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Versions: (draft-moran-suit-architecture) 00 01

SUIT                                                            B. Moran
Internet-Draft                                               Arm Limited
Intended status: Informational                                 M. Meriac
Expires: January 3, 2019                                      Consultant
                                                           H. Tschofenig
                                                             Arm Limited
                                                                D. Brown
                                                                  Linaro
                                                           July 02, 2018


     A Firmware Update Architecture for Internet of Things Devices
                    draft-ietf-suit-architecture-01

Abstract

   Vulnerabilities with Internet of Things (IoT) devices have raised the
   need for a solid and secure firmware update mechanism that is also
   suitable for constrained devices.  Incorporating such update
   mechanism to fix vulnerabilities, to update configuration settings as
   well as adding new functionality is recommended by security experts.

   This document lists requirements and describes an architecture for a
   firmware update mechanism suitable for IoT devices.  The architecture
   is agnostic to the transport of the firmware images and associated
   meta-data.

   This version of the document assumes asymmetric cryptography and a
   public key infrastructure.  Future versions may also describe a
   symmetric key approach for very constrained devices.

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 http://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 January 3, 2019.




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Copyright Notice

   Copyright (c) 2018 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
   (http://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 Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Conventions and Terminology . . . . . . . . . . . . . . . . .   3
   3.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .   6
     3.1.  Agnostic to how firmware images are distributed . . . . .   6
     3.2.  Friendly to broadcast delivery  . . . . . . . . . . . . .   6
     3.3.  Use state-of-the-art security mechanisms  . . . . . . . .   7
     3.4.  Rollback attacks must be prevented  . . . . . . . . . . .   7
     3.5.  High reliability  . . . . . . . . . . . . . . . . . . . .   7
     3.6.  Operate with a small bootloader . . . . . . . . . . . . .   8
     3.7.  Small Parsers . . . . . . . . . . . . . . . . . . . . . .   8
     3.8.  Minimal impact on existing firmware formats . . . . . . .   8
     3.9.  Robust permissions  . . . . . . . . . . . . . . . . . . .   8
     3.10. Operating modes . . . . . . . . . . . . . . . . . . . . .   9
   4.  Claims  . . . . . . . . . . . . . . . . . . . . . . . . . . .  11
   5.  Communication Architecture  . . . . . . . . . . . . . . . . .  11
   6.  Manifest  . . . . . . . . . . . . . . . . . . . . . . . . . .  14
   7.  Device Firmware Update Examples . . . . . . . . . . . . . . .  15
     7.1.  Single CPU SoC  . . . . . . . . . . . . . . . . . . . . .  16
     7.2.  Single CPU with Secure - Normal Mode Partitioning . . . .  16



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     7.3.  Dual CPU, shared memory . . . . . . . . . . . . . . . . .  16
     7.4.  Dual CPU, other bus . . . . . . . . . . . . . . . . . . .  16
   8.  Example Flow  . . . . . . . . . . . . . . . . . . . . . . . .  17
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  18
   11. Mailing List Information  . . . . . . . . . . . . . . . . . .  19
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  20
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  21
     13.2.  Informative References . . . . . . . . . . . . . . . . .  21
     13.3.  URIs . . . . . . . . . . . . . . . . . . . . . . . . . .  21
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   When developing IoT devices, one of the most difficult problems to
   solve is how to update the firmware on the device.  Once the device
   is deployed, firmware updates play a critical part in its lifetime,
   particularly when devices have a long lifetime, are deployed in
   remote or inaccessible areas or where manual intervention is cost
   prohibitive or otherwise difficult.  The need for a firmware update
   may be to fix bugs in software, to add new functionality, or to re-
   configure the device.

   The firmware update process, among other goals, has to ensure that

   -  The firmware image is authenticated and attempts to flash a
      malicious firmware image are prevented.

   -  The firmware image can be confidentiality protected so that
      attempts by an adversary to recover the plaintext binary can be
      prevented.  Obtaining the plaintext binary is often one of the
      first steps for an attack to mount an attack.

2.  Conventions and 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 RFC
   2119 [RFC2119].

   This document uses the following terms:

   -  Manifest: The manifest contains meta-data about the firmware
      image.  The manifest is protected against modification and
      provides information about the author.





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   -  Firmware Image: The firmware image is a binary that may contain
      the complete software of a device or a subset of it.  The firmware
      image may consist of multiple images, if the device contains more
      than one microcontroller.  The image may consist of a differential
      update for performance reasons.  Firmware is the more universal
      term.  Both terms are used in this document and are
      interchangeable.

   -  Bootloader: A bootloader is a piece of software that is executed
      once a microcontroller has been reset.  It is responsible for
      deciding whether to boot a firmware image that is present or
      whether to obtain and verify a new firmware image.  Since the
      bootloader is a security critical component its functionality may
      be split into separate stages.  Such a multi-stage bootloader may
      offer very basic functionality in the first stage and resides in
      ROM whereas the second stage may implement more complex
      functionality and resides in flash memory so that it can be
      updated in the future (in case bugs have been found).  The exact
      split of components into the different stages, the number of
      firmware images stored by an IoT device, and the detailed
      functionality varies throughout different implementations.

   The following entities are used:

   -  Author: The author is the entity that creates the firmware image.
      There may be multiple authors in a system either when a device
      consists of multiple micro-controllers or when the the final
      firmware image consists of software components from multiple
      companies.

   -  Device: The device is the recipient of the firmware image and the
      manifest.  The goal is to update the firmware of the device.  A
      single device may need to obtain more than one firmware image and
      manifest to succesfully perform an update.

   -  Communicator: The communicator component of the device interacts
      with the firmware update server.  It receives firmware images and
      triggers an update, if needed.  The communicator either polls a
      firmware update server for the most recent manifest/firmware or
      manifests/firmware images are pushed to it.  Note that the
      firmware update process may involve multiple stages since one or
      multiple manifests may need to be downloaded before the
      communicator can fetch one or multiple firmware images/software
      components.

   -  Status Tracker: The status tracker offers device management
      functionality that includes keep track of the firmware update
      process.  This includes fine-grained monitoring of changes at the



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      device, for example, what state of the firmware update cycle the
      device is currently in.

   -  Firmware Server: Entity that stores firmware images and manifests.
      Some deployments may require storage of the firmware images/
      manifests on more than one entities before they reach the device.

   -  Device Operator: The actor responsible for the day-to-day
      operation of a fleet of IoT devices.

   -  Network Operator: The actor responsible for the operation of a
      network to which IoT devices connect.

   In addition to the entities in the list above there is an orthogonal
   infrastructure with a Trust Provisioning Authority (TPA) distributing
   trust anchors and authorization permissions to various entities in
   the system.  The TPA may also delegate rights to install, update,
   enhance, or delete trust anchors and authorization permissions to
   other parties in the system.  This infrastructure overlaps the
   communication architecture and different deployments may empower
   certain entities while other deployments may not.  For example, in
   some cases, the Original Design Manufacturer (ODM), which is a
   company that designs and manufactures a product, may act as a TPA and
   may decide to remain in full control over the firmware update process
   of their products.

   The terms 'trust anchor' and 'trust anchor store' are defined in
   [RFC6024]:

   -  "A trust anchor represents an authoritative entity via a public
      key and associated data.  The public key is used to verify digital
      signatures, and the associated data is used to constrain the types
      of information for which the trust anchor is authoritative."

   -  "A trust anchor store is a set of one or more trust anchors stored
      in a device.  A device may have more than one trust anchor store,
      each of which may be used by one or more applications."

   Furthermore, the following abbreviations are used in this document:

   -  Microcontroller (MCU for microcontroller unit) is a small computer
      on a single integrated circuit, which is often used for mass
      volumne IoT devices.

   -  System on Chip (SoC) is an integrated circuit that integrates all
      components of a computer, such as CPU, memory, input/output ports,
      secondary storage, etc.




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   -  Homogeneous Storage Architecture (HoSA): A device that stores all
      firmware components in the same way, for example in a file system
      or in flash memory.

   -  Heterogeneous Storage Architecture (HeSA): A device that stores at
      least one firmware component differently from the rest, for
      example a device with an external, updatable radio, or a device
      with internal and external flash memory.

3.  Requirements

   The firmware update mechanism described in this specification was
   designed with the following requirements in mind:

   -  Agnostic to how firmware images are distributed

   -  Friendly to broadcast delivery

   -  Use state-of-the-art security mechanisms

   -  Rollback attacks must be prevented

   -  High reliability

   -  Operate with a small bootloader

   -  Small Parsers

   -  Minimal impact on existing firmware formats

   -  Robust permissions

   -  Diverse modes of operation

3.1.  Agnostic to how firmware images are distributed

   Firmware images can be conveyed to devices in a variety of ways,
   including USB, UART, WiFi, BLE, low-power WAN technologies, etc.  and
   use different protocols (e.g., CoAP, HTTP).  The specified mechanism
   needs to be agnostic to the distribution of the firmware images and
   manifests.

3.2.  Friendly to broadcast delivery

   This architecture does not specify any specific broadcast protocol
   however, given that broadcast may be desirable for some networks,
   updates must cause the least disruption possible both in metadata and
   payload transmission.



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   For an update to be broadcast friendly, it cannot rely on link layer,
   network layer, or transport layer security.  In addition, the same
   message must be deliverable to many devices, both those to which it
   applies and those to which it does not, without a chance that the
   wrong device will accept the update.  Considerations that apply to
   network broadcasts apply equally to the use of third-party content
   distribution networks for payload distribution.

3.3.  Use state-of-the-art security mechanisms

   End-to-end security between the author and the device, as shown in
   Section 5, is used to ensure that the device can verify firmware
   images and manifests produced by authorized authors.

   The use of post-quantum secure signature mechanisms, such as hash-
   based signatures, should be explored.  A migration to post-quantum
   secure signatures would require significant effort, therefore,
   mandatory-to-implement support for post-quantum secure signatures is
   a goal.

   A mandatory-to-implement set of algorithms has to be defined offering
   a key length of 112-bit symmetric key or security or more, as
   outlined in Section 20 of RFC 7925 [RFC7925].  This corresponds to a
   233 bit ECC key or a 2048 bit RSA key.

   If the firmware image is to be encrypted, it must be done in such a
   way that every intended recipient can decrypt it.  The information
   that is encrypted individually for each device must be an absolute
   minimum, for example AES Key Wrap [RFC5649], in order to maintain
   friendliness to Content Distribution Networks, bulk storage, and
   broadcast protocols.

3.4.  Rollback attacks must be prevented

   A device presented with an old, but valid manifest and firmware must
   not be tricked into installing such firmware since a vulnerability in
   the old firmware image may allow an attacker to gain control of the
   device.

3.5.  High reliability

   A power failure at any time must not cause a failure of the device.
   A failure to validate any part of an update must not cause a failure
   of the device.  One way to achieve this functionality is to provide a
   minimum of two storage locations for firmware and one bootable
   location for firmware.  An alternative approach is to use a 2nd stage
   bootloader with build-in full featured firmware update functionality




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   such that it is possible to return to the update process after power
   down.

   Note: This is an implementation requirement rather than a requirement
   on the manifest format.

3.6.  Operate with a small bootloader

   The bootloader must be minimal, containing only flash support,
   cryptographic primitives and optionally a recovery mechanism.  The
   recovery mechanism is used in case the update process failed and may
   include support for firmware updates over serial, USB or even a
   limited version of wireless connectivity standard like a limited
   Bluetooth Smart.  Such a recovery mechanism must provide security at
   least at the same level as the full featured firmware update
   functionalities.

   The bootloader needs to verify the received manifest and to install
   the bootable firmware image.  The bootloader should not require
   updating since a failed update poses a risk in reliability.  If more
   functionality is required in the bootloader, it must use a two-stage
   bootloader, with the first stage comprising the functionality defined
   above.

   All information necessary for a device to make a decision about the
   installation of a firmware update must fit into the available RAM of
   a constrained IoT device.  This prevents flash write exhaustion.

   Note: This is an implementation requirement.

3.7.  Small Parsers

   Since parsers are known sources of bugs they must be minimal.
   Additionally, it must be easy to parse only those fields that are
   required to validate at least one signature or MAC with minimal
   exposure.

3.8.  Minimal impact on existing firmware formats

   The design of the firmware update mechanism must not require changes
   to existing firmware formats.

3.9.  Robust permissions

   When a device obtains a monolithic firmware image from a single
   author without any additional approval steps then the authorization
   flow is relatively simple.  There are, however, other cases where




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   more complex policy decisions need to be made before updating a
   device.

   In this architecture the authorization policy is separated from the
   underlying communication architecture.  This is accomplished by
   separating the entities from their permissions.  For example, an
   author may not have the authority to install a firmware image on a
   device in critical infrastructure without the authorization of a
   device operator.  In this case, the device may be programmed to
   reject firmware updates unless they are signed both by the firmware
   author and by the device operator.

   Alternatively, a device may trust precisely one entity, which does
   all permission management and coordination.  This entity allows the
   device to offload complex permissions calculations for the device.

3.10.  Operating modes

   There are three broad classifications of update operating modes.

   -  Client-initiated Update

   -  Server-initiated Update

   -  Hybrid Update

   Client-initiated updates take the form of a communicator on a device
   proactively checking for new firmware imagines provided by firmware
   servers.

   Server-initiated updates are important to consider because timing of
   updates may need to be tightly controlled in some high- reliability
   environments.  In this case the communicator, potentially in
   coordination with the status tracker, determines what devices qualify
   for a firmware update.  Once those devices have been selected the
   firmware server distributes updates to those devices.

   Note: This assumes that the firmware server is able to reach the
   device, which may require devices to keep reachability information at
   the communicator and / or at the firmware server up-to-date.  This
   may also require keeping state at NATs and stateful packet filtering
   firewalls alive.

   Hybrid updates are those that require an interaction between the
   device and the firmware server / communicator.  The communicator
   pushes notifications of availability of an update to the device, and
   the device then downloads the image from the firmware server when it
   wants.



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   An alternative approach is to consider the steps a device has to go
   through in the course of an update:

   -  Notification

   -  Pre-authorisation

   -  Dependency resolution

   -  Download

   -  Installation

   The notification step consists of the communicator informing the
   device that an update is available.  This can be accomplished via
   polling (client-initiated), push notifications (server-initiated), or
   more complex mechanisms.

   The pre-authorisation step involves verifying whether the entity
   signing the manifest is indeed authorized to perform an update.  The
   device must also determine whether it should fetching and processing
   of the firmware image (unless it has been attached already to the
   manifest itself).

   A dependency resolution phase is needed when more than one component
   can be updated or when a differential update is used.  The necessary
   dependencies must be available prior to installation.

   The download step is the process of acquiring a local copy of the
   firmware image.  When the download is client-initiated, this means
   that the device chooses when a download occurs and initiates the
   download process.  When a download is server-party initiated, this
   means that either the communicator / firmware server tells the device
   when to download or that it initiates the transfer directly to the
   device.  For example, a download from an HTTP-based firmware server
   is client-initiated.  A transfer to a LwM2M Firmware Update resource
   [LwM2M] is server-initiated.

   If the Device has downloaded a new firmware image and is ready to
   install it it may need to wait for a trigger from a Communicator to
   install the firmware update, may trigger the update automatically, or
   may go through a more complex decision making process to determine
   the appropriate timing for an update (such as delaying the update
   process to a later time when end users are less impacted by the
   update process).






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   Installation is the act of processing the payload into a format that
   the IoT device can recognise and the bootloader is responsible for
   then booting from the newly installed firmware image.

   Each of these steps may require different permissions.

4.  Claims

   Claims in the manifest offer a way to convey instructions to a device
   that impact the firmware update process.  To have any value the
   manifest containing those claims must be authenticated and integrity
   protected.  The credential used to must be directly or indirectly
   related to the trust anchor installed at the device by the Trust
   Provisioning Authority.

   The baseline claims for all manifests are described in
   [I-D.ietf-suit-information-model].  For example, there are:

   -  Do not install firmware with earlier metadata than the current
      metadata.

   -  Only install firmware with a matching vendor, model, hardware
      revision, software version, etc.

   -  Only install firmware that is before its best-before timestamp.

   -  Only allow a firmware installation if dependencies have been met.

   -  Choose the mechanism to install the firmware, based on the type of
      firmware it is.

5.  Communication Architecture

   Figure 1 shows the communication architecture where a firmware image
   is created by an author, and uploaded to a firmware server.  The
   firmware image/manifest is distributed to the device either in a push
   or pull manner using the communicator residing on the device.  The
   device operator keeps track of the process using the status tracker.
   This allows the device operator to know and control what devices have
   received an update and which of them are still pending an update.











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               Firmware +  +----------+       Firmware + +-----------+
               Manifest    |          |-+     Manifest   |           |-+
                +--------->| Firmware | |<---------------|           | |
                |          | Server   | |                |  Author   | |
                |          |          | |                |           | |
                |          +----------+ |                +-----------+ |
                |            +----------+                  +-----------+
                |
                |
                |
               -+--                                  ------
          ----  |  ----                          ----      ----
        //      |      \\                      //              \\
       /        |        \                    /                  \
      /         |         \                  /                    \
     /          |          \                /                      \
    /           |           \              /                        \
   |            v            |            |                          |
   |     +------------+                                              |
   |     |Communicator|      |            |                          |
  |      +--------+---+       | Device    |       +--------+          |
  |      |        |           | Management|       |        |          |
  |      | Device |<----------------------------->| Status |          |
  |      |        |           |          |        | Tracker|          |
  |      +--------+           |          ||       |        |         |
   |                         |           ||       +--------+         |
   |                         |            |                          |
   |                         |             \                        /
    \                       /               \                      /
     \                     /                 \      Device        /
      \     Network       /                   \     Operator     /
       \   Operator      /                     \\              //
        \\             //                        ----      ----
          ----     ----                              ------
              -----

                          Figure 1: Architecture.

   End-to-end security mechanisms are used to protect the firmware image
   and the manifest although Figure 2 does not show the manifest itself
   since it may be distributed independently.










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                              +-----------+
  +--------+                  |           |                   +--------+
  |        |  Firmware Image  | Firmware  |   Firmware Image  |        |
  | Device |<-----------------| Server    |<------------------| Author |
  |        |                  |           |                   |        |
  +--------+                  +-----------+                   +--------+
       ^                                                          *
       *                                                          *
       ************************************************************
                          End-to-End Security

                      Figure 2: End-to-End Security.

   Whether the firmware image and the manifest is pushed to the device
   or fetched by the device is a deployment specific decision.

   The following assumptions are made to allow the device to verify the
   received firmware image and manifest before updating software:

   -  To accept an update, a device needs to verify the signature
      covering the manifest.  There may be one or multiple manifests
      that need to be validated, potentially signed by different
      parties.  The device needs to be in possession of the trust
      anchors to verify those signatures.  Installing trust anchors to
      devices via the Trust Provisioning Authority happens in an out-of-
      band fashion prior to the firmware update process.

   -  Not all entities creating and signing manifests have the same
      permissions.  A device needs to determine whether the requested
      action is indeed covered by the permission of the party that
      signed the manifest.  Informing the device about the permissions
      of the different parties also happens in an out-of-band fashion
      and is also a duty of the Trust Provisioning Authority.

   -  For confidentiality protection of firmware images the author needs
      to be in possession of the certificate/public key or a pre-shared
      key of a device.  The use of confidentiality protection of
      firmware images is deployment specific.

   There are different types of delivery modes, which are illustrated
   based on examples below.

   There is an option for embedding a firmware image into a manifest.
   This is a useful approach for deployments where devices are not
   connected to the Internet and cannot contact a dedicated server for
   download of the firmware.  It is also applicable when the firmware
   update happens via a USB stick or via Bluetooth Smart.  Figure 3
   shows this delivery mode graphically.



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                /------------\                 /------------\
               /Manifest with \               /Manifest with \
               |attached      |               |attached      |
               \firmware image/               \firmware image/
                \------------/  +-----------+  \------------/
    +--------+                  |           |                 +--------+
    |        |<.................| Firmware  |<................|        |
    | Device |                  | Server    |                 | Author |
    |        |                  |           |                 |        |
    +--------+                  +-----------+                 +--------+

                Figure 3: Manifest with attached firmware.

   Figure 4 shows an option for remotely updating a device where the
   device fetches the firmware image from some file server.  The
   manifest itself is delivered independently and provides information
   about the firmware image(s) to download.

                                /------------\
                               /              \
                               |   Manifest   |
                               \              /
    +--------+                  \------------/                +--------+
    |        |<..............................................>|        |
    | Device |                                             -- | Author |
    |        |<-                                         ---  |        |
    +--------+  --                                     ---    +--------+
                  --                                 ---
                    ---                            ---
                       --       +-----------+    --
                         --     |           |  --
          /------------\   --   | Firmware  |<-    /------------\
         /              \    -- | Server    |     /              \
         |   Firmware   |       |           |     |   Firmware   |
         \              /       +-----------+     \              /
          \------------/                           \------------/

          Figure 4: Independent retrieval of the firmware image.

   This architecture does not mandate a specific delivery mode but a
   solution must support both types.

6.  Manifest

   In order for a device to apply an update, it has to make several
   decisions about the update:

   -  Does it trust the author of the update?



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   -  Has the firmware been corrupted?

   -  Does the firmware update apply to this device?

   -  Is the update older than the active firmware?

   -  When should the device apply the update?

   -  How should the device apply the update?

   -  What kind of firmware binary is it?

   -  Where should the update be obtained?

   -  Where should the firmware be stored?

   The manifest encodes the information that devices need in order to
   make these decisions.  It is a data structure that contains the
   following information:

   -  information about the device(s) the firmware image is intended to
      be applied to,

   -  information about when the firmware update has to be applied,

   -  information about when the manifest was created,

   -  dependencies on other manifests,

   -  pointers to the firmware image and information about the format,

   -  information about where to store the firmware image,

   -  cryptographic information, such as digital signatures or message
      authentication codes (MACs).

   The manifest information model is described in
   [I-D.ietf-suit-information-model].

7.  Device Firmware Update Examples

   Although these documents attempt to define a firmware update
   architecture that is applicable to both existing systems, as well as
   yet-to-be-conceived systems; it is still helpful to consider existing
   architectures.






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7.1.  Single CPU SoC

   The simplest, and currently most common, architecture consists of a
   single MCU along with its own peripherals.  These SoCs generally
   contain some amount of flash memory for code and fixed data, as well
   as RAM for working storage.  These systems either have a single
   firmware image, or an immutable bootloader that runs a single image.
   A notable characteristic of these SoCs is that the primary code is
   generally execute in place (XIP).  Combined with the non-relocatable
   nature of the code, firmware updates need to be done in place.

7.2.  Single CPU with Secure - Normal Mode Partitioning

   Another configuration consists of a similar architecture to the
   previous, with a single CPU.  However, this CPU supports a security
   partitioning scheme that allows memory (in addition to other things)
   to be divided into secure and normal mode.  There will generally be
   two images, one for secure mode, and one for normal mode.  In this
   configuration, firmware upgrades will generally be done by the CPU in
   secure mode, which is able to write to both areas of the flash
   device.  In addition, there are requirements to be able to update
   either image independently, as well as to update them together
   atomically, as specified in the associated manifests.

7.3.  Dual CPU, shared memory

   This configuration has two or more CPUs in a single SoC that share
   memory (flash and RAM).  Generally, they will be a protection
   mechanism to prevent one CPU from accessing the other's memory.
   Upgrades in this case will typically be done by one of the CPUs, and
   is similar to the single CPU with secure mode.

7.4.  Dual CPU, other bus

   This configuration has two or more CPUs, each having their own
   memory.  There will be a communication channel between them, but it
   will be used as a peripheral, not via shared memory.  In this case,
   each CPU will have to be responsible for its own firmware upgrade.
   It is likely that one of the CPUs will be considered a master, and
   will direct the other CPU to do the upgrade.  This configuration is
   commonly used to offload specific work to other CPUs.  Firmware
   dependencies are similar to the other solutions above, sometimes
   allowing only one image to be upgraded, other times requiring several
   to be upgraded atomically.  Because the updates are happening on
   multiple CPUs, upgrading the two images atomically is challenging.






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8.  Example Flow

   The following example message flow illustrates the interaction for
   distributing a firmware image to a device starting with an author
   uploading the new firmware to Firmware Server and creating a
   manifest.  The firmware and manifest are stored on the same Firmware
   Server.

   +--------+    +-----------------+      +------------+ +----------+
   | Author |    | Firmware Server |      |Communicator| |Bootloader|
   +--------+    +-----------------+      +------------+ +----------+
     |                   |                     |                +
     | Create Firmware   |                     |                |
     |---------------    |                     |                |
     |              |    |                     |                |
     |<--------------    |                     |                |
     |                   |                     |                |
     | Upload Firmware   |                     |                |
     |------------------>|                     |                |
     |                   |                     |                |
     | Create Manifest   |                     |                |
     |----------------   |                     |                |
     |               |   |                     |                |
     |<---------------   |                     |                |
     |                   |                     |                |
     | Sign Manifest     |                     |                |
     |--------------     |                     |                |
     |             |     |                     |                |
     |<-------------     |                     |                |
     |                   |                     |                |
     | Upload Manifest   |                     |                |
     |------------------>|                     |                |
     |                   |                     |                |
     |                   |   Query Manifest    |                |
     |                   |<--------------------|                |
     |                   |                     |                |
     |                   |   Send Manifest     |                |
     |                   |-------------------->|                |
     |                   |                     | Validate       |
     |                   |                     | Manifest       |
     |                   |                     |---------+      |
     |                   |                     |         |      |
     |                   |                     |<--------+      |
     |                   |                     |                |
     |                   |  Request Firmware   |                |
     |                   |<--------------------|                |
     |                   |                     |                |
     |                   | Send Firmware       |                |



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     |                   |-------------------->|                |
     |                   |                     | Verify         |
     |                   |                     | Firmware       |
     |                   |                     |--------------- |
     |                   |                     |              | |
     |                   |                     |<-------------- |
     |                   |                     |                |
     |                   |                     | Store          |
     |                   |                     | Firmware       |
     |                   |                     |--------------  |
     |                   |                     |             |  |
     |                   |                     |<-------------  |
     |                   |                     |                |
     |                   |                     |                |
     |                   |                     | Reboot         |
     |                   |                     |--------------->|
     |                   |                     |                |
     |                   |                     | Validate       |
     |                   |                     | Firmware       |
     |                   |                     | ---------------|
     |                   |                     | |              |
     |                   |                     | -------------->|
     |                   |                     |                |
     |                   |                     | Activate new   |
     |                   |                     | Firmware       |
     |                   |                     | ---------------|
     |                   |                     | |              |
     |                   |                     | -------------->|
     |                   |                     |                |
     |                   |                     | Boot new       |
     |                   |                     | Firmware       |
     |                   |                     | ---------------|
     |                   |                     | |              |
     |                   |                     | -------------->|
     |                   |                     |                |

               Figure 5: Example Flow for a Firmware Upate.

9.  IANA Considerations

   This document does not require any actions by IANA.

10.  Security Considerations

   Firmware updates fix security vulnerabilities and are considered to
   be an important building block in securing IoT devices.  Due to the
   importance of firmware updates for IoT devices the Internet
   Architecture Board (IAB) organized a 'Workshop on Internet of Things



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   (IoT) Software Update (IOTSU)', which took place at Trinity College
   Dublin, Ireland on the 13th and 14th of June, 2016 to take a look at
   the big picture.  A report about this workshop can be found at
   [RFC8240].  A standardized firmware manifest format providing end-to-
   end security from the author to the device will be specified in a
   separate document.

   There are, however, many other considerations raised during the
   workshop.  Many of them are outside the scope of standardization
   organizations since they fall into the realm of product engineering,
   regulatory frameworks, and business models.  The following
   considerations are outside the scope of this document, namely

   -  installing firmware updates in a robust fashion so that the update
      does not break the device functionality of the environment this
      device operates in.

   -  installing firmware updates in a timely fashion considering the
      complexity of the decision making process of updating devices,
      potential re-certification requirements, and the need for user
      consent to install updates.

   -  the distribution of the actual firmware update, potentially in an
      efficient manner to a large number of devices without human
      involvement.

   -  energy efficiency and battery lifetime considerations.

   -  key management required for verifying the digital signature
      protecting the manifest.

   -  incentives for manufacturers to offer a firmware update mechanism
      as part of their IoT products.

11.  Mailing List Information

   The discussion list for this document is located at the e-mail
   address suit@ietf.org [1].  Information on the group and information
   on how to subscribe to the list is at
   https://www1.ietf.org/mailman/listinfo/suit

   Archives of the list can be found at: https://www.ietf.org/mail-
   archive/web/suit/current/index.html








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12.  Acknowledgements

   We would like to thank the following persons for their feedback:

   -  Geraint Luff

   -  Amyas Phillips

   -  Dan Ros

   -  Thomas Eichinger

   -  Michael Richardson

   -  Emmanuel Baccelli

   -  Ned Smith

   -  Jim Schaad

   -  Carsten Bormann

   -  Cullen Jennings

   -  Olaf Bergmann

   -  Suhas Nandakumar

   -  Phillip Hallam-Baker

   -  Marti Bolivar

   -  Andrzej Puzdrowski

   -  Markus Gueller

   -  Henk Birkholz

   -  Jintao Zhu

   We would also like to thank the WG chairs, Russ Housley, David
   Waltermire, Dave Thaler for their support and their reviews.
   Kathleen Moriarty was the responsible security area director when
   this work was started.







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13.  References

13.1.  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>.

   [RFC7925]  Tschofenig, H., Ed. and T. Fossati, "Transport Layer
              Security (TLS) / Datagram Transport Layer Security (DTLS)
              Profiles for the Internet of Things", RFC 7925,
              DOI 10.17487/RFC7925, July 2016, <https://www.rfc-
              editor.org/info/rfc7925>.

13.2.  Informative References

   [I-D.ietf-suit-information-model]
              Moran, B., Tschofenig, H., Birkholz, H., and J. Jimenez,
              "Firmware Updates for Internet of Things Devices - An
              Information Model for Manifests", draft-ietf-suit-
              information-model-00 (work in progress), June 2018.

   [LwM2M]    OMA, ., "Lightweight Machine to Machine Technical
              Specification, Version 1.0.2", February 2018,
              <http://www.openmobilealliance.org/release/LightweightM2M/
              V1_0_2-20180209-A/
              OMA-TS-LightweightM2M-V1_0_2-20180209-A.pdf>.

   [RFC5649]  Housley, R. and M. Dworkin, "Advanced Encryption Standard
              (AES) Key Wrap with Padding Algorithm", RFC 5649,
              DOI 10.17487/RFC5649, September 2009, <https://www.rfc-
              editor.org/info/rfc5649>.

   [RFC6024]  Reddy, R. and C. Wallace, "Trust Anchor Management
              Requirements", RFC 6024, DOI 10.17487/RFC6024, October
              2010, <https://www.rfc-editor.org/info/rfc6024>.

   [RFC8240]  Tschofenig, H. and S. Farrell, "Report from the Internet
              of Things Software Update (IoTSU) Workshop 2016",
              RFC 8240, DOI 10.17487/RFC8240, September 2017,
              <https://www.rfc-editor.org/info/rfc8240>.

13.3.  URIs

   [1] mailto:suit@ietf.org





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Authors' Addresses

   Brendan Moran
   Arm Limited

   EMail: Brendan.Moran@arm.com


   Milosch Meriac
   Consultant

   EMail: milosch@meriac.com


   Hannes Tschofenig
   Arm Limited

   EMail: hannes.tschofenig@gmx.net


   David Brown
   Linaro

   EMail: david.brown@linaro.org



























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