SUIT                                                            B. MoranInternet-Draft                                               Arm LimitedUpdates: draft-ietf-suit-manifest (if approved)            H. TschofenigIntended status: Standards Track                            3 March 2025Expires: 4 September 2025          Strong Assertions of IoT Network Access Requirements                         draft-ietf-suit-mud-10Abstract   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 4 September 2025.Moran & Tschofenig      Expires 4 September 2025                [Page 1]Internet-Draft              SUIT MUD Linkage                  March 2025Copyright Notice   Copyright (c) 2025 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject to BCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents (https://trustee.ietf.org/   license-info) in effect on the date of publication of this document.   Please review these documents carefully, as they describe your rights   and restrictions with respect to this document.  Code Components   extracted from this document must include Revised BSD License text as   described in Section 4.e of the Trust Legal Provisions and are   provided without warranty as described in the Revised BSD License.Table of Contents   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4   3.  Workflow  . . . . . . . . . . . . . . . . . . . . . . . . . .   4   4.  Operational Considerations  . . . . . . . . . . . . . . . . .   6     4.1.  Pros  . . . . . . . . . . . . . . . . . . . . . . . . . .   6     4.2.  Cons  . . . . . . . . . . . . . . . . . . . . . . . . . .   7   5.  Extensions to SUIT  . . . . . . . . . . . . . . . . . . . . .   8   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  . . . . . . . . . . . . . . . . . . . . . . .  111.  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.Moran & Tschofenig      Expires 4 September 2025                [Page 2]Internet-Draft              SUIT MUD Linkage                  March 2025       .......................................       .                      ____________   .           _____________       .                     |            |  .          |             |       .                     |    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   *  Device Certificates, such as IEEE 802.1X, whereby the URL to the      MUD File would be contained in the certificate.   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.Moran & Tschofenig      Expires 4 September 2025                [Page 3]Internet-Draft              SUIT MUD Linkage                  March 2025   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 has 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.Moran & Tschofenig      Expires 4 September 2025                [Page 4]Internet-Draft              SUIT MUD Linkage                  March 2025                           ____________                          |            |                          |  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 authenticationMoran & Tschofenig      Expires 4 September 2025                [Page 5]Internet-Draft              SUIT MUD Linkage                  March 2025      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.Moran & Tschofenig      Expires 4 September 2025                [Page 6]Internet-Draft              SUIT MUD Linkage                  March 2025   *  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.   *  MUD Manager must be aware of the Status Tracker or vice versa so      that the MUD Manager can obtain MUD URLs and MUD Signer SKIs from      the Status Tracker.  This implies a new API in the MUD manager or      Status Tracker.   *  The MUD manager requires a failover mechanism to trigger the      status tracker to obtain a copy of the SUIT Manifest in order to      extract the MUD URL if it is not already aware of a device.  This      could be done, for example, as a part of an onboarding flow.   *  Attestation Evidence may convey the SUIT Manifest, in which case      the Status Tracker becomes a Relying Party since it depends on      Attestation Evidence.  This workflow is expected, however.   *  This approach explicitly moves the decisions about device      behaviour away from the Network Operator and towards the Manifest      Author.  While this is appropriate when the Manifest Author is      trusted, not all IoT devices are fully trusted, and MUD files      enable a Network Operator to restrict their capabilities.  For aMoran & Tschofenig      Expires 4 September 2025                [Page 7]Internet-Draft              SUIT MUD Linkage                  March 2025      Network Operator to override a device's manufacturer-provided MUD      URL will require the MUD manager to have a mechanism to enable      this override, which adds complexity5.  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.   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 hasMoran & Tschofenig      Expires 4 September 2025                [Page 8]Internet-Draft              SUIT MUD Linkage                  March 2025   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.   *  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.  References8.1.  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-              06, 6 September 2024,              <https://datatracker.ietf.org/doc/html/draft-ietf-cose-              key-thumbprint-06>.Moran & Tschofenig      Expires 4 September 2025                [Page 9]Internet-Draft              SUIT MUD Linkage                  March 2025   [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-31, 6              September 2024, <https://datatracker.ietf.org/doc/html/              draft-ietf-rats-eat-31>.   [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-33, 24 February              2025, <https://datatracker.ietf.org/doc/html/draft-ietf-              suit-manifest-33>.   [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/>.Moran & Tschofenig      Expires 4 September 2025               [Page 10]Internet-Draft              SUIT MUD Linkage                  March 2025   [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-08, 21              October 2024, <https://datatracker.ietf.org/doc/html/              draft-fossati-tls-attestation-08>.   [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-12, 6 September 2024,              <https://datatracker.ietf.org/doc/html/draft-ietf-opsawg-              mud-acceptable-urls-12>.   [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.comMoran & Tschofenig      Expires 4 September 2025               [Page 11]Internet-Draft              SUIT MUD Linkage                  March 2025   Hannes Tschofenig   Email: hannes.tschofenig@gmx.netMoran & Tschofenig      Expires 4 September 2025               [Page 12]