Constrained Join Proxy for Bootstrapping Protocols

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Table of Contents

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, this solution is intended to support mesh networks as described in [RFC4944].¶

The constrained Join Proxy as specified in this document is one of the Join Proxy options referred to in [RFC8995] section 2.5.2 as future work.¶

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 [RFC9148].¶

The [I-D.ietf-anima-constrained-voucher] extends [RFC9148] with BRSKI artifacts such as voucher, request voucher, and the protocol extensions for constrained Pledges that use CoAP.¶

However, in networks that require authentication, such as those using [RFC4944], 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 that is operating the constrained Join Proxy, which forwards DTLS protected messages between Pledge and 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.¶

An enrolled Pledge can act as constrained Join Proxy between other Pledges and the enrolling Registrar.¶

Two modes of the constrained Join Proxy are specified:¶

1 A stateful Join Proxy that locally stores UDP connection state:
  IP addresses (link-local with interface and non-link-local and UDP port-numbers)
  during the connection.

2 A stateless Join Proxy where the connection state
  is replaced by a second layer of CoAP header in the
  UDP messages between constrained Join Proxy and Registrar.

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.¶

Similar to the difference between storing and non-storing Modes of Operations (MOP) in RPL [RFC6550], the stateful and stateless modes differ in the way that they store the state required to forward the return packet to the Pledge. 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 using the CoAP extended token in a way identical to that described in [RFC9031].¶

2. Terminology

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

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 "Constrained Join Proxy" enables a Pledge that is multiple hops away from the Registrar, to execute the BRSKI protocol [RFC8995] using a secure channel.¶

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

The [RFC8995] Circuit Proxy is referred to as a TCP circuit Join Proxy.¶

3. constrained Join Proxy functionality

As depicted in the Figure 1, the Pledge (P), in a network such as 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 link-local IPv6 addresses. However, the Pledge needs to communicate using end-to-end security with a Registrar in order to onboard, authenticate and get the relevant system/network parameters. If the Pledge, knowing the IP-address of the Registrar, initiates a DTLS connection to the Registrar, then the packets are dropped at the constrained Join Proxy since the Pledge is not yet admitted to the network or there is no IP routability to the Pledge for any returned messages from the Registrar.¶

Without a routeable IPv6 address, the Pledge (P) cannot exchange IPv6/UDP/DTLS traffic with the Registrar (R), over multiple hops in the network.¶

Furthermore, the Pledge may not even be able to discover the IP address of the Registrar over multiple hops to initiate a DTLS connection and perform authentication.¶

To overcome the problems with non-routability of DTLS packets and the discovery of the destination address of the Registrar, the constrained Join Proxy is introduced. This constrained Join Proxy functionality is also (auto) 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 4 specifies the two constrained Join Proxy modes. A comparison is presented in Section 6.¶

When a mesh network is set up, it consists of a Registrar and a set of connected Pledges. No constrained Join Proxies are present. Only some of these Pledges may be neighbors of the Registrar. Others would need for their traffic to be routed across one or more enrolled devices to reach the Registrar.¶

The desired state of the installation is a network with a Registrar and all Pledges becoming enrolled devices. Some of these enrolled devices can act as constrained Join Proxies. Pledges can only employ link-local communication until 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 neighbors of the Registrar will discover the Registrar and be enrolled following the constrained 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 constrained 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 neighboring constrained Join Proxy or the Registrar, and all Pledges are enrolled.¶

The constrained Join Proxy is as a packet-by-packet proxy for UDP packets between Pledge and Registrar. The constrained BRSKI protocol between Pledge and Registrar described in [I-D.ietf-anima-constrained-voucher] which this Join Proxy supports uses UDP messages with DTLS payload, but the Join Proxy as described here is unaware of this payload. It can therefore potentially also work for other UDP based protocols as long as they are agnostic to (or can be made to work with) the change of IP header by the constrained Join Proxy.¶

In both Stateless and Stateful mode, the Join Proxy needs to be configured with or dynamically discover a Registrar to perform its service. This specification does not discuss how a constrained Join Proxy selects a Registrar when it discovers 2 or more.¶

4. constrained Join Proxy specification

A Join Proxy can operate in two modes:¶

• Stateful mode¶
• Stateless mode¶

The advantages and disadvantages of the two modes are presented in Section 6.¶

A Registrar MUST implement both the stateful mode and the Stateless mode, but an operator MAY configure it to announce only one. A Join Proxy MUST implement the stateless mode, but SHOULD implement the stateful mode if it has sufficient memory.¶

For a Join Proxy to be operational, the node on which it is running has to be able to talk to a Registrar (exchange UDP messages with it). This can happen fully automatically by the Join Proxy node first enrolling itself as a Pledge, and then learning the IP address, the UDP port and the mode(s) (Stateful and/or Stateless) of the Registrar, through a discovery mechanism such as those described in Section 6.¶

In mesh LLN networks like those based upon RPL ([RFC6550]), it would not be unusual for the 6LBR (the DODAG root) to have a wired network interface on which the Registrar can be found. Or the Registrar may in fact be co-located with the 6LBR. This 6LBR then becomes the first Join Proxy, and additional nodes attach to it in a concentric fashion.¶

Other methods, such as provisioning the Join Proxy are out of scope of this document but equally feasible.¶

Once the Join Proxy is operational, its mode is determined by the mode of the Registrar. If the Registrar offers both Stateful and Stateless mode, the Join Proxy MUST use the stateless mode.¶

Independent of the mode of the Join Proxy, the Pledge first discovers (see Section 6) and selects the most appropriate Join Proxy. From the discovery, the Pledge learns the Join Proxies link-local scope IP address and UDP (join) port. This discovery can also be based upon [RFC8995] section 4.1. If the discovery method does not support discovery of the join-port, then the Pledge assumes the default CoAP over DTLS UDP port (5683).¶

+------------+------------+-------------+--------------------------+
|   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

+--------------+------------+---------------+-----------------------+
|    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

    pledge_context_message = [
      family:  uint .bits 1,
      ifindex: uint .bits 8,
      srcport: uint .bits 16,
      iid:     bstr .bits 64,
    ]

5. Discovery

  REQ: GET /.well-known/core?rt=brski.rjp

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

  REQ: GET /.well-known/core?rt=brski*

  RES: 2.05 Content
  <coap://[2001:db8:0:abcd::52]:7634>;rt=brski.rjp,
  <coaps://[2001:db8:0:abcd::52]/.well-known/brski>;rt=brski,
  <coaps://[2001:db8:0:abcd::52]/.well-known/brski/rv>;rt=brski.rv;ct=836,
  <coaps://[2001:db8:0:abcd::52]/.well-known/brski/vs>;rt=brski.vs;ct="50 60",
  <coaps://[2001:db8:0:abcd::52]/.well-known/brski/es>;rt=brski.es;ct="50 60",

   [M_FLOOD, 51804231, h'fda379a6f6ee00000200000064000001', 180000,
   [["AN_join_registrar", 4, 255, "BRSKI_RJP"],
    [O_IPv6_LOCATOR,
     h'fda379a6f6ee00000200000064000001', IPPROTO_UDP, 5685]]]

   [M_FLOOD, 51840231, h'fda379a6f6ee00000200000064000001', 180000,
   [["AN_join_registrar", 4, 255, ""],
    [O_IPv6_LOCATOR,
     h'fda379a6f6ee00000200000064000001', IPPROTO_TCP, 8443],
    ["AN_join_registrar", 4, 255, "BRSKI_JP"],
    [O_IPv6_LOCATOR,
     h'fda379a6f6ee00000200000064000001', IPPROTO_UDP, 5684],
    ["AN_join_registrar", 4, 255, "BRSKI_RJP"],
    [O_IPv6_LOCATOR,
     h'fda379a6f6ee00000200000064000001', IPPROTO_UDP, 5685]]]

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

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

     [M_FLOOD, 12340815, h'fe800000000000000000000000000001', 180000,
     [["AN_Proxy", 4, 1, ""],
     [O_IPv6_LOCATOR,
     h'fe800000000000000000000000000001', IPPROTO_UDP, 5684]]]

6. 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 operators to make a choice between the two modes based on the available device resources and network bandwidth.¶

7. 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 because successful enrollment is deemed 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. This, too, 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 Registrar to deviate traffic from the intended Pledge. These concerns are also expressed in [RFC8974]. It is also pointed out that the encryption by the Join Proxy 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.¶

8. IANA Considerations

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]

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]

9. Acknowledgements

Many thanks for the comments by Carsten Bormann, Brian Carpenter, Spencer Dawkins, Esko Dijk, Toerless Eckert, Russ Housley, Ines Robles, Rich Salz, Jürgen Schönwälder, Mališa Vučinić, and Rob Wilton.¶

10. 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.¶

11. Changelog

* jpy message encrypted and no longer standardized

* many typos fixes and text re-organized
* core of GRASP and CoAP discovery moved to contrained-voucher document, only stateless extensions remain

* Join-Proxy and Registrar discovery merged
* GRASP discovery updated
* ARTART review
* TSVART review

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

 * typos

 * AD review changes

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

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

* mail address and reference

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

Appendix A. Stateless CoAP payload examples

This section shows how the CoAP header is arranged by the stateless proxy.¶

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Ver|T=0|TKL=13 | Code=0.02 POST|          Message ID           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0 0 0 0 0 0 1 1|
   |               16-bytes of extended token                      |
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1 1 0 1 0 1 0 0|0 0 0 1 1 0 1 0| four bytes: "coap"            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1 1 1 1 1 1 1 1|    Payload (DTLS contents)
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

The Option is Proxy-Scheme, with a value 39, and must be encoded as an Option Delta of 13, followed by a single byte of (39-13=) 26.¶

The total size of the header is 4,1+16,6,1 is 28 bytes. A CBOR header would have taken 4+16 bytes or 20 bytes, for a difference of 8 bytes.¶

Appendix B. 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.¶

NOTE THESE ARE OLD.¶

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