based 
               Architecture", RFC 5886, June 2010.

     [RFC6007] Nishioka, I., King, D., "Use of the Synchronization 
               VECtor (SVEC) List for Synchronized Dependent Path
               Computations", RFC6007, September 2010.

     [G-8080]  ITU-T Recommendation G.8080/Y.1304, Architecture for
               the automatically switched optical network (ASON).

     [G-7715]  ITU-T Recommendation G.7715 (2002), Architecture


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               and Requirements for the Automatically Switched
               Optical Network (ASON).

     [G-7715-2] ITU-T Recommendation G.7715.2 (2007), ASON routing
               architecture and requirements for remote route query.
               
     [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.

     [RFC7334] Zhao, Q., Dhody, D., Ali Z.,  King, D., 
               Casellas, R., "PCE-based Computation                
               Procedure To Compute Shortest Constrained 
               P2MP Inter-domain Traffic Engineering Label Switched 
               Paths", August 2014.
               
     [RFC7420] Stephan, E., Koushik, K., Zhao, Q., King, D., "PCE
               Communication Protocol (PCEP) Management Information
               Base", December 2014.

     [RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
               "Recommendations for Secure Use of Transport Layer
               Security (TLS) and Datagram Transport Layer Security
               (DTLS)", BCP 195, RFC 7525, May 2015.

     [RFC7752] Gredler, H., Medved, J., Previdi, S., Farrel, A., and
               S. Ray, "North-Bound Distribution of Link-State and TE
               Information using BGP", March 2016.
               
     [RFC7897] Dhody, D., Palle, U., and R. Casellas, "Domain Subobjects
               for the Path Computation Element Communication Protocol
               (PCEP)", June 2016. 
               
     [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, October 2017.
               
     [RFC8453] Ceccarelli, D., Lee, Y. et al., "Framework for 
               Abstraction and Control of TE Networks (ACTN)", RFC8453, 
               August 2018.

   [PCEP-YANG] Dhody, D., Hardwick, J., Beeram, V., and J. Tantsura, "A
               YANG Data Model for Path Computation Element             
               Communications Protocol (PCEP)", work in progress, 
               October 2018.



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17. Contributors

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

   Email: dhruv.ietf@gmail.com   

   Quintin Zhao
   Huawei Technology
   125 Nagog Technology Park
   Acton, MA  01719
   US
   
   Email: qzhao@huawei.com

   Julien Meuric
   France Telecom
   2, avenue Pierre-Marzin
   22307 Lannion Cedex
   
   Email: julien.meuric@orange-ftgroup.com

   Olivier Dugeon
   France Telecom
   2, avenue Pierre-Marzin
   22307 Lannion Cedex
   
   Email: olivier.dugeon@orange-ftgroup.com
   
   Jon Hardwick
   Metaswitch Networks
   100 Church Street
   Enfield, Middlesex
   United Kingdom

   Email: jonathan.hardwick@metaswitch.com

   Oscar Gonzalez de Dios  
   Telefonica I+D
   Emilio Vargas 6, Madrid
   Spain
   
   Email: ogondio@tid.es
   



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18. Author's Addresses 

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

   Haomian Zheng
   Huawei Technologies
   F3 R&D Center, Huawei Industrial Base, Bantian, Longgang District
   Shenzhen, Guangdong  518129
   P.R.China

   Email: zhenghaomian@huawei.com



































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