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PROPOSED STANDARD
Internet Engineering Task Force (IETF) C. Bormann
Request for Comments: 8323 Universitaet Bremen TZI
Updates: 7641, 7959 S. Lemay
Category: Standards Track Zebra Technologies
ISSN: 2070-1721 H. Tschofenig
ARM Ltd.
K. Hartke
Universitaet Bremen TZI
B. Silverajan
Tampere University of Technology
B. Raymor, Ed.
February 2018
CoAP (Constrained Application Protocol) over TCP, TLS, and WebSockets
Abstract
The Constrained Application Protocol (CoAP), although inspired by
HTTP, was designed to use UDP instead of TCP. The message layer of
CoAP over UDP includes support for reliable delivery, simple
congestion control, and flow control.
Some environments benefit from the availability of CoAP carried over
reliable transports such as TCP or Transport Layer Security (TLS).
This document outlines the changes required to use CoAP over TCP,
TLS, and WebSockets transports. It also formally updates RFC 7641
for use with these transports and RFC 7959 to enable the use of
larger messages over a reliable transport.
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/rfc8323.
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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
2. Conventions and Terminology .....................................6
3. CoAP over TCP ...................................................7
3.1. Messaging Model ............................................7
3.2. Message Format .............................................9
3.3. Message Transmission ......................................11
3.4. Connection Health .........................................12
4. CoAP over WebSockets ...........................................13
4.1. Opening Handshake .........................................15
4.2. Message Format ............................................15
4.3. Message Transmission ......................................16
4.4. Connection Health .........................................17
5. Signaling ......................................................17
5.1. Signaling Codes ...........................................17
5.2. Signaling Option Numbers ..................................18
5.3. Capabilities and Settings Messages (CSMs) .................18
5.4. Ping and Pong Messages ....................................20
5.5. Release Messages ..........................................21
5.6. Abort Messages ............................................23
5.7. Signaling Examples ........................................24
6. Block-Wise Transfer and Reliable Transports ....................25
6.1. Example: GET with BERT Blocks .............................27
6.2. Example: PUT with BERT Blocks .............................27
7. Observing Resources over Reliable Transports ...................28
7.1. Notifications and Reordering ..............................28
7.2. Transmission and Acknowledgments ..........................28
7.3. Freshness .................................................28
7.4. Cancellation ..............................................29
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8. CoAP over Reliable Transport URIs ..............................29
8.1. coap+tcp URI Scheme .......................................30
8.2. coaps+tcp URI Scheme ......................................31
8.3. coap+ws URI Scheme ........................................32
8.4. coaps+ws URI Scheme .......................................33
8.5. Uri-Host and Uri-Port Options .............................33
8.6. Decomposing URIs into Options .............................34
8.7. Composing URIs from Options ...............................35
9. Securing CoAP ..................................................35
9.1. TLS Binding for CoAP over TCP .............................36
9.2. TLS Usage for CoAP over WebSockets ........................37
10. Security Considerations .......................................37
10.1. Signaling Messages .......................................37
11. IANA Considerations ...........................................38
11.1. Signaling Codes ..........................................38
11.2. CoAP Signaling Option Numbers Registry ...................38
11.3. Service Name and Port Number Registration ................40
11.4. Secure Service Name and Port Number Registration .........40
11.5. URI Scheme Registration ..................................41
11.6. Well-Known URI Suffix Registration .......................43
11.7. ALPN Protocol Identifier .................................44
11.8. WebSocket Subprotocol Registration .......................44
11.9. CoAP Option Numbers Registry .............................44
12. References ....................................................45
12.1. Normative References .....................................45
12.2. Informative References ...................................47
Appendix A. Examples of CoAP over WebSockets ......................49
Acknowledgments ...................................................52
Contributors ......................................................52
Authors' Addresses ................................................53
1. Introduction
The Constrained Application Protocol (CoAP) [RFC7252] was designed
for Internet of Things (IoT) deployments, assuming that UDP [RFC768]
can be used unimpeded as can the Datagram Transport Layer Security
(DTLS) protocol [RFC6347] over UDP. The use of CoAP over UDP is
focused on simplicity, has a low code footprint, and has a small
over-the-wire message size.
The primary reason for introducing CoAP over TCP [RFC793] and TLS
[RFC5246] is that some networks do not forward UDP packets. Complete
blocking of UDP happens in between about 2% and 4% of terrestrial
access networks, according to [EK2016]. UDP impairment is especially
concentrated in enterprise networks and networks in geographic
regions with otherwise challenged connectivity. Some networks also
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rate-limit UDP traffic, as reported in [BK2015], and deployment
investigations related to the standardization of Quick UDP Internet
Connections (QUIC) revealed numbers around 0.3% [SW2016].
The introduction of CoAP over TCP also leads to some additional
effects that may be desirable in a specific deployment:
o Where NATs are present along the communication path, CoAP over TCP
leads to different NAT traversal behavior than CoAP over UDP.
NATs often calculate expiration timers based on the
transport-layer protocol being used by application protocols.
Many NATs maintain TCP-based NAT bindings for longer periods based
on the assumption that a transport-layer protocol, such as TCP,
offers additional information about the session lifecycle. UDP,
on the other hand, does not provide such information to a NAT and
timeouts tend to be much shorter [HomeGateway]. According to
[HomeGateway], the mean for TCP and UDP NAT binding timeouts is
386 minutes (TCP) and 160 seconds (UDP). Shorter timeout values
require keepalive messages to be sent more frequently. Hence, the
use of CoAP over TCP requires less-frequent transmission of
keepalive messages.
o TCP utilizes mechanisms for congestion control and flow control
that are more sophisticated than the default mechanisms provided
by CoAP over UDP; these TCP mechanisms are useful for the transfer
of larger payloads. (However, work is ongoing to add advanced
congestion control to CoAP over UDP as well; see [CoCoA].)
Note that the use of CoAP over UDP (and CoAP over DTLS over UDP) is
still the recommended transport for use in constrained node networks,
particularly when used in concert with block-wise transfer. CoAP
over TCP is applicable for those cases where the networking
infrastructure leaves no other choice. The use of CoAP over TCP
leads to a larger code size, more round trips, increased RAM
requirements, and larger packet sizes. Developers implementing CoAP
over TCP are encouraged to consult [TCP-in-IoT] for guidance on
low-footprint TCP implementations for IoT devices.
Standards based on CoAP, such as Lightweight Machine to Machine
[LWM2M], currently use CoAP over UDP as a transport; adding support
for CoAP over TCP enables them to address the issues above for
specific deployments and to protect investments in existing CoAP
implementations and deployments.
Although HTTP/2 could also potentially address the need for
enterprise firewall traversal, there would be additional costs and
delays introduced by such a transition from CoAP to HTTP/2.
Currently, there are also fewer HTTP/2 implementations available for
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constrained devices in comparison to CoAP. Since CoAP also supports
group communication using IP-layer multicast and unreliable
communication, IoT devices would have to support HTTP/2 in addition
to CoAP.
Furthermore, CoAP may be integrated into a web environment where the
front end uses CoAP over UDP from IoT devices to a cloud
infrastructure and then CoAP over TCP between the back-end services.
A TCP-to-UDP gateway can be used at the cloud boundary to communicate
with the UDP-based IoT device.
Finally, CoAP applications running inside a web browser may be
without access to connectivity other than HTTP. In this case, the
WebSocket Protocol [RFC6455] may be used to transport CoAP requests
and responses, as opposed to cross-proxying them via HTTP to an
HTTP-to-CoAP cross-proxy. This preserves the functionality of CoAP
without translation -- in particular, the Observe Option [RFC7641].
To address the above-mentioned deployment requirements, this document
defines how to transport CoAP over TCP, CoAP over TLS, and CoAP over
WebSockets. For these cases, the reliability offered by the
transport protocol subsumes the reliability functions of the message
layer used for CoAP over UDP. (Note that for both a reliable
transport and the message layer for CoAP over UDP, the reliability
offered is per transport hop: where proxies -- see Sections 5.7 and
10 of [RFC7252] -- are involved, that layer's reliability function
does not extend end to end.) Figure 1 illustrates the layering:
+--------------------------------+
| Application |
+--------------------------------+
+--------------------------------+
| Requests/Responses/Signaling | CoAP (RFC 7252) / This Document
|--------------------------------|
| Message Framing | This Document
+--------------------------------+
| Reliable Transport |
+--------------------------------+
Figure 1: Layering of CoAP over Reliable Transports
This document specifies how to access resources using CoAP requests
and responses over the TCP, TLS, and WebSocket protocols. This
allows connectivity-limited applications to obtain end-to-end CoAP
connectivity either (1) by communicating CoAP directly with a CoAP
server accessible over a TCP, TLS, or WebSocket connection or (2) via
a CoAP intermediary that proxies CoAP requests and responses between
different transports, such as between WebSockets and UDP.
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Section 7 updates [RFC7641] ("Observing Resources in the Constrained
Application Protocol (CoAP)") for use with CoAP over reliable
transports. [RFC7641] is an extension to CoAP that enables CoAP
clients to "observe" a resource on a CoAP server. (The CoAP client
retrieves a representation of a resource and registers to be notified
by the CoAP server when the representation is updated.)
2. Conventions and 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.
This document assumes that readers are familiar with the terms and
concepts that are used in [RFC6455], [RFC7252], [RFC7641], and
[RFC7959].
The term "reliable transport" is used only to refer to transport
protocols, such as TCP, that provide reliable and ordered delivery of
a byte stream.
Block-wise Extension for Reliable Transport (BERT):
Extends [RFC7959] to enable the use of larger messages over a
reliable transport.
BERT Option:
A Block1 or Block2 option that includes an SZX (block size)
value of 7.
BERT Block:
The payload of a CoAP message that is affected by a BERT Option in
descriptive usage (see Section 2.1 of [RFC7959]).
Transport Connection:
Underlying reliable byte-stream connection, as directly provided
by TCP or indirectly provided via TLS or WebSockets.
Connection:
Transport Connection, unless explicitly qualified otherwise.
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Connection Initiator:
The peer that opens a Transport Connection, i.e., the TCP active
opener, TLS client, or WebSocket client.
Connection Acceptor:
The peer that accepts the Transport Connection opened by the other
peer, i.e., the TCP passive opener, TLS server, or WebSocket
server.
3. CoAP over TCP
The request/response interaction model of CoAP over TCP is the same
as CoAP over UDP. The primary differences are in the message layer.
The message layer of CoAP over UDP supports optional reliability by
defining four types of messages: Confirmable, Non-confirmable,
Acknowledgment, and Reset. In addition, messages include a
Message ID to relate Acknowledgments to Confirmable messages and to
detect duplicate messages.
Management of the transport connections is left to the application,
i.e., the present specification does not describe how an application
decides to open a connection or to reopen another one in the presence
of failures (or what it would deem to be a failure; see also
Section 5.4). In particular, the Connection Initiator need not be
the client of the first request placed on the connection. Some
implementations will want to implement dynamic connection management
similar to the technique described in Section 6 of [RFC7230] for
HTTP: opening a connection when the first client request is ready to
be sent, reusing that connection for subsequent messages until no
more messages are sent for a certain time period and no requests are
outstanding (possibly with a configurable idle time), and then
starting a release process (orderly shutdown) (see Section 5.5). In
implementations of this kind, connection releases or aborts may not
be indicated as errors to the application but may simply be handled
by automatic reconnection once the need arises again. Other
implementations may be based on configured connections that are kept
open continuously and lead to management system notifications on
release or abort. The protocol defined in the present specification
is intended to work with either model (or other, application-specific
connection management models).
3.1. Messaging Model
Conceptually, CoAP over TCP replaces most of the message layer of
CoAP over UDP with a framing mechanism on top of the byte stream
provided by TCP/TLS, conveying the length information for each
message that, on datagram transports, is provided by the UDP/DTLS
datagram layer.
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TCP ensures reliable message transmission, so the message layer of
CoAP over TCP is not required to support Acknowledgment messages or
to detect duplicate messages. As a result, both the Type and
Message ID fields are no longer required and are removed from the
message format for CoAP over TCP.
Figure 2 illustrates the difference between CoAP over UDP and CoAP
over reliable transports. The removed Type and Message ID fields are
indicated by dashes.
CoAP Client CoAP Server CoAP Client CoAP Server
| | | |
| CON [0xbc90] | | (-------) [------] |
| GET /temperature | | GET /temperature |
| (Token 0x71) | | (Token 0x71) |
+------------------->| +------------------->|
| | | |
| ACK [0xbc90] | | (-------) [------] |
| 2.05 Content | | 2.05 Content |
| (Token 0x71) | | (Token 0x71) |
| "22.5 C" | | "22.5 C" |
|<-------------------+ |<-------------------+
| | | |
CoAP over UDP CoAP over reliable
transports
Figure 2: Comparison between CoAP over Unreliable Transports and
CoAP over Reliable Transports
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3.2. Message Format
The CoAP message format defined in [RFC7252], as shown in Figure 3,
relies on the datagram transport (UDP, or DTLS over UDP) for keeping
the individual messages separate and for providing length
information.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|Ver| T | TKL | Code | Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Token (if any, TKL bytes) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1| Payload (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: CoAP Message Format as Defined in RFC 7252
The message format for CoAP over TCP is very similar to the format
specified for CoAP over UDP. The differences are as follows:
o Since the underlying TCP connection provides retransmissions and
deduplication, there is no need for the reliability mechanisms
provided by CoAP over UDP. The Type (T) and Message ID fields in
the CoAP message header are elided.
o The Version (Vers) field is elided as well. In contrast to the
message format of CoAP over UDP, the message format for CoAP over
TCP does not include a version number. CoAP is defined in
[RFC7252] with a version number of 1. At this time, there is no
known reason to support version numbers different from 1. If
version negotiation needs to be addressed in the future,
Capabilities and Settings Messages (CSMs) (see Section 5.3) have
been specifically designed to enable such a potential feature.
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o In a stream-oriented transport protocol such as TCP, a form of
message delimitation is needed. For this purpose, CoAP over TCP
introduces a length field with variable size. Figure 4 shows the
adjusted CoAP message format with a modified structure for the
fixed header (first 4 bytes of the header for CoAP over UDP),
which includes the length information of variable size.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Len | TKL | Extended Length (if any, as chosen by Len) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Token (if any, TKL bytes) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1| Payload (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: CoAP Frame for Reliable Transports
Length (Len): 4-bit unsigned integer. A value between 0 and 12
inclusive indicates the length of the message in bytes, starting
with the first bit of the Options field. Three values are
reserved for special constructs:
13: An 8-bit unsigned integer (Extended Length) follows the
initial byte and indicates the length of options/payload
minus 13.
14: A 16-bit unsigned integer (Extended Length) in network byte
order follows the initial byte and indicates the length of
options/payload minus 269.
15: A 32-bit unsigned integer (Extended Length) in network byte
order follows the initial byte and indicates the length of
options/payload minus 65805.
The encoding of the Length field is modeled after the Option Length
field of the CoAP Options (see Section 3.1 of [RFC7252]).
For simplicity, a Payload Marker (0xFF) is shown in Figure 4; the
Payload Marker indicates the start of the optional payload and is
absent for zero-length payloads (see Section 3 of [RFC7252]). (If
present, the Payload Marker is included in the message length, which
counts from the start of the Options field to the end of the Payload
field.)
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For example, a CoAP message just containing a 2.03 code with the
Token 7f and no options or payload is encoded as shown in Figure 5.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x01 | 0x43 | 0x7f |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Len = 0 ------> 0x01
TKL = 1 ___/
Code = 2.03 --> 0x43
Token = 0x7f
Figure 5: CoAP Message with No Options or Payload
The semantics of the other CoAP header fields are left unchanged.
3.3. Message Transmission
Once a Transport Connection is established, each endpoint MUST send a
CSM (see Section 5.3) as its first message on the connection. This
message establishes the initial settings and capabilities for the
endpoint, such as maximum message size or support for block-wise
transfers. The absence of options in the CSM indicates that base
values are assumed.
To avoid a deadlock, the Connection Initiator MUST NOT wait for the
Connection Acceptor to send its initial CSM before sending its own
initial CSM. Conversely, the Connection Acceptor MAY wait for the
Connection Initiator to send its initial CSM before sending its own
initial CSM.
To avoid unnecessary latency, a Connection Initiator MAY send
additional messages after its initial CSM without waiting to receive
the Connection Acceptor's CSM; however, it is important to note that
the Connection Acceptor's CSM might indicate capabilities that impact
how the Connection Initiator is expected to communicate with the
Connection Acceptor. For example, the Connection Acceptor's CSM
could indicate a Max-Message-Size Option (see Section 5.3.1) that is
smaller than the base value (1152) in order to limit both buffering
requirements and head-of-line blocking.
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Endpoints MUST treat a missing or invalid CSM as a connection error
and abort the connection (see Section 5.6).
CoAP requests and responses are exchanged asynchronously over the
Transport Connection. A CoAP client can send multiple requests
without waiting for a response, and the CoAP server can return
responses in any order. Responses MUST be returned over the same
connection as the originating request. Each concurrent request is
differentiated by its Token, which is scoped locally to the
connection.
The Transport Connection is bidirectional, so requests can be sent by
both the entity that established the connection (Connection
Initiator) and the remote host (Connection Acceptor). If one side
does not implement a CoAP server, an error response MUST be returned
for all CoAP requests from the other side. The simplest approach is
to always return 5.01 (Not Implemented). A more elaborate mock
server could also return 4.xx responses such as 4.04 (Not Found) or
4.02 (Bad Option) where appropriate.
Retransmission and deduplication of messages are provided by TCP.
3.4. Connection Health
Empty messages (Code 0.00) can always be sent and MUST be ignored by
the recipient. This provides a basic keepalive function that can
refresh NAT bindings.
If a CoAP client does not receive any response for some time after
sending a CoAP request (or, similarly, when a client observes a
resource and it does not receive any notification for some time), it
can send a CoAP Ping Signaling message (see Section 5.4) to test the
Transport Connection and verify that the CoAP server is responsive.
When the underlying Transport Connection is closed or reset, the
signaling state and any observation state (see Section 7.4)
associated with the connection are removed. Messages that are
in flight may or may not be lost.
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4. CoAP over WebSockets
CoAP over WebSockets is intentionally similar to CoAP over TCP;
therefore, this section only specifies the differences between the
transports.
CoAP over WebSockets can be used in a number of configurations. The
most basic configuration is a CoAP client retrieving or updating a
CoAP resource located on a CoAP server that exposes a WebSocket
endpoint (see Figure 6). The CoAP client acts as the WebSocket
client, establishes a WebSocket connection, and sends a CoAP request,
to which the CoAP server returns a CoAP response. The WebSocket
connection can be used for any number of requests.
___________ ___________
| | | |
| _|___ requests ___|_ |
| CoAP / \ \ -------------> / / \ CoAP |
| Client \__/__/ <------------- \__\__/ Server |
| | responses | |
|___________| |___________|
WebSocket =============> WebSocket
Client Connection Server
Figure 6: CoAP Client (WebSocket Client) Accesses CoAP Server
(WebSocket Server)
The challenge with this configuration is how to identify a resource
in the namespace of the CoAP server. When the WebSocket Protocol is
used by a dedicated client directly (i.e., not from a web page
through a web browser), the client can connect to any WebSocket
endpoint. Sections 8.3 and 8.4 define new URI schemes that enable
the client to identify both a WebSocket endpoint and the path and
query of the CoAP resource within that endpoint.
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Another possible configuration is to set up a CoAP forward proxy at
the WebSocket endpoint. Depending on what transports are available
to the proxy, it could forward the request to a CoAP server with a
CoAP UDP endpoint (Figure 7), an SMS endpoint (a.k.a. mobile phone),
or even another WebSocket endpoint. The CoAP client specifies the
resource to be updated or retrieved in the Proxy-Uri Option.
___________ ___________ ___________
| | | | | |
| _|___ ___|_ _|___ ___|_ |
| CoAP / \ \ ---> / / \ CoAP / \ \ ---> / / \ CoAP |
| Client \__/__/ <--- \__\__/ Proxy \__/__/ <--- \__\__/ Server |
| | | | | |
|___________| |___________| |___________|
WebSocket ===> WebSocket UDP UDP
Client Server Client Server
Figure 7: CoAP Client (WebSocket Client) Accesses CoAP Server
(UDP Server) via a CoAP Proxy (WebSocket Server / UDP Client)
A third possible configuration is a CoAP server running inside a web
browser (Figure 8). The web browser initially connects to a
WebSocket endpoint and is then reachable through the WebSocket
server. When no connection exists, the CoAP server is unreachable.
Because the WebSocket server is the only way to reach the CoAP
server, the CoAP proxy should be a reverse-proxy.
___________ ___________ ___________
| | | | | |
| _|___ ___|_ _|___ ___|_ |
| CoAP / \ \ ---> / / \ CoAP / / \ ---> / \ \ CoAP |
| Client \__/__/ <--- \__\__/ Proxy \__\__/ <--- \__/__/ Server |
| | | | | |
|___________| |___________| |___________|
UDP UDP WebSocket <=== WebSocket
Client Server Server Client
Figure 8: CoAP Client (UDP Client) Accesses CoAP Server (WebSocket
Client) via a CoAP Proxy (UDP Server / WebSocket Server)
Further configurations are possible, including those where a
WebSocket connection is established through an HTTP proxy.
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4.1. Opening Handshake
Before CoAP requests and responses are exchanged, a WebSocket
connection is established as defined in Section 4 of [RFC6455].
Figure 9 shows an example.
The WebSocket client MUST include the subprotocol name "coap" in the
list of protocols; this indicates support for the protocol defined in
this document.
The WebSocket client includes the hostname of the WebSocket server in
the Host header field of its handshake as per [RFC6455]. The Host
header field also indicates the default value of the Uri-Host Option
in requests from the WebSocket client to the WebSocket server.
GET /.well-known/coap HTTP/1.1
Host: example.org
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Sec-WebSocket-Protocol: coap
Sec-WebSocket-Version: 13
HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=
Sec-WebSocket-Protocol: coap
Figure 9: Example of an Opening Handshake
4.2. Message Format
Once a WebSocket connection is established, CoAP requests and
responses can be exchanged as WebSocket messages. Since CoAP uses a
binary message format, the messages are transmitted in binary data
frames as specified in Sections 5 and 6 of [RFC6455].
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The message format shown in Figure 10 is the same as the message
format for CoAP over TCP (see Section 3.2), with one change: the
Length (Len) field MUST be set to zero, because the WebSocket frame
contains the length.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Len=0 | TKL | Code | Token (TKL bytes) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1| Payload (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: CoAP Message Format over WebSockets
As with CoAP over TCP, the message format for CoAP over WebSockets
eliminates the Version field defined in CoAP over UDP. If CoAP
version negotiation is required in the future, CoAP over WebSockets
can address the requirement by defining a new subprotocol identifier
that is negotiated during the opening handshake.
Requests and responses can be fragmented as specified in Section 5.4
of [RFC6455], though typically they are sent unfragmented, as they
tend to be small and fully buffered before transmission. The
WebSocket Protocol does not provide means for multiplexing. If it is
not desirable for a large message to monopolize the connection,
requests and responses can be transferred in a block-wise fashion as
defined in [RFC7959].
4.3. Message Transmission
As with CoAP over TCP, each endpoint MUST send a CSM (see
Section 5.3) as its first message on the WebSocket connection.
CoAP requests and responses are exchanged asynchronously over the
WebSocket connection. A CoAP client can send multiple requests
without waiting for a response, and the CoAP server can return
responses in any order. Responses MUST be returned over the same
connection as the originating request. Each concurrent request is
differentiated by its Token, which is scoped locally to the
connection.
The connection is bidirectional, so requests can be sent by both the
entity that established the connection and the remote host.
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As with CoAP over TCP, retransmission and deduplication of messages
are provided by the WebSocket Protocol. CoAP over WebSockets
therefore does not make a distinction between Confirmable messages
and Non-confirmable messages and does not provide Acknowledgment or
Reset messages.
4.4. Connection Health
As with CoAP over TCP, a CoAP client can test the health of the
connection for CoAP over WebSockets by sending a CoAP Ping Signaling
message (Section 5.4). To ensure that redundant maintenance traffic
is not transmitted, WebSocket Ping and unsolicited Pong frames
(Section 5.5 of [RFC6455]) SHOULD NOT be used.
5. Signaling
Signaling messages are specifically introduced only for CoAP over
reliable transports to allow peers to:
o Learn related characteristics, such as maximum message size for
the connection.
o Shut down the connection in an orderly fashion.
o Provide diagnostic information when terminating a connection in
response to a serious error condition.
Signaling is a third basic kind of message in CoAP, after requests
and responses. Signaling messages share a common structure with the
existing CoAP messages. There are a code, a Token, options, and an
optional payload.
(See Section 3 of [RFC7252] for the overall structure of the message
format, option format, and option value formats.)
5.1. Signaling Codes
A code in the 7.00-7.31 range indicates a Signaling message. Values
in this range are assigned by the "CoAP Signaling Codes" subregistry
(see Section 11.1).
For each message, there are a sender and a peer receiving the
message.
Payloads in Signaling messages are diagnostic payloads as defined in
Section 5.5.2 of [RFC7252], unless otherwise defined by a Signaling
message option.
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5.2. Signaling Option Numbers
Option Numbers for Signaling messages are specific to the message
code. They do not share the number space with CoAP options for
request/response messages or with Signaling messages using other
codes.
Option Numbers are assigned by the "CoAP Signaling Option Numbers"
subregistry (see Section 11.2).
Signaling Options are elective or critical as defined in
Section 5.4.1 of [RFC7252]. If a Signaling Option is critical and
not understood by the receiver, it MUST abort the connection (see
Section 5.6). If the option is understood but cannot be processed,
the option documents the behavior.
5.3. Capabilities and Settings Messages (CSMs)
CSMs are used for two purposes:
o Each capability option indicates one capability of the sender to
the recipient.
o Each setting option indicates a setting that will be applied by
the sender.
One CSM MUST be sent by each endpoint at the start of the Transport
Connection. Additional CSMs MAY be sent at any other time by either
endpoint over the lifetime of the connection.
Both capability options and setting options are cumulative. A CSM
does not invalidate a previously sent capability indication or
setting even if it is not repeated. A capability message without any
option is a no-operation (and can be used as such). An option that
is sent might override a previous value for the same option. The
option defines how to handle this case if needed.
Base values are listed below for CSM options. These are the values
for the capability and settings before any CSMs send a modified
value.
These are not default values (as defined in Section 5.4.4 in
[RFC7252]) for the option. Default values apply on a per-message
basis and are thus reset when the value is not present in a
given CSM.
CSMs are indicated by the 7.01 (CSM) code; see Table 1
(Section 11.1).
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5.3.1. Max-Message-Size Capability Option
The sender can use the elective Max-Message-Size Option to indicate
the maximum size of a message in bytes that it can receive. The
message size indicated includes the entire message, starting from the
first byte of the message header and ending at the end of the message
payload.
(Note that there is no relationship of the message size to the
overall request or response body size that may be achievable in
block-wise transfer. For example, the exchange depicted in Figure 13
(Section 6.1) can be performed if the CoAP client indicates a value
of around 6000 bytes for the Max-Message-Size Option, even though the
total body size transferred to the client is 3072 + 5120 + 4711 =
12903 bytes.)
+---+---+---+---------+------------------+--------+--------+--------+
| # | C | R | Applies | Name | Format | Length | Base |
| | | | to | | | | Value |
+---+---+---+---------+------------------+--------+--------+--------+
| 2 | | | CSM | Max-Message-Size | uint | 0-4 | 1152 |
+---+---+---+---------+------------------+--------+--------+--------+
C=Critical, R=Repeatable
As per Section 4.6 of [RFC7252], the base value (and the value used
when this option is not implemented) is 1152.
The active value of the Max-Message-Size Option is replaced each time
the option is sent with a modified value. Its starting value is its
base value.
5.3.2. Block-Wise-Transfer Capability Option
+---+---+---+---------+------------------+--------+--------+--------+
| # | C | R | Applies | Name | Format | Length | Base |
| | | | to | | | | Value |
+---+---+---+---------+------------------+--------+--------+--------+
| 4 | | | CSM | Block-Wise- | empty | 0 | (none) |
| | | | | Transfer | | | |
+---+---+---+---------+------------------+--------+--------+--------+
C=Critical, R=Repeatable
A sender can use the elective Block-Wise-Transfer Option to indicate
that it supports the block-wise transfer protocol [RFC7959].
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If the option is not given, the peer has no information about whether
block-wise transfers are supported by the sender or not. An
implementation wishing to offer block-wise transfers to its peer
therefore needs to indicate so via the Block-Wise-Transfer Option.
If a Max-Message-Size Option is indicated with a value that is
greater than 1152 (in the same CSM or a different CSM), the
Block-Wise-Transfer Option also indicates support for BERT (see
Section 6). Subsequently, if the Max-Message-Size Option is
indicated with a value equal to or less than 1152, BERT support is no
longer indicated. (Note that the indication of BERT support does not
oblige either peer to actually choose to make use of BERT.)
Implementation note: When indicating a value of the Max-Message-Size
Option with an intention to enable BERT, the indicating
implementation may want to (1) choose a particular BERT block size it
wants to encourage and (2) add a delta for the header and any options
that may also need to be included in the message with a BERT block of
that size. Section 4.6 of [RFC7252] adds 128 bytes to a maximum
block size of 1024 to arrive at a default message size of 1152. A
BERT-enabled implementation may want to indicate a BERT block size of
2048 or a higher multiple of 1024 and at the same time be more
generous with the size of the header and options added (say, 256 or
512). However, adding 1024 or more to the base BERT block size may
encourage the peer implementation to vary the BERT block size based
on the size of the options included; this type of scenario might make
it harder to establish interoperability.
5.4. Ping and Pong Messages
In CoAP over reliable transports, Empty messages (Code 0.00) can
always be sent and MUST be ignored by the recipient. This provides a
basic keepalive function. In contrast, Ping and Pong messages are a
bidirectional exchange.
Upon receipt of a Ping message, the receiver MUST return a Pong
message with an identical Token in response. Unless the Ping carries
an option with delaying semantics such as the Custody Option, it
SHOULD respond as soon as practical. As with all Signaling messages,
the recipient of a Ping or Pong message MUST ignore elective options
it does not understand.
Ping and Pong messages are indicated by the 7.02 code (Ping) and
the 7.03 code (Pong).
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Note that, as with similar mechanisms defined in [RFC6455] and
[RFC7540], the present specification does not define any specific
maximum time that the sender of a Ping message has to allow when
waiting for a Pong reply. Any limitations on patience for this reply
are a matter of the application making use of these messages, as is
any approach to recover from a failure to respond in time.
5.4.1. Custody Option
+---+---+---+----------+----------------+--------+--------+---------+
| # | C | R | Applies | Name | Format | Length | Base |
| | | | to | | | | Value |
+---+---+---+----------+----------------+--------+--------+---------+
| 2 | | | Ping, | Custody | empty | 0 | (none) |
| | | | Pong | | | | |
+---+---+---+----------+----------------+--------+--------+---------+
C=Critical, R=Repeatable
When responding to a Ping message, the receiver can include an
elective Custody Option in the Pong message. This option indicates
that the application has processed all the request/response messages
received prior to the Ping message on the current connection. (Note
that there is no definition of specific application semantics for
"processed", but there is an expectation that the receiver of a Pong
message with a Custody Option should be able to free buffers based on
this indication.)
A sender can also include an elective Custody Option in a Ping
message to explicitly request the inclusion of an elective Custody
Option in the corresponding Pong message. In that case, the receiver
SHOULD delay its Pong message until it finishes processing all the
request/response messages received prior to the Ping message on the
current connection.
5.5. Release Messages
A Release message indicates that the sender does not want to continue
maintaining the Transport Connection and opts for an orderly
shutdown, but wants to leave it to the peer to actually start closing
the connection. The details are in the options. A diagnostic
payload (see Section 5.5.2 of [RFC7252]) MAY be included.
A peer will normally respond to a Release message by closing the
Transport Connection. (In case that does not happen, the sender of
the release may want to implement a timeout mechanism if getting rid
of the connection is actually important to it.)
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Messages may be in flight or responses outstanding when the sender
decides to send a Release message (which is one reason the sender had
decided to wait before closing the connection). The peer responding
to the Release message SHOULD delay the closing of the connection
until it has responded to all requests received by it before the
Release message. It also MAY wait for the responses to its own
requests.
It is NOT RECOMMENDED for the sender of a Release message to continue
sending requests on the connection it already indicated to be
released: the peer might close the connection at any time and miss
those requests. The peer is not obligated to check for this
condition, though.
Release messages are indicated by the 7.04 code (Release).
Release messages can indicate one or more reasons using elective
options. The following options are defined:
+---+---+---+---------+------------------+--------+--------+--------+
| # | C | R | Applies | Name | Format | Length | Base |
| | | | to | | | | Value |
+---+---+---+---------+------------------+--------+--------+--------+
| 2 | | x | Release | Alternative- | string | 1-255 | (none) |
| | | | | Address | | | |
+---+---+---+---------+------------------+--------+--------+--------+
C=Critical, R=Repeatable
The elective Alternative-Address Option requests the peer to instead
open a connection of the same scheme as the present connection to the
alternative transport address given. Its value is in the form
"authority" as defined in Section 3.2 of [RFC3986]. (Existing state
related to the connection is not transferred from the present
connection to the new connection.)
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The Alternative-Address Option is a repeatable option as defined in
Section 5.4.5 of [RFC7252]. When multiple occurrences of the option
are included, the peer can choose any of the alternative transport
addresses.
+---+---+---+---------+-----------------+--------+--------+---------+
| # | C | R | Applies | Name | Format | Length | Base |
| | | | to | | | | Value |
+---+---+---+---------+-----------------+--------+--------+---------+
| 4 | | | Release | Hold-Off | uint | 0-3 | (none) |
+---+---+---+---------+-----------------+--------+--------+---------+
C=Critical, R=Repeatable
The elective Hold-Off Option indicates that the server is requesting
that the peer not reconnect to it for the number of seconds given in
the value.
5.6. Abort Messages
An Abort message indicates that the sender is unable to continue
maintaining the Transport Connection and cannot even wait for an
orderly release. The sender shuts down the connection immediately
after the Abort message (and may or may not wait for a Release
message, Abort message, or connection shutdown in the inverse
direction). A diagnostic payload (see Section 5.5.2 of [RFC7252])
SHOULD be included in the Abort message. Messages may be in flight
or responses outstanding when the sender decides to send an Abort
message. The general expectation is that these will NOT be
processed.
Abort messages are indicated by the 7.05 code (Abort).
Abort messages can indicate one or more reasons using elective
options. The following option is defined:
+---+---+---+---------+-----------------+--------+--------+---------+
| # | C | R | Applies | Name | Format | Length | Base |
| | | | to | | | | Value |
+---+---+---+---------+-----------------+--------+--------+---------+
| 2 | | | Abort | Bad-CSM-Option | uint | 0-2 | (none) |
+---+---+---+---------+-----------------+--------+--------+---------+
C=Critical, R=Repeatable
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Bad-CSM-Option, which is elective, indicates that the sender is
unable to process the CSM option identified by its Option Number,
e.g., when it is critical and the Option Number is unknown by the
sender, or when there is a parameter problem with the value of an
elective option. More detailed information SHOULD be included as a
diagnostic payload.
For CoAP over UDP, messages that contain syntax violations are
processed as message format errors. As described in Sections 4.2 and
4.3 of [RFC7252], such messages are rejected by sending a matching
Reset message and otherwise ignoring the message.
For CoAP over reliable transports, the recipient rejects such
messages by sending an Abort message and otherwise ignoring (not
processing) the message. No specific Option has been defined for the
Abort message in this case, as the details are best left to a
diagnostic payload.
5.7. Signaling Examples
An encoded example of a Ping message with a non-empty Token is shown
in Figure 11.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x01 | 0xe2 | 0x42 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Len = 0 -------> 0x01
TKL = 1 ___/
Code = 7.02 Ping --> 0xe2
Token = 0x42
Figure 11: Ping Message Example
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An encoded example of the corresponding Pong message is shown in
Figure 12.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x01 | 0xe3 | 0x42 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Len = 0 -------> 0x01
TKL = 1 ___/
Code = 7.03 Pong --> 0xe3
Token = 0x42
Figure 12: Pong Message Example
6. Block-Wise Transfer and Reliable Transports
The message size restrictions defined in Section 4.6 of [RFC7252] to
avoid IP fragmentation are not necessary when CoAP is used over a
reliable transport. While this suggests that the block-wise transfer
protocol [RFC7959] is also no longer needed, it remains applicable
for a number of cases:
o Large messages, such as firmware downloads, may cause undesired
head-of-line blocking when a single transport connection is used.
o A UDP-to-TCP gateway may simply not have the context to convert a
message with a Block Option into the equivalent exchange without
any use of a Block Option (it would need to convert the entire
block-wise exchange from start to end into a single exchange).
BERT extends the block-wise transfer protocol to enable the use of
larger messages over a reliable transport.
The use of this new extension is signaled by sending Block1 or Block2
Options with SZX == 7 (a "BERT Option"). SZX == 7 is a reserved
value in [RFC7959].
In control usage, a BERT Option is interpreted in the same way as the
equivalent Option with SZX == 6, except that it also indicates the
capability to process BERT blocks. As with the basic block-wise
transfer protocol, the recipient of a CoAP request with a BERT Option
in control usage is allowed to respond with a different SZX value,
e.g., to send a non-BERT block instead.
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In descriptive usage, a BERT Option is interpreted in the same way as
the equivalent Option with SZX == 6, except that the payload is also
allowed to contain multiple blocks. For non-final BERT blocks, the
payload is always a multiple of 1024 bytes. For final BERT blocks,
the payload is a multiple (possibly 0) of 1024 bytes plus a partial
block of less than 1024 bytes.
The recipient of a non-final BERT block (M=1) conceptually partitions
the payload into a sequence of 1024-byte blocks and acts exactly as
if it had received this sequence in conjunction with block numbers
starting at, and sequentially increasing from, the block number given
in the Block Option. In other words, the entire BERT block is
positioned at the byte position that results from multiplying the
block number by 1024. The position of further blocks to be
transferred is indicated by incrementing the block number by the
number of elements in this sequence (i.e., the size of the payload
divided by 1024 bytes).
As with SZX == 6, the recipient of a final BERT block (M=0) simply
appends the payload at the byte position that is indicated by the
block number multiplied by 1024.
The following examples illustrate BERT Options. A value of SZX == 7
is labeled as "BERT" or as "BERT(nnn)" to indicate a payload of
size nnn.
In all these examples, a Block Option is decomposed to indicate the
kind of Block Option (1 or 2) followed by a colon, the block number
(NUM), the more bit (M), and the block size (2**(SZX + 4)) separated
by slashes. For example, a Block2 Option value of 33 would be shown
as 2:2/0/32), or a Block1 Option value of 59 would be shown as
1:3/1/128.
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6.1. Example: GET with BERT Blocks
Figure 13 shows a GET request with a response that is split into
three BERT blocks. The first response contains 3072 bytes of
payload; the second, 5120; and the third, 4711. Note how the block
number increments to move the position inside the response body
forward.
CoAP Client CoAP Server
| |
| GET, /status ------> |
| |
| <------ 2.05 Content, 2:0/1/BERT(3072) |
| |
| GET, /status, 2:3/0/BERT ------> |
| |
| <------ 2.05 Content, 2:3/1/BERT(5120) |
| |
| GET, /status, 2:8/0/BERT ------> |
| |
| <------ 2.05 Content, 2:8/0/BERT(4711) |
Figure 13: GET with BERT Blocks
6.2. Example: PUT with BERT Blocks
Figure 14 demonstrates a PUT exchange with BERT blocks.
CoAP Client CoAP Server
| |
| PUT, /options, 1:0/1/BERT(8192) ------> |
| |
| <------ 2.31 Continue, 1:0/1/BERT |
| |
| PUT, /options, 1:8/1/BERT(16384) ------> |
| |
| <------ 2.31 Continue, 1:8/1/BERT |
| |
| PUT, /options, 1:24/0/BERT(5683) ------> |
| |
| <------ 2.04 Changed, 1:24/0/BERT |
| |
Figure 14: PUT with BERT Blocks
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7. Observing Resources over Reliable Transports
This section describes how the procedures defined in [RFC7641] for
observing resources over CoAP are applied (and modified, as needed)
for reliable transports. In this section, "client" and "server"
refer to the CoAP client and CoAP server.
7.1. Notifications and Reordering
When using the Observe Option [RFC7641] with CoAP over UDP,
notifications from the server set the option value to an increasing
sequence number for reordering detection on the client, since
messages can arrive in a different order than they were sent. This
sequence number is not required for CoAP over reliable transports,
since TCP ensures reliable and ordered delivery of messages. The
value of the Observe Option in 2.xx notifications MAY be empty on
transmission and MUST be ignored on reception.
Implementation note: This means that a proxy from a reordering
transport to a reliable (in-order) transport (such as a UDP-to-TCP
proxy) needs to process the Observe Option in notifications according
to the rules in Section 3.4 of [RFC7641].
7.2. Transmission and Acknowledgments
For CoAP over UDP, server notifications to the client can be
Confirmable or Non-confirmable. A Confirmable message requires the
client to respond with either an Acknowledgment message or a Reset
message. An Acknowledgment message indicates that the client is
alive and wishes to receive further notifications. A Reset message
indicates that the client does not recognize the Token; this causes
the server to remove the associated entry from the list of observers.
Since TCP eliminates the need for the message layer to support
reliability, CoAP over reliable transports does not support
Confirmable or Non-confirmable message types. All notifications are
delivered reliably to the client with positive acknowledgment of
receipt occurring at the TCP level. If the client does not recognize
the Token in a notification, it MAY immediately abort the connection
(see Section 5.6).
7.3. Freshness
For CoAP over UDP, if a client does not receive a notification for
some time, it can send a new GET request with the same Token as the
original request to re-register its interest in a resource and verify
that the server is still responsive. For CoAP over reliable
transports, it is more efficient to check the health of the
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connection (and all its active observations) by sending a single CoAP
Ping Signaling message (Section 5.4) rather than individual requests
to confirm each active observation. (Note that such a Ping/Pong only
confirms a single hop: a proxy is not obligated or expected to react
to a Ping by checking all its own registered interests or all the
connections, if any, underlying them. A proxy MAY maintain its own
schedule for confirming the interests that it relies on being
registered toward the origin server; however, it is generally
inadvisable for a proxy to generate a large number of outgoing checks
based on a single incoming check.)
7.4. Cancellation
For CoAP over UDP, a client that is no longer interested in receiving
notifications can "forget" the observation and respond to the next
notification from the server with a Reset message to cancel the
observation.
For CoAP over reliable transports, a client MUST explicitly
deregister by issuing a GET request that has the Token field set to
the Token of the observation to be canceled and includes an Observe
Option with the value set to 1 (deregister).
If the client observes one or more resources over a reliable
transport, then the CoAP server (or intermediary in the role of the
CoAP server) MUST remove all entries associated with the client
endpoint from the lists of observers when the connection either
times out or is closed.
8. CoAP over Reliable Transport URIs
CoAP over UDP [RFC7252] defines the "coap" and "coaps" URI schemes.
This document introduces four additional URI schemes for identifying
CoAP resources and providing a means of locating the resource:
o The "coap+tcp" URI scheme for CoAP over TCP.
o The "coaps+tcp" URI scheme for CoAP over TCP secured by TLS.
o The "coap+ws" URI scheme for CoAP over WebSockets.
o The "coaps+ws" URI scheme for CoAP over WebSockets secured by TLS.
Resources made available via these schemes have no shared identity
even if their resource identifiers indicate the same authority (the
same host listening to the same TCP port). They are hosted in
distinct namespaces because each URI scheme implies a distinct origin
server.
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In this section, the syntax for the URI schemes is specified using
the Augmented Backus-Naur Form (ABNF) [RFC5234]. The definitions of
"host", "port", "path-abempty", and "query" are adopted from
[RFC3986].
Section 8 ("Multicast CoAP") in [RFC7252] is not applicable to these
schemes.
As with the "coap" and "coaps" schemes defined in [RFC7252], all URI
schemes defined in this section also support the path prefix
"/.well-known/" as defined by [RFC5785] for "well-known locations" in
the namespace of a host. This enables discovery as per Section 7 of
[RFC7252].
8.1. coap+tcp URI Scheme
The "coap+tcp" URI scheme identifies CoAP resources that are intended
to be accessible using CoAP over TCP.
coap-tcp-URI = "coap+tcp:" "//" host [ ":" port ]
path-abempty [ "?" query ]
The syntax defined in Section 6.1 of [RFC7252] applies to this URI
scheme, with the following change:
o The port subcomponent indicates the TCP port at which the CoAP
Connection Acceptor is located. (If it is empty or not given,
then the default port 5683 is assumed, as with UDP.)
Encoding considerations: The scheme encoding conforms to the
encoding rules established for URIs in [RFC3986].
Interoperability considerations: None.
Security considerations: See Section 11.1 of [RFC7252].
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8.2. coaps+tcp URI Scheme
The "coaps+tcp" URI scheme identifies CoAP resources that are
intended to be accessible using CoAP over TCP secured with TLS.
coaps-tcp-URI = "coaps+tcp:" "//" host [ ":" port ]
path-abempty [ "?" query ]
The syntax defined in Section 6.2 of [RFC7252] applies to this URI
scheme, with the following changes:
o The port subcomponent indicates the TCP port at which the TLS
server for the CoAP Connection Acceptor is located. If it is
empty or not given, then the default port 5684 is assumed.
o If a TLS server does not support the Application-Layer Protocol
Negotiation (ALPN) extension [RFC7301] or wishes to accommodate
TLS clients that do not support ALPN, it MAY offer a coaps+tcp
endpoint on TCP port 5684. This endpoint MAY also be ALPN
enabled. A TLS server MAY offer coaps+tcp endpoints on ports
other than TCP port 5684, which MUST be ALPN enabled.
o For TCP ports other than port 5684, the TLS client MUST use the
ALPN extension to advertise the "coap" protocol identifier (see
Section 11.7) in the list of protocols in its ClientHello. If the
TCP server selects and returns the "coap" protocol identifier
using the ALPN extension in its ServerHello, then the connection
succeeds. If the TLS server either does not negotiate the ALPN
extension or returns a no_application_protocol alert, the TLS
client MUST close the connection.
o For TCP port 5684, a TLS client MAY use the ALPN extension to
advertise the "coap" protocol identifier in the list of protocols
in its ClientHello. If the TLS server selects and returns the
"coap" protocol identifier using the ALPN extension in its
ServerHello, then the connection succeeds. If the TLS server
returns a no_application_protocol alert, then the TLS client MUST
close the connection. If the TLS server does not negotiate the
ALPN extension, then coaps+tcp is implicitly selected.
o For TCP port 5684, if the TLS client does not use the ALPN
extension to negotiate the protocol, then coaps+tcp is implicitly
selected.
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Encoding considerations: The scheme encoding conforms to the
encoding rules established for URIs in [RFC3986].
Interoperability considerations: None.
Security considerations: See Section 11.1 of [RFC7252].
8.3. coap+ws URI Scheme
The "coap+ws" URI scheme identifies CoAP resources that are intended
to be accessible using CoAP over WebSockets.
coap-ws-URI = "coap+ws:" "//" host [ ":" port ]
path-abempty [ "?" query ]
The port subcomponent is OPTIONAL. The default is port 80.
The WebSocket endpoint is identified by a "ws" URI that is composed
of the authority part of the "coap+ws" URI and the well-known path
"/.well-known/coap" [RFC5785] [RFC8307]. Within the endpoint
specified in a "coap+ws" URI, the path and query parts of the URI
identify a resource that can be operated on by the methods defined
by CoAP:
coap+ws://example.org/sensors/temperature?u=Cel
\______ ______/\___________ ___________/
\/ \/
Uri-Path: "sensors"
ws://example.org/.well-known/coap Uri-Path: "temperature"
Uri-Query: "u=Cel"
Figure 15: The "coap+ws" URI Scheme
Encoding considerations: The scheme encoding conforms to the
encoding rules established for URIs in [RFC3986].
Interoperability considerations: None.
Security considerations: See Section 11.1 of [RFC7252].
Bormann, et al. Standards Track [Page 32]
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8.4. coaps+ws URI Scheme
The "coaps+ws" URI scheme identifies CoAP resources that are intended
to be accessible using CoAP over WebSockets secured by TLS.
coaps-ws-URI = "coaps+ws:" "//" host [ ":" port ]
path-abempty [ "?" query ]
The port subcomponent is OPTIONAL. The default is port 443.
The WebSocket endpoint is identified by a "wss" URI that is composed
of the authority part of the "coaps+ws" URI and the well-known path
"/.well-known/coap" [RFC5785] [RFC8307]. Within the endpoint
specified in a "coaps+ws" URI, the path and query parts of the URI
identify a resource that can be operated on by the methods defined
by CoAP:
coaps+ws://example.org/sensors/temperature?u=Cel
\______ ______/\___________ ___________/
\/ \/
Uri-Path: "sensors"
wss://example.org/.well-known/coap Uri-Path: "temperature"
Uri-Query: "u=Cel"
Figure 16: The "coaps+ws" URI Scheme
Encoding considerations: The scheme encoding conforms to the
encoding rules established for URIs in [RFC3986].
Interoperability considerations: None.
Security considerations: See Section 11.1 of [RFC7252].
8.5. Uri-Host and Uri-Port Options
CoAP over reliable transports maintains the property from
Section 5.10.1 of [RFC7252]:
The default values for the Uri-Host and Uri-Port Options are
sufficient for requests to most servers.
Unless otherwise noted, the default value of the Uri-Host Option is
the IP literal representing the destination IP address of the request
message. The default value of the Uri-Port Option is the destination
TCP port.
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For CoAP over TLS, these default values are the same, unless Server
Name Indication (SNI) [RFC6066] is negotiated. In this case, the
default value of the Uri-Host Option in requests from the TLS client
to the TLS server is the SNI host.
For CoAP over WebSockets, the default value of the Uri-Host Option in
requests from the WebSocket client to the WebSocket server is
indicated by the Host header field from the WebSocket handshake.
8.6. Decomposing URIs into Options
The steps are the same as those specified in Section 6.4 of
[RFC7252], with minor changes:
This step from [RFC7252]:
3. If |url| does not have a <scheme> component whose value, when
converted to ASCII lowercase, is "coap" or "coaps", then fail
this algorithm.
is updated to:
3. If |url| does not have a <scheme> component whose value, when
converted to ASCII lowercase, is "coap+tcp", "coaps+tcp",
"coap+ws", or "coaps+ws", then fail this algorithm.
This step from [RFC7252]:
7. If |port| does not equal the request's destination UDP port,
include a Uri-Port Option and let that option's value be |port|.
is updated to:
7. If |port| does not equal the request's destination TCP port,
include a Uri-Port Option and let that option's value be |port|.
Bormann, et al. Standards Track [Page 34]
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8.7. Composing URIs from Options
The steps are the same as those specified in Section 6.5 of
[RFC7252], with minor changes:
This step from [RFC7252]:
1. If the request is secured using DTLS, let |url| be the string
"coaps://". Otherwise, let |url| be the string "coap://".
is updated to:
1. For CoAP over TCP, if the request is secured using TLS, let |url|
be the string "coaps+tcp://". Otherwise, let |url| be the string
"coap+tcp://". For CoAP over WebSockets, if the request is
secured using TLS, let |url| be the string "coaps+ws://".
Otherwise, let |url| be the string "coap+ws://".
This step from [RFC7252]:
4. If the request includes a Uri-Port Option, let |port| be that
option's value. Otherwise, let |port| be the request's
destination UDP port.
is updated to:
4. If the request includes a Uri-Port Option, let |port| be that
option's value. Otherwise, let |port| be the request's
destination TCP port.
9. Securing CoAP
"Security Challenges For the Internet Of Things" [SecurityChallenges]
recommends the following:
... it is essential that IoT protocol suites specify a mandatory
to implement but optional to use security solution. This will
ensure security is available in all implementations, but
configurable to use when not necessary (e.g., in closed
environment). ... even if those features stretch the capabilities
of such devices.
A security solution MUST be implemented to protect CoAP over reliable
transports and MUST be enabled by default. This document defines the
TLS binding, but alternative solutions at different layers in the
protocol stack MAY be used to protect CoAP over reliable transports
Bormann, et al. Standards Track [Page 35]
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when appropriate. Note that there is ongoing work to support a data-
object-based security model for CoAP that is independent of transport
(see [OSCORE]).
9.1. TLS Binding for CoAP over TCP
The TLS usage guidance in [RFC7925] applies, including the guidance
about cipher suites in that document that are derived from the
mandatory-to-implement cipher suites defined in [RFC7252].
This guidance assumes implementation in a constrained device or for
communication with a constrained device. However, CoAP over TCP/TLS
has a wider applicability. It may, for example, be implemented on a
gateway or on a device that is less constrained (such as a smart
phone or a tablet), for communication with a peer that is likewise
less constrained, or within a back-end environment that only
communicates with constrained devices via proxies. As an exception
to the previous paragraph, in this case, the recommendations in
[RFC7525] are more appropriate.
Since the guidance offered in [RFC7925] differs from the guidance
offered in [RFC7525] in terms of algorithms and credential types, it
is assumed that an implementation of CoAP over TCP/TLS that needs to
support both cases implements the recommendations offered by both
specifications.
During the provisioning phase, a CoAP device is provided with the
security information that it needs, including keying materials,
access control lists, and authorization servers. At the end of the
provisioning phase, the device will be in one of four security modes:
NoSec: TLS is disabled.
PreSharedKey: TLS is enabled. The guidance in Section 4.2 of
[RFC7925] applies.
RawPublicKey: TLS is enabled. The guidance in Section 4.3 of
[RFC7925] applies.
Certificate: TLS is enabled. The guidance in Section 4.4 of
[RFC7925] applies.
The "NoSec" mode is optional to implement. The system simply sends
the packets over normal TCP; this is indicated by the "coap+tcp"
scheme and the TCP CoAP default port. The system is secured only by
keeping attackers from being able to send or receive packets from the
network with the CoAP nodes.
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"PreSharedKey", "RawPublicKey", or "Certificate" is mandatory to
implement for the TLS binding, depending on the credential type used
with the device. These security modes are achieved using TLS and
are indicated by the "coaps+tcp" scheme and TLS-secured CoAP
default port.
9.2. TLS Usage for CoAP over WebSockets
A CoAP client requesting a resource identified by a "coaps+ws" URI
negotiates a secure WebSocket connection to a WebSocket server
endpoint with a "wss" URI. This is described in Section 8.4.
The client MUST perform a TLS handshake after opening the connection
to the server. The guidance in Section 4.1 of [RFC6455] applies.
When a CoAP server exposes resources identified by a "coaps+ws" URI,
the guidance in Section 4.4 of [RFC7925] applies towards mandatory-
to-implement TLS functionality for certificates. For the server-side
requirements for accepting incoming connections over an HTTPS
(HTTP over TLS) port, the guidance in Section 4.2 of [RFC6455]
applies.
Note that the guidance above formally inherits the mandatory-to-
implement cipher suites defined in [RFC5246]. However, modern
browsers usually implement cipher suites that are more recent; these
cipher suites are then automatically picked up via the JavaScript
WebSocket API. WebSocket servers that provide secure CoAP over
WebSockets for the browser use case will need to follow the browser
preferences and MUST follow [RFC7525].
10. Security Considerations
The security considerations of [RFC7252] apply. For CoAP over
WebSockets and CoAP over TLS-secured WebSockets, the security
considerations of [RFC6455] also apply.
10.1. Signaling Messages
The guidance given by an Alternative-Address Option cannot be
followed blindly. In particular, a peer MUST NOT assume that a
successful connection to the Alternative-Address inherits all the
security properties of the current connection.
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11. IANA Considerations
11.1. Signaling Codes
IANA has created a third subregistry for values of the Code field in
the CoAP header (Section 12.1 of [RFC7252]). The name of this
subregistry is "CoAP Signaling Codes".
Each entry in the subregistry must include the Signaling Code in the
range 7.00-7.31, its name, and a reference to its documentation.
Initial entries in this subregistry are as follows:
+------+---------+-----------+
| Code | Name | Reference |
+------+---------+-----------+
| 7.01 | CSM | RFC 8323 |
| | | |
| 7.02 | Ping | RFC 8323 |
| | | |
| 7.03 | Pong | RFC 8323 |
| | | |
| 7.04 | Release | RFC 8323 |
| | | |
| 7.05 | Abort | RFC 8323 |
+------+---------+-----------+
Table 1: CoAP Signaling Codes
All other Signaling Codes are Unassigned.
The IANA policy for future additions to this subregistry is
"IETF Review" or "IESG Approval" as described in [RFC8126].
11.2. CoAP Signaling Option Numbers Registry
IANA has created a subregistry for Option Numbers used in CoAP
Signaling Options within the "Constrained RESTful Environments (CoRE)
Parameters" registry. The name of this subregistry is "CoAP
Signaling Option Numbers".
Each entry in the subregistry must include one or more of the codes
in the "CoAP Signaling Codes" subregistry (Section 11.1), the number
for the Option, the name of the Option, and a reference to the
Option's documentation.
Bormann, et al. Standards Track [Page 38]
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Initial entries in this subregistry are as follows:
+------------+--------+---------------------+-----------+
| Applies to | Number | Name | Reference |
+------------+--------+---------------------+-----------+
| 7.01 | 2 | Max-Message-Size | RFC 8323 |
| | | | |
| 7.01 | 4 | Block-Wise-Transfer | RFC 8323 |
| | | | |
| 7.02, 7.03 | 2 | Custody | RFC 8323 |
| | | | |
| 7.04 | 2 | Alternative-Address | RFC 8323 |
| | | | |
| 7.04 | 4 | Hold-Off | RFC 8323 |
| | | | |
| 7.05 | 2 | Bad-CSM-Option | RFC 8323 |
+------------+--------+---------------------+-----------+
Table 2: CoAP Signaling Option Codes
The IANA policy for future additions to this subregistry is based on
number ranges for the option numbers, analogous to the policy defined
in Section 12.2 of [RFC7252]. (The policy is analogous rather than
identical because the structure of this subregistry includes an
additional column ("Applies to"); however, the value of this column
has no influence on the policy.)
The documentation for a Signaling Option Number should specify the
semantics of an option with that number, including the following
properties:
o Whether the option is critical or elective, as determined by the
Option Number.
o Whether the option is repeatable.
o The format and length of the option's value.
o The base value for the option, if any.
Bormann, et al. Standards Track [Page 39]
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11.3. Service Name and Port Number Registration
IANA has assigned the port number 5683 and the service name "coap",
in accordance with [RFC6335].
Service Name:
coap
Transport Protocol:
tcp
Assignee:
IESG <iesg@ietf.org>
Contact:
IETF Chair <chair@ietf.org>
Description:
Constrained Application Protocol (CoAP)
Reference:
RFC 8323
Port Number:
5683
11.4. Secure Service Name and Port Number Registration
IANA has assigned the port number 5684 and the service name "coaps",
in accordance with [RFC6335]. The port number is to address the
exceptional case of TLS implementations that do not support the ALPN
extension [RFC7301].
Service Name:
coaps
Transport Protocol:
tcp
Assignee:
IESG <iesg@ietf.org>
Contact:
IETF Chair <chair@ietf.org>
Description:
Constrained Application Protocol (CoAP)
Bormann, et al. Standards Track [Page 40]
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Reference:
[RFC7301], RFC 8323
Port Number:
5684
11.5. URI Scheme Registration
URI schemes are registered within the "Uniform Resource Identifier
(URI) Schemes" registry maintained at [IANA.uri-schemes].
Note: The following has been added as a note for each of the URI
schemes defined in this document:
CoAP registers different URI schemes for accessing CoAP resources
via different protocols. This approach runs counter to the WWW
principle that a URI identifies a resource and that multiple URIs
for identifying the same resource should be avoided
<https://www.w3.org/TR/webarch/#avoid-uri-aliases>.
This is not a problem for many of the usage scenarios envisioned for
CoAP over reliable transports; additional URI schemes can be
introduced to address additional usage scenarios (as being prepared,
for example, in [Multi-Transport-URIs] and [CoAP-Alt-Transports]).
11.5.1. coap+tcp
IANA has registered the URI scheme "coap+tcp". This registration
request complies with [RFC7595].
Scheme name:
coap+tcp
Status:
Permanent
Applications/protocols that use this scheme name:
The scheme is used by CoAP endpoints to access CoAP resources
using TCP.
Contact:
IETF Chair <chair@ietf.org>
Change controller:
IESG <iesg@ietf.org>
Reference:
Section 8.1 in RFC 8323
Bormann, et al. Standards Track [Page 41]
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11.5.2. coaps+tcp
IANA has registered the URI scheme "coaps+tcp". This registration
request complies with [RFC7595].
Scheme name:
coaps+tcp
Status:
Permanent
Applications/protocols that use this scheme name:
The scheme is used by CoAP endpoints to access CoAP resources
using TLS.
Contact:
IETF Chair <chair@ietf.org>
Change controller:
IESG <iesg@ietf.org>
Reference:
Section 8.2 in RFC 8323
11.5.3. coap+ws
IANA has registered the URI scheme "coap+ws". This registration
request complies with [RFC7595].
Scheme name:
coap+ws
Status:
Permanent
Applications/protocols that use this scheme name:
The scheme is used by CoAP endpoints to access CoAP resources
using the WebSocket Protocol.
Contact:
IETF Chair <chair@ietf.org>
Change controller:
IESG <iesg@ietf.org>
Reference:
Section 8.3 in RFC 8323
Bormann, et al. Standards Track [Page 42]
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11.5.4. coaps+ws
IANA has registered the URI scheme "coaps+ws". This registration
request complies with [RFC7595].
Scheme name:
coaps+ws
Status:
Permanent
Applications/protocols that use this scheme name:
The scheme is used by CoAP endpoints to access CoAP resources
using the WebSocket Protocol secured with TLS.
Contact:
IETF Chair <chair@ietf.org>
Change controller:
IESG <iesg@ietf.org>
References:
Section 8.4 in RFC 8323
11.6. Well-Known URI Suffix Registration
IANA has registered "coap" in the "Well-Known URIs" registry. This
registration request complies with [RFC5785].
URI suffix:
coap
Change controller:
IETF
Specification document(s):
RFC 8323
Related information:
None.
Bormann, et al. Standards Track [Page 43]
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11.7. ALPN Protocol Identifier
IANA has assigned the following value in the "Application-Layer
Protocol Negotiation (ALPN) Protocol IDs" registry created by
[RFC7301]. The "coap" string identifies CoAP when used over TLS.
Protocol:
CoAP
Identification Sequence:
0x63 0x6f 0x61 0x70 ("coap")
Reference:
RFC 8323
11.8. WebSocket Subprotocol Registration
IANA has registered the WebSocket CoAP subprotocol in the "WebSocket
Subprotocol Name Registry":
Subprotocol Identifier:
coap
Subprotocol Common Name:
Constrained Application Protocol (CoAP)
Subprotocol Definition:
RFC 8323
11.9. CoAP Option Numbers Registry
IANA has added this document as a reference for the following entries
registered by [RFC7959] in the "CoAP Option Numbers" subregistry
defined by [RFC7252]:
+--------+--------+--------------------+
| Number | Name | Reference |
+--------+--------+--------------------+
| 23 | Block2 | RFC 7959, RFC 8323 |
| | | |
| 27 | Block1 | RFC 7959, RFC 8323 |
+--------+--------+--------------------+
Table 3: CoAP Option Numbers
Bormann, et al. Standards Track [Page 44]
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12. References
12.1. Normative References
[RFC793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, DOI 10.17487/RFC0793, September 1981,
<https://www.rfc-editor.org/info/rfc793>.
[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>.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, DOI 10.17487/RFC3986, January 2005,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234,
DOI 10.17487/RFC5234, January 2008,
<https://www.rfc-editor.org/info/rfc5234>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<https://www.rfc-editor.org/info/rfc5246>.
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785,
DOI 10.17487/RFC5785, April 2010,
<https://www.rfc-editor.org/info/rfc5785>.
[RFC6066] Eastlake 3rd, D., "Transport Layer Security (TLS)
Extensions: Extension Definitions", RFC 6066,
DOI 10.17487/RFC6066, January 2011,
<https://www.rfc-editor.org/info/rfc6066>.
[RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol",
RFC 6455, DOI 10.17487/RFC6455, December 2011,
<https://www.rfc-editor.org/info/rfc6455>.
[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>.
Bormann, et al. Standards Track [Page 45]
RFC 8323 TCP/TLS/WebSockets Transports for CoAP February 2018
[RFC7301] Friedl, S., Popov, A., Langley, A., and E. Stephan,
"Transport Layer Security (TLS) Application-Layer Protocol
Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
July 2014, <https://www.rfc-editor.org/info/rfc7301>.
[RFC7525] Sheffer, Y., Holz, R., and P. Saint-Andre,
"Recommendations for Secure Use of Transport Layer
Security (TLS) and Datagram Transport Layer Security
(DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525,
May 2015, <https://www.rfc-editor.org/info/rfc7525>.
[RFC7595] Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
and Registration Procedures for URI Schemes", BCP 35,
RFC 7595, DOI 10.17487/RFC7595, June 2015,
<https://www.rfc-editor.org/info/rfc7595>.
[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>.
[RFC7925] Tschofenig, H., Ed., and T. Fossati, "Transport Layer
Security (TLS) / Datagram Transport Layer Security (DTLS)
Profiles for the Internet of Things", RFC 7925,
DOI 10.17487/RFC7925, July 2016,
<https://www.rfc-editor.org/info/rfc7925>.
[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>.
[RFC8126] Cotton, M., Leiba, B., and T. Narten, "Guidelines for
Writing an IANA Considerations Section in RFCs", BCP 26,
RFC 8126, DOI 10.17487/RFC8126, June 2017,
<https://www.rfc-editor.org/info/rfc8126>.
[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>.
[RFC8307] Bormann, C., "Well-Known URIs for the WebSocket Protocol",
RFC 8307, DOI 10.17487/RFC8307, January 2018,
<https://www.rfc-editor.org/info/rfc8307>.
Bormann, et al. Standards Track [Page 46]
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12.2. Informative References
[BK2015] Byrne, C. and J. Kleberg, "Advisory Guidelines for UDP
Deployment", Work in Progress, draft-byrne-opsec-udp-
advisory-00, July 2015.
[CoAP-Alt-Transports]
Silverajan, B. and T. Savolainen, "CoAP Communication with
Alternative Transports", Work in Progress,
draft-silverajan-core-coap-alternative-transports-10,
July 2017.
[CoCoA] Bormann, C., Betzler, A., Gomez, C., and I. Demirkol,
"CoAP Simple Congestion Control/Advanced", Work in
Progress, draft-ietf-core-cocoa-02, October 2017.
[EK2016] Edeline, K., Kuehlewind, M., Trammell, B., Aben, E., and
B. Donnet, "Using UDP for Internet Transport Evolution",
arXiv preprint 1612.07816, December 2016,
<https://arxiv.org/abs/1612.07816>.
[HomeGateway]
Haetoenen, S., Nyrhinen, A., Eggert, L., Strowes, S.,
Sarolahti, P., and N. Kojo, "An experimental study of home
gateway characteristics", Proceedings of the 10th ACM
SIGCOMM conference on Internet measurement,
DOI 10.1145/1879141.1879174, November 2010.
[IANA.uri-schemes]
IANA, "Uniform Resource Identifier (URI) Schemes",
<https://www.iana.org/assignments/uri-schemes>.
[LWM2M] Open Mobile Alliance, "Lightweight Machine to Machine
Technical Specification Version 1.0", February 2017,
<http://www.openmobilealliance.org/release/LightweightM2M/
V1_0-20170208-A/
OMA-TS-LightweightM2M-V1_0-20170208-A.pdf>.
[Multi-Transport-URIs]
Thaler, D., "Using URIs With Multiple Transport Stacks",
Work in Progress, draft-thaler-appsawg-multi-transport-
uris-01, July 2017.
[OSCORE] Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", Work in Progress, draft-ietf-core-object-
security-08, January 2018.
Bormann, et al. Standards Track [Page 47]
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[RFC768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011,
<https://www.rfc-editor.org/info/rfc6335>.
[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>.
[RFC7230] Fielding, R., Ed., and J. Reschke, Ed., "Hypertext
Transfer Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<https://www.rfc-editor.org/info/rfc7230>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<https://www.rfc-editor.org/info/rfc7540>.
[SecurityChallenges]
Polk, T. and S. Turner, "Security Challenges For the
Internet Of Things", Interconnecting Smart Objects with
the Internet / IAB Workshop, February 2011,
<https://www.iab.org/wp-content/IAB-uploads/2011/03/
Turner.pdf>.
[SW2016] Swett, I., "QUIC Deployment Experience @Google", IETF 96
Proceedings, Berlin, Germany, July 2016,
<https://www.ietf.org/proceedings/96/slides/
slides-96-quic-3.pdf>.
[TCP-in-IoT]
Gomez, C., Crowcroft, J., and M. Scharf, "TCP Usage
Guidance in the Internet of Things (IoT)", Work in
Progress, draft-ietf-lwig-tcp-constrained-node-
networks-01, October 2017.
Bormann, et al. Standards Track [Page 48]
RFC 8323 TCP/TLS/WebSockets Transports for CoAP February 2018
Appendix A. Examples of CoAP over WebSockets
This appendix gives examples for the first two configurations
discussed in Section 4.
An example of the process followed by a CoAP client to retrieve the
representation of a resource identified by a "coap+ws" URI might be
as follows. Figure 17 below illustrates the WebSocket and CoAP
messages exchanged in detail.
1. The CoAP client obtains the URI
<coap+ws://example.org/sensors/temperature?u=Cel>, for example,
from a resource representation that it retrieved previously.
2. The CoAP client establishes a WebSocket connection to the
endpoint URI composed of the authority "example.org" and the
well-known path "/.well-known/coap",
<ws://example.org/.well-known/coap>.
3. CSMs (Section 5.3) are exchanged (not shown).
4. The CoAP client sends a single-frame, masked, binary message
containing a CoAP request. The request indicates the target
resource with the Uri-Path ("sensors", "temperature") and
Uri-Query ("u=Cel") Options.
5. The CoAP client waits for the server to return a response.
6. The CoAP client uses the connection for further requests, or the
connection is closed.
Bormann, et al. Standards Track [Page 49]
RFC 8323 TCP/TLS/WebSockets Transports for CoAP February 2018
CoAP CoAP
Client Server
(WebSocket (WebSocket
Client) Server)
| |
| |
+=========>| GET /.well-known/coap HTTP/1.1
| | Host: example.org
| | Upgrade: websocket
| | Connection: Upgrade
| | Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
| | Sec-WebSocket-Protocol: coap
| | Sec-WebSocket-Version: 13
| |
|<=========+ HTTP/1.1 101 Switching Protocols
| | Upgrade: websocket
| | Connection: Upgrade
| | Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=
| | Sec-WebSocket-Protocol: coap
: :
:<-------->: Exchange of CSMs (not shown)
| |
+--------->| Binary frame (opcode=%x2, FIN=1, MASK=1)
| | +-------------------------+
| | | GET |
| | | Token: 0x53 |
| | | Uri-Path: "sensors" |
| | | Uri-Path: "temperature" |
| | | Uri-Query: "u=Cel" |
| | +-------------------------+
| |
|<---------+ Binary frame (opcode=%x2, FIN=1, MASK=0)
| | +-------------------------+
| | | 2.05 Content |
| | | Token: 0x53 |
| | | Payload: "22.3 Cel" |
| | +-------------------------+
: :
: :
+--------->| Close frame (opcode=%x8, FIN=1, MASK=1)
| |
|<---------+ Close frame (opcode=%x8, FIN=1, MASK=0)
| |
Figure 17: A CoAP Client Retrieves the Representation of a Resource
Identified by a "coap+ws" URI
Bormann, et al. Standards Track [Page 50]
RFC 8323 TCP/TLS/WebSockets Transports for CoAP February 2018
Figure 18 shows how a CoAP client uses a CoAP forward proxy with a
WebSocket endpoint to retrieve the representation of the resource
"coap://[2001:db8::1]/". The use of the forward proxy and the
address of the WebSocket endpoint are determined by the client from
local configuration rules. The request URI is specified in the
Proxy-Uri Option. Since the request URI uses the "coap" URI scheme,
the proxy fulfills the request by issuing a Confirmable GET request
over UDP to the CoAP server and returning the response over the
WebSocket connection to the client.
CoAP CoAP CoAP
Client Proxy Server
(WebSocket (WebSocket (UDP
Client) Server) Endpoint)
| | |
+--------->| | Binary frame (opcode=%x2, FIN=1, MASK=1)
| | | +------------------------------------+
| | | | GET |
| | | | Token: 0x7d |
| | | | Proxy-Uri: "coap://[2001:db8::1]/" |
| | | +------------------------------------+
| | |
| +--------->| CoAP message (Ver=1, T=Con, MID=0x8f54)
| | | +------------------------------------+
| | | | GET |
| | | | Token: 0x0a15 |
| | | +------------------------------------+
| | |
| |<---------+ CoAP message (Ver=1, T=Ack, MID=0x8f54)
| | | +------------------------------------+
| | | | 2.05 Content |
| | | | Token: 0x0a15 |
| | | | Payload: "ready" |
| | | +------------------------------------+
| | |
|<---------+ | Binary frame (opcode=%x2, FIN=1, MASK=0)
| | | +------------------------------------+
| | | | 2.05 Content |
| | | | Token: 0x7d |
| | | | Payload: "ready" |
| | | +------------------------------------+
| | |
Figure 18: A CoAP Client Retrieves the Representation of a Resource
Identified by a "coap" URI via a WebSocket-Enabled CoAP Proxy
Bormann, et al. Standards Track [Page 51]
RFC 8323 TCP/TLS/WebSockets Transports for CoAP February 2018
Acknowledgments
We would like to thank Stephen Berard, Geoffrey Cristallo, Olivier
Delaby, Esko Dijk, Christian Groves, Nadir Javed, Michael Koster,
Achim Kraus, David Navarro, Szymon Sasin, Goeran Selander, Zach
Shelby, Andrew Summers, Julien Vermillard, and Gengyu Wei for their
feedback.
Last Call reviews from Yoshifumi Nishida, Mark Nottingham, and Meral
Shirazipour as well as several IESG reviewers provided extensive
comments; from the IESG, we would like to specifically call out Ben
Campbell, Mirja Kuehlewind, Eric Rescorla, Adam Roach, and the
responsible AD Alexey Melnikov.
Contributors
Matthias Kovatsch
Siemens AG
Otto-Hahn-Ring 6
Munich D-81739
Germany
Phone: +49-173-5288856
Email: matthias.kovatsch@siemens.com
Teemu Savolainen
Nokia Technologies
Hatanpaan valtatie 30
Tampere FI-33100
Finland
Email: teemu.savolainen@nokia.com
Valik Solorzano Barboza
Zebra Technologies
820 W. Jackson Blvd. Suite 700
Chicago, IL 60607
United States of America
Phone: +1-847-634-6700
Email: vsolorzanobarboza@zebra.com
Bormann, et al. Standards Track [Page 52]
RFC 8323 TCP/TLS/WebSockets Transports for CoAP February 2018
Authors' Addresses
Carsten Bormann
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Simon Lemay
Zebra Technologies
820 W. Jackson Blvd. Suite 700
Chicago, IL 60607
United States of America
Phone: +1-847-634-6700
Email: slemay@zebra.com
Hannes Tschofenig
ARM Ltd.
110 Fulbourn Road
Cambridge CB1 9NJ
United Kingdom
Email: Hannes.tschofenig@gmx.net
URI: http://www.tschofenig.priv.at
Klaus Hartke
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63905
Email: hartke@tzi.org
Bormann, et al. Standards Track [Page 53]
RFC 8323 TCP/TLS/WebSockets Transports for CoAP February 2018
Bilhanan Silverajan
Tampere University of Technology
Korkeakoulunkatu 10
Tampere FI-33720
Finland
Email: bilhanan.silverajan@tut.fi
Brian Raymor (editor)
Email: brianraymor@hotmail.com
Bormann, et al. Standards Track [Page 54]
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