    Internet Engineering Task Force (IETF)                           E. Lear
    Request for Comments: 8520                                 Cisco Systems
    Category: Standards Track                                       R. Droms
    ISSN: 2070-1721                                                   Google
                                                                D. Romascanu
                                                                  March 2019

                  Manufacturer Usage Description Specification

    Abstract

       This memo specifies a component-based architecture for Manufacturer
       Usage Descriptions (MUDs).  The goal of MUD is to provide a means for
       end devices to signal to the network what sort of access and network
       functionality they require to properly function.  The initial focus
       is on access control.  Later work can delve into other aspects.

       This memo specifies two YANG modules, IPv4 and IPv6 DHCP options, a
       Link Layer Discovery Protocol (LLDP) TLV, a URL, an X.509 certificate
       extension, and a means to sign and verify the descriptions.

    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/rfc8520.

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    Copyright Notice

       Copyright (c) 2019 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 Simplified BSD License text as described in Section 4.e of
       the Trust Legal Provisions and are provided without warranty as
       described in the Simplified BSD License.

    Table of Contents

       1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   4
         1.1.  What MUD Doesn't Do . . . . . . . . . . . . . . . . . . .   5
         1.2.  A Simple Example  . . . . . . . . . . . . . . . . . . . .   5
         1.3.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   6
         1.4.  Determining Intended Use  . . . . . . . . . . . . . . . .   6
         1.5.  Finding a Policy: The MUD URL . . . . . . . . . . . . . .   7
         1.6.  Processing of the MUD URL . . . . . . . . . . . . . . . .   8
         1.7.  Types of Policies . . . . . . . . . . . . . . . . . . . .   8
         1.8.  The Manufacturer Usage Description Architecture . . . . .  10
         1.9.  Order of Operations . . . . . . . . . . . . . . . . . . .  12
       2.  The MUD Model and Semantic Meaning  . . . . . . . . . . . . .  12
         2.1.  The IETF-MUD YANG Module  . . . . . . . . . . . . . . . .  14
       3.  MUD Model Definitions for the Root "mud" Container  . . . . .  15
         3.1.  mud-version . . . . . . . . . . . . . . . . . . . . . . .  15
         3.2.  MUD URL . . . . . . . . . . . . . . . . . . . . . . . . .  15
         3.3.  to-device-policy and from-device-policy Containers  . . .  16
         3.4.  last-update . . . . . . . . . . . . . . . . . . . . . . .  16
         3.5.  cache-validity  . . . . . . . . . . . . . . . . . . . . .  16
         3.6.  is-supported  . . . . . . . . . . . . . . . . . . . . . .  16
         3.7.  systeminfo  . . . . . . . . . . . . . . . . . . . . . . .  16
         3.8.  mfg-name, software-rev, model-name, and firmware-rev  . .  17
         3.9.  extensions  . . . . . . . . . . . . . . . . . . . . . . .  17
       4.  Augmentation to the ACL Model . . . . . . . . . . . . . . . .  17
         4.1.  manufacturer  . . . . . . . . . . . . . . . . . . . . . .  17
         4.2.  same-manufacturer . . . . . . . . . . . . . . . . . . . .  17
         4.3.  documentation . . . . . . . . . . . . . . . . . . . . . .  18
         4.4.  model . . . . . . . . . . . . . . . . . . . . . . . . . .  18
         4.5.  local-networks  . . . . . . . . . . . . . . . . . . . . .  18
         4.6.  controller  . . . . . . . . . . . . . . . . . . . . . . .  18
         4.7.  my-controller . . . . . . . . . . . . . . . . . . . . . .  19
         4.8.  direction-initiated . . . . . . . . . . . . . . . . . . .  19

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       5.  Processing of the MUD File  . . . . . . . . . . . . . . . . .  19
       6.  What Does a MUD URL Look Like?  . . . . . . . . . . . . . . .  19
       7.  The MUD YANG Model  . . . . . . . . . . . . . . . . . . . . .  20
       8.  The Domain Name Extension to the ACL Model  . . . . . . . . .  26
         8.1.  src-dnsname . . . . . . . . . . . . . . . . . . . . . . .  27
         8.2.  dst-dnsname . . . . . . . . . . . . . . . . . . . . . . .  27
         8.3.  The ietf-acldns Model . . . . . . . . . . . . . . . . . .  28
       9.  MUD File Example  . . . . . . . . . . . . . . . . . . . . . .  30
       10. The MUD URL DHCP Option . . . . . . . . . . . . . . . . . . .  32
         10.1.  Client Behavior  . . . . . . . . . . . . . . . . . . . .  33
         10.2.  Server Behavior  . . . . . . . . . . . . . . . . . . . .  33
         10.3.  Relay Requirements . . . . . . . . . . . . . . . . . . .  33
       11. The Manufacturer Usage Description (MUD) URL X.509 Extension   34
       12. The Manufacturer Usage Description LLDP Extension . . . . . .  36
       13. The Creating and Processing of Signed MUD Files . . . . . . .  38
         13.1.  Creating a MUD File Signature  . . . . . . . . . . . . .  38
         13.2.  Verifying a MUD File Signature . . . . . . . . . . . . .  38
       14. Extensibility . . . . . . . . . . . . . . . . . . . . . . . .  39
       15. Deployment Considerations . . . . . . . . . . . . . . . . . .  39
       16. Security Considerations . . . . . . . . . . . . . . . . . . .  40
       17. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  43
         17.1.  YANG Module Registrations  . . . . . . . . . . . . . . .  43
         17.2.  URI Registrations  . . . . . . . . . . . . . . . . . . .  43
         17.3.  DHCPv4 and DHCPv6 Options  . . . . . . . . . . . . . . .  43
         17.4.  PKIX Extensions  . . . . . . . . . . . . . . . . . . . .  43
         17.5.  Media Type Registration for MUD Files  . . . . . . . . .  44
         17.6.  IANA LLDP TLV Subtype Registry . . . . . . . . . . . . .  45
         17.7.  The MUD Well-Known Universal Resource Name (URNs)  . . .  45
         17.8.  Extensions Registry  . . . . . . . . . . . . . . . . . .  46
       18. References  . . . . . . . . . . . . . . . . . . . . . . . . .  46
         18.1.  Normative References . . . . . . . . . . . . . . . . . .  46
         18.2.  Informative References . . . . . . . . . . . . . . . . .  49
       Appendix A.  Default MUD Nodes  . . . . . . . . . . . . . . . . .  52
       Appendix B.  A Sample Extension: DETNET-indicator . . . . . . . .  56
       Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  60
       Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  60

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    1.  Introduction

       The Internet has largely been constructed for general purpose
       computers, those devices that may be used for a purpose that is
       specified by those who own the device.  In [RFC1984], it was presumed
       that an end device would be most capable of protecting itself.  This
       made sense when the typical device was a workstation or a mainframe,
       and it continues to make sense for general purpose computing devices
       today, including laptops, smart phones, and tablets.

       [RFC7452] discusses design patterns for, and poses questions about,
       smart objects.  Let us then posit a group of objects that are
       specifically not intended to be used for general purpose computing
       tasks.  These devices, which this memo refers to as Things, have a
       specific purpose.  By definition, therefore, all other uses are not
       intended.  If a small number of communication patterns follows from
       those small number of uses, the combination of these two statements
       can be restated as a Manufacturer Usage Description (MUD) that can be
       applied at various points within a network.  MUD primarily addresses
       threats to the device rather than the device as a threat.  In some
       circumstances, however, MUD may offer some protection in the latter
       case, depending on how the MUD URL is communicated and how devices
       and their communications are authenticated.

       We use the notion of "manufacturer" loosely in this context to refer
       to the entity or organization that will state how a device is
       intended to be used.  For example, in the context of a light bulb,
       this might indeed be the light bulb manufacturer.  In the context of
       a smarter device that has a built in Linux stack, it might be an
       integrator of that device.  The key points are that the device itself
       is assumed to serve a limited purpose, and that there exists an
       organization in the supply chain of that device that will take
       responsibility for informing the network about that purpose.

       The intent of MUD is to provide the following:

       o  Substantially reduce the threat surface on a device to those
          communications intended by the manufacturer.

       o  Provide a means to scale network policies to the ever-increasing
          number of types of devices in the network.

       o  Provide a means to address at least some vulnerabilities in a way
          that is faster than the time it might take to update systems.
          This will be particularly true for systems that are no longer
          supported.

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       o  Keep the cost of implementation of such a system to the bare
          minimum.

       o  Provide a means of extensibility for manufacturers to express
          other device capabilities or requirements.

       MUD consists of three architectural building blocks:

       o  A URL that can be used to locate a description;

       o  The description itself, including how it is interpreted; and

       o  A means for local network management systems to retrieve the
          description.

       MUD is most effective when the network is able to identify in some
       way the remote endpoints that Things will talk to.

       In this specification, we describe each of these building blocks and
       how they are intended to be used together.  However, they may also be
       used separately, independent of this specification, by local
       deployments for their own purposes.

    1.1.  What MUD Doesn't Do

       MUD is not intended to address network authorization of general
       purpose computers, as their manufacturers cannot envision a specific
       communication pattern to describe.  In addition, even those devices
       that have a single or small number of uses might have very broad
       communication patterns.  MUD on its own is not for them either.

       Although MUD can provide network administrators with some additional
       protection when device vulnerabilities exist, it will never replace
       the need for manufacturers to patch vulnerabilities.

       Finally, no matter what the manufacturer specifies in a MUD file,
       these are not directives, but suggestions.  How they are instantiated
       locally will depend on many factors and will be ultimately up to the
       local network administrator, who must decide what is appropriate in a
       given circumstances.

    1.2.  A Simple Example

       A light bulb is intended to light a room.  It may be remotely
       controlled through the network, and it may make use of a rendezvous
       service (which could be accessed by an application on a smart phone).
       What we can say about that light bulb, then, is that all other
       network access is unwanted.  It will not contact a news service, nor

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       speak to the refrigerator, and it has no need of a printer or other
       devices.  It has no social networking friends.  Therefore, applying
       an access list to it that states it will only connect to the single
       rendezvous service will not impede performing its function; at the
       same time, this will allow the network to provide the light bulb and
       other devices an additional layer of protection.

    1.3.  Terminology

       MUD:  Manufacturer Usage Description.

       MUD file:  a file containing YANG-based JSON that describes a Thing
          and associated suggested specific network behavior.

       MUD file server:  a web server that hosts a MUD file.

       MUD manager:  the system that requests and receives the MUD file from
          the MUD server.  After it has processed a MUD file, it may direct
          changes to relevant network elements.

       MUD controller:  a synonym that has been used in the past for MUD
          manager.

       MUD URL:  a URL that can be used by the MUD manager to receive the
          MUD file.

       Thing:  the device emitting a MUD URL.

       Manufacturer:  the entity that configures the Thing to emit the MUD
          URL and the one who asserts a recommendation in a MUD file.  The
          manufacturer might not always be the entity that constructs a
          Thing.  It could, for instance, be a systems integrator, or even a
          component provider.

       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.

    1.4.  Determining Intended Use

       The notion of intended use is in itself not new.  Network
       administrators apply access lists every day to allow for only such
       use.  This notion of white listing was well described by Chapman and
       Zwicky in [FW95].  Profiling systems that make use of heuristics to
       identify types of systems have existed for years as well.

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       A Thing could just as easily tell the network what sort of access it
       requires without going into what sort of system it is.  This would,
       in effect, be the converse of [RFC7488].  In seeking a general
       solution, however, we assume that a device will implement
       functionality necessary to fulfill its limited purpose.  This is
       basic economic constraint.  Unless the network would refuse access to
       such a device, its developers would have no reason to provide the
       network any information.  To date, such an assertion has held true.

    1.5.  Finding a Policy: The MUD URL

       Our work begins with the device emitting a Universal Resource Locator
       (URL) [RFC3986].  This URL serves both to classify the device type
       and to provide a means to locate a policy file.

       MUD URLs MUST use the "https" scheme [RFC7230].

       In this memo, three means are defined to emit the MUD URL, as
       follows:

       o  A DHCP option [RFC2131] [RFC8415] that the DHCP client uses to
          inform the DHCP server.  The DHCP server may take further actions,
          such as acting as the MUD manager or passing the MUD URL along to
          the MUD manager.

       o  An X.509 constraint.  The IEEE has developed IEEE 802.1AR
          [IEEE8021AR] to provide a certificate-based approach to
          communicate device characteristics, which itself relies on
          [RFC5280].  The MUD URL extension is non-critical, as required by
          IEEE 802.1AR.  Various means may be used to communicate that
          certificate, including the Tunnel Extensible Authentication
          Protocol (TEAP) [RFC7170].

       o  Finally, a Link Layer Discovery Protocol (LLDP) frame is defined
          [IEEE8021AB].

       It is possible that there may be other means for a MUD URL to be
       learned by a network.  For instance, some devices may already be
       fielded or have very limited ability to communicate a MUD URL, and
       yet they can be identified through some means, such as a serial
       number or a public key.  In these cases, manufacturers may be able to
       map those identifiers to particular MUD URLs (or even the files
       themselves).  Similarly, there may be alternative resolution
       mechanisms available for situations where Internet connectivity is
       limited or does not exist.  Such mechanisms are not described in this
       memo, but they are possible.  Implementors are encouraged to allow
       for the flexibility of how MUD URLs may be learned.

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    1.6.  Processing of the MUD URL

       MUD managers that are able to do so SHOULD retrieve MUD URLs and
       signature files as per [RFC7230], using the GET method [RFC7231].
       They MUST validate the certificate using the rules in [RFC2818],
       Section 3.1.

       Requests for MUD URLs SHOULD include an "Accept" header field
       ([RFC7231], Section 5.3.2) containing "application/mud+json", an
       "Accept-Language" header field ([RFC7231], Section 5.3.5), and a
       "User-Agent" header field ([RFC7231], Section 5.5.3).

       MUD managers SHOULD automatically process 3xx response status codes.

       If a MUD manager is not able to fetch a MUD URL, other means MAY be
       used to import MUD files and associated signature files.  So long as
       the signature of the file can be validated, the file can be used.  In
       such environments, controllers SHOULD warn administrators when cache-
       validity expiry is approaching so that they may check for new files.

       It may not be possible for a MUD manager to retrieve a MUD file at
       any given time.  Should a MUD manager fail to retrieve a MUD file, it
       SHOULD consider the existing one safe to use, at least for a time.
       After some period, it SHOULD log that it has been unable to retrieve
       the file.  There may be very good reasons for such failures,
       including the possibility that the MUD manager is in an offline
       environment, the local Internet connection has failed, or the remote
       Internet connection has failed.  It is also possible that an attacker
       is attempting to interfere with the deployment of a device.  How to
       handle such circumstances is a local decision.

    1.7.  Types of Policies

       When the MUD URL is resolved, the MUD manager retrieves a file that
       describes what sort of communications a device is designed to have.
       The manufacturer may specify either specific hosts for cloud-based
       services or certain classes for access within an operational network.
       An example of a class might be "devices of a specified manufacturer
       type", where the manufacturer type itself is indicated simply by the
       authority component (e.g., the domain name) of the MUD URL.  Another
       example might be to allow or disallow local access.  Just like other
       policies, these may be combined.  For example:

       o  Allow access to devices of the same manufacturer

       o  Allow access to and from controllers via the Constrained
          Application Protocol (COAP) [RFC7252]

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       o  Allow access to local DNS/NTP

       o  Deny all other access

       A printer might have a description that states:

       o  Allow access for port IPP or port LPD

       o  Allow local access for port HTTP

       o  Deny all other access

       In this way, anyone can print to the printer, but local access would
       be required for the management interface.

       The files that are retrieved are intended to be closely aligned to
       existing network architectures so that they are easy to deploy.  We
       make use of YANG [RFC7950] because it provides accurate and adequate
       models for use by network devices.  JSON [RFC8259] is used as a
       serialization format for compactness and readability, relative to
       XML.  Other formats may be chosen with later versions of MUD.

       While the policy examples given here focus on access control, this is
       not intended to be the sole focus.  By structuring the model
       described in this document with clear extension points, other
       descriptions could be included.  One that often comes to mind is
       quality of service.

       The YANG modules specified here are extensions of [RFC8519].  The
       extensions to this model allow for a manufacturer to express classes
       of systems that a manufacturer would find necessary for the proper
       function of the device.  Two modules are specified.  The first module
       specifies a means for domain names to be used in Access Control Lists
       (ACLs) so that devices that have their controllers in the cloud may
       be appropriately authorized with domain names, where the mapping of
       those names to addresses may rapidly change.

       The other module abstracts away IP addresses into certain classes
       that are instantiated into actual IP addresses through local
       processing.  Through these classes, manufacturers can specify how the
       device is designed to communicate, so that network elements can be
       configured by local systems that have local topological knowledge.
       That is, the deployment populates the classes that the manufacturer
       specifies.  The abstractions below map to zero or more hosts, as
       follows:

       Manufacturer:  A device made by a particular manufacturer, as
          identified by the authority component of its MUD URL.

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       same-manufacturer:  Devices that have the same authority component of
          their MUD URL.

       controller:  Devices that the local network administrator admits to
          the particular class.

       my-controller:  Devices intended to serve as controllers for the MUD
          URL that the Thing emitted.

       local:  The class of IP addresses that are scoped within some
          administrative boundary.  By default, it is suggested that this be
          the local subnet.

       The "manufacturer" classes can be easily specified by the
       manufacturer, whereas controller classes are initially envisioned to
       be specified by the administrator.

       Because manufacturers do not know who will be using their devices, it
       is important for functionality referenced in usage descriptions to be
       relatively ubiquitous and mature.  For these reasons, the YANG-based
       configuration in a MUD file is limited to the modules either
       specified or referenced in this document, or specified in documented
       extensions.

    1.8.  The Manufacturer Usage Description Architecture

       With these components laid out, we now have the basis for an
       architecture.  This leads us to ASCII art.

        .......................................
        .                      ____________   .           _____________
        .                     |            |  .          |             |
        .                     |    MUD     |-->get URL-->|    MUD      |
        .                     |  Manager   |  .(https)   | File Server |
        .  End system network |____________|<-MUD file<-<|_____________|
        .                             .       .
        .                             .       .
        . _______                 _________   .
        .|       | (DHCP et al.) | router  |  .
        .| Thing |---->MUD URL-->|   or    |  .
        .|_______|               | switch  |  .
        .                        |_________|  .
        .......................................

                            Figure 1: MUD Architecture

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       In the above diagram, the switch or router collects MUD URLs and
       forwards them to the MUD manager (a network management system) for
       processing.  This happens in different ways, depending on how the URL
       is communicated.  For instance, in the case of DHCP, the DHCP server
       might receive the URL and then process it.  In the case of IEEE
       802.1X [IEEE8021X], the switch would carry the URL via a certificate
       to the authentication server via the Extensible Authentication
       Protocol (EAP) over Radius [RFC3748], which would then process it.
       One method to do this is TEAP, as described in [RFC7170].  The
       certificate extension is described below.

       The information returned by the MUD file server is valid for as long
       as the Thing is connected.  There is no expiry.  However, if the MUD
       manager has detected that the MUD file for a Thing has changed, it
       SHOULD update the policy expeditiously, taking into account whatever
       approval flow is required in a deployment.  In this way, new
       recommendations from the manufacturer can be processed in a timely
       fashion.

       The information returned by the MUD file server (a web server) is
       valid for the duration of the Thing's connection, or as specified in
       the description.  Thus, if the Thing is disconnected, any associated
       configuration in the switch can be removed.  Similarly, from time to
       time the description may be refreshed, based on new capabilities or
       communication patterns or vulnerabilities.

       The web server is typically run by or on behalf of the manufacturer.
       Its domain name is that of the authority found in the MUD URL.  For
       legacy cases where Things cannot emit a URL, if the switch is able to
       determine the appropriate URL, it may proxy it.  In a trivial case,
       it may hardcode a MUD URL on a switch port or a map from some
       available identifier such as an L2 address or certificate hash to a
       MUD URL.

       The role of the MUD manager in this environment is to do the
       following:

       o  receive MUD URLs,

       o  fetch MUD files,

       o  translate abstractions in the MUD files to specific network
          element configuration,

       o  maintain and update any required mappings of the abstractions, and

       o  update network elements with appropriate configuration.

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       A MUD manager may be a component of an Authentication, Authorization,
       and Accounting (AAA) system or a network management system.
       Communication within those systems and from those systems to network
       elements is beyond the scope of this memo.

    1.9.  Order of Operations

       As mentioned above, MUD contains architectural building blocks, so
       the order of operation may vary.  However, here is one clear intended
       example:

       1.  Thing emits a URL.

       2.  That URL is forwarded to a MUD manager by the nearest switch (how
           this happens depends on the way in which the MUD URL is emitted).

       3.  The MUD manager retrieves the MUD file and signature from the MUD
           file server, assuming it doesn't already have copies.  After
           validating the signature, it may test the URL against a web or
           domain reputation service, and it may test any hosts within the
           file against those reputation services, as it deems fit.

       4.  The MUD manager may query the administrator for permission to add
           the Thing and associated policy.  If the Thing is known or the
           Thing type is known, it may skip this step.

       5.  The MUD manager instantiates local configuration based on the
           abstractions defined in this document.

       6.  The MUD manager configures the switch nearest the Thing.  Other
           systems may be configured as well.

       7.  When the Thing disconnects, policy is removed.

    2.  The MUD Model and Semantic Meaning

       A MUD file consists of a YANG model instance that has been serialized
       in JSON [RFC7951].  For purposes of MUD, the nodes that can be
       modified are access lists as augmented by this model.  The MUD file
       is limited to the serialization of only the following YANG schema:

       o  ietf-access-control-list [RFC8519]

       o  ietf-mud (RFC 8520)

       o  ietf-acldns (RFC 8520)

    Lear, et al.                 Standards Track                   [Page 12]

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       Extensions may be used to add additional schema.  This is described
       further on.

       To provide the widest possible deployment, publishers of MUD files
       SHOULD make use of the abstractions in this memo and avoid the use of
       IP addresses.  A MUD manager SHOULD NOT automatically implement any
       MUD file that contains IP addresses, especially those that might have
       local significance.  The addressing of one side of an access list is
       implicit, based on whether it is applied as to-device-policy or
       from-device-policy.

       With the exceptions of the "name" of the ACL, "type", "name" of the
       Access Control Entry (ACE), and TCP and UDP source and destination
       port information, publishers of MUD files SHOULD limit the use of ACL
       model leaf nodes expressed to those found in this specification.
       Absent any extensions, MUD files are assumed to implement only the
       following ACL model features:

       o  match-on-ipv4, match-on-ipv6, match-on-tcp, match-on-udp,
          match-on-icmp

       Furthermore, only "accept" or "drop" actions SHOULD be included.  A
       MUD manager MAY choose to interpret "reject" as "drop".  A MUD
       manager SHOULD ignore all other actions.  This is because
       manufacturers do not have sufficient context within a local
       deployment to know whether reject is appropriate.  That is a decision
       that should be left to a network administrator.

       Given that MUD does not deal with interfaces, the support of the
       "ietf-interfaces" module [RFC8343] is not required.  Specifically,
       the support of interface-related features and branches (e.g.,
       interface-attachment and interface-stats) of the ACL YANG module is
       not required.

       In fact, MUD managers MAY ignore any particular component of a
       description or MAY ignore the description in its entirety, and they
       SHOULD carefully inspect all MUD descriptions.  Publishers of MUD
       files MUST NOT include other nodes except as described in
       Section 3.9.  See that section for more information.

    Lear, et al.                 Standards Track                   [Page 13]

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    2.1.  The IETF-MUD YANG Module

       This module is structured into three parts:

       o  The first component, the "mud" container, holds information that
          is relevant to retrieval and validity of the MUD file itself, as
          well as policy intended to and from the Thing.

       o  The second component augments the matching container of the ACL
          model to add several nodes that are relevant to the MUD URL, or
          they are otherwise abstracted for use within a local environment.

       o  The third component augments the tcp-acl container of the ACL
          model to add the ability to match on the direction of initiation
          of a TCP connection.

       A valid MUD file will contain two root objects: a "mud" container and
       an "acls" container.  Extensions may add additional root objects as
       required.  As a reminder, when parsing acls, elements within a
       "match" block are logically ANDed.  In general, a single abstraction
       in a match statement should be used.  For instance, it makes little
       sense to match both "my-controller" and "controller" with an
       argument, since they are highly unlikely to be the same value.

       A simplified graphical representation of the data models is used in
       this document.  The meaning of the symbols in these diagrams is
       explained in [RFC8340].

    Lear, et al.                 Standards Track                   [Page 14]

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       module: ietf-mud
         +--rw mud!
            +--rw mud-version           uint8
            +--rw mud-url               inet:uri
            +--rw last-update           yang:date-and-time
            +--rw mud-signature?        inet:uri
            +--rw cache-validity?       uint8
            +--rw is-supported          boolean
            +--rw systeminfo?           string
            +--rw mfg-name?             string
            +--rw model-name?           string
            +--rw firmware-rev?         string
            +--rw software-rev?         string
            +--rw documentation?        inet:uri
            +--rw extensions*           string
            +--rw from-device-policy
            |  +--rw acls
            |     +--rw access-list* [name]
            |        +--rw name    -> /acl:acls/acl/name
            +--rw to-device-policy
               +--rw acls
                  +--rw access-list* [name]
                     +--rw name    -> /acl:acls/acl/name

         augment /acl:acls/acl:acl/acl:aces/acl:ace/acl:matches:
           +--rw mud
              +--rw manufacturer?        inet:host
              +--rw same-manufacturer?   empty
              +--rw model?               inet:uri
              +--rw local-networks?      empty
              +--rw controller?          inet:uri
              +--rw my-controller?       empty
         augment
           /acl:acls/acl:acl/acl:aces/acl:ace/acl:matches
              /acl:l4/acl:tcp/acl:tcp:
           +--rw direction-initiated?   direction

    3.  MUD Model Definitions for the Root "mud" Container

    3.1.  mud-version

       This node specifies the integer version of the MUD specification.
       This memo specifies version 1.

    3.2.  MUD URL

       This URL identifies the MUD file.  This is useful when the file and
       associated signature are manually uploaded, say, in an offline mode.

    Lear, et al.                 Standards Track                   [Page 15]

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    3.3.  to-device-policy and from-device-policy Containers

       [RFC8519] describes access lists.  In the case of MUD, a MUD file
       must be explicit in describing the communication pattern of a Thing,
       and that includes indicating what is to be permitted or denied in
       either direction of communication.  Hence, each of these containers
       indicates the appropriate direction of a flow in association with a
       particular Thing.  They contain references to specific access lists.

    3.4.  last-update

       This is a date-and-time value of when the MUD file was generated.
       This is akin to a version number.  Its form is taken from [RFC6991].

    3.5.  cache-validity

       This uint8 is the period of time in hours that a network management
       station MUST wait since its last retrieval before checking for an
       update.  It is RECOMMENDED that this value be no less than 24, and it
       MUST NOT be more than 168 for any Thing that is supported.  This
       period SHOULD be no shorter than any period determined through HTTP
       caching directives (e.g., "cache-control" or "Expires").  N.B., the
       expiring of this timer does not require the MUD manager to discard
       the MUD file, nor terminate access to a Thing.  See Section 16 for
       more information.

    3.6.  is-supported

       This boolean is an indication from the manufacturer to the network
       administrator as to whether or not the Thing is supported.  In this
       context, a Thing is said to not be supported if the manufacturer
       intends never to issue a firmware or software update to the Thing or
       never to update the MUD file.  A MUD manager MAY still periodically
       check for updates.

    3.7.  systeminfo

       This is a textual UTF-8 description of the Thing to be connected.
       The intent is for administrators to be able to see a brief
       displayable description of the Thing.  It SHOULD NOT exceed 60
       characters worth of display space.

    Lear, et al.                 Standards Track                   [Page 16]

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    3.8.  mfg-name, software-rev, model-name, and firmware-rev

       These optional fields are filled in as specified by [RFC8348].  Note
       that firmware-rev and software-rev MUST NOT be populated in a MUD
       file if the device can be upgraded but the MUD URL cannot be.  This
       would be the case, for instance, with MUD URLs that are contained in
       802.1AR certificates.

    3.9.  extensions

       This optional leaf-list names MUD extensions that are used in the MUD
       file.  Note that MUD extensions MUST NOT be used in a MUD file
       without the extensions being declared.  Implementations MUST ignore
       any node in this file that they do not understand.

       Note that extensions can either extend the MUD file as described in
       the previous paragraph or reference other work.  An extension example
       can be found in Appendix B.

    4.  Augmentation to the ACL Model

       Note that in this section, when we use the term "match", we are
       referring to the ACL model "matches" node.

    4.1.  manufacturer

       This node consists of a hostname that would be matched against the
       authority component of another Thing's MUD URL.  In its simplest
       form, "manufacturer" and "same-manufacturer" may be implemented as
       access lists.  In more complex forms, additional network capabilities
       may be used.  For example, if one saw the line "manufacturer" :
       "flobbidy.example.com", then all Things that registered with a MUD
       URL that contained flobbity.example.com in its authority section
       would match.

    4.2.  same-manufacturer

       This null-valued node is an equivalent for when the manufacturer
       element is used to indicate that the authority found in another
       Thing's MUD URL matches that of the authority found in this Thing's
       MUD URL.  For example, if the Thing's MUD URL were
       "https://b1.example.com/ThingV1", then all devices that had a MUD URL
       with an authority section of b1.example.com would match.

    Lear, et al.                 Standards Track                   [Page 17]

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    4.3.  documentation

       This URI consists of a URL that points to documentation relating to
       the device and the MUD file.  This can prove particularly useful when
       the "controller" class is used, so that its use can be explained.

    4.4.  model

       This string matches the entire MUD URL, thus covering the model that
       is unique within the context of the authority.  It may contain not
       only model information, but versioning information as well, and any
       other information that the manufacturer wishes to add.  The intended
       use is for devices of this precise class to match, to permit or deny
       communication between one another.

    4.5.  local-networks

       This null-valued node expands to include local networks.  Its default
       expansion is that packets must not traverse toward a default route
       that is received from the router.  However, administrators may expand
       the expression as is appropriate in their deployments.

    4.6.  controller

       This URI specifies a value that a controller will register with the
       MUD manager.  The node then is expanded to the set of hosts that are
       so registered.  This node may also be a URN.  In this case, the URN
       describes a well-known service, such as DNS or NTP, that has been
       standardized.  Both of those URNs may be found in Section 17.7.

       When "my-controller" is used, it is possible that the administrator
       will be prompted to populate that class for each and every model.
       Use of "controller" with a named class allows the user to populate
       that class only once for many different models that a manufacturer
       may produce.

       Controller URIs MAY take the form of a URL (e.g., "http[s]://").
       However, MUD managers MUST NOT resolve and retrieve such files, and
       it is RECOMMENDED that there be no such file at this time, as their
       form and function may be defined at a point in the future.  For now,
       URLs should serve simply as class names and may be populated by the
       local deployment administrator.

       Great care should be taken by MUD managers when invoking the
       controller class in the form of URLs.  For one thing, it requires
       some understanding by the administrator as to when it is appropriate.

    Lear, et al.                 Standards Track                   [Page 18]

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       Pre-registration in such classes by controllers with the MUD server
       is encouraged.  The mechanism to do that is beyond the scope of this
       work.

    4.7.  my-controller

       This null-valued node signals to the MUD manager to use whatever
       mapping it has for this MUD URL to a particular group of hosts.  This
       may require prompting the administrator for class members.  Future
       work should seek to automate membership management.

    4.8.  direction-initiated

       This MUST only be applied to TCP.  This matches the direction in
       which a TCP connection is initiated.  When the direction initiated is
       "from-device", packets that are transmitted in the direction of a
       Thing MUST be dropped unless the Thing has first initiated a TCP
       connection.  By way of example, this node may be implemented in its
       simplest form by looking at naked SYN bits, but it may also be
       implemented through more stateful mechanisms.

       When applied, this matches packets when the flow was initiated in the
       corresponding direction.  [RFC6092] specifies IPv6 guidance best
       practices.  While that document is scoped specifically to IPv6, its
       contents are applicable for IPv4 as well.

    5.  Processing of the MUD File

       To keep things relatively simple in addition to whatever definitions
       exist, we also apply two additional default behaviors:

       o  Anything not explicitly permitted is denied.

       o  Local DNS and NTP are, by default, permitted to and from the
          Thing.

       An explicit description of the defaults can be found in Appendix A.
       These are applied AFTER all other explicit rules.  Thus, a default
       behavior can be changed with a "drop" action.

    6.  What Does a MUD URL Look Like?

       MUD URLs are required to use the "https" scheme, in order to
       establish the MUD file server's identity and assure integrity of the
       MUD file.

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       Any "https://" URL can be a MUD URL.  For example:

         https://things.example.org/product_abc123/v5
         https://www.example.net/mudfiles/temperature_sensor/
         https://example.com/lightbulbs/colour/v1

       A manufacturer may construct a MUD URL in any way, so long as it
       makes use of the "https" scheme.

    7.  The MUD YANG Model

       <CODE BEGINS>file "ietf-mud@2019-01-28.yang"
       module ietf-mud {
         yang-version 1.1;
         namespace "urn:ietf:params:xml:ns:yang:ietf-mud";
         prefix ietf-mud;

         import ietf-access-control-list {
           prefix acl;
         }
         import ietf-yang-types {
           prefix yang;
         }
         import ietf-inet-types {
           prefix inet;
         }

         organization
           "IETF OPSAWG (Operations and Management Area Working Group)";
         contact
           "WG Web: <https://datatracker.ietf.org/wg/opsawg/>
            WG List: opsawg@ietf.org

            Author: Eliot Lear
                    lear@cisco.com

            Author: Ralph Droms
                    rdroms@gmail.com

            Author: Dan Romascanu
                    dromasca@gmail.com
           ";
         description
           "This YANG module defines a component that augments the
            IETF description of an access list.  This specific module
            focuses on additional filters that include local, model,
            and same-manufacturer.

    Lear, et al.                 Standards Track                   [Page 20]

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            This module is intended to be serialized via JSON and stored
            as a file, as described in RFC 8520.

            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 (RFC 2119) (RFC 8174) when, and only when,
            they appear in all capitals, as shown here.

            Copyright (c) 2019 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 Simplified BSD License
            set forth in Section 4.c of the IETF Trust's Legal Provisions
            Relating to IETF Documents
            (http://trustee.ietf.org/license-info).

            This version of this YANG module is part of RFC 8520; see
            the RFC itself for full legal notices.";

         revision 2019-01-28 {
           description
             "Initial proposed standard.";
           reference
             "RFC 8520: Manufacturer Usage Description
              Specification";
         }

         typedef direction {
           type enumeration {
             enum to-device {
               description
                 "packets or flows destined to the target
                  Thing.";
             }
             enum from-device {
               description
                 "packets or flows destined from
                  the target Thing.";
             }
           }
           description
             "Which way are we talking about?";
         }

         container mud {

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           presence "Enabled for this particular MUD URL";
           description
             "MUD-related information, as specified
              by RFC 8520.";
           uses mud-grouping;
         }

         grouping mud-grouping {
           description
             "Information about when support ends (or ended)
              and when to refresh.";
           leaf mud-version {
             type uint8;
             mandatory true;
             description
               "This is the version of the MUD
                specification.  This memo specifies version 1.";
           }
           leaf mud-url {
             type inet:uri;
             mandatory true;
             description
               "This is the MUD URL associated with the entry found
                in a MUD file.";
           }
           leaf last-update {
             type yang:date-and-time;
             mandatory true;
             description
               "This is intended to be when the current MUD file
                was generated.  MUD managers SHOULD NOT check
                for updates between this time plus cache validity.";
           }
           leaf mud-signature {
             type inet:uri;
             description
               "A URI that resolves to a signature as
                described in this specification.";
           }
           leaf cache-validity {
             type uint8 {
               range "1..168";
             }
             units "hours";
             default "48";
             description
               "The information retrieved from the MUD server is
                valid for these many hours, after which it should

    Lear, et al.                 Standards Track                   [Page 22]

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                be refreshed.  N.B., MUD manager implementations
                need not discard MUD files beyond this period.";
           }
           leaf is-supported {
             type boolean;
             mandatory true;
             description
               "This boolean indicates whether or not the Thing is
                currently supported by the manufacturer.";
           }
           leaf systeminfo {
             type string;
             description
               "A UTF-8 description of this Thing.  This
                should be a brief description that may be
                displayed to the user to determine whether
                to allow the Thing on the
                network.";
           }
           leaf mfg-name {
             type string;
             description
               "Manufacturer name, as described in
                the ietf-hardware YANG module.";
           }
           leaf model-name {
             type string;
             description
               "Model name, as described in the
                ietf-hardware YANG module.";
           }
           leaf firmware-rev {
             type string;
             description
               "firmware-rev, as described in the
                ietf-hardware YANG module.  Note that this field
                MUST NOT be included when the device can be
                updated but the MUD URL cannot.";
           }
           leaf software-rev {
             type string;
             description
               "software-rev, as described in the
                ietf-hardware YANG module.  Note that this field
                MUST NOT be included when the device can be
                updated but the MUD URL cannot.";
           }
           leaf documentation {

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             type inet:uri;
             description
               "This URL points to documentation that
                relates to this device and any classes that it uses
                in its MUD file.  A caution: MUD managers need
                not resolve this URL on their own but rather simply
                provide it to the administrator.  Parsing HTML is
                not an intended function of a MUD manager.";
           }
           leaf-list extensions {
             type string {
               length "1..40";
             }
             description
               "A list of extension names that are used in this MUD
                file.  Each name is registered with the IANA and
                described in an RFC.";
           }
           container from-device-policy {
             description
               "The policies that should be enforced on traffic
                coming from the device.  These policies are not
                necessarily intended to be enforced at a single
                point but may be rendered by the controller to any
                relevant enforcement points in the network or
                elsewhere.";
             uses access-lists;
           }
           container to-device-policy {
             description
               "The policies that should be enforced on traffic
                going to the device.  These policies are not
                necessarily intended to be enforced at a single
                point but may be rendered by the controller to any
                relevant enforcement points in the network or
                elsewhere.";
             uses access-lists;
           }
         }

         grouping access-lists {
           description
             "A grouping for access lists in the context of device
              policy.";
           container access-lists {
             description
               "The access lists that should be applied to traffic
                to or from the device.";

    Lear, et al.                 Standards Track                   [Page 24]

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             list access-list {
               key "name";
               description
                 "Each entry on this list refers to an ACL that
                  should be present in the overall access list
                  data model.  Each ACL is identified by name and
                  type.";
               leaf name {
                 type leafref {
                   path "/acl:acls/acl:acl/acl:name";
                 }
                 description
                   "The name of the ACL for this entry.";
               }
             }
           }
         }

         augment "/acl:acls/acl:acl/acl:aces/acl:ace/acl:matches" {
           description
             "adding abstractions to avoid the need of IP addresses.";
           container mud {
             description
               "MUD-specific matches.";
             leaf manufacturer {
               type inet:host;
               description
                 "A domain that is intended to match the authority
                  section of the MUD URL.  This node is used to specify
                  one or more manufacturers a device should
                  be authorized to access.";
             }
             leaf same-manufacturer {
               type empty;
               description
                 "This node matches the authority section of the MUD URL
                  of a Thing.  It is intended to grant access to all
                  devices with the same authority section.";
             }
             leaf model {
               type inet:uri;
               description
                 "Devices of the specified model type will match if
                  they have an identical MUD URL.";
             }
             leaf local-networks {
               type empty;
               description

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                 "IP addresses will match this node if they are
                  considered local addresses.  A local address may be
                  a list of locally defined prefixes and masks
                  that indicate a particular administrative scope.";
             }
             leaf controller {
               type inet:uri;
               description
                 "This node names a class that has associated with it
                  zero or more IP addresses to match against.  These
                  may be scoped to a manufacturer or via a standard
                  URN.";
             }
             leaf my-controller {
               type empty;
               description
                 "This node matches one or more network elements that
                  have been configured to be the controller for this
                  Thing, based on its MUD URL.";
             }
           }
         }
         augment "/acl:acls/acl:acl/acl:aces/acl:ace/acl:matches"
               + "/acl:l4/acl:tcp/acl:tcp" {
           description
             "add direction-initiated";
           leaf direction-initiated {
             type direction;
             description
               "This node matches based on which direction a
                connection was initiated.  The means by which that
                is determined is discussed in this document.";
           }
         }
       }
       <CODE ENDS>

    8.  The Domain Name Extension to the ACL Model

       This module specifies an extension to the IETF-ACL model such that
       domain names may be referenced by augmenting the "matches" node.
       Different implementations may deploy differing methods to maintain
       the mapping between the IP address and domain name, if indeed any are
       needed.  However, the intent is that resources that are referred to
       using a name should be authorized (or not) within an access list.

    Lear, et al.                 Standards Track                   [Page 26]

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       The structure of the change is as follows:

       module: ietf-acldns
         augment /acl:acls/acl:acl/acl:aces/acl:ace/
           acl:matches/acl:l3/acl:ipv4/acl:ipv4:
           +--rw src-dnsname?   inet:host
           +--rw dst-dnsname?   inet:host
         augment /acl:acls/acl:acl/acl:aces/acl:ace/
           acl:matches/acl:l3/acl:ipv6/acl:ipv6:
           +--rw src-dnsname?   inet:host
           +--rw dst-dnsname?   inet:host

       The choice of these particular points in the access control list
       model is based on the assumption that we are in some way referring to
       IP-related resources, as that is what the DNS returns.  A domain name
       in our context is defined in [RFC6991].  The augmentations are
       replicated across IPv4 and IPv6 to allow MUD file authors the ability
       to control the IP version that the Thing may utilize.

       The following nodes are defined.

    8.1.  src-dnsname

       The argument corresponds to a domain name of a source as specified by
       inet:host.  A number of means may be used to resolve hosts.  What is
       important is that such resolutions be consistent with ACLs that are
       required by Things to properly operate.

    8.2.  dst-dnsname

       The argument corresponds to a domain name of a destination as
       specified by inet:host.  See the previous section (Section 8.1)
       relating to resolution.

       Note that when using either of these with a MUD file, because access
       is associated with a particular Thing, MUD files MUST NOT contain
       either a src-dnsname in an ACL associated with from-device-policy or
       a dst-dnsname associated with to-device-policy.

    Lear, et al.                 Standards Track                   [Page 27]

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    8.3.  The ietf-acldns Model

       <CODE BEGINS>file "ietf-acldns@2019-01-28.yang"
       module ietf-acldns {
         yang-version 1.1;
         namespace "urn:ietf:params:xml:ns:yang:ietf-acldns";
         prefix ietf-acldns;

         import ietf-access-control-list {
           prefix acl;
         }
         import ietf-inet-types {
           prefix inet;
         }

         organization
           "IETF OPSAWG (Operations and Management Area Working Group)";
         contact
           "WG Web: <https://datatracker.ietf.org/wg/opsawg/>
            WG List: opsawg@ietf.org

            Author: Eliot Lear
                    lear@cisco.com

            Author: Ralph Droms
                    rdroms@gmail.com

            Author: Dan Romascanu
                    dromasca@gmail.com
           ";
         description
           "This YANG module defines a component that augments the
            IETF description of an access list to allow DNS names
            as matching criteria.

            Copyright (c) 2019 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 Simplified BSD License
            set forth in Section 4.c of the IETF Trust's Legal Provisions
            Relating to IETF Documents
            (http://trustee.ietf.org/license-info).";

         revision 2019-01-28 {
           description
             "Base version of dnsname extension of the ACL model.";

    Lear, et al.                 Standards Track                   [Page 28]

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           reference
             "RFC 8520: Manufacturer Usage Description
              Specification";
         }

         grouping dns-matches {
           description
             "Domain names for matching.";
           leaf src-dnsname {
             type inet:host;
             description
               "domain name to be matched against.";
           }
           leaf dst-dnsname {
             type inet:host;
             description
               "domain name to be matched against.";
           }
         }

         augment "/acl:acls/acl:acl/acl:aces/acl:ace/acl:matches"
               + "/acl:l3/acl:ipv4/acl:ipv4" {
           description
             "Adding domain names to matching.";
           uses dns-matches;
         }
         augment "/acl:acls/acl:acl/acl:aces/acl:ace/acl:matches"
               + "/acl:l3/acl:ipv6/acl:ipv6" {
           description
             "Adding domain names to matching.";
           uses dns-matches;
         }
       }
       <CODE ENDS>

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    9.  MUD File Example

       This example contains two access lists that are intended to provide
       outbound access to a cloud service on TCP port 443.

       {
         "ietf-mud:mud": {
           "mud-version": 1,
           "mud-url": "https://lighting.example.com/lightbulb2000",
           "last-update": "2019-01-28T11:20:51+01:00",
           "cache-validity": 48,
           "is-supported": true,
           "systeminfo": "The BMS Example Light Bulb",
           "from-device-policy": {
             "access-lists": {
               "access-list": [
                 {
                   "name": "mud-76100-v6fr"
                 }
               ]
             }
           },
           "to-device-policy": {
             "access-lists": {
               "access-list": [
                 {
                   "name": "mud-76100-v6to"
                 }
               ]
             }
           }
         },
         "ietf-access-control-list:acls": {
           "acl": [
             {
               "name": "mud-76100-v6to",
               "type": "ipv6-acl-type",
               "aces": {
                 "ace": [
                   {
                     "name": "cl0-todev",
                     "matches": {
                       "ipv6": {
                         "ietf-acldns:src-dnsname": "test.example.com",
                         "protocol": 6
                       },
                       "tcp": {
                         "ietf-mud:direction-initiated": "from-device",

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                         "source-port": {
                           "operator": "eq",
                           "port": 443
                         }
                       }
                     },
                     "actions": {
                       "forwarding": "accept"
                     }
                   }
                 ]
               }
             },
             {
               "name": "mud-76100-v6fr",
               "type": "ipv6-acl-type",
               "aces": {
                 "ace": [
                   {
                     "name": "cl0-frdev",
                     "matches": {
                       "ipv6": {
                         "ietf-acldns:dst-dnsname": "test.example.com",
                         "protocol": 6
                       },
                       "tcp": {
                         "ietf-mud:direction-initiated": "from-device",
                         "destination-port": {
                           "operator": "eq",
                           "port": 443
                         }
                       }
                     },
                     "actions": {
                       "forwarding": "accept"
                     }
                   }
                 ]
               }
             }
           ]
         }
       }

       In this example, two policies are declared: one from the Thing and
       the other to the Thing.  Each policy names an access list that
       applies to the Thing and one that applies from the Thing.  Within
       each access list, access is permitted to packets flowing to or from

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       the Thing that can be mapped to the domain name of
       "service.bms.example.com".  For each access list, the enforcement
       point should expect that the Thing initiated the connection.

    10.  The MUD URL DHCP Option

       The IPv4 MUD URL client option has the following format:

         +------+-----+------------------------------
         | code | len |  MUDstring
         +------+-----+------------------------------

       Code OPTION_MUD_URL_V4 (161) has been assigned by IANA.  len is a
       single octet that indicates the length of the MUD string in octets.
       The MUDstring is defined as follows:

        MUDstring = mudurl [ " " reserved ]
        mudurl = URI; a URL [RFC3986] that uses the "https" scheme [RFC7230]
        reserved = 1*( OCTET ) ; from [RFC5234]

       The entire option MUST NOT exceed 255 octets.  If a space follows the
       MUD URL, a reserved string that will be defined in future
       specifications follows.  MUD managers that do not understand this
       field MUST ignore it.

       The IPv6 MUD URL client option has the following format:

          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
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |         OPTION_MUD_URL_V6     |        option-length          |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         |                            MUDstring                          |
         |                                                               |
         +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       OPTION_MUD_URL_V6 (112).

       option-length contains the length of the MUDstring, as defined above,
       in octets.

       The intent of this option is to provide both a new Thing classifier
       to the network as well as some recommended configuration to the
       routers that implement the policy.  However, it is entirely the
       purview of the network system as managed by the network administrator
       to decide what to do with this information.  The key function of this

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       option is simply to identify the type of Thing to the network in a
       structured way such that the policy can be easily found with existing
       toolsets.

    10.1.  Client Behavior

       A DHCPv4 client MAY emit a DHCPv4 option, and a DHCPv6 client MAY
       emit a DHCPv6 option.  These options are singletons, as specified in
       [RFC7227].  Because clients are intended to have at most one MUD URL
       associated with them, they may emit at most one MUD URL option via
       DHCPv4 and one MUD URL option via DHCPv6.  In the case where both v4
       and v6 DHCP options are emitted, the same URL MUST be used.

    10.2.  Server Behavior

       A DHCP server may ignore these options or take action based on
       receipt of these options.  When a server consumes this option, it
       will either forward the URL and relevant client information (such as
       the gateway address or giaddr and requested IP address, and lease
       length) to a network management system or retrieve the usage
       description itself by resolving the URL.

       DHCP servers may implement MUD functionality themselves or they may
       pass along appropriate information to a network management system or
       MUD manager.  A DHCP server that does process the MUD URL MUST adhere
       to the process specified in [RFC2818] and [RFC5280] to validate the
       TLS certificate of the web server hosting the MUD file.  Those
       servers will retrieve the file, process it, and create and install
       the necessary configuration on the relevant network element.  Servers
       SHOULD monitor the gateway for state changes on a given interface.  A
       DHCP server that does not provide MUD functionality and has forwarded
       a MUD URL to a MUD manager MUST notify the MUD manager of any
       corresponding change to the DHCP state of the client (such as
       expiration or explicit release of a network address lease).

       Should the DHCP server fail, in the case when it implements the MUD
       manager functionality, any backup mechanisms SHOULD include the MUD
       state, and the server SHOULD resolve the status of clients upon its
       restart, similar to what it would do absent MUD manager
       functionality.  In the case where the DHCP server forwards
       information to the MUD manager, the MUD manager will either make use
       of redundant DHCP servers for information or clear state based on
       other network information, such as monitoring port status on a switch
       via SNMP, Radius accounting, or similar mechanisms.

    10.3.  Relay Requirements

       There are no additional requirements for relays.

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    11.  The Manufacturer Usage Description (MUD) URL X.509 Extension

       This section defines an X.509 non-critical certificate extension that
       contains a single URL that points to an online Manufacturer Usage
       Description concerning the certificate subject.  The URI must be
       represented as described in Section 7.4 of [RFC5280].

       Any Internationalized Resource Identifiers (IRIs) MUST be mapped to
       URIs as specified in Section 3.1 of [RFC3987] before they are placed
       in the certificate extension.

       The semantics of the URL are defined Section 6 of this document.

       The choice of id-pe is based on guidance found in Section 4.2.2 of
       [RFC5280]:

             These extensions may be used to direct applications to on-line
             information about the issuer or the subject.

       The MUD URL is precisely that: online information about the
       particular subject.

       In addition, a separate new extension is defined as id-pe-mudsigner.
       This contains the subject field of the signing certificate of the MUD
       file.  Processing of this field is specified in Section 13.2.

       The purpose of this signature is to make a claim that the MUD file
       found on the server is valid for a given device, independent of any
       other factors.  There are several security considerations below in
       Section 16.

       A new content-type id-ct-mud is also defined.  While signatures are
       detached today, should a MUD file be transmitted as part of a
       Cryptographic Message Syntax (CMS) message, this content-type SHOULD
       be used.

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       This module imports from [RFC5912] and [RFC6268].  The new extension
       is identified as follows:

       <CODE BEGINS>
          MUDURLExtnModule-2016 { iso(1) identified-organization(3) dod(6)
                       internet(1) security(5) mechanisms(5) pkix(7)
                       id-mod(0) id-mod-mudURLExtn2016(88) }
           DEFINITIONS IMPLICIT TAGS ::= BEGIN

           -- EXPORTS ALL --

          IMPORTS

            -- RFC 5912
            EXTENSION
            FROM PKIX-CommonTypes-2009
                 { iso(1) identified-organization(3) dod(6) internet(1)
                   security(5) mechanisms(5) pkix(7) id-mod(0)
                   id-mod-pkixCommon-02(57) }

            -- RFC 5912
            id-ct
            FROM PKIXCRMF-2009
                 { iso(1) identified-organization(3) dod(6) internet(1)
                   security(5)  mechanisms(5) pkix(7) id-mod(0)
                   id-mod-crmf2005-02(55) }

            -- RFC 6268
            CONTENT-TYPE
            FROM CryptographicMessageSyntax-2010
              { iso(1) member-body(2) us(840) rsadsi(113549)
                pkcs(1) pkcs-9(9) smime(16) modules(0) id-mod-cms-2009(58) }

            -- RFC 5912
            id-pe, Name
            FROM PKIX1Explicit-2009
                  { iso(1) identified-organization(3) dod(6) internet(1)
                    security(5) mechanisms(5) pkix(7) id-mod(0)
                    id-mod-pkix1-explicit-02(51) } ;

           --
           -- Certificate Extensions
           --

           MUDCertExtensions EXTENSION ::=
              { ext-MUDURL | ext-MUDsigner, ... }

           ext-MUDURL EXTENSION ::=

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              { SYNTAX MUDURLSyntax IDENTIFIED BY id-pe-mud-url }

           id-pe-mud-url OBJECT IDENTIFIER ::= { id-pe 25 }

           MUDURLSyntax ::= IA5String

           ext-MUDsigner EXTENSION ::=
              { SYNTAX MUDsignerSyntax IDENTIFIED BY id-pe-mudsigner }

           id-pe-mudsigner OBJECT IDENTIFIER ::= { id-pe 30 }

           MUDsignerSyntax ::= Name

           --
           -- CMS Content Types
           --

           MUDContentTypes CONTENT-TYPE ::=
              { ct-mud, ... }

            ct-mud CONTENT-TYPE ::=
              { -- directly include the content
                IDENTIFIED BY id-ct-mudtype }
              -- The binary data that is in the form
              -- "application/mud+json" is directly encoded as the
              -- signed data.  No additional ASN.1 encoding is added.

           id-ct-mudtype OBJECT IDENTIFIER ::= { id-ct 41 }

           END
       <CODE ENDS>

       While this extension can appear in either an 802.AR manufacturer
       certificate (IDevID) or a deployment certificate (LDevID), of course
       it is not guaranteed in either, nor is it guaranteed to be carried
       over.  It is RECOMMENDED that MUD manager implementations maintain a
       table that maps a Thing to its MUD URL based on IDevIDs.

    12.  The Manufacturer Usage Description LLDP Extension

       The IEEE802.1AB Link Layer Discovery Protocol (LLDP) is a one-hop,
       vendor-neutral link-layer protocol used by end host network Things
       for advertising their identity, capabilities, and neighbors on an
       IEEE 802 local area network.  Its Type-Length-Value (TLV) design
       allows for "vendor-specific" extensions to be defined.  IANA has a
       registered IEEE 802 organizationally unique identifier (OUI) defined
       as documented in [RFC7042].  The MUD LLDP extension uses a subtype
       defined in this document to carry the MUD URL.

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       The LLDP vendor-specific frame has the following format:

       +--------+--------+----------+---------+--------------
       |TLV Type|  len   |   OUI    |subtype  | MUDString
       |  =127  |        |= 00 00 5E|  = 1    |
       |(7 bits)|(9 bits)|(3 octets)|(1 octet)|(1-255 octets)
       +--------+--------+----------+---------+--------------

       where:

       o  TLV Type = 127 indicates a vendor-specific TLV

       o  len = indicates the TLV string length

       o  OUI = 00 00 5E is the organizationally unique identifier of IANA

       o  subtype = 1 (as assigned by IANA for the MUDstring)

       o  MUDstring = the length MUST NOT exceed 255 octets

       The intent of this extension is to provide both a new Thing
       classifier to the network as well as some recommended configuration
       to the routers that implement the policy.  However, it is entirely
       the purview of the network system as managed by the network
       administrator to decide what to do with this information.  The key
       function of this extension is simply to identify the type of Thing to
       the network in a structured way such that the policy can be easily
       found with existing toolsets.

       Hosts, routers, or other network elements that implement this option
       are intended to have at most one MUD URL associated with them, so
       they may transmit at most one MUD URL value.

       Hosts, routers, or other network elements that implement this option
       may ignore these options or take action based on receipt of these
       options.  For example, they may fill in information in the respective
       extensions of the LLDP Management Information Base (MIB).  LLDP
       operates in a one-way direction.  Link Layer Discovery Protocol Data
       Units (LLDPDUs) are not exchanged as information requests by one
       Thing and responses sent by another Thing.  The other Things do not
       acknowledge LLDP information received from a Thing.  No specific
       network behavior is guaranteed.  When a Thing consumes this
       extension, it may either forward the URL and relevant remote Thing
       information to a MUD manager or retrieve the usage description by
       resolving the URL in accordance with normal HTTP semantics.

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    13.  The Creating and Processing of Signed MUD Files

       Because MUD files contain information that may be used to configure
       network access lists, they are sensitive.  To ensure that they have
       not been tampered with, it is important that they be signed.  We make
       use of DER-encoded Cryptographic Message Syntax (CMS) [RFC5652] for
       this purpose.

    13.1.  Creating a MUD File Signature

       A MUD file MUST be signed using CMS as an opaque binary object.  In
       order to make successful verification more likely, intermediate
       certificates SHOULD be included.  The signature is stored at the
       location specified in the MUD file.  Signatures are transferred using
       content-type "application/pkcs7-signature".

       For example:

       % openssl cms -sign -signer mancertfile -inkey mankey \
                     -in mudfile -binary -outform DER -binary \
                     -certfile intermediatecert -out mudfile.p7s

       Note: A MUD file may need to be re-signed if the signature expires.

    13.2.  Verifying a MUD File Signature

       Prior to processing the rest of a MUD file, the MUD manager MUST
       retrieve the MUD signature file by retrieving the value of "mud-
       signature" and validating the signature across the MUD file.  The Key
       Usage Extension in the signing certificate MUST be present and have
       the bit digitalSignature(0) set.  When the id-pe-mudsigner extension
       is present in a device's X.509 certificate, the MUD signature file
       MUST have been generated by a certificate whose subject matches the
       contents of that id-pe-mudsigner extension.  If these conditions are
       not met, or if it cannot validate the chain of trust to a known trust
       anchor, the MUD manager MUST cease processing the MUD file until an
       administrator has given approval.

       The purpose of the signature on the file is to assign accountability
       to an entity, whose reputation can be used to guide administrators on
       whether or not to accept a given MUD file.  It is already common
       place to check web reputation on the location of a server on which a
       file resides.  While it is likely that the manufacturer will be the
       signer of the file, this is not strictly necessary, and it may not be
       desirable.  For one thing, in some environments, integrators may
       install their own certificates.  For another, what is more important
       is the accountability of the recommendation, and not just the
       relationship between the Thing and the file.

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       An example:

       % openssl cms -verify -in mudfile.p7s -inform DER -content mudfile

       Note the additional step of verifying the common trust root.

    14.  Extensibility

       One of our design goals is to see that MUD files are able to be
       understood by as broad a cross-section of systems as is possible.
       Coupled with the fact that we have also chosen to leverage existing
       mechanisms, we are left with no ability to negotiate extensions and a
       limited desire for those extensions in any event.  As such, a two-
       tier extensibility framework is employed, as follows:

       1.  At a coarse grain, a protocol version is included in a MUD URL.
           This memo specifies MUD version 1.  Any and all changes are
           entertained when this version is bumped.  Transition approaches
           between versions would be a matter for discussion in future
           versions.

       2.  At a finer grain, only extensions that would not incur additional
           risk to the Thing are permitted.  Specifically, adding nodes to
           the mud container is permitted with the understanding that such
           additions will be ignored by unaware implementations.  Any such
           extensions SHALL be standardized through the IETF process and
           MUST be named in the "extensions" list.  MUD managers MUST ignore
           YANG nodes they do not understand and SHOULD create an exception
           to be resolved by an administrator, so as to avoid any policy
           inconsistencies.

    15.  Deployment Considerations

       Because MUD consists of a number of architectural building blocks, it
       is possible to assemble different deployment scenarios.  One key
       aspect is where to place policy enforcement.  In order to protect the
       Thing from other Things within a local deployment, policy can be
       enforced on the nearest switch or access point.  In order to limit
       unwanted traffic within a network, it may also be advisable to
       enforce policy as close to the Internet as possible.  In some
       circumstances, policy enforcement may not be available at the closest
       hop.  At that point, the risk of lateral infection (infection of
       devices that reside near one another) is increased to the number of
       Things that are able to communicate without protection.

       A caution about some of the classes: admission of a Thing into the
       "manufacturer" and "same-manufacturer" class may have impact on the
       access of other Things.  Put another way, the admission may grow the

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       access list on switches connected to other Things, depending on how
       access is managed.  Some care should be given on managing that access
       list growth.  Alternative methods such as additional network
       segmentation can be used to keep that growth within reason.

       Because as of this writing MUD is a new concept, one can expect a
       great many devices to not have implemented it.  It remains a local
       deployment decision as to whether a device that is first connected
       should be allowed broad or limited access.  Furthermore, as mentioned
       in the introduction, a deployment may choose to ignore a MUD policy
       in its entirety and simply take into account the MUD URL as a
       classifier to be used as part of a local policy decision.

       Finally, please see directly below information regarding device
       lifetimes and use of domain names.

    16.  Security Considerations

       Based on how a MUD URL is emitted, a Thing may be able to lie about
       what it is, thus gaining additional network access.  This can happen
       in a number of ways when a device emits a MUD URL using DHCP or LLDP,
       such as being inappropriately admitted to a class such as
       "same-manufacturer", being given access to a device such as
       "my-controller", or being permitted access to an Internet resource,
       where such access would otherwise be disallowed.  Whether that is the
       case will depend on the deployment.  Implementations SHOULD be
       configurable to disallow additive access for devices using MUD URLs
       that are not emitted in a secure fashion such as in a certificate.
       Similarly, implementations SHOULD NOT grant elevated permissions
       (beyond those of devices presenting no MUD policy) to devices that do
       not strongly bind their identity to their L2/L3 transmissions.  When
       insecure methods are used by the MUD manager, the classes SHOULD NOT
       contain devices that use both insecure and secure methods, in order
       to prevent privilege escalation attacks, and MUST NOT contain devices
       with the same MUD URL that are derived from both strong and weak
       authentication methods.

       Devices may forge source (L2/L3) information.  Deployments should
       apply appropriate protections to bind communications to the
       authentication that has taken place.  For 802.1X authentication, IEEE
       802.1AE (MACsec) [IEEE8021AE] is one means by which this may happen.
       A similar approach can be used with 802.11i (Wi-Fi Protected Access 2
       (WPA2)) [IEEE80211i].  Other means are available with other lower-
       layer technologies.  Implementations using session-oriented access
       that is not cryptographically bound should take care to remove state
       when any form of break in the session is detected.

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       A rogue certification authority (CA) may sign a certificate that
       contains the same subject name as is listed in the MUDsigner field in
       the manufacturer certificate, thus seemingly permitting a substitute
       MUD file for a device.  There are two mitigations available: First,
       if the signer changes, this may be flagged as an exception by the MUD
       manager.  Second, if the MUD file also changes, the MUD manager
       SHOULD seek administrator approval (it should do this in any case).
       In all circumstances, the MUD manager MUST maintain a cache of
       trusted CAs for this purpose.  When such a rogue is discovered, it
       SHOULD be removed.

       Additional mitigations are described below.

       When certificates are not present, Things claiming to be of a certain
       manufacturer SHOULD NOT be included in that manufacturer grouping
       without additional validation of some form.  This will be relevant
       when the MUD manager makes use of primitives such as "manufacturer"
       for the purpose of accessing Things of a particular type.  Similarly,
       network management systems may be able to fingerprint the Thing.  In
       such cases, the MUD URL can act as a classifier that can be proven or
       disproven.  Fingerprinting may have other advantages as well: when
       802.1AR certificates are used, because they themselves cannot change,
       fingerprinting offers the opportunity to add artifacts to the MUD
       string in the form of the reserved field discussed in Section 10.
       The meaning of such artifacts is left as future work.

       MUD managers SHOULD NOT accept a usage description for a Thing with
       the same Media Access Control (MAC) address that has indicated a
       change of the URL authority without some additional validation (such
       as review by a network administrator).  New Things that present some
       form of unauthenticated MUD URL SHOULD be validated by some external
       means when they would be given increased network access.

       It may be possible for a rogue manufacturer to inappropriately
       exercise the MUD file parser, in order to exploit a vulnerability.
       There are two recommended approaches to address this threat.  The
       first is to validate that the signer of the MUD file is known to and
       trusted by the MUD manager.  The second is to have a system do a
       primary scan of the file to ensure that it is both parseable and
       believable at some level.  MUD files will likely be relatively small,
       to start with.  The number of ACEs used by any given Thing should be
       relatively small as well.  It may also be useful to limit retrieval
       of MUD URLs to only those sites that are known to have decent web or
       domain reputations.

       Use of a URL necessitates the use of domain names.  If a domain name
       changes ownership, the new owner of that domain may be able to
       provide MUD files that MUD managers would consider valid.  MUD

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       managers SHOULD cache certificates used by the MUD file server.  When
       a new certificate is retrieved for whatever reason, the MUD manager
       should check to see if ownership of the domain has changed.  A fair
       programmatic approximation of this is when the name servers for the
       domain have changed.  If the actual MUD file has changed, the MUD
       manager MAY check the WHOIS database to see if registration ownership
       of a domain has changed.  If a change has occurred, or if for some
       reason it is not possible to determine whether ownership has changed,
       further review may be warranted.  Note, this remediation does not
       take into account the case of a Thing that was produced long ago and
       only recently fielded, or the case where a new MUD manager has been
       installed.

       The release of a MUD URL by a Thing reveals what the Thing is and
       provides an attacker with guidance on what vulnerabilities may be
       present.

       While the MUD URL itself is not intended to be unique to a specific
       Thing, the release of the URL may aid an observer in identifying
       individuals when combined with other information.  This is a privacy
       consideration.

       In addressing both of these concerns, implementors should take into
       account what other information they are advertising through
       mechanisms such as Multicast DNS (mDNS) [RFC6872]; how a Thing might
       otherwise be identified, perhaps through how it behaves when it is
       connected to the network; and whether a Thing is intended to be used
       by individuals or carry personal identifying information, and then
       apply appropriate data minimization techniques.  One approach is to
       make use of TEAP [RFC7170] as the means to share information with
       authorized components in the network.  Network elements may also
       assist in limiting access to the MUD URL through the use of
       mechanisms such as DHCPv6-Shield [RFC7610].

       There is the risk of the MUD manager itself being spied on to
       determine what things are connected to the network.  To address this
       risk, MUD managers may choose to make use of TLS proxies that they
       trust that would aggregate other information.

       Please note that the security considerations mentioned in Section 3.7
       of [RFC8407] are not applicable in this case because the YANG
       serialization is not intended to be accessed via NETCONF.  However,
       for those who try to instantiate this model in a network element via
       the Network Configuration Protocol (NETCONF), all objects in each
       model in this document exhibit similar security characteristics as
       [RFC8519].  The basic purpose of MUD is to configure access, so by
       its very nature, it can be disruptive if used by unauthorized
       parties.

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    17.  IANA Considerations

    17.1.  YANG Module Registrations

       The following YANG modules have been registered in the "YANG Module
       Names" registry:

          Name: ietf-mud
          URN: urn:ietf:params:xml:ns:yang:ietf-mud
          Prefix: ietf-mud
          Registrant contact: The IESG
          Reference: RFC 8520

          Name: ietf-acldns
          URI: urn:ietf:params:xml:ns:yang:ietf-acldns
          Prefix: ietf-acldns
          Registrant contact: The IESG
          Reference: RFC 8520

    17.2.  URI Registrations

       IANA has added the following entries to the "IETF XML registry":

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

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

    17.3.  DHCPv4 and DHCPv6 Options

       The IANA has allocated OPTION_MUD_URL_V4 (161) in the "Dynamic Host
       Configuration Protocol (DHCP) and Bootstrap Protocol (BOOTP)
       Parameters" registry, and OPTION_MUD_URL_V6 (112) in the "Dynamic
       Host Configuration Protocol for IPv6 (DHCPv6)" registry, as described
       in Section 10.

    17.4.  PKIX Extensions

       IANA has made the following assignments for:

       o  The MUDURLExtnModule-2016 ASN.1 module (88) in the "SMI Security
          for PKIX Module Identifier" registry (1.3.6.1.5.5.7.0).

       o  id-pe-mud-url object identifier (25) from the "SMI Security for
          PKIX Certificate Extension" registry (1.3.6.1.5.5.7.1).

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       o  id-pe-mudsigner object identifier (30) from the "SMI Security for
          PKIX Certificate Extension" registry.

       o  id-ct-mudtype object identifier (41) from the "SMI Security for
          S/MIME CMS Content Type" registry.

       o  The use of these values is specified in Section 11.

    17.5.  Media Type Registration for MUD Files

       The following media type is defined for the transfer of MUD files:

       o  Type name: application

       o  Subtype name: mud+json

       o  Required parameters: N/A

       o  Optional parameters: N/A

       o  Encoding considerations: 8bit; "application/mud+json" values are
          represented as JSON objects; UTF-8 encoding MUST be employed
          [RFC3629].

       o  Security considerations: See Security Considerations of RFC 8520
          and Section 12 of [RFC8259].

       o  Interoperability considerations: N/A

       o  Published specification: RFC 8520

       o  Applications that use this media type: MUD managers as specified
          by RFC 8520.

       o  Fragment identifier considerations: N/A

       o  Additional information:
          Magic number(s): N/A
          File extension(s): N/A
          Macintosh file type code(s): N/A

       o  Person & email address to contact for further information:
          Eliot Lear <lear@cisco.com>, Ralph Droms <rdroms@gmail.com>,
          Dan Romascanu <dromasca@gmail.com>

       o  Intended usage: COMMON

       o  Restrictions on usage: none

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       o  Author:
          Eliot Lear <lear@cisco.com>
          Ralph Droms <rdroms@gmail.com>
          Dan Romascanu <dromasca@gmail.com>

       o  Change controller: IESG

       o  Provisional registration? (standards tree only): No.

    17.6.  IANA LLDP TLV Subtype Registry

       IANA has created a new registry titled "IANA Link Layer Discovery
       Protocol (LLDP) TLV Subtypes" under "IEEE 802 Numbers".  The policy
       for this registry is Expert Review [RFC8126].  The maximum number of
       entries in the registry is 256.

       IANA has populated the initial registry as follows:

       LLDP subtype value: 1 (All the other 255 values are initially marked
       as "Unassigned".)

       Description: the Manufacturer Usage Description (MUD) Uniform
       Resource Locator (URL)

       Reference: RFC 8520

    17.7.  The MUD Well-Known Universal Resource Name (URNs)

       The following parameter registry has been added in accordance with
       [RFC3553].

          Registry name: MUD Well-Known Universal Resource Name (URN)
          Specification: RFC 8520
          Repository: https://www.iana.org/assignments/mud
          Index value:  Encoded identically to a TCP/UDP port service
                        name, as specified in Section 5.1 of [RFC6335]

       The following entries have been added to the "MUD Well-Known
       Universal Resource Name (URN)" registry:

       "urn:ietf:params:mud:dns" refers to the service specified by
       [RFC1123].  "urn:ietf:params:mud:ntp" refers to the service specified
       by [RFC5905].

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    17.8.  Extensions Registry

       The IANA has established a registry of extensions as follows:

          Registry name: MUD Extensions
          Registry policy: Standards Action
          Reference: RFC 8520
          Extension name: UTF-8-encoded string, not to exceed 40 characters.

       Each extension MUST follow the rules specified in this specification.
       As is usual, the IANA issues early allocations in accordance with
       [RFC7120].

    18.  References

    18.1.  Normative References

       [IEEE8021AB]
                  IEEE, "IEEE Standard for Local and Metropolitan Area
                  Networks-- Station and Media Access Control Connectivity
                  Discovery", IEEE 802.1AB.

       [RFC1123]  Braden, R., Ed., "Requirements for Internet Hosts -
                  Application and Support", STD 3, RFC 1123,
                  DOI 10.17487/RFC1123, October 1989,
                  <https://www.rfc-editor.org/info/rfc1123>.

       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119,
                  DOI 10.17487/RFC2119, March 1997,
                  <https://www.rfc-editor.org/info/rfc2119>.

       [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
                  RFC 2131, DOI 10.17487/RFC2131, March 1997,
                  <https://www.rfc-editor.org/info/rfc2131>.

       [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
                  DOI 10.17487/RFC2818, May 2000,
                  <https://www.rfc-editor.org/info/rfc2818>.

       [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
                  10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
                  2003, <https://www.rfc-editor.org/info/rfc3629>.

       [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
                  Levkowetz, Ed., "Extensible Authentication Protocol
                  (EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,
                  <https://www.rfc-editor.org/info/rfc3748>.

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       [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
                  Resource Identifier (URI): Generic Syntax", STD 66,
                  RFC 3986, DOI 10.17487/RFC3986, January 2005,
                  <https://www.rfc-editor.org/info/rfc3986>.

       [RFC3987]  Duerst, M. and M. Suignard, "Internationalized Resource
                  Identifiers (IRIs)", RFC 3987, DOI 10.17487/RFC3987,
                  January 2005, <https://www.rfc-editor.org/info/rfc3987>.

       [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
                  Specifications: ABNF", STD 68, RFC 5234,
                  DOI 10.17487/RFC5234, January 2008,
                  <https://www.rfc-editor.org/info/rfc5234>.

       [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
                  Housley, R., and W. Polk, "Internet X.509 Public Key
                  Infrastructure Certificate and Certificate Revocation List
                  (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
                  <https://www.rfc-editor.org/info/rfc5280>.

       [RFC5652]  Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
                  RFC 5652, DOI 10.17487/RFC5652, September 2009,
                  <https://www.rfc-editor.org/info/rfc5652>.

       [RFC5905]  Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
                  "Network Time Protocol Version 4: Protocol and Algorithms
                  Specification", RFC 5905, DOI 10.17487/RFC5905, June 2010,
                  <https://www.rfc-editor.org/info/rfc5905>.

       [RFC5912]  Hoffman, P. and J. Schaad, "New ASN.1 Modules for the
                  Public Key Infrastructure Using X.509 (PKIX)", RFC 5912,
                  DOI 10.17487/RFC5912, June 2010,
                  <https://www.rfc-editor.org/info/rfc5912>.

       [RFC6268]  Schaad, J. and S. Turner, "Additional New ASN.1 Modules
                  for the Cryptographic Message Syntax (CMS) and the Public
                  Key Infrastructure Using X.509 (PKIX)", RFC 6268,
                  DOI 10.17487/RFC6268, July 2011,
                  <https://www.rfc-editor.org/info/rfc6268>.

       [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
                  Cheshire, "Internet Assigned Numbers Authority (IANA)
                  Procedures for the Management of the Service Name and
                  Transport Protocol Port Number Registry", BCP 165,
                  RFC 6335, DOI 10.17487/RFC6335, August 2011,
                  <https://www.rfc-editor.org/info/rfc6335>.

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       [RFC6991]  Schoenwaelder, J., Ed., "Common YANG Data Types",
                  RFC 6991, DOI 10.17487/RFC6991, July 2013,
                  <https://www.rfc-editor.org/info/rfc6991>.

       [RFC7120]  Cotton, M., "Early IANA Allocation of Standards Track Code
                  Points", BCP 100, RFC 7120, DOI 10.17487/RFC7120, January
                  2014, <https://www.rfc-editor.org/info/rfc7120>.

       [RFC7227]  Hankins, D., Mrugalski, T., Siodelski, M., Jiang, S., and
                  S. Krishnan, "Guidelines for Creating New DHCPv6 Options",
                  BCP 187, RFC 7227, DOI 10.17487/RFC7227, May 2014,
                  <https://www.rfc-editor.org/info/rfc7227>.

       [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
                  Protocol (HTTP/1.1): Message Syntax and Routing",
                  RFC 7230, DOI 10.17487/RFC7230, June 2014,
                  <https://www.rfc-editor.org/info/rfc7230>.

       [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
                  Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
                  DOI 10.17487/RFC7231, June 2014,
                  <https://www.rfc-editor.org/info/rfc7231>.

       [RFC7610]  Gont, F., Liu, W., and G. Van de Velde, "DHCPv6-Shield:
                  Protecting against Rogue DHCPv6 Servers", BCP 199,
                  RFC 7610, DOI 10.17487/RFC7610, August 2015,
                  <https://www.rfc-editor.org/info/rfc7610>.

       [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>.

       [RFC7951]  Lhotka, L., "JSON Encoding of Data Modeled with YANG",
                  RFC 7951, DOI 10.17487/RFC7951, August 2016,
                  <https://www.rfc-editor.org/info/rfc7951>.

       [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
                  2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
                  May 2017, <https://www.rfc-editor.org/info/rfc8174>.

       [RFC8259]  Bray, T., Ed., "The JavaScript Object Notation (JSON) Data
                  Interchange Format", STD 90, RFC 8259,
                  DOI 10.17487/RFC8259, December 2017,
                  <https://www.rfc-editor.org/info/rfc8259>.

       [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>.

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       [RFC8348]  Bierman, A., Bjorklund, M., Dong, J., and D. Romascanu, "A
                  YANG Data Model for Hardware Management", RFC 8348,
                  DOI 10.17487/RFC8348, March 2018,
                  <https://www.rfc-editor.org/info/rfc8348>.

       [RFC8415]  Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
                  Richardson, M., Jiang, S., Lemon, T., and T. Winters,
                  "Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
                  RFC 8415, DOI 10.17487/RFC8415, November 2018,
                  <https://www.rfc-editor.org/info/rfc8415>.

       [RFC8519]  Jethanandani, M., Agarwal, S., Huang, L., and D. Blair,
                  "YANG Data Model for Network Access Control Lists (ACLs)",
                  RFC 8519, DOI 10.17487/RFC8519, March 2019,
                  <https://www.rfc-editor.org/info/rfc8519>.

    18.2.  Informative References

       [FW95]     Chapman, D. and E. Zwicky, "Building Internet Firewalls",
                  First Edition, November 1995.

       [IEEE80211i]
                  IEEE, "IEEE Standard for information technology-
                  Telecommunications and information exchange between
                  systems-Local and metropolitan area networks-Specific
                  requirements-Part 11: Wireless LAN Medium Access Control
                  (MAC) and Physical Layer (PHY) specifications: Amendment
                  6: Medium Access Control (MAC) Security Enhancements",
                  IEEE 802.11i.

       [IEEE8021AE]
                  IEEE, "IEEE Standard for Local and metropolitan area
                  networks-Media Access Control (MAC) Security",
                  IEEE 802.1AE.

       [IEEE8021AR]
                  IEEE, "IEEE Standard for Local and metropolitan area
                  networks - Secure Device Identity", IEEE 802.1AR.

       [IEEE8021X]
                  IEEE, "IEEE Standard for Local and metropolitan area
                  networks--Port-Based Network Access Control", IEEE 802.1X.

       [RFC1984]  IAB and IESG, "IAB and IESG Statement on Cryptographic
                  Technology and the Internet", BCP 200, RFC 1984,
                  DOI 10.17487/RFC1984, August 1996,
                  <https://www.rfc-editor.org/info/rfc1984>.

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       [RFC3553]  Mealling, M., Masinter, L., Hardie, T., and G. Klyne, "An
                  IETF URN Sub-namespace for Registered Protocol
                  Parameters", BCP 73, RFC 3553, DOI 10.17487/RFC3553, June
                  2003, <https://www.rfc-editor.org/info/rfc3553>.

       [RFC6092]  Woodyatt, J., Ed., "Recommended Simple Security
                  Capabilities in Customer Premises Equipment (CPE) for
                  Providing Residential IPv6 Internet Service", RFC 6092,
                  DOI 10.17487/RFC6092, January 2011,
                  <https://www.rfc-editor.org/info/rfc6092>.

       [RFC6872]  Gurbani, V., Ed., Burger, E., Ed., Anjali, T., Abdelnur,
                  H., and O. Festor, "The Common Log Format (CLF) for the
                  Session Initiation Protocol (SIP): Framework and
                  Information Model", RFC 6872, DOI 10.17487/RFC6872,
                  February 2013, <https://www.rfc-editor.org/info/rfc6872>.

       [RFC7042]  Eastlake 3rd, D. and J. Abley, "IANA Considerations and
                  IETF Protocol and Documentation Usage for IEEE 802
                  Parameters", BCP 141, RFC 7042, DOI 10.17487/RFC7042,
                  October 2013, <https://www.rfc-editor.org/info/rfc7042>.

       [RFC7170]  Zhou, H., Cam-Winget, N., Salowey, J., and S. Hanna,
                  "Tunnel Extensible Authentication Protocol (TEAP) Version
                  1", RFC 7170, DOI 10.17487/RFC7170, May 2014,
                  <https://www.rfc-editor.org/info/rfc7170>.

       [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
                  Application Protocol (CoAP)", RFC 7252,
                  DOI 10.17487/RFC7252, June 2014,
                  <https://www.rfc-editor.org/info/rfc7252>.

       [RFC7452]  Tschofenig, H., Arkko, J., Thaler, D., and D. McPherson,
                  "Architectural Considerations in Smart Object Networking",
                  RFC 7452, DOI 10.17487/RFC7452, March 2015,
                  <https://www.rfc-editor.org/info/rfc7452>.

       [RFC7488]  Boucadair, M., Penno, R., Wing, D., Patil, P., and T.
                  Reddy, "Port Control Protocol (PCP) Server Selection",
                  RFC 7488, DOI 10.17487/RFC7488, March 2015,
                  <https://www.rfc-editor.org/info/rfc7488>.

       [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,
                  <https://www.rfc-editor.org/info/rfc8126>.

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       [RFC8343]  Bjorklund, M., "A YANG Data Model for Interface
                  Management", RFC 8343, DOI 10.17487/RFC8343, March 2018,
                  <https://www.rfc-editor.org/info/rfc8343>.

       [RFC8407]  Bierman, A., "Guidelines for Authors and Reviewers of
                  Documents Containing YANG Data Models", BCP 216, RFC 8407,
                  DOI 10.17487/RFC8407, October 2018,
                  <https://www.rfc-editor.org/info/rfc8407>.

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    Appendix A.  Default MUD Nodes

       What follows is the portion of a MUD file that permits DNS traffic to
       a controller that is registered with the URN
       "urn:ietf:params:mud:dns" and traffic NTP to a controller that is
       registered with "urn:ietf:params:mud:ntp".  This is considered the
       default behavior, and the ACEs are in effect appended to whatever
       other "ace" entries that a MUD file contains.  To block DNS or NTP,
       one repeats the matching statement but replaces the "forwarding"
       action "accept" with "drop".  Because ACEs are processed in the order
       they are received, the defaults would not be reached.  A MUD manager
       might further decide to optimize to simply not include the defaults
       when they are overridden.

       Four "acl" list entries that implement default MUD nodes are listed
       below.  Two are for IPv4 and two are for IPv6 (one in each direction
       for both versions of IP).  Note that neither the access list name nor
       the ace name need be retained or used in any way by local
       implementations; they are simply there for the sake of completeness.

        "ietf-access-control-list:acls": {
           "acl": [
             {
               "name": "mud-59776-v4to",
               "type": "ipv4-acl-type",
               "aces": {
                 "ace": [
                   {
                     "name": "ent0-todev",
                     "matches": {
                       "ietf-mud:mud": {
                         "controller": "urn:ietf:params:mud:dns"
                       },
                       "ipv4": {
                         "protocol": 17
                       },
                       "udp": {
                         "source-port": {
                           "operator": "eq",
                           "port": 53
                         }
                       }
                     },
                     "actions": {
                       "forwarding": "accept"
                     }
                   },
                   {

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                     "name": "ent1-todev",
                     "matches": {
                       "ietf-mud:mud": {
                         "controller": "urn:ietf:params:mud:ntp"
                       },
                       "ipv4": {
                         "protocol": 17
                       },
                       "udp": {
                         "source-port": {
                           "operator": "eq",
                           "port": 123
                         }
                       }
                     },
                     "actions": {
                       "forwarding": "accept"
                     }
                   }
                 ]
               }
             },
             {
               "name": "mud-59776-v4fr",
               "type": "ipv4-acl-type",
               "aces": {
                 "ace": [
                   {
                     "name": "ent0-frdev",
                     "matches": {
                       "ietf-mud:mud": {
                         "controller": "urn:ietf:params:mud:dns"
                       },
                       "ipv4": {
                         "protocol": 17
                       },
                       "udp": {
                         "destination-port": {
                           "operator": "eq",
                           "port": 53
                         }
                       }
                     },
                     "actions": {
                       "forwarding": "accept"
                     }
                   },
                   {

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                     "name": "ent1-frdev",
                     "matches": {
                       "ietf-mud:mud": {
                         "controller": "urn:ietf:params:mud:ntp"
                       },
                       "ipv4": {
                         "protocol": 17
                       },
                       "udp": {
                         "destination-port": {
                           "operator": "eq",
                           "port": 123
                         }
                       }
                     },
                     "actions": {
                       "forwarding": "accept"
                     }
                   }
                 ]
               }
             },
             {
               "name": "mud-59776-v6to",
               "type": "ipv6-acl-type",
               "aces": {
                 "ace": [
                   {
                     "name": "ent0-todev",
                     "matches": {
                       "ietf-mud:mud": {
                         "controller": "urn:ietf:params:mud:dns"
                       },
                       "ipv6": {
                         "protocol": 17
                       },
                       "udp": {
                         "source-port": {
                           "operator": "eq",
                           "port": 53
                         }
                       }
                     },
                     "actions": {
                       "forwarding": "accept"
                     }
                   },
                   {

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                     "name": "ent1-todev",
                     "matches": {
                       "ietf-mud:mud": {
                         "controller": "urn:ietf:params:mud:ntp"
                       },
                       "ipv6": {
                         "protocol": 17
                       },
                       "udp": {
                         "source-port": {
                           "operator": "eq",
                           "port": 123
                         }
                       }
                     },
                     "actions": {
                       "forwarding": "accept"
                     }
                   }
                 ]
               }
             },
             {
               "name": "mud-59776-v6fr",
               "type": "ipv6-acl-type",
               "aces": {
                 "ace": [
                   {
                     "name": "ent0-frdev",
                     "matches": {
                       "ietf-mud:mud": {
                         "controller": "urn:ietf:params:mud:dns"
                       },
                       "ipv6": {
                         "protocol": 17
                       },
                       "udp": {
                         "destination-port": {
                           "operator": "eq",
                           "port": 53
                         }
                       }
                     },
                     "actions": {
                       "forwarding": "accept"
                     }
                   },
                   {

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                     "name": "ent1-frdev",
                     "matches": {
                       "ietf-mud:mud": {
                         "controller": "urn:ietf:params:mud:ntp"
                       },
                       "ipv6": {
                         "protocol": 17
                       },
                       "udp": {
                         "destination-port": {
                           "operator": "eq",
                           "port": 123
                         }
                       }
                     },
                     "actions": {
                       "forwarding": "accept"
                     }
                   }
                 ]
               }
             }
           ]
         }

    Appendix B.  A Sample Extension: DETNET-indicator

       In this sample extension, we augment the core MUD model to indicate
       whether the device implements DETNET.  If a device claims not to use
       DETNET, but then later attempts to do so, a notification or exception
       might be generated.  Note that this example is intended only for
       illustrative purposes.

     Extension Name: "Example-Extension" (to be used in the extensions list)
     Standard: RFC 8520 (but do not register the example)

       This extension augments the MUD model to include a single node, using
       the following sample module that has the following tree structure:

       module: ietf-mud-detext-example
         augment /ietf-mud:mud:
           +--rw is-detnet-required?   boolean

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       The model is defined as follows:

       <CODE BEGINS>file "ietf-mud-detext-example@2019-01-28.yang"
       module ietf-mud-detext-example {
         yang-version 1.1;
         namespace "urn:ietf:params:xml:ns:yang:ietf-mud-detext-example";
         prefix ietf-mud-detext-example;

         import ietf-mud {
           prefix ietf-mud;
         }

         organization
           "IETF OPSAWG (Operations and Management Area Working Group)";
         contact
           "WG Web: <https://datatracker.ietf.org/wg/opsawg/>
            WG List: opsawg@ietf.org

            Author: Eliot Lear
                    lear@cisco.com

            Author: Ralph Droms
                    rdroms@gmail.com

            Author: Dan Romascanu
                    dromasca@gmail.com
           ";
         description
           "Sample extension to a MUD module to indicate a need
            for DETNET support.";

         revision 2019-01-28 {
           description
             "Initial revision.";
           reference
             "RFC 8520: Manufacturer Usage Description
              Specification";
         }

         augment "/ietf-mud:mud" {
           description
             "This adds a simple extension for a manufacturer
               to indicate whether DETNET is required by a
              device.";
           leaf is-detnet-required {
             type boolean;
             description
               "This value will equal 'true' if a device requires

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                DETNET to properly function.";
           }
         }
       }
       <CODE ENDS>

       Using the previous example, we now show how the extension would be
       expressed:

       {
         "ietf-mud:mud": {
           "mud-version": 1,
           "mud-url": "https://lighting.example.com/lightbulb2000",
           "last-update": "2019-01-28T11:20:51+01:00",
           "cache-validity": 48,
           "extensions": [
               "ietf-mud-detext-example"
            ],
           "ietf-mud-detext-example:is-detnet-required": "false",
           "is-supported": true,
           "systeminfo": "The BMS Example Light Bulb",
           "from-device-policy": {
             "access-lists": {
               "access-list": [
                 {
                   "name": "mud-76100-v6fr"
                 }
               ]
             }
           },
           "to-device-policy": {
             "access-lists": {
               "access-list": [
                 {
                   "name": "mud-76100-v6to"
                 }
               ]
             }
           }
         },
         "ietf-access-control-list:acls": {
           "acl": [
             {
               "name": "mud-76100-v6to",
               "type": "ipv6-acl-type",
               "aces": {
                 "ace": [
                   {

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                     "name": "cl0-todev",
                     "matches": {
                       "ipv6": {
                         "ietf-acldns:src-dnsname": "test.example.com",
                         "protocol": 6
                       },
                       "tcp": {
                         "ietf-mud:direction-initiated": "from-device",
                         "source-port": {
                           "operator": "eq",
                           "port": 443
                         }
                       }
                     },
                     "actions": {
                       "forwarding": "accept"
                     }
                   }
                 ]
               }
             },
             {
               "name": "mud-76100-v6fr",
               "type": "ipv6-acl-type",
               "aces": {
                 "ace": [
                   {
                     "name": "cl0-frdev",
                     "matches": {
                       "ipv6": {
                         "ietf-acldns:dst-dnsname": "test.example.com",
                         "protocol": 6
                       },
                       "tcp": {
                         "ietf-mud:direction-initiated": "from-device",
                         "destination-port": {
                           "operator": "eq",
                           "port": 443
                         }
                       }
                     },
                     "actions": {
                       "forwarding": "accept"
                     }
                   }
                 ]
               }
             }

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    RFC 8520             Manufacturer Usage Descriptions          March 2019

           ]
         }
       }

    Acknowledgments

       The authors would like to thank Einar Nilsen-Nygaard, who
       singlehandedly updated the model to match the updated ACL model,
       Bernie Volz, Tom Gindin, Brian Weis, Sandeep Kumar, Thorsten Dahm,
       John Bashinski, Steve Rich, Jim Bieda, Dan Wing, Joe Clarke, Henk
       Birkholz, Adam Montville, Jim Schaad, and Robert Sparks for their
       valuable advice and reviews.  Russ Housley entirely rewrote
       Section 11 to be a complete module.  Adrian Farrel provided the basis
       for the privacy considerations text.  Kent Watsen provided a thorough
       review of the architecture and the YANG model.  The remaining errors
       in this work are entirely the responsibility of the authors.

    Authors' Addresses

       Eliot Lear
       Cisco Systems
       Richtistrasse 7
       Wallisellen  CH-8304
       Switzerland

       Phone: +41 44 878 9200
       Email: lear@cisco.com

       Ralph Droms
       Google
       355 Main St., 5th Floor
       Cambridge, MA  02142
       United States of America

       Phone: +1 978 376 3731
       Email: rdroms@gmail.com

       Dan Romascanu

       Phone: +972 54 5555347
       Email: dromasca@gmail.com

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