Constrained Join Proxy for Bootstrapping Protocols

Internet-Draft	Join Proxy	October 2022
Richardson, et al.	Expires 27 April 2023	[Page]

Abstract

This document extends the work of Bootstrapping Remote Secure Key Infrastructures (BRSKI) by replacing the (stateful) TLS Circuit proxy between Pledge and Registrar with a stateless or stateful Circuit proxy using CoAP which is called the 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.¶

Like the BRSKI Circuit proxy, this constrained Join Proxy eliminates the need of Pledges to have routeable IP addresses before enrolment by utilizing link-local addresses. Use of the constrained Join Proxy also eliminates the need of the Pledge to authenticate to the network or perform network-wide Registrar discover before enrolment.¶

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].¶

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.¶

Figure 1: multi-hop enrollment.

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).¶

4.1. Stateful Join Proxy

In stateful mode, the Join Proxy acts as a UDP "circuit" proxy that does not change the UDP payload (data octets according to [RFC768]) but only rewrites the IP and UDP headers of each packet it receives from Pledge and Registrar.¶

The stateful Join Proxy operates as a 'pseudo' UDP circuit proxy creating and utilizing connection mapping state to rewrite the IP address and UDP port number packet header fields of UDP packets that it forwards between Pledge and Registrar. Figure 2 depiects how this state is used.¶

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

Because UDP does not have the notion of a connection, this document calls this a 'pseudo' connection, whose establishment is solely triggered by receipt of a packet from a Pledge with an IP_p%IF:p_P source for which no mapping state exists, and that is termined by a connection expiry timer E.¶

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 proxy receives an ICMP error message from the Registrar or Plege, for which mapping state exist, the proxy SHOULD map the ICMP message as it would map a UDP message and forward the ICMP message to the Registrar / Pledge. Processing of ICMP messages SHOULD NOT reset the connection expiry timer.¶

To protect itself and the Registrar against malfunctioning Pledges and or denial of service attacks, the join proxy SHOULD limit the number of simultaneous mapping states on per ip address to 2 and the number of simultaneous mapping states per interface to 10. When mapping state can not be built due to exhausted state, the proxy SHOULD return an ICMP error (1), "Destination Port Unreachable" message with code (1), "Communication with destination administratively prohibited".¶

4.2. Stateless Join Proxy

Stateless Join Proxy operation eliminates the need and complexity to maintain per UDP connection mapping state on the proxy and the state machinery to build, maintain and remove this mapping state. It also removes the need to protect this mapping state against DoS attacks and may also reduce memory and CPU requirements on the proxy.¶

Stateless Join Proxy operations works by encapsulating the DTLS messages into a new CoAP header [RFC7252]. This new CoAP header is designed to be as minimalistic as possible. The use of CoAP here costs a XXX bytes more than a custom encapsulation, but simplies much of the operation, as well as permitting the result to pass through CoAP proxies, CoAP to HTTP proxies, and other mechanisms that might be introduced into a network. This also eliminates custom code that is only rarely used, which may reduce bugs.¶

The CoAP payload is configured much as [RFC9031], Section 8.1.1 specifies:¶

The request method is POST.¶
The type is Confirmable (CON).¶
The Proxy-Scheme option is set to "coap".¶
No Uri_Host option is included, as none is technically required.¶
No Uri-Path option is included.¶
The payload is the DTLS payload as received from the Pledge.¶
An extended token [RFC8974] is included to contain some encrypted state that allows replies to be returned to the Pledge.¶

Appendix A shows an example CoAP header, assuming a 16-byte extended token, with the resulting overhead of 28 bytes.¶

When the Join Proxy receives a UDP message from a Pledge, it encodes the Pledges link-local IP address, interface and UDP (source) port of the packet into the extended token. The result is sent to the Registrar from a fixed source UDP port.¶

As described in [RFC7252], Section 5.3.1, when the Registrar sends packets for the Pledge, it MUST return the token field unchanged. This allows the Join Proxy to decode the saved Pledge state, and reconstruct the Pledges link-local IP address, interace and UDP (destination) port for the return packet. Figure 3 shows this per-packet mapping on the Join Proxy.¶

The Registrar transiently stores the extended token field information in case it needs to generate additional messages as a result of DTLS processing.¶

The Registrar uses the payload field to execute the Registrar functionality.¶

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 CoAP message, the Join Proxy processes the CoAP header. It uses the extended token field to route the payload as a DTLS message to the Pledge.¶

In the stateless Join Proxy mode, both the Registrar and the Join Proxy use discoverable UDP join-ports. For the Join Proxy this may be a default CoAP port.¶

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

4.3. Constraucting the extended token

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

The extended token payload is to be reflected by the Registrar when sending reply packets to the Join Proxy. The extended token content is not standardized, but this section provides an non-normative example.¶

As explained in [RFC8974], Section 5.2, the Join Proxy SHOULD encrypt the extended token with a symmetric key known only to the Join Proxy. This key need not persist on a long term basis, and MAY be changed periodically.¶

This is intended to be identical to the mechanism described in Section 7.1 of [RFC9031]. However, since the CoAP layer is inside of the DTLS layer (which is between the Pledge and the Registrar), it is not possible for the Join Proxy to act as an actual CoAP proxy.¶

The context that is stored into the extended token might be constructed with the following CDDL grammar: (This is illustrative only: the contents are not subject to standardization)¶

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

This results in a total of 96 bits, or 12 bytes. The structure stores the srcport, the originating IPv6 Link-Local address, the IPv4/IPv6 family (as a single bit) and an ifindex to provide the link-local scope. This fits nicely into a single AES128 CBC block for instance, resulting in a 16 byte token. The Join Proxy MUST maintain the same context block for all communications from the same Pledge. This implies that any encryption key either does not change during the communication, or that when it does, it is acceptable to break any onboarding connections which have not yet completed. If using a context parameter like defined above, it should be easy for the Join Proxy to meet this requirement without maintaining any local state about the Pledge.¶

Note: when IPv6 is used only the lower 64-bits of the origin IP need to be recorded, because they are all IPv6 Link-Local addresses, so the upper 64-bits are just "fe80::". For IPv4, a Link-Local IPv4 address [RFC3927] would be used, and it would fit into 64-bits. On media where the IID is not 64-bits, a different arrangement will be necessary.¶

For the join messages relayed to a particular Registrar, the Join Proxy SHOULD use the same UDP source port for all messages related to all Pledges. A Join Proxy MAY change the UDP source port, but doing so creates more local state. But, a Join Proxy with multiple CPUs (unlikely in a constrained system, but possible in some future) could, for instance, use different source port numbers to demultiplex connections across CPUs.¶

4.3.1. Processing by Registrar

On reception of a CoAP encapsulated join message by the Registrar, the Registrar processes the CoAP header and extracts the extended token. The extended token will need to be provided as input to a DTLS library [RFC9147], as the 5-tuple of the UDP connection alone does not provide enough context for the Registrar to pick an appropriate context. Note that the socket will need to be used for multiple DTLS flows, which is atypical for how DTLS usually uses sockets.¶

As an alternative, the Registrar could split out the state processing from the DTLS processing, creating new sockets that it maintains, but this just duplicates state across many places. It may still be an advantage for some architectures.¶

Examples are shown in Appendix B.¶

At the CoAP level, within the Constrained BRSKI and the EST-COAP [RFC9148] level, the block option [RFC7959] is often used. The Registrar and the Pledge MUST select a block size that would allow the addition of the additional CoAP header without violating MTU sizes.¶

5. Discovery

5.1. Discovery operations by Join-Proxy

In order to accomodate automatic configuration of the Join-Proxy, it must discover the location and a capabilities of the Registar. Section 10.2 of [I-D.ietf-anima-constrained-voucher] explains the basic mechanism, and this section explains the extensions required to discover if stateless operation is supported.¶

5.1.1. CoAP discovery

Section 10.2.2 of [I-D.ietf-anima-constrained-voucher] describes how to use CoAP Discovery. The stateless Join Proxy requires a different end point that can accept the second CoAP header encapsulation and extended token.¶

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 a port that contain process the CoAP encalsulated DTLS messages.¶

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

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

In the [RFC6690] link format, and [RFC3986], Section 3.2, the authority attribute can not include a port number unless it also includes the IP address.¶

The returned join-port is expected to process the CoAP encapsulated DTLS messages described in section Section 4.3. The scheme is now CoAP, as the outside protocol is CoAP and could be subject to further CoAP operations.¶

An EST/Registrar server running at address 2001:db8:0:abcd::52, with the CoAP processing on port 7634, and the stateful Registrar on port 5683 could reply to a multicast query as follows:¶

  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",

5.1.2. GRASP discovery

Section 10.2.1 of [I-D.ietf-anima-constrained-voucher] describes how to use GRASP [RFC8990] discovery within the ACP to locate the stateful port of the Registrar.¶

A Join Proxy which supports a stateless mode of operation using the mechanism described in Section 4.3 must know where to send the encoded content from the Pledge. The Registrar announces its willingness to use the stateless mechanism by including an additional objective in it's M_FLOOD'ed AN_join_registrar announcements, but with a different objective value.¶

The following changes are necessary with respect to figure 10 of [RFC8995]:¶

The transport-proto is IPPROTO_UDP¶
the objective is AN_join_registrar, identical to [RFC8995].¶
the objective name is "BRSKI_RJP".¶

Here is an example M_FLOOD announcing the Registrar on example port 5685, which is a port number chosen by the Registrar.¶

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

Figure 4: Example of Registrar announcement message

Most Registrars will announce both a CoAP-stateless and stateful ports, and may also announce an HTTPS/TLS service:¶

   [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]]]

Figure 5: Example of Registrar announcing three services

5.2. Pledge discovers Join-Proxy

Regardless of whether the Join Proxy operates in stateful or stateless mode, the Join Proxy is discovered by the Pledge identically. When doing constrained onboarding with DTLS as security, the Pledge will always see an IPv6 Link-Local destination, with a single UDP port to which DTLS messages are to be sent.¶

5.2.1. 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 4.1 of [RFC9148]).¶

5.2.2. 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 following changes are necessary with respect to figure 10 of [RFC8995]:¶

The transport-proto is IPPROTO_UDP¶
the objective is AN_Proxy¶

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] .¶

Here is an example M_FLOOD announcing the Join-Proxy at fe80::1, on standard coaps port 5684.¶

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

Figure 6: Example of Registrar announcement message

5.2.3. 6tisch discovery

The discovery of Join-Proxy by the Pledge uses the enhanced beacons as discussed in [RFC9032]. 6tisch does not use DTLS and so this specification does not apply to it.¶

Table 1
Properties	Stateful mode	Stateless mode
State Information	The Join Proxy needs additional storage to maintain mapping between the address and port number of the Pledge and those of the Registrar.	No information is maintained by the Join Proxy. Registrar needs to store the packet header.
Packet size	The size of the forwarded message is the same as the original message.	Size of the forwarded message is bigger than the original, it includes additional information
Specification complexity	The Join Proxy needs additional functionality to maintain state information, and specify the source and destination addresses of the DTLS handshake messages	CoAP message to encapsulate DTLS payload. The Registrar and the Join Proxy have to understand the CoAP header in order to process it.
Ports	Join Proxy needs discoverable join-port	Join Proxy and Registrar need discoverable join-ports

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.¶

12. References

12.1. Normative References

[family]: "Address Family Numbers", IANA, 19 October 2021, <https://www.iana.org/assignments/address-family-numbers/address-family-numbers.xhtml>.
[I-D.ietf-anima-constrained-voucher]: Richardson, M., Van der Stok, P., Kampanakis, P., and E. Dijk, "Constrained Bootstrapping Remote Secure Key Infrastructure (BRSKI)", Work in Progress, Internet-Draft, draft-ietf-anima-constrained-voucher-18, 11 July 2022, <https://www.ietf.org/archive/id/draft-ietf-anima-constrained-voucher-18.txt>.
[ieee802-1AR]: "IEEE 802.1AR Secure Device Identifier", IEEE Standard, 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>.
[RFC6690]: Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, DOI 10.17487/RFC6690, August 2012, <https://www.rfc-editor.org/info/rfc6690>.
[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>.
[RFC768]: Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI 10.17487/RFC0768, August 1980, <https://www.rfc-editor.org/info/rfc768>.
[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>.
[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>.
[RFC9147]: Rescorla, E., Tschofenig, H., and N. Modadugu, "The Datagram Transport Layer Security (DTLS) Protocol Version 1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022, <https://www.rfc-editor.org/info/rfc9147>.
[RFC9148]: van der Stok, P., Kampanakis, P., Richardson, M., and S. Raza, "EST-coaps: Enrollment over Secure Transport with the Secure Constrained Application Protocol", RFC 9148, DOI 10.17487/RFC9148, April 2022, <https://www.rfc-editor.org/info/rfc9148>.

12.2. Informative References

[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., "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>.
[RFC3927]: Cheshire, S., Aboba, B., and E. Guttman, "Dynamic Configuration of IPv4 Link-Local Addresses", RFC 3927, DOI 10.17487/RFC3927, May 2005, <https://www.rfc-editor.org/info/rfc3927>.
[RFC3986]: Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005, <https://www.rfc-editor.org/info/rfc3986>.
[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>.
[RFC6550]: Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J., Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur, JP., and R. Alexander, "RPL: IPv6 Routing Protocol for Low-Power and Lossy Networks", RFC 6550, DOI 10.17487/RFC6550, March 2012, <https://www.rfc-editor.org/info/rfc6550>.
[RFC6775]: Shelby, Z., Ed., Chakrabarti, S., Nordmark, E., and C. Bormann, "Neighbor Discovery Optimization for IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs)", RFC 6775, DOI 10.17487/RFC6775, November 2012, <https://www.rfc-editor.org/info/rfc6775>.
[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>.
[RFC7959]: Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in the Constrained Application Protocol (CoAP)", RFC 7959, DOI 10.17487/RFC7959, August 2016, <https://www.rfc-editor.org/info/rfc7959>.
[RFC8610]: Birkholz, H., Vigano, C., and C. Bormann, "Concise Data Definition Language (CDDL): A Notational Convention to Express Concise Binary Object Representation (CBOR) and JSON Data Structures", RFC 8610, DOI 10.17487/RFC8610, June 2019, <https://www.rfc-editor.org/info/rfc8610>.
[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>.
[RFC9031]: Vučinić, M., Ed., Simon, J., Pister, K., and M. Richardson, "Constrained Join Protocol (CoJP) for 6TiSCH", RFC 9031, DOI 10.17487/RFC9031, May 2021, <https://www.rfc-editor.org/info/rfc9031>.
[RFC9032]: Dujovne, D., Ed. and M. Richardson, "Encapsulation of 6TiSCH Join and Enrollment Information Elements", RFC 9032, DOI 10.17487/RFC9032, May 2021, <https://www.rfc-editor.org/info/rfc9032>.