anima Working Group M. Richardson
Internet-Draft Sandelman Software Works
Intended status: Standards Track P. van der Stok
Expires: September 11, 2019 vanderstok consultancy
P. Kampanakis
Cisco Systems
March 10, 2019

Constrained Join Proxy for Bootstrapping Protocols


This document defines a protocol to securely assign a pledge to an owner, using an intermediary node between pledge and owner. This intermediary node is known as a "constrained-join-proxy".

This document extends the work of [ietf-anima-bootstrapping-keyinfra] by replacing the Circuit-proxy by a stateless constrained join-proxy, that transports routing information.

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

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 11, 2019.

Copyright Notice

Copyright (c) 2019 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 ( 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

Enrolment of new nodes into constrained networks with constrained nodes present is described in [I-D.ietf-anima-bootstrapping-keyinfra] and makes use of Enrolment over Secure Transport (EST) [RFC7030]. The specified solutions use https and may be too large in terms of code space or bandwidth required. Constrained devices in constrained networks [RFC7228] typically implement the IPv6 over Low-Power Wireless personal Area Networks (6LoWPAN) [RFC4944] and Constrained Application Protocol (CoAP) [RFC7252].

CoAP has chosen Datagram Transport Layer Security (DTLS) [RFC6347] as the preferred security protocol for authenticity and confidentiality of the messages. A constrained version of EST, using Coap and DTLS, is described in [I-D.ietf-ace-coap-est].

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 new "joining" device will not be IP routable until it is authenticated to the network. A new "joining" device can only initially use a link-local IPv6 address to communicate with a neighbour node using neighbour discovery [RFC6775] until it receives the necessary network configuration parameters. However, before the device can receive these configuration parameters, it needs to authenticate itself to the network to which it connects. In [I-D.ietf-anima-bootstrapping-keyinfra] Enrolment over Secure Transport (EST) [RFC7030] is used to authenticate the joining device. However, IPv6 routing is necessary to establish a connection between joining device and the EST server.

This document specifies a Join-proxy and protocol to act as intermediary between joining device and EST server to establish a connection between joining device and EST server.

This document is very much inspired by text published earlier in [I-D.kumar-dice-dtls-relay].

2. Terminology

The following terms are defined in [RFC8366], and are used identically as in that document: artifact, imprint, domain, Join Registrar/Coordinator (JRC), Manufacturer Authorized Signing Authority (MASA), pledge, Trust of First Use (TOFU), and Voucher.

3. Requirements Language

In this document, the key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" are to be interpreted as described in BCP 14, RFC 2119 [RFC2119] and indicate requirement levels for compliant STuPiD implementations.

4. Join Proxy functionality

As depicted in the Figure 1, the joining Device, or pledge (P), is more than one hop away from the EST server (E) and not yet authenticated into the network. At this stage, it can only communicate one-hop to its nearest neighbour, the Join proxy (J) using their link-local IPv6 addresses. However, the Pledge (P) needs to communicate with end-to-end security with a Registrar hosting the EST server (E) to authenticate and get the relevant system/network parameters. If the Pledge (P) initiates a DTLS connection to the EST server whose IP address has been pre-configured, then the packets are dropped at the 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.

                      |E |----       +--+        +--+
                      |  |    \      |J |........|P |
                      ++++     \-----|  |        |  |
                   EST server        +--+        +--+
                   REgistrar       Join Proxy   PLedge
                                                "Joining" Device

Figure 1: multi-hop enrolment.

Furthermore, the Pledge (P) may wish to establish a secure connection to the EST server (E) in the network assuming appropriate credentials are exchanged out-of-band, e.g. a hash of the Pledge (P)'s raw public key could be provided to the EST server (E). However, the Pledge (P) is unaware of the IP address of the EST-server (E) to initiate a DTLS connection and perform authentication with.

A DTLS connection is required between Pledge and EST server. To overcome the problems with non-routability of DTLS packets and/ or discovery of the destination address of the EST Server to contact, the Join Proxy is introduced. This Join-Proxy functionality is configured into all authenticated devices in the network which may act as the Join Proxy for newly joining nodes. The Join Proxy allows for routing of the packets from the Pledge using IP routing to the intended EST Server.

5. Join Proxy specification

The Join Proxy can operate in two modes:

In the statefull mode two configuration are envisaged:

5.1. Statefull Join Proxy

In stateful mode, the joining node forwards the DTLS messages to the EST Server.

Assume the Pledge knows the adddress of the EST server. The message is transmitted to the EST Server as if it originated from the joining node, by replacing the IP address and port of the Pledge to the DTLS IP address of the proxy and a randomly chosen port. The DTLS message itself is not modified. Consequently, the Join Proxy must track the ongoing DTLS connections based on the following 4-tuple stored locally:

The EST Server communicates with the Join Proxy as if it were communicating with the Pledge, without any modification required to the DTLS messages. On receiving a DTLS message from the EST Server, the Join Proxy looks up its locally stored 4-tuple array to identify to which Pledge (if multiple exist) the message belongs. The DTLS message's destination address and port are replaced with the link-local address and port of the corresponding Pledge and the DTLS message is then forwarded to the Pledge. The Join Proxy does not modify the DTLS packets and therefore the normal processing and security of DTLS is unaffected.

In Figure 2 the various steps of the process are shown where the EST Server address in known to the Pledge:

| EST Client | Join-Proxy |  EST Server |          Message         |
|    (P)     |     (J)    |     (E)     | Src_IP:port | Dst_IP:port|
|     --ClientHello-->                  |   IP_C:p_C  | IP_S:5684  |
|                    --ClientHello-->   |   IP_R:p_R  | IP_S:5684  |
|                                       |             |            |
|                    <--ServerHello--   |   IP_S:5684 | IP_R:p_R   |
|                            :          |             |            |
|      <--ServerHello--      :          |   IP_S:5684 | IP_C:p_C   |
|              :             :          |             |            |
|              :             :          |       :     |    :       |
|              :             :          |       :     |    :       |
|      --Finished-->                    |   IP_C:p_C  | IP_S:5684  |
|                      --Finished-->    |   IP_R:p_R  | IP_S:5684  |
|                                       |             |            |
|                      <--Finished--    |   IP_S:5684 | IP_R:p_R   |
|        <--Finished---                 |   IP_S:5684 | IP_C:p_C   |
|             :              :          |      :      |     :      |
IP_C:p_C = Link-local IP address and port of EST Client
IP_S:5684 = IP address and coaps port of EST Server
IP_R:p_R = IP address and port of Join Proxy

Figure 2: constrained statefull joining message flow with EST server address known to Join Proxy.

Assume that the pledge does not know the IP address of the EST Server it needs to contact. In that situation, the Join Proxy can be configured with the IP address of a default EST Server that an EST client needs to contact. The EST client initiates its request as if the Join Proxy is the intended EST Server. The Join Proxy changes the IP packet (without modifying the DTLS message) as in the previous case by modifying both the source and destination addresses to forward the message to the intended EST Server. The Join Proxy keeps a similar 4-tuple array to enable translation of the DTLS messages received from the EST Server and forwards it to the EST Client. In Figure 3 the various steps of the message flow are shown:

| EST Client | Join Proxy | EST Server  |          Message         |
|    (P)     |     (J)    |    (E)      | Src_IP:port | Dst_IP:port|
|      --ClientHello-->                 |   IP_C:p_C  | IP_Ra:5684 |
|                    --ClientHello-->   |   IP_Rb:p_Rb| IP_S:5684  |
|                                       |             |            | 
|                    <--ServerHello--   |   IP_S:5684 | IP_Rb:p_Rb |
|                            :          |             |            |
|       <--ServerHello--     :          |   IP_Ra:5684| IP_C:p_C   |
|               :            :          |             |            |
|               :            :          |       :     |    :       |
|               :            :          |       :     |    :       |
|        --Finished-->       :          |   IP_C:p_C  | IP_Ra:5684 |
|                      --Finished-->    |   IP_Rb:p_Rb| IP_S:5684  |
|                                       |             |            |
|                      <--Finished--    |   IP_S:5684 | IP_Rb:p_Rb |
|        <--Finished--                  |   IP_Ra:5684| IP_C:p_C   |
|              :             :          |      :      |     :      |
IP_C:p_C = Link-local IP address and port of DTLS Client
IP_S:5684 = IP address and coaps port of DTLS Server
IP_Ra:5684 = Link-local IP address and coaps port of DTLS Relay
IP_Rb:p_Rb = IP address (can be same as IP_Ra) and port of DTLS Relay

Figure 3: constrained statefull joining message flow with EST server address known to Join Proxy.

5.2. Stateless Join Proxy

The Join-proxy is stateless to minimize the requirements on the constrained Join-proxy device.

When a joining device as a client attempts a DTLS connection to the EST server, it uses its link-local IP address as its IP source address. This message is transmitted one-hop to a neighbour node. Under normal circumstances, this message would be dropped at the neighbour node since the joining device is not yet IP routable or it is not yet authenticated to send messages through the network. However, if the neighbour device has the Join Proxy functionality enabled, it routes the DTLS message to a specific EST Server. Additional security mechanisms need to exist to prevent this routing functionality being used by rogue nodes to bypass any network authentication procedures.

If an untrusted DTLS Client that can only use link-local addressing wants to contact a trusted end-point EST Server, it sends the DTLS message to the Join Proxy. The Join Proxy extends this message into a new type of message called Join ProxY (JPY) message and sends it on to the EST server. The JPY message payload consists of two parts:

On receiving the JPY message, the EST Server retrieves the two parts. The EST Server transiently stores the Header field information. The EST server uses the Contents field to execute the EST server functionality. However, when the EST Server replies, it also extends its DTLS message with the header field in a JPY message and sends it back to the Join Proxy. The Header contains the original source link-local address and port of the DTLS Client from the transient state stored earlier (which can now be discarded) and the Contents field contains the DTLS message.

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

The Figure 4 depicts the message flow diagram when the EST Server end-point address is known only to the Join Proxy:

| EST  Client  | Join Proxy |    EST server |        Message        |
|     (P)      |     (J)    |      (E)      |Src_IP:port|Dst_IP:port|
|      --ClientHello-->                     | IP_C:p_C  |IP_Ra:5684 |
|                    --JPY[H(IP_C:p_C),-->  | IP_Rb:p_Rb|IP_S:5684  |
|                          C(ClientHello)]  |           |           |
|                    <--JPY[H(IP_C:p_C),--  | IP_S:5684 |IP_Rb:p_Rb |
|                         C(ServerHello)]   |           |           |
|      <--ServerHello--                     | IP_Ra:5684|IP_C:p_C   |
|              :                            |           |           |
|              :                            |     :     |    :      |
|                                           |     :     |    :      |
|      --Finished-->                        | IP_C:p_C  |IP_Ra:5684 |
|                    --JPY[H(IP_C:p_C),-->  | IP_Rb:p_Rb|IP_S:5684  |
|                          C(Finished)]     |           |           |
|                    <--JPY[H(IP_C:p_C),--  | IP_S:5684 |IP_Rb:p_Rb |
|                         C(Finished)]      |           |           |
|      <--Finished--                        | IP_Ra:5684|IP_C:p_C   |
|              :                            |     :     |    :      |
IP_C:p_C = Link-local IP address and port of the Pledge
IP_S:5684 = IP address and coaps port of EST Server
IP_Ra:5684 = Link-local IP address and coaps port of Join Proxy
IP_Rb:p_Rb = IP address(can be same as IP_Ra) and port of Join Proxy

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

Figure 4: constrained stateless joining message flow.

5.3. Stateless Message structure

The JPY message is constructed as a payload with media-type application/multipart-core specified in [I-D.ietf-core-multipart-ct]. Header and Contents fields use different media formats:

  1. header field: application/CBOR containing a CBOR array [RFC7049] with the pledge IPv6 Link Local address as a 16-byte binary value, the pledge's UDP port number, if different from 5684, as a CBOR integer, and the proxy's ifindex or other identifier for the physical port on which the pledge is connected. Header is not DTLS encrypted.
  2. Content field: Any of the media types specified in [I-D.ietf-ace-coap-est] and [I-D.ietf-anima-constrained-voucher] dependent on the function that is requested:
 * application/pkcs7-mime; smime-type=server-generated-key
 * application/pkcs7-mime; smime-type=certs-only
 * application/voucher-cms+cbor
 * application/voucher-cose+cbor 
 * application/pkcs8
 * application/csrattrs
 * application/pkcs10 
 * application/pkix-cert  

Examples are shown in Appendix A. The content fields are DTLS encrypted.

6. Comparison of stateless and statefull 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 Proxy needs additional  | No information is      |
| Information |storage to maintain mapping | maintained by the Join |
|             |of the Pledge's address     | Proxy                  |
|             | with the port number       |                        |
|             |being used to communicate   |                        |
|             |with the Server.            |                        |
|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 source and   |
|             |                            |destination addresses.  |
|Specification|The additional functionality|New JPY message to      |
|complexity   |the Proxy to maintain state |encapsulate DTLS message|
|             |information, and modify     |The Server and the proxy|
|             |the source and destination  |have to understand the  |
|             |addresses of the DTLS       |JPY message in order    |
|             |handshake messages          |to process it.          |

Figure 5: Comparison between stateful and stateless mode

7. Discovery

It is assumed that Join-Proxy seamlessly provides a coaps connection between Pledge and coaps EST-server. An additional Registrar is needed to connect the Pledge to an http EST server, see section 8 of [I-D.ietf-ace-coap-est].

The Discovery of the coaps EST server by the Join Proxy follows section 6 of [I-D.ietf-ace-coap-est]. The discovery of the Join-Proxy by the Pledge is an extension to the discovery described in section 4 of [I-D.ietf-anima-bootstrapping-keyinfra]. In particular this section replaces section 4.2 of [I-D.ietf-anima-bootstrapping-keyinfra]. Three discovery cases are discussed: coap discovery, 6tisch discovery and GRASP discovery.

7.1. GRASP discovery

In the context of autonomous networks, discovery takes place via the GRASP protocol as described in [I-D.ietf-anima-bootstrapping-keyinfra]. The port number is.

EDNote: to be specified further

7.2. 6tisch discovery

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

7.3. Coaps discovery

In the context of a coap network without Autonomous 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 [I-D.ietf-anima-bootstrapping-keyinfra].

The presence and location of (path to) the join-proxy resource are discovered by sending a GET request to "/.well-known/core" including a resource type (rt) parameter with the value "brski-proxy" [RFC6690]. Upon success, the return payload will contain the root resource of the Join-Proxy resources. It is up to the implementation to choose its root resource; throughout this document the example root resource /est is used. The example below shows the discovery of the presence and location of join-proxy resources.

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

  RES: 2.05 Content
  </est>; rt="brski-proxy";ct=62

Port numbers, not returned in the example, are assumed to be the default numbers 5683 and 5684 for coap and coaps respectively (sections 12.6 and 12.7 of [RFC7252]. Discoverable port numbers MAY be returned in the <href> of the payload.

8. Security Considerations

It should be noted here that the contents of the CBOR map are not
protected, but that the communication is between the Proxy and a known registrar (a connected UDP socket), and that messages from other origins are ignored.

9. IANA Considerations

This document needs to create a registry for key indices in the CBOR map. It should be given a name, and the amending formula should be IETF Specification.

9.1. Resource Type registry

This specification registers a 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.

  rt="brski-proxy". This EST resource is used to query and return 
  the supported EST resource of a join-proxy placed between Pledge
  and EST server.

10. Acknowledgements

Many thanks for the comments by Brian Carpenter.

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. 00 to 01

12.2. 00 to 00

13. References

13.1. Normative References

[I-D.ietf-6tisch-enrollment-enhanced-beacon] Dujovne, D. and M. Richardson, "IEEE802.15.4 Informational Element encapsulation of 6tisch Join and Enrollment Information", Internet-Draft draft-ietf-6tisch-enrollment-enhanced-beacon-01, January 2019.
[I-D.ietf-ace-coap-est] Stok, P., Kampanakis, P., Richardson, M. and S. Raza, "EST over secure CoAP (EST-coaps)", Internet-Draft draft-ietf-ace-coap-est-10, March 2019.
[I-D.ietf-anima-bootstrapping-keyinfra] Pritikin, M., Richardson, M., Behringer, M., Bjarnason, S. and K. Watsen, "Bootstrapping Remote Secure Key Infrastructures (BRSKI)", Internet-Draft draft-ietf-anima-bootstrapping-keyinfra-19, March 2019.
[I-D.ietf-anima-constrained-voucher] Richardson, M., Stok, P. and P. Kampanakis, "Constrained Voucher Artifacts for Bootstrapping Protocols", Internet-Draft draft-ietf-anima-constrained-voucher-02, September 2018.
[I-D.ietf-core-multipart-ct] Fossati, T., Hartke, K. and C. Bormann, "Multipart Content-Format for CoAP", Internet-Draft draft-ietf-core-multipart-ct-02, August 2018.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049, October 2013.
[RFC8366] Watsen, K., Richardson, M., Pritikin, M. and T. Eckert, "A Voucher Artifact for Bootstrapping Protocols", RFC 8366, DOI 10.17487/RFC8366, May 2018.

13.2. Informative References

[duckling] Stajano, F. and R. Anderson, "The resurrecting duckling: security issues for ad-hoc wireless networks", 1999.
[I-D.kumar-dice-dtls-relay] Kumar, S., Keoh, S. and O. Garcia-Morchon, "DTLS Relay for Constrained Environments", Internet-Draft draft-kumar-dice-dtls-relay-02, October 2014.
[pledge], ., " Unabridged", 2015.
[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.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, DOI 10.17487/RFC6690, August 2012.
[RFC6775] Shelby, Z., 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.
[RFC7030] Pritikin, M., Yee, P. and D. Harkins, "Enrollment over Secure Transport", RFC 7030, DOI 10.17487/RFC7030, October 2013.
[RFC7228] Bormann, C., Ersue, M. and A. Keranen, "Terminology for Constrained-Node Networks", RFC 7228, DOI 10.17487/RFC7228, May 2014.
[RFC7252] Shelby, Z., Hartke, K. and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014.

Appendix A. Stateless Proxy payload examples

Examples are extensions of two examples shown in [I-D.ietf-ace-coap-est].

provisional stake holder examples to be improved and corrected.

A.1. cacerts

The request from Join-Proxy to EST-server looks like:

Get coaps://
(Accept: 62)
(Content-format: 62)
payload =
82                    # array(2)
18 3C                 # unsigned(60)
83                    # array(3)
69                    # text(9)
     464538303A3A414238 # "FE80::AB8"
19 237D               # unsigned(9085)
65                    # text(5)
     6964656E74       # "ident"

The response will then be

 2.05 Content
 (Content-format: 62)
   Payload =
 83                                # array(3)
 18 3C                             # unsigned(60)
 83                                # array(3)
 69                                # text(9)
     464538303A3A414238            # "FE80::AB8"
 19 237D                           # unsigned(9085)
 65                                # text(5)
     6964656E74                    # "ident"
 82                                # array(2)
 19 0119                           # unsigned(281)
 59 027F                           # bytes(639)

A.2. serverkeygen

The request from Join-Proxy to EST-server looks like:

Get coaps://
(Accept: 62)
(Content-Format: 62)
  Payload =
83                                # array(3)
18 3C                             # unsigned(60)
83                                # array(3)
69                                # text(9)
     464538303A3A414238           # "FE80::AB8"
19 237D                           # unsigned(9085)
65                                # text(5)
     6964656E74                   # "ident"
82                                # array(2)
19 011E                           # unsigned(286)
58 D2                             # bytes(210)

The response will then be

 2.05 Content
 (Content-format: 62)
   Payload =
 84                                # array(4)
 18 3C                             # unsigned(60)
 83                                # array(3)
 69                                # text(9)
     464538303A3A414238            # "FE80::AB8"
 19 237D                           # unsigned(9085)
 65                                # text(5)
     6964656E74                    # "ident"
 82                                # array(2)
 19 011E                           # unsigned(286)
 58 8A                             # bytes(138)
 19 0119                              # unsigned(281)
 59 01D3                              # bytes(467)

Authors' Addresses

Michael Richardson Sandelman Software Works EMail:
Peter van der Stok vanderstok consultancy EMail:
Panos Kampanakis Cisco Systems EMail: