Constrained Join Proxy for Bootstrapping Protocols
draft-ietf-anima-constrained-join-proxy-10

anima Working Group                                        M. Richardson
Internet-Draft                                  Sandelman Software Works
Intended status: Standards Track                         P. van der Stok
Expires: 16 October 2022                          vanderstok consultancy
                                                           P. Kampanakis
                                                           Cisco Systems
                                                           14 April 2022

           Constrained Join Proxy for Bootstrapping Protocols
               draft-ietf-anima-constrained-join-proxy-10

Abstract

   This document extends the work of Bootstrapping Remote Secure Key
   Infrastructures (BRSKI) by replacing the Circuit-proxy between Pledge
   and Registrar by a stateless/stateful constrained Join Proxy.  The
   constrained Join Proxy is a mesh neighbor of the Pledge and can relay
   a DTLS session originating from a Pledge with only link-local
   addresses to a Registrar which is not a mesh neighbor of the Pledge.

   This document defines a protocol to securely assign a Pledge to a
   domain, represented by a Registrar, using an intermediary node
   between Pledge and Registrar.  This intermediary node is known as a
   "constrained Join Proxy".  An enrolled Pledge can act as a
   constrained Join Proxy.

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
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   This Internet-Draft will expire on 16 October 2022.

Copyright Notice

   Copyright (c) 2022 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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 Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Requirements Language . . . . . . . . . . . . . . . . . . . .   5
   4.  constrained Join Proxy functionality  . . . . . . . . . . . .   5
   5.  constrained Join Proxy specification  . . . . . . . . . . . .   7
     5.1.  Stateful Join Proxy . . . . . . . . . . . . . . . . . . .   7
     5.2.  Stateless Join Proxy  . . . . . . . . . . . . . . . . . .   8
     5.3.  Stateless Message structure . . . . . . . . . . . . . . .  10
   6.  Discovery . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  Join Proxy discovers Registrar  . . . . . . . . . . . . .  12
       6.1.1.  CoAP discovery  . . . . . . . . . . . . . . . . . . .  13
       6.1.2.  GRASP discovery . . . . . . . . . . . . . . . . . . .  13
       6.1.3.  6tisch discovery  . . . . . . . . . . . . . . . . . .  13
     6.2.  Pledge discovers Registrar  . . . . . . . . . . . . . . .  13
       6.2.1.  CoAP discovery  . . . . . . . . . . . . . . . . . . .  13
       6.2.2.  GRASP discovery . . . . . . . . . . . . . . . . . . .  14
       6.2.3.  6tisch discovery  . . . . . . . . . . . . . . . . . .  14
     6.3.  Pledge discovers Join Proxy . . . . . . . . . . . . . . .  14
       6.3.1.  CoAP discovery  . . . . . . . . . . . . . . . . . . .  14
       6.3.2.  GRASP discovery . . . . . . . . . . . . . . . . . . .  15
       6.3.3.  6tisch discovery  . . . . . . . . . . . . . . . . . .  15
   7.  Comparison of stateless and stateful modes  . . . . . . . . .  15
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  17
     9.1.  Resource Type Attributes registry . . . . . . . . . . . .  17
     9.2.  service name and port number registry . . . . . . . . . .  17
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  18
   11. Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  18
   12. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . .  18
     12.1.  10 to 09 . . . . . . . . . . . . . . . . . . . . . . . .  18
     12.2.  09 to 07 . . . . . . . . . . . . . . . . . . . . . . . .  18
     12.3.  06 to 07 . . . . . . . . . . . . . . . . . . . . . . . .  18
     12.4.  05 to 06 . . . . . . . . . . . . . . . . . . . . . . . .  19
     12.5.  04 to 05 . . . . . . . . . . . . . . . . . . . . . . . .  19
     12.6.  03 to 04 . . . . . . . . . . . . . . . . . . . . . . . .  19
     12.7.  02 to 03 . . . . . . . . . . . . . . . . . . . . . . . .  19
     12.8.  01 to 02 . . . . . . . . . . . . . . . . . . . . . . . .  19

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     12.9.  00 to 01 . . . . . . . . . . . . . . . . . . . . . . . .  19
     12.10. 00 to 00 . . . . . . . . . . . . . . . . . . . . . . . .  19
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  20
     13.2.  Informative References . . . . . . . . . . . . . . . . .  21
   Appendix A.  Stateless Proxy payload examples . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction

   The Bootstrapping Remote Secure Key Infrastructure (BRSKI) protocol
   described in [RFC8995] provides a solution for a secure zero-touch
   (automated) bootstrap of new (unconfigured) devices.  In the context
   of BRSKI, new devices, called "Pledges", are equipped with a factory-
   installed Initial Device Identifier (IDevID) (see [ieee802-1AR]), and
   are enrolled into a network.  BRSKI makes use of Enrollment over
   Secure Transport (EST) [RFC7030] with [RFC8366] vouchers to securely
   enroll devices.  A Registrar provides the security anchor of the
   network to which a Pledge enrolls.  In this document, BRSKI is
   extended such that a Pledge connects to "Registrars" via a
   constrained Join Proxy.  In particular, the underlying IP network is
   assumed to be a mesh newtork as described in [RFC4944], although
   other IP-over-foo networks are not excluded.

   A complete specification of the terminology is pointed at in
   Section 2.

   The specified solutions in [RFC8995] and [RFC7030] are based on POST
   or GET requests to the EST resources (/cacerts, /simpleenroll,
   /simplereenroll, /serverkeygen, and /csrattrs), and the brski
   resources (/requestvoucher, /voucher_status, and /enrollstatus).
   These requests use https and may be too large in terms of code space
   or bandwidth required for constrained devices.  Constrained devices
   which may be part of constrained networks [RFC7228], typically
   implement the IPv6 over Low-Power Wireless personal Area Networks
   (6LoWPAN) [RFC4944] and Constrained Application Protocol (CoAP)
   [RFC7252].

   CoAP can be run with the Datagram Transport Layer Security (DTLS)
   [RFC6347] as a security protocol for authenticity and confidentiality
   of the messages.  This is known as the "coaps" scheme.  A constrained
   version of EST, using Coap and DTLS, is described in
   [I-D.ietf-ace-coap-est].  The [I-D.ietf-anima-constrained-voucher]
   extends [I-D.ietf-ace-coap-est] with BRSKI artifacts such as voucher,
   request voucher, and the protocol extensions for constrained Pledges.

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   DTLS is a client-server protocol relying on the underlying IP layer
   to perform the routing between the DTLS Client and the DTLS Server.
   However, the Pledge will not be IP routable over the mesh network
   until it is authenticated to the mesh network.  A new Pledge can only
   initially use a link-local IPv6 address to communicate with a mesh
   neighbor [RFC6775] until it receives the necessary network
   configuration parameters.  The Pledge receives these configuration
   parameters from the Registrar.  When the Registrar is not a direct
   neighbor of the Registrar but several hops away, the Pledge discovers
   a neighbor constrained Join Proxy, which transmits the DTLS protected
   request coming from the Pledge to the Registrar.  The constrained
   Join Proxy must be enrolled previously such that the message from
   constrained Join Proxy to Registrar can be routed over one or more
   hops.

   During enrollment, a DTLS connection is required between Pledge and
   Registrar.

   Once a Pledge is enrolled, it can act as constrained Join Proxy
   between other Pledges and the enrolling Registrar.

   This document specifies a new form of constrained Join Proxy and
   protocol to act as intermediary between Pledge and Registrar to relay
   DTLS messages between Pledge and Registrar.  Two modes of the
   constrained Join Proxy are specified:

   1 A stateful Join Proxy that locally stores IP addresses
     during the connection.
   2 A stateless Join Proxy that where the connection state
    is stored in the messages.

   This document is very much inspired by text published earlier in
   [I-D.kumar-dice-dtls-relay].
   [I-D.richardson-anima-state-for-joinrouter] outlined the various
   options for building a constrained Join Proxy.  [RFC8995] adopted
   only the Circuit Proxy method (1), leaving the other methods as
   future work.

   The stateful and stateless modes differ in the way that they store
   the state required to forward the return packet to the pledge.
   Similar to the difference between storing and non_storing Modes of
   Operations (MOP) in RPL [RFC6550].  In the stateful method, the
   return forward state is stored in the join proxy.  In the stateless
   method, the return forward state is stored in the network.

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2.  Terminology

   The following terms are defined in [RFC8366], and are used
   identically as in that document: artifact, imprint, domain, Join
   Registrar/Coordinator (JRC), Pledge, and Voucher.

   In this document, the term "Registrar" is used throughout instead of
   "Join Registrar/Coordinator (JRC)".

   The term "installation network" refers to all devices in the
   installation and the network connections between them.  The term
   "installation IP_address" refers to an address out of the set of
   addresses which are routable over the whole installation network.

   The "Constrained Join Proxy" enables a pledge that is multiple hops
   away from the Registrar, to securely execute the BRSKI protocol
   [RFC8995] over a secure channel.

   The term "join Proxy" is used interchangeably with the term
   "constrained Join Proxy" throughout this document.

3.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

4.  constrained Join Proxy functionality

   As depicted in the Figure 1, the Pledge (P), in a Low-Power and Lossy
   Network (LLN) mesh [RFC7102] can be more than one hop away from the
   Registrar (R) and not yet authenticated into the network.

   In this situation, the Pledge can only communicate one-hop to its
   nearest neighbor, the constrained Join Proxy (J) using their link-
   local IPv6 addresses.  However, the Pledge (P) needs to communicate
   with end-to-end security with a Registrar to authenticate and get the
   relevant system/network parameters.  If the Pledge (P), knowing the
   IP-address of the Registrar, initiates a DTLS connection to the
   Registrar, then the packets are dropped at the constrained Join Proxy
   (J) since the Pledge (P) is not yet admitted to the network or there
   is no IP routability to Pledge (P) for any returned messages from the
   Registrar.

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             ++++ multi-hop
             |R |---- mesh  +--+        +--+
             |  |    \      |J |........|P |
             ++++     \-----|  |        |  |
                            +--+        +--+
          Registrar       Join Proxy   Pledge

                      Figure 1: multi-hop enrollment.

   Without routing the Pledge (P) cannot establish a secure connection
   to the Registrar (R) over multiple hops in the network.

   Furthermore, the Pledge (P) cannot discover the IP address of the
   Registrar (R) over multiple hops to initiate a DTLS connection and
   perform authentication.

   To overcome the problems with non-routability of DTLS packets and/or
   discovery of the destination address of the Registrar, the
   constrained Join Proxy is introduced.  This constrained Join Proxy
   functionality is configured into all authenticated devices in the
   network which may act as a constrained Join Proxy for Pledges.  The
   constrained Join Proxy allows for routing of the packets from the
   Pledge using IP routing to the intended Registrar.  An authenticated
   constrained Join Proxy can discover the routable IP address of the
   Registrar over multiple hops.  The following Section 5 specifies the
   two constrained Join Proxy modes.  A comparison is presented in
   Section 7.

   When a mesh network is set up, it consists of a Registrar and a set
   of connected pledges.  No constrained Join Proxies are present.  The
   wanted end-state is a network with a Registrar and a set of enrolled
   devices.  Some of these enrolled devices can act as constrained Join
   Proxies.  Pledges can only employ link-local communication untill
   they are enrolled.  A Pledge will regularly try to discover a
   constrained Join Proxy or a Registrar with link-local discovery
   requests.  The Pledges which are neigbors of the Registrar will
   discover the Registrar and be enrolled following the BRSKI protocol.
   An enrolled device can act as constrained Join Proxy.  The Pledges
   which are not a neighbor of the Registrar will eventually discover a
   constrained Join Proxy and follow the BRSKI protocol to be enrolled.
   While this goes on, more and more constrained Join Proxies with a
   larger hop distance to the Registrar will emerge.  The network should
   be configured such that at the end of the enrollment process, all
   pledges have discovered a neigboring constrained Join Proxy or the
   Registrar, and all "legal" Pledges are enrolled.

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5.  constrained Join Proxy specification

   A Join Proxy can operate in two modes:

   *  Stateful mode

   *  Stateless mode

   A Join Proxy MAY implement both.  A mechanism to switch between modes
   is out of scope of this document.  It is recommended that a Join
   Proxy uses only one of these modes at any given moment during an
   installation lifetime.

5.1.  Stateful Join Proxy

   In stateful mode, the Join Proxy forwards the DTLS messages to the
   Registrar.

   Assume that the Pledge does not know the IP address of the Registrar
   it needs to contact.  The Join Proxy has been enrolled via the
   Registrar and learns the IP address and port of the Registrar, for
   example by using the discovery mechanism described in Section 6.  The
   Pledge first discovers (see Section 6) and selects the most
   appropriate Join Proxy.  (Discovery can also be based upon [RFC8995]
   section 4.1).  For service discovery via DNS-SD [RFC6763], this
   document specifies the service names in Section 9.2.  The Pledge
   initiates its request as if the Join Proxy is the intended Registrar.
   The Join Proxy receives the message at a discoverable join-port.  The
   Join Proxy constructs an IP packet by copying the DTLS payload from
   the message received from the Pledge, and provides source and
   destination addresses to forward the message to the intended
   Registrar.  The Join Proxy stores the 4-tuple array of the messages
   received from the Registrar and copies it back to the header of the
   message returned to the Pledge.

   In Figure 2 the various steps of the message flow are shown, with
   5684 being the standard coaps port:

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   +------------+------------+-------------+--------------------------+
   |   Pledge   | Join Proxy |  Registrar  |          Message         |
   |    (P)     |     (J)    |    (R)      | Src_IP:port | Dst_IP:port|
   +------------+------------+-------------+-------------+------------+
   |      --ClientHello-->                 |   IP_P:p_P  | IP_Jl:p_Jl |
   |                    --ClientHello-->   |   IP_Jr:p_Jr| IP_R:5684  |
   |                                       |             |            |
   |                    <--ServerHello--   |   IP_R:5684 | IP_Jr:p_Jr |
   |                            :          |             |            |
   |       <--ServerHello--     :          |   IP_Jl:p_Jl| IP_P:p_P   |
   |               :            :          |             |            |
   |              [DTLS messages]          |       :     |    :       |
   |               :            :          |       :     |    :       |
   |        --Finished-->       :          |   IP_P:p_P  | IP_Jl:p_Jl |
   |                      --Finished-->    |   IP_Jr:p_Jr| IP_R:5684  |
   |                                       |             |            |
   |                      <--Finished--    |   IP_R:5684 | IP_Jr:p_Jr |
   |        <--Finished--                  |   IP_Jl:p_Jl| IP_P:p_P   |
   |              :             :          |      :      |     :      |
   +---------------------------------------+-------------+------------+
   IP_P:p_P = Link-local IP address and port of Pledge (DTLS Client)
   IP_R:5684 = Routable IP address and coaps port of Registrar
   IP_Jl:p_Jl = Link-local IP address and join-port of Join Proxy
   IP_Jr:p_Jr = Routable IP address and client port of Join Proxy

          Figure 2: constrained stateful joining message flow with
                   Registrar address known to Join Proxy.

5.2.  Stateless Join Proxy

   The stateless Join Proxy aims to minimize the requirements on the
   constrained Join Proxy device.  Stateless operation requires no
   memory in the Join Proxy device, but may also reduce the CPU impact
   as the device does not need to search through a state table.

   If an untrusted Pledge that can only use link-local addressing wants
   to contact a trusted Registrar, and the Registrar is more than one
   hop away, it sends its DTLS messages to the Join Proxy.

   When a Pledge attempts a DTLS connection to the Join Proxy, it uses
   its link-local IP address as its IP source address.  This message is
   transmitted one-hop to a neighboring (Join Proxy) node.  Under normal
   circumstances, this message would be dropped at the neighbor node
   since the Pledge is not yet IP routable or is not yet authenticated
   to send messages through the network.  However, if the neighbor
   device has the Join Proxy functionality enabled; it routes the DTLS
   message to its Registrar of choice.

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   The Join Proxy transforms the DTLS message to a JPY message which
   includes the DTLS data as payload, and sends the JPY message to the
   join-port of the Registrar.

   The JPY message payload consists of two parts:

   *  Header (H) field: consisting of the source link-local address and
      port of the Pledge (P), and

   *  Contents (C) field: containing the original DTLS payload.

   On receiving the JPY message, the Registrar (or proxy) retrieves the
   two parts.

   The Registrar transiently stores the Header field information.  The
   Registrar uses the Contents field to execute the Registrar
   functionality.  However, when the Registrar replies, it also extends
   its DTLS message with the header field in a JPY message and sends it
   back to the Join Proxy.  The Registrar SHOULD NOT assume that it can
   decode the Header Field, it should simply repeat it when responding.
   The Header contains the original source link-local address and port
   of the Pledge from the transient state stored earlier and the
   Contents field contains the DTLS payload.

   On receiving the JPY message, the Join Proxy retrieves the two parts.
   It uses the Header field to route the DTLS message containing the
   DTLS payload retrieved from the Contents field to the Pledge.

   In this scenario, both the Registrar and the Join Proxy use
   discoverable join-ports, for the Join Proxy this may be a default
   CoAP port.

   The Figure 3 depicts the message flow diagram:

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   +--------------+------------+---------------+-----------------------+
   |    Pledge    | Join Proxy |    Registrar  |        Message        |
   |     (P)      |     (J)    |      (R)      |Src_IP:port|Dst_IP:port|
   +--------------+------------+---------------+-----------+-----------+
   |      --ClientHello-->                     | IP_P:p_P  |IP_Jl:p_Jl |
   |                    --JPY[H(IP_P:p_P),-->  | IP_Jr:p_Jr|IP_R:p_Ra  |
   |                          C(ClientHello)]  |           |           |
   |                    <--JPY[H(IP_P:p_P),--  | IP_R:p_Ra |IP_Jr:p_Jr |
   |                         C(ServerHello)]   |           |           |
   |      <--ServerHello--                     | IP_Jl:p_Jl|IP_P:p_P   |
   |              :                            |           |           |
   |          [ DTLS messages ]                |     :     |    :      |
   |              :                            |     :     |    :      |
   |      --Finished-->                        | IP_P:p_P  |IP_Jr:p_Jr |
   |                    --JPY[H(IP_P:p_P),-->  | IP_Jl:p_Jl|IP_R:p_Ra  |
   |                          C(Finished)]     |           |           |
   |                    <--JPY[H(IP_P:p_P),--  | IP_R:p_Ra |IP_Jr:p_Jr |
   |                         C(Finished)]      |           |           |
   |      <--Finished--                        | IP_Jl:p_Jl|IP_P:p_P   |
   |              :                            |     :     |    :      |
   +-------------------------------------------+-----------+-----------+
   IP_P:p_P = Link-local IP address and port of the Pledge
   IP_R:p_Ra = Routable IP address and join-port of Registrar
   IP_Jl:p_Jl = Link-local IP address and join-port of Join Proxy
   IP_Jr:p_Jr = Routable IP address and port of Join Proxy

   JPY[H(),C()] = Join Proxy message with header H and content C

           Figure 3: constrained stateless joining message flow.

5.3.  Stateless Message structure

   The JPY message is constructed as a payload with media-type
   application/cbor

   Header and Contents fields together are one CBOR array of 5 elements:

   1.  header field: containing a CBOR array [RFC8949] with the Pledge
       IPv6 Link Local address as a CBOR byte string, the Pledge's UDP
       port number as a CBOR integer, the IP address family (IPv4/IPv6)
       as a CBOR integer, and the proxy's ifindex or other identifier
       for the physical port as CBOR integer.  The header field is not
       DTLS encrypted.

   2.  Content field: containing the DTLS payload as a CBOR byte string.

   The address family integer is defined in [family] with:

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   1   IP (IP version 4)
   2   IP6 (IP version 6)

   The Join Proxy cannot decrypt the DTLS payload and has no knowledge
   of the transported media type.

       JPY_message =
       [
          ip      : bstr,
          port    : int,
          family  : int,
          index   : int
          content : bstr
       ]

                Figure 4: CDDL representation of JPY message

   The contents are DTLS encrypted.  In CBOR diagnostic notation the
   payload JPY[H(IP_P:p_P)], will look like:

         [h'IP_p', p_P, family, ident, h'DTLS-payload']

   On reception by the Registrar, the Registrar MUST verify that the
   number of array elements is larger than or equal to 5, and reject the
   message when the number of array elements is smaller than 5.  After
   replacing the 5th "content" element with the DTLS payload of the
   response message and leaving all other array elements unchanged, the
   Registrar returns the response message.

   Examples are shown in Appendix A.

   The header field is completely opaque to the receiver.  A Registrar
   MUST copy the header and return it unmodified in the return message.

6.  Discovery

   It is assumed that Join Proxy seamlessly provides a coaps connection
   between Pledge and Registrar.  In particular this section extends
   section 4.1 of [RFC8995] for the constrained case.

   The discovery follows two steps with two alternatives for step 1:

   *  Step 1.  Two alternatives exist (near and remote):

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      -  Near: the Pledge is one hop away from the Registrar.  The
         Pledge discovers the link-local address of the Registrar as
         described in [I-D.ietf-ace-coap-est].  From then on, it follows
         the BRSKI process as described in [I-D.ietf-ace-coap-est] and
         [I-D.ietf-anima-constrained-voucher], using link-local
         addresses.

      -  Remote: the Pledge is more than one hop away from a relevant
         Registrar, and discovers the link-local address and join-port
         of a Join Proxy.  The Pledge then follows the BRSKI procedure
         using the link-local address of the Join Proxy.

   *  Step 2.  The enrolled Join Proxy discovers the join-port of the
      Registrar.

   The order in which the two alternatives of step 1 are tried is
   installation dependent.  The trigger for discovery in Step 2 is
   implementation dependent.

   Once a Pledge is enrolled, it may function as Join Proxy.  The Join
   Proxy functions are advertised as described below.  In principle, the
   Join Proxy functions are offered via a join-port, and not the
   standard coaps port.  Also, the Registrar offers a join-port to which
   the stateless Join Proxy sends the JPY message.  The Join Proxy and
   Registrar show the extra join-port number when responding to a
   /.well-known/core discovery request addressed to the standard coap/
   coaps port.

   Three discovery cases are discussed: Join Proxy discovers Registrar,
   Pledge discovers Registrar, and Pledge discovers Join Proxy.  Each
   discovery case considers three alternatives: CoAP based discovery,
   GRASP Based discovery, and 6tisch based discovery.  The choice of
   discovery mechanism depends on the type of installation, and
   manufacturers can provide the pledge/Join Proxy with support for more
   than one discovery mechanism.  The pledge/Join Proxy can be designed
   to dynamically try different discovery mechanisms until a successful
   discovery mechanism is found, or the choice of discovery mechanism
   could be configured during device installation.

6.1.  Join Proxy discovers Registrar

   In this section, the Join Proxy and Registrar are assumed to
   communicate via Link-Local addresses.  This section describes the
   discovery of the Registrar by the Join Proxy.

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6.1.1.  CoAP discovery

   The discovery of the coaps Registrar, using coap discovery, by the
   Join Proxy follows sections 6.3 and 6.5.1 of
   [I-D.ietf-anima-constrained-voucher].  The stateless Join Proxy can
   discover the join-port of the Registrar by sending a GET request to
   "/.well-known/core" including a resource type (rt) parameter with the
   value "brski.rjp" [RFC6690].  Upon success, the return payload will
   contain the join-port of the Registrar.

     REQ: GET coap://[IP_address]/.well-known/core?rt=brski.rjp

     RES: 2.05 Content
     <coaps://[IP_address]:join-port>; rt="brski.rjp"

   The discoverable port numbers are usually returned for Join Proxy
   resources in the <URI-Reference> of the payload (see section 5.1 of
   [I-D.ietf-ace-coap-est]).

6.1.2.  GRASP discovery

   This section is normative for uses with an ANIMA ACP.  In the context
   of autonomic networks, the Join Proxy uses the DULL GRASP M_FLOOD
   mechanism to announce itself.  Section 4.1.1 of [RFC8995] discusses
   this in more detail.  The Registrar announces itself using ACP
   instance of GRASP using M_FLOOD messages.  Autonomic Network Join
   Proxies MUST support GRASP discovery of Registrar as described in
   section 4.3 of [RFC8995].

6.1.3.  6tisch discovery

   The discovery of the Registrar by the Join Proxy uses the enhanced
   beacons as discussed in [I-D.ietf-6tisch-enrollment-enhanced-beacon].

6.2.  Pledge discovers Registrar

   In this section, the Pledge and Registrar are assumed to communicate
   via Link-Local addresses.  This section describes the discovery of
   the Registrar by the Pledge.

6.2.1.  CoAP discovery

   The discovery of the coaps Registrar, using coap discovery, by the
   Pledge follows sections 6.3 and 6.5.1 of
   [I-D.ietf-anima-constrained-voucher].

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6.2.2.  GRASP discovery

   This section is normative for uses with an ANIMA ACP.  In the context
   of autonomic networks, the Pledge uses the DULL GRASP M_FLOOD
   mechanism to announce itself.  Section 4.1.1 of [RFC8995] discusses
   this in more detail.  The Registrar announces itself using ACP
   instance of GRASP using M_FLOOD messages.  Autonomic Network Join
   Proxies MUST support GRASP discovery of Registrar as described in
   section 4.3 of [RFC8995] .

6.2.3.  6tisch discovery

   The discovery of Registrar by the Pledge uses the enhanced beacons as
   discussed in [I-D.ietf-6tisch-enrollment-enhanced-beacon].

6.3.  Pledge discovers Join Proxy

   In this section, the Pledge and Join Proxy are assumed to communicate
   via Link-Local addresses.  This section describes the discovery of
   the Join Proxy by the Pledge.

6.3.1.  CoAP discovery

   In the context of a coap network without Autonomic Network support,
   discovery follows the standard coap policy.  The Pledge can discover
   a Join Proxy by sending a link-local multicast message to ALL CoAP
   Nodes with address FF02::FD.  Multiple or no nodes may respond.  The
   handling of multiple responses and the absence of responses follow
   section 4 of [RFC8995].

   The join-port of the Join Proxy is discovered by sending a GET
   request to "/.well-known/core" including a resource type (rt)
   parameter with the value "brski.jp" [RFC6690].  Upon success, the
   return payload will contain the join-port.

   The example below shows the discovery of the join-port of the Join
   Proxy.

     REQ: GET coap://[FF02::FD]/.well-known/core?rt=brski.jp

     RES: 2.05 Content
     <coaps://[IP_address]:join-port>; rt="brski.jp"

   Port numbers are assumed to be the default numbers 5683 and 5684 for
   coap and coaps respectively (sections 12.6 and 12.7 of [RFC7252])
   when not shown in the response.  Discoverable port numbers are
   usually returned for Join Proxy resources in the <URI-Reference> of
   the payload (see section 5.1 of [I-D.ietf-ace-coap-est]).

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6.3.2.  GRASP discovery

   This section is normative for uses with an ANIMA ACP.  The Pledge
   MUST listen for GRASP M_FLOOD [RFC8990] announcements of the
   objective: "AN_Proxy".  See section 4.1.1 [RFC8995] for the details
   of the objective.

6.3.3.  6tisch discovery

   The discovery of the Join Proxy by the Pledge uses the enhanced
   beacons as discussed in [I-D.ietf-6tisch-enrollment-enhanced-beacon].

7.  Comparison of stateless and stateful modes

   The stateful and stateless mode of operation for the Join Proxy have
   their advantages and disadvantages.  This section should enable to
   make a choice between the two modes based on the available device
   resources and network bandwidth.

   +-------------+----------------------------+------------------------+
   | Properties  |         Stateful mode      |     Stateless mode     |
   +-------------+----------------------------+------------------------+
   | State       |The Join Proxy needs        | No information is      |
   | Information |additional storage to       | maintained by the Join |
   |             |maintain mapping between    | Proxy. Registrar needs |
   |             |the address and port number | to store the packet    |
   |             |of the Pledge and those     | header.                |
   |             |of the Registrar.           |                        |
   +-------------+----------------------------+------------------------+
   |Packet size  |The size of the forwarded   |Size of the forwarded   |
   |             |message is the same as the  |message is bigger than  |
   |             |original message.           |the original,it includes|
   |             |                            |additional information  |
   +-------------+----------------------------+------------------------+
   |Specification|The Join Proxy needs        |New JPY message to      |
   |complexity   |additional functionality    |encapsulate DTLS payload|
   |             |to maintain state           |The Registrar           |
   |             |information, and specify    |and the Join Proxy      |
   |             |the source and destination  |have to understand the  |
   |             |addresses of the DTLS       |JPY message in order    |
   |             |handshake messages          |to process it.          |
   +-------------+----------------------------+------------------------+
   | Ports       | Join Proxy needs           |Join Proxy and Registrar|
   |             | discoverable join-port     |need discoverable       |
   |             |                            | join-ports             |
   +-------------+----------------------------+------------------------+

          Figure 5: Comparison between stateful and stateless mode

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8.  Security Considerations

   All the concerns in [RFC8995] section 4.1 apply.  The Pledge can be
   deceived by malicious Join Proxy announcements.  The Pledge will only
   join a network to which it receives a valid [RFC8366] voucher
   [I-D.ietf-anima-constrained-voucher].  Once the Pledge joined, the
   payload between Pledge and Registrar is protected by DTLS.

   A malicious constrained Join Proxy has a number of routing
   possibilities:

   *  It sends the message on to a malicious Registrar.  This is the
      same case as the presence of a malicious Registrar discussed in
      RFC 8995.

   *  It does not send on the request or does not return the response
      from the Registrar.  This is the case of the not responding or
      crashing Registrar discussed in RFC 8995.

   *  It uses the returned response of the Registrar to enroll itself in
      the network.  With very low probability it can decrypt the
      response.  Successful enrollment is deemed too unlikely.

   *  It uses the request from the pledge to appropriate the pledge
      certificate, but then it still needs to acquire the private key of
      the pledge.  Also this is assumed to be highly unlikely.

   *  A malicious node can construct an invalid Join Proxy message.
      Suppose, the destination port is the coaps port.  In that case, a
      Join Proxy can accept the message and add the routing addresses
      without checking the payload.  The Join Proxy then routes it to
      the Registrar.  In all cases, the Registrar needs to receive the
      message at the join-port, checks that the message consists of two
      parts and uses the DTLS payload to start the BRSKI procedure.  It
      is highly unlikely that this malicious payload will lead to node
      acceptance.

   *  A malicious node can sniff the messages routed by the constrained
      Join Proxy.  It is very unlikely that the malicious node can
      decrypt the DTLS payload.  A malicious node can read the header
      field of the message sent by the stateless Join Proxy.  This
      ability does not yield much more information than the visible
      addresses transported in the network packets.

   It should be noted here that the contents of the CBOR array used to
   convey return address information is not DTLS protected.  When the
   communication between JOIN Proxy and Registrar passes over an
   unsecure network, an attacker can change the CBOR array, causing the

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   Registrar to deviate traffic from the intended Pledge.  These
   concerns are also expressed in [RFC8974].  It is also pointed out
   that the encryption in the source is a local matter.  Similarly to
   [RFC8974], the use of AES-CCM [RFC3610] with a 64-bit tag is
   recommended, combined with a sequence number and a replay window.

   If such scenario needs to be avoided, the constrained Join Proxy MUST
   encrypt the CBOR array using a locally generated symmetric key.  The
   Registrar is not able to examine the encrypted result, but does not
   need to.  The Registrar stores the encrypted header in the return
   packet without modifications.  The constrained Join Proxy can decrypt
   the contents to route the message to the right destination.

   In some installations, layer 2 protection is provided between all
   member pairs of the mesh.  In such an enviroment encryption of the
   CBOR array is unnecessay because the layer 2 protection already
   provide it.

9.  IANA Considerations

9.1.  Resource Type Attributes registry

   This specification registers two new Resource Type (rt=) Link Target
   Attributes in the "Resource Type (rt=) Link Target Attribute Values"
   subregistry under the "Constrained RESTful Environments (CoRE)
   Parameters" registry per the [RFC6690] procedure.

   Attribute Value: brski.jp
   Description: This BRSKI resource type is used to query and return
                the supported BRSKI resources of the constrained
                Join Proxy.
   Reference: [this document]

   Attribute Value: brski.rjp
   Description: This BRSKI resource type is used for the constrained
                Join Proxy to query and return Join Proxy specific
                BRSKI resources of a Registrar.
   Reference: [this document]

9.2.  service name and port number registry

   This specification registers two service names under the "Service
   Name and Transport Protocol Port Number" registry.

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   Service Name: brski-jp
   Transport Protocol(s): udp
   Assignee:  IESG <iesg@ietf.org>
   Contact:  IESG <iesg@ietf.org>
   Description: Bootstrapping Remote Secure Key Infrastructure
                 constrained Join Proxy
   Reference: [this document]

   Service Name: brski-rjp
   Transport Protocol(s): udp
   Assignee:  IESG <iesg@ietf.org>
   Contact:  IESG <iesg@ietf.org>
   Description: Bootstrapping Remote Secure Key Infrastructure
                Registrar join-port used by stateless constrained
                Join Proxy
   Reference: [this document]

10.  Acknowledgements

   Many thanks for the comments by Cartsen, Bormann, Brian Carpenter,
   Esko Dijk, Toerless Eckert, Russ Housley, Ines Robles, Juergen
   Schoenwaelder, Malisa Vu&#269;ini&#263;, and Rob Wilton.

11.  Contributors

   Sandeep Kumar, Sye loong Keoh, and Oscar Garcia-Morchon are the co-
   authors of the draft-kumar-dice-dtls-relay-02.  Their draft has
   served as a basis for this document.  Much text from their draft is
   copied over to this draft.

12.  Changelog

12.1.  10 to 09

   * OPSDIR review
   * IANA review
   * SECDIR review
   * GENART review

12.2.  09 to 07

    * typos

12.3.  06 to 07

    * AD review changes

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12.4.  05 to 06

    * RT value change to brski.jp and brski.rjp
    * new registry values for IANA
    * improved handling of jpy header array

12.5.  04 to 05

    * Join Proxy and join-port consistent spelling
    * some nits removed
    * restructured discovery
    * section
    * rephrased parts of security section

12.6.  03 to 04

   * mail address and reference

12.7.  02 to 03

   * Terminology updated
   * Several clarifications on discovery and routability
   * DTLS payload introduced

12.8.  01 to 02

   *  Discovery of Join Proxy and Registrar ports

12.9.  00 to 01

   *  Registrar used throughout instead of EST server

   *  Emphasized additional Join Proxy port for Join Proxy and Registrar

   *  updated discovery accordingly

   *  updated stateless Join Proxy JPY header

   *  JPY header described with CDDL

   *  Example simplified and corrected

12.10.  00 to 00

   *  copied from vanderstok-anima-constrained-join-proxy-05

13.  References

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13.1.  Normative References

   [family]   "Address Family Numbers", 19 October 2021,
              <https://www.iana.org/assignments/address-family-numbers/
              address-family-numbers.xhtml>.

   [I-D.ietf-ace-coap-est]
              Stok, P. V. D., Kampanakis, P., Richardson, M. C., and S.
              Raza, "EST over secure CoAP (EST-coaps)", Work in
              Progress, Internet-Draft, draft-ietf-ace-coap-est-18, 6
              January 2020, <https://www.ietf.org/archive/id/draft-ietf-
              ace-coap-est-18.txt>.

   [I-D.ietf-anima-constrained-voucher]
              Richardson, M., Stok, P. V. D., Kampanakis, P., and E.
              Dijk, "Constrained Bootstrapping Remote Secure Key
              Infrastructure (BRSKI)", Work in Progress, Internet-Draft,
              draft-ietf-anima-constrained-voucher-17, 7 April 2022,
              <https://www.ietf.org/archive/id/draft-ietf-anima-
              constrained-voucher-17.txt>.

   [ieee802-1AR]
              "IEEE 802.1AR Secure Device Identifier", 2009,
              <https://standards.ieee.org/standard/802.1AR-2009.html>.

   [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>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8366]  Watsen, K., Richardson, M., Pritikin, M., and T. Eckert,
              "A Voucher Artifact for Bootstrapping Protocols",
              RFC 8366, DOI 10.17487/RFC8366, May 2018,
              <https://www.rfc-editor.org/info/rfc8366>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,
              <https://www.rfc-editor.org/info/rfc8949>.

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   [RFC8990]  Bormann, C., Carpenter, B., Ed., and B. Liu, Ed., "GeneRic
              Autonomic Signaling Protocol (GRASP)", RFC 8990,
              DOI 10.17487/RFC8990, May 2021,
              <https://www.rfc-editor.org/info/rfc8990>.

   [RFC8995]  Pritikin, M., Richardson, M., Eckert, T., Behringer, M.,
              and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructure (BRSKI)", RFC 8995, DOI 10.17487/RFC8995,
              May 2021, <https://www.rfc-editor.org/info/rfc8995>.

13.2.  Informative References

   [I-D.ietf-6tisch-enrollment-enhanced-beacon]
              (editor), D. D. and M. Richardson, "Encapsulation of
              6TiSCH Join and Enrollment Information Elements", Work in
              Progress, Internet-Draft, draft-ietf-6tisch-enrollment-
              enhanced-beacon-14, 21 February 2020,
              <https://www.ietf.org/archive/id/draft-ietf-6tisch-
              enrollment-enhanced-beacon-14.txt>.

   [I-D.kumar-dice-dtls-relay]
              Kumar, S. S., Keoh, S. L., and O. Garcia-Morchon, "DTLS
              Relay for Constrained Environments", Work in Progress,
              Internet-Draft, draft-kumar-dice-dtls-relay-02, 20 October
              2014, <https://www.ietf.org/archive/id/draft-kumar-dice-
              dtls-relay-02.txt>.

   [I-D.richardson-anima-state-for-joinrouter]
              Richardson, M. C., "Considerations for stateful vs
              stateless join router in ANIMA bootstrap", Work in
              Progress, Internet-Draft, draft-richardson-anima-state-
              for-joinrouter-03, 22 September 2020,
              <https://www.ietf.org/archive/id/draft-richardson-anima-
              state-for-joinrouter-03.txt>.

   [RFC3610]  Whiting, D., Housley, R., and N. Ferguson, "Counter with
              CBC-MAC (CCM)", RFC 3610, DOI 10.17487/RFC3610, September
              2003, <https://www.rfc-editor.org/info/rfc3610>.

   [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>.

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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>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <https://www.rfc-editor.org/info/rfc6690>.

   [RFC6763]  Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
              <https://www.rfc-editor.org/info/rfc6763>.

   [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>.

   [RFC7030]  Pritikin, M., Ed., Yee, P., Ed., and D. Harkins, Ed.,
              "Enrollment over Secure Transport", RFC 7030,
              DOI 10.17487/RFC7030, October 2013,
              <https://www.rfc-editor.org/info/rfc7030>.

   [RFC7102]  Vasseur, JP., "Terms Used in Routing for Low-Power and
              Lossy Networks", RFC 7102, DOI 10.17487/RFC7102, January
              2014, <https://www.rfc-editor.org/info/rfc7102>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <https://www.rfc-editor.org/info/rfc7228>.

   [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>.

   [RFC8974]  Hartke, K. and M. Richardson, "Extended Tokens and
              Stateless Clients in the Constrained Application Protocol
              (CoAP)", RFC 8974, DOI 10.17487/RFC8974, January 2021,
              <https://www.rfc-editor.org/info/rfc8974>.

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Appendix A.  Stateless Proxy payload examples

   The examples show the request "GET coaps://192.168.1.200:5965/est/
   crts" to a Registrar.  The header generated between Join Proxy and
   Registrar and from Registrar to Join Proxy are shown in detail.  The
   DTLS payload is not shown.

   The request from Join Proxy to Registrar looks like:

      85                                   # array(5)
         50                                # bytes(16)
            FE800000000000000000FFFFC0A801C8 #
         19 BDA7                           # unsigned(48551)
         01                                # unsigned(1) IP
         00                                # unsigned(0)
         58 2D                             # bytes(45)
      <cacrts DTLS encrypted request>

   In CBOR Diagnostic:

       [h'FE800000000000000000FFFFC0A801C8', 48551, 1, 0,
        h'<cacrts DTLS encrypted request>']

   The response is:

      85                                   # array(5)
         50                                # bytes(16)
            FE800000000000000000FFFFC0A801C8 #
         19 BDA7                           # unsigned(48551)
         01                                # unsigned(1) IP
         00                                # unsigned(0)
      59 026A                              # bytes(618)
         <cacrts DTLS encrypted response>

   In CBOR diagnostic:

       [h'FE800000000000000000FFFFC0A801C8', 48551, 1, 0,
       h'<cacrts DTLS encrypted response>']

Authors' Addresses

   Michael Richardson
   Sandelman Software Works
   Email: mcr+ietf@sandelman.ca

   Peter van der Stok
   vanderstok consultancy

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   Email: stokcons@bbhmail.nl

   Panos Kampanakis
   Cisco Systems
   Email: pkampana@cisco.com

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