CoRE Working Group                                            C. Amsuess
Internet-Draft
Updates: 7252 (if approved)                                  J. Mattsson
Intended status: Standards Track                             G. Selander
Expires: 13 January 7 April 2022                                        Ericsson AB
                                                            12 July
                                                          4 October 2021

             CoAP: Echo, Request-Tag, and Token Processing
                  draft-ietf-core-echo-request-tag-13
                  draft-ietf-core-echo-request-tag-14

Abstract

   This document specifies enhancements to the Constrained Application
   Protocol (CoAP) that mitigate security issues in particular use
   cases.  The Echo option enables a CoAP server to verify the freshness
   of a request or to force a client to demonstrate reachability at its
   claimed network address.  The Request-Tag option allows the CoAP
   server to match block-wise message fragments belonging to the same
   request.  This document updates RFC 7252 with respect to the client
   Token processing requirements, forbidding non-secure reuse of Tokens
   to ensure binding of response to request when CoAP is used with a
   security protocol, and with respect to amplification mitigation,
   where the use of Echo is now recommended.

Discussion Venues

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

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

   Source for this draft and an issue tracker can be found at
   https://github.com/core-wg/echo-request-tag.

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
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   This Internet-Draft will expire on 13 January 7 April 2022.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Request Freshness and the Echo Option . . . . . . . . . . . .   5
     2.1.  Request Freshness . . . . . . . . . . . . . . . . . . . .   5
     2.2.  The Echo Option . . . . . . . . . . . . . . . . . . . . .   5
       2.2.1.  Echo Option Format  . . . . . . . . . . . . . . . . .   6
     2.3.  Echo Processing . . . . . . . . . . . . . . . . . . . . .   7
     2.4.  Applications of the Echo Option . . . . . . . . . . . . .  10
     2.5.  Characterization of Echo Applications . . . . . . . . . .  13
       2.5.1.  Time versus Event Based Freshness . . . . . . . . . .  13
       2.5.2.  Authority over Used Information . . . . . . . . . . .  13
       2.5.3.  Protection by a Security Protocol . . . . . . . . . .  14
     2.6.  Updated Amplification Mitigation Requirements for
           Servers . . . . . . . . . . . . . . . . . . . . . . . . .  15
   3.  Protecting Message Bodies using Request Tags  . . . . . . . .  15
     3.1.  Fragmented Message Body Integrity . . . . . . . . . . . .  15
     3.2.  The Request-Tag Option  . . . . . . . . . . . . . . . . .  16
       3.2.1.  Request-Tag Option Format . . . . . . . . . . . . . .  16
     3.3.  Request-Tag Processing by Servers . . . . . . . . . . . .  17
     3.4.  Setting the Request-Tag . . . . . . . . . . . . . . . . .  18
     3.5.  Applications of the Request-Tag Option  . . . . . . . . .  19
       3.5.1.  Body Integrity Based on Payload Integrity . . . . . .  19
       3.5.2.  Multiple Concurrent Block-wise Operations . . . . . .  20
       3.5.3.  Simplified Block-Wise Handling for Constrained
               Proxies . . . . . . . . . . . . . . . . . . . . . . .  21

     3.6.  Rationale for the Option Properties . . . . . . . . . . .  21
     3.7.  Rationale for Introducing the Option  . . . . . . . . . .  22  21
     3.8.  Block2 / ETag Processing  . . . . . . . . . . . . . . . .  22
   4.  Token Processing for Secure Request-Response Binding  . . . .  22
     4.1.  Request-Response Binding  . . . . . . . . . . . . . . . .  22
     4.2.  Updated Token Processing Requirements for Clients . . . .  23
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  23
     5.1.  Token reuse . . . . . . . . . . . . . . . . . . . . . . .  25
   6.  Privacy Considerations  . . . . . . . . . . . . . . . . . . .  26
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  27
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  28  27
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  28
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  28
   Appendix A.  Methods for Generating Echo Option Values  . . . . .  30
   Appendix B.  Request-Tag Message Size Impact  . . . . . . . . . .  32
   Appendix C.  Change Log . . . . . . . . . . . . . . . . . . . . .  32
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  38  41
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  38  41

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

   [ Note to RFC editor: If C321 gets published before C280, then the
   [RFC6347] references can be upgraded to draft-ietf-tls-dtls13-43
   without the need for further changes; the reference is to 6347 here
   because that was the stable DTLS reference when the document was last
   touched by the authors. ]

   This document specifies two CoAP options, the Echo option and the
   Request-Tag option: The Echo option enables a CoAP server to verify
   the freshness of a request, which can be used to synchronize state,
   or to force a client to demonstrate reachability at its claimed
   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 block-wise 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
   ([RFC8613]), provide the additional security features.

   This document updates [RFC7252] with a recommendation that servers
   use the Echo option to mitigate amplification attacks.

   The document also updates the Token processing requirements for
   clients specified in [RFC7252].  The updated processing forbids non-
   secure reuse of Tokens to ensure binding of responses to requests
   when CoAP is used with security, thus mitigating error cases and
   attacks where the client may erroneously associate the wrong response
   to a request.

   Each of the following sections provides a more detailed introduction
   to the topic at hand in its first subsection.

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.

   Like [RFC7252], this document is relying on the Representational
   State Transfer [REST] architecture of the Web.

   Unless otherwise specified, the terms "client" and "server" refer 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.

   A message's "freshness" is a measure of when a message was sent on a
   time scale of the recipient.  A server that receives a request can
   either verify that the request is fresh or determine that it cannot
   be verified that the request is fresh.  What is considered a fresh
   message is application dependent; exemplary uses are "no more than
   one hour ago" or "after this server's last reboot".

   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 "block-wise operation".  A block-wise operation which is
   fragmenting the request body is called a "block-wise request
   operation".  A block-wise operation which is fragmenting the response
   body is called a "block-wise response operation".

   Two request messages are said to be "matchable" if they occur between
   the same endpoint pair, have the same code, and have the same set of
   options, with the exception that elective NoCacheKey options and
   options involved in block-wise transfer (Block1, Block2 and Request-
   Tag) need not be the same.  Two operations are said to be matchable
   if any of their messages are.

   Two matchable block-wise 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 block-wise request operations from a single endpoint are
   impossible with the options of [RFC7959] (see the last paragraphs of
   Sections 2.4 and 2.5) 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.  Request Freshness and the Echo Option

2.1.  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 perilous in the case of actuators
   ([I-D.mattsson-core-coap-attacks]).  Some attacks can be mitigated by
   establishing fresh session keys, e.g. performing a DTLS handshake for
   each request, 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.

2.2.  The Echo Option

   This document defines the Echo option, a lightweight challenge-
   response mechanism for CoAP that enables a CoAP server to verify the
   freshness of a request.  A fresh request is one whose age has not yet
   exceeded the freshness requirements set by the server.  The freshness
   requirements are application specific and may vary based on resource,
   method, and parameters outside of CoAP such as policies.  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.

   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, cryptographically verifying the aliveness of the
   client, or forcing a client to demonstrate reachability at its
   claimed network address.  The same functionality can be provided by
   echoing freshness indicators in CoAP payloads, but this only works
   for methods and response codes defined to have a payload.  The Echo
   option provides a convention to transfer freshness indicators that
   works for all methods and response codes.

2.2.1.  Echo 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 |
    +--------+---+---+---+---+-------------+--------+------+---------+
    | TBD252 |   |   | x |   | Echo        | opaque | 1-40 | (none)  |
    +--------+---+---+---+---+-------------+--------+------+---------+

          C = Critical, U = Unsafe, N = NoCacheKey, R = Repeatable

                       Figure 1: Echo Option Summary

   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 (a) was generated by the server
   or some other party that the server trusts, and (b) fulfills the
   freshness requirements of the application.  Depending on the
   freshness requirements the server may verify exactly when the Echo
   option value was generated (time-based freshness) or verify that the
   Echo option was generated after a specific event (event-based
   freshness).  As the request is bound to the Echo option value, the
   server can determine that the request is not older that the Echo
   option value.

   When the Echo option is used with OSCORE [RFC8613] it MAY be an Inner
   or Outer option, and the Inner and Outer values are independent.
   OSCORE servers MUST only produce Inner Echo options unless they are
   merely testing for reachability of the client (the same as proxies
   may do).  The Inner option is encrypted and integrity protected
   between the endpoints, whereas the Outer option is not protected by
   OSCORE.  As always with OSCORE, outer options are visible to (and may
   be acted on by) all proxies, and are visible on all links where no
   additional encryption (like TLS between client and proxy) is used.

2.3.  Echo Processing

   The Echo option MAY be included in any request or response (see
   Section 2.4 for different applications).

   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.

   If a certain request is required to be fresh, 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 response messages and to different
   clients.  Examples of this could be time-based freshness when several
   responses are sent closely after each other or event-based freshness
   with no event taking place between the responses.

   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 its claimed network address.  Note that
   the Echo option does not bind a request to any particular previous
   response, but provides an indication that the client had access to
   the previous response at the time when it created the request.

   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
   include the same Echo option value in several different requests to
   the server, or discard it at any time (especially to avoid tracking,
   see Section 6).

   A client MUST only send Echo values to endpoints it received them
   from (where as defined in [RFC7252] Section 1.2, the security
   association is part of the endpoint).  In OSCORE processing, that
   means sending Echo values from Outer options (or from non-OSCORE
   responses) back in Outer options, and those from Inner options in
   Inner options in the same security context.

   Upon receiving a request with the Echo option, the server determines
   if the request is required to be fresh.  If not, the Echo option MAY
   be ignored.  If the request is required to be fresh and the server
   cannot verify the freshness of the request in some other way, the
   server MUST use the Echo option to verify that the request is fresh.
   If the server cannot verify that the request is fresh, the request is
   not processed further, and an error message MAY be sent.  The error
   message SHOULD include a new Echo option.

   One way for the server to verify freshness is to bind the Echo value
   to a specific point in time and verify that the request is not older
   than a certain threshold T.  The server can verify this by checking
   that (t1 - t0) < T, where t1 is the request receive time and t0 is
   the time when the Echo option value was generated.  An example
   message flow over DTLS is shown Figure 2.

    Client   Server
       |       |
       +------>|        Code: 0.03 (PUT)
       |  PUT  |       Token: 0x41
       |       |    Uri-Path: lock
       |       |     Payload: 0 (Unlock)
       |       |
       |<------+        Code: 4.01 (Unauthorized)
       |  4.01 |       Token: 0x41
       |       |        Echo: 0x00000009437468756c687521 (t0 = 9, +MAC)
       |       |
       | ...   | The round trips take 1 second, time is now t1 = 10.
       |       |
       +------>|        Code: 0.03 (PUT)
       |  PUT  |       Token: 0x42
       |       |    Uri-Path: lock
       |       |        Echo: 0x00000009437468756c687521 (t0 = 9, +MAC)
       |       |     Payload: 0 (Unlock)
       |       |
       |       | Verify MAC, compare t1 - t0 = 1 < T => permitted.
       |       |
       |<------+        Code: 2.04 (Changed)
       |  2.04 |       Token: 0x42
       |       |

     Figure 2: Example Message Flow for Time-Based Freshness using the
         'Integrity Protected Timestamp' construction of Appendix A

   Another way for the server to verify freshness is to maintain a cache
   of values associated to events.  The size of the cache is defined by
   the application.  In the following we assume the cache size is 1, in
   which case freshness is defined as no new event has taken place.  At
   each event a new value is written into the cache.  The cache values
   MUST be different except with negligible probability.  The server
   verifies freshness by checking that e0 equals e1, where e0 is the
   cached value when the Echo option value was generated, and e1 is the
   cached value at the reception of the request.  An example message
   flow over DTLS is shown in Figure 3.

     Client   Server
        |       |
        +------>|        Code: 0.03 (PUT)
        |  PUT  |       Token: 0x41
        |       |    Uri-Path: lock
        |       |     Payload: 0 (Unlock)
        |       |
        |<------+        Code: 4.01 (Unauthorized)
        |  4.01 |       Token: 0x41
        |       |        Echo: 0x05 (e0 = 5, number of total lock
        |       |                            operations performed)
        |       |
        | ...   | No alterations happen to the lock state, e1 has the
        |       | same value e1 = 5.
        |       |
        +------>|        Code: 0.03 (PUT)
        |  PUT  |       Token: 0x42
        |       |    Uri-Path: lock
        |       |        Echo: 0x05
        |       |     Payload: 0 (Unlock)
        |       |
        |       | Compare e1 = e0 => permitted.
        |       |
        |<------+        Code: 2.04 (Changed)
        |  2.04 |       Token: 0x42
        |       |        Echo: 0x06 (e2 = 6, to allow later locking
        |       |                            without more round-trips)
        |       |

       Figure 3: Example Message Flow for Event-Based Freshness using
            the 'Persistent Counter' construction of Appendix A

   When used to serve freshness requirements (including client aliveness
   and state synchronizing), the Echo option value MUST be integrity
   protected between the intended endpoints, e.g. using DTLS, TLS, or an
   OSCORE Inner option ([RFC8613]).  When used to demonstrate
   reachability at a claimed network address, the Echo option SHOULD be
   a MAC of the claimed address, but MAY be unprotected.  Combining
   different Echo applications can necessitate different choices, see
   Appendix A item 2 for an example.

   An Echo option MAY be sent with a successful response, i.e., even
   though the request satisfied any freshness requirements on the
   operation.  This is occasionally called a "preemptive" Echo value, and useful when
   the server anticipates that the client will need to demonstrate
   freshness relative to the current response the near future.

   A CoAP-to-CoAP proxy MAY set an Echo option on responses, both on
   forwarded ones that had no Echo option or ones generated by the proxy
   (from cache or as an error).  If it does so, it MUST remove the Echo
   option it recognizes as one generated by itself on follow-up
   requests.  When it receives an Echo option in a response, it MAY
   forward it to the client (and, not recognizing it as an own in future
   requests, relay it in the other direction as well) or process it on
   its own.  If it does so, it MUST ensure that the client's request was
   generated (or is re-generated) after the Echo value used to send to
   the server was first seen.  (In most cases, this means that the proxy
   needs to ask the client to repeat the request with a new Echo value.)

   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 MUST respond to Echo challenges itself if the proxy knows
   from the recent establishing of the connection that the HTTP request
   is fresh.  Otherwise, it MUST NOT repeat an unsafe request and SHOULD
   respond with 503 Service Unavailable, Retry-After: 0 and terminate
   any underlying Keep-Alive connection.  If the HTTP request arrived in
   Early Data, the proxy SHOULD use a 425 Too Early response instead
   (see [RFC8470]).  They MAY also use other mechanisms to establish
   freshness of the HTTP request that are not specified here.

2.4.  Applications of the Echo Option

   Unless otherwise noted, all these applications require a security
   protocol to be used, and the Echo option to be protected by it.

   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 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 send preemptive Echo values in
          successful requests, irrespectively of whether the request
          contained an Echo option or not.  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 properties (such
       as state or time) with a requesting client.  A server MUST NOT
       synchronize a property with a client which is not the authority
       of the property being synchronized.  E.g. if access to a server
       resource is dependent on time, then server MUST NOT synchronize
       time with a client requesting access unless the client is time
       authority for the server.

       Note that the state to be synchronized is not carried inside the
       Echo option.  Any explicit state information needs to be carried
       along in the messages the Echo value is sent in; the Echo
       mechanism only provides a partial order on the messages'
       processing.

       *  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 as
          specified in Section 7.5 of [RFC8613].

       *  A device joining a CoAP group communication [RFC7390]
          [I-D.ietf-core-groupcomm-bis] protected with OSCORE
          [I-D.ietf-core-oscore-groupcomm] may be required to initially
          synchronize its replay window state 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 value in a subsequent unicast request to the
          responding server.

   3.  An attacker can perform a denial-of-service attack by putting a
       victim's address in the source address of a CoAP request and
       sending the request to a resource with a large amplification
       factor.  The amplification factor is the ratio between the size
       of the request and the total size of the response(s) to that
       request.  A server that provides a large amplification factor to
       an unauthenticated peer SHOULD mitigate amplification attacks as
       described in Section 11.3 of [RFC7252].  One way to mitigate such
       attacks is that the server responds to the alleged source address
       of the request with an Echo option in short response message
       (e.g. 4.01 Unauthorized), thereby requesting the client to verify
       its source address.  This needs to be done only once per endpoint
       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.  (This is formally recommended in Section 2.6).

       *  In the presence of a proxy, a server will not be able to
          distinguish different origin client endpoints.  Following from
          the recommendation above, a proxy that provides a large
          amplification factor to unauthenticated peers SHOULD mitigate
          amplification attacks.  The proxy SHOULD use Echo to verify
          origin reachability as described in Section 2.3.  The proxy
          MAY forward safe requests immediately to have a cached result
          available when the client's repeated request arrives.

       *  Amplification mitigation is a trade-off between giving
          leverage to an attacker and causing overhead.  An
          amplification factor of 3 (i.e., don't send more than three
          times the number of bytes received until the peer's address is
          confirmed) is considered acceptable for unconstrained
          applications in [RFC9000] Section 8.

          When that limit is applied and no further context is
          available, a safe default is sending initial responses no
          larger than 136 Bytes in CoAP serialization.  (The number is
          assuming a 14 + 40 + 8 Bytes Ethernet, IP and UDP header with
          4 Bytes added for the CoAP header.  Triple that minus the non-
          CoAP headers gives the 136 Bytes).  Given the token also takes
          up space in the request, responding with 132 Bytes after the
          token is safe as well.

       *  When an Echo response is sent to mitigate amplification, it
          MUST be sent as a piggybacked or Non-confirmable response,
          never as a separate one (which would cause amplification due
          to retransmission).

   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.

2.5.  Characterization of Echo Applications

   Use cases for the Echo option can be characterized by several
   criteria that help determine the required properties of the Echo
   value.  These criteria apply both to those listed in Section 2.4 and
   any novel applications.  They provide rationale for the statements in
   the former, and guidance for the latter.

2.5.1.  Time versus Event Based Freshness

   The property a client demonstrates by sending an Echo value is that
   the request was sent after a certain point in time, or after some
   event happened on the server.

   When events are counted, they form something that can be used as a
   monotonic but very non-uniform time line.  With highly regular events
   and low-resolution time, the distinction between time and event based
   freshness can be blurred: "No longer than a month ago" is similar to
   "since the last full moon".

   In an extreme form of event based freshness, the server can place an
   event whenever an Echo value is used.  This makes the Echo value
   effectively single-use.

   Event and time based freshness can be combined in a single Echo
   value, e.g. by encrypting a timestamp with a key that changes with
   every event to obtain "usable once but only for 5 minutes"-style
   semantics.

2.5.2.  Authority over Used Information

   The information extracted by

   Information conveyed to the server from in the request Echo value has
   different sources of truth authority depending on the application.  Understanding who
   or what is the authoritative source of that information helps the
   server implementer decide the necessary protection of the Echo value.

   If all that is conveyed to the server extracts is information which the client
   is authorized to provide arbitrarily, (which is another way of saying
   that the server has to trust the client on whatever Echo is being
   used for), then the server can issue Echo values that do not need to
   be protected on their own.  They still need to be covered by the
   security protocol that covers the rest of the message, but the Echo
   value can be just short enough to be unique between this server and
   client.

   For example, the client's OSCORE sender sequence number (as used in
   [RFC8613] Appendix B.1.2) is such information.

   In most other cases, there are properties extracted of is information conveyed for which the
   server is the authority ("The request must not be older than five
   minutes" is counted on the server's clock, not the client's) or which
   even involve the network (as when performing amplification
   mitigation).  In these cases, the Echo value itself needs to be
   protected against forgery by the client, e.g. by using a sufficiently
   large random value or a MAC as described in Appendix A items 1 and 2.

   For some applications, the server may be able to trust the client to
   also act as the authority (e.g. when using time based freshness
   purely to mitigate request delay attacks); these need careful case-
   by-case evaluation.

   To issue Echo values without own protection, the server needs to
   trust the client to never produce requests with attacker controlled
   Echo values.  The provisions of Section 2.3 (saying that an Echo
   value may only be sent as received from the same server) allow that.
   The requirement stated there for the client to treat the Echo value
   as opaque holds for these application like for all others.

   When the client is the sole authority over the synchronized property,
   the server can still use time or events to issue new Echo values.
   Then, the request's Echo value not so much proves the indicated
   freshness to the server, but reflects the client's intention to
   indicate reception of responses containing that value when sending
   the later ones.

   Note that a single Echo value can be used for multiple purposes (e.g.
   to get both the sequence number information and perform amplification
   mitigation); then,
   mitigation).  In this case the stricter protection requirements
   apply.

2.5.3.  Protection by a Security Protocol

   For meaningful results, the Echo option needs to be used in
   combination with a security protocol in almost all applications.

   When the information extracted by the server is only about a part of
   the system outside of any security protocol, then the Echo option can
   also be used without a security protocol (in case of OSCORE, as an
   outer option).

   The only known application satisfying this requirement is network
   address reachability, where unprotected Echo values are used both by
   servers (e.g. during setup of a security context) and proxies (which
   do not necessarily have a security association with their clients)
   for amplification mitigation.

2.6.  Updated Amplification Mitigation Requirements for Servers

   This section updates the amplification mitigation requirements for
   servers in [RFC7252] to recommend use of the Echo option to mitigate
   amplification attacks.  The requirements for clients are not updated.
   Section 11.3 of [RFC7252] is updated by adding the following text:

   A CoAP server SHOULD mitigate potential amplification attacks by
   responding to unauthenticated clients with 4.01 Unauthorized
   including an Echo option, as described in Section 2.4 item 3 of
   [[this document]].

3.  Protecting Message Bodies using Request Tags

3.1.  Fragmented Message Body Integrity

   CoAP was designed to work over unreliable transports, such as UDP,
   and includes 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.

   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 block-wise 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-attacks]), 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.

3.2.  The Request-Tag Option

   This document defines the Request-Tag option for identifying request
   bodies, similar to ETag, but ephemeral and set by the CoAP client.
   The Request-Tag is intended for use as a short-lived identifier for
   keeping apart distinct block-wise request operations on one resource
   from one client, addressing the issue described in Section 3.1.  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 block-wise request operations on a
   single resource.  The requests must be integrity protected if they
   should protect against interchange of blocks between different
   message bodies.  The Request-Tag option is only used in requests that
   carry the Block1 option, and in Block2 requests following these.

   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.

3.2.1.  Request-Tag Option Format

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

    +--------+---+---+---+---+-------------+--------+------+---------+
    | No.    | C | U | N | R | Name        | Format | Len. | Default |
    +--------+---+---+---+---+-------------+--------+------+---------+
    | TBD292 |   |   |   | x | Request-Tag | opaque |  0-8 | (none)  |
    +--------+---+---+---+---+-------------+--------+------+---------+

          C = Critical, U = Unsafe, N = NoCacheKey, R = Repeatable

                    Figure 4: Request-Tag Option Summary

   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 [RFC8613]):
   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.

   The Request-Tag option is only used in the request messages of block-
   wise 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.

3.3.  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 block-wise operations (which includes all
   options except elective NoCacheKey and except Block1/2), and thus the
   server cannot 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.  For one,
   a server may still treat them as independent messages when it sends
   2.01/2.04 responses for every block.  Also, a client that lost
   interest in an old operation but wants to start over can overwrite
   the server's old state with a new initial (num=0) Block1 request and
   the same Request-Tag under some circumstances.  Likewise, that
   results in the new message not being part of the old operation.

   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.

3.4.  Setting the Request-Tag

   For each separate block-wise request operation, the client can choose
   a Request-Tag value, or choose not to set a Request-Tag. It needs to
   be set to the same value (or unset) in all messages belonging to the
   same operation, as otherwise they are treated as separate operations
   by the server.

   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 that use Request-Tag for a particular purpose (like in
   Section 3.5) MUST NOT recycle a request tag unless the first
   operation has concluded.  What constitutes a concluded operation
   depends on the purpose, and is defined accordingly; see examples in
   Section 3.5.

   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 cannot be recycled.

   Note that Request-Tag options can be present in request messages that
   carry no Block option (for example, because a Request-Tag unaware
   proxy reassembled them), and MUST be ignored in those.

   The Request-Tag option MUST NOT be present in response messages.

3.5.  Applications of the Request-Tag Option

3.5.1.  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 an attacker to maliciously replace a later
   operation's blocks with an earlier operation's blocks (see
   Section 2.5 of [I-D.mattsson-core-coap-attacks]).  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:

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

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

      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 matchable if they
      were sent to _any_ endpoint address using the new operation's
      security context.

   *  The client MUST NOT regard a block-wise request operation as
      concluded unless all of the messages the client has sent in the
      operation would be regarded as invalid by the server if they were
      replayed.

      When security services are provided by OSCORE, these confirmations
      typically 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.

      When security services are provided by DTLS, this can only be
      confirmed if there was no CoAP retransmission of the request, the
      request was responded to, and the server performs uses replay protection.

   Authors of other documents (e.g. applications of [RFC8613]) are
   invited to mandate this subsection's behavior for clients that
   execute block-wise 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 have confidence in 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 cannot be
   replayed any more, so the client never needs to set the Request-Tag
   option unless it wants to perform concurrent operations.

   Body integrity only makes sense in applications that have stateful
   block-wise transfers.  On applications where all the state is in the
   application (e.g. because rather than POSTing a large representation
   to a collection in a stateful block-wise transfer, a collection item
   is created first, then written to once and available when written
   completely), clients need not concern themselves with body integrity
   and thus the Request-Tag.

   Body integrity is largely independent from replay protection: When no
   replay protection is available (it is optional in DTLS), a full
   block-wise operation may be replayed, but by adhering to the above,
   no operations will be mixed up.  The only link between body integrity
   and replay protection is that without replay protection, recycling is
   not possible.

3.5.2.  Multiple Concurrent Block-wise Operations

   CoAP clients, especially CoAP proxies, may initiate a block-wise
   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 block-wise operation, a client has to look at
   other active operations:

   *  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 (including the absent option) that is not in
      use by the other matchable operations for the new operation.

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

3.5.3.  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.

   In some cases, for example when forwarding block-wise request
   operations, appending a Request-Tag value unique to the client can
   satisfy the requirements on the proxy that come from the presence of
   a block option.

   This is particularly useful to proxies that strive for stateless
   operation as described in [RFC8974] Section 4.

   The precise classification of cases in which such a Request-Tag
   option is sufficient is not trivial, especially when both request and
   response body are fragmented, and out of scope for this document.

3.6.  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 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
   several cascaded stateless proxies to each put in an origin address.
   They can perform the steps of Section 3.5.3 without the need to
   create an option value that is the concatenation of the received
   option and their own value, and can simply add a new Request-Tag
   option unconditionally.

   In draft versions of this document, 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].

3.7.  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 3.5.1) 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-attacks]
   Section 2.5.2.

3.8.  Block2 / ETag Processing

   The same security properties as in Section 3.5.1 can be obtained for
   block-wise response operations.  The threat model here does not
   depend on an attacker: a client can construct a wrong representation
   by assembling it from blocks from different resource states.  That
   can happen when a resource is modified during a transfer, or when
   some blocks are still valid 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 3.5.1, 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.

4.  Token Processing for Secure Request-Response Binding

4.1.  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
   ensured, a client may erroneously associate the wrong response to a
   request.  The wrong response may be an old response for the same
   resource or a response for a completely different resource (see e.g.
   Section 2.3 of [I-D.mattsson-core-coap-attacks]).  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 HTTP/1.1, this type of binding is always 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 and where there can be any number of
   responses to a request (see e.g.  [RFC7641]).  In CoAP, concurrent
   requests are differentiated by their Token.  Note that the CoAP
   Message ID cannot be used for this purpose since those are typically
   different for REST request and corresponding response in case of
   "separate response", see Section 2.2 of [RFC7252].

   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 a
   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 not reuse
   Tokens until the traffic keys have been replaced.  The following
   section formalizes that.

4.2.  Updated Token Processing Requirements for Clients

   As described in Section 4.1, the client must be able to verify that a
   response corresponds to a particular request.  This section updates
   the Token processing requirements for clients in [RFC7252] to always
   assure a cryptographically secure binding of responses to requests
   for secure REST operations like "coaps".  The Token processing for
   servers is not updated.  Token processing in Section 5.3.1 of
   [RFC7252] is updated by adding the following text:

   When CoAP is used with a security protocol not providing bindings
   between requests and responses, the Tokens have cryptographic
   importance.  The client MUST make sure that Tokens are not used in a
   way so that responses risk being associated with the wrong request.

   One easy way to accomplish this is to implement the Token (or part of
   the Token) as a sequence number starting at zero for each new or
   rekeyed secure connection.  This approach SHOULD be followed.

5.  Security Considerations

   The freshness assertion of the Echo option comes from the client
   reproducing the same value of the Echo option in a request as it
   received in a previous response.  If the Echo value is a large random
   number then there is a high probability that the request is generated
   after having seen the response.  If the Echo value of the response
   can be guessed, e.g. if based on a small random number or a counter
   (see Appendix A), then it is possible to compose a request with the
   right Echo value ahead of time.  Using guessable Echo values is only
   permissible in a narrow set of cases described in Section 2.5.2.
   Echo values MUST be set by the CoAP server such that the risk
   associated with unintended reuse can be managed.

   If uniqueness of the Echo value is based on randomness, then 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.  As each pseudorandom
   number must only be used once, an implementation needs to get a new
   truly random seed after reboot, or continuously store state in
   nonvolatile memory.  See ([RFC8613], Appendix B.1.1) for issues and
   approaches for writing to nonvolatile memory.

   A single active Echo value with 64 (pseudo-)random bits gives the
   same theoretical security level as a 64-bit MAC (as used in e.g.
   AES_128_CCM_8).  If a random unique Echo value is intended, the Echo
   option value SHOULD contain 64 (pseudo-)random bits that are not
   predictable for any other party than the server.  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.

   Echo values without the protection of randomness or a MAC are limited
   to cases when the client is the trusted source of all derived
   properties (as per Section 2.5.2).  Using them needs per-application
   consideration of both the impact of a malicious client and of
   implementation errors in clients.  These Echo values are the only
   legitimate case for Echo values shorter than four bytes, which are
   not necessarily secret.  They MUST NOT be used unless the request
   Echo values are integrity protected as per Section 2.3.

   Servers SHOULD 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.  Non-monotonic clocks MAY be used as
   long as they have deviations that are acceptable given the freshness
   requirements.  If the deviations from a monotonic clock are known, it
   may be possible to adjust the threshold accordingly.

   An attacker may be able to affect the server's system time in various
   ways such as setting up a fake NTP server or broadcasting false time
   signals to radio-controlled clocks.

   For the purpose of generating timestamps for Echo a server MAY set a
   timer at reboot and use the time since reboot, choosing the
   granularity such that different requests arrive at different times.
   Servers MAY intermittently reset the timer and MAY generate a random
   offset applied to all timestamps.  When resetting the timer, the
   server MUST reject all Echo values that were 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.

5.1.  Token reuse

   Reusing Tokens in a way so that responses are guaranteed to not be
   associated with the wrong request is not trivial: The server may
   process requests in any order, and send multiple responses to the
   same request.  An attacker may block, delay, and reorder messages.
   The use of a sequence number is therefore recommended when CoAP is
   used with a security protocol that does not provide bindings between
   requests and responses such as DTLS or TLS.

   For a generic response to a Confirmable request over DTLS, binding
   can only be claimed without out-of-band knowledge if

   *  the original request was never retransmitted,

   *  the response was piggybacked in an Acknowledgement message (as a
      Confirmable or Non-confirmable response may have been transmitted
      multiple times), and

   *  if observation was used, the same holds for the registration, all
      re-registrations, and the cancellation.

   (In addition, for observations, any responses using that Token and a
   DTLS sequence number earlier than the cancellation Acknowledgement
   message need to be discarded.  This is typically not supported in
   DTLS implementations.)

   In some setups, Tokens can be reused without the above constraints,
   as a different component in the setup provides the associations:

   *  In CoAP over TLS, retransmissions are not handled by the CoAP
      layer and behaves like a replay window size of 1.  When a client
      is sending TLS-protected requests without Observe to a single
      server, the client can reuse a Token as soon as the previous
      response with that Token has been received.

   *  Requests whose responses are cryptographically bound to the
      requests (like in OSCORE) can reuse Tokens indefinitely.

   In all other cases, a sequence number approach is RECOMMENDED as per
   Section 4.

   Tokens that cannot be reused need to be handled appropriately.  This
   could be solved by increasing the Token as soon as the currently used
   Token cannot be reused, or by keeping a list of all Tokens unsuitable
   for reuse.

   When the Token (or part of the Token) contains a sequence number, the
   encoding of the sequence number has to be chosen in a way to avoid
   any collisions.  This is especially true when the Token contains more
   information than just the sequence number, e.g. serialized state as
   in [RFC8974].

6.  Privacy Considerations

   Implementations SHOULD NOT put any privacy-sensitive information in
   the Echo or Request-Tag option values.  Unencrypted timestamps could
   reveal information about the server such as location or time since
   reboot, or 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.

   Like HTTP cookies, the Echo option could potentially be abused as a
   tracking mechanism that identifies a client across requests.  This is
   especially true for preemptive Echo values.  Servers MUST NOT use the
   Echo option to correlate requests for other purposes than freshness
   and reachability.  Clients only send Echo values to the same server
   from which the values were received.  Compared to HTTP, CoAP clients
   are often authenticated and non-mobile, and servers can therefore
   often correlate requests based on the security context, the client
   credentials, or the network address.  Especially when the Echo option
   increases a server's ability to correlate requests, clients MAY
   discard all preemptive Echo values.

   Publicly visible generated identifiers, even when opaque (as all
   defined in this document are), can leak information as described in
   [I-D.irtf-pearg-numeric-ids-generation].  To avoid effects described
   there, the absent Request-Tag option should be recycled as much as
   possible.  (That is generally possible as long as a security
   mechanism is in place - even in the case of OSCORE outer block-wise
   transfers, as the OSCORE option's variation ensures that no matchable
   requests are created by different clients).  When an unprotected Echo
   option is used to demonstrate reachability, the recommended mechanism
   of Section 2.3 keeps the effects to a minimum.

7.  IANA Considerations

   IANA is requested to add the following option numbers to the "CoAP
   Option Numbers" registry defined by [RFC7252]:

   [

   The editor is asked to suggest the numbers after TBD, as those
   satisfy the construction requirements set out in RFC7252: Echo is
   NoCacheKey but not Unsafe or Critical, so it needs to end with 11100
   in binary representation; Request-Tag has no properties so it needs
   to end with 00 and not with 11100).

   Request-Tag was picked to not waste the precious space of less-than-
   one-byte options, but such that its offset from the Block1 option it
   regularly occurs with can still be expressed in an 1-byte offset (27
   + (13 + 255) > 292).

   Echo was picked to be the shortest it can be in an empty message as a
   NoCacheKey option (11100 in binary does not fit in a nibble, and two
   lower ones are already taken), and as high as possible to keep room
   for other options that might typically occur in pairs and might still
   use optimization around low numbers.

   ]

               +--------+-------------+-------------------+
               | Number | Name        | Reference         |
               +--------+-------------+-------------------+
               | TBD252 | Echo        | [[this document]] |
               |        |             |                   |
               | TBD292 | Request-Tag | [[this document]] |
               +--------+-------------+-------------------+

                       Figure 5: CoAP Option Numbers

8.  References
8.1.  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/rfc/rfc2119>.
              <https://doi.org/10.17487/RFC2119>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/rfc/rfc6347>. <https://doi.org/10.17487/RFC6347>.

   [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/rfc/rfc7252>.
              <https://doi.org/10.17487/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/rfc/rfc7959>.
              <https://doi.org/10.17487/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/rfc/rfc8174>. <https://doi.org/10.17487/RFC8174>.

   [RFC8470]  Thomson, M., Nottingham, M., and W. Tarreau, "Using Early
              Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September
              2018, <https://www.rfc-editor.org/rfc/rfc8470>. <https://doi.org/10.17487/RFC8470>.

   [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/rfc/rfc8613>.
              <https://doi.org/10.17487/RFC8613>.

8.2.  Informative References

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

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

   [I-D.irtf-pearg-numeric-ids-generation]
              Gont, F. and I. Arce, "On the Generation of Transient
              Numeric Identifiers", Work in Progress, Internet-Draft,
              draft-irtf-pearg-numeric-ids-generation-07, 2 February
              2021, <https://datatracker.ietf.org/doc/html/draft-irtf-
              pearg-numeric-ids-generation-07>.

   [I-D.mattsson-core-coap-attacks]
              Mattsson, J. P., Fornehed, J., Selander, G., Palombini,
              F., and C. Amsüss, "Summarizing Known Attacks on CoAP", "CoAP Attacks", Work in Progress,
              Internet-Draft, draft-mattsson-core-
              coap-attacks-00, 16 May draft-mattsson-core-coap-attacks-01, 27
              July 2021,
              <https://datatracker.ietf.org/doc/html/draft-mattsson-
              core-coap-attacks-00>. <https://datatracker.ietf.org/doc/html/draft-
              mattsson-core-coap-attacks-01>.

   [REST]     Fielding, R., "Architectural Styles and the Design of
              Network-based Software Architectures", 2000,
              <https://www.ics.uci.edu/~fielding/pubs/dissertation/
              fielding_dissertation.pdf>.

   [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/rfc/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/rfc/rfc7641>.
              <https://doi.org/10.17487/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/rfc/rfc8323>.
              <https://doi.org/10.17487/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/rfc/rfc8446>.
              <https://doi.org/10.17487/RFC8446>.

   [RFC8974]  Hartke, K. and M. Richardson, "Extended Tokens and
              Stateless Clients in the Constrained Application Protocol
              (CoAP)", RFC 8974, DOI 10.17487/RFC8974, January 2021,
              <https://www.rfc-editor.org/rfc/rfc8974>.
              <https://doi.org/10.17487/RFC8974>.

   [RFC9000]  Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
              Multiplexed and Secure Transport", RFC 9000,
              DOI 10.17487/RFC9000, May 2021,
              <https://www.rfc-editor.org/rfc/rfc9000>.
              <https://doi.org/10.17487/RFC9000>.

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 for
   time-based freshness and one for event-based freshness 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 called r.  The server caches a
   list containing the random byte strings and their transmission times.
   Assuming 72-bit random values and 32-bit timestamps, the size of the
   Echo option value is 9 bytes and the amount of server state is 13n
   bytes, where n is the number of active Echo Option values.  The
   security against an attacker guessing echo values is given by s = bit
   length of r - log2(n).  The length of r and the maximum allowed n
   should be set so that the security level is harmonized with other
   parts of the deployment, e.g., s >= 64.  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

   This method is suitable both for time time- and for event base event-based freshness
   (e.g. by clearing the cache when an event occurs), and independent of
   the client authority.

   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.  The security
   against an attacker guessing echo values is given by the MAC length.
   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 6 may apply to the timestamp.
   Therefore, it might be important to encrypt it.  Depending on the
   choice of encryption algorithms, this may require a nonce an initialization
   vector to be included in the Echo option value. value, see below.

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

   This method is suitable both for time time- and for event based event-based freshness
   (by the server remembering the time at which the event took place),
   and independent of the client authority.

   If this method is used to additionally obtain network reachability of
   the client, the server MUST use the client's network address too,
   e.g. as in "MAC(k, MAC(k, t0, apparent network address)". address).

   3.  Persistent Counter.  This can be used in OSCORE for sequence
   number recovery per Appendix B.1.2 of [RFC8613].  The Echo option
   value is a simple counter without integrity protection of its own,
   serialized in uint format.  The counter is incremented in a
   persistent way every time the state that needs to be synchronized is
   changed (in the B.1.2 case: when a reboot indicates that volatile
   state may have been lost).  An example of how such a persistent
   counter can be implemented efficiently is the OSCORE server Sender
   Sequence Number mechanism described in Appendix B.1.1 of [RFC8613].

         Echo option value: counter
         Server State: counter

   This method is suitable only if the client is the authority over the
   synchronized property.  Consequently, it cannot be used to show
   client aliveness.  It provides statements from the client similar to
   event based freshness (but without a proof of freshness).

   Other mechanisms complying with the security and privacy
   considerations may be used.  The use of encrypted timestamps in the
   Echo option increases security, provides additional protection, but typically requires an IV
   (Initialization Vector)
   initialization vector (a.k.a. nonce) as input to be included in the Echo option value, encryption
   algorithm, which adds overhead and makes the specification of such a mechanism
   slightly more complicated than what is described here. slight complication to the procedure as well
   as overhead.

Appendix B.  Request-Tag Message Size Impact

   In absence of concurrent operations, the Request-Tag mechanism for
   body integrity (Section 3.5.1) 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 packets are lost and
   replay protection is active), or when block-wise request operations
   happen rarely (in OSCORE, if there is always only one request block-
   wise 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 cannot 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:

   *  In DTLS, a new session can be established.

   *  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. ]

   *  Changes since draft-ietf-core-echo-request-tag-13

      -  Minor editorial fixes.

      -  Wording enhancements:

         o  nonce -> initialization vector

         o  information extracted by the sever -> information conveyed
            to the server

      -  Acknowledgements updated.

      -  Changelog for -13 added (previous upload just pointed to other
         resources).

      -  Short title is "Echo, Request-Tag, and Token Processing" now
         instead of outdated acronym.

      -  Informative reference to RFC 7390 is now to draft-ietf-core-
         groupcomm-bis

   *  Changes since draft-ietf-core-echo-request-tag-12 (addressing
      comments from IESG review)

      See CoRE point-to-point responses at https://github.com/core-wg/
      echo-request-tag/blob/master/point-to-point.md
      (https://github.com/core-wg/echo-request-tag/blob/master/point-to-
      point.md) and on CoRE mailing list.

   *  Changes since draft-ietf-core-echo-request-tag-11 (addressing
      GenART, TSVART, OpsDir comments)

      -  Explain the size permissible for responses before amplification
         mitigation by referring  Add subsection "Characterization of Echo Applications".

         o  Changes in applications and appendices to use the QUIC draft for an OK factor, and
         giving newly
            introduced terms.

         o  In particular, some of the remaining numbers that led to it.  The actual number
         is reduced legitimization for using short
            Echo values was drawn from 152 to 136 because the more conservative case applications being event
            based; the concept of the attacker not sending client being the "Authority over
            [the] Used Information" now legitimizes these more
            precisely.

      -  Add subsection "Updated Amplification Mitigation Requirements
         for Servers".  It contains the normative text updating RFC 7252
         w/rt recommended mitigation methods, which previously happened
         in passing.

      -  Amplification mitigation:

         o  Increase precision: Reachability check is performed once per
            endpoint (was: peer).

         o  State that amplification factor applies to the sum of all
            (previously: "the size of the", implicitly, single) returned
            packets.

         o  Fix odd wording around how the Echo value would "contain"
            the claimed address: was meant to contain in a cryptographic
            sense, now clarified in that a MAC is recommended

      -  Define "preemptive Echo value" that was previously used without
         definition; another occurrence of the concept was simplified
         using the term.

      -  Add considerations for the use of DTLS without replay
         protection.

      -  Privacy considerations: Address concerns raised in various
         numeric-ids documents.

      -  Explicitly state expected security modes for Echo applications
         and examples.

      -  Fix of requirements for H-C proxies: They _MUST NOT_ relay
         unsafe requests.  (Previously, it only said that they SHOULD
         use a particular method, but not clearly that some method is
         mandated.)

      -  Clarify that state synchonization is an application of the
         freshness results in combination with some transported
         application data, and not an immediate result of using Echo
         alone.

      -  Add text to tie together applications and suggested mechanisms

      -  Restrict C-C proxy allowed behavior: Only safe requests
         (incorrectly said "idempotent") may be used to proactively
         populate the proxy's cache.

      -  Justify some "SHOULD"s by outlining justification for different
         behavior.

         o  Normatively require H-C proxies to process Echo if they're
            justified to do so, as no alternatives are available.

      -  Reference updates:

         o  QUIC is now RFC9000; precise section given as amplification
            reference.

         o  Add note for RFC editor that RFC6347 can be upgraded to DTLS
            1.3 if C321 overtakes C280

         o  Follow the core-coap-actuators to core-coap-attacks update

         o  RFC8470 reference is now normative (as using what's defined
            there has been RECOMMENDED already)

      -  Editorial fixes

         o  Rewording of confusing sentences in amplification mitigation
            and inner-/outer Echo values

         o  Replace "blacklist" terminology with "deny-list" where left
            after other changes

         o  Removed sloppy use of Echo as a verb

         o  Minor clarifications

         o  Remove duplicate statements

         o  Typography and spelling fixes

      -  Fixes that are not editorial but minor

         o  Freshness is about time, of which round-trip time
            (specialization now removed) is only a part.

         o  Reference how HTTP _1.1_ does it when explaining token
            requirements, as that's an easily and widely understood
            baseline.

   *  Changes since draft-ietf-core-echo-request-tag-11 (addressing
      GenART, TSVART, OpsDir comments)

      -  Explain the size permissible for responses before amplification
         mitigation by referring to the QUIC draft for an OK factor, and
         giving the remaining numbers that led to it.  The actual number
         is reduced from 152 to 136 because the more conservative case
         of the attacker not sending a token is considered now.

      -  Added a definition for "freshness"

      -  Give more concrete example values in figures 2 and 3 (based on
         the appendix suggestions), highlighting the differences between
         the figures by telling how they are processed in the examples.

      -  Figure with option summary: E/U columns removed (for duplicate
         headers and generally not contributing)

      -  MAY capitalization changed for consistency.

      -  Editorial changes (IV acronym expanded, s/can not/cannot/g)

      -  Draft ietf-core-stateless has become RFC8974

   *  Changes since draft-ietf-core-echo-request-tag-10 (Barry's
      comments)

      -  Align terminology on attacker

      -  A number of clarifications and editorial fixes

      -  Promote DTLS and OSCORE to normative references

      -  Add counter-based version to the Methods for Generating Echo
         Option Values appendix

      -  Use 64-bit randomness recommendation throughout (but keep it as
         SHOULD so applications with strict requirements can reduce if
         if really needed)

      -  Speling and Capitalization

   *  Changes since draft-ietf-core-echo-request-tag-09:

      -  Allow intermediaries to do Echo processing, provided they ask
         at least as much freshness as they forward

      -  Emphasize that clients can forget Echo to further discourage
         abuse as cookies

      -  Emphasize that RESTful application design can avoid the need
         for a Request-Tag

      -  Align with core-oscore-groupcomm-09

      -  Add interaction with HTTP Early Data / 425 Too Early

      -  Abstract: Explicitly mention both updates to 7252

      -  Change requested option number of Echo to 252 (previous
         property calculation was erroneous)

   *  Changes since draft-ietf-core-echo-request-tag-08:

      -  Make amplification attack mitigation by Echo an RFC7252
         updating recommendation

      -  Give some more concrete guidance to that use case in terms of
         sizes and message types

      -  Allow short (1-3 byte) Echo values for deterministic cases,
         with according security considerations

      -  Point out the tricky parts around Request-Tag for stateless
         proxies, and make that purely an outlook example with out-of-
         scope details

      -  Lift ban on Request-Tag options without Block1 (as they can
         legitimately be generated by an unaware proxy)

      -  Suggest concrete numbers for the options

   *  Changes since draft-ietf-core-echo-request-tag-07 (largely
      addressing Francesca's review):

      -  Request tag: Explicitly limit "MUST NOT recycle" requirement to
         particular applications

      -  Token reuse: upper-case RECOMMEND sequence number approach

      -  Structure: Move per-topic introductions to respective chapters
         (this avoids long jumps by the reader)

      -  Structure: Group Block2 / ETag section inside new fragmentation
         (formerly Request-Tag) section

      -  More precise references into other documents

      -  "concurrent operations": Emphasise that all here only matters
         between endpoint pairs

      -  Freshness: Generalize wording away from time-based freshness

      -  Echo: Emphasise that no binding between any particular pair of
         responses and requests is established

      -  Echo: Add event-based example

      -  Echo: Clarify when protection is needed

      -  Request tag: Enhance wording around "not sufficient condition"

      -  Request tag: Explicitly state when a tag needs to be set

      -  Request tag: Clarification about permissibility of leaving the
         option absent

      -  Security considerations: wall clock time -> system time (and
         remove inaccurate explanations)

      -  Token reuse: describe deny-listing in a more implementation-
         independent way

   *  Changes since draft-ietf-core-echo-request-tag-06:

      -  Removed visible comment that should not be visible in Token
         reuse considerations.

   *  Changes since draft-ietf-core-echo-request-tag-05:

      -  Add privacy considerations on cookie-style use of Echo values

      -  Add security considerations for token reuse

      -  Add note in security considerations on use of nonvolatile
         memory when dealing with pseudorandom numbers

      -  Appendix on echo generation: add a few words on up- and
         downsides of the encrypted timestamp alternative

      -  Clarifications around Outer Echo:

         o  Could be generated by the origin server to prove network
            reachability (but for most applications it MUST be inner)

         o  Could be generated by intermediaries

         o  Is answered by the client to the endpoint from which it
            received it (ie.  Outer if received as Outer)

      -  Clarification that a server can send Echo preemtively

      -  Refer to stateless to explain what "more information than just
         the sequence number" could be

      -  Remove explanations around 0.00 empty messags

      -  Rewordings:

         o  the attack: from "forging" to "guessing"

         o  "freshness tokens" to "freshness indicators" (to avoid
            confusion with the Token)

      -  Editorial fixes:

         o  Abstract and introduction mention what is updated in RFC7252
         o  Reference updates

         o  Capitalization, spelling, terms from other documents

   *  Changes since draft-ietf-core-echo-request-tag-04:

      -  Editorial fixes

         o  Moved paragraph on collision-free encoding of data in the
            Token to Security Considerations and rephrased it

         o  "easiest" -> "one easy"

   *  Changes since draft-ietf-core-echo-request-tag-03:

      -  Mention Token processing changes in title

      -  Abstract reworded

      -  Clarify updates to Token processing

      -  Describe security levels from Echo length

      -  Allow non-monotonic clocks under certain conditions for
         freshness

      -  Simplify freshness expressions

      -  Describe when a Request-Tag can be set

      -  Add note on application-level freshness mechanisms

      -  Minor editorial changes

   *  Changes since draft-ietf-core-echo-request-tag-02:

      -  Define "freshness"

      -  Note limitations of "aliveness"

      -  Clarify proxy and OSCORE handling in presence of "echo"

      -  Clarify when Echo values may be reused

      -  Update security considerations

      -  Various minor clarifications
      -  Minor editorial changes

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

      -  Follow-up changes after the "relying on block-wise" change in
         -01:

         o  Simplify the description of Request-Tag and matchability

         o  Do not update RFC7959 any more

      -  Make Request-Tag repeatable.

      -  Add rationale on not relying purely on sequence numbers.

   *  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
         block-wise to treat it as part of the cache-key

      -  Dropped use case about OSCORE Outer-block-wise (the case went
         away when its Partial IV was moved into the Object-Security
         option)

   *  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 Carsten Bormann, Roman Danyliw, Benjamin
   Kaduk, Murray Kucherawy, Francesca Palombini, Palombini and Jim Schaad for
   providing valuable input to the draft.

Authors' Addresses

   Christian Amsüss

   Email: christian@amsuess.com

   John Preuß Mattsson
   Ericsson AB

   Email: john.mattsson@ericsson.com

   Göran Selander
   Ericsson AB

   Email: goran.selander@ericsson.com