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RFC 8323 - CoAP (Constrained Application Protocol) over TCP, TLS


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Internet Engineering Task Force (IETF)                        C. Bormann
Request for Comments: 8323                       Universitaet Bremen TZI
Updates: 7641, 7959                                             S. Lemay
Category: Standards Track                             Zebra Technologies
ISSN: 2070-1721                                            H. Tschofenig
                                                                ARM Ltd.
                                                               K. Hartke
                                                 Universitaet Bremen TZI
                                                           B. Silverajan
                                        Tampere University of Technology
                                                          B. Raymor, Ed.
                                                           February 2018

 CoAP (Constrained Application Protocol) over TCP, TLS, and WebSockets

Abstract

   The Constrained Application Protocol (CoAP), although inspired by
   HTTP, was designed to use UDP instead of TCP.  The message layer of
   CoAP over UDP includes support for reliable delivery, simple
   congestion control, and flow control.

   Some environments benefit from the availability of CoAP carried over
   reliable transports such as TCP or Transport Layer Security (TLS).
   This document outlines the changes required to use CoAP over TCP,
   TLS, and WebSockets transports.  It also formally updates RFC 7641
   for use with these transports and RFC 7959 to enable the use of
   larger messages over a reliable transport.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc8323.

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 ................................................53

1.  Introduction

   The Constrained Application Protocol (CoAP) [RFC7252] was designed
   for Internet of Things (IoT) deployments, assuming that UDP [RFC768]
   can be used unimpeded as can the Datagram Transport Layer Security
   (DTLS) protocol [RFC6347] over UDP.  The use of CoAP over UDP is
   focused on simplicity, has a low code footprint, and has a small
   over-the-wire message size.

   The primary reason for introducing CoAP over TCP [RFC793] and TLS
   [RFC5246] is that some networks do not forward UDP packets.  Complete
   blocking of UDP happens in between about 2% and 4% of terrestrial
   access networks, according to [EK2016].  UDP impairment is especially
   concentrated in enterprise networks and networks in geographic
   regions with otherwise challenged connectivity.  Some networks also

   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 Handshake

4.2.  Message Format

   Once a WebSocket connection is established, CoAP requests and
   responses can be exchanged as WebSocket messages.  Since CoAP uses a
   binary message format, the messages are transmitted in binary data
   frames as specified in Sections 5 and 6 of [RFC6455].

   The message format shown in Figure 10 is the same as the 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.

5.  Signaling

   Signaling messages are specifically introduced only for CoAP over
   reliable transports to allow peers to:

   o  Learn related characteristics, such as maximum message size for
      the connection.

   o  Shut down the connection in an orderly fashion.

   o  Provide diagnostic information when terminating a connection in
      response to a serious error condition.

   Signaling is a third basic kind of message in CoAP, after requests
   and responses.  Signaling messages share a common structure with the
   existing CoAP messages.  There are a code, a Token, options, and an
   optional payload.

   (See Section 3 of [RFC7252] for the overall structure of the message
   format, option format, and option value formats.)

5.1.  Signaling Codes

   A code in the 7.00-7.31 range indicates a Signaling message.  Values
   in this range are assigned by the "CoAP Signaling Codes" subregistry
   (see Section 11.1).

   For each message, there are a sender and a peer receiving the
   message.

   Payloads in Signaling messages are diagnostic payloads as defined in
   Section 5.5.2 of [RFC7252], unless otherwise defined by a Signaling
   message option.

5.2.  Signaling Option Numbers

   Option Numbers for Signaling messages are specific to the message
   code.  They do not share the number space with CoAP options for
   request/response messages or with Signaling messages using other
   codes.

   Option Numbers are assigned by the "CoAP Signaling Option Numbers"
   subregistry (see Section 11.2).

   Signaling Options are elective or critical as defined in
   Section 5.4.1 of [RFC7252].  If a Signaling Option is critical and
   not understood by the receiver, it MUST abort the connection (see
   Section 5.6).  If the option is understood but cannot be processed,
   the option documents the behavior.

5.3.  Capabilities and Settings Messages (CSMs)

   CSMs are used for two purposes:

   o  Each capability option indicates one capability of the sender to
      the recipient.

   o  Each setting option indicates a setting that will be applied by
      the sender.

   One CSM MUST be sent by each endpoint at the start of the Transport
   Connection.  Additional CSMs MAY be sent at any other time by either
   endpoint over the lifetime of the connection.

   Both capability options and setting options are cumulative.  A CSM
   does not invalidate a previously sent capability indication or
   setting even if it is not repeated.  A capability message without any
   option is a no-operation (and can be used as such).  An option that
   is sent might override a previous value for the same option.  The
   option defines how to handle this case if needed.

   Base values are listed below for CSM options.  These are the values
   for the capability and settings before any CSMs send a modified
   value.

   These are not default values (as defined in Section 5.4.4 in
   [RFC7252]) for the option.  Default values apply on a per-message
   basis and are thus reset when the value is not present in a
   given CSM.

   CSMs are indicated by the 7.01 (CSM) code; see Table 1
   (Section 11.1).

5.3.1.  Max-Message-Size Capability Option

   The sender can use the elective Max-Message-Size Option to indicate
   the maximum size of a message in bytes that it can receive.  The
   message size indicated includes the entire message, starting from the
   first byte of the message header and ending at the end of the message
   payload.

   (Note that there is no relationship of the message size to the
   overall request or response body size that may be achievable in
   block-wise transfer.  For example, the exchange depicted in Figure 13
   (Section 6.1) can be performed if the CoAP client indicates a value
   of around 6000 bytes for the Max-Message-Size Option, even though the
   total body size transferred to the client is 3072 + 5120 + 4711 =
   12903 bytes.)

   +---+---+---+---------+------------------+--------+--------+--------+
   | # | C | R | Applies | Name             | Format | Length | Base   |
   |   |   |   | to      |                  |        |        | Value  |
   +---+---+---+---------+------------------+--------+--------+--------+
   | 2 |   |   | CSM     | Max-Message-Size |   uint |    0-4 | 1152   |
   +---+---+---+---------+------------------+--------+--------+--------+

                         C=Critical, R=Repeatable

   As per Section 4.6 of [RFC7252], the base value (and the value used
   when this option is not implemented) is 1152.

   The active value of the Max-Message-Size Option is replaced each time
   the option is sent with a modified value.  Its starting value is its
   base value.

5.3.2.  Block-Wise-Transfer Capability Option

   +---+---+---+---------+------------------+--------+--------+--------+
   | # | C | R | Applies | Name             | Format | Length | Base   |
   |   |   |   | to      |                  |        |        | Value  |
   +---+---+---+---------+------------------+--------+--------+--------+
   | 4 |   |   | CSM     | Block-Wise-      |  empty |      0 | (none) |
   |   |   |   |         | Transfer         |        |        |        |
   +---+---+---+---------+------------------+--------+--------+--------+

                         C=Critical, R=Repeatable

   A sender can use the elective Block-Wise-Transfer Option to indicate
   that it supports the block-wise transfer protocol [RFC7959].

   If the option is not given, the peer has no information about whether
   block-wise transfers are supported by the sender or not.  An
   implementation wishing to offer block-wise transfers to its peer
   therefore needs to indicate so via the Block-Wise-Transfer Option.

   If a Max-Message-Size Option is indicated with a value that is
   greater than 1152 (in the same CSM or a different CSM), the
   Block-Wise-Transfer Option also indicates support for BERT (see
   Section 6).  Subsequently, if the Max-Message-Size Option is
   indicated with a value equal to or less than 1152, BERT support is no
   longer indicated.  (Note that the indication of BERT support does not
   oblige either peer to actually choose to make use of BERT.)

   Implementation note: When indicating a value of the Max-Message-Size
   Option with an intention to enable BERT, the indicating
   implementation may want to (1) choose a particular BERT block size it
   wants to encourage and (2) add a delta for the header and any options
   that may also need to be included in the message with a BERT block of
   that size.  Section 4.6 of [RFC7252] adds 128 bytes to a maximum
   block size of 1024 to arrive at a default message size of 1152.  A
   BERT-enabled implementation may want to indicate a BERT block size of
   2048 or a higher multiple of 1024 and at the same time be more
   generous with the size of the header and options added (say, 256 or
   512).  However, adding 1024 or more to the base BERT block size may
   encourage the peer implementation to vary the BERT block size based
   on the size of the options included; this type of scenario might make
   it harder to establish interoperability.

5.4.  Ping and Pong Messages

   In CoAP over reliable transports, Empty messages (Code 0.00) can
   always be sent and MUST be ignored by the recipient.  This provides a
   basic keepalive function.  In contrast, Ping and Pong messages are a
   bidirectional exchange.

   Upon receipt of a Ping message, the receiver MUST return a Pong
   message with an identical Token in response.  Unless the Ping carries
   an option with delaying semantics such as the Custody Option, it
   SHOULD respond as soon as practical.  As with all Signaling messages,
   the recipient of a Ping or Pong message MUST ignore elective options
   it does not understand.

   Ping and Pong messages are indicated by the 7.02 code (Ping) and
   the 7.03 code (Pong).

   Note that, as with similar mechanisms defined in [RFC6455] and
   [RFC7540], the present specification does not define any specific
   maximum time that the sender of a Ping message has to allow when
   waiting for a Pong reply.  Any limitations on patience for this reply
   are a matter of the application making use of these messages, as is
   any approach to recover from a failure to respond in time.

5.4.1.  Custody Option

   +---+---+---+----------+----------------+--------+--------+---------+
   | # | C | R | Applies  | Name           | Format | Length | Base    |
   |   |   |   | to       |                |        |        | Value   |
   +---+---+---+----------+----------------+--------+--------+---------+
   | 2 |   |   | Ping,    | Custody        |  empty |      0 | (none)  |
   |   |   |   | Pong     |                |        |        |         |
   +---+---+---+----------+----------------+--------+--------+---------+

                         C=Critical, R=Repeatable

   When responding to a Ping message, the receiver can include an
   elective Custody Option in the Pong message.  This option indicates
   that the application has processed all the request/response messages
   received prior to the Ping message on the current connection.  (Note
   that there is no definition of specific application semantics for
   "processed", but there is an expectation that the receiver of a Pong
   message with a Custody Option should be able to free buffers based on
   this indication.)

   A sender can also include an elective Custody Option in a Ping
   message to explicitly request the inclusion of an elective Custody
   Option in the corresponding Pong message.  In that case, the receiver
   SHOULD delay its Pong message until it finishes processing all the
   request/response messages received prior to the Ping message on the
   current connection.

5.5.  Release Messages

   A Release message indicates that the sender does not want to continue
   maintaining the Transport Connection and opts for an orderly
   shutdown, but wants to leave it to the peer to actually start closing
   the connection.  The details are in the options.  A diagnostic
   payload (see Section 5.5.2 of [RFC7252]) MAY be included.

   A peer will normally respond to a Release message by closing the
   Transport Connection.  (In case that does not happen, the sender of
   the release may want to implement a timeout mechanism if getting rid
   of the connection is actually important to it.)

   Messages may be in flight or responses outstanding when the sender
   decides to send a Release message (which is one reason the sender had
   decided to wait before closing the connection).  The peer responding
   to the Release message SHOULD delay the closing of the connection
   until it has responded to all requests received by it before the
   Release message.  It also MAY wait for the responses to its own
   requests.

   It is NOT RECOMMENDED for the sender of a Release message to continue
   sending requests on the connection it already indicated to be
   released: the peer might close the connection at any time and miss
   those requests.  The peer is not obligated to check for this
   condition, though.

   Release messages are indicated by the 7.04 code (Release).

   Release messages can indicate one or more reasons using elective
   options.  The following options are defined:

   +---+---+---+---------+------------------+--------+--------+--------+
   | # | C | R | Applies | Name             | Format | Length | Base   |
   |   |   |   | to      |                  |        |        | Value  |
   +---+---+---+---------+------------------+--------+--------+--------+
   | 2 |   | x | Release | Alternative-     | string |  1-255 | (none) |
   |   |   |   |         | Address          |        |        |        |
   +---+---+---+---------+------------------+--------+--------+--------+

                         C=Critical, R=Repeatable

   The elective Alternative-Address Option requests the peer to instead
   open a connection of the same scheme as the present connection to the
   alternative transport address given.  Its value is in the form
   "authority" as defined in Section 3.2 of [RFC3986].  (Existing state
   related to the connection is not transferred from the present
   connection to the new connection.)

   The Alternative-Address Option is a repeatable option as defined in
   Section 5.4.5 of [RFC7252].  When multiple occurrences of the option
   are included, the peer can choose any of the alternative transport
   addresses.

   +---+---+---+---------+-----------------+--------+--------+---------+
   | # | C | R | Applies | Name            | Format | Length | Base    |
   |   |   |   | to      |                 |        |        | Value   |
   +---+---+---+---------+-----------------+--------+--------+---------+
   | 4 |   |   | Release | Hold-Off        |   uint |    0-3 | (none)  |
   +---+---+---+---------+-----------------+--------+--------+---------+

                         C=Critical, R=Repeatable

   The elective Hold-Off Option indicates that the server is requesting
   that the peer not reconnect to it for the number of seconds given in
   the value.

5.6.  Abort Messages

   An Abort message indicates that the sender is unable to continue
   maintaining the Transport Connection and cannot even wait for an
   orderly release.  The sender shuts down the connection immediately
   after the Abort message (and may or may not wait for a Release
   message, Abort message, or connection shutdown in the inverse
   direction).  A diagnostic payload (see Section 5.5.2 of [RFC7252])
   SHOULD be included in the Abort message.  Messages may be in flight
   or responses outstanding when the sender decides to send an Abort
   message.  The general expectation is that these will NOT be
   processed.

   Abort messages are indicated by the 7.05 code (Abort).

   Abort messages can indicate one or more reasons using elective
   options.  The following option is defined:

   +---+---+---+---------+-----------------+--------+--------+---------+
   | # | C | R | Applies | Name            | Format | Length | Base    |
   |   |   |   | to      |                 |        |        | Value   |
   +---+---+---+---------+-----------------+--------+--------+---------+
   | 2 |   |   | Abort   | Bad-CSM-Option  |   uint |    0-2 | (none)  |
   +---+---+---+---------+-----------------+--------+--------+---------+

                         C=Critical, R=Repeatable

   Bad-CSM-Option, which is elective, indicates that the sender is
   unable to process the CSM option identified by its Option Number,
   e.g., when it is critical and the Option Number is unknown by the
   sender, or when there is a parameter problem with the value of an
   elective option.  More detailed information SHOULD be included as a
   diagnostic payload.

   For CoAP over UDP, messages that contain syntax violations are
   processed as message format errors.  As described in Sections 4.2 and
   4.3 of [RFC7252], such messages are rejected by sending a matching
   Reset message and otherwise ignoring the message.

   For CoAP over reliable transports, the recipient rejects such
   messages by sending an Abort message and otherwise ignoring (not
   processing) the message.  No specific Option has been defined for the
   Abort message in this case, as the details are best left to a
   diagnostic payload.

5.7.  Signaling Examples

   An encoded example of a Ping message with a non-empty Token is shown
   in Figure 11.

       0                   1                   2
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      0x01     |      0xe2     |      0x42     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Len   =    0 -------> 0x01
       TKL   =    1 ___/
       Code  = 7.02 Ping --> 0xe2
       Token =               0x42

                      Figure 11: Ping Message Example

   An encoded example of the corresponding Pong message is shown in
   Figure 12.

       0                   1                   2
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |      0x01     |      0xe3     |      0x42     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

       Len   =    0 -------> 0x01
       TKL   =    1 ___/
       Code  = 7.03 Pong --> 0xe3
       Token =               0x42

                      Figure 12: Pong Message Example

6.  Block-Wise Transfer and Reliable Transports

   The message size restrictions defined in Section 4.6 of [RFC7252] to
   avoid IP fragmentation are not necessary when CoAP is used over a
   reliable transport.  While this suggests that the block-wise transfer
   protocol [RFC7959] is also no longer needed, it remains applicable
   for a number of cases:

   o  Large messages, such as firmware downloads, may cause undesired
      head-of-line blocking when a single transport connection is used.

   o  A UDP-to-TCP gateway may simply not have the context to convert a
      message with a Block Option into the equivalent exchange without
      any use of a Block Option (it would need to convert the entire
      block-wise exchange from start to end into a single exchange).

   BERT extends the block-wise transfer protocol to enable the use of
   larger messages over a reliable transport.

   The use of this new extension is signaled by sending Block1 or Block2
   Options with SZX == 7 (a "BERT Option").  SZX == 7 is a reserved
   value in [RFC7959].

   In control usage, a BERT Option is interpreted in the same way as the
   equivalent Option with SZX == 6, except that it also indicates the
   capability to process BERT blocks.  As with the basic block-wise
   transfer protocol, the recipient of a CoAP request with a BERT Option
   in control usage is allowed to respond with a different SZX value,
   e.g., to send a non-BERT block instead.

   In descriptive usage, a BERT Option is interpreted in the same way as
   the equivalent Option with SZX == 6, except that the payload is also
   allowed to contain multiple blocks.  For non-final BERT blocks, the
   payload is always a multiple of 1024 bytes.  For final BERT blocks,
   the payload is a multiple (possibly 0) of 1024 bytes plus a partial
   block of less than 1024 bytes.

   The recipient of a non-final BERT block (M=1) conceptually partitions
   the payload into a sequence of 1024-byte blocks and acts exactly as
   if it had received this sequence in conjunction with block numbers
   starting at, and sequentially increasing from, the block number given
   in the Block Option.  In other words, the entire BERT block is
   positioned at the byte position that results from multiplying the
   block number by 1024.  The position of further blocks to be
   transferred is indicated by incrementing the block number by the
   number of elements in this sequence (i.e., the size of the payload
   divided by 1024 bytes).

   As with SZX == 6, the recipient of a final BERT block (M=0) simply
   appends the payload at the byte position that is indicated by the
   block number multiplied by 1024.

   The following examples illustrate BERT Options.  A value of SZX == 7
   is labeled as "BERT" or as "BERT(nnn)" to indicate a payload of
   size nnn.

   In all these examples, a Block Option is decomposed to indicate the
   kind of Block Option (1 or 2) followed by a colon, the block number
   (NUM), the more bit (M), and the block size (2**(SZX + 4)) separated
   by slashes.  For example, a Block2 Option value of 33 would be shown
   as 2:2/0/32), or a Block1 Option value of 59 would be shown as
   1:3/1/128.

6.1.  Example: GET with BERT Blocks

   Figure 13 shows a GET request with a response that is split into
   three BERT blocks.  The first response contains 3072 bytes of
   payload; the second, 5120; and the third, 4711.  Note how the block
   number increments to move the position inside the response body
   forward.

   CoAP Client                             CoAP Server
     |                                            |
     | GET, /status                       ------> |
     |                                            |
     | <------   2.05 Content, 2:0/1/BERT(3072)   |
     |                                            |
     | GET, /status, 2:3/0/BERT           ------> |
     |                                            |
     | <------   2.05 Content, 2:3/1/BERT(5120)   |
     |                                            |
     | GET, /status, 2:8/0/BERT          ------>  |
     |                                            |
     | <------   2.05 Content, 2:8/0/BERT(4711)   |

                      Figure 13: GET with BERT Blocks

6.2.  Example: PUT with BERT Blocks

   Figure 14 demonstrates a PUT exchange with BERT blocks.

   CoAP Client                             CoAP Server
     |                                             |
     | PUT, /options, 1:0/1/BERT(8192)     ------> |
     |                                             |
     | <------   2.31 Continue, 1:0/1/BERT         |
     |                                             |
     | PUT, /options, 1:8/1/BERT(16384)    ------> |
     |                                             |
     | <------   2.31 Continue, 1:8/1/BERT         |
     |                                             |
     | PUT, /options, 1:24/0/BERT(5683)    ------> |
     |                                             |
     | <------   2.04 Changed, 1:24/0/BERT         |
     |                                             |

                      Figure 14: PUT with BERT Blocks

7.  Observing Resources over Reliable Transports

   This section describes how the procedures defined in [RFC7641] for
   observing resources over CoAP are applied (and modified, as needed)
   for reliable transports.  In this section, "client" and "server"
   refer to the CoAP client and CoAP server.

7.1.  Notifications and Reordering

   When using the Observe Option [RFC7641] with CoAP over UDP,
   notifications from the server set the option value to an increasing
   sequence number for reordering detection on the client, since
   messages can arrive in a different order than they were sent.  This
   sequence number is not required for CoAP over reliable transports,
   since TCP ensures reliable and ordered delivery of messages.  The
   value of the Observe Option in 2.xx notifications MAY be empty on
   transmission and MUST be ignored on reception.

   Implementation note: This means that a proxy from a reordering
   transport to a reliable (in-order) transport (such as a UDP-to-TCP
   proxy) needs to process the Observe Option in notifications according
   to the rules in Section 3.4 of [RFC7641].

7.2.  Transmission and Acknowledgments

   For CoAP over UDP, server notifications to the client can be
   Confirmable or Non-confirmable.  A Confirmable message requires the
   client to respond with either an Acknowledgment message or a Reset
   message.  An Acknowledgment message indicates that the client is
   alive and wishes to receive further notifications.  A Reset message
   indicates that the client does not recognize the Token; this causes
   the server to remove the associated entry from the list of observers.

   Since TCP eliminates the need for the message layer to support
   reliability, CoAP over reliable transports does not support
   Confirmable or Non-confirmable message types.  All notifications are
   delivered reliably to the client with positive acknowledgment of
   receipt occurring at the TCP level.  If the client does not recognize
   the Token in a notification, it MAY immediately abort the connection
   (see Section 5.6).

7.3.  Freshness

   For CoAP over UDP, if a client does not receive a notification for
   some time, it can send a new GET request with the same Token as the
   original request to re-register its interest in a resource and verify
   that the server is still responsive.  For CoAP over reliable
   transports, it is more efficient to check the health of the

   connection (and all its active observations) by sending a single CoAP
   Ping Signaling message (Section 5.4) rather than individual requests
   to confirm each active observation.  (Note that such a Ping/Pong only
   confirms a single hop: a proxy is not obligated or expected to react
   to a Ping by checking all its own registered interests or all the
   connections, if any, underlying them.  A proxy MAY maintain its own
   schedule for confirming the interests that it relies on being
   registered toward the origin server; however, it is generally
   inadvisable for a proxy to generate a large number of outgoing checks
   based on a single incoming check.)

7.4.  Cancellation

   For CoAP over UDP, a client that is no longer interested in receiving
   notifications can "forget" the observation and respond to the next
   notification from the server with a Reset message to cancel the
   observation.

   For CoAP over reliable transports, a client MUST explicitly
   deregister by issuing a GET request that has the Token field set to
   the Token of the observation to be canceled and includes an Observe
   Option with the value set to 1 (deregister).

   If the client observes one or more resources over a reliable
   transport, then the CoAP server (or intermediary in the role of the
   CoAP server) MUST remove all entries associated with the client
   endpoint from the lists of observers when the connection either
   times out or is closed.

8.  CoAP over Reliable Transport URIs

   CoAP over UDP [RFC7252] defines the "coap" and "coaps" URI schemes.
   This document introduces four additional URI schemes for identifying
   CoAP resources and providing a means of locating the resource:

   o  The "coap+tcp" URI scheme for CoAP over TCP.

   o  The "coaps+tcp" URI scheme for CoAP over TCP secured by TLS.

   o  The "coap+ws" URI scheme for CoAP over WebSockets.

   o  The "coaps+ws" URI scheme for CoAP over WebSockets secured by TLS.

   Resources made available via these schemes have no shared identity
   even if their resource identifiers indicate the same authority (the
   same host listening to the same TCP port).  They are hosted in
   distinct namespaces because each URI scheme implies a distinct origin
   server.

   In this section, the syntax for the URI schemes is specified using
   the Augmented Backus-Naur Form (ABNF) [RFC5234].  The definitions of
   "host", "port", "path-abempty", and "query" are adopted from
   [RFC3986].

   Section 8 ("Multicast CoAP") in [RFC7252] is not applicable to these
   schemes.

   As with the "coap" and "coaps" schemes defined in [RFC7252], all URI
   schemes defined in this section also support the path prefix
   "/.well-known/" as defined by [RFC5785] for "well-known locations" in
   the namespace of a host.  This enables discovery as per Section 7 of
   [RFC7252].

8.1.  coap+tcp URI Scheme

   The "coap+tcp" URI scheme identifies CoAP resources that are intended
   to be accessible using CoAP over TCP.

     coap-tcp-URI = "coap+tcp:" "//" host [ ":" port ]
       path-abempty [ "?" query ]

   The syntax defined in Section 6.1 of [RFC7252] applies to this URI
   scheme, with the following change:

   o  The port subcomponent indicates the TCP port at which the CoAP
      Connection Acceptor is located.  (If it is empty or not given,
      then the default port 5683 is assumed, as with UDP.)

   Encoding considerations:  The scheme encoding conforms to the
      encoding rules established for URIs in [RFC3986].

   Interoperability considerations:  None.

   Security considerations:  See Section 11.1 of [RFC7252].

8.2.  coaps+tcp URI Scheme

   The "coaps+tcp" URI scheme identifies CoAP resources that are
   intended to be accessible using CoAP over TCP secured with TLS.

     coaps-tcp-URI = "coaps+tcp:" "//" host [ ":" port ]
       path-abempty [ "?" query ]

   The syntax defined in Section 6.2 of [RFC7252] applies to this URI
   scheme, with the following changes:

   o  The port subcomponent indicates the TCP port at which the TLS
      server for the CoAP Connection Acceptor is located.  If it is
      empty or not given, then the default port 5684 is assumed.

   o  If a TLS server does not support the Application-Layer Protocol
      Negotiation (ALPN) extension [RFC7301] or wishes to accommodate
      TLS clients that do not support ALPN, it MAY offer a coaps+tcp
      endpoint on TCP port 5684.  This endpoint MAY also be ALPN
      enabled.  A TLS server MAY offer coaps+tcp endpoints on ports
      other than TCP port 5684, which MUST be ALPN enabled.

   o  For TCP ports other than port 5684, the TLS client MUST use the
      ALPN extension to advertise the "coap" protocol identifier (see
      Section 11.7) in the list of protocols in its ClientHello.  If the
      TCP server selects and returns the "coap" protocol identifier
      using the ALPN extension in its ServerHello, then the connection
      succeeds.  If the TLS server either does not negotiate the ALPN
      extension or returns a no_application_protocol alert, the TLS
      client MUST close the connection.

   o  For TCP port 5684, a TLS client MAY use the ALPN extension to
      advertise the "coap" protocol identifier in the list of protocols
      in its ClientHello.  If the TLS server selects and returns the
      "coap" protocol identifier using the ALPN extension in its
      ServerHello, then the connection succeeds.  If the TLS server
      returns a no_application_protocol alert, then the TLS client MUST
      close the connection.  If the TLS server does not negotiate the
      ALPN extension, then coaps+tcp is implicitly selected.

   o  For TCP port 5684, if the TLS client does not use the ALPN
      extension to negotiate the protocol, then coaps+tcp is implicitly
      selected.

   Encoding considerations:  The scheme encoding conforms to the
      encoding rules established for URIs in [RFC3986].

   Interoperability considerations:  None.

   Security considerations:  See Section 11.1 of [RFC7252].

8.3.  coap+ws URI Scheme

   The "coap+ws" URI scheme identifies CoAP resources that are intended
   to be accessible using CoAP over WebSockets.

     coap-ws-URI = "coap+ws:" "//" host [ ":" port ]
       path-abempty [ "?" query ]

   The port subcomponent is OPTIONAL.  The default is port 80.

   The WebSocket endpoint is identified by a "ws" URI that is composed
   of the authority part of the "coap+ws" URI and the well-known path
   "/.well-known/coap" [RFC5785] [RFC8307].  Within the endpoint
   specified in a "coap+ws" URI, the path and query parts of the URI
   identify a resource that can be operated on by the methods defined
   by CoAP:

             coap+ws://example.org/sensors/temperature?u=Cel
                  \______  ______/\___________  ___________/
                         \/                   \/
                                            Uri-Path: "sensors"
       ws://example.org/.well-known/coap    Uri-Path: "temperature"
                                            Uri-Query: "u=Cel"

                    Figure 15: The "coap+ws" URI Scheme

   Encoding considerations:  The scheme encoding conforms to the
      encoding rules established for URIs in [RFC3986].

   Interoperability considerations:  None.

   Security considerations:  See Section 11.1 of [RFC7252].

8.4.  coaps+ws URI Scheme

   The "coaps+ws" URI scheme identifies CoAP resources that are intended
   to be accessible using CoAP over WebSockets secured by TLS.

     coaps-ws-URI = "coaps+ws:" "//" host [ ":" port ]
       path-abempty [ "?" query ]

   The port subcomponent is OPTIONAL.  The default is port 443.

   The WebSocket endpoint is identified by a "wss" URI that is composed
   of the authority part of the "coaps+ws" URI and the well-known path
   "/.well-known/coap" [RFC5785] [RFC8307].  Within the endpoint
   specified in a "coaps+ws" URI, the path and query parts of the URI
   identify a resource that can be operated on by the methods defined
   by CoAP:

             coaps+ws://example.org/sensors/temperature?u=Cel
                   \______  ______/\___________  ___________/
                          \/                   \/
                                            Uri-Path: "sensors"
       wss://example.org/.well-known/coap   Uri-Path: "temperature"
                                            Uri-Query: "u=Cel"

                   Figure 16: The "coaps+ws" URI Scheme

   Encoding considerations:  The scheme encoding conforms to the
      encoding rules established for URIs in [RFC3986].

   Interoperability considerations:  None.

   Security considerations:  See Section 11.1 of [RFC7252].

8.5.  Uri-Host and Uri-Port Options

   CoAP over reliable transports maintains the property from
   Section 5.10.1 of [RFC7252]:

      The default values for the Uri-Host and Uri-Port Options are
      sufficient for requests to most servers.

   Unless otherwise noted, the default value of the Uri-Host Option is
   the IP literal representing the destination IP address of the request
   message.  The default value of the Uri-Port Option is the destination
   TCP port.

   For CoAP over TLS, these default values are the same, unless Server
   Name Indication (SNI) [RFC6066] is negotiated.  In this case, the
   default value of the Uri-Host Option in requests from the TLS client
   to the TLS server is the SNI host.

   For CoAP over WebSockets, the default value of the Uri-Host Option in
   requests from the WebSocket client to the WebSocket server is
   indicated by the Host header field from the WebSocket handshake.

8.6.  Decomposing URIs into Options

   The steps are the same as those specified in Section 6.4 of
   [RFC7252], with minor changes:

   This step from [RFC7252]:

   3.  If |url| does not have a <scheme> component whose value, when
       converted to ASCII lowercase, is "coap" or "coaps", then fail
       this algorithm.

   is updated to:

   3.  If |url| does not have a <scheme> component whose value, when
       converted to ASCII lowercase, is "coap+tcp", "coaps+tcp",
       "coap+ws", or "coaps+ws", then fail this algorithm.

   This step from [RFC7252]:

   7.  If |port| does not equal the request's destination UDP port,
       include a Uri-Port Option and let that option's value be |port|.

   is updated to:

   7.  If |port| does not equal the request's destination TCP port,
       include a Uri-Port Option and let that option's value be |port|.

8.7.  Composing URIs from Options

   The steps are the same as those specified in Section 6.5 of
   [RFC7252], with minor changes:

   This step from [RFC7252]:

   1.  If the request is secured using DTLS, let |url| be the string
       "coaps://".  Otherwise, let |url| be the string "coap://".

   is updated to:

   1.  For CoAP over TCP, if the request is secured using TLS, let |url|
       be the string "coaps+tcp://".  Otherwise, let |url| be the string
       "coap+tcp://".  For CoAP over WebSockets, if the request is
       secured using TLS, let |url| be the string "coaps+ws://".
       Otherwise, let |url| be the string "coap+ws://".

   This step from [RFC7252]:

   4.  If the request includes a Uri-Port Option, let |port| be that
       option's value.  Otherwise, let |port| be the request's
       destination UDP port.

   is updated to:

   4.  If the request includes a Uri-Port Option, let |port| be that
       option's value.  Otherwise, let |port| be the request's
       destination TCP port.

9.  Securing CoAP

   "Security Challenges For the Internet Of Things" [SecurityChallenges]
   recommends the following:

      ... it is essential that IoT protocol suites specify a mandatory
      to implement but optional to use security solution.  This will
      ensure security is available in all implementations, but
      configurable to use when not necessary (e.g., in closed
      environment). ... even if those features stretch the capabilities
      of such devices.

   A security solution MUST be implemented to protect CoAP over reliable
   transports and MUST be enabled by default.  This document defines the
   TLS binding, but alternative solutions at different layers in the
   protocol stack MAY be used to protect CoAP over reliable transports

   when appropriate.  Note that there is ongoing work to support a data-
   object-based security model for CoAP that is independent of transport
   (see [OSCORE]).

9.1.  TLS Binding for CoAP over TCP

   The TLS usage guidance in [RFC7925] applies, including the guidance
   about cipher suites in that document that are derived from the
   mandatory-to-implement cipher suites defined in [RFC7252].

   This guidance assumes implementation in a constrained device or for
   communication with a constrained device.  However, CoAP over TCP/TLS
   has a wider applicability.  It may, for example, be implemented on a
   gateway or on a device that is less constrained (such as a smart
   phone or a tablet), for communication with a peer that is likewise
   less constrained, or within a back-end environment that only
   communicates with constrained devices via proxies.  As an exception
   to the previous paragraph, in this case, the recommendations in
   [RFC7525] are more appropriate.

   Since the guidance offered in [RFC7925] differs from the guidance
   offered in [RFC7525] in terms of algorithms and credential types, it
   is assumed that an implementation of CoAP over TCP/TLS that needs to
   support both cases implements the recommendations offered by both
   specifications.

   During the provisioning phase, a CoAP device is provided with the
   security information that it needs, including keying materials,
   access control lists, and authorization servers.  At the end of the
   provisioning phase, the device will be in one of four security modes:

   NoSec:  TLS is disabled.

   PreSharedKey:  TLS is enabled.  The guidance in Section 4.2 of
      [RFC7925] applies.

   RawPublicKey:  TLS is enabled.  The guidance in Section 4.3 of
      [RFC7925] applies.

   Certificate:  TLS is enabled.  The guidance in Section 4.4 of
      [RFC7925] applies.

   The "NoSec" mode is optional to implement.  The system simply sends
   the packets over normal TCP; this is indicated by the "coap+tcp"
   scheme and the TCP CoAP default port.  The system is secured only by
   keeping attackers from being able to send or receive packets from the
   network with the CoAP nodes.

   "PreSharedKey", "RawPublicKey", or "Certificate" is mandatory to
   implement for the TLS binding, depending on the credential type used
   with the device.  These security modes are achieved using TLS and
   are indicated by the "coaps+tcp" scheme and TLS-secured CoAP
   default port.

9.2.  TLS Usage for CoAP over WebSockets

   A CoAP client requesting a resource identified by a "coaps+ws" URI
   negotiates a secure WebSocket connection to a WebSocket server
   endpoint with a "wss" URI.  This is described in Section 8.4.

   The client MUST perform a TLS handshake after opening the connection
   to the server.  The guidance in Section 4.1 of [RFC6455] applies.
   When a CoAP server exposes resources identified by a "coaps+ws" URI,
   the guidance in Section 4.4 of [RFC7925] applies towards mandatory-
   to-implement TLS functionality for certificates.  For the server-side
   requirements for accepting incoming connections over an HTTPS
   (HTTP over TLS) port, the guidance in Section 4.2 of [RFC6455]
   applies.

   Note that the guidance above formally inherits the mandatory-to-
   implement cipher suites defined in [RFC5246].  However, modern
   browsers usually implement cipher suites that are more recent; these
   cipher suites are then automatically picked up via the JavaScript
   WebSocket API.  WebSocket servers that provide secure CoAP over
   WebSockets for the browser use case will need to follow the browser
   preferences and MUST follow [RFC7525].

10.  Security Considerations

   The security considerations of [RFC7252] apply.  For CoAP over
   WebSockets and CoAP over TLS-secured WebSockets, the security
   considerations of [RFC6455] also apply.

10.1.  Signaling Messages

   The guidance given by an Alternative-Address Option cannot be
   followed blindly.  In particular, a peer MUST NOT assume that a
   successful connection to the Alternative-Address inherits all the
   security properties of the current connection.

11.  IANA Considerations

11.1.  Signaling Codes

   IANA has created a third subregistry for values of the Code field in
   the CoAP header (Section 12.1 of [RFC7252]).  The name of this
   subregistry is "CoAP Signaling Codes".

   Each entry in the subregistry must include the Signaling Code in the
   range 7.00-7.31, its name, and a reference to its documentation.

   Initial entries in this subregistry are as follows:

                      +------+---------+-----------+
                      | Code | Name    | Reference |
                      +------+---------+-----------+
                      | 7.01 | CSM     | RFC 8323  |
                      |      |         |           |
                      | 7.02 | Ping    | RFC 8323  |
                      |      |         |           |
                      | 7.03 | Pong    | RFC 8323  |
                      |      |         |           |
                      | 7.04 | Release | RFC 8323  |
                      |      |         |           |
                      | 7.05 | Abort   | RFC 8323  |
                      +------+---------+-----------+

                       Table 1: CoAP Signaling Codes

   All other Signaling Codes are Unassigned.

   The IANA policy for future additions to this subregistry is
   "IETF Review" or "IESG Approval" as described in [RFC8126].

11.2.  CoAP Signaling Option Numbers Registry

   IANA has created a subregistry for Option Numbers used in CoAP
   Signaling Options within the "Constrained RESTful Environments (CoRE)
   Parameters" registry.  The name of this subregistry is "CoAP
   Signaling Option Numbers".

   Each entry in the subregistry must include one or more of the codes
   in the "CoAP Signaling Codes" subregistry (Section 11.1), the number
   for the Option, the name of the Option, and a reference to the
   Option's documentation.

   Initial entries in this subregistry are as follows:

         +------------+--------+---------------------+-----------+
         | Applies to | Number | Name                | Reference |
         +------------+--------+---------------------+-----------+
         | 7.01       |      2 | Max-Message-Size    |  RFC 8323 |
         |            |        |                     |           |
         | 7.01       |      4 | Block-Wise-Transfer |  RFC 8323 |
         |            |        |                     |           |
         | 7.02, 7.03 |      2 | Custody             |  RFC 8323 |
         |            |        |                     |           |
         | 7.04       |      2 | Alternative-Address |  RFC 8323 |
         |            |        |                     |           |
         | 7.04       |      4 | Hold-Off            |  RFC 8323 |
         |            |        |                     |           |
         | 7.05       |      2 | Bad-CSM-Option      |  RFC 8323 |
         +------------+--------+---------------------+-----------+

                   Table 2: CoAP Signaling Option Codes

   The IANA policy for future additions to this subregistry is based on
   number ranges for the option numbers, analogous to the policy defined
   in Section 12.2 of [RFC7252].  (The policy is analogous rather than
   identical because the structure of this subregistry includes an
   additional column ("Applies to"); however, the value of this column
   has no influence on the policy.)

   The documentation for a Signaling Option Number should specify the
   semantics of an option with that number, including the following
   properties:

   o  Whether the option is critical or elective, as determined by the
      Option Number.

   o  Whether the option is repeatable.

   o  The format and length of the option's value.

   o  The base value for the option, if any.

11.3.  Service Name and Port Number Registration

   IANA has assigned the port number 5683 and the service name "coap",
   in accordance with [RFC6335].

   Service Name:
      coap

   Transport Protocol:
      tcp

   Assignee:
      IESG <iesg@ietf.org>

   Contact:
      IETF Chair <chair@ietf.org>

   Description:
      Constrained Application Protocol (CoAP)

   Reference:
      RFC 8323

   Port Number:
      5683

11.4.  Secure Service Name and Port Number Registration

   IANA has assigned the port number 5684 and the service name "coaps",
   in accordance with [RFC6335].  The port number is to address the
   exceptional case of TLS implementations that do not support the ALPN
   extension [RFC7301].

   Service Name:
      coaps

   Transport Protocol:
      tcp

   Assignee:
      IESG <iesg@ietf.org>

   Contact:
      IETF Chair <chair@ietf.org>

   Description:
      Constrained Application Protocol (CoAP)

   Reference:
      [RFC7301], RFC 8323

   Port Number:
      5684

11.5.  URI Scheme Registration

   URI schemes are registered within the "Uniform Resource Identifier
   (URI) Schemes" registry maintained at [IANA.uri-schemes].

   Note: The following has been added as a note for each of the URI
   schemes defined in this document:

      CoAP registers different URI schemes for accessing CoAP resources
      via different protocols.  This approach runs counter to the WWW
      principle that a URI identifies a resource and that multiple URIs
      for identifying the same resource should be avoided
      <https://www.w3.org/TR/webarch/#avoid-uri-aliases>.

   This is not a problem for many of the usage scenarios envisioned for
   CoAP over reliable transports; additional URI schemes can be
   introduced to address additional usage scenarios (as being prepared,
   for example, in [Multi-Transport-URIs] and [CoAP-Alt-Transports]).

11.5.1.  coap+tcp

   IANA has registered the URI scheme "coap+tcp".  This registration
   request complies with [RFC7595].

   Scheme name:
      coap+tcp

   Status:
      Permanent

   Applications/protocols that use this scheme name:
      The scheme is used by CoAP endpoints to access CoAP resources
      using TCP.

   Contact:
      IETF Chair <chair@ietf.org>

   Change controller:
      IESG <iesg@ietf.org>

   Reference:
      Section 8.1 in RFC 8323

11.5.2.  coaps+tcp

   IANA has registered the URI scheme "coaps+tcp".  This registration
   request complies with [RFC7595].

   Scheme name:
      coaps+tcp

   Status:
      Permanent

   Applications/protocols that use this scheme name:
      The scheme is used by CoAP endpoints to access CoAP resources
      using TLS.

   Contact:
      IETF Chair <chair@ietf.org>

   Change controller:
      IESG <iesg@ietf.org>

   Reference:
      Section 8.2 in RFC 8323

11.5.3.  coap+ws

   IANA has registered the URI scheme "coap+ws".  This registration
   request complies with [RFC7595].

   Scheme name:
      coap+ws

   Status:
      Permanent

   Applications/protocols that use this scheme name:
      The scheme is used by CoAP endpoints to access CoAP resources
      using the WebSocket Protocol.

   Contact:
      IETF Chair <chair@ietf.org>

   Change controller:
      IESG <iesg@ietf.org>

   Reference:
      Section 8.3 in RFC 8323

11.5.4.  coaps+ws

   IANA has registered the URI scheme "coaps+ws".  This registration
   request complies with [RFC7595].

   Scheme name:
      coaps+ws

   Status:
      Permanent

   Applications/protocols that use this scheme name:
      The scheme is used by CoAP endpoints to access CoAP resources
      using the WebSocket Protocol secured with TLS.

   Contact:
      IETF Chair <chair@ietf.org>

   Change controller:
      IESG <iesg@ietf.org>

   References:
      Section 8.4 in RFC 8323

11.6.  Well-Known URI Suffix Registration

   IANA has registered "coap" in the "Well-Known URIs" registry.  This
   registration request complies with [RFC5785].

   URI suffix:
      coap

   Change controller:
      IETF

   Specification document(s):
      RFC 8323

   Related information:
      None.

11.7.  ALPN Protocol Identifier

   IANA has assigned the following value in the "Application-Layer
   Protocol Negotiation (ALPN) Protocol IDs" registry created by
   [RFC7301].  The "coap" string identifies CoAP when used over TLS.

   Protocol:
      CoAP

   Identification Sequence:
      0x63 0x6f 0x61 0x70 ("coap")

   Reference:
      RFC 8323

11.8.  WebSocket Subprotocol Registration

   IANA has registered the WebSocket CoAP subprotocol in the "WebSocket
   Subprotocol Name Registry":

   Subprotocol Identifier:
      coap

   Subprotocol Common Name:
      Constrained Application Protocol (CoAP)

   Subprotocol Definition:
      RFC 8323

11.9.  CoAP Option Numbers Registry

   IANA has added this document as a reference for the following entries
   registered by [RFC7959] in the "CoAP Option Numbers" subregistry
   defined by [RFC7252]:

                 +--------+--------+--------------------+
                 | Number | Name   | Reference          |
                 +--------+--------+--------------------+
                 | 23     | Block2 | RFC 7959, RFC 8323 |
                 |        |        |                    |
                 | 27     | Block1 | RFC 7959, RFC 8323 |
                 +--------+--------+--------------------+

                       Table 3: CoAP Option Numbers

12.  References

12.1.  Normative References

   [RFC793]   Postel, J., "Transmission Control Protocol", STD 7,
              RFC 793, DOI 10.17487/RFC0793, September 1981,
              <https://www.rfc-editor.org/info/rfc793>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, DOI 10.17487/RFC3986, January 2005,
              <https://www.rfc-editor.org/info/rfc3986>.

   [RFC5234]  Crocker, D., Ed., and P. Overell, "Augmented BNF for
              Syntax Specifications: ABNF", STD 68, RFC 5234,
              DOI 10.17487/RFC5234, January 2008,
              <https://www.rfc-editor.org/info/rfc5234>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <https://www.rfc-editor.org/info/rfc5246>.

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              DOI 10.17487/RFC5785, April 2010,
              <https://www.rfc-editor.org/info/rfc5785>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <https://www.rfc-editor.org/info/rfc6066>.

   [RFC6455]  Fette, I. and A. Melnikov, "The WebSocket Protocol",
              RFC 6455, DOI 10.17487/RFC6455, December 2011,
              <https://www.rfc-editor.org/info/rfc6455>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <https://www.rfc-editor.org/info/rfc7252>.

   [RFC7301]  Friedl, S., Popov, A., Langley, A., and E. Stephan,
              "Transport Layer Security (TLS) Application-Layer Protocol
              Negotiation Extension", RFC 7301, DOI 10.17487/RFC7301,
              July 2014, <https://www.rfc-editor.org/info/rfc7301>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525,
              May 2015, <https://www.rfc-editor.org/info/rfc7525>.

   [RFC7595]  Thaler, D., Ed., Hansen, T., and T. Hardie, "Guidelines
              and Registration Procedures for URI Schemes", BCP 35,
              RFC 7595, DOI 10.17487/RFC7595, June 2015,
              <https://www.rfc-editor.org/info/rfc7595>.

   [RFC7641]  Hartke, K., "Observing Resources in the Constrained
              Application Protocol (CoAP)", RFC 7641,
              DOI 10.17487/RFC7641, September 2015,
              <https://www.rfc-editor.org/info/rfc7641>.

   [RFC7925]  Tschofenig, H., Ed., and T. Fossati, "Transport Layer
              Security (TLS) / Datagram Transport Layer Security (DTLS)
              Profiles for the Internet of Things", RFC 7925,
              DOI 10.17487/RFC7925, July 2016,
              <https://www.rfc-editor.org/info/rfc7925>.

   [RFC7959]  Bormann, C. and Z. Shelby, Ed., "Block-Wise Transfers in
              the Constrained Application Protocol (CoAP)", RFC 7959,
              DOI 10.17487/RFC7959, August 2016,
              <https://www.rfc-editor.org/info/rfc7959>.

   [RFC8126]  Cotton, M., Leiba, B., and T. Narten, "Guidelines for
              Writing an IANA Considerations Section in RFCs", BCP 26,
              RFC 8126, DOI 10.17487/RFC8126, June 2017,
              <https://www.rfc-editor.org/info/rfc8126>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in
              RFC 2119 Key Words", BCP 14, RFC 8174,
              DOI 10.17487/RFC8174, May 2017,
              <https://www.rfc-editor.org/info/rfc8174>.

   [RFC8307]  Bormann, C., "Well-Known URIs for the WebSocket Protocol",
              RFC 8307, DOI 10.17487/RFC8307, January 2018,
              <https://www.rfc-editor.org/info/rfc8307>.

12.2.  Informative References

   [BK2015]   Byrne, C. and J. Kleberg, "Advisory Guidelines for UDP
              Deployment", Work in Progress, draft-byrne-opsec-udp-
              advisory-00, July 2015.

   [CoAP-Alt-Transports]
              Silverajan, B. and T. Savolainen, "CoAP Communication with
              Alternative Transports", Work in Progress,
              draft-silverajan-core-coap-alternative-transports-10,
              July 2017.

   [CoCoA]    Bormann, C., Betzler, A., Gomez, C., and I. Demirkol,
              "CoAP Simple Congestion Control/Advanced", Work in
              Progress, draft-ietf-core-cocoa-02, October 2017.

   [EK2016]   Edeline, K., Kuehlewind, M., Trammell, B., Aben, E., and
              B. Donnet, "Using UDP for Internet Transport Evolution",
              arXiv preprint 1612.07816, December 2016,
              <https://arxiv.org/abs/1612.07816>.

   [HomeGateway]
              Haetoenen, S., Nyrhinen, A., Eggert, L., Strowes, S.,
              Sarolahti, P., and N. Kojo, "An experimental study of home
              gateway characteristics", Proceedings of the 10th ACM
              SIGCOMM conference on Internet measurement,
              DOI 10.1145/1879141.1879174, November 2010.

   [IANA.uri-schemes]
              IANA, "Uniform Resource Identifier (URI) Schemes",
              <https://www.iana.org/assignments/uri-schemes>.

   [LWM2M]    Open Mobile Alliance, "Lightweight Machine to Machine
              Technical Specification Version 1.0", February 2017,
              <http://www.openmobilealliance.org/release/LightweightM2M/
              V1_0-20170208-A/
              OMA-TS-LightweightM2M-V1_0-20170208-A.pdf>.

   [Multi-Transport-URIs]
              Thaler, D., "Using URIs With Multiple Transport Stacks",
              Work in Progress, draft-thaler-appsawg-multi-transport-
              uris-01, July 2017.

   [OSCORE]   Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", Work in Progress, draft-ietf-core-object-
              security-08, January 2018.

   [RFC768]   Postel, J., "User Datagram Protocol", STD 6, RFC 768,
              DOI 10.17487/RFC0768, August 1980,
              <https://www.rfc-editor.org/info/rfc768>.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, DOI 10.17487/RFC6335, August 2011,
              <https://www.rfc-editor.org/info/rfc6335>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <https://www.rfc-editor.org/info/rfc6347>.

   [RFC7230]  Fielding, R., Ed., and J. Reschke, Ed., "Hypertext
              Transfer Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

   [SecurityChallenges]
              Polk, T. and S. Turner, "Security Challenges For the
              Internet Of Things", Interconnecting Smart Objects with
              the Internet / IAB Workshop, February 2011,
              <https://www.iab.org/wp-content/IAB-uploads/2011/03/
              Turner.pdf>.

   [SW2016]   Swett, I., "QUIC Deployment Experience @Google", IETF 96
              Proceedings, Berlin, Germany, July 2016,
              <https://www.ietf.org/proceedings/96/slides/
              slides-96-quic-3.pdf>.

   [TCP-in-IoT]
              Gomez, C., Crowcroft, J., and M. Scharf, "TCP Usage
              Guidance in the Internet of Things (IoT)", Work in
              Progress, draft-ietf-lwig-tcp-constrained-node-
              networks-01, October 2017.

Appendix A.  Examples of CoAP over WebSockets

   This appendix gives examples for the first two configurations
   discussed in Section 4.

   An example of the process followed by a CoAP client to retrieve the
   representation of a resource identified by a "coap+ws" URI might be
   as follows.  Figure 17 below illustrates the WebSocket and CoAP
   messages exchanged in detail.

   1.  The CoAP client obtains the URI
       <coap+ws://example.org/sensors/temperature?u=Cel>, for example,
       from a resource representation that it retrieved previously.

   2.  The CoAP client establishes a WebSocket connection to the
       endpoint URI composed of the authority "example.org" and the
       well-known path "/.well-known/coap",
       <ws://example.org/.well-known/coap>.

   3.  CSMs (Section 5.3) are exchanged (not shown).

   4.  The CoAP client sends a single-frame, masked, binary message
       containing a CoAP request.  The request indicates the target
       resource with the Uri-Path ("sensors", "temperature") and
       Uri-Query ("u=Cel") Options.

   5.  The CoAP client waits for the server to return a response.

   6.  The CoAP client uses the connection for further requests, or the
       connection is closed.

      CoAP        CoAP
     Client      Server
   (WebSocket  (WebSocket
     Client)     Server)

        |          |
        |          |
        +=========>|  GET /.well-known/coap HTTP/1.1
        |          |  Host: example.org
        |          |  Upgrade: websocket
        |          |  Connection: Upgrade
        |          |  Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
        |          |  Sec-WebSocket-Protocol: coap
        |          |  Sec-WebSocket-Version: 13
        |          |
        |<=========+  HTTP/1.1 101 Switching Protocols
        |          |  Upgrade: websocket
        |          |  Connection: Upgrade
        |          |  Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=
        |          |  Sec-WebSocket-Protocol: coap
        :          :
        :<-------->:  Exchange of CSMs (not shown)
        |          |
        +--------->|  Binary frame (opcode=%x2, FIN=1, MASK=1)
        |          |    +-------------------------+
        |          |    | GET                     |
        |          |    | Token: 0x53             |
        |          |    | Uri-Path: "sensors"     |
        |          |    | Uri-Path: "temperature" |
        |          |    | Uri-Query: "u=Cel"      |
        |          |    +-------------------------+
        |          |
        |<---------+  Binary frame (opcode=%x2, FIN=1, MASK=0)
        |          |    +-------------------------+
        |          |    | 2.05 Content            |
        |          |    | Token: 0x53             |
        |          |    | Payload: "22.3 Cel"     |
        |          |    +-------------------------+
        :          :
        :          :
        +--------->|  Close frame (opcode=%x8, FIN=1, MASK=1)
        |          |
        |<---------+  Close frame (opcode=%x8, FIN=1, MASK=0)
        |          |

    Figure 17: A CoAP Client Retrieves the Representation of a Resource
                       Identified by a "coap+ws" URI

   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

Acknowledgments

   We would like to thank Stephen Berard, Geoffrey Cristallo, Olivier
   Delaby, Esko Dijk, Christian Groves, Nadir Javed, Michael Koster,
   Achim Kraus, David Navarro, Szymon Sasin, Goeran Selander, Zach
   Shelby, Andrew Summers, Julien Vermillard, and Gengyu Wei for their
   feedback.

   Last Call reviews from Yoshifumi Nishida, Mark Nottingham, and Meral
   Shirazipour as well as several IESG reviewers provided extensive
   comments; from the IESG, we would like to specifically call out Ben
   Campbell, Mirja Kuehlewind, Eric Rescorla, Adam Roach, and the
   responsible AD Alexey Melnikov.

Contributors

   Matthias Kovatsch
   Siemens AG
   Otto-Hahn-Ring 6
   Munich  D-81739
   Germany

   Phone: +49-173-5288856
   Email: matthias.kovatsch@siemens.com

   Teemu Savolainen
   Nokia Technologies
   Hatanpaan valtatie 30
   Tampere  FI-33100
   Finland

   Email: teemu.savolainen@nokia.com

   Valik Solorzano Barboza
   Zebra Technologies
   820 W. Jackson Blvd. Suite 700
   Chicago, IL  60607
   United States of America

   Phone: +1-847-634-6700
   Email: vsolorzanobarboza@zebra.com

Authors' Addresses

   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63921
   Email: cabo@tzi.org

   Simon Lemay
   Zebra Technologies
   820 W. Jackson Blvd. Suite 700
   Chicago, IL  60607
   United States of America

   Phone: +1-847-634-6700
   Email: slemay@zebra.com

   Hannes Tschofenig
   ARM Ltd.
   110 Fulbourn Road
   Cambridge  CB1 9NJ
   United Kingdom

   Email: Hannes.tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at

   Klaus Hartke
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63905
   Email: hartke@tzi.org

   Bilhanan Silverajan
   Tampere University of Technology
   Korkeakoulunkatu 10
   Tampere  FI-33720
   Finland

   Email: bilhanan.silverajan@tut.fi

   Brian Raymor (editor)

   Email: brianraymor@hotmail.com

 

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