PCE Working Group H. Zheng, Ed. Internet-Draft Huawei Technologies Co., Ltd. Intended status: Standards Track Y. Lee Expires: 4 May 2027 CRU R. Casellas CTTC D. Ceccarelli Cisco 31 October 2026 PCEP Extension for Flexible Grid Networks draft-ietf-pce-flexible-grid-21 Abstract This document provides the Path Computation Element Communication Protocol (PCEP) extensions for the support of Routing and Spectrum Assignment (RSA) in Flexible Grid networks. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at https://datatracker.ietf.org/drafts/current/. 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." This Internet-Draft will expire on 4 May 2027. Copyright Notice Copyright (c) 2026 IETF Trust and the persons identified as the document authors. All rights reserved. Zheng, et al. Expires 4 May 2027 [Page 1] Internet-Draft PCEP Ext for Flexi-grid October 2026 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 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 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Requirements Language . . . . . . . . . . . . . . . . . . . . 4 4. Spectrum Assignment (SA) Object . . . . . . . . . . . . . . . 5 4.1. Spectrum Assignment Object . . . . . . . . . . . . . . . 6 4.2. Frequency Slot Selection TLV . . . . . . . . . . . . . . 7 4.3. Frequency Slot Restriction TLV . . . . . . . . . . . . . 8 5. Encoding of an RSA . . . . . . . . . . . . . . . . . . . . . 10 5.1. Spectrum Allocation TLV . . . . . . . . . . . . . . . . . 11 5.2. Error Indicator . . . . . . . . . . . . . . . . . . . . . 13 5.3. NO-PATH Indicator . . . . . . . . . . . . . . . . . . . . 14 6. Operational Considerations . . . . . . . . . . . . . . . . . 14 6.1. Control of Function and Policy . . . . . . . . . . . . . 14 6.2. Information and Data Models . . . . . . . . . . . . . . . 15 6.3. Liveness Detection and Monitoring . . . . . . . . . . . . 15 6.4. Verifying Correct Operation . . . . . . . . . . . . . . . 15 6.5. Requirements on Other Protocols and Functional Components . . . . . . . . . . . . . . . . . . . . . . . 15 6.6. Impact on Network Operation . . . . . . . . . . . . . . . 15 7. Security Considerations . . . . . . . . . . . . . . . . . . . 15 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16 8.1. New PCEP Object . . . . . . . . . . . . . . . . . . . . . 16 8.2. SA Object Flag Field . . . . . . . . . . . . . . . . . . 16 8.3. New PCEP TLVs . . . . . . . . . . . . . . . . . . . . . . 17 8.4. Frequency Slot Selection TLV Flag Field . . . . . . . . . 17 8.5. Frequency Slot Assignment Method Values . . . . . . . . . 17 8.6. Frequency Slot Restriction TLV Action Values . . . . . . 18 8.7. Spectrum Allocation TLV Flag Field . . . . . . . . . . . 18 8.8. New No-Path Reasons . . . . . . . . . . . . . . . . . . . 19 8.9. New Error-Types and Error-Values . . . . . . . . . . . . 19 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 19 10. Contributors' Address . . . . . . . . . . . . . . . . . . . . 19 11. References . . . . . . . . . . . . . . . . . . . . . . . . . 19 11.1. Normative References . . . . . . . . . . . . . . . . . . 20 11.2. Informative References . . . . . . . . . . . . . . . . . 22 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 23 Zheng, et al. Expires 4 May 2027 [Page 2] Internet-Draft PCEP Ext for Flexi-grid October 2026 1. Introduction [RFC4655] defines a Path Computation Element (PCE)-based path computation architecture and explains how a Path Computation Element (PCE) can compute Label Switched Paths (LSPs) in Multiprotocol Label Switching Traffic Engineering (MPLS-TE) and Generalized MPLS (GMPLS) networks at the request of Path Computation Clients (PCCs). A PCC is said to be any network component that makes such a request and can be, for instance, an Optical Switching Element within a Wavelength Division Multiplexing (WDM) network. The PCE, itself, can be located anywhere within the network, and can be within an optical switching element, a Network Management System (NMS), a Network Controller or Operational Support System (OSS), or can be an independent network server. The Path Computation Element Communication Protocol (PCEP) [RFC5440] is the communication protocol used between a PCC and a PCE, and can also be used between cooperating PCEs. [RFC4657] sets out the common protocol requirements for PCEP. Additional application-specific requirements for PCEP are deferred to separate documents. [RFC8780] provides the PCEP extensions for the support of Routing and Wavelength Assignment (RWA) in Wavelength Switched Optical Networks (WSON) based on the requirements specified in [RFC6163] and [RFC7449]. To allow efficient allocation of optical spectral bandwidth for systems that have high bit-rates, the International Telecommunication Union Telecommunication Standardization Sector (ITU-T) has extended its Recommendations [ITU-T_G.694.1] to include an enhanced Dense Wavelength Division Multiplexing (DWDM) grid by defining a set of nominal central frequencies, channel spacings, and the concept of the "frequency slot". In such an environment, a data-plane connection is switched based on allocated, variable-sized frequency ranges within the optical spectrum, creating what is known as a flexible grid (flexi-grid). [RFC7698] provides Framework and Requirements for GMPLS-Based Control of Flexi-Grid Dense Wavelength Division Multiplexing (DWDM) Networks. [RFC7792] and [RFC8363] define the extensions to the Resource Reservation Protocol - Traffic Engineering (RSVP-TE) signaling protocol and Open Shortest Path First - Traffic Engineering (OSPF-TE) to support Label Switched Paths (LSPs) in a GMPLS-controlled network that includes devices using the flexible optical grid. The term "Routing and Spectrum Assignment" (RSA) is introduced in [RFC7698] to refer to the process that determines a route and frequency slot for an LSP. Hence, when a path is computed, the spectrum assignment process determines the central frequency and slot Zheng, et al. Expires 4 May 2027 [Page 3] Internet-Draft PCEP Ext for Flexi-grid October 2026 width. The term "Spectrum Switched Optical Networks" is also introduced in [RFC7698] to refer to a flexi-grid enabled DWDM network, which can be controlled by a GMPLS or PCE control plane. This document provides PCEP extensions to support RSA in Flexi-grid networks. The extensions in this document apply to point-to-point LSPs; support for point-to-multipoint (P2MP) LSPs is out of scope for this document. Figure 1 shows one typical PCE-based implementation, which is referred to as the Combined Routing and Spectrum Assignment (RSA) [RFC7698]. With this architecture, the two processes of routing and spectrum assignment are accessed via a single PCE. This architecture is the base architecture from which the PCEP extensions are specified in this document. +----------------------------+ +-----+ | +-------+ +--+ | | | | |Routing| |SA| | | PCC |<----->| +-------+ +--+ | | | | | +-----+ | PCE | +----------------------------+ Figure 1: Combined Routing and Spectrum Assignment Architecture This document aligns with GMPLS extensions for PCEP [RFC8779] for generic properties such as label, label-set and label assignment, noting that flexi-grid frequency slot is a type of label per [RFC7699]. Frequency slots are encoded as Flexi-Grid Labels [RFC7699], and frequency slot restrictions reuse the field encodings of [RFC8363]. 2. Terminology This document uses the terminology defined in [RFC4655], [RFC5440], and [RFC7698]. 3. 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 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. Zheng, et al. Expires 4 May 2027 [Page 4] Internet-Draft PCEP Ext for Flexi-grid October 2026 4. Spectrum Assignment (SA) Object The SA Object enables a PCC to request spectrum assignment from the PCE and to express preferences such as assignment mode and frequency- slot restrictions. It thus allows a PCE to perform the combined RSA according to the PCC's policies. Spectrum allocation can be performed by the PCE by different means: a. By means of Explicit Label Control (ELC) [RFC3471], where the PCE allocates the frequency slot to use on each link along the path. The PCC requests this by setting the M bit of the SA Object to 1 (Section 4.1). b. By means of a set of candidate frequency slots, where the PCE provides for each link the frequency slots from which the slot is selected during signaling. The PCC requests this by setting the M bit of the SA Object to 0 (Section 4.1). Option b. allows distributed spectrum allocation (performed during signaling) to complete spectrum assignment. When the PCE performs the assignment (option a.), a PCReq can convey the method the PCE is to use to select among the feasible frequency slots in the Frequency Slot Selection TLV (Section 4.2); in option b., the selection is made by the nodes during signaling. The format Routing Backus-Naur Form (RBNF) [RFC5511] of a PCReq message per [RFC5440] after incorporating the Spectrum Assignment (SA) Object is as follows: ::= [] Where: ::=[] ::= [] [other optional objects...] Zheng, et al. Expires 4 May 2027 [Page 5] Internet-Draft PCEP Ext for Flexi-grid October 2026 If the SA Object is present in the PCReq message, it MUST be encoded after the END-POINTS Object. The slot width requested for the LSP is conveyed in the GENERALIZED-BANDWIDTH object (Section 2.3 of [RFC8779]), using Bandwidth Spec Type 8 (SSON), which carries the slot width parameter m defined in Section 4.1 of [RFC7792]. The Frequency Slot Restriction TLV (Section 4.3) constrains which frequency slots and slot widths can be used; it does not convey the requested slot width. 4.1. Spectrum Assignment Object This document specifies the use of the SA Object in the PCReq message. The use of the SA Object in other PCEP messages, such as the LSP Initiate Request (PCInitiate) message defined in [RFC8281], is out of scope for this document. The SA Object-Class is TBD1 (to be assigned by IANA). The SA Object- Type is 1. The format of the Spectrum Assignment (SA) Object body is as shown in Figure 2. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | Flags |M| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ // Optional TLVs // | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 2: SA Object Reserved (16 bits): the Reserved field MUST be set to zero by the sender and ignored by the receiver. Flags (16 bits): bits 0-14 are currently unassigned. They MUST be set to zero by the sender and ignored by the receiver. One flag bit is allocated as follows: Zheng, et al. Expires 4 May 2027 [Page 6] Internet-Draft PCEP Ext for Flexi-grid October 2026 M (Mode - bit 15): M bit is used to indicate the mode of spectrum assignment. When the M bit is set to 1, this indicates that the spectrum assigned by the PCE MUST be explicit. That is, the selected way to convey the allocated spectrum is by means of Explicit Label Control (ELC) [RFC3471]. Otherwise, when the M bit is set to 0, the spectrum assigned by the PCE does not need to be explicit, but it can be suggested in the Allocated Spectrum field of the Spectrum Allocation TLV, defined in Section 5 of this document. IANA maintains the "SA Object Flag Field" registry requested in Section 8.2. 4.2. Frequency Slot Selection TLV The Frequency Slot Selection TLV in the SA Object is used to indicate the frequency slot selection constraint in regard to the order of frequency slot assignment to be returned by the PCE. This TLV is only applied when the M bit is set to 1 in the SA Object specified in Section 4.1. The Frequency Slot Selection TLV SHOULD NOT be present and MUST be ignored when the M bit is set to 0. The Frequency Slot Selection TLV type is TBD2 (to be assigned by IANA). The TLVs carried in the SA Object use the PCEP TLV format defined in Section 7.1 of [RFC5440]; the Length of the Frequency Slot Selection TLV is 4. The Frequency Slot Selection TLV value field format is defined as: 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |S| Flag | FSA Method | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Flags (8 bits): bits 1-7 are currently unassigned. They MUST be set to zero by the sender and ignored by the receiver. S (Symmetry - 1 bit): This flag is only meaningful when the request is for a bidirectional LSP, i.e., when the B (Bi-directional) flag is set in the RP Object (see Section 7.4.1 of [RFC5440]) or B (Bidirectional LSP) flag is set in the LSP-EXTENDED-FLAG TLV in LSP Object (see Section 6.2.3 of [RFC9504]); otherwise it is ignored. The value 0 for S flag denotes requiring the same frequency slot in both directions; 1 denotes that different spectrum in the two directions is allowed. Frequency Slot Assignment (FSA) Method (8 bits): Zheng, et al. Expires 4 May 2027 [Page 7] Internet-Draft PCEP Ext for Flexi-grid October 2026 * 0: unspecified (any); This does not constrain the SA method used by a PCE. This value is implied when the Frequency Slot Selection TLV is absent. * 1: First-Fit. All the feasible frequency slots are numbered (based on 'n' parameter), and this SA method chooses the available frequency slot with the lowest index, where 'n' is the parameter in f = 193.1 THz + n x 0.00625 THz where 193.1THz is the ITU-T 'anchor frequency' and 'n' is a positive or negative integer including 0 [RFC7698]. * 2: Random. This SA method chooses a feasible frequency slot value of 'n' randomly. * 3-255: Unassigned. IANA maintains the "Frequency Slot Assignment Method Values" registry requested in Section 8.5. Reserved (16 bits): the Reserved field MUST be set to zero by the sender and ignored by the receiver. At most one instance of the Frequency Slot Selection TLV MAY be present in the SA Object. If more than one Frequency Slot Selection TLV is present, only the first instance is processed and subsequent instances MUST be ignored. 4.3. Frequency Slot Restriction TLV For any request that contains a frequency slot assignment, the requester (PCC) needs to be able to specify a restriction on the frequency slots to be used. This restriction is to be interpreted by the PCE as a constraint on the tuning ability of the origination laser transmitter or on any other maintenance related constraints. The Frequency Slot Restriction TLV type is TBD5 (to be assigned by IANA). This TLV MAY appear more than once in the SA Object to be able to specify multiple restrictions. When more than one Frequency Slot Restriction TLV applies to a link, the PCE MUST select a frequency slot that satisfies all of them. If no frequency slot does, the PCE responds with a NO-PATH object as described in Section 5.3. The TLV data is defined as shown in Figure 3. Zheng, et al. Expires 4 May 2027 [Page 8] Internet-Draft PCEP Ext for Flexi-grid October 2026 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Action | Count | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link Identifiers | | . . . | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Frequency Slot Restriction | // . . . . // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: Frequency Slot Restriction TLV value field Encoding The fields in the value field of Frequency Slot Restriction TLV are as follows: * Action (8 bits) The following values are defined: - 0 - Inclusive List indicates that one or more link identifiers are included in the Link Identifiers field. Each identifies a separate link that is part of the list. - 1 - Inclusive Range indicates that the Link Identifiers field defines a range of links. It contains two link identifiers. The first identifier indicates the start of the range (inclusive). The second identifier indicates the end of the range (inclusive). All links with numeric values between the bounds are considered to be part of the range. A value of zero in either position indicates that there is no bound on the corresponding portion of the range. The Action field MUST be set to 0 (Inclusive List) when unnumbered link identifiers are used, since a range of unnumbered link identifiers is not meaningful. * Count(8 bits): The number of the link identifiers . * Reserved: Reserved for future use (16 bits). It MUST be set to zero by the sender and ignored by the receiver. * Link Identifiers: Contains one or more Link Identifier Records. The interpretation of each record depends on the Action field. When Action=0 (Inclusive List), each record consists of a single link identifier. When Action=1 (Inclusive Range), there are exactly two records, indicating the start and end of the range (inclusive). All link identifiers in the same list MUST be of the same type. See Section 4.3.1 in [RFC8780] for Link Identifier Zheng, et al. Expires 4 May 2027 [Page 9] Internet-Draft PCEP Ext for Flexi-grid October 2026 encoding. If the Action value is unknown, if Action=1 and Count is not 2, or if the link identifiers in the same list are not of the same type, the PCE MUST reject the request rather than ignore the TLV. * Frequency Slot Restriction: this field is encoded as shown in Figure 4; its fields are described below. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | C.S. | Starting n | No. of Effective Bits | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Bitmap ... ~ ~ ... | padding bits ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | MBZ | C.F.G | S.W.G | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Min Slot Width | Max Slot Width | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 4: Frequency Slot Restriction Encoding The fields "C.S", "Starting n", "No. of Effective Bits", "Bitmap" and "padding bits" are as per Section 4.1.1. of [RFC8363]. Whereas the fields "C.F.G", "S.W.G", "Reserved", and "Min Slot Width" are as per Section 4.2. of [RFC8363]. The field "Max Slot Width" (16 bits) is a positive integer, whose value indicates the multiple of 2*C.S. (in GHz), in terms of the supported max slot width. The field MBZ (4-bits) MUST be set to zero and ignored on receipt. A PCC MAY add a frequency slot restriction that applies to all links by setting the Count field to zero and specifying just a set of frequency slots. IANA maintains the "Frequency Slot Restriction TLV Action Values" registry requested in Section 8.6. 5. Encoding of an RSA This section provides the encoding of an RSA, in the PCRep message or, for an LSP delegated to a stateful PCE, in the PCUpd message [RFC8231], for frequency slot allocation as discussed in Section 4. Zheng, et al. Expires 4 May 2027 [Page 10] Internet-Draft PCEP Ext for Flexi-grid October 2026 5.1. Spectrum Allocation TLV The Spectrum Allocation TLV type is TBD6 (to be assigned by IANA). The value portion of the TLV is defined as shown in Figure 5. The Spectrum Allocation TLV uses the Attribute TLV format of Section 3 of [RFC5420]: its Length field gives the total length of the TLV in octets, including the Type and Length fields. The size of the ERO Hop Attributes subobject limits a Hop Attributes TLV to 250 bytes (Section 2.1 of [RFC7570]); this bounds the number of Flexi-Grid Labels that a Spectrum Allocation TLV can carry. The Spectrum Allocation TLV is carried as an Attribute TLV [RFC5420] in the ERO Hop Attribute subobjects [RFC7570] in the Explicit Route Object (ERO) [RFC5440] to indicate the spectrum to be assigned on the identified link. 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved | Flags |M| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Link Identifier | | . . . | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Allocated Spectrum | // . . . . // +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 5: Spectrum Allocation TLV value field Encoding * Reserved (16 bits): The Reserved field MUST be set to zero by the sender and ignored by the receiver. * Flags (16 bits): bits 0-14 are unassigned. They MUST be set to zero by the sender and ignored by the receiver. * M (Mode - bit 15): 1 bit - 1 indicates the allocation is under Explicit Label Control: the Allocated Spectrum field carries exactly one Flexi-Grid Labels sub-field, which is the frequency slot (a single or a compound label) to be used on the link. Zheng, et al. Expires 4 May 2027 [Page 11] Internet-Draft PCEP Ext for Flexi-grid October 2026 - 0 indicates that the Allocated Spectrum field carries a set of candidates: the Allocated Spectrum field carries one or more Flexi-Grid Labels sub-fields, each of which is one candidate frequency slot (a single or a compound label) for the link, from which the slot is selected during signaling. * Link Identifier: Identifies the link ID for which the spectrum allocation is applied. See Section 4.3.1 in [RFC8780] for Link Identifier encoding. * Allocated Spectrum (variable): Indicates the spectrum allocated to the link associated with the link identifier. The field carries one or more Flexi-Grid Labels sub-fields, which encodes either a single label (see Section 4.1 of [RFC7699]), or a Composite label (see Section 4.3 of [RFC7699]). The encoding of the Flexi-Grid Labels sub-fields is as shown in Figure 6 below which matches the fields and description in Section 4.3 of [RFC7699]. Each 64-bit Flexi-Grid Label follows the encoding defined in Figure 1 of [RFC7699]. The Spectrum Allocation TLV is valid only in an ERO Hop Attributes subobject associated with an ERO subobject that identifies a link. The ERO Hop Attributes subobject containing the Spectrum Allocation TLV MUST be placed immediately after the link-identifying ERO subobject or after any Label subobjects associated with that link, otherwise, it is ignored. The ordering of the ERO Hop Attributes subobject relative to the Label subobjects associated with the same link is not significant for the interpretation of the Spectrum Allocation TLV. In either case, the Spectrum Allocation TLV specifies the spectrum allocation for the identified link. The Spectrum Allocation TLV specifies the spectrum allocation the PCE computed for the identified link. A node processing the ERO need not modify, add, or remove the Spectrum Allocation TLV. A PCE MAY provide a different spectrum allocation by generating a new or updated ERO using the applicable PCEP procedures. Processing of unrecognized Spectrum Allocation TLVs follows the procedures specified in Section 2.3 of [RFC7570], including the required and optional attribute processing rules of [RFC5420] according to the R bit of the enclosing ERO Hop Attributes subobject. Zheng, et al. Expires 4 May 2027 [Page 12] Internet-Draft PCEP Ext for Flexi-grid October 2026 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 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Length (4 + 8r) | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Grid | C.S. | Identifier | n | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | m | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ ~ ~ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Grid | C.S. | Identifier | n | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | m | Reserved | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 6: Flexi-Grid Labels sub-field Encoding The Length (16 bits) encodes the length of the Flexi-Grid Labels sub- field which is 4 plus 8 bytes for each 64-bit Flexi-Grid Label. The Reserved field (16-bits) MUST be set to zero by the sender and ignored by the receiver. The other fields are as per [RFC7699]. IANA maintains the "Spectrum Allocation TLV Flag Field" registry requested in Section 8.7. The Spectrum Allocation TLV is specific to PCEP. Its type is assigned from the "PCEP TLV Type Indicators" registry (Section 9.3), as for the Wavelength Allocation TLV of [RFC8780], and not from the RSVP-TE "Attributes TLV Space" registry. A PCC that uses the computed ERO for RSVP-TE signaling MUST NOT include the Spectrum Allocation TLV in the ERO that it signals; the allocated spectrum is signaled using the Label subobjects of Explicit Label Control or the flexi-grid signaling procedures of [RFC7792]. 5.2. Error Indicator To indicate errors associated with the RSA request, a new Error-Type (TBD7, Flexi-Grid RSA Error) and subsequent error-value are defined for inclusion in the PCEP-ERROR Object: * Error-Type=TBD7 (Flexi-Grid RSA Error); Error-value=1: if a PCE receives an RSA request and the PCE is not capable of RSA computation, the PCE MUST send a PCErr message with a PCEP-ERROR Object (Error-Type=TBD7) and an Error-value (Error-value=1). The PCE stops processing the request. The corresponding RSA computation MUST be cancelled at the PCC. Zheng, et al. Expires 4 May 2027 [Page 13] Internet-Draft PCEP Ext for Flexi-grid October 2026 Note that generic resource exhaustion at the PCE (e.g., insufficient memory) is not signaled via this Error-Type; the PCEP overload mechanism (see Section 7.14 of [RFC5440]) applies. 5.3. NO-PATH Indicator To communicate the reasons for not being able to find RSA for the path request, the NO-PATH Object can be used in the corresponding response. The format of the NO-PATH Object body is defined in [RFC5440]. The object MAY contain a NO-PATH-VECTOR TLV to provide additional information about why a path computation has failed. One new bit flag is defined to be carried in the Flags field in the NO-PATH-VECTOR TLV carried in the NO-PATH Object. * Bit TBD8: When set to 1, the PCE indicates no feasible path was found that meets all the optical spectrum constraints associated with the path computation request. This flag is specific to spectrum-related path computation failures; other path-computation failure reasons continue to use the existing PCEP NO-PATH mechanisms. 6. Operational Considerations Manageability of flexi-grid Routing and Spectrum Assignment (RSA) with PCE includes the following considerations: 6.1. Control of Function and Policy In addition to the parameters already listed in Section 8.1 of [RFC5440], a PCEP implementation SHOULD allow configuring the following PCEP session parameters on a PCC: * The ability to send a Flexi-Grid RSA request. In addition to the parameters already listed in Section 8.1 of [RFC5440], a PCEP implementation SHOULD allow configuring the following PCEP session parameters on a PCE: * The support for Flexi-Grid RSA. * A set of Flexi-Grid RSA specific policies (authorized sender, request rate limiter, etc). Zheng, et al. Expires 4 May 2027 [Page 14] Internet-Draft PCEP Ext for Flexi-grid October 2026 These parameters can be configured as default parameters for any PCEP session the PCEP speaker participates in, or can apply to a specific session with a given PCEP peer or a specific group of sessions with a specific group of PCEP peers. 6.2. Information and Data Models This document does not define YANG extensions. The PCEP YANG module [RFC9826] could be augmented to cover the Flexi-Grid RSA information introduced in this document, such as the S flag, the FSA method and the assignment mode; such extensions are outside the scope of this document. 6.3. Liveness Detection and Monitoring Liveness Detection and Monitoring Mechanisms defined in this document do not imply any new liveness detection and monitoring requirements in addition to those already listed in Section 8.3 of [RFC5440]. 6.4. Verifying Correct Operation Mechanisms defined in this document do not imply any new verification requirements in addition to those already listed in section 8.4 of [RFC5440]. 6.5. Requirements on Other Protocols and Functional Components The PCE Discovery mechanisms ([RFC5088] and [RFC5089]) can be used to advertise Flexi-Grid RSA path computation capabilities to PCCs. 6.6. Impact on Network Operation Mechanisms defined in this document do not imply any new network operation requirements in addition to those already listed in Section 8.6 of [RFC5440]. 7. Security Considerations The extensions defined in this document convey the frequency slots computed for an LSP on each link and the frequency slot restrictions of a PCC's transceivers. This information reveals spectrum usage and equipment capabilities that an operator might wish to keep private. An attacker able to modify PCEP messages could cause a frequency slot to be signaled that overlaps one already in use, disrupting existing optical connections on the affected links, or could cause requests to be rejected or mis-computed. Zheng, et al. Expires 4 May 2027 [Page 15] Internet-Draft PCEP Ext for Flexi-grid October 2026 These extensions are intended for use between PCEs and PCCs within a single administrative domain. To protect them against eavesdropping, modification, insertion and replay, PCEP sessions carrying them SHOULD be secured usingTransport Layer Security (TLS) [RFC8253] [RFC9916], as per the recommendations and the best current practices are in [RFC9325]. 8. IANA Considerations This document requests the following IANA actions. 8.1. New PCEP Object IANA is requested to allocate a new object class from the "PCEP Objects" registry (http://www.iana.org/assignments/pcep/ pcep.xhtml#pcep-objects) under the “Path Computation Element Protocol (PCEP) Numbers” registry group : +====================+======+========================+===========+ | Object-Class Value | Name | Object-Type | Reference | +====================+======+========================+===========+ | TBD1 | SA | 0: Reserved | | | | +------------------------| | | | | 1: Spectrum Assignment | [This.I-D]| | | +------------------------| | | | | 2-15: Unassigned | | +--------------------+------+------------------------+-----------+ 8.2. SA Object Flag Field IANA is requested to create the "SA Object Flag Field" registry within the "Path Computation Element Protocol (PCEP) Numbers" registry. New values are to be assigned by IETF Review [RFC8126]. Bits are numbered from bit 0 as the most significant bit. +=======+==========================+===========+ | Bit | Description | Reference | +=======+==========================+===========+ | 0-14 | Unassigned | | +-------+--------------------------|[This.I-D] | | 15 | Spectrum Assignment Mode | | +-------+--------------------------+-----------+ Zheng, et al. Expires 4 May 2027 [Page 16] Internet-Draft PCEP Ext for Flexi-grid October 2026 8.3. New PCEP TLVs IANA is requested to allocate three new TLV types, the Frequency Slot Selection TLV, the Frequency Slot Restriction TLV, the Spectrum Allocation TLV. All these TLVs are from the "PCEP TLV Type Indicators" registry (http://www.iana.org/assignments/pcep/ pcep.xhtml#pcep-tlv-type-indicators) under the “Path Computation Element Protocol (PCEP) Numbers” registry group. +=======+============================+===========+ | Value | Description | Reference | +=======+============================+===========+ | TBD2 | Frequency Slot Selection | [This.I-D]| +=======+============================+===========+ | TBD5 | Frequency Slot Restriction | [This.I-D]| +=======+============================+===========+ | TBD6 | Spectrum Allocation | [This.I-D]| +-------+----------------------------+-----------+ 8.4. Frequency Slot Selection TLV Flag Field IANA is requested to create the "Frequency Slot Selection TLV Flag Field" registry within the "Path Computation Element Protocol (PCEP) Numbers" registry. New values are to be assigned by IETF Review [RFC8126]. Bits are numbered from bit 0 as the most significant bit. +=======+==========================+===========+ | Bit | Description | Reference | +=======+==========================+===========+ | 0 | Symmetry | | +-------+--------------------------| [This.I-D]| | 1-7 | Unassigned | | +-------+--------------------------+-----------+ 8.5. Frequency Slot Assignment Method Values IANA is requested to create the "Frequency Slot Assignment Method Values" registry within the "Path Computation Element Protocol (PCEP) Numbers" registry. New values are to be assigned by IETF Review [RFC8126]. Zheng, et al. Expires 4 May 2027 [Page 17] Internet-Draft PCEP Ext for Flexi-grid October 2026 +=========+=============+===========+ | Value | Description | Reference | +=========+=============+===========+ | 0 | Unspecified | [This.I-D]| +---------+-------------+-----------+ | 1 | First-Fit | [This.I-D]| +---------+-------------+-----------+ | 2 | Random | [This.I-D]| +---------+-------------+-----------+ | 3-255 | Unassigned | | +---------+-------------+-----------+ 8.6. Frequency Slot Restriction TLV Action Values IANA is requested to create the "Frequency Slot Restriction TLV Action Values" registry within the "Path Computation Element Protocol (PCEP) Numbers" registry. New values are to be assigned by IETF Review [RFC8126]. +=======+=================+===========+ | Value | Description | Reference | +=======+=================+===========+ | 0 | Inclusive List | [This.I-D]| +-------+-----------------+-----------+ | 1 | Inclusive Range | [This.I-D]| +-------+-----------------+-----------+ | 2-255 | Unassigned | | +-------+-----------------+-----------+ 8.7. Spectrum Allocation TLV Flag Field IANA is requested to create the "Spectrum Allocation TLV Flag Field" registry within the "Path Computation Element Protocol (PCEP) Numbers" registry. New values are to be assigned by IETF Review [RFC8126]. Bits are numbered from bit 0 as the most significant bit. +=======+==========================+===========+ | Bit | Description | Reference | +=======+==========================+===========+ | 0-14 | Unassigned | | +-------+--------------------------| [This.I-D]| | 15 | Spectrum Allocation Mode | | +-------+--------------------------+-----------+ Zheng, et al. Expires 4 May 2027 [Page 18] Internet-Draft PCEP Ext for Flexi-grid October 2026 8.8. New No-Path Reasons IANA is requested to allocate a new bit flag from the PCEP "NO-PATH- VECTOR TLV Flag Field" registry (http://www.iana.org/assignments/pcep/pcep.xhtml#no-path-vector-tlv) under the “Path Computation Element Protocol (PCEP) Numbers” registry group. +=======+=============================+===========+ | Bit | Description | Reference | +=======+=============================+===========+ | TBD8 | No spectrum constraints met | [This.I-D]| +-------+-----------------------------+-----------+ 8.9. New Error-Types and Error-Values IANA is requested to allocate a new Error-Type and Error-value from the "PCEP-ERROR Object Error Types and Values" registry (http://www.iana.org/assignments/pcep/pcep.xhtml#pcep-error-object) under the “Path Computation Element Protocol (PCEP) Numbers” registry group. +============+================+====================+===========+ | Error-Type | Meaning | Error-value | Reference | +============+================+====================+===========+ | TBD7 | Flexi-Grid RSA | 0: Unassigned | [This.I-D]| | | Error | 1: RSA computation | | | | | not supported | | | | | 2: Unassgiend range| | +------------+----------------+--------------------+-----------+ 9. Acknowledgements Thanks to Francesco Lazzeri for the technical contribution, and Quan Xiong, Dhruv Dhody and Adrian Farrel for useful comments. Thanks to Ketan Talaulikar for his detailed AD review and helpful suggestions. 10. Contributors' Address Ricard Vilalta CTTC Spain Email: ricard.vilalta@cttc.es 11. References Zheng, et al. Expires 4 May 2027 [Page 19] Internet-Draft PCEP Ext for Flexi-grid October 2026 11.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [RFC3471] Berger, L., Ed., "Generalized Multi-Protocol Label Switching (GMPLS) Signaling Functional Description", RFC 3471, DOI 10.17487/RFC3471, February 2003, . [RFC5420] Farrel, A., Ed., Papadimitriou, D., Vasseur, JP., and A. Ayyangar, "Encoding of Attributes for MPLS LSP Establishment Using Resource Reservation Protocol Traffic Engineering (RSVP-TE)", RFC 5420, DOI 10.17487/RFC5420, February 2009, . [RFC5440] Vasseur, JP., Ed. and JL. Le Roux, Ed., "Path Computation Element (PCE) Communication Protocol (PCEP)", RFC 5440, DOI 10.17487/RFC5440, March 2009, . [RFC5511] Farrel, A., "Routing Backus-Naur Form (RBNF): A Syntax Used to Form Encoding Rules in Various Routing Protocol Specifications", RFC 5511, DOI 10.17487/RFC5511, April 2009, . [RFC7570] Margaria, C., Ed., Martinelli, G., Balls, S., and B. Wright, "Label Switched Path (LSP) Attribute in the Explicit Route Object (ERO)", RFC 7570, DOI 10.17487/RFC7570, July 2015, . [RFC7699] Farrel, A., King, D., Li, Y., and F. Zhang, "Generalized Labels for the Flexi-Grid in Lambda Switch Capable (LSC) Label Switching Routers", RFC 7699, DOI 10.17487/RFC7699, November 2015, . [RFC7792] Zhang, F., Zhang, X., Farrel, A., Gonzalez de Dios, O., and D. Ceccarelli, "RSVP-TE Signaling Extensions in Support of Flexi-Grid Dense Wavelength Division Multiplexing (DWDM) Networks", RFC 7792, DOI 10.17487/RFC7792, March 2016, . Zheng, et al. Expires 4 May 2027 [Page 20] Internet-Draft PCEP Ext for Flexi-grid October 2026 [RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for Writing an IANA Considerations Section in RFCs", BCP 26, RFC 8126, DOI 10.17487/RFC8126, June 2017, . [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, May 2017, . [RFC8231] Crabbe, E., Minei, I., Medved, J., and R. Varga, "Path Computation Element Communication Protocol (PCEP) Extensions for Stateful PCE", RFC 8231, DOI 10.17487/RFC8231, September 2017, . [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, . [RFC8363] Zhang, X., Zheng, H., Casellas, R., Gonzalez de Dios, O., and D. Ceccarelli, "GMPLS OSPF-TE Extensions in Support of Flexi-Grid Dense Wavelength Division Multiplexing (DWDM) Networks", RFC 8363, DOI 10.17487/RFC8363, May 2018, . [RFC8779] Margaria, C., Ed., Gonzalez de Dios, O., Ed., and F. Zhang, Ed., "Path Computation Element Communication Protocol (PCEP) Extensions for GMPLS", RFC 8779, DOI 10.17487/RFC8779, July 2020, . [RFC8780] Lee, Y., Ed. and R. Casellas, Ed., "The Path Computation Element Communication Protocol (PCEP) Extension for Wavelength Switched Optical Network (WSON) Routing and Wavelength Assignment (RWA)", RFC 8780, DOI 10.17487/RFC8780, July 2020, . [RFC9325] Sheffer, Y., Ed., Saint-Andre, P., Ed., and T. T. Fossati, Ed., "Recommendations for Secure Use of Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", BCP 195, RFC 9325, DOI 10.17487/RFC9325, November 2022, . Zheng, et al. Expires 4 May 2027 [Page 21] Internet-Draft PCEP Ext for Flexi-grid October 2026 [RFC9504] Lee, Y., Zheng, H., Gonzalez de Dios, O., Lopez, V., and Z. Ali, "Path Computation Element Communication Protocol (PCEP) Extensions for Stateful PCE Usage in GMPLS- Controlled Networks", RFC 9504, DOI 10.17487/RFC9504, December 2023, . [RFC9916] Dhody, D., Turner, S., and R. Housley, "Updates to the Usage of TLS to Provide a Secure Transport for the Path Computation Element Communication Protocol (PCEP)", RFC 9916, DOI 10.17487/RFC9916, July 2026, . 11.2. Informative References [ITU-T_G.694.1] ITU-, T., "SERIES G: TRANSMISSION SYSTEMS AND MEDIA, DIGITAL SYSTEMS AND NETWORKS; Digital networks; Spectral grids for WDM applications: DWDM frequency grid", ITU-T Rec. G.694.1 , October 2020, . [RFC4655] Farrel, A., Vasseur, J.-P., and J. Ash, "A Path Computation Element (PCE)-Based Architecture", RFC 4655, DOI 10.17487/RFC4655, August 2006, . [RFC4657] Ash, J., Ed. and J.L. Le Roux, Ed., "Path Computation Element (PCE) Communication Protocol Generic Requirements", RFC 4657, DOI 10.17487/RFC4657, September 2006, . [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, . [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, . [RFC6163] Lee, Y., Ed., Bernstein, G., Ed., and W. Imajuku, "Framework for GMPLS and Path Computation Element (PCE) Control of Wavelength Switched Optical Networks (WSONs)", RFC 6163, DOI 10.17487/RFC6163, April 2011, . Zheng, et al. Expires 4 May 2027 [Page 22] Internet-Draft PCEP Ext for Flexi-grid October 2026 [RFC7449] Lee, Y., Ed., Bernstein, G., Ed., Martensson, J., Takeda, T., Tsuritani, T., and O. Gonzalez de Dios, "Path Computation Element Communication Protocol (PCEP) Requirements for Wavelength Switched Optical Network (WSON) Routing and Wavelength Assignment", RFC 7449, DOI 10.17487/RFC7449, February 2015, . [RFC7698] Gonzalez de Dios, O., Ed., Casellas, R., Ed., Zhang, F., Fu, X., Ceccarelli, D., and I. Hussain, "Framework and Requirements for GMPLS-Based Control of Flexi-Grid Dense Wavelength Division Multiplexing (DWDM) Networks", RFC 7698, DOI 10.17487/RFC7698, November 2015, . [RFC8281] Crabbe, E., Minei, I., Sivabalan, S., and R. Varga, "Path Computation Element Communication Protocol (PCEP) Extensions for PCE-Initiated LSP Setup in a Stateful PCE Model", RFC 8281, DOI 10.17487/RFC8281, December 2017, . [RFC9826] Dhody, D., Beeram, V., Hardwick, J., and J. Tantsura, "A YANG Data Model for the Path Computation Element Communication Protocol (PCEP)", BCP 205, RFC 9826, DOI 10.17487/RFC9826, September 2025, . Authors' Addresses Haomian Zheng (editor) Huawei Technologies Co., Ltd. Email: zhenghaomian@huawei.com Young Lee CRU Email: younglee.tx@gmail.com Ramon Casellas CTTC Email: ramon.casellas@cttc.es Daniele Ceccarelli Cisco Email: dceccare@cisco.com Zheng, et al. Expires 4 May 2027 [Page 23]