draft-ietf-opsawg-sbom-access-02.txt   draft-ietf-opsawg-sbom-access-03.txt 
Network Working Group E. Lear Network Working Group E. Lear
Internet-Draft Cisco Systems Internet-Draft Cisco Systems
Intended status: Standards Track S. Rose Intended status: Standards Track S. Rose
Expires: 10 January 2022 NIST Expires: 27 April 2022 NIST
9 July 2021 24 October 2021
Discovering and Retrieving Software Transparency and Vulnerability Discovering and Retrieving Software Transparency and Vulnerability
Information Information
draft-ietf-opsawg-sbom-access-02 draft-ietf-opsawg-sbom-access-03
Abstract Abstract
To improve cybersecurity posture, automation is necessary to locate To improve cybersecurity posture, automation is necessary to locate
what software is running on a device, whether that software has known what software is running on a device, whether that software has known
vulnerabilities, and what, if any recommendations suppliers may have. vulnerabilities, and what, if any recommendations suppliers may have.
This memo specifies a model to provide access this information. It This memo specifies a model to provide access to this information.
may optionally be discovered through manufacturer usage descriptions. It may optionally be discovered through manufacturer usage
descriptions.
Status of This Memo Status of This Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/. Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on 10 January 2022. This Internet-Draft will expire on 27 April 2022.
Copyright Notice Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/ Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document. license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights Please review these documents carefully, as they describe your rights
skipping to change at page 2, line 24 skipping to change at page 2, line 24
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Cases Not Addressed . . . . . . . . . . . . . . . . . . . 4 1.1. Cases Not Addressed . . . . . . . . . . . . . . . . . . . 4
1.2. How This Information Is Retrieved . . . . . . . . . . . . 5 1.2. How This Information Is Retrieved . . . . . . . . . . . . 5
1.3. Formats . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.3. Formats . . . . . . . . . . . . . . . . . . . . . . . . . 5
1.4. Discussion points . . . . . . . . . . . . . . . . . . . . 5 1.4. Discussion points . . . . . . . . . . . . . . . . . . . . 5
2. The .well-known/transparency endpoint set . . . . . . . . . . 6 2. The .well-known/transparency endpoint set . . . . . . . . . . 6
3. The mud-transparency extension model extension . . . . . . . 6 3. The mud-transparency extension model extension . . . . . . . 6
4. The mud-sbom augmentation to the MUD YANG model . . . . . . . 7 4. The mud-sbom augmentation to the MUD YANG model . . . . . . . 7
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 11 5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.1. Without ACLS . . . . . . . . . . . . . . . . . . . . . . 11 5.1. Without ACLS . . . . . . . . . . . . . . . . . . . . . . 10
5.2. SBOM Located on the Device . . . . . . . . . . . . . . . 13 5.2. SBOM Located on the Device . . . . . . . . . . . . . . . 12
5.3. Further contact required. . . . . . . . . . . . . . . . . 14 5.3. Further contact required. . . . . . . . . . . . . . . . . 13
5.4. With ACLS . . . . . . . . . . . . . . . . . . . . . . . . 15 5.4. With ACLS . . . . . . . . . . . . . . . . . . . . . . . . 14
6. Security Considerations . . . . . . . . . . . . . . . . . . . 17 6. Security Considerations . . . . . . . . . . . . . . . . . . . 16
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 18 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
7.1. MUD Extension . . . . . . . . . . . . . . . . . . . . . . 18 7.1. MUD Extension . . . . . . . . . . . . . . . . . . . . . . 17
7.2. Well-Known Prefix . . . . . . . . . . . . . . . . . . . . 18 7.2. Well-Known Prefix . . . . . . . . . . . . . . . . . . . . 17
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 18 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.1. Normative References . . . . . . . . . . . . . . . . . . 18 8.1. Normative References . . . . . . . . . . . . . . . . . . 17
8.2. Informative References . . . . . . . . . . . . . . . . . 19 8.2. Informative References . . . . . . . . . . . . . . . . . 18
Appendix A. Changes from Earlier Versions . . . . . . . . . . . 19 Appendix A. Changes from Earlier Versions . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18
1. Introduction 1. Introduction
A number of activities have been working to improve visibility to A number of activities have been working to improve visibility to
what software is running on a system, and what vulnerabilities that what software is running on a system, and what vulnerabilities that
software may have. software may have.
Put simply, we seek to answer two classes of questions *at scale*: Put simply, we seek to answer two classes of questions *at scale*:
* Is this system vulnerable to a particular vulnerability? * Is this system vulnerable to a particular vulnerability?
* Which devices in a particular environment contain vulnerabilities * Which devices in a particular environment contain vulnerabilities
that require some action? that require some action?
Software bills of material (SBOMs) are descriptions of what software, Software bills of materials (SBOMs) are descriptions of what
including versioning and dependencies, a device contains. There are software, including versioning and dependencies, a device contains.
different SBOM formats such as Software Package Data Exchange [SPDX] There are different SBOM formats such as Software Package Data
or CycloneDX[CycloneDX12]. Exchange [SPDX] or CycloneDX[CycloneDX12].
System vulnerabilities may similarly be described using several data System vulnerabilities may similarly be described using several data
formats, including the aforementioned CycloneDX, Common Vulnerability formats, including the aforementioned CycloneDX, Common Vulnerability
Reporting Framework [CVRF], the Common Security Advisory Format Reporting Framework [CVRF], the Common Security Advisory Format
[CSAF]. This information is typically used to report to customers [CSAF]. This information is typically used to report to customers
the state of a system. the state of a system.
These two classes of information can be used in concert. For These two classes of information can be used in concert. For
instance, a network management tool may discover that a system makes instance, a network management tool may discover that a system makes
use of a particular software component that has a known use of a particular software component that has a known
vulnerability, and a vulnerability report may be used to indicate vulnerability, and a vulnerability report may be used to indicate
what if any versions of software correct that vulnerability, or what if any versions of software correct that vulnerability, or
whether the system exercises the vulnerable code at all. whether the system exercises the vulnerable code at all.
Both classes of information elements are optional under the model Both classes of information elements are optional under the model
specified in this memo. One can provide only an SBOM, only specified in this memo. One can provide only an SBOM, only
vulnerability information, or both an SBOM and vulnerability vulnerability information, or both an SBOM and vulnerability
information. information.
Note that SBOMs may also carry other information, the most common Note that SBOM formats may also carry other information, the most
being any licensing terms. Because this specification is neutral common being any licensing terms. Because this specification is
regarding content, it is left for format developers such as the Linux neutral regarding content, it is left for format developers such as
Foundation, OASIS, and ISO to decide what attributes they will the Linux Foundation, OASIS, and ISO to decide what attributes they
support. will support.
This memo specifies means by which both SBOMs and vulnerability This specification does not allow for vulnerability information to be
information can be advertised and retrieved through the use of a YANG retrieved directly from the endpoint. That's because vulnerability
augmentation of the Manufacturer User Description (MUD) model information changes occur at different rates to software updates.
[RFC8520]. Note that the schema creates a grouping that can also be
used independently of MUD. SBOMs and vulnerability information are advertised and retrieved
through the use of a YANG augmentation of the Manufacturer User
Description (MUD) model [RFC8520]. Note that the schema creates a
grouping that can also be used independently of MUD. Moreover, other
MUD features, such as access controls, needn't be present.
The mechanisms specified in this document are meant to satisfy The mechanisms specified in this document are meant to satisfy
several use cases: several use cases:
* A network-layer management system retrieving information from an * A network-layer management system retrieving information from an
IoT device as part of its ongoing lifecycle. Such devices may or IoT device as part of its ongoing lifecycle. Such devices may or
may not have query interfaces available. may not have query interfaces available.
* An application-layer management system retrieving vulnerability or * An application-layer management system retrieving vulnerability or
SBOM information in order to evaluate the posture of an SBOM information in order to evaluate the posture of an
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* Are there other retrieval mechanisms that need to be specified? * Are there other retrieval mechanisms that need to be specified?
* Do we need to be more specific in how to authenticate and retrieve * Do we need to be more specific in how to authenticate and retrieve
SBOMs? SBOMs?
* What are the implications if the MUD URL is an extension in a * What are the implications if the MUD URL is an extension in a
certificate (e.g. an IDevID cert)? certificate (e.g. an IDevID cert)?
2. The .well-known/transparency endpoint set 2. The .well-known/transparency endpoint set
Three well known endpoints are defined: Two well known endpoints are defined:
* "/.well-known/sbom" retrieves an SBOM. * "/.well-known/sbom" retrieves an SBOM.
* "/.well-known/vuln" retrieves vulnerability information.
* "/.well-known/openc2" is the HTTPS binding to OpenC2. * "/.well-known/openc2" is the HTTPS binding to OpenC2.
As discussed previously, the precise format of a response is based on As discussed previously, the precise format of a response is based on
the Content-type provided. the Content-type provided.
3. The mud-transparency extension model extension 3. The mud-transparency extension model extension
We now formally define this extension. This is done in two parts. We now formally define this extension. This is done in two parts.
First, the extension name "transparency" is listed in the First, the extension name "transparency" is listed in the
"extensions" array of the MUD file. N.B., this schema extension is "extensions" array of the MUD file. N.B., this schema extension is
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| | +--rw sboms* [version-info] | | +--rw sboms* [version-info]
| | +--rw version-info string | | +--rw version-info string
| | +--rw sbom-url? inet:uri | | +--rw sbom-url? inet:uri
| +--:(local-well-known) | +--:(local-well-known)
| | +--rw sbom-local-well-known? enumeration | | +--rw sbom-local-well-known? enumeration
| +--:(sbom-contact-info) | +--:(sbom-contact-info)
| +--rw sbom-contact-uri inet:uri | +--rw sbom-contact-uri inet:uri
+--rw (vuln-retrieval-method)? +--rw (vuln-retrieval-method)?
+--:(cloud) +--:(cloud)
| +--rw vuln-url? inet:uri | +--rw vuln-url? inet:uri
+--:(vuln-local-well-known)
| +--rw vuln-local-well-known? enumeration
+--:(vuln-contact-info) +--:(vuln-contact-info)
+--rw contact-uri inet:uri +--rw contact-uri inet:uri
4. The mud-sbom augmentation to the MUD YANG model 4. The mud-sbom augmentation to the MUD YANG model
<CODE BEGINS> <CODE BEGINS>
file "ietf-mud-transparency@2021-07-06.yang" file "ietf-mud-transparency@2021-10-22.yang"
module ietf-mud-transparency { module ietf-mud-transparency {
yang-version 1.1; yang-version 1.1;
namespace "urn:ietf:params:xml:ns:yang:ietf-mud-transparency"; namespace "urn:ietf:params:xml:ns:yang:ietf-mud-transparency";
prefix mud-transparency; prefix mud-transparency;
import ietf-inet-types { import ietf-inet-types {
prefix inet; prefix inet;
} reference "RFC 6991";
}
import ietf-mud { import ietf-mud {
prefix mud; prefix mud;
reference "RFC 8520";
} }
organization organization
"IETF OPSAWG (Ops Area) Working Group"; "IETF OPSAWG (Ops Area) Working Group";
contact contact
"WG "WG Web: http://tools.ietf.org/wg/opsawg/
Web: http://tools.ietf.org/wg/opsawg/
WG List: opsawg@ietf.org WG List: opsawg@ietf.org
Author: Eliot Lear lear@cisco.com
Author: Scott Rose scott.rose@nist.gov"; Editor: Eliot Lear lear@cisco.com
Editor: Scott Rose scott.rose@nist.gov";
description description
"This YANG module augments the ietf-mud model to provide for "This YANG module augments the ietf-mud model to provide for
reporting of SBOMs. reporting of SBOMs.
Copyright (c) 2020 IETF Trust and the persons identified as Copyright (c) 2020 IETF Trust and the persons identified as
authors of the code. All rights reserved. authors of the code. All rights reserved.
Redistribution and use in source and binary forms, with or Redistribution and use in source and binary forms, with or
without modification, is permitted pursuant to, and subject to without modification, is permitted pursuant to, and subject to
the license terms contained in, the Simplified BSD License set the license terms contained in, the Simplified BSD License set
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choice vuln-retrieval-method { choice vuln-retrieval-method {
description description
"How to find vulnerability information"; "How to find vulnerability information";
case cloud { case cloud {
leaf vuln-url { leaf vuln-url {
type inet:uri; type inet:uri;
description description
"A statically located URL."; "A statically located URL.";
} }
} }
case vuln-local-well-known {
leaf vuln-local-well-known {
type enumeration {
enum http {
description
"Use http (insecure) to retrieve vulnerability
information.";
}
enum https {
description
"Use https to retrieve vulnerability information.";
}
enum coap {
description
"Use COAP (insecure) to retrieve vulnerability
information";
}
enum coaps {
description
"Use COAPS to retrieve vulnerability information";
}
enum openc2 {
description
"Use OpenC2 endpoint.
This is https://{host}/.well-known/openc2";
}
}
description
"What communication protocol to use.";
}
}
case vuln-contact-info { case vuln-contact-info {
leaf contact-uri { leaf contact-uri {
type inet:uri; type inet:uri;
mandatory true; mandatory true;
description description
"This MUST be either a tel, http, https, or "This MUST be either a tel, http, https, or
mailto uri schema that customers can use to mailto uri schema that customers can use to
contact someone for vulnerability information."; contact someone for vulnerability information.";
} }
} }
} }
} }
} }
augment "/mud:mud" { augment "/mud:mud" {
description description
"Add extension for software transparency."; "Add extension for software transparency.";
uses transparency-extension; uses transparency-extension;
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This first MUD file demonstrates how to get SBOM and vulnerability This first MUD file demonstrates how to get SBOM and vulnerability
information without ACLs. information without ACLs.
{ {
"ietf-mud:mud": { "ietf-mud:mud": {
"mud-version": 1, "mud-version": 1,
"extensions": [ "extensions": [
"transparency" "transparency"
], ],
"transparency": { "ietf-mud-transparency:transparency": {
"sboms": [ "sboms": [
{ {
"version-info": "ExOS1.1", "version-info": "ExOS1.1",
"sbom-url": "https://iot.example.com/info/modelX/sbom.json" "sbom-url": "https://iot.example.com/info/modelX/sbom.json"
} }
], ],
"vuln-url": "https://iot.example.com/info/modelX/csaf.json" "vuln-url": "https://iot.example.com/info/modelX/csaf.json"
}, },
"mud-url": "https://iot.example.com/modelX.json", "mud-url": "https://iot.example.com/modelX.json",
"mud-signature": "https://iot.example.com/modelX.p7s", "mud-signature": "https://iot.example.com/modelX.p7s",
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The second example demonstrates that just SBOM information is The second example demonstrates that just SBOM information is
included. included.
{ {
"ietf-mud:mud": { "ietf-mud:mud": {
"mud-version": 1, "mud-version": 1,
"extensions": [ "extensions": [
"transparency" "transparency"
], ],
"transparency": { "ietf-mud-transparency:transparency": {
"sboms": [ "sboms": [
{ {
"version-info": "ExOS1.1", "version-info": "ExOS1.1",
"sbom-url": "https://iot.example.com/info/modelX/sbom.json" "sbom-url": "https://iot.example.com/info/modelX/sbom.json"
} }
] ]
}, },
"mud-url": "https://iot.example.com/modelX.json", "mud-url": "https://iot.example.com/modelX.json",
"mud-signature": "https://iot.example.com/modelX.p7s", "mud-signature": "https://iot.example.com/modelX.p7s",
"last-update": "2021-07-09T06:03:21+00:00", "last-update": "2021-07-09T06:03:21+00:00",
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"extensions": [ "extensions": [
"ol", "ol",
"transparency" "transparency"
], ],
"ol": { "ol": {
"owners": [ "owners": [
"Copyright (c) Example, Inc. 2021. All Rights Reserved" "Copyright (c) Example, Inc. 2021. All Rights Reserved"
], ],
"spdx-tag": "0BSD" "spdx-tag": "0BSD"
}, },
"transparency": { "ietf-mud-transparency:transparency": {
"sbom-local-well-known": "https", "sbom-local-well-known": "https",
"vuln-url": "https://iot-device.example.com/info/modelX/csaf.json" "vuln-url": "https://iot-device.example.com/info/modelX/csaf.json"
}, },
"mud-url": "https://iot-device.example.com/modelX.json", "mud-url": "https://iot-device.example.com/modelX.json",
"mud-signature": "https://iot-device.example.com/modelX.p7s", "mud-signature": "https://iot-device.example.com/modelX.p7s",
"last-update": "2021-07-09T06:06:13+00:00", "last-update": "2021-07-09T06:06:13+00:00",
"cache-validity": 48, "cache-validity": 48,
"is-supported": true, "is-supported": true,
"systeminfo": "retrieving vuln and SBOM info via a cloud service", "systeminfo": "retrieving vuln and SBOM info via a cloud service",
"mfg-name": "Example, Inc.", "mfg-name": "Example, Inc.",
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In this example, the network manager must take further steps to In this example, the network manager must take further steps to
retrieve SBOM information. Vulnerability information is still retrieve SBOM information. Vulnerability information is still
available. available.
{ {
"ietf-mud:mud": { "ietf-mud:mud": {
"mud-version": 1, "mud-version": 1,
"extensions": [ "extensions": [
"transparency" "transparency"
], ],
"transparency": { "ietf-mud-transparency:transparency": {
"contact-info": "https://iot-device.example.com/contact-info.html", "contact-info": "https://iot-device.example.com/contact-info.html",
"vuln-url": "https://iot-device.example.com/info/modelX/csaf.json" "vuln-url": "https://iot-device.example.com/info/modelX/csaf.json"
}, },
"mud-url": "https://iot-device.example.com/modelX.json", "mud-url": "https://iot-device.example.com/modelX.json",
"mud-signature": "https://iot-device.example.com/modelX.p7s", "mud-signature": "https://iot-device.example.com/modelX.p7s",
"last-update": "2021-07-09T06:16:42+00:00", "last-update": "2021-07-09T06:16:42+00:00",
"cache-validity": 48, "cache-validity": 48,
"is-supported": true, "is-supported": true,
"systeminfo": "retrieving vuln and SBOM info via a cloud service", "systeminfo": "retrieving vuln and SBOM info via a cloud service",
"mfg-name": "Example, Inc.", "mfg-name": "Example, Inc.",
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Finally, here is a complete example where the device provides SBOM Finally, here is a complete example where the device provides SBOM
and vulnerability information, as well as access-control information. and vulnerability information, as well as access-control information.
{ {
"ietf-mud:mud": { "ietf-mud:mud": {
"mud-version": 1, "mud-version": 1,
"extensions": [ "extensions": [
"transparency" "transparency"
], ],
"transparency": { "ietf-mud-transparency:transparency": {
"sboms": [ "sboms": [
{ {
"version-info": "ExOS1.1", "version-info": "ExOS1.1",
"sbom-url": "https://iot.example.com/info/modelX/sbom.json" "sbom-url": "https://iot.example.com/info/modelX/sbom.json"
} }
], ],
"vuln-url": "https://iot.example.com/info/modelX/csaf.json" "vuln-url": "https://iot.example.com/info/modelX/csaf.json"
}, },
"mud-url": "https://iot.example.com/modelX.json", "mud-url": "https://iot.example.com/modelX.json",
"mud-signature": "https://iot.example.com/modelX.p7s", "mud-signature": "https://iot.example.com/modelX.p7s",
skipping to change at page 17, line 44 skipping to change at page 16, line 44
HTTP. In particular, if a system attempts to retrieve an SBOM via HTTP. In particular, if a system attempts to retrieve an SBOM via
HTTP and the client is not authorized, the server MUST produce an HTTP and the client is not authorized, the server MUST produce an
appropriate error, with instructions on how to register a particular appropriate error, with instructions on how to register a particular
client. One example may be to issue a certificate to the client for client. One example may be to issue a certificate to the client for
this purpose after a registration process has taken place. Another this purpose after a registration process has taken place. Another
example would involve the use of OAUTH in combination with a example would involve the use of OAUTH in combination with a
federations of SBOM servers. federations of SBOM servers.
Another risk is a skew in the SBOM listing and the actual software Another risk is a skew in the SBOM listing and the actual software
inventory of a device/container. For example, a manufacturer may inventory of a device/container. For example, a manufacturer may
update the SBOM on its server, but an individual device has not be update the SBOM on its server, but an individual device has not been
upgraded yet. This may result in an incorrect policy being applied upgraded yet. This may result in an incorrect policy being applied
to a device. A unique mapping of a device's firmware version and its to a device. A unique mapping of a device's firmware version and its
SBOM can minimize this risk. SBOM can minimize this risk.
To further mitigate attacks against a device, manufacturers SHOULD To further mitigate attacks against a device, manufacturers SHOULD
recommend access controls through the normal MUD mechanism. recommend access controls through the normal MUD mechanism.
Vulnerability information is generally made available to such Vulnerability information is generally made available to such
databases as NIST's National Vulnerability Database. It is possible databases as NIST's National Vulnerability Database. It is possible
that vendor may wish to release information early to some customers. that vendor may wish to release information early to some customers.
We do not discuss here whether that is a good idea, but if it is We do not discuss here whether that is a good idea, but if it is
employed, then appropriate access controls and authoration would be employed, then appropriate access controls and authorization would be
applied to the vulnerability resource. applied to the vulnerability resource.
7. IANA Considerations 7. IANA Considerations
7.1. MUD Extension 7.1. MUD Extension
The IANA is requested to add "transparency" to the MUD extensions The IANA is requested to add "transparency" to the MUD extensions
registry as follows: registry as follows:
Extension Name: transparency Extension Name: transparency
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URI suffix: "sbom" URI suffix: "sbom"
Change controller: "IETF" Change controller: "IETF"
Specification document: This memo Specification document: This memo
Related information: See ISO/IEC 19970-2 and SPDX.org Related information: See ISO/IEC 19970-2 and SPDX.org
URI suffix: "openc2" URI suffix: "openc2"
Change controller: "IETF" Change controller: "IETF"
Specification document: This memo Specification document: This memo
Related information: OpenC2 Project Related information: OpenC2 Project
URI suffix: "vuln"
Change controller: "IETF"
Specification document: This memo
Related information: OASIS.ORG's CSAF project
8. References 8. References
8.1. Normative References 8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC6991] Schoenwaelder, J., Ed., "Common YANG Data Types",
RFC 6991, DOI 10.17487/RFC6991, July 2013,
<https://www.rfc-editor.org/info/rfc6991>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8520] Lear, E., Droms, R., and D. Romascanu, "Manufacturer Usage [RFC8520] Lear, E., Droms, R., and D. Romascanu, "Manufacturer Usage
Description Specification", RFC 8520, Description Specification", RFC 8520,
DOI 10.17487/RFC8520, March 2019, DOI 10.17487/RFC8520, March 2019,
<https://www.rfc-editor.org/info/rfc8520>. <https://www.rfc-editor.org/info/rfc8520>.
[RFC8615] Nottingham, M., "Well-Known Uniform Resource Identifiers [RFC8615] Nottingham, M., "Well-Known Uniform Resource Identifiers
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