Echo and Request-Tag
draft-ietf-core-echo-request-tag-01

CoRE Working Group                                            C. Amsuess
Internet-Draft
Updates: 7252, 7959 (if approved)                            J. Mattsson
Intended status: Standards Track                             G. Selander
Expires: September 6, 2018                                   Ericsson AB
                                                          March 05, 2018

                          Echo and Request-Tag
                  draft-ietf-core-echo-request-tag-01

Abstract

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

Status of This Memo

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

   Internet-Drafts are working documents of the Internet Engineering
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 6, 2018.

Copyright Notice

   Copyright (c) 2018 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Request Freshness . . . . . . . . . . . . . . . . . . . .   3
     1.2.  Fragmented Message Body Integrity . . . . . . . . . . . .   3
     1.3.  Request-Response Binding  . . . . . . . . . . . . . . . .   4
     1.4.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   5
   2.  The Echo Option . . . . . . . . . . . . . . . . . . . . . . .   5
     2.1.  Option Format . . . . . . . . . . . . . . . . . . . . . .   5
     2.2.  Echo Processing . . . . . . . . . . . . . . . . . . . . .   6
     2.3.  Applications  . . . . . . . . . . . . . . . . . . . . . .   8
   3.  The Request-Tag Option  . . . . . . . . . . . . . . . . . . .   9
     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
     3.4.  Rationale for the option properties . . . . . . . . . . .  13
   4.  Block2 / ETag Processing  . . . . . . . . . . . . . . . . . .  14
   5.  Token Processing  . . . . . . . . . . . . . . . . . . . . . .  14
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  14
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  14
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  15
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  15
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  16
   Appendix A.  Methods for Generating Echo Option Values  . . . . .  17
   Appendix B.  Request-Tag Message Size Impact  . . . . . . . . . .  18
   Appendix C.  Change Log . . . . . . . . . . . . . . . . . . . . .  18
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  19

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 several
   security enhancements to the Constrained Application Protocol (CoAP).

   This document specifies two server-oriented CoAP options, the Echo
   option and the Request-Tag option, mainly addressing the security
   features request freshness and fragmented message body integrity,

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   respectively.  The Echo option enables a CoAP server to verify the
   freshness of a request, verify the aliveness of a client, synchronize
   state, or force a client to demonstrate reachability at its apparent
   network address.  The Request-Tag option allows the CoAP server to
   match message fragments belonging to the same request, fragmented
   using the CoAP Block-Wise Transfer mechanism, which mitigates attacks
   and enables concurrent blockwise operations.  These options in
   themselves do not replace the need for a security protocol; they
   specify the format and processing of data which, when integrity
   protected using e.g.  DTLS ([RFC6347]), TLS ([RFC5246]), or OSCORE
   ([I-D.ietf-core-object-security]), provide the additional security
   features.

   The document also updates the processing requirements on the Block1
   option, the Block2 option, and the Token.  The updated blockwise
   processing secure blockwise operations with multiple representations
   of a particular resource.  The updated Token processing ensures
   secure binding of responses to requests.

1.1.  Request Freshness

   A CoAP server receiving a request is in general not able to verify
   when the request was sent by the CoAP client.  This remains true even
   if the request was protected with a security protocol, such as DTLS.
   This makes CoAP requests vulnerable to certain delay attacks which
   are particularly incriminating in the case of actuators
   ([I-D.mattsson-core-coap-actuators]).  Some attacks are possible to
   mitigate by establishing fresh session keys (e.g. performing 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 and to verify aliveness of the
   client.

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

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   must allow out-of-order delivery of messages using e.g. a sliding
   replay window such as described in Section 4.1.2.6 of DTLS
   ([RFC6347]).

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

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

1.3.  Request-Response Binding

   A fundamental requirement of secure REST operations is that the
   client can bind a response to a particular request.  In HTTPS this is
   assured by the ordered and reliable delivery as well as mandating
   that the server sends responses in the same order that the requests
   were received.

   The same is not true for CoAP where the server can return responses
   in any order.  Concurrent requests are instead differentiated by
   their Token.  Unfortunately, CoAP [RFC7252] does not treat Token as a
   cryptographically important value and does not give stricter
   guidelines than that the tokens currently "in use" SHOULD (not SHALL)
   be unique.  If used with security protocol not providing bindings
   between requests and responses (e.g.  DTLS and TLS) token reuse may
   result in situations where a client matches a response to the wrong
   request (see e.g.  Section 2.3 of
   [I-D.mattsson-core-coap-actuators]).  Note that mismatches can also
   happen for other reasons than a malicious attacker, e.g. delayed
   delivery or a server sending notifications to an uninterested client.

   A straightforward mitigation is to mandate clients to never reuse
   tokens until the traffic keys have been replaced.  As there may be

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   any number of responses to a request (see e.g.  [RFC7641]), the
   easiest way to accomplish this is to implement the token as a counter
   and never reuse any tokens at all.  This document updates the Token
   processing in [RFC7252] to always assure a cryptographically secure
   binding of responses to requests.

1.4.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "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
   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 one or more 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.

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

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

                       Figure 1: Echo Option Summary

   [Note to RFC editor: If this document is not released together with
   OSCORE but before it, the following paragraph and the "E" column
   above need to move into OSCORE.]

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

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

2.2.  Echo Processing

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

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

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

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

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   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 a next request to the server.  The client MAY
   include the same Echo option value in several different requests to
   the server.

   Upon receiving a request with the Echo option, the server determines
   if the request has freshness requirement.  If the request does not
   have freshness requirements, the Echo option MAY be ignored.  If the
   request has freshness requirements and the server cannot verify the
   freshness of the request in some other way, the server MUST verify
   that the Echo option value was generated by the server; otherwise the
   request is not processed further.  The server MUST then calculate the
   round-trip time RTT = (t1 - t0), where t1 is the request receive time
   and t0 is the transmit time of the response that included the
   specific Echo option value.  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.

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

                Figure 2: Example Echo Option Message Flow

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   When used to serve freshness requirements (including client aliveness
   and state synchronizing), CoAP requests containing the Echo option
   MUST be integrity protected, e.g. using DTLS, TLS, or OSCORE
   ([I-D.ietf-core-object-security]).  When used to demonstrate
   reachability at their apparent network address, the Echo option MAY
   be used without protection.

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

   CoAP-CoAP proxies MUST relay the Echo option unmodified.  The CoAP
   server side of CoAP-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-CoAP-Proxies SHOULD respond
   to Echo challenges themselves if they know from the recent
   establishing of the connection that the HTTP request is fresh.
   Otherwise, they SHOULD respond with 503 Service Unavailable, Retry-
   After: 0 and terminate any underlying Keep-Alive connection.  They
   MAY also use other mechanisms to establish freshness of the HTTP
   request that are not specified here.

2.3.  Applications

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

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

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       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.

   6.  A server may want to verify the aliveness of a client by
       responding with an Echo option.

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.

3.1.  Option Format

   The Request-Tag option is not critical, safe to forward, and part of
   the cache key as illustrated in Figure 3.

   +-----+---+---+---+---+-------------+--------+--------+---------+---+
   | No. | C | U | N | R | Name        | Format | Length | Default | E |
   +-----+---+---+---+---+-------------+--------+--------+---------+---+
   | TBD |   |   |   |   | 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

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   [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 block options, 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 the request messages of
   blockwise request operations.

   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 (precisely: target the same
   endpoint and cache-key except for cache-key options that are related
   to blockwise), 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 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-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
   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

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   option (if not, 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 SHOULD respond to the
   second request with a 5.03 (Service Unavailable) response code and
   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].  (Alternatively, the server can cancel
   the original operation, especially if it is already likely to time
   out.  Cancelling it unconditionally is the behavior that could be
   expected of a Request-Tag unaware server.)

   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.

   Note that RFC 7959 already implies that the cache key is the element
   that binds exchanges together to operations (together with the
   request's source endpoint), but is not explicit about it; therefore,
   the above rules are spelt out here.

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 2.5 of [I-D.mattsson-core-coap-actuators]).  Therefore, the
   integrity protection of each block does not extend to the operation's
   request body.

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

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   o  The individual exchanges 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.

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

   Note that this mechanism is implicitly implemented when the security
   layer guarantees ordered delivery (e.g.  CoAP over TLS [RFC8323]).
   This is because with each message, any earlier 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.  A CoAP
   proxy would be in such a situation when it forwards operations with
   the same cache-key options but possibly different payloads.

   When a client fragments an initial 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,

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   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 second concurrent blockwise operations can then continue.

      The first operation might have been cancelled by that (typical of
      servers that only support a single blockwise operation), in which
      case its resumption will result in a 4.08 Request Entity
      Incomplete error.

   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.

      In this, the proxy can indicate the anticipated delay by sending a
      5.03 Service Unavailable response itself.

   Note that a correctly implemented Request-Tag unaware proxy in the
   same situation would need to make a choice to either send a 5.03 with
   Max-Age by itself (holding off the second operation), or to commence
   the second operation and reject any further requests on the first
   operation with 4.08 Request Entity Incompelte errors by itself
   without forwarding them.

3.4.  Rationale for the option properties

   The Request-Tag option used to be critical and unsafe to forward in
   earlier revisions of this draft.

   Given that supporting it will be mandated for where it is used for
   its security properties, the choice of whether it is mandatory or
   safe to forward can be made as required for the multiple concurrent
   operations use case.  For those cases, Request-Tag is the proxy-safe
   elective option suggested in [RFC7959] Section 2.4 last paragraph.

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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.  Token Processing

   This section updates the Token processing in Section 5.3.1 of
   [RFC7252] by adding the following text:

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

6.  IANA Considerations

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

                   +--------+-------------+------------+
                   | Number | Name        | Reference  |
                   +--------+-------------+------------+
                   | TBD1   | Echo        | [RFC XXXX] |
                   |        |             |            |
                   | TBD2   | Request-Tag | [RFC XXXX] |
                   +--------+-------------+------------+

                       Figure 4: CoAP Option Numbers

7.  Security Considerations

   Servers SHOULD NOT put any privacy sensitive information in the Echo
   or Request-Tag option values.  Unencrypted timestamps MAY reveal
   information about the server such as its wall clock time or location.
   Servers MUST use a monotonic clock to generate timestamps and compute
   round-trip times.  Servers SHOULD NOT use wall clock time for

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   timestamps, as wall clock time is not monotonic, may reveal that the
   server will accept expired certificates, or reveal the server's
   location.  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.  An attacker may be able to affect the
   server's wall clock time in various ways such as setting up a fake
   NTP server or broadcasting false time signals to radio-controlled
   clocks.  Servers SHOULD use the time since reboot measured in some
   unit of time.  Servers MAY reset the timer periodically even when not
   rebooting.

   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.

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

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

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

8.  References

8.1.  Normative References

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

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

8.2.  Informative References

   [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-09 (work in
              progress), March 2018.

   [I-D.mattsson-core-coap-actuators]
              Mattsson, J., Fornehed, J., Selander, G., Palombini, F.,
              and C. Amsuess, "Controlling Actuators with CoAP", draft-
              mattsson-core-coap-actuators-04 (work in progress), March
              2018.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [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>.

   [RFC8323]  Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
              Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
              Application Protocol) over TCP, TLS, and WebSockets",
              RFC 8323, DOI 10.17487/RFC8323, February 2018,
              <https://www.rfc-editor.org/info/rfc8323>.

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Appendix A.  Methods for Generating Echo Option Values

   The content and structure of the Echo option value are implementation
   specific and determined by the server.  Use of one of the mechanisms
   outlined in this section is RECOMMENDED.

   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.

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

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

   o  List of Cached Random Values and Timestamps.  An alternative
      method is to construct the Echo option value as a (pseudo-)random
      byte string.  The server caches a list containing the random byte
      strings and their transmission times.  Assuming 64-bit random
      values and 32-bit timestamps, the size of the Echo option value is
      8 bytes and the amount of server state is 12n bytes, where n is
      the number of active Echo Option values.  If the server loses time
      synchronization, 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

   A server MAY use different methods and security levels for different
   uses cases (client aliveness, request freshness, state
   synchronization, network address reachability, etc.).

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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
   used-up, 256 values from one byte long options (2 Bytes overhead) are
   available.

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

   o  In DTLS, a new session can be established.

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

Appendix C.  Change Log

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

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

      *  Reworded the Echo section.

      *  Added rules for Token processing.

      *  Added security considerations.

      *  Added actual IANA section.

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

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

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

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

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

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

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

Authors' Addresses

   Christian Amsuess

   Email: christian@amsuess.com

   John Mattsson
   Ericsson AB

   Email: john.mattsson@ericsson.com

   Goeran Selander
   Ericsson AB

   Email: goran.selander@ericsson.com

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