Authorized update to MUD URLs
draft-ietf-opsawg-mud-acceptable-urls-11

OPSAWG Working Group                                       M. Richardson
Internet-Draft                                  Sandelman Software Works
Updates: 8520 (if approved)                                       W. Pan
Intended status: Standards Track                     Huawei Technologies
Expires: 2 September 2024                                        E. Lear
                                                           Cisco Systems
                                                            1 March 2024

                     Authorized update to MUD URLs
                draft-ietf-opsawg-mud-acceptable-urls-11

Abstract

   This document provides a way for an RFC8520 Manufacturer Usage
   Description (MUD) definitions to declare what are acceptable
   replacement MUD URLs for a device.

   RFCEDITOR-please-remove: this document is being worked on at:
   https://github.com/mcr/iot-mud-acceptable-urls

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on 2 September 2024.

Copyright Notice

   Copyright (c) 2024 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
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   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

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   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Possible issues with updating the MUD files in place  . . . .   3
     3.1.  Adding capabilities . . . . . . . . . . . . . . . . . . .   4
     3.2.  Removing capabilities . . . . . . . . . . . . . . . . . .   4
     3.3.  Significant changes to protocols  . . . . . . . . . . . .   5
     3.4.  Motivation for updating MUD URLs  . . . . . . . . . . . .   5
   4.  Updating the MUD URLs . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Leveraging the manufacturer signature . . . . . . . . . .   6
     4.2.  Concerns about same-signer mechanism  . . . . . . . . . .   7
   5.  Proposed mechanism for updating MUD URLs  . . . . . . . . . .   7
     5.1.  Small Changes to the MUD URL  . . . . . . . . . . . . . .   8
     5.2.  Big Changes to the MUD URL  . . . . . . . . . . . . . . .   9
     5.3.  Merger, Acquisitions and Key Changes  . . . . . . . . . .   9
       5.3.1.  Changing file structure . . . . . . . . . . . . . . .   9
       5.3.2.  Changing hosting URLs . . . . . . . . . . . . . . . .  10
       5.3.3.  Changing Signing Authority  . . . . . . . . . . . . .  10
   6.  Polling for changes in MUD files  . . . . . . . . . . . . . .  11
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  11
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  12
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  12
     9.1.  Updating files vs Updating MUD URLs . . . . . . . . . . .  13
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     10.2.  Informative References . . . . . . . . . . . . . . . . .  13
   Appendix A.  Appendices . . . . . . . . . . . . . . . . . . . . .  15
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  15
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   [RFC8520] provides a standardized way to describe how a specific
   purpose device makes use of Internet resources and associated
   suggested network behavior.  The behaviors are described in a MUD
   file hosted in its manufacturer's server.  The device provides a MUD
   URL to the MUD controller, which can locate this MUD file and
   determine the required network authorization of the device.

   In some cases, e.g., the firmware update, the network behaviors of
   the device may change, and the description in the original MUD file
   will no longer apply.  To solve this problem, there are two common
   ways which the manufacturer can use.

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   One is to change what is in the MUD file, i.e., update the MUD file
   in place, whenever the behavior of the firmware changes.  Section 3
   discusses three scenarios for updating the MUD file and the
   corresponding potential issues.

   The other is to change which MUD file is processed by changing the
   MUD URL.  Section 4 describes the common sources of MUD URLs and the
   problems and threats faced by each type of source when updating the
   MUD URL.  This document proposes an enhanced mechanism of how to
   securely update the MUD URL in Section 5.

   There are also some assumptions and prerequisites in this document.

   While MUD files may include signatures, [RFC8520] does not mandate
   checking them, and there is not a clear way to connect the entity
   which signed the MUD file to the device itself.  This document limits
   itself to situations in which the MUD file is signed, and that the
   MUD controller has been configured to always check the signatures,
   rejecting files whose signatures do not match.

   [RFC8520] does not specify how MUD controllers establish their trust
   in the manufacturers' signing key: there are many possible solutions
   from manual configuration of trust anchors, some kind of automatic
   configuration during onboarding, or a Trust on First Use (TOFU)
   mechanism that accepts the signer on first use.  How this initial
   trust is established is not important for this document, it is
   sufficient that some satisfactory initial trust is established.

2.  Terminology

   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.

3.  Possible issues with updating the MUD files in place

   Three scenarios for updating the MUD file and the corresponding
   potential issues are discussed below.

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3.1.  Adding capabilities

   For situations where new capabilities are added to the firmware, the
   MUD file will detail the new access that the new firmware requires.
   This may involve new incoming or outgoing connections that should be
   authorized.  Devices that have been upgraded to the new firmware will
   make use of the new features.  Devices that have not been upgraded to
   the new firmware may have new connections that are authorized, but
   which the device does not use (outgoing connections), or which the
   device is not prepared to respond to (new incoming connections).

   It is possible that older versions of the firmware have
   vulnerabilities that were not easily exploitable due to the MUD file
   preventing particular kinds of access.  For example, an older
   firmware could have no credentials required (or default credentials)
   access via telnet on port 23 or HTTP on port 80.  The MUD file
   protected the device such that it could either not be accessed at
   all, or access was restricted to connections from a controller only.

   Useful and needed upgrades to the firmware could add credentials to
   that service, allowing it to be opened up for more general access.
   The new MUD file would provide for such access, but when combined
   with the weak security of the old firmware, it results in a
   compromised device.

   While there is an argument that old firmware was insecure and should
   be replaced, it is often the case that the upgrade process involves
   downtime, or can introduce risks due to needed evaluations not having
   been completed yet.  As an example: moving vehicles (cars, airplanes,
   etc.) should not perform upgrades while in motion!  It is probably
   undesirable to perform any upgrade to an airplane outside the service
   facility.  A vehicle owner may desire only to perform software
   upgrades when they are at their residence.  Should there be a
   problem, they could make alternate arrangements for transportation.
   This contrasts with an alternative situation where the vehicle is
   parked at, for instance, a remote cabin, and where an upgrade failure
   could cause a much greater inconvenience.

   The situation for upgrades of medical devices has even more
   considerations involving regulatory compliance.

3.2.  Removing capabilities

   For situations where existing capabilities prove to be a problem and
   are to be turned off or removed in subsequent versions of the
   firmware, the MUD file will be updated to disallow connections that
   previously were allowed.

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   In this case, the new MUD file will forbid some connections, which
   the old firmware still expects to do.  As explained in the previous
   section, upgrades may not always occur immediately upon releasing the
   new firmware.

   In this case, the old device will be performing unwanted connections,
   and the MUD controller will be alerting the network owner that the
   device is misbehaving rather than not being upgraded.  This causes a
   false-positive situation (see [boycrieswolf]), leading to real
   security issues being ignored.  This is a serious issue as documented
   also in [boywolfinfosec], and [falsemalware].

3.3.  Significant changes to protocols

   [I-D.ietf-opsawg-mud-tls] suggests MUD definitions to allow
   examination of TLS protocol details.  Such a profile may be very
   specific to the TLS library which is shipped in a device.  Changes to
   the library (including bug fixes) may cause significant changes to
   the profile, requiring changes to the profile described in the MUD
   file.  Such changes are likely neither forward nor backward
   compatible with other versions, and in place updates to MUD files are
   therefore not advised.

3.4.  Motivation for updating MUD URLs

   While many small tweaks to a MUD file can be done in place, the
   situation described above, particularly when it comes to removing
   capabilities will suggest that changes to the MUD URL are in order.
   A strategy for doing this securely is needed, and the rest of this
   document provides a mechanism to do this securely.

4.  Updating the MUD URLs

   MUD URLs can come from a number of sources:

   *  IDevID Extensions

   *  DHCP option

   *  LLDP TLV

   *  [I-D.richardson-mud-qrcode] proposes to scan them from QRcodes.

   The IDevID mechanism provides a URL that is asserted
   cryptographically by a manufacturer.  However, it is difficult for
   manufacturers to update the IDevID of a device which is already in a
   box.

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   The DHCP and LLDP mechanisms are not signed, but are asserted by the
   device.  A firmware update may update what MUD URL is emitted.
   Sufficiently well targeted malware would also be able to change the
   MUD URL that is emitted.

   The QRcode mechanism is usually done via paper/stickers, and is
   typically not under the control of the device itself at all.
   However, being applied by a human and not easily changed, a MUD URL
   obtained in this fashion is likely as trustworthy as the rest of the
   vendors packaging.  (It may not, due to mixups in labeling represent
   the correct device, but this is a human coordination issue, and is
   out of scope for this document.)

   The manufacturer can use all the four mechanisms above when
   manufacturing the device.  But when considering updating the
   firmware, it seems like only the DHCP and LLDP mechanisms are
   sufficiently easy to send the new MUD URL.  Because of that
   sensitivity, they may also be easily changed by malware!

   There are mitigating mechanisms which may be enough to deal with this
   problem when MUD files are signed by the manufacturer.

   [RFC8520], Section 13.2 explains how to verify MUD File Signatures.
   That document does not define a way for a MUD controller to determine
   who should sign the MUD file for a particular device.

   [RFC8520] leaves this for a local policy.  This document establishes
   one such local policy.  There are a number of other processes that
   could be used, it is expected that many such industrial vertical will
   work out supply chain arrangements or other heuristics to supply
   appropriate anchors.

4.1.  Leveraging the manufacturer signature

   The first time a signature of the MUD file related to a particular
   device-type is verified by the MUD controller, the identity of the
   signing authority is recorded.  That it, the signing authority is
   pinned.  This policy means that subsequent MUD files must be signed
   by the same entity in order to be accepted.

   The trust and acceptance of the first signer may come from many
   sources.  The first signature could be from a manually configured
   trust anchor in the MUD controller.  The first signature could be
   Trust on First Use (TOFU), with the URL coming from a trusted IDevID
   certificate.

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   Based upon this process, an update to the MUD URL would be valid if
   the new MUD file was signed by the same entity that signed the
   previous entry.  This mechanism permits a replacement URL to be any
   URL that the same manufacturer can provide.

4.2.  Concerns about same-signer mechanism

   There is still a potential threat: a manufacturer which has many
   products may have a MUD definition for another product that has the
   privileges that the malware desires.

   The malware could simply change the expressed MUD URL to that of the
   other product, and it will be accepted by the MUD controller as
   valid.

   This works as long as manufacturers use a single key to sign all
   products.  Some manufacturers could sign each product with a
   different key.  Such manufacturers would probably then collect all
   the signing keys into a certificate infrastructure (PKI), with a
   single manufacturer CA key.

   In this case, the question then becomes whether the MUD controller
   should pin the End-Entity (EE) certificate, or the CA certificate.

   Pinning the End-Entity (EE) certificate defends against malware that
   changes the product type, but prevents the manufacturer from being
   able to cycle the validity of the End-Entity certificate for
   cryptographic hygiene reasons.

   Pinning the CA certificate allows the EE certificate to change, but
   may not defend against product type changes.

   It is possible to invent policy mechanisms that would link the EE
   certificate to a value that is in the MUD file.  This could be a
   policy OID, or could involve some content in a subjectAltName.
   Future work could go in that direction.  This document proposes a
   simpler solution.

5.  Proposed mechanism for updating MUD URLs

   The document proposes to limit what MUD URLs are considered valid
   from the device, limiting new MUD URLs to be variations of the
   initial (presumed to be secure) URL.

   The first MUD file which is defined for a device can come from an
   IDevID (which is considered more secure), or via Trust on First Use
   with DHCP or LLDP or other mechanisms.  This first, initially
   trusted, MUD file will be called the "root" MUD file.

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   A MUD file contains a self-referential MUD-URL attribute that points
   to the MUD file itself located on the vendor's website.  While the
   IDevID, DHCP and LLDP mechanisms only transmit a URL, there are some
   newer, not yet standardized proposals that would fetch an entire MUD
   file from the device, such as [I-D.jimenez-t2trg-mud-coap].

   The MUD-URL MUST always be an Absolute URI: see [RFC3986] section
   4.3.

   The URL found in the MUD-URL attribute is to be called the canonical
   MUD URL for the device.

   The MUD-SIGNATURE attribute in the MUD file SHOULD be a relative URI
   (see [RFC3986] section 4.2) with the (hierarchical) base URI for this
   reference being the MUD-URL attribute.

   When pinning the signature, the MUD manager SHOULD pin the lowest
   Certification Authority (CA) that was used in the validation of the
   CMS structure, along with the chain of Subject Names leading to the
   signature.  The MUD manager may need additional trust anchors
   (including previously pinned ones) in order to verify that CA
   certificate.

5.1.  Small Changes to the MUD URL

   Subsequent MUD files are considered valid if:

   *  they have the same initial Base-URI as the MUD-URL, but may have a
      different final part

   *  they are signed by an equivalent End Entity (same trusted CA and
      same Subject Name) as the "root" MUD file.

   Section 5.2 of [RFC3986] details many cases for calculating the Base-
   URI.

   Section 3.3 of [RFC3986] explains how the different parts of the URL
   are described.  As explained in that section, a _path_ component
   consists of a series of _segment_ seperated by slash ("/")
   characters.  The new URL is considered acceptable if it contains the
   same series of segments in its path, excepting that the last segment
   may be different.

   For a simple example, if the canonical MUD-URL is
   http://example.com/hello/there/file.json then any URL that starts
   with http://example.com/hello/there/ would be acceptable, such as
   http://example.com/hello/there/revision2.json.

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   One problem with these small changes is that malware could still
   express a MUD file that was previously valid, but which should no
   longer considered accurate.  This is a rollback attack.  This might
   result in the malware being able to reach destinations that turned
   out to be a mistake; a security fault.  In order to combat this, MUD
   managers SHOULD keep track of the list of MUD-URLs that they have
   successfully retrieved, and if a device ever suggests a URL that was
   previously used, then the MUD manager should suspect that is a
   rollback attack.  MUD managers are not typically resource
   constrained, and while the list of URLs could grow without bound, it
   is unlikely to be a burden.  A site with thousands of similar devices
   could keep a common list of URLs.

5.2.  Big Changes to the MUD URL

   Once a new MUD file is accepted, either by reloading an existing file
   from the same URL, or via the Small Changes mechanism described
   above, then the MUD-URL attribute in this file becomes the new
   canonical MUD file.  The contained MUD-URL attribute in the file need
   not be related in any way to the existing MUD-URL.

   As a result, any subsequent updates MUST be relative to the new MUD-
   URL in this file.

   This rule enables the location of the MUD file to change over time
   based upon the needs of the organization.

5.3.  Merger, Acquisitions and Key Changes

   The above process allows for a manufacturer to rework its file
   structure.  They can change web server host names, so long as they
   retain the old structure long enough for all devices to upgrade at
   least once.

   The process also allows a manufacturer to change the EE certificate
   and Certification Authority used for signing.

5.3.1.  Changing file structure

   A manufacturer has been hosting a MUD file at
   https://example.com/household/products/mudfiles/toaster.json and
   wishes to move it to https://example.com/mudfiles/toasters/model1945/
   mud.json

   The manufacturer creates a new MUD file at the new location.

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   Then the manufacturer changes the MUD-URL contained with the files at
   the old location to have a value of
   https://example.com/mudfiles/toasters/model1945/mud.json.  Note that
   in order for MUD controllers to reload the old file, it MUST have
   been served with an appropriate ETag, and appropriate Expires or
   Cache Control headers [RFC9111], Section 5.3.  If control over
   caching is not possible for the manufacturer, then they need to do
   this in two steps, with the first step creating a new MUD file at an
   acceptable location (in the above example, perhaps:
   https://example.com/household/products/mudfiles/toaster0.json ).  The
   device then will have to do two firmware updates: one to switch to
   the intermediate URL, and a second one to switch to the desired final
   URL.

   The manufacturer must continue to serve the files from the old
   location for some time, or to return an HTTP 301 (Moved Permanently)
   redirecting to the new location.

5.3.2.  Changing hosting URLs

   A manufacturer has been hosting a MUD file at
   https://example.com/household/products/mudfiles/toaster.json and
   wishes to move it to https://mud.example/hosthold/products/mudfiles/
   toaster.json

   The scenario is much the same as for Section 5.3.1, and can be
   handled in the same fashion.  This situation is likely to occur when
   one company acquires another.

   Note, however, that a 301 Redirect that changed the hostname SHOULD
   NOT be accepted by MUD controllers.

5.3.3.  Changing Signing Authority

   A manufacturer has been signing MUD files using an EE Certificate
   with subjectAltName foo.example, issued by an internal Certification
   Authority BAZ.

   The manufacturer wishes to begin signing with an EE Certificate with
   subjectAltname foo.example, but now signed by a public CA (call it:
   Fluffy).

   The manufacturer first creates a new MUD file with a new detached
   signature file.  Within this signature file, the manufacturer places
   a certificate chain: Internal-CA BAZ->Fluffy, and then the Fluffy
   Certificate, and then the foo.example certificate issued from Fluffy.

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   This supports changing certification authorities, but it does not
   support changing the Subject Name of the signing entity.

6.  Polling for changes in MUD files

   The MUD file update mechanisms described in Section 3 requires that
   the MUD controller poll for updates.  The MUD controller will receive
   no signal about a change from the device because the URL will not
   have changed.

   The manufacturer SHOULD serve MUD files from a source for which ETag
   Section 2.3 of [RFC7232] may be generated.  Static files on disk
   satisfy this requirement.  MUD files generated from a database
   process might not.  The use of ETag allows a MUD controller to more
   efficiently poll for changes in the file.

   A manufacturer should also serve MUD files with an HTTP Max-Age
   header as per Section 5.2.2.8 of [RFC7234].

   The MUD controller should take the Max-Age as an indication of when
   to next poll for updates to the MUD file.  Values of less than 1
   hour, or more than 1 month should be considered out of range, and
   clamped into the range (1 hour, 1 month).

   MUD controllers SHOULD add some random jitter to the timing of their
   requests.  MUD controllers MAY use a single HTTP(S)/1.1 connection to
   retrieve all resources at the same destination.

7.  Privacy Considerations

   The MUD URL could contain sensitive information such as the model
   number and even firmware revision numbers.  Thus, the MUD URL may
   identify the make, model and revision of a device.

   [RFC8520] already identifies this privacy concern, and suggests use
   of TLS so that the HTTP requests that retrieve the MUD file do not
   divulge that level of detail.

   The requirement for the MUD controller to poll for changes to MUD
   files results in multiple interactions between the MUD controller and
   the manufacturer whereas a more naive implementation might only
   interact once.  Even if HTTPS used, an observer of the traffic to
   that manufacturer will be revealing, and [RFC8520] goes on to suggest
   use of a proxy as well.

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8.  IANA Considerations

   This document makes no requests to IANA.

9.  Security Considerations

   Prior to the standardization of the process in this document, if a
   device was infiltrated by malware, and said malware wished to make
   accesses beyond what the current MUD file allowed, the malware would
   have to:

   1.  arrange for an equivalent MUD file to be visible somewhere on the
       Internet

   2.  depend upon the MUD controller either not checking signatures, or

   3.  somehow get the manufacturer to sign the alternate MUD file

   4.  announce this new URL via DHCP or LLDP, updating the MUD
       controller with the new permissions.

   One way to accomplish (3) is to leverage the existence of MUD files
   created by the manufacturer for different classes of devices.  Such
   files would already be signed by the same manufacturer, eliminating
   the need to spoof a signature.

   With the standardization of the process in this document, then the
   attacker can no longer point to arbitrary MUD files in step 4, but
   can only make use of MUD files that the manufacturer has already
   provided for this device.

   Manufacturers are advised to maintain an orderly layout of MUD files
   in their web servers, with each unique product having its own
   directory/pathname.

   The process described updates only MUD controllers and the processes
   that manufacturers use to manage the location of their MUD files.

   A manufacturer which has not managed their MUD files in the way
   described here can deploy new directories of per-product MUD files,
   and then can update the existing MUD files in place to point to the
   new URLs using the MUD-URL attribute.

   There is therefore no significant flag day: MUD controllers may
   implement the new policy without significant concern about backwards
   compatibility.

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9.1.  Updating files vs Updating MUD URLs

   Device developers need to consider whether to make a change by
   updating a MUD file, or updating the MUD URL.

   MUD URLs can only be updated by shipping a new firmware.  It is
   reasonable to update the MUD URL whenever a new firmware release
   causes new connectivity to be required.  The updated mechanism
   defined in this document makes this a secure operation, and there is
   no practical limitation on the number of files that a web server can
   hold.

   In place updates to a MUD file should be restricted to cases where it
   turns out that the description was inaccurate: a missing connection,
   an inadvertent one authorized, or just incorrect information.

   Developers should be aware that many enterprise websites use
   outsourced content distribution networks, and MUD controllers are
   likely to cache files for some time.  Changes to MUD files will take
   some time to propagate through the various caches.  An updated MUD
   URL will however, not experience any cache issues, but can not be
   deployed with a firmware update.

10.  References

10.1.  Normative References

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

   [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>.

   [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>.

   [RFC8520]  Lear, E., Droms, R., and D. Romascanu, "Manufacturer Usage
              Description Specification", RFC 8520,
              DOI 10.17487/RFC8520, March 2019,
              <https://www.rfc-editor.org/info/rfc8520>.

10.2.  Informative References

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   [boycrieswolf]
              "The Boy Who Cried Wolf", 18 January 2020,
              <https://fablesofaesop.com/the-boy-who-cried-wolf.html>.

   [boywolfinfosec]
              "Security Alerts - A Case of the Boy Who Cried Wolf?", 18
              January 2020, <https://www.infosecurity-
              magazine.com/opinions/security-alerts-boy-cried-wolf/>.

   [falsemalware]
              "False malware alerts cost organizations $1.27M annually,
              report says", 18 January 2020,
              <https://www.scmagazine.com/home/security-news/false-
              malware-alerts-cost-organizations-1-27m-annually-report-
              says/ and http://go.cyphort.com/Ponemon-Report-Page.html>.

   [I-D.ietf-opsawg-mud-tls]
              Reddy.K, T., Wing, D., and B. Anderson, "Manufacturer
              Usage Description (MUD) (D)TLS Profiles for IoT Devices",
              Work in Progress, Internet-Draft, draft-ietf-opsawg-mud-
              tls-13, 23 January 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-opsawg-
              mud-tls-13>.

   [I-D.jimenez-t2trg-mud-coap]
              Jimenez, J., "Using MUD on CoAP environments", Work in
              Progress, Internet-Draft, draft-jimenez-t2trg-mud-coap-00,
              9 March 2020, <https://datatracker.ietf.org/doc/html/
              draft-jimenez-t2trg-mud-coap-00>.

   [I-D.richardson-mud-qrcode]
              Richardson, M., Latour, J., and H. H. Gharakheili,
              "Loading Manufacturer Usage Description (MUD) URLs from QR
              Codes", Work in Progress, Internet-Draft, draft-
              richardson-mud-qrcode-07, 21 March 2022,
              <https://datatracker.ietf.org/doc/html/draft-richardson-
              mud-qrcode-07>.

   [RFC7232]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Conditional Requests", RFC 7232,
              DOI 10.17487/RFC7232, June 2014,
              <https://www.rfc-editor.org/info/rfc7232>.

   [RFC7234]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "Hypertext Transfer Protocol (HTTP/1.1): Caching",
              RFC 7234, DOI 10.17487/RFC7234, June 2014,
              <https://www.rfc-editor.org/info/rfc7234>.

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   [RFC9111]  Fielding, R., Ed., Nottingham, M., Ed., and J. Reschke,
              Ed., "HTTP Caching", STD 98, RFC 9111,
              DOI 10.17487/RFC9111, June 2022,
              <https://www.rfc-editor.org/info/rfc9111>.

Appendix A.  Appendices

Contributors

   Jie Yang
   Email: jay.yang@huawei.com

   Tianqing Tang
   Email: tangtianqing@huawei.com

Authors' Addresses

   Michael Richardson
   Sandelman Software Works
   Email: mcr+ietf@sandelman.ca

   Wei Pan
   Huawei Technologies
   Email: william.panwei@huawei.com

   Eliot Lear
   Cisco Systems
   Email: lear@cisco.com

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