RFC 8694 | Applicability of PCE to MPLS and GMPLS | December 2019
King & Zheng | Informational | [Page]

Stream:
  Internet Engineering Task Force (IETF)
RFC:
  8694
Category:
  Informational
Published:
  December 2019
ISSN:
  2070-1721
Authors:
  D. King
  Old Dog Consulting
  郑好棉 (H. Zheng)
  华为技术有限公司 (Huawei Technologies)

# RFC 8694

# Applicability of the Path Computation Element to Inter-area and Inter-AS MPLS and GMPLS Traffic Engineering

## Abstract

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

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

## Status of This Memo

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

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

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8694.¶

## Copyright Notice

Copyright (c) 2019 IETF Trust and the persons identified as the document authors. All rights reserved.¶

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.¶

## 1. Introduction

Computing paths across large multi-domain environments may require special computational components and cooperation between entities in different domains capable of complex path computation.¶

Issues that may exist when routing in multi-domain networks include the following:¶

- There is often a lack of full topology and TE information across domains.¶
- No single node has the full visibility to determine an optimal or even feasible end-to-end path across domains.¶
- Knowing how to evaluate and select the exit point and next domain boundary from a domain.¶
- Understanding how the ingress node determines which domains should be used for the end-to-end path.¶

An information exchange across multiple domains is often limited due to the lack of trust relationship, security issues, or scalability issues, even if there is a trust relationship between domains.¶

The Path Computation Element (PCE) [RFC4655] provides an architecture and a set of functional components to address the problem space and the issues highlighted above.¶

A PCE may be used to compute end-to-end paths across multi-domain environments using a per-domain path computation technique [RFC5152]. The so-called backward recursive PCE-based computation (BRPC) mechanism [RFC5441] defines a path computation procedure to compute inter-domain constrained Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS) Traffic-Engineered (TE) networks. However, both per-domain and BRPC techniques assume that the sequence of domains to be crossed from source to destination is known, either fixed by the network operator or obtained by other means.¶

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

This document describes the processes and procedures available when using the PCE architecture and protocols for computing inter-area and inter-AS MPLS and GMPLS Traffic-Engineered paths.¶

The scope of this document does not include discussions of deployment scenarios for stateful PCE, active PCE, remotely initiated PCE, or PCE as a central controller (PCECC).¶

### 1.1. Domains

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

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

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

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

### 1.2. Path Computation

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

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

#### 1.2.1. PCE-Based Path Computation Procedure

As highlighted, the PCE is an entity capable of computing an inter-domain TE path upon receiving a request from a PCC. There could be a single PCE per domain or a single PCE responsible for all domains. A PCE may or may not reside on the same node as the requesting PCC. A path may be computed by either a single PCE node or a set of distributed PCE nodes that collaborate during path computation.¶

According to [RFC4655], a PCC should send a path computation request to a particular PCE using [RFC5440] (PCC-to-PCE communication). This negates the need to broadcast a request to all the PCEs. Each PCC can maintain information about the computation capabilities of the PCEs it is aware of. The PCC-PCE capability awareness can be configured using static configurations or by automatic and dynamic PCE discovery procedures.¶

If a network path is required, the PCC will send a path computation request to the PCE. A PCE may then compute the end-to-end path if it is aware of the topology and TE information required to compute the entire path. If the PCE is unable to compute the entire path, the PCE architecture provides cooperative PCE mechanisms for the resolution of path computation requests when an individual PCE does not have sufficient TE visibility.¶

End-to-end path segments may be kept confidential through the application of Path-Keys to protect partial or full path information. A Path-Key is a token that replaces a path segment in an explicit route. The Path-Key mechanism is described in [RFC5520].¶

### 1.3. Traffic Engineering Aggregation and Abstraction

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

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

The PCE architecture and associated mechanisms provide a solution to avoid the use of TE aggregation and abstraction.¶

### 1.4. Traffic-Engineered Label Switched Paths

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

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

- Contiguous LSP¶
- Stitched LSP¶
- Nested LSP¶

All three signaling methods are applicable to the architectures and procedures discussed in this document.¶

### 1.5. Inter-area and Inter-AS-capable PCE Discovery

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

### 1.6. Objective Functions

An Objective Function (OF) [RFC5541] or a set of OFs specifies the intentions of the path computation and so defines the "optimality" in the context of the computation request.¶

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

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

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

## 2. Terminology

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

ABR:
  IGP Area Border Router -- a router that is attached to more than one IGP area.¶
ASBR:
  Autonomous System Border Router -- a router used to connect together ASes of a different or the same Service Provider via one or more inter-AS links.¶
Inter-area TE LSP:
  A TE LSP whose path transits through two or more IGP areas.¶
Inter-AS MPLS TE LSP:
  A TE LSP whose path transits through two or more ASes or sub-ASes (BGP confederations)¶
SRLG:
  Shared Risk Link Group.¶
TED:
  Traffic Engineering Database, which contains the topology and resource information of the domain. The TED may be fed by Interior Gateway Protocol (IGP) extensions or potentially by other means.¶

## 3. Issues and Considerations

### 3.1. Multihoming

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

### 3.2. Destination Location

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

### 3.3. Domain Confidentiality

When the end-to-end path crosses multiple domains, it may be possible that each domain (AS or area) is administered by separate Service Providers. Thus, if a PCE supplies a path segment to a PCE in another domain, it may break confidentiality rules and could disclose AS-internal topology information.¶

If confidentiality is required between domains (ASes and areas) belonging to different Service Providers, then cooperating PCEs cannot exchange path segments; otherwise, the receiving PCE or PCC will be able to see the individual hops through another domain.¶

This topic is discussed further in Section 8 of this document.¶

## 4. Domain Topologies

Constraint-based inter-domain path computation is a fundamental requirement for operating traffic-engineered MPLS [RFC3209] and GMPLS [RFC3473] networks in inter-area and inter-AS (multi-domain) environments. Path computation across multi-domain networks is complex and requires computational cooperational entities like the PCE.¶

### 4.1. Selecting Domain Paths

Where the sequence of domains is known a priori, various techniques can be employed to derive an optimal multi-domain path. If the domains are connected to a simple path with no branches and single links between all domains or if the preferred points of interconnection are also known, the per-domain path computation [RFC5152] technique may be used. Where there are multiple connections between domains and there is no preference for the choice of points of interconnection, BRPC [RFC5441] can be used to derive an optimal path.¶

When the sequence of domains is not known in advance or the end-to-end path will have to navigate a mesh of small domains (especially typical in optical networks), the optimum path may be derived through the application of a hierarchical PCE [RFC6805].¶

### 4.2. Domain Sizes

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

### 4.3. Domain Diversity

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

### 4.4. Synchronized Path Computations

In some scenarios, it would be beneficial for the operator to rely on the capability of the PCE to perform synchronized path computation.¶

Synchronized path computations, known as Synchronization VECtors (SVECs), are used for dependent path computations. SVECs are defined in [RFC5440], and [RFC6007] provides an overview of the use of the PCE SVEC list for synchronized path computations when computing dependent requests.¶

In hierarchical PCE (H-PCE) deployments, a child PCE will be able to request both dependent and synchronized domain-diverse end-to-end paths from its parent PCE.¶

### 4.5. Domain Inclusion or Exclusion

A domain sequence is an ordered sequence of domains traversed to reach the destination domain. A domain sequence may be supplied during path computation to guide the PCEs or are derived via the use of hierarchical PCE (H-PCE).¶

During multi-domain path computation, a PCC may request specific domains to be included or excluded in the domain sequence using the Include Route Object (IRO) [RFC5440] and Exclude Route Object (XRO) [RFC5521]. The use of Autonomous Number (AS) as an abstract node representing a domain is defined in [RFC3209]. [RFC7897] specifies new subobjects to include or exclude domains such as an IGP area or a 4-byte AS number.¶

An operator may also need to avoid a path that uses specified nodes for administrative reasons. If a specific connectivity service is required to have a 1+1 protection capability, two separate disjoint paths must be established. A mechanism known as Shared Risk Link Group (SRLG) information may be used to ensure path diversity.¶

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

As networks increase in size and complexity, it may be required to introduce scaling methods to reduce the amount of information flooded within the network and make the network more manageable. An IGP hierarchy is designed to improve IGP scalability by dividing the IGP domain into areas and limiting the flooding scope of topology information to within area boundaries. This restricts visibility of the area to routers in a single area. If a router needs to compute the route to a destination located in another area, a method would be required to compute a path across area boundaries.¶

In order to support multiple vendors in a network in cases where data or control-plane technologies cannot interoperate, it is useful to divide the network into vendor domains. Each vendor domain is an IGP area, and the flooding scope of the topology (as well as any other relevant information) is limited to the area boundaries.¶

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

- This method does not guarantee an optimal constrained path.¶
- The method may require several crankback signaling messages, as per [RFC4920], increasing signaling traffic and delaying the LSP setup.¶

PCE-based architecture [RFC4655] is designed to solve inter-area path computation problems. The issue of limited topology visibility is resolved by introducing path computation entities that are able to cooperate in order to establish LSPs with the source and destinations located in different areas.¶

### 5.1. Inter-area Routing

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

#### 5.1.1. Area Inclusion and Exclusion

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

#### 5.1.2. Strict Explicit Path and Loose Path

A strict explicit path is defined as a set of strict hops, while a loose path is defined as a set of at least one loose hop and zero or more strict hops. It may be useful to indicate whether a strict explicit path is required during the path computation request. An inter-area path may be strictly explicit or loose (e.g., a list of ABRs as loose hops).¶

A PCC request to a PCE does allow indication of whether a strict explicit path across specific areas ([RFC7897]) is required or desired or whether the path request is loose.¶

#### 5.1.3. Inter-Area Diverse Path Computation

It may be necessary to compute a path that is partially or entirely diverse from a previously computed path to avoid fate sharing of a primary service with a corresponding backup service. There are various levels of diversity in the context of an inter-area network:¶

- Per-area diversity (the intra-area path segments are a link, node, or SRLG disjoint).¶
- Inter-area diversity (the end-to-end inter-area paths are a link, node, or SRLG disjoint).¶

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

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

As discussed in Section 5 (Applicability of the PCE to Inter-area Traffic Engineering), it is necessary to divide the network into smaller administrative domains, or ASes. If an LSR within an AS needs to compute a path across an AS boundary, it must also use an inter-AS computation technique. [RFC5152] defines mechanisms for the computation of inter-domain TE LSPs using network elements along the signaling paths to compute per-domain constrained path segments.¶

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

### 6.1. Inter-AS Routing

#### 6.1.1. AS Inclusion and Exclusion

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

### 6.2. Inter-AS Bandwidth Guarantees

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

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

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

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

### 6.3. Inter-AS Recovery

During a path computation process, a PCC request may contain the requirement to compute a backup LSP for protecting the primary LSP, such as 1+1 protection. A single LSP or multiple backup LSPs may also be used for a group of primary LSPs; this is typically known as m:n protection.¶

Other inter-AS recovery mechanisms include [RFC4090], which adds Fast Reroute (FRR) protection to an LSP. So, the PCE could be used to trigger computation of backup tunnels in order to protect inter-AS connectivity.¶

Inter-AS recovery clearly requires backup LSPs for service protection, but it would also be advisable to have multiple PCEs deployed for path computation redundancy, especially for service restoration in the event of catastrophic network failure.¶

### 6.4. Inter-AS PCE Peering Policies

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

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

## 7. Multi-domain PCE Deployment Options

### 7.1. Traffic Engineering Database and Synchronization

An optimal path computation requires knowledge of the available network resources, including nodes and links, constraints, link connectivity, available bandwidth, and link costs. The PCE operates on a view of the network topology as presented by a TED. As discussed in [RFC4655], the TED used by a PCE may be learned by the relevant IGP extensions.¶

Thus, the PCE may operate its TED by participating in the IGP running in the network. In an MPLS-TE network, this would require OSPF-TE [RFC3630] or ISIS-TE [RFC5305]. In a GMPLS network, it would utilize the GMPLS extensions to OSPF and IS-IS defined in [RFC4203] and [RFC5307]. Inter-AS connectivity information may be populated via [RFC5316] and [RFC5392].¶

An alternative method to providing network topology and resource information is offered by [RFC7752], which is described in the following section.¶

#### 7.1.1. Applicability of BGP-LS to PCE

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

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

### 7.2. Pre-planning and Management-Based Solutions

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

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

The management system may also use a PCE and BRPC to pre-plan an AS sequence, and the source domain PCE and per-domain path computation to be used when the actual end-to-end path is required. This model may also be used where the operator wishes to retain full manual control of the placement of LSPs, using the PCE only as a computation tool to assist the operator and not as part of an automated network.¶

In environments where operators peer with each other to provide end-to-end paths, the operator responsible for each domain must agree on the extent to which paths must be pre-planned or manually controlled.¶

## 8. Domain Confidentiality

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

Confidentiality typically applies to inter-provider (inter-AS) PCE communication. Where the TE LSP crosses multiple domains (ASes or areas), the path may be computed by multiple PCEs that cooperate together, with each local PCE responsible for computing a segment of the path. With each local PCE responsible for computing a segment of the path.¶

In situations where ASes are administered by separate Service Providers, it would break confidentiality rules for a PCE to supply path segment details to a PCE responsible for another domain, thus disclosing AS-internal or area topology information.¶

### 8.1. Loose Hops

A method for preserving the confidentiality of the path segment is for the PCE to return a path containing a loose hop in place of the segment that must be kept confidential. The concept of loose and strict hops for the route of a TE LSP is described in [RFC3209].¶

[RFC5440] supports the use of paths with loose hops; whether it returns a full explicit path with strict hops or uses loose hops is a local policy decision at a PCE. A path computation request may require an explicit path with strict hops or may allow loose hops, as detailed in [RFC5440].¶

### 8.2. Confidential Path Segments and Path-Keys

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

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

## 9. Point to Multipoint

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

## 10. Optical Domains

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

In the ASON framework, a path computation request is termed a route query. This query is executed before signaling is used to establish an LSP, which is termed a Switched Connection (SC) or a Soft Permanent Connection (SPC). [G-7715-2] defines the requirements and architecture for the functions performed by Routing Controllers (RC) during the operation of remote route queries. An RC is synonymous with a PCE.¶

In the ASON routing environment, an RC responsible for an RA may communicate with its neighbor RC to request the computation of an end-to-end path across several RAs. The path computation components and sequences are defined as follows:¶

- Remote route query. An operation where a Routing Controller communicates with another Routing Controller, which does not have the same set of layer resources, in order to compute a routing path in a collaborative manner.¶
- Route query requester. The connection controller or RC that sends a route query message to a Routing Controller that requests one or more routing paths satisfying a set of routing constraints.¶
- Route query responder. An RC that performs the path computation upon reception of a route query message from a Routing Controller or connection controller, and sends a response back at the end of the computation.¶

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

### 10.1. Abstraction and Control of TE Networks (ACTN)

Where a single operator operates multiple TE domains (including optical environments), an Abstraction and Control of TE Networks (ACTN) framework [RFC8453] may be used to create an abstracted (virtualized network) view of underlay-interconnected domains. This underlay connectivity is then exposed to higher-layer control entities and applications.¶

ACTN describes the method and procedure for coordinating the underlay per-domain Provisioning Network Controllers (PNCs), which may be PCEs, via a hierarchical model to facilitate setup of end-to-end connections across interconnected TE domains.¶

## 11. Policy

Policy is important in the deployment of new services and the operation of the network. [RFC5394] provides a framework for PCE-based policy-enabled path computation. This framework is based on the Policy Core Information Model (PCIM) as defined in [RFC3060] and further extended by [RFC3460].¶

When using a PCE to compute inter-domain paths, policy may be invoked by specifying the following:¶

- Each PCC must select which computations it will request from a PCE.¶
- Each PCC must select which PCEs it will use.¶
- Each PCE must determine which PCCs are allowed to use its services and for what computations.¶
- The PCE must determine how to collect the information in its TED, whom to trust for that information, and how to refresh/update the information.¶
- Each PCE must determine which objective functions and algorithms to apply.¶

## 12. Manageability Considerations

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

### 12.1. Control of Function and Policy

As per [RFC5440], PCEP implementation allows the user to configure a number of PCEP session parameters. These are detailed in Section 8.1 of [RFC5440].¶

In H-PCE deployments, the administrative entity responsible for the management of the parent PCEs for multi-areas would typically be a single service provider. In multiple ASes (managed by different service providers), it may be necessary for a third party to manage the parent PCE.¶

### 12.2. Information and Data Models

A PCEP MIB module is defined in [RFC7420], which describes managed objects for modeling PCEP communication, including:¶

- PCEP client configuration and status.¶
- PCEP peer configuration and information.¶
- PCEP session configuration and information.¶
- Notifications to indicate PCEP session changes.¶

A YANG module for PCEP has also been proposed [PCEP-YANG].¶

An H-PCE MIB module or YANG data model will be required to report parent PCE and child PCE information, including:¶

- Parent PCE configuration and status.¶
- Child PCE configuration and information.¶
- Notifications to indicate session changes between parent PCEs and child PCEs.¶
- Notification of parent PCE TED updates and changes.¶

### 12.3. Liveness Detection and Monitoring

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

### 12.4. Verifying Correct Operation

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

### 12.5. Impact on Network Operation

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

## 13. Security Considerations

PCEP security considerations are discussed in [RFC5440] and [RFC6952]. Potential vulnerabilities include spoofing, snooping, falsification, and using PCEP as a mechanism for denial of service attacks.¶

As PCEP operates over TCP, it may make use of TCP security encryption mechanisms, such as Transport Layer Security (TLS) and TCP Authentication Option (TCP-AO). Usage of these security mechanisms for PCEP is described in [RFC8253], and recommendations and best current practices are described in [RFC7525].¶

### 13.1. Multi-domain Security

Any multi-domain operation necessarily involves the exchange of information across domain boundaries. This represents a significant security and confidentiality risk.¶

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

## 14. IANA Considerations

This document has no IANA actions.¶

## 15. References

### 15.1. Normative References

[RFC3209]
  Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan, V., and G. Swallow, "RSVP-TE: Extensions to RSVP for LSP Tunnels", RFC 3209, DOI 10.17487/RFC3209, December 2001, <https://www.rfc-editor.org/info/rfc3209>.
[RFC3473]
  Berger, L., Ed., "Generalized Multi-Protocol Label Switching (GMPLS) Signaling Resource ReserVation Protocol-Traffic Engineering (RSVP-TE) Extensions", RFC 3473, DOI 10.17487/RFC3473, January 2003, <https://www.rfc-editor.org/info/rfc3473>.
[RFC4216]
  Zhang, R., Ed. and J.-P. Vasseur, Ed., "MPLS Inter-Autonomous System (AS) Traffic Engineering (TE) Requirements", RFC 4216, DOI 10.17487/RFC4216, November 2005, <https://www.rfc-editor.org/info/rfc4216>.
[RFC4655]
  Farrel, A., Vasseur, J.-P., and J. Ash, "A Path Computation Element (PCE)-Based Architecture", RFC 4655, DOI 10.17487/RFC4655, August 2006, <https://www.rfc-editor.org/info/rfc4655>.
[RFC4726]
  Farrel, A., Vasseur, J.-P., and A. Ayyangar, "A Framework for Inter-Domain Multiprotocol Label Switching Traffic Engineering", RFC 4726, DOI 10.17487/RFC4726, November 2006, <https://www.rfc-editor.org/info/rfc4726>.
[RFC5152]
  Vasseur, JP., Ed., Ayyangar, A., Ed., and R. Zhang, "A Per-Domain Path Computation Method for Establishing Inter-Domain Traffic Engineering (TE) Label Switched Paths (LSPs)", RFC 5152, DOI 10.17487/RFC5152, February 2008, <https://www.rfc-editor.org/info/rfc5152>.
[RFC5440]
  Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation Element (PCE) Communication Protocol (PCEP)", RFC 5440, DOI 10.17487/RFC5440, March 2009, <https://www.rfc-editor.org/info/rfc5440>.
[RFC5441]
  Vasseur, JP., Ed., Zhang, R., Bitar, N., and JL. Le Roux, "A Backward-Recursive PCE-Based Computation (BRPC) Procedure to Compute Shortest Constrained Inter-Domain Traffic Engineering Label Switched Paths", RFC 5441, DOI 10.17487/RFC5441, April 2009, <https://www.rfc-editor.org/info/rfc5441>.
[RFC5520]
  Bradford, R., Ed., Vasseur, JP., and A. Farrel, "Preserving Topology Confidentiality in Inter-Domain Path Computation Using a Path-Key-Based Mechanism", RFC 5520, DOI 10.17487/RFC5520, April 2009, <https://www.rfc-editor.org/info/rfc5520>.
[RFC5541]
  Le Roux, JL., Vasseur, JP., and Y. Lee, "Encoding of Objective Functions in the Path Computation Element Communication Protocol (PCEP)", RFC 5541, DOI 10.17487/RFC5541, June 2009, <https://www.rfc-editor.org/info/rfc5541>.
[RFC6805]
  King, D., Ed. and A. Farrel, Ed., "The Application of the Path Computation Element Architecture to the Determination of a Sequence of Domains in MPLS and GMPLS", RFC 6805, DOI 10.17487/RFC6805, November 2012, <https://www.rfc-editor.org/info/rfc6805>.

### 15.2. Informative References

[RFC3060]
  Moore, B., Ellesson, E., Strassner, J., and A. Westerinen, "Policy Core Information Model -- Version 1 Specification", RFC 3060, DOI 10.17487/RFC3060, February 2001, <https://www.rfc-editor.org/info/rfc3060>.
[RFC3460]
  Moore, B., Ed., "Policy Core Information Model (PCIM) Extensions", RFC 3460, DOI 10.17487/RFC3460, January 2003, <https://www.rfc-editor.org/info/rfc3460>.
[RFC3630]
  Katz, D., Kompella, K., and D. Yeung, "Traffic Engineering (TE) Extensions to OSPF Version 2", RFC 3630, DOI 10.17487/RFC3630, September 2003, <https://www.rfc-editor.org/info/rfc3630>.
[RFC4090]
  Pan, P., Ed., Swallow, G., Ed., and A. Atlas, Ed., "Fast Reroute Extensions to RSVP-TE for LSP Tunnels", RFC 4090, DOI 10.17487/RFC4090, May 2005, <https://www.rfc-editor.org/info/rfc4090>.
[RFC4203]
  Kompella, K., Ed. and Y. Rekhter, Ed., "OSPF Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS)", RFC 4203, DOI 10.17487/RFC4203, October 2005, <https://www.rfc-editor.org/info/rfc4203>.
[RFC4920]
  Farrel, A., Ed., Satyanarayana, A., Iwata, A., Fujita, N., and G. Ash, "Crankback Signaling Extensions for MPLS and GMPLS RSVP-TE", RFC 4920, DOI 10.17487/RFC4920, July 2007, <https://www.rfc-editor.org/info/rfc4920>.
[RFC5088]
  Le Roux, JL., Ed., Vasseur, JP., Ed., Ikejiri, Y., and R. Zhang, "OSPF Protocol Extensions for Path Computation Element (PCE) Discovery", RFC 5088, DOI 10.17487/RFC5088, January 2008, <https://www.rfc-editor.org/info/rfc5088>.
[RFC5089]
  Le Roux, JL., Ed., Vasseur, JP., Ed., Ikejiri, Y., and R. Zhang, "IS-IS Protocol Extensions for Path Computation Element (PCE) Discovery", RFC 5089, DOI 10.17487/RFC5089, January 2008, <https://www.rfc-editor.org/info/rfc5089>.
[RFC5305]
  Li, T. and H. Smit, "IS-IS Extensions for Traffic Engineering", RFC 5305, DOI 10.17487/RFC5305, October 2008, <https://www.rfc-editor.org/info/rfc5305>.
[RFC5307]
  Kompella, K., Ed. and Y. Rekhter, Ed., "IS-IS Extensions in Support of Generalized Multi-Protocol Label Switching (GMPLS)", RFC 5307, DOI 10.17487/RFC5307, October 2008, <https://www.rfc-editor.org/info/rfc5307>.
[RFC5316]
  Chen, M., Zhang, R., and X. Duan, "ISIS Extensions in Support of Inter-Autonomous System (AS) MPLS and GMPLS Traffic Engineering", RFC 5316, DOI 10.17487/RFC5316, December 2008, <https://www.rfc-editor.org/info/rfc5316>.
[RFC5392]
  Chen, M., Zhang, R., and X. Duan, "OSPF Extensions in Support of Inter-Autonomous System (AS) MPLS and GMPLS Traffic Engineering", RFC 5392, DOI 10.17487/RFC5392, January 2009, <https://www.rfc-editor.org/info/rfc5392>.
[RFC5394]
  Bryskin, I., Papadimitriou, D., Berger, L., and J. Ash, "Policy-Enabled Path Computation Framework", RFC 5394, DOI 10.17487/RFC5394, December 2008, <https://www.rfc-editor.org/info/rfc5394>.
[RFC5521]
  Oki, E., Takeda, T., and A. Farrel, "Extensions to the Path Computation Element Communication Protocol (PCEP) for Route Exclusions", RFC 5521, DOI 10.17487/RFC5521, April 2009, <https://www.rfc-editor.org/info/rfc5521>.
[RFC5886]
  Vasseur, JP., Ed., Le Roux, JL., and Y. Ikejiri, "A Set of Monitoring Tools for Path Computation Element (PCE)-Based Architecture", RFC 5886, DOI 10.17487/RFC5886, June 2010, <https://www.rfc-editor.org/info/rfc5886>.
[RFC6007]
  Nishioka, I. and D. King, "Use of the Synchronization VECtor (SVEC) List for Synchronized Dependent Path Computations", RFC 6007, DOI 10.17487/RFC6007, September 2010, <https://www.rfc-editor.org/info/rfc6007>.
[G-8080]
  ITU-T, "Architecture for the automatically switched optical network", ITU-T Recommendation G.8080/Y.1304, February 2012.
[G-7715]
  ITU-T, "Architecture and requirements for routing in the automatically switched optical networks", ITU-T Recommendation G.7715/Y.1706, June 2002.
[G-7715-2]
  ITU-T, "ASON routing architecture and requirements for remote route query", ITU-T Recommendation G.7715.2/Y.1706.2, February 2007.
[RFC6952]
  Jethanandani, M., Patel, K., and L. Zheng, "Analysis of BGP, LDP, PCEP, and MSDP Issues According to the Keying and Authentication for Routing Protocols (KARP) Design Guide", RFC 6952, DOI 10.17487/RFC6952, May 2013, <https://www.rfc-editor.org/info/rfc6952>.
[RFC7334]
  Zhao, Q., Dhody, D., King, D., Ali, Z., and R. Casellas, "PCE-Based Computation Procedure to Compute Shortest Constrained Point-to-Multipoint (P2MP) Inter-Domain Traffic Engineering Label Switched Paths", RFC 7334, DOI 10.17487/RFC7334, August 2014, <https://www.rfc-editor.org/info/rfc7334>.
[RFC7420]
  Koushik, A., Stephan, E., Zhao, Q., King, D., and J. Hardwick, "Path Computation Element Communication Protocol (PCEP) Management Information Base (MIB) Module", RFC 7420, DOI 10.17487/RFC7420, December 2014, <https://www.rfc-editor.org/info/rfc7420>.
[RFC7525]
  Sheffer, Y., Holz, R., and P. Saint-Andre, "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May 2015, <https://www.rfc-editor.org/info/rfc7525>.
[RFC7752]
  Gredler, H., Ed., Medved, J., Previdi, S., Farrel, A., and S. Ray, "North-Bound Distribution of Link-State and Traffic Engineering (TE) Information Using BGP", RFC 7752, DOI 10.17487/RFC7752, March 2016, <https://www.rfc-editor.org/info/rfc7752>.
[RFC7897]
  Dhody, D., Palle, U., and R. Casellas, "Domain Subobjects for the Path Computation Element Communication Protocol (PCEP)", RFC 7897, DOI 10.17487/RFC7897, June 2016, <https://www.rfc-editor.org/info/rfc7897>.
[RFC8253]
  Lopez, D., Gonzalez de Dios, O., Wu, Q., and D. Dhody, "PCEPS: Usage of TLS to Provide a Secure Transport for the Path Computation Element Communication Protocol (PCEP)", RFC 8253, DOI 10.17487/RFC8253, October 2017, <https://www.rfc-editor.org/info/rfc8253>.
[RFC8453]
  Ceccarelli, D., Ed. and Y. Lee, Ed., "Framework for Abstraction and Control of TE Networks (ACTN)", RFC 8453, DOI 10.17487/RFC8453, August 2018, <https://www.rfc-editor.org/info/rfc8453>.
[PCEP-YANG]
  Dhody, D., Hardwick, J., Beeram, V., and J. Tantsura, "A YANG Data Model for Path Computation Element Communications Protocol (PCEP)", Work in Progress, Internet-Draft, draft-ietf-pce-pcep-yang-13, 31 October 2019, <https://tools.ietf.org/html/draft-ietf-pce-pcep-yang-13>.

## Acknowledgements

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

## Contributors

## Authors' Addresses