CoRE Working Group C. Amsuess
Internet-Draft
Updates: 7252 (if approved) J. Mattsson
Intended status: Standards Track G. Selander
Expires: September 10, 2020 Ericsson AB
March 09, 2020
CoAP: Echo, Request-Tag, and Token Processing
draft-ietf-core-echo-request-tag-09
Abstract
This document specifies enhancements to the Constrained Application
Protocol (CoAP) that mitigate security issues in particular use
cases. The Echo option enables a CoAP server to verify the freshness
of a request or to force a client to demonstrate reachability at its
claimed network address; it is now the recommeded way to mitigate
amplification attacks. The Request-Tag option allows the CoAP server
to match block-wise message fragments belonging to the same request.
The update to the client Token processing requirements of RFC 7252
forbids non-secure reuse of Tokens to ensure binding of responses to
requests when CoAP is used with security.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 10, 2020.
Copyright Notice
Copyright (c) 2020 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
2. Request Freshness and the Echo Option . . . . . . . . . . . . 4
2.1. Request Freshness . . . . . . . . . . . . . . . . . . . . 4
2.2. The Echo Option . . . . . . . . . . . . . . . . . . . . . 5
2.2.1. Echo Option Format . . . . . . . . . . . . . . . . . 5
2.3. Echo Processing . . . . . . . . . . . . . . . . . . . . . 6
2.4. Applications of the Echo Option . . . . . . . . . . . . . 10
3. Protecting Message Bodies using Request Tags . . . . . . . . 11
3.1. Fragmented Message Body Integrity . . . . . . . . . . . . 11
3.2. The Request-Tag Option . . . . . . . . . . . . . . . . . 12
3.2.1. Request-Tag Option Format . . . . . . . . . . . . . . 12
3.3. Request-Tag Processing by Servers . . . . . . . . . . . . 13
3.4. Setting the Request-Tag . . . . . . . . . . . . . . . . . 14
3.5. Applications of the Request-Tag Option . . . . . . . . . 15
3.5.1. Body Integrity Based on Payload Integrity . . . . . . 15
3.5.2. Multiple Concurrent Block-wise Operations . . . . . . 16
3.5.3. Simplified Block-Wise Handling for Constrained
Proxies . . . . . . . . . . . . . . . . . . . . . . . 16
3.6. Rationale for the Option Properties . . . . . . . . . . . 17
3.7. Rationale for Introducing the Option . . . . . . . . . . 17
3.8. Block2 / ETag Processing . . . . . . . . . . . . . . . . 18
4. Token Processing for Secure Request-Response Binding . . . . 18
4.1. Request-Response Binding . . . . . . . . . . . . . . . . 18
4.2. Updated Token Processing Requirements for Clients . . . . 19
5. Security Considerations . . . . . . . . . . . . . . . . . . . 19
5.1. Token reuse . . . . . . . . . . . . . . . . . . . . . . . 20
6. Privacy Considerations . . . . . . . . . . . . . . . . . . . 21
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 23
8.1. Normative References . . . . . . . . . . . . . . . . . . 23
8.2. Informative References . . . . . . . . . . . . . . . . . 23
Appendix A. Methods for Generating Echo Option Values . . . . . 24
Appendix B. Request-Tag Message Size Impact . . . . . . . . . . 25
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 26
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 30
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 30
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1. Introduction
The initial Constrained Application Protocol (CoAP) suite of
specifications ([RFC7252], [RFC7641], and [RFC7959]) was designed
with the assumption that security could be provided on a separate
layer, in particular by using DTLS ([RFC6347]). However, for some
use cases, additional functionality or extra processing is needed to
support secure CoAP operations. This document specifies security
enhancements to the Constrained Application Protocol (CoAP).
This document specifies two CoAP options, the Echo option and the
Request-Tag option: The Echo option enables a CoAP server to verify
the freshness of a request, synchronize state, or force a client to
demonstrate reachability at its claimed network address. The
Request-Tag option allows the CoAP server to match message fragments
belonging to the same request, fragmented using the CoAP block-wise
Transfer mechanism, which mitigates attacks and enables concurrent
block-wise operations. These options in themselves do not replace
the need for a security protocol; they specify the format and
processing of data which, when integrity protected using e.g. DTLS
([RFC6347]), TLS ([RFC8446]), or OSCORE ([RFC8613]), provide the
additional security features.
This document updates [RFC7252] with a recommendation that servers
use the Echo option to mitigate amplification attacks.
The document also updates the Token processing requirements for
clients specified in [RFC7252]. The updated processing forbids non-
secure reuse of Tokens to ensure binding of responses to requests
when CoAP is used with security, thus mitigating error cases and
attacks where the client may erroneously associate the wrong response
to a request.
Each of the following sections provides a more detailed introduction
to the topic at hand in its first subsection.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
Unless otherwise specified, the terms "client" and "server" refers to
"CoAP client" and "CoAP server", respectively, as defined in
[RFC7252]. The term "origin server" is used as in [RFC7252]. The
term "origin client" is used in this document to denote the client
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from which a request originates; to distinguish from clients in
proxies.
The terms "payload" and "body" of a message are used as in [RFC7959].
The complete interchange of a request and a response body is called a
(REST) "operation". An operation fragmented using [RFC7959] is
called a "block-wise operation". A block-wise operation which is
fragmenting the request body is called a "block-wise request
operation". A block-wise operation which is fragmenting the response
body is called a "block-wise response operation".
Two request messages are said to be "matchable" if they occur between
the same endpoint pair, have the same code and the same set of
options except for elective NoCacheKey options and options involved
in block-wise transfer (Block1, Block2 and Request-Tag). Two
operations are said to be matchable if any of their messages are.
Two matchable block-wise operations are said to be "concurrent" if a
block of the second request is exchanged even though the client still
intends to exchange further blocks in the first operation.
(Concurrent block-wise request operations from a single endpoint are
impossible with the options of [RFC7959] (see the last paragraphs of
Sections 2.4 and 2.5) because the second operation's block overwrites
any state of the first exchange.).
The Echo and Request-Tag options are defined in this document.
2. Request Freshness and the Echo Option
2.1. Request Freshness
A CoAP server receiving a request is in general not able to verify
when the request was sent by the CoAP client. This remains true even
if the request was protected with a security protocol, such as DTLS.
This makes CoAP requests vulnerable to certain delay attacks which
are particularly perilous in the case of actuators
([I-D.mattsson-core-coap-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
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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-
key, and not repeatable, see Figure 1, which extends Table 4 of
[RFC7252]).
+--------+---+---+---+---+-------------+--------+------+---------+---+---+
| No. | C | U | N | R | Name | Format | Len. | Default | E | U |
+--------+---+---+---+---+-------------+--------+------+---------+---+---+
| TBD248 | | | x | | Echo | opaque | 1-40 | (none) | x | x |
+--------+---+---+---+---+-------------+--------+------+---------+---+---+
C = Critical, U = Unsafe, N = NoCacheKey, R = Repeatable,
E = Encrypt and Integrity Protect (when using OSCORE)
Figure 1: Echo Option Summary
The Echo option value is generated by a server, and its content and
structure are implementation specific. Different methods for
generating Echo option values are outlined in Appendix A. Clients
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and intermediaries MUST treat an Echo option value as opaque and make
no assumptions about its content or structure.
When receiving an Echo option in a request, the server MUST be able
to verify that the Echo option value (a) was generated by the server
or some other party that the server trusts, and (b) fulfills the
freshness requirements of the application. Depending on the
freshness requirements the server may verify exactly when the Echo
option value was generated (time-based freshness) or verify that the
Echo option was generated after a specific event (event-based
freshness). As the request is bound to the Echo option value, the
server can determine that the request is not older that the Echo
option value.
When the Echo option is used with OSCORE [RFC8613] it MAY be an Inner
or Outer option, and the Inner and Outer values are independent.
OSCORE servers MUST only produce Inner Echo options unless they are
merely testing for reachability of the client (the same as proxies
may do). The Inner option is encrypted and integrity protected
between the endpoints, whereas the Outer option is not protected by
OSCORE and visible between the endpoints to the extent it is not
protected by some other security protocol. E.g. in the case of DTLS
hop-by-hop between the endpoints, the Outer option is visible to
proxies along the path.
2.3. Echo Processing
The Echo option MAY be included in any request or response (see
Section 2.4 for different applications).
The application decides under what conditions a CoAP request to a
resource is required to be fresh. These conditions can for example
include what resource is requested, the request method and other data
in the request, and conditions in the environment such as the state
of the server or the time of the day.
If a certain request is required to be fresh, the request does not
contain a fresh Echo option value, and the server cannot verify the
freshness of the request in some other way, the server MUST NOT
process the request further and SHOULD send a 4.01 Unauthorized
response with an Echo option. The server MAY include the same Echo
option value in several different response messages and to different
clients. Examples of this could be time-based freshness when several
responses are sent closely after each other or event-based freshness
with no event taking place between the responses.
The server may use request freshness provided by the Echo option to
verify the aliveness of a client or to synchronize state. The server
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may also include the Echo option in a response to force a client to
demonstrate reachability at its claimed network address. Note that
the Echo option does not bind a request to any particular previous
response, but provides an indication that the client had access to
the previous response at the time when it created the request.
Upon receiving a 4.01 Unauthorized response with the Echo option, the
client SHOULD resend the original request with the addition of an
Echo option with the received Echo option value. The client MAY send
a different request compared to the original request. Upon receiving
any other response with the Echo option, the client SHOULD echo the
Echo option value in the next request to the server. The client MAY
include the same Echo option value in several different requests to
the server.
A client MUST only send Echo values to endpoints it received them
from (where as defined in [RFC7252] Section 1.2, the security
association is part of the endpoint). In OSCORE processing, that
means sending Echo values from Outer options (or from non-OSCORE
responses) back in Outer options, and those from Inner options in
Inner options in the same security context.
Upon receiving a request with the Echo option, the server determines
if the request is required to be fresh. If not, the Echo option MAY
be ignored. If the request is required to be fresh and the server
cannot verify the freshness of the request in some other way, the
server MUST use the Echo option to verify that the request is fresh.
If the server cannot verify that the request is fresh, the request is
not processed further, and an error message MAY be sent. The error
message SHOULD include a new Echo option.
One way for the server to verify freshness is that to bind the Echo
value to a specific point in time and verify that the request is not
older than a certain threshold T. The server can verify this by
checking that (t1 - t0) < T, where t1 is the request receive time and
t0 is the time when the Echo option value was generated. An example
message flow is shown in Figure 2.
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Client Server
| |
+------>| Code: 0.03 (PUT)
| PUT | Token: 0x41
| | Uri-Path: lock
| | Payload: 0 (Unlock)
| |
|<------+ Code: 4.01 (Unauthorized)
| 4.01 | Token: 0x41
| | Echo: 0x437468756c687521 (t0)
| |
+------>| t1 Code: 0.03 (PUT)
| PUT | Token: 0x42
| | Uri-Path: lock
| | Echo: 0x437468756c687521 (t0)
| | Payload: 0 (Unlock)
| |
|<------+ Code: 2.04 (Changed)
| 2.04 | Token: 0x42
| |
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.
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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
and state synchronizing), the Echo option value MUST be integrity
protected between the intended endpoints, e.g. using DTLS, TLS, or an
OSCORE Inner option ([RFC8613]). When used to demonstrate
reachability at a claimed network address, the Echo option SHOULD
contain the client's network address, but MAY be unprotected.
A CoAP-to-CoAP proxy MAY set an Echo option on responses, both on
forwarded ones that had no Echo option or ones generated by the proxy
(from cache or as an error). If it does so, it MUST remove the Echo
option it recognizes as one generated by itself on follow-up
requests. However, it MUST relay the Echo option of responses
unmodified, and MUST relay the Echo option of requests it does not
recognize as generated by itself unmodified.
The CoAP server side of CoAP-to-HTTP proxies MAY request freshness,
especially if they have reason to assume that access may require it
(e.g. because it is a PUT or POST); how this is determined is out of
scope for this document. The CoAP client side of HTTP-to-CoAP
proxies SHOULD respond to Echo challenges themselves if they know
from the recent establishing of the connection that the HTTP request
is fresh. Otherwise, they SHOULD respond with 503 Service
Unavailable, Retry-After: 0 and terminate any underlying Keep-Alive
connection. They MAY also use other mechanisms to establish
freshness of the HTTP request that are not specified here.
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2.4. Applications of the Echo Option
1. Actuation requests often require freshness guarantees to avoid
accidental or malicious delayed actuator actions. In general,
all non-safe methods (e.g. POST, PUT, DELETE) may require
freshness guarantees for secure operation.
* The same Echo value may be used for multiple actuation
requests to the same server, as long as the total round-trip
time since the Echo option value was generated is below the
freshness threshold.
* For actuator applications with low delay tolerance, to avoid
additional round-trips for multiple requests in rapid
sequence, the server may include the Echo option with a new
value even in a successful response to a request,
irrespectively of whether the request contained an Echo option
or not. The client then uses the Echo option with the new
value in the next actuation request, and the server compares
the receive time accordingly.
2. A server may use the Echo option to synchronize properties (such
as state or time) with a requesting client. A server MUST NOT
synchronize a property with a client which is not the authority
of the property being synchronized. E.g. if access to a server
resource is dependent on time, then server MUST NOT synchronize
time with a client requesting access unless it is time authority
for the server.
* If a server reboots during operation it may need to
synchronize state or time before continuing the interaction.
For example, with OSCORE it is possible to reuse a partly
persistently stored security context by synchronizing the
Partial IV (sequence number) using the Echo option, see
Section 7.5 of [RFC8613].
* A device joining a CoAP group communication [RFC7390]
protected with OSCORE [I-D.ietf-core-oscore-groupcomm] may be
required to initially verify freshness and synchronize state
or time with a client by using the Echo option in a unicast
response to a multicast request. The client receiving the
response with the Echo option includes the Echo option with
the same value in a request, either in a unicast request to
the responding server, or in a subsequent group request. In
the latter case, the Echo option will be ignored except by the
responding server.
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3. A server that sends large responses to unauthenticated peers
SHOULD mitigate amplification attacks such as described in
Section 11.3 of [RFC7252] (where an attacker would put a victim's
address in the source address of a CoAP request). The
RECOMMENDED way to do this is to ask a client to Echo its request
to verify its source address. This needs to be done only once
per peer and limits the range of potential victims from the
general Internet to endpoints that have been previously in
contact with the server. For this application, the Echo option
can be used in messages that are not integrity protected, for
example during discovery.
* In the presence of a proxy, a server will not be able to
distinguish different origin client endpoints. Following from
the recommendation above, a proxy that sends large responses
to unauthenticated peers SHOULD mitigate amplification
attacks. The proxy SHOULD use Echo to verify origin
reachability as described in Section 2.3. The proxy MAY
forward idempotent requests immediately to have a cached
result available when the client's Echoed request arrives.
* Amplification mitigation should be used when the the response
would be more than three times the size of the request,
considering the complete frame on the wire as it is typically
sent across the Internet. In practice, this allows UDP data
of at least 152 Bytes without further checks.
* When an Echo response is sent to mitigate amplification, it
MUST be sent as a piggybacked or non-confirmable response,
never as a separate one (which would cause amplification due
to retransmission).
4. A server may want to use the request freshness provided by the
Echo to verify the aliveness of a client. Note that in a
deployment with hop-by-hop security and proxies, the server can
only verify aliveness of the closest proxy.
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]).
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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
keeping apart distinct block-wise request operations on one resource
from one client, addressing the issue described in Section 3.1. It
enables the receiving server to reliably assemble request payloads
(blocks) to their message bodies, and, if it chooses to support it,
to reliably process simultaneous block-wise request operations on a
single resource. The requests must be integrity protected if they
should protect against interchange of blocks between different
message bodies. The Request-Tag option is only used in requests that
carry the Block1 option, and in Block2 requests following these.
In essence, it is an implementation of the "proxy-safe elective
option" used just to "vary the cache key" as suggested in [RFC7959]
Section 2.4.
3.2.1. Request-Tag Option Format
The Request-Tag option is not critical, is safe to forward,
repeatable, and part of the cache key, see Figure 4, which extends
Table 4 of [RFC7252]).
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+--------+---+---+---+---+-------------+--------+------+---------+---+---+
| No. | C | U | N | R | Name | Format | Len. | Default | E | U |
+--------+---+---+---+---+-------------+--------+------+---------+---+---+
| TBD292 | | | | x | Request-Tag | opaque | 0-8 | (none) | x | x |
+--------+---+---+---+---+-------------+--------+------+---------+---+---+
C = Critical, U = Unsafe, N = NoCacheKey, R = Repeatable,
E = Encrypt and Integrity Protect (when using OSCORE)
Figure 4: Request-Tag Option Summary
Request-Tag, like the block options, is both a class E and a class U
option in terms of OSCORE processing (see Section 4.1 of [RFC8613]):
The Request-Tag MAY be an Inner or Outer option. It influences the
Inner or Outer block operation, respectively. The Inner and Outer
values are therefore independent of each other. The Inner option is
encrypted and integrity protected between client and server, and
provides message body identification in case of end-to-end
fragmentation of requests. The Outer option is visible to proxies
and labels message bodies in case of hop-by-hop fragmentation of
requests.
The Request-Tag option is only used in the request messages of block-
wise operations.
The Request-Tag mechanism can be applied independently on the server
and client sides of CoAP-to-CoAP proxies as are the block options,
though given it is safe to forward, a proxy is free to just forward
it when processing an operation. CoAP-to-HTTP proxies and HTTP-to-
CoAP proxies can use Request-Tag on their CoAP sides; it is not
applicable to HTTP requests.
3.3. Request-Tag Processing by Servers
The Request-Tag option does not require any particular processing on
the server side outside of the processing already necessary for any
unknown elective proxy-safe cache-key option: The option varies the
properties that distinguish block-wise operations (which includes all
options except elective NoCacheKey and except Block1/2), and thus the
server can not treat messages with a different list of Request-Tag
options as belonging to the same operation.
To keep utilizing the cache, a server (including proxies) MAY discard
the Request-Tag option from an assembled block-wise request when
consulting its cache, as the option relates to the operation-on-the-
wire and not its semantics. For example, a FETCH request with the
same body as an older one can be served from the cache if the older's
Max-Age has not expired yet, even if the second operation uses a
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Request-Tag and the first did not. (This is similar to the situation
about ETag in that it is formally part of the cache key, but
implementations that are aware of its meaning can cache more
efficiently, see [RFC7252] Section 5.4.2).
A server receiving a Request-Tag MUST treat it as opaque and make no
assumptions about its content or structure.
Two messages carrying the same Request-Tag is a necessary but not
sufficient condition for being part of the same operation. For one,
a server may still treat them as independent messages when it sends
2.01/2.04 responses for every block. Also, a client that lost
interest in an old operation but wants to start over can overwrite
the server's old state with a new initial (num=0) Block1 request and
the same Request-Tag under some circumstances. Likewise, that
results in the new message not being part of he old operation.
As it has always been, a server that can only serve a limited number
of block-wise operations at the same time can delay the start of the
operation by replying with 5.03 (Service unavailable) and a Max-Age
indicating how long it expects the existing operation to go on, or it
can forget about the state established with the older operation and
respond with 4.08 (Request Entity Incomplete) to later blocks on the
first operation.
3.4. Setting the Request-Tag
For each separate block-wise request operation, the client can choose
a Request-Tag value, or choose not to set a Request-Tag. It needs to
be set to the same value (or unset) in all messages belonging to the
same operation, as otherwise they are treated as separate operations
by the server.
Starting a request operation matchable to a previous operation and
even using the same Request-Tag value is called request tag
recycling. The absence of a Request-Tag option is viewed as a value
distinct from all values with a single Request-Tag option set;
starting a request operation matchable to a previous operation where
neither has a Request-Tag option therefore constitutes request tag
recycling just as well (also called "recycling the absent option").
Clients that use Request-Tag for a particular purpose (like in
Section 3.5) MUST NOT recycle a request tag unless the first
operation has concluded. What constitutes a concluded operation
depends on that purpose, and is defined there.
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
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the request would have a different set of options and would not be
recognized any more.
Clients are encouraged to generate compact messages. This means
sending messages without Request-Tag options whenever possible, and
using short values when the absent option can not be recycled.
The Request-Tag options MAY be present in request messages that carry
no Block option (for example, because a Request-Tag unaware proxy
reassembled them), and MUST be ignored in those.
The Request-Tag option MUST NOT be present in response messages.
3.5. Applications of the Request-Tag Option
3.5.1. Body Integrity Based on Payload Integrity
When a client fragments a request body into multiple message
payloads, even if the individual messages are integrity protected, it
is still possible for a man-in-the-middle to maliciously replace a
later operation's blocks with an earlier operation's blocks (see
Section 2.5 of [I-D.mattsson-core-coap-actuators]). Therefore, the
integrity protection of each block does not extend to the operation's
request body.
In order to gain that protection, use the Request-Tag mechanism as
follows:
o The individual exchanges MUST be integrity protected end-to-end
between client and server.
o The client MUST NOT recycle a request tag in a new operation
unless the previous operation matchable to the new one has
concluded.
If any future security mechanisms allow a block-wise transfer to
continue after an endpoint's details (like the IP address) have
changed, then the client MUST consider messages sent to _any_
endpoint address within the new operation's security context.
o The client MUST NOT regard a block-wise request operation as
concluded unless all of the messages the client previously sent in
the operation have been confirmed by the message integrity
protection mechanism, or are considered invalid by the server if
replayed.
Typically, in OSCORE, these confirmations can result either from
the client receiving an OSCORE response message matching the
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request (an empty ACK is insufficient), or because the message's
sequence number is old enough to be outside the server's receive
window.
In DTLS, this can only be confirmed if the request message was not
retransmitted, and was responded to.
Authors of other documents (e.g. applications of [RFC8613]) are
invited to mandate this behavior for clients that execute block-wise
interactions over secured transports. In this way, the server can
rely on a conforming client to set the Request-Tag option when
required, and thereby conclude on the integrity of the assembled
body.
Note that this mechanism is implicitly implemented when the security
layer guarantees ordered delivery (e.g. CoAP over TLS [RFC8323]).
This is because with each message, any earlier message can not be
replayed any more, so the client never needs to set the Request-Tag
option unless it wants to perform concurrent operations.
3.5.2. Multiple Concurrent Block-wise Operations
CoAP clients, especially CoAP proxies, may initiate a block-wise
request operation to a resource, to which a previous one is already
in progress, which the new request should not cancel. A CoAP proxy
would be in such a situation when it forwards operations with the
same cache-key options but possibly different payloads.
For those cases, Request-Tag is the proxy-safe elective option
suggested in [RFC7959] Section 2.4 last paragraph.
When initializing a new block-wise operation, a client has to look at
other active operations:
o If any of them is matchable to the new one, and the client neither
wants to cancel the old one nor postpone the new one, it can pick
a Request-Tag value (including the absent option) that is not in
use by the other matchable operations for the new operation.
o Otherwise, it can start the new operation without setting the
Request-Tag option on it.
3.5.3. Simplified Block-Wise Handling for Constrained Proxies
The Block options were defined to be unsafe to forward because a
proxy that would forward blocks as plain messages would risk mixing
up clients' requests.
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In some cases, for example when forwarding block-wise request
operations, appending a Request-Tag value unique to the client can
satisfy the requirements on the proxy that come from the presence of
a block option.
This is particularly useful to proxies that strive for stateless
operation as described in [I-D.ietf-core-stateless] Section 4.
The precise classification of cases in which such a Request-Tag
option is sufficient is not trivial, especially when both request and
response body are fragmented, and out of scope for this document.
3.6. Rationale for the Option Properties
The Request-Tag option can be elective, because to servers unaware of
the Request-Tag option, operations with differing request tags will
not be matchable.
The Request-Tag option can be safe to forward but part of the cache
key, because to proxies unaware of the Request-Tag option will
consider operations with differing request tags unmatchable but can
still forward them.
The Request-Tag option is repeatable because this easily allows
stateless proxies to "chain" their origin address. They can perform
the steps of Section 3.5.3 without the need to create an option value
that is the concatenation of the received option and their own value,
and can simply add a new Request-Tag option unconditionally.
In draft versions of this document, the Request-Tag option used to be
critical and unsafe to forward. That design was based on an
erroneous understanding of which blocks could be composed according
to [RFC7959].
3.7. Rationale for Introducing the Option
An alternative that was considered to the Request-Tag option for
coping with the problem of fragmented message body integrity
(Section 3.5.1) was to update [RFC7959] to say that blocks could only
be assembled if their fragments' order corresponded to the sequence
numbers.
That approach would have been difficult to roll out reliably on DTLS
where many implementations do not expose sequence numbers, and would
still not prevent attacks like in [I-D.mattsson-core-coap-actuators]
Section 2.5.2.
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3.8. Block2 / ETag Processing
The same security properties as in Section 3.5.1 can be obtained for
blockwise response operations. The threat model here is not an
attacker (because the response is made sure to belong to the current
request by the security layer), but blocks in the client's cache.
Rules stating that response body reassembly is conditional on
matching ETag values are already in place from Section 2.4 of
[RFC7959].
To gain equivalent protection to Section 3.5.1, a server MUST use the
Block2 option in conjunction with the ETag option ([RFC7252],
Section 5.10.6), and MUST NOT use the same ETag value for different
representations of a resource.
4. Token Processing for Secure Request-Response Binding
4.1. Request-Response Binding
A fundamental requirement of secure REST operations is that the
client can bind a response to a particular request. If this is not
ensured, a client may erroneously associate the wrong response to a
request. The wrong response may be an old response for the same
resource or 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
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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
the Token processing requirements for clients in [RFC7252] to always
assure a cryptographically secure binding of responses to requests
for secure REST operations like "coaps". The Token processing for
servers is not updated. Token processing in Section 5.3.1 of
[RFC7252] is updated by adding the following text:
When CoAP is used with a security protocol not providing bindings
between requests and responses, the Tokens have cryptographic
importance. The client MUST make sure that Tokens are not used in a
way so that responses risk being associated with the wrong request.
One easy way to accomplish this is to implement the Token (or part of
the Token) as a sequence number starting at zero for each new or
rekeyed secure connection, this approach SHOULD be followed.
5. Security Considerations
The availability of a secure pseudorandom number generator and truly
random seeds are essential for the security of the Echo option
(except when using counting Echo values). If no true random number
generator is available, a truly random seed must be provided from an
external source. As each pseudoranom number must only be used once,
an implementation need to get a new truly random seed after reboot,
or continously store state in nonvolatile memory, see ([RFC8613],
Appendix B.1.1) for issues and solution approaches for writing to
nonvolatile memory.
A single active Echo value with 64 (pseudo-)random bits gives the
same theoretical security level as a 64-bit MAC (as used in e.g.
AES_128_CCM_8). Unless a counting Echo value is used, the Echo
option value MUST contain 32 (pseudo-)random bits that are not
predictable for any other party than the server, and SHOULD contain
64 (pseudo-)random bits. A server MAY use different security levels
for different uses cases (client aliveness, request freshness, state
synchronization, network address reachability, etc.).
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The security provided by the Echo and Request-Tag options depends on
the security protocol used. CoAP and HTTP proxies require (D)TLS to
be terminated at the proxies. The proxies are therefore able to
manipulate, inject, delete, or reorder options or packets. The
security claims in such architectures only hold under the assumption
that all intermediaries are fully trusted and have not been
compromised.
Counting Echo values can only be used to show freshness relative to
numbered events, and are the legitimate case for Echo values shorter
than four bytes, which are not necessarily secret. They MUST only be
used when the request Echo values are integrity protected.
Servers SHOULD use a monotonic clock to generate timestamps and
compute round-trip times. Use of non-monotonic clocks is not secure
as the server will accept expired Echo option values if the clock is
moved backward. The server will also reject fresh Echo option values
if the clock is moved forward. Non-monotonic clocks MAY be used as
long as they have deviations that are acceptable given the freshness
requirements. If the deviations from a monotonic clock are known, it
may be possible to adjust the threshold accordingly.
An attacker may be able to affect the server's system time in various
ways such as setting up a fake NTP server or broadcasting false time
signals to radio-controlled clocks.
Servers MAY use the time since reboot measured in some unit of time.
Servers MAY reset the timer at certain times and MAY generate a
random offset applied to all timestamps. When resetting the timer,
the server MUST reject all Echo values that was created before the
reset.
Servers that use the List of Cached Random Values and Timestamps
method described in Appendix A may be vulnerable to resource
exhaustion attacks. One way to minimize state is to use the
Integrity Protected Timestamp method described in Appendix A.
5.1. Token reuse
Reusing Tokens in a way so that responses are guaranteed to not be
associated with the wrong request is not trivial as on-path attackers
may block, delay, and reorder messages, requests may be sent to
several servers, and servers may process requests in any order and
send many responses to the same request. The use of a sequence
number is therefore recommended when CoAP is used with a security
protocol that does not providing bindings between requests and
responses such as DTLS or TLS.
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For a generic response to a confirmable request over DTLS, binding
can only be claimed without out-of-band knowledge if
o the original request was never retransmitted,
o the response was piggybacked in an Acknowledgement message (as a
confirmable or non-confirmable response may have been transmitted
multiple times), and
o if observation was used, the same holds for the registration, all
re-registrations, and the cancellation.
(In addition, for observations, any responses using that Token and a
DTLS sequence number earlier than the cancellation Acknowledgement
message need to be discarded. This is typically not supported in
DTLS implementations.)
In some setups, Tokens can be reused without the above constraints,
as a different component in the setup provides the associations:
o In CoAP over TLS, retransmissions are not handled by the CoAP
layer and the replay window size is always exactly 1. When a
client is sending TLS protected requests without Observe to a
single server, the client can reuse a Token as soon as the
previous response with that Token has been received.
o Requests whose responses are cryptographically bound to the
requests (like in OSCORE) can reuse Tokens indefinitely.
In all other cases, a sequence number approach is RECOMMENDED as per
Section 4.
Tokens that cannot be reused need to be handled appropriately. This
could be solved by increasing the Token as soon as the currently used
Token cannot be reused, or by keeping a list of all blacklisted
Tokens.
When the Token (or part of the Token) contains a sequence number, the
encoding of the sequence number has to be chosen in a way to avoid
any collisions. This is especially true when the Token contains more
information than just the sequence number, e.g. serialized state as
in [I-D.ietf-core-stateless].
6. Privacy Considerations
Implementations SHOULD NOT put any privacy sensitive information in
the Echo or Request-Tag option values. Unencrypted timestamps MAY
reveal information about the server such as location or time since
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reboot, or that the server will accept expired certificates.
Timestamps MAY be used if Echo is encrypted between the client and
the server, e.g. in the case of DTLS without proxies or when using
OSCORE with an Inner Echo option.
Like HTTP cookies, the Echo option could potentially be abused as a
tracking mechanism to link to different requests to the same client.
This is especially true for pre-emptive Echo values. Servers MUST
NOT use the Echo option to correlate requests for other purposes than
freshness and reachability. Clients only send Echo to the same from
which they were received. Compared to HTTP, CoAP clients are often
authenticated and non-mobile, and servers can therefore often
correlate requests based on the security context, the client
credentials, or the network address. When the Echo option increases
a server's ability to correlate requests, clients MAY discard all
pre-emptive Echo values.
7. IANA Considerations
IANA is requested to add the following option numbers to the "CoAP
Option Numbers" registry defined by [RFC7252]:
[
The editor is asked to suggest the numbers after TBD, as those satisfy the construction requirements set out in RFC7252:
Echo is NoCacheKey but not Unsafe or Critical, so it needs to end with 11100 in binary representation;
Request-Tag has no properties so it needs to end with 00 and not with 11100).
Request-Tag are picked to not waste the precious space of less-than-one-byte options,
but such that its offset from the Block1 option it regularly occurs with can still be expressed in an 1-byte offset (27 + (13 + 255) > 292).
Echo was picked to be the shortest it can be in an empty message as a NoCacheKey option
(11100 in binary does not fit in a nibble, and two lower ones are already taken),
and as high as possible to keep room for other options that might typically occur in pairs and might still use optimization around low numbers.
]
+--------+-------------+-------------------+
| Number | Name | Reference |
+--------+-------------+-------------------+
| TBD248 | Echo | [[this document]] |
| | | |
| TBD292 | Request-Tag | [[this document]] |
+--------+-------------+-------------------+
Figure 5: CoAP Option Numbers
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8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC7959] Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
the Constrained Application Protocol (CoAP)", RFC 7959,
DOI 10.17487/RFC7959, August 2016,
<https://www.rfc-editor.org/info/rfc7959>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
8.2. Informative References
[I-D.ietf-core-oscore-groupcomm]
Tiloca, M., Selander, G., Palombini, F., and J. Park,
"Group OSCORE - Secure Group Communication for CoAP",
draft-ietf-core-oscore-groupcomm-06 (work in progress),
November 2019.
[I-D.ietf-core-stateless]
Hartke, K., "Extended Tokens and Stateless Clients in the
Constrained Application Protocol (CoAP)", draft-ietf-core-
stateless-04 (work in progress), December 2019.
[I-D.mattsson-core-coap-actuators]
Mattsson, J., Fornehed, J., Selander, G., Palombini, F.,
and C. Amsuess, "Controlling Actuators with CoAP", draft-
mattsson-core-coap-actuators-06 (work in progress),
September 2018.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
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[RFC7390] Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
the Constrained Application Protocol (CoAP)", RFC 7390,
DOI 10.17487/RFC7390, October 2014,
<https://www.rfc-editor.org/info/rfc7390>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>.
[RFC8323] Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
Silverajan, B., and B. Raymor, Ed., "CoAP (Constrained
Application Protocol) over TCP, TLS, and WebSockets",
RFC 8323, DOI 10.17487/RFC8323, February 2018,
<https://www.rfc-editor.org/info/rfc8323>.
[RFC8446] Rescorla, E., "The Transport Layer Security (TLS) Protocol
Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
<https://www.rfc-editor.org/info/rfc8446>.
[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/info/rfc8613>.
Appendix A. Methods for Generating Echo Option Values
The content and structure of the Echo option value are implementation
specific and determined by the server. Two simple mechanisms for
time-based freshness are outlined in this section, the first is
RECOMMENDED in general, and the second is RECOMMENDED in case the
Echo option is encrypted between the client and the server.
Different mechanisms have different tradeoffs between the size of the
Echo option value, the amount of server state, the amount of
computation, and the security properties offered. A server MAY use
different methods and security levels for different uses cases
(client aliveness, request freshness, state synchronization, network
address reachability, etc.).
1. List of Cached Random Values and Timestamps. The Echo option
value is a (pseudo-)random byte string. The server caches a list
containing the random byte strings and their transmission times.
Assuming 72-bit random values and 32-bit timestamps, the size of the
Echo option value is 9 bytes and the amount of server state is 13n
bytes, where n is the number of active Echo Option values. The
security against an attacker guessing echo values is given by s = bit
length of r - log2(n). The length of r and the maximum allowed n
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should be set so that the security level is harmonized with other
parts of the deployment, e.g., s >= 64. If the server loses time
continuity, e.g. due to reboot, the entries in the old list MUST be
deleted.
Echo option value: random value r
Server State: random value r, timestamp t0
2. Integrity Protected Timestamp. The Echo option value is an
integrity protected timestamp. The timestamp can have different
resolution and range. A 32-bit timestamp can e.g. give a resolution
of 1 second with a range of 136 years. The (pseudo-)random secret
key is generated by the server and not shared with any other party.
The use of truncated HMAC-SHA-256 is RECOMMENDED. With a 32-bit
timestamp and a 64-bit MAC, the size of the Echo option value is 12
bytes and the Server state is small and constant. The security
against an attacker guessing echo values is given by the MAC length.
If the server loses time continuity, e.g. due to reboot, the old key
MUST be deleted and replaced by a new random secret key. Note that
the privacy considerations in Section 6 may apply to the timestamp.
A server MAY want to encrypt its timestamps, and, depending on the
choice of encryption algorithms, this may require a nonce to be
included in the Echo option value.
Echo option value: timestamp t0, MAC(k, t0)
Server State: secret key k
Other mechanisms complying with the security and privacy
considerations may be used. The use of encrypted timestamps in the
Echo option increases security, but typically requires an IV to be
included in the Echo option value, which adds overhead and makes the
specification of such a mechanism slightly more complicated than the
two mechanisms specified here.
Appendix B. Request-Tag Message Size Impact
In absence of concurrent operations, the Request-Tag mechanism for
body integrity (Section 3.5.1) incurs no overhead if no messages are
lost (more precisely: in OSCORE, if no operations are aborted due to
repeated transmission failure; in DTLS, if no packages are lost), or
when block-wise request operations happen rarely (in OSCORE, if there
is always only one request block-wise operation in the replay
window).
In those situations, no message has any Request-Tag option set, and
that can be recycled indefinitely.
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When the absence of a Request-Tag option can not be recycled any more
within a security context, the messages with a present but empty
Request-Tag option can be used (1 Byte overhead), and when that is
used-up, 256 values from one byte long options (2 Bytes overhead) are
available.
In situations where those overheads are unacceptable (e.g. because
the payloads are known to be at a fragmentation threshold), the
absent Request-Tag value can be made usable again:
o In DTLS, a new session can be established.
o In OSCORE, the sequence number can be artificially increased so
that all lost messages are outside of the replay window by the
time the first request of the new operation gets processed, and
all earlier operations can therefore be regarded as concluded.
Appendix C. Change Log
[ The editor is asked to remove this section before publication. ]
o Changes since draft-ietf-core-echo-request-tag-08:
* Make amplification attack mitigation by Echo an RFC7252
updating recommendation
* Give some more concrete guidance to that use case in terms of
sizes and message types
* Allow short (1-3 byte) Echo values for deterministic cases,
with according security considerations
* Point out the tricky parts around Request-Tag for stateless
proxies, and make that purely an outlook example with out-of-
scope details
* Lift ban on Request-Tag options without Block1 (as they can
legitimately be generated by an unaware proxy)
* Suggest concrete numbers for the options
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
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* 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:
* Removed visible comment that should not be visible in Token
reuse considerations.
o Changes since draft-ietf-core-echo-request-tag-05:
* Add privacy considerations on cookie-style use of Echo values
* Add security considerations for token reuse
* Add note in security considerations on use of nonvolatile
memory when dealing with pseudorandom numbers
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* Appendix on echo generation: add a few words on up- and
downsides of the encrypted timestamp alternative
* Clarifications around Outer Echo:
+ Could be generated by the origin server to prove network
reachability (but for most applications it MUST be inner)
+ Could be generated by intermediaries
+ Is answered by the client to the endpoint from which it
received it (ie. Outer if received as Outer)
* Clarification that a server can send Echo preemtively
* Refer to stateless to explain what "more information than just
the sequence number" could be
* Remove explanations around 0.00 empty messags
* Rewordings:
+ the attack: from "forging" to "guessing"
+ "freshness tokens" to "freshness indicators" (to avoid
confusion with the Token)
* Editorial fixes:
+ Abstract and introduction mention what is updated in RFC7252
+ Reference updates
+ Capitalization, spelling, terms from other documents
o Changes since draft-ietf-core-echo-request-tag-04:
* Editorial fixes
+ Moved paragraph on collision-free encoding of data in the
Token to Security Considerations and rephrased it
+ "easiest" -> "one easy"
o Changes since draft-ietf-core-echo-request-tag-03:
* Mention Token processing changes in title
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* Abstract reworded
* Clarify updates to Token processing
* Describe security levels from Echo length
* Allow non-monotonic clocks under certain conditions for
freshness
* Simplify freshness expressions
* Describe when a Request-Tag can be set
* Add note on application-level freshness mechanisms
* Minor editorial changes
o Changes since draft-ietf-core-echo-request-tag-02:
* Define "freshness"
* Note limitations of "aliveness"
* Clarify proxy and OSCORE handling in presence of "echo"
* Clarify when Echo values may be reused
* Update security considerations
* Various minor clarifications
* Minor editorial changes
o Major changes since draft-ietf-core-echo-request-tag-01:
* Follow-up changes after the "relying on block-wise" change in
-01:
+ Simplify the description of Request-Tag and matchability
+ Do not update RFC7959 any more
* Make Request-Tag repeatable.
* Add rationale on not relying purely on sequence numbers.
o Major changes since draft-ietf-core-echo-request-tag-00:
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* Reworded the Echo section.
* Added rules for Token processing.
* Added security considerations.
* Added actual IANA section.
* Made Request-Tag optional and safe-to-forward, relying on
block-wise to treat it as part of the cache-key
* Dropped use case about OSCORE Outer-block-wise (the case went
away when its Partial IV was moved into the Object-Security
option)
o Major changes since draft-amsuess-core-repeat-request-tag-00:
* The option used for establishing freshness was renamed from
"Repeat" to "Echo" to reduce confusion about repeatable
options.
* The response code that goes with Echo was changed from 4.03 to
4.01 because the client needs to provide better credentials.
* The interaction between the new option and (cross) proxies is
now covered.
* Two messages being "Request-Tag matchable" was introduced to
replace the older concept of having a request tag value with
its slightly awkward equivalence definition.
Acknowledgments
The authors want to thank Carsten Bormann, Francesca Palombini, and
Jim Schaad for providing valuable input to the draft.
Authors' Addresses
Christian Amsuess
Email: christian@amsuess.com
John Preuss Mattsson
Ericsson AB
Email: john.mattsson@ericsson.com
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Goeran Selander
Ericsson AB
Email: goran.selander@ericsson.com
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