Internet DRAFT - draft-ietf-6tisch-minimal-security
draft-ietf-6tisch-minimal-security
6TiSCH Working Group M. Vucinic, Ed.
Internet-Draft Inria
Intended status: Standards Track J. Simon
Expires: June 12, 2020 Analog Devices
K. Pister
University of California Berkeley
M. Richardson
Sandelman Software Works
December 10, 2019
Constrained Join Protocol (CoJP) for 6TiSCH
draft-ietf-6tisch-minimal-security-15
Abstract
This document describes the minimal framework required for a new
device, called "pledge", to securely join a 6TiSCH (IPv6 over the
TSCH mode of IEEE 802.15.4e) network. The framework requires that
the pledge and the JRC (join registrar/coordinator, a central
entity), share a symmetric key. How this key is provisioned is out
of scope of this document. Through a single CoAP (Constrained
Application Protocol) request-response exchange secured by OSCORE
(Object Security for Constrained RESTful Environments), the pledge
requests admission into the network and the JRC configures it with
link-layer keying material and other parameters. The JRC may at any
time update the parameters through another request-response exchange
secured by OSCORE. This specification defines the Constrained Join
Protocol and its CBOR (Concise Binary Object Representation) data
structures, and describes how to configure the rest of the 6TiSCH
communication stack for this join process to occur in a secure
manner. Additional security mechanisms may be added on top of this
minimal framework.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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material or to cite them other than as "work in progress."
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This Internet-Draft will expire on June 12, 2020.
Copyright Notice
Copyright (c) 2019 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Provisioning Phase . . . . . . . . . . . . . . . . . . . . . 5
4. Join Process Overview . . . . . . . . . . . . . . . . . . . . 7
4.1. Step 1 - Enhanced Beacon . . . . . . . . . . . . . . . . 8
4.2. Step 2 - Neighbor Discovery . . . . . . . . . . . . . . . 9
4.3. Step 3 - Constrained Join Protocol (CoJP) Execution . . . 9
4.4. The Special Case of the 6LBR Pledge Joining . . . . . . . 10
5. Link-layer Configuration . . . . . . . . . . . . . . . . . . 10
5.1. Distribution of Time . . . . . . . . . . . . . . . . . . 11
6. Network-layer Configuration . . . . . . . . . . . . . . . . . 12
6.1. Identification of Unauthenticated Traffic . . . . . . . . 13
7. Application-level Configuration . . . . . . . . . . . . . . . 14
7.1. Statelessness of the JP . . . . . . . . . . . . . . . . . 15
7.2. Recommended Settings . . . . . . . . . . . . . . . . . . 16
7.3. OSCORE . . . . . . . . . . . . . . . . . . . . . . . . . 16
8. Constrained Join Protocol (CoJP) . . . . . . . . . . . . . . 19
8.1. Join Exchange . . . . . . . . . . . . . . . . . . . . . . 20
8.2. Parameter Update Exchange . . . . . . . . . . . . . . . . 21
8.3. Error Handling . . . . . . . . . . . . . . . . . . . . . 23
8.4. CoJP Objects . . . . . . . . . . . . . . . . . . . . . . 25
8.5. Recommended Settings . . . . . . . . . . . . . . . . . . 39
9. Security Considerations . . . . . . . . . . . . . . . . . . . 39
10. Privacy Considerations . . . . . . . . . . . . . . . . . . . 41
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 42
11.1. CoJP Parameters Registry . . . . . . . . . . . . . . . . 42
11.2. CoJP Key Usage Registry . . . . . . . . . . . . . . . . 43
11.3. CoJP Unsupported Configuration Code Registry . . . . . . 44
12. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 44
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13. References . . . . . . . . . . . . . . . . . . . . . . . . . 45
13.1. Normative References . . . . . . . . . . . . . . . . . . 45
13.2. Informative References . . . . . . . . . . . . . . . . . 46
Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . 48
Appendix B. Lightweight Implementation Option . . . . . . . . . 51
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 52
1. Introduction
This document defines a "secure join" solution for a new device,
called "pledge", to securely join a 6TiSCH network. The term "secure
join" refers to network access authentication, authorization and
parameter distribution, as defined in [I-D.ietf-6tisch-architecture].
The Constrained Join Protocol (CoJP) defined in this document handles
parameter distribution needed for a pledge to become a joined node.
Mutual authentication during network access and implicit
authorization are achieved through the use of a secure channel, as
configured by this document. This document also specifies a
configuration of different layers of the 6TiSCH protocol stack that
reduces the Denial of Service (DoS) attack surface during the join
process.
This document presumes a 6TiSCH network as described by [RFC7554] and
[RFC8180]. By design, nodes in a 6TiSCH network [RFC7554] have their
radio turned off most of the time, to conserve energy. As a
consequence, the link used by a new device for joining the network
has limited bandwidth [RFC8180]. The secure join solution defined in
this document therefore keeps the number of over-the-air exchanges to
a minimum.
The micro-controllers at the heart of 6TiSCH nodes have a small
amount of code memory. It is therefore paramount to reuse existing
protocols available as part of the 6TiSCH stack. At the application
layer, the 6TiSCH stack already relies on CoAP [RFC7252] for web
transfer, and on OSCORE [RFC8613] for its end-to-end security. The
secure join solution defined in this document therefore reuses those
two protocols as its building blocks.
CoJP is a generic protocol that can be used as-is in all modes of
IEEE Std 802.15.4 [IEEE802.15.4], including the Time-Slotted Channel
Hopping (TSCH) mode 6TiSCH is based on. CoJP may as well be used in
other (low-power) networking technologies where efficiency in terms
of communication overhead and code footprint is important. In such a
case, it may be necessary to define configuration parameters specific
to the technology in question, through companion documents. The
overall process described in Section 4 and the configuration of the
stack is specific to 6TiSCH.
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CoJP assumes the presence of a Join Registrar/Coordinator (JRC), a
central entity. The configuration defined in this document assumes
that the pledge and the JRC share a unique symmetric cryptographic
key, called PSK (pre-shared key). The PSK is used to configure
OSCORE to provide a secure channel to CoJP. How the PSK is installed
is out of scope of this document: this may happen during the
provisioning phase or by a key exchange protocol that may precede the
execution of CoJP.
When the pledge seeks admission to a 6TiSCH network, it first
synchronizes to it, by initiating the passive scan defined in
[IEEE802.15.4]. The pledge then exchanges CoJP messages with the
JRC; for this end-to-end communication to happen, messages are
forwarded by nodes already part of the 6TiSCH network, called Join
Proxies. The messages exchanged allow the JRC and the pledge to
mutually authenticate, based on the properties provided by OSCORE.
They also allow the JRC to configure the pledge with link-layer
keying material, short identifier and other parameters. After this
secure join process successfully completes, the joined node can
interact with its neighbors to request additional bandwidth using the
6top Protocol [RFC8480] and start sending application traffic.
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
BCP14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
The reader is expected to be familiar with the terms and concepts
defined in [I-D.ietf-6tisch-architecture], [RFC7252], [RFC8613], and
[RFC8152].
The specification also includes a set of informative specifications
using the Concise data definition language (CDDL)
[I-D.ietf-cbor-cddl].
The following terms defined in [I-D.ietf-6tisch-architecture] are
used extensively throughout this document:
o pledge
o joined node
o join proxy (JP)
o join registrar/coordinator (JRC)
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o enhanced beacon (EB)
o join protocol
o join process
The following terms defined in [RFC8505] are also used throughout
this document:
o 6LoWPAN Border Router (6LBR)
o 6LoWPAN Node (6LN)
The term "6LBR" is used interchangeably with the term "DODAG root"
defined in [RFC6550], on the assumption that the two entities are co-
located, as recommended by [I-D.ietf-6tisch-architecture].
The term "pledge", as used throughout the document, explicitly
denotes non-6LBR devices attempting to join the network using their
IEEE Std 802.15.4 network interface. The device that attempts to
join as the 6LBR of the network and does so over another network
interface is explicitly denoted as the "6LBR pledge". When the text
equally applies to the pledge and the 6LBR pledge, the "(6LBR)
pledge" form is used.
In addition, we use generic terms "pledge identifier" and "network
identifier". See Section 3.
3. Provisioning Phase
The (6LBR) pledge is provisioned with certain parameters before
attempting to join the network, and the same parameters are
provisioned to the JRC. There are many ways by which this
provisioning can be done. Physically, the parameters can be written
into the (6LBR) pledge using a number of mechanisms, such as a JTAG
interface, a serial (craft) console interface, pushing buttons
simultaneously on different devices, over-the-air configuration in a
Faraday cage, etc. The provisioning can be done by the vendor, the
manufacturer, the integrator, etc.
Details of how this provisioning is done is out of scope of this
document. What is assumed is that there can be a secure, private
conversation between the JRC and the (6LBR) pledge, and that the two
devices can exchange the parameters.
Parameters that are provisioned to the (6LBR) pledge include:
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o pledge identifier. The pledge identifier identifies the (6LBR)
pledge. The pledge identifier MUST be unique in the set of all
pledge identifiers managed by a JRC. The pledge identifier
uniqueness is an important security requirement, as discussed in
Section 9. The pledge identifier is typically the globally unique
64-bit Extended Unique Identifier (EUI-64) of the IEEE Std
802.15.4 device, in which case it is provisioned by the hardware
manufacturer. The pledge identifier is used to generate the IPv6
addresses of the (6LBR) pledge and to identify it during the
execution of the join protocol. Depending on the configuration,
the pledge identifier may also be used after the join process to
identify the joined node. For privacy reasons (see Section 10),
it is possible to use a pledge identifier different from the EUI-
64. For example, a pledge identifier may be a random byte string,
but care needs to be taken that such a string meets the uniqueness
requirement.
o Pre-Shared Key (PSK). A symmetric cryptographic key shared
between the (6LBR) pledge and the JRC. To look up the PSK for a
given pledge, the JRC additionally needs to store the
corresponding pledge identifier. Each (6LBR) pledge MUST be
provisioned with a unique PSK. The PSK MUST be a
cryptographically strong key, with at least 128 bits of entropy,
indistinguishable by feasible computation from a random uniform
string of the same length. How the PSK is generated and/or
provisioned is out of scope of this specification. This could be
done during a provisioning step or companion documents can specify
the use of a key agreement protocol. Common pitfalls when
generating PSKs are discussed in Section 9. In case of device re-
commissioning to a new owner, the PSK MUST be changed. Note that
the PSK is different from the link-layer keys K1 and K2 specified
in [RFC8180]. The PSK is a long-term secret used to protect the
execution of the secure join protocol specified in this document
whose one output are link-layer keys.
o Optionally, a network identifier. The network identifier
identifies the 6TiSCH network. The network identifier MUST be
carried within Enhanced Beacon (EB) frames. Typically, the 16-bit
Personal Area Network Identifier (PAN ID) defined in
[IEEE802.15.4] is used as the network identifier. However, PAN ID
is not considered a stable network identifier as it may change
during network lifetime if a collision with another network is
detected. Companion documents can specify the use of a different
network identifier for join purposes, but this is out of scope of
this specification. Provisioning the network identifier to a
pledge is RECOMMENDED. However, due to operational constraints,
the network identifier may not be known at the time when the
provisioning is done. In case this parameter is not provisioned
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to the pledge, the pledge attempts to join one advertised network
at a time, which significantly prolongs the join process. This
parameter MUST be provisioned to the 6LBR pledge.
o Optionally, any non-default algorithms. The default algorithms
are specified in Section 7.3.3. When algorithm identifiers are
not provisioned, the use of these default algorithms is implied.
Additionally, the 6LBR pledge that is not co-located with the JRC
needs to be provisioned with:
o Global IPv6 address of the JRC. This address is used by the 6LBR
pledge to address the JRC during the join process. The 6LBR
pledge may also obtain the IPv6 address of the JRC through other
available mechanisms, such as DHCPv6 [RFC8415], GRASP
[I-D.ietf-anima-grasp], mDNS [RFC6762], the use of which is out of
scope of this document. Pledges do not need to be provisioned
with this address as they discover it dynamically through CoJP.
4. Join Process Overview
This section describes the steps taken by a pledge in a 6TiSCH
network. When a pledge seeks admission to a 6TiSCH network, the
following exchange occurs:
1. The pledge listens for an Enhanced Beacon (EB) frame
[IEEE802.15.4]. This frame provides network synchronization
information, and tells the device when it can send a frame to the
node sending the beacons, which acts as a Join Proxy (JP) for the
pledge, and when it can expect to receive a frame. The Enhanced
Beacon provides the link-layer address of the JP and it may also
provide its link-local IPv6 address.
2. The pledge configures its link-local IPv6 address and advertises
it to the JP using Neighbor Discovery. The advertisement step
may be omitted if the link-local address has been derived from a
known unique interface identifier, such as an EUI-64 address.
3. The pledge sends a Join Request to the JP in order to securely
identify itself to the network. The Join Request is forwarded to
the JRC.
4. In case of successful processing of the request, the pledge
receives a Join Response from the JRC (via the JP). The Join
Response contains configuration parameters necessary for the
pledge to join the network.
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From the pledge's perspective, joining is a local phenomenon - the
pledge only interacts with the JP, and it needs not know how far it
is from the 6LBR, or how to route to the JRC. Only after
establishing one or more link-layer keys does it need to know about
the particulars of a 6TiSCH network.
The join process is shown as a transaction diagram in Figure 1:
+--------+ +-------+ +--------+
| pledge | | JP | | JRC |
| | | | | |
+--------+ +-------+ +--------+
| | |
|<---Enhanced Beacon (1)---| |
| | |
|<-Neighbor Discovery (2)->| |
| | |
|-----Join Request (3a)----|----Join Request (3a)---->| \
| | | | CoJP
|<----Join Response (3b)---|----Join Response (3b)----| /
| | |
Figure 1: Overview of a successful join process.
As for other nodes in the network, the 6LBR node may act as the JP.
The 6LBR may in addition be co-located with the JRC.
The details of each step are described in the following sections.
4.1. Step 1 - Enhanced Beacon
The pledge synchronizes to the network by listening for, and
receiving, an Enhanced Beacon (EB) sent by a node already in the
network. This process is entirely defined by [IEEE802.15.4], and
described in [RFC7554].
Once the pledge hears an EB, it synchronizes to the joining schedule
using the cells contained in the EB. The pledge can hear multiple
EBs; the selection of which EB to use is out of the scope for this
document, and is discussed in [RFC7554]. Implementers should make
use of information such as: what network identifier the EB contains,
the value of the Join Metric field within EBs, whether the source
link-layer address of the EB has been tried before, what signal
strength the different EBs were received at, etc. In addition, the
pledge may be pre-configured to search for EBs with a specific
network identifier.
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If the pledge is not provisioned with the network identifier, it
attempts to join one network at a time, as described in
Section 8.1.1.
Once the pledge selects the EB, it synchronizes to it and transitions
into a low-power mode. It follows the schedule information contained
in the EB which indicates the slots that the pledge may use for the
join process. During the remainder of the join process, the node
that has sent the EB to the pledge acts as the JP.
At this point, the pledge may proceed to step 2, or continue to
listen for additional EBs.
4.2. Step 2 - Neighbor Discovery
The pledge forms its link-local IPv6 address based on the interface
identifier, as per [RFC4944]. The pledge MAY perform the Neighbor
Solicitation / Neighbor Advertisement exchange with the JP, as per
Section 5.6 of [RFC8505]. As per [RFC8505], there is no need to
perform duplicate address detection for the link-local address. The
pledge and the JP use their link-local IPv6 addresses for all
subsequent communication during the join process.
Note that Neighbor Discovery exchanges at this point are not
protected with link-layer security as the pledge is not in possession
of the keys. How the JP accepts these unprotected frames is
discussed in Section 5.
4.3. Step 3 - Constrained Join Protocol (CoJP) Execution
The pledge triggers the join exchange of the Constrained Join
Protocol (CoJP). The join exchange consists of two messages: the
Join Request message (Step 3a), and the Join Response message
conditioned on the successful security processing of the request
(Step 3b).
All CoJP messages are exchanged over a secure end-to-end channel that
provides confidentiality, data authenticity and replay protection.
Frames carrying CoJP messages are not protected with link-layer
security when exchanged between the pledge and the JP as the pledge
is not in possession of the link-layer keys in use. How JP and
pledge accept these unprotected frames is discussed in Section 5.
When frames carrying CoJP messages are exchanged between nodes that
have already joined the network, the link-layer security is applied
according to the security configuration used in the network.
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4.3.1. Step 3a - Join Request
The Join Request is a message sent from the pledge to the JP, and
which the JP forwards to the JRC. The pledge indicates in the Join
Request the role it requests to play in the network, as well as the
identifier of the network it requests to join. The JP forwards the
Join Request to the JRC on the existing links. How exactly this
happens is out of scope of this document; some networks may wish to
dedicate specific link layer resources for this join traffic.
4.3.2. Step 3b - Join Response
The Join Response is sent by the JRC to the pledge, and is forwarded
through the JP. The packet containing the Join Response travels from
the JRC to the JP using the operating routes in the network. The JP
delivers it to the pledge. The JP operates as an application-layer
proxy, see Section 7.
The Join Response contains different parameters needed by the pledge
to become a fully operational network node. These parameters include
the link-layer key(s) currently in use in the network, the short
address assigned to the pledge, the IPv6 address of the JRC needed by
the pledge to operate as the JP, among others.
4.4. The Special Case of the 6LBR Pledge Joining
The 6LBR pledge performs Section 4.3 of the join process described
above, just as any other pledge, albeit over a different network
interface. There is no JP intermediating the communication between
the 6LBR pledge and the JRC, as described in Section 6. The other
steps of the described join process do not apply to the 6LBR pledge.
How the 6LBR pledge obtains an IPv6 address and triggers the
execution of the CoJP protocol is out of scope of this document.
5. Link-layer Configuration
In an operational 6TiSCH network, all frames use link-layer frame
security [RFC8180]. The IEEE Std 802.15.4 security attributes
include frame authenticity, and optionally frame confidentiality
(i.e. encryption).
Any node sending EB frames MUST be prepared to act as a JP for
potential pledges.
The pledge does not initially do any authenticity check of the EB
frames, as it does not possess the link-layer key(s) in use. The
pledge is still able to parse the contents of the received EBs and
synchronize to the network, as EBs are not encrypted [RFC8180].
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When sending frames during the join process, the pledge sends
unencrypted and unauthenticated frames at the link layer. In order
for the join process to be possible, the JP must accept these
unsecured frames for the duration of the join process. This behavior
may be implemented by setting the "secExempt" attribute in the IEEE
Std 802.15.4 security configuration tables. It is expected that the
lower layer provides an interface to indicate to the upper layer that
unsecured frames are being received from a device, and that the upper
layer can use that information to make a determination that a join
process is in place and the unsecured frames should be processed.
How the JP makes such a determination and interacts with the lower
layer is out of scope of this specification. The JP can additionally
make use of information such as the value of the join rate parameter
(Section 8.4.2) set by the JRC, physical button press, etc.
When the pledge initially synchronizes to the network, it has no
means of verifying the authenticity of EB frames. As an attacker can
craft a frame that looks like a legitimate EB frame this opens up a
DoS vector, as discussed in Section 9.
5.1. Distribution of Time
Nodes in a 6TiSCH network keep a global notion of time known as the
absolute slot number. Absolute slot number is used in the
construction of the link-layer nonce, as defined in [IEEE802.15.4].
The pledge initially synchronizes to the EB frame sent by the JP, and
uses the value of the absolute slot number found in the TSCH
Synchronization Information Element. At the time of the
synchronization, the EB frame can neither be authenticated nor its
freshness verified. During the join process, the pledge sends frames
that are unprotected at the link-layer and protected end-to-end
instead. The pledge does not obtain the time information as the
output of the join process as this information is local to the
network and may not be known at the JRC.
This enables an attack on the pledge where the attacker replays to
the pledge legitimate EB frames obtained from the network and acts as
a man-in-the-middle between the pledge and the JP. The EB frames
will make the pledge believe that the replayed absolute slot number
value is the current notion of time in the network. By forwarding
the join traffic to the legitimate JP, the attacker enables the
pledge to join the network. Under different conditions relating to
the reuse of the pledge's short address by the JRC or its attempt to
rejoin the network, this may cause the pledge to reuse the link-layer
nonce in the first frame it sends protected after the join process is
completed.
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For this reason, all frames originated at the JP and destined to the
pledge during the join process MUST be authenticated at the link-
layer using the key that is normally in use in the network. Link-
layer security processing at the pledge for these frames will fail as
the pledge is not yet in possession of the key. The pledge
acknowledges these frames without link-layer security, and JP accepts
the unsecured acknowledgment due to the secExempt attribute set for
the pledge. The frames should be passed to the upper layer for
processing using the promiscuous mode of [IEEE802.15.4] or another
appropriate mechanism. When the upper layer processing on the pledge
is completed and the link-layer keys are configured, the upper layer
MUST trigger the security processing of the corresponding frame.
Once the security processing of the frame carrying the Join Response
message is successful, the current absolute slot number kept locally
at the pledge SHALL be declared as valid.
6. Network-layer Configuration
The pledge and the JP SHOULD keep a separate neighbor cache for
untrusted entries and use it to store each other's information during
the join process. Mixing neighbor entries belonging to pledges and
nodes that are part of the network opens up the JP to a DoS attack,
as the attacker may fill JP's neighbor table and prevent the
discovery of legitimate neighbors.
Once the pledge obtains link-layer keys and becomes a joined node, it
is able to securely communicate with its neighbors, obtain the
network IPv6 prefix and form its global IPv6 address. The joined
node then undergoes an independent process to bootstrap its neighbor
cache entries, possibly with a node that formerly acted as a JP,
following [RFC8505]. From the point of view of the JP, there is no
relationship between the neighbor cache entry belonging to a pledge
and the joined node that formerly acted as a pledge.
The pledge does not communicate with the JRC at the network layer.
This allows the pledge to join without knowing the IPv6 address of
the JRC. Instead, the pledge communicates with the JP at the network
layer using link-local addressing, and with the JRC at the
application layer, as specified in Section 7.
The JP communicates with the JRC over global IPv6 addresses. The JP
discovers the network IPv6 prefix and configures its global IPv6
address upon successful completion of the join process and the
obtention of link-layer keys. The pledge learns the IPv6 address of
the JRC from the Join Response, as specified in Section 8.1.2; it
uses it once joined in order to operate as a JP.
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As a special case, the 6LBR pledge may have an additional network
interface that it uses in order to obtain the configuration
parameters from the JRC and start advertising the 6TiSCH network.
This additional interface needs to be configured with a global IPv6
address, by a mechanism that is out of scope of this document. The
6LBR pledge uses this interface to directly communicate with the JRC
using global IPv6 addressing.
The JRC can be co-located on the 6LBR. In this special case, the
IPv6 address of the JRC can be omitted from the Join Response message
for space optimization. The 6LBR then MUST set the DODAGID field in
the RPL DIOs [RFC6550] to its IPv6 address. The pledge learns the
address of the JRC once joined and upon the reception of the first
RPL DIO message, and uses it to operate as a JP.
6.1. Identification of Unauthenticated Traffic
The traffic that is proxied by the Join Proxy (JP) comes from
unauthenticated pledges, and there may be an arbitrary amount of it.
In particular, an attacker may send fraudulent traffic in an attempt
to overwhelm the network.
When operating as part of a [RFC8180] 6TiSCH minimal network using
distributed scheduling algorithms, the traffic from unauthenticated
pledges may cause intermediate nodes to request additional bandwidth.
An attacker could use this property to cause the network to
overcommit bandwidth (and energy) to the join process.
The Join Proxy is aware of what traffic originates from
unauthenticated pledges, and so can avoid allocating additional
bandwidth itself. The Join Proxy implements a data cap on outgoing
join traffic by implementing the recommendation of 1 packet per 3
seconds in Section 3.1.3 of [RFC8085]. This can be achieved with the
congestion control mechanism specified in Section 4.7 of [RFC7252].
This cap will not protect intermediate nodes as they can not tell
join traffic from regular traffic. Despite the data cap implemented
separately on each Join Proxy, the aggregate join traffic from many
Join Proxies may cause intermediate nodes to decide to allocate
additional cells. It is undesirable to do so in response to the
traffic originated at unauthenticated pledges. In order to permit
the intermediate nodes to avoid this, the traffic needs to be tagged.
[RFC2597] defines a set of per-hop behaviors that may be encoded into
the Diffserv Code Points (DSCPs). Based on the DSCP, intermediate
nodes can decide whether to act on a given packet.
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6.1.1. Traffic from JP to JRC
The Join Proxy SHOULD set the DSCP of packets that it produces as
part of the forwarding process to AF43 code point (See Section 6 of
[RFC2597]). A Join Proxy that does not require a specific DSCP value
on traffic forwarded should set it to zero so that it is compressed
out.
A Scheduling Function (SF) running on 6TiSCH nodes SHOULD NOT
allocate additional cells as a result of traffic with code point
AF43. Companion SF documents SHOULD specify how this recommended
behavior is achieved. One example is the 6TiSCH Minimal Scheduling
Function [I-D.ietf-6tisch-msf].
6.1.2. Traffic from JRC to JP
The JRC SHOULD set the DSCP of join response packets addressed to the
Join Proxy to AF42 code point. AF42 has lower drop probability than
AF43, giving this traffic priority in buffers over the traffic going
towards the JRC.
The 6LBR links are often the most congested within a DODAG, and from
that point down there is progressively less (or equal) congestion.
If the 6LBR paces itself when sending join response traffic then it
ought to never exceed the bandwidth allocated to the best effort
traffic cells. If the 6LBR has the capacity (if it is not
constrained) then it should provide some buffers in order to satisfy
the Assured Forwarding behavior.
Companion SF documents SHOULD specify how traffic with code point
AF42 is handled with respect to cell allocation. In case the
recommended behavior described in this section is not followed, the
network may become prone to the attack discussed in Section 6.1.
7. Application-level Configuration
The CoJP join exchange in Figure 1 is carried over CoAP [RFC7252] and
the secure channel provided by OSCORE [RFC8613]. The (6LBR) pledge
acts as a CoAP client; the JRC acts as a CoAP server. The JP
implements CoAP forward proxy functionality [RFC7252]. Because the
JP can also be a constrained device, it cannot implement a cache.
The pledge designates a JP as a proxy by including the Proxy-Scheme
option in CoAP requests it sends to the JP. The pledge also includes
in the requests the Uri-Host option with its value set to the well-
known JRC's alias, as specified in Section 8.1.1.
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The JP resolves the alias to the IPv6 address of the JRC that it
learned when it acted as a pledge, and joined the network. This
allows the JP to reach the JRC at the network layer and forward the
requests on behalf of the pledge.
7.1. Statelessness of the JP
The CoAP proxy defined in [RFC7252] keeps per-client state
information in order to forward the response towards the originator
of the request. This state information includes at least the CoAP
token, the IPv6 address of the client, and the UDP source port
number. Since the JP can be a constrained device that acts as a CoAP
proxy, memory limitations make it prone to a Denial-of-Service (DoS)
attack.
This DoS vector on the JP can be mitigated by making the JP act as a
stateless CoAP proxy, where "state" encompasses the information
related to individual pledges. The JP can wrap the state it needs to
keep for a given pledge throughout the network stack in a "state
object" and include it as a CoAP token in the forwarded request to
the JRC. The JP may use the CoAP token as defined in [RFC7252], if
the size of the serialized state object permits, or use the extended
CoAP token defined in [I-D.ietf-core-stateless], to transport the
state object. The JRC and any other potential proxy on the JP - JRC
path MUST support extended token lengths, as defined in
[I-D.ietf-core-stateless]. Since the CoAP token is echoed back in
the response, the JP is able to decode the state object and configure
the state needed to forward the response to the pledge. The
information that the JP needs to encode in the state object to
operate in a fully stateless manner with respect to a given pledge is
implementation specific.
It is RECOMMENDED that the JP operates in a stateless manner and
signals the per-pledge state within the CoAP token, for every request
it forwards into the network on behalf of unauthenticated pledges.
When the JP is operating in a stateless manner, the security
considerations from [I-D.ietf-core-stateless] apply and the type of
the CoAP message that the JP forwards on behalf of the pledge MUST be
non-confirmable (NON), regardless of the message type received from
the pledge. The use of a non-confirmable message by the JP
alleviates the JP from keeping CoAP message exchange state. The
retransmission burden is then entirely shifted to the pledge. A JP
that operates in a stateless manner still needs to keep congestion
control state with the JRC, see Section 9. Recommended values of
CoAP settings for use during the join process, both by the pledge and
the JP, are given in Section 7.2.
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Note that in some networking stack implementations, a fully (per-
pledge) stateless operation of the JP may be challenging from the
implementation's point of view. In those cases, the JP may operate
as a statefull proxy that stores the per-pledge state until the
response is received or timed out, but this comes at a price of a DoS
vector.
7.2. Recommended Settings
This section gives RECOMMENDED values of CoAP settings during the
join process.
+-------------------+---------------+
| Name | Default Value |
+-------------------+---------------+
| ACK_TIMEOUT | 10 seconds |
| | |
| ACK_RANDOM_FACTOR | 1.5 |
| | |
| MAX_RETRANSMIT | 4 |
| | |
| NSTART | 1 |
| | |
| DEFAULT_LEISURE | 5 seconds |
| | |
| PROBING_RATE | 1 byte/second |
+-------------------+---------------+
Recommended CoAP settings.
These values may be configured to values specific to the deployment.
The default values have been chosen to accommodate a wide range of
deployments, taking into account dense networks.
The PROBING_RATE value at the JP is controlled by the join rate
parameter, see Section 8.4.2. Following [RFC7252], the average data
rate in sending to the JRC must not exceed PROBING_RATE. For
security reasons, the average data rate SHOULD be measured over a
rather short window, e.g. ACK_TIMEOUT, see Section 9.
7.3. OSCORE
Before the (6LBR) pledge and the JRC start exchanging CoAP messages
protected with OSCORE, they need to derive the OSCORE security
context from the provisioned parameters, as discussed in Section 3.
The OSCORE security context MUST be derived as per Section 3 of
[RFC8613].
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o the Master Secret MUST be the PSK.
o the Master Salt MUST be the empty byte string.
o the ID Context MUST be set to the pledge identifier.
o the ID of the pledge MUST be set to the empty byte string. This
identifier is used as the OSCORE Sender ID of the pledge in the
security context derivation, since the pledge initially acts as a
CoAP client.
o the ID of the JRC MUST be set to the byte string 0x4a5243 ("JRC"
in ASCII). This identifier is used as the OSCORE Recipient ID of
the pledge in the security context derivation, as the JRC
initially acts as a CoAP server.
o the Algorithm MUST be set to the value from [RFC8152], agreed out-
of-band by the same mechanism used to provision the PSK. The
default is AES-CCM-16-64-128.
o the Key Derivation Function MUST be agreed out-of-band by the same
mechanism used to provision the PSK. Default is HKDF SHA-256
[RFC5869].
Since the pledge's OSCORE Sender ID is the empty byte string, when
constructing the OSCORE option, the pledge sets the k bit in the
OSCORE flag byte, but indicates a 0-length kid. The pledge
transports its pledge identifier within the kid context field of the
OSCORE option. The derivation in [RFC8613] results in OSCORE keys
and a common IV for each side of the conversation. Nonces are
constructed by XOR'ing the common IV with the current sequence
number. For details on nonce and OSCORE option construction, refer
to [RFC8613].
Implementations MUST ensure that multiple CoAP requests, including to
different JRCs, are properly incrementing the sequence numbers, so
that the same sequence number is never reused in distinct requests
protected under the same PSK. The pledge typically sends requests to
different JRCs if it is not provisioned with the network identifier
and attempts to join one network at a time. Failure to comply will
break the security guarantees of the Authenticated Encryption with
Associated Data (AEAD) algorithm because of nonce reuse.
This OSCORE security context is used for initial joining of the
(6LBR) pledge, where the (6LBR) pledge acts as a CoAP client, as well
as for any later parameter updates, where the JRC acts as a CoAP
client and the joined node as a CoAP server, as discussed in
Section 8.2. Note that when the (6LBR) pledge and the JRC change
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roles between CoAP client and CoAP server, the same OSCORE security
context as initially derived remains in use and the derived
parameters are unchanged, for example Sender ID when sending and
Recipient ID when receiving (see Section 3.1 of [RFC8613]). A (6LBR)
pledge is expected to have exactly one OSCORE security context with
the JRC.
7.3.1. Replay Window and Persistency
Both (6LBR) pledge and the JRC MUST implement a replay protection
mechanism. The use of the default OSCORE replay protection mechanism
specified in Section 3.2.2 of [RFC8613] is RECOMMENDED.
Implementations MUST ensure that mutable OSCORE context parameters
(Sender Sequence Number, Replay Window) are stored in persistent
memory. A technique detailed in Appendix B.1.1 of [RFC8613] that
prevents reuse of sequence numbers MUST be implemented. Each update
of the OSCORE Replay Window MUST be written to persistent memory.
This is an important security requirement in order to guarantee nonce
uniqueness and resistance to replay attacks across reboots and
rejoins. Traffic between the (6LBR) pledge and the JRC is rare,
making security outweigh the cost of writing to persistent memory.
7.3.2. OSCORE Error Handling
Errors raised by OSCORE during the join process MUST be silently
dropped, with no error response being signaled. The pledge MUST
silently discard any response not protected with OSCORE, including
error codes.
Such errors may happen for a number of reasons, including failed
lookup of an appropriate security context (e.g. the pledge attempting
to join a wrong network), failed decryption, positive replay window
lookup, formatting errors (possibly due to malicious alterations in
transit). Silently dropping OSCORE messages prevents a DoS attack on
the pledge where the attacker could send bogus error responses,
forcing the pledge to attempt joining one network at a time, until
all networks have been tried.
7.3.3. Mandatory to Implement Algorithms
The mandatory to implement AEAD algorithm for use with OSCORE is AES-
CCM-16-64-128 from [RFC8152]. This is the algorithm used for
securing IEEE Std 802.15.4 frames, and hardware acceleration for it
is present in virtually all compliant radio chips. With this choice,
CoAP messages are protected with an 8-byte CCM authentication tag,
and the algorithm uses 13-byte long nonces.
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The mandatory to implement hash algorithm is SHA-256 [RFC4231]. The
mandatory to implement key derivation function is HKDF [RFC5869],
instantiated with a SHA-256 hash. See Appendix B for implementation
guidance when code footprint is important.
8. Constrained Join Protocol (CoJP)
The Constrained Join Protocol (CoJP) is a lightweight protocol over
CoAP [RFC7252] and a secure channel provided by OSCORE [RFC8613].
CoJP allows a (6LBR) pledge to request admission into a network
managed by the JRC. It enables the JRC to configure the pledge with
the necessary parameters. The JRC may update the parameters at any
time, by reaching out to the joined node that formerly acted as a
(6LBR) pledge. For example, network-wide rekeying can be implemented
by updating the keying material on each node.
CoJP relies on the security properties provided by OSCORE. This
includes end-to-end confidentiality, data authenticity, replay
protection, and a secure binding of responses to requests.
+-----------------------------------+
| Constrained Join Protocol (CoJP) |
+-----------------------------------+
+-----------------------------------+ \
| Requests / Responses | |
|-----------------------------------| |
| OSCORE | | CoAP
|-----------------------------------| |
| Messaging Layer | |
+-----------------------------------+ /
+-----------------------------------+
| UDP |
+-----------------------------------+
Figure 2: Abstract layering of CoJP.
When a (6LBR) pledge requests admission to a given network, it
undergoes the CoJP join exchange that consists of:
o the Join Request message, sent by the (6LBR) pledge to the JRC,
potentially proxied by the JP. The Join Request message and its
mapping to CoAP is specified in Section 8.1.1.
o the Join Response message, sent by the JRC to the (6LBR) pledge,
if the JRC successfully processes the Join Request using OSCORE
and it determines through a mechanism that is out of scope of this
specification that the (6LBR) pledge is authorized to join the
network. The Join Response message is potentially proxied by the
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JP. The Join Response message and its mapping to CoAP is
specified in Section 8.1.2.
When the JRC needs to update the parameters of a joined node that
formerly acted as a (6LBR) pledge, it executes the CoJP parameter
update exchange that consists of:
o the Parameter Update message, sent by the JRC to the joined node
that formerly acted as a (6LBR) pledge. The Parameter Update
message and its mapping to CoAP is specified in Section 8.2.1.
The payload of CoJP messages is encoded with CBOR [RFC7049]. The
CBOR data structures that may appear as the payload of different CoJP
messages are specified in Section 8.4.
8.1. Join Exchange
This section specifies the messages exchanged when the (6LBR) pledge
requests admission and configuration parameters from the JRC.
8.1.1. Join Request Message
The Join Request message that the (6LBR) pledge sends SHALL be mapped
to a CoAP request:
o The request method is POST.
o The type is Confirmable (CON).
o The Proxy-Scheme option is set to "coap".
o The Uri-Host option is set to "6tisch.arpa". This is an anycast
type of identifier of the JRC that is resolved to its IPv6 address
by the JP or the 6LBR pledge.
o The Uri-Path option is set to "j".
o The OSCORE option SHALL be set according to [RFC8613]. The OSCORE
security context used is the one derived in Section 7.3. The
OSCORE kid context allows the JRC to retrieve the security context
for a given pledge.
o The payload is a Join_Request CBOR object, as defined in
Section 8.4.1.
Since the Join Request is a confirmable message, the transmission at
(6LBR) pledge will be controlled by CoAP's retransmission mechanism.
The JP, when operating in a stateless manner, forwards this Join
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Request as a non-confirmable (NON) CoAP message, as specified in
Section 7. If the CoAP implementation at (6LBR) pledge declares the
message transmission as failure, the (6LBR) pledge SHOULD attempt to
join a 6TiSCH network advertised with a different network identifier.
See Section 7.2 for recommended values of CoAP settings to use during
the join exchange.
If all join attempts to advertised networks have failed, the (6LBR)
pledge SHOULD signal the presence of an error condition, through some
out-of-band mechanism.
BCP190 [RFC7320] provides guidelines on URI design and ownership. It
recommends that whenever a third party wants to mandate a URL to web
authority that it SHOULD go under "/.well-known" (as per [RFC5785]).
In the case of CoJP, the Uri-Host option is always set to
"6tisch.arpa", and based upon the recommendations in the Introduction
of [RFC7320], it is asserted that this document is the owner of the
CoJP service. As such, the concerns of [RFC7320] do not apply, and
thus the Uri-Path is only "/j".
8.1.2. Join Response Message
The Join Response message that the JRC sends SHALL be mapped to a
CoAP response:
o The response Code is 2.04 (Changed).
o The payload is a Configuration CBOR object, as defined in
Section 8.4.2.
8.2. Parameter Update Exchange
During the network lifetime, parameters returned as part of the Join
Response may need to be updated. One typical example is the update
of link-layer keying material for the network, a process known as
rekeying. This section specifies a generic mechanism when this
parameter update is initiated by the JRC.
At the time of the join, the (6LBR) pledge acts as a CoAP client and
requests the network parameters through a representation of the "/j"
resource, exposed by the JRC. In order for the update of these
parameters to happen, the JRC needs to asynchronously contact the
joined node. The use of the CoAP Observe option for this purpose is
not feasible due to the change in the IPv6 address when the pledge
becomes the joined node and obtains a global address.
Instead, once the (6LBR) pledge receives and successfully validates
the Join Response and so becomes a joined node, it becomes a CoAP
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server. The joined node creates a CoAP service at the Uri-Host value
of "6tisch.arpa", and the joined node exposes the "/j" resource that
is used by the JRC to update the parameters. Consequently, the JRC
operates as a CoAP client when updating the parameters. The request/
response exchange between the JRC and the (6LBR) pledge happens over
the already-established OSCORE secure channel.
8.2.1. Parameter Update Message
The Parameter Update message that the JRC sends to the joined node
SHALL be mapped to a CoAP request:
o The request method is POST.
o The type is Confirmable (CON).
o The Uri-Host option is set to "6tisch.arpa".
o The Uri-Path option is set to "j".
o The OSCORE option SHALL be set according to [RFC8613]. The OSCORE
security context used is the one derived in Section 7.3. When a
joined node receives a request with the Sender ID set to 0x4a5243
(ID of the JRC), it is able to correctly retrieve the security
context with the JRC.
o The payload is a Configuration CBOR object, as defined in
Section 8.4.2.
The JRC has implicit knowledge on the global IPv6 address of the
joined node, as it knows the pledge identifier that the joined node
used when it acted as a pledge, and the IPv6 network prefix. The JRC
uses this implicitly derived IPv6 address of the joined node to
directly address CoAP messages to it.
In case the JRC does not receive a response to a Parameter Update
message, it attempts multiple retransmissions, as configured by the
underlying CoAP retransmission mechanism triggered for confirmable
messages. Finally, if the CoAP implementation declares the
transmission as failure, the JRC may consider this as a hint that the
joined node is no longer in the network. How the JRC decides when to
stop attempting to contact a previously joined node is out of scope
of this specification but security considerations on the reuse of
assigned resources apply, as discussed in Section 9.
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8.3. Error Handling
8.3.1. CoJP CBOR Object Processing
CoJP CBOR objects are transported within both CoAP requests and
responses. This section describes handling in case certain CoJP CBOR
object parameters are not supported by the implementation or their
processing fails. See Section 7.3.2 for the handling of errors that
may be raised by the underlying OSCORE implementation.
When such a parameter is detected in a CoAP request (Join Request
message, Parameter Update message), a Diagnostic Response message
MUST be returned. A Diagnostic Response message maps to a CoAP
response and is specified in Section 8.3.2.
When a parameter that cannot be acted upon is encountered while
processing a CoJP object in a CoAP response (Join Response message),
a (6LBR) pledge SHOULD reattempt to join. In this case, the (6LBR)
pledge SHOULD include the Unsupported Configuration CBOR object
within the Join Request object in the following Join Request message.
The Unsupported Configuration CBOR object is self-contained and
enables the (6LBR) pledge to signal any parameters that the
implementation of the networking stack may not support. A (6LBR)
pledge MUST NOT attempt more than COJP_MAX_JOIN_ATTEMPTS number of
attempts to join if the processing of the Join Response message fails
each time. If COJP_MAX_JOIN_ATTEMPTS number of attempts is reached
without success, the (6LBR) pledge SHOULD signal the presence of an
error condition, through some out-of-band mechanism.
Note that COJP_MAX_JOIN_ATTEMPTS relates to the application-level
handling of the CoAP response and is different from CoAP's
MAX_RETRANSMIT setting that drives the retransmission mechanism of
the underlying CoAP message.
8.3.2. Diagnostic Response Message
The Diagnostic Response message is returned for any CoJP request when
the processing of the payload failed. The Diagnostic Response
message is protected by OSCORE as any other CoJP protocol message.
The Diagnostic Response message SHALL be mapped to a CoAP response:
o The response Code is 4.00 (Bad Request).
o The payload is an Unsupported Configuration CBOR object, as
defined in Section 8.4.5, containing more information about the
parameter that triggered the sending of this message.
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8.3.3. Failure Handling
The Parameter Update exchange may be triggered at any time during the
network lifetime, which may span several years. During this period,
it may occur that a joined node or the JRC experience unexpected
events such as reboots or complete failures.
This document mandates that the mutable parameters in the security
context are written to persistent memory (see Section 7.3.1) by both
the JRC and pledges (joined nodes). As the joined node (pledge) is
typically a constrained device that handles the write operations to
persistent memory in a predictable manner, the retrieval of mutable
security context parameters is feasible across reboots such that
there is no risk of AEAD nonce reuse due to reinitialized Sender
Sequence numbers, or of a replay attack due to the reinitialized
replay window. JRC may be hosted on a generic machine where the
write operation to persistent memory may lead to unpredictable delays
due to caching. In case of a reboot event at JRC occurring before
the cached data is written to persistent memory, the loss of mutable
security context parameters is likely which consequently poses the
risk of AEAD nonce reuse.
In the event of a complete device failure, where the mutable security
context parameters cannot be retrieved, it is expected that a failed
joined node is replaced with a new physical device, using a new
pledge identifier and a PSK. When such a failure event occurs at the
JRC, it is possible that the static information on provisioned
pledges, like PSKs and pledge identifiers, can be retrieved through
available backups. However, it is likely that the information about
joined nodes, their assigned short identifiers and mutable security
context parameters is lost. If this is the case, during the process
of JRC reinitialization, the network administrator MUST force through
out-of-band means all the networks managed by the failed JRC to
rejoin, through e.g. the reinitialization of the 6LBR nodes and
freshly generated dynamic cryptographic keys and other parameters
that have influence on the security properties of the network.
In order to recover from such a failure event, the reinitialized JRC
can trigger the renegotiation of the OSCORE security context through
the procedure described in Appendix B.2 of [RFC8613]. Aware of the
failure event, the reinitialized JRC responds to the first join
request of each pledge it is managing with a 4.01 Unauthorized error
and a random nonce. The pledge verifies the error response and then
initiates the CoJP join exchange using a new OSCORE security context
derived from an ID Context consisting of the concatenation of two
nonces, one that it received from the JRC and the other that the
pledge generates locally. After verifying the join request with the
new ID Context and the derived OSCORE security context, the JRC
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should consequently take action in mapping the new ID Context with
the previously used pledge identifier. How JRC handles this mapping
is out of scope of this document.
The described procedure is specified in Appendix B.2 of [RFC8613] and
is RECOMMENDED in order to handle the failure events or any other
event that may lead to the loss of mutable security context
parameters. The length of nonces exchanged using this procedure MUST
be at least 8 bytes.
The procedure does require both the pledge and the JRC to have good
sources of randomness. While this is typically not an issue at the
JRC side, the constrained device hosting the pledge may pose
limitations in this regard. If the procedure outlined in
Appendix B.2 of [RFC8613] is not supported by the pledge, the network
administrator MUST take action in reprovisioning the concerned
devices with freshly generated parameters, through out-of-band means.
8.4. CoJP Objects
This section specifies the structure of CoJP CBOR objects that may be
carried as the payload of CoJP messages. Some of these objects may
be received both as part of the CoJP join exchange when the device
operates as a (CoJP) pledge, or the parameter update exchange, when
the device operates as a joined (6LBR) node.
8.4.1. Join Request Object
The Join_Request structure is built on a CBOR map object.
The set of parameters that can appear in a Join_Request object is
summarized below. The labels can be found in the "CoJP Parameters"
registry Section 11.1.
o role: The identifier of the role that the pledge requests to play
in the network once it joins, encoded as an unsigned integer.
Possible values are specified in Table 2. This parameter MAY be
included. In case the parameter is omitted, the default value of
0, i.e. the role "6TiSCH Node", MUST be assumed.
o network identifier: The identifier of the network, as discussed in
Section 3, encoded as a CBOR byte string. When present in the
Join_Request, it hints to the JRC the network that the pledge is
requesting to join, enabling the JRC to manage multiple networks.
The pledge obtains the value of the network identifier from the
received EB frames. This parameter MUST be included in a
Join_Request object regardless of the role parameter value.
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o unsupported configuration: The identifier of the parameters that
are not supported by the implementation, encoded as an
Unsupported_Configuration object described in Section 8.4.5. This
parameter MAY be included. If a (6LBR) pledge previously
attempted to join and received a valid Join Response message over
OSCORE, but failed to act on its payload (Configuration object),
it SHOULD include this parameter to facilitate the recovery and
debugging.
Table 1 summarizes the parameters that may appear in a Join_Request
object.
+---------------------------+-------+------------------+
| Name | Label | CBOR Type |
+---------------------------+-------+------------------+
| role | 1 | unsigned integer |
| | | |
| network identifier | 5 | byte string |
| | | |
| unsupported configuration | 8 | array |
+---------------------------+-------+------------------+
Table 1: Summary of Join_Request parameters.
The CDDL fragment that represents the text above for the Join_Request
follows.
Join_Request = {
? 1 : uint, ; role
5 : bstr, ; network identifier
? 8 : Unsupported_Configuration ; unsupported configuration
}
+--------+-------+-------------------------------------+------------+
| Name | Value | Description | Reference |
+--------+-------+-------------------------------------+------------+
| 6TiSCH | 0 | The pledge requests to play the | [[this |
| Node | | role of a regular 6TiSCH node, i.e. | document]] |
| | | non-6LBR node. | |
| | | | |
| 6LBR | 1 | The pledge requests to play the | [[this |
| | | role of 6LoWPAN Border Router | document]] |
| | | (6LBR). | |
+--------+-------+-------------------------------------+------------+
Table 2: Role values.
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8.4.2. Configuration Object
The Configuration structure is built on a CBOR map object. The set
of parameters that can appear in a Configuration object is summarized
below. The labels can be found in "CoJP Parameters" registry
Section 11.1.
o link-layer key set: An array encompassing a set of cryptographic
keys and their identifiers that are currently in use in the
network, or that are scheduled to be used in the future. The
encoding of individual keys is described in Section 8.4.3. The
link-layer key set parameter MAY be included in a Configuration
object. When present, the link-layer key set parameter MUST
contain at least one key. This parameter is also used to
implement rekeying in the network. How the keys are installed and
used differs for the 6LBR and other (regular) nodes, and this is
explained in Section 8.4.3.1 and Section 8.4.3.2.
o short identifier: a compact identifier assigned to the pledge.
The short identifier structure is described in Section 8.4.4. The
short identifier parameter MAY be included in a Configuration
object.
o JRC address: the IPv6 address of the JRC, encoded as a byte
string, with the length of 16 bytes. If the length of the byte
string is different from 16, the parameter MUST be discarded. If
the JRC is not co-located with the 6LBR and has a different IPv6
address than the 6LBR, this parameter MUST be included. In the
special case where the JRC is co-located with the 6LBR and has the
same IPv6 address as the 6LBR, this parameter MAY be included. If
the JRC address parameter is not present in the Configuration
object, this indicates that the JRC has the same IPv6 address as
the 6LBR. The joined node can then discover the IPv6 address of
the JRC through network control traffic. See Section 6.
o blacklist: An array encompassing a list of pledge identifiers that
are blacklisted by the JRC, with each pledge identifier encoded as
a byte string. The blacklist parameter MAY be included in a
Configuration object. When present, the array MUST contain zero
or more byte strings encoding pledge identifiers. The joined node
MUST silently drop any link-layer frames originating from the
pledge identifiers enclosed in the blacklist parameter. When this
parameter is received, its value MUST overwrite any previously set
values. This parameter allows the JRC to configure the node
acting as a JP to filter out traffic from misconfigured or
malicious pledges before their traffic is forwarded into the
network. If the JRC decides to remove a given pledge identifier
from a blacklist, it omits the pledge identifier in the blacklist
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parameter value it sends next. Since the blacklist parameter
carries the pledge identifiers, privacy considerations apply. See
Section 10.
o join rate: Average data rate (in units of bytes/second) of join
traffic forwarded into the network that should not be exceeded
when a joined node operates as a JP, encoded as an unsigned
integer. The join rate parameter MAY be included in a
Configuration object. This parameter allows the JRC to configure
different nodes in the network to operate as JP, and act in case
of an attack by throttling the rate at which JP forwards
unauthenticated traffic into the network. When this parameter is
present in a Configuration object, the value MUST be used to set
the PROBING_RATE of CoAP at the joined node for communication with
the JRC. In case this parameter is set to zero, a joined node
MUST silently drop any join traffic coming from unauthenticated
pledges. In case this parameter is omitted, the value of positive
infinity SHOULD be assumed. Node operating as a JP MAY use
another mechanism that is out of scope of this specification to
configure PROBING_RATE of CoAP in the absence of a join rate
parameter from the Configuration object.
Table 3 summarizes the parameters that may appear in a Configuration
object.
+--------------------+-------+------------------+
| Name | Label | CBOR Type |
+--------------------+-------+------------------+
| link-layer key set | 2 | array |
| | | |
| short identifier | 3 | array |
| | | |
| JRC address | 4 | byte string |
| | | |
| blacklist | 6 | array |
| | | |
| join rate | 7 | unsigned integer |
+--------------------+-------+------------------+
Table 3: Summary of Configuration parameters.
The CDDL fragment that represents the text above for the
Configuration follows. Structures Link_Layer_Key and
Short_Identifier are specified in Section 8.4.3 and Section 8.4.4.
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Configuration = {
? 2 : [ +Link_Layer_Key ], ; link-layer key set
? 3 : Short_Identifier, ; short identifier
? 4 : bstr, ; JRC address
? 6 : [ *bstr ], ; blacklist
? 7 : uint ; join rate
}
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+---------------+-------+----------+-------------------+------------+
| Name | Label | CBOR | Description | Reference |
| | | type | | |
+---------------+-------+----------+-------------------+------------+
| role | 1 | unsigned | Identifies the | [[this |
| | | integer | role parameter | document]] |
| | | | | |
| link-layer | 2 | array | Identifies the | [[this |
| key set | | | array carrying | document]] |
| | | | one or more link- | |
| | | | level | |
| | | | cryptographic | |
| | | | keys | |
| | | | | |
| short | 3 | array | Identifies the | [[this |
| identifier | | | assigned short | document]] |
| | | | identifier | |
| | | | | |
| JRC address | 4 | byte | Identifies the | [[this |
| | | string | IPv6 address of | document]] |
| | | | the JRC | |
| | | | | |
| network | 5 | byte | Identifies the | [[this |
| identifier | | string | network | document]] |
| | | | identifier | |
| | | | parameter | |
| | | | | |
| blacklist | 6 | array | Identifies the | [[this |
| | | | blacklist | document]] |
| | | | parameter | |
| | | | | |
| join rate | 7 | unsigned | Identifier the | [[this |
| | | integer | join rate | document]] |
| | | | parameter | |
| | | | | |
| unsupported | 8 | array | Identifies the | [[this |
| configuration | | | unsupported | document]] |
| | | | configuration | |
| | | | parameter | |
+---------------+-------+----------+-------------------+------------+
Table 4: CoJP parameters map labels.
8.4.3. Link-Layer Key
The Link_Layer_Key structure encompasses the parameters needed to
configure the link-layer security module: the key identifier; the
value of the cryptographic key; the link-layer algorithm identifier
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and the security level and the frame types that it should be used
with, both for outgoing and incoming security operations; and any
additional information that may be needed to configure the key.
For encoding compactness, the Link_Layer_Key object is not enclosed
in a top-level CBOR object. Rather, it is transported as a sequence
of CBOR elements [I-D.ietf-cbor-sequence], some being optional.
The set of parameters that can appear in a Link_Layer_Key object is
summarized below, in order:
o key_id: The identifier of the key, encoded as a CBOR unsigned
integer. This parameter MUST be included. If the decoded CBOR
unsigned integer value is larger than the maximum link-layer key
identifier, the key is considered invalid. In case the key is
considered invalid, the key MUST be discarded and the
implementation MUST signal the error as specified in
Section 8.3.1.
o key_usage: The identifier of the link-layer algorithm, security
level and link-layer frame types that can be used with the key,
encoded as an integer. This parameter MAY be included. Possible
values and the corresponding link-layer settings are specified in
IANA "CoJP Key Usage" registry (Section 11.2). In case the
parameter is omitted, the default value of 0 (6TiSCH-K1K2-ENC-
MIC32) from Table 5 MUST be assumed. This default value has been
chosen such that it results in byte savings in the most
constrained settings but does not imply a recommendation for its
general usage.
o key_value: The value of the cryptographic key, encoded as a byte
string. This parameter MUST be included. If the length of the
byte string is different than the corresponding key length for a
given algorithm specified by the key_usage parameter, the key MUST
be discarded and the implementation MUST signal the error as
specified in Section 8.3.1.
o key_addinfo: Additional information needed to configure the link-
layer key, encoded as a byte string. This parameter MAY be
included. The processing of this parameter is dependent on the
link-layer technology in use and a particular keying mode.
To be able to decode the keys that are present in the link-layer key
set, and to identify individual parameters of a single Link_Layer_Key
object, the CBOR decoder needs to differentiate between elements
based on the CBOR type. For example, a uint that follows a byte
string signals to the decoder that a new Link_Layer_Key object is
being processed.
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The CDDL fragment that represents the text above for the
Link_Layer_Key follows.
Link_Layer_Key = (
key_id : uint,
? key_usage : int,
key_value : bstr,
? key_addinfo : bstr,
)
+-----------------+-----+------------------+-------------+----------+
| Name | Val | Algorithm | Description | Referenc |
| | ue | | | e |
+-----------------+-----+------------------+-------------+----------+
| 6TiSCH-K1K2 | 0 | IEEE802154-AES- | Use MIC-32 | [[this d |
| -ENC-MIC32 | | CCM-128 | for EBs, | ocument] |
| | | | ENC-MIC-32 | ] |
| | | | for DATA | |
| | | | and ACKNOWL | |
| | | | EDGMENT. | |
| | | | | |
| 6TiSCH-K1K2 | 1 | IEEE802154-AES- | Use MIC-64 | [[this d |
| -ENC-MIC64 | | CCM-128 | for EBs, | ocument] |
| | | | ENC-MIC-64 | ] |
| | | | for DATA | |
| | | | and ACKNOWL | |
| | | | EDGMENT. | |
| | | | | |
| 6TiSCH-K1K2 | 2 | IEEE802154-AES- | Use MIC-128 | [[this d |
| -ENC-MIC128 | | CCM-128 | for EBs, | ocument] |
| | | | ENC-MIC-128 | ] |
| | | | for DATA | |
| | | | and ACKNOWL | |
| | | | EDGMENT. | |
| | | | | |
| 6TiSCH- | 3 | IEEE802154-AES- | Use MIC-32 | [[this d |
| K1K2-MIC32 | | CCM-128 | for EBs, | ocument] |
| | | | DATA and AC | ] |
| | | | KNOWLEDGMEN | |
| | | | T. | |
| | | | | |
| 6TiSCH- | 4 | IEEE802154-AES- | Use MIC-64 | [[this d |
| K1K2-MIC64 | | CCM-128 | for EBs, | ocument] |
| | | | DATA and AC | ] |
| | | | KNOWLEDGMEN | |
| | | | T. | |
| | | | | |
| 6TiSCH- | 5 | IEEE802154-AES- | Use MIC-128 | [[this d |
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| K1K2-MIC128 | | CCM-128 | for EBs, | ocument] |
| | | | DATA and AC | ] |
| | | | KNOWLEDGMEN | |
| | | | T. | |
| | | | | |
| 6TiSCH-K1-MIC32 | 6 | IEEE802154-AES- | Use MIC-32 | [[this d |
| | | CCM-128 | for EBs. | ocument] |
| | | | | ] |
| | | | | |
| 6TiSCH-K1-MIC64 | 7 | IEEE802154-AES- | Use MIC-64 | [[this d |
| | | CCM-128 | for EBs. | ocument] |
| | | | | ] |
| | | | | |
| 6TiSCH-K1-MIC12 | 8 | IEEE802154-AES- | Use MIC-128 | [[this d |
| 8 | | CCM-128 | for EBs. | ocument] |
| | | | | ] |
| | | | | |
| 6TiSCH-K2-MIC32 | 9 | IEEE802154-AES- | Use MIC-32 | [[this d |
| | | CCM-128 | for DATA | ocument] |
| | | | and ACKNOWL | ] |
| | | | EDGMENT. | |
| | | | | |
| 6TiSCH-K2-MIC64 | 10 | IEEE802154-AES- | Use MIC-64 | [[this d |
| | | CCM-128 | for DATA | ocument] |
| | | | and ACKNOWL | ] |
| | | | EDGMENT. | |
| | | | | |
| 6TiSCH-K2-MIC12 | 11 | IEEE802154-AES- | Use MIC-128 | [[this d |
| 8 | | CCM-128 | for DATA | ocument] |
| | | | and ACKNOWL | ] |
| | | | EDGMENT. | |
| | | | | |
| 6TiSCH-K2-ENC- | 12 | IEEE802154-AES- | Use ENC- | [[this d |
| MIC32 | | CCM-128 | MIC-32 for | ocument] |
| | | | DATA and AC | ] |
| | | | KNOWLEDGMEN | |
| | | | T. | |
| | | | | |
| 6TiSCH-K2-ENC- | 13 | IEEE802154-AES- | Use ENC- | [[this d |
| MIC64 | | CCM-128 | MIC-64 for | ocument] |
| | | | DATA and AC | ] |
| | | | KNOWLEDGMEN | |
| | | | T. | |
| | | | | |
| 6TiSCH-K2-ENC- | 14 | IEEE802154-AES- | Use ENC- | [[this d |
| MIC128 | | CCM-128 | MIC-128 for | ocument] |
| | | | DATA and AC | ] |
| | | | KNOWLEDGMEN | |
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| | | | T. | |
+-----------------+-----+------------------+-------------+----------+
Table 5: Key Usage values.
8.4.3.1. Rekeying of (6LoWPAN) Border Routers (6LBR)
When the 6LoWPAN Border Router (6LBR) receives the Configuration
object containing a link-layer key set, it MUST immediately install
and start using the new keys for all outgoing traffic, and remove any
old keys it has installed from the previous key set after a delay of
COJP_REKEYING_GUARD_TIME has passed. This mechanism is used by the
JRC to force the 6LBR to start sending traffic with the new key. The
decision is taken by the JRC when it has determined that the new key
has been made available to all (or some overwhelming majority) of
nodes. Any node that the JRC has not yet reached at that point is
either non-functional or in extended sleep such that it will not be
reached. To get the key update, such node needs to go through the
join process anew.
8.4.3.2. Rekeying of regular (6LoWPAN) Nodes (6LN)
When a regular 6LN node receives the Configuration object with a
link-layer key set, it MUST install the new keys. The 6LN will use
both the old and the new keys to decrypt and authenticate any
incoming traffic that arrives based upon the key identifier in the
packet. It MUST continue to use the old keys for all outgoing
traffic until it has detected that the network has switched to the
new key set.
The detection of network switch is based upon the receipt of traffic
secured with the new keys. Upon reception and successful security
processing of a link-layer frame secured with a key from the new key
set, a 6LN node MUST then switch to sending outgoing traffic using
the keys from the new set for all outgoing traffic. The 6LN node
MUST remove any old keys it has installed from the previous key set
after a delay of COJP_REKEYING_GUARD_TIME has passed after it starts
using the new key set.
Sending of traffic with the new keys signals to other downstream
nodes to switch to their new key, and the effect is that there is a
ripple of key updates around each 6LBR.
8.4.3.3. Use in IEEE Std 802.15.4
When Link_Layer_Key is used in the context of [IEEE802.15.4], the
following considerations apply.
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Signaling of different keying modes of [IEEE802.15.4] is done based
on the parameter values present in a Link_Layer_Key object. For
instance, the value of the key_id parameter in combination with
key_addinfo denotes which of the four Key ID modes of [IEEE802.15.4]
is used and how.
o Key ID Mode 0x00 (Implicit, pairwise): key_id parameter MUST be
set to 0. key_addinfo parameter MUST be present. key_addinfo
parameter MUST be set to the link-layer address(es) of a single
peer with whom the key should be used. Depending on the
configuration of the network, key_addinfo may carry the peer's
long link-layer address (i.e. pledge identifier), short link-layer
address, or their concatenation with the long address being
encoded first. Which address type(s) is carried is determined
from the length of the byte string.
o Key ID Mode 0x01 (Key Index): key_id parameter MUST be set to a
value different than 0. key_addinfo parameter MUST NOT be present.
o Key ID Mode 0x02 (4-byte Explicit Key Source): key_id parameter
MUST be set to a value different than 0. key_addinfo parameter
MUST be present. key_addinfo parameter MUST be set to a byte
string, exactly 4 bytes long. key_addinfo parameter carries the
Key Source parameter used to configure [IEEE802.15.4].
o Key ID Mode 0x03 (8-byte Explicit Key Source): key_id parameter
MUST be set to a value different than 0. key_addinfo parameter
MUST be present. key_addinfo parameter MUST be set to a byte
string, exactly 8 bytes long. key_addinfo parameter carries the
Key Source parameter used to configure [IEEE802.15.4].
In all cases, key_usage parameter determines how a particular key
should be used in respect to incoming and outgoing security policies.
For Key ID Modes 0x01 - 0x03, parameter key_id sets the "secKeyIndex"
parameter of [IEEE802.15.4] that is signaled in all outgoing frames
secured with a given key. The maximum value key_id can have is 254.
The value of 255 is reserved in [IEEE802.15.4] and is therefore
considered invalid.
Key ID Mode 0x00 (Implicit, pairwise) enables the JRC to act as a
trusted third party and assign pairwise keys between nodes in the
network. How JRC learns about the network topology is out of scope
of this specification, but could be done through 6LBR - JRC signaling
for example. Pairwise keys could also be derived through a key
agreement protocol executed between the peers directly, where the
authentication is based on the symmetric cryptographic material
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provided to both peers by the JRC. Such a protocol is out of scope
of this specification.
Implementations MUST use different link-layer keys when using
different authentication tag (MIC) lengths, as using the same key
with different authentication tag lengths might be unsafe. For
example, this prohibits the usage of the same key for both MIC-32 and
MIC-64 levels. See Annex B.4.3 of [IEEE802.15.4] for more
information.
8.4.4. Short Identifier
The Short_Identifier object represents an identifier assigned to the
pledge. It is encoded as a CBOR array object, containing, in order:
o identifier: The short identifier assigned to the pledge, encoded
as a byte string. This parameter MUST be included. The
identifier MUST be unique in the set of all identifiers assigned
in a network that is managed by a JRC. In case the identifier is
invalid, the decoder MUST silently ignore the Short_Identifier
object.
o lease_time: The validity of the identifier in hours after the
reception of the CBOR object, encoded as a CBOR unsigned integer.
This parameter MAY be included. The node MUST stop using the
assigned short identifier after the expiry of the lease_time
interval. It is up to the JRC to renew the lease before the
expiry of the previous interval. The JRC updates the lease by
executing the Parameter Update exchange with the node and
including the Short_Identifier in the Configuration object, as
described in Section 8.2. In case the lease expires, the node
SHOULD initiate a new join exchange, as described in Section 8.1.
In case this parameter is omitted, the value of positive infinity
MUST be assumed, meaning that the identifier is valid for as long
as the node participates in the network.
The CDDL fragment that represents the text above for the
Short_Identifier follows.
Short_Identifier = [
identifier : bstr,
? lease_time : uint
]
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8.4.4.1. Use in IEEE Std 802.15.4
When Short_Identifier is used in the context of [IEEE802.15.4], the
following considerations apply.
The identifier MUST be used to set the short address of IEEE Std
802.15.4 module. When operating in TSCH mode, the identifier MUST be
unique in the set of all identifiers assigned in multiple networks
that share link-layer key(s). If the length of the byte string
corresponding to the identifier parameter is different than 2, the
identifier is considered invalid. The values 0xfffe and 0xffff are
reserved by [IEEE802.15.4] and their use is considered invalid.
The security properties offered by the [IEEE802.15.4] link-layer in
TSCH mode are conditioned on the uniqueness requirement of the short
identifier (i.e. short address). The short address is one of the
inputs in the construction of the nonce, which is used to protect
link-layer frames. If a misconfiguration occurs, and the same short
address is assigned twice under the same link-layer key, the loss of
security properties is imminent. For this reason, practices where
the pledge generates the short identifier locally are not safe and
are likely to result in the loss of link-layer security properties.
The JRC MUST ensure that at any given time there are never two same
short identifiers being used under the same link-layer key. If the
lease_time parameter of a given Short_Identifier object is set to
positive infinity, care needs to be taken that the corresponding
identifier is not assigned to another node until the JRC is certain
that it is no longer in use, potentially through out-of-band
signaling. If the lease_time parameter expires for any reason, the
JRC should take into consideration potential ongoing transmissions by
the joined node, which may be hanging in the queues, before assigning
the same identifier to another node.
Care needs to be taken on how the pledge (joined node) configures the
expiration of the lease. Since units of the lease_time parameter are
in hours after the reception of the CBOR object, the pledge needs to
convert the received time to the corresponding absolute slot number
in the network. The joined node (pledge) MUST only use the absolute
slot number as the appropriate reference of time to determine whether
the assigned short identifier is still valid.
8.4.5. Unsupported Configuration Object
The Unsupported_Configuration object is encoded as a CBOR array,
containing at least one Unsupported_Parameter object. Each
Unsupported_Parameter object is a sequence of CBOR elements without
an enclosing top-level CBOR object for compactness. The set of
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parameters that appear in an Unsupported_Parameter object is
summarized below, in order:
o code: Indicates the capability of acting on the parameter signaled
by parameter_label, encoded as an integer. This parameter MUST be
included. Possible values of this parameter are specified in the
IANA "CoJP Unsupported Configuration Code Registry"
(Section 11.3).
o parameter_label: Indicates the parameter. This parameter MUST be
included. Possible values of this parameter are specified in the
label column of the IANA "CoJP Parameters" registry
(Section 11.1).
o parameter_addinfo: Additional information about the parameter that
cannot be acted upon. This parameter MUST be included. In case
the code is set to "Unsupported", parameter_addinfo gives
additional information to the JRC. If the parameter indicated by
parameter_label cannot be acted upon regardless of its value,
parameter_addinfo MUST be set to null, signaling to the JRC that
it SHOULD NOT attempt to configure the parameter again. If the
pledge can act on the parameter, but cannot configure the setting
indicated by the parameter value, the pledge can hint this to the
JRC. In this case, parameter_addinfo MUST be set to the value of
the parameter that cannot be acted upon following the normative
parameter structure specified in this document. For example, it
is possible to include the link-layer key set object, signaling a
subset of keys that cannot be acted upon, or the entire key set
that was received. In that case, the value of the
parameter_addinfo follows the link-layer key set structure defined
in Section 8.4.2. In case the code is set to "Malformed",
parameter_addinfo MUST be set to null, signaling to the JRC that
it SHOULD NOT attempt to configure the parameter again.
The CDDL fragment that represents the text above for
Unsupported_Configuration and Unsupported_Parameter objects follows.
Unsupported_Configuration = [
+ parameter : Unsupported_Parameter
]
Unsupported_Parameter = (
code : int,
parameter_label : int,
parameter_addinfo : nil / any
)
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+-------------+-------+--------------------------------+------------+
| Name | Value | Description | Reference |
+-------------+-------+--------------------------------+------------+
| Unsupported | 0 | The indicated setting is not | [[this |
| | | supported by the networking | document]] |
| | | stack implementation. | |
| | | | |
| Malformed | 1 | The indicated parameter value | [[this |
| | | is malformed. | document]] |
+-------------+-------+--------------------------------+------------+
Table 6: Unsupported Configuration code values.
8.5. Recommended Settings
This section gives RECOMMENDED values of CoJP settings.
+--------------------------+---------------+
| Name | Default Value |
+--------------------------+---------------+
| COJP_MAX_JOIN_ATTEMPTS | 4 |
| | |
| COJP_REKEYING_GUARD_TIME | 12 seconds |
+--------------------------+---------------+
Recommended CoJP settings.
The COJP_REKEYING_GUARD_TIME value SHOULD take into account possible
retransmissions at the link layer due to imperfect wireless links.
9. Security Considerations
Since this document uses the pledge identifier to set the ID Context
parameter of OSCORE, an important security requirement is that the
pledge identifier is unique in the set of all pledge identifiers
managed by a JRC. The uniqueness of the pledge identifier ensures
unique (key, nonce) pairs for AEAD algorithm used by OSCORE. It also
allows the JRC to retrieve the correct security context, upon the
reception of a Join Request message. The management of pledge
identifiers is simplified if the globally unique EUI-64 is used, but
this comes with privacy risks, as discussed in Section 10.
This document further mandates that the (6LBR) pledge and the JRC are
provisioned with unique PSKs. While the process of provisioning PSKs
to all pledges can result in a substantial operational overhead, it
is vital to do so for the security properties of the network. The
PSK is used to set the OSCORE Master Secret during security context
derivation. This derivation process results in OSCORE keys that are
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important for mutual authentication of the (6LBR) pledge and the JRC.
The resulting security context shared between the pledge (joined
node) and the JRC is used for the purpose of joining and is long-
lived in that it can be used throughout the lifetime of a joined node
for parameter update exchanges. Should an attacker come to know the
PSK, then a man-in-the-middle attack is possible.
Note that while OSCORE provides replay protection, it does not
provide an indication of freshness in the presence of an attacker
that can drop/reorder traffic. Since the join request contains no
randomness, and the sequence number is predictable, the JRC could in
principle anticipate a join request from a particular pledge and pre-
calculate the response. In such a scenario, the JRC does not have to
be alive at the time when the request is received. This could be
relevant in case the JRC was temporarily compromised and control
subsequently regained by the legitimate owner.
It is of utmost importance to avoid unsafe practices when generating
and provisioning PSKs. The use of a single PSK shared among a group
of devices is a common pitfall that results in poor security. In
this case, the compromise of a single device is likely to lead to a
compromise of the entire batch, with the attacker having the ability
to impersonate a legitimate device and join the network, generate
bogus data and disturb the network operation. Additionally, some
vendors use methods such as scrambling or hashing of device serial
numbers or their EUI-64 to generate "unique" PSKs. Without any
secret information involved, the effort that the attacker needs to
invest into breaking these unsafe derivation methods is quite low,
resulting in the possible impersonation of any device from the batch,
without even needing to compromise a single device. The use of
cryptographically secure random number generators to generate the PSK
is RECOMMENDED, see [NIST800-90A] for different mechanisms using
deterministic methods.
The JP forwards the unauthenticated join traffic into the network. A
data cap on the JP prevents it from forwarding more traffic than the
network can handle and enables throttling in case of an attack. Note
that this traffic can only be directed at the JRC so that the JRC
needs to be prepared to handle such unsanitized inputs. The data cap
can be configured by the JRC by including a join rate parameter in
the Join Response and it is implemented through the CoAP's
PROBING_RATE setting. The use of a data cap at a JP forces attackers
to use more than one JP if they wish to overwhelm the network.
Marking the join traffic packets with a non-zero DSCP allows the
network to carry the traffic if it has capacity, but encourages the
network to drop the extra traffic rather than add bandwidth due to
that traffic.
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The shared nature of the "minimal" cell used for the join traffic
makes the network prone to a DoS attack by congesting the JP with
bogus traffic. Such an attacker is limited by its maximum transmit
power. The redundancy in the number of deployed JPs alleviates the
issue and also gives the pledge a possibility to use the best
available link for joining. How a network node decides to become a
JP is out of scope of this specification.
At the beginning of the join process, the pledge has no means of
verifying the content in the EB, and has to accept it at "face
value". In case the pledge tries to join an attacker's network, the
Join Response message will either fail the security check or time
out. The pledge may implement a temporary blacklist in order to
filter out undesired EBs and try to join using the next seemingly
valid EB. This blacklist alleviates the issue, but is effectively
limited by the node's available memory. Note that this temporary
blacklist is different from the one communicated as part of the CoJP
Configuration object as it helps pledge fight a DoS attack. The
bogus beacons prolong the join time of the pledge, and so the time
spent in "minimal" [RFC8180] duty cycle mode. The blacklist
communicated as part of the CoJP Configuration object helps JP fight
a DoS attack by a malicious pledge.
During the network lifetime, the JRC may at any time initiate a
Parameter Update exchange with a joined node. The Parameter Update
message uses the same OSCORE security context as is used for the join
exchange, except that the server/client roles are interchanged. As a
consequence, each Parameter Update message carries the well-known
OSCORE Sender ID of the JRC. A passive attacker may use the OSCORE
Sender ID to identify the Parameter Update traffic in case the link-
layer protection does not provide confidentiality. A countermeasure
against such traffic analysis attack is to use encryption at the
link-layer. Note that the join traffic does not undergo link-layer
protection at the first hop, as the pledge is not yet in possession
of cryptographic keys. Similarly, enhanced beacon traffic in the
network is not encrypted. This makes it easy for a passive attacker
to identify these types of traffic.
10. Privacy Considerations
The join solution specified in this document relies on the uniqueness
of the pledge identifier in the set of all pledge identifiers managed
by a JRC. This identifier is transferred in clear as an OSCORE kid
context. The use of the globally unique EUI-64 as pledge identifier
simplifies the management but comes with certain privacy risks. The
implications are thoroughly discussed in [RFC7721] and comprise
correlation of activities over time, location tracking, address
scanning and device-specific vulnerability exploitation. Since the
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join process occurs rarely compared to the network lifetime, long-
term threats that arise from using EUI-64 as the pledge identifier
are minimal. However, the use of EUI-64 after the join process
completes, in the form of a layer-2 or layer-3 address, extends the
aforementioned privacy threats to long term.
As an optional mitigation technique, the Join Response message may
contain a short address which is assigned by the JRC to the (6LBR)
pledge. The assigned short address SHOULD be uncorrelated with the
long-term pledge identifier. The short address is encrypted in the
response. Once the join process completes, the new node may use the
short addresses for all further layer-2 (and layer-3) operations.
This reduces the privacy threats as the short layer-2 address
(visible even when the network is encrypted) does not disclose the
manufacturer, as is the case of EUI-64. However, an eavesdropper
with access to the radio medium during the join process may be able
to correlate the assigned short address with the extended address
based on timing information with a non-negligible probability. This
probability decreases with an increasing number of pledges joining
concurrently.
11. IANA Considerations
Note to RFC Editor: Please replace all occurrences of "[[this
document]]" with the RFC number of this specification.
This document allocates a well-known name under the .arpa name space
according to the rules given in [RFC3172]. The name "6tisch.arpa" is
requested. No subdomains are expected, and addition of any such
subdomains requires the publication of an IETF standards-track RFC.
No A, AAAA or PTR record is requested.
11.1. CoJP Parameters Registry
This section defines a sub-registry within the "IPv6 over the TSCH
mode of IEEE 802.15.4e (6TiSCH) parameters" registry with the name
"Constrained Join Protocol Parameters Registry".
The columns of the registry are:
Name: This is a descriptive name that enables an easier reference to
the item. It is not used in the encoding.
Label: The value to be used to identify this parameter. The label is
an integer.
CBOR type: This field contains the CBOR type for the field.
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Description: This field contains a brief description for the field.
Reference: This field contains a pointer to the public specification
for the field, if one exists.
This registry is to be populated with the values in Table 4.
The amending formula for this sub-registry is: Different ranges of
values use different registration policies [RFC8126]. Integer values
from -256 to 255 are designated as Standards Action. Integer values
from -65536 to -257 and from 256 to 65535 are designated as
Specification Required. Integer values greater than 65535 are
designated as Expert Review. Integer values less than -65536 are
marked as Private Use.
11.2. CoJP Key Usage Registry
This section defines a sub-registry within the "IPv6 over the TSCH
mode of IEEE 802.15.4e (6TiSCH) parameters" registry with the name
"Constrained Join Protocol Key Usage Registry".
The columns of this registry are:
Name: This is a descriptive name that enables easier reference to the
item. The name MUST be unique. It is not used in the encoding.
Value: This is the value used to identify the key usage setting.
These values MUST be unique. The value is an integer.
Algorithm: This is a descriptive name of the link-layer algorithm in
use and uniquely determines the key length. The name is not used in
the encoding.
Description: This field contains a description of the key usage
setting. The field should describe in enough detail how the key is
to be used with different frame types, specific for the link-layer
technology in question.
Reference: This contains a pointer to the public specification for
the field, if one exists.
This registry is to be populated with the values in Table 5.
The amending formula for this sub-registry is: Different ranges of
values use different registration policies [RFC8126]. Integer values
from -256 to 255 are designated as Standards Action. Integer values
from -65536 to -257 and from 256 to 65535 are designated as
Specification Required. Integer values greater than 65535 are
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designated as Expert Review. Integer values less than -65536 are
marked as Private Use.
11.3. CoJP Unsupported Configuration Code Registry
This section defines a sub-registry within the "IPv6 over the TSCH
mode of IEEE 802.15.4e (6TiSCH) parameters" registry with the name
"Constrained Join Protocol Unsupported Configuration Code Registry".
The columns of this registry are:
Name: This is a descriptive name that enables easier reference to the
item. The name MUST be unique. It is not used in the encoding.
Value: This is the value used to identify the diagnostic code. These
values MUST be unique. The value is an integer.
Description: This is a descriptive human-readable name. The
description MUST be unique. It is not used in the encoding.
Reference: This contains a pointer to the public specification for
the field, if one exists.
This registry is to be populated with the values in Table 6.
The amending formula for this sub-registry is: Different ranges of
values use different registration policies [RFC8126]. Integer values
from -256 to 255 are designated as Standards Action. Integer values
from -65536 to -257 and from 256 to 65535 are designated as
Specification Required. Integer values greater than 65535 are
designated as Expert Review. Integer values less than -65536 are
marked as Private Use.
12. Acknowledgments
The work on this document has been partially supported by the
European Union's H2020 Programme for research, technological
development and demonstration under grant agreements: No 644852,
project ARMOUR; No 687884, project F-Interop and open-call project
SPOTS; No 732638, project Fed4FIRE+ and open-call project SODA.
The following individuals provided input to this document (in
alphabetic order): Christian Amsuss, Tengfei Chang, Klaus Hartke,
Tero Kivinen, Jim Schaad, Goeran Selander, Yasuyuki Tanaka, Pascal
Thubert, William Vignat, Xavier Vilajosana, Thomas Watteyne.
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13. References
13.1. Normative References
[I-D.ietf-6tisch-architecture]
Thubert, P., "An Architecture for IPv6 over the TSCH mode
of IEEE 802.15.4", draft-ietf-6tisch-architecture-28 (work
in progress), October 2019.
[I-D.ietf-core-stateless]
Hartke, K., "Extended Tokens and Stateless Clients in the
Constrained Application Protocol (CoAP)", draft-ietf-core-
stateless-03 (work in progress), October 2019.
[IEEE802.15.4]
IEEE standard for Information Technology, ., "IEEE Std
802.15.4 Standard for Low-Rate Wireless Networks", n.d..
[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>.
[RFC2597] Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
"Assured Forwarding PHB Group", RFC 2597,
DOI 10.17487/RFC2597, June 1999,
<https://www.rfc-editor.org/info/rfc2597>.
[RFC3172] Huston, G., Ed., "Management Guidelines & Operational
Requirements for the Address and Routing Parameter Area
Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172,
September 2001, <https://www.rfc-editor.org/info/rfc3172>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/info/rfc5869>.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
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[RFC7320] Nottingham, M., "URI Design and Ownership", BCP 190,
RFC 7320, DOI 10.17487/RFC7320, July 2014,
<https://www.rfc-editor.org/info/rfc7320>.
[RFC7554] Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using
IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the
Internet of Things (IoT): Problem Statement", RFC 7554,
DOI 10.17487/RFC7554, May 2015,
<https://www.rfc-editor.org/info/rfc7554>.
[RFC8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC8085,
March 2017, <https://www.rfc-editor.org/info/rfc8085>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC8180] Vilajosana, X., Ed., Pister, K., and T. Watteyne, "Minimal
IPv6 over the TSCH Mode of IEEE 802.15.4e (6TiSCH)
Configuration", BCP 210, RFC 8180, DOI 10.17487/RFC8180,
May 2017, <https://www.rfc-editor.org/info/rfc8180>.
[RFC8505] Thubert, P., Ed., Nordmark, E., Chakrabarti, S., and C.
Perkins, "Registration Extensions for IPv6 over Low-Power
Wireless Personal Area Network (6LoWPAN) Neighbor
Discovery", RFC 8505, DOI 10.17487/RFC8505, November 2018,
<https://www.rfc-editor.org/info/rfc8505>.
[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/info/rfc8613>.
13.2. Informative References
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[I-D.ietf-6tisch-msf]
Chang, T., Vucinic, M., Vilajosana, X., Duquennoy, S., and
D. Dujovne, "6TiSCH Minimal Scheduling Function (MSF)",
draft-ietf-6tisch-msf-08 (work in progress), November
2019.
[I-D.ietf-anima-grasp]
Bormann, C., Carpenter, B., and B. Liu, "A Generic
Autonomic Signaling Protocol (GRASP)", draft-ietf-anima-
grasp-15 (work in progress), July 2017.
[I-D.ietf-cbor-cddl]
Birkholz, H., Vigano, C., and C. Bormann, "Concise data
definition language (CDDL): a notational convention to
express CBOR and JSON data structures", draft-ietf-cbor-
cddl-08 (work in progress), March 2019.
[I-D.ietf-cbor-sequence]
Bormann, C., "Concise Binary Object Representation (CBOR)
Sequences", draft-ietf-cbor-sequence-02 (work in
progress), September 2019.
[NIST800-90A]
NIST Special Publication 800-90A, Revision 1, ., Barker,
E., and J. Kelsey, "Recommendation for Random Number
Generation Using Deterministic Random Bit Generators",
2015.
[RFC4231] Nystrom, M., "Identifiers and Test Vectors for HMAC-SHA-
224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512",
RFC 4231, DOI 10.17487/RFC4231, December 2005,
<https://www.rfc-editor.org/info/rfc4231>.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<https://www.rfc-editor.org/info/rfc4944>.
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785,
DOI 10.17487/RFC5785, April 2010,
<https://www.rfc-editor.org/info/rfc5785>.
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[RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
Low-Power and Lossy Networks", RFC 6550,
DOI 10.17487/RFC6550, March 2012,
<https://www.rfc-editor.org/info/rfc6550>.
[RFC6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC6762, February 2013,
<https://www.rfc-editor.org/info/rfc6762>.
[RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
Considerations for IPv6 Address Generation Mechanisms",
RFC 7721, DOI 10.17487/RFC7721, March 2016,
<https://www.rfc-editor.org/info/rfc7721>.
[RFC8415] Mrugalski, T., Siodelski, M., Volz, B., Yourtchenko, A.,
Richardson, M., Jiang, S., Lemon, T., and T. Winters,
"Dynamic Host Configuration Protocol for IPv6 (DHCPv6)",
RFC 8415, DOI 10.17487/RFC8415, November 2018,
<https://www.rfc-editor.org/info/rfc8415>.
[RFC8480] Wang, Q., Ed., Vilajosana, X., and T. Watteyne, "6TiSCH
Operation Sublayer (6top) Protocol (6P)", RFC 8480,
DOI 10.17487/RFC8480, November 2018,
<https://www.rfc-editor.org/info/rfc8480>.
Appendix A. Example
Figure 3 illustrates a successful join protocol exchange. The pledge
instantiates the OSCORE context and derives the OSCORE keys and
nonces from the PSK. It uses the instantiated context to protect the
Join Request addressed with a Proxy-Scheme option, the well-known
host name of the JRC in the Uri-Host option, and its EUI-64 as pledge
identifier and OSCORE kid context. Triggered by the presence of a
Proxy-Scheme option, the JP forwards the request to the JRC and sets
the CoAP token to the internally needed state. The JP has learned
the IPv6 address of the JRC when it acted as a pledge and joined the
network. Once the JRC receives the request, it looks up the correct
context based on the kid context parameter. The OSCORE data
authenticity verification ensures that the request has not been
modified in transit. In addition, replay protection is ensured
through persistent handling of mutable context parameters.
Once the JP receives the Join Response, it authenticates the state
within the CoAP token before deciding where to forward. The JP sets
its internal state to that found in the token, and forwards the Join
Response to the correct pledge. Note that the JP does not possess
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the key to decrypt the CoJP object (configuration) present in the
payload. The Join Response is matched to the Join Request and
verified for replay protection at the pledge using OSCORE processing
rules. In this example, the Join Response does not contain the IPv6
address of the JRC, the pledge hence understands the JRC is co-
located with the 6LBR.
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<---E2E OSCORE-->
Client Proxy Server
Pledge JP JRC
| | |
| Join | | Code: 0.02 (POST)
| Request | | Token: -
+--------->| | Proxy-Scheme: coap
| | | Uri-Host: 6tisch.arpa
| | | OSCORE: kid: -,
| | | kid_context: EUI-64,
| | | Partial IV: 1
| | | Payload: { Code: 0.02 (POST),
| | | Uri-Path: "j",
| | | join_request, <Tag> }
| | |
| | Join | Code: 0.02 (POST)
| | Request | Token: opaque state
| +--------->| OSCORE: kid: -,
| | | kid_context: EUI-64,
| | | Partial IV: 1
| | | Payload: { Code: 0.02 (POST),
| | | Uri-Path: "j",
| | | join_request, <Tag> }
| | |
| | |
| | Join | Code: 2.04 (Changed)
| | Response | Token: opaque state
| |<---------+ OSCORE: -
| | | Payload: { Code: 2.04 (Changed),
| | | configuration, <Tag> }
| | |
| | |
| Join | | Code: 2.04 (Changed)
| Response | | Token: -
|<---------+ | OSCORE: -
| | | Payload: { Code: 2.04 (Changed),
| | | configuration, <Tag> }
| | |
Figure 3: Example of a successful join protocol exchange. { ... }
denotes authenticated encryption, <Tag> denotes the authentication
tag.
Where the join_request object is:
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join_request:
{
5 : h'cafe' / PAN ID of the network pledge is attempting to join /
}
Since the role parameter is not present, the default role of "6TiSCH
Node" is implied.
The join_request object encodes to h'a10542cafe' with a size of 5
bytes.
And the configuration object is:
configuration:
{
2 : [ / link-layer key set /
1, / key_id /
h'e6bf4287c2d7618d6a9687445ffd33e6' / key_value /
],
3 : [ / short identifier /
h'af93' / assigned short address /
]
}
Since the key_usage parameter is not present in the link-layer key
set object, the default value of "6TiSCH-K1K2-ENC-MIC32" is implied.
Since key_addinfo parameter is not present and key_id is different
than 0, Key ID Mode 0x01 (Key Index) is implied. Similarly, since
the lease_time parameter is not present in the short identifier
object, the default value of positive infinity is implied.
The configuration object encodes to
h'a202820150e6bf4287c2d7618d6a9687445ffd33e6038142af93' with a size
of 26 bytes.
Appendix B. Lightweight Implementation Option
In environments where optimizing the implementation footprint is
important, it is possible to implement this specification without
having the implementations of HKDF [RFC5869] and SHA [RFC4231] on
constrained devices. HKDF and SHA are used during the OSCORE
security context derivation phase. This derivation can also be done
by the JRC or a provisioning device, on behalf of the (6LBR) pledge
during the provisioning phase. In that case, the derived OSCORE
security context parameters are written directly into the (6LBR)
pledge, without requiring the PSK be provisioned to the (6LBR)
pledge.
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The use of HKDF to derive OSCORE security context parameters ensures
that the resulting OSCORE keys have good security properties, and are
unique as long as the input for different pledges varies. This
specification ensures the uniqueness by mandating unique pledge
identifiers and a unique PSK for each (6LBR) pledge. From the AEAD
nonce reuse viewpoint, having a unique pledge identifier is a
sufficient condition. However, as discussed in Section 9, the use of
a single PSK shared among many devices is a common security pitfall.
The compromise of this shared PSK on a single device would lead to
the compromise of the entire batch. When using the implementation/
deployment scheme outlined above, the PSK does not need to be written
to individual pledges. As a consequence, even if a shared PSK is
used, the scheme offers a comparable level of security as in the
scenario where each pledge is provisioned with a unique PSK. In this
case, there is still a latent risk of the shared PSK being
compromised from the provisioning device, which would compromise all
devices in the batch.
Authors' Addresses
Malisa Vucinic (editor)
Inria
2 Rue Simone Iff
Paris 75012
France
Email: malisa.vucinic@inria.fr
Jonathan Simon
Analog Devices
32990 Alvarado-Niles Road, Suite 910
Union City, CA 94587
USA
Email: jonathan.simon@analog.com
Kris Pister
University of California Berkeley
512 Cory Hall
Berkeley, CA 94720
USA
Email: pister@eecs.berkeley.edu
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Michael Richardson
Sandelman Software Works
470 Dawson Avenue
Ottawa, ON K1Z5V7
Canada
Email: mcr+ietf@sandelman.ca
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