Proxy Operations for CoAP Group Communication
draft-tiloca-core-groupcomm-proxy-00

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CoRE Working Group                                             M. Tiloca
Internet-Draft                                                   RISE AB
Intended status: Standards Track                                 E. Dijk
Expires: September 10, 2020                            IoTconsultancy.nl
                                                          March 09, 2020

             Proxy Operations for CoAP Group Communication
                  draft-tiloca-core-groupcomm-proxy-00

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 originator servers, by acquiring their addressing information.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on September 10, 2020.

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   to this document.  Code Components extracted from this document must
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  The Multicast-Signaling Option  . . . . . . . . . . . . . . .   3
   3.  The Response-Forwarding Option  . . . . . . . . . . . . . . .   4
   4.  Requirements and Objectives . . . . . . . . . . . . . . . . .   5
   5.  Protocol Description  . . . . . . . . . . . . . . . . . . . .   6
     5.1.  Request Sending . . . . . . . . . . . . . . . . . . . . .   6
     5.2.  Request Processing at the Proxy . . . . . . . . . . . . .   7
     5.3.  Request and Response Processing at the Server . . . . . .   8
     5.4.  Response Processing at the Proxy  . . . . . . . . . . . .   8
     5.5.  Response Processing at the Client . . . . . . . . . . . .   9
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  10
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10
     7.1.  CoAP Option Numbers Registry  . . . . . . . . . . . . . .  10
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

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.dijk-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.dijk-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

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   specific aggregation content-format, which is not available yet.
   Both these approaches have open issues.

   This specification considers the former approach of how 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.dijk-core-groupcomm-bis].

   To this end, a dedicated signaling protocol is defined, using two new
   CoAP options.  In particular, the client can explicitly confirm its
   support for receiving multiple responses to a proxied unicast
   request, i.e. one per originator server, and for how long it is
   willing to wait for responses.  Also, each server originating a
   response indicates to the client its own addressing information.
   This enables the client to distinguish the multiple, diffent
   responses by origin and to possibly contact one or more of the
   individual servers by a unicast request, 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.

   Readers are expected to be familiar with terms and concepts defined
   in CoAP [RFC7252], Group Communication for CoAP
   [I-D.dijk-core-groupcomm-bis], 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].  The option is intended only for CoAP requests.

   Since the option is not Safe-to-Forward, the column "N" indicates a
   dash for "not applicable".  The Multicast-Signaling Option contains a
   timeout value in seconds, encoded as a CBOR [RFC7049] unsigned
   integer.

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     +------+---+---+---+---+------------+--------+--------+---------+
     | No.  | C | U | N | R | Name       | Format | Length | Default |
     +------+---+---+---+---+------------+--------+--------+---------+
     |      |   |   |   |   |            |        |        |         |
     | TBD1 | X | x | - |   | Multicast- | uint   | 1-5 B  | (none)  |
     |      |   |   |   |   | Signaling  |        |        |         |
     |      |   |   |   |   |            |        |        |         |
     +------+---+---+---+---+------------+--------+--------+---------+
                C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable
                (*) See below.

                 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 originator 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 in 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 I for OSCORE
   [RFC8613][I-D.ietf-core-oscore-groupcomm], in order to allow the
   proxy to access its value as intended consumer.

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 not Safe-to-Forward and is intended only for
   responses, the column "N" indicates a dash.

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     +------+---+---+---+---+------------+--------+--------+---------+
     | No.  | C | U | N | R | Name       | Format | Length | Default |
     +------+---+---+---+---+------------+--------+--------+---------+
     |      |   |   |   |   |            |        |        |         |
     | TBD2 | X | x | - |   | Response-  | (*)    | 8-20 B | (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
   server, when it receives a request originated by a client and
   forwarded by a proxy over IP multicast.  The server uses the option
   to indicate its own addressing information to the originator client,
   when sending its own response to the proxy (see Section 5).

   When replying to a multicast request received via a proxy, the server
   includes the Response-Forwarding Option in the response sent to the
   client via that proxy.  The option value includes addressing
   information of the server, that the client can use to identify the
   response originator and possibly send later unicast requests to
   directly, or via the proxy as CoAP unicast request.

   The value of the option is the unicast IP address of the server,
   encoded as a CBOR byte string.  The byte string is in turn tagged and
   identified by the CBOR tag 260 "Network Address (IPv4 or IPv6 or MAC
   Address)".

   The Response-Forwarding Option is of class E for OSCORE
   [RFC8613][I-D.ietf-core-oscore-groupcomm].

4.  Requirements and Objectives

   This specification assumes that the following requirements are
   fulfilled.

   o  REQ1.  The CoAP proxy is explicitly configured (white-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 that the client is white-
      listed to do so.  This can rely for example on using a (D)TLS
      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.

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   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.dijk-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.

   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 originator servers.

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

5.  Protocol Description

5.1.  Request Sending

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

   1.  The client prepares a request addressed to the 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.dijk-core-groupcomm-bis]).

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   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.dijk-core-groupcomm-bis].  In particular, it picks an amount
       of time T before freeing up the Token value, such that:

       *  T is smaller than the amount of time Tr it may pick for
          potentially reusing the Token value.

       *  T includes the expected worst-case time taken by the request
          and response processing on the forward-proxy plus the servers
          in the addressed CoAP group.

       *  T includes the expected worst-case round-trip delay between
          client and proxy, and between proxy and servers.

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

   4.  The rest of the request processing occurs as defined in
       [I-D.dijk-core-groupcomm-bis], and in
       [I-D.ietf-core-oscore-groupcomm] when secure group communication
       is used.

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

5.2.  Request Processing at the Proxy

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

   1.  The proxy identifies the client and verifies that it is in fact
       white-listed for proxy requests to CoAP group URIs.

   2.  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.

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

5.3.  Request and Response Processing at the Server

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

   1.  Thanks to the Multicast-Signaling Option, the server understands
       that the original request originator is in fact a client behind a
       proxy.

   2.  The rest of the request processing occurs as defined in
       [I-D.dijk-core-groupcomm-bis], and in
       [I-D.ietf-core-oscore-groupcomm] when secure group communication
       is used.

   3.  When preparing its response to the proxy, to be forwarded back to
       the client, the server includes the Response-Forwarding Option
       defined in Section 3.  The server specifies as option value its
       own addressing information, i.e. its unicast IP address, encoded
       as defined in Section 3.  The server MUST include its IPv6
       address if the multicast request was destined to an IPv6
       multicast address and MUST include its IPv4 address if the
       multicast request was destined to an IPv4 address.

   4.  When using Observe [RFC7641], the server includes the Response-
       Forwarding Option also in every notification, including non-2.xx
       notifications resulting in removing the proxy from the list of
       observers.

   5.  The rest of the response processing occurs as defined in
       [I-D.dijk-core-groupcomm-bis], and in
       [I-D.ietf-core-oscore-groupcomm] when secure group communication
       is used.

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 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

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   same group request will be discarded and not forwarded back to the
   client.

   When using CoAP Observe [RFC7641], the Token value is freed up only
   if, after the timeout expiration, no 2.xx (Success) responses
   matching the group request and also including an Observe option have
   been received.  Then, 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.

5.5.  Response Processing at the Client

   Upon receiving from the proxy a response that matches the original
   unicast request, i.e. before the amount of time T has elapsed, the
   client is able to identify the originator server, whose addressing
   information is specified as value of the Response-Forwarding Option.

   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, directly
   (bypassing the proxy) or indirectly (via a proxied CoAP unicast
   request).  Note that the client already knows the destination port
   number to use for sending unicast requests to the server, i.e. 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 rest of the response processing occurs as defined in
   [I-D.dijk-core-groupcomm-bis], and in
   [I-D.ietf-core-oscore-groupcomm] when secure group communication is
   used.

   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 enabling
   Token reusage expires, following late responses to the same request
   forwarded by the proxy will be discarded, as not matching any active
   request Token from the client.

   When using CoAP Observe [RFC7641], the Token value is freed up only
   if, after the timeout expiration, no 2.xx (Success) responses
   matching the original unicast and also including an Observe option
   have been received.  If at least one such response has been received,
   then for as long as the observation for the target resource of the
   original unicast request is active, the client receives those
   notifications as forwarded by the proxy.

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6.  Security Considerations

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

   The Multicast-Signaling Option is of class I for OSCORE
   [RFC8613][I-D.ietf-core-oscore-groupcomm].  While this allows the
   proxy to access the option value and retrieve the timeout value T',
   the proxy is not able to remove the option altogether without this
   being noted by the servers.  This ensures that the servers include
   their addressing information as value of the Response-Forwarding
   Option.

   Besides, this prevents further possible intermediaries as well as on-
   path active adversaries to remove the option or alter its content.
   However, intermediares as well as on-path passive adversaries are
   able to access the option content, and thus learn for how long
   clients are willing to receive responses from the servers in the
   group via the proxy.

   If no secure group communication is enforced end-to-end between the
   client and the servers (see Section 5.1 of
   [I-D.dijk-core-groupcomm-bis]), the proxy or any other on-path active
   intermediary is able to undetectably remove the Multicast-Signaling
   Option, i.e. to not include it in the group request sent to the
   servers in the group over multicast.  As a consequence, the servers
   will not include the Response-Forwarding Option in their response,
   thus preventing the clients to distinguish the different responses
   and their corresponding originator server.  The same result is
   achievable by removing the Response-Forwarding Option in the
   individual response of specific servers.

7.  IANA Considerations

   This document has the following actions for IANA.

7.1.  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]] |
           +--------+---------------------+-------------------+

8.  References

8.1.  Normative References

   [I-D.dijk-core-groupcomm-bis]
              Dijk, E., Wang, C., and M. Tiloca, "Group Communication
              for the Constrained Application Protocol (CoAP)", draft-
              dijk-core-groupcomm-bis-03 (work in progress), March
              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-07 (work in progress),
              March 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>.

   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,
              October 2013, <https://www.rfc-editor.org/info/rfc7049>.

   [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>.

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   [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>.

8.2.  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-05 (work in progress), March 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-dtls13-37 (work in progress), March
              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-05 (work in progress), March
              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>.

Acknowledgments

   The work on this document has been partly supported by VINNOVA and
   the Celtic-Next project CRITISEC.

Authors' Addresses

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

   Email: marco.tiloca@ri.se

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Internet-Draft  Proxy Operations for Group Communication      March 2020

   Esko Dijk
   IoTconsultancy.nl
   \________________\
   Utrecht
   The Netherlands

   Email: esko.dijk@iotconsultancy.nl

Tiloca & Dijk          Expires September 10, 2020              [Page 13]