Network Working Group D. King Internet-Draft Old Dog Consulting Intended Status: Informational A. Farrel Created: October 25, 2009 Old Dog Consulting Expires: March 25, 2010 The Application of the Path Computation Element Architecture to the Determination of a Sequence of Domains in MPLS & GMPLS draft-king-pce-hierarchy-fwk-02.txt Abstract Computing optimum routes for Label Switched Paths (LSPs) across multiple domains in Multiprotocol Label Switching Traffic Engineering (MPLS-TE) and Generalized MPLS (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 provides mechanisms that allow the optimum sequence of domains to be selected, and the optimum end-to-end path to be derived. Status of this Memo This Internet-Draft is submitted to IETF in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. King and Farrel [Page 1]
draft-king-hierarchy-fwk-01.txt October 2009 Copyright Notice Copyright (c) 2009 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents in effect on the date of publication of this document (http://trustee.ietf.org/license-info). Please review these documents carefully, as they describe your rights and restrictions with respect to this document. King and Farrel [Page 2]
draft-king-hierarchy-fwk-01.txt October 2009 Contents 1. Introduction..................................................4 1.1 Problem Statement............................................4 1.2 Definition of a Domain............. .........................5 1.3 Requirements.................................................5 1.3.1 Metric Objectives..........................................6 1.3.2 Domain Diversity...........................................6 1.3.3 Domain Path Diversity......................................7 1.3.4 Existing Traffic Engineering Constraints...................7 1.3.5 Commercial Constraints.....................................7 1.3.6 Domain Confidentiality.....................................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..........................................11 5.2 Maintaining Domain Confidentiality...........................12 5.3 PCE Discovery................................................12 5.4 Domain Traffic Engineering Abstraction.......................13 5.5 Determination of Destination Domain .........................13 5.6 Hierarchical PCE Examples....................................14 5.6.1 Hierarchical PCE Initial Information Exchange..............16 5.6.2 Hierarchical PCE End-to-End Path Computation Procedure Example..........................................................16 6. Hierarchical PCE Applicability................................17 6.1 Antonymous Systems...........................................18 6.2 ASON architecture (G-7715-2).................................18 6.3 IGP Areas....................................................18 7. Management Considerations ....................................18 7.1 Control of Function and Policy...............................18 7.1.1 Child PCE..................................................18 7.1.2 Parent PCE.................................................19 7.1.3 Policy Control.............................................19 7.2 Information and Data Models..................................19 7.3 Liveness Detection and Monitoring............................19 7.4 Verifying Correct Operation..................................19 7.5. Impact on Network Operation.................................20 8. Security Considerations ......................................20 9. IANA Considerations ..........................................20 10. Acknowledgements ............................................20 11. References ..................................................20 11.1. Normative References.......................................20 11.2. Informative References ....................................20 12. Authors' Addresses ..........................................22 12. Intellectual Property Consideration .........................22 13. Disclaimer of Validity ......................................22 14. Full Copyright Statement ....................................22 King and Farrel [Page 3]
draft-king-hierarchy-fwk-01.txt October 2009 1. Introduction The capability to compute, establish and control end-to-end inter- domain Multiprotocol Label Switching (MPLS) Traffic Engineering (TE) and Generalized Multiprotocol Label Switching (GMPLS) paths, using a Path Computation Element is well known. The Path Computation Element (PCE) architecture is defined in [RFC4655]. The methods for establishing and controlling inter-domain MPLS and GMPLS are documented in [RFC4726] In a multi-domain environment, the determination of an en-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. 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. This document examines techniques to establish the optimum path when the sequence of domains is not known in advance. It describes the architecture and mechanisms necessary 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 is applicable to environments with small groups of domains (where visibility is limited from the ingress Label Switching Router - LSR). Applying the hierarchical PCE model to large groups of domains such as the Internet, is not feasible or relevant. 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. King and Farrel [Page 4]
draft-king-hierarchy-fwk-01.txt October 2009 Both techniques, assume that the sequence of domains to be crossed from source to destination is well known. No explanation is given 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. 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 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]. 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 and-to-end path is not always well-known. King and Farrel [Page 5]
draft-king-hierarchy-fwk-01.txt October 2009 The traffic engineering properties of a domain cannot be seen from outside the domain. Traffic engineering aggregation or abstraction, hides information and can leads 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 internal domain topology confidentiality while providing the optimal end-to-end path. 1.3.1 Metric Objectives The definition of optimality is dependent on policy and will be based on a single objective or a group objectives. An objective is expressed as an objective function [RFC5541] and may be requested on a path computation request. The following objective functions are identified in this document for manipulating the path metrics for inter-domain path computation, but the list may be expanded in future versions of this document: * Minimize the cost of the path [RFC5541] * Select a path using links with the minimal load [RFC5541] * Select a path that leaves the maximum residual bandwidth [RFC5541] * Minimize aggregate bandwidth consumption [RFC5541] * Minimize the Load of the most loaded Link [RFC5541] * Minimize the Cumulative Cost of a set of paths [RFC5541] * Minimize the number of border routers used * Limit the number of domains crossed 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 Label Switched Paths (LSPs). King and Farrel [Page 6]
draft-king-hierarchy-fwk-01.txt October 2009 This, domain path selection should provide the capability to include or exclude specific domains and specific border nodes. 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 serves. 1.3.6 Limiting Resource 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 information. 1.3.7 Domain Interconnection Discovery To support large mesh domain 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. King and Farrel [Page 7]
draft-king-hierarchy-fwk-01.txt October 2009 Ingress Domain: The domain that includes the ingress LSR of a path. 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 either be Area Border Routers (ABRs) or Autonomous System Border Routers (ASBRs). 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- connectivity. 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, etc.). 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 a section of the LSP that crosses the domain or for requesting that a PCE for that domain computes the path. King and Farrel [Page 8]
draft-king-hierarchy-fwk-01.txt October 2009 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). Ultimately per domain path computation may lead to sub-optimal paths. In the case that the domain path (in particular, the sequence of border 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 number of significant crankback routing attempts to establish even a sub-optimal path. Note also that the PCEs in each domain may have different computation capabilities, run different path computation algorithms, and apply different sets of constraints and optimization criteria, etc. This can result in the end-to-end path being inconsistent and sub- optimal. 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 the 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 ingrss PCC (Path Computation Client) sends a path computation requests 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 Virtual Shortest Path Tree (VSPT) (a tree of potential paths), 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 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 which it returns to the initiating PCC. King and Farrel [Page 9]
draft-king-hierarchy-fwk-01.txt October 2009 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. - PCC sends a request to the PCE for the ingress domain (the ingress PCE) - The ingress PCE sends the path computation request direct to the PCE responsible for the domain containing the destination node (egress PCE) - The egress PCE computes an egress VSPT and passes it to a PCE responsible for each of the adjacent (potentially upstream) domains. - Each PCE in turn constructs a VSPT and passes it on to all of its neighboring PCEs. - 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 (hierarchical) PCE maintains a domain topology map. The domain topology map 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. King and Farrel [Page 10]
draft-king-hierarchy-fwk-01.txt October 2009 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 domains 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 beyond the connectivity of the domains it serves to their immediate neighbors. 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 traffic engineering (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 5.3. When a multi-domain path is needed, the ingress PCE sends a request to the parent PCE (using the PCE Protocol [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 its path segment or a set of candidate segments 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 request. 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 in order to define the policy being applied to the domain path computation. 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: - Minimum cost path - Minimum load path - Maximum residual bandwidth path - Minimize aggregate bandwidth consumption - Minimize the number of border routers used King and Farrel [Page 11]
draft-king-hierarchy-fwk-01.txt October 2009 The number of domains crossed - Minimum number of transit domains 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 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. This is the case for reasons of scaling and confidentiality. Information about the content of Child domains is not shared for scaling and confidentiality reasons. This means that one Child PCE cannot know the 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 domain). Consideration of discovery of the relationships, between Parent PCEs and Child PCEs, is for future study. Mechanisms that rely on advertising or querying PCE locations across domain or provider boundaries are undesirable for security, scaling, commercial, and confidentiality reasons. King and Farrel [Page 12]
draft-king-hierarchy-fwk-01.txt October 2009 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 Domain Traffic Engineering Abstraction 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 right. The mechanism for building the parent TED is likely to rely heavily on administrative configuration and commercial issues. However in models such as ASON, 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. King and Farrel [Page 13]
draft-king-hierarchy-fwk-01.txt October 2009 This topic will require further study. 5.6 Hierarchical PCE Examples Figure 1 shows four interconnected domains within a fifth Parent domain. Each domain contains a PCE. - Domain 1 is the ingress domain and Child PCE 1 is able to compute paths within the domain. Its neighbors are Domain 2 (with Child PCE 2) and Domain 4 (with Child PCE 4. The domain also contains the source LSR (S) and three egress border nodes (BN11, BN12, and BN13). - Domain 2 is served by Child PCE 2. Its neighbors are Domain 1 (with Child PCE 1) and Domain 3 (with Child PCE 3). The domain also contains four border nodes (BN21, BN22BN23, and BN24). - Domain 3 is the egress domain and is served by Child PCE 3. Its neighbors are Domain 2 (with Child PCE 2) and Domain 4 (with Child PCE 4). The domain also contains the destination LSR (D) and three ingress border nodes (BN31, BN32, and BN33). - Domain 4 is served by Child PCE 4. Its neighbors are Domain 2 (with Child PCE 2) and Domain 3 (with Child PCE 3). The domain also contains two border nodes (BN41 and BN42). All of these domains are encompassed within Domain 5 which is served by the Parent PCE (PCE 5). King and Farrel [Page 14]
draft-king-hierarchy-fwk-01.txt October 2009 ----------------------------------------------------------------- | 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 domain. King and Farrel [Page 15]
draft-king-hierarchy-fwk-01.txt October 2009 ---------------------------- | 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. 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 PCE (i.e., PCE5). 5. Child PCE 1 reports any change in the resource availability on its inter-domain links to its Parent PCE (i.e., PCE5). Each Child PCE performs steps 1 through 5 so that the Parent PCE can create an abstracted 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 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 abstracted domain topology view as described in Section 5.6.1. King and Farrel [Page 16]
draft-king-hierarchy-fwk-01.txt October 2009 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. 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. 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 BN33-D). 9. PCE 5 correlates all the computation responses from each Child PCE 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). 6. Hierarchical PCE Applicability As per [RFC4655], PCE can inherently support inter-domain path computation. A domain represents any collection of entities that are grouped for a particular purpose, e.g., geography, technology, policy, and routing responsibility. Examples of domains include IGP areas, Autonomous Systems (ASes). 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 border nodes. King and Farrel [Page 17]
draft-king-hierarchy-fwk-01.txt October 2009 6.1 Antonymous Systems TBD 6.2 ASON architecture (G-7715-2) The Parent PCE mechanism fits well within the ASON routing architecture. It can be used to provide paths across subnetworks, and to determine end-to-end paths in networks constructed from multiple subnetworks or Routing Areas (RAs). The Routing Controllers each implement a PCE that can be queried by any Network Element (NE) within the RA for which the routing controller has responsibility. When an inter-domain path is needed, the routing controllers use the hierarchical PCE model to fully match the hierarchical routing model of ASON. This topic will require further study and updated accordingly. 6.3 IGP Areas. TBD 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 management responsibility of the Parent PCEs must be determined. In the case of multi-domains (e.g., IGP areas or multiple ASes) within one service provider network, the management responsibility of the Parent PCEs might be handled by the service provider. In the case of multiple ASes within different service provider networks, it may be necessary for a third-party to manage the Parent PCEs according to commercial and policy agreements from each of the participating service providers. The following management consideration sections require continued consideration and will be discussed in further revisions of this document. 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. King and Farrel [Page 18]
draft-king-hierarchy-fwk-01.txt October 2009 7.1.2 Parent PCE The Parent PCE must only accept path computation requests from 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 TBD (monitoring of parent/child relationships, the use of PCEP between parent and child, and the parent TED) 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: 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. King and Farrel [Page 19]
draft-king-hierarchy-fwk-01.txt October 2009 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 acknowledge Fatai Zhang and Quintin Zhao for their valuable comments and suggestions. 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. 11.2. Informative References [RFC5152] Vasseur, J.P., Ed., Ayyangar, A., Ed., and R. Zhang, "A Per-domain path computation method for computing Inter-domain Traffic Engineering (TE) Label Switched Path (LSP)", King and Farrel [Page 20]
draft-king-hierarchy-fwk-01.txt October 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. [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)", RFC5440, March 2009. [RFC4726] Farrel, A., Vasseur, J., and A. Ayyangar, "A Framework for Inter-Domain Multiprotocol Label Switching Traffic Engineering", RFC 4726, November 2006. [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. [RFC5394] Bryskin, I., Papadimitriou, D., Berger, L., and J. Ash, "Policy-Enabled Path Computation Framework", RFC 5394, December 2008. [RFC5541] Roux, J., Vasseur, J., and Y. Lee, "Encoding of Objective Functions in the Path Computation Element Communication Protocol (PCEP)", RFC5541, December 2008. [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. King and Farrel [Page 21]
draft-king-hierarchy-fwk-01.txt October 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 query. 12. Authors' Addresses Daniel King Old Dog Consulting Email: daniel@olddog.co.uk Adrian Farrel Old Dog Consulting Email: adrian@olddog.co.uk King and Farrel [Page 22]
draft-king-hierarchy-fwk-01.txt October 2009