draft-ietf-core-echo-request-tag-07.txt   draft-ietf-core-echo-request-tag-08.txt 
CoRE Working Group C. Amsuess CoRE Working Group C. Amsuess
Internet-Draft Internet-Draft
Updates: 7252 (if approved) J. Mattsson Updates: 7252 (if approved) J. Mattsson
Intended status: Standards Track G. Selander Intended status: Standards Track G. Selander
Expires: March 22, 2020 Ericsson AB Expires: May 7, 2020 Ericsson AB
September 19, 2019 November 04, 2019
CoAP: Echo, Request-Tag, and Token Processing CoAP: Echo, Request-Tag, and Token Processing
draft-ietf-core-echo-request-tag-07 draft-ietf-core-echo-request-tag-08
Abstract Abstract
This document specifies enhancements to the Constrained Application This document specifies enhancements to the Constrained Application
Protocol (CoAP) that mitigate security issues in particular use Protocol (CoAP) that mitigate security issues in particular use
cases. The Echo option enables a CoAP server to verify the freshness 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 of a request or to force a client to demonstrate reachability at its
claimed network address. The Request-Tag option allows the CoAP claimed network address. The Request-Tag option allows the CoAP
server to match block-wise message fragments belonging to the same server to match block-wise message fragments belonging to the same
request. The update to the client Token processing requirements of request. The update to the client Token processing requirements of
skipping to change at page 1, line 40 skipping to change at page 1, line 40
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 22, 2020. This Internet-Draft will expire on May 7, 2020.
Copyright Notice Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Request Freshness . . . . . . . . . . . . . . . . . . . . 3 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Fragmented Message Body Integrity . . . . . . . . . . . . 4 2. Request Freshness and the Echo Option . . . . . . . . . . . . 4
1.3. Request-Response Binding . . . . . . . . . . . . . . . . 5 2.1. Request Freshness . . . . . . . . . . . . . . . . . . . . 4
1.4. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5 2.2. The Echo Option . . . . . . . . . . . . . . . . . . . . . 5
2. The Echo Option . . . . . . . . . . . . . . . . . . . . . . . 6 2.2.1. Echo Option Format . . . . . . . . . . . . . . . . . 5
2.1. Option Format . . . . . . . . . . . . . . . . . . . . . . 7 2.3. Echo Processing . . . . . . . . . . . . . . . . . . . . . 6
2.2. Echo Processing . . . . . . . . . . . . . . . . . . . . . 8 2.4. Applications of the Echo Option . . . . . . . . . . . . . 10
2.3. Applications . . . . . . . . . . . . . . . . . . . . . . 10 3. Protecting Message Bodies using Request Tags . . . . . . . . 11
3. The Request-Tag Option . . . . . . . . . . . . . . . . . . . 11 3.1. Fragmented Message Body Integrity . . . . . . . . . . . . 11
3.1. Option Format . . . . . . . . . . . . . . . . . . . . . . 12 3.2. The Request-Tag Option . . . . . . . . . . . . . . . . . 12
3.2. Request-Tag Processing by Servers . . . . . . . . . . . . 12 3.2.1. Request-Tag Option Format . . . . . . . . . . . . . . 12
3.3. Setting the Request-Tag . . . . . . . . . . . . . . . . . 13 3.3. Request-Tag Processing by Servers . . . . . . . . . . . . 13
3.4. Applications . . . . . . . . . . . . . . . . . . . . . . 14 3.4. Setting the Request-Tag . . . . . . . . . . . . . . . . . 14
3.4.1. Body Integrity Based on Payload Integrity . . . . . . 14 3.5. Applications of the Request-Tag Option . . . . . . . . . 15
3.4.2. Multiple Concurrent Block-wise Operations . . . . . . 15 3.5.1. Body Integrity Based on Payload Integrity . . . . . . 15
3.4.3. Simplified Block-Wise Handling for Constrained 3.5.2. Multiple Concurrent Block-wise Operations . . . . . . 16
3.5.3. Simplified Block-Wise Handling for Constrained
Proxies . . . . . . . . . . . . . . . . . . . . . . . 16 Proxies . . . . . . . . . . . . . . . . . . . . . . . 16
3.5. Rationale for the Option Properties . . . . . . . . . . . 16 3.6. Rationale for the Option Properties . . . . . . . . . . . 16
3.6. Rationale for Introducing the Option . . . . . . . . . . 16 3.7. Rationale for Introducing the Option . . . . . . . . . . 17
4. Block2 / ETag Processing . . . . . . . . . . . . . . . . . . 17 3.8. Block2 / ETag Processing . . . . . . . . . . . . . . . . 17
5. Token Processing . . . . . . . . . . . . . . . . . . . . . . 17 4. Token Processing for Secure Request-Response Binding . . . . 18
6. Security Considerations . . . . . . . . . . . . . . . . . . . 17 4.1. Request-Response Binding . . . . . . . . . . . . . . . . 18
6.1. Token reuse . . . . . . . . . . . . . . . . . . . . . . . 18 4.2. Updated Token Processing Requirements for Clients . . . . 18
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 19 5. Security Considerations . . . . . . . . . . . . . . . . . . . 19
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 20 5.1. Token reuse . . . . . . . . . . . . . . . . . . . . . . . 20
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 20 6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 21
9.1. Normative References . . . . . . . . . . . . . . . . . . 20 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
9.2. Informative References . . . . . . . . . . . . . . . . . 21 8. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
Appendix A. Methods for Generating Echo Option Values . . . . . 22 8.1. Normative References . . . . . . . . . . . . . . . . . . 22
Appendix B. Request-Tag Message Size Impact . . . . . . . . . . 23 8.2. Informative References . . . . . . . . . . . . . . . . . 22
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 24 Appendix A. Methods for Generating Echo Option Values . . . . . 23
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 27 Appendix B. Request-Tag Message Size Impact . . . . . . . . . . 25
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 27 Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 25
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 29
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 29
1. Introduction 1. Introduction
The initial Constrained Application Protocol (CoAP) suite of The initial Constrained Application Protocol (CoAP) suite of
specifications ([RFC7252], [RFC7641], and [RFC7959]) was designed specifications ([RFC7252], [RFC7641], and [RFC7959]) was designed
with the assumption that security could be provided on a separate with the assumption that security could be provided on a separate
layer, in particular by using DTLS ([RFC6347]). However, for some layer, in particular by using DTLS ([RFC6347]). However, for some
use cases, additional functionality or extra processing is needed to use cases, additional functionality or extra processing is needed to
support secure CoAP operations. This document specifies security support secure CoAP operations. This document specifies security
enhancements to the Constrained Application Protocol (CoAP). enhancements to the Constrained Application Protocol (CoAP).
skipping to change at page 3, line 35 skipping to change at page 3, line 35
([RFC6347]), TLS ([RFC8446]), or OSCORE ([RFC8613]), provide the ([RFC6347]), TLS ([RFC8446]), or OSCORE ([RFC8613]), provide the
additional security features. additional security features.
The document also updates the Token processing requirements for The document also updates the Token processing requirements for
clients specified in [RFC7252]. The updated processing forbids non- clients specified in [RFC7252]. The updated processing forbids non-
secure reuse of Tokens to ensure binding of responses to requests secure reuse of Tokens to ensure binding of responses to requests
when CoAP is used with security, thus mitigating error cases and when CoAP is used with security, thus mitigating error cases and
attacks where the client may erroneously associate the wrong response attacks where the client may erroneously associate the wrong response
to a request. to a request.
1.1. Request Freshness Each of the following sections provides a more detailed introduction
to the topic at hand in its first subsection.
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-actuators]). 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. The Echo option, defined in this document, specifies
such a mechanism which thereby enables a CoAP server to verify the
freshness of a request. This mechanism is not only important in the
case of actuators, or other use cases where the CoAP operations
require freshness of requests, but also in general for synchronizing
state between CoAP client and server, cryptographically verify the
aliveness of the client, or force 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.
1.2. Fragmented Message Body Integrity
CoAP was designed to work over unreliable transports, such as UDP,
and include a lightweight reliability feature to handle messages
which are lost or arrive out of order. In order for a security
protocol to support CoAP operations over unreliable transports, it
must allow out-of-order delivery of messages using e.g. a sliding
replay window such as described in Section 4.1.2.6 of DTLS
([RFC6347]).
The block-wise transfer mechanism [RFC7959] extends CoAP by defining
the transfer of a large resource representation (CoAP message body)
as a sequence of blocks (CoAP message payloads). The mechanism uses
a pair of CoAP options, Block1 and Block2, pertaining to the request
and response payload, respectively. The 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-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 request bodies, similar to ETag, but ephemeral and set by
the CoAP client. The Request-Tag option is only used in requests
that carry the Block1 option, and in Block2 requests following these.
1.3. Request-Response Binding
A fundamental requirement of secure REST operations is that the
client can bind a response to a particular request. If this is not
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 for a completely different resource (see e.g.
Section 2.3 of [I-D.mattsson-core-coap-actuators]). For example, a
request for the alarm status "GET /status" may be associated to a
prior response "on", instead of the correct response "off".
In HTTPS, 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. One easy way to
accomplish this is to implement the Token as a counter starting at
zero for each new or rekeyed secure connection. This document
updates the Token processing in [RFC7252] to always assure a
cryptographically secure binding of responses to requests for secure
REST operations like "coaps".
1.4. Terminology 1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here. capitals, as shown here.
Unless otherwise specified, the terms "client" and "server" refers to Unless otherwise specified, the terms "client" and "server" refers to
"CoAP client" and "CoAP server", respectively, as defined in "CoAP client" and "CoAP server", respectively, as defined in
[RFC7252]. The term "origin server" is used as in [RFC7252]. The [RFC7252]. The term "origin server" is used as in [RFC7252]. The
skipping to change at page 6, line 29 skipping to change at page 4, line 22
Two request messages are said to be "matchable" if they occur between Two request messages are said to be "matchable" if they occur between
the same endpoint pair, have the same code and the same set of the same endpoint pair, have the same code and the same set of
options except for elective NoCacheKey options and options involved options except for elective NoCacheKey options and options involved
in block-wise transfer (Block1, Block2 and Request-Tag). Two in block-wise transfer (Block1, Block2 and Request-Tag). Two
operations are said to be matchable if any of their messages are. operations are said to be matchable if any of their messages are.
Two matchable block-wise operations are said to be "concurrent" if a Two matchable block-wise operations are said to be "concurrent" if a
block of the second request is exchanged even though the client still block of the second request is exchanged even though the client still
intends to exchange further blocks in the first operation. intends to exchange further blocks in the first operation.
(Concurrent block-wise request operations are impossible with the (Concurrent block-wise request operations from a single endpoint are
options of [RFC7959] because the second operation's block overwrites 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.). any state of the first exchange.).
The Echo and Request-Tag options are defined in this document. The Echo and Request-Tag options are defined in this document.
2. The Echo Option 2. Request Freshness and the Echo Option
A fresh request is one whose age has not yet exceeded the freshness 2.1. Request 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 is a
lightweight challenge-response mechanism for CoAP, motivated by a
need for a server to verify freshness of a request as described in
Section 1.1. 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. The Echo option provides a convention
to transfer freshness indicators that works for all CoAP methods and
response codes.
2.1. Option Format 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-actuators]). 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 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. The Echo option provides a convention to transfer
freshness indicators that works for all CoAP methods and response
codes.
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 verify the aliveness of the
client, or force 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- 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 key, and not repeatable, see Figure 1, which extends Table 4 of
[RFC7252]). [RFC7252]).
+-----+---+---+---+---+-------------+--------+------+---------+---+---+ +-----+---+---+---+---+-------------+--------+------+---------+---+---+
| No. | C | U | N | R | Name | Format | Len. | Default | E | U | | No. | C | U | N | R | Name | Format | Len. | Default | E | U |
+-----+---+---+---+---+-------------+--------+------+---------+---+---+ +-----+---+---+---+---+-------------+--------+------+---------+---+---+
| TBD | | | x | | Echo | opaque | 4-40 | (none) | x | x | | TBD | | | x | | Echo | opaque | 4-40 | (none) | x | x |
+-----+---+---+---+---+-------------+--------+------+---------+---+---+ +-----+---+---+---+---+-------------+--------+------+---------+---+---+
skipping to change at page 7, line 29 skipping to change at page 6, line 6
Figure 1: Echo Option Summary Figure 1: Echo Option Summary
The Echo option value is generated by a server, and its content and The Echo option value is generated by a server, and its content and
structure are implementation specific. Different methods for structure are implementation specific. Different methods for
generating Echo option values are outlined in Appendix A. Clients generating Echo option values are outlined in Appendix A. Clients
and intermediaries MUST treat an Echo option value as opaque and make and intermediaries MUST treat an Echo option value as opaque and make
no assumptions about its content or structure. no assumptions about its content or structure.
When receiving an Echo option in a request, the server MUST be able When receiving an Echo option in a request, the server MUST be able
to verify when the Echo option value was generated. This implies to verify that the Echo option value (a) was generated by the server
that the server MUST be able to verify that the Echo option value was or some other party that the server trusts, and (b) fulfills the
generated by the server or some other party that the server trusts. freshness requirements of the application. Depending on the
Depending on the freshness requirements the server may verify exactly freshness requirements the server may verify exactly when the Echo
when the Echo option value was generated (time-based freshness) or option value was generated (time-based freshness) or verify that the
verify that the Echo option was generated after a specific event Echo option was generated after a specific event (event-based
(event-based freshness). As the request is bound to the Echo option freshness). As the request is bound to the Echo option value, the
value, the server can determine that the request is not older that server can determine that the request is not older that the Echo
the Echo option value. option value.
When the Echo option is used with OSCORE [RFC8613] it MAY be an Inner 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. or Outer option, and the Inner and Outer values are independent.
OSCORE servers MUST only produce Inner Echo options unless they are OSCORE servers MUST only produce Inner Echo options unless they are
merely testing for reachability of the client (the same as proxies merely testing for reachability of the client (the same as proxies
may do). The Inner option is encrypted and integrity protected may do). The Inner option is encrypted and integrity protected
between the endpoints, whereas the Outer option is not protected by between the endpoints, whereas the Outer option is not protected by
OSCORE and visible between the endpoints to the extent it is not OSCORE and visible between the endpoints to the extent it is not
protected by some other security protocol. E.g. in the case of DTLS protected by some other security protocol. E.g. in the case of DTLS
hop-by-hop between the endpoints, the Outer option is visible to hop-by-hop between the endpoints, the Outer option is visible to
proxies along the path. proxies along the path.
2.2. Echo Processing 2.3. Echo Processing
The Echo option MAY be included in any request or response (see The Echo option MAY be included in any request or response (see
Section 2.3 for different applications). Section 2.4 for different applications).
The application decides under what conditions a CoAP request to a The application decides under what conditions a CoAP request to a
resource is required to be fresh. These conditions can for example resource is required to be fresh. These conditions can for example
include what resource is requested, the request method and other data include what resource is requested, the request method and other data
in the request, and conditions in the environment such as the state in the request, and conditions in the environment such as the state
of the server or the time of the day. of the server or the time of the day.
If a certain request is required to be fresh, the request does not 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 contain a fresh Echo option value, and the server cannot verify the
freshness of the request in some other way, the server MUST NOT freshness of the request in some other way, the server MUST NOT
process the request further and SHOULD send a 4.01 Unauthorized process the request further and SHOULD send a 4.01 Unauthorized
response with an Echo option. The server MAY include the same Echo response with an Echo option. The server MAY include the same Echo
option value in several different responses and to different clients. 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 The server may use request freshness provided by the Echo option to
verify the aliveness of a client or to synchronize state. The server 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 may also include the Echo option in a response to force a client to
demonstrate reachability at its claimed network address. 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 Upon receiving a 4.01 Unauthorized response with the Echo option, the
client SHOULD resend the original request with the addition of an client SHOULD resend the original request with the addition of an
Echo option with the received Echo option value. The client MAY send Echo option with the received Echo option value. The client MAY send
a different request compared to the original request. Upon receiving a different request compared to the original request. Upon receiving
any other response with the Echo option, the client SHOULD echo the 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 Echo option value in the next request to the server. The client MAY
include the same Echo option value in several different requests to include the same Echo option value in several different requests to
the server. the server.
skipping to change at page 8, line 48 skipping to change at page 7, line 27
from (where as defined in [RFC7252] Section 1.2, the security from (where as defined in [RFC7252] Section 1.2, the security
association is part of the endpoint). In OSCORE processing, that association is part of the endpoint). In OSCORE processing, that
means sending Echo values from Outer options (or from non-OSCORE means sending Echo values from Outer options (or from non-OSCORE
responses) back in Outer options, and those from Inner options in responses) back in Outer options, and those from Inner options in
Inner options in the same security context. Inner options in the same security context.
Upon receiving a request with the Echo option, the server determines 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 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 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 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 server MUST use the Echo option to verify that the request is fresh.
enough. If the server cannot verify that the request is fresh If the server cannot verify that the request is fresh, the request is
enough, the request is not processed further, and an error message not processed further, and an error message MAY be sent. The error
MAY be sent. The error message SHOULD include a new Echo option. message SHOULD include a new Echo option.
One way for the server to verify freshness is that to bind the Echo One way for the server to verify freshness is that to bind the Echo
value to a specific point in time and verify that the request is not 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 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 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 t0 is the time when the Echo option value was generated. An example
message flow is illustrated in Figure 2. message flow is shown in Figure 2.
Client Server Client Server
| | | |
+------>| Code: 0.03 (PUT) +------>| Code: 0.03 (PUT)
| PUT | Token: 0x41 | PUT | Token: 0x41
| | Uri-Path: lock | | Uri-Path: lock
| | Payload: 0 (Unlock) | | Payload: 0 (Unlock)
| | | |
|<------+ Code: 4.01 (Unauthorized) |<------+ Code: 4.01 (Unauthorized)
| 4.01 | Token: 0x41 | 4.01 | Token: 0x41
skipping to change at page 9, line 33 skipping to change at page 8, line 26
+------>| t1 Code: 0.03 (PUT) +------>| t1 Code: 0.03 (PUT)
| PUT | Token: 0x42 | PUT | Token: 0x42
| | Uri-Path: lock | | Uri-Path: lock
| | Echo: 0x437468756c687521 (t0) | | Echo: 0x437468756c687521 (t0)
| | Payload: 0 (Unlock) | | Payload: 0 (Unlock)
| | | |
|<------+ Code: 2.04 (Changed) |<------+ Code: 2.04 (Changed)
| 2.04 | Token: 0x42 | 2.04 | Token: 0x42
| | | |
Figure 2: Example Echo Option Message Flow Figure 2: Example Message Flow for Time-Based Freshness
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 for all practical purposes. 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 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: 0x436F6D69632053616E73 (e0)
| |
+------>| e1 Code: 0.03 (PUT)
| PUT | Token: 0x42
| | Uri-Path: lock
| | Echo: 0x436F6D69632053616E73 (e0)
| | Payload: 0 (Unlock)
| |
|<------+ Code: 2.04 (Changed)
| 2.04 | Token: 0x42
| |
Figure 3: Example Message Flow for Event-Based Freshness
When used to serve freshness requirements (including client aliveness When used to serve freshness requirements (including client aliveness
and state synchronizing), CoAP messages containing the Echo option and state synchronizing), the Echo option value MUST be integrity
MUST be integrity protected between the intended endpoints, e.g. protected between the intended endpoints, e.g. using DTLS, TLS, or an
using DTLS, TLS, or an OSCORE Inner option ([RFC8613]). When used to OSCORE Inner option ([RFC8613]). When used to demonstrate
demonstrate reachability at a claimed network address, the Echo reachability at a claimed network address, the Echo option SHOULD
option SHOULD contain the client's network address, but MAY be contain the client's network address, but MAY be unprotected.
unprotected.
A CoAP-to-CoAP proxy MAY set an Echo option on responses, both on 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 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 (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 option it recognizes as one generated by itself on follow-up
requests. However, it MUST relay the Echo option of responses requests. However, it MUST relay the Echo option of responses
unmodified, and MUST relay the Echo option of requests it does not unmodified, and MUST relay the Echo option of requests it does not
recognize as generated by itself unmodified. recognize as generated by itself unmodified.
The CoAP server side of CoAP-to-HTTP proxies MAY request freshness, The CoAP server side of CoAP-to-HTTP proxies MAY request freshness,
especially if they have reason to assume that access may require it 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 (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 scope for this document. The CoAP client side of HTTP-to-CoAP
proxies SHOULD respond to Echo challenges themselves if they know proxies SHOULD respond to Echo challenges themselves if they know
from the recent establishing of the connection that the HTTP request from the recent establishing of the connection that the HTTP request
is fresh. Otherwise, they SHOULD respond with 503 Service is fresh. Otherwise, they SHOULD respond with 503 Service
Unavailable, Retry-After: 0 and terminate any underlying Keep-Alive Unavailable, Retry-After: 0 and terminate any underlying Keep-Alive
connection. They MAY also use other mechanisms to establish connection. They MAY also use other mechanisms to establish
freshness of the HTTP request that are not specified here. freshness of the HTTP request that are not specified here.
2.3. Applications 2.4. Applications of the Echo Option
1. Actuation requests often require freshness guarantees to avoid 1. Actuation requests often require freshness guarantees to avoid
accidental or malicious delayed actuator actions. In general, accidental or malicious delayed actuator actions. In general,
all non-safe methods (e.g. POST, PUT, DELETE) may require all non-safe methods (e.g. POST, PUT, DELETE) may require
freshness guarantees for secure operation. freshness guarantees for secure operation.
* The same Echo value may be used for multiple actuation * The same Echo value may be used for multiple actuation
requests to the same server, as long as the total round-trip requests to the same server, as long as the total round-trip
time since the Echo option value was generated is below the time since the Echo option value was generated is below the
freshness threshold. freshness threshold.
* For actuator applications with low delay tolerance, to avoid * For actuator applications with low delay tolerance, to avoid
additional round-trips for multiple requests in rapid additional round-trips for multiple requests in rapid
sequence, the server may include the Echo option with a new sequence, the server may include the Echo option with a new
value even in a successful response to a request, value even in a successful response to a request,
irrespectively of whether the request contained an Echo option irrespectively of whether the request contained an Echo option
or not. The client then uses the Echo option with the new or not. The client then uses the Echo option with the new
value in the next actuation request, and the server compares value in the next actuation request, and the server compares
the receive time accordingly. the receive time accordingly.
2. A server may use the Echo option to synchronize state or time 2. A server may use the Echo option to synchronize properties (such
with a requesting client. A server MUST NOT synchronize state or as state or time) with a requesting client. A server MUST NOT
time with clients which are not the authority of the property synchronize a property with a client which is not the authority
being synchronized. E.g. if access to a server resource is of the property being synchronized. E.g. if access to a server
dependent on time, then the client MUST NOT set the time of the resource is dependent on time, then server MUST NOT synchronize
server. time with a client requesting access unless it is time authority
for the server.
* If a server reboots during operation it may need to * If a server reboots during operation it may need to
synchronize state or time before continuing the interaction. synchronize state or time before continuing the interaction.
For example, with OSCORE it is possible to reuse a partly For example, with OSCORE it is possible to reuse a partly
persistently stored security context by synchronizing the persistently stored security context by synchronizing the
Partial IV (sequence number) using the Echo option, see Partial IV (sequence number) using the Echo option, see
Section 7.5 of [RFC8613]. Section 7.5 of [RFC8613].
* A device joining a CoAP group communication [RFC7390] * A device joining a CoAP group communication [RFC7390]
protected with OSCORE [I-D.ietf-core-oscore-groupcomm] may be protected with OSCORE [I-D.ietf-core-oscore-groupcomm] may be
skipping to change at page 11, line 28 skipping to change at page 11, line 22
to endpoints that have been previously in contact with the to endpoints that have been previously in contact with the
server. For this application, the Echo option can be used in server. For this application, the Echo option can be used in
messages that are not integrity protected, for example during messages that are not integrity protected, for example during
discovery. discovery.
* In the presence of a proxy, a server will not be able to * In the presence of a proxy, a server will not be able to
distinguish different origin client endpoints. Following from distinguish different origin client endpoints. Following from
the recommendation above, a proxy that sends large responses the recommendation above, a proxy that sends large responses
to unauthenticated peers SHOULD mitigate amplification to unauthenticated peers SHOULD mitigate amplification
attacks. The proxy MAY use Echo to verify origin reachability attacks. The proxy MAY use Echo to verify origin reachability
as described in Section 2.2. The proxy MAY forward idempotent as described in Section 2.3. The proxy MAY forward idempotent
requests immediately to have a cached result available when requests immediately to have a cached result available when
the client's Echoed request arrives. the client's Echoed request arrives.
4. A server may want to use the request freshness provided by the 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 Echo to verify the aliveness of a client. Note that in a
deployment with hop-by-hop security and proxies, the server can deployment with hop-by-hop security and proxies, the server can
only verify aliveness of the closest proxy. only verify aliveness of the closest proxy.
3. The Request-Tag Option 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 include a lightweight reliability feature to handle messages
which are lost or arrive out of order. In order for a security
protocol to support CoAP operations over unreliable transports, it
must allow out-of-order delivery of messages using e.g. a sliding
replay window such as described in Section 4.1.2.6 of DTLS
([RFC6347]).
The block-wise transfer mechanism [RFC7959] extends CoAP by defining
the transfer of a large resource representation (CoAP message body)
as a sequence of blocks (CoAP message payloads). The mechanism uses
a pair of CoAP options, Block1 and Block2, pertaining to the request
and response payload, respectively. The 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-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.
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 The Request-Tag is intended for use as a short-lived identifier for
keeping apart distinct block-wise request operations on one resource keeping apart distinct block-wise request operations on one resource
from one client, addressing the issue described in Section 1.2. It from one client, addressing the issue described in Section 3.1. It
enables the receiving server to reliably assemble request payloads enables the receiving server to reliably assemble request payloads
(blocks) to their message bodies, and, if it chooses to support it, (blocks) to their message bodies, and, if it chooses to support it,
to reliably process simultaneous block-wise request operations on a to reliably process simultaneous block-wise request operations on a
single resource. The requests must be integrity protected if they single resource. The requests must be integrity protected if they
should protect against interchange of blocks between different should protect against interchange of blocks between different
message bodies. 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 In essence, it is an implementation of the "proxy-safe elective
option" used just to "vary the cache key" as suggested in [RFC7959] option" used just to "vary the cache key" as suggested in [RFC7959]
Section 2.4. Section 2.4.
3.1. Option Format 3.2.1. Request-Tag Option Format
The Request-Tag option is not critical, is safe to forward, The Request-Tag option is not critical, is safe to forward,
repeatable, and part of the cache key, see Figure 3, which extends repeatable, and part of the cache key, see Figure 4, which extends
Table 4 of [RFC7252]). Table 4 of [RFC7252]).
+-----+---+---+---+---+-------------+--------+------+---------+---+---+ +-----+---+---+---+---+-------------+--------+------+---------+---+---+
| No. | C | U | N | R | Name | Format | Len. | Default | E | U | | No. | C | U | N | R | Name | Format | Len. | Default | E | U |
+-----+---+---+---+---+-------------+--------+------+---------+---+---+ +-----+---+---+---+---+-------------+--------+------+---------+---+---+
| TBD | | | | x | Request-Tag | opaque | 0-8 | (none) | x | x | | TBD | | | | x | Request-Tag | opaque | 0-8 | (none) | x | x |
+-----+---+---+---+---+-------------+--------+------+---------+---+---+ +-----+---+---+---+---+-------------+--------+------+---------+---+---+
C = Critical, U = Unsafe, N = NoCacheKey, R = Repeatable, C = Critical, U = Unsafe, N = NoCacheKey, R = Repeatable,
E = Encrypt and Integrity Protect (when using OSCORE) E = Encrypt and Integrity Protect (when using OSCORE)
Figure 3: Request-Tag Option Summary Figure 4: Request-Tag Option Summary
Request-Tag, like the block options, is both a class E and a class U 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]): 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 The Request-Tag MAY be an Inner or Outer option. It influences the
Inner or Outer block operation, respectively. The Inner and Outer Inner or Outer block operation, respectively. The Inner and Outer
values are therefore independent of each other. The Inner option is values are therefore independent of each other. The Inner option is
encrypted and integrity protected between client and server, and encrypted and integrity protected between client and server, and
provides message body identification in case of end-to-end provides message body identification in case of end-to-end
fragmentation of requests. The Outer option is visible to proxies fragmentation of requests. The Outer option is visible to proxies
and labels message bodies in case of hop-by-hop fragmentation of and labels message bodies in case of hop-by-hop fragmentation of
requests. requests.
The Request-Tag option is only used in the request messages of block- The Request-Tag option is only used in the request messages of block-
skipping to change at page 12, line 43 skipping to change at page 13, line 24
The Request-Tag option is only used in the request messages of block- The Request-Tag option is only used in the request messages of block-
wise operations. wise operations.
The Request-Tag mechanism can be applied independently on the server The Request-Tag mechanism can be applied independently on the server
and client sides of CoAP-to-CoAP proxies as are the block options, 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 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- 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 CoAP proxies can use Request-Tag on their CoAP sides; it is not
applicable to HTTP requests. applicable to HTTP requests.
3.2. Request-Tag Processing by Servers 3.3. Request-Tag Processing by Servers
The Request-Tag option does not require any particular processing on The Request-Tag option does not require any particular processing on
the server side outside of the processing already necessary for any the server side outside of the processing already necessary for any
unknown elective proxy-safe cache-key option: The option varies the unknown elective proxy-safe cache-key option: The option varies the
properties that distinguish block-wise operations (which includes all properties that distinguish block-wise operations (which includes all
options except elective NoCacheKey and except Block1/2), and thus the options except elective NoCacheKey and except Block1/2), and thus the
server can not treat messages with a different list of Request-Tag server can not treat messages with a different list of Request-Tag
options as belonging to the same operation. options as belonging to the same operation.
To keep utilizing the cache, a server (including proxies) MAY discard To keep utilizing the cache, a server (including proxies) MAY discard
skipping to change at page 13, line 20 skipping to change at page 13, line 49
Max-Age has not expired yet, even if the second operation uses a 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 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 about ETag in that it is formally part of the cache key, but
implementations that are aware of its meaning can cache more implementations that are aware of its meaning can cache more
efficiently, see [RFC7252] Section 5.4.2). efficiently, see [RFC7252] Section 5.4.2).
A server receiving a Request-Tag MUST treat it as opaque and make no A server receiving a Request-Tag MUST treat it as opaque and make no
assumptions about its content or structure. assumptions about its content or structure.
Two messages carrying the same Request-Tag is a necessary but not Two messages carrying the same Request-Tag is a necessary but not
sufficient condition for being part of the same operation. They can sufficient condition for being part of the same operation. For one,
still be treated as independent messages by the server (e.g. when it a server may still treat them as independent messages when it sends
sends 2.01/2.04 responses for every block), or initiate a new 2.01/2.04 responses for every block. Also, a client that lost
operation (overwriting kept context) when the later message carries interest in an old operation but wants to start over can overwrite
Block1 number 0. 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 he old operation.
As it has always been, a server that can only serve a limited number 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 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 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 indicating how long it expects the existing operation to go on, or it
can forget about the state established with the older operation and can forget about the state established with the older operation and
respond with 4.08 (Request Entity Incomplete) to later blocks on the respond with 4.08 (Request Entity Incomplete) to later blocks on the
first operation. first operation.
3.3. Setting the Request-Tag 3.4. Setting the Request-Tag
For each separate block-wise request operation, the client can choose For each separate block-wise request operation, the client can choose
a Request-Tag value, or choose not to set a Request-Tag. Starting a a Request-Tag value, or choose not to set a Request-Tag. It needs to
request operation matchable to a previous operation and even using be set to the same value (or unset) in all messages belonging to the
the same Request-Tag value is called request tag recycling. The same operation, as otherwise they are treated as separate operations
absence of a Request-Tag option is viewed as a value distinct from by the server.
all values with a single Request-Tag option set; starting a request
operation matchable to a previous operation where neither has a
Request-Tag option therefore constitutes request tag recycling just
as well (also called "recycling the absent option").
Clients MUST NOT recycle a request tag unless the first operation has Starting a request operation matchable to a previous operation and
concluded. What constitutes a concluded operation depends on the even using the same Request-Tag value is called request tag
application, and is outlined individually in Section 3.4. 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 that purpose, and is defined there.
When Block1 and Block2 are combined in an operation, the Request-Tag 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 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 the request would have a different set of options and would not be
recognized any more. recognized any more.
Clients are encouraged to generate compact messages. This means Clients are encouraged to generate compact messages. This means
sending messages without Request-Tag options whenever possible, and sending messages without Request-Tag options whenever possible, and
using short values when the absent option can not be recycled. using short values when the absent option can not be recycled.
The Request-Tag options MAY be present in request messages that carry The Request-Tag options MAY be present in request messages that carry
a Block2 option even if those messages are not part of a blockwise a Block2 option even if those messages are not part of a blockwise
request operation (this is to allow the operation described in request operation (this is to allow the operation described in
Section 3.4.3). The Request-Tag option MUST NOT be present in Section 3.5.3). The Request-Tag option MUST NOT be present in
response messages, and MUST NOT be present if neither the Block1 nor response messages, and MUST NOT be present if neither the Block1 nor
the Block2 option is present. the Block2 option is present.
3.4. Applications 3.5. Applications of the Request-Tag Option
3.4.1. Body Integrity Based on Payload Integrity 3.5.1. Body Integrity Based on Payload Integrity
When a client fragments a request body into multiple message When a client fragments a request body into multiple message
payloads, even if the individual messages are integrity protected, it payloads, even if the individual messages are integrity protected, it
is still possible for a man-in-the-middle to maliciously replace a is still possible for a man-in-the-middle to maliciously replace a
later operation's blocks with an earlier operation's blocks (see later operation's blocks with an earlier operation's blocks (see
Section 2.5 of [I-D.mattsson-core-coap-actuators]). Therefore, the Section 2.5 of [I-D.mattsson-core-coap-actuators]). Therefore, the
integrity protection of each block does not extend to the operation's integrity protection of each block does not extend to the operation's
request body. request body.
In order to gain that protection, use the Request-Tag mechanism as In order to gain that protection, use the Request-Tag mechanism as
skipping to change at page 15, line 27 skipping to change at page 16, line 11
rely on a conforming client to set the Request-Tag option when rely on a conforming client to set the Request-Tag option when
required, and thereby conclude on the integrity of the assembled required, and thereby conclude on the integrity of the assembled
body. body.
Note that this mechanism is implicitly implemented when the security Note that this mechanism is implicitly implemented when the security
layer guarantees ordered delivery (e.g. CoAP over TLS [RFC8323]). layer guarantees ordered delivery (e.g. CoAP over TLS [RFC8323]).
This is because with each message, any earlier message can not be This is because with each message, any earlier message can not be
replayed any more, so the client never needs to set the Request-Tag replayed any more, so the client never needs to set the Request-Tag
option unless it wants to perform concurrent operations. option unless it wants to perform concurrent operations.
3.4.2. Multiple Concurrent Block-wise Operations 3.5.2. Multiple Concurrent Block-wise Operations
CoAP clients, especially CoAP proxies, may initiate a block-wise CoAP clients, especially CoAP proxies, may initiate a block-wise
request operation to a resource, to which a previous one is already request operation to a resource, to which a previous one is already
in progress, which the new request should not cancel. A CoAP proxy in progress, which the new request should not cancel. A CoAP proxy
would be in such a situation when it forwards operations with the would be in such a situation when it forwards operations with the
same cache-key options but possibly different payloads. same cache-key options but possibly different payloads.
For those cases, Request-Tag is the proxy-safe elective option For those cases, Request-Tag is the proxy-safe elective option
suggested in [RFC7959] Section 2.4 last paragraph. suggested in [RFC7959] Section 2.4 last paragraph.
When initializing a new block-wise operation, a client has to look at When initializing a new block-wise operation, a client has to look at
other active operations: other active operations:
o If any of them is matchable to the new one, and the client neither o If any of them is matchable to the new one, and the client neither
wants to cancel the old one nor postpone the new one, it can pick wants to cancel the old one nor postpone the new one, it can pick
a Request-Tag value that is not in use by the other matchable a Request-Tag value (including the absent option) that is not in
operations for the new operation. use by the other matchable operations for the new operation.
o Otherwise, it can start the new operation without setting the o Otherwise, it can start the new operation without setting the
Request-Tag option on it. Request-Tag option on it.
3.4.3. Simplified Block-Wise Handling for Constrained Proxies 3.5.3. Simplified Block-Wise Handling for Constrained Proxies
The Block options were defined to be unsafe to forward because a The Block options were defined to be unsafe to forward because a
proxy that would forward blocks as plain messages would risk mixing proxy that would forward blocks as plain messages would risk mixing
up clients' requests. up clients' requests.
The Request-Tag option provides a very simple way for a proxy to keep The Request-Tag option provides a very simple way for a proxy to keep
them separate: if it appends a Request-Tag that is particular to the them separate: if it appends a Request-Tag that is particular to the
requesting endpoint to all request carrying any Block option, it does requesting endpoint to all request carrying any Block option, it does
not need to keep track of any further block state. not need to keep track of any further block state.
This is particularly useful to proxies that strive for stateless This is particularly useful to proxies that strive for stateless
operation as described in [I-D.ietf-core-stateless] Section 3.1. operation as described in [I-D.ietf-core-stateless] Section 3.1.
3.5. Rationale for the Option Properties 3.6. Rationale for the Option Properties
The Request-Tag option can be elective, because to servers unaware of The Request-Tag option can be elective, because to servers unaware of
the Request-Tag option, operations with differing request tags will the Request-Tag option, operations with differing request tags will
not be matchable. not be matchable.
The Request-Tag option can be safe to forward but part of the cache The Request-Tag option can be safe to forward but part of the cache
key, because to proxies unaware of the Request-Tag option will key, because to proxies unaware of the Request-Tag option will
consider operations with differing request tags unmatchable but can consider operations with differing request tags unmatchable but can
still forward them. still forward them.
The Request-Tag option is repeatable because this easily allows The Request-Tag option is repeatable because this easily allows
stateless proxies to "chain" their origin address. They can perform stateless proxies to "chain" their origin address. They can perform
the steps of Section 3.4.3 without the need to create an option value 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, that is the concatenation of the received option and their own value,
and can simply add a new Request-Tag option unconditionally. and can simply add a new Request-Tag option unconditionally.
In draft versions of this document, the Request-Tag option used to be In draft versions of this document, the Request-Tag option used to be
critical and unsafe to forward. That design was based on an critical and unsafe to forward. That design was based on an
erroneous understanding of which blocks could be composed according erroneous understanding of which blocks could be composed according
to [RFC7959]. to [RFC7959].
3.6. Rationale for Introducing the Option 3.7. Rationale for Introducing the Option
An alternative that was considered to the Request-Tag option for An alternative that was considered to the Request-Tag option for
coping with the problem of fragmented message body integrity coping with the problem of fragmented message body integrity
(Section 3.4.1) was to update [RFC7959] to say that blocks could only (Section 3.5.1) was to update [RFC7959] to say that blocks could only
be assembled if their fragments' order corresponded to the sequence be assembled if their fragments' order corresponded to the sequence
numbers. numbers.
That approach would have been difficult to roll out reliably on DTLS That approach would have been difficult to roll out reliably on DTLS
where many implementations do not expose sequence numbers, and would where many implementations do not expose sequence numbers, and would
still not prevent attacks like in [I-D.mattsson-core-coap-actuators] still not prevent attacks like in [I-D.mattsson-core-coap-actuators]
Section 2.5.2. Section 2.5.2.
4. Block2 / ETag Processing 3.8. Block2 / ETag Processing
The same security properties as in Section 3.4.1 can be obtained for The same security properties as in Section 3.5.1 can be obtained for
blockwise response operations. The threat model here is not an blockwise response operations. The threat model here is not an
attacker (because the response is made sure to belong to the current attacker (because the response is made sure to belong to the current
request by the security layer), but blocks in the client's cache. request by the security layer), but blocks in the client's cache.
Rules stating that response body reassembly is conditional on Rules stating that response body reassembly is conditional on
matching ETag values are already in place from Section 2.4 of matching ETag values are already in place from Section 2.4 of
[RFC7959]. [RFC7959].
To gain equivalent protection to Section 3.4.1, a server MUST use the To gain equivalent protection to Section 3.5.1, a server MUST use the
Block2 option in conjunction with the ETag option ([RFC7252], Block2 option in conjunction with the ETag option ([RFC7252],
Section 5.10.6), and MUST NOT use the same ETag value for different Section 5.10.6), and MUST NOT use the same ETag value for different
representations of a resource. representations of a resource.
5. Token Processing 4. Token Processing for Secure Request-Response Binding
As described in Section 1.3, the client must be able to verify that a 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 for a completely different resource (see e.g.
Section 2.3 of [I-D.mattsson-core-coap-actuators]). For example, a
request for the alarm status "GET /status" may be associated to a
prior response "on", instead of the correct response "off".
In HTTPS, 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. One easy way to
accomplish this is to implement the Token as a counter starting at
zero for each new or rekeyed secure connection.
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 response corresponds to a particular request. This section updates
the CoAP Token processing requirements for clients. The Token the Token processing requirements for clients in [RFC7252] to always
processing for servers is not updated. Token processing in assure a cryptographically secure binding of responses to requests
Section 5.3.1 of [RFC7252] is updated by adding the following text: 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 When CoAP is used with a security protocol not providing bindings
between requests and responses, the Tokens have cryptographic between requests and responses, the Tokens have cryptographic
importance. The client MUST make sure that Tokens are not used in a importance. The client MUST make sure that Tokens are not used in a
way so that responses risk being associated with the wrong request. 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 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 the Token) as a sequence number starting at zero for each new or
rekeyed secure connection, this approach SHOULD be followed. rekeyed secure connection, this approach SHOULD be followed.
6. Security Considerations 5. Security Considerations
The availability of a secure pseudorandom number generator and truly The availability of a secure pseudorandom number generator and truly
random seeds are essential for the security of the Echo option. If random seeds are essential for the security of the Echo option. If
no true random number generator is available, a truly random seed no true random number generator is available, a truly random seed
must be provided from an external source. As each pseudoranom number must be provided from an external source. As each pseudoranom number
must only be used once, an implementation need to get a new truly must only be used once, an implementation need to get a new truly
random seed after reboot, or continously store state in nonvolatile random seed after reboot, or continously store state in nonvolatile
memory, see ([RFC8613], Appendix B.1.1) for issues and solution memory, see ([RFC8613], Appendix B.1.1) for issues and solution
approaches for writing to nonvolatile memory. approaches for writing to nonvolatile memory.
skipping to change at page 18, line 25 skipping to change at page 19, line 50
Servers SHOULD use a monotonic clock to generate timestamps and Servers SHOULD use a monotonic clock to generate timestamps and
compute round-trip times. Use of non-monotonic clocks is not secure 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 as the server will accept expired Echo option values if the clock is
moved backward. The server will also reject fresh Echo option values moved backward. The server will also reject fresh Echo option values
if the clock is moved forward. Non-monotonic clocks MAY be used as if the clock is moved forward. Non-monotonic clocks MAY be used as
long as they have deviations that are acceptable given the freshness long as they have deviations that are acceptable given the freshness
requirements. If the deviations from a monotonic clock are known, it requirements. If the deviations from a monotonic clock are known, it
may be possible to adjust the threshold accordingly. may be possible to adjust the threshold accordingly.
Servers SHOULD NOT use wall clock time for timestamps, as wall clock An attacker may be able to affect the server's system time in various
time have large deviations from a monotonic clock. Furthermore, an ways such as setting up a fake NTP server or broadcasting false time
attacker may be able to affect the server's wall clock time in signals to radio-controlled clocks.
various ways such as setting up a fake NTP server or broadcasting
false time signals to radio-controlled clocks.
Servers MAY use the time since reboot measured in some unit of time. Servers MAY use the time since reboot measured in some unit of time.
Servers MAY reset the timer at certain times and MAY generate a Servers MAY reset the timer at certain times and MAY generate a
random offset applied to all timestamps. When resetting the timer, random offset applied to all timestamps. When resetting the timer,
the server MUST reject all Echo values that was created before the the server MUST reject all Echo values that was created before the
reset. reset.
Servers that use the List of Cached Random Values and Timestamps Servers that use the List of Cached Random Values and Timestamps
method described in Appendix A may be vulnerable to resource method described in Appendix A may be vulnerable to resource
exhaustion attacks. One way to minimize state is to use the exhaustion attacks. One way to minimize state is to use the
Integrity Protected Timestamp method described in Appendix A. Integrity Protected Timestamp method described in Appendix A.
6.1. Token reuse 5.1. Token reuse
Reusing Tokens in a way so that responses are guaranteed to not be Reusing Tokens in a way so that responses are guaranteed to not be
associated with the wrong request is not trivial as on-path attackers associated with the wrong request is not trivial as on-path attackers
may block, delay, and reorder messages, requests may be sent to may block, delay, and reorder messages, requests may be sent to
several servers, and servers may process requests in any order and several servers, and servers may process requests in any order and
send many responses to the same request. The use of a sequence send many responses to the same request. The use of a sequence
number is therefore recommended when CoAP is used with a security number is therefore recommended when CoAP is used with a security
protocol that does not providing bindings between requests and protocol that does not providing bindings between requests and
responses such as DTLS or TLS. responses such as DTLS or TLS.
skipping to change at page 19, line 19 skipping to change at page 20, line 41
o the response was piggybacked in an Acknowledgement message (as a o the response was piggybacked in an Acknowledgement message (as a
confirmable or non-confirmable response may have been transmitted confirmable or non-confirmable response may have been transmitted
multiple times), and multiple times), and
o if observation was used, the same holds for the registration, all o if observation was used, the same holds for the registration, all
re-registrations, and the cancellation. re-registrations, and the cancellation.
(In addition, for observations, any responses using that Token and a (In addition, for observations, any responses using that Token and a
DTLS sequence number earlier than the cancellation Acknowledgement DTLS sequence number earlier than the cancellation Acknowledgement
message must be discarded. This is typically not supported in DTLS message need to be discarded. This is typically not supported in
implementations.) DTLS implementations.)
In some setups, Tokens can be reused without the above constraints, In some setups, Tokens can be reused without the above constraints,
as a different component in the setup provides the associations: as a different component in the setup provides the associations:
o In CoAP over TLS, retransmissions are not handled by the CoAP o In CoAP over TLS, retransmissions are not handled by the CoAP
layer and the replay window size is always exactly 1. When a layer and the replay window size is always exactly 1. When a
client is sending TLS protected requests without Observe to a client is sending TLS protected requests without Observe to a
single server, the client can reuse a Token as soon as the single server, the client can reuse a Token as soon as the
previous response with that Token has been received. previous response with that Token has been received.
o Requests whose responses are cryptographically bound to the o Requests whose responses are cryptographically bound to the
requests (like in OSCORE) can reuse Tokens indefinitely. requests (like in OSCORE) can reuse Tokens indefinitely.
In all other cases, a sequence number approach is recommended as per In all other cases, a sequence number approach is RECOMMENDED as per
Section 5. Section 4.
Tokens that cannot be reused need to be blacklisted. This could be Tokens that cannot be reused need to be handled appropriately. This
solved by increasing the Token as soon as the currently used Token could be solved by increasing the Token as soon as the currently used
cannot be reused, or by keeping a list of all blacklisted Tokens. Token cannot be reused, or by keeping a list of all blacklisted
Tokens.
When the Token (or part of the Token) contains a sequence number, the 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 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 any collisions. This is especially true when the Token contains more
information than just the sequence number, e.g. serialized state as information than just the sequence number, e.g. serialized state as
in [I-D.ietf-core-stateless]. in [I-D.ietf-core-stateless].
7. Privacy Considerations 6. Privacy Considerations
Implementations SHOULD NOT put any privacy sensitive information in Implementations SHOULD NOT put any privacy sensitive information in
the Echo or Request-Tag option values. Unencrypted timestamps MAY the Echo or Request-Tag option values. Unencrypted timestamps MAY
reveal information about the server such as location or time since reveal information about the server such as location or time since
reboot. The use of wall clock time is not allowed (see Section 6) reboot, or that the server will accept expired certificates.
and there also privacy reasons, e.g. it may reveal that the server Timestamps MAY be used if Echo is encrypted between the client and
will accept expired certificates. Timestamps MAY be used if Echo is the server, e.g. in the case of DTLS without proxies or when using
encrypted between the client and the server, e.g. in the case of DTLS OSCORE with an Inner Echo option.
without proxies or when using OSCORE with an Inner Echo option.
Like HTTP cookies, the Echo option could potentially be abused as a Like HTTP cookies, the Echo option could potentially be abused as a
tracking mechanism to link to different requests to the same client. tracking mechanism to link to different requests to the same client.
This is especially true for pre-emptive Echo values. Servers MUST This is especially true for pre-emptive Echo values. Servers MUST
NOT use the Echo option to correlate requests for other purposes than NOT use the Echo option to correlate requests for other purposes than
freshness and reachability. Clients only send Echo to the same from freshness and reachability. Clients only send Echo to the same from
which they were received. Compared to HTTP, CoAP clients are often which they were received. Compared to HTTP, CoAP clients are often
authenticated and non-mobile, and servers can therefore often authenticated and non-mobile, and servers can therefore often
correlate requests based on the security context, the client correlate requests based on the security context, the client
credentials, or the network address. When the Echo option increases credentials, or the network address. When the Echo option increases
a server's ability to correlate requests, clients MAY discard all a server's ability to correlate requests, clients MAY discard all
pre-emptive Echo values. pre-emptive Echo values.
8. IANA Considerations 7. IANA Considerations
This document adds the following option numbers to the "CoAP Option This document adds the following option numbers to the "CoAP Option
Numbers" registry defined by [RFC7252]: Numbers" registry defined by [RFC7252]:
+--------+-------------+-------------------+ +--------+-------------+-------------------+
| Number | Name | Reference | | Number | Name | Reference |
+--------+-------------+-------------------+ +--------+-------------+-------------------+
| TBD1 | Echo | [[this document]] | | TBD1 | Echo | [[this document]] |
| | | | | | | |
| TBD2 | Request-Tag | [[this document]] | | TBD2 | Request-Tag | [[this document]] |
+--------+-------------+-------------------+ +--------+-------------+-------------------+
Figure 4: CoAP Option Numbers Figure 5: CoAP Option Numbers
9. References 8. References
9.1. Normative References 8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>. <https://www.rfc-editor.org/info/rfc2119>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained [RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252, Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014, DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>. <https://www.rfc-editor.org/info/rfc7252>.
[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in [RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959, the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016, DOI 10.17487/RFC7959, August 2016,
<https://www.rfc-editor.org/info/rfc7959>. <https://www.rfc-editor.org/info/rfc7959>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
9.2. Informative References 8.2. Informative References
[I-D.ietf-core-oscore-groupcomm] [I-D.ietf-core-oscore-groupcomm]
Tiloca, M., Selander, G., Palombini, F., and J. Park, Tiloca, M., Selander, G., Palombini, F., and J. Park,
"Group OSCORE - Secure Group Communication for CoAP", "Group OSCORE - Secure Group Communication for CoAP",
draft-ietf-core-oscore-groupcomm-05 (work in progress), draft-ietf-core-oscore-groupcomm-05 (work in progress),
July 2019. July 2019.
[I-D.ietf-core-stateless] [I-D.ietf-core-stateless]
Hartke, K., "Extended Tokens and Stateless Clients in the Hartke, K., "Extended Tokens and Stateless Clients in the
Constrained Application Protocol (CoAP)", draft-ietf-core- Constrained Application Protocol (CoAP)", draft-ietf-core-
stateless-01 (work in progress), March 2019. stateless-03 (work in progress), October 2019.
[I-D.mattsson-core-coap-actuators] [I-D.mattsson-core-coap-actuators]
Mattsson, J., Fornehed, J., Selander, G., Palombini, F., Mattsson, J., Fornehed, J., Selander, G., Palombini, F.,
and C. Amsuess, "Controlling Actuators with CoAP", draft- and C. Amsuess, "Controlling Actuators with CoAP", draft-
mattsson-core-coap-actuators-06 (work in progress), mattsson-core-coap-actuators-06 (work in progress),
September 2018. September 2018.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer [RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>. January 2012, <https://www.rfc-editor.org/info/rfc6347>.
skipping to change at page 22, line 17 skipping to change at page 23, line 43
<https://www.rfc-editor.org/info/rfc8446>. <https://www.rfc-editor.org/info/rfc8446>.
[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz, [RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments "Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019, (OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/info/rfc8613>. <https://www.rfc-editor.org/info/rfc8613>.
Appendix A. Methods for Generating Echo Option Values Appendix A. Methods for Generating Echo Option Values
The content and structure of the Echo option value are implementation The content and structure of the Echo option value are implementation
specific and determined by the server. Two simple mechanisms are specific and determined by the server. Two simple mechanisms for
outlined in this section, the first is RECOMMENDED in general, and time-based freshness are outlined in this section, the first is
the second is RECOMMENDED in case the Echo option is encrypted RECOMMENDED in general, and the second is RECOMMENDED in case the
between the client and the server. Echo option is encrypted between the client and the server.
Different mechanisms have different tradeoffs between the size of the Different mechanisms have different tradeoffs between the size of the
Echo option value, the amount of server state, the amount of Echo option value, the amount of server state, the amount of
computation, and the security properties offered. A server MAY use computation, and the security properties offered. A server MAY use
different methods and security levels for different uses cases different methods and security levels for different uses cases
(client aliveness, request freshness, state synchronization, network (client aliveness, request freshness, state synchronization, network
address reachability, etc.). address reachability, etc.).
1. List of Cached Random Values and Timestamps. The Echo option 1. List of Cached Random Values and Timestamps. The Echo option
value is a (pseudo-)random byte string. The server caches a list value is a (pseudo-)random byte string. The server caches a list
skipping to change at page 23, line 7 skipping to change at page 24, line 34
integrity protected timestamp. The timestamp can have different integrity protected timestamp. The timestamp can have different
resolution and range. A 32-bit timestamp can e.g. give a resolution 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 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. 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 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 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 bytes and the Server state is small and constant. The security
against an attacker guessing echo values is given by the MAC length. 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 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 MUST be deleted and replaced by a new random secret key. Note that
the privacy considerations in Section 7 may apply to the timestamp. the privacy considerations in Section 6 may apply to the timestamp.
A server MAY want to encrypt its timestamps, and, depending on the A server MAY want to encrypt its timestamps, and, depending on the
choice of encryption algorithms, this may require a nonce to be choice of encryption algorithms, this may require a nonce to be
included in the Echo option value. included in the Echo option value.
Echo option value: timestamp t0, MAC(k, t0) Echo option value: timestamp t0, MAC(k, t0)
Server State: secret key k Server State: secret key k
Other mechanisms complying with the security and privacy Other mechanisms complying with the security and privacy
considerations may be used. The use of encrypted timestamps in the considerations may be used. The use of encrypted timestamps in the
Echo option increases security, but typically requires an IV to be Echo option increases security, but typically requires an IV to be
included in the Echo option value, which adds overhead and makes the included in the Echo option value, which adds overhead and makes the
specification of such a mechanism slightly more complicated than the specification of such a mechanism slightly more complicated than the
two mechanisms specified here. two mechanisms specified here.
Appendix B. Request-Tag Message Size Impact Appendix B. Request-Tag Message Size Impact
In absence of concurrent operations, the Request-Tag mechanism for In absence of concurrent operations, the Request-Tag mechanism for
body integrity (Section 3.4.1) incurs no overhead if no messages are 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 lost (more precisely: in OSCORE, if no operations are aborted due to
repeated transmission failure; in DTLS, if no packages are lost), or repeated transmission failure; in DTLS, if no packages are lost), or
when block-wise request operations happen rarely (in OSCORE, if there when block-wise request operations happen rarely (in OSCORE, if there
is always only one request block-wise operation in the replay is always only one request block-wise operation in the replay
window). window).
In those situations, no message has any Request-Tag option set, and In those situations, no message has any Request-Tag option set, and
that can be recycled indefinitely. that can be recycled indefinitely.
When the absence of a Request-Tag option can not be recycled any more When the absence of a Request-Tag option can not be recycled any more
skipping to change at page 24, line 9 skipping to change at page 25, line 39
o In OSCORE, the sequence number can be artificially increased so o In OSCORE, the sequence number can be artificially increased so
that all lost messages are outside of the replay window by the that all lost messages are outside of the replay window by the
time the first request of the new operation gets processed, and time the first request of the new operation gets processed, and
all earlier operations can therefore be regarded as concluded. all earlier operations can therefore be regarded as concluded.
Appendix C. Change Log Appendix C. Change Log
[ The editor is asked to remove this section before publication. ] [ The editor is asked to remove this section before publication. ]
o 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 blacklisting in a more implementation-
independent way
o Changes since draft-ietf-core-echo-request-tag-06: o Changes since draft-ietf-core-echo-request-tag-06:
* Removed visible comment that should not be visible in Token * Removed visible comment that should not be visible in Token
reuse considerations. reuse considerations.
o Changes since draft-ietf-core-echo-request-tag-05: o Changes since draft-ietf-core-echo-request-tag-05:
* Add privacy considerations on cookie-style use of Echo values * Add privacy considerations on cookie-style use of Echo values
* Add security considerations for token reuse * Add security considerations for token reuse
skipping to change at page 27, line 7 skipping to change at page 29, line 30
* The interaction between the new option and (cross) proxies is * The interaction between the new option and (cross) proxies is
now covered. now covered.
* Two messages being "Request-Tag matchable" was introduced to * Two messages being "Request-Tag matchable" was introduced to
replace the older concept of having a request tag value with replace the older concept of having a request tag value with
its slightly awkward equivalence definition. its slightly awkward equivalence definition.
Acknowledgments Acknowledgments
The authors want to thank Jim Schaad and Carsten Bormann for The authors want to thank Carsten Bormann, Francesca Palombini, and
providing valuable input to the draft. Jim Schaad for providing valuable input to the draft.
Authors' Addresses Authors' Addresses
Christian Amsuess Christian Amsuess
Email: christian@amsuess.com Email: christian@amsuess.com
John Preuss Mattsson John Preuss Mattsson
Ericsson AB Ericsson AB
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