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Versions: (draft-amsuess-core-repeat-request-tag) 00 01 02 03 04 05 06 07 08

CoRE Working Group                                            C. Amsuess
Internet-Draft
Updates: 7252 (if approved)                                  J. Mattsson
Intended status: Standards Track                             G. Selander
Expires: April 25, 2019                                      Ericsson AB
                                                        October 22, 2018


                          Echo and Request-Tag
                  draft-ietf-core-echo-request-tag-03

Abstract

   This document specifies security enhancements to the Constrained
   Application Protocol (CoAP).  Two optional extensions are defined:
   the Echo option and the Request-Tag option.  Each of these options
   provide additional features to CoAP and protects against certain
   attacks.  The document also updates the processing requirements on
   the Token of RFC 7252.  The updated Token processing ensures secure
   binding of responses to requests.

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 April 25, 2019.

Copyright Notice

   Copyright (c) 2018 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  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Request Freshness . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Fragmented Message Body Integrity . . . . . . . . . . . .   4
     1.3.  Request-Response Binding  . . . . . . . . . . . . . . . .   4
     1.4.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  The Echo Option . . . . . . . . . . . . . . . . . . . . . . .   6
     2.1.  Option Format . . . . . . . . . . . . . . . . . . . . . .   6
     2.2.  Echo Processing . . . . . . . . . . . . . . . . . . . . .   7
     2.3.  Applications  . . . . . . . . . . . . . . . . . . . . . .   9
   3.  The Request-Tag Option  . . . . . . . . . . . . . . . . . . .  11
     3.1.  Option Format . . . . . . . . . . . . . . . . . . . . . .  11
     3.2.  Request-Tag Processing by Servers . . . . . . . . . . . .  12
     3.3.  Setting the Request-Tag . . . . . . . . . . . . . . . . .  13
     3.4.  Applications  . . . . . . . . . . . . . . . . . . . . . .  13
       3.4.1.  Body Integrity Based on Payload Integrity . . . . . .  13
       3.4.2.  Multiple Concurrent Blockwise Operations  . . . . . .  14
       3.4.3.  Simplified Block-Wise Handling for Constrained
               Proxies . . . . . . . . . . . . . . . . . . . . . . .  15
     3.5.  Rationale for the Option Properties . . . . . . . . . . .  15
     3.6.  Rationale for Introducing the Option  . . . . . . . . . .  16
   4.  Block2 / ETag Processing  . . . . . . . . . . . . . . . . . .  16
   5.  Token Processing  . . . . . . . . . . . . . . . . . . . . . .  16
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  16
   7.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  17
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18
   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  18
     9.1.  Normative References  . . . . . . . . . . . . . . . . . .  18
     9.2.  Informative References  . . . . . . . . . . . . . . . . .  18
   Appendix A.  Methods for Generating Echo Option Values  . . . . .  20
   Appendix B.  Request-Tag Message Size Impact  . . . . . . . . . .  21
   Appendix C.  Change Log . . . . . . . . . . . . . . . . . . . . .  21
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

1.  Introduction

   The initial Constrained Application Protocol (CoAP) suite of
   specifications ([RFC7252], [RFC7641], and [RFC7959]) was designed
   with the assumption that security could be provided on a separate
   layer, in particular by using DTLS ([RFC6347]).  However, for some
   use cases, additional functionality or extra processing is needed to



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   support secure CoAP operations.  This document specifies security
   enhancements to the Constrained Application Protocol (CoAP).

   This document specifies two server-oriented CoAP options, the Echo
   option and the Request-Tag option: The Echo option enables a CoAP
   server to verify the freshness of a request, synchronize state, or
   force a client to demonstrate reachability at its apparent network
   address.  The Request-Tag option allows the CoAP server to match
   message fragments belonging to the same request, fragmented using the
   CoAP Block-Wise Transfer mechanism, which mitigates attacks and
   enables concurrent blockwise operations.  These options in themselves
   do not replace the need for a security protocol; they specify the
   format and processing of data which, when integrity protected using
   e.g.  DTLS ([RFC6347]), TLS ([RFC8446]), or OSCORE
   ([I-D.ietf-core-object-security]), provide the additional security
   features.

   The document also updates the processing requirements on the Token.
   The updated processing ensures secure binding of responses to
   requests, thus mitigating error cases and attacks where the client
   may erroneously associate the wrong response to a request.

1.1.  Request Freshness

   A CoAP server receiving a request is in general not able to verify
   when the request was sent by the CoAP client.  This remains true even
   if the request was protected with a security protocol, such as DTLS.
   This makes CoAP requests vulnerable to certain delay attacks which
   are particularly incriminating in the case of actuators
   ([I-D.mattsson-core-coap-actuators]).  Some attacks are possible to
   mitigate by establishing fresh session keys, e.g. performing a DTLS
   handshake for each actuation, but in general this is not a solution
   suitable for constrained environments, for example, due to increased
   message overhead and latency.  Additionally, if there are proxies,
   fresh DTLS session keys between server and proxy does not say
   anything about when the client made the request.  In a general hop-
   by-hop setting, freshness may need to be verified in each hop.

   A straightforward mitigation of potential delayed requests is that
   the CoAP server rejects a request the first time it appears and asks
   the CoAP client to prove that it intended to make the request at this
   point in time.  The Echo option, defined in this document, specifies
   such a mechanism which thereby enables a CoAP server to verify the
   freshness of a request.  This mechanism is not only important in the
   case of actuators, or other use cases where the CoAP operations
   require freshness of requests, but also in general for synchronizing
   state between CoAP client and server and to verify aliveness of the
   client.



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1.2.  Fragmented Message Body Integrity

   CoAP was designed to work over unreliable transports, such as UDP,
   and include a lightweight reliability feature to handle messages
   which are lost or arrive out of order.  In order for a security
   protocol to support CoAP operations over unreliable transports, it
   must allow out-of-order delivery of messages using e.g. a sliding
   replay window such as described in Section 4.1.2.6 of DTLS
   ([RFC6347]).

   The Block-Wise Transfer mechanism [RFC7959] extends CoAP by defining
   the transfer of a large resource representation (CoAP message body)
   as a sequence of blocks (CoAP message payloads).  The mechanism uses
   a pair of CoAP options, Block1 and Block2, pertaining to the request
   and response payload, respectively.  The blockwise functionality does
   not support the detection of interchanged blocks between different
   message bodies to the same resource having the same block number.
   This remains true even when CoAP is used together with a security
   protocol such as DTLS or OSCORE, within the replay window
   ([I-D.mattsson-core-coap-actuators]), which is a vulnerability of
   CoAP when using RFC7959.

   A straightforward mitigation of mixing up blocks from different
   messages is to use unique identifiers for different message bodies,
   which would provide equivalent protection to the case where the
   complete body fits into a single payload.  The ETag option [RFC7252],
   set by the CoAP server, identifies a response body fragmented using
   the Block2 option.  This document defines the Request-Tag option for
   identifying the request body fragmented using the Block1 option,
   similar to ETag, but ephemeral and set by the CoAP client.

1.3.  Request-Response Binding

   A fundamental requirement of secure REST operations is that the
   client can bind a response to a particular request.  If this is not
   valid a client may erroneously associate the wrong response to a
   request.  The wrong response may be an old response for the same
   resource or for a completely different resource (see e.g.
   Section 2.3 of [I-D.mattsson-core-coap-actuators]).  For example a
   request for the alarm status "GET /status" may be associated to a
   prior response "on", instead of the correct response "off".

   In HTTPS, binding is assured by the ordered and reliable delivery as
   well as mandating that the server sends responses in the same order
   that the requests were received.  The same is not true for CoAP where
   the server (or an attacker) can return responses in any order.
   Concurrent requests are instead differentiated by their Token.  Note
   that the CoAP Message ID cannot be used for this purpose since those



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   are typically different for REST request and corresponding response
   in case of "separate response", see Section 2.2 of [RFC7252].

   Unfortunately, CoAP [RFC7252] does not treat Token as a
   cryptographically important value and does not give stricter
   guidelines than that the tokens currently "in use" SHOULD (not SHALL)
   be unique.  If used with security protocol not providing bindings
   between requests and responses (e.g.  DTLS and TLS) token reuse may
   result in situations where a client matches a response to the wrong
   request.  Note that mismatches can also happen for other reasons than
   a malicious attacker, e.g. delayed delivery or a server sending
   notifications to an uninterested client.

   A straightforward mitigation is to mandate clients to never reuse
   tokens until the AEAD keys have been replaced.  As there may be any
   number of responses to a request (see e.g.  [RFC7641]), the easiest
   way to accomplish this is to implement the token as a counter and
   never reuse any tokens at all.  This document updates the Token
   processing in [RFC7252] to always assure a cryptographically secure
   binding of responses to requests.

1.4.  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.

   Unless otherwise specified, the terms "client" and "server" refers to
   "CoAP client" and "CoAP server", respectively, as defined in
   [RFC7252].  The term "origin server" is used as in [RFC7252].  The
   term "origin client" is used in this document to denote the client
   from which a request originates; to distinguish from clients in
   proxies.

   The terms "payload" and "body" of a message are used as in [RFC7959].
   The complete interchange of a request and a response body is called a
   (REST) "operation".  An operation fragmented using [RFC7959] is
   called a "blockwise operation".  A blockwise operation which is
   fragmenting the request body is called a "blockwise request
   operation".  A blockwise operation which is fragmenting the response
   body is called a "blockwise response operation".

   Two request messages are said to be "matchable" if they occur between
   the same endpoint pair, have the same code and the same set of
   options except for elective NoCacheKey options and options involved




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   in block-wise transfer (Block1, Block2 and Request-Tag).  Two
   operations are said to be matchable if any of their messages are.

   Two matchable blockwise operations are said to be "concurrent" if a
   block of the second request is exchanged even though the client still
   intends to exchange further blocks in the first operation.
   (Concurrent blockwise request operations are impossible with the
   options of [RFC7959] because the second operation's block overwrites
   any state of the first exchange.).

   The Echo and Request-Tag options are defined in this document.

2.  The Echo Option

   The Echo option is a lightweight server-driven challenge-response
   mechanism for CoAP, motivated by the need for a server to verify
   freshness of a request as described in Section 1.1.  With request
   freshness we mean that the server can determine that the client (or
   in the case of hop-by-hop security the proxy) sent the request
   recently.  The time threshold for being fresh is application
   specific.  The Echo option value is a challenge from the server to
   the client included in a CoAP response and echoed back to the server
   in one or more CoAP requests.

2.1.  Option Format

   The Echo Option is elective, safe-to-forward, not part of the cache-
   key, and not repeatable, see Figure 1, which extends Table 4 of
   [RFC7252]).

 +-----+---+---+---+---+-------------+--------+------+---------+---+---+
 | No. | C | U | N | R | Name        | Format | Len. | Default | E | U |
 +-----+---+---+---+---+-------------+--------+------+---------+---+---+
 | TBD |   |   | x |   | Echo        | opaque | 4-40 | (none)  | x | x |
 +-----+---+---+---+---+-------------+--------+------+---------+---+---+

       C = Critical, U = Unsafe, N = NoCacheKey, R = Repeatable,
       E = Encrypt and Integrity Protect (when using OSCORE)

                       Figure 1: Echo Option Summary

   [ Note to RFC editor: If this document is released before core-
   object-security, then the following paragraph and the "E"/"U" columns
   above need to move into core-object-security, as they are defined in
   that draft. ]

   The Echo option MAY be an Inner or Outer option
   [I-D.ietf-core-object-security], and the Inner and Outer values are



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   independent.  The Inner option is encrypted and integrity protected
   between the endpoints, whereas the Outer option is not protected by
   OSCORE and visible between the endpoints to the extent it is not
   protected by some other security protocol.  E.g. in the case of DTLS
   hop-by-hop between the endpoints, the Outer option is visible to
   proxies along the path.

   The Echo option value is generated by a server, and its content and
   structure are implementation specific.  Different methods for
   generating Echo option values are outlined in Appendix A.  Clients
   and intermediaries MUST treat an Echo option value as opaque and make
   no assumptions about its content or structure.

   When receiving an Echo option in a request, the server MUST be able
   to verify that the Echo option value was generated by the server as
   well as the point in time when the Echo option value was generated.

2.2.  Echo Processing

   The Echo option MAY be included in any request or response (see
   Section 2.3 for different applications), but the Echo option MUST NOT
   be used with empty CoAP requests (i.e.  Code=0.00).

   If a server receives a request which has freshness requirements, the
   request does not contain a fresh Echo option value, and the server
   cannot verify the freshness of the request in some other way, the
   server MUST NOT process the request further and SHOULD send a 4.01
   Unauthorized response with an Echo option.  The server MAY include
   the same Echo option value in several different responses and to
   different clients.

   The application decides under what conditions a CoAP request to a
   resource is required to be fresh.  These conditions can for example
   include what resource is requested, the request method and other data
   in the request, and conditions in the environment such as the state
   of the server or the time of the day.

   The server may use request freshness provided by the Echo option to
   verify the aliveness of a client or to synchronize state.  The server
   may also include the Echo option in a response to force a client to
   demonstrate reachability at their apparent network address.

   Upon receiving a 4.01 Unauthorized response with the Echo option, the
   client SHOULD resend the original request with the addition of an
   Echo option with the received Echo option value.  The client MAY send
   a different request compared to the original request.  Upon receiving
   any other response with the Echo option, the client SHOULD echo the
   Echo option value in the next request to the server.  The client MAY



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   include the same Echo option value in several different requests to
   the server.

   Upon receiving a request with the Echo option, the server determines
   if the request has freshness requirements.  If the request does not
   have freshness requirements, the Echo option MAY be ignored.  If the
   request has freshness requirements and the server cannot verify the
   freshness of the request in some other way, the server MUST verify
   that the Echo option value was generated by the server; otherwise the
   request is not processed further.  The server MUST then calculate the
   round-trip time RTT = (t1 - t0), where t1 is the request receive time
   and t0 is the time when the Echo option value was generated.  The
   server MUST only accept requests with a round-trip time below a
   certain threshold T, i.e. RTT < T.  If the server cannot verify that
   the Echo option value was generated by the server or the round-trip
   time is not below the threshold the request is not processed further,
   and an error message MAY be sent.  The error message SHOULD include a
   new Echo option.  The threshold T is application specific, its value
   depends e.g. on the freshness requirements of the request.  An
   example message flow is illustrated in Figure 2.

               Client   Server
                  |       |
                  +------>|        Code: 0.03 (PUT)
                  |  PUT  |       Token: 0x41
                  |       |    Uri-Path: lock
                  |       |     Payload: 0 (Unlock)
                  |       |
                  |<------+ t0     Code: 4.01 (Unauthorized)
                  |  4.01 |       Token: 0x41
                  |       |        Echo: 0x437468756c687521
                  |       |
                  +------>| t1     Code: 0.03 (PUT)
                  |  PUT  |       Token: 0x42
                  |       |    Uri-Path: lock
                  |       |        Echo: 0x437468756c687521
                  |       |     Payload: 0 (Unlock)
                  |       |
                  |<------+        Code: 2.04 (Changed)
                  |  2.04 |       Token: 0x42
                  |       |

                Figure 2: Example Echo Option Message Flow

   Note that the server does not have to synchronize the time used for
   the Echo timestamps with any other party.  However, if the server
   loses time continuity, e.g. due to reboot, it MUST reject all Echo
   values that was created before time continuity was lost.



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   When used to serve freshness requirements (including client aliveness
   and state synchronizing), CoAP messages containing the Echo option
   MUST be integrity protected between the intended endpoints, e.g.
   using DTLS, TLS, or an OSCORE Inner option
   ([I-D.ietf-core-object-security]).  When used to demonstrate
   reachability at their apparent network address, the Echo option MAY
   be unprotected.

   A CoAP-to-CoAP proxy MAY respond to requests with 4.01 with an Echo
   option to ensure the client's reachability at its apparent address,
   and MUST remove the Echo option it recognizes as one generated by
   itself on follow-up requests.  However, it MUST relay the Echo option
   of responses unmodified, and MUST relay the Echo option of requests
   it does not recognize as generated by itself unmodified.

   The CoAP server side of CoAP-to-HTTP proxies MAY request freshness,
   especially if they have reason to assume that access may require it
   (e.g. because it is a PUT or POST); how this is determined is out of
   scope for this document.  The CoAP client side of HTTP-to-CoAP
   proxies SHOULD respond to Echo challenges themselves if they know
   from the recent establishing of the connection that the HTTP request
   is fresh.  Otherwise, they SHOULD respond with 503 Service
   Unavailable, Retry-After: 0 and terminate any underlying Keep-Alive
   connection.  They MAY also use other mechanisms to establish
   freshness of the HTTP request that are not specified here.

2.3.  Applications

   1.  Actuation requests often require freshness guarantees to avoid
       accidental or malicious delayed actuator actions.  In general,
       all non-safe methods (e.g.  POST, PUT, DELETE) may require
       freshness guarantees for secure operation.

       *  The same Echo value may be used for multiple actuation
          requests to the same server, as long as the total round-trip
          time since the Echo option value was generated is below the
          freshness threshold.

       *  For actuator applications with low delay tolerance, to avoid
          additional round-trips for multiple requests in rapid
          sequence, the server may include the Echo option with a new
          value in response to a request containing the Echo option.
          The client then uses the Echo option with the new value in the
          next actuation request, and the server compares the receive
          time accordingly.

   2.  A server may use the Echo option to synchronize state or time
       with a requesting client.  A server MUST NOT synchronize state or



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       time with clients which are not the authority of the property
       being synchronized.  E.g. if access to a server resource is
       dependent on time, then the client MUST NOT set the time of the
       server.

       *  If a server reboots during operation it may need to
          synchronize state or time before continuing the interaction.
          For example, with OSCORE it is possible to reuse a partly
          persistently stored security context by synchronizing the
          Partial IV (sequence number) using the Echo option, see
          Section 7.5 of [I-D.ietf-core-object-security].

       *  A device joining a CoAP group communication [RFC7390]
          protected with OSCORE [I-D.ietf-core-oscore-groupcomm] may be
          required to initially verify freshness and synchronize state
          or time with a client by using the Echo option in a unicast
          response to a multicast request.  The client receiving the
          response with the Echo option includes the Echo option with
          the same value in a request, either in a unicast request to
          the responding server, or in a subsequent group request.  In
          the latter case, the Echo option will be ignored expect by
          responding server.

   3.  A server that sends large responses to unauthenticated peers
       SHOULD mitigate amplification attacks such as described in
       Section 11.3 of [RFC7252] (where an attacker would put a victim's
       address in the source address of a CoAP request).  For this
       purpose, a server MAY ask a client to Echo its request to verify
       its source address.  This needs to be done only once per peer and
       limits the range of potential victims from the general Internet
       to endpoints that have been previously in contact with the
       server.  For this application, the Echo option can be used in
       messages that are not integrity protected, for example during
       discovery.

       *  In the presence of a proxy, a server will not be able to
          distiguish different origin client endpoints.  Following from
          the recommendation above, a proxy that sends large responses
          to unauthenticatied peers SHOULD mitigate amplification
          attacks.  The proxy MAY use Echo to verify origin reachability
          as described in Section 2.2.  The proxy MAY forward idempotent
          requests immediately to have a cached result available when
          the client's Echoed request arrives.

   4.  A server may want to use the request freshness provided by the
       Echo to verify the aliveness of a client.  Note that in a
       deployment with hop-by-hop security and proxies, the server can
       only verify aliveness of the closest proxy.



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3.  The Request-Tag Option

   The Request-Tag is intended for use as a short-lived identifier for
   keeping apart distinct blockwise request operations on one resource
   from one client, addressing the issue described in Section 1.2.  It
   enables the receiving server to reliably assemble request payloads
   (blocks) to their message bodies, and, if it chooses to support it,
   to reliably process simultaneous blockwise request operations on a
   single resource.  The requests must be integrity protected in order
   to protect against interchange of blocks between different message
   bodies.

   In essence, it is an implementation of the "proxy-safe elective
   option" used just to "vary the cache key" as suggested in [RFC7959]
   Section 2.4.

3.1.  Option Format

   The Request-Tag option is not critical, is safe to forward,
   repeatable, and part of the cache key, see Figure 3, which extends
   Table 4 of [RFC7252]).

 +-----+---+---+---+---+-------------+--------+------+---------+---+---+
 | No. | C | U | N | R | Name        | Format | Len. | Default | E | U |
 +-----+---+---+---+---+-------------+--------+------+---------+---+---+
 | TBD |   |   |   | x | Request-Tag | opaque |  0-8 | (none)  | x | x |
 +-----+---+---+---+---+-------------+--------+------+---------+---+---+

       C = Critical, U = Unsafe, N = NoCacheKey, R = Repeatable,
       E = Encrypt and Integrity Protect (when using OSCORE)

                   Figure 3: Request-Tag Option Summary

   [ Note to RFC editor: If this document is released before core-
   object-security, then the following paragraph and the "E"/"U" columns
   above need to move into core-object-security, as they are defined in
   that draft. ]

   Request-Tag, like the block options, is both a class E and a class U
   option in terms of OSCORE processing (see Section 4.1 of
   [I-D.ietf-core-object-security]): The Request-Tag MAY be an inner or
   outer option.  It influences the inner or outer block operation,
   respectively.  The inner and outer values are therefore independent
   of each other.  The inner option is encrypted and integrity protected
   between client and server, and provides message body identification
   in case of end-to-end fragmentation of requests.  The outer option is
   visible to proxies and labels message bodies in case of hop-by-hop
   fragmentation of requests.



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   The Request-Tag option is only used in the request messages of
   blockwise operations.

   The Request-Tag mechanism can be applied independently on the server
   and client sides of CoAP-to-CoAP proxies as are the block options,
   though given it is safe to forward, a proxy is free to just forward
   it when processing an operation.  CoAP-to-HTTP proxies and HTTP-to-
   CoAP proxies can use Request-Tag on their CoAP sides; it is not
   applicable to HTTP requests.

3.2.  Request-Tag Processing by Servers

   The Request-Tag option does not require any particular processing on
   the server side outside of the processing already necessary for any
   unknown elective proxy-safe cache-key option: The option varies the
   properties that distinguish blockwise operations (which includes all
   options except elective NoCacheKey and except Block1/2), and thus the
   server can not treat messages with a different list of Request-Tag
   options as belonging to the same operation.

   To keep utilizing the cache, a server (including proxies) MAY discard
   the Request-Tag option from an assembled block-wise request when
   consulting its cache, as the option relates to the operation-on-the-
   wire and not its semantics.  For example, a FETCH request with the
   same body as an older one can be served from the cache if the older's
   Max-Age has not expired yet, even if the second operation uses a
   Request-Tag and the first did not.  (This is similar to the situation
   about ETag in that it is formally part of the cache key, but
   implementations that are aware of its meaning can cache more
   efficiently, see [RFC7252] Section 5.4.2).

   A server receiving a Request-Tag MUST treat it as opaque and make no
   assumptions about its content or structure.

   Two messages carrying the same Request-Tag is a necessary but not
   sufficient condition for being part of the same operation.  They can
   still be treated as independent messages by the server (e.g. when it
   sends 2.01/2.04 responses for every block), or initiate a new
   operation (overwriting kept context) when the later message carries
   Block1 number 0.

   As it has always been, a server that can only serve a limited number
   of block-wise operations at the same time can delay the start of the
   operation by replying with 5.03 (Service unavailable) and a Max-Age
   indicating how long it expects the existing operation to go on, or it
   can forget about the state established with the older operation and
   respond with 4.08 (Request Entity Incomplete) to later blocks on the
   first operation.



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3.3.  Setting the Request-Tag

   For each separate blockwise request operation, the client can choose
   a Request-Tag value, or choose not to set a Request-Tag.  Starting a
   request operation matchable to a previous operation and even using
   the same Request-Tag value is called request tag recycling.  The
   absence of a Request-Tag option is viewed as a value distinct from
   all values with a single Request-Tag option set; starting a request
   operation matchable to a previous operation where neither has a
   Request-Tag option therefore constitutes request tag recycling just
   as well (also called "recycling the absent option").

   Clients MUST NOT recycle a request tag unless the first operation has
   concluded.  What constitutes a concluded operation depends on the
   application, and is outlined individually in Section 3.4.

   When Block1 and Block2 are combined in an operation, the Request-Tag
   of the Block1 phase is set in the Block2 phase as well for otherwise
   the request would have a different set of options and would not be
   recognized any more.

   Clients are encouraged to generate compact messages.  This means
   sending messages without Request-Tag options whenever possible, and
   using short values when the absent option can not be recycled.

3.4.  Applications

3.4.1.  Body Integrity Based on Payload Integrity

   When a client fragments a request body into multiple message
   payloads, even if the individual messages are integrity protected, it
   is still possible for a man-in-the-middle to maliciously replace a
   later operation's blocks with an earlier operation's blocks (see
   Section 2.5 of [I-D.mattsson-core-coap-actuators]).  Therefore, the
   integrity protection of each block does not extend to the operation's
   request body.

   In order to gain that protection, use the Request-Tag mechanism as
   follows:

   o  The individual exchanges MUST be integrity protected end-to-end
      between client and server.

   o  The client MUST NOT recycle a request tag in a new operation
      unless the previous operation matchable to the new one has
      concluded.





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      If any future security mechanisms allow a block-wise transfer to
      continue after an endpoint's details (like the IP address) have
      changed, then the client MUST consider messages sent to _any_
      endpoint address within the new operation's security context.

   o  The client MUST NOT regard a blockwise request operation as
      concluded unless all of the messages the client previously sent in
      the operation have been confirmed by the message integrity
      protection mechanism, or are considered invalid by the server if
      replayed.

      Typically, in OSCORE, these confirmations can result either from
      the client receiving an OSCORE response message matching the
      request (an empty ACK is insufficient), or because the message's
      sequence number is old enough to be outside the server's receive
      window.

      In DTLS, this can only be confirmed if the request message was not
      retransmitted, and was responded to.

   Authors of other documents (e.g.  [I-D.ietf-core-object-security])
   are invited to mandate this behavior for clients that execute
   blockwise interactions over secured transports.  In this way, the
   server can rely on a conforming client to set the Request-Tag option
   when required, and thereby conclude on the integrity of the assembled
   body.

   Note that this mechanism is implicitly implemented when the security
   layer guarantees ordered delivery (e.g.  CoAP over TLS [RFC8323]).
   This is because with each message, any earlier message can not be
   replayed any more, so the client never needs to set the Request-Tag
   option unless it wants to perform concurrent operations.

3.4.2.  Multiple Concurrent Blockwise Operations

   CoAP clients, especially CoAP proxies, may initiate a blockwise
   request operation to a resource, to which a previous one is already
   in progress, which the new request should not cancel.  A CoAP proxy
   would be in such a situation when it forwards operations with the
   same cache-key options but possibly different payloads.

   For those cases, Request-Tag is the proxy-safe elective option
   suggested in [RFC7959] Section 2.4 last paragraph.

   When initializing a new blockwise operation, a client has to look at
   other active operations:





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   o  If any of them is matchable to the new one, and the client neither
      wants to cancel the old one nor postpone the new one, it can pick
      a Request-Tag value that is not in use by the other matchable
      operations for the new operation.

   o  Otherwise, it can start the new operation without setting the
      Request-Tag option on it.

3.4.3.  Simplified Block-Wise Handling for Constrained Proxies

   The Block options were defined to be unsafe to forward because a
   proxy that would forward blocks as plain messages would risk mixing
   up clients' requests.

   The Request-Tag option provides a very simple way for a proxy to keep
   them separate: if it appends a Request-Tag that is particular to the
   requesting endpoint to all request carrying any Block option, it does
   not need to keep track of any further block state.

   This is particularly useful to proxies that strive for stateless
   operation as described in [I-D.hartke-core-stateless] Section 3.1.

3.5.  Rationale for the Option Properties

   The Request-Tag option can be elective, because to servers unaware of
   the Request-Tag option, operations with differing request tags will
   not be matchable.

   The Request-Tag option can be safe to forward but part of the cache
   key, because to proxies unaware of the Request-Tag option will
   consider operations with differing request tags unmatchable but can
   still forward them.

   The Request-Tag option is repeatable because this easily allows
   stateless proxies to "chain" their origin address.  Were it a single
   option, they would need to employ some length/value scheme to avoid
   confusing requests without a Request-Tag option with requests that
   carry a zero-length request tag.

   In earlier versions of this draft, the Request-Tag option used to be
   critical and unsafe to forward.  That design was based on an
   erroneous understanding of which blocks could be composed according
   to [RFC7959].








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3.6.  Rationale for Introducing the Option

   An alternative that was considered to the Request-Tag option for
   coping with the problem of fragmented message body integrity
   (Section 3.4.1) was to update [RFC7959] to say that blocks could only
   be assembled if their fragments' order corresponded to the sequence
   numbers.

   That approach would have been difficult to roll out reliably on DTLS
   where many implementations do not expose sequence numbers, and would
   still not prevent attacks like in [I-D.mattsson-core-coap-actuators]
   Section 2.5.2.

4.  Block2 / ETag Processing

   The same security properties as in Section 3.4.1 can be obtained for
   blockwise response operations.  The threat model here is not an
   attacker (because the response is made sure to belong to the current
   request by the security layer), but blocks in the client's cache.

   Rules stating that response body reassembly is conditional on
   matching ETag values are already in place from Section 2.4 of
   [RFC7959].

   To gain equivalent protection to Section 3.4.1, a server MUST use the
   Block2 option in conjunction with the ETag option ([RFC7252],
   Section 5.10.6), and MUST NOT use the same ETag value for different
   representations of a resource.

5.  Token Processing

   As described in Section 1.3, the client must be able to verify that a
   response corresponds to a particular request.  This section updates
   the Token processing in Section 5.3.1 of [RFC7252] by adding the
   following text:

   When CoAP is used with a security protocol not providing bindings
   between requests and responses, the client MUST NOT reuse tokens
   until the traffic keys have been replaced.  The easiest way to
   accomplish this is to implement the Token as a counter, this approach
   SHOULD be followed.

6.  Security Considerations

   The availability of a secure pseudorandom number generator and truly
   random seeds are essential for the security of the Echo option.  If
   no true random number generator is available, a truly random seed
   must be provided from an external source.



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   An Echo value with 64 (pseudo-)random bits gives the same theoretical
   security level against forgeries as a 64-bit MAC (as used in e.g.
   AES_128_CCM_8).  In practice, forgery of an Echo option value is much
   harder as an attacker must also forge the MAC in the security
   protocol.  The Echo option value MUST contain 32 (pseudo-)random bits
   that are not predictable for any other party than the server, and
   SHOULD contain 64 (pseudo-)random bits.  A server MAY use different
   security levels for different uses cases (client aliveness, request
   freshness, state synchronization, network address reachability,
   etc.).

   The security provided by the Echo and Request-Tag options depends on
   the security protocol used.  CoAP and HTTP proxies require (D)TLS to
   be terminated at the proxies.  The proxies are therefore able to
   manipulate, inject, delete, or reorder options or packets.  The
   security claims in such architectures only hold under the assumption
   that all intermediaries are fully trusted and have not been
   compromised.

   Servers MUST use a monotonic clock to generate timestamps and compute
   round-trip times.  Use of non-monotonic clocks is not secure as the
   server will accept expired Echo option values if the clock is moved
   backward.  The server will also reject fresh Echo option values if
   the clock is moved forward.

   Servers are not allowed to use wall clock time for timestamps, as
   wall clock time is not monotonic.  Furthermore, an attacker may be
   able to affect the server's wall clock time in various ways such as
   setting up a fake NTP server or broadcasting false time signals to
   radio-controlled clocks.

   Servers MAY use the time since reboot measured in some unit of time.
   Servers MAY reset the timer at certain times and MAY generate a
   random offset applied to all timestamps.  When resetting the timer,
   the server MUST reject all Echo values that was created before the
   reset.

   Servers that use the List of Cached Random Values and Timestamps
   method described in Appendix A may be vulnerable to resource
   exhaustion attacks.  One way to minimize state is to use the
   Integrity Protected Timestamp method described in Appendix A.

7.  Privacy Considerations

   Implementations SHOULD NOT put any privacy sensitive information in
   the Echo or Request-Tag option values.  Unencrypted timestamps MAY
   reveal information about the server such as location or time since
   reboot.  The use of wall clock time is not allowed (see Section 6)



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   and there also privacy reasons, e.g. it may reveal that the server
   will accept expired certificates.  Timestamps MAY be used if Echo is
   encrypted between the client and the server, e.g. in the case of DTLS
   without proxies or when using OSCORE with an Inner Echo option.

8.  IANA Considerations

   This document adds the following option numbers to the "CoAP Option
   Numbers" registry defined by [RFC7252]:

               +--------+-------------+-------------------+
               | Number | Name        | Reference         |
               +--------+-------------+-------------------+
               | TBD1   | Echo        | [[this document]] |
               |        |             |                   |
               | TBD2   | Request-Tag | [[this document]] |
               +--------+-------------+-------------------+

                       Figure 4: CoAP Option Numbers

9.  References

9.1.  Normative References

   [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>.

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,
              <https://www.rfc-editor.org/info/rfc7959>.

   [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>.

9.2.  Informative References







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   [I-D.hartke-core-stateless]
              Hartke, K., "Extended Tokens and Stateless Clients in the
              Constrained Application Protocol (CoAP)", draft-hartke-
              core-stateless-01 (work in progress), September 2018.

   [I-D.ietf-core-object-security]
              Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", draft-ietf-core-object-security-15 (work in
              progress), August 2018.

   [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-03 (work in progress),
              October 2018.

   [I-D.mattsson-core-coap-actuators]
              Mattsson, J., Fornehed, J., Selander, G., Palombini, F.,
              and C. Amsuess, "Controlling Actuators with CoAP", draft-
              mattsson-core-coap-actuators-06 (work in progress),
              September 2018.

   [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>.

   [RFC7390]  Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
              the Constrained Application Protocol (CoAP)", RFC 7390,
              DOI 10.17487/RFC7390, October 2014,
              <https://www.rfc-editor.org/info/rfc7390>.

   [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>.

   [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>.

   [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>.





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Appendix A.  Methods for Generating Echo Option Values

   The content and structure of the Echo option value are implementation
   specific and determined by the server.  Two simple mechanisms are
   outlined in this section, the first is RECOMMENDED in general, and
   the second is RECOMMENDED in case the Echo option is encrypted
   between the client and the server.

   Different mechanisms have different tradeoffs between the size of the
   Echo option value, the amount of server state, the amount of
   computation, and the security properties offered.  A server MAY use
   different methods and security levels for different uses cases
   (client aliveness, request freshness, state synchronization, network
   address reachability, etc.).

   1.  List of Cached Random Values and Timestamps.  The Echo option
   value is a (pseudo-)random byte string.  The server caches a list
   containing the random byte strings and their transmission times.
   Assuming 64-bit random values and 32-bit timestamps, the size of the
   Echo option value is 8 bytes and the amount of server state is 12n
   bytes, where n is the number of active Echo Option values.  If the
   server loses time continuity, e.g. due to reboot, the entries in the
   old list MUST be deleted.

         Echo option value: random value r
         Server State: random value r, timestamp t0

   2.  Integrity Protected Timestamp.  The Echo option value is an
   integrity protected timestamp.  The timestamp can have different
   resolution and range.  A 32-bit timestamp can e.g. give a resolution
   of 1 second with a range of 136 years.  The (pseudo-)random secret
   key is generated by the server and not shared with any other party.
   The use of truncated HMAC-SHA-256 is RECOMMENDED.  With a 32-bit
   timestamp and a 64-bit MAC, the size of the Echo option value is 12
   bytes and the Server state is small and constant.  If the server
   loses time continuity, e.g. due to reboot, the old key MUST be
   deleted and replaced by a new random secret key.  Note that the
   privacy considerations in Section 7 may apply to the timestamp.  A
   server MAY want to encrypt its timestamps, and, depending on the
   choice of encryption algorithms, this may require a nonce to be
   included in the Echo option value.

         Echo option value: timestamp t0, MAC(k, t0)
         Server State: secret key k

   Other mechanisms complying with the security and privacy
   considerations may be used.  The use of encrypted timestamps in the
   Echo option typically requires an IV to be included in the Echo



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   option value, which adds overhead and makes the specification of such
   a mechanims slightly more complicated than the two mechanisms
   specified here.

Appendix B.  Request-Tag Message Size Impact

   In absence of concurrent operations, the Request-Tag mechanism for
   body integrity (Section 3.4.1) incurs no overhead if no messages are
   lost (more precisely: in OSCORE, if no operations are aborted due to
   repeated transmission failure; in DTLS, if no packages are lost), or
   when blockwise request operations happen rarely (in OSCORE, if there
   is always only one request blockwise operation in the replay window).

   In those situations, no message has any Request-Tag option set, and
   that can be recycled indefinitely.

   When the absence of a Request-Tag option can not be recycled any more
   within a security context, the messages with a present but empty
   Request-Tag option can be used (1 Byte overhead), and when that is
   used-up, 256 values from one byte long options (2 Bytes overhead) are
   available.

   In situations where those overheads are unacceptable (e.g. because
   the payloads are known to be at a fragmentation threshold), the
   absent Request-Tag value can be made usable again:

   o  In DTLS, a new session can be established.

   o  In OSCORE, the sequence number can be artificially increased so
      that all lost messages are outside of the replay window by the
      time the first request of the new operation gets processed, and
      all earlier operations can therefore be regarded as concluded.

Appendix C.  Change Log

   [ The editor is asked to remove this section before publication. ]

   o  Major changes since draft-ietf-core-echo-request-tag-01:

      *  Follow-up changes after the "relying on blockwise" change in
         -01:

         +  Simplify the description of Request-Tag and matchability

         +  Do not update RFC7959 any more

      *  Make Request-Tag repeatable.




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      *  Add rationale on not relying purely on sequence numbers.

   o  Major changes since draft-ietf-core-echo-request-tag-00:

      *  Reworded the Echo section.

      *  Added rules for Token processing.

      *  Added security considerations.

      *  Added actual IANA section.

      *  Made Request-Tag optional and safe-to-forward, relying on
         blockwise to treat it as part of the cache-key

      *  Dropped use case about OSCORE outer-blockwise (the case went
         away when its Partial IV was moved into the Object-Security
         option)

   o  Major changes since draft-amsuess-core-repeat-request-tag-00:

      *  The option used for establishing freshness was renamed from
         "Repeat" to "Echo" to reduce confusion about repeatable
         options.

      *  The response code that goes with Echo was changed from 4.03 to
         4.01 because the client needs to provide better credentials.

      *  The interaction between the new option and (cross) proxies is
         now covered.

      *  Two messages being "Request-Tag matchable" was introduced to
         replace the older concept of having a request tag value with
         its slightly awkward equivalence definition.

Acknowledgments

   The authors want to thank Jim Schaad for providing valuable input to
   the draft.

Authors' Addresses

   Christian Amsuess

   Email: christian@amsuess.com






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   John Mattsson
   Ericsson AB

   Email: john.mattsson@ericsson.com


   Goeran Selander
   Ericsson AB

   Email: goran.selander@ericsson.com









































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