CoRE Working Group                                         B. Silverajan
Internet-Draft                          Tampere University of Technology
Intended status: Informational                             T. Savolainen
Expires: January 16, 2014                                          Nokia
                                                           July 15, 2013

             CoAP Communication with Alternative Transports


   CoAP is being standardised as an application level REST-based
   protocol.  A single CoAP message is typically encapsulated and
   transmitted using UDP.  This draft examines the requirements and
   possible solutions for conveying CoAP packets to end points over
   alternative transports to UDP.  UDP remains the optimal solution for
   CoAP use in IP-based constrained environments and nodes.  However the
   need for M2M communication using non-IP networks, improved transport
   level end-to-end reliability and security, NAT and firewall traversal
   issues, and mechanisms possibly incurring a lower overhead to CoAP/
   HTTP translation gateways provide compelling motivation for
   understanding how CoAP can operate in various other environments.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   This Internet-Draft will expire on January 16, 2014.

Copyright Notice

   Copyright (c) 2013 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

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   ( 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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Using Alternative Transports  . . . . . . . . . . . . . .   3
     1.2.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  CoAP Transport URI  . . . . . . . . . . . . . . . . . . . . .   5
   3.  Alternative Transport Analysis and Properties . . . . . . . .   6
     3.1.  Other Considerations  . . . . . . . . . . . . . . . . . .   9
   4.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .   9
   6.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   7.  Informative References  . . . . . . . . . . . . . . . . . . .  10
   Appendix A.  Expressing transport in the URI in other ways  . . .  12
     A.1.  Transport in URI path or query component  . . . . . . . .  12
     A.2.  Transport in the URI authority component  . . . . . . . .  13
     A.3.  Transport as part of a 'service:' URL scheme  . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

1.  Introduction

   The Constrained Application Protocol (CoAP) [I-D.ietf-core-coap] is
   being standardised by the CoRE WG as a lightweight, HTTP-like
   protocol providing a request/response model that constrained nodes
   can use to communicate with other nodes, be those servers, proxies,
   gateways, less constrained nodes, or other constrained nodes.

   As the Internet continues taking shape by integrating new kinds of
   networks, services and devices, the need for a consistent,
   lightweight method for resource representation, retrieval and
   manipulation becomes evident.  Owing to its simplicity and low
   overhead, CoAP is a highly suitable protocol for this purpose.
   However, the CoAP endpoint can reside in a non-IP network, be
   separated from its peer by NATs and firewalls or simply has no
   possibility to communicate over UDP.  Consequently in addition to
   UDP, alternative transport channels for conveying CoAP packets could
   be considered.

   This document looks at how CoAP can be used by nodes for resource
   retrieval, in an end-to-end manner regardless of the transport
   channel available.  It looks at current usage of CoAP in this regard

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   today and provides other possible scenarios.  Then the document looks
   at how resources using CoAP can contain resource information that
   provide endpoint as well as transport identifiers, without imposing
   incompatibilities with [I-D.ietf-core-coap] and maintaining
   conformance to [RFC3986].

   This draft does not discuss on application QoS requirements, user
   policies or network adaptation, nor does it advocate replacing the
   current practice of UDP-based CoAP communication.  The scope of this
   draft is limited towards a description and a requirements capture of
   how CoAP packets can be transmitted over alternative transports,
   especially how such protocols can be expressed at the CoAP layer, as
   well as how CoAP packets can be mapped at transport level payloads.

1.1.  Using Alternative Transports

   Extending CoAP's REST-based usage over alternative transports allows
   CoAP implementations to have a significantly larger relevance in
   constrained as well as non-constrained networked environments.  It
   leads to better code optimisation in constrained nodes and
   implementation reuse across new transport channels.  As opposed to
   implementing new resource retrieval schemes, an application in an
   end-node can continue relying on using CoAP for this purpose, but
   lets CoAP take into account the change in end point identification
   and transport protocol.  This simplifies development and memory
   requirements.  Resource representations are also visible in an end-
   to-end manner for any CoAP client.

   Inevitably, if two CoAP endpoints reside in distinctly separate
   networks with orthogonal transports, a CoAP proxy node is needed
   between the 2 networks so that CoAP Requests and Responses can be
   fulfilled properly.  The processing and computational overhead for
   conveying CoAP packets from one underlying transport to another,
   would be less than that of an application-level gateway performing
   individual packet-based, protocol translation between CoAP to another
   resource retrieval scheme.

1.2.  Use Cases

   CoAP has been designed to work on top of UDP, that is, on top of a
   transport that can lose, reorder, and duplicate packets.  UDP has
   been chosen as the transport protocol over IP due its lightweight
   nature and connectionless characteristics.  In addition to point-to-
   point communication, this allows multicast and group communications
   [I-D.ietf-core-groupcomm].  Moreover, DTLS can be employed to secure
   CoAP communication.

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   At this time of writing, the use of CoAP is also being specified for
   other environments as follows:

   1.  CoAP Request and Response messages can be sent via SMS or USSD
       between CoAP end-points in a cellular network
       [I-D.becker-core-coap-sms-gprs].  A CoAP Request message can also
       be sent via SMS from a CoAP client to a sleeping CoAP Server as a
       wake-up mechanism for subsequent communication via GPRS.  The
       Open Mobile Alliance (OMA) specifies both UDP and SMS as
       transports for M2M communication in cellular networks.  The OMA
       Lightweight M2M protocol being drafted uses CoAP, and as
       transports, specifies both UDP binding as well as Short Message
       Service (SMS) bindings [OMALWM2M] for the same reason.

   2.  The WebSocket protocol is being used as a transport channel
       between WebSocket enabled CoAP end-points on the Internet
       [I-D.savolainen-core-coap-websockets].  This is particularly
       useful as a means for web browsers, particularly in smart
       devices, to allow embedded client side scripts to upgrade an
       existing HTTP connection to a WebSocket connection through which
       CoAP Request and Response messages can be exchanged with a
       WebSocket-enabled server.  This also allows a browser containing
       an embedded CoAP server to behave as a WebSocket client by
       opening a connection to a WebSocket enabled CoAP Mirror Server to
       register and update its resources.

   3.  [I-D.jimenez-p2psip-coap-reload] specifices how CoAP nodes can
       use a peer-to-peer overlay network called RELOAD, as a resource
       caching facility for storing wireless sensor data.  When a CoAP
       node registers its resources with a RELOAD Proxy Node (PN), the
       node computes a hash value from the CoAP URI and stores it as a
       structure together with the PN's Node ID as well as the
       resources.  Resource retrieval by CoAP nodes is accomplished by
       computing the hash key over the Request URI,opening a connection
       to the overlay and using its message routing system to contact
       the CoAP server via its PN.

   We also envisage CoAP being extended atop other transport channels,
   such as:

   1.  Using TCP to facilitate the traversal of CoAP Request and
       Response messages.  This allows easier communication between CoAP
       clients and servers separated by firewalls and NATS.  This also
       allows CoAP messages to be transported over push notification
       services from a notification server to a client app on a
       smartphone, that may previously have subscribed to receive change
       notifications of CoAP resource representations, possibly by using
       CoAP Observe-functionality [I-D.ietf-core-observe].

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   2.  The transportation of CoAP messages in Delay-Tolerant Networks
       [RFC4838], using the Bundle Protocol [RFC5050] for reaching
       sensors in extremely challenging environments such as acoustic,
       underwater and deep space networks.

   3.  Any type of non-IP networks supporting constrained nodes and low-
       energy sensors, such as Bluetooth and Bluetooth Low Energy
       (either through L2CAP or with GATT), ZigBee, Z-Wave, 1-Wire,
       DASH7 and so on.

   4.  Instant Messaging and Social Networking channels, such as Jabber
       and Twitter.

2.  CoAP Transport URI

   CoAP is logically divided into 2 sublayers, whereby a request/
   response layer is responsible for the protocol functionality of
   exchanging request and response messages, while the messaging layer
   is bound to UDP.  These 2 sublayers are tightly coupled, both being
   responsible for properly encoding the header and body of the CoAP
   message.  The COAP URI is used by both logical sublayers.  For a URI
   that is expressed generically as

   URI = scheme ":" "//" authority path-abempty ["?"query ]

   A simple example COAP URI, "coap://
   temperature" can be interpreted as follows:

               coap :// /sensors/temperature
               \___/    \______  ________/  \______  _________/
                 |             \/                  \/
              protocol      endpoint          parameterised
             identifier    identifier           resource

                       Figure 1: The CoAP URI format

   The resource path is explicitly expressed, and the endpoint
   identifier, which contains the host address at the network-level is
   also directly bound to the scheme name containing the application-
   level protocol identifier.  The choice of a specific transport for a
   scheme, however, cannot be embedded with a URI, but is defined by
   convention or standardisation of the protocol using the scheme.  As
   examples, [RFC5092] defines the 'imap' scheme for the IMAP protocol
   over TCP, while [RFC2818] requires that the 'https' protocol

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   identifier be used to differentiate using HTTP over TLS instead of

   To express an alternative transport binding to CoAP, a scheme name
   can follow a convention of the form "coap+<transport-name>", where
   the name of the transport is clearly and unambiguously described.
   Each scheme name formed in this manner can be used to differentiate
   the use of CoAP over an alternative transport instead of the use of
   CoAP over UDP or DTLS.  The endpoint identifier, path and query
   components together with each scheme name would be used to uniquely
   identify each resource.

   Examples of such URIs are:

   o  coap:// for using CoAP over

   o  coaps:// for using CoAP over

   o  coap+tel://+15105550101/sensors/temperature for using CoAP over
      SMS or USSD with the endpoint identifier being a telephone
      subscriber number

   o  coap+ws://
      for using CoAP over WebSockets with the endpoint at ws://

   o  coap+ble.l2cap://[12:34:56:78:90:AB]:4/pulse where the scheme name
      can possibly be used to describe the future use of CoAP over L2CAP
      using Bluetooth Low Energy, but not L2CAP using classic Bluetooth.

   When such a URI is provided from an end-application to its CoAP
   implementation, the scheme name can be checked to allow the CoAP to
   use the appropriate transport for the specified endpoint identifier.
   The CoAP Transport URI can also be supplied as a Proxy-Uri option by
   a CoAP end-point to a CoAP forward proxy in order to communicate with
   a CoAP end-point residing in a network using a different transport.
   Section 6.4 of [I-D.ietf-core-coap] provides an algorithm for parsing
   the received URI to obtain the request's options.

3.  Alternative Transport Analysis and Properties

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   In this section we consider the various characteristics of
   alternative transports for successfully supporting various kinds of
   functionality for CoAP.  CoAP factors lossiness, unreliability, small
   packet sizes and connection statelessness into its protocol logic.
   We discuss general transport differences and their impact on carrying
   CoAP packets here.  Note that Properties 1, 2, and 3 are related.

   Property 1: Uniqueness of an end-point identifier.

   Transport protocols providing non-unique end-point IDs for nodes may
   only convey a subset of the CoAP functionality.  Such nodes may only
   serve as CoAP servers that announce data at specific intervals to a
   pre-specified end point, or to a shared medium.

   Property 2: Unidirectional or bidirectional CoAP communication

   This refers to the ability of the CoAP end-point to use a single
   transport channel for both request and response messages.  Depending
   on the scenario, having a unidirectional transport layer would mean
   the CoAP end-point might utilise it only for outgoing data or
   incoming data.  Should both functionalities be needed, 2
   unidirectional transport channels would be necessary.

   Property 3: 1:N communication support.

   This refers to the ability of the transport protocol to support
   broadcast and multicast communication.  CoAP's request/response
   behaviour depends on unicast messaging.  Group communication in CoAP
   is bound to using multicasting.  Therefore a protocol such as TCP
   would be ill-suited for group communications using multicast.
   Anycast support, where a message is sent to a well defined
   destination address to which several nodes belong, on the other hand,
   is supported by TCP.

   Property 4: Transport-level reliability.

   This refers to the ability of the transport protocol to provide a
   guarantee of reliability against packet loss, ensuring ordered packet
   delivery and having error control.  When CoAP Request and Response
   messages are delivered over such transports, the CoAP implementations
   elide certain fields in the packet header.  As an example, if the
   usage of a connection-oriented transport renders it unnecessary to
   specify the various CoAP message types, the Type field can be elided.
   For some connection-oriented transports, such as WebSockets, the
   version of CoAP being used can be negotiated during the opening
   transfer.  Consequently, the Version field in CoAP packets can also
   be elided.

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   Property 5: Message encoding.

   While parts of the CoAP payload are human readable or are transmitted
   in XML, JSON or SenML format, CoAP is essentially a low overhead
   binary protocol.  Efficient transmission of such packets would
   therefore be met with a transport offering binary encoding support,
   although techniques exist in allowing binary payloads to be
   transferred over text-based transport protocols such as base-64
   encoding.  A fuller discussion about performing CoAP message encoding
   for SMS can be found in Appendix A.5 of [I-D.bormann-coap-misc]

   Property 6: Network byte order.

   CoAP, as well as transports based on the IP stack use a Big Endian
   byte order for transmitting packets over the air or wire, while
   transports based on Bluetooth and Zigbee prefer Little Endian byte
   ordering for packet fields and transmission.  Any CoAP implementation
   that potentially uses multiple transports has to ensure correct byte
   ordering for the transport used.

   Property 7: MTU correlation with CoAP PDU size.

   Section 4.6 of [I-D.ietf-core-coap] discusses the avoidance of IP
   fragmentation by ensuring CoAP message fit into a single UDP
   datagram.  End-points on constrained networks using 6LoWPAN may use
   blockwise transfers to accommodate even smaller packet sizes to avoid
   fragmentation.  The MTU sizes for Bluetooth Low Energy as well as
   Classic Bluetooth are provided in Section 2.4 of
   [I-D.ietf-6lowpan-btle].  Transport MTU correlation with CoAP
   messages helps ensure minimal to no fragmentation at the transport
   layer.  On the other hand, allowing a CoAP message to be delivered
   using a delay-tolerant transport service such as the Bundle Protocol
   [RFC5050] would imply that the CoAP message may be fragmented (or
   reconstituted) along various nodes in the DTN as various sized
   bundles and bundle fragments.

   Property 8: Transport latency.

   A confirmable CoAP request would be retransmitted by a CoAP end-point
   if a response is not obtained within a certain time.  A CoAP end-
   point registering to a Resource Directory uses a POST message that
   could include a lifetime value.  A sleepy end-point similarly uses a
   lifetime value to indicate the freshness of the data to a CoAP Mirror
   Server.  Care needs to be exercised to ensure the latency of the
   transport being used to carry CoAP packets is small enough not to
   interfere with these values for the proper operation of these

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3.1.  Other Considerations

   This section outlines miscellaneous considerations concerning
   transport bindings with the CoAP URI.

   1.  A CoAP endpoint using a connection-oriented transport should be
       responsible for proper connection establishment prior to sending
       a CoAP Request message.  Both communicating endpoints may monitor
       the connection health during the Data Transfer phase.  Finally,
       once data transfer is complete, at least one end point should
       perform connection teardown gracefully.

   2.  A CoAP server, such as a sensor, may make its data available over
       multiple types of transports concurrently.  For example, this can
       be done to allow the value to be retrieved via UDP as well as
       TCP.  However, this could be carried out only when neccesary to
       avoid a resource being identified by more than one URI.  On the
       other hand, when using only a single underlying transport, URI
       aliasing should not be practised [WWWArchv1].  For some scenarios
       where there is an availability of DNS for lookups as well as
       updates, SRV records can be used.  In these cases, the "_service"
       field can be "coap", and the "_proto" field carries the name of
       the transport to be used.

   3.  As the usage of each alternative transport results in an entirely
       new scheme, IANA intervention is required for the registration of
       each scheme name.  The registration process follows the
       guidelines stipulated in [RFC4395], particularly where permanent
       URI scheme registration is concerned.

4.  IANA Considerations

   This memo includes no request to IANA.

5.  Security Considerations

   While we envisage no new security risks simply from the introduction
   of support for alternative transports, end-applications and CoAP
   implementations should take note if certain transports require
   privacy trade-offs that may arise if identifiers such as MAC
   addresses or phone numbers are made public in addition to FQDNs.

6.  Acknowledgements

   Discussions with Klaus Hartke, Graham Klyne, Markus Becker and Golnaz
   Karbaschi provided useful insights and ideas for this draft.

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7.  Informative References

              BLUETOOTH Special Interest Group, "BLUETOOTH Specification
              Version 4.0 ", June 2010.

              Becker, M., Li, K., Kuladinithi, K., and T. Poetsch,
              "Transport of CoAP over SMS, USSD and GPRS", draft-becker-
              core-coap-sms-gprs-03 (work in progress), February 2013.

              Bormann, C. and K. Hartke, "Miscellaneous additions to
              CoAP", draft-bormann-coap-misc-25 (work in progress), May

              Nieminen, J., Savolainen, T., Isomaki, M., Patil, B.,
              Shelby, Z., and C. Gomez, "Transmission of IPv6 Packets
              over BLUETOOTH Low Energy", draft-ietf-6lowpan-btle-12
              (work in progress), February 2013.

              Shelby, Z., Hartke, K., and C. Bormann, "Constrained
              Application Protocol (CoAP)", draft-ietf-core-coap-18
              (work in progress), June 2013.

              Rahman, A. and E. Dijk, "Group Communication for CoAP",
              draft-ietf-core-groupcomm-10 (work in progress), July

              Hartke, K., "Observing Resources in CoAP", draft-ietf-
              core-observe-08 (work in progress), February 2013.

              Jimenez, J., Lopez-Vega, J., Maenpaa, J., and G.
              Camarillo, "A Constrained Application Protocol (CoAP)
              Usage for REsource LOcation And Discovery (RELOAD)",
              draft-jimenez-p2psip-coap-reload-03 (work in progress),
              February 2013.

              Savolainen, T., Hartke, K., and B. Silverajan, "CoAP over
              WebSockets", draft-savolainen-core-coap-websockets-00
              (work in progress), July 2013.

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              paparazzi, "Resource Identifier (RI) Scheme name:
              paparazzi ", September 2012.

              Open Mobile Alliance (OMA), "Lightweight Machine to
              Machine Technical Specification ", 2013.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2609]  Guttman, E., Perkins, C., and J. Kempf, "Service Templates
              and Service: Schemes", RFC 2609, June 1999.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66, RFC
              3986, January 2005.

   [RFC4395]  Hansen, T., Hardie, T., and L. Masinter, "Guidelines and
              Registration Procedures for New URI Schemes", BCP 35, RFC
              4395, February 2006.

   [RFC4838]  Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst,
              R., Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant
              Networking Architecture", RFC 4838, April 2007.

   [RFC5050]  Scott, K. and S. Burleigh, "Bundle Protocol
              Specification", RFC 5050, November 2007.

   [RFC5092]  Melnikov, A. and C. Newman, "IMAP URL Scheme", RFC 5092,
              November 2007.

   [RFC6455]  Fette, I. and A. Melnikov, "The WebSocket Protocol", RFC
              6455, December 2011.

   [RFC6568]  Kim, E., Kaspar, D., and JP. Vasseur, "Design and
              Application Spaces for IPv6 over Low-Power Wireless
              Personal Area Networks (6LoWPANs)", RFC 6568, April 2012.

   [RFC6733]  Fajardo, V., Arkko, J., Loughney, J., and G. Zorn,
              "Diameter Base Protocol", RFC 6733, October 2012.

    , "Architecture
              of the World Wide Web, Volume One ", December 2004.

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Appendix A.  Expressing transport in the URI in other ways

   Other means of indicating the transport as a distinguishable
   component within the CoAP URI are possible, but have been deemed
   unsuitable by working group consensus or because they violate
   [RFC3986], and are incompatible with existing practices outlined in
   [I-D.ietf-core-coap].  They are however, retained in this section for
   historical documentation and completeness.

A.1.  Transport in URI path or query component

   For a URI that is expressed generically as

   URI = scheme ":" "//" authority path-abempty [ "?"query ]

   The transport protocol can alternatively be provided as a path or
   query component.  The Diameter Base Protocol [RFC6733] is one example
   of a protocol that uses the "aaa" and "aaas" URI scheme names to
   reflect whether transport security is used, and at the same time
   provides the actual transport protocol to be used as a ";transport="
   path component.  Example valid Diameter URIs are aaa://;transport=sctp and aaas://;transport=tcp

   Adopting such a procedure for CoAP can be done in two ways.  The
   first is to provide the transport as a path component, similar to the
   Diameter protocol.  An example resulting URI could be coap://;transport=tcp/.well-known/core?rt=core-rd specifying
   a CoAP endpoint discovering a Resource Directory and its base RD
   resource while using TCP as a transport instead of UDP.  A URI-Path
   option would then be used to encode the transport used.

   An alternative means of expressing the transport protocol used is to
   encode the transport as a query component instead.  In this case, the
   resulting URI would then be coap://
   core?rt=core-rd?tt=tcp where "tt" refers to the transport type.  Such
   a scheme would mean that the CoAP implementation encodes two URI-
   query components.

   It is also conceivable that an end point may wish to register its
   available transports and associated end point identifiers in a CoAP
   resource directory, and periodically update them.  A "core-transport"
   resource type or a "tt" link target attribute may then need to be

   Encoding the transport as part of the URI path or query provides an
   advantage in that IANA registration is not required, as opposed to
   introducing new URI scheme names.  New transports can be easily

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   introduced into the CoAP URI.  As both the URI-Path and the URI-Query
   options fall into the "critical" class of options, caution must be
   exercised if an endpoint does not recognise them.  In such cases,
   section 5.4.1 in [I-D.ietf-core-coap] provides handling guidelines.

A.2.  Transport in the URI authority component

   An application-specific, provisional resource identifier registered
   with IANA, has been done so by specifying the transport to be used as
   part of the authority [IANA-paparazzi-uri]:

   paparazzi:[options] http:[//host>[:[port][transport]]/

   While the URI is used by the application to obtain a screenshot of a
   non-secure webpage, usage of the transport parameter is unclear and
   if it is at all used.

A.3.  Transport as part of a 'service:' URL scheme

   The "service:" URL scheme name was introduced in [RFC2609] and forms
   the basis of service description used primarily by the Service
   Location Protocol.  An abstract service type URI would have the form


   where <abstract-type> refers to a service type name that can be
   associated with a variety of protocols, while the <concrete-type>
   then providing the specific details of the protocol used, authority
   and other URI components.

   Adopting the "service:" URL scheme to describe CoAP usage over
   alternative transports would be rather trivial.  To use a previous
   example, a CoAP service to discover a Resource Directory and its base
   RD resource using TCP would take the form


   The syntax of the "service:" URL scheme differs from the generic URI
   syntax and therefore such a representation should be treated as an
   opaque URI as Section 2.1 of [RFC2609] recommends.

Authors' Addresses

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


   Teemu Savolainen
   Hermiankatu 12 D
   FI-33720 Tampere


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