wdiff draft-ietf-6tisch-minimal-security-04.txt draft-ietf-6tisch-minimal-security-05.txt

6TiSCH Working Group M. Vucinic, Ed.
Internet-Draft University of Montenegro
Intended status: Standards Track J. Simon
Expires: May 3, September 6, 2018 Analog Devices
K. Pister
University of California Berkeley
M. Richardson
Sandelman Software Works
October 30, 2017
March 05, 2018

Minimal Security Framework for 6TiSCH
draft-ietf-6tisch-minimal-security-04
draft-ietf-6tisch-minimal-security-05

Abstract

This document describes the minimal configuration framework required for a new
device, called "pledge", to securely join a 6TiSCH (IPv6 over the
TSCH mode of IEEE 802.15.4e) network. The entities involved use CoAP
(Constrained Application Protocol) and OSCORE (Object Security for
Constrained RESTful Environments). The configuration 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. The result of Through a single CoAP (Constrained
Application Protocol) request-response exchange secured by OSCORE
(Object Security for Constrained RESTful Environments), the joining process is that pledge
requests admission into the network and the JRC configures the pledge it with
link-layer keying material and a short link-layer address. This
specification also defines the message format, a new Stateless-Proxy CoAP option.
option, and configures 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

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provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on May 3, September 6, 2018.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 4
3. One-Touch Assumption Identifiers . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Pre-Shared Key . 4
4. One-Touch Assumption . . . . . . . . . . . . . . . . . . . . 4
4. 5
5. Join Overview . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1.
5.1. Step 1 - Enhanced Beacon . . . . . . . . . . . . . . . . 6
4.2. 7
5.2. Step 2 - Neighbor Discovery . . . . . . . . . . . . . . . 7
4.3.
5.3. Step 3 - Join Request . . . . . . . . . . . . . . . . . . 7
4.4. 8
5.4. Step 4 - Join Response . . . . . . . . . . . . . . . . . 8
5. Architectural Overview and Communication through Join Proxy . 8
5.1. Stateless-Proxy CoAP Option .
6. Link-layer Configuration . . . . . . . . . . . . . . 9
6. OSCORE Security Context . . . . 9
7. Network-layer Configuration . . . . . . . . . . . . . . . 10
6.1. Persistency . . 9
7.1. Identification of Join Request Traffic . . . . . . . . . 10
7.2. Identification of Join Response Traffic . . . . . . . . . 11
8. Application-level Configuration . . . 11
7. Specification of Join Request . . . . . . . . . . . . 11
8.1. OSCORE Security Context . . . . 11
8. Specification of Join Response . . . . . . . . . . . . . 12
9. 6TiSCH Join Protocol . . 11
8.1. Link-layer Keys Transported in COSE Key Set . . . . . . . 12
8.2. Short Address . . . . . . . . . . . 13
9.1. Specification of the Join Request . . . . . . . . . . . 12
9. Error Handling and Retransmission . 14
9.2. Specification of the Join Response . . . . . . . . . . . 15
9.3. Error Handling and Retransmission . . 13
10. Parameters . . . . . . . . . . 17
9.4. Rekeying and Rejoining . . . . . . . . . . . . . . . 14
11. Mandatory to Implement Algorithms . . 18
9.5. Parameters . . . . . . . . . . . . 14
12. Link-layer Requirements . . . . . . . . . . . 18
9.6. Mandatory to Implement Algorithms . . . . . . . . 14
13. Rekeying and Rejoin . . . . 18
10. Stateless-Proxy CoAP Option . . . . . . . . . . . . . . . . . 15
14. 19
11. Security Considerations . . . . . . . . . . . . . . . . . . . 15
15. 20
12. Privacy Considerations . . . . . . . . . . . . . . . . . . . 16
16. 21
13. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
16.1. 21
13.1. CoAP Option Numbers Registry . . . . . . . . . . . . . . 16
17. 21
14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 17
18. 22
15. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
18.1. 22
15.1. Normative References . . . . . . . . . . . . . . . . . . 17
18.2. 22
15.2. Informative References . . . . . . . . . . . . . . . . . 18 23

Appendix A. Example . . . . . . . . . . . . . . . . . . . . . . 19 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 22 27

1. Introduction

This document presumes a 6TiSCH network as described by [RFC7554],
[RFC8180], [I-D.ietf-6tisch-6top-protocol], [RFC7554] and
[I-D.ietf-6tisch-terminology].
[RFC8180]. By design, nodes in a 6TiSCH network [RFC7554] have their
radio turned off most of the time, to conserve energy. As a
consequence, the link used by a new device for joining the network
has limited bandwidth [RFC8180]. The secure join solution defined in
this document therefore keeps the number of over-
the-air over-the-air exchanges
for join purposes to a minimum.

The micro-controllers at the heart of 6TiSCH nodes have a small
amount of code memory. It is therefore paramount to reuse existing
protocols available as part of the 6TiSCH stack. At the application
layer, the 6TiSCH stack already relies on CoAP [RFC7252] for web
transfer, and on OSCORE [I-D.ietf-core-object-security] for its end-
to-end security. The secure join solution defined in this document
therefore reuses those two protocols as its building blocks.

This document defines a secure join solution for a new device, called
"pledge", to securely join a 6TiSCH network. The specification
configures different layers of the
defines a 6TiSCH protocol stack Join Protocol (6JP) used by the pledge to request
admission into a network managed by the JRC, and also
defines for the JRC to
configure the pledge with the necessary parameters, a new CoAP option.
option, and configures different layers of the 6TiSCH protocol stack
for the join process to occur in a secure manner.

It assumes the presence of a JRC (join registrar/coordinator), a
central entity. It further assumes that the pledge and the JRC share
a symmetric key, called PSK (pre-shared key). The PSK is used to
configure OSCORE to provide a secure channel to 6JP. How the PSK is
installed is out of scope of this document.

When the pledge seeks admission to a 6TiSCH network, it first
synchronizes to it, by initiating the passive scan defined in
[IEEE802.15.4-2015]. The pledge then exchanges messages with the
JRC; these messages can be forwarded by nodes already part of the
6TiSCH network. The messages exchanged allow the JRC and the pledge
to mutually authenticate, based on the PSK. They also allow the JRC
to configure the pledge with link-layer keying material and a short
link-layer address. After this secure joining join process successfully
completes, the joined node can establish an end-to-end secure session
with an Internet host. The joined node can also interact with its neighbors to request
additional bandwidth using the 6top Protocol
[I-D.ietf-6tisch-6top-protocol].
[I-D.ietf-6tisch-6top-protocol] and start sending the application
traffic.

2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. These
words may also appear in this document in lowercase, absent their
normative meanings.

The reader is expected to be familiar with the terms and concepts
defined in [I-D.ietf-6tisch-terminology], [RFC7252],
[I-D.ietf-core-object-security], and [RFC8152].

The specification also includes a set of informative examples using
the CBOR diagnostic notation [I-D.ietf-cbor-cddl].

The following terms defined in [I-D.ietf-6tisch-terminology] are used
extensively throughout this document:

pledge: The new device that wishes to join a 6TiSCH network.

o pledge

o joined node: The new device, after having completed the join
process, often just called a node. node

o join proxy (JP): A node already part of (JP)

o join registrar/coordinator (JRC)

o enhanced beacon (EB)

o join protocol

o join process

In addition, we use the generic terms "network identifier" and
"pledge identifier". See Section 3.

3. Identifiers

The "network identifier" uniquely identifies the 6TiSCH network that
serves as in
the namespace managed by a relay JRC. Typically, this is the 16-bit
Personal Area Network Identifier (PAN ID) defined in
[IEEE802.15.4-2015]. Companion documents can specify the use of a
different network identifier for join purposes, but this is out of
scope of this specification. Such identifier needs to provide connectivity between be carried
within Enhanced Beacon (EB) frames.

The "pledge identifier" uniquely identifies the pledge and in the
namespace managed by a JRC.

join registrar/coordinator (JRC): A central entity responsible for The pledge identifier is typically the authentication, authorization
globally unique 64-bit Extended Unique Identifier (EUI-64) of the
IEEE 802.15.4 device. This identifier is used to generate the IPv6
addresses of the pledge and configuration to identify it during the execution of
the pledge.

3. join protocol. For privacy reasons, it is possible to use an
identifier different from the EUI-64 (e.g. a random string). See
Section 12.

4. One-Touch Assumption

This document assumes a one-touch scenario. The pledge is
provisioned with a PSK certain parameters before attempting to join the
network, and the same PSK (as well as the uniquer identifier of the pledge) is parameters are provisioned on to the JRC.

There are many ways by which this provisioning can be done.
Physically, the PSK parameters can be written into the pledge using a
number of mechanisms, such as a JTAG interface, a serial (craft)
console interface, pushing buttons simultaneously on different
devices, over-
the-air over-the-air configuration in a Faraday cage, etc. The
provisioning can be done by the vendor, the manufacturer, the
integrator, etc.

Details of how this provisioning is done is out of scope of this
document. What is assumed is that there can be a secure, private
conversation between the JRC and the pledge, and that the two devices
can exchange the PSK.

3.1. parameters.

Parameters that are provisioned to the pledge include:

o Pre-Shared Key (PSK). The JRC additionally needs to store the
identifier of the pledge bound to the given PSK. The PSK SHOULD
be at least 128 bits in length, generated uniformly at random. It
is RECOMMENDED to generate the PSK with a cryptographically secure
pseudorandom number generator. Each pledge SHOULD be provisioned
with a unique PSK.

o Optionally, a network identifier. Provisioning the network
identifier to the pledge is RECOMMENDED, as it significantly
speeds up the join process. In case this parameter is not
provisioned, the pledge attempts to join one network at a time.

o Optionally, any non-default algorithms. Mandatory to implement
and default algorithms are specified in Section 9.6.

5. Join 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
[IEEE802.15.4-2015]. This frame provides network synchronization
information, and tells the device when it can send a frame to the
node sending the beacons, which plays the role of join proxy (JP)
for the pledge, and when it can expect to receive a frame.

2. The pledge configures its link-local IPv6 address and advertises
it to the join proxy (JP).

3. The pledge sends a Join Request to the JP in order to securely
identify itself to the network. The Join Request is directed to
the JRC, which may be co-located on the JP or another device.

4. In case of successful processing of the request, the pledge
receives a join response from JRC (via the JP) that sets up one
or more link-layer keys used to authenticate and encrypt
subsequent transmissions to peers, and a short link-layer address
for the pledge.

From the pledge's perspective, minimal joining is a local phenomenon - the
pledge only interacts with the JP, and it need 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 (3)-----|------Join Request (3a)-->| \
| | | | 6JP
|<---Join Response (4)-----|-----Join Response (4a)---| /
| | |

Figure 1: Overview of a successful join process. 6JP stands for
6TiSCH Join Protocol.

The details of each step are described in the following sections.

4.1.

5.1. Step 1 - Enhanced Beacon

The pledge synchronizes to the network by listening for, and
receiving, an Enhanced Beacon (EB) sent by a node already in the
network. This process is entirely defined by [IEEE802.15.4-2015],
and described in [RFC7554].

Once the pledge hears an EB, it synchronizes to the joining schedule
using the cells contained in the EB. The pledge can hear multiple
EBs; the selection of which EB to use is out of the scope for this
document, and is discussed in [RFC7554]. Implementers SHOULD should make
use of information such as: what Personal Area Network Identifier
(PAN ID) [IEEE802.15.4-2015] network identifier the EB contains,
whether the source link-layer address of the EB has been tried
before, what signal strength the different EBs were received at, etc.
In addition, the pledge may be pre-configured to search for EBs with
a specific PAN
ID. 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 9.3.

Once the pledge selects the EB, it synchronizes to it and transitions
into a low-power mode. It deeply duty cycles its radio, switching
the radio on when the provided schedule indicates slots which 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 plays the
role of JP.

At this point, the pledge may proceed to step 2, or continue to
listen for additional EBs.

4.2.

5.2. Step 2 - Neighbor Discovery

The pledge forms its link-local IPv6 address based on EUI-64, the interface
identifier, as per [RFC4944]. The Neighbor Discovery exchange shown in Figure 1 refers
to a single round trip pledge MAY perform the Neighbor
Solicitation / Neighbor Advertisement exchange between the pledge and with the JP (Section JP, as per
Section 5.5.1 of [RFC6775]). [RFC6775]. The pledge uses and the link-local JP use their link-
local IPv6 address addresses for all subsequent communication with the JP during the join
process.

Note that ND Neighbor Discovery exchanges at this point are not
protected with link-
layer link-layer security as the pledge is not in possession
of the keys. How JP accepts these unprotected frames is discussed in
Section 12. 6.

5.3. Step 3 - Join Request

The pledge and the JP SHOULD keep Join Request is a separate neighbor cache for
untrusted entries and use it to store each other's information during message sent from the join process. Mixing neighbor entries belonging pledge to pledges and
nodes that are part of the network opens up JP, and
which the JP forwards to a DoS attack.
How the pledge and JRC. The JP decide to transition each other from untrusted
to trusted cache, once the join process completes, is out of scope.
One implementation technique is to use the information whether the
incoming frames are secured at the link layer.

4.3. Step 3 - Join Request

The Join Request is a message sent from the pledge to the JP using
the shared slot as described in the EB, and which the JP forwards to
the JRC. The JP forwards the Join Request forwards the Join Request
to the JRC on the existing 6TiSCH network. How exactly this happens
is out of scope of this document; some networks may wish to dedicate
specific slots for this join traffic.

The Join Request is authenticated/encrypted end-to-end using an AEAD
(Authenticated Encryption with Associated Data) algorithm from
[RFC8152] and a key derived from the PSK, the pledge's
EUI-64 pledge identifier and a
request-specific constant value. Algorithms which MUST be
implemented are specified in Section 11. 9.6.

The nonce used when securing the Join Request is derived from the
PSK, the pledge's EUI-64 pledge identifier and a monotonically increasing counter
initialized to 0 when first starting.

Join Request construction message is specified in Section 7, 9.1, while the details
on security processing can be found in Section 7 of
[I-D.ietf-core-object-security].

4.4.

5.4. Step 4 - Join Response

The Join Response is sent by the JRC to the pledge, and is forwarded
through the JP as it serves as a stateless relay. The packet
containing the Join Response travels from the JRC to JP using the
operating routes in the 6TiSCH network. The JP delivers it to the
pledge using the slot information it has indicated in the EB it sent.
pledge. The JP operates as the application-layer proxy, and does not
keep any state to relay the message. It uses information sent in the
clear within the Join Response to decide where to forward to.

The Join Response is authenticated/encrypted end-to-end using an AEAD
algorithm from [RFC8152]. The key used to protect the response is
different from the one used to protect the request (both are derived
from the PSK, as explained in Section 6). 8.1). The response is
protected using the same nonce as in the request.

The Join Response contains one or more link-layer key(s) that the
pledge will use for subsequent communication. Each key that is
provided by the JRC is associated with an 802.15.4 key identifier.
In other link-layer technologies, a different identifier may be
substituted. The Join Response also contains an IEEE 802.15.4 short
address [IEEE802.15.4-2015] assigned by the JRC to the pledge, and
optionally the IPv6 address of the JRC.

Join Response construction message is specified in Section 8, 9.2, while the details
on security processing can be found in Section 7 of
[I-D.ietf-core-object-security].

5. Architectural Overview and Communication through Join Proxy

6. Link-layer Configuration

In an operational 6TiSCH network, all frames MUST use link-layer
frame security [RFC8180]. The Join Request/Join Response exchange in Figure 1 is carried over
CoAP [RFC7252] IEEE 802.15.4 security attributes MUST
include frame authenticity, and secured using OSCORE
[I-D.ietf-core-object-security]. The pledge plays MAY include frame confidentiality
(i.e. encryption).

As specified in [RFC8180], the role of network uses a CoAP
client; the JRC plays the role of key termed as K1 to
authenticate EBs and a CoAP server. key termed as K2 to authenticate and
optionally encrypt DATA and ACKNOWLEDGMENT frames. The JP implements
CoAP forward proxy functionality [RFC7252]. Because the JP can also keys K1 and
K2 MAY be a constrained device, it cannot implement a cache. Rather, the JP
processes forwarding-related CoAP options same key (same value and makes requests on
behalf of IEEE 802.15.4 index). How the pledge, in a stateless manner.

The pledge
JRC communicates with a JP over link-local IPv6 addresses. these keys to 6LBR is out of scope of this
specification.

The pledge designates a JP does not initially do any authenticity check of the EB
frames, as a proxy by including it does not know the Proxy-Scheme
option with value "coap" (CoAP-to-CoAP proxy) in CoAP requests it
sends to the JP. K1 key. The pledge MUST include the Uri-Host option with
its value set is still able to
parse the well-known JRC's alias "6tisch.arpa". This
allows contents of the pledge received EBs and synchronize to the
network, as EBs are not encrypted [RFC8180].

When sending frames during the join without knowing process, the IPv6 address pledge sends
unencrypted and unauthenticated frames. The JP accepts these frames
(using the "exempt mode" in 802.15.4) for the duration of the
JRC. The pledge join
process. How the JP learns whether the actual IPv6 address join process is ongoing is
out of scope of this specification.

As the JRC from EB itself cannot be authenticated by the
Join Response; it uses it once joined in order to operate as pledge, an attacker
may craft a JP.

The JRC can frame that appears to be co-located on the 6LBR. Before a valid EB, since the 6TiSCH network is
started, pledge can
neither know the 6LBR MUST be provisioned with ASN a priori nor verify the IPv6 address of the
JRC.

5.1. Stateless-Proxy CoAP Option

The CoAP proxy defined JP. This
opens up a possibility of DoS attack, as discussed in [RFC7252] keeps per-client state
information Section 11.
Beacon authentication keys are discussed in order to forward [RFC8180].

7. Network-layer Configuration

The pledge and the response towards JP SHOULD keep a separate neighbor cache for
untrusted entries and use it to store each other's information during
the originator join process. Mixing neighbor entries belonging to pledges and
nodes that are part of the request. This state information includes at least network opens up the CoAP
token, JP to a DoS attack.
How the IPv6 address of the host, and the UDP source port number.
If pledge and the JP used the stateful CoAP proxy defined in [RFC7252], it would
be prone decide to Denial-of-Service (DoS) attacks, due transition each other from
untrusted to its limited
memory.

The Stateless-Proxy CoAP option Figure 2 allows trusted cache, once the JP join process completes, is out
of scope. One implementation technique is to be entirely
stateless. This option inserts, in the request, use the state information needed for relaying
whether the response back to incoming frames are secured at the client. The
proxy still keeps some general state (e.g. for congestion control or
request retransmission), but no per-client state. link layer.

The Stateless-Proxy CoAP option is critical, Safe-to-Forward, pledge does not
part communicate with the JRC at the network layer.
This allows the pledge to join without knowing the IPv6 address of
the cache key, not repeatable JRC. Instead, the pledge communicates with the JP at the network
layer, and opaque. When processed by
OSCORE, with the Stateless-Proxy option is neither encrypted nor integrity
protected.

+-----+---+---+---+---+-----------------+--------+--------+
| No. | C | U | N | R | Name | Format | Length |
+-----+---+---+---+---+-----------------+--------+--------|
| TBD | x | | x | | Stateless-Proxy | opaque | 1-255 |
+-----+---+---+---+---+-----------------+--------+--------+
C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable

Figure 2: Stateless-Proxy CoAP Option

Upon reception of a Stateless-Proxy option, JRC at the CoAP server MUST echo
it application layer, as specified in the response.
Section 8.

The value of the Stateless-Proxy option is
internal proxy state that is opaque to JP communicates with the server. Example state
information includes JRC over global IPv6 addresses. The JP
discovers the network prefix and configures its global IPv6 address
upon successful completion of the client, its UDP source
port, join process and the CoAP token. For security reasons, the state
information MUST be authenticated, MUST include a freshness indicator
(e.g. a sequence number or timestamp) and MAY be encrypted. obtention of
link-layer keys. The
proxy may use an appropriate COSE structure [RFC8152] to wrap the
state information as pledge learns the value actual IPv6 address of the Stateless-Proxy option. The
key used for encryption/authentication of
JRC from the state information may
be known only Join Response, as specified in Section 9.2; it uses it
once joined in order to operate as a JP.

The JRC can be co-located on the proxy.

Once the proxy has received 6LBR. In this special case, the CoAP response with Stateless-Proxy
option present, it decrypts/authenticates it, checks
IPv6 address of the freshness
indicator and constructs JRC can be omitted from the response Join Response message
for space optimization. The 6LBR then MUST set the client, based on the
information present DODAGID field in the option value.

Note that a CoAP proxy using the Stateless-Proxy option is not able
RPL DIOs [RFC6550] to return a 5.04 Gateway Timeout Response Code in case its IPv6 address. The pledge learns the request to
address of the server times out. Likewise, if JRC once joined and upon the response reception of a first RPL
DIO message, and uses it to operate as a JP.

Before the proxy's
request does not contain the Stateless-Proxy option, for example when
the option is not supported by the server, the proxy 6TiSCH network is not able to
return the response to started, the client.

6. OSCORE Security Context

The OSCORE security context 6LBR MUST be derived at provisioned
with the pledge and IPv6 address of the JRC
as per Section 3 JRC.

7.1. Identification of [I-D.ietf-core-object-security].

o the Master Secret MUST be the PSK.

o Join Request Traffic

The join request traffic that is proxied by the Master Salt MUST Join Proxy comes from
unauthenticated nodes, and there may be pledge's EUI-64.

o the Sender ID an arbitrary amount of the pledge MUST be set it.
In particular, an attacker may send fraudulent traffic in attempt to byte string 0x00.

o
overwhelm the Recipient ID (ID network.

When operating as part of a [RFC8180] 6TiSCH minimal network using
reasonable scheduling algorithms, the JRC) MUST be set join request traffic present
may cause intermediate nodes to byte string 0x01.

o the Algorithm MUST be set request additional bandwidth. An
attacker could use this property to cause the value from [RFC8152], agreed out-
of-band by the same mechanism used network to overcommit
bandwidth (and energy) to provision the PSK. join process.

The
default is AES-CCM-16-64-128.

o the Key derivation function MUST be agreed out-of-band. Default Join Proxy is HKDF SHA-256.

The derivation in [I-D.ietf-core-object-security] results in aware of what traffic
keys is join request traffic, and
so can avoid allocating additional bandwidth itself. The Join Proxy
SHOULD implement a common IV for each side of the conversation. Nonces are
constructed by XOR'ing the common IV with bandwidth cap on outgoing join request traffic.
This cap will not protect intermediate nodes as they can not tell
join request traffic from regular traffic. Despite the current sequence number
and sender identifier. For details bandwidth cap
implemented separately on nonce construction, refer each Join Proxy, the aggregate join request
traffic from many Join Proxies may cause intermediate nodes to
[I-D.ietf-core-object-security]. decide
to allocate additional cells. It is RECOMMENDED that a PAN ID be provisioned undesirable to to so in response
to the pledge out-of-
band by join request traffic. In order to permit the same mechanism used intermediate
nodes to provision avoid this, the PSK. This prevents
the pledge from attempting traffic needs to join be tagged in some way.

[RFC2597] defines a wrong network. If the pledge is
not provisioned with set of per-hop behaviors that may be encoded into
the PAN ID, it Diffserv Code Points (DSCPs). The Join Proxy SHOULD attempt to set the DSCP
of join one
network at a time. In that case, implementations MUST ensure request packets that
multiple CoAP requests to different JRCs result in it produces as part of the use relay process
to AF43 code point (See Section 6 of [RFC2597]).

A Join Proxy that does not set the
same OSCORE context DSCP on traffic forwarded should
set it to zero so that sequence numbers are properly incremented
for each request.

6.1. Persistency

Implementations MUST ensure that mutable OSCORE context parameters
(Sender Sequence Number, Replay Window) are stored in persistent
memory. it is compressed out.

A technique that prevents reuse Scheduling Function (SF) running on 6TiSCH nodes SHOULD NOT
allocate additional cells as a result of sequence numbers,
detailed in Section 6.5.1 traffic with code point
AF43. Companion SF documents SHOULD specify how this recommended
behavior is achieved.

7.2. Identification of [I-D.ietf-core-object-security], MUST be
implemented. Each update Join Response Traffic

The JRC SHOULD set the DSCP of join response packets addressed to the OSCORE Replay Window MUST be written
Join Proxy to persistent memory.

This is AF42 code point. Join response traffic can not be
induced by an important security requirement attacker as it is generated only in order to guarantee nonce
uniqueness and resistance response to replay attacks across reboots and
rejoins. Traffic between the pledge and
legitimate pledges (see Section 9.3). AF42 has lower drop
probability than AF43, giving join response traffic priority in
buffers over join request traffic.

When the JRC is rare, making
security outweigh not co-located with the cost of writing 6LBR, then the code point
provides a clear indication to persistent memory.

7. Specification of Join Request

The Join Request the pledge sends SHALL be mapped to a CoAP request:

o The request method is POST.

o The type is Non-confirmable (NON).

o The Proxy-Scheme option 6LBR that this is set join response
traffic.

Due to "coap".

o The Uri-Host option the convergecast nature of the DODAG, the 6LBR links are often
the most congested, and from that point down there is set progressively
less (or equal) congestion. If the 6LBR paces itself when sending
join response traffic then it ought to "6tisch.arpa".

o The Uri-Path option never exceed the bandwidth
allocated to the best effort traffic cells. If the 6LBR has the
capacity (if it is set not constrained) then it should provide some
buffers in order to "j".

o The Object-Security option SHALL be set according satisfy the Assured Forwarding behavior.

Companion SF documents SHOULD specify how traffic with code point
AF42 is handled with respect to
[I-D.ietf-core-object-security]. cell allocation.

8. Application-level Configuration

The Join Request/Join Response exchange in Figure 1 is carried over
CoAP [RFC7252] and secured using OSCORE Context Hint SHALL be
set to pledge's EUI-64.
[I-D.ietf-core-object-security]. The OSCORE Context Hint allows pledge plays the role of a CoAP
client; the JRC to
retrieve plays the security context for role of a given pledge.

o CoAP server. The payload is empty.

8. Specification of Join Response

If JP implements
CoAP forward proxy functionality [RFC7252]. Because the JRC successfully JP can also
be a constrained device, it cannot implement a cache. Rather, the JP
processes forwarding-related CoAP options and makes requests on
behalf of the Join Request pledge, in a stateless manner by using OSCORE, and
if the Stateless-
Proxy option defined in this document.

The pledge is authorized to join the network, the Join Response designates a JP as a proxy by including the JRC Proxy-Scheme
option in CoAP requests it sends back to the pledge SHALL be mapped to a CoAP response:

o JP. The response Code is 2.04 (Changed).

o The payload is a CBOR [RFC7049] array containing, pledge also includes
in order:

* the COSE Key Set, specified in [RFC8152], containing one or
more link-layer keys. The mapping of individual keys to
802.15.4-specific parameters is described in Section 8.1.

* requests the link-layer short address Uri-Host option with its value set to be used by the pledge. well-
known JRC's alias, as specified in Section 9.1.

The
format of JP resolves the short address follows Section 8.2.

* optionally, alias to the IPv6 address of the JRC transported that it
learned when it acted as a byte
string. If the IPv6 address of pledge, and joined the JRC is not present in network. This
allows the
Join Response, this indicates JP to reach the JRC is co-located with 6LBR, at the network layer and has forward the same IPv6 address as
requests on behalf of the 6LBR. pledge.

The address of JP MUST add a Stateless-Proxy option to all the
6LBR can then be learned from DODAGID field in RPL DIOs
[RFC6550].

response_payload = [
COSE_KeySet,
short_address,
? JRC_address : bstr,
]

8.1. Link-layer Keys Transported in COSE Key Set

Each key in requests that it
forwards on behalf of the COSE Key Set [RFC8152] SHALL be a symmetric key. If pledge as part of the "kid" parameter join process.

The value of the COSE Key structure Stateless-Proxy option is present, the
corresponding keys SHALL belong set to an IEEE 802.15.4 KeyIdMode 0x01
class. In that case, parameter "kid" of the COSE Key structure SHALL
be used internal JP
state, needed to carry forward the IEEE 802.15.4 KeyIndex value. If Join Response message to the "kid"
parameter pledge.
The Stateless-Proxy option handling is not present defined in Section 10.

The JP also tags all packets carrying the transported key, Join Request message at the application
SHALL consider
network layer, as specified in Section 7.1.

8.1. OSCORE Security Context

Before the key pledge and the JRC may start exchanging CoAP messages
protected with OSCORE, they need to derive the OSCORE security
context from the parameters provisioned out-of-band, as discussed in
Section 4.

The OSCORE security context MUST be an IEEE 802.15.4 KeyIdMode 0x00
(implicit) key. This document does not support IEEE 802.15.4
KeyIdMode 0x02 derived at the pledge and 0x03 class keys.

8.2. Short Address

The "short_address" structure transported the JRC
as part per Section 3 of [I-D.ietf-core-object-security].

o the join
response payload represents the IEEE 802.15.4 short address assigned
to Master Secret MUST be the pledge. It is encoded as a CBOR array object, containing, in
order: PSK.

o Byte string, containing the 16-bit address. Master Salt MUST be the pledge identifier.

o Optionally, the lease time parameter, "lease_asn". The value Sender ID of the "lease_asn" parameter is the 5-byte Absolute Slot Number (ASN)
corresponding pledge MUST be set to its expiration, carried as a byte string in
network byte order.

short_address = [
address : bstr,
? lease_asn : bstr,
]
It is up 0x00.

o the Recipient ID (ID of the JRC) MUST be set to byte string 0x01.

o the joined node Algorithm MUST be set to request a new short address before the
expiry of its previous address. The mechanism value from [RFC8152], agreed out-
of-band by which the node
requests renewal is the same as during join procedure, as described
in Section 13. The assigned short address is mechanism used for configuring
both link-layer short address and IPv6 addresses.

9. Error Handling and Retransmission

Since to provision the Join Request PSK. The
default is mapped to a Non-confirmable CoAP message,
OSCORE processing at JRC will silently drop AES-CCM-16-64-128.

o the request Key Derivation Function MUST be agreed out-of-band. Default
is HKDF SHA-256 [RFC5869].

The derivation in case of [I-D.ietf-core-object-security] results in traffic
keys and a
failure. This may happen common IV for a number of reasons, including failed
lookup each side of an appropriate security context, failed decryption,
positive replay window lookup, formatting errors possibly due the conversation. Nonces are
constructed by XOR'ing the common IV with the current sequence number
and sender identifier. For details on nonce construction, refer to
malicious alterations
[I-D.ietf-core-object-security].

Implementations MUST ensure that multiple CoAP requests to different
JRCs result in transit. Silent drop at JRC prevents a DoS
attack where an attacker could force the use of the same OSCORE context, so that the
sequence numbers are properly incremented for each request. The
pledge typically sends requests to attempt joining different JRCs if it is not
provisioned with the network identifier and attempts to join one
network at a time, until all networks have been tried.

Using Non-confirmable CoAP message time. A simple implementation technique is to transport Join Request also
helps minimize the required CoAP state at
instantiate the pledge OSCORE security context with a given PSK only once
and the Join
Proxy, keeping use it for all subsequent requests. Failure to a minimum typically needed to perform CoAP
congestion control. It does, however, introduce complexity at comply will break
the
application layer, as confidentiality property of the pledge needs to implement a retransmission
mechanism.

The following binary exponential back-off AEAD algorithm is inspired by
the one described in [RFC7252]. For each Join Request the pledge
sends while waiting for a Join Response, due to the pledge nonce
reuse.

8.1.1. Persistency

Implementations MUST keep track
of a timeout and a retransmission counter. For a new Join Request, ensure that mutable OSCORE context parameters
(Sender Sequence Number, Replay Window) are stored in persistent
memory. A technique that prevents reuse of sequence numbers,
detailed in Section 6.5.1 of [I-D.ietf-core-object-security], MUST be
implemented. Each update of the timeout OSCORE Replay Window MUST be written
to persistent memory.

This is set an important security requirement in order to a random value between TIMEOUT and (TIMEOUT *
TIMEOUT_RANDOM_FACTOR), guarantee nonce
uniqueness and the retransmission counter is set resistance to 0.
When replay attacks across reboots and
rejoins. Traffic between the timeout is triggered pledge and the retransmission counter JRC is less
than MAX_RETRANSMIT, rare, making
security outweigh the cost of writing to persistent memory.

9. 6TiSCH Join Request Protocol

6TiSCH Join Protocol (6JP) is retransmitted, a lightweight protocol over CoAP
[RFC7252] and a secure channel provided by OSCORE
[I-D.ietf-core-object-security]. 6JP allows the
retransmission counter is incremented, pledge to request
admission into a network managed by the JRC, and for the timeout is doubled.
Note that JRC to
configure the retransmitted Join Request passes new OSCORE
processing, such that pledge with the sequence number parameters necessary for joining the
network. These parameters are: link-layer keys in use, IEEE 802.15.4
short address assigned to the OSCORE context is
properly incremented. If pledge, and the retransmission counter reaches
MAX_RETRANSMIT IPv6 address of the
JRC.

This section specifies the 6JP bindings to CoAP and OSCORE, 6JP
message formats and the semantics of different fields.

6JP relies on a timeout, the pledge SHOULD attempt security properties provided by OSCORE. This
includes end-to-end confidentiality, data authenticity, replay
protection, and a secure binding of responses to join requests.

+-----------------------------------+
| 6TiSCH Join Protocol (6JP) |
+-----------------------------------+
+-----------------------------------+ \
| Requests / Responses | |
|-----------------------------------| |
| OSCORE | | CoAP
|-----------------------------------| |
| Messaging Layer / Message Framing | |
+-----------------------------------+ /
+-----------------------------------+
| UDP |
+-----------------------------------+

Figure 2: Abstract layering of 6JP.

6JP consists of two messages:

o the Join Request message, sent by the pledge to the JRC, proxied
by the JP. The Join Request message and its mapping to CoAP is
specified in Section 9.1.

o the Join Response message, sent by the JRC to the 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 pledge is authorized to join the network.
The Join Response message is proxied by the JP. The Join Response
message and its mapping to CoAP is specified in Section 9.2.

The payload of 6JP messages is encoded with CBOR [RFC7049], with some
parameters being optional. The first byte of the CBOR-encoded byte
string contains the CBOR major type and additional information (e.g.
the number of elements in an array). In case of an array, the CBOR
decoder decides based on this additional information if a certain
optional parameter is present or not.

9.1. Specification of the Join Request

The Join Request the pledge sends SHALL be mapped to a CoAP request:

o The request method is POST.

o The type is Non-confirmable (NON).

o The Proxy-Scheme option is set to "coap".

o The Uri-Host option is set to "6tisch.arpa".

o The Uri-Path option is set to "j".

o The Object-Security option SHALL be set according to
[I-D.ietf-core-object-security]. The OSCORE security context used
is the one derived in Section 8.1. The OSCORE Context Hint SHALL
be set to the pledge identifier. The OSCORE Context Hint allows
the JRC to retrieve the security context for a given pledge.

o The payload is a CBOR array [RFC7049] containing, in order:

* Byte string, containing the identifier of the network that the
pledge is attempting to join. This enables the JRC to manage
multiple 6TiSCH networks.

request_payload = [
network_identifier : bstr,
]

9.2. Specification of the Join Response

The Join Response the JRC sends SHALL be mapped to a CoAP response:

o The response Code is 2.04 (Changed).

o The payload is a CBOR array [RFC7049] containing, in order:

* the COSE Key Set, specified in [RFC8152], containing one or
more link-layer keys. The mapping of individual keys to
802.15.4-specific parameters is described in Section 9.2.1.

* the link-layer short address to be used by the pledge. The
format of the short address follows Section 9.2.2.

* optionally, the IPv6 address of the JRC, encoded as a byte
string, with the length of 16 bytes. If the IPv6 address of
the JRC is not present in the Join Response, this indicates the
JRC is co-located with the 6LBR, and has the same IPv6 address
as the 6LBR. See Section 7.

response_payload = [
COSE_KeySet,
short_address,
? JRC_address : bstr,
]

9.2.1. Link-layer Keys Transported in the COSE Key Set

Each key in the COSE Key Set [RFC8152] SHALL be a symmetric key. The
first key in the COSE Key Set SHALL be used as the K1 key from
[RFC8180]. The second key in the COSE Key Set SHALL be used as the
K2 key from [RFC8180]. In the case where the network uses the same
key for K1 and K2, the COSE Key Set SHALL carry a single key.

If the COSE Key Set carries more than 2 keys, the implementation
SHOULD consider the response as malformed.

If the "kid" parameter of the COSE Key structure is present, the
corresponding key SHALL be used as IEEE 802.15.4 KeyIdMode 0x01
(index). In that case, parameter "kid" of the COSE Key structure
SHALL be used to carry the IEEE 802.15.4 KeyIndex value.

If the length of the "kid" parameter is more than 1 byte (length
defined by [IEEE802.15.4-2015]), the implementation SHOULD consider
the response as malformed.

If the "kid" parameter is not present in the transported key, the
implementation SHALL consider the key to be an IEEE 802.15.4
KeyIdMode 0x00 (implicit) key.

This document does not support IEEE 802.15.4 KeyIdMode 0x02 and 0x03
class keys. In the case that the response is considered malformed,
the implementation SHOULD indicate to the user through an out-of-band
mechanism the presence of an error condition.

9.2.2. Short Address

The "short_address" structure transported as part of the join
response payload represents the IEEE 802.15.4 short address assigned
to the pledge. It is encoded as a CBOR array object, containing, in
order:

o Byte string, containing the 16-bit address.

o Optionally, the lease time parameter, "lease_asn". The value of
the "lease_asn" parameter is the 5-byte Absolute Slot Number (ASN)
corresponding to its expiration, carried as a byte string in
network byte order.

short_address = [
address : bstr,
? lease_asn : bstr,
]
It is up to the joined node to request a new short address before the
expiry of its previous address. The mechanism by which the node
requests renewal is the same as during join procedure, as described
in Section 9.4.

9.3. Error Handling and Retransmission

Since the Join Request is mapped to a Non-confirmable CoAP message,
OSCORE processing at the JRC will silently drop the request in case
of a failure. This 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 the Join Request at the
JRC prevents a DoS attack where an attacker could force the pledge to
attempt joining one network at a time, until all networks have been
tried.

Using a Non-confirmable CoAP message to transport the Join Request
also helps minimize the required CoAP state at the pledge and the
Join Proxy, keeping it to a minimum typically needed to perform CoAP
congestion control. It does, however, introduce some complexity as
the pledge needs to implement a retransmission mechanism.

The following binary exponential back-off algorithm is inspired by
the one described in [RFC7252]. For each Join Request the pledge
sends while waiting for a Join Response, the pledge MUST keep track
of a timeout and a retransmission counter. For a new Join Request,
the timeout is set to a random value between TIMEOUT_BASE and
(TIMEOUT_BASE * TIMEOUT_RANDOM_FACTOR), and the retransmission
counter is set to 0. When the timeout is triggered and the
retransmission counter is less than MAX_RETRANSMIT, the Join Request
is retransmitted, the retransmission counter is incremented, and the
timeout is doubled. Note that the retransmitted Join Request passes
new OSCORE processing, such that the sequence number in the OSCORE
context is properly incremented. If the retransmission counter
reaches MAX_RETRANSMIT on a timeout, the pledge SHOULD attempt to
join the next advertised 6TiSCH network. If the pledge receives a
Join Response that successfully passes OSCORE processing, it cancels
the pending timeout and processes the response. The pledge MUST
silently discard any response not protected with OSCORE, including
error codes. For default values of retransmission parameters, see
Section 9.5.

If all join attempts to advertised networks have failed, the pledge
SHOULD signal to the user the presence of an error condition, through
some out-of-band mechanism.

9.4. Rekeying and Rejoining

This specification handles initial keying of the pledge. For reasons
such as rejoining after a long sleep, expiry of the short address, or
node-initiated rekeying, the joined node MAY send a new Join Request
using the already-established OSCORE security context. The JRC then
responds with up-to-date keys and a (possibly new) short address.
How the joined node decides when to rekey is out of scope of this
document. Mechanisms for rekeying the network are defined in
companion specifications.

9.5. Parameters

6JP uses the following parameters:

+-----------------------+----------------+
| Name | Default Value |
+-----------------------+----------------+
| TIMEOUT_BASE | 10 s |
+-----------------------+----------------+
| TIMEOUT_RANDOM_FACTOR | 1.5 |
+-----------------------+----------------+
| MAX_RETRANSMIT | 4 |
+----------------------------------------+

The values of TIMEOUT_BASE, TIMEOUT_RANDOM_FACTOR, MAX_RETRANSMIT 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.

9.6. Mandatory to Implement Algorithms

The mandatory to implement AEAD algorithm for use with OSCORE is AES-
CCM-16-64-128 from [RFC8152]. This is the algorithm used for
securing 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 [RFC4231].

The mandatory to implement key derivation function is HKDF [RFC5869],
instantiated with a SHA-256 hash.

10. Stateless-Proxy CoAP Option

The CoAP proxy defined in [RFC7252] keeps per-client state
information in order to forward the response towards the originator
of the request. This state information includes at least the
next advertised 6TiSCH network. If CoAP
token, the pledge receives a Join
Response that successfully passed OSCORE processing, it cancels IPv6 address of the
pending timeout host, and processes the response. The pledge MUST silently
discard any response not protected with OSCORE, including error
codes. For default values of retransmission parameters, see
Section 10. UDP source port number.
If all join attempts the JP used the stateful CoAP proxy defined in [RFC7252], it would
be prone to advertised networks have failed, Denial-of-Service (DoS) attacks, due to its limited
memory.

The Stateless-Proxy CoAP option Figure 3 allows the pledge
SHOULD signal JP to be entirely
stateless. This option inserts, in the user request, the presence of an error condition, through state
information needed for relaying the response back to the client. The
proxy still keeps some out-of-band mechanism.

10. Parameters

This specification uses general state (e.g. for congestion control or
request retransmission), but no per-client state.

The Stateless-Proxy CoAP option is critical, Safe-to-Forward, not
part of the following parameters:

+-----------------------+----------------+ cache key, not repeatable and opaque. When processed by
OSCORE, the Stateless-Proxy option is neither encrypted nor integrity
protected.

+-----+---+---+---+---+-----------------+--------+--------+
| No. | C | U | N | R | Name | Default Value Format |
+-----------------------+----------------+ Length | TIMEOUT
+-----+---+---+---+---+-----------------+--------+--------|
| 10 s TBD |
+-----------------------+----------------+ x | TIMEOUT_RANDOM_FACTOR | 1.5 x |
+-----------------------+----------------+ | MAX_RETRANSMIT Stateless-Proxy | 4 opaque |
+----------------------------------------+

The values of TIMEOUT, TIMEOUT_RANDOM_FACTOR, MAX_RETRANSMIT 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.

11. Mandatory to Implement Algorithms 1-255 |
+-----+---+---+---+---+-----------------+--------+--------+
C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable

Figure 3: Stateless-Proxy CoAP Option

Upon reception of a Stateless-Proxy option, the CoAP server MUST echo
it in the response. The mandatory to implement AEAD algorithm for use with OSCORE value of the Stateless-Proxy option is AES-
CCM-16-64-128 from [RFC8152]. This
internal proxy state that is opaque to the algorithm used for
securing 802.15.4 frames, server. Example state
information includes the IPv6 address of the client, its UDP source
port, and hardware acceleration for it is present
in virtually all compliant radio chips. With this choice, the CoAP
messages are protected with an 8-byte CCM authentication tag, and token. For security reasons, the
algorithm uses 13-byte long nonces.

The mandatory to implement hash algorithm is SHA-256 [RFC4231].

12. Link-layer Requirements

In an operational 6TiSCH network, all frames state
information MUST use link-layer
frame security [RFC8180]. The frame security options be authenticated, MUST include
frame authentication, a freshness indicator
(e.g. a sequence number or timestamp) and MAY include frame encryption. be encrypted. The pledge does not initially do any authentication of
proxy may use an appropriate COSE structure [RFC8152] to wrap the EB frames,
state information as it does not know the K1 key [RFC8180]. When sending frames, value of the
pledge sends unencrypted and unauthenticated frames. Stateless-Proxy option. The JP accepts
these frames (using the "exempt mode" in 802.15.4)
key used for the duration
of the join process. How the JP learns whether the join process is
ongoing is out of scope encryption/authentication of this specification.

As the EB itself cannot be authenticated by the pledge, an attacker state information may craft a frame that appears to
be a valid EB, since the pledge can
neither know known only to the ASN a priori nor verify proxy.

Once the address of proxy has received the JP. This
opens up CoAP response with a possibility of DoS attack, as discussed in Section 14.
Beacon authentication keys are discussed in [RFC8180].

13. Rekeying Stateless-Proxy
option present, it decrypts/authenticates it, checks the freshness
indicator and Rejoin

This specification handles initial keying of constructs the pledge. For reasons
such as rejoining after a long sleep, expiry of response for the short address, or
node-initiated rekeying, client, based on the joined node MAY send
information present in the option value.

Note that a new Join Request CoAP proxy using the already-established OSCORE security context. The JRC then
responds with up-to-date keys and Stateless-Proxy option is not able
to return a (possibly new) short address.
How 5.04 Gateway Timeout Response Code in case the joined node decides when request to rekey is out of scope of this
document. Mechanisms
the server times out. Likewise, if the response to the proxy's
request does not contain the Stateless-Proxy option, for rekeying example when
the network are defined in
companion specifications, such as
[I-D.richardson-6tisch-minimal-rekey].

14. option is not supported by the server, the proxy is not able to
return the response to the client.

11. Security Considerations

This document recommends that the pledge and JRC are provisioned with
unique PSKs. The request nonce used for the Join Request and the response nonce are Join
Response is the same, but used under a different key. The design
differentiates between keys derived for requests and keys derived for
responses by different sender identifiers (0x00 for pledge and 0x01
for JRC). Note that the address of the JRC does not take part in
nonce or key construction. Even in the case of a misconfiguration in
which the same PSK is used for several nodes, pledges, the keys used to
protect the requests/responses from/
towards from/towards different pledges are
different, as they are derived using the pledge's EUI-64 pledge identifier as Master
Salt. The PSK is still important for mutual authentication of the
pledge and the JRC. Should an attacker come to know the PSK, then a
man-in-the-middle attack is possible. The well-known problem with
Bluetooth headsets with a "0000" pin applies here.

Being a stateless relay, the JP blindly forwards the join traffic
into the network. While the exchange between pledge and JP takes
place over a shared 6TiSCH cell, join traffic is forwarded using
dedicated cells on the JP to JRC multi-hop path. In case of
distributed scheduling, the join traffic may therefore cause
intermediate nodes to request additional bandwidth. Because the
relay operation of

Being a stateless relay, the JP is implemented at the application layer, blindly forwards the JP is join traffic
into the only hop network. A simple bandwidth cap on the JP-6LBR path that JP prevents it from
forwarding more traffic than the network can distinguish handle. This forces
attackers to use more than one Join Proxy if they wish to overwhelm
the network. Marking the join traffic from regular IP packets with a non-zero DSCP
allows the network to carry the traffic in if it has capacity, but
encourages the network. It is therefore
recommended network to implement stateless rate limiting at JP; a simple drop the extra traffic rather than add
bandwidth cap would be appropriate. due to that traffic.

The shared nature of the "minimal" cell used for the join traffic
makes the network prone to DoS attacks by congesting the JP with
bogus radio traffic. As such Such an attacker is limited by its emitted
radio power, the maximum transmit
power. The redundancy in the number of deployed JPs alleviates the
issue and also gives the pledge a possibility to use the best
available link for joining. How a network node decides to become a
JP is out of scope of this specification.

At the beginning of the join process, the pledge has no means of
verifying the content in the EB, and has to accept it at "face
value". In case the pledge tries to join an attacker's network, the
Join Response message will either fail the security check or time
out. The pledge may implement a 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. Bogus beacons prolong the join time of
the pledge, and so the time spent in "minimal" [RFC8180] duty cycle
mode.

15.

12. Privacy Considerations

This specification

The join solution specified in this document relies on the uniqueness
of the node's EUI-64 that pledge identifier within the namespace managed by the JRC.
This identifier is transferred in clear as an OSCORE Context Hint. Privacy
implications
The use of using such long-term the globally unique EUI-64 as pledge identifier simplifies
the management but comes with certain privacy risks. The
implications are thoroughly discussed in [RFC7721] and comprise
correlation of activities over time, location tracking, address
scanning and device-specific vulnerability exploitation. Since the
join protocol is executed rarely compared to the network lifetime,
long-term threats that arise from using EUI-64 as the pledge
identifier are minimal. In addition, the Join Response message
contains a short address which is assigned by the JRC to the pledge.
The assigned short address SHOULD be uncorrelated with the long-term EUI-64
pledge identifier. The short address is encrypted in the response. Use of short
addresses once
Once the join protocol completes process completes, the new node uses the short
addresses for all further layer 2 (and layer-3) operations. This
mitigates the aforementioned privacy risks.

16. risks as the short layer-2
address (visible even when the network is encrypted) is not traceable
between locations and does not disclose the manufacturer, as is the
case of EUI-64.

13. IANA Considerations

Note to RFC Editor: Please replace all occurrences of "[[this
document]]" with the RFC number of this specification.

This document allocates a well-known name under the .arpa name space
according to the rules given in: in [RFC3172]. The name "6tisch.arpa" is
requested. No subdomains are expected. No A, AAAA or PTR record is
requested.

16.1.

13.1. CoAP Option Numbers Registry

The Stateless-Proxy option is added to the CoAP Option Numbers
registry:

+--------+-----------------+-------------------+
| Number | Name | Reference |
+--------+-----------------+-------------------+
| TBD | Stateless-Proxy | [[this document]] |
+--------+-----------------+-------------------+

17.

14. 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 agreement No 644852,
project ARMOUR.

The authors are grateful to Thomas Watteyne and Goeran Selander for
reviewing, and to Klaus Hartke for providing input on the Stateless-
Proxy CoAP option. The authors would also like to thank Francesca
Palombini, Ludwig Seitz and John Mattsson for participating in the
discussions that have helped shape the document.

18.

15. References

18.1.

15.1. Normative References

[I-D.ietf-core-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", draft-ietf-core-object-security-06 draft-ietf-core-object-security-08 (work in
progress), October 2017. January 2018.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.

[RFC2597] Heinanen, J., Baker, F., Weiss, W., and J. Wroclawski,
"Assured Forwarding PHB Group", RFC 2597,
DOI 10.17487/RFC2597, June 1999,
<https://www.rfc-editor.org/info/rfc2597>.

[RFC3172] Huston, G., Ed., "Management Guidelines & Operational
Requirements for the Address and Routing Parameter Area
Domain ("arpa")", BCP 52, RFC 3172, DOI 10.17487/RFC3172,
September 2001, <https://www.rfc-editor.org/info/rfc3172>.

[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
October 2013, <https://www.rfc-editor.org/info/rfc7049>.

[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.

[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.

18.2.

15.2. Informative References

[I-D.ietf-6tisch-6top-protocol]
Wang, Q., Vilajosana, X., and T. Watteyne, "6top Protocol
(6P)", draft-ietf-6tisch-6top-protocol-09 (work in
progress), October 2017.

[I-D.ietf-6tisch-terminology]
Palattella, M., Thubert, P., Watteyne, T., and Q. Wang,
"Terminology in IPv6 over the TSCH mode of IEEE
802.15.4e", draft-ietf-6tisch-terminology-09 (work in
progress), June 2017.

[I-D.ietf-cbor-cddl]
Birkholz, H., Vigano, C., and C. Bormann, "Concise data
definition language (CDDL): a notational convention to
express CBOR data structures", draft-ietf-cbor-cddl-00 draft-ietf-cbor-cddl-02
(work in progress), July 2017. February 2018.

[I-D.richardson-6tisch-minimal-rekey]
Richardson, M., "Minimal Security rekeying mechanism for
6TiSCH", draft-richardson-6tisch-minimal-rekey-02 (work in
progress), August 2017.

[IEEE802.15.4-2015]
IEEE standard for Information Technology, ., "IEEE Std
802.15.4-2015 Standard for Low-Rate Wireless Personal Area
Networks (WPANs)", 2015.

[RFC4231] Nystrom, M., "Identifiers and Test Vectors for HMAC-SHA-
224, HMAC-SHA-256, HMAC-SHA-384, and HMAC-SHA-512",
RFC 4231, DOI 10.17487/RFC4231, December 2005,
<https://www.rfc-editor.org/info/rfc4231>.

[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007,
<https://www.rfc-editor.org/info/rfc4944>.

[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<https://www.rfc-editor.org/info/rfc5869>.

[RFC6550] Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
Low-Power and Lossy Networks", RFC 6550,
DOI 10.17487/RFC6550, March 2012,
<https://www.rfc-editor.org/info/rfc6550>.

[RFC6775] Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C.
Bormann, "Neighbor Discovery Optimization for IPv6 over
Low-Power Wireless Personal Area Networks (6LoWPANs)",
RFC 6775, DOI 10.17487/RFC6775, November 2012,
<https://www.rfc-editor.org/info/rfc6775>.

[RFC7554] Watteyne, T., Ed., Palattella, M., and L. Grieco, "Using
IEEE 802.15.4e Time-Slotted Channel Hopping (TSCH) in the
Internet of Things (IoT): Problem Statement", RFC 7554,
DOI 10.17487/RFC7554, May 2015,
<https://www.rfc-editor.org/info/rfc7554>.

[RFC7721] Cooper, A., Gont, F., and D. Thaler, "Security and Privacy
Considerations for IPv6 Address Generation Mechanisms",
RFC 7721, DOI 10.17487/RFC7721, March 2016,
<https://www.rfc-editor.org/info/rfc7721>.

[RFC8180] Vilajosana, X., Ed., Pister, K., and T. Watteyne, "Minimal
IPv6 over the TSCH Mode of IEEE 802.15.4e (6TiSCH)
Configuration", BCP 210, RFC 8180, DOI 10.17487/RFC8180,
May 2017, <https://www.rfc-editor.org/info/rfc8180>.

Appendix A. Example

Figure 3 4 illustrates a successful join protocol exchange. The pledge
instantiates the OSCORE context and derives the traffic keys and
nonces from the PSK. It uses the instantiated context to protect the
Join Request addressed with a Proxy-Scheme option, the well-known
host name of the JRC in the Uri-Host option, and its EUI-64
identifier as pledge
identifier and OSCORE Context Hint. Triggered by the presence of a
Proxy-Scheme option, the JP forwards the request to the JRC and adds
the Stateless-Proxy option with value set to the internally needed
state, authentication tag, and a freshness indicator. The JP has
learned the IPv6 address of the JRC when it acted as a pledge and
joined the network. Once the JRC receives the request, it looks up
the correct context based on the Context Hint parameter. It
reconstructs OSCORE's external Additional Authenticated Data (AAD)
needed for verification based on:

o the Version of the received CoAP header.

o the Algorithm value agreed out-of-band, default being AES-CCM-
16-64-128 from [RFC8152].

o the Request ID being set to the value of the "kid" field of the
received COSE object.

o the Join Request sequence number set to the value of "Partial IV"
field of the received COSE object.

o Integrity-protected options received as part of the request.

Replay protection is ensured by OSCORE and the tracking through persistent
handling of sequence
numbers at each side. mutable context parameters. Once the JP receives the
Join Response, it authenticates the Stateless-Proxy option before
deciding where to forward. The JP sets its internal state to that
found in the Stateless-Proxy option, and forwards the Join Response
to the correct pledge. Note that the JP does not possess the key to
decrypt the COSE object (join_response) present in the payload. The
Join Response is matched to the Join Request and verified for replay
protection at the pledge using OSCORE processing rules. In this
example, the Join Response does not contain the IPv6 address of the
JRC, the pledge hence understands the JRC is co-located with the
6LBR.

<---E2E OSCORE-->
Client Proxy Server
Pledge JP JRC
| | |
+------>| | Code: { 0.02 } (POST)
| GET | | Token: 0x8c
| | | Proxy-Scheme: [ coap ]
| | | Uri-Host: [ 6tisch.arpa ]
| | | Object-Security: [ kid: 0 ]
| | | Payload: Context-Hint: EUI-64
| | | [ Partial IV: 1,
| | | { Uri-Path:"j" Uri-Path:"j",
| | | join_request },
| | | <Tag> ]
| | |
| +------>| Code: { 0.01 } (GET)
| | GET | Token: 0x7b
| | | Uri-Host: [ 6tisch.arpa ]
| | | Object-Security: [ kid: 0 ]
| | | Stateless-Proxy: opaque state
| | | Payload: Context-Hint: EUI-64
| | | [ Partial IV: 1,
| | | { Uri-Path:"j" Uri-Path:"j",
| | | join_request },
| | | <Tag> ]
| | |
| |<------+ Code: { 2.05 } (Content)
| | 2.05 | Token: 0x7b
| | | Object-Security: -
| | | Stateless-Proxy: opaque state
| | | Payload: [ { join_response }, <Tag> ]
| | |
|<------+ | Code: { 2.05 } (Content)
| 2.05 | | Token: 0x8c
| | | Object-Security: -
| | | Payload: [ { join_response }, <Tag> ]
| | |

Figure 3: 4: Example of a successful join protocol exchange. { ... }
denotes encryption and authentication, [ ... ] denotes
authentication.

Where join_response join_request is:

join_request:
[
h'cafe' / PAN ID of the network pledge is as follows. attempting to join /
]
The join_request encodes to h'8142cafe' with a size of 4 bytes.

And join_response is:

join_response:
[
[ / COSE Key Set array with a single key /
{
1 : 4, / key type symmetric /
2 : h'01', / key id /
-1 : h'e6bf4287c2d7618d6a9687445ffd33e6' / key value /
}
],
[
h'af93' / assigned short address /
]
]

Encodes

The join_response encodes to
h'8281a301040241012050e6bf4287c2d7618d6a9687445ffd33e68142af93' with
a size of 30 bytes.

Authors' Addresses

Malisa Vucinic (editor)
University of Montenegro
Dzordza Vasingtona bb
Podgorica 81000
Montenegro

Email: malisav@ac.me

Jonathan Simon
Analog Devices
32990 Alvarado-Niles Road, Suite 910
Union City, CA 94587
USA

Email: jonathan.simon@analog.com
Kris Pister
University of California Berkeley
512 Cory Hall
Berkeley, CA 94720
USA

Email: pister@eecs.berkeley.edu

Michael Richardson
Sandelman Software Works
470 Dawson Avenue
Ottawa, ON K1Z5V7
Canada

Email: mcr+ietf@sandelman.ca