Nokia

600 March Road Kanata Ontario K2K 2T6 Canada andrew.stone@nokia.com

Nokia

600 March Road Kanata Ontario K2K 2T6 Canada mustapha.aissaoui@nokia.com

Cisco Systems, Inc.

Eurovea Central 3 Pribinova 10 Bratislava 811 09 Slovakia ssidor@cisco.com

Ciena Corporation

385 Terry Fox Drive Kanata Ontario K2K 0L1 Canada ssivabal@ciena.com

rtg pce segment routing adjacency sid sla fast reroute ti-lfa This document updates RFC 5440 to clarify usage of the Local Protection Desired bit signaled in the Path Computation Element Communication Protocol (PCEP). This document also introduces a new flag for signaling protection enforcement in PCEP.

Status of This Memo This is an Internet Standards Track document. 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). Further information on Internet Standards is available in 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 .

Copyright Notice Copyright (c) 2023 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. 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 Revised BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Revised BSD License.

Table of Contents

• .  Introduction
• .  Requirements Language
• .  Terminology
• .  Motivation
  • .  Implementation Differences
  • .  SLA Enforcement
• .  Protection Enforcement Flag (E Flag)
  • .  Backwards Compatibility
• .  Security Considerations
• .  IANA Considerations
• .  References
  • .  Normative References
  • .  Informative References
• Acknowledgements
• Authors' Addresses

Introduction The Path Computation Element Communication Protocol (PCEP) enables the communication between a Path Computation Client (PCC) and a PCE or between two PCEs based on the PCE architecture . PCEP utilizes flags, values, and concepts previously defined in RSVP-TE Extensions and Fast Reroute Extensions to RSVP-TE . One such concept in PCEP is the Local Protection Desired (L) flag in the LSP Attributes (LSPA) object in , which was originally defined in the Session Attribute object in . In RSVP, this flag signals to downstream routers that they may use a local repair mechanism. The headend router calculating the path does not know if a downstream router will or will not protect a hop during its calculation. Therefore, the L flag does not require the transit router to satisfy protection in order to establish the RSVP-signaled path. This flag is signaled in PCEP as an attribute of the Label Switched Path (LSP) via the LSPA object. PCEP Extensions for Segment Routing extends support in PCEP for Segment Routing paths. The path list is encoded with Segment Identifiers (SIDs), each of which might offer local protection. The PCE may discover the protection eligibility for a SID via the Border Gateway Protocol - Link State (BGP-LS) and take the protection into consideration as a path constraint. It is desirable for an operator to be able to define the enforcement of the protection requirement. This document updates by further describing the behavior of the Local Protection Desired (L) flag and extends on it with the introduction of the Protection Enforcement (E) flag. The document contains descriptions in the context of Segment Routing; however, the content described is agnostic in regard to path setup type and data plane technology.

Requirements Language The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Terminology This document uses the following terminology:

PROTECTION MANDATORY:

The path MUST have protection eligibility on all links.

UNPROTECTED MANDATORY:

The path MUST NOT have protection eligibility on all links.

PROTECTION PREFERRED:

The path should have protection eligibility on all links but might contain links that do not have protection eligibility.

UNPROTECTED PREFERRED:

The path should not have protection eligibility on all links but might contain links that have protection eligibility.

PCC:

Path Computation Client. Any client application requesting a path computation to be performed by a Path Computation Element.

PCE:

Path Computation Element. 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.

PCEP:

Path Computation Element Communication Protocol

LSPA:

LSP Attributes object

Motivation

Implementation Differences As defined in , the mechanism to signal protection enforcement in PCEP is the previously mentioned L flag defined in the LSPA object. The name of the flag uses the term "Desired", which by definition means "strongly wished for or intended". The use case for this flag originated in RSVP. For RSVP-signaled paths, local protection is not within control of the PCE. However, does state that "[w]hen set, this means that the computed path must include links protected with Fast Reroute as defined in ." Implementations that use PCEP have interpreted the L flag as either PROTECTION MANDATORY or PROTECTION PREFERRED, leading to operational differences.

SLA Enforcement The L flag is a boolean bit and thus unable to distinguish between the different options of PROTECTION MANDATORY, UNPROTECTED MANDATORY, PROTECTION PREFERRED, and UNPROTECTED PREFERRED. Selecting one of these options is typically dependent on the Service Level Agreement (SLA) the operator wishes to impose on the LSP. A network may be providing transit to multiple SLA definitions against the same base topology network, whose behavior could vary, such as wanting local protection to be invoked on some LSPs and not wanting local protection on others. When enforcement is used, the resulting shortest path calculation is impacted. For example, PROTECTION MANDATORY is for use cases in which an operator may need the LSP to follow a path that has local protection provided along the full path, ensuring that traffic will be fast rerouted at the point if there is a failure anywhere along the path. As another example, UNPROTECTED MANDATORY is for use cases in which an operator may intentionally prefer an LSP to not be locally protected and thus would rather local failures cause the LSP to go down. An example scenario is one where an LSP is protected via a secondary diverse LSP. Each LSP is traffic engineered to follow specific traffic-engineered criteria computed by the PCE to satisfy the SLA. Upon a failure, if local protection is invoked on the active LSP traffic, the traffic may temporarily traverse links that violate the TE requirements and could negatively impact the resources being traversed (e.g., insufficient bandwidth). In addition, depending on the network topological scenario, it may not be feasible for the PCE to reroute the LSP while respecting the TE requirements, which include path diversity; this results in the LSP being torn down and switched to the protected path anyways. In such scenarios, it is desirable for the LSP to be simply torn down immediately and not rerouted through local protection, so that traffic may be forwarded through an already-established traffic-engineered secondary path. Both the UNPROTECTED PREFERRED and PROTECTED PREFERRED options provide a relaxation of the protection constraint. These options can be used when an operator does not require protection enforcement. Regardless of the option selected, the protection status of a resource does not influence whether the link must be pruned during a path calculation. Furthermore, the selection of either option indicates a priority selection to the PCE when there is an option to choose a protected or unprotected instruction associated with a resource, ensuring consistent PCE behavior across different implementations. When used with Segment Routing, an adjacency may have both a protected SID and an unprotected SID. If the UNPROTECTED PREFERRED option is selected, the PCE chooses the unprotected SID. Alternatively, if the PROTECTED PREFERRED option is selected, the PCE chooses the protected SID.

Protection Enforcement Flag (E Flag) describes the encoding of the Local Protection Desired (L) flag. The Protection Enforcement (E) flag, which extends the L flag, is specified below. Codespace of the Flag Field (LSPA Object)

Bit	Description	Reference
6	Protection Enforcement	RFC 9488
7	Local Protection Desired	RFC 5440

The following shows the format of the LSPA object as defined in with the addition of the E flag defined in this document: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Exclude-any | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Include-any | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Include-all | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Setup Prio | Holding Prio | Flags |E|L| Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | // Optional TLVs // | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Flags (8 bits):

L (Local Protection Desired):

This flag is defined in and further updated by this document. When set to 1, protection is desired. When set to 0, protection is not desired. The enforcement of the protection is identified via the E flag.

E (Protection Enforcement):

This flag controls the strictness with which the PCE must apply the L flag. When set to 1, the value of the L flag needs to be respected during resource selection by the PCE. When the E flag is set to 0, an attempt to respect the value of the L flag is made; however, the PCE could relax or ignore the L flag when computing a path. The statements below indicate preference when the E flag is set to 0 in combination with the L flag value.

When both the L flag and E flag are set to 1, then the PCE MUST consider the protection eligibility as a PROTECTION MANDATORY constraint. When the L flag is set to 1 and the E flag is set to 0, then the PCE MUST consider the protection eligibility as a PROTECTION PREFERRED constraint. When both the L flag and E flag are set to 0, then the PCE SHOULD consider the protection eligibility as an UNPROTECTED PREFERRED constraint but MAY consider the protection eligibility as an UNPROTECTED MANDATORY constraint. An example of when the latter behavior might be chosen is if the PCE has some means (outside the scope of this document) to detect that it is interacting with a legacy PCC that expects the legacy behavior. When the L flag is set to 0 and the E flag is set to 1, then the PCE MUST consider the protection eligibility as an UNPROTECTED MANDATORY constraint. If a PCE is unable to infer the protection status of a resource, the PCE MAY use local policy to define protected status assumptions. When computing a Segment Routing path, it is RECOMMENDED that a PCE assume a Node SID is protected. It is also RECOMMENDED that a PCE assume an Adjacency SID is protected if the backup flag advertised with the Adjacency SID is set.

Backwards Compatibility This section outlines considerations for the E flag bit in the message passing between the PCC and the PCE that are not supported by the entity. The requirements for the PCE and the PCC implementing this document are described at the end. For a PCC or PCE that does not yet support this document, the E flag is ignored and set to 0 in PCRpt and/or PCUpd messages as per for PCC-initiated LSPs or as per for PCE-initiated LSPs. It is important to note that and permit the LSPA object to be included in PCUpd messages for PCC-initiated and PCE-initiated LSPs. For PCC-initiated LSPs, the E flag (and L flag) in a PCUpd message is an echo from the previous PCRpt message; however, the bit value is ignored on the PCE from the previous PCRpt message, so the E flag value set in the PCUpd message is 0. A PCE that does not support this document sends PCUpd messages with the E flag set to 0 for PCC-initiated LSPs even if set to 1 in the prior PCReq or PCRpt message. A PCC that does not support this document sends PCRpt messages with the E flag set to 0 for PCE-initiated LSPs even if set to 1 in the prior PCInitiate or PCUpd message. For a PCC that does support this document, the E flag MAY be set to 1 depending on local configuration. If communicating with a PCE that does not yet support this document, the PCE follows the behavior specified in and ignores the E flag. Thus, a computed path might not respect the enforcement constraint. For PCC-initiated LSPs, the PCC SHOULD ignore the E flag value received from the PCE in a PCUpd message as it may be communicating with a PCE that does not support this document. For PCE-initiated LSPs, the PCC MAY process the E flag value received from the PCE in a PCUpd message. The PCE SHOULD ignore the E flag value received from the PCC in a PCRpt message as it may be communicating with a PCC that does not support this document.

Security Considerations This document clarifies the behavior of an existing flag and introduces a new flag to provide further control of that existing behavior. The introduction of this new flag and the behavior clarification do not create any new sensitive information. No additional security measure is required. Securing the PCEP session using Transport Layer Security (TLS) , as per the recommendations and best current practices in , is RECOMMENDED.

IANA Considerations This document defines a new bit value in the subregistry "LSPA Object Flag Field" in the "Path Computation Element Protocol (PCEP) Numbers" registry. IANA has made the following codepoint allocation. Addition to LSPA Object Flag Field Registry

Bit	Description	Reference
6	Protection Enforcement	RFC 9488

References Normative References In many standards track documents several words are used to signify the requirements in the specification. These words are often capitalized. This document defines these words as they should be interpreted in IETF documents. This document specifies an Internet Best Current Practices for the Internet Community, and requests discussion and suggestions for improvements. This document describes the use of RSVP (Resource Reservation Protocol), including all the necessary extensions, to establish label-switched paths (LSPs) in MPLS (Multi-Protocol Label Switching). Since the flow along an LSP is completely identified by the label applied at the ingress node of the path, these paths may be treated as tunnels. A key application of LSP tunnels is traffic engineering with MPLS as specified in RFC 2702. [STANDARDS-TRACK] This document defines RSVP-TE extensions to establish backup label-switched path (LSP) tunnels for local repair of LSP tunnels. These mechanisms enable the re-direction of traffic onto backup LSP tunnels in 10s of milliseconds, in the event of a failure. Two methods are defined here. The one-to-one backup method creates detour LSPs for each protected LSP at each potential point of local repair. The facility backup method creates a bypass tunnel to protect a potential failure point; by taking advantage of MPLS label stacking, this bypass tunnel can protect a set of LSPs that have similar backup constraints. Both methods can be used to protect links and nodes during network failure. The described behavior and extensions to RSVP allow nodes to implement either method or both and to interoperate in a mixed network. [STANDARDS-TRACK] This document specifies the Path Computation Element (PCE) Communication Protocol (PCEP) for communications between a Path Computation Client (PCC) and a PCE, or between two PCEs. Such interactions include path computation requests and path computation replies as well as notifications of specific states related to the use of a PCE in the context of Multiprotocol Label Switching (MPLS) and Generalized MPLS (GMPLS) Traffic Engineering. PCEP is designed to be flexible and extensible so as to easily allow for the addition of further messages and objects, should further requirements be expressed in the future. [STANDARDS-TRACK] RFC 2119 specifies common key words that may be used in protocol specifications. This document aims to reduce the ambiguity by clarifying that only UPPERCASE usage of the key words have the defined special meanings. The Path Computation Element Communication Protocol (PCEP) provides mechanisms for Path Computation Elements (PCEs) to perform path computations in response to Path Computation Client (PCC) requests. Although PCEP explicitly makes no assumptions regarding the information available to the PCE, it also makes no provisions for PCE control of timing and sequence of path computations within and across PCEP sessions. This document describes a set of extensions to PCEP to enable stateful control of MPLS-TE and GMPLS Label Switched Paths (LSPs) via PCEP. The Path Computation Element Communication Protocol (PCEP) defines the mechanisms for the communication between a Path Computation Client (PCC) and a Path Computation Element (PCE), or among PCEs. This document describes PCEPS -- the usage of Transport Layer Security (TLS) to provide a secure transport for PCEP. The additional security mechanisms are provided by the transport protocol supporting PCEP; therefore, they do not affect the flexibility and extensibility of PCEP. This document updates RFC 5440 in regards to the PCEP initialization phase procedures. The Path Computation Element Communication Protocol (PCEP) provides mechanisms for Path Computation Elements (PCEs) to perform path computations in response to Path Computation Client (PCC) requests. The extensions for stateful PCE provide active control of Multiprotocol Label Switching (MPLS) Traffic Engineering Label Switched Paths (TE LSPs) via PCEP, for a model where the PCC delegates control over one or more locally configured LSPs to the PCE. This document describes the creation and deletion of PCE-initiated LSPs under the stateful PCE model. Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS) are used to protect data exchanged over a wide range of application protocols and can also form the basis for secure transport protocols. Over the years, the industry has witnessed several serious attacks on TLS and DTLS, including attacks on the most commonly used cipher suites and their modes of operation. This document provides the latest recommendations for ensuring the security of deployed services that use TLS and DTLS. These recommendations are applicable to the majority of use cases. RFC 7525, an earlier version of the TLS recommendations, was published when the industry was transitioning to TLS 1.2. Years later, this transition is largely complete, and TLS 1.3 is widely available. This document updates the guidance given the new environment and obsoletes RFC 7525. In addition, this document updates RFCs 5288 and 6066 in view of recent attacks. Informative References Constraint-based path computation is a fundamental building block for traffic engineering systems such as Multiprotocol Label Switching (MPLS) and Generalized Multiprotocol Label Switching (GMPLS) networks. Path computation in large, multi-domain, multi-region, or multi-layer networks is complex and may require special computational components and cooperation between the different network domains. This document specifies the architecture for a Path Computation Element (PCE)-based model to address this problem space. This document does not attempt to provide a detailed description of all the architectural components, but rather it describes a set of building blocks for the PCE architecture from which solutions may be constructed. This memo provides information for the Internet community. Segment Routing (SR) enables any head-end node to select any path without relying on a hop-by-hop signaling technique (e.g., LDP or RSVP-TE). It depends only on "segments" that are advertised by link-state Interior Gateway Protocols (IGPs). An SR path can be derived from a variety of mechanisms, including an IGP Shortest Path Tree (SPT), an explicit configuration, or a Path Computation Element (PCE). This document specifies extensions to the Path Computation Element Communication Protocol (PCEP) that allow a stateful PCE to compute and initiate Traffic-Engineering (TE) paths, as well as a Path Computation Client (PCC) to request a path subject to certain constraints and optimization criteria in SR networks. This document updates RFC 8408. Segment Routing (SR) allows for a flexible definition of end-to-end paths by encoding paths as sequences of topological subpaths, called "segments". These segments are advertised by routing protocols, e.g., by the link-state routing protocols (IS-IS, OSPFv2, and OSPFv3) within IGP topologies. This document defines extensions to the Border Gateway Protocol - Link State (BGP-LS) address family in order to carry SR information via BGP.

Acknowledgements Thanks to , , and for reviewing and providing very valuable feedback and discussions on this document. Thanks to for shepherding this document.

Authors' Addresses Nokia

600 March Road Kanata Ontario K2K 2T6 Canada andrew.stone@nokia.com

Nokia

600 March Road Kanata Ontario K2K 2T6 Canada mustapha.aissaoui@nokia.com

Cisco Systems, Inc.

Eurovea Central 3 Pribinova 10 Bratislava 811 09 Slovakia ssidor@cisco.com

Ciena Corporation

385 Terry Fox Drive Kanata Ontario K2K 0L1 Canada ssivabal@ciena.com