YANG Data Model for Maximum Segment Identifier (SID) Depth (MSD) Types and MPLS MSD

YANG Data Model for Maximum Segment Identifier (SID) Depth (MSD) Types and MPLS MSD Futurewei Technologies

yingzhen.ietf@gmail.com

LabN Consulting, L.L.C.

acee.ietf@gmail.com

Cisco Systems

slitkows.ietf@gmail.com

Nvidia

jefftant.ietf@gmail.com

RTG mpls example This document defines two YANG modules. The first module is the initial version of the IANA-maintained YANG module for Maximum Segment Identifier (SID) Depth (MSD) Types, which includes identities for both the MPLS data plane and Segment Routing over IPv6 (SRv6) data plane. The second module augments the IETF MPLS YANG data model to provide support for MPLS MSDs as defined in RFCs 8476 and 8491.

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) 2025 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

. Overview
. Design of the Model
- . IANA MSD Types Module
- . IETF MPLS MSD Module
. Tree for IETF MPLS MSD Types YANG Module
. YANG Modules
- . IANA-Maintained Module for MSD-Types
- . MPLS MSD YANG
. Security Considerations
. IANA Considerations
- . Registering YANG Modules
- . IANA MSD-Types Module
. References
- . Normative References
- . Informative References
Acknowledgements
Authors' Addresses

Overview There are two YANG modules defined in this document. Module iana-msd-types defines the identities for Maximum SID Depth (MSD) Types as per the "IGP MSD-Types" IANA registry , which includes MSD types for both the MPLS and SRv6 data planes. This document also defines module ietf-mpls-msd augmenting the IETF MPLS YANG data model , which augments the routing RIB data model to provide operational state for various MSDs for the MPLS data plane. The module augments the base MPLS model with a list of various types of Node MSDs as well as various types of Link MSDs. The YANG modules in this document conform to the Network Management Datastore Architecture (NMDA) .

Design of the Model

IANA MSD Types Module IANA has created a registry titled "IGP MSD-Types" under the "Interior Gateway Protocol (IGP) Parameters" registry group to identify MSD-Types. Module iana-msd-types is an IANA-maintained module, which defines the identities for the MSD-Types as in the IANA "IGP MSD-Types" registry. This module references , , , , , , and . On top of the base identity "msd-base", identity "msd-base-mpls" is defined to serve as the base for MSD types for the MPLS data plane, and identity "msd-base-srh" is defined to serve as the base for MSD types for the Segment Routing Header (SRH) in the IPv6 data plane. This module is maintained by IANA and will be updated if and when there is any change to the registry.

IETF MPLS MSD Module Module ietf-mpls-msd augments the base MPLS model , and it provides support of different types of MSDs in the MPLS data plane. As defined in , a Link MSD is the number of SIDs supported by a node's link, while a Node MSD is the smallest MSD supported by the node across all its links. The module defines lists of MSDs and their MSD Types for a node and its links. Please note that these are read-only data nodes exposing a node's hardware capability.

Tree for IETF MPLS MSD Types YANG Module This document uses the graphical representation of data models per . module: ietf-mpls-msd augment /rt:routing/mpls:mpls: +--ro node-msds +--ro node-msd* [msd-type] +--ro msd-type identityref +--ro msd-value? uint8 augment /rt:routing/mpls:mpls/mpls:interfaces/mpls:interface: +--ro link-msds +--ro link-msd* [msd-type] +--ro msd-type identityref +--ro msd-value? uint8

YANG Modules There are two YANG modules defined in this document.

IANA-Maintained Module for MSD-Types This document defines the initial version of the IANA-maintained YANG module for MSD Types that mirrors the IANA "IGP MSD-Types" registry . module iana-msd-types { yang-version 1.1; namespace "urn:ietf:params:xml:ns:yang:iana-msd-types"; prefix iana-msd-types; organization "Internet Assigned Numbers Authority (IANA)"; contact "Internet Assigned Numbers Authority ICANN 12025 Waterfront Drive, Suite 300 Los Angeles, CA 90094-2536 United States of America Tel: +1 310 301 5800 <mailto:iana@iana.org>"; description "The YANG module defines the identities for Maximum Segment Identifier (SID) Depth (MSD) Types. This YANG module is maintained by IANA and reflects the 'IGP MSD-Types' registry. Copyright (c) 2025 IETF Trust and the persons identified as authors of the code. All rights reserved. Redistribution and use in source and binary forms, with or without modification, is permitted pursuant to, and subject to the license terms contained in, the Revised BSD License set forth in Section 4.c of the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info). This initial version of this YANG module is part of RFC 9702 (https://www.rfc-editor.org/info/rfc9702); see the RFC itself for full legal notices. The latest version of this YANG module is available at https://www.iana.org/assignments/yang-parameters."; revision 2025-01-10 { description "Initial Version"; reference "RFC 9702: YANG Data Model for Maximum Segment Identifier (SID) Depth Types and MPLS Maximum SID Depth"; } identity msd-base { description "Base identity for Maximum SID Depth (MSD) Type. The MSD type definition is defined in the IANA 'IGP MSD-Types' registry."; } identity msd-base-mpls { base msd-base; description "Base identity of MSD types for the MPLS data plane."; } identity base-mpls-imposition-msd { base msd-base-mpls; description "Base MPLS Imposition MSD."; reference "RFC 8491: Signaling Maximum SID Depth (MSD) Using IS-IS RFC 8476: Signaling Maximum SID Depth (MSD) Using OSPF RFC 8664: Path Computation Element Communication Protocol (PCEP) Extensions for Segment Routing RFC 8814: Signaling Maximum SID Depth (MSD) Using the Border Gateway Protocol - Link State"; } identity erld-msd { base msd-base-mpls; description "msd-erld is defined to advertise the Entropy Readable Label Depth (ERLD)."; reference "RFC 8662: Entropy Label for Source Packet Routing in Networking (SPRING) Tunnels RFC 9088: Signaling Entropy Label Capability and Entropy Readable Label Depth Using IS-IS"; } identity msd-base-srh { base msd-base; description "Base identity of MSD types for Segment Routing Header (SRH)."; } identity srh-max-sl { base msd-base-srh; description "The Maximum Segments Left MSD type."; reference "RFC 9352: IS-IS Extensions to Support Segment Routing over the IPv6 Data Plane"; } identity srh-max-end-pop { base msd-base-srh; description "The Maximum End Pop MSD Type."; reference "RFC 9352: IS-IS Extensions to Support Segment Routing over the IPv6 Data Plane"; } identity srh-max-h-encaps { base msd-base-srh; description "The Maximum H.Encaps MSD Type."; reference "RFC 9352: IS-IS Extensions to Support Segment Routing over the IPv6 Data Plane"; } identity srh-max-end-d { base msd-base-srh; description "The Maximum End D MSD Type."; reference "RFC 9352: IS-IS Extensions to Support Segment Routing over the IPv6 Data Plane"; } }

MPLS MSD YANG This document also defines a YANG module for MSD extensions to the MPLS base model as defined in . module ietf-mpls-msd { yang-version 1.1; namespace "urn:ietf:params:xml:ns:yang:ietf-mpls-msd"; prefix mpls-msd; import ietf-routing { prefix rt; reference "RFC 8349: A YANG Data Model for Routing Management (NMDA Version)"; } import ietf-mpls { prefix mpls; reference "RFC 8960: A YANG Data Model for MPLS Base"; } import iana-msd-types { prefix iana-msd-types; } organization "IETF Multiprotocol Label Switching (MPLS) Working Group"; contact "WG Web: <https://datatracker.ietf.org/wg/mpls/> WG List: <mailto:mpls@ietf.org> Author: Yingzhen Qu <mailto:yingzhen.ietf@gmail.com> Author: Acee Lindem <mailto:acee.ietf@gmail.com> Author: Stephane Litkowski <mailto:slitkows.ietf@gmail.com> Author: Jeff Tantsura <mailto:jefftant.ietf@gmail.com> "; description "This YANG module augments the base MPLS model, and it is to provide support for different types of Maximum SID Depth (MSD). This YANG module conforms to the Network Management Datastore Architecture (NMDA) as described in RFC 8342. Copyright (c) 2025 IETF Trust and the persons identified as authors of the code. All rights reserved. Redistribution and use in source and binary forms, with or without modification, is permitted pursuant to, and subject to the license terms contained in, the Revised BSD License set forth in Section 4.c of the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info). This version of this YANG module is part of RFC 9702; see the RFC itself for full legal notices."; revision 2025-01-10 { description "Initial Version"; reference "RFC 9702: YANG Data Model for Maximum Segment Identifier (SID) Depth Types and MPLS Maximum SID Depth"; } grouping msd-type-value { description "Grouping for MSD type and value."; leaf msd-type { type identityref { base iana-msd-types:msd-base-mpls; } description "MSD types. The MSD type is defined in IANA 'IGP MSD-Types' registry."; } leaf msd-value { type uint8; description "MSD value, in the range of 0-255. 0 represents the lack of ability to support a SID stack of any depth."; } } augment "/rt:routing/mpls:mpls" { description "This module augments MPLS data model (RFC 8960) with Node MSD."; container node-msds { config false; description "Maximum SID Depth (MSD) of a node."; list node-msd { key "msd-type"; uses msd-type-value; description "List of different types of Node MSDs. For the same type, the value of Node MSD is the smallest among Link MSD values."; } } } augment "/rt:routing/mpls:mpls/mpls:interfaces/mpls:interface" { description "This module augments MPLS data model (RFC 8960) with Link MSD."; container link-msds { config false; description "Maximum SID Depth (MSD) of an interface."; list link-msd { key "msd-type"; uses msd-type-value; description "List of different types of MSDs on the link."; } } } }

Security Considerations The YANG modules specified in this document define a schema for data that is designed to be accessed via network management protocols such as NETCONF or RESTCONF . The lowest NETCONF layer is the secure transport layer, and the mandatory-to-implement secure transport is Secure Shell (SSH) . The lowest RESTCONF layer is HTTPS, and the mandatory-to-implement secure transport is TLS . The Network Configuration Access Control Model (NACM) provides the means to restrict access for particular NETCONF or RESTCONF users to a pre-configured subset of all available NETCONF or RESTCONF protocol operations and content. Some of the readable data nodes in the ietf-mpls-msd YANG module may be considered sensitive or vulnerable in some network environments. It is thus important to control read access (e.g., via get, get-config, or notification) to these data nodes. These are the subtrees and data nodes and their sensitivity/vulnerability: /rt:routing/mpls:mpls/msd/node-msds /rt:routing/mpls:mpls/msd/link-msds Exposure of the node's maximum SID depth may be useful in mounting a Denial-of-Service (DoS) attack by sending packets to the node that the router can't process. The iana-msd-types YANG module defines a set of identities that mirror the IANA "IGP MSD-Types" registry. These identities are intended to be reused by other YANG modules. The module by itself does not expose any data nodes that are writable or readable. As such, there are no additional security issues related to this YANG module that need to be considered.

IANA Considerations

Registering YANG Modules This document registers URIs in the IETF XML registry as defined in . The following registrations have been made:

URI:: urn:ietf:params:xml:ns:yang:iana-msd-types
Registrant Contact:: The IESG.
XML:: N/A; the requested URI is an XML namespace.

URI:: urn:ietf:params:xml:ns:yang:ietf-mpls-msd
Registrant Contact:: The IESG.
XML:: N/A; the requested URI is an XML namespace.

This document registers the YANG modules in the "YANG Module Names" registry as defined in .

Name:: iana-msd-types
Maintained by IANA?: Y
Namespace:: urn:ietf:params:xml:ns:yang:iana-msd-types
Prefix:: iana-msd-types
Reference:: RFC 9702

Name:: ietf-mpls-msd
Maintained by IANA?: N
Namespace:: urn:ietf:params:xml:ns:yang:ietf-mpls-msd
Prefix:: mpls-msd
Reference:: RFC 9702

IANA MSD-Types Module This document defines the initial version of the IANA-maintained "iana-msd-types" YANG module (). The latest version of the YANG module is available from the "YANG Parameters" registry . IANA has added this note to the "YANG Module Names" registry:

New values must not be directly added to the "iana-msd-types" YANG module. They must instead be added to the "IGP MSD-Types" registry in the "Interior Gateway Protocol (IGP) Parameters" registry group .

The identities defined in the iana-msd-types YANG module are organized hierarchically based on the data plane. In this initial version, only MPLS and SRv6 data planes are supported, hence "msd-base-mpls" and "msd-base-srh" are defined. When a new data plane is added to the "IGP MSD-Types" registry, a new "identity" statement should be added to the "iana-msd-types" YANG module. The name of the "identity" is the prefix "msd-base-" plus a lowercase version of the data plane name. The identity statement should have the following sub-statements defined:

"base":: Contains 'msd-base'.
"description":: Replicates the description from "msd-base-mpls" and changes the corresponding data plane name.
"reference":: Replicates the reference from the registry with the title of the document added.

When an MSD type is added to the "IGP MSD-Types" registry, a new "identity" statement must be added to the "iana-msd-types" YANG module. The name of the "identity" is a lowercase version of the type name provided in the name with the space replaced with "-". The "identity" statement should have the following sub-statements defined:

"base":: Contains the base name of the corresponding data plane.
"description":: Replicates the name from the registry.
"reference":: Replicates the reference from the registry with the title of the document added.

Unassigned or reserved values are not present in the module. When the "iana-msd-types" YANG module is updated, a new "revision" statement with a unique revision date must be added in front of the existing revision statements. IANA has added a "Data Plane" column to the "IGP MSD-Types" registry. This will unequivocally identify the base identity for MSD-Types. The values for the "Data Plane" column for existing MSD-Types are:

Value	Name	Data Plane
0	Reserved
1	Base MPLS Imposition MSD	MPLS
2	ERLD-MSD	MPLS
3-40	Unassigned
41	SRH Max SL	SRv6
42	SRH Max End Pop	SRv6
43	Unassigned
44	SRH Max H.encaps	SRv6
45	SRH Max End	SRv6
46-250	Unassigned
251-254	Experimental Use
255	Reserved

IANA has added this note to :

When this registry is modified, the YANG module "iana-msd-types" must be updated as defined in RFC 9702.

References Normative References IGP MSD-Types IANA YANG Module Names IANA The IETF XML Registry This document describes an IANA maintained registry for IETF standards which use Extensible Markup Language (XML) related items such as Namespaces, Document Type Declarations (DTDs), Schemas, and Resource Description Framework (RDF) Schemas. YANG - A Data Modeling Language for the Network Configuration Protocol (NETCONF) YANG is a data modeling language used to model configuration and state data manipulated by the Network Configuration Protocol (NETCONF), NETCONF remote procedure calls, and NETCONF notifications. [STANDARDS-TRACK] Network Configuration Protocol (NETCONF) The Network Configuration Protocol (NETCONF) defined in this document provides mechanisms to install, manipulate, and delete the configuration of network devices. It uses an Extensible Markup Language (XML)-based data encoding for the configuration data as well as the protocol messages. The NETCONF protocol operations are realized as remote procedure calls (RPCs). This document obsoletes RFC 4741. [STANDARDS-TRACK] Using the NETCONF Protocol over Secure Shell (SSH) This document describes a method for invoking and running the Network Configuration Protocol (NETCONF) within a Secure Shell (SSH) session as an SSH subsystem. This document obsoletes RFC 4742. [STANDARDS-TRACK] The YANG 1.1 Data Modeling Language YANG is a data modeling language used to model configuration data, state data, Remote Procedure Calls, and notifications for network management protocols. This document describes the syntax and semantics of version 1.1 of the YANG language. YANG version 1.1 is a maintenance release of the YANG language, addressing ambiguities and defects in the original specification. There are a small number of backward incompatibilities from YANG version 1. This document also specifies the YANG mappings to the Network Configuration Protocol (NETCONF). RESTCONF Protocol This document describes an HTTP-based protocol that provides a programmatic interface for accessing data defined in YANG, using the datastore concepts defined in the Network Configuration Protocol (NETCONF). Network Configuration Access Control Model The standardization of network configuration interfaces for use with the Network Configuration Protocol (NETCONF) or the RESTCONF protocol requires a structured and secure operating environment that promotes human usability and multi-vendor interoperability. There is a need for standard mechanisms to restrict NETCONF or RESTCONF protocol access for particular users to a preconfigured subset of all available NETCONF or RESTCONF protocol operations and content. This document defines such an access control model. This document obsoletes RFC 6536. Network Management Datastore Architecture (NMDA) Datastores are a fundamental concept binding the data models written in the YANG data modeling language to network management protocols such as the Network Configuration Protocol (NETCONF) and RESTCONF. This document defines an architectural framework for datastores based on the experience gained with the initial simpler model, addressing requirements that were not well supported in the initial model. This document updates RFC 7950. A YANG Data Model for Routing Management (NMDA Version) This document specifies three YANG modules and one submodule. Together, they form the core routing data model that serves as a framework for configuring and managing a routing subsystem. It is expected that these modules will be augmented by additional YANG modules defining data models for control-plane protocols, route filters, and other functions. The core routing data model provides common building blocks for such extensions -- routes, Routing Information Bases (RIBs), and control-plane protocols. The YANG modules in this document conform to the Network Management Datastore Architecture (NMDA). This document obsoletes RFC 8022. The Transport Layer Security (TLS) Protocol Version 1.3 This document specifies version 1.3 of the Transport Layer Security (TLS) protocol. TLS allows client/server applications to communicate over the Internet in a way that is designed to prevent eavesdropping, tampering, and message forgery. This document updates RFCs 5705 and 6066, and obsoletes RFCs 5077, 5246, and 6961. This document also specifies new requirements for TLS 1.2 implementations. Signaling Maximum SID Depth (MSD) Using OSPF This document defines a way for an Open Shortest Path First (OSPF) router to advertise multiple types of supported Maximum SID Depths (MSDs) at node and/or link granularity. Such advertisements allow entities (e.g., centralized controllers) to determine whether a particular Segment Identifier (SID) stack can be supported in a given network. This document only refers to the Signaling MSD as defined in RFC 8491, but it defines an encoding that can support other MSD types. Here, the term "OSPF" means both OSPFv2 and OSPFv3. Signaling Maximum SID Depth (MSD) Using IS-IS This document defines a way for an Intermediate System to Intermediate System (IS-IS) router to advertise multiple types of supported Maximum SID Depths (MSDs) at node and/or link granularity. Such advertisements allow entities (e.g., centralized controllers) to determine whether a particular Segment ID (SID) stack can be supported in a given network. This document only defines one type of MSD: Base MPLS Imposition. However, it defines an encoding that can support other MSD types. This document focuses on MSD use in a network that is Segment Routing (SR) enabled, but MSD may also be useful when SR is not enabled. A YANG Data Model for MPLS Base This document contains a specification of the MPLS base YANG data model. The MPLS base YANG data model serves as a base framework for configuring and managing an MPLS switching subsystem on an MPLS-enabled router. It is expected that other MPLS YANG data models (e.g., MPLS Label Switched Path (LSP) static, LDP, or RSVP-TE YANG data models) will augment the MPLS base YANG data model. Signaling Entropy Label Capability and Entropy Readable Label Depth Using IS-IS Multiprotocol Label Switching (MPLS) has defined a mechanism to load-balance traffic flows using Entropy Labels (EL). An ingress Label Switching Router (LSR) cannot insert ELs for packets going into a given Label Switched Path (LSP) unless an egress LSR has indicated via signaling that it has the capability to process ELs, referred to as the Entropy Label Capability (ELC), on that LSP. In addition, it would be useful for ingress LSRs to know each LSR's capability for reading the maximum label stack depth and performing EL-based load-balancing, referred to as Entropy Readable Label Depth (ERLD). This document defines a mechanism to signal these two capabilities using IS-IS and Border Gateway Protocol - Link State (BGP-LS). IS-IS Extensions to Support Segment Routing over the IPv6 Data Plane The Segment Routing (SR) architecture allows a flexible definition of the end-to-end path by encoding it as a sequence of topological elements called "segments". It can be implemented over the MPLS or the IPv6 data plane. This document describes the IS-IS extensions required to support SR over the IPv6 data plane. This document updates RFC 7370 by modifying an existing registry. Informative References YANG Tree Diagrams This document captures the current syntax used in YANG module tree diagrams. The purpose of this document is to provide a single location for this definition. This syntax may be updated from time to time based on the evolution of the YANG language. Entropy Label for Source Packet Routing in Networking (SPRING) Tunnels Segment Routing (SR) leverages the source-routing paradigm. A node steers a packet through an ordered list of instructions, called segments. Segment Routing can be applied to the Multiprotocol Label Switching (MPLS) data plane. Entropy labels (ELs) are used in MPLS to improve load-balancing. This document examines and describes how ELs are to be applied to Segment Routing MPLS. Path Computation Element Communication Protocol (PCEP) Extensions for Segment Routing 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. Signaling Maximum SID Depth (MSD) Using the Border Gateway Protocol - Link State This document defines a way for a Border Gateway Protocol - Link State (BGP-LS) speaker to advertise multiple types of supported Maximum SID Depths (MSDs) at node and/or link granularity. Such advertisements allow entities (e.g., centralized controllers) to determine whether a particular Segment Identifier (SID) stack can be supported in a given network.

Acknowledgements The YANG data model was developed using the suite of YANG tools written and maintained by numerous authors. We would like to thank , , , , and for their reviews and comments.