RFC 8323
CoAP (Constrained Application Protocol) over TCP, TLS, and WebSockets
Internet Engineering Task Force (IETF) C. Bormann Request for Comments: 8323 Universitaet Bremen TZI Updates: 7641, 7959 S. Lemay Category: Standards Track Zebra Technologies ISSN: 2070-1721 H. Tschofenig ARM Ltd. K. Hartke Universitaet Bremen TZI B. Silverajan Tampere University of Technology B. Raymor, Ed. February 2018 CoAP (Constrained Application Protocol) over TCP, TLS, and WebSocketsAbstract
The Constrained Application Protocol (CoAP), although inspired by HTTP, was designed to use UDP instead of TCP. The message layer of CoAP over UDP includes support for reliable delivery, simple congestion control, and flow control. Some environments benefit from the availability of CoAP carried over reliable transports such as TCP or Transport Layer Security (TLS). This document outlines the changes required to use CoAP over TCP, TLS, and WebSockets transports. It also formally updates RFC 7641 for use with these transports and RFC 7959 to enable the use of larger messages over a reliable transport. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 7841. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at https://www.rfc-editor.org/info/rfc8323.
Copyright Notice Copyright (c) 2018 IETF Trust and the persons identified as the document authors. All rights reserved. This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (https://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.Table of Contents
1. Introduction ....................................................3 2. Conventions and Terminology .....................................6 3. CoAP over TCP ...................................................7 3.1. Messaging Model ............................................7 3.2. Message Format .............................................9 3.3. Message Transmission ......................................11 3.4. Connection Health .........................................12 4. CoAP over WebSockets ...........................................13 4.1. Opening Handshake .........................................15 4.2. Message Format ............................................15 4.3. Message Transmission ......................................16 4.4. Connection Health .........................................17 5. Signaling ......................................................17 5.1. Signaling Codes ...........................................17 5.2. Signaling Option Numbers ..................................18 5.3. Capabilities and Settings Messages (CSMs) .................18 5.4. Ping and Pong Messages ....................................20 5.5. Release Messages ..........................................21 5.6. Abort Messages ............................................23 5.7. Signaling Examples ........................................24 6. Block-Wise Transfer and Reliable Transports ....................25 6.1. Example: GET with BERT Blocks .............................27 6.2. Example: PUT with BERT Blocks .............................27 7. Observing Resources over Reliable Transports ...................28 7.1. Notifications and Reordering ..............................28 7.2. Transmission and Acknowledgments ..........................28 7.3. Freshness .................................................28 7.4. Cancellation ..............................................29
8. CoAP over Reliable Transport URIs ..............................29 8.1. coap+tcp URI Scheme .......................................30 8.2. coaps+tcp URI Scheme ......................................31 8.3. coap+ws URI Scheme ........................................32 8.4. coaps+ws URI Scheme .......................................33 8.5. Uri-Host and Uri-Port Options .............................33 8.6. Decomposing URIs into Options .............................34 8.7. Composing URIs from Options ...............................35 9. Securing CoAP ..................................................35 9.1. TLS Binding for CoAP over TCP .............................36 9.2. TLS Usage for CoAP over WebSockets ........................37 10. Security Considerations .......................................37 10.1. Signaling Messages .......................................37 11. IANA Considerations ...........................................38 11.1. Signaling Codes ..........................................38 11.2. CoAP Signaling Option Numbers Registry ...................38 11.3. Service Name and Port Number Registration ................40 11.4. Secure Service Name and Port Number Registration .........40 11.5. URI Scheme Registration ..................................41 11.6. Well-Known URI Suffix Registration .......................43 11.7. ALPN Protocol Identifier .................................44 11.8. WebSocket Subprotocol Registration .......................44 11.9. CoAP Option Numbers Registry .............................44 12. References ....................................................45 12.1. Normative References .....................................45 12.2. Informative References ...................................47 Appendix A. Examples of CoAP over WebSockets ......................49 Acknowledgments ...................................................52 Contributors ......................................................52 Authors' Addresses ................................................531. Introduction
The Constrained Application Protocol (CoAP) [RFC7252] was designed for Internet of Things (IoT) deployments, assuming that UDP [RFC768] can be used unimpeded as can the Datagram Transport Layer Security (DTLS) protocol [RFC6347] over UDP. The use of CoAP over UDP is focused on simplicity, has a low code footprint, and has a small over-the-wire message size. The primary reason for introducing CoAP over TCP [RFC793] and TLS [RFC5246] is that some networks do not forward UDP packets. Complete blocking of UDP happens in between about 2% and 4% of terrestrial access networks, according to [EK2016]. UDP impairment is especially concentrated in enterprise networks and networks in geographic regions with otherwise challenged connectivity. Some networks also
rate-limit UDP traffic, as reported in [BK2015], and deployment investigations related to the standardization of Quick UDP Internet Connections (QUIC) revealed numbers around 0.3% [SW2016]. The introduction of CoAP over TCP also leads to some additional effects that may be desirable in a specific deployment: o Where NATs are present along the communication path, CoAP over TCP leads to different NAT traversal behavior than CoAP over UDP. NATs often calculate expiration timers based on the transport-layer protocol being used by application protocols. Many NATs maintain TCP-based NAT bindings for longer periods based on the assumption that a transport-layer protocol, such as TCP, offers additional information about the session lifecycle. UDP, on the other hand, does not provide such information to a NAT and timeouts tend to be much shorter [HomeGateway]. According to [HomeGateway], the mean for TCP and UDP NAT binding timeouts is 386 minutes (TCP) and 160 seconds (UDP). Shorter timeout values require keepalive messages to be sent more frequently. Hence, the use of CoAP over TCP requires less-frequent transmission of keepalive messages. o TCP utilizes mechanisms for congestion control and flow control that are more sophisticated than the default mechanisms provided by CoAP over UDP; these TCP mechanisms are useful for the transfer of larger payloads. (However, work is ongoing to add advanced congestion control to CoAP over UDP as well; see [CoCoA].) Note that the use of CoAP over UDP (and CoAP over DTLS over UDP) is still the recommended transport for use in constrained node networks, particularly when used in concert with block-wise transfer. CoAP over TCP is applicable for those cases where the networking infrastructure leaves no other choice. The use of CoAP over TCP leads to a larger code size, more round trips, increased RAM requirements, and larger packet sizes. Developers implementing CoAP over TCP are encouraged to consult [TCP-in-IoT] for guidance on low-footprint TCP implementations for IoT devices. Standards based on CoAP, such as Lightweight Machine to Machine [LWM2M], currently use CoAP over UDP as a transport; adding support for CoAP over TCP enables them to address the issues above for specific deployments and to protect investments in existing CoAP implementations and deployments. Although HTTP/2 could also potentially address the need for enterprise firewall traversal, there would be additional costs and delays introduced by such a transition from CoAP to HTTP/2. Currently, there are also fewer HTTP/2 implementations available for
constrained devices in comparison to CoAP. Since CoAP also supports group communication using IP-layer multicast and unreliable communication, IoT devices would have to support HTTP/2 in addition to CoAP. Furthermore, CoAP may be integrated into a web environment where the front end uses CoAP over UDP from IoT devices to a cloud infrastructure and then CoAP over TCP between the back-end services. A TCP-to-UDP gateway can be used at the cloud boundary to communicate with the UDP-based IoT device. Finally, CoAP applications running inside a web browser may be without access to connectivity other than HTTP. In this case, the WebSocket Protocol [RFC6455] may be used to transport CoAP requests and responses, as opposed to cross-proxying them via HTTP to an HTTP-to-CoAP cross-proxy. This preserves the functionality of CoAP without translation -- in particular, the Observe Option [RFC7641]. To address the above-mentioned deployment requirements, this document defines how to transport CoAP over TCP, CoAP over TLS, and CoAP over WebSockets. For these cases, the reliability offered by the transport protocol subsumes the reliability functions of the message layer used for CoAP over UDP. (Note that for both a reliable transport and the message layer for CoAP over UDP, the reliability offered is per transport hop: where proxies -- see Sections 5.7 and 10 of [RFC7252] -- are involved, that layer's reliability function does not extend end to end.) Figure 1 illustrates the layering: +--------------------------------+ | Application | +--------------------------------+ +--------------------------------+ | Requests/Responses/Signaling | CoAP (RFC 7252) / This Document |--------------------------------| | Message Framing | This Document +--------------------------------+ | Reliable Transport | +--------------------------------+ Figure 1: Layering of CoAP over Reliable Transports This document specifies how to access resources using CoAP requests and responses over the TCP, TLS, and WebSocket protocols. This allows connectivity-limited applications to obtain end-to-end CoAP connectivity either (1) by communicating CoAP directly with a CoAP server accessible over a TCP, TLS, or WebSocket connection or (2) via a CoAP intermediary that proxies CoAP requests and responses between different transports, such as between WebSockets and UDP.
Section 7 updates [RFC7641] ("Observing Resources in the Constrained Application Protocol (CoAP)") for use with CoAP over reliable transports. [RFC7641] is an extension to CoAP that enables CoAP clients to "observe" a resource on a CoAP server. (The CoAP client retrieves a representation of a resource and registers to be notified by the CoAP server when the representation is updated.)2. Conventions and Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here. This document assumes that readers are familiar with the terms and concepts that are used in [RFC6455], [RFC7252], [RFC7641], and [RFC7959]. The term "reliable transport" is used only to refer to transport protocols, such as TCP, that provide reliable and ordered delivery of a byte stream. Block-wise Extension for Reliable Transport (BERT): Extends [RFC7959] to enable the use of larger messages over a reliable transport. BERT Option: A Block1 or Block2 option that includes an SZX (block size) value of 7. BERT Block: The payload of a CoAP message that is affected by a BERT Option in descriptive usage (see Section 2.1 of [RFC7959]). Transport Connection: Underlying reliable byte-stream connection, as directly provided by TCP or indirectly provided via TLS or WebSockets. Connection: Transport Connection, unless explicitly qualified otherwise.
Connection Initiator:
The peer that opens a Transport Connection, i.e., the TCP active
opener, TLS client, or WebSocket client.
Connection Acceptor:
The peer that accepts the Transport Connection opened by the other
peer, i.e., the TCP passive opener, TLS server, or WebSocket
server.
3. CoAP over TCP
The request/response interaction model of CoAP over TCP is the same
as CoAP over UDP. The primary differences are in the message layer.
The message layer of CoAP over UDP supports optional reliability by
defining four types of messages: Confirmable, Non-confirmable,
Acknowledgment, and Reset. In addition, messages include a
Message ID to relate Acknowledgments to Confirmable messages and to
detect duplicate messages.
Management of the transport connections is left to the application,
i.e., the present specification does not describe how an application
decides to open a connection or to reopen another one in the presence
of failures (or what it would deem to be a failure; see also
Section 5.4). In particular, the Connection Initiator need not be
the client of the first request placed on the connection. Some
implementations will want to implement dynamic connection management
similar to the technique described in Section 6 of [RFC7230] for
HTTP: opening a connection when the first client request is ready to
be sent, reusing that connection for subsequent messages until no
more messages are sent for a certain time period and no requests are
outstanding (possibly with a configurable idle time), and then
starting a release process (orderly shutdown) (see Section 5.5). In
implementations of this kind, connection releases or aborts may not
be indicated as errors to the application but may simply be handled
by automatic reconnection once the need arises again. Other
implementations may be based on configured connections that are kept
open continuously and lead to management system notifications on
release or abort. The protocol defined in the present specification
is intended to work with either model (or other, application-specific
connection management models).
3.1. Messaging Model
Conceptually, CoAP over TCP replaces most of the message layer of
CoAP over UDP with a framing mechanism on top of the byte stream
provided by TCP/TLS, conveying the length information for each
message that, on datagram transports, is provided by the UDP/DTLS
datagram layer.
TCP ensures reliable message transmission, so the message layer of CoAP over TCP is not required to support Acknowledgment messages or to detect duplicate messages. As a result, both the Type and Message ID fields are no longer required and are removed from the message format for CoAP over TCP. Figure 2 illustrates the difference between CoAP over UDP and CoAP over reliable transports. The removed Type and Message ID fields are indicated by dashes. CoAP Client CoAP Server CoAP Client CoAP Server | | | | | CON [0xbc90] | | (-------) [------] | | GET /temperature | | GET /temperature | | (Token 0x71) | | (Token 0x71) | +------------------->| +------------------->| | | | | | ACK [0xbc90] | | (-------) [------] | | 2.05 Content | | 2.05 Content | | (Token 0x71) | | (Token 0x71) | | "22.5 C" | | "22.5 C" | |<-------------------+ |<-------------------+ | | | | CoAP over UDP CoAP over reliable transports Figure 2: Comparison between CoAP over Unreliable Transports and CoAP over Reliable Transports
3.2. Message Format
The CoAP message format defined in [RFC7252], as shown in Figure 3, relies on the datagram transport (UDP, or DTLS over UDP) for keeping the individual messages separate and for providing length information. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Ver| T | TKL | Code | Message ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Token (if any, TKL bytes) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options (if any) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1 1 1 1 1 1 1 1| Payload (if any) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 3: CoAP Message Format as Defined in RFC 7252 The message format for CoAP over TCP is very similar to the format specified for CoAP over UDP. The differences are as follows: o Since the underlying TCP connection provides retransmissions and deduplication, there is no need for the reliability mechanisms provided by CoAP over UDP. The Type (T) and Message ID fields in the CoAP message header are elided. o The Version (Vers) field is elided as well. In contrast to the message format of CoAP over UDP, the message format for CoAP over TCP does not include a version number. CoAP is defined in [RFC7252] with a version number of 1. At this time, there is no known reason to support version numbers different from 1. If version negotiation needs to be addressed in the future, Capabilities and Settings Messages (CSMs) (see Section 5.3) have been specifically designed to enable such a potential feature.
o In a stream-oriented transport protocol such as TCP, a form of
message delimitation is needed. For this purpose, CoAP over TCP
introduces a length field with variable size. Figure 4 shows the
adjusted CoAP message format with a modified structure for the
fixed header (first 4 bytes of the header for CoAP over UDP),
which includes the length information of variable size.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Len | TKL | Extended Length (if any, as chosen by Len) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code | Token (if any, TKL bytes) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1| Payload (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: CoAP Frame for Reliable Transports
Length (Len): 4-bit unsigned integer. A value between 0 and 12
inclusive indicates the length of the message in bytes, starting
with the first bit of the Options field. Three values are
reserved for special constructs:
13: An 8-bit unsigned integer (Extended Length) follows the
initial byte and indicates the length of options/payload
minus 13.
14: A 16-bit unsigned integer (Extended Length) in network byte
order follows the initial byte and indicates the length of
options/payload minus 269.
15: A 32-bit unsigned integer (Extended Length) in network byte
order follows the initial byte and indicates the length of
options/payload minus 65805.
The encoding of the Length field is modeled after the Option Length
field of the CoAP Options (see Section 3.1 of [RFC7252]).
For simplicity, a Payload Marker (0xFF) is shown in Figure 4; the
Payload Marker indicates the start of the optional payload and is
absent for zero-length payloads (see Section 3 of [RFC7252]). (If
present, the Payload Marker is included in the message length, which
counts from the start of the Options field to the end of the Payload
field.)
For example, a CoAP message just containing a 2.03 code with the Token 7f and no options or payload is encoded as shown in Figure 5. 0 1 2 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | 0x01 | 0x43 | 0x7f | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Len = 0 ------> 0x01 TKL = 1 ___/ Code = 2.03 --> 0x43 Token = 0x7f Figure 5: CoAP Message with No Options or Payload The semantics of the other CoAP header fields are left unchanged.3.3. Message Transmission
Once a Transport Connection is established, each endpoint MUST send a CSM (see Section 5.3) as its first message on the connection. This message establishes the initial settings and capabilities for the endpoint, such as maximum message size or support for block-wise transfers. The absence of options in the CSM indicates that base values are assumed. To avoid a deadlock, the Connection Initiator MUST NOT wait for the Connection Acceptor to send its initial CSM before sending its own initial CSM. Conversely, the Connection Acceptor MAY wait for the Connection Initiator to send its initial CSM before sending its own initial CSM. To avoid unnecessary latency, a Connection Initiator MAY send additional messages after its initial CSM without waiting to receive the Connection Acceptor's CSM; however, it is important to note that the Connection Acceptor's CSM might indicate capabilities that impact how the Connection Initiator is expected to communicate with the Connection Acceptor. For example, the Connection Acceptor's CSM could indicate a Max-Message-Size Option (see Section 5.3.1) that is smaller than the base value (1152) in order to limit both buffering requirements and head-of-line blocking.
Endpoints MUST treat a missing or invalid CSM as a connection error and abort the connection (see Section 5.6). CoAP requests and responses are exchanged asynchronously over the Transport Connection. A CoAP client can send multiple requests without waiting for a response, and the CoAP server can return responses in any order. Responses MUST be returned over the same connection as the originating request. Each concurrent request is differentiated by its Token, which is scoped locally to the connection. The Transport Connection is bidirectional, so requests can be sent by both the entity that established the connection (Connection Initiator) and the remote host (Connection Acceptor). If one side does not implement a CoAP server, an error response MUST be returned for all CoAP requests from the other side. The simplest approach is to always return 5.01 (Not Implemented). A more elaborate mock server could also return 4.xx responses such as 4.04 (Not Found) or 4.02 (Bad Option) where appropriate. Retransmission and deduplication of messages are provided by TCP.3.4. Connection Health
Empty messages (Code 0.00) can always be sent and MUST be ignored by the recipient. This provides a basic keepalive function that can refresh NAT bindings. If a CoAP client does not receive any response for some time after sending a CoAP request (or, similarly, when a client observes a resource and it does not receive any notification for some time), it can send a CoAP Ping Signaling message (see Section 5.4) to test the Transport Connection and verify that the CoAP server is responsive. When the underlying Transport Connection is closed or reset, the signaling state and any observation state (see Section 7.4) associated with the connection are removed. Messages that are in flight may or may not be lost.
4. CoAP over WebSockets
CoAP over WebSockets is intentionally similar to CoAP over TCP; therefore, this section only specifies the differences between the transports. CoAP over WebSockets can be used in a number of configurations. The most basic configuration is a CoAP client retrieving or updating a CoAP resource located on a CoAP server that exposes a WebSocket endpoint (see Figure 6). The CoAP client acts as the WebSocket client, establishes a WebSocket connection, and sends a CoAP request, to which the CoAP server returns a CoAP response. The WebSocket connection can be used for any number of requests. ___________ ___________ | | | | | _|___ requests ___|_ | | CoAP / \ \ -------------> / / \ CoAP | | Client \__/__/ <------------- \__\__/ Server | | | responses | | |___________| |___________| WebSocket =============> WebSocket Client Connection Server Figure 6: CoAP Client (WebSocket Client) Accesses CoAP Server (WebSocket Server) The challenge with this configuration is how to identify a resource in the namespace of the CoAP server. When the WebSocket Protocol is used by a dedicated client directly (i.e., not from a web page through a web browser), the client can connect to any WebSocket endpoint. Sections 8.3 and 8.4 define new URI schemes that enable the client to identify both a WebSocket endpoint and the path and query of the CoAP resource within that endpoint.
Another possible configuration is to set up a CoAP forward proxy at the WebSocket endpoint. Depending on what transports are available to the proxy, it could forward the request to a CoAP server with a CoAP UDP endpoint (Figure 7), an SMS endpoint (a.k.a. mobile phone), or even another WebSocket endpoint. The CoAP client specifies the resource to be updated or retrieved in the Proxy-Uri Option. ___________ ___________ ___________ | | | | | | | _|___ ___|_ _|___ ___|_ | | CoAP / \ \ ---> / / \ CoAP / \ \ ---> / / \ CoAP | | Client \__/__/ <--- \__\__/ Proxy \__/__/ <--- \__\__/ Server | | | | | | | |___________| |___________| |___________| WebSocket ===> WebSocket UDP UDP Client Server Client Server Figure 7: CoAP Client (WebSocket Client) Accesses CoAP Server (UDP Server) via a CoAP Proxy (WebSocket Server / UDP Client) A third possible configuration is a CoAP server running inside a web browser (Figure 8). The web browser initially connects to a WebSocket endpoint and is then reachable through the WebSocket server. When no connection exists, the CoAP server is unreachable. Because the WebSocket server is the only way to reach the CoAP server, the CoAP proxy should be a reverse-proxy. ___________ ___________ ___________ | | | | | | | _|___ ___|_ _|___ ___|_ | | CoAP / \ \ ---> / / \ CoAP / / \ ---> / \ \ CoAP | | Client \__/__/ <--- \__\__/ Proxy \__\__/ <--- \__/__/ Server | | | | | | | |___________| |___________| |___________| UDP UDP WebSocket <=== WebSocket Client Server Server Client Figure 8: CoAP Client (UDP Client) Accesses CoAP Server (WebSocket Client) via a CoAP Proxy (UDP Server / WebSocket Server) Further configurations are possible, including those where a WebSocket connection is established through an HTTP proxy.
4.1. Opening Handshake
Before CoAP requests and responses are exchanged, a WebSocket connection is established as defined in Section 4 of [RFC6455]. Figure 9 shows an example. The WebSocket client MUST include the subprotocol name "coap" in the list of protocols; this indicates support for the protocol defined in this document. The WebSocket client includes the hostname of the WebSocket server in the Host header field of its handshake as per [RFC6455]. The Host header field also indicates the default value of the Uri-Host Option in requests from the WebSocket client to the WebSocket server. GET /.well-known/coap HTTP/1.1 Host: example.org Upgrade: websocket Connection: Upgrade Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ== Sec-WebSocket-Protocol: coap Sec-WebSocket-Version: 13 HTTP/1.1 101 Switching Protocols Upgrade: websocket Connection: Upgrade Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo= Sec-WebSocket-Protocol: coap Figure 9: Example of an Opening Handshake4.2. 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 message format for CoAP over TCP (see Section 3.2), with one change: the Length (Len) field MUST be set to zero, because the WebSocket frame contains the length. 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Len=0 | TKL | Code | Token (TKL bytes) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options (if any) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1 1 1 1 1 1 1 1| Payload (if any) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Figure 10: CoAP Message Format over WebSockets As with CoAP over TCP, the message format for CoAP over WebSockets eliminates the Version field defined in CoAP over UDP. If CoAP version negotiation is required in the future, CoAP over WebSockets can address the requirement by defining a new subprotocol identifier that is negotiated during the opening handshake. Requests and responses can be fragmented as specified in Section 5.4 of [RFC6455], though typically they are sent unfragmented, as they tend to be small and fully buffered before transmission. The WebSocket Protocol does not provide means for multiplexing. If it is not desirable for a large message to monopolize the connection, requests and responses can be transferred in a block-wise fashion as defined in [RFC7959].4.3. Message Transmission
As with CoAP over TCP, each endpoint MUST send a CSM (see Section 5.3) as its first message on the WebSocket connection. CoAP requests and responses are exchanged asynchronously over the WebSocket connection. A CoAP client can send multiple requests without waiting for a response, and the CoAP server can return responses in any order. Responses MUST be returned over the same connection as the originating request. Each concurrent request is differentiated by its Token, which is scoped locally to the connection. The connection is bidirectional, so requests can be sent by both the entity that established the connection and the remote host.
As with CoAP over TCP, retransmission and deduplication of messages are provided by the WebSocket Protocol. CoAP over WebSockets therefore does not make a distinction between Confirmable messages and Non-confirmable messages and does not provide Acknowledgment or Reset messages.4.4. Connection Health
As with CoAP over TCP, a CoAP client can test the health of the connection for CoAP over WebSockets by sending a CoAP Ping Signaling message (Section 5.4). To ensure that redundant maintenance traffic is not transmitted, WebSocket Ping and unsolicited Pong frames (Section 5.5 of [RFC6455]) SHOULD NOT be used.
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