Network Working Group                                      D. King (Ed.)
Internet-Draft                                        Old Dog Consulting
Intended Status: Informational                           A. Farrel (Ed.)
Expires: April 4, 2012                               Old Dog Consulting
                                                         October 4, 2011

   The Application of the Path Computation Element Architecture to the
        Determination of a Sequence of Domains in MPLS and GMPLS



   Computing optimum routes for Label Switched Paths (LSPs) across
   multiple domains in MPLS Traffic Engineering (MPLS-TE) and GMPLS
   networks presents a problem because no single point of path
   computation is aware of all of the links and resources in each
   domain. A solution may be achieved using the Path Computation
   Element (PCE) architecture.

   Where the sequence of domains is known a priori, various techniques
   can be employed to derive an optimum path. If the domains are
   simply-connected, or if the preferred points of interconnection are
   also known, the Per-Domain Path Computation technique can be used.
   Where there are multiple connections between domains and there is
   no preference for the choice of points of interconnection, the
   Backward Recursive Path Computation Procedure (BRPC) can be used to
   derive an optimal path.

   This document examines techniques to establish the optimum path when
   the sequence of domains is not known in advance. The document
   shows how the PCE architecture can be extended to allow the optimum
   sequence of domains to be selected, and the optimum end-to-end path
   to be derived through the use of a hierarchical relationship between

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   described in the Simplified BSD License.


   1. Introduction..................................................3
      1.1 Problem Statement.........................................4
      1.2 Definition of a Domain............. ......................5
      1.3 Assumptions and Requirements..............................5
          1.3.1 Metric Objectives...................................6
          1.3.2 Domain Diversity....................................6
          1.3.3 Existing Traffic Engineering Constraints............7
          1.3.4 Commercial Constraints..............................7
          1.3.5 Domain Confidentiality..............................7
          1.3.6 Limiting Information Aggregation....................7
          1.3.7 Domain Interconnection Discovery....................7
      1.4 Terminology...............................................7
   2. Per Domain Path Computation...................................8
   3. Backward Recursive Path Computation...........................9
      3.1. Applicability of BRPC when the Domain Path is not Known..10
   4. Hierarchical PCE..............................................10
   5. Hierarchical PCE Procedures...................................11
      5.1 Objective Functions and Policy............................11
      5.2 Maintaining Domain Confidentiality........................12
      5.3 PCE Discovery.............................................12
      5.4 Parent Domain Traffic Engineering Database................13
      5.5 Determination of Destination Domain ......................14

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      5.6 Hierarchical PCE Examples.................................14
          5.6.1 Hierarchical PCE Initial Information Exchange.......17
          5.6.2 Hierarchical PCE End-to-End Path Computation
          Procedure Example.........................................17
      5.7 Hierarchical PCE Error Handling...........................17
      5.8 Hierarchical PCEP Protocol Extensions.....................18
          5.8.1 PCEP Request Qualifiers.............................18
          5.8.2 Indication of H-PCE Capability......................18
          5.8.3 Intention to Utilize Parent PCE Capabilities........19
          5.8.4 Communication of Domain Connectivity Information....19
          5.8.5 Domain Identifiers..................................19
   6. Hierarchical PCE Applicability................................20
      6.1 Antonymous Systems and Areas..............................20
      6.2 ASON architecture (G-7715-2)..............................20
          6.2.1 Implicit Consistency Between Hierarchical PCE and
          6.2.2 Benefits of Hierarchical PCEs in ASON...............23
   7. Management Considerations ....................................23
      7.1 Control of Function and Policy............................23
          7.1.1 Child PCE...........................................23
          7.1.2 Parent PCE..........................................23
          7.1.3 Policy Control......................................24
      7.2 Information and Data Models...............................24
      7.3 Liveness Detection and Monitoring.........................24
      7.4 Verifying Correct Operation...............................24
      7.5. Impact on Network Operation..............................25
   8. Security Considerations ......................................25
   9. IANA Considerations ..........................................25
   10. Acknowledgements ............................................25
   11. References ..................................................26
      11.1. Normative References....................................26
      11.2. Informative References .................................26
   12. Authors' Addresses ..........................................27

1. Introduction

   The capability to compute the routes of end-to-end inter-domain MPLS
   Traffic Engineering (TE) and GMPLS Label Switched Paths (LSPs) may be
   provided by a Path Computation Element (PCE). The PCE architecture is
   defined in [RFC4655]. The methods for establishing and controlling
   inter-domain MPLS-TE and GMPLS LSPs are documented in [RFC4726].

   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 Antonymous System
   (AS) or an Interior Gateway Protocol (IGP) area [RFC4726] and
   [RFC4655]. Domains are connected through ingress and egress
   boundary nodes (BNs). A more detailed definition is given in
   Section 1.2.

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   In a multi-domain environment, the determination of an end-to-end
   traffic engineered path is a problem because no single point of path
   computation is aware of all of the links and resources in each
   domain. PCEs can be used to compute end-to-end paths using a per-
   domain path computation technique [RFC5152]. Alternatively, the
   backward recursive path computation (BRPC) mechanism [RFC5441]
   allows multiple PCEs to collaborate in order to select an optimal
   end-to-end path that crosses multiple domains. Both mechanisms
   assume that the sequence of domains to be crossed between ingress
   and egress in known in advance.

   This document examines techniques to establish the optimum path when
   the sequence of domains is not known in advance. It shows how the PCE
   architecture can be extended to allow the optimum sequence of domains
   to be selected, and the optimum end-to-end path to be derived.

   The model described in this document introduces a hierarchical
   relationship between domains. It is applicable to environments with
   small groups of domains where visibility from the ingress Label
   Switching Router (LSR) is limited. Applying the hierarchical PCE
   model to large groups of domains such as the Internet, is not
   considered feasible or desirable, and is out of scope for this

1.1 Problem Statement

   Using a PCE to compute a path between nodes within a single domain is
   relatively straightforward. Computing an end-to-end path when the
   source and destination nodes are located in different domains
   requires co-operation between multiple PCEs, each responsible for
   its own domain.

   Techniques for inter-domain path computation described so far
   ([RFC5152] and [RFC5441]) assume that the sequence of domains to be
   crossed from source to destination is well known. No explanation is
   given (for example, in [RFC4655]) of how this sequence is generated
   or what criteria may be used for the selection of paths between
   domains. In small clusters of domains, such as simple cooperation
   between adjacent ISPs, this selection process is not complex. In more
   advanced deployments (such as optical networks constructed from
   multiple sub-domains, or multi-AS environments) the choice of domains
   in the end-to-end domain sequence can be critical to the
   determination of an optimum end-to-end path.

   This document introduces the concept of a hierarchical PCE
   architecture and shows how to coordinate PCEs in peer domains in
   order to derive an optimal end-to-end path.

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   The work is currently scoped to operate with a small group of domains
   and there is no intent to apply this model to a large group of
   domains, e.g., to the Internet.

1.2 Definition of a Domain

   A domain is defined in [RFC4726] as any collection of network
   elements within a common sphere of address management or path
   computational responsibility. Examples of such domains include
   IGP areas and Autonomous Systems. 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].

   See Section 6.2 for a further discussion of the applicability to the
   ASON architecture.

   This document assumes that the selection of a sequence of domains for
   an end-to-end path is in some sense a hierarchical path computation
   problem. That is, where one mechanism is used to determine a path
   across a domain, a separate mechanism (or at least a separate set
   of paradigms) is used to determine the sequence of domains.

1.3 Assumptions and Requirements

   Networks are often constructed from multiple domains. These
   domains are often interconnected via multiple interconnect points.
   Its assumed that the sequence of domains for an end-to-end path is
   not always well known; that is, an application requesting end-to-end
   connectivity has no preference for, or no ability to specify, the
   sequence of domains to be crossed by the path.

   The traffic engineering properties of a domain cannot be seen from
   outside the domain. Traffic engineering aggregation or abstraction,
   hides information and can lead to failed path setup 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 primary goal of this document is to define how to derive optimal
   end-to-end, multi-domain paths when the sequence of domains is not
   known in advance. The solution needs to be scalable and to maintain

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   internal domain topology confidentiality while providing the optimal
   end-to-end path. It cannot rely on the exchange of TE information
   between domains, and it cannot utilise a computation element that has
   universal knowledge of TE properties and topology of all domains.

   The sub-sections that follow set out the primary objectives and
   requirements to be satisfied by a PCE solution to multi-domain path

1.3.1 Metric Objectives

   The definition of optimality is dependent on policy, and is based on
   a single objective or a group objectives. An objective is expressed
   as an objective function [RFC5541] and may be specified on a path
   computation request. The following objective functions are identified
   in this document. They define how the path metrics and TE link
   qualities are manipulated during inter-domain path computation. The
   list is not proscriptive and may be expanded in other documents.

   o Minimize the cost of the path [RFC5541]
   o Select a path using links with the minimal load [RFC5541]
   o Select a path that leaves the maximum residual bandwidth [RFC5541]
   o Minimize aggregate bandwidth consumption [RFC5541]
   o Minimize the Load of the most loaded Link [RFC5541]
   o Minimize the Cumulative Cost of a set of paths [RFC5541]
   o Minimize the number of boundary nodes used
   o Limit the number of domains crossed
   o Disallow domain re-entry

   See Section 5.1 for further discussion of objective functions.

1.3.2 Domain Diversity

   A pair of paths are domain-diverse if they do not transit any of the
   same domains. A pair of paths that share a common ingress and egress
   are domain-diverse if they only share the same domains at the ingress
   and egress (the ingress and egress domains). Domain diversity may be
   maximized for a pair of paths by selecting paths that have the
   smallest number of shared domains. (Note that this is not the same
   as finding paths with the greatest number of distinct domains!)

   Path computation should facilitate the selection of paths that share
   ingress and egress domains, but do not share any transit domains.
   This provides a way to reduce the risk of shared failure along any
   path, and automatically helps to ensure path diversity for most of
   the route of a pair of LSPs.

   Thus, domain path selection should provide the capability to include
   or exclude specific domains and specific boundary nodes.

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1.3.3 Existing Traffic Engineering Constraints

   Any solution should take advantage of typical traffic engineering
   constraints (hop count, bandwidth, lambda continuity, path cost,
   etc.) to meet the service demands expressed in the path computation
   request [RFC4655].

1.3.4 Commercial Constraints

   The solution should provide the capability to include commercially
   relevant constraints such as policy, SLAs, security, peering
   preferences, and dollar costs.

   Additionally it may be necessary for the service provider to
   request that specific domains are included or excluded based on
   commercial relationships, security implications, and reliability.

1.3.5 Domain Confidentiality

   A key requirement is the ability to maintain domain confidentiality
   when computing inter-domain end-to-end paths. When required by local
   policy, a PCE should not need to disclose to any other PCE the intra-
   domain paths it computes or the internal topology of the domain it

1.3.6 Limiting Information Aggregation

   It is important to minimise the amount of aggregation within the
   solution. There should be no associated computation burden or
   requirement to aggregate and abstract traffic engineering link

1.3.7 Domain Interconnection Discovery

   To support domain mesh topologies, the solution should allow the
   discovery and selection of domain inter-connections. Pre-
   configuration of preferred domain interconnections should also be
   supported for network operators that have bilateral agreement, and
   preference for the choice of points of interconnection.

1.4 Terminology

   This document uses PCE terminology defined in [RFC4655], [RFC4875],
   and [RFC5440]. Additional terms are defined below.

   Domain Path: The sequence of domains for a path.

   Ingress Domain: The domain that includes the ingress LSR of a path.

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   Transit Domain: A domain that has an upstream and downstream
   neighbor domain for a specific path.

   Egress Domain: The domain that includes the egress LSR of a path.

   Boundary Nodes: Each Domain has entry LSRs and exit LSRs that could
   be Area Border Routers (ABRs) or Autonomous System Border Routers
   (ASBRs) depending on the type of domain. They are defined here more
   generically as Boundary Nodes (BNs).

   Entry BN of domain(n): a BN connecting domain(n-1) to domain(n)
   on a path.

   Exit BN of domain(n): a BN connecting domain(n) to domain(n+1)
   on a path.

   Parent Domain: A domain higher up in a domain hierarchy such
   that it contains other domains (child domains) and potentially other
   links and nodes.

   Child Domain: A domain lower in a domain hierarchy such that it has
   a parent domain.

   Parent PCE: A PCE responsible for selecting a path across a parent
   domain and any number of child domains by coordinating with child
   PCEs and examining a topology map that shows domain inter-

   Child PCE: A PCE responsible for computing the path across one or
   more specific (child) domains. A child PCE maintains a relationship
   with at least one parent PCE.

   OF: Objective Function: A set of one or more optimization
   criteria used for the computation of a single path (e.g., path cost
   minimization), or the synchronized computation of a set of paths
   (e.g., aggregate bandwidth consumption minimization). See [RFC4655]
   and [RFC5541].

2. Per-Domain Path Computation

   The per-domain path computation method for establishing inter-domain
   TE-LSPs [RFC5152] defines a technique whereby the path is computed
   during the signalling process on a per-domain basis. The entry BN of
   each domain is responsible for performing the path computation for
   the section of the LSP that crosses the domain, or for requesting
   that a PCE for that domain computes that piece of the path.

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   During per-domain path computation, each computation results in the
   best path across the domain to provide connectivity to the next
   domain in the domain sequence (usually indicated in signalling by an
   identifier of the next domain or the identity of the next entry BN).

   Per-domain path computation may lead to sub-optimal end-to-end paths
   because the most optimal path in one domain may lead to the choice of
   an entry BN for the next domain that results in a very poor path
   across that next domain.

   In the case that the domain path (in particular, the sequence of
   boundary nodes) is not known, the PCE must select an exit BN based on
   some determination of how to reach the destination that is outside
   the domain for which the PCE has computational responsibility.
   [RFC5152] suggest that this might be achieved using the IP shortest
   path as advertise by BGP. Note, however, that the existence of an IP
   forwarding path does guarantee the presence of sufficient bandwidth,
   let alone an optimal TE path. Furthermore, in many GMPLS systems
   inter-domain IP routing will not be present. Thus, per-domain path
   computation may require a significant number of crankback routing
   attempts to establish even a sub-optimal path.

   Note also that the PCEs in each domain may have different computation
   capabilities, may run different path computation algorithms, and may
   apply different sets of constraints and optimization criteria, etc.
   This can result in the end-to-end path being inconsistent and sub-

   Per-domain path computation can suit simply-connected domains where
   the preferred points of interconnection are known.

3. Backward Recursive Path Computation

   The Backward Recursive Path Computation (BRPC) [RFC5441] procedure
   involves cooperation and communication between PCEs in order to
   compute an optimal end-to-end path across multiple domains. The
   sequence of domains to be traversed can either be determined before
   or during the path computation. In the case where the sequence of
   domains is known, the ingress Path Computation Client (PCC) sends a
   path computation request to the PCE responsible for the ingress
   domain. This request is forwarded between PCEs, domain-by-domain, to
   the PCE responsible for the egress domain. The PCE in the egress
   domain creates a set of optimal paths from all of the domain entry
   BNs to the egress LSR. This set is represented as a tree of potential
   paths called a Virtual Shortest Path Tree (VSPT), and the PCE passes
   it back to the previous PCE on the domain path. As the VSPT is passed
   back toward the ingress domain, each PCE computes the optimal paths
   from its entry BNs to its exit BNs that connect to the rest of the

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   tree. It adds these paths to the VSPT and passes the VSPT on until
   the PCE for the ingress domain is reached and computes paths from the
   ingress LSR to connect to the rest of the tree. The ingress PCE then
   selects the optimal end-to-end path from the tree, and returns the
   path to the initiating PCC.

   BRPC may suit environments where multiple connections exist between
   domains and there is no preference for the choice of points of
   interconnection. It is best suited to scenarios where the domain
   path is known in advance, but can also be used when the domain path
   is not known.

3.1. Applicability of BRPC when the Domain Path is Not Known

   As described above BRPC can be used to determine an optimal inter-
   domain path when the sequence is known. Even when the sequence of
   domains is not known BRPC could be used as follows.

   o The PCC sends a request to the PCE for the ingress domain (the
     ingress PCE).

   o The ingress PCE sends the path computation request direct to the
     PCE responsible for the domain containing the destination node (the
     egress PCE).

   o The egress PCE computes an egress VSPT and passes it to a PCE
     responsible for each of the adjacent (potentially upstream)

   o Each PCE in turn constructs a VSPT and passes it on to all of its
     neighboring PCEs.

   o When the ingress PCE has received a VSPT from each of its
     neighboring domains it is able to select the optimum path.

   Clearly this mechanism (which could be called path computation
   flooding) has significant scaling issues. It could be improved by
   the application of policy and filtering, but such mechanisms are not
   simple and would still leave scaling concerns.

4. Hierarchical PCE

   In the hierarchical PCE architecture, a parent PCE maintains a domain
   topology map that contains the child domains (seen as vertices in the
   topology) and their interconnections (links in the topology). The
   parent PCE has no information about the content of the child domains;
   that is, the parent PCE does not know about the resource availability
   within the child domains, nor about the availability of connectivity
   across each domain. The parent PCE is aware of the TE capabilities of
   the interconnections between child domains as these interconnections
   are links in its own topology map.

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   Note that in the case that the domains are IGP areas, there is no
   link between the domains (the ABRs have a presence in both
   neighboring areas). The parent domain may choose to represent this in
   its TED as a virtual link that is unconstrained and has zero cost,
   but this is entirely an implementation issue.

   Each child domain has at least one PCE capable of computing paths
   across the domain. These PCEs are known as child PCEs and have a
   relationship with the parent PCE. Each child PCE also knows the
   identity of the domains that neighbor its own domain. A child PCE
   only knows the topology of the domain that it serves and does not
   know the topology of other child domains. Child PCEs are also not
   aware of the general domain mesh connectivity (i.e., the domain
   topology map) beyond the connectivity to the immediate neighbor
   domains of the domain it serves.

   The parent PCE builds the domain topology map either from
   configuration or from information received from each child PCE. This
   tells it how the domains are interconnected including the TE
   properties of the domain interconnections. But the parent PCE does
   not know the contents of the child domains. Discovery of the domain
   topology and domain interconnections is discussed further in Section

   When a multi-domain path is needed, the ingress PCE sends a request
   to the parent PCE (using the path computation element protocol, PCEP
   [RFC5440]). The parent PCE selects a set of candidate domain paths
   based on the domain topology and the state of the inter-domain links.
   It then sends computation requests to the child PCEs responsible for
   each of the domains on the candidate domain paths.

   Each child PCE computes a set of candidate path segments across its
   domain and sends the results to the parent PCE. The parent PCE uses
   this information to select path segments and concatenate them to
   derive the optimal end-to-end inter-domain path. The end-to-end path
   is then sent to the child PCE which received the initial path request
   and this passes the path on to the PCC that issues the original

5. Hierarchical PCE Procedures

5.1 Objective Functions and Policy

   Deriving the optimal end-to-end domain path sequence is dependent on
   the policy applied during domain path computation. An Objective
   Function (OF) [RFC5541], or set of OFs, may be applied to define the
   policy being applied to the domain path computation.

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   The OF specifies the desired outcome of the computation. It does
   not describe the algorithm to use. When computing end-to-end inter-
   domain paths, required OFs may include (see Section 1.3.1):

   o Minimum cost path
   o Minimum load path
   o Maximum residual bandwidth path
   o Minimize aggregate bandwidth consumption
   o Minimize the number of boundary nodes used
   o Minimize the number of transit domains
   o Disallow domain re-entry

   The objective function may be requested by the PCC, the ingress
   domain PCE (according to local policy), or maybe applied by the
   parent PCE according to inter-domain policy.

5.2 Maintaining Domain Confidentiality

   Information about the content of child domains is not shared for
   scaling and confidentiality reasons. This means that a parent PCE is
   aware of the domain topology and the nature of the connections
   between domains, but is not aware of the content of the domains.
   Similarly, a child PCE cannot know the internal topology of another
   child domain. Child PCEs also do not know the general domain mesh
   connectivity, this information is only known by the parent PCE.

   As described in the earlier sections of this document, PCEs can
   exchange path information in order to construct an end-to-end inter-
   domain path. Each per-domain path fragment reveals information about
   the topology and resource availability within a domain. Some
   management domains or ASes will not want to share this information
   outside of the domain (even with a trusted parent PCE). In order to
   conceal the information, a PCE may replace a path segment with a
   path-key [RFC5520]. This mechanism effectively hides the content of a
   segment of a path.

5.3 PCE Discovery

   It is a simple matter for each child PCE to be configured with the
   address of its parent PCE. Typically, there will only be one or two
   parents of any child.

   The parent PCE also needs to be aware of the child PCEs for all child
   domains that it can see. This information is most likely to be
   configured (as part of the administrative definition of each

   Discovery of the relationships between parent PCEs and child PCEs
   does not form part of the H-PCE architecture. Mechanisms that rely on
   advertising or querying PCE locations across domain or provider
   boundaries are undesirable for security, scaling, commercial, and
   confidentiality reasons.

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   The parent PCE also needs to know the inter-domain connectivity.
   This information could be configured with suitable policy and
   commercial rules, or could be learned from the child PCEs as
   described in Section 4.

   In order for the parent PCE to learn about domain interconnection
   the child PCE will report the identity of its neighbor domains. The
   IGP in each neighbor domain can advertise its inter-domain TE
   link capabilities [RFC5316], [RFC5392]. This information can be
   collected by the child PCEs and forwarded to the parent PCE, or the
   parent PCE could participate in the IGP in the child domains.

5.4 Parent Domain Traffic Engineering Database

   The parent PCE maintains a domain topology map of the child domains
   and their interconnectivity. Where inter-domain connectivity is
   provided by TE links the capabilities of those links must also be
   known to the parent PCE. Furthermore the parent domain
   may contain nodes and links in its own right. Therefore, the
   parent PCE maintains a traffic engineering database (TED) for
   the parent domain in the same way that any PCE does.

   The parent domain may just be the collection of child domains and the
   inter-domain links, or it may contain nodes and links in its own

   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.

   In practice, certain information may be passed from the child domains
   to the parent PCE to help build the parent TED. In theory, the parent
   PCE could listen to the routing protocols in the child domains, but
   this would violate the confidentiality and scaling issues that may be
   responsible for the partition of the network into domains. So it is
   much more likely that a suitable solution will involve specific
   communication from an entity in the child domain (such as the child
   PCE) to convey the necessary information. As already mentioned, the
   "necessary information" relates to how the child domains are inter-
   connected. The topology and available resources within the child
   domain do not need to be communicated to the parent PCE: doing so
   would violate the PCE architecture. Mechanisms for reporting this
   information are described in the examples in Section 5.6 in abstract
   terms as "a child PCE reports its neighbor domain connectivity to its
   parent PCE"; the specifics of a solution are out of scope of this
   document, but the requirements are indicated in Section 5.8.

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   In models such as ASON (see Section 6.2), it is possible to consider
   a separate instance of an IGP running within the parent domain where
   the participating protocol speakers are the nodes directly present in
   that domain and the PCEs (routing controllers) responsible for each
   of the child domains.

5.5 Determination of Destination Domain

   The PCC asking for an inter-domain path computation is aware of the
   identity of the destination node by definition. If it knows the
   egress domain it can supply this information as part of the path
   computation request. However, if it does not know the egress domain
   this information must be determined by the parent PCE.

   In some specialist topologies the parent PCE could determine the
   destination domain based on the destination address, for example from
   configuration. However, this is not appropriate for many multi-domain
   addressing scenarios. In IP-based multi-domain networks the
   parent PCE may be able to determine the destination domain by
   participating in inter-domain routing. Finally, the parent PCE could
   issue specific requests to the child PCEs to discover if they contain
   the destination node, but this has scaling implications.

5.6 Hierarchical PCE Examples

   The following example describes the hierarchical domain topology.
   Figure 1 (sample hierarchical domain topology) demonstrates four
   interconnected domains within a fifth parent domain. Each domain
   contains a single PCE:

   o Domain 1 is the ingress domain and child PCE 1 is able to compute
     paths within the domain. Its neighbors are Domain 2 and Domain 4.
     The domain also contains the source LSR (S) and three egress
     boundary nodes (BN11, BN12, and BN13).

   o Domain 2 is served by child PCE 2. Its neighbors are Domain 1 and
     Domain 3. The domain also contains four boundary nodes (BN21, BN22,
     BN23, and BN24).

   o Domain 3 is the egress domain and is served by child PCE 3. Its
     neighbors are Domain 2 and Domain 4. The domain also contains the
     destination LSR (D) and three ingress boundary nodes (BN31, BN32,
     and BN33).

   o Domain 4 is served by child PCE 4. Its neighbors are Domain 2 and
     Domain 3. The domain also contains two boundary nodes (BN41 and

   All of these domains are encompassed within Domain 5 which is served
   by the parent PCE (PCE 5).

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

                 Figure 1 : Sample Hierarchical Domain Topology

   Figure 2, shows the view of the domain topology as seen by the parent
   PCE (PCE 5). This view is an abstracted topology; PCE 5 is aware of
   domain connectivity, but not of the internal topology within each

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                      | Domain 5                   |
                      |            ----            |
                      |           |PCE5|           |
                      |            ----            |
                      |                            |
                      |   ----     ----     ----   |
                      |  |    |---|    |---|    |  |
                      |  | D1 |   | D2 |   | D3 |  |
                      |  |    |---|    |---|    |  |
                      |   ----     ----     ----   |
                      |     \      ----      /     |
                      |      \    |    |    /      |
                      |       ----| D4 |----       |
                      |           |    |           |
                      |            ----            |
                      |                            |

      Figure 2 : Abstract Domain Topology as Seen by the Parent PCE

5.6.1 Hierarchical PCE Initial Information Exchange

   Based on the Figure 1 topology, the following is an illustration of
   the initial hierarchical PCE information exchange.

   1. Child PCE 1, the PCE responsible for Domain 1, is configured
      with the location of its parent PCE (PCE5).

   2. Child PCE 1 establishes contact with its parent PCE. The parent
      applies policy to ensure that communication with PCE 1 is allowed.

   3. Child PCE 1 listens to the IGP in its domain and learns its
      inter-domain connectivity. That is, it learns about the links
      BN11-BN21, BN12-BN22, and BN13-BN41.

   4. Child PCE 1 reports its neighbor domain connectivity to its parent

   5. Child PCE 1 reports any change in the resource availability on its
      inter-domain links to its parent PCE.

   Each child PCE performs steps 1 through 5 so that the parent PCE can
   create a domain topology view as shown in Figure 2.

5.6.2 Hierarchical PCE End-to-End Path Computation Procedure

   The procedure below is an example of a source PCC requesting an

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   end-to-end path in a multi-domain environment. The topology is
   represented in Figure 1. It is assumed that the each child PCE has
   connected to its parent PCE and exchanged the initial information
   required for the parent PCE to create its domain topology view as
   described in Section 5.6.1.

   1.  The source PCC (the ingress LSR in our example), sends a request
       to the PCE responsible for its domain (PCE1) for a path to the
       destination LSR.

   2.  PCE 1 determines the destination, is not in domain 1.

   3.  PCE 1 sends a computation request to its parent PCE (PCE 5).

   4.  The parent PCE determines that the destination is in Domain 3.
       (See Section 5.5).

   5.  PCE 5 determines the likely domain paths according to the domain
       interconnectivity and TE capabilities between the domains. For
       example, three domain paths (S-BN11-BN21-D2-BN23-BN31-D, S-BN11-
       BN21-D2-BN24-BN32-D, and S-BN13-BN41-D4-BN42-BN33-D) are
       determined (assuming the link BN12-BN22 is not suitable for the
       requested path).

   6.  PCE 5 sends edge-to-edge path computation requests to PCE 2
       which is responsible for Domain 2 (e.g., BN21-BN23 and BN21-BN24)
       and to PCE 4 for Domain 4 (e.g., BN41-BN42).

   7.  PCE 5 sends source-to-edge path computation requests to PCE 1
       which is responsible for Domain 1 (e.g., S-BN11 and S-BN13).

   8.  PCE 5 sends edge-to-egress path computation requests to PCE3
       which is responsible for Domain 3 (e.g., BN31-D, BN32-D, and

   9.  PCE 5 correlates all the computation responses from each child
       PCE, adds in the information about the inter-domain links, and
       applies any requested and locally configured policies.

   10. PCE 5 then selects the optimal end-to-end multi-domain path
       that meets the policies and objective functions, and supplies the
       resulting path to PCE 1.

   11. PCE 1 forwards the path to the PCC (the ingress LSR).

5.7 Hierarchical PCE Error Handling

   In the event that a child PCE in a domain cannot find a suitible
   path to the egress. The child PCE should return the relevent
   error notifying the parent PCE. Depending on the error response the
   parent PCE can elect to:

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   o Cancel the request and send the relevent response back to the
     intial child PCE requesting an end-to-end path.
   o Relax the contraints associated with the intial path request;
   o Select another candidate domain and send the path request to the
     child PCE responsible for the domain.

   If the parent PCE does not recieve a response from a child PCE within
   an alloted time period. The parent PCE can either:

   o Send the path request to another child PCE in the same domain, if a
     secoundary child PCE exists;
   o Select another candidate domain and send the path request to the
     child PCE responsible for that domain.

5.8 Requirements for Hierarchical PCEP Protocol Extensions

   This section lists the high-level requirements for extensions to the
   PCEP to support the hierachical PCE model.

   [Editors Note: This section may be expanded as work progresses.]

5.8.1 PCEP Request Qualifiers

   PCEP request (PCReq) messages are used by a PCC or a PCE to make a
   computation request or enquiry to a PCE. The requests are qualified
   so that the PCE knows what type of action is required.

   Support of the H-PCE architecture will introduce two new
   qualifications as follows:

   o It must be possible for a child PCE to indicate that the request it
     sends to a parent PCE shold be satisfied by a domain sequence only,
     that is, not by a full end-to-end path. This allows the child PCE
     to initiate per-domain or backward recursive path computation.

   o A parent PCE needs to be able to ask a child PCE whether a
     particular node address (the destination of an end-to-end path) is
     present in the domain that the child PCE serves.

   In PCEP, such request qualifications are carried as bit-flags in the
   RP object carried within the PCReq message.

5.8.2 Indication of H-PCE Capability

   Although parent/child PCE relationships are likely configured, it
   assist network operations if the parent PCE is able to indicate to
   the child that it really is capable of acting as a parent PCE. This
   will help to trap misconfigurations.

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   A parent PCE needs a way to indicate that is capable of acting as a
   parent PCE, and should also be able to indicate the identity of the
   parent domain. This informaiton is most obviously carried in the Open
   Object within the Open message.

5.8.3 Intention to Utilize Parent PCE Capabilities

   A PCE that is capable of acting as a parent PCE might not be
   configured or willing to act as the parent for a specific child PCE.
   This fact could be determined when the child sends a PCReq that
   requires parental activity (such as querying other child PCEs), and
   could result in a negative response in a PCEP Error (PCErr) message.

   However, the expense of a poorly targetted PCReq can be avoided if
   the child PCE indicates that it might wish to use the parent as a
   parent (for example, on the Open message), and if the parent
   determines at that time whether it is willing to act as a parent to
   this child.

5.8.4 Communication of Domain Connectivity Information

   Section 5.4 describes how the parent PCE needs a parent TED and
   indicates that the information might be supplied from the child PCEs
   in each domain. This requires a mechanism whereby information about
   inter-domain links can be supplied by a child PCE to a parent PCE,
   for example on a PCEP Notify (PCNtf) message.

   The information that would be exchanged includes:

   o Identifier of advertising child PCE
   o Identifier of PCE's domain
   o Identifier of the link
   o TE properties of the link (metrics, bandwidth)
   o Other properties of the link (technology-specific)
   o Identifier of link end-points
   o Identifier of adjacent domain

   It may be desirable for this information to be periodically updated,
   for example, when available bandwidth changes. In this case, the
   parent PCE might be given the ability to configure thresholds in the
   child PCE to prevent flapping of information.

5.8.5 Domain Identifiers

   Domain identifiers are already needed to allow a PCE to indicate
   which domains it serves, and to allow the representation of domains
   as abstract nodes in paths. The wider use of domains in the context
   of this work on H-PCE will require that domains can be identified in
   more places within objects in PCEP messages. This should pose no

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   However, more attention may need to be applied to the precision of
   domian identifier definitions.

6. Hierarchical PCE Applicability

   As per [RFC4655], PCE can inherently support inter-domain path
   computation for any definition of a domain as set out in Section 1.2.

   Hierarchical PCE can be applied to inter-domain environments,
   including Antonymous Systems and IGP areas. The hierarchical PCE
   procedures make no distinction between, Antonymous Systems and IGP
   area applications, although it should be noted that the TED
   maintained by a parent PCE must be able to support the concept of
   child domains connected by inter-domain links or directly connected
   at boundary nodes (see Section 4).

   This section sets out the applicability of hierarchical PCE to three

   o MPLS traffic engineering across multiple Autonomous Systems
   o MPLS traffic engineering across multiple IGP areas
   o GMPLS traffic engineering in the ASON architecture

6.1 Antonymous Systems and Areas

   Networks are comprised of domains. A domain can be considered to be
   a collection of network elements within an AS or area that has a
   common sphere of address management or path computational

   As networks increase in size and complexity it may be required to
   introduce scaling methods to reduce the amount 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 a
   route to destination located in another AS or area a method is
   required to compute a path across teh AS and area boundaries.

   When an LSR within an AS or area needs to compute a path across an
   area or AS boundary it must also use an inter-AS computation
   technique. Hierachical PCE is equally applicable to computing
   inter-area and inter-AS MPLS and GMPLS paths across domain

6.2 ASON Architecture

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   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.
   Note that the RA hierarchy can be recursive.

   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. For an end-to-end connection, the route may be computed
   by a single RC or multiple RCs in a collaborative manner (i.e., RC
   federations). In the case of RC federations, [G-7715-2] describes
   three styles during remote route query operation:

   o Step-by-step remote path computation
   o Hierarchical remote path computation
   o A combination of the above.

   In a hierarchical ASON routing environment, a child RC may
   communicate with its parent RC (at the next higher level of the ASON
   routing hierarchy) to request the computation of an end-to-end path
   across several RAs. It does this using a route query message (known
   as the abstract message RI_QUERY). The corresponding parent RC may
   communicate with other child RCs that belong to other child RAs at
   the next lower hierarchical level. Thus, a parent RC can act as
   either a Route Query Requester or Route Query Responder.

   It can be seen that the hierarchical PCE architecture fits the
   hierarchical ASON routing architecture well. It can be used to
   provide paths across subnetworks, and to determine end-to-end paths
   in networks constructed from multiple subnetworks or RAs.

   When hierarchical PCE is applied to implement hierarchical remote
   path computation in [G-7715-2], it is very important for operators to
   understand the different terminology and implicit consistency
   between hierarchical PCE and [G-7715-2].

6.2.1 Implicit Consistency Between Hierarchical PCE and G.7715.2

   This section highlights the correspondence between features of the
   hierarchical PCE architecture and the ASON routing architecture.

   (1) RC (Routing Controller) and PCE (Path Computation Element)

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       [G-8080] describes the Routing Controller Component as an
       abstract entity, which is responsible for responding to requests
       for path (route) information and topology information. It can be
       implemented as a single entity, or as a distributed set of
       entities that make up a cooperative federation.

       [RFC4655] describes PCE (Path Computation Element) is an entity
       (component, application, or network node) that is capable of
       computing a network path or route based on a network graph and
       applying computational constraints.

       Therefore, in the ASON architecture, a PCE can be regarded as a
       realizations of the RC.

   (2) Route Query Requester/Route Query Responder and PCC/PCE

       [G-7715-2] describes the Route Query Requester as a Connection
       Controller or Routing Controller that sends a route query message
       to a Routing Controller requesting for one or more paths that
       satisfy a set of routing constraints. The Route Query Responder
       is a Routing Controller that performs path computation upon
       receipt of a route query message from a Route Query Requester,
       sending a response back at the end of the path computation.

       In the context of ASON, a signaling controller initiates and
       processes signaling messages and closely coupled to a signaling
       protocol speaker. A routing controller makes routing decisions
       and is usually coupled to configuration entities and/or routing a
       protocol speaker.

       It can be seen that a PCC corresponds to a Route Query Requester,
       and a PCE corresponds to a Route Query Responder. A PCE/RC can
       also act as a Route Query Requester sending requests to another
       Route Query Responder.

       The PCEP path computation request (PCReq) and path computation
       reply (PCRep) messages between PCC and PCE correspond to the
       RI_QUERY and RI_UPDATE messages in [G-7715-2].

   (3) Routing Area Hierarchy and Hierarchical Domain

       The ASON routing hierarchy model is shown in Figure 6 of
       [G-7715] through an example that illustrates routing area levels.
       If the hierarchical remote path computation mechanism of
       [G-7715-2] is applied in this scenario, each routing area should
       have at least one RC for route query function and there is a
       parent RC for the child RCs in each routing area.

       According to [G-8080], the parent RC has visibility of the
       structure of the lower level, so it knows the interconnectivity
       of the RAs in the lower level. Each child RC can compute edge-to-
       edge paths across its own child RA.

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       Thus, an RA corresponds to a domain, and the hierarchical
       relationship between RAs corresponds to the hierarchical
       relationship between domains. Furthermore, a parent PCE in a
       parent domain can be regarded as parent RC in a higher routing
       level, and a child PCE in a child domain can be regarded as child
       RC in a lower routing level.

6.2.2 Benefits of Hierarchical PCEs in ASON

   RCs in an ASON environment can use the hierarchical PCE model to
   fully match the ASON hierarchical routing model, so the hierarchical
   PCE mechanisms can be applied to fully satisfy the architecture and
   requirements of [G-7715-2] without any changes. If the hierarchical
   PCE mechanism is applied in ASON, it can be used to determine end-to-
   end optimized paths across sub-networks and RAs before initiating
   signaling to create the connection. It can also improve the
   efficiency of connection setup to avoid crankback.

7. Management Considerations

   General PCE management considerations are discussed in [RFC4655]. In
   the case of the hierarchical PCE architecture, there are additional
   management considerations.

   The administrative entity responsible for the management of the
   parent PCEs must be determined. 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

7.1 Control of Function and Policy

7.1.1 Child PCE

   Support of the hierarchical procedure will be controlled by the
   management organization responsible for each child PCE. A child PCE
   must be configured with the address of its parent PCE in order for
   it to interact with its parent PCE. The child PCE must also be
   authorized to peer with the parent PCE.

7.1.2 Parent PCE

   The parent PCE must only accept path computation requests from

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   authorized child PCEs. If a parent PCE receives requests from an
   unauthorized child PCE, the request should be dropped.

   This means that a parent PCE must 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.

7.1.3 Policy Control

   It may be necessary to maintain a policy module on the parent PCE
   [RFC5394]. This would allow the parent PCE to apply commercially
   relevant constraints such as SLAs, security, peering preferences, and
   dollar costs.

   It may also be necessary for the parent PCE to limit end-to-end path
   selection by including or excluding specific domains based on
   commercial relationships, security implications, and reliability.

7.2 Information and Data Models

   A PCEP MIB module is defined in [PCEP-MIB] that describes managed
   objects for modeling of PCEP communication. An additional PCEP MIB
   will be required to report parent PCE and child PCE information,

   o Parent PCE configuration and status,

   o Child PCE configuration and information,

   o Notifications to indicate session changes between parent PCEs and
     child PCEs.

   o Notification of parent PCE TED updates and changes.

7.3 Liveness Detection and Monitoring

   The hierarchical procedure requires interaction with multiple PCEs.
   Once a child PCE requests an end-to-end path, a sequence of events
   occurs that requires interaction between the parent PCE and each
   child  PCE. If a child PCE is not operational, and an alternate
   transit domain is not available, then a failure must be reported.

7.4 Verifying Correct Operation

   Verifying the correct operation of a parent PCE can be performed by
   monitoring a set of parameters. The parent PCE implementation should
   provide the following parameters:

   Parameters monitored by the parent PCE:

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   o  Number of child PCE requests.

   o  Number of successful hierarchical PCE procedures completions on a
      per-PCE-peer basis.

   o  Number of hierarchical PCE procedure completion failures on a per-
      PCE-peer basis.

   o  Number of hierarchical PCE procedure requests from unauthorized
      child PCEs.

7.5. Impact on Network Operation

   The hierarchical PCE procedure is a multiple-PCE path computation
   scheme. Subsequent requests to and from the child and parent PCEs do
   not differ from other path computation requests and should not have
   any significant impact on network operations.

8. Security Considerations

   The hierarchical PCE procedure relies on PCEP and inherits the
   security requirements defined [RFC5440]. Any multi-domain
   operation necessarily involves the exchange of information across
   domain boundaries. This is bound to represent a significant
   security and confidentiality risk especially when the child
   domains are controlled by different commercial concerns.

   The hierarchical PCE architecture makes use of PCE policy
   [RFC5394] and the security aspects of the PCE communication protocol
   documented in [RFC5440]. It is expected that the parent PCE will
   require all child PCEs to use full security when communicating with
   the parent and that security will be maintained by not supporting the
   discovery by a parent of child PCEs.

   Confidentiality may be enhanced by the use of Path Keys [RFC5520].

   Further considerations of the security issues related to inter-AS
   path computation see [RFC5376].

9. IANA Considerations

   This document makes no requests for IANA action.

10. Acknowledgements

   The authors would like to thank David Amzallag, Oscar Gonzalez de
   Diosm and Franz Rambach for their comments and suggestions.

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11. References

11.1 Normative References

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

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

   [RFC5394]  Bryskin, I., Papadimitriou, D., Berger, L., and J. Ash,
              "Policy-Enabled Path Computation Framework", RFC 5394,
              December 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", RFC
              5441, April 2009.

   [RFC5520]  Brandford, R., Vasseur J.P., and Farrel A., "Preserving
              Topology Confidentiality in Inter-Domain Path
              Computation Using a Key-Based Mechanism
              RFC5520, April 2009.

   [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
              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

11.2. Informative References

   [RFC4726]  Farrel, A., Vasseur, J., and A. Ayyangar, "A Framework
              for Inter-Domain Multiprotocol Label Switching Traffic
              Engineering", RFC 4726, November 2006.

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   [RFC4875]  Aggarwal, R., Papadimitriou, D., and Yasukawa, S.,
              "Extensions to Resource Reservation Protocol - Traffic
              Engineering (RSVP-TE) for Point-to-Multipoint TE Label
              Switched Paths (LSPs)", RFC 4875, May 2007.

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

   [RFC5316]  Chen, M., Zhang, R., and X. Duan, "ISIS Extensions in
              Support of Inter-Autonomous System (AS) MPLS and GMPLS
              Traffic Engineering", RFC 5316, December 2008.

   [RFC5376]  Bitar, N., et al., "Inter-AS Requirements for the
              Path Computation Element Communication Protocol
              (PCECP)", RFC 5376, November 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.

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

   [PCEP-MIB] Stephan, E., K. Koushik, Q. Zhao, and D. King, "PCE
              communication protocol (PCEP) Management Information
              Base", Work in Progress, June 2010

12. Authors' Addresses

   Daniel King
   Old Dog Consulting

   Adrian Farrel
   Old Dog Consulting

   Quintin Zhao
   Huawei Technology
   125 Nagog Technology Park
   Acton, MA  01719

King & Farrel, et al.                                         [Page 27]

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   Fatai Zhang
   Huawei Technologies
   F3-5-B R&D Center, Huawei Base
   Bantian, Longgang District
   Shenzhen 518129 P.R.China

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