Strong Assertions of IoT Network Access Requirements

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Table of Contents

1. Introduction

Under [RFC8520], devices report a MUD URL to a MUD Manager in the network, which then interacts with a MUD File Server to ultimately obtain the MUD file. The following figure shows the MUD architecture.¶

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

RFC 8520 envisions different approaches for conveying the MUD URL from the device to the network such as:¶

• DHCP,¶
• IEEE802.1AB 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 uses the MUD URL to fetch the MUD file, which contains connectivity-related functionality required for a device to properly function.¶

The MUD Manager must trust the MUD File Server from which the MUD file is fetched to return an authentic copy of the MUD file. This concern may be mitigated using the optional signature reference in the MUD file. The MUD Manager must also trust the device to report a correct MUD URL. In case of DHCP and LLDP the URL is likely unprotected.¶

When the MUD URL is included in a certificate then it is authenticated and integrity protected. A certificate created for use with network access authentication is typically not signed by the entity that wrote the software and configured the device, which leads to a conflation of rights.¶

There is a need to bind the entity that creates the software/configuration to the MUD file because only that entity can attest the connectivity requirements of the device.¶

This specification defines an extension to the Software Updates for Internet of Things (SUIT) manifest format [I-D.ietf-suit-manifest] to include a MUD URL. When combining a MUD URL with a manifest used for software/firmware updates then a network operator can get more confidence in the description of the connectivity 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.¶

3. Workflow

The intended workflow is as follows:¶

• At the time of onboarding, devices report their manifest in use to the MUD Manager via attestation evidence in the Entity Attestation Token (EAT) [I-D.ietf-rats-eat].¶

  • Among other claims, the device will report its software digest(s), and the manifest URI in the EAT "manifests" claim to the MUD Manager. This approach assumes that attestation evidence includes a link to the SUIT manifest via the "manifests" claim (see Section 4.2.15 of [I-D.ietf-rats-eat]) and that this evidence can be carried in either a network access authentication protocol (for eample an EAP method) or some onboarding protocol like FIDO Device Onboard (FDO).¶
  • The MUD Manager can then (with the help of the Verifier) validate the evidence in order 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.¶
• If the SUIT_MUD_container, see Section 5, has been severed, the MUD Manager can use the suit-reference-uri to retrieve the complete SUIT manifest.¶
• The manifest authenticity is verified by the MUD Manager, which enforces that the MUD file presented is also authentic and as intended by the device software vendor.¶
• The MUD Manager acquires the MUD file from the MUD URL found in the SUIT manifest.¶
• 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

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). 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].¶

The following CDDL describes the extension to the SUIT_Manifest structure:¶

$$severable-manifest-members-choice-extensions //= (
  suit-manifest-mud => SUIT_Digest / SUIT_MUD_container
)

The SUIT_Envelope is also amended:¶

$$SUIT_severable-members-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.¶

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]]¶

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

• Label: TBD2 [[Value allocated from the standards action address range]]¶
• Name: Manufacturer Usage Description (MUD)¶
• Reference: [[TBD: This document]]¶

Authors' Addresses