wdiff rfc9175v5.txt rfc9175.txt

Internet Engineering Task Force (IETF) C. Amsüss
Request for Comments: 9175
Updates: 7252 J. Preuß Mattsson
Category: Standards Track G. Selander
ISSN: 2070-1721 Ericsson AB
January
February 2022

Constrained Application Protocol (CoAP): Echo, Request-Tag, and Token
Processing

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. This document updates RFC 7252 with respect to the
following: processing requirements for client Tokens, forbidding non-
secure reuse of Tokens to ensure response-to-request binding when
CoAP is used with a security protocol, and amplification mitigation
(where the use of the Echo option is now recommended).

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.

Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9175.

This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(https://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Revised BSD License text as described in Section 4.e of the
Trust Legal Provisions and are provided without warranty as described
in the Revised BSD License.

Table of Contents

1. Introduction
1.1. Terminology
2. Request Freshness and the Echo Option
2.1. Request Freshness
2.2. The Echo Option
2.2.1. Echo Option Format
2.3. Echo Processing
2.4. Applications of the Echo Option
2.5. Characterization of Echo Applications
2.5.1. Time-Based versus Event-Based Freshness
2.5.2. Authority over Used Information
2.5.3. Protection by a Security Protocol
2.6. Updated Amplification Mitigation Requirements for Servers
3. Protecting Message Bodies Using Request Tags
3.1. Fragmented Message Body Integrity
3.2. The Request-Tag Option
3.2.1. Request-Tag Option Format
3.3. Request-Tag Processing by Servers
3.4. Setting the Request-Tag
3.5. Applications of the Request-Tag Option
3.5.1. Body Integrity Based on Payload Integrity
3.5.2. Multiple Concurrent Block-Wise Operations
3.5.3. Simplified Block-Wise Handling for Constrained Proxies
3.6. Rationale for the Option Properties
3.7. Rationale for Introducing the Option
3.8. Block2 and ETag Processing
4. Token Processing for Secure Request-Response Binding
4.1. Request-Response Binding
4.2. Updated Token Processing Requirements for Clients
5. Security Considerations
5.1. Token Reuse
6. Privacy Considerations
7. IANA Considerations
8. References
8.1. Normative References
8.2. Informative References
Appendix A. Methods for Generating Echo Option Values
Appendix B. Request-Tag Message Size Impact
Acknowledgements
Authors' Addresses

1. Introduction

The initial suite of specifications for the Constrained Application
Protocol (CoAP) ([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 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, which can be used to synchronize state,
or to 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 that, when integrity
protected using, e.g., DTLS [RFC6347], TLS [RFC8446], or Object
Security for Constrained RESTful Environments (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.

Like [RFC7252], this document relies on the Representational State
Transfer [REST] architecture of the Web.

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

A message's "freshness" is a measure of when a message was sent on a
timescale of the recipient. A server that receives a request can
either verify that the request is fresh or determine that it cannot
be verified that the request is fresh. What is considered a fresh
message is application dependent; exemplary uses are "no more than 42
seconds ago" or "after this server's last reboot".

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 that is
fragmenting the request body is called a "block-wise request
operation". A block-wise operation that 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 have the same set of
options, with the exception that elective NoCacheKey options and
options involved in block-wise transfer (Block1, Block2, and Request-
Tag) need not be the same. Two blockwise request operations are said
to be matchable if their request messages are matchable.

Two matchable block-wise request 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 that are
particularly perilous in the case of actuators [COAP-ATTACKS]. 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 the server and the
proxy do not say anything about when the client made the request. In
a general hop-by-hop setting, freshness may need to be verified in
each hop.

A straightforward mitigation of potential delayed requests is that
the CoAP server rejects a request the first time it appears and asks
the CoAP client to prove that it intended to make the request at this
point in time.

2.2. The Echo Option

This document defines the Echo option, a 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.

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 a CoAP
client and server, cryptographically verifying the aliveness of the
client or forcing 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 Table 1, which extends Table 4 of
[RFC7252]).

+=====+===+===+===+===+======+========+========+=========+
| No. | C | U | N | R | Name | Format | Length | Default |
+=====+===+===+===+===+======+========+========+=========+
| 252 | | | x | | Echo | opaque | 1-40 | (none) |
+-----+---+---+---+---+------+--------+--------+---------+

Table 1: Echo Option Summary

C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable

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 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 than 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. As always with OSCORE, Outer options are
visible to (and may be acted on by) all proxies and are visible on
all links where no additional encryption (like TLS between client and
proxy) is used.

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
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 or discard it at any time (especially to avoid tracking;
see Section 6).

A client MUST only send Echo option values to endpoints it received
them from (where, as defined in Section 1.2 of [RFC7252], the
security association is part of the endpoint). In OSCORE processing,
that means sending Echo option values from Outer options (or from
non-OSCORE responses) back in Outer options and sending 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 to bind the Echo option
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 over DTLS is shown Figure 1.

Figure 1: Example Message Flow for Time-Based Freshness Using the
'Integrity-Protected Timestamp' Construction of Appendix A

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 or chosen in a way so the probability for
collisions is negligible. 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 over DTLS is shown in Figure 2.

Figure 2: Example Message Flow for Event-Based Freshness Using
the 'Persistent Counter' Construction of Appendix A

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 be
a Message Authentication Code (MAC) of the claimed address but MAY be
unprotected. Combining different Echo applications can necessitate
different choices; see Appendix A, item 2 for an example.

An Echo option MAY be sent with a successful response, i.e., even
though the request satisfied any freshness requirements on the
operation. This is called a "preemptive" Echo option value and is
useful when the server anticipates that the client will need to
demonstrate freshness relative to the current response in the near
future.

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. When it receives an Echo option in a response, it MAY
forward it to the client (and, not recognizing it as its own in
future requests, relay it in the other direction as well) or process
it on its own. If it does so, it MUST ensure that the client's
request was generated (or is regenerated) after the Echo option value
used to send to the server was first seen. (In most cases, this
means that the proxy needs to ask the client to repeat the request
with a new Echo option value.)

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 MUST respond to Echo challenges itself if the proxy knows
from the recent establishing of the connection that the HTTP request
is fresh. Otherwise, it MUST NOT repeat an unsafe request and SHOULD
respond with a 503 (Service Unavailable) with a Retry-After value of
0 seconds and terminate any underlying Keep-Alive connection. If the
HTTP request arrived in early data, the proxy SHOULD use a 425 (Too
Early) response instead (see [RFC8470]). The proxy MAY also use
other mechanisms to establish freshness of the HTTP request that are
not specified here.

2.4. Applications of the Echo Option

Unless otherwise noted, all these applications require a security
protocol to be used and the Echo option to be protected by it.

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

* The same Echo option value may be used for multiple actuation
requests to the same server, as long as the total 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 send preemptive Echo option values in
successful requests, irrespectively of whether or not the
request contained an 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 properties (such
as state or time) with a requesting client. A server MUST NOT
synchronize a property with a client that is not the authority of
the property being synchronized. For example, if access to a
server resource is dependent on time, then the server MUST NOT
synchronize time with a client requesting access unless the
client is a time authority for the server.

Note that the state to be synchronized is not carried inside the
Echo option. Any explicit state information needs to be carried
along in the messages the Echo option value is sent in; the Echo
mechanism only provides a partial order on the messages'
processing.

* 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, as
specified in Section 7.5 of [RFC8613].

* A device joining a CoAP group communication [GROUP-COAP]
protected with OSCORE [GROUP-OSCORE] may be required to
initially synchronize its replay window state 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 value in a subsequent unicast
request to the responding server.

3. An attacker can perform a denial-of-service attack by putting a
victim's address in the source address of a CoAP request and
sending the request to a resource with a large amplification
factor. The amplification factor is the ratio between the size
of the request and the total size of the response(s) to that
request. A server that provides a large amplification factor to
an unauthenticated peer SHOULD mitigate amplification attacks, as
described in Section 11.3 of [RFC7252]. One way to mitigate such
attacks is for the server to respond to the alleged source
address of the request with an Echo option in a short response
message (e.g., 4.01 (Unauthorized)), thereby requesting the
client to verify its source address. This needs to be done only
once per endpoint 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. (This is formally recommended in
Section 2.6.)

* In the presence of a proxy, a server will not be able to
distinguish different origin client endpoints, i.e., the
client from which a request originates. Following from the
recommendation above, a proxy that provides a large
amplification factor to unauthenticated peers SHOULD mitigate
amplification attacks. The proxy SHOULD use the Echo option
to verify origin reachability, as described in Section 2.3.
The proxy MAY forward safe requests immediately to have a
cached result available when the client's repeated request
arrives.

* Amplification mitigation is a trade-off between giving
leverage to an attacker and causing overhead. An
amplification factor of 3 (i.e., don't send more than three
times the number of bytes received until the peer's address is
confirmed) is considered acceptable for unconstrained
applications in [RFC9000], Section 8.

When that limit is applied and no further context is
available, a safe default is sending initial responses no
larger than 136 bytes in CoAP serialization. (The number is
assuming Ethernet, IP, and UDP headers of 14, 40, and 8 bytes,
respectively, with 4 bytes added for the CoAP header. Triple
that minus the non-CoAP headers gives the 136 bytes.) Given
the token also takes up space in the request, responding with
132 bytes after the token is safe as well.

* 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 option 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.

2.5. Characterization of Echo Applications

Use cases for the Echo option can be characterized by several
criteria that help determine the required properties of the Echo
option value. These criteria apply both to those listed in
Section 2.4 and any novel applications. They provide rationale for
the statements in the former and guidance for the latter.

2.5.1. Time-Based versus Event-Based Freshness

The property a client demonstrates by sending an Echo option value is
that the request was sent after a certain point in time or after some
event happened on the server.

When events are counted, they form something that can be used as a
monotonic but very non-uniform time line. With highly regular events
and low-resolution time, the distinction between time-based and
event-based freshness can be blurred: "no longer than a month ago" is
similar to "since the last full moon".

In an extreme form of event-based freshness, the server can place an
event whenever an Echo option value is used. This makes the Echo
option value effectively single use.

Event-based and time-based freshness can be combined in a single Echo
option value, e.g., by encrypting a timestamp with a key that changes
with every event to obtain semantics in the style of "usable once but
only for 5 minutes".

2.5.2. Authority over Used Information

Information conveyed to the server in the request Echo option value
has different authority depending on the application. Understanding
who or what is the authoritative source of that information helps the
server implementor decide the necessary protection of the Echo option
value.

If all that is conveyed to the server is information that the client
is authorized to provide arbitrarily (which is another way of saying
that the server has to trust the client on whatever the Echo option
is being used for), then the server can issue Echo option values that
do not need to be protected on their own. They still need to be
covered by the security protocol that covers the rest of the message,
but the Echo option value can be just short enough to be unique
between this server and client.

For example, the client's OSCORE Sender Sequence Number (as used in
[RFC8613], Appendix B.1.2) is such information.

In most other cases, there is information conveyed for which the
server is the authority ("the request must not be older than five
minutes" is counted on the server's clock, not the client's) or which
even involve the network (as when performing amplification
mitigation). In these cases, the Echo option value itself needs to
be protected against forgery by the client, e.g., by using a
sufficiently large, random value or a MAC, as described in
Appendix A, items 1 and 2.

For some applications, the server may be able to trust the client to
also act as the authority (e.g., when using time-based freshness
purely to mitigate request delay attacks); these need careful case-
by-case evaluation.

To issue Echo option values without integrity protection of its own,
the server needs to trust the client to never produce requests with
attacker-controlled Echo option values. The provisions of
Section 2.3 (saying that an Echo option value may only be sent as
received from the same server) allow that. The requirement stated
there for the client to treat the Echo option value as opaque holds
for these applications like for all others.

When the client is the sole authority over the synchronized property,
the server can still use time or events to issue new Echo option
values. Then, the request's Echo option value not so much proves the
indicated freshness to the server but reflects the client's intention
to indicate reception of responses containing that value when sending
the later ones.

Note that a single Echo option value can be used for multiple
purposes (e.g., to both get the sequence number information and
perform amplification mitigation). In this case, the stricter
protection requirements apply.

2.5.3. Protection by a Security Protocol

For meaningful results, the Echo option needs to be used in
combination with a security protocol in almost all applications.

When the information extracted by the server is only about a part of
the system outside of any security protocol, then the Echo option can
also be used without a security protocol (in case of OSCORE, as an
Outer option).

The only known application satisfying this requirement is network
address reachability, where unprotected Echo option values are used
both by servers (e.g., during setup of a security context) and
proxies (which do not necessarily have a security association with
their clients) for amplification mitigation.

2.6. Updated Amplification Mitigation Requirements for Servers

This section updates the amplification mitigation requirements for
servers in [RFC7252] to recommend the use of the Echo option to
mitigate amplification attacks. The requirements for clients are not
updated. Section 11.3 of [RFC7252] is updated by adding the
following text:

| A CoAP server SHOULD mitigate potential amplification attacks by
| responding to unauthenticated clients with 4.01 (Unauthorized)
| including an Echo option, as described in item 3 in Section 2.4 of
| RFC 9175.

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 includes a lightweight reliability feature to handle messages
that 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.

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
[COAP-ATTACKS], which is a vulnerability of the block-wise
functionality of CoAP [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 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 mainly 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 elective, safe to forward, repeatable, and
part of the cache key (see Table 2, which extends Table 4 of
[RFC7252]).

+=====+===+===+===+===+=============+========+========+=========+
| No. | C | U | N | R | Name | Format | Length | Default |
+=====+===+===+===+===+=============+========+========+=========+
| 292 | | | | x | Request-Tag | opaque | 0-8 | (none) |
+-----+---+---+---+---+-------------+--------+--------+---------+

Table 2: Request-Tag Option Summary

C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable

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 operations, respectively. The Inner and Outer
values are therefore independent of each other. The Inner option is
encrypted and integrity protected between the client and server, and
it 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.
However, 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 Block1, Block2, and all operations that are elective
NoCacheKey). Thus, the server cannot 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
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 (Created) and 2.04 (Changed) 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 the 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; 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 the purpose and is defined accordingly; see examples in
Section 3.5.

When Block1 and Block2 are combined in an operation, the Request-Tag
of the Block1 phase is set in the Block2 phase as well; otherwise,
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 cannot be recycled.

Note that Request-Tag options can be present in request messages that
carry no Block options (for example, because a proxy unaware of
Request-Tag reassembled them).

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 an attacker to maliciously replace a later
operation's blocks with an earlier operation's blocks (see
Section 2.5 of [COAP-ATTACKS]). 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:

* The individual exchanges MUST be integrity protected end to end
between the client and server.

* 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 matchable if they
were sent to any endpoint address using the new operation's
security context.

* The client MUST NOT regard a block-wise request operation as
concluded unless all of the messages the client has sent in the
operation would be regarded as invalid by the server if they were
replayed.

When security services are provided by OSCORE, these confirmations
typically result either from the client receiving an OSCORE
response message matching the request (an empty Acknowledgement
(ACK) is insufficient) or because the message's sequence number is
old enough to be outside the server's receive window.

When security services are provided by DTLS, this can only be
confirmed if there was no CoAP retransmission of the request, the
request was responded to, and the server uses replay protection.

Authors of other documents (e.g., applications of [RFC8613]) are
invited to mandate this subsection's 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 have confidence in 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 cannot be
replayed any more, so the client never needs to set the Request-Tag
option unless it wants to perform concurrent operations.

Body integrity only makes sense in applications that have stateful
block-wise transfers. On applications where all the state is in the
application (e.g., because rather than POSTing a large representation
to a collection in a stateful block-wise transfer, a collection item
is created first, then written to once and available when written
completely), clients need not concern themselves with body integrity
and thus the Request-Tag.

Body integrity is largely independent from replay protection. When
no replay protection is available (it is optional in DTLS), a full
block-wise operation may be replayed, but, by adhering to the above,
no operations will be mixed up. The only link between body integrity
and replay protection is that, without replay protection, recycling
is not possible.

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 the last paragraph of Section 2.4 of [RFC7959].

When initializing a new block-wise operation, a client has to look at
other active operations:

* 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.

* 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.

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
operations, as described in [RFC8974], 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 is 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 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
several cascaded stateless proxies to each put in an 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 Section 2.5.2 of [COAP-ATTACKS].

3.8. Block2 and ETag Processing

The same security properties as in Section 3.5.1 can be obtained for
block-wise response operations. The threat model here does not
depend on an attacker; a client can construct a wrong representation
by assembling it from blocks from different resource states. That
can happen when a resource is modified during a transfer or when some
blocks are still valid 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 protection equivalent to that described in Section 3.5.1, a
server MUST use the Block2 option in conjunction with the ETag Option 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 a response for a completely different resource (e.g., see
Section 2.3 of [COAP-ATTACKS]). 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 HTTP/1.1, 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 (e.g., see [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 the REST request and corresponding response in case of
"separate response" (see Section 2.2 of [RFC7252]).

CoAP [RFC7252] does not treat the 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. The following
section formalizes that.

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 freshness assertion of the Echo option comes from the client
reproducing the same value of the Echo option in a request as it
received in a previous response. If the Echo option value is a large
random number, then there is a high probability that the request is
generated after having seen the response. If the Echo option value
of the response can be guessed, e.g., if based on a small random
number or a counter (see Appendix A), then it is possible to compose
a request with the right Echo option value ahead of time. Using
guessable Echo option values is only permissible in a narrow set of
cases described in Section 2.5.2. Echo option values MUST be set by
the CoAP server such that the risk associated with unintended reuse
can be managed.

If uniqueness of the Echo option value is based on randomness, then
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. As each pseudorandom
number must only be used once, an implementation needs to get a new
truly random seed after reboot or continuously store the state in
nonvolatile memory. See [RFC8613], Appendix B.1.1 for issues and
approaches for writing to nonvolatile memory.

A single active Echo option 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). If a random unique Echo option value is
intended, the Echo option value SHOULD contain 64 (pseudo)random bits
that are not predictable for any other party than the server. A
server MAY use different security levels for different use cases
(client aliveness, request freshness, state synchronization, network
address reachability, etc.).

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

Echo option values without the protection of randomness or a MAC are
limited to cases when the client is the trusted source of all derived
properties (as per Section 2.5.2). Using them needs per-application
consideration of both the impact of a malicious client and of
implementation errors in clients. These Echo option values are the
only legitimate case for Echo option values shorter than four bytes,
which are not necessarily secret. They MUST NOT be used unless the
Echo option values in the request are integrity protected, as per
Section 2.3.

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.

For the purpose of generating timestamps for the Echo option, a
server MAY set a timer at reboot and use the time since reboot,
choosing the granularity such that different requests arrive at
different times. Servers MAY intermittently reset the timer and MAY
generate a random offset applied to all timestamps. When resetting
the timer, the server MUST reject all Echo option values that were
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 the 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. The server may
process requests in any order and send multiple responses to the same
request. An attacker may block, delay, and reorder messages. The
use of a sequence number is therefore recommended when CoAP is used
with a security protocol that does not provide bindings between
requests and responses, such as DTLS or TLS.

For a generic response to a Confirmable request over DTLS, binding
can only be claimed without out-of-band knowledge if:

* the original request was never retransmitted and

* the response was piggybacked in an Acknowledgement message (as a
Confirmable or Non-confirmable response may have been transmitted
multiple times).

If observation was used, the same holds for the registration, all
reregistrations, 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:

* In CoAP over TLS, retransmissions are not handled by the CoAP
layer and behave like a replay window size of 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.

* 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 Tokens unsuitable
for reuse.

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., the serialized
state, as in [RFC8974].

6. Privacy Considerations

Implementations SHOULD NOT put any privacy-sensitive information in
the Echo or Request-Tag option values. Unencrypted timestamps could
reveal information about the server, such as location, time since
reboot, or that the server will accept expired certificates.
Timestamps MAY be used if the Echo option 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 that identifies a client across requests. This is
especially true for preemptive Echo option values. Servers MUST NOT
use the Echo option to correlate requests for other purposes than
freshness and reachability. Clients only send Echo option values to
the same server from which the values 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. Especially
when the Echo option increases a server's ability to correlate
requests, clients MAY discard all preemptive Echo option values.

Publicly visible generated identifiers, even when opaque (as all
defined in this document are), can leak information as described in
[NUMERIC-IDS]. To avoid the effects described there, the absent
Request-Tag option should be recycled as much as possible. (That is
generally possible as long as a security mechanism is in place --
even in the case of OSCORE outer block-wise transfers, as the OSCORE
option's variation ensures that no matchable requests are created by
different clients.) When an unprotected Echo option is used to
demonstrate reachability, the recommended mechanism of Section 2.3
keeps the effects to a minimum.

7. IANA Considerations

IANA has added the following option numbers to the "CoAP Option
Numbers" registry defined by [RFC7252]:

+========+=============+===========+
| Number | Name | Reference |
+========+=============+===========+
| 252 | Echo | RFC 9175 |
+--------+-------------+-----------+
| 292 | Request-Tag | RFC 9175 |
+--------+-------------+-----------+

Table 3: Additions to CoAP
Option Numbers Registry

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>.

[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>.

[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>.

[RFC8470] Thomson, M., Nottingham, M., and W. Tarreau, "Using Early
Data in HTTP", RFC 8470, DOI 10.17487/RFC8470, September
2018, <https://www.rfc-editor.org/info/rfc8470>.

[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>.

8.2. Informative References

[COAP-ATTACKS]
Preuß Mattsson, J., Fornehed, J., Selander, G., Palombini,
F., and C. Amsüss, "CoAP Attacks", "Attacks on the Constrained Application
Protocol (CoAP)", Work in Progress, Internet-Draft, draft-
mattsson-core-coap-attacks-01, 27 July 2021, <https://datatracker.ietf.org/doc/html/draft-
mattsson-core-coap-attacks-01>.
<https://datatracker.ietf.org/doc/html/draft-mattsson-
core-coap-attacks-01>.

[GROUP-COAP]
Dijk, E., Wang, C., and M. Tiloca, "Group Communication
for the Constrained Application Protocol (CoAP)", Work in
Progress, Internet-Draft, draft-ietf-core-groupcomm-bis-
05, 25 October 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
groupcomm-bis-05>.

[GROUP-OSCORE]
Tiloca, M., Selander, G., Palombini, F., Preuß Mattsson,
J., and J. Park, "Group OSCORE - Secure Group
Communication for CoAP", Work in Progress, Internet-Draft,
draft-ietf-core-oscore-groupcomm-13, 25 October 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-core-
oscore-groupcomm-13>.

[NUMERIC-IDS]
Gont, F. and I. Arce, "On the Generation of Transient
Numeric Identifiers", Work in Progress, Internet-Draft,
draft-irtf-pearg-numeric-ids-generation-08, 31 January
2022, <https://datatracker.ietf.org/doc/html/draft-irtf-
pearg-numeric-ids-generation-08>.

[REST] Fielding, R., "Architectural Styles and the Design of
Network-based Software Architectures", 2000,
<https://www.ics.uci.edu/~fielding/pubs/dissertation/
fielding_dissertation.pdf>.

[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>.

[RFC8974] Hartke, K. and M. Richardson, "Extended Tokens and
Stateless Clients in the Constrained Application Protocol
(CoAP)", RFC 8974, DOI 10.17487/RFC8974, January 2021,
<https://www.rfc-editor.org/info/rfc8974>.

[RFC9000] Iyengar, J., Ed. and M. Thomson, Ed., "QUIC: A UDP-Based
Multiplexed and Secure Transport", RFC 9000,
DOI 10.17487/RFC9000, May 2021,
<https://www.rfc-editor.org/info/rfc9000>.

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 and one for event-based freshness are outlined
in this appendix. The "List of Cached Random Values and Timestamps"
mechanism is RECOMMENDED in general. The "Integrity-Protected
Timestamp" mechanism is RECOMMENDED in case the Echo option is
encrypted between the client and the server.

Different mechanisms have different trade-offs 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 use 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 called r. The server
caches a list containing the random byte strings and their
initial 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 option 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

This method is suitable for both time-based and event-based
freshness (e.g., by clearing the cache when an event occurs) and
is independent of the client authority.

2. Integrity-Protected Timestamp. The Echo option value is an
integrity-protected timestamp. The timestamp can have a
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 option 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.
Therefore, it might be important to encrypt it. Depending on the
choice of encryption algorithms, this may require an
initialization vector to be included in the Echo option value
(see below).

Echo option value: timestamp t0, MAC(k, t0)

Server State: secret key k

This method is suitable for both time-based and event-based
freshness (by the server remembering the time at which the event
took place) and independent of the client authority.

If this method is used to additionally obtain network
reachability of the client, the server MUST use the client's
network address too, e.g., as in MAC(k, t0, apparent claimed network
address).

3. Persistent Counter. This can be used in OSCORE for sequence
number recovery, per Appendix B.1.2 of [RFC8613]. The Echo
option value is a simple counter without integrity protection of
its own, serialized in uint format. The counter is incremented
in a persistent way every time the state that needs to be
synchronized is changed (in the case described in Appendix B.1.2
of [RFC8613], when a reboot indicates that volatile state may
have been lost). An example of how such a persistent counter can
be implemented efficiently is the OSCORE server Sender Sequence
Number mechanism described in Appendix B.1.1 of [RFC8613].

Echo option value: counter

Server State: counter

This method is suitable only if the client is the authority over
the synchronized property. Consequently, it cannot be used to
show client aliveness. It provides statements from the client
similar to event-based freshness (but without a proof of
freshness).

Other mechanisms complying with the security and privacy
considerations may be used. The use of encrypted timestamps in the
Echo option provides additional protection but typically requires an
initialization vector (a.k.a. nonce) as input to the encryption
algorithm, which adds a slight complication to the procedure as well
as overhead.

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 and, in DTLS, if no packets are lost
and replay protection is active) 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
the Request-Tag value can be recycled indefinitely.

When the absence of a Request-Tag option cannot 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 1-byte options (2 bytes overhead) are
available.

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

* In DTLS, a new session can be established.

* 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.

Acknowledgements

The authors want to thank Carsten Bormann, Roman Danyliw, Benjamin
Kaduk, Murray Kucherawy, Francesca Palombini, and Jim Schaad for
providing valuable input to the document.

Authors' Addresses

Christian Amsüss
Email: christian@amsuess.com

John Preuß Mattsson
Ericsson AB
Email: john.mattsson@ericsson.com

Göran Selander
Ericsson AB
Email: goran.selander@ericsson.com