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Updates:
Keywords: 6tisch, minimal-security
Internet Engineering Task Force (IETF) K. Hartke
Request for Comments: 8974 Ericsson
Updates: 7252, 8323 M. Richardson
Category: Standards Track Sandelman
ISSN: 2070-1721 January 2021
Extended Tokens and Stateless Clients
in the Constrained Application Protocol (CoAP)
Abstract
This document provides considerations for alleviating Constrained
Application Protocol (CoAP) clients and intermediaries of keeping
per-request state. To facilitate this, this document additionally
introduces a new, optional CoAP protocol extension for extended token
lengths.
This document updates RFCs 7252 and 8323 with an extended definition
of the "TKL" field in the CoAP message header.
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/rfc8974.
Copyright Notice
Copyright (c) 2021 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
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 Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction
1.1. Terminology
2. Extended Tokens
2.1. Extended Token Length (TKL) Field
2.2. Discovering Support
2.2.1. Extended-Token-Length Capability Option
2.2.2. Trial and Error
2.3. Intermediaries
3. Stateless Clients
3.1. Serializing Client State
3.2. Using Extended Tokens
3.3. Transmitting Messages
4. Stateless Intermediaries
4.1. Observing Resources
4.2. Block-Wise Transfers
4.3. Gateway Timeouts
4.4. Extended Tokens
5. Security Considerations
5.1. Extended Tokens
5.2. Stateless Clients and Intermediaries
6. IANA Considerations
6.1. CoAP Signaling Option Number
7. References
7.1. Normative References
7.2. Informative References
Appendix A. Updated Message Formats
A.1. CoAP over UDP
A.2. CoAP over TCP/TLS
A.3. CoAP over WebSockets
Acknowledgements
Authors' Addresses
1. Introduction
The Constrained Application Protocol (CoAP) [RFC7252] is a RESTful
application-layer protocol for constrained environments [RFC7228].
In CoAP, clients (or intermediaries in the client role) make requests
to servers (or intermediaries in the server role), which satisfy the
requests by returning responses.
While a request is ongoing, a client typically needs to keep some
state that it requires for processing the response when that arrives.
Identification of this state is done in CoAP by means of a token: an
opaque sequence of bytes that is chosen by the client and included in
the CoAP request and that is returned by the server verbatim in any
resulting CoAP response (Figure 1).
+-----------------+ request with +------------+
| | | state identifier | |
| | | as token | |
| .-<-+->------|--------------------->|------. |
| _|_ | | | |
| / \ stored | | | |
| \___/ state | | | |
| | | | | |
| '->-+-<------|<---------------------|------' |
| | | response with | |
| v | token echoed back | |
+-----------------+ +------------+
Client Server
Figure 1: Token as an Identifier for Request State
In some scenarios, it can be beneficial to reduce the amount of state
that is stored at the client at the cost of increased message sizes.
A client can opt into this by serializing (parts of) its state into
the token itself and then recovering this state from the token in the
response (Figure 2).
+-----------------+ request with +------------+
| | | serialized state | |
| | | as token | |
| +--------|=====================>|------. |
| | | | |
| look ma, | | | |
| no state! | | | |
| | | | |
| +--------|<=====================|------' |
| | | response with | |
| v | token echoed back | |
+-----------------+ +------------+
Client Server
Figure 2: Token as Serialization of Request State
Section 3 of this document provides considerations for clients
becoming "stateless" in this way. (The term "stateless" is in quotes
here, because it's a bit oversimplified. Such clients still need to
maintain per-server state and other kinds of state. So it would be
more accurate to just say that the clients are avoiding per-request
state.)
Section 4 of this document extends the considerations for clients to
intermediaries, which may want to avoid keeping state for not only
the requests they send to servers but also the requests they receive
from clients.
The serialization of state into tokens is limited by the fact that
both CoAP over UDP [RFC7252] and CoAP over reliable transports
[RFC8323] restrict the maximum token length to 8 bytes. To overcome
this limitation, Section 2 of this document introduces a CoAP
protocol extension for extended token lengths.
While the use case (avoiding per-request state) and the mechanism
(extended token lengths) presented in this document are closely
related, each can be used independently of the other. Some
implementations may be able to fit their state in just 8 bytes; some
implementations may have other use cases for extended token lengths.
1.1. Terminology
In this document, the term "stateless" refers to an implementation
strategy for a client (or intermediary in the client role) that does
not require it to keep state for the individual requests it sends to
a server (or intermediary in the server role). The client still
needs to keep state for each server it communicates with (e.g., for
token generation, message retransmission, and congestion control).
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.
2. Extended Tokens
This document updates the message formats defined for CoAP over UDP
[RFC7252] and CoAP over TCP, TLS, and WebSockets [RFC8323] with a new
definition of the "TKL" field.
2.1. Extended Token Length (TKL) Field
The definition of the "TKL" field is updated as follows:
Token Length (TKL): 4-bit unsigned integer. A value between 0 and
12, inclusive, indicates the length of the variable-length "Token"
field in bytes. The other three values are reserved for special
constructs:
13: An 8-bit unsigned integer directly precedes the "Token"
field and indicates the length of the "Token" field minus
13.
14: A 16-bit unsigned integer in network byte order directly
precedes the "Token" field and indicates the length of the
"Token" field minus 269.
15: Reserved. This value MUST NOT be sent and MUST be processed
as a message-format error.
All other fields retain their definitions.
The updated message formats are illustrated in Appendix A.
The new definition of the "TKL" field increases the maximum token
length that can be represented in a message to 65804 bytes. However,
the maximum token length that sender and recipient implementations
support may be shorter. For example, a constrained node of Class 1
[RFC7228] might support extended token lengths only up to 32 bytes.
In CoAP over UDP, it is often beneficial to keep CoAP messages small
enough to avoid IP fragmentation. The maximum practical token length
may therefore also be influenced by the Path MTU (PMTU). See
Section 4.6 of [RFC7252] for details.
2.2. Discovering Support
Extended token lengths require support from server implementations.
Support can be discovered by a client implementation in one of two
ways:
* Where Capabilities and Settings Messages (CSMs) are available,
such as in CoAP over TCP, support can be discovered using the
Extended-Token-Length Capability Option defined in Section 2.2.1.
* Otherwise, such as in CoAP over UDP, support can only be
discovered by trial and error, as described in Section 2.2.2.
2.2.1. Extended-Token-Length Capability Option
A server can use the elective Extended-Token-Length Capability Option
to indicate the maximum token length it can accept in requests.
+=+=+===+=========+=======================+========+========+=======+
|#|C| R | Applies | Name | Format | Length | Base |
| | | | to | | | | Value |
+=+=+===+=========+=======================+========+========+=======+
|6| | | CSM | Extended-Token-Length | uint | 0-3 | 8 |
+-+-+---+---------+-----------------------+--------+--------+-------+
Table 1: The Extended-Token-Length Capability Option
C=Critical, R=Repeatable
As per Section 3 of [RFC7252], the base value (and the value used
when this option is not implemented) is 8.
The active value of the Extended-Token-Length Option is replaced each
time the option is sent with a modified value. Its starting value is
its base value.
The option value MUST NOT be less than 8 or greater than 65804. If
an option value less than 8 is received, the option MUST be ignored.
If an option value greater than 65804 is received, the option value
MUST be set to 65804.
Any option value greater than 8 implies support for the new
definition of the "TKL" field specified in Section 2.1. Indication
of support by a server does not oblige a client to actually make use
of token lengths greater than 8.
If a server receives a request with a token of a length greater than
what it indicated in its Extended-Token-Length Option, it MUST handle
the request as a message-format error.
If a server receives a request with a token of a length less than, or
equal to, what it indicated in its Extended-Token-Length Option but
is unwilling or unable to handle the token at that time, it MUST NOT
handle the request as a message-format error. Instead, it SHOULD
return a 5.03 (Service Unavailable) response.
The Extended-Token-Length Capability Option does not apply to
responses. The sender of a request is simply expected not to use a
token of a length greater than it is willing to accept in a response.
2.2.2. Trial and Error
A server implementation that does not support the updated definition
of the "TKL" field specified in Section 2.1 will consider a request
with a "TKL" field value outside the range 0 to 8 to be a message-
format error and reject it (Section 3 of [RFC7252]). A client can
therefore determine support by sending a request with an extended
token length and checking whether or not it is rejected by the
server.
In CoAP over UDP, the way a request message is rejected depends on
the message type. A Confirmable message with a message-format error
is rejected with a Reset message (Section 4.2 of [RFC7252]). A Non-
confirmable message with a message-format error is either rejected
with a Reset message or just silently ignored (Section 4.3 of
[RFC7252]). To reliably get a Reset message, it is therefore
REQUIRED that clients use a Confirmable message for determining
support.
As per RFC 7252, Reset messages are empty and do not contain a token;
they only return the Message ID (Figure 3). They also do not contain
any indication of what caused a message-format error. To avoid any
ambiguity, it is therefore RECOMMENDED that clients use a request
that has no potential message-format error other than the extended
token length.
+-----------------+ request message +------------+
| | | with extended | |
| | | token length | |
| .-<-+->------|--------------------->|------. |
| _|_ | | | |
| / \ stored | | | |
| \___/ state | | | |
| | | | | |
| '->-+-<------|<---------------------|------' |
| | | Reset message | |
| v | with only message | |
+-----------------+ ID echoed back +------------+
Client Server
Figure 3: A Confirmable Request with an Extended Token Is
Rejected with a Reset Message If the Server Does Not Have Support
An example of a suitable request is a GET request in a Confirmable
message that includes only an If-None-Match option and a token of the
greatest length that the client intends to use. Any response with
the same token echoed back indicates that tokens up to that length
are supported by the server.
Since network addresses may change, a client SHOULD NOT assume that
extended token lengths are supported by a server for an unlimited
duration. Unless additional information is available, the client
should assume that addresses (and therefore extended token lengths)
are valid for a minimum of 1800 s and a maximum of 86400 s (1 day).
A client may use additional forms of input into this determination.
For instance, a client may assume a server that is in the same subnet
as the client has a similar address lifetime as the client. The
client may use DHCP lease times or Router Advertisements to set the
limits. For servers that are not local, if the server was looked up
using DNS, then the DNS resource record will have a Time To Live
(TTL), and the extended token length should be kept for only that
amount of time.
If a server supports extended token lengths but receives a request
with a token of a length it is unwilling or unable to handle, it MUST
NOT reject the message, as that would imply that extended token
lengths are not supported at all. Instead, if the server cannot
handle the request at the time, it SHOULD return a 5.03 (Service
Unavailable) response; if the server will never be able to handle the
request (e.g., because the token is too large), it SHOULD return a
4.00 (Bad Request) response.
| Design Note: The requirement to return an error response when a
| token cannot be handled might seem somewhat contradictory, as
| returning the error response requires the server also to return
| the token it cannot handle. However, processing a request
| usually involves a number of steps from receiving the message
| to passing it to application logic. The idea is that a server
| implementing this extension supports large tokens at least in
| its first few processing steps, enough to return an error
| response rather than a Reset message.
| Design Note: To prevent the trial-and-error-based discovery
| from becoming too complicated, no effort is made to indicate
| the maximum supported token length. A client implementation
| would probably already choose the shortest token possible for
| the task (such as being stateless, as described in Section 3),
| so it would probably not be able to reduce the length any
| further anyway should a server indicate a lower limit.
2.3. Intermediaries
Tokens are a hop-by-hop feature: if there are one or more
intermediaries between a client and a server, every token is scoped
to the exchange between a node in the client role and the node in the
server role that it is immediately interacting with.
When an intermediary receives a request, the only requirement is that
it echoes the token back in any resulting response. There is no
requirement or expectation that an intermediary passes a client's
token on to a server or that an intermediary uses extended token
lengths itself in its request to satisfy a request with an extended
token length. Discovery needs to be performed for each hop where
extended token lengths are to be used.
3. Stateless Clients
A client can be alleviated of keeping per-request state as follows:
1. The client serializes (parts of) its per-request state into a
sequence of bytes and sends those bytes as the token of its
request to the server.
2. The server returns the token verbatim in the response to the
client, which allows the client to recover the state and process
the response as if it had kept the state locally.
As servers are just expected to return any token verbatim to the
client, this implementation strategy for clients does not impact the
interoperability of client and server implementations. However,
there are a number of significant, nonobvious implications (e.g.,
related to security and other CoAP protocol features) that client
implementations need take into consideration.
The following subsections discuss some of these considerations.
3.1. Serializing Client State
The format of the serialized state is generally an implementation
detail of the client and opaque to the server. However, serialized
state information is an attractive target for both unwanted nodes
(e.g., on-path attackers) and wanted nodes (e.g., any configured
forward proxy) on the path. The serialization format therefore needs
to include security measures such as the following:
* A client SHOULD protect the integrity of the state information
serialized in a token.
* Even when the integrity of the serialized state is protected, an
attacker may still replay a response, making the client believe it
sent the same request twice. For this reason, the client SHOULD
implement replay protection (e.g., by using sequence numbers and a
replay window). For replay protection, integrity protection is
REQUIRED.
* If processing a response without keeping request state is
sensitive to the time elapsed since sending the request, then the
client SHOULD include freshness information (e.g., a timestamp) in
the serialized state and reject any response where the freshness
information is insufficiently fresh.
* Information in the serialized state may be privacy sensitive. A
client SHOULD encrypt the serialized state if it contains privacy-
sensitive information that an attacker would not get otherwise.
* When a client changes the format of the serialized state, it
SHOULD prevent false interoperability with the previous format
(e.g., by changing the key used for integrity protection or
changing a field in the serialized state).
3.2. Using Extended Tokens
A client that depends on support for extended token lengths
(Section 2) from the server to avoid keeping request state needs to
perform a discovery of support (Section 2.2) before it can be
stateless.
This discovery MUST be performed in a stateful way, i.e., keeping
state for the request (Figure 4). If the client was stateless from
the start, and the server does not support extended tokens, then no
error message could be processed, since the state would neither be
present at the client nor returned in the Reset message (Figure 5).
+-----------------+ dummy request +------------+
| | | with extended | |
| | | token | |
| .-<-+->------|=====================>|------. |
| _|_ | | | |
| / \ stored | | | |
| \___/ state | | | |
| | | | | |
| '->-+-<------|<=====================|------' |
| | | response with | |
| | | extended token | |
| | | echoed back | |
| | | | |
| | | | |
| | | request with | |
| | | serialized state | |
| | | as token | |
| +--------|=====================>|------. |
| | | | |
| look ma, | | | |
| no state! | | | |
| | | | |
| +--------|<=====================|------' |
| | | response with | |
| v | token echoed back | |
+-----------------+ +------------+
Client Server
Figure 4: Depending on Extended Tokens for Being Stateless First
Requires a Successful Stateful Discovery of Support
+-----------------+ dummy request +------------+
| | | with extended | |
| | | token | |
| +--------|=====================>|------. |
| | | | |
| | | | |
| | | | |
| | | | |
| ???|<---------------------|------' |
| | Reset message | |
| | with only message | |
+-----------------+ ID echoed back +------------+
Client Server
Figure 5: Stateless Discovery of Support Does Not Work
In environments where support can be reliably discovered through some
other means, the discovery of support is OPTIONAL. An example for
this is the Constrained Join Protocol (CoJP) in a 6TiSCH network
[6TISCH-MIN-SEC], where support for extended tokens is required from
all relevant parties.
3.3. Transmitting Messages
In CoAP over UDP [RFC7252], a client has the choice between
Confirmable and Non-confirmable messages for requests. When using
Non-confirmable messages, a client does not have to keep any message-
exchange state, which can help in the goal of avoiding state. When
using Confirmable messages, a client needs to keep message-exchange
state for performing retransmissions and handling Acknowledgement and
Reset messages, however. Non-confirmable messages are therefore
better suited for avoiding state. In any case, a client still needs
to keep congestion-control state, i.e., maintain state for each node
it communicates with and enforce limits like NSTART.
As per Section 5.2 of [RFC7252], a client must be prepared to receive
a response as a piggybacked response, a separate response, or a Non-
confirmable response, regardless of the message type used for the
request. A stateless client MUST handle these response types as
follows:
* If a piggybacked response passes the checks for token integrity
and freshness (Section 3.1), the client processes the message as
specified in RFC 7252; otherwise, it processes the acknowledgement
portion of the message as specified in RFC 7252 and silently
discards the response portion.
* If a separate response passes the checks for token integrity and
freshness, the client processes the message as specified in RFC
7252; otherwise, it rejects the message as specified in
Section 4.2 of [RFC7252].
* If a Non-confirmable response passes the checks for token
integrity and freshness, the client processes the message as
specified in RFC 7252; otherwise, it rejects the message as
specified in Section 4.3 of [RFC7252].
4. Stateless Intermediaries
Tokens are a hop-by-hop feature. If a client makes a request to an
intermediary, that intermediary needs to store the client's token
(along with the client's transport address) while it makes its own
request towards the origin server and waits for the response. When
the intermediary receives the response, it looks up the client's
token and transport address for the received request and sends an
appropriate response to the client.
An intermediary might want to be "stateless" not only in its role as
a client but also in its role as a server, i.e., be alleviated of
storing the client information for the requests it receives.
Such an intermediary can be implemented by serializing the client
information along with the request state into the token towards the
origin server. When the intermediary receives the response, it can
recover the client information from the token and use it to satisfy
the client's request; therefore, the intermediary doesn't need to
store the information itself.
The following subsections discuss some considerations for this
approach.
4.1. Observing Resources
One drawback of the approach is that an intermediary, without keeping
request state, is unable to aggregate multiple requests for the same
target resource, which can significantly reduce efficiency. In
particular, when clients observe [RFC7641] the same resource,
aggregating requests is REQUIRED (Section 3.1 of [RFC7641]). This
requirement cannot be satisfied without keeping request state.
Furthermore, an intermediary that does not keep track of the clients
observing a resource is not able to determine whether these clients
are still interested in receiving further notifications (Section 3.5
of [RFC7641]) or want to cancel an observation (Section 3.6 of
[RFC7641]).
Therefore, an intermediary MUST NOT include an Observe Option in
requests it sends without keeping both the request state for the
requests it sends and the client information for the requests it
receives.
4.2. Block-Wise Transfers
When using block-wise transfers [RFC7959], a server might not be able
to distinguish blocks originating from different clients once they
have been forwarded by an intermediary. Intermediaries need to
ensure that this does not lead to inconsistent resource state by
keeping distinct block-wise request operations on the same resource
apart, e.g., utilizing the Request-Tag Option [ECHO-REQUEST-TAG].
4.3. Gateway Timeouts
As per Section 5.7.1 of [RFC7252], an intermediary is REQUIRED to
return a 5.04 (Gateway Timeout) response if it cannot obtain a
response within a timeout. However, if an intermediary does not keep
the client information for the requests it receives, it cannot return
such a response. Therefore, in this case, the gateway cannot return
such a response and as such cannot implement such a timeout.
4.4. Extended Tokens
A client may make use of extended token lengths in a request to an
intermediary that wants to be "stateless". This means that such an
intermediary may have to serialize potentially very large client
information into its token towards the origin server. The tokens can
grow even further when it progresses along a chain of intermediaries
that all want to be "stateless".
Intermediaries SHOULD limit the size of client information they are
serializing into their own tokens. An intermediary can do this, for
example, by limiting the extended token lengths it accepts from its
clients (see Section 2.2) or by keeping the client information
locally when the client information exceeds the limit (i.e., not
being "stateless").
5. Security Considerations
5.1. Extended Tokens
Tokens significantly larger than the 8 bytes specified in RFC 7252
have implications -- in particular, for nodes with constrained memory
size -- that need to be mitigated. A node in the server role
supporting extended token lengths may be vulnerable to a denial of
service when an attacker (either on-path or a malicious client) sends
large tokens to fill up the memory of the node. Implementations need
to be prepared to handle such messages.
5.2. Stateless Clients and Intermediaries
Transporting the state needed by a client to process a response as
serialized state information in the token has several significant and
nonobvious security and privacy implications that need to be
mitigated; see Section 3.1 for recommendations.
In addition to the format requirements outlined there,
implementations need to ensure that they are not vulnerable to
maliciously crafted, delayed, or replayed tokens.
It is generally expected that the use of encryption, integrity
protection, and replay protection for serialized state is
appropriate.
In the absence of integrity and replay protection, an on-path
attacker or rogue server/intermediary could return a state (either
one modified in a reply, or an unsolicited one) that could alter the
internal state of the client.
It is for this reason that at least the use of integrity protection
on the token is always recommended.
It may be that in some very specific cases, as a result of a careful
and detailed analysis of any potential attacks, it is decided that
such cryptographic protections do not add value. The authors of this
document have not found such a use case as yet, but this is a local
decision.
It should further be emphasized that the encrypted state is created
by the sending node and decrypted by the same node when receiving a
response. The key is not shared with any other system. Therefore,
the choice of encryption scheme and the generation of the key for
this system is purely a local matter.
When encryption is used, the use of AES-CCM [RFC3610] with a 64-bit
tag is recommended, combined with a sequence number and a replay
window. This choice is informed by available hardware acceleration
of on many constrained systems. If a different algorithm is
available accelerated on the sender, with similar or stronger
strength, then it SHOULD be preferred. Where privacy of the state is
not required, and encryption is not needed, HMAC-SHA-256 [RFC6234],
combined with a sequence number and a replay window, may be used.
This size of the replay window depends upon the number of requests
that need to be outstanding. This can be determined from the rate at
which new ones are made and the expected time period during which
responses are expected.
For instance, given a CoAP MAX_TRANSMIT_WAIT of 93 s (Section 4.8.2
of [RFC7252]), any request that is not answered within 93 s will be
considered to have failed. At a request rate of one request per 10
s, at most 10 (ceil(9.3)) requests can be outstanding at a time, and
any convenient replay window larger than 20 will work. As replay
windows are often implemented with a sliding window and a bit, the
use of a 32-bit window would be sufficient.
For use cases where requests are being relayed from another node, the
request rate may be estimated by the total link capacity allocated
for that kind of traffic. An alternate view would consider how many
IPv6 Neighbor Cache Entries (NCEs) the system can afford to allocate
for this use.
When using an encryption mode that depends on a nonce, such as AES-
CCM, repeated use of the same nonce under the same key causes the
cipher to fail catastrophically.
If a nonce is ever used for more than one encryption operation with
the same key, then the same key stream gets used to encrypt both
plaintexts, and the confidentiality guarantees are voided. Devices
with low-quality entropy sources -- as is typical with constrained
devices, which incidentally happen to be a natural candidate for the
stateless mechanism described in this document -- need to carefully
pick a nonce-generation mechanism that provides the above uniqueness
guarantee.
[RFC8613], Appendix B.1.1 ("Sender Sequence Number") provides a model
for how to maintain nonrepeating nonces without causing excessive
wear of flash memory.
6. IANA Considerations
6.1. CoAP Signaling Option Number
The following entry has been added to the "CoAP Signaling Option
Numbers" registry within the "CoRE Parameters" registry.
+============+========+=======================+===========+
| Applies to | Number | Name | Reference |
+============+========+=======================+===========+
| 7.01 | 6 | Extended-Token-Length | RFC 8974 |
+------------+--------+-----------------------+-----------+
Table 2: CoAP Signaling Option Number
7. References
7.1. 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>.
[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>.
[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>.
[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>.
7.2. Informative References
[6TISCH-MIN-SEC]
Vucinic, M., Simon, J., Pister, K., and M. Richardson,
"Constrained Join Protocol (CoJP) for 6TiSCH", Work in
Progress, Internet-Draft, draft-ietf-6tisch-minimal-
security-15, 10 December 2019,
<https://tools.ietf.org/html/draft-ietf-6tisch-minimal-
security-15>.
[ECHO-REQUEST-TAG]
Amsüss, C., Mattsson, J. P., and G. Selander, "CoAP: Echo,
Request-Tag, and Token Processing", Work in Progress,
Internet-Draft, draft-ietf-core-echo-request-tag-11, 2
November 2020, <https://tools.ietf.org/html/draft-ietf-
core-echo-request-tag-11>.
[RFC3610] Whiting, D., Housley, R., and N. Ferguson, "Counter with
CBC-MAC (CCM)", RFC 3610, DOI 10.17487/RFC3610, September
2003, <https://www.rfc-editor.org/info/rfc3610>.
[RFC6234] Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
(SHA and SHA-based HMAC and HKDF)", RFC 6234,
DOI 10.17487/RFC6234, May 2011,
<https://www.rfc-editor.org/info/rfc6234>.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<https://www.rfc-editor.org/info/rfc7228>.
[RFC8613] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", RFC 8613, DOI 10.17487/RFC8613, July 2019,
<https://www.rfc-editor.org/info/rfc8613>.
Appendix A. Updated Message Formats
In Section 2, this document updates the CoAP message formats by
specifying a new definition of the "TKL" field in the message header.
As an alternative presentation of this update, this appendix shows
the CoAP message formats for CoAP over UDP [RFC7252] and CoAP over
TCP, TLS, and WebSockets [RFC8323] with the new definition applied.
A.1. CoAP over UDP
0 1 2 3 4 5 6 7
+-------+-------+---------------+
| | | |
| Ver | T | TKL | 1 byte
| | | |
+-------+-------+---------------+
| |
| Code | 1 byte
| |
+-------------------------------+
| |
| |
| |
+- Message ID -+ 2 bytes
| |
| |
| |
+-------------------------------+
\ \
/ TKL / 0-2 bytes
\ (extended) \
+-------------------------------+
\ \
/ Token / 0-65804 bytes
\ \
+-------------------------------+
\ \
/ /
\ \
/ Options / 0 or more bytes
\ \
/ /
\ \
+---------------+---------------+
| | |
| 15 | 15 | 1 byte (if payload)
| | |
+---------------+---------------+
\ \
/ /
\ \
/ Payload / 0 or more bytes
\ \
/ /
\ \
+-------------------------------+
A.2. CoAP over TCP/TLS
0 1 2 3 4 5 6 7
+---------------+---------------+
| | |
| Len | TKL | 1 byte
| | |
+---------------+---------------+
\ \
/ Len / 0-4 bytes
\ (extended) \
+-------------------------------+
| |
| Code | 1 byte
| |
+-------------------------------+
\ \
/ TKL / 0-2 bytes
\ (extended) \
+-------------------------------+
\ \
/ Token / 0-65804 bytes
\ \
+-------------------------------+
\ \
/ /
\ \
/ Options / 0 or more bytes
\ \
/ /
\ \
+---------------+---------------+
| | |
| 15 | 15 | 1 byte (if payload)
| | |
+---------------+---------------+
\ \
/ /
\ \
/ Payload / 0 or more bytes
\ \
/ /
\ \
+-------------------------------+
A.3. CoAP over WebSockets
0 1 2 3 4 5 6 7
+---------------+---------------+
| | |
| 0 | TKL | 1 byte
| | |
+---------------+---------------+
| |
| Code | 1 byte
| |
+-------------------------------+
\ \
/ TKL / 0-2 bytes
\ (extended) \
+-------------------------------+
\ \
/ Token / 0-65804 bytes
\ \
+-------------------------------+
\ \
/ /
\ \
/ Options / 0 or more bytes
\ \
/ /
\ \
+---------------+---------------+
| | |
| 15 | 15 | 1 byte (if payload)
| | |
+---------------+---------------+
\ \
/ /
\ \
/ Payload / 0 or more bytes
\ \
/ /
\ \
+-------------------------------+
Acknowledgements
This document is based on the requirements of, and work on,
"Constrained Join Protocol (CoJP) for 6TiSCH" (January 2020) by
Mališa Vučinić, Jonathan Simon, Kris Pister, and Michael Richardson.
Thanks to Christian Amsüss, Carsten Bormann, Roman Danyliw, Christer
Holmberg, Benjamin Kaduk, Ari Keränen, Erik Kline, Murray Kucherawy,
Warren Kumari, Barry Leiba, David Mandelberg, Dan Romascanu, Jim
Schaad, Göran Selander, Mališa Vučinić, Éric Vyncke, and Robert
Wilton for helpful comments and discussions that have shaped the
document.
Special thanks to John Mattsson for his contributions to the security
considerations of the document, and to Thomas Fossati for his in-
depth review, copious comments, and suggested text.
Authors' Addresses
Klaus Hartke
Ericsson
Torshamnsgatan 23
SE-16483 Stockholm
Sweden
Email: klaus.hartke@ericsson.com
Michael C. Richardson
Sandelman Software Works
Email: mcr+ietf@sandelman.ca
URI: http://www.sandelman.ca/