CORE C. Bormann
Internet-Draft Universitaet Bremen TZI
Intended status: Standards Track S. Lemay
Expires: February 25, 2017 Zebra Technologies
H. Tschofenig
ARM Ltd.
K. Hartke
Universitaet Bremen TZI
B. Silverajan
Tampere University of Technology
B. Raymor, Ed.
Microsoft
August 24, 2016
CoAP (Constrained Application Protocol) over TCP, TLS, and WebSockets
draft-ietf-core-coap-tcp-tls-04
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.
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 http://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 February 25, 2017.
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Copyright Notice
Copyright (c) 2016 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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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. CoAP over TCP . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Messaging Model . . . . . . . . . . . . . . . . . . . . . 5
2.2. UDP-to-TCP gateways . . . . . . . . . . . . . . . . . . . 6
2.3. Opening Handshake . . . . . . . . . . . . . . . . . . . . 6
2.4. Message Format . . . . . . . . . . . . . . . . . . . . . 6
2.5. Message Transmission . . . . . . . . . . . . . . . . . . 10
3. CoAP over WebSockets . . . . . . . . . . . . . . . . . . . . 10
3.1. Opening Handshake . . . . . . . . . . . . . . . . . . . . 12
3.2. Message Format . . . . . . . . . . . . . . . . . . . . . 13
3.3. Message Transmission . . . . . . . . . . . . . . . . . . 14
3.4. Connection Health . . . . . . . . . . . . . . . . . . . . 14
3.5. Closing the Connection . . . . . . . . . . . . . . . . . 15
4. Signaling . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.1. Signaling Codes . . . . . . . . . . . . . . . . . . . . . 15
4.2. Signaling Option Numbers . . . . . . . . . . . . . . . . 16
4.3. Capability and Settings Messages (CSM) . . . . . . . . . 16
4.4. Ping and Pong Messages . . . . . . . . . . . . . . . . . 18
4.5. Release Messages . . . . . . . . . . . . . . . . . . . . 19
4.6. Abort Messages . . . . . . . . . . . . . . . . . . . . . 20
4.7. Capability and Settings examples . . . . . . . . . . . . 21
5. Block-wise Transfer and Reliable Transports . . . . . . . . . 21
5.1. Example: GET with BERT Blocks . . . . . . . . . . . . . . 23
5.2. Example: PUT with BERT Blocks . . . . . . . . . . . . . . 23
6. CoAP URIs . . . . . . . . . . . . . . . . . . . . . . . . . . 24
6.1. CoAP over TCP and TLS URIs . . . . . . . . . . . . . . . 24
6.2. CoAP over WebSockets URIs . . . . . . . . . . . . . . . . 25
7. Security Considerations . . . . . . . . . . . . . . . . . . . 26
7.1. Signaling Messages . . . . . . . . . . . . . . . . . . . 27
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 27
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8.1. Signaling Codes . . . . . . . . . . . . . . . . . . . . . 27
8.2. CoAP Signaling Option Numbers Registry . . . . . . . . . 28
8.3. Service Name and Port Number Registration . . . . . . . . 29
8.4. Secure Service Name and Port Number Registration . . . . 30
8.5. URI Scheme Registration . . . . . . . . . . . . . . . . . 30
8.6. Well-Known URI Suffix Registration . . . . . . . . . . . 33
8.7. ALPN Protocol Identifier . . . . . . . . . . . . . . . . 33
8.8. WebSocket Subprotocol Registration . . . . . . . . . . . 33
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 34
9.1. Normative References . . . . . . . . . . . . . . . . . . 34
9.2. Informative References . . . . . . . . . . . . . . . . . 35
Appendix A. Negotiating Protocol Versions . . . . . . . . . . . 36
Appendix B. CoAP over WebSocket Examples . . . . . . . . . . . . 36
Appendix C. Change Log . . . . . . . . . . . . . . . . . . . . . 40
C.1. Since draft-core-coap-tcp-tls-02 . . . . . . . . . . . . 40
C.2. Since draft-core-coap-tcp-tls-03 . . . . . . . . . . . . 40
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 40
Contributors . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 41
1. Introduction
The Constrained Application Protocol (CoAP) [RFC7252] was designed
for Internet of Things (IoT) deployments, assuming that UDP [RFC0768]
or DTLS [RFC6347] over UDP can be used unimpeded. UDP is a good
choice for transferring small amounts of data across networks that
follow the IP architecture.
Some CoAP deployments need to integrate well with existing enterprise
infrastructures, where UDP-based protocols may not be well-received
or may even be blocked by firewalls. Middleboxes that are unaware of
CoAP usage for IoT can make the use of UDP brittle, resulting in lost
or malformed packets.
Emerging standards such as Lightweight Machine to Machine [LWM2M]
currently use CoAP over UDP as a transport and require support for
CoAP over TCP to address the issues above and to protect investments
in existing CoAP implementations and deployments. Although HTTP/2
could also potentially address these requirements, there would be
additional costs and delays introduced by such a transition.
Currently, there are also fewer HTTP/2 implementations available for
constrained devices in comparison to CoAP.
To address these requirements, this document defines how to transport
CoAP over TCP, CoAP over TLS, and CoAP over WebSockets. Figure 1
illustrates the layering:
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+--------------------------------+
| Application |
+--------------------------------+
+--------------------------------+
| Requests/Responses/Signaling | CoAP (RFC 7252) / This Document
|--------------------------------|
| Message Framing | This Document
+--------------------------------+
| Reliable Transport |
+--------------------------------+
Figure 1: Layering of CoAP over Reliable Transports
Where NATs are present, CoAP over TCP can help with their traversal.
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 life cycle. UDP, on the other hand,
does not provide such information to a NAT and timeouts tend to be
much shorter [HomeGateway].
Some environments may also benefit from the ability of TCP to
exchange larger payloads, such as firmware images, without
application layer segmentation and to utilize the more sophisticated
congestion control capabilities provided by many TCP implementations.
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.
To allow IoT devices to better communicate in these demanding
environments, CoAP needs to support different transport protocols,
namely TCP [RFC0793], in some situations secured by TLS [RFC5246].
In addition, some corporate networks only allow Internet access via a
HTTP proxy. In this case, the best transport for CoAP would be the
WebSocket Protocol [RFC6455]. The WebSocket protocol provides two-
way communication between a client and a server after upgrading an
HTTP/1.1 [RFC7230] connection and may be available in an environment
that blocks CoAP over UDP. Another scenario for CoAP over WebSockets
is a CoAP application running inside a web browser without access to
connectivity other than HTTP and WebSockets.
This document specifies how to access resources using CoAP requests
and responses over the TCP/TLS and WebSocket protocols. This allows
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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
intermediary that proxies CoAP requests and responses between
different transports, such as between WebSockets and UDP.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
This document assumes that readers are familiar with the terms and
concepts that are used in [RFC6455] and [RFC7252].
The term "reliable transport" only refers to stream-based transport
protocols such as TCP.
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 (Section 2.1 of [I-D.ietf-core-block]).
2. 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.
CoAP over UDP supports optional reliability by defining four types of
messages: Confirmable, Non-confirmable, Acknowledgement, and Reset.
TCP eliminates the need for the message layer to support reliability.
As a result, message types are not defined in CoAP over TCP.
2.1. Messaging Model
Conceptually, CoAP over TCP replaces most of the CoAP over UDP
message layer 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 CoAP over TCP
messaging layer is not required to support acknowledgements or
detection of duplicate messages. As a result, both the Type and
Message ID fields are no longer required and are removed from the
CoAP over TCP message format. All messages are also untyped.
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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.
Client Server Client 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.
2.2. UDP-to-TCP gateways
A UDP-to-TCP gateway MUST discard all Empty messages (Code 0.00)
after processing at the message layer. For Confirmable (CON), Non-
Confirmable (NOM), and Acknowledgement (ACK) messages that are not
Empty, their contents are repackaged into untyped messages.
2.3. Opening Handshake
Both the client and the server MUST send a Capability and Settings
message (CSM see Section 4.3) 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.
Clients and servers MUST treat a missing or invalid CSM as a
connection error and abort the connection (see Section 4.6).
2.4. 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
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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.
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 "T" and "Message ID" fields in the
CoAP message header are elided.
o The "Ver" field is elided as well. In constrast to the UDP
message layer for UDP and DTLS, the CoAP over TCP message layer
does not send a version number in each message. If required in
the future, a new Capability and Settings Option (See Appendix A)
could be defined to support version negotiation.
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, shown here as an
8-bit length.
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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=13 | TKL |Extended Length| Code | TKL bytes ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1| Payload (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: CoAP frame with 8-bit Extended Length field.
Length (Len): 4-bit unsigned integer. A value between 0 and 12
directly 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 on CoAP Options (see
section 3.1 of [RFC7252]).
The following figures show the message format for the 0-bit, 16-bit,
and the 32-bit variable length cases.
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 | Code | Token (if any, TKL bytes) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1| Payload (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: CoAP message format without an Extended Length 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 would be encoded as shown in
Figure 6.
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 6: CoAP message with no options or payload.
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=14 | TKL | Extended Length (16 bits) | Code |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Token (if any, TKL bytes) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1| Payload (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: CoAP message format with 16-bit Extended Length field.
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=15 | TKL | Extended Length (32 bits)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| | Code | Token (if any, TKL bytes) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1| Payload (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 8: CoAP message format with 32-bit Extended Length field.
The semantics of the other CoAP header fields are left unchanged.
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2.5. Message Transmission
CoAP requests and responses are exchanged asynchronously over the
TCP/TLS 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.
Retransmission and deduplication of messages is provided by the TCP/
TLS protocol.
Since the TCP protocol provides ordered delivery of messages, the
mechanism for reordering detection when observing resources [RFC7641]
is not needed. The value of the Observe Option in notifications MAY
be empty on transmission and MUST be ignored on reception.
3. CoAP over WebSockets
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 at a CoAP server that exposes a WebSocket
endpoint (Figure 9). 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 9: 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. This means it is necessary for the client to identify both
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the WebSocket endpoint (identified by a "ws" or "wss" URI) and the
path and query of the CoAP resource within that endpoint from the
same URI. When the WebSocket protocol is used from a web page, the
choices are more limited [RFC6454], but the challenge persists.
Section 6.2 defines a new "coap+ws" URI scheme that identifies both a
WebSocket endpoint and a resource within that endpoint as follows:
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 10: The "coap+ws" URI Scheme
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 11), an SMS endpoint (a.k.a. mobile phone),
or even another WebSocket endpoint. The 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 11: 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 12). 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.
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___________ ___________ ___________
| | | | | |
| _|___ ___|_ _|___ ___|_ |
| CoAP / \ \ ---> / / \ CoAP / / \ ---> / \ \ CoAP |
| Client \__/__/ <--- \__\__/ Proxy \__\__/ <--- \__/__/ Server |
| | | | | |
|___________| |___________| |___________|
UDP UDP WebSocket <=== WebSocket
Client Server Server Client
Figure 12: CoAP Client (UDP client) accesses sleepy 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.
CoAP over WebSockets is intentionally very similar to CoAP over UDP.
Therefore, instead of presenting CoAP over WebSockets as a new
protocol, this document specifies it as a series of deltas from
[RFC7252].
3.1. Opening Handshake
Before CoAP requests and responses are exchanged, a WebSocket
connection is established as defined in Section 4 of [RFC6455].
Figure 13 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. Any later, incompatible versions of CoAP or CoAP
over WebSockets will use a different subprotocol name.
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.
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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 13: Example of an Opening Handshake
3.2. 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 14 is the same as the CoAP over
TCP message format (see Section 2.4) with one restriction. 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 | TKL | Code | Token (TKL bytes) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1| Payload (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14: CoAP Message Format over WebSockets
The CoAP over TCP message format 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.
Requests and response messages can be fragmented as specified in
Section 5.4 of [RFC6455], though typically they are sent unfragmented
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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 [I-D.ietf-core-block].
Empty messages (Code 0.00) MUST be ignored by the recipient (see also
Section 4.4).
3.3. Message Transmission
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.
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.
Since the WebSocket protocol provides ordered delivery of messages,
the mechanism for reordering detection when observing resources
[RFC7641] is not needed. The value of the Observe Option in
notifications MAY be empty on transmission and MUST be ignored on
reception.
3.4. Connection Health
When a 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), the
connection between the WebSocket client and the WebSocket server may
be lost or temporarily disrupted without the client being aware of
it.
To check the health of the WebSocket connection (and thereby of all
active requests, if any), a client can send a CoAP Ping Signaling
message (Section 4.4). WebSocket Ping and unsolicited Pong frames as
specified in Section 5.5 of [RFC6455] SHOULD NOT be used to ensure
that redundant maintenance traffic is not transmitted.
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There is no way to retransmit a request without creating a new one.
Re-registering interest in a resource is permitted, but entirely
unnecessary.
3.5. Closing the Connection
The WebSocket connection is closed as specified in Section 7 of
[RFC6455].
All requests for which the CoAP client has not received a response
yet are cancelled when the connection is closed. If the client
observes one or more resources over the WebSocket connection, then
the CoAP server (or intermediary in the role of the CoAP server) MUST
remove all entries associated with the client from the lists of
observers when the connection is closed.
4. Signaling
Signaling messages are introduced to allow peers to:
o Share characteristics such as maximum message size for the
connection
o Shutdown the connection in an ordered fashion
o Terminate the 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, as adapted to
the specific transport.)
4.1. Signaling Codes
A code in the 7.01-7.31 range indicates a Signaling message. Values
in this range are assigned by the "CoAP Signaling Codes" sub-registry
(see Section 8.1).
For each message, there is a sender and a peer receiving the message.
Payloads in Signaling messages are diagnostic payloads (see
Section 5.5.2 of [RFC7252]), unless otherwise defined by a Signaling
message option.
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4.2. 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 8.2).
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 4.6). If the option is understood but cannot be processed,
the option documents the behavior.
4.3. Capability and Settings Messages (CSM)
Capability and Settings messages (CSM) are used for two purposes:
o Each capability option advertises one capability of the sender to
the recipient.
o Setting options indicate a setting that will be applied by the
sender.
A Capability and Settings message MUST be sent by both endpoints at
the start of the connection and MAY be sent at any other time by
either endpoint over the lifetime of the connection.
Both capability and settings options are cumulative. A Capability
and Settings message 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 Capability and Settings
messages send a modified value.
These are not default values for the option as defined in
Section 5.4.4 in [RFC7252]. A default value would mean that an empty
Capability and Settings message would result in the option being set
to its default value.
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Capability and Settings messages are indicated by the 7.01 code
(CSM).
4.3.1. Server-Name Setting Option
+--------+------------+-------------+--------+--------+-------------+
| Number | Applies to | Name | Format | Length | Base Value |
+--------+------------+-------------+--------+--------+-------------+
| 1 | CSM | Server-Name | string | 1-255 | (see below) |
+--------+------------+-------------+--------+--------+-------------+
A client can use the Server-Name critical option to indicate the
default value for the Uri-Host Options in the messages that it sends
to the server. It has the same restrictions as the Uri-Host Option
(Section 5.10 of [RFC7252].
For TLS, the initial value for the Server-Name Option is given by the
SNI value.
For Websockets, the initial value for the Server-Name Option is given
by the HTTP Host header field.
4.3.2. Max-Message-Size Capability Option
The sender can use the Max-Message-Size elective option to indicate
the maximum message size in bytes that it can receive.
+--------+-----------+------------------+--------+--------+---------+
| Number | Applies | Name | Format | Length | Base |
| | to | | | | Value |
+--------+-----------+------------------+--------+--------+---------+
| 2 | CSM | Max-Message-Size | uint | 0-4 | 1152 |
+--------+-----------+------------------+--------+--------+---------+
As per Section 4.6 of [RFC7252], the base value (and the value used
when this option is not implemented) is 1152. A peer that relies on
this option being indicated with a certain minimum value will enjoy
limited interoperability.
4.3.3. Block-wise Transfer Capability Option
+--------+-----------+----------------+--------+--------+-----------+
| Number | Applies | Name | Format | Length | Base |
| | to | | | | Value |
+--------+-----------+----------------+--------+--------+-----------+
| 4 | CSM | Block-wise | empty | 0 | (none) |
| | | Transfer | | | |
+--------+-----------+----------------+--------+--------+-----------+
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A sender can use the Block-wise Transfer elective Option to indicate
that it supports the block-wise transfer protocol
[I-D.ietf-core-block].
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 that supports block-wise transfers SHOULD 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 5).
4.4. Ping and Pong Messages
In CoAP over TCP, 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. In contrast, Ping and Pong
messages are a bidirectional exchange.
Upon receipt of a Ping message, a single Pong message is returned
with the identical token. 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).
4.4.1. Custody Option
+--------+------------+---------+--------+--------+------------+
| Number | Applies to | Name | Format | Length | Base Value |
+--------+------------+---------+--------+--------+------------+
| 2 | Ping, Pong | Custody | empty | 0 | (none) |
+--------+------------+---------+--------+--------+------------+
A peer replying to a Ping message can add a Custody elective option
to the Pong message it returns. This option indicates that the
application has processed all request/response messages that it has
received in the present connection ahead of the Ping message that
prompted the Pong message. (Note that there is no definition of
specific application semantics of "processed", but there is an
expectation that the sender of the Ping leading to the Pong with a
Custody Option should be able to free buffers based on this
indication.)
A Custody elective option can also be sent in a Ping message to
explicitly request the return of a Custody Option in the Pong
message. A peer is always free to indicate that it has finished
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processing all previous request/response messages by sending a
Custody Option in a Pong message. A peer is also free NOT to send a
Custody Option in case it is still processing previous request/
response messages; however, it SHOULD delay its response to a Ping
with a Custody Option until it can also return one.
4.5. Release Messages
A release message indicates that the sender does not want to continue
maintaining the connection and opts for an orderly shutdown; the
details are in the options. A diagnostic payload MAY be included. A
release message will normally be replied to by the peer by closing
the TCP/TLS connection. Messages may be in flight when the sender
decides to send a Release message. The general expectation is that
these will still be processed.
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:
+--------+-----------+-----------------+--------+--------+----------+
| Number | Applies | Name | Format | Length | Base |
| | to | | | | Value |
+--------+-----------+-----------------+--------+--------+----------+
| 2 | Release | Bad-Server-Name | empty | 0 | (none) |
+--------+-----------+-----------------+--------+--------+----------+
The Bad-Server-Name elective option indicates that the default
indicated by the CSM Server-Name Option is unlikely to be useful for
this server.
+--------+----------+-------------------+--------+--------+---------+
| Number | Applies | Name | Format | Length | Base |
| | to | | | | Value |
+--------+----------+-------------------+--------+--------+---------+
| 4 | Release | Alternate-Address | string | 1-255 | (none) |
+--------+----------+-------------------+--------+--------+---------+
The Alternative-Address elective option requests the peer to instead
open a connection of the same kind 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].
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+--------+------------+----------+--------+--------+------------+
| Number | Applies to | Name | Format | Length | Base Value |
+--------+------------+----------+--------+--------+------------+
| 6 | Release | Hold-Off | uint | 0-3 | (none) |
+--------+------------+----------+--------+--------+------------+
The Hold-Off elective option indicates that the server is requesting
that the peer not reconnect to it for the number of seconds given in
the value.
4.6. Abort Messages
An abort message indicates that the sender is unable to continue
maintaining the 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
SHOULD be included in the Abort message. Messages may be in flight
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:
+--------+-----------+----------------+--------+--------+-----------+
| Number | Applies | Name | Format | Length | Base |
| | to | | | | Value |
+--------+-----------+----------------+--------+--------+-----------+
| 2 | Abort | Bad-CSM-Option | uint | 0-2 | (none) |
+--------+-----------+----------------+--------+--------+-----------+
The 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.
One reason for an sender to generate an abort message is a general
syntax error in the byte stream received. No specific option has
been defined for this, as the details of that syntax error are best
left to a diagnostic payload.
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4.7. Capability and Settings examples
An encoded example of a Ping message with a non-empty token is shown
in Figure 15.
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 15: Ping Message Example
An encoded example of the corresponding Pong message is shown in
Figure 16.
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 16: Pong Message Example
5. 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 byte stream transport. While this suggests that
the Block-wise transfer protocol [I-D.ietf-core-block] 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 TCP 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
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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 [I-D.ietf-core-block].
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
allowed to contain a multiple of 1024 bytes (non-final BERT block) or
more than 1024 bytes (final BERT block).
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 exponent (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.
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5.1. Example: GET with BERT Blocks
Figure 17 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.
CLIENT 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 17: GET with BERT blocks.
5.2. Example: PUT with BERT Blocks
Figure 18 demonstrates a PUT exchange with BERT blocks.
CLIENT 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 18: PUT with BERT blocks.
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6. CoAP URIs
CoAP over UDP [RFC7252] defines the "coap" and "coaps" URI schemes
for identifying CoAP resources and providing a means of locating the
resource.
6.1. CoAP over TCP and TLS URIs
CoAP over TCP uses the "coap+tcp" URI scheme. CoAP over TLS uses the
"coaps+tcp" scheme. The rules from Section 6 of [RFC7252] apply to
both of these URI schemes.
[RFC7252], Section 8 (Multicast CoAP) is not applicable to these
schemes.
Resources made available via one of the "coap+tcp" or "coaps+tcp"
schemes have no shared identity with the other scheme or with the
"coap" or "coaps" scheme, even if their resource identifiers indicate
the same authority (the same host listening to the same port). The
schemes constitute distinct namespaces and, in combination with the
authority, are considered to be distinct origin servers.
6.1.1. coap+tcp URI scheme
coap-tcp-URI = "coap+tcp:" "//" host [ ":" port ] path-abempty
[ "?" query ]
The semantics defined in [RFC7252], Section 6.1, apply to this URI
scheme, with the following changes:
o The port subcomponent indicates the TCP port at which the CoAP
server is located. (If it is empty or not given, then the default
port 5683 is assumed, as with UDP.)
6.1.2. coaps+tcp URI scheme
coaps-tcp-URI = "coaps+tcp:" "//" host [ ":" port ] path-abempty
[ "?" query ]
The semantics defined in [RFC7252], Section 6.2, apply to this URI
scheme, with the following changes:
o The port subcomponent indicates the TCP port at which the TLS
server for the CoAP server is located. If it is empty or not
given, then the default port 443 is assumed (this is different
from the default port for "coaps", i.e., CoAP over DTLS over UDP).
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o If a server does not support the Application-Layer Protocol
Negotiation Extension (ALPN) [RFC7301] or wishes to accommodate
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 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 client MUST use the ALPN
extension to advertise the "coap" protocol identifier (see
Section 8.7) in the list of protocols in its ClientHello. If the
server selects and returns the "coap" protocol identifier using
the ALPN extension in its ServerHello, then the connection
succeeds. If the server either does not negotiate the ALPN
extension or returns a no_application_protocol alert, the client
MUST close the connection.
o For TCP port 5684, a client MAY use the ALPN extension to
advertise the "coap" protocol identifier in the list of protocols
in its ClientHello. If the server selects and returns the "coap"
protocol identifier using the ALPN extension in its ServerHello,
then the connection succeeds. If the server returns a
no_application_protocol alert, then the client MUST close the
connection. If the server does not negotiate the ALPN extension,
then coaps+tcp is implicitly selected.
o For TCP port 5684, if the client does not use the ALPN extension
to negotiate the protocol, then coaps+tcp is implicitly selected.
6.2. CoAP over WebSockets URIs
For the first configuration discussed in Section 3, this document
defines two new URIs schemes that can be used for identifying CoAP
resources and providing a means of locating these resources:
"coap+ws" and "coap+wss".
Similar to the "coap" and "coaps" schemes, the "coap+ws" and
"coap+wss" schemes organize resources hierarchically under a CoAP
origin server. The key difference is that the server is potentially
reachable on a WebSocket endpoint instead of a UDP endpoint.
The WebSocket endpoint is identified by a "ws" or "wss" URI that is
composed of the authority part of the "coap+ws" or "coap+wss" URI,
respectively, and the well-known path "/.well-known/coap" [RFC5785].
The path and query parts of a "coap+ws" or "coap+wss" URI identify a
resource within the specified endpoint which can be operated on by
the methods defined by CoAP.
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The syntax of the "coap+ws" and "coap+wss" URI schemes is specified
below in Augmented Backus-Naur Form (ABNF) [RFC5234]. The
definitions of "host", "port", "path-abempty" and "query" are the
same as in [RFC3986].
coap-ws-URI =
"coap+ws:" "//" host [ ":" port ] path-abempty [ "?" query ]
coap-wss-URI =
"coap+wss:" "//" host [ ":" port ] path-abempty [ "?" query ]
The port component is OPTIONAL; the default for "coap+ws" is port 80,
while the default for "coap+wss" is port 443.
Fragment identifiers are not part of the request URI and thus MUST
NOT be transmitted in a WebSocket handshake or in the URI options of
a CoAP request.
6.2.1. Decomposing and Composing URIs
The steps for decomposing a "coap+ws" or "coap+wss" URI into CoAP
options are the same as specified in Section 6.4 of [RFC7252] with
the following changes:
o The <scheme> component MUST be "coap+ws" or "coap+wss" when
converted to ASCII lowercase.
o A Uri-Host Option MUST only be included in a request when the
<host> component does not equal the uri-host component in the Host
header field in the WebSocket handshake.
o A Uri-Port Option MUST only be included in a request if |port|
does not equal the port component in the Host header field in the
WebSocket handshake.
The steps to construct a URI from a request's options are changed
accordingly.
7. Security Considerations
The security considerations of [RFC7252] apply.
TLS version 1.2 or higher is mandatory-to-implement and MUST be
enabled by default. An endpoint MAY immediately abort a CoAP over
TLS connection that does not meet this requirement (see Section 4.6)
and SHOULD include a diagnostic payload.
The TLS usage guidance in [RFC7925] SHOULD be followed.
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TLS does not protect the TCP header. This may, for example, allow an
on-path adversary to terminate a TCP connection prematurely by
spoofing a TCP reset message.
CoAP over WebSockets and CoAP over TLS-secured WebSockets do not
introduce additional security issues beyond CoAP and DTLS-secured
CoAP respectively [RFC7252]. The security considerations of
[RFC6455] apply.
7.1. Signaling Messages
o 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.
o SNI vs. Server-Name: Any security negotiated in the TLS handshake
is for the SNI name exchanged in the TLS handshake and checked
against the certificate provided by the server. The Server-Name
Option cannot be used to extend these security properties to the
additional server name.
8. IANA Considerations
8.1. 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 7.01-7.31, its name, and a reference to its documentation.
Initial entries in this sub-registry are as follows:
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+------+---------+-----------+
| 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 [RFC5226].
8.2. CoAP Signaling Option Numbers Registry
IANA is requested to create a sub-registry for signaling options
similar to the CoAP Option Numbers Registry (Section 12.2 of
[RFC7252]), with the single change that a fourth column is added to
the sub-registry that is one of the codes in the Signaling Codes
subregistry (Section 8.1).
The name of this sub-registry is "CoAP Signaling Option Numbers".
Initial entries in this sub-registry are as follows:
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+--------+------------+---------------------+-----------+
| Number | Applies to | Name | Reference |
+--------+------------+---------------------+-----------+
| 1 | CSM | Server-Name | [RFCthis] |
| | | | |
| 2 | CSM | Max-Message-Size | [RFCthis] |
| | | | |
| 4 | CSM | Block-wise-Transfer | [RFCthis] |
| | | | |
| 2 | Ping, Pong | Custody | [RFCthis] |
| | | | |
| 2 | Release | Bad-Server-Name | [RFCthis] |
| | | | |
| 4 | Release | Alternative-Address | [RFCthis] |
| | | | |
| 6 | Release | Hold-Off | [RFCthis] |
| | | | |
| 2 | Abort | 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].
8.3. 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
Assignee.
IESG <iesg@ietf.org>
Contact.
IETF Chair <chair@ietf.org>
Description.
Constrained Application Protocol (CoAP)
Reference.
[RFCthis]
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Port Number.
5683
8.4. 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
8.5. URI Scheme Registration
This document registers two new URI schemes, namely "coap+tcp" and
"coaps+tcp", for the use of CoAP over TCP and for CoAP over TLS over
TCP, respectively. The "coap+tcp" and "coaps+tcp" URI schemes can
thus be compared to the "http" and "https" URI schemes.
The syntax of the "coap" and "coaps" URI schemes is specified in
Section 6 of [RFC7252] and the present document re-uses their
semantics for "coap+tcp" and "coaps+tcp", respectively, with the
exception that TCP, or TLS over TCP is used as a transport protocol.
IANA is requested to add these new URI schemes to the registry
established with [RFC7595].
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8.5.1. coap+ws
This document requests the registration of the Uniform Resource
Identifier (URI) scheme "coap+ws". The registration request complies
with [RFC4395].
URL scheme name.
coap+ws
Status.
Permanent
URI scheme syntax.
Defined in Section N of [RFCthis]
URI scheme semantics.
The "coap+ws" URI scheme provides a way to identify resources that
are potentially accessible over the Constrained Application
Protocol (CoAP) using the WebSocket protocol.
Encoding considerations.
The scheme encoding conforms to the encoding rules established for
URIs in [RFC3986], i.e., internationalized and reserved characters
are expressed using UTF-8-based percent-encoding.
Applications/protocols that use this URI scheme name.
The scheme is used by CoAP endpoints to access CoAP resources
using the WebSocket protocol.
Interoperability considerations.
None.
Security Considerations.
See Section N of [RFCthis]
Contact.
IETF chair <chair@ietf.org>
Author/Change controller.
IESG <iesg@ietf.org>
References.
[RFCthis]
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8.5.2. coap+wss
This document requests the registration of the Uniform Resource
Identifier (URI) scheme "coap+wss". The registration request
complies with [RFC4395].
URL scheme name.
coap+wss
Status.
Permanent
URI scheme syntax.
Defined in Section N of [RFCthis]
URI scheme semantics.
The "coap+ws" URI scheme provides a way to identify resources that
are potentially accessible over the Constrained Application
Protocol (CoAP) using the WebSocket protocol secured with
Transport Layer Security (TLS).
Encoding considerations.
The scheme encoding conforms to the encoding rules established for
URIs in [RFC3986], i.e., internationalized and reserved characters
are expressed using UTF-8-based percent-encoding.
Applications/protocols that use this URI scheme name.
The scheme is used by CoAP endpoints to access CoAP resources
using the WebSocket protocol secured with TLS.
Interoperability considerations.
None.
Security Considerations.
See Section N of [RFCthis]
Contact.
IETF chair <chair@ietf.org>
Author/Change controller.
IESG <iesg@ietf.org>
References.
[RFCthis]
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8.6. 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.
8.7. 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]
8.8. WebSocket Subprotocol Registration
IANA is requested to register the WebSocket CoAP subprotocol under
the "WebSocket Subprotocol Name Registry":
Subprotocol Identifier.
coap
Subprotocol Common Name.
Constrained Application Protocol (CoAP)
Subprotocol Definition.
[RFCthis]
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9. References
9.1. Normative References
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC
793, DOI 10.17487/RFC0793, September 1981,
<http://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,
<http://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,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC4395] Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
Registration Procedures for New URI Schemes", RFC 4395,
DOI 10.17487/RFC4395, February 2006,
<http://www.rfc-editor.org/info/rfc4395>.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
DOI 10.17487/RFC5226, May 2008,
<http://www.rfc-editor.org/info/rfc5226>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, DOI 10.17487/
RFC5246, August 2008,
<http://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,
<http://www.rfc-editor.org/info/rfc5785>.
[RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol", RFC
6455, DOI 10.17487/RFC6455, December 2011,
<http://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,
<http://www.rfc-editor.org/info/rfc7252>.
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[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, <http://www.rfc-editor.org/info/rfc7301>.
[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,
<http://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,
<http://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,
<http://www.rfc-editor.org/info/rfc7925>.
9.2. Informative References
[HomeGateway]
Eggert, L., "An experimental study of home gateway
characteristics", Proceedings of the 10th annual
conference on Internet measurement, 2010.
[I-D.becker-core-coap-sms-gprs]
Becker, M., Li, K., Kuladinithi, K., and T. Poetsch,
"Transport of CoAP over SMS", draft-becker-core-coap-sms-
gprs-05 (work in progress), August 2014.
[I-D.ietf-core-block]
Bormann, C. and Z. Shelby, "Block-wise transfers in CoAP",
draft-ietf-core-block-21 (work in progress), July 2016.
[LWM2M] Open Mobile Alliance, "Lightweight Machine to Machine
Technical Specification Candidate Version 1.0", April
2016, <http://technical.openmobilealliance.org/Technical/R
elease_Program/docs/LightweightM2M/V1_0-20160407-C/
OMA-TS-LightweightM2M-V1_0-20160407-C.pdf>.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI
10.17487/RFC0768, August 1980,
<http://www.rfc-editor.org/info/rfc768>.
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[RFC5234] Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/
RFC5234, January 2008,
<http://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,
<http://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, <http://www.rfc-editor.org/info/rfc6347>.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, DOI
10.17487/RFC6454, December 2011,
<http://www.rfc-editor.org/info/rfc6454>.
[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,
<http://www.rfc-editor.org/info/rfc7230>.
Appendix A. Negotiating Protocol Versions
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.
In contrast to the message layer for UDP and DTLS, the CoAP over TCP
message format does not include a version number. If version
negotiation needs to be addressed in the future, then Capability and
Settings have been specifically designed to enable such a potential
feature.
Appendix B. CoAP over WebSocket Examples
This section gives examples for the first two configurations
discussed in Section 3.
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 19 below illustrates the WebSocket and CoAP
messages exchanged in detail.
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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. 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.
4. It waits for the server to return a response.
5. 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
| |
| |
+--------->| 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 19: A CoAP client retrieves the representation of a resource
identified by a "coap+ws" URI
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Figure 20 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 20: A CoAP client retrieves the representation of a resource
identified by a "coap" URI via a WebSockets-enabled CoAP proxy
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Appendix C. Change Log
The RFC Editor is requested to remove this section at publication.
C.1. Since draft-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
C.2. Since draft-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
Acknowledgements
We would like to thank Stephen Berard, Geoffrey Cristallo, Olivier
Delaby, Christian Groves, Nadir Javed, Michael Koster, Matthias
Kovatsch, Achim Kraus, David Navarro, Szymon Sasin, Zach Shelby,
Andrew Summers, Julien Vermillard, and Gengyu Wei for their feedback.
Contributors
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
Teemu Savolainen
Nokia Technologies
Hatanpaan valtatie 30
Tampere FI-33100
Finland
Email: teemu.savolainen@nokia.com
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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
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
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Bilhanan Silverajan
Tampere University of Technology
Korkeakoulunkatu 10
Tampere FI-33720
Finland
Email: bilhanan.silverajan@tut.fi
Brian Raymor (editor)
Microsoft
One Microsoft Way
Redmond 98052
United States of America
Email: brian.raymor@microsoft.com
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