Internet-Draft SUIT MUD Linkage March 2024
Moran & Tschofenig Expires 5 September 2024 [Page]
draft-ietf-suit-manifest (if approved)
Intended Status:
Standards Track
B. Moran
Arm Limited
H. Tschofenig

Strong Assertions of IoT Network Access Requirements


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

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 5 September 2024.

Table of Contents

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.

    .                      ____________   .           _____________
    .                     |            |  .          |             |
    .                     |    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:

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. 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.

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.

                       |            |
                       |  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          / \
                                                  /  \
Figure 2: SUIT-MUD Architecture.

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

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.

  • 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.

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.

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:

7. IANA Considerations

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

8. References

8.1. Normative References

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-04, , <>.
Lundblade, L., Mandyam, G., O'Donoghue, J., and C. Wallace, "The Entity Attestation Token (EAT)", Work in Progress, Internet-Draft, draft-ietf-rats-eat-25, , <>.
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, , <>.
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <>.
Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, , <>.
Lear, E., Droms, R., and D. Romascanu, "Manufacturer Usage Description Specification", RFC 8520, DOI 10.17487/RFC8520, , <>.
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, , <>.
Birkholz, H., Thaler, D., Richardson, M., Smith, N., and W. Pan, "Remote ATtestation procedureS (RATS) Architecture", RFC 9334, DOI 10.17487/RFC9334, , <>.

8.2. Informative References

FIDO Alliance, "FIDO Device Onboard Specification 1.1", , <>.
Tschofenig, H., Sheffer, Y., Howard, P., Mihalcea, I., Deshpande, Y., and A. Niemi, "Using Attestation in Transport Layer Security (TLS) and Datagram Transport Layer Security (DTLS)", Work in Progress, Internet-Draft, draft-fossati-tls-attestation-05, , <>.
Richardson, M., Pan, W., and E. Lear, "Authorized update to MUD URLs", Work in Progress, Internet-Draft, draft-ietf-opsawg-mud-acceptable-urls-11, , <>.
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, , <>.
Pritikin, M., Richardson, M., Eckert, T., Behringer, M., and K. Watsen, "Bootstrapping Remote Secure Key Infrastructure (BRSKI)", RFC 8995, DOI 10.17487/RFC8995, , <>.
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, , <>.


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
Hannes Tschofenig