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IETF RFC 9031
Last modified on Sunday, May 30th, 2021
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Internet Engineering Task Force (IETF) M. Vučinić, Ed.
Request for Comments: 9031 Inria
Category: Standards Track J. Simon
ISSN: 2070-1721 Analog Devices
K. Pister
University of California Berkeley
M. Richardson
Sandelman Software Works
May 2021
Constrained Join Protocol (CoJP) for 6TiSCH
 Abstract
This document describes the minimal framework required for a new
device, called a "pledge", to securely join a 6TiSCH (IPv6 over the
Time-Slotted Channel Hopping mode of IEEE 802.15.4) 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 it
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 is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/RFC 9031.
Copyright Notice
Copyright (c) 2021 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
2. Terminology
3. Provisioning Phase
4. Join Process Overview
4.1. Step 1 - Enhanced Beacon
4.2. Step 2 - Neighbor Discovery
4.3. Step 3 - Constrained Join Protocol (CoJP) Execution
4.4. The Special Case of the 6LBR Pledge Joining
5. Link-Layer Configuration
5.1. Distribution of Time
6. Network-Layer Configuration
6.1. Identification of Unauthenticated Traffic
7. Application-Layer Configuration
7.1. Statelessness of the JP
7.2. Recommended Settings
7.3. OSCORE
8. Constrained Join Protocol (CoJP)
8.1. Join Exchange
8.2. Parameter Update Exchange
8.3. Error Handling
8.4. CoJP Objects
8.5. Recommended Settings
9. Security Considerations
10. Privacy Considerations
11. IANA Considerations
11.1. Constrained Join Protocol (CoJP) Parameters
11.2. Constrained Join Protocol (CoJP) Key Usage
11.3. Constrained Join Protocol (CoJP) Unsupported Configuration
Codes
12. References
12.1. Normative References
12.2. Informative References
Appendix A. Example
Appendix B. Lightweight Implementation Option
Acknowledgments
Authors' Addresses
1. Introduction
This document defines a "secure join" solution for a new device,
called a "pledge", to securely join a 6TiSCH network. The term
"secure join" refers to network access authentication, authorization,
and parameter distribution as defined in [RFC 9030]. 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 according
to 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 [RFC 7554] and
[RFC 8180]. By design, nodes in a 6TiSCH network [RFC 7554] have their
radio turned off most of the time in order to conserve energy. As a
consequence, the link used by a new device for joining the network
has limited bandwidth [RFC 8180]. The secure join solution defined in
this document therefore keeps the number of over-the-air exchanges to
a minimum.
The microcontrollers at the heart of 6TiSCH nodes have small amounts
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 [RFC 7252] for web
transfer and on OSCORE [RFC 8613] 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 on which 6TiSCH is based. CoJP may also 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 through companion
documents the configuration parameters specific to the technology in
question. The overall process is described in Section 4, and the
configuration of the stack is specific to 6TiSCH.
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, the messages are
forwarded by nodes, called Join Proxies, that are already part of the
6TiSCH network. 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, a 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
6TiSCH Operation Sublayer (6top) Protocol [RFC 8480] and can 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 BCP
14 [RFC 2119] [RFC 8174] 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 [RFC 9030], [RFC 7252], [RFC 8613], and [RFC 8152].
The specification also includes a set of informative specifications
using the Concise Data Definition Language (CDDL) [RFC 8610].
The following terms defined in [RFC 9030] are used extensively
throughout this document:
* pledge
* joined node
* Join Proxy (JP)
* Join Registrar/Coordinator (JRC)
* Enhanced Beacon (EB)
* join protocol
* join process
The following terms defined in [RFC 8505] are also used throughout
this document:
* 6LoWPAN Border Router (6LBR)
* 6LoWPAN Node (6LN)
The term "6LBR" is used interchangeably with the term "DODAG root"
defined in [RFC 6550] on the assumption that the two entities are co-
located, as recommended by [RFC 9030].
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
applies equally 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 with a number of mechanisms, such as using a
JTAG (Joint Test Action Group) interface, using a serial (craft)
console interface, pushing buttons simultaneously on different
devices, configuring over-the-air 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 are 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:
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.
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 the case of recommissioning a device 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 [RFC 8180]. The
PSK is a long-term secret used to protect the execution of the
secure join protocol specified in this document; the link-layer
keys are transported as part of the secure join protocol.
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 of provisioning. If this parameter
is not provisioned to the pledge, the pledge will attempt to join
one advertised network at a time, which significantly prolongs the
join process. This parameter MUST be provisioned to the 6LBR
pledge.
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 the following:
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 [RFC 8415], Generic Autonomic
Signaling Protocol (GRASP) [RFC 8990], or Multicast DNS (mDNS)
[RFC 6762]; the use of these mechanisms 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, telling 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 EB
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 the 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.
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 EB sent by a node already in the network. This process
is entirely defined by [IEEE802.15.4] and described in [RFC 7554].
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 [RFC 7554]. Implementers should make use
of information such as the following: which 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, at which
signal strength the different EBs were received, etc. In addition,
the pledge may be preconfigured to search for EBs with a specific
network identifier.
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 either 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 per [RFC 4944]. The pledge MAY perform the Neighbor
Solicitation / Neighbor Advertisement exchange with the JP per
Section 5.6 of [RFC 8505]. Per [RFC 8505], 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 (Section 4.3.1)) and the Join Response
message, conditioned on the successful security processing of the
request (Step 3b (Section 4.3.2)).
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 the 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.
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 it 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 various 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 just like
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 CoJP is out of
scope of this document.
5. Link-Layer Configuration
In an operational 6TiSCH network, all frames use link-layer frame
security [RFC 8180]. 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 perform an authenticity check of the EB
frames because 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 [RFC 8180].
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. The upper layer
can use that information to determine that a join process is in place
and that 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 use 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 with the network, it has no
means of verifying the authenticity of EB frames. Because 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. The Absolute Slot Number is used in the
construction of the link-layer nonce, as defined in [IEEE802.15.4].
The pledge initially synchronizes with 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.
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 the 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 [RFC 8505]. 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.
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 to 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 DODAG Information Objects (DIOs) [RFC 6550] 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 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 6TiSCH minimal network [RFC 8180] 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 JP is aware of what traffic originates from unauthenticated
pledges, and so can avoid allocating additional bandwidth itself.
The JP implements a data cap on outgoing join traffic by implementing
the recommendation of 1 packet per 3 seconds in Section 3.1.3 of
[RFC 8085]. This can be achieved with the congestion control
mechanism specified in Section 4.7 of [RFC 7252]. This cap will not
protect intermediate nodes as they cannot tell join traffic from
regular traffic. Despite the data cap implemented separately on each
JP, the aggregate join traffic from many JPs may cause intermediate
nodes to decide to allocate additional cells. It is undesirable to
do so in response to the traffic originated from unauthenticated
pledges. In order to permit the intermediate nodes to avoid this,
the traffic needs to be tagged. [RFC 2597] 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.
6.1.1. Traffic from JP to JRC
The JP SHOULD set the DSCP of packets that it produces as part of the
forwarding process to AF43 code point (See Section 6 of [RFC 2597]).
A JP that does not require a specific DSCP value on forwarded traffic
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.
6.1.2. Traffic from JRC to JP
The JRC SHOULD set the DSCP of Join Response packets addressed to the
JP to the 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. If 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-Layer Configuration
The CoJP join exchange in Figure 1 is carried over CoAP [RFC 7252] and
the secure channel provided by OSCORE [RFC 8613]. The (6LBR) pledge
acts as a CoAP client; the JRC acts as a CoAP server. The JP
implements CoAP forward proxy functionality [RFC 7252]. 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 the CoAP requests that 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.
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 [RFC 7252] 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 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 [RFC 7252], if
the size of the serialized state object permits, or use the extended
CoAP token defined in [RFC 8974] 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 [RFC 8974]. 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
that it forwards into the network on behalf of unauthenticated
pledges. When the JP is operating in a stateless manner, the
security considerations from [RFC 8974] 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.
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 stateful 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 |
+-------------------+---------------+
Table 1: 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 [RFC 7252], 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 per Section 3 of
[RFC 8613].
* The Master Secret MUST be the PSK.
* The Master Salt MUST be the empty byte string.
* The ID Context MUST be set to the pledge identifier.
* 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.
* 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.
* The Algorithm MUST be set to the value from [RFC 8152], agreed to
out-of-band by the same mechanism used to provision the PSK. The
default is AES-CCM-16-64-128.
* The key derivation function MUST be agreed out-of-band by the same
mechanism used to provision the PSK. Default is HKDF SHA-256
[RFC 5869].
Since the pledge's OSCORE Sender ID is the empty byte string, when
constructing the OSCORE option, the pledge sets the 'kid' flag in the
OSCORE flag bits but indicates a 0-length 'kid'. The pledge
transports its pledge identifier within the 'kid context' field of
the OSCORE option. The derivation in [RFC 8613] results in OSCORE
keys and a Common Initialization Vector (IV) for each side of the
conversation. Nonces are constructed by XORing the Common IV with
the current sequence number. For details on nonce and OSCORE option
construction, refer to [RFC 8613].
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 the 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
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 [RFC 8613]). A (6LBR)
pledge is expected to have exactly one OSCORE security context with
the JRC.
7.3.1. Replay Window and Persistency
Both the (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 [RFC 8613] 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 [RFC 8613] 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 [RFC 8152]. 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.
The mandatory-to-implement hash algorithm is SHA-256 [RFC 4231]. The
mandatory-to-implement key derivation function is HKDF [RFC 5869],
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 [RFC 7252] and a secure channel provided by OSCORE [RFC 8613].
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:
* 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.
* 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
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 the following:
* 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 [RFC 8949]. 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:
* The request method is POST.
* The type is Confirmable (CON).
* The Proxy-Scheme option is set to "coap".
* 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.
* The Uri-Path option is set to "j".
* The OSCORE option SHALL be set according to [RFC 8613]. 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.
* 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
Request as a non-confirmable (NON) CoAP message, as specified in
Section 7. If the CoAP implementation at the (6LBR) pledge declares
the message transmission a 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.
BCP 190 [RFC 8820] provides guidelines on URI design and ownership.
It recommends that whenever a third party wants to mandate a URI to
web authority that it SHOULD go under "/.well-known" (per [RFC 8615]).
In the case of CoJP, the Uri-Host option is always set to
"6tisch.arpa", and based upon the recommendations in Section 1 of
[RFC 8820], it is asserted that this document is the owner of the CoJP
service. As such, the concerns of [RFC 8820] 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:
* The Response Code is 2.04 (Changed).
* 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
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:
* The request method is POST.
* The type is Confirmable (CON).
* The Uri-Host option is set to "6tisch.arpa".
* The Uri-Path option is set to "j".
* The OSCORE option SHALL be set according to [RFC 8613]. 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.
* The payload is a Configuration CBOR object, as defined in
Section 8.4.2.
The JRC has implicit knowledge of 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.
If 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 a
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 the security considerations on the reuse of
assigned resources apply, as discussed in Section 9.
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 the cases in which
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 the 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-layer
handling of the CoAP response and is different from CoAP's
MAX_RETRANSMIT setting, which 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 message.
The Diagnostic Response message SHALL be mapped to a CoAP response:
* The Response Code is 4.00 (Bad Request).
* 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.
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,
a joined node or the JRC may 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 pledge (joined node) 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. The JRC may be hosted on a generic machine where the
write operation to persistent memory may lead to unpredictable delays
due to caching. If a reboot event occurs at the JRC 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 will be 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,
the network administrator MUST force all the networks managed by the
failed JRC to rejoin through out-of-band means during the process of
JRC reinitialization, e.g., reinitialize the 6LBR nodes and freshly
generate dynamic cryptographic keys and other parameters that
influence 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 [RFC 8613]. 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
should consequently map the new ID Context to the previously used
pledge identifier. How the JRC handles this mapping is out of scope
of this document.
The use of the procedure specified in Appendix B.2 of [RFC 8613] 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 requires 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 [RFC 8613] is not supported by the pledge, the network
administrator MUST reprovision 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 as part of 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 "Constrained Join
Protocol (CoJP) Parameters" registry, Section 11.1.
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 3. This parameter MAY be
included. If the parameter is omitted, the default value of 0,
i.e., the role "6TiSCH Node", MUST be assumed.
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 which network 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.
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 2 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 2: 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 | RFC 9031 |
| Node | | the role of a regular 6TiSCH | |
| | | node, i.e., non-6LBR node. | |
+--------+-------+------------------------------+-----------+
| 6LBR | 1 | The pledge requests to play | RFC 9031 |
| | | the role of 6LoWPAN Border | |
| | | Router (6LBR). | |
+--------+-------+------------------------------+-----------+
Table 3: Role values.
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 "Constrained Join Protocol (CoJP)
Parameters" registry, Section 11.1.
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. The installation and use of
keys differs for the 6LBR and other (regular) nodes, and this is
explained in Sections 8.4.3.1 and 8.4.3.2.
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.
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.
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
parameter value it sends next. Since the blacklist parameter
carries the pledge identifiers, privacy considerations apply. See
Section 10.
join rate: The 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 to 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. If this parameter is set to zero, a joined node MUST
silently drop any join traffic coming from unauthenticated
pledges. If this parameter is omitted, the value of positive
infinity SHOULD be assumed. A node operating as a JP MAY use
another mechanism that is out of scope of this specification to
configure the PROBING_RATE of CoAP in the absence of a join rate
parameter from the Configuration object.
Table 4 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 4: Summary of Configuration parameters.
The CDDL fragment that represents the text above for the
Configuration follows. The structures Link_Layer_Key and
Short_Identifier are specified in Sections 8.4.3 and 8.4.4,
respectively.
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
}
+===============+=======+==========+====================+===========+
| Name | Label | CBOR | Description | Reference |
| | | type | | |
+===============+=======+==========+====================+===========+
| role | 1 | unsigned | Identifies the | RFC 9031 |
| | | integer | role parameter | |
+---------------+-------+----------+--------------------+-----------+
| link-layer | 2 | array | Identifies the | RFC 9031 |
| key set | | | array carrying | |
| | | | one or more | |
| | | | link-layer | |
| | | | cryptographic | |
| | | | keys | |
+---------------+-------+----------+--------------------+-----------+
| short | 3 | array | Identifies the | RFC 9031 |
| identifier | | | assigned short | |
| | | | identifier | |
+---------------+-------+----------+--------------------+-----------+
| JRC address | 4 | byte | Identifies the | RFC 9031 |
| | | string | IPv6 address | |
| | | | of the JRC | |
+---------------+-------+----------+--------------------+-----------+
| network | 5 | byte | Identifies the | RFC 9031 |
| identifier | | string | network | |
| | | | identifier | |
| | | | parameter | |
+---------------+-------+----------+--------------------+-----------+
| blacklist | 6 | array | Identifies the | RFC 9031 |
| | | | blacklist | |
| | | | parameter | |
+---------------+-------+----------+--------------------+-----------+
| join rate | 7 | unsigned | Identifier the | RFC 9031 |
| | | integer | join rate | |
| | | | parameter | |
+---------------+-------+----------+--------------------+-----------+
| unsupported | 8 | array | Identifies the | RFC 9031 |
| configuration | | | unsupported | |
| | | | configuration | |
| | | | parameter | |
+---------------+-------+----------+--------------------+-----------+
Table 5: 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
and the security level and the frame types with which it should be
used for both 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 [RFC 8742], some being optional.
The set of parameters that can appear in a Link_Layer_Key object is
summarized below, in order:
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. If the key is
considered invalid, the key MUST be discarded, and the
implementation MUST signal the error as specified in
Section 8.3.1.
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
the IANA "Constrained Join Protocol (CoJP) Key Usage" registry
(Section 11.2). If the parameter is omitted, the default value of
0 (6TiSCH-K1K2-ENC-MIC32) from Table 6 MUST be assumed. This
default value has been chosen because it results in byte savings
in the most constrained settings; its selection does not imply a
recommendation for its general usage.
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.
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.
The CDDL fragment for the Link_Layer_Key that represents the text
above follows:
Link_Layer_Key = (
key_id : uint,
? key_usage : int,
key_value : bstr,
? key_addinfo : bstr,
)
+======================+=====+======================+===============+
|Name |Value|Algorithm |Description |
+======================+=====+======================+===============+
|6TiSCH-K1K2-ENC-MIC32 |0 |IEEE802154-AES-CCM-128|Use MIC-32 for |
| | | |EBs, ENC-MIC-32|
| | | |for DATA and |
| | | |ACKNOWLEDGMENT.|
+----------------------+-----+----------------------+---------------+
|6TiSCH-K1K2-ENC-MIC64 |1 |IEEE802154-AES-CCM-128|Use MIC-64 for |
| | | |EBs, ENC-MIC-64|
| | | |for DATA and |
| | | |ACKNOWLEDGMENT.|
+----------------------+-----+----------------------+---------------+
|6TiSCH-K1K2-ENC-MIC128|2 |IEEE802154-AES-CCM-128|Use MIC-128 for|
| | | |EBs, ENC- |
| | | |MIC-128 for |
| | | |DATA and |
| | | |ACKNOWLEDGMENT.|
+----------------------+-----+----------------------+---------------+
|6TiSCH-K1K2-MIC32 |3 |IEEE802154-AES-CCM-128|Use MIC-32 for |
| | | |EBs, DATA and |
| | | |ACKNOWLEDGMENT.|
+----------------------+-----+----------------------+---------------+
|6TiSCH-K1K2-MIC64 |4 |IEEE802154-AES-CCM-128|Use MIC-64 for |
| | | |EBs, DATA and |
| | | |ACKNOWLEDGMENT.|
+----------------------+-----+----------------------+---------------+
|6TiSCH-K1K2-MIC128 |5 |IEEE802154-AES-CCM-128|Use MIC-128 for|
| | | |EBs, DATA and |
| | | |ACKNOWLEDGMENT.|
+----------------------+-----+----------------------+---------------+
|6TiSCH-K1-MIC32 |6 |IEEE802154-AES-CCM-128|Use MIC-32 for |
| | | |EBs. |
+----------------------+-----+----------------------+---------------+
|6TiSCH-K1-MIC64 |7 |IEEE802154-AES-CCM-128|Use MIC-64 for |
| | | |EBs. |
+----------------------+-----+----------------------+---------------+
|6TiSCH-K1-MIC128 |8 |IEEE802154-AES-CCM-128|Use MIC-128 for|
| | | |EBs. |
+----------------------+-----+----------------------+---------------+
|6TiSCH-K2-MIC32 |9 |IEEE802154-AES-CCM-128|Use MIC-32 for |
| | | |DATA and |
| | | |ACKNOWLEDGMENT.|
+----------------------+-----+----------------------+---------------+
|6TiSCH-K2-MIC64 |10 |IEEE802154-AES-CCM-128|Use MIC-64 for |
| | | |DATA and |
| | | |ACKNOWLEDGMENT.|
+----------------------+-----+----------------------+---------------+
|6TiSCH-K2-MIC128 |11 |IEEE802154-AES-CCM-128|Use MIC-128 for|
| | | |DATA and |
| | | |ACKNOWLEDGMENT.|
+----------------------+-----+----------------------+---------------+
|6TiSCH-K2-ENC-MIC32 |12 |IEEE802154-AES-CCM-128|Use ENC-MIC-32 |
| | | |for DATA and |
| | | |ACKNOWLEDGMENT.|
+----------------------+-----+----------------------+---------------+
|6TiSCH-K2-ENC-MIC64 |13 |IEEE802154-AES-CCM-128|Use ENC-MIC-64 |
| | | |for DATA and |
| | | |ACKNOWLEDGMENT.|
+----------------------+-----+----------------------+---------------+
|6TiSCH-K2-ENC-MIC128 |14 |IEEE802154-AES-CCM-128|Use ENC-MIC-128|
| | | |for DATA and |
| | | |ACKNOWLEDGMENT.|
+----------------------+-----+----------------------+---------------+
Table 6: Key Usage values.
8.4.3.1. Rekeying of 6LBRs
When the 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 made 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 nonfunctional or in extended sleep such that it will not be
reached. To get the key update, such a node will need to go through
the join process anew.
8.4.3.2. Rekeying of 6LNs
When a regular 6LN 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 the network switch is based upon the receipt of
traffic secured with the new keys. Upon the reception and the
successful security processing of a link-layer frame secured with a
key from the new key set, a 6LN MUST then switch to sending all
outgoing traffic using the keys from the new set. The 6LN MUST
remove any keys it had installed from the previous key set after
waiting COJP_REKEYING_GUARD_TIME since it started using the new key
set.
Sending traffic with the new keys signals to other downstream nodes
to switch to their new key, causing 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.
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.
Key ID Mode 0x00 (Implicit, pairwise): The key_id parameter MUST be
set to 0. The key_addinfo parameter MUST be present. The
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.
Key ID Mode 0x01 (Key Index): The key_id parameter MUST be set to a
value different from 0. The key_addinfo parameter MUST NOT be
present.
Key ID Mode 0x02 (4-byte Explicit Key Source): The key_id parameter
MUST be set to a value different from 0. The key_addinfo
parameter MUST be present. The key_addinfo parameter MUST be set
to a byte string, exactly 4 bytes long. The key_addinfo parameter
carries the Key Source parameter used to configure [IEEE802.15.4].
Key ID Mode 0x03 (8-byte Explicit Key Source): The key_id parameter
MUST be set to a value different from 0. The key_addinfo
parameter MUST be present. The key_addinfo parameter MUST be set
to a byte string, exactly 8 bytes long. The key_addinfo parameter
carries the Key Source parameter used to configure [IEEE802.15.4].
In all cases, the key_usage parameter determines how a particular key
should be used with respect to incoming and outgoing security
policies.
For Key ID Modes 0x01 through 0x03, the key_id parameter sets the
"secKeyIndex" parameter of [IEEE802.15.4] that is signaled in all
outgoing frames secured with a given key. The maximum value that
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 the JRC learns about the network topology is out of
scope of this specification, but it 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
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 and contains, in order:
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. If the identifier is invalid, the
decoder MUST silently ignore the Short_Identifier object.
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. If the lease expires, then the node
SHOULD initiate a new join exchange, as described in Section 8.1.
If this parameter is omitted, then 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 for the Short_Identifier that represents the text
above follows:
Short_Identifier = [
identifier : bstr,
? lease_time : uint
]
8.4.4.1. Use in IEEE Std 802.15.4
When the 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 the 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 from 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 of the
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
parameters that appear in an Unsupported_Parameter object is
summarized below, in order:
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 "Constrained Join Protocol (CoJP) Unsupported Configuration
Codes" registry (Section 11.3).
parameter_label: Indicates the parameter. This parameter MUST be
included. Possible values of this parameter are specified in the
label column of the IANA "Constrained Join Protocol (CoJP)
Parameters" registry" (Section 11.1).
parameter_addinfo: Additional information about the parameter that
cannot be acted upon. This parameter MUST be included. If 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
that either a subset or the entire key set that was received
cannot be acted upon. In that case, the value of
parameter_addinfo follows the link-layer key set structure defined
in Section 8.4.2. If 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 for the Unsupported_Configuration and
Unsupported_Parameter objects that represents the text above follows:
Unsupported_Configuration = [
+ parameter : Unsupported_Parameter
]
Unsupported_Parameter = (
code : int,
parameter_label : int,
parameter_addinfo : nil / any
)
+=============+=======+==============================+===========+
| Name | Value | Description | Reference |
+=============+=======+==============================+===========+
| Unsupported | 0 | The indicated setting is not | RFC 9031 |
| | | supported by the networking | |
| | | stack implementation. | |
+-------------+-------+------------------------------+-----------+
| Malformed | 1 | The indicated parameter | RFC 9031 |
| | | value is malformed. | |
+-------------+-------+------------------------------+-----------+
Table 7: 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 |
+--------------------------+---------------+
Table 8: 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 the 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
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 and/or 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 the request is received. This
could be relevant in the case when the JRC was temporarily
compromised and control was 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 device serial
numbers or their EUI-64 identifiers 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 nonzero DSCP allows the
network to carry the traffic if it has capacity, but it encourages
the network to drop the extra traffic rather than add bandwidth due
to that traffic.
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 the 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".
If 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 the pledge fight a DoS attack. The bogus beacons
prolong the join time of the pledge and so does the time spent in
"minimal" duty cycle mode [RFC 8180]. The blacklist communicated as
part of the CoJP Configuration object helps the 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 and 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 if the
link-layer protection does not provide confidentiality. A
countermeasure against such a 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, EB 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 the 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 [RFC 7721] and
comprise correlation of activities over time, location tracking,
address scanning, and device-specific vulnerability exploitation.
Since the 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, after the join process
completes, the use of EUI-64 in the form of a Layer 2 or Layer 3
address extends the aforementioned privacy threats to the long term.
As an optional mitigation technique, the Join Response message may
contain a short address that 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
This document allocates a well-known name under the .arpa name space
according to the rules given in [RFC 3172] and [RFC 6761]. 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. Constrained Join Protocol (CoJP) Parameters
This section defines a subregistry within the "IPv6 Over the TSCH
Mode of IEEE 802.15.4 (6TiSCH)" registry with the name "Constrained
Join Protocol (CoJP) Parameters".
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. The name MUST be
unique.
Label: The value to be used to identify this parameter. The label
is an integer. The label MUST be unique.
CBOR Type: This field contains the CBOR type for the field.
Description: This field contains a brief description for the field.
The description MUST be unique.
Reference: This field contains a pointer to the public specification
for the field, if one exists.
This registry is populated with the values in Table 5.
The amending formula for this subregistry is: Different ranges of
values use different registration policies [RFC 8126]. 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. Constrained Join Protocol (CoJP) Key Usage
This section defines a subregistry within the "IPv6 Over the TSCH
Mode of IEEE 802.15.4 (6TiSCH)" registry with the name "Constrained
Join Protocol (CoJP) Key Usage".
The columns of this registry are:
Name: This is a descriptive name that enables easier reference to
the item. It is not used in the encoding. The name MUST be
unique.
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. The algorithm MUST be unique.
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. The description MUST be unique.
Reference: This contains a pointer to the public specification for
the field, if one exists.
This registry is populated with the values in Table 6.
The amending formula for this subregistry is: Different ranges of
values use different registration policies [RFC 8126]. 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.
11.3. Constrained Join Protocol (CoJP) Unsupported Configuration Codes
This section defines a subregistry within the "IPv6 Over the TSCH
Mode of IEEE 802.15.4 (6TiSCH)" registry with the name "Constrained
Join Protocol (CoJP) Unsupported Configuration Codes".
The columns of this registry are:
Name: This is a descriptive name that enables easier reference to
the item. It is not used in the encoding. The name MUST be
unique.
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 7.
The amending formula for this subregistry is: Different ranges of
values use different registration policies [RFC 8126]. 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. References
12.1. Normative References
[IEEE802.15.4]
IEEE, "IEEE Standard for Low-Rate Wireless Networks", IEEE
Standard 802.15.4-2015, DOI 10.1109/IEEESTD.2016.7460875,
April 2016,
<https://ieeexplore.ieee.org/document/7460875>.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC 2119, March 1997,
<https://www.rfc-editor.org/info/RFC 2119>.
[RFC 2597] Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
"Assured Forwarding PHB Group", RFC 2597,
DOI 10.17487/RFC 2597, June 1999,
<https://www.rfc-editor.org/info/RFC 2597>.
[RFC 3172] Huston, G., Ed., "Management Guidelines & Operational
Requirements for the Address and Routing Parameter Area
Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC 3172,
September 2001, <https://www.rfc-editor.org/info/RFC 3172>.
[RFC 5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC 5869, May 2010,
<https://www.rfc-editor.org/info/RFC 5869>.
[RFC 6761] Cheshire, S. and M. Krochmal, "Special-Use Domain Names",
RFC 6761, DOI 10.17487/RFC 6761, February 2013,
<https://www.rfc-editor.org/info/RFC 6761>.
[RFC 7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC 7252, June 2014,
<https://www.rfc-editor.org/info/RFC 7252>.
[RFC 7554] 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/RFC 7554, May 2015,
<https://www.rfc-editor.org/info/RFC 7554>.
[RFC 8085] Eggert, L., Fairhurst, G., and G. Shepherd, "UDP Usage
Guidelines", BCP 145, RFC 8085, DOI 10.17487/RFC 8085,
March 2017, <https://www.rfc-editor.org/info/RFC 8085>.
[RFC 8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC 8126, June 2017,
<https://www.rfc-editor.org/info/RFC 8126>.
[RFC 8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC 8152, July 2017,
<https://www.rfc-editor.org/info/RFC 8152>.
[RFC 8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC 8174,
May 2017, <https://www.rfc-editor.org/info/RFC 8174>.
[RFC 8180] 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/RFC 8180,
May 2017, <https://www.rfc-editor.org/info/RFC 8180>.
[RFC 8505] 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/RFC 8505, November 2018,
<https://www.rfc-editor.org/info/RFC 8505>.
[RFC 8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC 8613, July 2019,
<https://www.rfc-editor.org/info/RFC 8613>.
[RFC 8820] Nottingham, M., "URI Design and Ownership", BCP 190,
RFC 8820, DOI 10.17487/RFC 8820, June 2020,
<https://www.rfc-editor.org/info/RFC 8820>.
[RFC 8949] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", STD 94, RFC 8949,
DOI 10.17487/RFC 8949, December 2020,
<https://www.rfc-editor.org/info/RFC 8949>.
[RFC 8974] Hartke, K. and M. Richardson, "Extended Tokens and
Stateless Clients in the Constrained Application Protocol
(CoAP)", RFC 8974, DOI 10.17487/RFC 8974, January 2021,
<https://www.rfc-editor.org/info/RFC 8974>.
[RFC 9030] Thubert, P., Ed., "An Architecture for IPv6 over the Time-
Slotted Channel Hopping Mode of IEEE 802.15.4 (6TiSCH)",
RFC 9030, DOI 10.17487/RFC 9030, May 2021,
<https://www.rfc-editor.org/info/RFC 9030>.
12.2. Informative References
[NIST800-90A]
National Institute of Standards and Technology,
"Recommendation for Random Number Generation Using
Deterministic Random Bit Generators", Special Publication
800-90A, Revision 1, DOI 10.6028/NIST.SP.800-90Ar1, June
2015, <https://doi.org/10.6028/NIST.SP.800-90Ar1>.
[RFC 4231] 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/RFC 4231, December 2005,
<https://www.rfc-editor.org/info/RFC 4231>.
[RFC 4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC 4944, September 2007,
<https://www.rfc-editor.org/info/RFC 4944>.
[RFC 6550] 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/RFC 6550, March 2012,
<https://www.rfc-editor.org/info/RFC 6550>.
[RFC 6762] Cheshire, S. and M. Krochmal, "Multicast DNS", RFC 6762,
DOI 10.17487/RFC 6762, February 2013,
<https://www.rfc-editor.org/info/RFC 6762>.
[RFC 7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
Considerations for IPv6 Address Generation Mechanisms",
RFC 7721, DOI 10.17487/RFC 7721, March 2016,
<https://www.rfc-editor.org/info/RFC 7721>.
[RFC 8415] 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/RFC 8415, November 2018,
<https://www.rfc-editor.org/info/RFC 8415>.
[RFC 8480] Wang, Q., Ed., Vilajosana, X., and T. Watteyne, "6TiSCH
Operation Sublayer (6top) Protocol (6P)", RFC 8480,
DOI 10.17487/RFC 8480, November 2018,
<https://www.rfc-editor.org/info/RFC 8480>.
[RFC 8610] Birkholz, H., Vigano, C., and C. Bormann, "Concise Data
Definition Language (CDDL): A Notational Convention to
Express Concise Binary Object Representation (CBOR) and
JSON Data Structures", RFC 8610, DOI 10.17487/RFC 8610,
June 2019, <https://www.rfc-editor.org/info/RFC 8610>.
[RFC 8615] Nottingham, M., "Well-Known Uniform Resource Identifiers
(URIs)", RFC 8615, DOI 10.17487/RFC 8615, May 2019,
<https://www.rfc-editor.org/info/RFC 8615>.
[RFC 8742] Bormann, C., "Concise Binary Object Representation (CBOR)
Sequences", RFC 8742, DOI 10.17487/RFC 8742, February 2020,
<https://www.rfc-editor.org/info/RFC 8742>.
[RFC 8990] Bormann, C., Carpenter, B., Ed., and B. Liu, Ed., "GeneRic
Autonomic Signaling Protocol (GRASP)", RFC 8990,
DOI 10.17487/RFC 8990, May 2021,
<https://www.rfc-editor.org/info/RFC 8990>.
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 it uses 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
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
the key to decrypt the CoJP object (configuration) present in the
payload. At the pledge, the Join Response is matched to the Join
Request and verified for replay protection using OSCORE processing
rules. In this example, the Join Response does not contain the IPv6
address of the JRC, hence the pledge understands that the JRC is co-
located with the 6LBR.
<-----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:
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 is converted to h'a10542cafe' with a size of
5 bytes.
And the configuration object is the following:
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 the key_addinfo parameter is not present and key_id is
different from 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 is converted to the following:
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 [RFC 5869] and SHA [RFC 4231] 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 to be provisioned to the (6LBR)
pledge.
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 varies for different pledges. 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 level of security comparable to the
scenario in which each pledge is provisioned with a unique PSK. In
this case, there is still a latent risk of the shared PSK being
compromised on the provisioning device, which would compromise all
devices in the batch.
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 Amsüss, Tengfei Chang, Roman Danyliw,
Linda Dunbar, Vijay Gurbani, Klaus Hartke, Barry Leiba, Benjamin
Kaduk, Tero Kivinen, Mirja Kühlewind, John Mattsson, Hilarie Orman,
Alvaro Retana, Adam Roach, Jim Schaad, Göran Selander, Yasuyuki
Tanaka, Pascal Thubert, William Vignat, Xavier Vilajosana, Éric
Vyncke, and Thomas Watteyne.
Authors' Addresses
Mališa Vučinić (editor)
Inria
2 Rue Simone Iff
75012 Paris
France
Email: malisa.vucinic@inria.fr
Jonathan Simon
Analog Devices
32990 Alvarado-Niles Road, Suite 910
Union City, CA 94587
United States of America
Email: jonathan.simon@analog.com
Kris Pister
University of California Berkeley
512 Cory Hall
Berkeley, CA 94720
United States of America
Email: pister@eecs.berkeley.edu
Michael Richardson
Sandelman Software Works
470 Dawson Avenue
Ottawa ON K1Z5V7
Canada
Email: mcr+ietf@sandelman.ca
RFC TOTAL SIZE: 121196 bytes
PUBLICATION DATE: Sunday, May 30th, 2021
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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