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Versions: (draft-tschofenig-core-coap-tcp-tls) 00 01 02 03 04 05 06 07 08 09 10

CORE                                                          C. Bormann
Internet-Draft                                   Universitaet Bremen TZI
Updates: 7641, 7959 (if approved)                               S. Lemay
Intended status: Standards Track                      Zebra Technologies
Expires: May 3, 2018                                       H. Tschofenig
                                                                ARM Ltd.
                                                               K. Hartke
                                                 Universitaet Bremen TZI
                                                           B. Silverajan
                                        Tampere University of Technology
                                                          B. Raymor, Ed.
                                                               Microsoft
                                                        October 30, 2017


 CoAP (Constrained Application Protocol) over TCP, TLS, and WebSockets
                  draft-ietf-core-coap-tcp-tls-10

Abstract

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

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

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on May 3, 2018.



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Copyright Notice

   Copyright (c) 2017 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   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  . . . . . . . . . . . . . . . . . . . . .   8
     3.3.  Message Transmission  . . . . . . . . . . . . . . . . . .  10
     3.4.  Connection Health . . . . . . . . . . . . . . . . . . . .  11
   4.  CoAP over WebSockets  . . . . . . . . . . . . . . . . . . . .  11
     4.1.  Opening Handshake . . . . . . . . . . . . . . . . . . . .  13
     4.2.  Message Format  . . . . . . . . . . . . . . . . . . . . .  14
     4.3.  Message Transmission  . . . . . . . . . . . . . . . . . .  15
     4.4.  Connection Health . . . . . . . . . . . . . . . . . . . .  15
   5.  Signaling . . . . . . . . . . . . . . . . . . . . . . . . . .  15
     5.1.  Signaling Codes . . . . . . . . . . . . . . . . . . . . .  16
     5.2.  Signaling Option Numbers  . . . . . . . . . . . . . . . .  16
     5.3.  Capabilities and Settings Messages (CSM)  . . . . . . . .  16
     5.4.  Ping and Pong Messages  . . . . . . . . . . . . . . . . .  18
     5.5.  Release Messages  . . . . . . . . . . . . . . . . . . . .  20
     5.6.  Abort Messages  . . . . . . . . . . . . . . . . . . . . .  21
     5.7.  Signaling examples  . . . . . . . . . . . . . . . . . . .  22
   6.  Block-wise Transfer and Reliable Transports . . . . . . . . .  22
     6.1.  Example: GET with BERT Blocks . . . . . . . . . . . . . .  24
     6.2.  Example: PUT with BERT Blocks . . . . . . . . . . . . . .  24
   7.  Observing Resources over Reliable Transports  . . . . . . . .  25
     7.1.  Notifications and Reordering  . . . . . . . . . . . . . .  25
     7.2.  Transmission and Acknowledgements . . . . . . . . . . . .  25
     7.3.  Freshness . . . . . . . . . . . . . . . . . . . . . . . .  25
     7.4.  Cancellation  . . . . . . . . . . . . . . . . . . . . . .  26
   8.  CoAP over Reliable Transport URIs . . . . . . . . . . . . . .  26
     8.1.  coap+tcp URI scheme . . . . . . . . . . . . . . . . . . .  27
     8.2.  coaps+tcp URI scheme  . . . . . . . . . . . . . . . . . .  27



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     8.3.  coap+ws URI scheme  . . . . . . . . . . . . . . . . . . .  28
     8.4.  coaps+ws URI scheme . . . . . . . . . . . . . . . . . . .  29
     8.5.  Uri-Host and Uri-Port Options . . . . . . . . . . . . . .  30
     8.6.  Decomposing URIs into Options . . . . . . . . . . . . . .  30
     8.7.  Composing URIs from Options . . . . . . . . . . . . . . .  31
   9.  Securing CoAP . . . . . . . . . . . . . . . . . . . . . . . .  32
     9.1.  TLS binding for CoAP over TCP . . . . . . . . . . . . . .  32
     9.2.  TLS usage for CoAP over WebSockets  . . . . . . . . . . .  33
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  33
     10.1.  Signaling Messages . . . . . . . . . . . . . . . . . . .  34
   11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  34
     11.1.  Signaling Codes  . . . . . . . . . . . . . . . . . . . .  34
     11.2.  CoAP Signaling Option Numbers Registry . . . . . . . . .  34
     11.3.  Service Name and Port Number Registration  . . . . . . .  36
     11.4.  Secure Service Name and Port Number Registration . . . .  36
     11.5.  URI Scheme Registration  . . . . . . . . . . . . . . . .  37
     11.6.  Well-Known URI Suffix Registration . . . . . . . . . . .  39
     11.7.  ALPN Protocol Identifier . . . . . . . . . . . . . . . .  39
     11.8.  WebSocket Subprotocol Registration . . . . . . . . . . .  40
     11.9.  CoAP Option Numbers Registry . . . . . . . . . . . . . .  40
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  40
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  40
     12.2.  Informative References . . . . . . . . . . . . . . . . .  42
   Appendix A.  CoAP over WebSocket Examples . . . . . . . . . . . .  44
   Appendix B.  Change Log . . . . . . . . . . . . . . . . . . . . .  47
     B.1.  Since draft-ietf-core-coap-tcp-tls-02 . . . . . . . . . .  47
     B.2.  Since draft-ietf-core-coap-tcp-tls-03 . . . . . . . . . .  47
     B.3.  Since draft-ietf-core-coap-tcp-tls-04 . . . . . . . . . .  47
     B.4.  Since draft-ietf-core-coap-tcp-tls-05 . . . . . . . . . .  47
     B.5.  Since draft-ietf-core-coap-tcp-tls-06 . . . . . . . . . .  48
     B.6.  Since draft-ietf-core-coap-tcp-tls-07 . . . . . . . . . .  48
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  48
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .  48
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  49

1.  Introduction

   The Constrained Application Protocol (CoAP) [RFC7252] was designed
   for Internet of Things (IoT) deployments, assuming that UDP [RFC0768]
   can be used unimpeded, as can the Datagram Transport Layer Security
   protocol (DTLS [RFC6347]) over UDP.  The use of CoAP over UDP is
   focused on simplicity, has a low code footprint, and a small over-
   the-wire message size.

   The primary reason for introducing CoAP over TCP [RFC0793] and TLS
   [RFC5246] is that some networks do not forward UDP packets.  Complete
   blocking of UDP happens in between about 2% and 4% of terrestrial
   access networks, according to [EK2016].  UDP impairment is especially



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   concentrated in enterprise networks and networks in geographic
   regions with otherwise challenged connectivity.  Some networks also
   rate-limit UDP traffic, as reported in [BK2015] and deployment
   investigations related to the standardization of QUIC revealed
   numbers around 0.3 % [SW2016].

   The introduction of CoAP over TCP also leads to some additional
   effects that may be desirable in a specific deployment:

   o  Where NATs are present along the communication path, CoAP over TCP
      leads to different NAT traversal behavior than CoAP over UDP.
      NATs often calculate expiration timers based on the transport
      layer protocol being used by application protocols.  Many NATs
      maintain TCP-based NAT bindings for longer periods based on the
      assumption that a transport layer protocol, such as TCP, offers
      additional information about the session lifecycle.  UDP, on the
      other hand, does not provide such information to a NAT and
      timeouts tend to be much shorter [HomeGateway].  According to
      [HomeGateway] the mean for TCP and UDP NAT binding timeouts is 386
      minutes (TCP) and 160 seconds (UDP).  Shorter timeout values
      require keepalive messages to be sent more frequently.  Hence, the
      use of CoAP over TCP requires less frequent transmission of keep-
      alive messages.

   o  TCP utilizes more sophisticated congestion and flow control
      mechanisms than the default mechanisms provided by CoAP over UDP,
      which is useful for the transfer of larger payloads.  (Work is,
      however, ongoing to add advanced congestion control to CoAP over
      UDP as well, see [I-D.ietf-core-cocoa].)

   Note that the use of CoAP over UDP (and CoAP over DTLS over UDP) is
   still the recommended transport for use in constrained node networks,
   particularly when used in concert with blockwise transfer.  CoAP over
   TCP is applicable for those cases where the networking infrastructure
   leaves no other choice.  The use of CoAP over TCP leads to a larger
   code size, more roundtrips, increased RAM requirements and larger
   packet sizes.  Developers implementing CoAP over TCP are encouraged
   to consult [I-D.gomez-lwig-tcp-constrained-node-networks] for
   guidance on low-footprint TCP implementations for IoT devices.

   Standards based on CoAP such as Lightweight Machine to Machine
   [LWM2M] currently use CoAP over UDP as a transport; adding support
   for CoAP over TCP enables them to address the issues above for
   specific deployments and to protect investments in existing CoAP
   implementations and deployments.

   Although HTTP/2 could also potentially address the need for
   enterprise firewall traversal, there would be additional costs and



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   delays introduced by such a transition from CoAP to HTTP/2.
   Currently, there are also fewer HTTP/2 implementations available for
   constrained devices in comparison to CoAP.  Since CoAP also support
   group communication using IP layer multicast and unreliable
   communication IoT devices would have to support HTTP/2 in addition to
   CoAP.

   Furthermore, CoAP may be integrated into a Web environment where the
   front-end uses CoAP over UDP from IoT devices to a cloud
   infrastructure and then CoAP over TCP between the back-end services.
   A TCP-to-UDP gateway can be used at the cloud boundary to communicate
   with the UDP-based IoT device.

   Finally, CoAP applications running inside a web browser may be
   without access to connectivity other than HTTP.  In this case, the
   WebSocket protocol [RFC6455] may be used to transport CoAP requests
   and responses, as opposed to cross-proxying them via HTTP to an HTTP-
   to-CoAP cross-proxy.  This preserves the functionality of CoAP
   without translation, in particular the Observe mechanism [RFC7641].

   To address the above-mentioned deployment requirements, this document
   defines how to transport CoAP over TCP, CoAP over TLS, and CoAP over
   WebSockets.  For these cases, the reliability offered by the
   transport protocol subsumes the reliability functions of the message
   layer used for CoAP over UDP.  (Note that both for a reliable
   transport and the CoAP over UDP message layer, the reliability
   offered is per transport hop: where proxies -- see Sections 5.7 and
   10 of [RFC7252] -- are involved, that layer's reliability function
   does not extend end-to-end.)  Figure 1 illustrates the layering:

     +--------------------------------+
     |          Application           |
     +--------------------------------+
     +--------------------------------+
     |  Requests/Responses/Signaling  |  CoAP (RFC 7252) / This Document
     |--------------------------------|
     |        Message Framing         |  This Document
     +--------------------------------+
     |      Reliable Transport        |
     +--------------------------------+

            Figure 1: Layering of CoAP over Reliable Transports

   This document specifies how to access resources using CoAP requests
   and responses over the TCP, TLS and WebSocket protocols.  This allows
   connectivity-limited applications to obtain end-to-end CoAP
   connectivity either by communicating CoAP directly with a CoAP server
   accessible over a TCP, TLS or WebSocket connection or via a CoAP



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   intermediary that proxies CoAP requests and responses between
   different transports, such as between WebSockets and UDP.

   Section 7 updates the "Observing Resources in the Constrained
   Application Protocol" [RFC7641] specification for use with CoAP over
   reliable transports.  [RFC7641] is an extension to the CoAP protocol
   that enables CoAP clients to "observe" a resource on a CoAP server.
   (The CoAP client retrieves a representation of a resource and
   registers to be notified by the CoAP server when the representation
   is updated.)

2.  Conventions and Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].

   This document assumes that readers are familiar with the terms and
   concepts that are used in [RFC6455], [RFC7252], [RFC7641], and
   [RFC7959].

   The term "reliable transport" is used only to refer to transport
   protocols, such as TCP, which provide reliable and ordered delivery
   of a byte-stream.

   Block-wise Extension for Reliable Transport (BERT):
      BERT extends [RFC7959] to enable the use of larger messages over a
      reliable transport.

   BERT Option:
      A Block1 or Block2 option that includes an SZX value of 7.

   BERT Block:
      The payload of a CoAP message that is affected by a BERT Option in
      descriptive usage (see Section 2.1 of [RFC7959]).

   Connection Initiator:
      The peer that opens a reliable byte stream connection, i.e., the
      TCP active opener, TLS client, or WebSocket client.

   Connection Acceptor:
      The peer that accepts the reliable byte stream connection opened
      by the other peer, i.e., the TCP passive opener, TLS server, or
      WebSocket server.






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3.  CoAP over TCP

   The request/response interaction model of CoAP over TCP is the same
   as CoAP over UDP.  The primary differences are in the message layer.
   The message layer of CoAP over UDP supports optional reliability by
   defining four types of messages: Confirmable, Non-confirmable,
   Acknowledgement, and Reset.  In addition, messages include a Message
   ID to relate Acknowledgments to Confirmable messages and to detect
   duplicate messages.

   The management of the connections is left to the application, i.e.,
   the present specification does not describe how an application
   decides to open a connection or to re-open another one in the
   presence of failures (or what it would deem to be a failure, see also
   Section 5.4).  In particular, the Connection Initiator need not be
   the client of the first request placed on the connection.

3.1.  Messaging Model

   Conceptually, CoAP over TCP replaces most of the message layer of
   CoAP over UDP with a framing mechanism on top of the byte-stream
   provided by TCP/TLS, conveying the length information for each
   message that on datagram transports is provided by the UDP/DTLS
   datagram layer.

   TCP ensures reliable message transmission, so the message layer of
   CoAP over TCP is not required to support acknowledgements or to
   detect duplicate messages.  As a result, both the Type and Message ID
   fields are no longer required and are removed from the CoAP over TCP
   message format.

   Figure 2 illustrates the difference between CoAP over UDP and CoAP
   over reliable transport.  The removed Type and Message ID fields are
   indicated by dashes.

















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       CoAP Client       CoAP Server  CoAP Client       CoAP Server
           |                    |         |                    |
           |   CON [0xbc90]     |         | (-------) [------] |
           | GET /temperature   |         | GET /temperature   |
           |   (Token 0x71)     |         |   (Token 0x71)     |
           +------------------->|         +------------------->|
           |                    |         |                    |
           |   ACK [0xbc90]     |         | (-------) [------] |
           |   2.05 Content     |         |   2.05 Content     |
           |   (Token 0x71)     |         |   (Token 0x71)     |
           |     "22.5 C"       |         |     "22.5 C"       |
           |<-------------------+         |<-------------------+
           |                    |         |                    |

               CoAP over UDP                CoAP over reliable
                                                transport

      Figure 2: Comparison between CoAP over unreliable and reliable
                                 transport

3.2.  Message Format

   The CoAP message format defined in [RFC7252], as shown in Figure 3,
   relies on the datagram transport (UDP, or DTLS over UDP) for keeping
   the individual messages separate and for providing length
   information.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |Ver| T |  TKL  |      Code     |          Message ID           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Token (if any, TKL bytes) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Options (if any) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1 1 1 1 1 1 1 1|    Payload (if any) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 3: RFC 7252 defined CoAP Message Format

   The CoAP over TCP message format is very similar to the format
   specified for CoAP over UDP.  The differences are as follows:

   o  Since the underlying TCP connection provides retransmissions and
      deduplication, there is no need for the reliability mechanisms
      provided by CoAP over UDP.  The Type (T) and Message ID fields in
      the CoAP message header are elided.



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   o  The Version (Vers) field is elided as well.  In contrast to the
      message format of CoAP over UDP, the message format for CoAP over
      TCP does not include a version number.  CoAP is defined in
      [RFC7252] with a version number of 1.  At this time, there is no
      known reason to support version numbers different from 1.  If
      version negotiation needs to be addressed in the future, then
      Capabilities and Settings Messages (CSM see Section 5.3) have been
      specifically designed to enable such a potential feature.

   o  In a stream oriented transport protocol such as TCP, a form of
      message delimitation is needed.  For this purpose, CoAP over TCP
      introduces a length field with variable size.  Figure 4 shows the
      adjusted CoAP message format with a modified structure for the
      fixed header (first 4 bytes of the CoAP over UDP header), which
      includes the length information of variable size.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Len  |  TKL  | Extended Length (if any, as chosen by Len) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      Code     | Token (if any, TKL bytes) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Options (if any) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1 1 1 1 1 1 1 1|    Payload (if any) ...
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

               Figure 4: CoAP frame for reliable transports

   Length (Len):  4-bit unsigned integer.  A value between 0 and 12
      inclusive indicates the length of the message in bytes starting
      with the first bit of the Options field.  Three values are
      reserved for special constructs:

      13:  An 8-bit unsigned integer (Extended Length) follows the
         initial byte and indicates the length of options/payload minus
         13.

      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.





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   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 connection is established, each endpoint MUST send a
   Capabilities and Settings message (CSM see Section 5.3) as their
   first message on the connection.  This message establishes the
   initial settings and capabilities for the endpoint, such as maximum
   message size or support for block-wise transfers.  The absence of
   options in the CSM indicates that base values are assumed.

   To avoid a deadlock, the Connection Initiator MUST NOT wait for the
   Connection Acceptor to send its initial CSM message before sending
   its own initial CSM message.  Conversely, the Connection Acceptor MAY
   wait for the Connection Initiator to send its initial CSM message
   before sending its own initial CSM message.

   To avoid unnecessary latency, a Connection Initiator MAY send
   additional messages after its initial CSM without waiting to receive
   the Connection Acceptor's CSM; however, it is important to note that
   the Connection Acceptor's CSM might indicate capabilities that impact
   how the initiator is expected to communicate with the acceptor.  For



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   example, the acceptor CSM could indicate a Max-Message-Size option
   (see Section 5.3.1) that is smaller than the base value (1152) in
   order to limit both buffering requirements and head-of-line blocking.

   Endpoints MUST treat a missing or invalid CSM as a connection error
   and abort the connection (see Section 5.6).

   CoAP requests and responses are exchanged asynchronously over the
   TCP/TLS connection.  A CoAP client can send multiple requests without
   waiting for a response and the CoAP server can return responses in
   any order.  Responses MUST be returned over the same connection as
   the originating request.  Concurrent requests are differentiated by
   their Token, which is scoped locally to the connection.

   The connection is bi-directional, so requests can be sent both by the
   entity that established the connection (Connection Initiator) and the
   remote host (Connection Acceptor).  If one side does not implement a
   CoAP server, an error response MUST be returned for all CoAP requests
   from the other side.  The simplest approach is to always return 5.01
   (Not Implemented).  A more elaborate mock server could also return
   4.xx responses such as 4.04 (Not Found) or 4.02 (Bad Option) where
   appropriate.

   Retransmission and deduplication of messages is provided by the TCP
   protocol.

3.4.  Connection Health

   Empty messages (Code 0.00) can always be sent and MUST be ignored by
   the recipient.  This provides a basic keep-alive function that can
   refresh NAT bindings.

   If a CoAP client does not receive any response for some time after
   sending a CoAP request (or, similarly, when a client observes a
   resource and it does not receive any notification for some time), it
   can send a CoAP Ping Signaling message (see Section 5.4) to test the
   connection and verify that the CoAP server is responsive.

   When the underlying TCP connection is closed or reset, the signaling
   state and any observation state (see Section 7.4) associated with the
   reliable connection are removed.  In flight messages may or may not
   be lost.

4.  CoAP over WebSockets

   CoAP over WebSockets is intentionally similar to CoAP over TCP;
   therefore, this section only specifies the differences between the
   transports.



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   CoAP over WebSockets can be used in a number of configurations.  The
   most basic configuration is a CoAP client retrieving or updating a
   CoAP resource located on a CoAP server that exposes a WebSocket
   endpoint (see Figure 6).  The CoAP client acts as the WebSocket
   client, establishes a WebSocket connection, and sends a CoAP request,
   to which the CoAP server returns a CoAP response.  The WebSocket
   connection can be used for any number of requests.

            ___________                            ___________
           |           |                          |           |
           |          _|___      requests      ___|_          |
           |   CoAP  /  \  \  ------------->  /  /  \  CoAP   |
           |  Client \__/__/  <-------------  \__\__/ Server  |
           |           |         responses        |           |
           |___________|                          |___________|
                   WebSocket  =============>  WebSocket
                     Client     Connection     Server

       Figure 6: CoAP Client (WebSocket client) accesses CoAP Server
                            (WebSocket server)

   The challenge with this configuration is how to identify a resource
   in the namespace of the CoAP server.  When the WebSocket protocol is
   used by a dedicated client directly (i.e., not from a web page
   through a web browser), the client can connect to any WebSocket
   endpoint.  Section 8.3 and Section 8.4 define new URI schemes that
   enable the client to identify both a WebSocket endpoint and the path
   and query of the CoAP resource within that endpoint.

   Another possible configuration is to set up a CoAP forward proxy at
   the WebSocket endpoint.  Depending on what transports are available
   to the proxy, it could forward the request to a CoAP server with a
   CoAP UDP endpoint (Figure 7), an SMS endpoint (a.k.a. mobile phone),
   or even another WebSocket endpoint.  The CoAP client specifies the
   resource to be updated or retrieved in the Proxy-Uri Option.

     ___________                ___________                ___________
    |           |              |           |              |           |
    |          _|___        ___|_         _|___        ___|_          |
    |   CoAP  /  \  \ ---> /  /  \ CoAP  /  \  \ ---> /  /  \  CoAP   |
    |  Client \__/__/ <--- \__\__/ Proxy \__/__/ <--- \__\__/ Server  |
    |           |              |           |              |           |
    |___________|              |___________|              |___________|
            WebSocket ===> WebSocket      UDP            UDP
              Client        Server      Client          Server

    Figure 7: CoAP Client (WebSocket client) accesses CoAP Server (UDP
          server) via a CoAP proxy (WebSocket server/UDP client)



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   A third possible configuration is a CoAP server running inside a web
   browser (Figure 8).  The web browser initially connects to a
   WebSocket endpoint and is then reachable through the WebSocket
   server.  When no connection exists, the CoAP server is unreachable.
   Because the WebSocket server is the only way to reach the CoAP
   server, the CoAP proxy should be a reverse-proxy.

     ___________                ___________                ___________
    |           |              |           |              |           |
    |          _|___        ___|_         _|___        ___|_          |
    |   CoAP  /  \  \ ---> /  /  \ CoAP  /  /  \ ---> /  \  \  CoAP   |
    |  Client \__/__/ <--- \__\__/ Proxy \__\__/ <--- \__/__/ Server  |
    |           |              |           |              |           |
    |___________|              |___________|              |___________|
               UDP            UDP      WebSocket <=== WebSocket
             Client          Server      Server        Client

    Figure 8: CoAP Client (UDP client) accesses CoAP Server (WebSocket
          client) via a CoAP proxy (UDP server/WebSocket server)

   Further configurations are possible, including those where a
   WebSocket connection is established through an HTTP proxy.

4.1.  Opening Handshake

   Before CoAP requests and responses are exchanged, a WebSocket
   connection is established as defined in Section 4 of [RFC6455].
   Figure 9 shows an example.

   The WebSocket client MUST include the subprotocol name "coap" in the
   list of protocols, which indicates support for the protocol defined
   in this document.

   The WebSocket client includes the hostname of the WebSocket server in
   the Host header field of its handshake as per [RFC6455].  The Host
   header field also indicates the default value of the Uri-Host Option
   in requests from the WebSocket client to the WebSocket server.














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            GET /.well-known/coap HTTP/1.1
            Host: example.org
            Upgrade: websocket
            Connection: Upgrade
            Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
            Sec-WebSocket-Protocol: coap
            Sec-WebSocket-Version: 13

            HTTP/1.1 101 Switching Protocols
            Upgrade: websocket
            Connection: Upgrade
            Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=
            Sec-WebSocket-Protocol: coap

                 Figure 9: Example of an Opening Handshake

4.2.  Message Format

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

   The message format shown in Figure 10 is the same as the CoAP over
   TCP message format (see Section 3.2) with one change.  The Length
   (Len) field MUST be set to zero because the WebSockets frame contains
   the length.

       0                   1                   2                   3
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     | Len=0 |  TKL  |      Code     |    Token (TKL bytes) ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |   Options (if any) ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |1 1 1 1 1 1 1 1|    Payload (if any) ...
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

              Figure 10: CoAP Message Format over WebSockets

   As with CoAP over TCP, the message format for CoAP over WebSockets
   eliminates the Version field defined in CoAP over UDP.  If CoAP
   version negotiation is required in the future, CoAP over WebSockets
   can address the requirement by the definition of a new subprotocol
   identifier that is negotiated during the opening handshake.

   Requests and response messages can be fragmented as specified in
   Section 5.4 of [RFC6455], though typically they are sent unfragmented



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   as they tend to be small and fully buffered before transmission.  The
   WebSocket protocol does not provide means for multiplexing.  If it is
   not desirable for a large message to monopolize the connection,
   requests and responses can be transferred in a block-wise fashion as
   defined in [RFC7959].

4.3.  Message Transmission

   As with CoAP over TCP, each endpoint MUST send a Capabilities and
   Settings message (CSM see Section 5.3) as their first message on the
   WebSocket connection.

   CoAP requests and responses are exchanged asynchronously over the
   WebSocket connection.  A CoAP client can send multiple requests
   without waiting for a response and the CoAP server can return
   responses in any order.  Responses MUST be returned over the same
   connection as the originating request.  Concurrent requests are
   differentiated by their Token, which is scoped locally to the
   connection.

   The connection is bi-directional, so requests can be sent both by the
   entity that established the connection and the remote host.

   As with CoAP over TCP, retransmission and deduplication of messages
   is provided by the WebSocket protocol.  CoAP over WebSockets
   therefore does not make a distinction between Confirmable or Non-
   Confirmable messages, and does not provide Acknowledgement or Reset
   messages.

4.4.  Connection Health

   As with CoAP over TCP, a CoAP client can test the health of the CoAP
   over WebSocket connection by sending a CoAP Ping Signaling message
   (Section 5.4).  WebSocket Ping and unsolicited Pong frames
   (Section 5.5 of [RFC6455]) SHOULD NOT be used to ensure that
   redundant maintenance traffic is not transmitted.

5.  Signaling

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

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

   o  Shut down the connection in an orderly fashion





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

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

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

5.1.  Signaling Codes

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

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

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

5.2.  Signaling Option Numbers

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

   Option numbers are assigned by the "CoAP Signaling Option Numbers"
   sub-registry (see Section 11.2).

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

5.3.  Capabilities and Settings Messages (CSM)

   Capabilities and Settings messages (CSM) are used for two purposes:

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

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



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

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

   Base values are listed below for CSM Options.  These are the values
   for the capability and setting before any Capabilities and Settings
   messages send a modified value.

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

   Capabilities and Settings messages are indicated by the 7.01 code
   (CSM).

5.3.1.  Max-Message-Size Capability Option

   The sender can use the elective Max-Message-Size Option to indicate
   the maximum size of a message in bytes that it can receive.  The
   message size indicated includes the entire message, starting from the
   first byte of the message header and ending at the end of the message
   payload (there is no relationship of the message size to the overall
   request or response body size that may be achievable in block-wise
   transfer.)

   +---+---+---+---------+------------------+--------+--------+--------+
   | # | 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.



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5.3.2.  Block-wise Transfer Capability Option

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

                         C=Critical, R=Repeatable

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

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

   If a Max-Message-Size Option is indicated with a value that is
   greater than 1152 (in the same or a different CSM message), the
   Block-wise Transfer Option also indicates support for BERT (see
   Section 6).  Subsequently, if the Max-Message-Size Option is
   indicated with a value equal to or less than 1152, BERT support is no
   longer indicated.  (Note that indication of BERT support obliges
   neither peer to actually choose to make use of BERT.)

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

5.4.  Ping and Pong Messages

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



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

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

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

5.4.1.  Custody Option

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

                         C=Critical, R=Repeatable

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

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







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5.5.  Release Messages

   A Release message indicates that the sender does not want to continue
   maintaining the connection and opts for an orderly shutdown.  The
   details are in the options.  A diagnostic payload (see Section 5.5.2
   of [RFC7252]) MAY be included.  A peer will normally respond to a
   Release message by closing the TCP/TLS connection.  Messages may be
   in flight or responses outstanding when the sender decides to send a
   Release message.  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.

   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



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

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

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

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

                         C=Critical, R=Repeatable

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

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

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




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

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

   o  a UDP-to-TCP gateway may simply not have the context to convert a
      message with a Block Option into the equivalent exchange without




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      any use of a Block Option (it would need to convert the entire
      blockwise exchange from start to end into a single exchange)

   The 'Block-wise Extension for Reliable Transport (BERT)' extends the
   Block protocol to enable the use of larger messages over a reliable
   transport.

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

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

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

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

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

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

   In all these examples, a Block Option is decomposed to indicate the
   kind of Block Option (1 or 2) followed by a colon, the block number
   (NUM), more bit (M), and block size (2**(SZX+4)) separated by
   slashes.  E.g., a Block2 Option value of 33 would be shown as



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   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




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

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

7.2.  Transmission and Acknowledgements

   For CoAP over UDP, server notifications to the client can be
   confirmable or non-confirmable.  A confirmable message requires the
   client to either respond with an acknowledgement message or a reset
   message.  An acknowledgement message indicates that the client is
   alive and wishes to receive further notifications.  A reset message
   indicates that the client does not recognize the token which causes
   the server to remove the associated entry from the list of observers.

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

7.3.  Freshness

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



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   connection (and all its active observations) by sending a single CoAP
   Ping Signaling message (Section 5.4) rather than individual requests
   to confirm each active observation.  (Note that such a Ping/Pong only
   confirms a single hop: there is no obligation, and no expectation, of
   a proxy to react to a Ping by checking all its onward observations or
   all the connections, if any, underlying them.  A proxy MAY maintain
   its own schedule for confirming the onward observations it relies on;
   it is however generally inadvisable for a proxy to generate a large
   number of outgoing checks based on a single incoming check.)

7.4.  Cancellation

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

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

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

8.  CoAP over Reliable Transport URIs

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

   o  the "coap+tcp" URI scheme for CoAP over TCP

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

   o  the "coap+ws" URI scheme for CoAP over WebSockets

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

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




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

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

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

8.1.  coap+tcp URI scheme

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

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

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

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

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

   Interoperability considerations:  None.

   Security considerations:  See Section 11.1 of [RFC7252].

8.2.  coaps+tcp URI scheme

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

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

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





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

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




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

   The WebSocket endpoint is identified by a "ws" URI that is composed
   of the authority part of the "coap+ws" URI and the well-known path
   "/.well-known/coap" [RFC5785] [I-D.bormann-hybi-ws-wk].  The path and
   query parts of a "coap+ws" URI identify a resource within the
   specified endpoint which can be operated on by the methods defined by
   CoAP:

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

                    Figure 15: The "coap+ws" URI Scheme

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

   Interoperability considerations:  None.

   Security considerations:  See Section 11.1 of [RFC7252].

8.4.  coaps+ws URI scheme

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

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

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

   The WebSocket endpoint is identified by a "wss" URI that is composed
   of the authority part of the "coaps+ws" URI and the well-known path
   "/.well-known/coap" [RFC5785] [I-D.bormann-hybi-ws-wk].  The path and
   query parts of a "coaps+ws" URI identify a resource within the
   specified endpoint which can be operated on by the methods defined by
   CoAP.










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             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 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.



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   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 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.






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9.  Securing CoAP

   Security Challenges for the Internet of Things [SecurityChallenges]
   recommends:

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

   A security solution MUST be implemented to protect CoAP over reliable
   transports and MUST be enabled by default.  This document defines the
   TLS binding, but alternative solutions at different layers in the
   protocol stack MAY be used to protect CoAP over reliable transports
   when appropriate.  Note that there is ongoing work to support a data
   object-based security model for CoAP that is independent of transport
   (see [I-D.ietf-core-object-security]).

9.1.  TLS binding for CoAP over TCP

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

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

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

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

   NoSec:  TLS is disabled.




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

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

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

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

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

9.2.  TLS usage for CoAP over WebSockets

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

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

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

10.  Security Considerations

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




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10.1.  Signaling Messages

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

11.  IANA Considerations

11.1.  Signaling Codes

   IANA is requested to create a third sub-registry for values of the
   Code field in the CoAP header (Section 12.1 of [RFC7252]).  The name
   of this sub-registry is "CoAP Signaling Codes".

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

   Initial entries in this sub-registry are as follows:

                      +------+---------+-----------+
                      | Code | Name    | Reference |
                      +------+---------+-----------+
                      | 7.01 | CSM     | [RFCthis] |
                      |      |         |           |
                      | 7.02 | Ping    | [RFCthis] |
                      |      |         |           |
                      | 7.03 | Pong    | [RFCthis] |
                      |      |         |           |
                      | 7.04 | Release | [RFCthis] |
                      |      |         |           |
                      | 7.05 | Abort   | [RFCthis] |
                      +------+---------+-----------+

                        Table 1: CoAP Signal Codes

   All other Signaling Codes are Unassigned.

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

11.2.  CoAP Signaling Option Numbers Registry

   IANA is requested to create a sub-registry for Options Numbers used
   in CoAP signaling options within the "CoRE Parameters" registry.  The
   name of this sub-registry is "CoAP Signaling Option Numbers".





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   Each entry in the sub-registry must include one or more of the codes
   in the Signaling Codes subregistry (Section 11.1), the option number,
   the name of the option, and a reference to the option's
   documentation.

   Initial entries in this sub-registry are as follows:

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

                     Table 2: CoAP Signal Option Codes

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

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

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

   o  Whether the option is repeatable.

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

   o  The base value for the option, if any.







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11.3.  Service Name and Port Number Registration

   IANA is requested to assign the port number 5683 and the service name
   "coap+tcp", in accordance with [RFC6335].

   Service Name.
      coap+tcp

   Transport Protocol.
      tcp

   Assignee.
      IESG <iesg@ietf.org>

   Contact.
      IETF Chair <chair@ietf.org>

   Description.
      Constrained Application Protocol (CoAP)

   Reference.
      [RFCthis]

   Port Number.
      5683

11.4.  Secure Service Name and Port Number Registration

   IANA is requested to assign the port number 5684 and the service name
   "coaps+tcp", in accordance with [RFC6335].  The port number is
   requested to address the exceptional case of TLS implementations that
   do not support the "Application-Layer Protocol Negotiation Extension"
   [RFC7301].

   Service Name.
      coaps+tcp

   Transport Protocol.
      tcp

   Assignee.
      IESG <iesg@ietf.org>

   Contact.
      IETF Chair <chair@ietf.org>

   Description.
      Constrained Application Protocol (CoAP)



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   Reference.
      [RFC7301], [RFCthis]

   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].

11.5.1.  coap+tcp

   IANA is requested to register the Uniform Resource Identifier (URI)
   scheme "coap+tcp".  This registration request complies with
   [RFC7595].

   Scheme name:
      coap+tcp

   Status:
      Permanent

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

   Contact:
      IETF chair <chair@ietf.org>

   Change controller:
      IESG <iesg@ietf.org>

   Reference:
      Section 8.1 in [RFCthis]

11.5.2.  coaps+tcp

   IANA is requested to register the Uniform Resource Identifier (URI)
   scheme "coaps+tcp".  This registration request complies with
   [RFC7595].

   Scheme name:
      coaps+tcp

   Status:
      Permanent




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   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 [RFCthis]

11.5.3.  coap+ws

   IANA is requested to register the Uniform Resource Identifier (URI)
   scheme "coap+ws".  This registration request complies with [RFC7595].

   Scheme name:
      coap+ws

   Status:
      Permanent

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

   Contact:
      IETF chair <chair@ietf.org>

   Change controller:
      IESG <iesg@ietf.org>

   Reference:
      Section 8.3 in [RFCthis]

11.5.4.  coaps+ws

   IANA is requested to register the Uniform Resource Identifier (URI)
   scheme "coaps+ws".  This registration request complies with
   [RFC7595].

   Scheme name:
      coaps+ws

   Status:
      Permanent



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   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 [RFCthis]

11.6.  Well-Known URI Suffix Registration

   IANA is requested to register the 'coap' well-known URI in the "Well-
   Known URIs" registry.  This registration request complies with
   [RFC5785]:

   URI Suffix.
      coap

   Change controller.
      IETF

   Specification document(s).
      [RFCthis]

   Related information.
      None.

11.7.  ALPN Protocol Identifier

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

   Protocol.
      CoAP

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

   Reference.
      [RFCthis]






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11.8.  WebSocket Subprotocol Registration

   IANA is requested to register the WebSocket CoAP subprotocol under
   the "WebSocket Subprotocol Name Registry":

   Subprotocol Identifier.
      coap

   Subprotocol Common Name.
      Constrained Application Protocol (CoAP)

   Subprotocol Definition.
      [RFCthis]

11.9.  CoAP Option Numbers Registry

   IANA is requested to add [RFCthis] to the references for the
   following entries registered by [RFC7959] in the "CoAP Option
   Numbers" sub-registry defined by [RFC7252]:

                 +--------+--------+---------------------+
                 | Number | Name   | Reference           |
                 +--------+--------+---------------------+
                 | 23     | Block2 | RFC 7959, [RFCthis] |
                 |        |        |                     |
                 | 27     | Block1 | RFC 7959, [RFCthis] |
                 +--------+--------+---------------------+

                       Table 3: CoAP Option Numbers

12.  References

12.1.  Normative References

   [I-D.bormann-hybi-ws-wk]
              Bormann, C., "Well-known URIs for the WebSocket Protocol",
              draft-bormann-hybi-ws-wk-00 (work in progress), May 2017.

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

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





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

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

   [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>.

   [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>.







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   [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>.

12.2.  Informative References

   [BK2015]   Byrne, C. and J. Kleberg, "Advisory Guidelines for UDP
              Deployment", Proceedings draft-byrne-opsec-udp-advisory-00
              (expired), 2015.

   [EK2016]   Edeline, K., Kuehlewind, M., Trammell, B., Aben, E., and
              B. Donnet, "Using UDP for Internet Transport Evolution",
              Proceedings arXiv preprint 1612.07816, 2016.

   [HomeGateway]
              Eggert, L., "An experimental study of home gateway
              characteristics", Proceedings of the 10th annual
              conference on Internet measurement , 2010.

   [I-D.gomez-lwig-tcp-constrained-node-networks]
              Gomez, C., Crowcroft, J., and M. Scharf, "TCP over
              Constrained-Node Networks", draft-gomez-lwig-tcp-
              constrained-node-networks-03 (work in progress), June
              2017.





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   [I-D.ietf-core-cocoa]
              Bormann, C., Betzler, A., Gomez, C., and I. Demirkol,
              "CoAP Simple Congestion Control/Advanced", draft-ietf-
              core-cocoa-01 (work in progress), March 2017.

   [I-D.ietf-core-object-security]
              Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", draft-ietf-core-object-security-06 (work in
              progress), October 2017.

   [IANA.uri-schemes]
              IANA, "Uniform Resource Identifier (URI) Schemes",
              <http://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>.

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

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

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

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

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




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

Appendix A.  CoAP over WebSocket Examples

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

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

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

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

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

   4.  It waits for the server to return a response.

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




















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

        |          |
        |          |
        +=========>|  GET /.well-known/coap HTTP/1.1
        |          |  Host: example.org
        |          |  Upgrade: websocket
        |          |  Connection: Upgrade
        |          |  Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
        |          |  Sec-WebSocket-Protocol: coap
        |          |  Sec-WebSocket-Version: 13
        |          |
        |<=========+  HTTP/1.1 101 Switching Protocols
        |          |  Upgrade: websocket
        |          |  Connection: Upgrade
        |          |  Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=
        |          |  Sec-WebSocket-Protocol: coap
        |          |
        |          |
        +--------->|  Binary frame (opcode=%x2, FIN=1, MASK=1)
        |          |    +-------------------------+
        |          |    | GET                     |
        |          |    | Token: 0x53             |
        |          |    | Uri-Path: "sensors"     |
        |          |    | Uri-Path: "temperature" |
        |          |    | Uri-Query: "u=Cel"      |
        |          |    +-------------------------+
        |          |
        |<---------+  Binary frame (opcode=%x2, FIN=1, MASK=0)
        |          |    +-------------------------+
        |          |    | 2.05 Content            |
        |          |    | Token: 0x53             |
        |          |    | Payload: "22.3 Cel"     |
        |          |    +-------------------------+
        :          :
        :          :
        |          |
        +--------->|  Close frame (opcode=%x8, FIN=1, MASK=1)
        |          |
        |<---------+  Close frame (opcode=%x8, FIN=1, MASK=0)
        |          |

    Figure 17: A CoAP client retrieves the representation of a resource
                       identified by a "coap+ws" URI




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   Figure 18 shows how a CoAP client uses a CoAP forward proxy with a
   WebSocket endpoint to retrieve the representation of the resource
   "coap://[2001:db8::1]/".  The use of the forward proxy and the
   address of the WebSocket endpoint are determined by the client from
   local configuration rules.  The request URI is specified in the
   Proxy-Uri Option.  Since the request URI uses the "coap" URI scheme,
   the proxy fulfills the request by issuing a Confirmable GET request
   over UDP to the CoAP server and returning the response over the
   WebSocket connection to the client.

     CoAP        CoAP       CoAP
    Client      Proxy      Server
  (WebSocket  (WebSocket    (UDP
    Client)     Server)   Endpoint)

       |          |          |
       +--------->|          |  Binary frame (opcode=%x2, FIN=1, MASK=1)
       |          |          |    +------------------------------------+
       |          |          |    | GET                                |
       |          |          |    | Token: 0x7d                        |
       |          |          |    | Proxy-Uri: "coap://[2001:db8::1]/" |
       |          |          |    +------------------------------------+
       |          |          |
       |          +--------->|  CoAP message (Ver=1, T=Con, MID=0x8f54)
       |          |          |    +------------------------------------+
       |          |          |    | GET                                |
       |          |          |    | Token: 0x0a15                      |
       |          |          |    +------------------------------------+
       |          |          |
       |          |<---------+  CoAP message (Ver=1, T=Ack, MID=0x8f54)
       |          |          |    +------------------------------------+
       |          |          |    | 2.05 Content                       |
       |          |          |    | Token: 0x0a15                      |
       |          |          |    | Payload: "ready"                   |
       |          |          |    +------------------------------------+
       |          |          |
       |<---------+          |  Binary frame (opcode=%x2, FIN=1, MASK=0)
       |          |          |    +------------------------------------+
       |          |          |    | 2.05 Content                       |
       |          |          |    | Token: 0x7d                        |
       |          |          |    | Payload: "ready"                   |
       |          |          |    +------------------------------------+
       |          |          |

    Figure 18: A CoAP client retrieves the representation of a resource
       identified by a "coap" URI via a WebSocket-enabled CoAP proxy





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Appendix B.  Change Log

   The RFC Editor is requested to remove this section at publication.

B.1.  Since draft-ietf-core-coap-tcp-tls-02

   Merged draft-savolainen-core-coap-websockets-07 Merged draft-bormann-
   core-block-bert-01 Merged draft-bormann-core-coap-sig-02

B.2.  Since draft-ietf-core-coap-tcp-tls-03

   Editorial updates

   Added mandatory exchange of Capabilities and Settings messages after
   connecting

   Added support for coaps+tcp port 5684 and more details on
   Application-Layer Protocol Negotiation (ALPN)

   Added guidance on CoAP Signaling Ping-Pong versus WebSocket Ping-Pong

   Updated references and requirements for TLS security considerations

B.3.  Since draft-ietf-core-coap-tcp-tls-04

   Updated references

   Added Appendix: Updates to RFC7641 Observing Resources in the
   Constrained Application Protocol (CoAP)

   Updated Capability and Settings Message (CSM) exchange in the Opening
   Handshake to allow initiator to send messages before receiving
   acceptor CSM

B.4.  Since draft-ietf-core-coap-tcp-tls-05

   Addressed feedback from Working Group Last Call

   Added Securing CoAP section and informative reference to OSCOAP

   Removed the Server-Name and Bad-Server-Name Options

   Clarified the Capability and Settings Message (CSM) exchange

   Updated Pong response requirements

   Added Connection Initiator and Connection Acceptor terminology where
   appropriate



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   Updated LWM2M 1.0 informative reference

B.5.  Since draft-ietf-core-coap-tcp-tls-06

   Addressed feedback from second Working Group Last Call

B.6.  Since draft-ietf-core-coap-tcp-tls-07

   Addressed feedback from IETF Last Call

   Addressed feedback from ARTART review

   Addressed feedback from GENART review

   Addressed feedback from TSVART review

   Added fragment identifiers to URI schemes

   Added "Updates RFC7959" for BERT

   Added "Updates RFC6455" to extend well-known URI mechanism to ws and
   wss

   Clarified well-known URI mechanism use for all URI schemes

   Changed NoSec to optional-to-implement

Acknowledgements

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

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

Contributors









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

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


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

   Email: teemu.savolainen@nokia.com


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

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

Authors' Addresses

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

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


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

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




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   Hannes Tschofenig
   ARM Ltd.
   110 Fulbourn Rd
   Cambridge  CB1 9NJ
   Great Britain

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


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

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


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

   Email: bilhanan.silverajan@tut.fi


   Brian Raymor (editor)
   Microsoft
   One Microsoft Way
   Redmond  98052
   United States of America

   Email: brian.raymor@microsoft.com















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