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


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

Abstract

   This document describes the minimal framework required for a new
   device, called "pledge", to securely join a 6TiSCH (IPv6 over the
   TSCH mode of IEEE 802.15.4e) network.  The framework requires that
   the pledge and the JRC (join registrar/coordinator, a central
   entity), share a symmetric key.  How this key is provisioned is out
   of scope of this document.  Through a single CoAP (Constrained
   Application Protocol) request-response exchange secured by OSCORE
   (Object Security for Constrained RESTful Environments), the pledge
   requests admission into the network and the JRC configures it with
   link-layer keying material and a short link-layer address.  This
   specification defines the message format, a new Stateless-Proxy CoAP
   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

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

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




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Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Identifiers . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  One-Touch Assumption  . . . . . . . . . . . . . . . . . . . .   5
   5.  Join Overview . . . . . . . . . . . . . . . . . . . . . . . .   5
     5.1.  Step 1 - Enhanced Beacon  . . . . . . . . . . . . . . . .   7
     5.2.  Step 2 - Neighbor Discovery . . . . . . . . . . . . . . .   7
     5.3.  Step 3 - Join Request . . . . . . . . . . . . . . . . . .   8
     5.4.  Step 4 - Join Response  . . . . . . . . . . . . . . . . .   8
   6.  Link-layer Configuration  . . . . . . . . . . . . . . . . . .   9
   7.  Network-layer Configuration . . . . . . . . . . . . . . . . .   9
     7.1.  Identification of Join Request Traffic  . . . . . . . . .  10
     7.2.  Identification of Join Response Traffic . . . . . . . . .  11
   8.  Application-level Configuration . . . . . . . . . . . . . . .  11
     8.1.  OSCORE Security Context . . . . . . . . . . . . . . . . .  12
   9.  6TiSCH Join Protocol  . . . . . . . . . . . . . . . . . . . .  13
     9.1.  Specification of the Join Request . . . . . . . . . . . .  14
     9.2.  Specification of the Join Response  . . . . . . . . . . .  15
     9.3.  Error Handling and Retransmission . . . . . . . . . . . .  17
     9.4.  Rekeying and Rejoining  . . . . . . . . . . . . . . . . .  18
     9.5.  Parameters  . . . . . . . . . . . . . . . . . . . . . . .  18
     9.6.  Mandatory to Implement Algorithms . . . . . . . . . . . .  18
   10. Stateless-Proxy CoAP Option . . . . . . . . . . . . . . . . .  19
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  20
   12. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  21
   13. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
     13.1.  CoAP Option Numbers Registry . . . . . . . . . . . . . .  21
   14. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  22
   15. References  . . . . . . . . . . . . . . . . . . . . . . . . .  22
     15.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     15.2.  Informative References . . . . . . . . . . . . . . . . .  23



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

1.  Introduction

   This document presumes a 6TiSCH network as described by [RFC7554] and
   [RFC8180].  By design, nodes in a 6TiSCH network [RFC7554] have their
   radio turned off most of the time, to conserve energy.  As a
   consequence, the link used by a new device for joining the network
   has limited bandwidth [RFC8180].  The secure join solution defined in
   this document therefore keeps the number of over-the-air exchanges
   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
   defines a 6TiSCH Join Protocol (6JP) used by the pledge to request
   admission into a network managed by the JRC, and for the JRC to
   configure the pledge with the necessary parameters, a new CoAP
   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 join process successfully
   completes, the joined node can interact with its neighbors to request
   additional bandwidth using the 6top Protocol
   [I-D.ietf-6tisch-6top-protocol] and start sending the application
   traffic.




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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:

   o  pledge

   o  joined node

   o  join proxy (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 in
   the namespace managed by a 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 be carried
   within Enhanced Beacon (EB) frames.

   The "pledge identifier" uniquely identifies the pledge in the
   namespace managed by a JRC.  The pledge identifier is typically the
   globally unique 64-bit Extended Unique Identifier (EUI-64) of the



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   IEEE 802.15.4 device.  This identifier is used to generate the IPv6
   addresses of the pledge and to identify it during the execution of
   the 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 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 pledge using a
   number of mechanisms, such as a JTAG interface, a serial (craft)
   console interface, pushing buttons simultaneously on different
   devices, over-the-air configuration in a Faraday cage, etc.  The
   provisioning can be done by the vendor, the manufacturer, the
   integrator, etc.

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




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   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, 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 (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.






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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 make
   use of information such as: what 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 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.

5.2.  Step 2 - Neighbor Discovery

   The pledge forms its link-local IPv6 address based on the interface
   identifier, as per [RFC4944].  The pledge MAY perform the Neighbor
   Solicitation / Neighbor Advertisement exchange with the JP, as per
   Section 5.5.1 of [RFC6775].  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 JP accepts these unprotected frames is discussed in
   Section 6.








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5.3.  Step 3 - Join Request

   The Join Request is a message sent from the pledge to the JP, and
   which the JP forwards to the JRC.  The JP 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 identifier and a
   request-specific constant value.  Algorithms which MUST be
   implemented are specified in Section 9.6.

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

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

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.  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 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.






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

6.  Link-layer Configuration

   In an operational 6TiSCH network, all frames MUST use link-layer
   frame security [RFC8180].  The IEEE 802.15.4 security attributes MUST
   include frame authenticity, and MAY include frame confidentiality
   (i.e. encryption).

   As specified in [RFC8180], the network uses a key termed as K1 to
   authenticate EBs and a key termed as K2 to authenticate and
   optionally encrypt DATA and ACKNOWLEDGMENT frames.  The keys K1 and
   K2 MAY be the same key (same value and IEEE 802.15.4 index).  How the
   JRC communicates these keys to 6LBR is out of scope of this
   specification.

   The pledge does not initially do any authenticity check of the EB
   frames, as it does not know the K1 key.  The pledge is still able to
   parse the contents of the received EBs and synchronize to the
   network, as EBs are not encrypted [RFC8180].

   When sending frames during the join process, the pledge sends
   unencrypted and unauthenticated frames.  The JP accepts these frames
   (using the "exempt mode" in 802.15.4) for the duration of the join
   process.  How the JP learns whether the join process is ongoing is
   out of scope of this specification.

   As the EB itself cannot be authenticated by the pledge, an attacker
   may craft a frame that appears to be a valid EB, since the pledge can
   neither know the ASN a priori nor verify the address of the JP.  This
   opens up a possibility of DoS attack, as discussed in Section 11.
   Beacon authentication keys are discussed in [RFC8180].

7.  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.
   How the pledge and 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.

   The pledge does not communicate with the JRC at the network layer.
   This allows the pledge to join without knowing the IPv6 address of



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   the JRC.  Instead, the pledge communicates with the JP at the network
   layer, and with the JRC at the application layer, as specified in
   Section 8.

   The JP communicates with the JRC over global IPv6 addresses.  The JP
   discovers the network 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 actual IPv6 address of the
   JRC from the 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 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
   RPL DIOs [RFC6550] to its IPv6 address.  The pledge learns the
   address of the JRC once joined and upon the reception of a first RPL
   DIO message, and uses it to operate as a JP.

   Before the 6TiSCH network is started, the 6LBR MUST be provisioned
   with the IPv6 address of the JRC.

7.1.  Identification of Join Request Traffic

   The join request traffic that is proxied by the Join Proxy comes from
   unauthenticated nodes, and there may be an arbitrary amount of it.
   In particular, an attacker may send fraudulent traffic in attempt to
   overwhelm the network.

   When operating as part of a [RFC8180] 6TiSCH minimal network using
   reasonable scheduling algorithms, the join request traffic present
   may cause intermediate nodes to request additional bandwidth.  An
   attacker could use this property to cause the network to overcommit
   bandwidth (and energy) to the join process.

   The Join Proxy is aware of what traffic is join request traffic, and
   so can avoid allocating additional bandwidth itself.  The Join Proxy
   SHOULD implement a 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 bandwidth cap
   implemented separately on each Join Proxy, the aggregate join request
   traffic from many Join Proxies may cause intermediate nodes to decide
   to allocate additional cells.  It is undesirable to to so in response
   to the join request traffic.  In order to permit the intermediate
   nodes to avoid this, the traffic needs to be tagged in some way.

   [RFC2597] defines a set of per-hop behaviors that may be encoded into
   the Diffserv Code Points (DSCPs).  The Join Proxy SHOULD set the DSCP




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   of join request packets that it produces as part of the relay process
   to AF43 code point (See Section 6 of [RFC2597]).

   A Join Proxy that does not set the DSCP on traffic forwarded should
   set it to zero so that it is compressed out.

   A Scheduling Function (SF) running on 6TiSCH nodes SHOULD NOT
   allocate additional cells as a result of traffic with code point
   AF43.  Companion SF documents SHOULD specify how this recommended
   behavior is achieved.

7.2.  Identification of Join Response Traffic

   The JRC SHOULD set the DSCP of join response packets addressed to the
   Join Proxy to AF42 code point.  Join response traffic can not be
   induced by an attacker as it is generated only in response to
   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 not co-located with the 6LBR, then the code point
   provides a clear indication to the 6LBR that this is join response
   traffic.

   Due to the convergecast nature of the DODAG, the 6LBR links are often
   the most congested, 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.

8.  Application-level Configuration

   The Join Request/Join Response exchange in Figure 1 is carried over
   CoAP [RFC7252] and secured using OSCORE
   [I-D.ietf-core-object-security].  The pledge plays the role of a CoAP
   client; the JRC plays the role of a CoAP server.  The JP implements
   CoAP forward proxy functionality [RFC7252].  Because the JP can also
   be a constrained device, it cannot implement a cache.  Rather, the JP
   processes forwarding-related CoAP options and makes requests on
   behalf of the pledge, in a stateless manner by using the Stateless-
   Proxy option defined in this document.





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   The pledge designates a JP as a proxy by including the Proxy-Scheme
   option in CoAP requests it sends to the JP.  The pledge also includes
   in the requests the Uri-Host option with its value set to the well-
   known JRC's alias, as specified in Section 9.1.

   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.

   The JP MUST add a Stateless-Proxy option to all the requests that it
   forwards on behalf of the pledge as part of the join process.

   The value of the Stateless-Proxy option is set to the internal JP
   state, needed to forward the Join Response message to the pledge.
   The Stateless-Proxy option handling is defined in Section 10.

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

8.1.  OSCORE Security Context

   Before the 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 derived at the pledge and the JRC
   as per Section 3 of [I-D.ietf-core-object-security].

   o  the Master Secret MUST be the PSK.

   o  the Master Salt MUST be the pledge identifier.

   o  the Sender ID of the pledge MUST be set to byte string 0x00.

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

   o  the Algorithm MUST be set to the value from [RFC8152], agreed out-
      of-band by the same mechanism used to provision the PSK.  The
      default is AES-CCM-16-64-128.

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

   The derivation in [I-D.ietf-core-object-security] results in traffic
   keys and a common IV for each side of the conversation.  Nonces are
   constructed by XOR'ing the common IV with the current sequence number



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   and sender identifier.  For details on nonce construction, refer to
   [I-D.ietf-core-object-security].

   Implementations MUST ensure that multiple CoAP requests to different
   JRCs result in the use of the same OSCORE context, so that the
   sequence numbers are properly incremented for each request.  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.  A simple implementation technique is to
   instantiate the OSCORE security context with a given PSK only once
   and use it for all subsequent requests.  Failure to comply will break
   the confidentiality property of the AEAD algorithm due to the nonce
   reuse.

8.1.1.  Persistency

   Implementations MUST 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 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 pledge and the JRC is rare, making
   security outweigh the cost of writing to persistent memory.

9.  6TiSCH Join Protocol

   6TiSCH Join Protocol (6JP) is a lightweight protocol over CoAP
   [RFC7252] and a secure channel provided by OSCORE
   [I-D.ietf-core-object-security].  6JP allows the pledge to request
   admission into a network managed by the JRC, and for the JRC to
   configure the pledge with the parameters necessary for joining the
   network.  These parameters are: link-layer keys in use, IEEE 802.15.4
   short address assigned to the pledge, and the 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 the security properties provided by OSCORE.  This
   includes end-to-end confidentiality, data authenticity, replay
   protection, and a secure binding of responses to requests.






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               +-----------------------------------+
               |     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".




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   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,
   ]







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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,
   ]




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   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.




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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.








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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 CoAP
   token, the IPv6 address of the host, and the UDP source port number.
   If the JP used the stateful CoAP proxy defined in [RFC7252], it would
   be prone to Denial-of-Service (DoS) attacks, due to its limited
   memory.

   The Stateless-Proxy CoAP option Figure 3 allows the JP to be entirely
   stateless.  This option inserts, in the request, the state
   information needed for relaying the response back to the client.  The
   proxy still keeps some 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 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            | Format | Length |
        +-----+---+---+---+---+-----------------+--------+--------|
        | TBD | x |   | x |   | Stateless-Proxy | opaque | 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 value of the Stateless-Proxy option is
   internal proxy state that is opaque to the server.  Example state
   information includes the IPv6 address of the client, its UDP source
   port, 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.  The
   proxy may use an appropriate COSE structure [RFC8152] to wrap the
   state information as the value of the Stateless-Proxy option.  The
   key used for encryption/authentication of the state information may
   be known only to the proxy.

   Once the proxy has received the CoAP response with a Stateless-Proxy
   option present, it decrypts/authenticates it, checks the freshness
   indicator and constructs the response for the client, based on the
   information present in the option value.





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   Note that a CoAP proxy using the Stateless-Proxy option is not able
   to return a 5.04 Gateway Timeout Response Code in case the request to
   the server times out.  Likewise, if the response to the proxy's
   request does not contain the Stateless-Proxy option, for example when
   the 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 nonce used for the Join Request and the 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 pledges, the keys used to
   protect the requests/responses from/towards different pledges are
   different, as they are derived using the 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.  A simple bandwidth cap on the JP prevents it from
   forwarding more traffic than the network can handle.  This forces
   attackers to use more than one Join Proxy if they wish to overwhelm
   the network.  Marking the join traffic packets with a non-zero DSCP
   allows the network to carry the traffic if it has capacity, but
   encourages the network to drop the extra traffic rather than add
   bandwidth due to that traffic.

   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 traffic.  Such an attacker is limited by its maximum transmit
   power.  The redundancy in the number of deployed JPs alleviates the
   issue and also gives the pledge a possibility to use the best
   available link for joining.  How a network node decides to become a
   JP is out of scope of this specification.

   At the beginning of the join process, the pledge has no means of
   verifying the content in the EB, and has to accept it at "face
   value".  In case the pledge tries to join an attacker's network, the
   Join Response message will either fail the security check or time
   out.  The pledge may implement a 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



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   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.

12.  Privacy Considerations

   The join solution specified in this document relies on the uniqueness
   of the pledge identifier within the namespace managed by the JRC.
   This identifier is transferred in clear as an OSCORE Context Hint.
   The use of the globally unique EUI-64 as pledge identifier simplifies
   the management but comes with certain privacy risks.  The
   implications are thoroughly discussed in [RFC7721] and comprise
   correlation of activities over time, location tracking, address
   scanning and device-specific vulnerability exploitation.  Since the
   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
   pledge identifier.  The short address is encrypted in the response.
   Once the join process completes, the new node uses the short
   addresses for all further layer 2 (and layer-3) operations.  This
   mitigates the aforementioned privacy 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 [RFC3172].  The name "6tisch.arpa" is
   requested.  No subdomains are expected.  No A, AAAA or PTR record is
   requested.

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]] |
             +--------+-----------------+-------------------+




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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.

15.  References

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-08 (work in
              progress), 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>.





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

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-02
              (work in progress), 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>.




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

   [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 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 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.




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   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 through persistent
   handling of 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.




























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       <---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",
       |       |       |                      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",
       |       |       |                     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 4: Example of a successful join protocol exchange. { ... }
          denotes encryption and authentication, [ ... ] denotes
                              authentication.

   Where join_request is:

   join_request:
   [
       h'cafe' / PAN ID of the network pledge is attempting to join /
   ]



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   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 /
       ]
   ]

   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











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   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



































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