Extended Tokens and Stateless Clients in the Constrained Application Protocol (CoAP)
draft-hartke-core-stateless-00
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft whose latest revision state is "Replaced".
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|---|---|---|---|
| Author | Klaus Hartke | ||
| Last updated | 2018-09-10 | ||
| Replaced by | draft-ietf-core-stateless, RFC 8974 | ||
| RFC stream | (None) | ||
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| Additional resources | |||
| Stream | Stream state | (No stream defined) | |
| Consensus boilerplate | Unknown | ||
| RFC Editor Note | (None) | ||
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draft-hartke-core-stateless-00
CoRE Working Group K. Hartke
Internet-Draft Ericsson
Updates: 7252, 8323 (if approved) September 10, 2018
Intended status: Standards Track
Expires: March 14, 2019
Extended Tokens and Stateless Clients
in the Constrained Application Protocol (CoAP)
draft-hartke-core-stateless-00
Abstract
This document provides considerations for alleviating CoAP clients
and intermediaries of maintaining per-request state. Additionally,
it introduces a new, optional CoAP protocol extension for extended
tokens.
[[The present draft presents two alternative variants for extending
CoAP with extended tokens. Variant A makes use of the unused values
in the TKL field of the CoAP header. Variant B duplicates the
functionality of the token field of the CoAP header in a new CoAP
option, "Extended-Token". The considerations for stateless clients
making use of either variant are the same.]]
This document updates RFCs 7252 and 8323 (if Variant A).
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on March 14, 2019.
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Copyright Notice
Copyright (c) 2018 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 . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Notation . . . . . . . . . . . . . . . . . . 4
2. (Variant A:) Extended Tokens . . . . . . . . . . . . . . . . 4
2.1. Extended Token Length (TKL) Field . . . . . . . . . . . . 4
2.2. Discovering Support . . . . . . . . . . . . . . . . . . . 5
2.2.1. Extended-Token-Lengths Capability Option . . . . . . 5
2.2.2. Trial and Error . . . . . . . . . . . . . . . . . . . 6
3. (Variant B:) Extended Tokens . . . . . . . . . . . . . . . . 6
3.1. Extended-Token Option . . . . . . . . . . . . . . . . . . 6
3.2. Discovering Support . . . . . . . . . . . . . . . . . . . 7
3.2.1. Extended-Token-Option Capability Option . . . . . . . 7
3.2.2. Trial and Error . . . . . . . . . . . . . . . . . . . 8
4. Stateless Clients . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Intermediaries . . . . . . . . . . . . . . . . . . . . . 9
4.2. Extended Tokens . . . . . . . . . . . . . . . . . . . . . 9
4.3. Message Transmission . . . . . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. (Variant A:) IANA Considerations . . . . . . . . . . . . . . 11
6.1. CoAP Signaling Option Number . . . . . . . . . . . . . . 11
7. (Variant B:) IANA Considerations . . . . . . . . . . . . . . 11
7.1. CoAP Option Number . . . . . . . . . . . . . . . . . . . 11
7.2. CoAP Signaling Option Number . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
8.1. Normative References . . . . . . . . . . . . . . . . . . 12
8.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. (Variant A:) Updated Message Formats . . . . . . . . 13
A.1. CoAP over UDP . . . . . . . . . . . . . . . . . . . . . . 14
A.2. CoAP over TCP . . . . . . . . . . . . . . . . . . . . . . 15
A.3. CoAP over WebSockets . . . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 17
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1. Introduction
The Constrained Application Protocol (CoAP) [RFC7252] is a RESTful
application-layer protocol for constrained environments [RFC7228].
In CoAP, clients (or proxies in the client role) make requests to
servers (or proxies in the server role), which are serving these
requests by returning responses.
While a request is outstanding, a client typically maintains some
state that it requires for processing the response when it arrives.
Identification of this state is done by means of a _token_ in CoAP,
an opaque sequence of bytes chosen by the client and in the CoAP
request. The server returns it 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
stored at the client at the cost of increased message sizes. Client
implementations can accomplish this by serializing their state into
the token and recovering the state from the token in the response
when it comes back (Figure 2).
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+-----------------+ request with +------------+
| | | serialized state | |
| | | as token | |
| +--------|=====================>|------. |
| | | | |
| look, no | | | |
| state, ma! | | | |
| | | | |
| +--------|<=====================|------' |
| | | response with | |
| v | token echoed back | |
+-----------------+ +------------+
Client Server
Figure 2: Token as Serialization of Request State
Section 4 of this document provides considerations for making clients
"stateless" in this way, i.e., avoiding per-request state. (They'll
still need to maintain per-server state and other kinds of state,
though, so they're not entirely stateless.)
Serializing state into tokens is complicated by the fact that both
CoAP over UDP [RFC7252] and CoAP over reliable transports [RFC8323]
limit the maximum token length to 8 bytes. To overcome this
limitation, Section 2 (Variant A) / Section 3 (Variant B) of this
document first introduces a CoAP protocol extension for extended
tokens.
1.1. Requirements Notation
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. (Variant A:) Extended Tokens
2.1. Extended Token Length (TKL) Field
The message formats defined for CoAP over UDP [RFC7252] and CoAP over
TCP, TLS, and WebSockets [RFC8323] are updated with the following new
definition of the TKL field:
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. Three values are reserved for special constructs:
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13: An 8-bit unsigned integer precedes the Token field and
indicates the length of the Token field minus 13.
14: A 16-bit unsigned integer in network byte order 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.
2.2. Discovering Support
Extended token lengths require support from the server or, if there
are intermediaries between the client and the origin server, the
intermediary in the server role that the client is interacting with.
Support can be discovered by a client (or intermediary in the client
role) in one of two ways: If Capabilities and Settings Messages
(CSMs) are available, such as in CoAP over TCP, then support can be
discovered using the new Extended-Token-Lengths Capability Option
defined in Section 2.2.1. Otherwise, such as in CoAP over UDP,
support can only be discovered by trial and error, which is described
in Section 2.2.2.
2.2.1. Extended-Token-Lengths Capability Option
A sender can use the elective Extended-Token-Lengths Capability
Option to indicate its support for the updated definition of the TKL
field as specified above.
+----+---+---+-------+--------------------+-------+--------+--------+
| # | C | R | Appli | Name | Forma | Length | Base |
| | | | es to | | t | | Value |
+----+---+---+-------+--------------------+-------+--------+--------+
| TB | | | CSM | Extended-Token- | empty | 0 | (none) |
| D | | | | Lengths | | | |
+----+---+---+-------+--------------------+-------+--------+--------+
C=Critical, R=Repeatable
Table 1: The Extended-Token-Lengths Capability Option
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2.2.2. Trial and Error
A request with a TKL field value outside the range from 0 to 8 will
be considered a message format error (Section 3 of RFC 7252) and be
rejected by the next hop if that does not support extended token
lengths. A client can therefore determine support by sending a
request with an extended token length and checking if the next hop
rejects the message.
For CoAP over UDP, the recipient rejects a malformed confirmable
message by sending an Reset message and otherwise ignoring the
message (Section 4.2 of RFC 7252). In case of a non-confirmable
message, the Reset message is optional (Section 4.3 of RFC 7252), so
it is RECOMMENDED that the client uses a confirmable message. As per
RFC 7252, a Reset message does not contain a token and only echoes
back the Message ID of the confirmable or non-confirmable message
(Figure 3).
+-----------------+ 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 Next Hop Does Not Support It
For CoAP over TCP, TLS, and WebSockets, the recipient rejects a
malformed message by sending an Abort message and shutting down the
connection (Section 5.6 of RFC 8323). There is no specific Abort
message option indicating the reason defined for this case.
3. (Variant B:) Extended Tokens
3.1. Extended-Token Option
The Extended-Token Option contains an extension of the token that can
additionally be used for matching a response with a request.
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+-----+---+---+---+---+----------------+--------+---------+---------+
| # | C | U | N | R | Name | Format | Length | Default |
+-----+---+---+---+---+----------------+--------+---------+---------+
| TBD | x | x | | | Extended-Token | opaque | 0-65535 | (none) |
+-----+---+---+---+---+----------------+--------+---------+---------+
C=Critical, U=Unsafe, N=NoCacheKey, R=Repeatable
Table 2: The Extended-Token Option
A client (or intermediary in the role of a client) can include the
Extended-Token Option in a request with any client-generated value.
A server (or intermediary in the role of a server) MUST echo the
value without modification in an Extended-Token Option in any
resulting response.
It is entirely up to the client how it splits the token between the
Token field in the message header and the Extended-Token Option.
3.2. Discovering Support
The Extended-Token Option requires support from the server or, if
there are intermediaries between the client and the origin server,
the intermediary in the server role that the client is interacting
with.
Support can be discovered by a client (or intermediary in the client
role) in one of two ways: If Capabilities and Settings Messages
(CSMs) are available, such as in CoAP over TCP, then support can be
discovered using the new Extended-Token-Option Capability Option
defined in Section 3.2.1. Otherwise, such as in CoAP over UDP,
support can only be discovered by trial and error, which is described
in Section 3.2.2.
3.2.1. Extended-Token-Option Capability Option
A sender can use the elective Extended-Token-Option Capability Option
to indicate its support for the Extended-Token Option specified
above.
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+----+---+---+--------+-------------------+-------+--------+--------+
| # | C | R | Applie | Name | Forma | Length | Base |
| | | | s to | | t | | Value |
+----+---+---+--------+-------------------+-------+--------+--------+
| TB | | | CSM | Extended-Token- | empty | 0 | (none) |
| D | | | | Option | | | |
+----+---+---+--------+-------------------+-------+--------+--------+
C=Critical, R=Repeatable
Table 3: The Extended-Token-Option Capability Option
3.2.2. Trial and Error
A request with a critical, unsafe-to-forward option such as the
Extended-Token Option will cause the return of a 4.02 (Bad Option)
response from the next hop if that does not support the option. A
client can therefore determine support by sending a request with an
Extended-Token Option to next hop and checking if that returns such a
response. As per RFC 7252, the response will not contain an
Extended-Token Option, so only the regular token can be used for
matching the response to the request.
Note that a 4.02 (Bad Option) response does not necessarily mean that
the Extended-Token Option is unsupported if there is another
unsupported critical option in the request.
+-----------------+ request message +------------+
| | | with Extended- | |
| | | Token option | |
| .-<-+->------|--------------------->|------. |
| _|_ | | | |
| / \ stored | | | |
| \___/ state | | | |
| | | | | |
| '->-+-<------|<---------------------|------' |
| | | 4.02 (Bad Option) | |
| v | response without | |
+-----------------+ Extended-Token +------------+
Client Option Server
Figure 4: A Request With an Extended-Token Option Results in a 4.02
(Bad Option) Response if the Next Hop Does Not Support It
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4. Stateless Clients
A client can be alleviated of keeping request state by serializing
the state into a sequence of bytes and sending this as the token in a
request. The server will return the token to the client verbatim in
the response, so that the client can recover the state and process
the response as if it had kept the state locally.
The format of the serialized state is client implementation specific
and opaque to any server implementation. For security, the
serialized state MUST be integrity protected, MUST include a
freshness indicator (e.g., a sequence number or a timestamp), and MAY
additionally be encrypted. These requirement can be satisfied, for
example, by using COSE [RFC8152] with a key only known to the client.
4.1. 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 a request
to the next hop towards the origin server and waits for the response.
An intermediary can be alleviated of storing the client's token and
transport address by serializing this information along the request
state into its own token to the next hop. When the next hop returns
the response, the intermediary can recover the information from the
token and satisfy the client's outstanding request.
The downside of this approach is that the intermediary, without
keeping request state, is unable to use a cacheable response to
satisfy multiple outstanding requests, which degrades efficiency.
It is also unable to aggregate requests when it acts on behalf of
multiple clients observing [RFC7641] the same resource (which is
REQUIRED by RFC 7641).
When using blockwise transfers [RFC7959], servers might not be able
to distinguish blocks originating from different clients once they
have been forwarded by an intermediary. To ensure that this does not
lead inconsistent resource state, a stateless intermediary MUST
include the Request-Tag Option [I-D.ietf-core-echo-request-tag] with
a value that uniquely identifies the originating endpoint in the
intermediary namespace.
4.2. Extended Tokens
A client or intermediary in the client role that depends on support
for extended tokens from the next hop to avoid keeping request state
MUST perform a discovery of support (Section 2.2/Section 3.2) before
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it can be stateless. This discovery MUST be performed in a stateful
way, i.e., keeping state for the request (Figure 5): If the client
was stateless from the start and the next hop doesn't support
extended tokens, then any error message couldn't be processed as the
state would neither be present at the client nor echoed back in the
error message.
+-----------------+ dummy request +------------+
| | | with extended | |
| | | token | |
| .-<-+->------|=====================>|------. |
| _|_ | | | |
| / \ stored | | | |
| \___/ state | | | |
| | | | | |
| '->-+-<------|<=====================|------' |
| | | response with | |
| | | extended token | |
| | | echoed back | |
| | | | |
| | | | |
| | | request with | |
| | | serialized state | |
| | | as token | |
| +--------|=====================>|------. |
| | | | |
| look, no | | | |
| state, ma! | | | |
| | | | |
| +--------|<=====================|------' |
| | | response with | |
| v | token echoed back | |
+-----------------+ +------------+
Client Server
Figure 5: Depending on Extended Tokens for Being Stateless First
Requires a Successful Stateful Discovery of Support
4.3. Message Transmission
In the case of CoAP over UDP [RFC7252], a client can use confirmable
or non-confirmable messages for requests. When using confirmable
messages, it needs to keep message exchange state for performing
retransmissions and for handling Acknowledgement and Reset messages.
When using non-confirmable messages, it can keep no message exchange
state. However, in either case the client needs to keep congestion
control state. That is, it needs to maintain state for each node it
communicates with and, e.g., enforce NSTART.
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In accordance with RFC 7252, a client must always be prepared to
receive a separate response or non-confirmable response in reply to a
request, regardless of the message type it used for that request. If
a separate response contains a valid authentication tag and freshness
indicator in the token, the client MUST process the message as
specified in Section 4.2 of RFC 7252; otherwise, it SHOULD reject the
response as specified in Section 4.2 of RFC 7252.
5. Security Considerations
TODO.
6. (Variant A:) IANA Considerations
6.1. CoAP Signaling Option Number
The following entries are added to the "CoAP Signaling Option
Numbers" registry within the "CoRE Parameters" registry.
+------------+--------+------------------------+-------------------+
| Applies to | Number | Name | Reference |
+------------+--------+------------------------+-------------------+
| 7.01 | TBD | Extended-Token-Lengths | [[this document]] |
+------------+--------+------------------------+-------------------+
[[The three lowest available option numbers with the properties
specified in Table 1 are 6, 8, and 10 at the time of writing.]]
7. (Variant B:) IANA Considerations
7.1. CoAP Option Number
The following entries are added to the "CoAP Option Numbers" registry
within the "CoRE Parameters" registry.
+--------+----------------+-------------------+
| Number | Name | Reference |
+--------+----------------+-------------------+
| TBD | Extended-Token | [[this document]] |
+--------+----------------+-------------------+
[[The three lowest available option numbers with the properties
specified in Table 2 are 19, 31, and 43 at the time of writing.]]
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7.2. CoAP Signaling Option Number
The following entries are added to the "CoAP Signaling Option
Numbers" registry within the "CoRE Parameters" registry.
+------------+--------+-----------------------+-------------------+
| Applies to | Number | Name | Reference |
+------------+--------+-----------------------+-------------------+
| 7.01 | TBD | Extended-Token-Option | [[this document]] |
+------------+--------+-----------------------+-------------------+
[[The three lowest available option numbers with the properties
specified in Table 3 are 6, 8, and 10 at the time of writing.]]
8. References
8.1. Normative References
[I-D.ietf-core-echo-request-tag]
Amsuess, C., Mattsson, J., and G. Selander, "Echo and
Request-Tag", draft-ietf-core-echo-request-tag-02 (work in
progress), June 2018.
[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>.
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[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>.
8.2. Informative References
[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>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
Appendix A. (Variant A:) Updated Message Formats
This appendix illustrates the CoAP message formats updated with the
new definition of the TKL field (Section 2).
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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 or more bytes
\ \
+-------------------------------+
\ \
/ /
\ \
/ Options / 0 or more bytes
\ \
/ /
\ \
+---------------+---------------+
| | |
| 15 | 15 | 1 byte (if payload)
| | |
+---------------+---------------+
\ \
/ /
\ \
/ Payload / 0 or more bytes
\ \
/ /
\ \
+-------------------------------+
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A.2. CoAP over TCP
0 1 2 3 4 5 6 7
+---------------+---------------+
| | |
| Len | TKL | 1 byte
| | |
+---------------+---------------+
\ \
/ Len / 0-2 bytes
\ (extended) \
+-------------------------------+
| |
| Code | 1 byte
| |
+-------------------------------+
\ \
/ TKL / 0-2 bytes
\ (extended) \
+-------------------------------+
\ \
/ Token / 0 or more bytes
\ \
+-------------------------------+
\ \
/ /
\ \
/ Options / 0 or more bytes
\ \
/ /
\ \
+---------------+---------------+
| | |
| 15 | 15 | 1 byte (if payload)
| | |
+---------------+---------------+
\ \
/ /
\ \
/ Payload / 0 or more bytes
\ \
/ /
\ \
+-------------------------------+
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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 or more bytes
\ \
+-------------------------------+
\ \
/ /
\ \
/ Options / 0 or more bytes
\ \
/ /
\ \
+---------------+---------------+
| | |
| 15 | 15 | 1 byte (if payload)
| | |
+---------------+---------------+
\ \
/ /
\ \
/ Payload / 0 or more bytes
\ \
/ /
\ \
+-------------------------------+
Hartke Expires March 14, 2019 [Page 16]
Internet-DraftExtended Tokens and Stateless Clients in CoASeptember 2018
Author's Address
Klaus Hartke
Ericsson
Torshamnsgatan 23
Stockholm SE-16483
Sweden
Email: klaus.hartke@ericsson.com
Hartke Expires March 14, 2019 [Page 17]