wdiff draft-ietf-suit-architecture-00.txt draft-ietf-suit-architecture-01.txt

SUIT B. Moran
Internet-Draft Arm Limited
Intended status: Informational M. Meriac
Expires: December 5, 2018 January 3, 2019 Consultant
H. Tschofenig
Arm Limited
June 03,
D. Brown
Linaro
July 02, 2018

A Firmware Update Architecture for Internet of Things Devices
draft-ietf-suit-architecture-00
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

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provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on December 5, 2018. January 3, 2019.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Terminology . . . . . . . . . . . . . . . . . 3
3. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 4 6
3.1. Agnostic to how firmware images are distributed . . . . . 5 6
3.2. Friendly to broadcast delivery . . . . . . . . . . . . . 5 6
3.3. Use state-of-the-art security mechanisms . . . . . . . . 5 7
3.4. Rollback attacks must be prevented . . . . . . . . . . . 6 7
3.5. High reliability . . . . . . . . . . . . . . . . . . . . 6 7
3.6. Operate with a small bootloader . . . . . . . . . . . . . 6 8
3.7. Small Parsers . . . . . . . . . . . . . . . . . . . . . . 7 8
3.8. Minimal impact on existing firmware formats . . . . . . . 7 8
3.9. Robust permissions . . . . . . . . . . . . . . . . . . . 7 8
3.10. Operating modes . . . . . . . . . . . . . . . . . . . . . 8 9
4. Claims . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 11
5. Communication Architecture . . . . . . . . . . . . . . . . . 11
6. Manifest . . . . . . . 10
6. Manifest . . . . . . . . . . . . . . . . . . . 14
7. Device Firmware Update Examples . . . . . . . 12
7. . . . . . . . . 15
7.1. Single CPU SoC . . . . . . . . . . . . . . . . . . . . . 16
7.2. Single CPU with Secure - Normal Mode Partitioning . . . . 16
7.3. Dual CPU, shared memory . . . . . . . . . . . . . . . . . 16
7.4. Dual CPU, other bus . . . . . . . . . . . . . . . . . . . 16
8. Example Flow . . . . . . . . . . . . . . . . . . . . . . . . 13
8. 17
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
9. 18
10. Security Considerations . . . . . . . . . . . . . . . . . . . 15
10. 18
11. Mailing List Information . . . . . . . . . . . . . . . . . . 16
11. 19
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
12. 20
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
12.1. 21
13.1. Normative References . . . . . . . . . . . . . . . . . . 17
12.2. 21
13.2. Informative References . . . . . . . . . . . . . . . . . 17
12.3. 21
13.3. URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 18 21
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 18 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 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.

- 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,
signs and/or encrypts it. The author is most likely image.
There may be multiple authors in a developer
using system either when a set device
consists of tools. 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.

- Untrusted Storage: Firmware 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 stored on
untrusted fileservers downloaded before the
communicator can fetch one or cloud storage infrastructure. 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
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
to be stored images/
manifests on various 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.

- 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.

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
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 may have many modules that require updating individually.
It may also 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
more complex policy decisions need to trust several actors in order to authorize an
update. These actors might include be made before updating a
device.

In this architecture the following (this authorization policy is not a
comprehensive list).

* A firmware author
* A device OEM
* A device operator
* A network operator
* A device owner

These actors exert their authority on separated from the device
underlying communication architecture. This is accomplished by making claims (as
in Section 4).
separating the entities from their permissions. For example, a firmware 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. To facilitate
complex use-cases such as this, updates require several claims.

Alternatively, a device may trust precisely one authority, entity, which does
all permission management and coordination. Effectively, the
authority 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.

* Self initiated
* Third-party initiated
*

- Client-initiated Update

- Server-initiated Update

- Hybrid

Self initiated Update

Client-initiated updates take the form of a proactive IoT communicator on a device that
checks
proactively checking for updates. Third-party initiated updates are triggered new firmware imagines provided by
an actor other than the IoT device, be it a server, a peer, or a
user. Hybrid updates are those that require agreement from both the
target IoT device and another actor.

Third-party initiated firmware
servers.

Server-initiated updates are important to consider because timing of
updates may need to be tightly controlled in some high- reliability
environments.

An IoT device goes through several steps In this case the communicator, potentially in
coordination with the course of an update,
each of 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 can be self-initiated or third-party initiated, may require devices to keep reachability information at
the communicator and / or
hybrid. 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.

An IoT alternative approach is to consider the steps a device may has to go
through in the following steps, though
this is not a comprehensive list.

* course of an update:

- Notification
*

- Pre-authorisation
*

- Dependency resolution
*

- Download
*

- Installation

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

The pre-authorisation step involves verifying whether the update authority
and making a determination that entity
signing the device manifest is prepared indeed authorized to initiate
the perform an update. The
device must also determine whether it should fetching and processing
of updates. If the device firmware image (unless it has all
information that is necessary been attached already to make this determination, then the
pre-authorisation may be self-initiated. However, the device can
wait for instruction to begin (third-party initiated). Hybrid
approaches are possible as well.
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
payload.
firmware image. When the download is self-initiated, client-initiated, this means
that the
IoT device chooses when a download occurs and initiates the
download process. When a download is third-party server-party initiated, this
means that either the remote service communicator / firmware server tells the IoT device
when to download or that it initiates the transfer directly to the IoT
device. For example, a download from an HTTP HTTP-based firmware server
is initiated locally. client-initiated. A transfer to a LwM2M Firmware Update resource
[LwM2M] is initiated
remotely. 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).

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

Each of these steps may require different permissions expressed in
claims and may be implemented in a variety of ways. permissions.

4. Claims

When a simple set of permissions fails to encapsulate

Claims in the rules
required for manifest offer a device way to convey instructions to make decisions about firmware, claims can be
used instead. Claims represent a form of policy. Several device
that impact the firmware update process. To have any value the
manifest containing those claims can must be authenticated and integrity
protected. The credential used together, when multiple actors should have the rights to set
policies.

Some example claims are:

- Trust the actor identified by the referenced public key.

- Trust the actor with access must be directly or indirectly
related to the referenced shared secret (MAC).

- Three actors are trusted identified by their public keys.
Signatures from at least two of these actors are required to trust
a manifest.

- The actor identified anchor installed at the device by the referenced public key is authorized to
create secondary policies Trust
Provisioning Authority.

The baseline claims for all manifests are described in [SUIT-IM]. In
summary, they
[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 install firmware with metadata signed/authenticated by a
trusted actor.

- Only allow an actor to exercise rights on the device via a
manifest if that actor has signed the manifest.

- Only allow a firmware installation if all required rights dependencies have been met through signatures/MACs (one or more) or manifest
dependencies (one or more).

- Use the instructions provided by the manifest to install the
firmware.

- Install any and all firmware images that are linked together with
manifest dependencies. met.

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

5. Communication Architecture

We start the architectural description with the security model. It
is based on end-to-end security. In

Figure 1 shows the communication architecture where a firmware image
is created by an author, sent to the device and subsequently installed.
When the author is ready uploaded to distribute the a firmware image it server. The
firmware image/manifest is
conveyed using some communication channel distributed to the device, which will
typically involve device either in a push
or pull manner using the use communicator residing on the device. The
device operator keeps track of untrusted storage. Examples the process using the status tracker.
This allows the device operator to know and control what devices have
received an update and which of
untrusted storage them are FTP servers, Web servers or USB sticks. End-
to-end still pending an update.

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. image
and the manifest although Figure 1 2 does not show the manifest itself, which provides the meta-
data about the firmware image and offers the security protection. It itself
since it may bundled with the firmware image or travel as a standalone item. be distributed independently.

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

Figure 1: 2: End-to-End Security.

Whether the firmware image and the manifest is pushed to the device
or fetched by the device is outside the scope of this work and
existing device management protocols can be used for efficiently
distributing this information. 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 decide whether the author
signing the firmware image and verify the manifest is authorized to make signature
covering the updates. We use public key cryptography manifest. There may be one or multiple manifests
that need to accomplish this. be validated, potentially signed by different
parties. The device verifies the signature covering needs to be in possession of the manifest using a
digital signature algorithm OR trust
anchors to verify those signatures. Installing trust anchors to
devices via the device verifies Trust Provisioning Authority happens in an out-of-
band fashion prior to the MAC
covering firmware update process.

- Not all entities creating and signing manifests have the manifest using a MAC algorithm. The same
permissions. A device is
provisioned with a trust anchor that is used needs to validate determine whether the
digital signature or MAC produced requested
action is indeed covered by the author. This trust
anchor is potentially different from permission of the trust anchor used to
validate party that
signed the digital signature produced for other protocols (such
as device management protocols). This trust anchor may be
provisioned to manifest. Informing the device during manufacturing or during
commissioning. 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 2 3
shows this delivery mode graphically.

Figure 2: 3: Manifest with attached firmware.

Figure 3 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 |
| |<- --- | |
+--------+ -- --- +--------+
-- ---
--- ---
-- +-----------+ --
-- | | --
/------------\ -- | Untrusted Firmware |<- /------------\
/ \ -- | Storage Server | / \
| Firmware | | | | Firmware |
\ / +-----------+ \ /
\------------/ \------------/

Figure 3: 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?
- 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 [SUIT-IM].
[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.

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.

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 untrusted storage Firmware Server and creating a
manifest. The firmware and manifest are stored on the same untrusted
storage. Firmware
Server.

+--------+ +-----------------+ +------+ +------------+ +----------+
| Author | |Untrusted Storage| |Device| | Firmware Server | |Communicator| |Bootloader|
+--------+ +-----------------+ +------+ +------------+ +----------+
| | | +
| Create Firmware | | |
|--------------- | | |
| | | | |
|<-------------- | | |
| | | |
| Upload Firmware | | |
|------------------>| | |
| | | |
| Create Manifest | | |
|---------------- | | |
| | | | |
|<--------------- | | |
| | | |
| Sign Manifest | | |
|-------------- | | |
| | | | |
|<------------- | | |
| | | |
| Upload Manifest | | |
|------------------>| | |
| | | |
| | Query Manifest | |
| |<--------------------| |
| | | |
| | Send Manifest | |
| |-------------------->| |
| | | Validate |
| | | Validate Manifest |
| |------------------ | |---------+ |
| | | | |<----------------- |
| | |<--------+ |
| | | |
| | Request Firmware | |
| |<--------------------| |
| | | |
| | Send Firmware | |
| |-------------------->| |
| | | Verify |
| | | Verify Firmware |
| | |--------------- |
| | | | |
| | |<-------------- |
| | | |
| | | Store |
| | | Firmware |
| | |-------------- |
| | | | |
| | |<------------- |
| | | |
| Reboot | | |------- |
| | | Reboot |
| | |--------------->|
| | |<------ | |
| | | Validate | Bootloader validates
| | | Firmware |
| |---------------------- | | ---------------|
| | | | | |<---------------------
| | | -------------->|
| | | Bootloader activates |
| | | Activate new |
| | | Firmware |
| |---------------------- | | ---------------|
| | | | |<--------------------- |
| | | -------------->|
| | Bootloader transfers | |
| control to | | Boot new |
| | | Firmware |
| |---------------------- | | ---------------|
| | | | |
| |<--------------------- | | -------------->|
| | | |

Figure 4: 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
(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.

10.

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

11.

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

- David Brown

- 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.

12.

13. References

12.1.

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>.

12.2.

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>.

[SUIT-IM] Moran, B., Tschofenig, H., Birkholz, H., and J. Jimenez,
"Firmware Updates for Internet of Things Devices - An
Information Model for Manifests", June 2018.

12.3.

13.3. URIs

[1] mailto:suit@ietf.org

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