| CORE | C. Bormann |
| Internet-Draft | Universitaet Bremen TZI |
| Updates: 7641, 7959 (if approved) | S. Lemay |
| Intended status: Standards Track | Zebra Technologies |
| Expires: June 21, 2018 | H. Tschofenig |
| ARM Ltd. | |
| K. Hartke | |
| Universitaet Bremen TZI | |
| B. Silverajan | |
| Tampere University of Technology | |
| B. Raymor, Ed. | |
| December 18, 2017 |
CoAP (Constrained Application Protocol) over TCP, TLS, and WebSockets
draft-ietf-core-coap-tcp-tls-11
The Constrained Application Protocol (CoAP), although inspired by HTTP, was designed to use UDP instead of TCP. The message layer of the CoAP over UDP protocol includes support for reliable delivery, simple congestion control, and flow control.
Some environments benefit from the availability of CoAP carried over reliable transports such as TCP or TLS. This document outlines the changes required to use CoAP over TCP, TLS, and WebSockets transports. It also formally updates RFC 7641 for use with these transports and RFC 7959 to enable the use of larger messages over a reliable transport.
This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 21, 2018.
Copyright (c) 2017 IETF Trust and the persons identified as the document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal 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.
The Constrained Application Protocol (CoAP) was designed for Internet of Things (IoT) deployments, assuming that UDP [RFC0768] can be used unimpeded, as can the Datagram Transport Layer Security protocol (DTLS [RFC6347]) over UDP. The use of CoAP over UDP is focused on simplicity, has a low code footprint, and a small over-the-wire message size.
The primary reason for introducing CoAP over TCP [RFC0793] and TLS [RFC5246] is that some networks do not forward UDP packets. Complete blocking of UDP happens in between about 2% and 4% of terrestrial access networks, according to [EK2016]. UDP impairment is especially concentrated in enterprise networks and networks in geographic regions with otherwise challenged connectivity. Some networks also rate-limit UDP traffic, as reported in [BK2015] and deployment investigations related to the standardization of QUIC revealed numbers around 0.3 % [SW2016].
The introduction of CoAP over TCP also leads to some additional effects that may be desirable in a specific deployment:
Note that the use of CoAP over UDP (and CoAP over DTLS over UDP) is still the recommended transport for use in constrained node networks, particularly when used in concert with blockwise transfer. CoAP over TCP is applicable for those cases where the networking infrastructure leaves no other choice. The use of CoAP over TCP leads to a larger code size, more roundtrips, increased RAM requirements and larger packet sizes. Developers implementing CoAP over TCP are encouraged to consult [I-D.gomez-lwig-tcp-constrained-node-networks] for guidance on low-footprint TCP implementations for IoT devices.
Standards based on CoAP such as Lightweight Machine to Machine [LWM2M] currently use CoAP over UDP as a transport; adding support for CoAP over TCP enables them to address the issues above for specific deployments and to protect investments in existing CoAP implementations and deployments.
Although HTTP/2 could also potentially address the need for enterprise firewall traversal, there would be additional costs and delays introduced by such a transition from CoAP to HTTP/2. Currently, there are also fewer HTTP/2 implementations available for constrained devices in comparison to CoAP. Since CoAP also support group communication using IP layer multicast and unreliable communication IoT devices would have to support HTTP/2 in addition to CoAP.
Furthermore, CoAP may be integrated into a Web environment where the front-end uses CoAP over UDP from IoT devices to a cloud infrastructure and then CoAP over TCP between the back-end services. A TCP-to-UDP gateway can be used at the cloud boundary to communicate with the UDP-based IoT device.
Finally, CoAP applications running inside a web browser may be without access to connectivity other than HTTP. In this case, the WebSocket protocol [RFC6455] may be used to transport CoAP requests and responses, as opposed to cross-proxying them via HTTP to an HTTP-to-CoAP cross-proxy. This preserves the functionality of CoAP without translation, in particular the Observe mechanism [RFC7641].
To address the above-mentioned deployment requirements, this document defines how to transport CoAP over TCP, CoAP over TLS, and CoAP over WebSockets. For these cases, the reliability offered by the transport protocol subsumes the reliability functions of the message layer used for CoAP over UDP. (Note that both for a reliable transport and the CoAP over UDP message layer, the reliability offered is per transport hop: where proxies — see Sections 5.7 and 10 of [RFC7252] — are involved, that layer’s reliability function does not extend end-to-end.) Figure 1 illustrates the layering:
+--------------------------------+ | Application | +--------------------------------+ +--------------------------------+ | Requests/Responses/Signaling | CoAP (RFC 7252) / This Document |--------------------------------| | Message Framing | This Document +--------------------------------+ | Reliable Transport | +--------------------------------+
Figure 1: Layering of CoAP over Reliable Transports
This document specifies how to access resources using CoAP requests and responses over the TCP, TLS and WebSocket protocols. This allows connectivity-limited applications to obtain end-to-end CoAP connectivity either by communicating CoAP directly with a CoAP server accessible over a TCP, TLS or WebSocket connection or via a CoAP intermediary that proxies CoAP requests and responses between different transports, such as between WebSockets and UDP.
Section 7 updates the “Observing Resources in the Constrained Application Protocol” specification for use with CoAP over reliable transports. [RFC7641] is an extension to the CoAP protocol that enables CoAP clients to “observe” a resource on a CoAP server. (The CoAP client retrieves a representation of a resource and registers to be notified by the CoAP server when the representation is updated.)
The key words “MUST”, “MUST NOT”, “REQUIRED”, “SHALL”, “SHALL NOT”, “SHOULD”, “SHOULD NOT”, “RECOMMENDED”, “NOT RECOMMENDED”, “MAY”, and “OPTIONAL” in this document are to be interpreted as described in [RFC2119].
This document assumes that readers are familiar with the terms and concepts that are used in [RFC6455], [RFC7252], [RFC7641], and [RFC7959].
The term “reliable transport” is used only to refer to transport protocols, such as TCP, which provide reliable and ordered delivery of a byte-stream.
The request/response interaction model of CoAP over TCP is the same as CoAP over UDP. The primary differences are in the message layer. The message layer of CoAP over UDP supports optional reliability by defining four types of messages: Confirmable, Non-confirmable, Acknowledgement, and Reset. In addition, messages include a Message ID to relate Acknowledgments to Confirmable messages and to detect duplicate messages.
The management of the transport connections is left to the application, i.e., the present specification does not describe how an application decides to open a connection or to re-open another one in the presence of failures (or what it would deem to be a failure, see also Section 5.4). In particular, the Connection Initiator need not be the client of the first request placed on the connection. Some implementations will want to implement a dynamic connection management similar to the one described in Section 6 of [RFC7230] for HTTP, opening a connection when the first client request is ready to be sent and reusing that for further messages for a while, until no message is sent for a certain time and no requests are outstanding (possibly with a configurable idle time) and a release process is started (Section 5.5). In implementations of this kind, connection releases or aborts may not be indicated as errors to the application but may simply be handled by automatic reconnection once the need arises again. Other implementations may be based on configured connections that are kept open continuously and lead to management system notifications on release or abort. The protocol defined in the present specification is intended to work with either model (or other, application-specific connection management models).
Conceptually, CoAP over TCP replaces most of the message layer of CoAP over UDP with a framing mechanism on top of the byte-stream provided by TCP/TLS, conveying the length information for each message that on datagram transports is provided by the UDP/DTLS datagram layer.
TCP ensures reliable message transmission, so the message layer of CoAP over TCP is not required to support acknowledgements or to detect duplicate messages. As a result, both the Type and Message ID fields are no longer required and are removed from the CoAP over TCP message format.
Figure 2 illustrates the difference between CoAP over UDP and CoAP over reliable transport. The removed Type and Message ID fields are indicated by dashes.
CoAP Client CoAP Server CoAP Client CoAP Server
| | | |
| CON [0xbc90] | | (-------) [------] |
| GET /temperature | | GET /temperature |
| (Token 0x71) | | (Token 0x71) |
+------------------->| +------------------->|
| | | |
| ACK [0xbc90] | | (-------) [------] |
| 2.05 Content | | 2.05 Content |
| (Token 0x71) | | (Token 0x71) |
| "22.5 C" | | "22.5 C" |
|<-------------------+ |<-------------------+
| | | |
CoAP over UDP CoAP over reliable
transport
Figure 2: Comparison between CoAP over unreliable and reliable transport
The CoAP message format defined in [RFC7252], as shown in Figure 3, relies on the datagram transport (UDP, or DTLS over UDP) for keeping the individual messages separate and for providing length information.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |Ver| T | TKL | Code | Message ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Token (if any, TKL bytes) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options (if any) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1 1 1 1 1 1 1 1| Payload (if any) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3: RFC 7252 defined CoAP Message Format
The CoAP over TCP message format is very similar to the format specified for CoAP over UDP. The differences are as follows:
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
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.
Once a transport connection is established, each endpoint MUST send a Capabilities and Settings message (CSM, see Section 5.3) as their first message on the connection. This message establishes the initial settings and capabilities for the endpoint, such as maximum message size or support for block-wise transfers. The absence of options in the CSM indicates that base values are assumed.
To avoid a deadlock, the Connection Initiator MUST NOT wait for the Connection Acceptor to send its initial CSM message before sending its own initial CSM message. Conversely, the Connection Acceptor MAY wait for the Connection Initiator to send its initial CSM message before sending its own initial CSM message.
To avoid unnecessary latency, a Connection Initiator MAY send additional messages after its initial CSM without waiting to receive the Connection Acceptor’s CSM; however, it is important to note that the Connection Acceptor’s CSM might indicate capabilities that impact how the initiator is expected to communicate with the acceptor. For example, the acceptor CSM could indicate a Max-Message-Size option (see Section 5.3.1) that is smaller than the base value (1152) in order to limit both buffering requirements and head-of-line blocking.
Endpoints MUST treat a missing or invalid CSM as a connection error and abort the connection (see Section 5.6).
CoAP requests and responses are exchanged asynchronously over the transport connection. A CoAP client can send multiple requests without waiting for a response and the CoAP server can return responses in any order. Responses MUST be returned over the same connection as the originating request. Concurrent requests are differentiated by their Token, which is scoped locally to the connection.
The transport connection is bi-directional, so requests can be sent both by the entity that established the connection (Connection Initiator) and the remote host (Connection Acceptor). If one side does not implement a CoAP server, an error response MUST be returned for all CoAP requests from the other side. The simplest approach is to always return 5.01 (Not Implemented). A more elaborate mock server could also return 4.xx responses such as 4.04 (Not Found) or 4.02 (Bad Option) where appropriate.
Retransmission and deduplication of messages is provided by the TCP protocol.
Empty messages (Code 0.00) can always be sent and MUST be ignored by the recipient. This provides a basic keep-alive function that can refresh NAT bindings.
If a CoAP client does not receive any response for some time after sending a CoAP request (or, similarly, when a client observes a resource and it does not receive any notification for some time), it can send a CoAP Ping Signaling message (see Section 5.4) to test the transport connection and verify that the CoAP server is responsive.
When the underlying transport connection is closed or reset, the signaling state and any observation state (see Section 7.4) associated with the connection are removed. In flight messages may or may not be lost.
CoAP over WebSockets is intentionally similar to CoAP over TCP; therefore, this section only specifies the differences between the transports.
CoAP over WebSockets can be used in a number of configurations. The most basic configuration is a CoAP client retrieving or updating a CoAP resource located on a CoAP server that exposes a WebSocket endpoint (see Figure 6). The CoAP client acts as the WebSocket client, establishes a WebSocket connection, and sends a CoAP request, to which the CoAP server returns a CoAP response. The WebSocket connection can be used for any number of requests.
___________ ___________
| | | |
| _|___ requests ___|_ |
| CoAP / \ \ -------------> / / \ CoAP |
| Client \__/__/ <------------- \__\__/ Server |
| | responses | |
|___________| |___________|
WebSocket =============> WebSocket
Client Connection Server
Figure 6: CoAP Client (WebSocket client) accesses CoAP Server (WebSocket server)
The challenge with this configuration is how to identify a resource in the namespace of the CoAP server. When the WebSocket protocol is used by a dedicated client directly (i.e., not from a web page through a web browser), the client can connect to any WebSocket endpoint. Section 8.3 and Section 8.4 define new URI schemes that enable the client to identify both a WebSocket endpoint and the path and query of the CoAP resource within that endpoint.
Another possible configuration is to set up a CoAP forward proxy at the WebSocket endpoint. Depending on what transports are available to the proxy, it could forward the request to a CoAP server with a CoAP UDP endpoint (Figure 7), an SMS endpoint (a.k.a. mobile phone), or even another WebSocket endpoint. The CoAP client specifies the resource to be updated or retrieved in the Proxy-Uri Option.
___________ ___________ ___________
| | | | | |
| _|___ ___|_ _|___ ___|_ |
| 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.
Before CoAP requests and responses are exchanged, a WebSocket connection is established as defined in Section 4 of [RFC6455]. Figure 9 shows an example.
The WebSocket client MUST include the subprotocol name “coap” in the list of protocols, which indicates support for the protocol defined in this document.
The WebSocket client includes the hostname of the WebSocket server in the Host header field of its handshake as per [RFC6455]. The Host header field also indicates the default value of the Uri-Host Option in requests from the WebSocket client to the WebSocket server.
GET /.well-known/coap HTTP/1.1 Host: example.org Upgrade: websocket Connection: Upgrade Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ== Sec-WebSocket-Protocol: coap Sec-WebSocket-Version: 13 HTTP/1.1 101 Switching Protocols Upgrade: websocket Connection: Upgrade Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo= Sec-WebSocket-Protocol: coap
Figure 9: Example of an Opening Handshake
Once a WebSocket connection is established, CoAP requests and responses can be exchanged as WebSocket messages. Since CoAP uses a binary message format, the messages are transmitted in binary data frames as specified in Sections 5 and 6 of [RFC6455].
The message format shown in Figure 10 is the same as the CoAP over TCP message format (see Section 3.2) with one change. The Length (Len) field MUST be set to zero because the WebSockets frame contains the length.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Len=0 | TKL | Code | Token (TKL bytes) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Options (if any) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1 1 1 1 1 1 1 1| Payload (if any) ... +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10: CoAP Message Format over WebSockets
As with CoAP over TCP, the message format for CoAP over WebSockets eliminates the Version field defined in CoAP over UDP. If CoAP version negotiation is required in the future, CoAP over WebSockets can address the requirement by the definition of a new subprotocol identifier that is negotiated during the opening handshake.
Requests and response messages can be fragmented as specified in Section 5.4 of [RFC6455], though typically they are sent unfragmented as they tend to be small and fully buffered before transmission. The WebSocket protocol does not provide means for multiplexing. If it is not desirable for a large message to monopolize the connection, requests and responses can be transferred in a block-wise fashion as defined in [RFC7959].
As with CoAP over TCP, each endpoint MUST send a Capabilities and Settings message (CSM see Section 5.3) as their first message on the WebSocket connection.
CoAP requests and responses are exchanged asynchronously over the WebSocket connection. A CoAP client can send multiple requests without waiting for a response and the CoAP server can return responses in any order. Responses MUST be returned over the same connection as the originating request. Concurrent requests are differentiated by their Token, which is scoped locally to the connection.
The connection is bi-directional, so requests can be sent both by the entity that established the connection and the remote host.
As with CoAP over TCP, retransmission and deduplication of messages is provided by the WebSocket protocol. CoAP over WebSockets therefore does not make a distinction between Confirmable or Non-Confirmable messages, and does not provide Acknowledgement or Reset messages.
As with CoAP over TCP, a CoAP client can test the health of the CoAP over WebSocket connection by sending a CoAP Ping Signaling message (Section 5.4). WebSocket Ping and unsolicited Pong frames (Section 5.5 of [RFC6455]) SHOULD NOT be used to ensure that redundant maintenance traffic is not transmitted.
Signaling messages are specifically introduced only for CoAP over reliable transports to allow peers to:
Signaling is a third basic kind of message in CoAP, after requests and responses. Signaling messages share a common structure with the existing CoAP messages. There is a code, a token, options, and an optional payload.
(See Section 3 of [RFC7252] for the overall structure of the message format, option format, and option value format.)
A code in the 7.00-7.31 range indicates a Signaling message. Values in this range are assigned by the “CoAP Signaling Codes” sub-registry (see Section 11.1).
For each message, there is a sender and a peer receiving the message.
Payloads in Signaling messages are diagnostic payloads as defined in Section 5.5.2 of [RFC7252]), unless otherwise defined by a Signaling message option.
Option numbers for Signaling messages are specific to the message code. They do not share the number space with CoAP options for request/response messages or with Signaling messages using other codes.
Option numbers are assigned by the “CoAP Signaling Option Numbers” sub-registry (see Section 11.2).
Signaling options are elective or critical as defined in Section 5.4.1 of [RFC7252]. If a Signaling option is critical and not understood by the receiver, it MUST abort the connection (see Section 5.6). If the option is understood but cannot be processed, the option documents the behavior.
Capabilities and Settings messages (CSM) are used for two purposes:
One CSM MUST be sent by each endpoint at the start of the transport connection. Further CSM MAY be sent at any other time by either endpoint over the lifetime of the connection.
Both capability and setting options are cumulative. A CSM does not invalidate a previously sent capability indication or setting even if it is not repeated. A capability message without any option is a no-operation (and can be used as such). An option that is sent might override a previous value for the same option. The option defines how to handle this case if needed.
Base values are listed below for CSM Options. These are the values for the capability and setting before any Capabilities and Settings messages send a modified value.
These are not default values for the option, as defined in Section 5.4.4 in [RFC7252]. Default values apply on a per-message basis and thus reset when the value is not present in a given Capabilities and Settings message.
Capabilities and Settings messages are indicated by the 7.01 code (CSM).
The sender can use the elective Max-Message-Size Option to indicate the maximum size of a message in bytes that it can receive. The message size indicated includes the entire message, starting from the first byte of the message header and ending at the end of the message payload.
(Note that there is no relationship of the message size to the overall request or response body size that may be achievable in block-wise transfer. For example, the exchange depicted further down in Figure 13 can be performed if the CoAP client indicates a value of around 6000 bytes for the Max-Message-Size option, even though the total body size transferred to the client is 3072 + 5120 + 4711 = 12903 bytes.)
| # | C | R | Applies to | Name | Format | Length | Base 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.
The active value of the Max-Message-Size Option is replaced each time the option is sent with a modified value. Its starting value is its base value.
| # | C | R | Applies to | Name | Format | Length | Base Value |
|---|---|---|---|---|---|---|---|
| 4 | CSM | Block-Wise-Transfer | empty | 0 | (none) |
A sender can use the elective Block-Wise-Transfer Option to indicate that it supports the block-wise transfer protocol [RFC7959].
If the option is not given, the peer has no information about whether block-wise transfers are supported by the sender or not. An implementation wishing to offer block-wise transfers to its peer therefore needs to indicate the Block-Wise-Transfer Option.
If a Max-Message-Size Option is indicated with a value that is greater than 1152 (in the same or a different CSM message), the Block-Wise-Transfer Option also indicates support for BERT (see Section 6). Subsequently, if the Max-Message-Size Option is indicated with a value equal to or less than 1152, BERT support is no longer indicated. (Note that indication of BERT support obliges neither peer to actually choose to make use of BERT.)
Implementation note: When indicating a value of the Max-Message-Size option with an intention to enable BERT, the indicating implementation may want to choose a BERT size message it wants to encourage and add a delta for the header and any options that also need to be included in the message. Section 4.6 of [RFC7252] adds 128 bytes to a maximum block size of 1024 to arrive at a default message size of 1152. A BERT-enabled implementation may want to indicate a BERT block size of 2048 or a higher multiple of 1024, and at the same time be more generous for the size of header and options added (say, 256 or 512). Adding 1024 or more however to the base BERT block size may encourage the peer implementation to vary the BERT block size based on the size of the options included, which can be harder to establish interoperability for.
In CoAP over reliable transports, Empty messages (Code 0.00) can always be sent and MUST be ignored by the recipient. This provides a basic keep-alive function. In contrast, Ping and Pong messages are a bidirectional exchange.
Upon receipt of a Ping message, the receiver MUST return a Pong message with an identical token in response. Unless the Ping carries an option with delaying semantics such as the Custody Option, it SHOULD respond as soon as practical. As with all Signaling messages, the recipient of a Ping or Pong message MUST ignore elective options it does not understand.
Ping and Pong messages are indicated by the 7.02 code (Ping) and the 7.03 code (Pong).
Note that, as with similar mechanisms defined in [RFC6455] and [RFC7540], the present specification does not define any specific maximum time that the sender of a Ping message has to allow waiting for a Pong reply. Any limitations on the patience for this reply are a matter of the application making use of these messages, as is any approach to recover from a failure to respond in time.
| # | C | R | Applies to | Name | Format | Length | Base Value |
|---|---|---|---|---|---|---|---|
| 2 | Ping, Pong | Custody | empty | 0 | (none) |
When responding to a Ping message, the receiver can include an elective Custody Option in the Pong message. This option indicates that the application has processed all the request/response messages received prior to the Ping message on the current connection. (Note that there is no definition of specific application semantics for “processed”, but there is an expectation that the receiver of a Pong Message with a Custody Option should be able to free buffers based on this indication.)
A sender can also include an elective Custody Option in a Ping message to explicitly request the inclusion of an elective Custody Option in the corresponding Pong message. In that case, the receiver SHOULD delay its Pong message until it finishes processing all the request/response messages received prior to the Ping message on the current connection.
A Release message indicates that the sender does not want to continue maintaining the transport connection and opts for an orderly shutdown, but wants to leave it to the peer to actually start closing the connection. The details are in the options. A diagnostic payload (see Section 5.5.2 of [RFC7252]) MAY be included.
A peer will normally respond to a Release message by closing the transport connection. (In case that does not happen, the sender of the release may want to implement a timeout mechanism if getting rid of the connection is actually important to it.)
Messages may be in flight or responses outstanding when the sender decides to send a Release message (which is one reason the sender had decided to wait with closing the connection). The peer responding to the Release message SHOULD delay the closing of the connection until it has responded to all requests received by it before the Release message. It also MAY wait for the responses to its own requests.
It is NOT RECOMMENDED for the sender of a Release message to continue sending requests on the connection it already indicated to be released: the peer might close the connection at any time and miss those requests. There is no obligation for the peer to check for this condition, though.
Release messages are indicated by the 7.04 code (Release).
Release messages can indicate one or more reasons using elective options. The following options are defined:
| # | C | R | Applies to | Name | Format | Length | Base Value |
|---|---|---|---|---|---|---|---|
| 2 | x | Release | Alternative-Address | string | 1-255 | (none) |
The elective Alternative-Address Option requests the peer to instead open a connection of the same scheme as the present connection to the alternative transport address given. Its value is in the form “authority” as defined in Section 3.2 of [RFC3986]. (Existing state related to the connection is not transferred from the present connection to the new connection.)
The Alternative-Address Option is a repeatable option as defined in Section 5.4.5 of [RFC7252]. When multiple occurrences of the option are included, the peer can choose any of the alternative transport addresses.
| # | C | R | Applies to | Name | Format | Length | Base Value |
|---|---|---|---|---|---|---|---|
| 4 | Release | Hold-Off | uint | 0-3 | (none) |
The elective Hold-Off Option indicates that the server is requesting that the peer not reconnect to it for the number of seconds given in the value.
An Abort message indicates that the sender is unable to continue maintaining the transport connection and cannot even wait for an orderly release. The sender shuts down the connection immediately after the abort (and may or may not wait for a Release or Abort message or connection shutdown in the inverse direction). A diagnostic payload (see Section 5.5.2 of [RFC7252]) SHOULD be included in the Abort message. Messages may be in flight or responses outstanding when the sender decides to send an Abort message. The general expectation is that these will NOT be processed.
Abort messages are indicated by the 7.05 code (Abort).
Abort messages can indicate one or more reasons using elective options. The following option is defined:
| # | C | R | Applies to | Name | Format | Length | Base Value |
|---|---|---|---|---|---|---|---|
| 2 | Abort | Bad-CSM-Option | uint | 0-2 | (none) |
The elective Bad-CSM-Option Option indicates that the sender is unable to process the CSM option identified by its option number, e.g. when it is critical and the option number is unknown by the sender, or when there is parameter problem with the value of an elective option. More detailed information SHOULD be included as a diagnostic payload.
For CoAP over UDP, messages which contain syntax violations are processed as message format errors. As described in Sections 4.2 and 4.3 of [RFC7252], such messages are rejected by sending a matching Reset message and otherwise ignoring the message.
For CoAP over reliable transports, the recipient rejects such messages by sending an Abort message and otherwise ignoring (not processing) the message. No specific option has been defined for the Abort message in this case, as the details are best left to a diagnostic payload.
An encoded example of a Ping message with a non-empty token is shown in Figure 11.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x01 | 0xe2 | 0x42 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Len = 0 -------> 0x01
TKL = 1 ___/
Code = 7.02 Ping --> 0xe2
Token = 0x42
Figure 11: Ping Message Example
An encoded example of the corresponding Pong message is shown in Figure 12.
0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| 0x01 | 0xe3 | 0x42 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Len = 0 -------> 0x01
TKL = 1 ___/
Code = 7.03 Pong --> 0xe3
Token = 0x42
Figure 12: Pong Message Example
The message size restrictions defined in Section 4.6 of CoAP [RFC7252] to avoid IP fragmentation are not necessary when CoAP is used over a reliable transport. While this suggests that the Block-wise transfer protocol [RFC7959] is also no longer needed, it remains applicable for a number of cases:
The ‘Block-wise Extension for Reliable Transport (BERT)’ extends the Block protocol to enable the use of larger messages over a reliable transport.
The use of this new extension is signaled by sending Block1 or Block2 Options with SZX == 7 (a “BERT option”). SZX == 7 is a reserved value in [RFC7959].
In control usage, a BERT option is interpreted in the same way as the equivalent Option with SZX == 6, except that it also indicates the capability to process BERT blocks. As with the basic Block protocol, the recipient of a CoAP request with a BERT option in control usage is allowed to respond with a different SZX value, e.g. to send a non-BERT block instead.
In descriptive usage, a BERT Option is interpreted in the same way as the equivalent Option with SZX == 6, except that the payload is also allowed to contain multiple blocks. For non-final BERT blocks, the payload is always a multiple of 1024 bytes. For final BERT blocks, the payload is a multiple (possibly 0) of 1024 bytes plus a partial block of less than 1024 bytes.
The recipient of a non-final BERT block (M=1) conceptually partitions the payload into a sequence of 1024-byte blocks and acts exactly as if it had received this sequence in conjunction with block numbers starting at, and sequentially increasing from, the block number given in the Block Option. In other words, the entire BERT block is positioned at the byte position that results from multiplying the block number with 1024. The position of further blocks to be transferred is indicated by incrementing the block number by the number of elements in this sequence (i.e., the size of the payload divided by 1024 bytes).
As with SZX == 6, the recipient of a final BERT block (M=0) simply appends the payload at the byte position that is indicated by the block number multiplied with 1024.
The following examples illustrate BERT options. A value of SZX == 7 is labeled as “BERT” or as “BERT(nnn)” to indicate a payload of size nnn.
In all these examples, a Block Option is decomposed to indicate the kind of Block Option (1 or 2) followed by a colon, the block number (NUM), more bit (M), and block size (2**(SZX+4)) separated by slashes. E.g., a Block2 Option value of 33 would be shown as 2:2/0/32), or a Block1 Option value of 59 would be shown as 1:3/1/128.
Figure 13 shows a GET request with a response that is split into three BERT blocks. The first response contains 3072 bytes of payload; the second, 5120; and the third, 4711. Note how the block number increments to move the position inside the response body forward.
CoAP Client CoAP Server | | | GET, /status ------> | | | | <------ 2.05 Content, 2:0/1/BERT(3072) | | | | GET, /status, 2:3/0/BERT ------> | | | | <------ 2.05 Content, 2:3/1/BERT(5120) | | | | GET, /status, 2:8/0/BERT ------> | | | | <------ 2.05 Content, 2:8/0/BERT(4711) |
Figure 13: GET with BERT blocks
Figure 14 demonstrates a PUT exchange with BERT blocks.
CoAP Client CoAP Server | | | PUT, /options, 1:0/1/BERT(8192) ------> | | | | <------ 2.31 Continue, 1:0/1/BERT | | | | PUT, /options, 1:8/1/BERT(16384) ------> | | | | <------ 2.31 Continue, 1:8/1/BERT | | | | PUT, /options, 1:24/0/BERT(5683) ------> | | | | <------ 2.04 Changed, 1:24/0/BERT | | |
Figure 14: PUT with BERT blocks
This section describes how the procedures defined in [RFC7641] for observing resources over CoAP are applied (and modified, as needed) for reliable transports. In this section, “client” and “server” refer to the CoAP client and CoAP server.
When using the Observe Option with CoAP over UDP, notifications from the server set the option value to an increasing sequence number for reordering detection on the client since messages can arrive in a different order than they were sent. This sequence number is not required for CoAP over reliable transports since the TCP protocol ensures reliable and ordered delivery of messages. The value of the Observe Option in 2.xx notifications MAY be empty on transmission and MUST be ignored on reception.
Implementation note: This means that a proxy from a reordering transport to a reliable (in-order) transport (such as a UDP-to-TCP proxy) needs to process the Observe Option in notifications according to the rules in Section 3.4 of [RFC7641].
For CoAP over UDP, server notifications to the client can be confirmable or non-confirmable. A confirmable message requires the client to either respond with an acknowledgement message or a reset message. An acknowledgement message indicates that the client is alive and wishes to receive further notifications. A reset message indicates that the client does not recognize the token which causes the server to remove the associated entry from the list of observers.
Since TCP eliminates the need for the message layer to support reliability, CoAP over reliable transports does not support confirmable or non-confirmable message types. All notifications are delivered reliably to the client with positive acknowledgement of receipt occurring at the TCP level. If the client does not recognize the token in a notification, it MAY immediately abort the connection (see Section 5.6).
For CoAP over UDP, if a client does not receive a notification for some time, it MAY send a new GET request with the same token as the original request to re-register its interest in a resource and verify that the server is still responsive. For CoAP over reliable transports, it is more efficient to check the health of the connection (and all its active observations) by sending a single CoAP Ping Signaling message (Section 5.4) rather than individual requests to confirm each active observation. (Note that such a Ping/Pong only confirms a single hop: there is no obligation, and no expectation, of a proxy to react to a Ping by checking all its onward observations or all the connections, if any, underlying them. A proxy MAY maintain its own schedule for confirming the onward observations it relies on; it is however generally inadvisable for a proxy to generate a large number of outgoing checks based on a single incoming check.)
For CoAP over UDP, a client that is no longer interested in receiving notifications can “forget” the observation and respond to the next notification from the server with a reset message to cancel the observation.
For CoAP over reliable transports, a client MUST explicitly deregister by issuing a GET request that has the Token field set to the token of the observation to be cancelled and includes an Observe Option with the value set to 1 (deregister).
If the client observes one or more resources over a reliable transport, then the CoAP server (or intermediary in the role of the CoAP server) MUST remove all entries associated with the client endpoint from the lists of observers when the connection is either closed or times out.
CoAP over UDP [RFC7252] defines the “coap” and “coaps” URI schemes. This document introduces four additional URI schemes for identifying CoAP resources and providing a means of locating the resource:
Resources made available via these schemes have no shared identity even if their resource identifiers indicate the same authority (the same host listening to the same TCP port). They are hosted in distinct namespaces because each URI scheme implies a distinct origin server.
The syntax for the URI schemes in this section are specified using Augmented Backus-Naur Form (ABNF) [RFC5234]. The definitions of “host”, “port”, “path-abempty”, and “query” are adopted from [RFC3986].
Section 8 (Multicast CoAP) in [RFC7252] is not applicable to these schemes.
As with the “coap” and “coaps” schemes defined in [RFC7252], all URI schemes defined in this section also support the path prefix “/.well-known/” defined by [RFC5785] for “well-known locations” in the namespace of a host. This enables discovery as per Section 7 of [RFC7252].
The “coap+tcp” URI scheme identifies CoAP resources that are intended to be accessible using CoAP over TCP.
coap-tcp-URI = "coap+tcp:" "//" host [ ":" port ]
path-abempty [ "?" query ]
The syntax defined in Section 6.1 of [RFC7252] applies to this URI scheme with the following changes:
The “coaps+tcp” URI scheme identifies CoAP resources that are intended to be accessible using CoAP over TCP secured with TLS.
coaps-tcp-URI = "coaps+tcp:" "//" host [ ":" port ]
path-abempty [ "?" query ]
The syntax defined in Section 6.2 of [RFC7252] applies to this URI scheme, with the following changes:
The “coap+ws” URI scheme identifies CoAP resources that are intended to be accessible using CoAP over WebSockets.
coap-ws-URI = "coap+ws:" "//" host [ ":" port ]
path-abempty [ "?" query ]
The port subcomponent is OPTIONAL. The default is port 80.
The WebSocket endpoint is identified by a “ws” URI that is composed of the authority part of the “coap+ws” URI and the well-known path “/.well-known/coap” [RFC5785] [I-D.bormann-hybi-ws-wk]. The path and query parts of a “coap+ws” URI identify a resource within the specified endpoint which can be operated on by the methods defined by CoAP:
coap+ws://example.org/sensors/temperature?u=Cel
\______ ______/\___________ ___________/
\/ \/
Uri-Path: "sensors"
ws://example.org/.well-known/coap Uri-Path: "temperature"
Uri-Query: "u=Cel"
Figure 15: The "coap+ws" URI Scheme
The “coaps+ws” URI scheme identifies CoAP resources that are intended to be accessible using CoAP over WebSockets secured by TLS.
coaps-ws-URI = "coaps+ws:" "//" host [ ":" port ]
path-abempty [ "?" query ]
The port subcomponent is OPTIONAL. The default is port 443.
The WebSocket endpoint is identified by a “wss” URI that is composed of the authority part of the “coaps+ws” URI and the well-known path “/.well-known/coap” [RFC5785] [I-D.bormann-hybi-ws-wk]. The path and query parts of a “coaps+ws” URI identify a resource within the specified endpoint which can be operated on by the methods defined by CoAP.
coaps+ws://example.org/sensors/temperature?u=Cel
\______ ______/\___________ ___________/
\/ \/
Uri-Path: "sensors"
wss://example.org/.well-known/coap Uri-Path: "temperature"
Uri-Query: "u=Cel"
Figure 16: The "coaps+ws" URI Scheme
CoAP over reliable transports maintains the property from Section 5.10.1 of [RFC7252]:
Unless otherwise noted, the default value of the Uri-Host Option is the IP literal representing the destination IP address of the request message. The default value of the Uri-Port Option is the destination TCP port.
For CoAP over TLS, these default values are the same unless Server Name Indication (SNI) [RFC6066] is negotiated. In this case, the default value of the Uri-Host Option in requests from the TLS client to the TLS server is the SNI host.
For CoAP over WebSockets, the default value of the Uri-Host Option in requests from the WebSocket client to the WebSocket server is indicated by the Host header field from the WebSocket handshake.
The steps are the same as specified in Section 6.4 of [RFC7252] with minor changes.
This step from [RFC7252]:
3. If |url| does not have a <scheme> component whose value, when
converted to ASCII lowercase, is "coap" or "coaps", then fail
this algorithm.
is updated to:
3. If |url| does not have a <scheme> component whose value, when
converted to ASCII lowercase, is "coap+tcp", "coaps+tcp",
"coap+ws", or "coaps+ws", then fail this algorithm.
This step from [RFC7252]:
7. If |port| does not equal the request's destination UDP port,
include a Uri-Port Option and let that option's value be |port|.
is updated to:
7. If |port| does not equal the request's destination TCP port,
include a Uri-Port Option and let that option's value be |port|.
The steps are the same as specified in Section 6.5 of [RFC7252] with minor changes.
This step from [RFC7252]:
1. If the request is secured using DTLS, let |url| be the string
"coaps://". Otherwise, let |url| be the string "coap://".
is updated to:
1. For CoAP over TCP, if the request is secured using TLS, let |url|
be the string "coaps+tcp://". Otherwise, let |url| be the string
"coap+tcp://". For CoAP over WebSockets, if the request is
secured using TLS, let |url| be the string "coaps+ws://".
Otherwise, let |url| be the string "coap+ws://".
This step from [RFC7252]:
4. If the request includes a Uri-Port Option, let |port| be that
option's value. Otherwise, let |port| be the request's
destination UDP port.
is updated to:
4. If the request includes a Uri-Port Option, let |port| be that
option's value. Otherwise, let |port| be the request's
destination TCP port.
Security Challenges for the Internet of Things [SecurityChallenges] recommends:
A security solution MUST be implemented to protect CoAP over reliable transports and MUST be enabled by default. This document defines the TLS binding, but alternative solutions at different layers in the protocol stack MAY be used to protect CoAP over reliable transports when appropriate. Note that there is ongoing work to support a data object-based security model for CoAP that is independent of transport (see [I-D.ietf-core-object-security]).
The TLS usage guidance in [RFC7925] applies, including the guidance about cipher suites in that document that are derived from the mandatory-to-implement (MTI) cipher suites defined in [RFC7252].
This guidance assumes implementation in a constrained device or for communication with a constrained device. CoAP over TCP/TLS has, however, a wider applicability. It may, for example, be implemented on a gateway or on a device that is less constrained (such as a smart phone or a tablet), for communication with a peer that is likewise less constrained, or within a backend environment that only communicates with constrained devices via proxies. As an exception to the previous paragraph, in this case, the recommendations in [RFC7525] are more appropriate.
Since the guidance offered in [RFC7925] and [RFC7525] differs in terms of algorithms and credential types, it is assumed that a CoAP over TCP/TLS implementation that needs to support both cases implements the recommendations offered by both specifications.
During the provisioning phase, a CoAP device is provided with the security information that it needs, including keying materials, access control lists, and authorization servers. At the end of the provisioning phase, the device will be in one of four security modes:
The “NoSec” mode is optional-to-implement. The system simply sends the packets over normal TCP which is indicated by the “coap+tcp” scheme and the TCP CoAP default port. The system is secured only by keeping attackers from being able to send or receive packets from the network with the CoAP nodes.
“PreSharedKey”, “RawPublicKey”, or “Certificate” is mandatory-to-implement for the TLS binding depending on the credential type used with the device. These security modes are achieved using TLS and are indicated by the “coaps+tcp” scheme and TLS-secured CoAP default port.
A CoAP client requesting a resource identified by a “coaps+ws” URI negotiates a secure WebSocket connection to a WebSocket server endpoint with a “wss” URI. This is described in Section 8.4.
The client MUST perform a TLS handshake after opening the connection to the server. The guidance in Section 4.1 of [RFC6455] applies. When a CoAP server exposes resources identified by a “coaps+ws” URI, the guidance in Section 4.4 of [RFC7925] applies towards mandatory-to-implement TLS functionality for certificates. For the server-side requirements in accepting incoming connections over a HTTPS (HTTP-over-TLS) port, the guidance in Section 4.2 of [RFC6455] applies.
Note that this formally inherits the mandatory-to-implement cipher suites defined in [RFC5246]. However, usually modern browsers implement more recent cipher suites that then are automatically picked up via the JavaScript WebSocket API. WebSocket Servers that provide Secure CoAP over WebSockets for the browser use case will need to follow the browser preferences and MUST follow [RFC7525].
The security considerations of [RFC7252] apply. For CoAP over WebSockets and CoAP over TLS-secured WebSockets, the security considerations of [RFC6455] also apply.
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.
IANA is requested to create a third sub-registry for values of the Code field in the CoAP header (Section 12.1 of [RFC7252]). The name of this sub-registry is “CoAP Signaling Codes”.
Each entry in the sub-registry must include the Signaling Code in the range 7.00-7.31, its name, and a reference to its documentation.
Initial entries in this sub-registry are as follows:
| Code | Name | Reference |
|---|---|---|
| 7.01 | CSM | [RFCthis] |
| 7.02 | Ping | [RFCthis] |
| 7.03 | Pong | [RFCthis] |
| 7.04 | Release | [RFCthis] |
| 7.05 | Abort | [RFCthis] |
All other Signaling Codes are Unassigned.
The IANA policy for future additions to this sub-registry is “IETF Review or IESG Approval” as described in [RFC8126].
IANA is requested to create a sub-registry for Options Numbers used in CoAP signaling options within the “CoRE Parameters” registry. The name of this sub-registry is “CoAP Signaling Option Numbers”.
Each entry in the sub-registry must include one or more of the codes in the Signaling Codes subregistry (Section 11.1), the option number, the name of the option, and a reference to the option’s documentation.
Initial entries in this sub-registry are as follows:
| Applies to | Number | Name | Reference |
|---|---|---|---|
| 7.01 | 2 | Max-Message-Size | [RFCthis] |
| 7.01 | 4 | Block-Wise-Transfer | [RFCthis] |
| 7.02, 7.03 | 2 | Custody | [RFCthis] |
| 7.04 | 2 | Alternative-Address | [RFCthis] |
| 7.04 | 4 | Hold-Off | [RFCthis] |
| 7.05 | 2 | Bad-CSM-Option | [RFCthis] |
The IANA policy for future additions to this sub-registry is based on number ranges for the option numbers, analogous to the policy defined in Section 12.2 of [RFC7252]. (The policy is analogous rather than identical because the structure of the subregistry includes an additional column; however, the value of this column has no influence on the policy.)
The documentation for a Signaling Option Number should specify the semantics of an option with that number, including the following properties:
IANA is requested to assign the port number 5683 and the service name “coap+tcp”, in accordance with [RFC6335].
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].
URI schemes are registered within the “Uniform Resource Identifier (URI) Schemes” registry maintained at [IANA.uri-schemes].
IANA is requested to register the Uniform Resource Identifier (URI) scheme “coap+tcp”. This registration request complies with [RFC7595].
IANA is requested to register the Uniform Resource Identifier (URI) scheme “coaps+tcp”. This registration request complies with [RFC7595].
IANA is requested to register the Uniform Resource Identifier (URI) scheme “coap+ws”. This registration request complies with [RFC7595].
IANA is requested to register the Uniform Resource Identifier (URI) scheme “coaps+ws”. This registration request complies with [RFC7595].
IANA is requested to register the ‘coap’ well-known URI in the “Well-Known URIs” registry. This registration request complies with [RFC5785]:
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.
IANA is requested to register the WebSocket CoAP subprotocol under the “WebSocket Subprotocol Name Registry”:
IANA is requested to add [RFCthis] to the references for the following entries registered by [RFC7959] in the “CoAP Option Numbers” sub-registry defined by [RFC7252]:
| Number | Name | Reference |
|---|---|---|
| 23 | Block2 | RFC 7959, [RFCthis] |
| 27 | Block1 | RFC 7959, [RFCthis] |
| [BK2015] | Byrne, C. and J. Kleberg, "Advisory Guidelines for UDP Deployment", Proceedings draft-byrne-opsec-udp-advisory-00 (expired), 2015. |
| [EK2016] | Edeline, K., Kuehlewind, M., Trammell, B., Aben, E. and B. Donnet, "Using UDP for Internet Transport Evolution", Proceedings arXiv preprint 1612.07816, 2016. |
| [HomeGateway] | Eggert, L., "An experimental study of home gateway characteristics", Proceedings of the 10th annual conference on Internet measurement , 2010. |
| [I-D.gomez-lwig-tcp-constrained-node-networks] | Gomez, C., Crowcroft, J. and M. Scharf, "TCP over Constrained-Node Networks", Internet-Draft draft-gomez-lwig-tcp-constrained-node-networks-03, June 2017. |
| [I-D.ietf-core-cocoa] | Bormann, C., Betzler, A., Gomez, C. and I. Demirkol, "CoAP Simple Congestion Control/Advanced", Internet-Draft draft-ietf-core-cocoa-02, October 2017. |
| [I-D.ietf-core-object-security] | Selander, G., Mattsson, J., Palombini, F. and L. Seitz, "Object Security for Constrained RESTful Environments (OSCORE)", Internet-Draft draft-ietf-core-object-security-07, November 2017. |
| [IANA.uri-schemes] | IANA, "Uniform Resource Identifier (URI) Schemes" |
| [LWM2M] | Open Mobile Alliance, "Lightweight Machine to Machine Technical Specification Version 1.0", February 2017. |
| [RFC0768] | Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI 10.17487/RFC0768, August 1980. |
| [RFC5234] | Crocker, D. and P. Overell, "Augmented BNF for Syntax Specifications: ABNF", STD 68, RFC 5234, DOI 10.17487/RFC5234, January 2008. |
| [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. |
| [RFC6347] | Rescorla, E. and N. Modadugu, "Datagram Transport Layer Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347, January 2012. |
| [RFC7230] | Fielding, R. and J. Reschke, "Hypertext Transfer Protocol (HTTP/1.1): Message Syntax and Routing", RFC 7230, DOI 10.17487/RFC7230, June 2014. |
| [RFC7540] | Belshe, M., Peon, R. and M. Thomson, "Hypertext Transfer Protocol Version 2 (HTTP/2)", RFC 7540, DOI 10.17487/RFC7540, May 2015. |
| [SecurityChallenges] | Polk, T. and S. Turner, "Security Challenges for the Internet of Things", Interconnecting Smart Objects with the Internet / IAB Workshop , February 2011. |
| [SW2016] | Swett, I., "QUIC Deployment Experience @Google", Proceedings https://www.ietf.org/proceedings/96/slides/slides-96-quic-3.pdf, 2016. |
This section gives examples for the first two configurations discussed in Section 4.
An example of the process followed by a CoAP client to retrieve the representation of a resource identified by a “coap+ws” URI might be as follows. Figure 17 below illustrates the WebSocket and CoAP messages exchanged in detail.
CoAP CoAP
Client Server
(WebSocket (WebSocket
Client) Server)
| |
| |
+=========>| GET /.well-known/coap HTTP/1.1
| | Host: example.org
| | Upgrade: websocket
| | Connection: Upgrade
| | Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
| | Sec-WebSocket-Protocol: coap
| | Sec-WebSocket-Version: 13
| |
|<=========+ HTTP/1.1 101 Switching Protocols
| | Upgrade: websocket
| | Connection: Upgrade
| | Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=
| | Sec-WebSocket-Protocol: coap
: :
:<-------->: Exchange of CSM messages (not shown)
| |
+--------->| Binary frame (opcode=%x2, FIN=1, MASK=1)
| | +-------------------------+
| | | GET |
| | | Token: 0x53 |
| | | Uri-Path: "sensors" |
| | | Uri-Path: "temperature" |
| | | Uri-Query: "u=Cel" |
| | +-------------------------+
| |
|<---------+ Binary frame (opcode=%x2, FIN=1, MASK=0)
| | +-------------------------+
| | | 2.05 Content |
| | | Token: 0x53 |
| | | Payload: "22.3 Cel" |
| | +-------------------------+
: :
: :
+--------->| Close frame (opcode=%x8, FIN=1, MASK=1)
| |
|<---------+ Close frame (opcode=%x8, FIN=1, MASK=0)
| |
Figure 17: A CoAP client retrieves the representation of a resource identified by a "coap+ws" URI
Figure 18 shows how a CoAP client uses a CoAP forward proxy with a WebSocket endpoint to retrieve the representation of the resource coap://[2001:db8::1]/. The use of the forward proxy and the address of the WebSocket endpoint are determined by the client from local configuration rules. The request URI is specified in the Proxy-Uri Option. Since the request URI uses the “coap” URI scheme, the proxy fulfills the request by issuing a Confirmable GET request over UDP to the CoAP server and returning the response over the WebSocket connection to the client.
CoAP CoAP CoAP
Client Proxy Server
(WebSocket (WebSocket (UDP
Client) Server) Endpoint)
| | |
+--------->| | Binary frame (opcode=%x2, FIN=1, MASK=1)
| | | +------------------------------------+
| | | | GET |
| | | | Token: 0x7d |
| | | | Proxy-Uri: "coap://[2001:db8::1]/" |
| | | +------------------------------------+
| | |
| +--------->| CoAP message (Ver=1, T=Con, MID=0x8f54)
| | | +------------------------------------+
| | | | GET |
| | | | Token: 0x0a15 |
| | | +------------------------------------+
| | |
| |<---------+ CoAP message (Ver=1, T=Ack, MID=0x8f54)
| | | +------------------------------------+
| | | | 2.05 Content |
| | | | Token: 0x0a15 |
| | | | Payload: "ready" |
| | | +------------------------------------+
| | |
|<---------+ | Binary frame (opcode=%x2, FIN=1, MASK=0)
| | | +------------------------------------+
| | | | 2.05 Content |
| | | | Token: 0x7d |
| | | | Payload: "ready" |
| | | +------------------------------------+
| | |
Figure 18: A CoAP client retrieves the representation of a resource identified by a "coap" URI via a WebSocket-enabled CoAP proxy
The RFC Editor is requested to remove this section at publication.
Merged draft-savolainen-core-coap-websockets-07 Merged draft-bormann-core-block-bert-01 Merged draft-bormann-core-coap-sig-02
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
Updated references
Added Appendix: Updates to RFC7641 Observing Resources in the Constrained Application Protocol (CoAP)
Updated Capability and Settings Message (CSM) exchange in the Opening Handshake to allow initiator to send messages before receiving acceptor CSM
Addressed feedback from Working Group Last Call
Added Securing CoAP section and informative reference to OSCOAP
Removed the Server-Name and Bad-Server-Name Options
Clarified the Capability and Settings Message (CSM) exchange
Updated Pong response requirements
Added Connection Initiator and Connection Acceptor terminology where appropriate
Updated LWM2M 1.0 informative reference
Addressed feedback from second Working Group Last Call
Addressed feedback from IETF Last Call
Addressed feedback from ARTART review
Addressed feedback from GENART review
Addressed feedback from TSVART review
Added fragment identifiers to URI schemes
Added “Updates RFC7959” for BERT
Added “Updates RFC6455” to extend well-known URI mechanism to ws and wss
Clarified well-known URI mechanism use for all URI schemes
Changed NoSec to optional-to-implement
We would like to thank Stephen Berard, Geoffrey Cristallo, Olivier Delaby, Esko Dijk, Christian Groves, Nadir Javed, Michael Koster, Matthias Kovatsch, Achim Kraus, David Navarro, Szymon Sasin, Goran Selander, Zach Shelby, Andrew Summers, Julien Vermillard, and Gengyu Wei for their feedback.
Last-call reviews from Yoshifumi Nishida, Mark Nottingham, and Meral Shirazipour as well as several IESG reviewers provided extensive comments; from the IESG, we would like to specifically call out Ben Campbell, Mirja Kühlewind, Eric Rescorla, Adam Roach, and the responsible AD Alexey Melnikov.
Matthias Kovatsch Siemens AG Otto-Hahn-Ring 6 Munich D-81739 Phone: +49-173-5288856 EMail: matthias.kovatsch@siemens.com Teemu Savolainen Nokia Technologies Hatanpaan valtatie 30 Tampere FI-33100 Finland Email: teemu.savolainen@nokia.com Valik Solorzano Barboza Zebra Technologies 820 W. Jackson Blvd. Suite 700 Chicago 60607 United States of America Phone: +1-847-634-6700 Email: vsolorzanobarboza@zebra.com