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CoRE Working Group                                             M. Tiloca
Internet-Draft                                                   RISE AB
Intended status: Standards Track                                 E. Dijk
Expires: May 6, 2021                                   IoTconsultancy.nl
                                                       November 02, 2020


             Proxy Operations for CoAP Group Communication
                  draft-tiloca-core-groupcomm-proxy-02

Abstract

   This document specifies the operations performed by a forward-proxy,
   when using the Constrained Application Protocol (CoAP) in group
   communication scenarios.  Proxy operations involve the processing of
   individual responses from servers, as reply to a single request sent
   by the client over unicast to the proxy, and then distributed by the
   proxy over IP multicast to the servers.  When receiving the different
   responses via the proxy, the client is able to distinguish them and
   their origin servers, by acquiring their addressing information.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at 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."

   This Internet-Draft will expire on May 6, 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect



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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  The Multicast-Signaling Option  . . . . . . . . . . . . . . .   4
   3.  The Response-Forwarding Option  . . . . . . . . . . . . . . .   5
   4.  Requirements and Objectives . . . . . . . . . . . . . . . . .   6
   5.  Protocol Description  . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Request Sending . . . . . . . . . . . . . . . . . . . . .   7
       5.1.1.  Supporting Observe  . . . . . . . . . . . . . . . . .   9
     5.2.  Request Processing at the Proxy . . . . . . . . . . . . .   9
       5.2.1.  Supporting Observe  . . . . . . . . . . . . . . . . .  10
     5.3.  Request and Response Processing at the Server . . . . . .  10
       5.3.1.  Supporting Observe  . . . . . . . . . . . . . . . . .  10
     5.4.  Response Processing at the Proxy  . . . . . . . . . . . .  10
       5.4.1.  Supporting Observe  . . . . . . . . . . . . . . . . .  11
     5.5.  Response Processing at the Client . . . . . . . . . . . .  12
       5.5.1.  Supporting Observe  . . . . . . . . . . . . . . . . .  13
   6.  Chain of Proxies  . . . . . . . . . . . . . . . . . . . . . .  13
     6.1.  Request Processing at the Proxy . . . . . . . . . . . . .  14
       6.1.1.  Supporting Observe  . . . . . . . . . . . . . . . . .  15
     6.2.  Response Processing at the Proxy  . . . . . . . . . . . .  16
       6.2.1.  Supporting Observe  . . . . . . . . . . . . . . . . .  16
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
     7.1.  Client Authentication . . . . . . . . . . . . . . . . . .  17
     7.2.  Multicast-Signaling Option  . . . . . . . . . . . . . . .  18
     7.3.  Response-Forwarding Option  . . . . . . . . . . . . . . .  19
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
     8.1.  CoAP Option Numbers Registry  . . . . . . . . . . . . . .  19
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  20
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  21
   Appendix A.  Using OSCORE Between Client and Proxy  . . . . . . .  21
     A.1.  Protecting the Request  . . . . . . . . . . . . . . . . .  22
     A.2.  Verifying the Request . . . . . . . . . . . . . . . . . .  22
     A.3.  Protecting the Response . . . . . . . . . . . . . . . . .  23
     A.4.  Verifying the Response  . . . . . . . . . . . . . . . . .  23
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  23
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23







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1.  Introduction

   The Constrained Application Protocol (CoAP) [RFC7252] allows the
   presence of forward-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 2.3.3 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
   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 2.3.3 of [I-D.ietf-core-groupcomm-bis].  To this end, a
   dedicated signaling protocol is defined, using two new CoAP options.

   In particular, the client explicitly confirms its support for
   receiving multiple responses to a proxied unicast request, i.e. one
   per origin server, and 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 the multiple, diffent responses by
   origin and to possibly contact one or more of the servers by sending
   individual unicast requests, optionally bypassing the forward-proxy.

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.





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   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 [I-D.ietf-cbor-7049bis], OSCORE
   [RFC8613] and Group OSCORE [I-D.ietf-core-oscore-groupcomm].

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].

   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, 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 and Section 5.2).

   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 CoAP Observe is used [RFC7641].
   This allows the intermediary proxy to stop forwarding responses back
   to the client, if received from the servers later than a timeout
   expiration.

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





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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 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".

     +------+---+---+---+---+------------+--------+--------+---------+
     | No.  | C | U | N | R | Name       | Format | Length | Default |
     +------+---+---+---+---+------------+--------+--------+---------+
     |      |   |   |   |   |            |        |        |         |
     | TBD2 |   |   | - |   | Response-  |  (*)   |  9-24  | (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 forwards a request originated by a client over IP multicast.

   Upon receiving a response to that 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
   to the client the addressing information of the server generating the
   response.

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

   The option value is the byte serialization of a CBOR array
   'srv_info', which includes the following elements.

   o  'srv_addr': this element is a CBOR byte string, with value 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.





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   o  'srv_port': this element is a CBOR unsigned integer, with value
      the destination port number where to send unicast requests to the
      server.  This element MAY be present.

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

   srv_info = [
     srv_addr : #6.260(bstr),  ; IP address of the server
     ? srv_port : uint,        ; Port number of the server
   ]

   If the 'srv_info' array does not include the element 'srv_port', it
   is assumed that the port number where to send unicast requests to the
   server 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).

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

4.  Requirements and Objectives

   This specification assumes that the following requirements are
   fulfilled.

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

   o  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.

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

      *  A pairwise OSCORE Security Context between the client and the
         proxy, as described in Appendix A.

   o  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 5.2 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



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      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.

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

   o  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 CoAP Observe is used
      [RFC7641]).

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

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

   o  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

   In order to send a request addressed to a group of servers via the
   CoAP proxy, the client proceeds as follows.

   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 2.3.3 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 2.3.1 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 2.3.1 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, as well as between
          the proxy and 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.  If OSCORE is used to protect the leg between the client and the
       proxy (see REQ2 in Section 4), the client (further) protects the
       unicast request as resulting at the end of step 4.  In
       particular, the client uses the pairwise OSCORE Security Context
       it has with the proxy, as described in Appendix A.1.

   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.

5.1.1.  Supporting Observe

   When using CoAP Observe [RFC7641], the client follows what is
   specified in Section 2.3.5 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 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

   Upon receiving the request from the client, the proxy proceeds as
   follows.

   1.  If OSCORE is used to protect the leg between the client and the
       proxy (see REQ2 in Section 4), the proxy decrypts the request
       using the pairwise OSCORE Security Context it has with the
       client, as described in Appendix A.2.

   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
       diagnostic payload, specifying that the Multicast-Signaling
       Option was missing and has to be included.

   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.



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   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.

5.2.1.  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 2.3.5 of [I-D.ietf-core-groupcomm-bis], by
   forwarding the group request to the servers over IP multicast, as
   defined in Section 5.2 of this specification.

5.3.  Request and Response Processing at the Server

   Upon receiving the group request from the proxy, a server proceeds as
   follows.

   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.

   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.

5.3.1.  Supporting Observe

   When using CoAP Observe [RFC7641], the server especially follows what
   is specified in Section 2.3.5 of [I-D.ietf-core-groupcomm-bis] and
   Section 5 of [RFC7641].

5.4.  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.




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   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).  Otherwise, the 'srv_port'
          element MAY be omitted.

   2.  If OSCORE is used to protect the leg between the client and the
       proxy (see REQ2 in Section 4), the proxy (further) protects the
       response using the pairwise OSCORE Security Context it has with
       the client, as described in Appendix A.3.

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

   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.

5.4.1.  Supporting Observe

   When using CoAP Observe [RFC7641], the proxy acts as a client
   registered with the servers, as described earlier in Section 5.2.1.

   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 2.3.5 of
   [I-D.ietf-core-groupcomm-bis] and Section 5 of [RFC7641], with the
   following additions.

   o  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.




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   o  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.

   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 2.3.5 of
   [I-D.ietf-core-groupcomm-bis], with more details available in
   Section 4.5 of [RFC7641].

5.5.  Response Processing at the Client

   Upon receiving from the proxy a response matching the original
   unicast request before the amount of time T has elapsed, the client
   proceeds as follows.

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

   2.  If OSCORE is used to protect the leg between the client and the
       proxy (see REQ2 in Section 4), the client decrypts the response
       using the pairwise OSCORE Security Context it has with the proxy,
       as described in Appendix A.4.

   3.  If secure group communication is used between the client and the
       servers, the client processes the response, possibly as outcome
       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 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).

   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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   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).  Thus, until the actual amount of time before
   enabling Token reusage has elapsed, following late responses to the
   same request forwarded by the proxy will be discarded, as not
   matching (by Token) any active request from the client.

5.5.1.  Supporting Observe

   When using CoAP Observe [RFC7641], the client frees up its Token
   value only if, after the timeout 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.

6.  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 (multicast) 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 takes the role of server towards the
   previous proxy closer to the origin client.

   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 the origin client.  In
   either case, a proxy can identify the sender of a group request by
   the same means mentioned in Section 4.



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6.1.  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.

   o  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.

   o  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 defined at step 2 of Section 5.1
          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.





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          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
          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.

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

   o  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.

6.1.1.  Supporting Observe

   When using CoAP Observe [RFC7641], what defined in Section 5.2.1
   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].






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6.2.  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.

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

   o  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.

   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.

6.2.1.  Supporting Observe

   When using CoAP Observe [RFC7641], what defined in Section 5.4.1
   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.

   o  The proxy acts as a client registered with the next hop towards
      the origin servers, as described earlier in Section 6.1.1.

   o  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].

   o  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



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      back to the (previous hop proxy closer to the) origin client, as
      defined in Section 6.2.

   o  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].

7.  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 6), 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.

7.1.  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.  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 7.2 and
   Section 7.3), this prevents the proxy from forwarding replayed group
   requests with a valid counter signature, as possibly injected by an
   active, on-path adversary.




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   The same considerations apply when a chain of proxies is used (see
   Section 6), with each proxy but the last one in the chain acting as
   client with the next hop towards the origin servers.

7.2.  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 6), 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 6.1).

   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, further intermediaries
   between the two as well as 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 (further) protects the unicast request sent to the
   proxy using OSCORE (see Appendix A) and/or with DTLS, 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 6), 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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7.3.  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 or 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 (further) protects the response forwarded back to the
   client using OSCORE (see Appendix A) and/or with DTLS, 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 6), with each proxy but the
   last one in the chain acting as client with the next hop towards the
   origin servers.

8.  IANA Considerations

   This document has the following actions for IANA.

8.1.  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.

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

9.  References







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9.1.  Normative References

   [I-D.ietf-cbor-7049bis]
              Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", draft-ietf-cbor-7049bis-16 (work
              in progress), September 2020.

   [I-D.ietf-core-groupcomm-bis]
              Dijk, E., Wang, C., and M. Tiloca, "Group Communication
              for the Constrained Application Protocol (CoAP)", draft-
              ietf-core-groupcomm-bis-02 (work in progress), November
              2020.

   [I-D.ietf-core-oscore-groupcomm]
              Tiloca, M., Selander, G., Palombini, F., and J. Park,
              "Group OSCORE - Secure Group Communication for CoAP",
              draft-ietf-core-oscore-groupcomm-10 (work in progress),
              November 2020.

   [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>.

   [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>.



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9.2.  Informative References

   [I-D.bormann-coap-misc]
              Bormann, C. and K. Hartke, "Miscellaneous additions to
              CoAP", draft-bormann-coap-misc-27 (work in progress),
              November 2014.

   [I-D.ietf-ace-key-groupcomm-oscore]
              Tiloca, M., Park, J., and F. Palombini, "Key Management
              for OSCORE Groups in ACE", draft-ietf-ace-key-groupcomm-
              oscore-09 (work in progress), November 2020.

   [I-D.ietf-tls-dtls13]
              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-ietf-tls-dtls13-38 (work in progress), May
              2020.

   [I-D.tiloca-core-oscore-discovery]
              Tiloca, M., Amsuess, C., and P. Stok, "Discovery of OSCORE
              Groups with the CoRE Resource Directory", draft-tiloca-
              core-oscore-discovery-07 (work in progress), November
              2020.

   [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>.

Appendix A.  Using OSCORE Between Client and Proxy

   This section describes how OSCORE is used to protect messages
   exchanged by an origin client and a proxy, using their pairwise
   OSCORE Security Context.

   This is especially convenient for the communication scenario
   addressed in this document, when the origin client already supports
   and uses Group OSCORE [I-D.ietf-core-oscore-groupcomm] to protect
   messages end-to-end with the origin servers.

   The following focuses on the origin client originating the group
   request and a single proxy as its immediate next hop.  When a chain
   of proxies is used (see Section 6), the same independently applies
   between each pair of proxies in the chain, where the proxy forwarding



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   the group request acts as client and the next hop towards the origin
   servers acts as server.

A.1.  Protecting the Request

   Before sending the CoAP request to the proxy, the origin client
   protects it using the pairwise OSCORE Security Context it has with
   the proxy.

   To this end, the origin client processes the CoAP request as defined
   in Section 8.1 of [RFC8613], with the following differences.

   o  The Proxy-Uri option, if present, is not decomposed and recomposed
      as defined in Section 4.1.3.3 of [RFC8613].

   o  The following options, if present, are processed as Class E.

      *  Proxy-Uri, Proxy-Scheme, Uri-Host and Uri-Port, defined in
         [RFC7252].

      *  OSCORE, defined in [RFC8613], which is present if Group OSCORE
         is used between the origin client and the origin servers, to
         achieve end-to-end secure group communication.

      *  Multicast-Signaling Option, defined in Section 2 of this
         specification.

   As per [RFC8613], the resulting message includes an outer OSCORE
   Option, which reflects the usage of the pairwise OSCORE Security
   Context between the origin client and the proxy.

A.2.  Verifying the Request

   The proxy verifies the CoAP request as defined in Section 8.2 of
   [RFC8613].  Note that the Multicast-Signaling Option is retrieved
   during the decryption process, and added to the decrypted request.

   If secure group communication is also used between the origin client
   and the origin servers, the request resulting from the previous step
   and to be forwarded to the origin servers is also already protected
   with Group OSCORE [I-D.ietf-core-oscore-groupcomm].  Consequently, it
   includes an outer OSCORE Option, which reflects the usage of the
   group OSCORE Security Context between the origin client and the
   origin servers.







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A.3.  Protecting the Response

   The proxy protects the CoAP response received from a server, using
   the pairwise OSCORE Security Context it has with the origin client.

   To this end, the proxy processes the CoAP response as defined in
   Section 8.3 of [RFC8613], with the difference that the OSCORE Option,
   if present, is processed as Class E.  This is the case if Group
   OSCORE is used between the origin client and the origin servers, to
   achieve end-to-end secure group communication.

   Furthermore, the Response-Forwarding Option defined in Section 3 of
   this specification is also processed as Class E.

   As per [RFC8613], the resulting message to be forwarded back to the
   origin client includes an outer OSCORE Option, which reflects the
   usage of the pairwise OSCORE Security Context between the origin
   client and the proxy.

A.4.  Verifying the Response

   The origin client verifies the CoAP response received from the proxy
   as defined in Section 8.4 of [RFC8613].  Note that, the Response-
   Forwarding Option is retrieved during the decryption process, and
   added to the decrypted response.

   If secure group communication is also used between the origin client
   and the origin servers, the response resulting from the previous step
   is protected with Group OSCORE [I-D.ietf-core-oscore-groupcomm].
   Consequently, it includes an outer OSCORE Option, which reflects the
   usage of the group OSCORE Security Context between the origin client
   and the origin servers.

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








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

   Email: marco.tiloca@ri.se


   Esko Dijk
   IoTconsultancy.nl
   \________________\
   Utrecht
   The Netherlands

   Email: esko.dijk@iotconsultancy.nl



































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