Echo and Request-Tag
draft-ietf-core-echo-request-tag-00

The information below is for an old version of the document
Document Type Active Internet-Draft (core WG)
Last updated 2017-10-31 (latest revision 2017-10-30)
Replaces draft-amsuess-core-repeat-request-tag
Stream IETF
Intended RFC status (None)
Formats plain text pdf html bibtex
Stream WG state WG Document
Document shepherd No shepherd assigned
IESG IESG state I-D Exists
Consensus Boilerplate Unknown
Telechat date
Responsible AD (None)
Send notices to (None)
CoRE Working Group                                            C. Amsuess
Internet-Draft                               Energy Harvesting Solutions
Updates: 7959 (if approved)                                  J. Mattsson
Intended status: Standards Track                             G. Selander
Expires: May 3, 2018                                         Ericsson AB
                                                        October 30, 2017

                          Echo and Request-Tag
                  draft-ietf-core-echo-request-tag-00

Abstract

   This document defines two optional extensions to the Constrained
   Application Protocol (CoAP): the Echo option and the Request-Tag
   option.  Each of these options when integrity protected, such as with
   DTLS or OSCORE, protects against certain attacks on CoAP message
   exchanges.

   The Echo option enables a CoAP server to verify the freshness of a
   request by requiring the CoAP client to make another request and
   include a server-provided challenge.  The Request-Tag option allows
   the CoAP server to match message fragments belonging to the same
   request message, fragmented using the CoAP Block-Wise Transfer
   mechanism.  This document also specifies additional processing
   requirements on Block1 and Block2 options.

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

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

   This Internet-Draft will expire on May 3, 2018.

Amsuess, et al.            Expires May 3, 2018                  [Page 1]
Internet-Draft            Echo and Request-Tag              October 2017

Copyright Notice

   Copyright (c) 2017 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
   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 . . . . . . . . . . . .   3
     1.3.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  The Echo Option . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Option Format . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Echo Processing . . . . . . . . . . . . . . . . . . . . .   5
     2.3.  Applications  . . . . . . . . . . . . . . . . . . . . . .   7
   3.  The Request-Tag Option  . . . . . . . . . . . . . . . . . . .   8
     3.1.  Option Format . . . . . . . . . . . . . . . . . . . . . .   9
     3.2.  Request-Tag Processing  . . . . . . . . . . . . . . . . .  10
     3.3.  Applications  . . . . . . . . . . . . . . . . . . . . . .  11
       3.3.1.  Body Integrity Based on Payload Integrity . . . . . .  11
       3.3.2.  Multiple Concurrent Blockwise Operations  . . . . . .  12
   4.  Block2 / ETag Processing  . . . . . . . . . . . . . . . . . .  13
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  13
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Appendix A.  Performance Impact When Using the Echo Option  . . .  14
   Appendix B.  Request-Tag Message Size Impact  . . . . . . . . . .  15
   Appendix C.  Change Log . . . . . . . . . . . . . . . . . . . . .  16
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  Introduction

   The initial CoAP suite of specifications ([RFC7252], [RFC7641],
   [RFC7959]) was designed with the assumption that security could be
   provided on a separate layer, in particular by using DTLS

Amsuess, et al.            Expires May 3, 2018                  [Page 2]
Internet-Draft            Echo and Request-Tag              October 2017

   ([RFC6347]).  However, for some use cases, additional functionality
   or extra processing is needed to support secure CoAP operations.

   This document specifies two server-oriented CoAP options, the Echo
   option and the Request-Tag option, addressing the security features
   request freshness and fragmented message body integrity,
   respectively.  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 in a message, e.g. using DTLS
   ([RFC6347]) or OSCORE ([I-D.ietf-core-object-security]), provide
   those security features.  The Request-Tag option and also the ETag
   option are mandatory to use with Block1 and Block2, respectively, to
   secure blockwise operations with multiple representations of a
   particular resource as is specified in this document.

   Additional applications of the options are introduced.  For example,
   Echo can be used to mitigate amplification attacks.

1.1.  Request Freshness

   A CoAP server receiving a request may not be 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 the DTLS
   handshake) for each actuation, but in general this is not a solution
   suitable for constrained environments.

   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 the 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.

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

Amsuess, et al.            Expires May 3, 2018                  [Page 3]
Internet-Draft            Echo and Request-Tag              October 2017

   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 endpoint 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.amsuess-core-request-tag]), 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.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].

   Unless otherwise specified, the terms "client" and "server" refers to
   "CoAP client" and "CoAP server", respectively, as defined in
   [RFC7252].

   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 blockwise operations between the same endpoint pair on the same
   resource 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

Amsuess, et al.            Expires May 3, 2018                  [Page 4]
Internet-Draft            Echo and Request-Tag              October 2017

   second operation's block overwrites any state of the first
   exchange.).

   The Echo and Request-Tag options are defined in this document.  The
   concept of two messages being "Request-Tag-matchable" is defined in
   Section 3.1.

2.  The Echo Option

   The Echo option is a server-driven challenge-response mechanism for
   CoAP.  The Echo option value is a challenge from the server to the
   client included in a CoAP response and echoed in a CoAP request.

2.1.  Option Format

   The Echo Option is elective, safe-to-forward, not part of the cache-
   key, and not repeatable, see Figure 1.

   +-----+---+---+---+---+-------------+--------+--------+---------+---+
   | No. | C | U | N | R | Name        | Format | Length | Default | E |
   +-----+---+---+---+---+-------------+--------+--------+---------+---+
   | TBD |   |   |   |   | Echo        | opaque |   8-40 | (none)  | x |
   +-----+---+---+---+---+-------------+--------+--------+---------+---+

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

                       Figure 1: Echo Option Summary

   The value of the Echo option MUST be a (pseudo-)random bit string of
   a length of at least 64 bits.  A new (pseudo-)random bit string MUST
   be generated by the server for each use of the Echo option.

2.2.  Echo Processing

   It is important to identify 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.

   A server MAY include the Echo option in a response.  The Echo option
   MUST NOT be used with empty CoAP requests (i.e.  Code=0.00).  If the
   server receives a request which has freshness requirements, and the
   request does not contain the Echo option, the server SHOULD send a
   4.01 Unauthorized response with a Echo option.  The server SHOULD

Amsuess, et al.            Expires May 3, 2018                  [Page 5]
Internet-Draft            Echo and Request-Tag              October 2017

   cache the transmitted Echo option value and the response transmit
   time (here denoted t0).

   Upon receiving a response with the Echo option within the
   EXCHANGE_LIFETIME ([RFC7252]) of the original request, the client
   SHOULD echo the Echo option with the same value in a new request to
   the server.  Upon receiving a 4.01 Unauthorized response with the
   Echo option in response to a request within the EXCHANGE_LIFETIME of
   the original request, the client SHOULD resend the original request.
   The client MAY send a different request compared to the original
   request.

   If the server receives a request which has freshness requirements,
   and the request contains the Echo option, the server MUST verify that
   the option value equals a cached value; otherwise the request is not
   processed further.  The server MUST calculate the round-trip time RTT
   = (t1 - t0), where t1 is the request receive time.  The server MUST
   only accept requests with a round-trip time below a certain threshold
   T, i.e. RTT < T, otherwise the request is not processed further, and
   an error message MAY be sent.  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.

   When used to serve freshness requirements, CoAP messages containing
   the Echo option MUST be integrity protected, e.g. using DTLS or
   OSCORE ([I-D.ietf-core-object-security]).

   If the server loses time synchronization, e.g. due to reboot, it MUST
   delete all cached Echo option values and response transmission times.

Amsuess, et al.            Expires May 3, 2018                  [Page 6]
Internet-Draft            Echo and Request-Tag              October 2017

                   Client  Server
                      |      |
                      +----->|        Code: 0.03 (PUT)
                      | PUT  |       Token: 0x41
                      |      |    Uri-Path: lock
                      |      |     Payload: 0 (Unlock)
                      |      |
                      |<-----+ t0     Code: 4.01 (Unauthorized)
                      | 4.03 |       Token: 0x41
                      |      |        Echo: 0x6c880d41167ba807
                      |      |
                      +----->| t1     Code: 0.03 (PUT)
                      | PUT  |       Token: 0x42
                      |      |    Uri-Path: lock
                      |      |        Echo: 0x6c880d41167ba807
                      |      |     Payload: 0 (Unlock)
                      |      |
                      |<-----+        Code: 2.04 (Changed)
                      | 2.04 |       Token: 0x42
                      |      |

                    Figure 2: Echo option message flow

   Constrained server implementations can use the mechanisms outlined in
   Appendix A to minimize the memory impact of having many unanswered
   Echo responses.

   CoAP-CoAP proxies MUST relay the Echo option unmodified, and SHOULD
   NOT cache responses when a Echo option is present in request or
   response for more than the exchange.  CoAP-HTTP proxies MAY request
   freshness, especially if they have reason to assume that access may
   require it (eg. because it is a PUT or POST); how this is determined
   is out of scope for this document.  HTTP-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.  It 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.

   2.  To avoid additional roundtrips for applications with multiple
       actuator requests in rapid sequence between the same client and

Amsuess, et al.            Expires May 3, 2018                  [Page 7]
Internet-Draft            Echo and Request-Tag              October 2017

       server, the server may use 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.

   3.  If a server reboots during operation it may need to synchronize
       state with requesting clients before continuing the interaction.
       For example, with OSCORE it is possible to reuse a persistently
       stored security context by synchronizing the Partial IV (sequence
       number) using the Echo option.

   4.  When a device joins a multicast/broadcast group the device may
       need to synchronize state or time with the sender to ensure that
       the received message is fresh.  By synchronizing time with the
       broadcaster, time can be used for synchronizing subsequent
       broadcast messages.  A server MUST NOT synchronize state or 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.

   5.  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, the 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.

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.  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.

Amsuess, et al.            Expires May 3, 2018                  [Page 8]
Internet-Draft            Echo and Request-Tag              October 2017

3.1.  Option Format

   The Request-Tag option has the same properties as the Block1 option:
   it is critical, unsafe, not part of the cache-key, and not
   repeatable, see Figure 3.

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

               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 not released together with
   OSCORE but before it, the following paragraph and the "E" column
   above need to move into OSCORE.]

   Request-Tag, like the Block1 option, is a special class E option in
   terms of OSCORE processing (see Section 4.3.1.2 of
   [I-D.ietf-core-object-security]): The Request-Tag MAY be an inner or
   outer option.  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 request messages, and only in
   conjunction with the Block1 option.

   Two messages are defined to be Request-Tag-matchable if and only if
   they are sent from and to the same end points (including security
   associations), and target the same URI, and if either neither carries
   a Request-Tag option, or both carry exactly one Request-Tag option
   and the option values are of same length and content.

   The Request-Tag mechanism is applied independently on the server and
   client sides of CoAP-CoAP proxies.  CoAP-HTTP proxies and HTTP-CoAP
   proxies can use Request-Tag on their CoAP sides; it is not applicable
   to HTTP requests.

   For each separate blockwise request operation, the client can choose
   a Request-Tag value, or choose not to set a Request-Tag.  Creating a
   new request operation whose messages are Request-Tag-matchable to a

Amsuess, et al.            Expires May 3, 2018                  [Page 9]
Internet-Draft            Echo and Request-Tag              October 2017

   previous operation is called request tag recycling.  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 3.3.

   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.

3.2.  Request-Tag Processing

   A server MUST NOT act on any two blocks in the same blockwise request
   operation that are not Request-Tag-matchable.  This rule applies
   independent of whether the request actually carries a Request-Tag
   option (in this case, the request can only be acted on together with
   other messages not carrying the option, as per matchability
   definition).

   As not all messages from the same source can be combined any more, a
   block not matchable to the first Block1 cannot overwrite context kept
   for an operation under a different tag (cf.  [RFC7959] Section 2.5).
   The server is still under no obligation to keep state of more than
   one transaction.  When an operation is in progress and a second one
   cannot be served at the same time, the server MUST respond to the
   second request with a 5.03 (Service Unavailable) response code and
   SHOULD indicate the time it is willing to wait for additional blocks
   in the first operation using the Max-Age option, as specified in
   Section 5.9.3.4 of [RFC7252].

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

   Two messages being Request-Tag-matchable 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.

   If a request that uses Request-Tag is rejected with 4.02 Bad Option,
   the client MAY retry the operation without it, but then it MUST
   serialize all operations that affect the same resource.  Security
   requirements can forbid dropping the use of Request-Tag mechanism.

Amsuess, et al.            Expires May 3, 2018                 [Page 10]
Internet-Draft            Echo and Request-Tag              October 2017

3.3.  Applications

3.3.1.  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
   later operation's blocks with earlier operation's blocks (see
   Section 3.2 of [I-D.amsuess-core-request-tag]).  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 message payloads MUST be integrity protected end-to-end
      between client and server.

   o  The client MUST NOT recycle a request tag unless the previous
      blockwise request operation that used matchable Request-Tags has
      concluded.

   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.

   o  The client MUST NOT fall back to not using the Request-Tag
      mechanisms when receiving a 4.02 Bad Option response.

   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.

Amsuess, et al.            Expires May 3, 2018                 [Page 11]
Internet-Draft            Echo and Request-Tag              October 2017

   Note that this mechanism is implicitly implemented when the security
   layer guarantees ordered delivery (e.g.  CoAP over TLS
   [I-D.tschofenig-core-coap-tcp-tls]).  This is because with each
   message, any earlier operation can be regarded as concluded by the
   client, so it never needs to set the Request-Tag option unless it
   wants to perform concurrent operations.

3.3.2.  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, and which the new request should not cancel.  One
   example is when a CoAP proxy fragments an OSCORE messages using
   blockwise (so-called "outer" blockwise, see Section 4.3.1. of
   [I-D.ietf-core-object-security])), where the Uri-Path is hidden
   inside the encrypted message, and all the proxy sees is the server's
   "/" path.

   When a client fragments a message as part of a blockwise request
   operation, it can do so without a Request-Tag option set.  For this
   application, an operation can be regarded as concluded when a final
   Block1 option has been sent and acknowledged, or when the client
   chose not to continue with the operation (e.g. by user choice, or in
   the case of a proxy when it decides not to take any further messages
   in the operation due to a timeout).  When another concurrent
   blockwise request operation is made (i.e. before the operation is
   concluded), the client can not recycle the request tag, and has to
   pick a new one.  The possible outcomes are:

   o  The server responds with a successful code.

      The concurrent blockwise operations can then continue.

   o  The server responds 4.02 Bad Option.

      This can indicate that the server does not support Request-Tag.
      The client should wait for the first operation to conclude, and
      then try the same request without a Request-Tag option.

   o  The server responds 5.03 Service Unavailable with a Max-Age option
      to indicate when it is likely to be available again.

      This can indicate that the server supports Request-Tag, but still
      is not prepared to handle concurrent requests.  The client should
      wait for as long as the response is valid, and then retry the
      operation, which may not need to carry a Request-Tag option by
      then any more.

Amsuess, et al.            Expires May 3, 2018                 [Page 12]
Internet-Draft            Echo and Request-Tag              October 2017

   In the cases where a CoAP proxy receives an error code, it can
   indicate the anticipated delay by sending a 5.03 Service Unavailable
   response itself.  Note that this behavior is no different from what a
   CoAP proxy would need to do were it unaware of the Request-Tag
   option.

4.  Block2 / ETag Processing

   The same security properties as in Section 3.3.1 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.

   Analogous rules to Section 3.2 are already in place for assembling a
   response body in Section 2.4 of [RFC7959].

   To gain equivalent protection to Section 3.3.1, 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.  IANA Considerations

   [TBD: Fill out the option templates for Echo and Request-Tag]

6.  Security Considerations

   Servers that store a Echo challenge per client can be attacked for
   resource exhaustion, and should consider minimizing the state kept
   per client, e.g. using a mechanism as described in Appendix A.

7.  References

7.1.  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>.

Amsuess, et al.            Expires May 3, 2018                 [Page 13]
Internet-Draft            Echo and Request-Tag              October 2017

   [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>.

7.2.  Informative References

   [I-D.amsuess-core-request-tag]
              Amsuess, C., "Request-Tag option", draft-amsuess-core-
              request-tag-00 (work in progress), March 2017.

   [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-06 (work in
              progress), October 2017.

   [I-D.mattsson-core-coap-actuators]
              Mattsson, J., Fornehed, J., Selander, G., and F.
              Palombini, "Controlling Actuators with CoAP", draft-
              mattsson-core-coap-actuators-02 (work in progress),
              November 2016.

   [I-D.tschofenig-core-coap-tcp-tls]
              Bormann, C., Lemay, S., Technologies, Z., and H.
              Tschofenig, "A TCP and TLS Transport for the Constrained
              Application Protocol (CoAP)", draft-tschofenig-core-coap-
              tcp-tls-05 (work in progress), November 2015.

   [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>.

   [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>.

Appendix A.  Performance Impact When Using the Echo Option

   The Echo option requires the server to keep some state in order to
   later verify the echoed request.

   Instead of caching Echo option values and response transmission
   times, the server MAY use the encryption of the response transmit
   time t0 as Echo option value.  Such a scheme needs to ensure that the
   server can detect a replay of a previous encrypted response transmit
   time.

Amsuess, et al.            Expires May 3, 2018                 [Page 14]
Internet-Draft            Echo and Request-Tag              October 2017

   For example, the server MAY encrypt t0 with AES-CCM-128-64-64 using a
   (pseudo-)random secret key k generated and cached by the server.  A
   unique IV MUST be used with each encryption, e.g. using a sequence
   number.  If the server loses time synchronization, e.g. due to
   reboot, then k MUST be deleted and replaced by a new random secret
   key.  When using encrypted response transmit times, the Echo
   processing is modified in the following way: The verification of
   cached option value in the server processing is replaced by the
   verification of the integrity of the encrypted option value using the
   cached key and IV (e.g. sequence number).

   The two methods - (a) the list of cached values, and (b) the
   encryption of transmit time - have different impact on the
   implementation:

   o  size of cached data (list of cached values vs. key and IV)

   o  size of message (typically larger with encrypted time)

   o  computation (encryption + decryption vs. generation new nonce +
      cache + lookup)

   In general, the encryption of transmission times is most useful if
   the number of concurrent requests is high.

   A hybrid scheme is also possible: the first Echo option values are
   cached, and if the number of concurrent requests reach a certain
   threshold, then encrypted times are used until there is space for
   storing new values in the list.  In that case, the server may need to
   make both verifications - either that the Echo value is in the list,
   or that it verifies in decryption - and in either case that the
   transmission time is valid.

Appendix B.  Request-Tag Message Size Impact

   In absence of concurrent operations, the Request-Tag mechanism for
   body integrity (Section 3.3.1) 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 only
   one request operation with losses within 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

Amsuess, et al.            Expires May 3, 2018                 [Page 15]
Internet-Draft            Echo and Request-Tag              October 2017

   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-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.

Authors' Addresses

   Christian Amsuess
   Energy Harvesting Solutions

   Email: c.amsuess@energyharvesting.at

   John Mattsson
   Ericsson AB

   Email: john.mattsson@ericsson.com

Amsuess, et al.            Expires May 3, 2018                 [Page 16]
Internet-Draft            Echo and Request-Tag              October 2017

   Goeran Selander
   Ericsson AB

   Email: goran.selander@ericsson.com

Amsuess, et al.            Expires May 3, 2018                 [Page 17]