]> Cisco Systems, Inc.

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

Airtel India

Praveen.Maheshwari@airtel.com

Nokia

andrew.stone@nokia.com

Verizon

luay.jalil@verizon.com

Huawei Technologies

pengshuping@huawei.com

PCE Working Group Segment Routing (SR) enables a node to steer packet flows along a specified path without the need for intermediate per-path states, due to the utilization of source routing. An SR Policy comprises a sequence of segments, which are essentially instructions that define a source-routed policy This document proposes a set of extensions to the Path Computation Element Communication Protocol (PCEP) for Segment Routing Policies that are designed to satisfy requirements for connection-oriented transport services (Circuit-Style SR policies). They include the ability to control path recomputation and the option to request path with strict hops only and are also applicable for generic SR policy use cases where controlling path recomputation or distinct hop requirements are applicable.

Introduction Segment Routing (SR) leverages the source routing paradigm, where the sender of a packet defines the path that the packet takes through the network. This is achieved by encoding the path information as a sequence of segments within the packet header. SR can be applied to both MPLS and IPv6 data planes, providing a flexible and scalable method for traffic engineering. The Path Computation Element (PCE) is a network component, application, or node that is capable of computing a network path or route based on a network graph and applying computational constraints. The PCE Communication Protocol (PCEP) enables communication between a PCE and Path Computation Clients (PCCs), facilitating the computation of optimal paths for traffic flows. introduces the concept of Segment Routing Policy (SR Policy), which is a set of candidate paths that can be used to steer traffic through a network. Each candidate path is represented by a list of segments, and the path can be dynamically adjusted based on network conditions and requirements. In connection-oriented transport services, such as those defined in , there is a need for path persistency and per-hop behavior for PCE-computed paths. This ensures that the paths remain stable and predictable, which is crucial for services that require high reliability and performance guarantees. To support the requirements of connection-oriented transport services, this document specifies extensions to PCEP to enable the use of Circuit Style Policies. These extensions allow for the request of strict paths from the PCE, the encoding of information to disable path recomputation for specific paths, and the clarification of the usage of existing flags within PCEP messages. The PCEP extensions described in this document are designed to be compatible with any Path Setup Type and are not limited to Circuit Style SR policies, ensuring broad applicability across different network environments and use cases.

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 terms defined in : ERO, LSPA, PCC, PCE, PCEP, PCEP Peer, and PCEP speaker. This document uses the following term defined in : LSP.

Overview of Extensions to PCEP

New Flags in STATEFUL-PCE-CAPABILITY TLV The STATEFUL-PCE-CAPABILITY TLV is an optional TLV introduced in in the OPEN object for stateful PCEP peer capability advertisement. This document defines the following new flags in that TLV:

• STRICT-PATH-CAPABILITY - 1 bit (Bit Position 18) - If set to 1, it indicates support for Strict-Path flag in LSP-EXTENDED-FLAG TLV. See for details.
• PATH-RECOMPUTATION-CAPABILITY - 1 bit (Bit Position 19) - If set to 1, it indicates support for PATH-RECOMPUTATION TLV. See for details.

New Flag in the LSP-EXTENDED-FLAG TLV O-flag is proposed in the LSP-EXTENDED-FLAG TLV, which was introduced in 3.1 of . O (Strict-Path) - 1 bit (Bit Position 4): If set to 1, this indicates to the PCE that a path exclusively made of strict hops is required. Strict hop definition is described in Section 4.1

PATH-RECOMPUTATION TLV This document defines new TLV for the LSPA Object for encoding information whether path recomputation is allowed for delegated LSP. The TLV is optional. If the TLV is included in LSPA object, the PCE MUST NOT recompute path in cases specified by flags in the TLV. Only the first instance of this TLV MUST be processed, subsequent instances MUST be ignored. Type (16 bits): 72. Length (16 bits): 4.

Reserved (16 bits):

MUST be set to zero by the sender and MUST be ignored by the receiver.

Flags (16 bits):

This document defines the following flag bits. The other bits MUST be set to zero by the sender and MUST be ignored by the receiver.

• P (Permanent): If set to 1, the PCE MUST NOT recompute path even if current path is not satisfying path computation constraints. If this flag is cleared, then the PCE SHOULD recompute path if original path is invalidated.
• F (Force): If set to 1, the PCE MUST NOT update path (exceptions description in Section 4.2). If flag is cleared, the PCE MAY update path based on explicit request from operator.

Operation

Strict Path Enforcement PCC MAY set the O flag in LSP-EXTENDED-FLAG TLV in PCRpt message sent to the PCE to indicate that a path exclusively made of strict hops is required. It MUST NOT be set to 1 if one or both PCEP speakers have not set the STRICT-PATH-CAPABILITY flag to 1 in the STATEFUL-PCE-CAPABILITY TLV. If PCEP peer received LSP-EXTENDED-FLAG TLV with O flag set, but it does not support that flag, it MUST send PCErr with Error-Type = 2 (Capability not supported). O flag cleared or LSP-EXTENDED-FLAG TLV not included indicates that a loose path is acceptable. In PCUpd or PCInitiate messages, PCE MAY set O bit if strict path is provided. The flag is applicable only for stateful messages. Existing O flag in RP object MAY be used to indicate similar behavior in PCReq and PCRep messages as described in as described in Section 7.4.1 of . If the O flag is set to 1 for both stateful and stateless messages for SR paths introduced in , the PCE MUST use only Segment Identifiers (SIDs) that explicitly specify adjacencies for packet forwarding. For example Adjacency SIDs MAY be used, but Prefix SIDs MUST NOT be used (even if there is only one adjacency).

Path Recomputation PCC MAY set flags in PATH-RECOMPUTATION TLV to control path computation behavior on PCE side. If TLV is not included, then the PCE MAY use local policy to trigger path-computation or LSP path update. If a PCEP speaker does not recognize the PATH-RECOMPUTATION TLV, it MUST ignore the TLV based on . If a PCEP speaker recognizes the TLV but does not support the TLV, it MUST send PCErr with Error-Type = 2 (Capability not supported). The presence of the TLV is blocking path recomputation based on various triggers like topology update, any periodic update or changed state of other LSPs in the network. The LSP path MAY be modified if forwarded packets will still use the same path. For example, if the same path can be encoded using Adjacency, Binding, Prefix, or other SIDs, then PCE MAY switch between various representations of the same path. If P flag is cleared, the PCE MAY recompute if current path is not considered valid, for example after topology update resulting in path not satisfying LSP's path constraints, but it MUST NOT recompute path if current path is not optimal. If P flag is set, the PCE MUST NOT recompute path during LSP lifetime even if path is invalidated. Only exception is explicit request from operator to recompute path If F flag is cleared, path update triggered manually by operator or any northbound interface of PCE MAY be done. If flag is set then PCE can update path only to tear down LSP by sending PCUpd message with empty ERO ERO or to bring it up again with path, which was used before LSP was tore down. TLV MAY be included in PCInitiate and PCUpd messages to indicate, which triggers will be disabled on the PCE. PCC should reflect flag values in PCRpt messages to forward requirement to other PCEs in the network.

All manageability requirements and considerations listed in , and apply to PCEP protocol extensions defined in this document. In addition, requirements and considerations listed in this section apply.

A PCE or PCC implementation MAY allow the capability of supporting PCEP extensions introduced in this document to be enabled/disabled as part of the global configuration.

An implementation SHOULD allow an operator to view the PCEP peer capability defined in this document. Section 4.1 and 4.1.1 of should be extended to include that capability for PCEP peer. Section 4.2 of module should be extended to add notification for blocked recomputation that satisfies specified constraints if recomputation is blocked using the PATH-RECOMPUTATION TLV.

Circuit-Style Policy draft is already describing connectivity verification and path validity considerations for Circuit Style Policies.

A PCE implementation SHOULD notify the operator in case of blocked recomputation for an LSPthat no longer satisfies specified constraints. It SHOULD also allow the operator to view LSPs on the PCE that do not satisfy specified constraints.

The PCEP extensions defined in this document do not imply any new requirements on other protocols. Overall concept of Circuit Style policies requires interaction with other protocols, but those requirements are already described in .

The mechanisms defined in , , and also apply to the PCEP extensions defined in this document.

[Note to the RFC Editor - remove this section before publication, as well as remove the reference to RFC 7942.] This section records the status of known implementations of the protocol defined by this specification at the time of posting of this Internet-Draft, and is based on a proposal described in . The description of implementations in this section is intended to assist the IETF in its decision processes in progressing drafts to RFCs. Please note that the listing of any individual implementation here does not imply endorsement by the IETF. Furthermore, no effort has been spent to verify the information presented here that was supplied by IETF contributors. This is not intended as, and must not be construed to be, a catalog of available implementations or their features. Readers are advised to note that other implementations may exist. According to , "this will allow reviewers and working groups to assign due consideration to documents that have the benefit of running code, which may serve as evidence of valuable experimentation and feedback that have made the implemented protocols more mature. It is up to the individual working groups to use this information as they see fit".

• Organization: Cisco Systems
• Implementation: IOS-XR PCC and PCE.
• Description: PCEP extensions supported using VENDOR-INFORMATION Object.
• Maturity Level: Production.
• Coverage: Partial.
• Contact: ssidor@cisco.com

The security considerations described in , , , and in itself. Note that this specification introduces the possibility to block path recomputation after various topology events. This creates an additional vulnerability if the security mechanisms of , , and are not used. If there is no integrity protection on the session, then an attacker could block path updates from PCE potentially resulting in traffic drop. As per it is RECOMMENDED that these PCEP extensions only be activated on authenticated and encrypted sessions across PCEs and PCCs using Transport Layer Security (TLS) , as per the recommendations and best current practices in RFC 9325 (unless explicitly set aside in ).

IANA Considerations

STATEFUL-PCE-CAPABILITY defines the STATEFUL-PCE-CAPABILITY. IANA is requested to confirm the following allocation in the "STATEFUL-PCE-CAPABILITY TLV Flag Field" within the "Path Computation Element Protocol (PCEP) Numbers" registry group:

Bit	Description	Reference
18	STRICT-PATH-CAPABILITY	This document
19	PATH-RECOMPUTATION-CAPABILITY	This document

LSP-EXTENDED-FLAG TLV defines the LSP-EXTENDED-FLAG TLV. IANA is requested to confirm the following allocation in the "LSP-EXTENDED-FLAG TLV Flag Field" within the "Path Computation Element Protocol (PCEP) Numbers" registry group:

Bit	Description	Reference
4	Strict-Path Flag (O)	This document

PATH-RECOMPUTATION TLV IANA is requested to confirm the following allocation in the "PCEP TLV Type Indicators" within the "Path Computation Element Protocol (PCEP) Numbers" registry group:

TLV Type	TLV Name	Reference
72	PATH-RECOMPUTATION TLV	This document

PATH-RECOMPUTATION TLV Flag Field IANA has created a new registry named "PATH-RECOMPUTATION TLV Flag Field" within the "Path Computation Element Protocol (PCEP) Numbers" registry group. New values are to be assigned by "IETF Review" . Each bit should be tracked with the following qualities:

• Bit number (count from 0 as the most significant bit)
• Description
• Reference

The registry contains the following codepoints, with initial values, to be assigned by IANA with the reference set to this document:

Bit	Description	Reference
0-13	Unassigned
14	Permanent	This document
15	Force	This document

References Normative References This document formally deprecates Transport Layer Security (TLS) versions 1.0 (RFC 2246) and 1.1 (RFC 4346). Accordingly, those documents have been moved to Historic status. These versions lack support for current and recommended cryptographic algorithms and mechanisms, and various government and industry profiles of applications using TLS now mandate avoiding these old TLS versions. TLS version 1.2 became the recommended version for IETF protocols in 2008 (subsequently being obsoleted by TLS version 1.3 in 2018), providing sufficient time to transition away from older versions. Removing support for older versions from implementations reduces the attack surface, reduces opportunity for misconfiguration, and streamlines library and product maintenance. This document also deprecates Datagram TLS (DTLS) version 1.0 (RFC 4347) but not DTLS version 1.2, and there is no DTLS version 1.1. This document updates many RFCs that normatively refer to TLS version 1.0 or TLS version 1.1, as described herein. This document also updates the best practices for TLS usage in RFC 7525; hence, it is part of BCP 195. 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

Contributors Bell Canada

daniel.voyer@bell.ca

Ciena

rrokui@ciena.com

Cisco Systems, Inc.

tsaad.net@gmail.com

Cisco Systems, Inc.

zali@cisco.com

ZTE Corporation

chen.ran@zte.com.cn

ZTE Corporation

xiong.quan@zte.com.cn

Huawei

dhruv.ietf@gmail.com

Cisco Systems, Inc.

cschmutz@cisco.com