RFC 9702 PROPOSED STANDARD YANG · MPLS · SEGMENT ROUTING
MSD YANG Model: YANG Data Model for Maximum Segment Identifier (SID) Depth (MSD) Types and MPLS MSD
Two YANG modules that let routers report how many segment labels they can handle, with types kept in step with an IANA registry.
RFC EDITOR
Qu, Y., Lindem, A., Litkowski, S., and J. Tantsura, "YANG Data Model for Maximum Segment Identifier (SID) Depth (MSD) Types and MPLS MSD", RFC 9702, DOI 10.17487/RFC9702, January 2025, <https://www.rfc-editor.org/info/rfc9702>.
BIBTEX
@misc{rfc9702,
series = {Request for Comments},
number = 9702,
howpublished = {RFC 9702},
publisher = {RFC Editor},
doi = {10.17487/RFC9702},
url = {https://www.rfc-editor.org/info/rfc9702},
author = {Y. Qu and A. Lindem and S. Litkowski and J. Tantsura},
title = {{YANG Data Model for Maximum Segment Identifier (SID) Depth (MSD) Types and MPLS MSD}},
pagetotal = 14,
year = 2025,
month = jan,
} RFC text
Contents
Internet Engineering Task Force (IETF) Y. Qu
Request for Comments: 9702 Futurewei Technologies
Category: Standards Track A. Lindem
ISSN: 2070-1721 LabN Consulting, L.L.C.
S. Litkowski
Cisco Systems
J. Tantsura
Nvidia
January 2025
YANG Data Model for Maximum Segment Identifier (SID) Depth (MSD) Types
and MPLS MSD
Abstract
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
https://www.rfc-editor.org/info/rfc9702.
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
(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. Overview
2. Design of the Model
2.1. IANA MSD Types Module
2.2. IETF MPLS MSD Module
3. Tree for IETF MPLS MSD Types YANG Module
4. YANG Modules
4.1. IANA-Maintained Module for MSD-Types
4.2. MPLS MSD YANG
5. Security Considerations
6. IANA Considerations
6.1. Registering YANG Modules
6.2. IANA MSD-Types Module
7. References
7.1. Normative References
7.2. Informative References
Acknowledgements
Authors' Addresses
1. Overview
There are two YANG modules [RFC7950] 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
[IANA-IGP-MSD-Types], 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 [RFC8960], which augments
the routing RIB data model [RFC8349] to provide operational state for
various MSDs [RFC8662] 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) [RFC8342].
2. Design of the Model
2.1. 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 [RFC8476],
[RFC8491], [RFC8662], [RFC8664], [RFC8814], [RFC9088], and [RFC9352].
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.
2.2. IETF MPLS MSD Module
Module ietf-mpls-msd augments the base MPLS model [RFC8960], and it
provides support of different types of MSDs in the MPLS data plane.
As defined in [RFC8491], 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.
3. Tree for IETF MPLS MSD Types YANG Module
This document uses the graphical representation of data models per
[RFC8340].
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
4. YANG Modules
There are two YANG modules defined in this document.
4.1. 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
[IANA-IGP-MSD-Types].
<CODE BEGINS> file "iana-msd-types@2025-01-10.yang"
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";
}
}
<CODE ENDS>
4.2. MPLS MSD YANG
This document also defines a YANG module for MSD extensions [RFC8476]
[RFC8491] to the MPLS base model as defined in [RFC8960].
<CODE BEGINS> file "ietf-mpls-msd@2025-01-10.yang"
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.";
}
}
}
}
<CODE ENDS>
5. 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 [RFC6241] or RESTCONF [RFC8040]. The lowest NETCONF layer
is the secure transport layer, and the mandatory-to-implement secure
transport is Secure Shell (SSH) [RFC6242]. The lowest RESTCONF layer
is HTTPS, and the mandatory-to-implement secure transport is TLS
[RFC8446].
The Network Configuration Access Control Model (NACM) [RFC8341]
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.
6. IANA Considerations
6.1. Registering YANG Modules
This document registers URIs in the IETF XML registry as defined in
[RFC3688]. 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 [RFC6020].
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
6.2. IANA MSD-Types Module
This document defines the initial version of the IANA-maintained
"iana-msd-types" YANG module (Section 4.1). The latest version of
the YANG module is available from the "YANG Parameters" registry
[IANA-YANG-Parameters].
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 [IANA-IGP-MSD-Types].
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 | Reference |
+=========+==========================+============+===========+
| 0 | Reserved | | [RFC8491] |
+---------+--------------------------+------------+-----------+
| 1 | Base MPLS Imposition MSD | MPLS | [RFC8491] |
+---------+--------------------------+------------+-----------+
| 2 | ERLD-MSD | MPLS | [RFC9088] |
+---------+--------------------------+------------+-----------+
| 3-40 | Unassigned | | |
+---------+--------------------------+------------+-----------+
| 41 | SRH Max SL | SRv6 | [RFC9352] |
+---------+--------------------------+------------+-----------+
| 42 | SRH Max End Pop | SRv6 | [RFC9352] |
+---------+--------------------------+------------+-----------+
| 43 | Unassigned | | |
+---------+--------------------------+------------+-----------+
| 44 | SRH Max H.encaps | SRv6 | [RFC9352] |
+---------+--------------------------+------------+-----------+
| 45 | SRH Max End | SRv6 | [RFC9352] |
+---------+--------------------------+------------+-----------+
| 46-250 | Unassigned | | |
+---------+--------------------------+------------+-----------+
| 251-254 | Experimental Use | | [RFC8491] |
+---------+--------------------------+------------+-----------+
| 255 | Reserved | | [RFC8491] |
+---------+--------------------------+------------+-----------+
Table 1
IANA has added this note to [IANA-IGP-MSD-Types]:
| When this registry is modified, the YANG module "iana-msd-types"
| must be updated as defined in RFC 9702.
7. References
7.1. Normative References
[IANA-IGP-MSD-Types]
IANA, "IGP MSD-Types",
<https://www.iana.org/assignments/igp-parameters>.
[IANA-YANG-Parameters]
IANA, "YANG Module Names",
<https://www.iana.org/assignments/yang-parameters>.
[RFC3688] Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
DOI 10.17487/RFC3688, January 2004,
<https://www.rfc-editor.org/info/rfc3688>.
[RFC6020] Bjorklund, M., Ed., "YANG - A Data Modeling Language for
the Network Configuration Protocol (NETCONF)", RFC 6020,
DOI 10.17487/RFC6020, October 2010,
<https://www.rfc-editor.org/info/rfc6020>.
[RFC6241] Enns, R., Ed., Bjorklund, M., Ed., Schoenwaelder, J., Ed.,
and A. Bierman, Ed., "Network Configuration Protocol
(NETCONF)", RFC 6241, DOI 10.17487/RFC6241, June 2011,
<https://www.rfc-editor.org/info/rfc6241>.
[RFC6242] Wasserman, M., "Using the NETCONF Protocol over Secure
Shell (SSH)", RFC 6242, DOI 10.17487/RFC6242, June 2011,
<https://www.rfc-editor.org/info/rfc6242>.
[RFC7950] Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language",
RFC 7950, DOI 10.17487/RFC7950, August 2016,
<https://www.rfc-editor.org/info/rfc7950>.
[RFC8040] Bierman, A., Bjorklund, M., and K. Watsen, "RESTCONF
Protocol", RFC 8040, DOI 10.17487/RFC8040, January 2017,
<https://www.rfc-editor.org/info/rfc8040>.
[RFC8341] Bierman, A. and M. Bjorklund, "Network Configuration
Access Control Model", STD 91, RFC 8341,
DOI 10.17487/RFC8341, March 2018,
<https://www.rfc-editor.org/info/rfc8341>.
[RFC8342] Bjorklund, M., Schoenwaelder, J., Shafer, P., Watsen, K.,
and R. Wilton, "Network Management Datastore Architecture
(NMDA)", RFC 8342, DOI 10.17487/RFC8342, March 2018,
<https://www.rfc-editor.org/info/rfc8342>.
[RFC8349] Lhotka, L., Lindem, A., and Y. Qu, "A YANG Data Model for
Routing Management (NMDA Version)", RFC 8349,
DOI 10.17487/RFC8349, March 2018,
<https://www.rfc-editor.org/info/rfc8349>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8476] Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak,
"Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476,
DOI 10.17487/RFC8476, December 2018,
<https://www.rfc-editor.org/info/rfc8476>.
[RFC8491] Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg,
"Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491,
DOI 10.17487/RFC8491, November 2018,
<https://www.rfc-editor.org/info/rfc8491>.
[RFC8960] Saad, T., Raza, K., Gandhi, R., Liu, X., and V. Beeram, "A
YANG Data Model for MPLS Base", RFC 8960,
DOI 10.17487/RFC8960, December 2020,
<https://www.rfc-editor.org/info/rfc8960>.
[RFC9088] Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S.,
and M. Bocci, "Signaling Entropy Label Capability and
Entropy Readable Label Depth Using IS-IS", RFC 9088,
DOI 10.17487/RFC9088, August 2021,
<https://www.rfc-editor.org/info/rfc9088>.
[RFC9352] Psenak, P., Ed., Filsfils, C., Bashandy, A., Decraene, B.,
and Z. Hu, "IS-IS Extensions to Support Segment Routing
over the IPv6 Data Plane", RFC 9352, DOI 10.17487/RFC9352,
February 2023, <https://www.rfc-editor.org/info/rfc9352>.
7.2. Informative References
[RFC8340] Bjorklund, M. and L. Berger, Ed., "YANG Tree Diagrams",
BCP 215, RFC 8340, DOI 10.17487/RFC8340, March 2018,
<https://www.rfc-editor.org/info/rfc8340>.
[RFC8662] Kini, S., Kompella, K., Sivabalan, S., Litkowski, S.,
Shakir, R., and J. Tantsura, "Entropy Label for Source
Packet Routing in Networking (SPRING) Tunnels", RFC 8662,
DOI 10.17487/RFC8662, December 2019,
<https://www.rfc-editor.org/info/rfc8662>.
[RFC8664] Sivabalan, S., Filsfils, C., Tantsura, J., Henderickx, W.,
and J. Hardwick, "Path Computation Element Communication
Protocol (PCEP) Extensions for Segment Routing", RFC 8664,
DOI 10.17487/RFC8664, December 2019,
<https://www.rfc-editor.org/info/rfc8664>.
[RFC8814] Tantsura, J., Chunduri, U., Talaulikar, K., Mirsky, G.,
and N. Triantafillis, "Signaling Maximum SID Depth (MSD)
Using the Border Gateway Protocol - Link State", RFC 8814,
DOI 10.17487/RFC8814, August 2020,
<https://www.rfc-editor.org/info/rfc8814>.
Acknowledgements
The YANG data model was developed using the suite of YANG tools
written and maintained by numerous authors.
We would like to thank Jan Lindblad, Tom Petch, Dhruv Dhody, Mohamed
Boucadair, and Susan Hares for their reviews and comments.
Authors' Addresses
Yingzhen Qu
Futurewei Technologies
Email: yingzhen.ietf@gmail.com
Acee Lindem
LabN Consulting, L.L.C.
Email: acee.ietf@gmail.com
Stephane Litkowski
Cisco Systems
Email: slitkows.ietf@gmail.com
Jeff Tantsura
Nvidia
Email: jefftant.ietf@gmail.com
AI ELI5
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PROBLEMRouters can only push or process a limited number of segment identifiers (SIDs) on a packet, and there was no standard YANG way to type or read that Maximum SID Depth (MSD).
MECHANISMThe RFC defines iana-msd-types, a module of MSD type identities that mirrors the IANA 'IGP MSD-Types' registry, and ietf-mpls-msd, which augments the MPLS base model with read-only node-msds and link-msds lists.
WHY IT MATTERSManagement systems get a common, machine-readable way to read a node's MSD capabilities for the MPLS data plane, and the type list can be updated by IANA as the registry grows.
§1Overview of the Two Modules
RFC 9702 defines two YANG modules (YANG is the data modeling language from RFC 7950). The first, iana-msd-types, defines identities for Maximum SID Depth (MSD) types, taken from the 'IGP MSD-Types' IANA registry. That registry covers both the MPLS and the Segment Routing over IPv6 (SRv6) data planes.
The second, ietf-mpls-msd, augments the IETF MPLS YANG model (RFC 8960), which in turn augments the routing RIB model (RFC 8349). It exposes operational state for MSDs on the MPLS data plane, as a list of Node MSDs and a list of Link MSDs. Both modules conform to the Network Management Datastore Architecture (RFC 8342).
There are two rule sheets. One is a list of the kinds of limits a router can have. The other is a place on the router to read those limits.
§2Design of the Model
IANA created the 'IGP MSD-Types' registry under the 'Interior Gateway Protocol (IGP) Parameters' registry group. The iana-msd-types module is IANA-maintained and will be updated if and when the registry changes.
On top of the base identity msd-base, the identity msd-base-mpls is the base for MSD types in the MPLS data plane, and msd-base-srh is the base for MSD types for the Segment Routing Header (SRH) in the IPv6 data plane.
As defined in RFC 8491, a Link MSD is the number of SIDs supported by a node's link, and a Node MSD is the smallest MSD supported by the node across all its links. The module defines lists of MSDs and their types for a node and for its links. These are read-only data nodes that expose a node's hardware capability.
Each link on a router has its own limit, and the whole router's limit is the smallest of its links, like a chain that is only as strong as its weakest link.
§3The MPLS MSD Data Tree
The tree has two augmentations, both read-only (config false). The first adds a node-msds container under the MPLS container, holding a node-msd list. The second adds a link-msds container under each MPLS interface, holding a link-msd list.
Each list entry is keyed by msd-type, an identityref based on msd-base-mpls, and carries an optional msd-value of type uint8. The value is in the range 0 to 255, and 0 represents the lack of ability to support a SID stack of any depth. For the same type, the Node MSD value is the smallest among the Link MSD values.
Each entry is a label saying what kind of limit it is, plus a number. A zero means the router cannot take any stack at all.
- /rt:routing/mpls:mpls
- node-msds (ro)
- node-msd* [msd-type]
- msd-type: identityref
- msd-value?: uint8
- node-msd* [msd-type]
- interfaces/interface
- link-msds (ro)
- link-msd* [msd-type]
- link-msds (ro)
- node-msds (ro)
§4Keeping the Module in Sync with IANA
IANA added a 'Data Plane' column to the 'IGP MSD-Types' registry so the base identity for each type is unambiguous. IANA also added a note saying new values must not be added directly to the iana-msd-types module. They must be added to the registry, and the module is then updated.
When a new MSD type is added, a new identity statement must be added. Its name is the lowercase type name with spaces replaced by a hyphen. Its base is the base of the matching data plane, its description replicates the registry name, and its reference replicates the registry reference with the document title. A new data plane gets an identity named msd-base- plus the lowercase data plane name. Unassigned or reserved values do not appear in the module, and every update adds a new revision statement in front of the existing ones.
The RFC also registers the two XML namespace URIs and the two module names (iana-msd-types and ietf-mpls-msd) with prefixes iana-msd-types and mpls-msd.
IANA keeps the master list. Whenever a new kind of limit is added there, the matching entry has to be added to the module too, so the two lists never disagree.
- 1Base MPLS Imposition MSD — MPLS data plane.
- 2ERLD-MSD — MPLS data plane.
- 41SRH Max SL — SRv6 data plane.
- 42SRH Max End Pop — SRv6 data plane.
- 44SRH Max H.encaps — SRv6 data plane.
- 45SRH Max End — SRv6 data plane.
- 251-254Experimental Use — No data plane listed.
§5Security Considerations
The modules are meant to be accessed through network management protocols such as NETCONF (RFC 6241) or RESTCONF (RFC 8040). Their mandatory-to-implement secure transports are SSH and TLS respectively, and the Network Configuration Access Control Model (RFC 8341) can restrict access.
Some readable nodes in ietf-mpls-msd may be sensitive in some networks, so read access to node-msds and link-msds should be controlled. The reason given is that exposing a node's maximum SID depth may help mount a Denial-of-Service attack, by sending the node packets it cannot process. The iana-msd-types module exposes no readable or writable data nodes, so it raises no additional issues.
Telling strangers exactly how much a router can handle could help them overload it, so only trusted people should be allowed to look.
§6References
[RFC7950]RFC 7950Bjorklund, M., Ed., "The YANG 1.1 Data Modeling Language", RFC 7950, August 2016.August 2016[RFC8349]RFC 8349Lhotka, L., Lindem, A., and Y. Qu, "A YANG Data Model for Routing Management (NMDA Version)", RFC 8349, March 2018.March 2018[RFC8476]RFC 8476Tantsura, J., Chunduri, U., Aldrin, S., and P. Psenak, "Signaling Maximum SID Depth (MSD) Using OSPF", RFC 8476, December 2018.December 2018[RFC8491]RFC 8491Tantsura, J., Chunduri, U., Aldrin, S., and L. Ginsberg, "Signaling Maximum SID Depth (MSD) Using IS-IS", RFC 8491, November 2018.November 2018[RFC8960]RFC 8960Saad, T., Raza, K., Gandhi, R., Liu, X., and V. Beeram, "A YANG Data Model for MPLS Base", RFC 8960, December 2020.December 2020[RFC9088]RFC 9088Xu, X., Kini, S., Psenak, P., Filsfils, C., Litkowski, S., and M. Bocci, "Signaling Entropy Label Capability and Entropy Readable Label Depth Using IS-IS", RFC 9088, August 2021.August 2021[RFC9352]RFC 9352Psenak, P., Ed., Filsfils, C., Bashandy, A., Decraene, B., and Z. Hu, "IS-IS Extensions to Support Segment Routing over the IPv6 Data Plane", RFC 9352, February 2023.February 2023