Internet DRAFT - draft-ietf-core-coap-tcp-tls
draft-ietf-core-coap-tcp-tls
CORE C. Bormann
Internet-Draft Universitaet Bremen TZI
Updates: 7641, 7959 (if approved) S. Lemay
Intended status: Standards Track Zebra Technologies
Expires: June 21, 2018 H. Tschofenig
ARM Ltd.
K. Hartke
Universitaet Bremen TZI
B. Silverajan
Tampere University of Technology
B. Raymor, Ed.
December 18, 2017
CoAP (Constrained Application Protocol) over TCP, TLS, and WebSockets
draft-ietf-core-coap-tcp-tls-11
Abstract
The Constrained Application Protocol (CoAP), although inspired by
HTTP, was designed to use UDP instead of TCP. The message layer of
the CoAP over UDP protocol 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 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 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 June 21, 2018.
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Copyright Notice
Copyright (c) 2017 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
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(https://trustee.ietf.org/license-info) in effect on the date of
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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 . . . . . . . . . . . . . . . . . . . . . 8
3.3. Message Transmission . . . . . . . . . . . . . . . . . . 10
3.4. Connection Health . . . . . . . . . . . . . . . . . . . . 11
4. CoAP over WebSockets . . . . . . . . . . . . . . . . . . . . 12
4.1. Opening Handshake . . . . . . . . . . . . . . . . . . . . 13
4.2. Message Format . . . . . . . . . . . . . . . . . . . . . 14
4.3. Message Transmission . . . . . . . . . . . . . . . . . . 15
4.4. Connection Health . . . . . . . . . . . . . . . . . . . . 15
5. Signaling . . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.1. Signaling Codes . . . . . . . . . . . . . . . . . . . . . 16
5.2. Signaling Option Numbers . . . . . . . . . . . . . . . . 16
5.3. Capabilities and Settings Messages (CSM) . . . . . . . . 16
5.4. Ping and Pong Messages . . . . . . . . . . . . . . . . . 19
5.5. Release Messages . . . . . . . . . . . . . . . . . . . . 20
5.6. Abort Messages . . . . . . . . . . . . . . . . . . . . . 21
5.7. Signaling examples . . . . . . . . . . . . . . . . . . . 22
6. Block-wise Transfer and Reliable Transports . . . . . . . . . 23
6.1. Example: GET with BERT Blocks . . . . . . . . . . . . . . 24
6.2. Example: PUT with BERT Blocks . . . . . . . . . . . . . . 25
7. Observing Resources over Reliable Transports . . . . . . . . 25
7.1. Notifications and Reordering . . . . . . . . . . . . . . 26
7.2. Transmission and Acknowledgements . . . . . . . . . . . . 26
7.3. Freshness . . . . . . . . . . . . . . . . . . . . . . . . 26
7.4. Cancellation . . . . . . . . . . . . . . . . . . . . . . 27
8. CoAP over Reliable Transport URIs . . . . . . . . . . . . . . 27
8.1. coap+tcp URI scheme . . . . . . . . . . . . . . . . . . . 28
8.2. coaps+tcp URI scheme . . . . . . . . . . . . . . . . . . 28
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8.3. coap+ws URI scheme . . . . . . . . . . . . . . . . . . . 29
8.4. coaps+ws URI scheme . . . . . . . . . . . . . . . . . . . 30
8.5. Uri-Host and Uri-Port Options . . . . . . . . . . . . . . 31
8.6. Decomposing URIs into Options . . . . . . . . . . . . . . 31
8.7. Composing URIs from Options . . . . . . . . . . . . . . . 32
9. Securing CoAP . . . . . . . . . . . . . . . . . . . . . . . . 32
9.1. TLS binding for CoAP over TCP . . . . . . . . . . . . . . 33
9.2. TLS usage for CoAP over WebSockets . . . . . . . . . . . 34
10. Security Considerations . . . . . . . . . . . . . . . . . . . 34
10.1. Signaling Messages . . . . . . . . . . . . . . . . . . . 34
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 34
11.1. Signaling Codes . . . . . . . . . . . . . . . . . . . . 34
11.2. CoAP Signaling Option Numbers Registry . . . . . . . . . 35
11.3. Service Name and Port Number Registration . . . . . . . 36
11.4. Secure Service Name and Port Number Registration . . . . 37
11.5. URI Scheme Registration . . . . . . . . . . . . . . . . 38
11.6. Well-Known URI Suffix Registration . . . . . . . . . . . 40
11.7. ALPN Protocol Identifier . . . . . . . . . . . . . . . . 40
11.8. WebSocket Subprotocol Registration . . . . . . . . . . . 40
11.9. CoAP Option Numbers Registry . . . . . . . . . . . . . . 41
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 41
12.1. Normative References . . . . . . . . . . . . . . . . . . 41
12.2. Informative References . . . . . . . . . . . . . . . . . 43
Appendix A. CoAP over WebSocket Examples . . . . . . . . . . . . 45
Appendix B. Change Log . . . . . . . . . . . . . . . . . . . . . 48
B.1. Since draft-ietf-core-coap-tcp-tls-02 . . . . . . . . . . 48
B.2. Since draft-ietf-core-coap-tcp-tls-03 . . . . . . . . . . 48
B.3. Since draft-ietf-core-coap-tcp-tls-04 . . . . . . . . . . 48
B.4. Since draft-ietf-core-coap-tcp-tls-05 . . . . . . . . . . 48
B.5. Since draft-ietf-core-coap-tcp-tls-06 . . . . . . . . . . 49
B.6. Since draft-ietf-core-coap-tcp-tls-07 . . . . . . . . . . 49
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 49
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 49
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 50
1. Introduction
The Constrained Application Protocol (CoAP) [RFC7252] was designed
for Internet of Things (IoT) deployments, assuming that UDP [RFC0768]
can be used unimpeded, as can the Datagram Transport Layer Security
protocol (DTLS [RFC6347]) over UDP. The use of CoAP over UDP is
focused on simplicity, has a low code footprint, and a small over-
the-wire message size.
The primary reason for introducing CoAP over TCP [RFC0793] 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
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concentrated in enterprise networks and networks in geographic
regions with otherwise challenged connectivity. Some networks also
rate-limit UDP traffic, as reported in [BK2015] and deployment
investigations related to the standardization of 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 keep-
alive messages.
o TCP utilizes more sophisticated congestion and flow control
mechanisms than the default mechanisms provided by CoAP over UDP,
which is useful for the transfer of larger payloads. (Work is,
however, ongoing to add advanced congestion control to CoAP over
UDP as well, see [I-D.ietf-core-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 blockwise 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 roundtrips, increased RAM requirements and larger
packet sizes. Developers implementing CoAP over TCP are encouraged
to consult [I-D.gomez-lwig-tcp-constrained-node-networks] 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
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delays introduced by such a transition from CoAP to HTTP/2.
Currently, there are also fewer HTTP/2 implementations available for
constrained devices in comparison to CoAP. Since CoAP also support
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 mechanism [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 both for a reliable
transport and the CoAP over UDP message layer, 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 by communicating CoAP directly with a CoAP server
accessible over a TCP, TLS or WebSocket connection or via a CoAP
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intermediary that proxies CoAP requests and responses between
different transports, such as between WebSockets and UDP.
Section 7 updates the "Observing Resources in the Constrained
Application Protocol" [RFC7641] specification for use with CoAP over
reliable transports. [RFC7641] is an extension to the CoAP protocol
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
[RFC2119].
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, which provide reliable and ordered delivery
of a byte-stream.
Block-wise Extension for Reliable Transport (BERT):
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 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 via TLS or WebSockets.
Connection:
Transport Connection, unless explicitly qualified otherwise.
Connection Initiator:
The peer that opens a Transport Connection, i.e., the TCP active
opener, TLS client, or WebSocket client.
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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,
Acknowledgement, and Reset. In addition, messages include a Message
ID to relate Acknowledgments to Confirmable messages and to detect
duplicate messages.
The 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 re-open 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 a dynamic connection
management similar to the one described in Section 6 of [RFC7230] for
HTTP, opening a connection when the first client request is ready to
be sent and reusing that for further messages for a while, until no
message is sent for a certain time and no requests are outstanding
(possibly with a configurable idle time) and a release process is
started (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.
TCP ensures reliable message transmission, so the message layer of
CoAP over TCP is not required to support acknowledgements or to
detect duplicate messages. As a result, both the Type and Message ID
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fields are no longer required and are removed from the CoAP over TCP
message format.
Figure 2 illustrates the difference between CoAP over UDP and CoAP
over reliable transport. 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
transport
Figure 2: Comparison between CoAP over unreliable and reliable
transport
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: RFC 7252 defined CoAP Message Format
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The CoAP over TCP message format 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, then
Capabilities and Settings Messages (CSM see Section 5.3) have been
specifically designed to enable such a potential feature.
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 CoAP over UDP header), 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.
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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.)
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
Capabilities and Settings message (CSM, see Section 5.3) as their
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 message before sending
its own initial CSM message. Conversely, the Connection Acceptor MAY
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wait for the Connection Initiator to send its initial CSM message
before sending its own initial CSM message.
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 initiator is expected to communicate with the acceptor. For
example, the acceptor 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.
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. Concurrent requests are
differentiated by their Token, which is scoped locally to the
connection.
The transport connection is bi-directional, so requests can be sent
both by 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 is provided by the TCP
protocol.
3.4. Connection Health
Empty messages (Code 0.00) can always be sent and MUST be ignored by
the recipient. This provides a basic keep-alive 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.
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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. In flight messages may
or may not be lost.
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. Section 8.3 and Section 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.
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.
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___________ ___________ ___________
| | | | | |
| _|___ ___|_ _|___ ___|_ |
| 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.
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, which 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
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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].
The message format shown in Figure 10 is the same as the CoAP over
TCP message format (see Section 3.2) with one change. The Length
(Len) field MUST be set to zero because the WebSockets 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 the definition of a new subprotocol
identifier that is negotiated during the opening handshake.
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Requests and response messages 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 Capabilities and
Settings message (CSM see Section 5.3) as their 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. Concurrent requests are
differentiated by their Token, which is scoped locally to the
connection.
The connection is bi-directional, so requests can be sent both by the
entity that established the connection and the remote host.
As with CoAP over TCP, retransmission and deduplication of messages
is provided by the WebSocket protocol. CoAP over WebSockets
therefore does not make a distinction between Confirmable or Non-
Confirmable messages, and does not provide Acknowledgement or Reset
messages.
4.4. Connection Health
As with CoAP over TCP, a CoAP client can test the health of the CoAP
over WebSocket connection by sending a CoAP Ping Signaling message
(Section 5.4). WebSocket Ping and unsolicited Pong frames
(Section 5.5 of [RFC6455]) SHOULD NOT be used to ensure that
redundant maintenance traffic is not transmitted.
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
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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 is 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 format.)
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" sub-registry
(see Section 11.1).
For each message, there is 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.
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"
sub-registry (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 (CSM)
Capabilities and Settings messages (CSM) 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.
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One CSM MUST be sent by each endpoint at the start of the transport
connection. Further CSM MAY be sent at any other time by either
endpoint over the lifetime of the connection.
Both capability 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 setting before any Capabilities and Settings
messages send a modified value.
These are not default values for the option, as defined in
Section 5.4.4 in [RFC7252]. Default values apply on a per-message
basis and thus reset when the value is not present in a given
Capabilities and Settings message.
Capabilities and Settings messages are indicated by the 7.01 code
(CSM).
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 further down
in Figure 13 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
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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].
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 the Block-Wise-Transfer Option.
If a Max-Message-Size Option is indicated with a value that is
greater than 1152 (in the same or a different CSM message), 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 indication of BERT support obliges
neither 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 choose a BERT size message it wants to
encourage and add a delta for the header and any options that also
need to be included in the message. 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 for the size of header and options
added (say, 256 or 512). Adding 1024 or more however 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, which can
be harder to establish interoperability for.
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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 keep-alive 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).
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 waiting
for a Pong reply. Any limitations on the 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
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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.)
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 with 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. There is no obligation for the peer 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
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"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.)
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 (and may or may not wait for a Release or 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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The elective Bad-CSM-Option Option 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 parameter problem with the value of an
elective option. More detailed information SHOULD be included as a
diagnostic payload.
For CoAP over UDP, messages which 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
An encoded example of the corresponding Pong message is shown in
Figure 12.
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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 CoAP
[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
blockwise exchange from start to end into a single exchange)
The 'Block-wise Extension for Reliable Transport (BERT)' extends the
Block 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 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.
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,
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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 with 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 with 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), more bit (M), and block size (2**(SZX+4)) separated by
slashes. E.g., 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.
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.
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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
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.
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7.1. Notifications and Reordering
When using the Observe Option 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 the TCP protocol
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 Acknowledgements
For CoAP over UDP, server notifications to the client can be
confirmable or non-confirmable. A confirmable message requires the
client to either respond with an acknowledgement message or a reset
message. An acknowledgement 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 which 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 acknowledgement 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 MAY 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
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: there is no obligation, and no expectation, of
a proxy to react to a Ping by checking all its onward observations or
all the connections, if any, underlying them. A proxy MAY maintain
its own schedule for confirming the onward observations it relies on;
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it is however 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 cancelled 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 is either
closed or times out.
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.
The syntax for the URI schemes in this section are specified using
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.
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As with the "coap" and "coaps" schemes defined in [RFC7252], all URI
schemes defined in this section also support the path prefix "/.well-
known/" 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 changes:
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].
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 Extension (ALPN) [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
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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.
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] [I-D.bormann-hybi-ws-wk]. The path and
query parts of a "coap+ws" URI identify a resource within the
specified endpoint which can be operated on by the methods defined by
CoAP:
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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].
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] [I-D.bormann-hybi-ws-wk]. The path and
query parts of a "coaps+ws" URI identify a resource within the
specified endpoint which 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].
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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.
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 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|.
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8.7. Composing URIs from Options
The steps are the same as 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:
... 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
when appropriate. Note that there is ongoing work to support a data
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object-based security model for CoAP that is independent of transport
(see [I-D.ietf-core-object-security]).
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 (MTI) cipher suites defined in [RFC7252].
This guidance assumes implementation in a constrained device or for
communication with a constrained device. CoAP over TCP/TLS has,
however, 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 backend 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] and [RFC7525] differs in
terms of algorithms and credential types, it is assumed that a CoAP
over TCP/TLS implementation 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 which 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.
"PreSharedKey", "RawPublicKey", or "Certificate" is mandatory-to-
implement for the TLS binding depending on the credential type used
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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 in accepting incoming connections over a HTTPS (HTTP-
over-TLS) port, the guidance in Section 4.2 of [RFC6455] applies.
Note that this formally inherits the mandatory-to-implement cipher
suites defined in [RFC5246]. However, usually modern browsers
implement more recent cipher suites that then are 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.
11. IANA Considerations
11.1. Signaling Codes
IANA is requested to create a third sub-registry for values of the
Code field in the CoAP header (Section 12.1 of [RFC7252]). The name
of this sub-registry is "CoAP Signaling Codes".
Each entry in the sub-registry must include the Signaling Code in the
range 7.00-7.31, its name, and a reference to its documentation.
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Initial entries in this sub-registry are as follows:
+------+---------+-----------+
| Code | Name | Reference |
+------+---------+-----------+
| 7.01 | CSM | [RFCthis] |
| | | |
| 7.02 | Ping | [RFCthis] |
| | | |
| 7.03 | Pong | [RFCthis] |
| | | |
| 7.04 | Release | [RFCthis] |
| | | |
| 7.05 | Abort | [RFCthis] |
+------+---------+-----------+
Table 1: CoAP Signal Codes
All other Signaling Codes are Unassigned.
The IANA policy for future additions to this sub-registry is "IETF
Review or IESG Approval" as described in [RFC8126].
11.2. CoAP Signaling Option Numbers Registry
IANA is requested to create a sub-registry for Options Numbers used
in CoAP signaling options within the "CoRE Parameters" registry. The
name of this sub-registry is "CoAP Signaling Option Numbers".
Each entry in the sub-registry must include one or more of the codes
in the Signaling Codes subregistry (Section 11.1), the option number,
the name of the option, and a reference to the option's
documentation.
Initial entries in this sub-registry are as follows:
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+------------+--------+---------------------+-----------+
| Applies to | Number | Name | Reference |
+------------+--------+---------------------+-----------+
| 7.01 | 2 | Max-Message-Size | [RFCthis] |
| | | | |
| 7.01 | 4 | Block-Wise-Transfer | [RFCthis] |
| | | | |
| 7.02, 7.03 | 2 | Custody | [RFCthis] |
| | | | |
| 7.04 | 2 | Alternative-Address | [RFCthis] |
| | | | |
| 7.04 | 4 | Hold-Off | [RFCthis] |
| | | | |
| 7.05 | 2 | Bad-CSM-Option | [RFCthis] |
+------------+--------+---------------------+-----------+
Table 2: CoAP Signal Option Codes
The IANA policy for future additions to this sub-registry 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 the subregistry includes an
additional column; 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.
11.3. Service Name and Port Number Registration
IANA is requested to assign the port number 5683 and the service name
"coap+tcp", in accordance with [RFC6335].
Service Name.
coap+tcp
Transport Protocol.
tcp
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Assignee.
IESG <iesg@ietf.org>
Contact.
IETF Chair <chair@ietf.org>
Description.
Constrained Application Protocol (CoAP)
Reference.
[RFCthis]
Port Number.
5683
11.4. Secure Service Name and Port Number Registration
IANA is requested to assign the port number 5684 and the service name
"coaps+tcp", in accordance with [RFC6335]. The port number is
requested to address the exceptional case of TLS implementations that
do not support the "Application-Layer Protocol Negotiation Extension"
[RFC7301].
Service Name.
coaps+tcp
Transport Protocol.
tcp
Assignee.
IESG <iesg@ietf.org>
Contact.
IETF Chair <chair@ietf.org>
Description.
Constrained Application Protocol (CoAP)
Reference.
[RFC7301], [RFCthis]
Port Number.
5684
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11.5. URI Scheme Registration
URI schemes are registered within the "Uniform Resource Identifier
(URI) Schemes" registry maintained at [IANA.uri-schemes].
11.5.1. coap+tcp
IANA is requested to register the Uniform Resource Identifier (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 [RFCthis]
11.5.2. coaps+tcp
IANA is requested to register the Uniform Resource Identifier (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>
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Change controller:
IESG <iesg@ietf.org>
Reference:
Section 8.2 in [RFCthis]
11.5.3. coap+ws
IANA is requested to register the Uniform Resource Identifier (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 [RFCthis]
11.5.4. coaps+ws
IANA is requested to register the Uniform Resource Identifier (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>
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Change controller:
IESG <iesg@ietf.org>
References:
Section 8.4 in [RFCthis]
11.6. Well-Known URI Suffix Registration
IANA is requested to register the 'coap' well-known URI in the "Well-
Known URIs" registry. This registration request complies with
[RFC5785]:
URI Suffix.
coap
Change controller.
IETF
Specification document(s).
[RFCthis]
Related information.
None.
11.7. ALPN Protocol Identifier
IANA is requested to assign the following value in the registry
"Application Layer Protocol Negotiation (ALPN) Protocol IDs" created
by [RFC7301]. The "coap" string identifies CoAP when used over TLS.
Protocol.
CoAP
Identification Sequence.
0x63 0x6f 0x61 0x70 ("coap")
Reference.
[RFCthis]
11.8. WebSocket Subprotocol Registration
IANA is requested to register the WebSocket CoAP subprotocol under
the "WebSocket Subprotocol Name Registry":
Subprotocol Identifier.
coap
Subprotocol Common Name.
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Constrained Application Protocol (CoAP)
Subprotocol Definition.
[RFCthis]
11.9. CoAP Option Numbers Registry
IANA is requested to add [RFCthis] to the references for the
following entries registered by [RFC7959] in the "CoAP Option
Numbers" sub-registry defined by [RFC7252]:
+--------+--------+---------------------+
| Number | Name | Reference |
+--------+--------+---------------------+
| 23 | Block2 | RFC 7959, [RFCthis] |
| | | |
| 27 | Block1 | RFC 7959, [RFCthis] |
+--------+--------+---------------------+
Table 3: CoAP Option Numbers
12. References
12.1. Normative References
[I-D.bormann-hybi-ws-wk]
Bormann, C., "Well-known URIs for the WebSocket Protocol",
draft-bormann-hybi-ws-wk-00 (work in progress), May 2017.
[RFC0793] 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>.
[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>.
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[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>.
[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>.
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[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>.
12.2. Informative References
[BK2015] Byrne, C. and J. Kleberg, "Advisory Guidelines for UDP
Deployment", Proceedings draft-byrne-opsec-udp-advisory-00
(expired), 2015.
[EK2016] Edeline, K., Kuehlewind, M., Trammell, B., Aben, E., and
B. Donnet, "Using UDP for Internet Transport Evolution",
Proceedings arXiv preprint 1612.07816, 2016.
[HomeGateway]
Eggert, L., "An experimental study of home gateway
characteristics", Proceedings of the 10th annual
conference on Internet measurement , 2010.
[I-D.gomez-lwig-tcp-constrained-node-networks]
Gomez, C., Crowcroft, J., and M. Scharf, "TCP over
Constrained-Node Networks", draft-gomez-lwig-tcp-
constrained-node-networks-03 (work in progress), June
2017.
[I-D.ietf-core-cocoa]
Bormann, C., Betzler, A., Gomez, C., and I. Demirkol,
"CoAP Simple Congestion Control/Advanced", draft-ietf-
core-cocoa-02 (work in progress), October 2017.
[I-D.ietf-core-object-security]
Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
"Object Security for Constrained RESTful Environments
(OSCORE)", draft-ietf-core-object-security-07 (work in
progress), November 2017.
[IANA.uri-schemes]
IANA, "Uniform Resource Identifier (URI) Schemes",
<http://www.iana.org/assignments/uri-schemes>.
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[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>.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
DOI 10.17487/RFC0768, August 1980,
<https://www.rfc-editor.org/info/rfc768>.
[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>.
[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,
<http://www.iab.org/wp-content/IAB-uploads/2011/03/
Turner.pdf>.
[SW2016] Swett, I., "QUIC Deployment Experience @Google",
Proceedings
https://www.ietf.org/proceedings/96/slides/slides-96-quic-
3.pdf, 2016.
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Appendix A. CoAP over WebSocket Examples
This section 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. It 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. CSM messages (Section 5.3) are exchanged (not shown for lack of
space).
4. It 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. It waits for the server to return a response.
6. The CoAP client uses the connection for further requests, or the
connection is closed.
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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 CSM messages (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
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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
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Appendix B. Change Log
The RFC Editor is requested to remove this section at publication.
B.1. Since draft-ietf-core-coap-tcp-tls-02
Merged draft-savolainen-core-coap-websockets-07 Merged draft-bormann-
core-block-bert-01 Merged draft-bormann-core-coap-sig-02
B.2. Since draft-ietf-core-coap-tcp-tls-03
Editorial updates
Added mandatory exchange of Capabilities and Settings messages after
connecting
Added support for coaps+tcp port 5684 and more details on
Application-Layer Protocol Negotiation (ALPN)
Added guidance on CoAP Signaling Ping-Pong versus WebSocket Ping-Pong
Updated references and requirements for TLS security considerations
B.3. Since draft-ietf-core-coap-tcp-tls-04
Updated references
Added Appendix: Updates to RFC7641 Observing Resources in the
Constrained Application Protocol (CoAP)
Updated Capability and Settings Message (CSM) exchange in the Opening
Handshake to allow initiator to send messages before receiving
acceptor CSM
B.4. Since draft-ietf-core-coap-tcp-tls-05
Addressed feedback from Working Group Last Call
Added Securing CoAP section and informative reference to OSCOAP
Removed the Server-Name and Bad-Server-Name Options
Clarified the Capability and Settings Message (CSM) exchange
Updated Pong response requirements
Added Connection Initiator and Connection Acceptor terminology where
appropriate
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Updated LWM2M 1.0 informative reference
B.5. Since draft-ietf-core-coap-tcp-tls-06
Addressed feedback from second Working Group Last Call
B.6. Since draft-ietf-core-coap-tcp-tls-07
Addressed feedback from IETF Last Call
Addressed feedback from ARTART review
Addressed feedback from GENART review
Addressed feedback from TSVART review
Added fragment identifiers to URI schemes
Added "Updates RFC7959" for BERT
Added "Updates RFC6455" to extend well-known URI mechanism to ws and
wss
Clarified well-known URI mechanism use for all URI schemes
Changed NoSec to optional-to-implement
Acknowledgements
We would like to thank Stephen Berard, Geoffrey Cristallo, Olivier
Delaby, Esko Dijk, Christian Groves, Nadir Javed, Michael Koster,
Matthias Kovatsch, Achim Kraus, David Navarro, Szymon Sasin, Goran
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
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Matthias Kovatsch
Siemens AG
Otto-Hahn-Ring 6
Munich D-81739
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 60607
United States of America
Phone: +1-847-634-6700
Email: vsolorzanobarboza@zebra.com
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 60607
United States of America
Phone: +1-847-634-6700
Email: slemay@zebra.com
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Hannes Tschofenig
ARM Ltd.
110 Fulbourn Rd
Cambridge CB1 9NJ
Great Britain
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
Bilhanan Silverajan
Tampere University of Technology
Korkeakoulunkatu 10
Tampere FI-33720
Finland
Email: bilhanan.silverajan@tut.fi
Brian Raymor (editor)
Email: brianraymor@hotmail.com
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