Proxy Operations for CoAP Group Communication
draft-tiloca-core-groupcomm-proxy-04

Document Type Active Internet-Draft (individual)
Authors Marco Tiloca  , Esko Dijk 
Last updated 2021-07-12
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CoRE Working Group                                             M. Tiloca
Internet-Draft                                                   RISE AB
Updates: 7252 (if approved)                                      E. Dijk
Intended status: Standards Track                       IoTconsultancy.nl
Expires: 13 January 2022                                    12 July 2021

             Proxy Operations for CoAP Group Communication
                  draft-tiloca-core-groupcomm-proxy-04

Abstract

   This document specifies the operations performed by a forward-proxy
   or reverse-proxy, when using the Constrained Application Protocol
   (CoAP) in group communication scenarios.  Such CoAP proxy processes a
   single request, sent by a CoAP client over unicast, and distributes
   the request over IP multicast to a group of CoAP servers.  It then
   collects the individual responses from these CoAP servers and sends
   these responses to the CoAP client, in a way that allows the client
   to distinguish the responses and their origin servers through
   addressing information.  This document updates RFC7252.

Discussion Venues

   This note is to be removed before publishing as an RFC.

   Discussion of this document takes place on the Constrained RESTful
   Environments Working Group mailing list (core@ietf.org), which is
   archived at https://mailarchive.ietf.org/arch/browse/core/.

   Source for this draft and an issue tracker can be found at
   https://gitlab.com/crimson84/draft-tiloca-core-groupcomm-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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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

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   This Internet-Draft will expire on 13 January 2022.

Copyright Notice

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   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
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   Please review these documents carefully, as they describe your rights
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   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  The Multicast-Signaling Option  . . . . . . . . . . . . . . .   4
   3.  The Response-Forwarding Option  . . . . . . . . . . . . . . .   5
     3.1.  Encoding of Server Address  . . . . . . . . . . . . . . .   8
     3.2.  Default Values of the Server Port Number  . . . . . . . .   8
   4.  Requirements and Objectives . . . . . . . . . . . . . . . . .   9
   5.  Protocol Description  . . . . . . . . . . . . . . . . . . . .  10
     5.1.  Request Sending at the Client . . . . . . . . . . . . . .  10
       5.1.1.  Request Sending . . . . . . . . . . . . . . . . . . .  10
       5.1.2.  Supporting Observe  . . . . . . . . . . . . . . . . .  12
     5.2.  Request Processing at the Proxy . . . . . . . . . . . . .  12
       5.2.1.  Request Processing  . . . . . . . . . . . . . . . . .  12
       5.2.2.  Supporting Observe  . . . . . . . . . . . . . . . . .  13
     5.3.  Request and Response Processing at the Server . . . . . .  13
       5.3.1.  Request and Response Processing . . . . . . . . . . .  13
       5.3.2.  Supporting Observe  . . . . . . . . . . . . . . . . .  14
     5.4.  Response Processing at the Proxy  . . . . . . . . . . . .  14
       5.4.1.  Response Processing . . . . . . . . . . . . . . . . .  14
       5.4.2.  Supporting Observe  . . . . . . . . . . . . . . . . .  15
     5.5.  Response Processing at the Client . . . . . . . . . . . .  16
       5.5.1.  Response Processing . . . . . . . . . . . . . . . . .  16
       5.5.2.  Supporting Observe  . . . . . . . . . . . . . . . . .  17
     5.6.  Example . . . . . . . . . . . . . . . . . . . . . . . . .  17
   6.  Reverse-Proxies . . . . . . . . . . . . . . . . . . . . . . .  19
     6.1.  Processing on the Client Side . . . . . . . . . . . . . .  20
     6.2.  Processing on the Proxy Side  . . . . . . . . . . . . . .  20
   7.  Caching . . . . . . . . . . . . . . . . . . . . . . . . . . .  20
     7.1.  Freshness Model . . . . . . . . . . . . . . . . . . . . .  21
     7.2.  Validation Model  . . . . . . . . . . . . . . . . . . . .  23

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       7.2.1.  Proxy-Servers Revalidation with Unicast Requests  . .  23
       7.2.2.  Proxy-Servers Revalidation with Group Requests  . . .  23
     7.3.  Client-Proxy Revalidation with Group Requests . . . . . .  23
     7.4.  Caching of End-To-End Protected Responses at Proxies  . .  25
       7.4.1.  Deterministic Requests to Achieve Cacheability  . . .  25
       7.4.2.  Validation of Responses . . . . . . . . . . . . . . .  26
   8.  Chain of Proxies  . . . . . . . . . . . . . . . . . . . . . .  27
     8.1.  Request Processing at the Proxy . . . . . . . . . . . . .  27
       8.1.1.  Supporting Observe  . . . . . . . . . . . . . . . . .  29
     8.2.  Response Processing at the Proxy  . . . . . . . . . . . .  29
       8.2.1.  Supporting Observe  . . . . . . . . . . . . . . . . .  30
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  31
     9.1.  Client Authentication . . . . . . . . . . . . . . . . . .  31
     9.2.  Multicast-Signaling Option  . . . . . . . . . . . . . . .  32
     9.3.  Response-Forwarding Option  . . . . . . . . . . . . . . .  33
     9.4.  Group-ETag Option . . . . . . . . . . . . . . . . . . . .  33
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  34
     10.1.  CoAP Option Numbers Registry . . . . . . . . . . . . . .  34
     10.2.  CoAP Transport Information Registry  . . . . . . . . . .  35
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  35
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  35
     11.2.  Informative References . . . . . . . . . . . . . . . . .  37
   Appendix A.  Examples with Reverse-Proxy  . . . . . . . . . . . .  38
     A.1.  Example 1 . . . . . . . . . . . . . . . . . . . . . . . .  39
     A.2.  Example 2 . . . . . . . . . . . . . . . . . . . . . . . .  41
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  43
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  43

1.  Introduction

   The Constrained Application Protocol (CoAP) [RFC7252] allows the
   presence of forward-proxies and reverse-proxies, as intermediary
   entities supporting clients to perform requests on their behalf.

   CoAP supports also group communication over IP multicast
   [I-D.ietf-core-groupcomm-bis], where a group request can be addressed
   to multiple recipient servers, each of which may reply with an
   individual unicast response.  As discussed in Section 3.5 of
   [I-D.ietf-core-groupcomm-bis], this group communication scenario
   poses a number of issues and limitations to proxy operations.

   In particular, the client sends a single unicast request to the
   proxy, which the proxy forwards to a group of servers over IP
   multicast.  Later on, the proxy delivers back to the client multiple
   responses to the original unicast request.  As defined by [RFC7252],
   the multiple responses are delivered to the client inside separate
   CoAP messages, all matching (by Token) to the client's original
   unicast request.  A possible alternative approach of performing

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   aggregation of responses into a single CoAP response would require a
   specific aggregation content-format, which is not available yet.
   Both these approaches have open issues.

   This specification considers the former approach, i.e., the proxy
   forwards the individual responses to a CoAP group request back to the
   client.  The described method addresses all the related issues raised
   in Section 3.5 of [I-D.ietf-core-groupcomm-bis].  To this end, a
   dedicated signaling protocol is defined, using two new CoAP options.

   Using this protocol, the client explicitly confirms its intent to
   perform a proxied group request and its support for receiving
   multiple responses as a result, i.e., one per origin server.  It also
   signals for how long it is willing to wait for responses.  Also, when
   forwarding a response to the client, the proxy indicates the
   addressing information of the origin server.  This enables the client
   to distinguish multiple, diffent responses by origin and to possibly
   contact one or more of the respective servers by sending individual
   unicast request(s) to the indicated address(es).  In doing these
   follow-up unicast requests, the client may optionally bypass the
   proxy.

   Furthermore, this document defines a caching model for proxies and
   specifies how they can serve a group request by using cached
   responses.  Therefore, this document updates [RFC7252].

1.1.  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.

   Readers are expected to be familiar with terms and concepts defined
   in CoAP [RFC7252], Group Communication for CoAP
   [I-D.ietf-core-groupcomm-bis], CBOR [RFC8949], OSCORE [RFC8613] and
   Group OSCORE [I-D.ietf-core-oscore-groupcomm].

   Unless specified otherwise, the term "proxy" refers to a CoAP-to-CoAP
   forward-proxy, as defined in Section 5.7.2 of [RFC7252].

2.  The Multicast-Signaling Option

   The Multicast-Signaling Option defined in this section has the
   properties summarized in Figure 1, which extends Table 4 of
   [RFC7252].

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   Since the option is not Safe-to-Forward, the column "N" indicates a
   dash for "not applicable".  The value of the Multicast-Signaling
   Option specifies a timeout value in seconds, encoded as an unsigned
   integer (see Section 3.2 of [RFC7252]).

     +------+---+---+---+---+------------+--------+--------+---------+
     | No.  | C | U | N | R | Name       | Format | Length | Default |
     +------+---+---+---+---+------------+--------+--------+---------+
     |      |   |   |   |   |            |        |        |         |
     | TBD1 |   | x | - |   | Multicast- |  uint  |  0-5   | (none)  |
     |      |   |   |   |   | Signaling  |        |        |         |
     |      |   |   |   |   |            |        |        |         |
     +------+---+---+---+---+------------+--------+--------+---------+
                C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable

                 Figure 1: The Multicast-Signaling Option.

   This document specifically defines how this option is used by a
   client in a CoAP request, to indicate to a forward-proxy its support
   for and interest in receiving multiple responses to a proxied CoAP
   group request, i.e., one per origin server, and for how long it is
   willing to wait for receiving responses via that proxy (see
   Section 5.1.1 and Section 5.2.1).

   The client, when sending a CoAP group request to a proxy via IP
   unicast, to be forwarded by the proxy to a targeted group of servers,
   includes the Multicast-Signaling Option into the request.  The option
   value indicates after what time period in seconds the client will
   stop accepting responses matching its original unicast request, with
   the exception of notifications if the CoAP Observe Option [RFC7641]
   is used in the same request.  Signaling the time period allows the
   proxy to stop forwarding responses back to the client, that are
   received from servers after the end of the time period.

   The Multicast-Signaling Option is of class U in terms of OSCORE
   processing (see Section 4.1 of [RFC8613]).

3.  The Response-Forwarding Option

   The Response-Forwarding Option defined in this section has the
   properties summarized in Figure 2, which extends Table 4 of
   [RFC7252].  The option is intended only for inclusion in CoAP
   responses, and builds on the Base-Uri option from Section 3 of
   [I-D.bormann-coap-misc].

   Since the option is intended only for responses, the column "N"
   indicates a dash for "not applicable".

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     +------+---+---+---+---+------------+--------+--------+---------+
     | No.  | C | U | N | R | Name       | Format | Length | Default |
     +------+---+---+---+---+------------+--------+--------+---------+
     |      |   |   |   |   |            |        |        |         |
     | TBD2 |   |   | - |   | Response-  |  (*)   | 10-25  | (none)  |
     |      |   |   |   |   | Forwarding |        |        |         |
     |      |   |   |   |   |            |        |        |         |
     +------+---+---+---+---+------------+--------+--------+---------+
                C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable

     (*) See below.

                 Figure 2: The Response-Forwarding Option.

   This document specifically defines how this option is used by a proxy
   that can perform proxied CoAP group communication requests.

   Upon receiving a response to such request from a server, the proxy
   includes the Response-Forwarding Option into the response sent to the
   origin client (see Section 5).  The proxy uses the option to indicate
   the addressing information where the client can send an individual
   request intended to that origin server.

   In particular, the client can use the addressing information
   specified in the option to identify the response originator and
   possibly send it individual requests later on, either directly, or
   indirectly via the proxy, as CoAP unicast requests.

   The option value is set to the byte serialization of the CBOR array
   'tp_info' defined in Section 2.2.1 of
   [I-D.ietf-core-observe-multicast-notifications], including only the
   set of elements 'srv_addr'.  In turn, the set includes the integer
   'tp_id' identifying the used transport protocol, and further elements
   whose number, format and encoding depend on the value of 'tp_id'.

   The value of 'tp_id' MUST be taken from the "Value" column of the
   "CoAP Transport Information" Registry defined in Section 14.4 of
   [I-D.ietf-core-observe-multicast-notifications].  The elements of
   'srv_addr' following 'tp_id' are specified in the corresponding entry
   of the Registry, under the "Server Addr" column.

   If the server is reachable through CoAP transported over UDP, the
   'tp_info' array includes the following elements, encoded as defined
   in Section 2.2.1.1 of
   [I-D.ietf-core-observe-multicast-notifications].

   *  'tp_id': the CBOR integer with value 1.  This element MUST be
      present.

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   *  'srv_host': a CBOR byte string, encoding the unicast IP address of
      the server.  This element is tagged and identified by the CBOR tag
      260 "Network Address (IPv4 or IPv6 or MAC Address)".  This element
      MUST be present.

   *  'srv_port': a CBOR unsigned integer or the CBOR simple value Null.
      This element MAY be present.

      If present as a CBOR unsigned integer, it has as value the
      destination UDP port number to use for individual requests to the
      server.

      If present as the CBOR simple value Null, the client MUST assume
      that the default port number 5683 defined in [RFC7252] can be used
      as the destination UDP port number for individual requests to the
      server.

      If not present, the client MUST assume that the same port number
      specified in the group URI of the original unicast CoAP group
      request sent to the proxy (see Section 5.1.1) can be used for
      individual requests to the server.

   The CDDL notation [RFC8610] provided below describes the 'tp_info'
   CBOR array using the format defined above.

   tp_info = [
          tp_id : 1,             ; UDP as transport protocol
       srv_host : #6.260(bstr),  ; IP address where to reach the server
     ? srv_port : uint / null    ; Port number where to reach the server
   ]

   At present, 'tp_id' is expected to take only value 1 (UDP) when using
   forward proxies, UDP being the only currently available transport for
   CoAP to work over IP multicast.  While additional multicast-friendly
   transports may be defined in the future, other current tranport
   protocols can still be useful in applications relying on a reverse-
   proxy (see Section 6).

   The rest of this section considers the new values of 'tp_id'
   registered by this document (see Section 10.2), and specifies:

   *  The encoding for the elements of 'tp_info' following 'tp_id' (see
      Section 3.1).

   *  The port number assumed by the client if 'srv_port' in 'tp_info'
      specifies the CBOR simple value Null (see Section 3.2).

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   The Response-Forwarding Option is of class U in terms of OSCORE
   processing (see Section 4.1 of [RFC8613]).

3.1.  Encoding of Server Address

   This specification defines some values used as transport protocol
   identifiers, whose respective new entries are included in the "CoAP
   Transport Information" Registry defined in Section 14.4 of
   [I-D.ietf-core-observe-multicast-notifications].

   For each of these values, the following table summarizes the elements
   specified under the "Srv Addr" and "Req Info" columns of the
   registry, together with their CBOR encoding and short description.

   While not listed here for brevity, the element 'tp_id' is always
   present as a CBOR integer in the element set "Srv Addr".

   +----------+-------------+----------+--------------+---------------+
   | 'tp_id'  | Element Set | Element  | CBOR Type    | Description   |
   | Values   |             |          |              |               |
   +----------+-------------+----------+--------------+---------------+
   | 2, 3, 4, | Srv Addr    | srv_host | #6.260(bstr) | Address of    |
   | 5, 6     |             |          |     (*)      | the server    |
   |          |             +----------+--------------+---------------+
   |          |             | srv_port | uint / null  | Port number   |
   |          |             |          |              | of the server |
   |          +-------------+----------+--------------+---------------+
   |          | Req Info    | cli_host | #6.260(bstr) | Address of    |
   |          |             |          |     (*)      | the client    |
   |          |             +----------+--------------+---------------+
   |          |             | cli_port | uint         | Port number   |
   |          |             |          |              | of the client |
   +----------+-------------+----------+--------------+---------------+

   * The CBOR byte string is tagged and identified by the
     CBOR tag 260 "Network Address (IPv4 or IPv6 or MAC Address)".

3.2.  Default Values of the Server Port Number

   If the 'srv_port' element in the 'tp_info' array specifies the CBOR
   simple value Null, the client MUST assume the following value as port
   number where to send individual requests intended to the server,
   based on the value of 'tp_id'.

   *  If 'tp_id' is equal to 2, i.e., CoAP over UDP secured with DTLS,
      the default port number 5684 as defined in [RFC7252].

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   *  If 'tp_id' is equal to 3, i.e., CoAP over TCP, the default port
      number 5683 as defined in [RFC8323].

   *  If 'tp_id' is equal to 4, i.e., CoAP over TCP secured with TLS,
      the default port number 5684 as defined in [RFC8323].

   *  If 'tp_id' is equal to 5, i.e., CoAP over WebSockets, the default
      port number 80 as defined in [RFC8323].

   *  If 'tp_id' is equal to 6, i.e., CoAP over WebSockets secured with
      TLS, the default port number 443 as defined in [RFC8323].

4.  Requirements and Objectives

   This specification assumes that the following requirements are
   fulfilled.

   *  REQ1.  The CoAP proxy is explicitly configured (allow-list) to
      allow proxied CoAP group requests from specific client(s).

   *  REQ2.  The CoAP proxy MUST identify a client sending a CoAP group
      request, in order to verify whether the client is allowed-listed
      to do so.  For example, this can rely on one of the following
      security associations.

      -  A TLS [RFC8446] or DTLS [RFC6347][I-D.ietf-tls-dtls13] channel
         between the client and the proxy, where the client has been
         authenticated during the secure channel establishment.

      -  A pairwise OSCORE [RFC8613] Security Context between the client
         and the proxy, as defined in
         [I-D.tiloca-core-oscore-capable-proxies].

   *  REQ3.  If secure, end-to-end communication is required between the
      client and the servers in the CoAP group, exchanged messages MUST
      be protected by using Group OSCORE
      [I-D.ietf-core-oscore-groupcomm], as discussed in Section 5 of
      [I-D.ietf-core-groupcomm-bis].  This requires the client and the
      servers to have previously joined the correct OSCORE group, for
      instance by using the approach described in
      [I-D.ietf-ace-key-groupcomm-oscore].  The correct OSCORE group to
      join can be pre-configured or alternatively discovered, for
      instance by using the approach described in
      [I-D.tiloca-core-oscore-discovery].

   This specification defines how to achieve the following objectives.

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   *  OBJ1.  The CoAP proxy gets an indication from the client that it
      is in fact interested in and capable to receive multiple responses
      to its unicast request containing a CoAP group URI.

   *  OBJ2.  The CoAP proxy learns how long it should wait for responses
      to a proxied request, before starting to ignore following
      responses (except for notifications, if a CoAP Observe Option is
      used [RFC7641]).

   *  OBJ3.  The CoAP proxy returns individual unicast responses to the
      client, each of which matches the original unicast request made to
      the proxy.

   *  OBJ4.  The CoAP client is able to distinguish the different
      responses to the original unicast request, as well as their
      corresponding origin servers.

   *  OBJ5.  The CoAP client is enabled to optionally contact one or
      more of the responding origin servers in the future, either
      directly or via the CoAP proxy.

5.  Protocol Description

   This section specifies the steps of the signaling protocol.

5.1.  Request Sending at the Client

   This section defines the operations that the client performs for
   sending a request addressed to a group of servers via the CoAP proxy.

5.1.1.  Request Sending

   The client proceeds according to the following steps.

   1.  The client prepares a request addressed to the CoAP proxy.  The
       request specifies the group URI as a string in the Proxi-URI
       option, or by using the Proxy-Scheme option with the group URI
       constructed from the URI-* options (see Section 3.5.1 of
       [I-D.ietf-core-groupcomm-bis]).

   2.  The client MUST retain the Token value used for this original
       unicast request beyond the reception of a first response matching
       it.  To this end, the client follows the same rules for Token
       retention defined for multicast requests in Section 3.1.5 of
       [I-D.ietf-core-groupcomm-bis].

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       In particular, the client picks an amount of time T it is fine to
       wait for before freeing up the Token value.  Specifically, the
       value of T MUST be such that:

       *  T < T_r , where T_r is the amount of time that the client is
          fine to wait for before potentially reusing the Token value.
          Note that T_r MUST NOT be less than MIN_TOKEN_REUSE_TIME
          defined in Section 3.1.5 of [I-D.ietf-core-groupcomm-bis].

       *  T should be at least the expected worst-case time taken by the
          request and response processing on the forward-proxy and on
          the servers in the addressed CoAP group.

       *  T should be at least the expected worst-case round-trip delay
          between the client and the forward-proxy plus the worst-case
          round-trip delay between the proxy and any one of the origin
          servers.

   3.  The client MUST include the Multicast-Signaling Option defined in
       Section 2 into the unicast request to send to the proxy.  The
       option value specifies an amount of time T' < T.  The difference
       (T - T') should be at least the expected worst-case round-trip
       time between the client and the forward-proxy.

       The client can specify T' = 0 as option value, thus indicating to
       be not interested in receiving responses from the origin servers
       through the proxy.  In such a case, the client SHOULD also
       include a No-Response Option [RFC7967] with value 26 (suppress
       all response codes), if it supports the option.

       Consistently, if the unicast request to send to the proxy already
       included a No-Response Option with value 26, the client SHOULD
       specify T' = 0 as value of the Multicast-Signaling Option.

   4.  The client processes the request as defined in
       [I-D.ietf-core-groupcomm-bis], and also as in
       [I-D.ietf-core-oscore-groupcomm] when secure group communication
       is used between the client and the servers.

   5.  The client protects the unicast request resulting at the end of
       step 4, according to the security association it has with the
       proxy.

   6.  The client sends the request to the proxy as a unicast CoAP
       message.

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   The exact method that the client uses to estimate the worst-case
   processing times and round-trip delays mentioned above is out of the
   scope of this specification.  However, such a method is expected to
   be already used by the client when generally determining a good Token
   lifetime and reuse interval.

5.1.2.  Supporting Observe

   When using CoAP Observe [RFC7641], the client follows what is
   specified in Section 3.7 of [I-D.ietf-core-groupcomm-bis], with the
   difference that it sends a unicast request to the proxy, to be
   forwarded to the group of servers, as defined in Section 5.1.1 of
   this specification.

   Furthermore, the client especially follows what is specified in
   Section 5 of [RFC7641], i.e., it registers its interest to be an
   observer with the proxy, as if it was communicating with the servers.

5.2.  Request Processing at the Proxy

   This section defines the operations that the proxy performs when
   receiving a request addressed to a group of servers.

5.2.1.  Request Processing

   Upon receiving the request from the client, the proxy proceeds
   according to the following steps.

   1.  The proxy decrypts the request, according to the security
       association it has with the client.

   2.  The proxy identifies the client, and verifies that the client is
       in fact allowed-listed to have its requests proxyied to CoAP
       group URIs.

   3.  The proxy verifies the presence of the Multicast-Signaling
       Option, as a confirmation that the client is fine to receive
       multiple responses matching the same original request.

       If the Multicast-Signaling Option is not present, the proxy MUST
       stop processing the request and MUST reply to the client with a
       4.00 (Bad Request) response.  The response MUST include a
       Multicast-Signaling Option with an empty (zero-length) value,
       specifying that the Multicast-Signaling Option was missing and
       has to be included in the request.  As per Section 5.9.2 of
       [RFC7252] The response SHOULD include a diagnostic payload.

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   4.  The proxy retrieves the value T' from the Multicast-Signaling
       Option, and then removes the option from the client's request.

   5.  The proxy forwards the client's request to the group of servers.
       In particular, the proxy sends it as a CoAP group request over IP
       multicast, addressed to the group URI specified by the client.

   6.  The proxy sets a timeout with the value T' retrieved from the
       Multicast-Signaling Option of the original unicast request.

       In case T' > 0, the proxy will ignore responses to the forwarded
       group request coming from servers, if received after the timeout
       expiration, with the exception of Observe notifications (see
       Section 5.4).

       In case T' = 0, the proxy will ignore all responses to the
       forwarded group request coming from servers.

   If the proxy supports caching of responses, it can serve the original
   unicast request also by using cached responses, as per Section 7.

5.2.2.  Supporting Observe

   When using CoAP Observe [RFC7641], the proxy takes the role of the
   client and registers its own interest to observe the target resource
   with the servers as per Section 5 of [RFC7641].

   When doing so, the proxy especially follows what is specified for the
   client in Section 3.7 of [I-D.ietf-core-groupcomm-bis], by forwarding
   the group request to the servers over IP multicast, as defined in
   Section 5.2.1 of this specification.

5.3.  Request and Response Processing at the Server

   This section defines the operations that the server performs when
   receiving a group request from the proxy.

5.3.1.  Request and Response Processing

   Upon receiving the request from the proxy, the server proceeds
   according to the following steps.

   1.  The server processes the group request as defined in
       [I-D.ietf-core-groupcomm-bis], and also as in
       [I-D.ietf-core-oscore-groupcomm] when secure group communication
       is used between the client and the server.

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   2.  The server processes the response to be forwarded back to the
       client as defined in [I-D.ietf-core-groupcomm-bis], and also as
       in [I-D.ietf-core-oscore-groupcomm] when secure group
       communication is used between the client and the server.

5.3.2.  Supporting Observe

   When using CoAP Observe [RFC7641], the server especially follows what
   is specified in Section 3.7 of [I-D.ietf-core-groupcomm-bis] and
   Section 5 of [RFC7641].

5.4.  Response Processing at the Proxy

   This section defines the operations that the proxy performs when
   receiving a response matching a forwarded group request.

5.4.1.  Response Processing

   Upon receiving a response matching the group request before the
   amount of time T' has elapsed, the proxy proceeds according to the
   following steps.

   1.  The proxy MUST include the Response-Forwarding Option defined in
       Section 3 into the response.  The proxy specifies as option value
       the addressing information of the server generating the response,
       encoded as defined in Section 3.  In particular:

       *  The 'srv_addr' element of the 'srv_info' array MUST specify
          the server IPv6 address if the multicast request was destined
          for an IPv6 multicast address, and MUST specify the server
          IPv4 address if the multicast request was destined for an IPv4
          address.

       *  If present, the 'srv_port' element of the 'srv_info' array
          MUST specify the port number of the server as the source port
          number of the response.  This element MUST be present if the
          source port number of the response differs from the port
          number specified in the group URI of the original unicast CoAP
          group request (see Section 5.1.1).  Otherwise, the 'srv_port'
          element MAY be omitted.

   2.  The proxy protects the response according to the security
       association it has with the client.

   3.  The proxy forwards the response back to the client.

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   As discussed in Section 3.1.6 of [I-D.ietf-core-groupcomm-bis], it is
   possible that a same server replies with multiple responses to the
   same group request, i.e., with the same Token.  As long as the proxy
   forwards responses to a group request back to the origin client, the
   proxy MUST follow the steps defined above and forward also such
   multiple responses "as they come".

   Upon timeout expiration, i.e., T' seconds after having sent the group
   request over IP multicast, the proxy frees up its local Token value
   associated to that request.  Thus, following late responses to the
   same group request will be discarded and not forwarded back to the
   client.

5.4.2.  Supporting Observe

   When using CoAP Observe [RFC7641], the proxy acts as a client
   registered with the servers, as described earlier in Section 5.2.2.

   Furthermore, the proxy takes the role of a server when forwarding
   notifications from origin servers back to the client.  To this end,
   the proxy follows what is specified in Section 3.7 of
   [I-D.ietf-core-groupcomm-bis] and Section 5 of [RFC7641], with the
   following additions.

   *  At step 1 in Section 5.4, the proxy includes the Response-
      Forwarding Option in every notification, including non-2.xx
      notifications resulting in removing the proxy from the list of
      observers of the origin server.

   *  The proxy frees up its Token value used for a group observation
      only if, after the timeout expiration, no 2.xx (Success) responses
      matching the group request and also including an Observe option
      have been received from any origin server.  After that, as long as
      observations are active with servers in the group for the target
      resource of the group request, notifications from those servers
      are forwarded back to the client, as defined in Section 5.4, and
      the Token value used for the group observation is not freed during
      this time.

   Finally, the proxy SHOULD regularly verify that the client is still
   interested in receiving observe notifications for a group
   observation.  To this end, the proxy can rely on the same approach
   discussed for servers in Section 3.7 of
   [I-D.ietf-core-groupcomm-bis], with more details available in
   Section 4.5 of [RFC7641].

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5.5.  Response Processing at the Client

   This section defines the operations that the client performs when
   receiving a response matching a request addressed to a group of
   servers via the CoAP proxy.

5.5.1.  Response Processing

   Upon receiving from the proxy a response matching the original
   unicast request before the amount of time T has elapsed, the client
   proceeds according to the following steps.

   1.  The client processes the response as defined in
       [I-D.ietf-core-groupcomm-bis].

   2.  The client decrypts the response, according to the security
       association it has with the proxy.

   3.  If secure group communication is used end-to-end between the
       client and the servers, the client processes the response
       resulting at the end of step 2, as defined in
       [I-D.ietf-core-oscore-groupcomm].

   4.  The client identifies the origin server, whose addressing
       information is specified as value of the Response-Forwarding
       Option.  If the port number is omitted in the value of the
       Response-Forwarding Option, the client MUST assume that the port
       number where to send unicast requests to the server -- in case
       this is needed -- is the same port number specified in the group
       URI of the original unicast CoAP group request sent to the proxy
       (see Section 5.1.1).

       In particular, the client is able to distinguish different
       responses as originated by different servers.  Optionally, the
       client may contact one or more of those servers individually,
       i.e., directly (bypassing the proxy) or indirectly (via a proxied
       CoAP unicast request).

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       In order to individually reach an origin server again through the
       proxy, the client is not required to understand or support the
       transport protocol indicated in the Response-Forwarding Option,
       as used between the proxy and the origin server, in case it
       differs from "UDP" (1).  That is, using the IPv4/IPv6 address
       value and optional port value from the Response-Forwarding
       Option, the client simply creates the correct URI for the
       individual request, by means of the Proxy-Uri or Uri-Scheme
       Option in the unicast request to the proxy.  The client uses the
       transport protocol it knows, and has used before, to send the
       request to the proxy.

   As discussed in Section 3.1.6 of [I-D.ietf-core-groupcomm-bis], it is
   possible that the client receives multiple responses to the same
   group request, i.e., with the same Token, from the same origin
   server.  The client normally processes at the CoAP layer each of
   those responses from the same origin server, and decides how to
   exactly handle them depending on its available context information
   (see Section 3.1.6 of [I-D.ietf-core-groupcomm-bis]).

   Upon the timeout expiration, i.e., T seconds after having sent the
   original unicast request to the proxy, the client frees up its local
   Token value associated to that request.  Note that, upon this timeout
   expiration, the Token value is not eligible for possible reuse yet
   (see Section 5.1.1).  Thus, until the actual amount of time before
   enabling Token reusage has elapsed, any following late responses to
   the same request forwarded by the proxy will be discarded, as these
   are not matching (by Token) any active request from the client.

5.5.2.  Supporting Observe

   When using CoAP Observe [RFC7641], the client frees up its Token
   value only if, after the timeout T expiration, no 2.xx (Success)
   responses matching the original unicast request and also including an
   Observe option have been received.

   Instead, if at least one such response has been received, the client
   continues receiving those notifications as forwarded by the proxy, as
   long as the observation for the target resource of the original
   unicast request is active.

5.6.  Example

   The example in this section refers to the following actors.

   *  One origin client C, with address C_ADDR and port number C_PORT.

   *  One proxy P, with address P_ADDR and port number P_PORT.

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   *  Two origin servers S1 and S2, where the server Sx has address
      Sx_ADDR and port number Sx_PORT.

   The origin servers are members of a CoAP group with IP multicast
   address G_ADDR and port number G_PORT.  Also, the origin servers are
   members of a same application group, and share the same resource /r.

   The communication between C and P is based on CoAP over UDP, as per
   [RFC7252].  The communication between P and the origin servers is
   based on CoAP over UDP and IP multicast, as per
   [I-D.ietf-core-groupcomm-bis].

   Finally, 'bstr(X)' denotes a CBOR byte string with value the byte
   serialization of X.

   C                          P                      S1           S2
   |                          |                      |             |
   |------------------------->|                      |             |
   | Src: C_ADDR:C_PORT       |                      |             |
   | Dst: P_ADDR:P_PORT       |                      |             |
   | Proxi-URI {              |                      |             |
   |  coap://G_ADDR:G_PORT/r  |                      |             |
   | }                        |                      |             |
   | Multicast-Signaling: 60  |                      |             |
   |                          |                      |             |
   |                          | Src: P_ADDR:P_PORT   |             |
   |                          | Dst: G_ADDR:G_PORT   |             |
   |                          | Uri-Path: /r         |             |
   |                          |---------------+----->|             |
   |                          |                \     |             |
   |                          |                 +----------------->|
   |                          |                      |             |
   |                          | /* t = 0 : P starts  |             |
   |                          | accepting responses  |             |
   |                          | for this request */  |             |
   |                          |                      |             |
   |                          |                      |             |
   |                          |<---------------------|             |
   |                          | Src: S1_ADDR:G_PORT  |             |
   |                          | Dst: P_ADDR:P_PORT   |             |
   |                          |                      |             |
   |                          |                      |             |
   |<-------------------------|                      |             |
   | Src: P_ADDR:P_PORT       |                      |             |
   | Dst: C_ADDR:C_PORT       |                      |             |
   | Response-Forwarding {    |                      |             |
   |  [1, /*CoAP over UDP*/   |                      |             |
   |   #6.260(bstr(S1_ADDR))  |                      |             |

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   |  ]                       |                      |             |
   | }                        |                      |             |
   |                          |<-----------------------------------|
   |                          |               Src: S2_ADDR:S2_PORT |
   |                          |               Dst: P_ADDR:P_PORT   |
   |                          |                      |             |
   |                          |                      |             |
   |                          |                      |             |
   |<-------------------------|                      |             |
   | Src: P_ADDR:P_PORT       |                      |             |
   | Dst: C_ADDR:C_PORT       |                      |             |
   | Response-Forwarding {    |                      |             |
   |  [1, /*CoAP over UDP*/   |                      |             |
   |   #6.260(bstr(S2_ADDR)), |                      |             |
   |   S2_PORT                |                      |             |
   |  ]                       |                      |             |
   | }                        |                      |             |
   |            /* At t = 60, P stops accepting      |             |
   |            responses for this request */        |             |
   |                          |                      |             |

              Figure 3: Workflow example with a forward-proxy

6.  Reverse-Proxies

   The use of reverse-proxies in group communication scenarios is
   defined in Section 3.5.2 of [I-D.ietf-core-groupcomm-bis].

   This section clarifies how the Multicast-Signaling Option is
   effective also in such a context, in order for:

   *  The proxy to explictly reveal itself as a reverse-proxy to the
      client.

   *  The client to indicate to the proxy of being aware that it is
      communicating with a reverse-proxy, and for how long it is willing
      to receive responses to a proxied request.

   This practically addresses the addional issues compared to the case
   with a forward-proxy, as compiled in Section 3.5.2 of
   [I-D.ietf-core-groupcomm-bis].  A reverse-proxy may also operate
   without support of the Multicast-Signaling Option, as defined in that
   section.

   Appendix A provides examples with a reverse-proxy.

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6.1.  Processing on the Client Side

   If a client sends a request intended to a group of servers and is
   aware of actually communicating with a reverse-proxy, then the client
   MUST perform the steps defined in Section 5.1.1.  In particular, this
   results in a request sent to the proxy including a Multicast-
   Signaling Option.

   The client processes the responses forwarded back by the proxy as
   defined in Section 5.5.

6.2.  Processing on the Proxy Side

   If the proxy receives a request and determines that it should forward
   it to a group of servers over IP multicast, then the proxy MUST
   perform the steps defined in Section 5.2.

   In particular, when such request does not include a Multicast-
   Signaling Option, the proxy explicitly reveals itself as a reverse-
   proxy, by sending a 4.00 (Bad Request) response including a
   Multicast-Signaling Option with empty (zero-length) value.

7.  Caching

   A proxy MAY cache responses to a group request, as defined in
   Section 5.7.1 of [RFC7252].  In particular, these same rules apply to
   determine the set of request options used as "Cache-Key", and to
   determine the max-age values offered for responses served from the
   cache.

   A cache entry is associated to one server and stores one response
   from that server, regardless whether it is a response to a unicast
   request or to a group request.  The following two types of requests
   can produce a hit to a cache entry.

   *  A matching request intended to that server, i.e., to the
      corresponding unicast URI.

      When the stored response is a response to a unicast request to the
      server, the unicast URI of the matching request is the same target
      URI used for the original unicast request.

      When the stored response is a response to a group request to the
      CoAP group, the unicast URI of the matching request is the target
      URI obtained by replacing the authority part of the group URI in
      the original group request with the transport-layer source address
      and port number of the response.

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   *  A matching group request intended to the CoAP group, i.e., to the
      corresponding group URI.

      That is, a matching group request produces a hit to multiple cache
      entries, each of which associated to one of the CoAP servers
      currently member of the CoAP group.

      Note that, as per the freshness model defined in Section 7.1, the
      proxy might serve a group request exclusively from its cached
      responses only when it knows all the CoAP servers that are current
      members of the CoAP group and it has a valid cache entry for each
      of them.

   When forwarding a GET or FETCH group request to the servers in the
   CoAP group, a proxy behaves like a CoAP client as defined in
   Section 3.2 of [I-D.ietf-core-groupcomm-bis], with the following
   additions.

   *  As discussed in Section 5.4.1, the proxy can receive multiple
      responses to the same group request from a same origin server, and
      forwards them back to the origin client "as they come".  When this
      happens, each of such multiple responses is stored in the cache
      entry associated to the server "as it comes", possibly replacing
      an already stored response from that server.

   *  As discussed in Section 7.4, when communications in the group are
      secured with Group OSCORE [I-D.ietf-core-oscore-groupcomm],
      additional means are required to enable cacheability of responses
      at the proxy.

   The following subsections define the freshness model and validation
   model that the proxy uses for cached responses.

7.1.  Freshness Model

   The proxy relies on the same freshness model defined in Section 3.2.1
   of [I-D.ietf-core-groupcomm-bis], by taking the role of a CoAP client
   with respect to the servers in the CoAP group.

   In particular, when receiving a group request, the proxy MAY serve
   the request by using exclusively cached responses without forwarding
   the group request to the servers in the CoAP group, but only if both
   the following conditions hold.

   *  The proxy knows all the CoAP servers that are currently members of
      the CoAP group for which the group request is intended to.

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   *  The proxy's cache currently stores a fresh response for each of
      those CoAP servers.

   The specific way that the proxy uses to determine the CoAP servers
   currently members of the target CoAP group is out of scope for this
   document.  As possible examples, the proxy can synchronize with a
   group manager server; rely on well-known time patterns used in the
   application or in the network for the addition of new CoAP group
   members; observe group join requests or IGMP/MLD multicast group join
   messages, e.g., if embedded in a multicast router.

   When forwarding the group request to the servers, the proxy may have
   fresh responses stored in its cache for (some of) those servers.  In
   such a case, the proxy uses (also) those cached responses to serve
   the original unicast request, as defined below.

   *  The request processing in Section 5.2.1 is extended as follows.

      After setting the timeout with value T' > 0 in step 6, the proxy
      checks whether its cache currently stores fresh responses to the
      group request.  For each of such responses, the proxy compares the
      residual lifetime L of the corresponding cache entry against the
      value T'.

      If a cached response X is such that L < T', then the proxy
      forwards X back to the client at its earliest convenience.
      Otherwise, the proxy does not forward X back to the client right
      away, and rather waits for approaching the timeout expiration, as
      discussed in the next point.

   *  The response processing in Section 5.4.1 is extended as follows.

      Before the timeout with original value T' > 0 expires and the
      proxy stops accepting responses to the group request, the proxy
      checks whether it stores in its cache any fresh response X to the
      group request such that both the following conditions hold.

      -  The cache entry E storing X was already existing when
         forwarding the group request.

      -  The proxy has received no response to the forwarded group
         request from the server associated to E.

      Then, the proxy forwards back to the client each response X stored
      in its cache and selected as above, before the timeout expires.

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      Note that, from the forwarding of the group request until the
      timeout expiration, the proxy still forwards responses to the
      group request back to the client as they come (see Section 5.4.1).
      Also, such responses possibly refresh older responses from the
      same servers that the proxy has stored in its cache, as defined
      earlier in Section 7.

7.2.  Validation Model

   This section defines the revalidation of responses, separately
   between the proxy and the origin servers, as well as between the
   origin client and the proxy.

7.2.1.  Proxy-Servers Revalidation with Unicast Requests

   The proxy MAY revalidate a cached response by making a GET or FETCH
   request on the related unicast request URI, i.e., by taking the role
   of a CoAP client with respect to a server in the CoAP group.

   As discussed in Section 7.4, this is however not possible for the
   proxy if communications in the group are secured end-to-end between
   origin client and origin servers by using Group OSCORE
   [I-D.ietf-core-oscore-groupcomm].

7.2.2.  Proxy-Servers Revalidation with Group Requests

   When forwarding a group request to the servers in the CoAP group, the
   proxy MAY revalidate one of more stored responses it has cached.

   To this end, the proxy relies on the same validation model defined in
   Section 3.2.2 of [I-D.ietf-core-groupcomm-bis] and using the ETag
   Option, by taking the role of a CoAP client with respect to the
   servers in the CoAP group.

   As discussed in Section 7.4, this is however not possible for the
   proxy if communications in the group are secured end-to-end between
   origin client and origin servers by using Group OSCORE
   [I-D.ietf-core-oscore-groupcomm].

7.3.  Client-Proxy Revalidation with Group Requests

   A client MAY revalidate the full set of responses to a group request
   by leveraging the corresponding cache entries at the proxy.  To this
   end, this specification defines the new Group-ETag Option.

   The Group-ETag Option has the properties summarized in Figure 4,
   which extends Table 4 of [RFC7252].  The Group-ETag Option is
   elective, safe to forward, part of the cache key, and repeatable.

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   The option is intended for group requests sent to a proxy, as well as
   for the associated responses.

     +------+---+---+---+---+------------+--------+--------+---------+
     | No.  | C | U | N | R | Name       | Format | Length | Default |
     +------+---+---+---+---+------------+--------+--------+---------+
     |      |   |   |   |   |            |        |        |         |
     | TBD3 |   |   |   | x | Group-ETag | opaque |  1-8   | (none)  |
     |      |   |   |   |   |            |        |        |         |
     +------+---+---+---+---+------------+--------+--------+---------+
                C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable

                      Figure 4: The Group-ETag Option.

   The Group-ETag Option has the same properties of the ETag Option
   defined in Section 5.10.6 of [RFC7252].

   The Group-ETag Option is of class U in terms of OSCORE processing
   (see Section 4.1 of [RFC8613]).

   A proxy MUST NOT provide this form of validation if it is not in a
   position to serve a group request by using exclusively cached
   responses, i.e., without forwarding the group request to the servers
   in the CoAP group (see Section 7.1).

   If the proxy supports this form of response revalidation, the
   following applies.

   *  The proxy defines J as a joint set including all the cache entries
      currently storing fresh responses that satisfy a group request.  A
      set J is "complete" if it includes a valid cache entry for each of
      the CoAP servers currently members of the CoAP group.

   *  When the set J becomes "complete", the proxy assigns it an entity-
      tag value.  The proxy MUST update the current entity-tag value,
      when J is "complete" and one of its cache entry is updated.

   *  When returning a 2.05 (Content) response to a GET or FETCH group
      request, the proxy MAY include one Group-ETag Option, in case the
      set J is "complete".  Such a response MUST NOT include more than
      one Group-ETag Option.  The option value specifies the entity-tag
      value currently associated to the set J.

   When sending a GET or FETCH group request to the proxy, to be
   forwarded to a CoAP group, the client MAY include one or more Group-
   ETag Options.  Each option specifies one entity-tag value, applicable
   to the set J of cache entries that can be hit by the group request.

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   The proxy MAY perform the following actions, in case the group
   request produces a hit to the cache entry of each CoAP server
   currently member of the CoAP group, i.e., the set J associated to the
   group request is "complete".

   *  The proxy checks whether the current entity-tag value of the set J
      matches with one of the entity-tag values specified in the Group-
      ETag Options of the group request.

   *  In case of positive match, the proxy replies with a single 2.03
      (Valid) response.  This response has no payload and MUST include
      one Group-ETag Option, specifying the current entity-tag value of
      the set J.

   That is, the 2.03 (Valid) response from the proxy indicates to the
   client that the stored responses idenfied by the entity-tag given in
   the response's Group-ETag Option can be reused, after updating each
   of them as described in Section 5.9.1.3 of [RFC7252].  In effect, the
   client can determine if any of the stored representations from the
   respective cache entries at the proxy is current, without needing to
   transfer any of them again.

7.4.  Caching of End-To-End Protected Responses at Proxies

   When using Group OSCORE [I-D.ietf-core-oscore-groupcomm] to protect
   communications end-to-end between a client and multiple servers in
   the group, it is normally not possible for an intermediary proxy to
   cache protected responses.

   In fact, when starting from the same plain CoAP message, different
   clients generate different protected requests to send on the wire.
   This prevents different clients to generate potential cache hits, and
   thus makes response caching at the proxy pointless.

7.4.1.  Deterministic Requests to Achieve Cacheability

   For application scenarios that use secure group communication, it is
   still possible to achieve cacheability of responses at proxies, by
   using the approach defined in [I-D.amsuess-core-cachable-oscore]
   which is based on Deterministic Requests protected with the pairwise
   mode of Group OSCORE.  This approach is limited to group requests
   that are safe (in the RESTful sense) to process and do not yield side
   effects at the server.  As for any protected group request, it
   requires the clients and all the servers in the CoAP group to have
   already joined the correct OSCORE group.

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   Starting from the same plain CoAP request, this allows different
   clients in the OSCORE group to deterministically generate a same
   request protected with Group OSCORE, which is sent to the proxy for
   being forwarded to the CoAP group.  The proxy can now effectively
   cache the resulting responses from the servers in the CoAP group,
   since the same plain CoAP request will result again in the same
   Deterministic Request and thus will produce a cache hit.

   When caching of Group OSCORE secured responses is enabled at the
   proxy, the same as defined in Section 7 applies, with respect to
   cache entries and their lifetimes.

   Note that different Deterministic Requests result in different cache
   entries at the proxy.  This includes the case where different plain
   group requests differ only in their set of ETag Options, as defined
   in Section 3.2.2 of [I-D.ietf-core-groupcomm-bis].

   That is, even though the servers would produce the same plain CoAP
   responses in reply to two different Deterministic Requests, those
   will result in different protected responses to each respective
   Deterministic Request, hence in different cache entries at the proxy.

   Thus, given a plain group request, a client needs to reuse the same
   set of ETag Options, in order to send that group request as a
   Deterministic Request that can actually produce a cache hit at the
   proxy.  However, while this would prevent the caching at the proxy to
   be inefficient and unnecessarily redundant, it would also limit the
   flexibility of end-to-end response revalidation for a client.

7.4.2.  Validation of Responses

   Response revalidation remains possible end-to-end between the client
   and the servers in the group, by means of including inner ETag
   Option(s) as defined in Sections 3.2 and 3.2.2 of
   [I-D.ietf-core-groupcomm-bis].

   Furthermore, it remains possible for a client to attempt revalidating
   responses to a group request from a "complete" set of cache entries
   at the proxy, by using the Group-ETag Option as defined in
   Section 7.3.

   When directly interacting with the servers in the CoAP group to
   refresh its cache entries, the proxy cannot rely on response
   revalidation anymore.  This applies to both the case where the
   request is addressed to a single server and sent to the related
   unicast URI (see Section 7.2.1) or instead is a group request
   addressed to the CoAP group and sent to the related group URI (see
   Section 7.2.2).

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8.  Chain of Proxies

   A client may be interested to access a resource at a group of origin
   servers which is reached through a chain of two or more proxies.

   That is, these proxies are configured into a chain, where each non-
   last proxy is configured to forward CoAP (group) requests to the next
   hop towards the origin servers.  Also, each non-first proxy is
   configured to forward back CoAP responses to (the previous hop proxy
   towards) the origin client.

   This section specifies how the signaling protocol defined in
   Section 5 is used in that setting.  Except for the last proxy before
   the origin servers, every other proxy in the chain takes the role of
   client with respect to the next hop towards the origin servers.
   Also, every proxy in the chain except the first takes the role of
   server towards the previous proxy closer to the origin client.

   Accordingly, possible caching of responses at each proxy works as
   defined in Section 7 and Section 7.4.  Also, possible revalidation of
   responses cached ad each proxy and based on the Group-ETag option
   works as defined in Section 7.3 and Section 7.4.2.

   The requirements REQ1 and REQ2 defined in Section 4 MUST be fulfilled
   for each proxy in the chain.  That is, every proxy in the chain has
   to be explicitly configured (allow-list) to allow proxied group
   requests from specific senders, and MUST identify those senders upon
   receiving their group request.  For the first proxy in the chain,
   that sender is the origin client.  For each other proxy in the chain,
   that sender is the previous hop proxy closer to the origin client.
   In either case, a proxy can identify the sender of a group request by
   the same means mentioned in Section 4.

8.1.  Request Processing at the Proxy

   Upon receiving a group request to be forwarded to a CoAP group URIs,
   a proxy proceed as follows.

   If the proxy is the last one in the chain, i.e., it is the last hop
   before the origin servers, the proxy performs the steps defined in
   Section 5.2, with no modifications.

   Otherwise, the proxy performs the steps defined in Section 5.2, with
   the following differences.

   *  At steps 1-3, "client" refers to the origin client for the first
      proxy in the chain; or to the previous hop proxy closer to the
      origin client, otherwise.

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   *  At step 4, the proxy rather performs the following actions.

      1.  The proxy retrieves the value T' from the Multicast-Signaling
          Option, and does not remove the option.

      2.  In case T' > 0, the proxy picks an amount of time T it is fine
          to wait for before freeing up its local Token value to use
          with the next hop towards the origin servers.  To this end,
          the proxy MUST follow what is defined at step 2 of
          Section 5.1.1 for the origin client, with the following
          differences.

          -  T MUST be greater than the retrieved value T', i.e., T' <
             T.

          -  The worst-case message processing time takes into account
             all the next hops towards the origin servers, as well as
             the origin servers themselves.

          -  The worst-case round-trip delay takes into account all the
             legs between the proxy and the origin servers.

      3.  In case T' > 0, the proxy replaces the value of the Multicast-
          Signaling Option with a new value T'', such that:

          -  T'' < T.  The difference (T - T'') should be at least the
             expected worst-case round-trip time between the proxy and
             the next hop towards the origin servers.

          -  T'' < T'.  The difference (T' - T'') should be at least the
             expected worst-case round-trip time between the proxy and
             the (previous hop proxy closer to the) origin client.

          If the proxy is not able to determine a value T'' that
          fulfills both the requirements above, the proxy MUST stop
          processing the request and MUST respond with a 5.05 (Proxying
          Not Supported) error response to the previous hop proxy closer
          to the origin client.  The proxy SHOULD include a Multicast-
          Signaling Option, set to the minimum value T' that would be
          acceptable in the Multicast-Signaling Option of a request to
          forward.

          Upon receiving such an error response, any proxy in the chain
          MAY send an updated request to the next hop towards the origin
          servers, specifying in the Multicast-Signaling Option a value
          T' greater than in the previous request.  If this does not
          happen, the proxy receiving the error response MUST also send
          a 5.05 (Proxying Not Supported) error response to the previous

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          hop proxy closer to the origin client.  Like the received one,
          also this error response SHOULD include a Multicast-Signaling
          Option, set to the minimum value T' acceptable by the proxy
          sending the error response.

   *  At step 5, the proxy forwards the request to the next hop towards
      the origin servers.

   *  At step 6, the proxy sets a timeout with the value T' retrieved
      from the Multicast-Signaling Option of the request received from
      the (previous hop proxy closer to the) origin client.

      In case T' > 0, the proxy will ignore responses to the forwarded
      group request coming from the (next hop towards the) origin
      servers, if received after the timeout expiration, with the
      exception of Observe notifications (see Section 5.4).

      In case T' = 0, the proxy will ignore all responses to the
      forwarded group request coming from the (next hop towards the)
      origin servers.

8.1.1.  Supporting Observe

   When using CoAP Observe [RFC7641], what is defined in Section 5.2.2
   applies for the last proxy in the chain, i.e., the last hop before
   the origin servers.

   Any other proxy in the chain acts as a client and registers its own
   interest to observe the target resource with the next hop towards the
   origin servers, as per Section 5 of [RFC7641].

8.2.  Response Processing at the Proxy

   Upon receiving a response matching the group request before the
   amount of time T' has elapsed, the proxy proceeds as follows.

   If the proxy is the last one in the chain, i.e., it is the last hop
   before the origin servers, the proxy performs the steps defined in
   Section 5.4, with no modifications.

   Otherwise, the proxy performs the steps defined in Section 5.4, with
   the following differences.

   *  The proxy skips step 1.  In particular, the proxy MUST NOT remove,
      alter or replace the Response-Forwarding Option.

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   *  At steps 2-3, "client" refers to the origin client for the first
      proxy in the chain; or to the previous hop proxy closer to the
      origin client, otherwise.

   As to the possible reception of multiple responses to the same group
   request from the same (next hop proxy towards the) origin server, the
   same as defined in Section 5.4.1 applies.  That is, as long as the
   proxy forwards responses to a group request back to the (previous hop
   proxy closer to the) origin client, the proxy MUST follow the steps
   above and forward also such multiple responses "as they come".

   Upon timeout expiration, i.e., T seconds after having sent the group
   request to the next hop towards the origin servers, the proxy frees
   up its local Token value associated to that request.  Thus, following
   late responses to the same group request will be discarded and not
   forwarded back to the (previous hop proxy closer to the) origin
   client.

8.2.1.  Supporting Observe

   When using CoAP Observe [RFC7641], what is defined in Section 5.4.2
   applies for the last proxy in the chain, i.e., the last hop before
   the origin servers.

   As to any other proxy in the chain, the following applies.

   *  The proxy acts as a client registered with the next hop towards
      the origin servers, as described earlier in Section 8.1.1.

   *  The proxy takes the role of a server when forwarding notifications
      from the next hop to the origin servers back to the (previous hop
      proxy closer to the) origin client, as per Section 5 of [RFC7641].

   *  The proxy frees up its Token value used for a group observation
      only if, after the timeout expiration, no 2.xx (Success) responses
      matching the group request and also including an Observe option
      have been received from the next hop towards the origin servers.
      After that, as long as the observation for the target resource of
      the group request is active with the next hop towards the origin
      servers in the group, notifications from that hop are forwarded
      back to the (previous hop proxy closer to the) origin client, as
      defined in Section 8.2.

   *  The proxy SHOULD regularly verify that the (previous hop proxy
      closer to the) origin client is still interested in receiving
      observe notifications for a group observation.  To this end, the
      proxy can rely on the same approach defined in Section 4.5 of
      [RFC7641].

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9.  Security Considerations

   The security considerations from [RFC7252][I-D.ietf-core-groupcomm-bi
   s][RFC8613][I-D.ietf-core-oscore-groupcomm] hold for this document.

   When a chain of proxies is used (see Section 8), the secure
   communication between any two adjacent hops is independent.

   When Group OSCORE is used for end-to-end secure group communication
   between the origin client and the origin servers, this security
   association is unaffected by the possible presence of a proxy or a
   chain of proxies.

   Furthermore, the following additional considerations hold.

9.1.  Client Authentication

   As per the requirement REQ2 (see Section 4), the client has to
   authenticate to the proxy when sending a group request to forward.
   This leverages an established security association between the client
   and the proxy, that the client uses to protect the group request,
   before sending it to the proxy.

   Note that, if the group request is (also) protected with Group
   OSCORE, i.e., end-to-end between the client and the servers, the
   proxy can authenticate the client by successfully verifying the
   counter signature embedded in the group request.  However, this
   requires that, for each client to authenticate, the proxy stores the
   public key used by that client in the OSCORE group, which in turn
   would require a form of active synchronization between the proxy and
   the Group Manager for that group [I-D.ietf-core-oscore-groupcomm].

   Nevertheless, the client and the proxy SHOULD still rely on a full-
   fledged, pairwise secure association.  In addition to ensuring the
   integrity of group requests sent to the proxy (see Section 9.2,
   Section 9.3 and Section 9.4), this prevents the proxy from forwarding
   replayed group requests with a valid counter signature, as possibly
   injected by an active, on-path adversary.

   The same considerations apply when a chain of proxies is used (see
   Section 8), with each proxy but the last one in the chain acting as
   client with the next hop towards the origin servers.

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9.2.  Multicast-Signaling Option

   The Multicast-Signaling Option is of class U for OSCORE [RFC8613].
   Hence, also when Group OSCORE is used between the client and the
   servers [I-D.ietf-core-oscore-groupcomm], a proxy is able to access
   the option value and retrieve the timeout value T', as well as to
   remove the option altogether before forwarding the group request to
   the servers.  When a chain of proxies is used (see Section 8), this
   also allows each proxy but the last one in the chain to update the
   option value, as an indication for the next hop towards the origin
   servers (see Section 8.1).

   The security association between the client and the proxy MUST
   provide message integrity, so that further intermediaries between the
   two as well as on-path active adversaries are not able to remove the
   option or alter its content, before the group request reaches the
   proxy.  Removing the option would otherwise result in not forwarding
   the group request to the servers.  Instead, altering the option
   content would result in the proxy accepting and forwarding back
   responses for an amount of time different than the one actually
   indicated by the client.

   The security association between the client and the proxy SHOULD also
   provide message confidentiality.  Otherwise, any further
   intermediaries between the two as well as any on-path passive
   adversaries would be able to simply access the option content, and
   thus learn for how long the client is willing to receive responses
   from the servers in the group via the proxy.  This may in turn be
   used to perform a more efficient, selective suppression of responses
   from the servers.

   When the client protects the unicast request sent to the proxy using
   OSCORE (see [I-D.tiloca-core-oscore-capable-proxies]) and/or with
   (D)TLS, both message integrity and message confidentiality are
   achieved in the leg between the client and the proxy.

   The same considerations above about security associations apply when
   a chain of proxies is used (see Section 8), with each proxy but the
   last one in the chain acting as client with the next hop towards the
   origin servers.

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9.3.  Response-Forwarding Option

   The Response-Forwarding Option is of class U for OSCORE [RFC8613].
   Hence, also when Group OSCORE is used between the client and the
   servers [I-D.ietf-core-oscore-groupcomm], the proxy that has
   forwarded the group request to the servers is able to include the
   option into a server response, before forwarding this response back
   to the (previous hop proxy closer to the) origin client.

   Since the security association between the client and the proxy
   provides message integrity, any further intermediaries between the
   two as well as any on-path active adversaries are not able to
   undetectably remove the Response-Forwarding Option from a forwarded
   server response.  This ensures that the client can correctly
   distinguish the different responses and identify their corresponding
   origin server.

   When the proxy protects the response forwarded back to the client
   using OSCORE (see [I-D.tiloca-core-oscore-capable-proxies]) and/or
   (D)TLS, message integrity is achieved in the leg between the client
   and the proxy.

   The same considerations above about security associations apply when
   a chain of proxies is used (see Section 8), with each proxy but the
   last one in the chain acting as client with the next hop towards the
   origin servers.

9.4.  Group-ETag Option

   The Group-ETag Option is of class U for OSCORE [RFC8613].  Hence,
   also when Group OSCORE is used between the client and the servers
   [I-D.ietf-core-oscore-groupcomm], a proxy is able to access the
   option value and use it to possibly perform response revalidation at
   its cache entries associated to the servers in the CoAP group, as
   well as to remove the option altogether before forwarding the group
   request to the servers.  When a chain of proxies is used (see
   Section 8), this also allows each proxy but the last one in the chain
   to update the option value, to possibly ask the next hop towards the
   origin servers to perform response revalidation at its cache entries.

   The security association between the client and the proxy MUST
   provide message integrity, so that further intermediaries between the
   two as well as on-path active adversaries are not able to remove the
   option or alter its content, before the group request reaches the
   proxy.  Removing the option would otherwise result in the proxy not
   performing response revalidation at its cache entries associated to
   the servers in the CoAP group, even though that was what the client
   asked for.

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   Altering the option content in a group request would result in the
   proxy replying with 2.05 (Content) responses conveying the full
   resource representations from its cache entries, rather than with a
   single 2.03 (Valid) response.  Instead, altering the option content
   in a 2.03 (Valid) or 2.05 (Content) response would result in the
   client wrongly believing that the already stored or the just received
   representation, respectively, is also the current one, as per the
   entity value of the tampered Group-ETag option.

   The security association between the client and the proxy SHOULD also
   provide message confidentiality.  Otherwise, any further
   intermediaries between the two as well as any on-path passive
   adversaries would be able to simply access the option content, and
   thus learn the rate and pattern according to which the group resource
   in question changes over time, as inferable from the entity values
   read over time.

   When the client protects the unicast request sent to the proxy using
   OSCORE (see [I-D.tiloca-core-oscore-capable-proxies]) and/or (D)TLS,
   both message integrity and message confidentiality are achieved in
   the leg between the client and the proxy.

   The same considerations above about security associations apply when
   a chain of proxies is used (see Section 8), with each proxy but the
   last one in the chain acting as client with the next hop towards the
   origin servers.

   When caching of Group OSCORE secured responses is enabled at the
   proxy, the same as defined in Section 7 applies, with respect to
   cache entries and the way they are maintained.

10.  IANA Considerations

   This document has the following actions for IANA.

10.1.  CoAP Option Numbers Registry

   IANA is asked to enter the following option numbers to the "CoAP
   Option Numbers" registry defined in [RFC7252] within the "CoRE
   Parameters" registry.

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           +--------+---------------------+-------------------+
           | Number |        Name         |     Reference     |
           +--------+---------------------+-------------------+
           |  TBD1  | Multicast-Signaling | [[this document]] |
           +--------+---------------------+-------------------+
           |  TBD2  | Response-Forwarding | [[this document]] |
           +--------+---------------------+-------------------+
           |  TBD3  |     Group-ETag      | [[this document]] |
           +--------+---------------------+-------------------+

10.2.  CoAP Transport Information Registry

   IANA is asked to enter the following entries to the "CoAP Transport
   Information" Registry defined in Section 14.4 of
   [I-D.ietf-core-observe-multicast-notifications].

 +------------+-------------+-------+----------+-----------+-----------+
 | Transport  | Description | Value | Srv Addr | Req Info  | Reference |
 | Protocol   |             |       |          |           |           |
 +------------+-------------+-------+----------+-----------+-----------+
 | UDP        | UDP with    | 2     | tp_id    |  token    | [This     |
 | secured    | DTLS is     |       | srv_host |  cli_host | document] |
 | with DTLS  | used as per |       | srv_port | ?cli_port |           |
 |            | RFC8323     |       |          |           |           |
 +------------+-------------+-------+----------+-----------+-----------+
 | TCP        | TCP is used | 3     | tp_id    |  token    | [This     |
 |            | as per      |       | srv_host |  cli_host | document] |
 |            | RFC8323     |       | srv_port | ?cli_port |           |
 +------------+-------------+-------+----------+-----------+-----------+
 | TCP        | TCP with    | 4     | tp_id    |  token    | [This     |
 | secured    | TLS is      |       | srv_host |  cli_host | document] |
 | with TLS   | used as per |       | srv_port | ?cli_port |           |
 |            | RFC8323     |       |          |           |           |
 +------------+-------------+-------+----------+-----------+-----------+
 | WebSockets | WebSockets  | 5     | tp_id    |  token    | [This     |
 |            | are used as |       | srv_host |  cli_host | document] |
 |            | per RFC8323 |       | srv_port | ?cli_port |           |
 +------------+-------------+-------+----------+-----------+-----------+
 | WebSockets | WebSockets  | 6     | tp_id    |  token    | [This     |
 | secured    | with TLS    |       | srv_host |  cli_host | document] |
 | with TLS   | are used as |       | srv_port | ?cli_port |           |
 |            | per RFC8323 |       |          |           |           |
 +------------+-------------+-------+----------+-----------+-----------+

11.  References

11.1.  Normative References

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   [I-D.ietf-core-groupcomm-bis]
              Dijk, E., Wang, C., and M. Tiloca, "Group Communication
              for the Constrained Application Protocol (CoAP)", Work in
              Progress, Internet-Draft, draft-ietf-core-groupcomm-bis-
              04, 12 July 2021, <https://www.ietf.org/archive/id/
              draft-ietf-core-groupcomm-bis-04.txt>.

   [I-D.ietf-core-observe-multicast-notifications]
              Tiloca, M., Hoeglund, R., Amsuess, C., and F. Palombini,
              "Observe Notifications as CoAP Multicast Responses", Work
              in Progress, Internet-Draft, draft-ietf-core-observe-
              multicast-notifications-01, 12 July 2021,
              <https://www.ietf.org/archive/id/draft-ietf-core-observe-
              multicast-notifications-01.txt>.

   [I-D.ietf-core-oscore-groupcomm]
              Tiloca, M., Selander, G., Palombini, F., Mattsson, J. P.,
              and J. Park, "Group OSCORE - Secure Group Communication
              for CoAP", Work in Progress, Internet-Draft, draft-ietf-
              core-oscore-groupcomm-12, 12 July 2021,
              <https://www.ietf.org/archive/id/draft-ietf-core-oscore-
              groupcomm-12.txt>.

   [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>.

   [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>.

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <https://www.rfc-editor.org/info/rfc7641>.

   [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>.

   [RFC8323]  Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
              Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
              Application Protocol) over TCP, TLS, and WebSockets",
              RFC 8323, DOI 10.17487/RFC8323, February 2018,
              <https://www.rfc-editor.org/info/rfc8323>.

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   [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>.

   [RFC8613]  Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
              <https://www.rfc-editor.org/info/rfc8613>.

   [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>.

11.2.  Informative References

   [I-D.amsuess-core-cachable-oscore]
              Amsuess, C. and M. Tiloca, "Cacheable OSCORE", Work in
              Progress, Internet-Draft, draft-amsuess-core-cachable-
              oscore-01, 22 February 2021,
              <https://www.ietf.org/archive/id/draft-amsuess-core-
              cachable-oscore-01.txt>.

   [I-D.bormann-coap-misc]
              Bormann, C. and K. Hartke, "Miscellaneous additions to
              CoAP", Work in Progress, Internet-Draft, draft-bormann-
              coap-misc-27, 14 November 2014,
              <https://www.ietf.org/archive/id/draft-bormann-coap-misc-
              27.txt>.

   [I-D.ietf-ace-key-groupcomm-oscore]
              Tiloca, M., Park, J., and F. Palombini, "Key Management
              for OSCORE Groups in ACE", Work in Progress, Internet-
              Draft, draft-ietf-ace-key-groupcomm-oscore-11, 12 July
              2021, <https://www.ietf.org/archive/id/draft-ietf-ace-key-
              groupcomm-oscore-11.txt>.

   [I-D.ietf-tls-dtls13]
              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", Work in Progress, Internet-Draft, draft-ietf-tls-
              dtls13-43, 30 April 2021, <https://www.ietf.org/internet-
              drafts/draft-ietf-tls-dtls13-43.txt>.

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   [I-D.tiloca-core-oscore-capable-proxies]
              Tiloca, M. and R. Hoeglund, "OSCORE-capable Proxies", Work
              in Progress, Internet-Draft, draft-tiloca-core-oscore-
              capable-proxies-00, 12 July 2021,
              <https://www.ietf.org/archive/id/draft-tiloca-core-oscore-
              capable-proxies-00.txt>.

   [I-D.tiloca-core-oscore-discovery]
              Tiloca, M., Amsuess, C., and P. V. D. Stok, "Discovery of
              OSCORE Groups with the CoRE Resource Directory", Work in
              Progress, Internet-Draft, draft-tiloca-core-oscore-
              discovery-09, 12 July 2021,
              <https://www.ietf.org/archive/id/draft-tiloca-core-oscore-
              discovery-09.txt>.

   [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>.

   [RFC7967]  Bhattacharyya, A., Bandyopadhyay, S., Pal, A., and T.
              Bose, "Constrained Application Protocol (CoAP) Option for
              No Server Response", RFC 7967, DOI 10.17487/RFC7967,
              August 2016, <https://www.rfc-editor.org/info/rfc7967>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

Appendix A.  Examples with Reverse-Proxy

   The examples in this section refer to the following actors.

   *  One origin client C, with address C_ADDR and port number C_PORT.

   *  One proxy P, with address P_ADDR and port number P_PORT.

   *  Two origin servers S1 and S2, where the server Sx has address
      Sx_ADDR and port number Sx_PORT.

   The origin servers are members of a CoAP group with IP multicast
   address G_ADDR and port number G_PORT.  Also, the origin servers are
   members of a same application group, and share the same resource /r.

   The communication between C and P is based on CoAP over TCP, as per
   [RFC8323].  The communication between P and the origin servers is
   based on CoAP over UDP and IP multicast, as per
   [I-D.ietf-core-groupcomm-bis].

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   Finally, 'bstr(X)' denotes a CBOR byte string with value the byte
   serialization of X.

A.1.  Example 1

   The example shown in Figure 5 considers a reverse-proxy that stands
   in for both the whole group of servers and for each of those servers
   (e.g., acting as a firewall).

   In particular:

   *  The address 'group1.com' resolves to P_ADDR.  The proxy forwards
      an incoming request to that address, for any resource i.e., URI
      path, towards the CoAP group at G_ADDR:G_PORT leaving the URI path
      unchanged.

   *  The address Dx_ADDR and port number Dx_PORT are used by the proxy,
      which forwards an incoming request to that address towards the
      server at Sx_ADDR:Sx_PORT.

   Note that this type of reverse-proxy implementation requires the
   proxy to use (potentially) a large number of distinct IP addresses,
   so it is not very scalable.

   C                              P                      S1           S2
   |                              |                      |             |
   |----------------------------->| /* C is not aware    |             |
   | Src: C_ADDR:C_PORT           | that P is in fact    |             |
   | Dst: group1.com:P_PORT       | a reverse-proxy */   |             |
   | Uri-Path: /r                 |                      |             |
   |                              |                      |             |
   |<-----------------------------|                      |             |
   | Src: group1.com:P_PORT       |                      |             |
   | Dst: C_ADDR:C_PORT           |                      |             |
   | 4.00 Bad Request             |                      |             |
   | Multicast-Signaling: (empty) |                      |             |
   | Payload: "Please use         |                      |             |
   |     Multicast-Signaling"     |                      |             |
   |                              |                      |             |
   |----------------------------->|                      |             |
   | Src: C_ADDR:C_PORT           |                      |             |
   | Dst: group1.com:P_PORT       |                      |             |
   | Multicast-Signaling: 60      |                      |             |
   | Uri-Path: /r                 |                      |             |
   |                              |                      |             |
   |                              |                      |             |
   |                              |                      |             |
   |                              |                      |             |

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   |                              |                      |             |
   |                              | Src: P_ADDR:P_PORT   |             |
   |                              | Dst: G_ADDR:G_PORT   |             |
   |                              | Uri-Path: /r         |             |
   |                              |---------------+----->|             |
   |                              |                \     |             |
   |                              |                 +----------------->|
   |                              |                      |             |
   |                              | /* t = 0 : P starts  |             |
   |                              | accepting responses  |             |
   |                              | for this request */  |             |
   |                              |                      |             |
   |                              |                      |             |
   |                              |                      |             |
   |                              |<---------------------|             |
   |                              | Src: S1_ADDR:S1_PORT |             |
   |                              | Dst: P_ADDR:P_PORT   |             |
   |                              |                      |             |
   |<-----------------------------|                      |             |
   | Src: group1.com:P_PORT       |                      |             |
   | Dst: C_ADDR:C_PORT           |                      |             |
   | Response-Forwarding {        |                      |             |
   |  [3, /*CoAP over TCP*/       |                      |             |
   |   #6.260(bstr(D1_ADDR)),     |                      |             |
   |   D1_PORT                    |                      |             |
   |  ]                           |                      |             |
   | }                            |                      |             |
   |                              |                      |             |
   |                              |<-----------------------------------|
   |                              |               Src: S2_ADDR:S2_PORT |
   |                              |               Dst: P_ADDR:P_PORT   |
   |                              |                      |             |
   |<-----------------------------|                      |             |
   | Src: group1.com:P_PORT       |                      |             |
   | Dst: C_ADDR:C_PORT           |                      |             |
   | Response-Forwarding {        |                      |             |
   |  [3, /*CoAP over TCP*/       |                      |             |
   |   #6.260(bstr(D2_ADDR)),     |                      |             |
   |   D2_PORT                    |                      |             |
   |  ]                           |                      |             |
   | }                            |                      |             |
   |                              |                      |             |
   |                /* At t = 60, P stops accepting      |             |
   |                responses for this request */        |             |
   |                              |                      |             |
   |                              |                      |             |
   |                              |                      |             |

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   |                              |                      |             |
   |----------------------------->| /* Request intended  |             |
   | Src: C_ADDR:C_PORT           | only to S1 */        |             |
   | Dst: D1_ADDR:D1_PORT         |                      |             |
   | Uri-Path: /r                 |                      |             |
   |                              |                      |             |
   |                              | Src: P_ADDR:P_PORT   |             |
   |                              | Dst: S1_ADDR:S1_PORT |             |
   |                              | Uri-Path: /r         |             |
   |                              |--------------------->|             |
   |                              |                      |             |
   |                              |                      |             |
   |                              |<---------------------|             |
   |                              | Src: S1_ADDR:S1_PORT |             |
   |                              | Dst: P_ADDR:P_PORT   |             |
   |                              |                      |             |
   |<-----------------------------|                      |             |
   | Src: D1_ADDR:D1_PORT         |                      |             |
   | Dst: C_ADDR:C_PORT           |                      |             |
   |                              |                      |             |

       Figure 5: Workflow example with reverse-proxy standing in for
         both the whole group of servers and each individual server

A.2.  Example 2

   The example shown in Figure 6 builds on the example in Appendix A.1.

   However, it considers a reverse-proxy that stands in for only the
   whole group of servers, but not for each individual server.

   The final exchange between C and S1 occurs with CoAP over UDP.

   C                              P                      S1           S2
   |                              |                      |             |
   |----------------------------->| /* C is not aware    |             |
   | Src: C_ADDR:C_PORT           | that P is in fact    |             |
   | Dst: group1.com:P_PORT       | a reverse-proxy */   |             |
   | Uri-Path: /r                 |                      |             |
   |                              |                      |             |
   |<-----------------------------|                      |             |
   | Src: group1.com:P_PORT       |                      |             |
   | Dst: C_ADDR:C_PORT           |                      |             |
   | 4.00 Bad Request             |                      |             |
   | Multicast-Signaling: (empty) |                      |             |
   | Payload: "Please use         |                      |             |
   |     Multicast-Signaling"     |                      |             |
   |                              |                      |             |

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   |                              |                      |             |
   |----------------------------->|                      |             |
   | Src: C_ADDR:C_PORT           |                      |             |
   | Dst: group1.com:P_PORT       |                      |             |
   | Multicast-Signaling: 60      |                      |             |
   | Uri-Path: /r                 |                      |             |
   |                              |                      |             |
   |                              | Src: P_ADDR:P_PORT   |             |
   |                              | Dst: G_ADDR:G_PORT   |             |
   |                              | Uri-Path: /r         |             |
   |                              |---------------+----->|             |
   |                              |                \     |             |
   |                              |                 +----------------->|
   |                              |                      |             |
   |                              |                      |             |
   |                              | /* t = 0 : P starts  |             |
   |                              | accepting responses  |             |
   |                              | for this request */  |             |
   |                              |                      |             |
   |                              |                      |             |
   |                              |<---------------------|             |
   |                              | Src: S1_ADDR:S1_PORT |             |
   |                              | Dst: P_ADDR:P_PORT   |             |
   |                              |                      |             |
   |<-----------------------------|                      |             |
   | Dst: group1.com:P_PORT       |                      |             |
   | Dst: C_ADDR:C_PORT           |                      |             |
   | Response-Forwarding {        |                      |             |
   |  [1, /*CoAP over UDP*/       |                      |             |
   |   #6.260(bstr(S1_ADDR)),     |                      |             |
   |   S1_PORT                    |                      |             |
   |  ]                           |                      |             |
   | }                            |                      |             |
   |                              |                      |             |
   |                              |<-----------------------------------|
   |                              |               Src: S2_ADDR:S2_PORT |
   |                              |               Dst: P_ADDR:P_PORT   |
   |                              |                      |             |
   |<-----------------------------|                      |             |
   | Dst: group1.com:P_PORT       |                      |             |
   | Dst: C_ADDR:C_PORT           |                      |             |
   | Response-Forwarding {        |                      |             |
   |  [1, /*CoAP over UDP*/       |                      |             |
   |   #6.260(bstr(S2_ADDR)),     |                      |             |
   |   S2_PORT                    |                      |             |
   |  ]                           |                      |             |
   | }                            |                      |             |
   |                              |                      |             |

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   |                              |                      |             |
   |                /* At t = 60, P stops accepting      |             |
   |                responses for this request */        |             |
   |                              |                      |             |
   |                              |                      |             |
   |                              |                      |             |
   |---------------------------------------------------->|             |
   | Src: C_ADDR:C_PORT           | /* Request intended  |             |
   | Dst: S1.ADDR:S1_PORT         | only to S1 */        |             |
   | Uri-Path: /r                 |                      |             |
   |                              |                      |             |
   |                              |                      |             |
   |<----------------------------------------------------|             |
   | Src: S1.ADDR:S1_PORT         |                      |             |
   | Dst: C_ADDR:C_PORT           |                      |             |
   |                              |                      |             |

       Figure 6: Workflow example with reverse-proxy standing in for
        only the whole group of servers, but not for each individual
                                   server

Acknowledgments

   The authors sincerely thank Christian Amsuess, Jim Schaad and Goeran
   Selander for their comments and feedback.

   The work on this document has been partly supported by VINNOVA and
   the Celtic-Next project CRITISEC; and by the H2020 project SIFIS-Home
   (Grant agreement 952652).

Authors' Addresses

   Marco Tiloca
   RISE AB
   Isafjordsgatan 22
   SE-16440 Stockholm Kista
   Sweden

   Email: marco.tiloca@ri.se

   Esko Dijk
   IoTconsultancy.nl
   \________________\
   Utrecht

   Email: esko.dijk@iotconsultancy.nl

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