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Strong Assertions of IoT Network Access Requirements
draft-ietf-suit-mud-09

Document Type Active Internet-Draft (suit WG)
Authors Brendan Moran , Hannes Tschofenig
Last updated 2024-07-23 (Latest revision 2024-07-08)
Replaces draft-moran-suit-mud
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
Formats
Reviews
Additional resources GitHub Repository
Mailing list discussion
Stream WG state Submitted to IESG for Publication
Associated WG milestones
Dec 2021
Adopt SUIT Manifest MUD extension document as WG item
Dec 2022
Submit SUIT Manifest MUD extension document to the IESG for publication as a Proposed Standard
Document shepherd Russ Housley
Shepherd write-up Show Last changed 2023-09-05
IESG IESG state Approved-announcement to be sent::External Party
Action Holders
(None)
Consensus boilerplate Yes
Telechat date (None)
Responsible AD Roman Danyliw
Send notices to housley@vigilsec.com
IANA IANA review state Version Changed - Review Needed
draft-ietf-suit-mud-09
SUIT                                                            B. Moran
Internet-Draft                                               Arm Limited
Updates: draft-ietf-suit-manifest (if approved)            H. Tschofenig
Intended status: Standards Track                             8 July 2024
Expires: 9 January 2025

          Strong Assertions of IoT Network Access Requirements
                         draft-ietf-suit-mud-09

Abstract

   The Manufacturer Usage Description (MUD) specification describes the
   access and network functionality required for a device to properly
   function.  This description has to reflect the software running on
   the device and its configuration.  Because of this, the most
   appropriate entity for describing device network access requirements
   is the same as the entity developing the software and its
   configuration.

   A network presented with a MUD file by a device allows detection of
   misbehavior by the device software and configuration of access
   control.

   This document defines a way to link the Software Updates for Internet
   of Things (SUIT) manifest to a MUD file offering a stronger binding
   between the two.

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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 9 January 2025.

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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
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Workflow  . . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Operational Considerations  . . . . . . . . . . . . . . . . .   6
     4.1.  Pros  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Cons  . . . . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Extensions to SUIT  . . . . . . . . . . . . . . . . . . . . .   7
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .   8
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .   9
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  10
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   A Manufacturer Usage Description (MUD) file describes what sort of
   network communication behavior a device is designed to have.  For
   example, a manufacturer may use a MUD file to describe that a device
   uses HTTP, DNS and NTP communication but no other protocols.  The
   communication patterns are described in a JSON-based format in the
   MUD file.

   The MUD files do, however, need to be presented by the device to a
   MUD manager in the operational network where the device is deployed.
   Under [RFC8520], devices report a MUD URL to a MUD manager in the
   operational network.  The MUD URL is a URL that can be used by the
   MUD manager to receive the MUD file from a MUD file server to
   ultimately obtain the MUD file.

   Figure 1 shows the MUD architecture, as defined in RFC 8520.

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       .......................................
       .                      ____________   .           _____________
       .                     |            |  .          |             |
       .                     |    MUD     |-->get URL-->|    MUD      |
       .                     |  Manager   |  .(https)   | File Server |
       .  End system network |____________|<-MUD file<-<|_____________|
       .                             .       .
       .                             .       .
       . ________                _________   .
       .|        |              | router  |  .
       .| Device |--->MUD URL-->|   or    |  .
       .|________|              | switch  |  .
       .                        |_________|  .
       .......................................

                  Figure 1: MUD Architecture per RFC 8520.

   RFC 8520 envisions different approaches for conveying the MUD URL
   from the device to the operational network.  Section 4 of
   [I-D.ietf-opsawg-mud-acceptable-urls] provides additional description
   of the MUD URLs sources, which include:

   *  DHCP,

   *  IEEE 802.1AB Link Layer Discovery Protocol (LLDP), and

   *  IEEE 802.1X whereby the URL to the MUD file would be contained in
      the certificate used in an EAP method.

   The MUD manager must trust the MUD file server from which the MUD
   file is fetched to return the most up-to-date MUD file.  It must also
   trust the device to report the correct MUD URL.  In case of DHCP and
   LLDP the URL is unprotected and not bound to the device itself.

   When the MUD URL is included in a certificate then it is
   authenticated and integrity protected.  However, the certificate only
   proves possession of a private key and endorsements by the
   certificate issuer.  This does not prove what software is in use, nor
   does it prove that the MUD file is the correct file for the deployed
   software: instead, the responsibility falls on the certificate issuer
   to identify the MUD URL correctly and to supply a MUD Signer
   correctly.  There is a need to bind the entity that creates the
   software and configuration to the MUD file.  The developer is in the
   best position to describe the communication requirements of the
   software it developed and configured for a device.

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   This specification defines an extension to the Software Updates for
   Internet of Things (SUIT) manifest [I-D.ietf-suit-manifest] to
   include a MUD URL.  A SUIT manifest is a bundle of metadata about
   code/data for an IoT device, where to find the code/data, the devices
   to which it applies, and cryptographic information protecting the
   manifest.

   When combining a MUD URL with a manifest used for software/firmware
   updates then a network operator can gain more confidence in the
   description of the communication requirements for a device to
   properly function.

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.

   This document re-uses the terms defined in [RFC9334] related to
   remote attestation.  Readers of this document are assumed to be
   familiar with the following terms: Evidence, Claim, Attester,
   Verifier, and Relying Party (RP).

   This document also uses terms defined in [RFC8520], such as MUD, MUD
   file, MUD manager, MUD URL, etc.

3.  Workflow

   Figure 2 shows the architectural extensions introduced by combining
   SUIT and MUD.  The key elements are that the developer, who produces
   the firmware is also generating a manifest and the MUD file.
   Information about the MUD file is embedded into the SUIT manifest and
   provided to the device via firmware update mechanism.  Once this
   information is available on the device it can be presented during
   device onboarding, during network access authentication, or as part
   of other interactions that involve the conveyance of Evidence to the
   operational network.  After retrieving the manifest, the MUD file can
   be obtained as well.

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                           ____________
                          |            |
                          |  Manifest  |
                          | Repository |
                          |____________|
                     get URL ^      | SUIT manifest
    .........................|......|..........
    .                      __|______v__       .       _____________
    .                     |            |      .      |             |
    .                     |    MUD     |-->get URL-->|    MUD      |
    .                     |  Manager   |  .(https)   | File Server |
    .  End system network |____________|<-MUD file<-<|             |
    .                             ^       +Signature |_____________|
    .                             .           .
    .                             .           .
    .                             .           .
    . ________                _____________   .
    .|        | Attestation  | NAS, AAA or |  .
    .| Device |-->Evidence-->| Onboarding  |  .
    .|________| (+ Manifest  | Server      |  .
    .     ^      Claim)      |_____________|  .
    ......*....................................
          *                                         //-\\
          *                                          \-/
          *                        SUIT Manifest      |
          +************************(+ MUD URL)    ----*-----
                                   Firmware          / \
                                                     /  \
                                                  Developer

                      Figure 2: SUIT-MUD Architecture.

   The intended workflow is as follows, and assumes an attestation
   mechanism between the device and the MUD Manager:

   *  At the time of onboarding, devices report their manifest in use to
      the MUD Manager via some form of attestation Evidence and a
      conveyance protocol.  The device thereby acts as an Attester.  The
      normative specification of these mechanisms is out of scope for
      this document.

   *  An example of an Evidence format is the Entity Attestation Token
      (EAT) [I-D.ietf-rats-eat], which offers a rich set of claims.
      This specification assumes that Evidence includes a link to the
      SUIT manifest via the "manifests" claim (see Section 4.2.15 of
      [I-D.ietf-rats-eat]) or that the manifest itself is embedded in
      the Evidence.  This Evidence is conveyed to the operational
      network via some protocol, such as network access authentication

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      protocol (for example using the EAP-TLS 1.3 method [RFC9190]
      utilizing the attestation extensions
      [I-D.fossati-tls-attestation]) or an onboarding protocol like FIDO
      Device Onboard (FDO) [FDO] or Bootstrapping Remote Secure Key
      Infrastructure (BRSKI) [RFC8995].

   *  The MUD Manager, acting as a Relying Party, relays the Evidence to
      the Verifier and receives an Attestation Result in response.  This
      allows the MUD Manager to check that the device is operating with
      the expected version of software and configuration.

   *  Since a URL to the manifest is contained in the Evidence, the MUD
      Manager can look up the corresponding manifest.

   *  The MUD Manager acquires the MUD file from the MUD URL found in
      the SUIT manifest.  The SUIT manifest contains the MUD URL and not
      the MUD file primarily to due the size of the MUD file.  This also
      allows the MUD file to be updated rapidly in response to evolving
      threats.

   *  The MUD Manager verifies the MUD file signature using the Subject
      Key Identifier (SKI) provided in the SUIT manifest.

   *  Then, the MUD Manager can apply any appropriate policy as
      described by the MUD file.

   Each time a device is updated, rebooted, or otherwise substantially
   changed, it will execute the remote attestation procedures again.

4.  Operational Considerations

   This specification assumes that the software/firmware author provides
   a MUD file that describes the behavior of the software running on a
   device.

4.1.  Pros

   The approach described in this document has several advantages over
   the RFC 8520 MUD URL reporting mechanisms:

   *  The MUD URL is tightly coupled to device software/firmware
      version.

   *  The device does not report the MUD URL, so the device cannot
      tamper with the MUD URL.

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   *  The author explicitly authorizes a key to sign MUD files,
      providing a tight coupling between the party that knows device
      behavior best (the author of the software/firmware) and the party
      that declares device behavior (MUD file signer).

   *  Network operators do not need to know, a priori, which MUD URL to
      use for each device; this can be harvested from the device's
      manifest and only replaced if necessary.

   *  A network operator can still replace a MUD URL in a SUIT manifest:

      -  By providing a SUIT manifest that overrides the MUD URL.

      -  By replacing the MUD URL in their network infrastructure.

   *  Devices can be quarantined if the Attestation Result indicates
      that an out-dated or compromised software/firmware version has
      been used.

   *  Devices cannot lie about which MUD URL to use.

4.2.  Cons

   This mechanism relies on the use of SUIT manifests to encode the MUD
   URL.  Conceptually, the MUD file is similar to a Software Bill of
   Material (SBOM) but focuses on the external visible communication
   behavior, which is essential for network operators, rather than
   describing the software libraries contained within the device itself.
   The SUIT manifest must then be conveyed to the network during
   onboarding or during the network access authentication step.
   Attestation Evidence is used to convey the SUIT manifest.

5.  Extensions to SUIT

   To enable strong assertions about the network access requirements
   that a device should have for a particular software/configuration
   pair a MUD URL is added to the SUIT manifest along with a subject key
   identifier (ski).  Note that the subject key identifier refers to a
   more generic version of SubjectPublicKeyInfo defined in [RFC5280],
   which refers to an X.509-based ski.  The subject key identifier MUST
   be generated according to the process defined in
   [I-D.ietf-cose-key-thumbprint] and the SUIT_Digest structure MUST be
   populated with the selected hash algorithm and obtained fingerprint.
   The subject key identifier corresponds to the key used in the MUD
   signature file described in Section 13.2 of [RFC8520].

   Note: A key need not be in COSE Key format to create a COSE Key
   Thumbprint of it.

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   The following Concise Data Definition Language (CDDL) [RFC8610]
   describes the extension to the SUIT_Manifest structure:

   The extension to the SUIT_Manifest is described here:

   $$unseverable-manifest-member-extensions //= (
     suit-manifest-mud => bstr .cbor SUIT_MUD_container
   )

   The SUIT_MUD_container structure is defined as follows:

   SUIT_MUD_container = {
       suit-mud-url => #6.32(tstr),
       suit-mud-ski => SUIT_Digest,
   }

6.  Security Considerations

   This specification links MUD files to SUIT manifests for improving
   security protection and ease of use.  By including MUD URLs in SUIT
   manifests an extra layer of protection has been created and
   synchronization risks can be minimized.

   Used in this way, the MUD manager presents an additional layer of
   security on networks where they are enabled.  The MUD manager
   configures the L2/L3 infrastructure of a Local Area Network to apply
   restrictive policies to certain devices.  The MUD manager only has
   the ability to elevate or restrict the network privileges of a
   device.  Therefore, attacks on the MUD Manager cannot compromise
   devices, they can only enable a compromised device to access more of
   the network.  Further security considerations related to the MUD
   Manager are covered in [RFC8520].

   If the MUD file and the software/firmware loaded onto the device gets
   out-of-sync a device may be firewalled and, with firewalling by
   networks in place, the device may stop functioning.  This is,
   however, not a concern specific to this specification but rather to
   the use of MUD in general.  Below are two mitigations:

   *  A manufacturer must update the MUD file in advance of network
      service or product changes so that the new services can be
      supported.  Because the MUD file is accessed by a URL means that
      it can be subsequently updated.  This requires a MUD file being
      retrieved again.  This handles the case when the device is already
      deployed and in use.

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   *  There is a possibility that an IoT device has remained on-shelf
      inventory for an extended period, resulting in its MUD file being
      inaccessible at its previous location.  This necessitates a
      decision on how to implement a fail-safe tailored to the
      particular environment.

7.  IANA Considerations

   IANA is requested to add a new value to the SUIT manifest elements
   registry created with [I-D.ietf-suit-manifest]:

   *  Label: TBD1 [[Value allocated from the standards action address
      range]]

   *  Name: Manufacturer Usage Description (MUD)

   *  Reference: [[TBD: This document]]

8.  References

8.1.  Normative References

   [I-D.ietf-cose-key-thumbprint]
              Isobe, K., Tschofenig, H., and O. Steele, "CBOR Object
              Signing and Encryption (COSE) Key Thumbprint", Work in
              Progress, Internet-Draft, draft-ietf-cose-key-thumbprint-
              05, 8 July 2024, <https://datatracker.ietf.org/doc/html/
              draft-ietf-cose-key-thumbprint-05>.

   [I-D.ietf-rats-eat]
              Lundblade, L., Mandyam, G., O'Donoghue, J., and C.
              Wallace, "The Entity Attestation Token (EAT)", Work in
              Progress, Internet-Draft, draft-ietf-rats-eat-29, 8 July
              2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
              rats-eat-29>.

   [I-D.ietf-suit-manifest]
              Moran, B., Tschofenig, H., Birkholz, H., Zandberg, K., and
              O. Rønningstad, "A Concise Binary Object Representation
              (CBOR)-based Serialization Format for the Software Updates
              for Internet of Things (SUIT) Manifest", Work in Progress,
              Internet-Draft, draft-ietf-suit-manifest-25, 5 February
              2024, <https://datatracker.ietf.org/doc/html/draft-ietf-
              suit-manifest-25>.

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   [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/rfc/rfc2119>.

   [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/rfc/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/rfc/rfc8520>.

   [RFC8610]  Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
              Definition Language (CDDL): A Notational Convention to
              Express Concise Binary Object Representation (CBOR) and
              JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610,
              June 2019, <https://www.rfc-editor.org/rfc/rfc8610>.

   [RFC9334]  Birkholz, H., Thaler, D., Richardson, M., Smith, N., and
              W. Pan, "Remote ATtestation procedureS (RATS)
              Architecture", RFC 9334, DOI 10.17487/RFC9334, January
              2023, <https://www.rfc-editor.org/rfc/rfc9334>.

8.2.  Informative References

   [FDO]      FIDO Alliance, "FIDO Device Onboard Specification 1.1",
              April 2022, <https://fidoalliance.org/specifications/
              download-iot-specifications/>.

   [I-D.fossati-tls-attestation]
              Tschofenig, H., Sheffer, Y., Howard, P., Mihalcea, I.,
              Deshpande, Y., Niemi, A., and T. Fossati, "Using
              Attestation in Transport Layer Security (TLS) and Datagram
              Transport Layer Security (DTLS)", Work in Progress,
              Internet-Draft, draft-fossati-tls-attestation-07, 8 July
              2024, <https://datatracker.ietf.org/doc/html/draft-
              fossati-tls-attestation-07>.

   [I-D.ietf-opsawg-mud-acceptable-urls]
              Richardson, M., Pan, W., and E. Lear, "Authorized update
              to MUD URLs", Work in Progress, Internet-Draft, draft-
              ietf-opsawg-mud-acceptable-urls-11, 1 March 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-opsawg-
              mud-acceptable-urls-11>.

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   [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/rfc/rfc5280>.

   [RFC8995]  Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
              and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructure (BRSKI)", RFC 8995, DOI 10.17487/RFC8995,
              May 2021, <https://www.rfc-editor.org/rfc/rfc8995>.

   [RFC9190]  Preuß Mattsson, J. and M. Sethi, "EAP-TLS 1.3: Using the
              Extensible Authentication Protocol with TLS 1.3",
              RFC 9190, DOI 10.17487/RFC9190, February 2022,
              <https://www.rfc-editor.org/rfc/rfc9190>.

Acknowledgements

   We would like to thank Roman Danyliw for his excellent review as the
   responsible security area director, Bahcet Sarikaya for his Genart
   review, Michael Richardson for his IoT directorate review and Susan
   Hares for her Opsdir review.  During the IESG review Robert Wilton,
   Eliot Lear, Zaheduzzaman Sarker, Francesca Palombini, John Scudder,
   Paul Wouters, Éric Vyncke, and Murray Kucherawy.

Authors' Addresses

   Brendan Moran
   Arm Limited
   Email: brendan.moran.ietf@gmail.com

   Hannes Tschofenig
   Email: hannes.tschofenig@gmx.net

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