CoRE Working Group C. Amsuess
Internet-Draft
Updates: 7252 (if approved) J. Mattsson
Intended status: Standards Track G. Selander
Expires: November 7, 2019 Ericsson AB
May 06, 2019
CoAP: Echo, Request-Tag, and Token Processing
draft-ietf-core-echo-request-tag-05
Abstract
This document specifies enhancements to the Constrained Application
Protocol (CoAP) that mitigate security issues in particular use
cases. The Echo option enables a CoAP server to verify the freshness
of a request or to force a client to demonstrate reachability at its
claimed network address. The Request-Tag option allows the CoAP
server to match Block-Wise message fragments belonging to the same
request. The updated Token processing requirements for clients
ensure secure binding of responses to requests when CoAP is used with
security.
Status of This Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on November 7, 2019.
Copyright Notice
Copyright (c) 2019 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
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publication of this document. Please review these documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Request Freshness . . . . . . . . . . . . . . . . . . . . 3
1.2. Fragmented Message Body Integrity . . . . . . . . . . . . 4
1.3. Request-Response Binding . . . . . . . . . . . . . . . . 4
1.4. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. The Echo Option . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Option Format . . . . . . . . . . . . . . . . . . . . . . 6
2.2. Echo Processing . . . . . . . . . . . . . . . . . . . . . 7
2.3. Applications . . . . . . . . . . . . . . . . . . . . . . 10
3. The Request-Tag Option . . . . . . . . . . . . . . . . . . . 11
3.1. Option Format . . . . . . . . . . . . . . . . . . . . . . 11
3.2. Request-Tag Processing by Servers . . . . . . . . . . . . 12
3.3. Setting the Request-Tag . . . . . . . . . . . . . . . . . 13
3.4. Applications . . . . . . . . . . . . . . . . . . . . . . 14
3.4.1. Body Integrity Based on Payload Integrity . . . . . . 14
3.4.2. Multiple Concurrent Blockwise Operations . . . . . . 15
3.4.3. Simplified Block-Wise Handling for Constrained
Proxies . . . . . . . . . . . . . . . . . . . . . . . 16
3.5. Rationale for the Option Properties . . . . . . . . . . . 16
3.6. Rationale for Introducing the Option . . . . . . . . . . 16
4. Block2 / ETag Processing . . . . . . . . . . . . . . . . . . 17
5. Token Processing . . . . . . . . . . . . . . . . . . . . . . 17
6. Security Considerations . . . . . . . . . . . . . . . . . . . 17
7. Privacy Considerations . . . . . . . . . . . . . . . . . . . 18
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 19
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 19
9.1. Normative References . . . . . . . . . . . . . . . . . . 19
9.2. Informative References . . . . . . . . . . . . . . . . . 19
Appendix A. Methods for Generating Echo Option Values . . . . . 21
Appendix B. Request-Tag Message Size Impact . . . . . . . . . . 22
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 22
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 24
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
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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
blockwise operations. These options in themselves do not replace the
need for a security protocol; they specify the format and processing
of data which, when integrity protected using e.g. DTLS ([RFC6347]),
TLS ([RFC8446]), or OSCORE ([I-D.ietf-core-object-security]), provide
the additional security features.
The document also updates the Token processing requirements for
clients specified in [RFC7252]. The updated processing ensures
secure 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.
1.1. Request Freshness
A CoAP server receiving a request is in general not able to verify
when the request was sent by the CoAP client. This remains true even
if the request was protected with a security protocol, such as DTLS.
This makes CoAP requests vulnerable to certain delay attacks which
are particularly 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
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state between CoAP client and server, verify 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
tokens 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 tokens 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 blockwise functionality does
not support the detection of interchanged blocks between different
message bodies to the same resource having the same block number.
This remains true even when CoAP is used together with a security
protocol such as DTLS or OSCORE, within the replay window
([I-D.mattsson-core-coap-actuators]), which is a vulnerability of
CoAP when using RFC7959.
A straightforward mitigation of mixing up blocks from different
messages is to use unique identifiers for different message bodies,
which would provide equivalent protection to the case where the
complete body fits into a single payload. The ETag option [RFC7252],
set by the CoAP server, identifies a response body fragmented using
the Block2 option. This document defines the Request-Tag option for
identifying 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
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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
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
from which a request originates; to distinguish from clients in
proxies.
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The terms "payload" and "body" of a message are used as in [RFC7959].
The complete interchange of a request and a response body is called a
(REST) "operation". An operation fragmented using [RFC7959] is
called a "blockwise operation". A blockwise operation which is
fragmenting the request body is called a "blockwise request
operation". A blockwise operation which is fragmenting the response
body is called a "blockwise response operation".
Two 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 blockwise 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 blockwise request operations are impossible with the
options of [RFC7959] because the second operation's block overwrites
any state of the first exchange.).
The Echo and Request-Tag options are defined in this document.
2. The Echo Option
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 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 tokens that works for all CoAP methods and
response codes.
2.1. 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]).
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+-----+---+---+---+---+-------------+--------+------+---------+---+---+
| No. | C | U | N | R | Name | Format | Len. | Default | E | U |
+-----+---+---+---+---+-------------+--------+------+---------+---+---+
| TBD | | | x | | Echo | opaque | 4-40 | (none) | x | x |
+-----+---+---+---+---+-------------+--------+------+---------+---+---+
C = Critical, U = Unsafe, N = NoCacheKey, R = Repeatable,
E = Encrypt and Integrity Protect (when using OSCORE)
Figure 1: Echo Option Summary
[ Note to RFC editor: If this document is released before core-
object-security, then the following paragraph and the "E"/"U" columns
above need to move into core-object-security, as they are defined in
that draft. ]
The Echo option value is generated by a server, and its content and
structure are implementation specific. Different methods for
generating Echo option values are outlined in Appendix A. Clients
and intermediaries MUST treat an Echo option value as opaque and make
no assumptions about its content or structure.
When receiving an Echo option in a request, the server MUST be able
to verify when the Echo option value was generated. This implies
that the server MUST be able to verify that the Echo option value was
generated by the server or some other party that the server trusts.
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
[I-D.ietf-core-object-security] it MAY be an Inner or Outer option,
and the Inner and Outer values are independent. 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.
2.2. Echo Processing
The Echo option MAY be included in any request or response (see
Section 2.3 for different applications), but the Echo option MUST NOT
be used with empty CoAP requests (i.e., Code=0.00).
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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 responses and to different clients.
The server may use request freshness provided by the Echo option to
verify the aliveness of a client or to synchronize state. The server
may also include the Echo option in a response to force a client to
demonstrate reachability at its claimed network address.
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.
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
enough. If the server cannot verify that the request is fresh
enough, 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 illustrated 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 Echo Option Message Flow
When used to serve freshness requirements (including client aliveness
and state synchronizing), CoAP messages containing the Echo option
MUST be integrity protected between the intended endpoints, e.g.
using DTLS, TLS, or an OSCORE Inner option
([I-D.ietf-core-object-security]). 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 respond to requests with 4.01 with an Echo
option to ensure the client's reachability at its claimed address,
and 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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2.3. Applications
1. Actuation requests often require freshness guarantees to avoid
accidental or malicious delayed actuator actions. In general,
all non-safe methods (e.g. POST, PUT, DELETE) may require
freshness guarantees for secure operation.
* 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 in response to a request containing the Echo option.
The client then uses the Echo option with the new value in the
next actuation request, and the server compares the receive
time accordingly.
2. A server may use the Echo option to synchronize state or time
with a requesting client. A server MUST NOT synchronize state or
time with clients which are not the authority of the property
being synchronized. E.g. if access to a server resource is
dependent on time, then the client MUST NOT set the time of the
server.
* 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 [I-D.ietf-core-object-security].
* 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 expect by
responding server.
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
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address in the source address of a CoAP request). For this
purpose, a server MAY ask a client to Echo its request to verify
its source address. This needs to be done only once per peer and
limits the range of potential victims from the general Internet
to endpoints that have been previously in contact with the
server. For this application, the Echo option can be used in
messages that are not integrity protected, for example during
discovery.
* 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 MAY use Echo to verify origin reachability
as described in Section 2.2. The proxy MAY forward idempotent
requests immediately to have a cached result available when
the client's Echoed request arrives.
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. The Request-Tag Option
The Request-Tag is intended for use as a short-lived identifier for
keeping apart distinct blockwise request operations on one resource
from one client, addressing the issue described in Section 1.2. It
enables the receiving server to reliably assemble request payloads
(blocks) to their message bodies, and, if it chooses to support it,
to reliably process simultaneous blockwise request operations on a
single resource. The requests must be integrity protected in order
to protect against interchange of blocks between different message
bodies.
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.
3.1. Option Format
The Request-Tag option is not critical, is safe to forward,
repeatable, and part of the cache key, see Figure 3, which extends
Table 4 of [RFC7252]).
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+-----+---+---+---+---+-------------+--------+------+---------+---+---+
| No. | C | U | N | R | Name | Format | Len. | Default | E | U |
+-----+---+---+---+---+-------------+--------+------+---------+---+---+
| TBD | | | | 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 3: Request-Tag Option Summary
[ Note to RFC editor: If this document is released before core-
object-security, then the following paragraph and the "E"/"U" columns
above need to move into core-object-security, as they are defined in
that draft. ]
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
[I-D.ietf-core-object-security]): 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
blockwise 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.
3.2. 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 blockwise 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.
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To keep utilizing the cache, a server (including proxies) MAY discard
the Request-Tag option from an assembled block-wise request when
consulting its cache, as the option relates to the operation-on-the-
wire and not its semantics. For example, a FETCH request with the
same body as an older one can be served from the cache if the older's
Max-Age has not expired yet, even if the second operation uses a
Request-Tag and the first did not. (This is similar to the situation
about ETag in that it is formally part of the cache key, but
implementations that are aware of its meaning can cache more
efficiently, see [RFC7252] Section 5.4.2).
A server receiving a Request-Tag MUST treat it as opaque and make no
assumptions about its content or structure.
Two messages carrying the same Request-Tag is a necessary but not
sufficient condition for being part of the same operation. They can
still be treated as independent messages by the server (e.g. when it
sends 2.01/2.04 responses for every block), or initiate a new
operation (overwriting kept context) when the later message carries
Block1 number 0.
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.
3.3. Setting the Request-Tag
For each separate blockwise request operation, the client can choose
a Request-Tag value, or choose not to set a Request-Tag. 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 MUST NOT recycle a request tag unless the first operation has
concluded. What constitutes a concluded operation depends on the
application, and is outlined individually in Section 3.4.
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
a Block2 option even if those messages are not part of a blockwise
request operation (this is to allow the operation described in
Section 3.4.3). The Request-Tag option MUST NOT be present in
response messages, and MUST NOT be present if neither the Block1 nor
the Block2 option is present.
3.4. Applications
3.4.1. 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 blockwise request operation as
concluded unless all of the messages the client previously sent in
the operation have been confirmed by the message integrity
protection mechanism, or are considered invalid by the server if
replayed.
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Typically, in OSCORE, these confirmations can result either from
the client receiving an OSCORE response message matching the
request (an empty ACK is insufficient), or because the message's
sequence number is old enough to be outside the server's receive
window.
In DTLS, this can only be confirmed if the request message was not
retransmitted, and was responded to.
Authors of other documents (e.g. [I-D.ietf-core-object-security])
are invited to mandate this behavior for clients that execute
blockwise interactions over secured transports. In this way, the
server can rely on a conforming client to set the Request-Tag option
when required, and thereby conclude on the integrity of the assembled
body.
Note that this mechanism is implicitly implemented when the security
layer guarantees ordered delivery (e.g. CoAP over TLS [RFC8323]).
This is because with each message, any earlier 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.
3.4.2. Multiple Concurrent Blockwise Operations
CoAP clients, especially CoAP proxies, may initiate a blockwise
request operation to a resource, to which a previous one is already
in progress, 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 blockwise 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 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.
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3.4.3. 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.
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
requesting endpoint to all request carrying any Block option, it does
not need to keep track of any further block state.
This is particularly useful to proxies that strive for stateless
operation as described in [I-D.hartke-core-stateless] Section 3.1.
3.5. 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 3.4.3 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].
3.6. 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 3.4.1) 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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4. Block2 / ETag Processing
The same security properties as in Section 3.4.1 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 3.4.1, a server MUST use the
Block2 option in conjunction with the ETag option ([RFC7252],
Section 5.10.6), and MUST NOT use the same ETag value for different
representations of a resource.
5. Token Processing
As described in Section 1.3, the client must be able to verify that a
response corresponds to a particular request. This section updates
the CoAP Token processing requirements for clients. 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.
6. Security Considerations
The availability of a secure pseudorandom number generator and truly
random seeds are essential for the security of the Echo option. If
no true random number generator is available, a truly random seed
must be provided from an external source.
A single active Echo value with 64 (pseudo-)random bits gives the
same theoretical security level against forgeries as a 64-bit MAC (as
used in e.g. AES_128_CCM_8). In practice, forgery of an Echo option
value is much harder as an attacker must also forge the MAC in the
security protocol. The Echo option value MUST contain 32
(pseudo-)random bits that are not predictable for any other party
than the server, and SHOULD contain 64 (pseudo-)random bits. A
server MAY use different security levels for different uses cases
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(client aliveness, request freshness, state synchronization, network
address reachability, etc.).
The security provided by the Echo and Request-Tag options depends on
the security protocol used. CoAP and HTTP proxies require (D)TLS to
be terminated at the proxies. The proxies are therefore able to
manipulate, inject, delete, or reorder options or packets. The
security claims in such architectures only hold under the assumption
that all intermediaries are fully trusted and have not been
compromised.
Servers 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.
Servers SHOULD NOT use wall clock time for timestamps, as wall clock
time have large deviations from a monotonic clock. Furthermore, an
attacker may be able to affect the server's wall clock time in
various ways such as setting up a fake NTP server or broadcasting
false time signals to radio-controlled clocks.
Servers 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.
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).
7. 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
reboot. The use of wall clock time is not allowed (see Section 6)
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and there also privacy reasons, e.g. it may reveal 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.
8. IANA Considerations
This document adds the following option numbers to the "CoAP Option
Numbers" registry defined by [RFC7252]:
+--------+-------------+-------------------+
| Number | Name | Reference |
+--------+-------------+-------------------+
| TBD1 | Echo | [[this document]] |
| | | |
| TBD2 | Request-Tag | [[this document]] |
+--------+-------------+-------------------+
Figure 4: CoAP Option Numbers
9. References
9.1. 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>.
9.2. Informative References
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[I-D.hartke-core-stateless]
Hartke, K., "Extended Tokens and Stateless Clients in the
Constrained Application Protocol (CoAP)", draft-hartke-
core-stateless-02 (work in progress), October 2018.
[I-D.ietf-core-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", draft-ietf-core-object-security-16 (work in
progress), March 2019.
[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-04 (work in progress),
March 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>.
[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>.
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Appendix A. Methods for Generating Echo Option Values
The content and structure of the Echo option value are implementation
specific and determined by the server. Two simple mechanisms 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 forged echo values is given by s = bit length of r -
log2(n). The length of r and the maximum allowed n should be set so
that the security level is harmonized with other parts of the
deployment, e.g., s >= 64. If the server loses time continuity, e.g.
due to reboot, the entries in the old list MUST be deleted.
Echo option value: random value r
Server State: random value r, timestamp t0
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 forged 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 7 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
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Other mechanisms complying with the security and privacy
considerations may be used. The use of encrypted timestamps in the
Echo option 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 3.4.1) incurs no overhead if no messages are
lost (more precisely: in OSCORE, if no operations are aborted due to
repeated transmission failure; in DTLS, if no packages are lost), or
when blockwise request operations happen rarely (in OSCORE, if there
is always only one request blockwise operation in the replay window).
In those situations, no message has any Request-Tag option set, and
that can be recycled indefinitely.
When the absence of a Request-Tag option 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-04:
* Editorial fixes
+ Moved paragraph on collision-free encoding of data in the
token to Security Considerations and rephrased it
+ "easiest" -> "one easy"
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o Changes since draft-ietf-core-echo-request-tag-03:
* Mention token processing changes in title
* Abstract reworded
* Clarify updates to token processing
* Describe security levels from Echo length
* Allow non-monotonic clocks under certain conditions for
freshness
* Simplify freshness expressions
* Describe when a Request-Tag can be set
* Add note on application-level freshness mechanisms
* Minor editorial changes
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 blockwise" change in
-01:
+ Simplify the description of Request-Tag and matchability
+ Do not update RFC7959 any more
* Make Request-Tag repeatable.
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* Add rationale on not relying purely on sequence numbers.
o Major changes since draft-ietf-core-echo-request-tag-00:
* Reworded the Echo section.
* Added rules for Token processing.
* Added security considerations.
* Added actual IANA section.
* Made Request-Tag optional and safe-to-forward, relying on
blockwise to treat it as part of the cache-key
* Dropped use case about OSCORE outer-blockwise (the case went
away when its Partial IV was moved into the Object-Security
option)
o Major changes since draft-amsuess-core-repeat-request-tag-00:
* The option used for establishing freshness was renamed from
"Repeat" to "Echo" to reduce confusion about repeatable
options.
* The response code that goes with Echo was changed from 4.03 to
4.01 because the client needs to provide better credentials.
* The interaction between the new option and (cross) proxies is
now covered.
* Two messages being "Request-Tag matchable" was introduced to
replace the older concept of having a request tag value with
its slightly awkward equivalence definition.
Acknowledgments
The authors want to thank Jim Schaad and Carsten Bormann for
providing valuable input to the draft.
Authors' Addresses
Christian Amsuess
Email: christian@amsuess.com
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John Mattsson
Ericsson AB
Email: john.mattsson@ericsson.com
Goeran Selander
Ericsson AB
Email: goran.selander@ericsson.com
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