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
draft-silverajan-core-coap-alternative-transports-02
Abstract
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
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 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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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://server.example.com/sensors/
temperature" can be interpreted as follows:
coap :// server.example.com /sensors/temperature
\___/ \______ ________/ \______ _________/
| \/ \/
protocol endpoint parameterised
identifier identifier resource
identifier
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
TCP.
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://server.example.com/sensors/temperature for using CoAP over
UDP
o coaps://server.example.com/sensors/temperature for using CoAP over
DTLS
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://www.example.com/WebSocket?/.well-known/core?rt=core.ms
for using CoAP over WebSockets with the endpoint at ws://
www.example.com/WebSocket
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
support.
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
functionalities.
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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
[BTCorev4.0]
BLUETOOTH Special Interest Group, "BLUETOOTH Specification
Version 4.0 ", June 2010.
[I-D.becker-core-coap-sms-gprs]
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.
[I-D.bormann-coap-misc]
Bormann, C. and K. Hartke, "Miscellaneous additions to
CoAP", draft-bormann-coap-misc-25 (work in progress), May
2013.
[I-D.ietf-6lowpan-btle]
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.
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., and C. Bormann, "Constrained
Application Protocol (CoAP)", draft-ietf-core-coap-18
(work in progress), June 2013.
[I-D.ietf-core-groupcomm]
Rahman, A. and E. Dijk, "Group Communication for CoAP",
draft-ietf-core-groupcomm-10 (work in progress), July
2013.
[I-D.ietf-core-observe]
Hartke, K., "Observing Resources in CoAP", draft-ietf-
core-observe-08 (work in progress), February 2013.
[I-D.jimenez-p2psip-coap-reload]
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.
[I-D.savolainen-core-coap-websockets]
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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[IANA-paparazzi-uri]
https://www.iana.org/assignments/uri-schemes/prov/
paparazzi, "Resource Identifier (RI) Scheme name:
paparazzi ", September 2012.
[OMALWM2M]
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.
[WWWArchv1]
http://www.w3.org/TR/webarch/#uri-aliases, "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://
host.example.com;transport=sctp and aaas://
host.example.com:6666;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://
host.example.com;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://host.example.com/.well-known/
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
registered.
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
"service:<abstract-type>:<concrete-type>"
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
service:coap:tcp://host.example.com/.well-known/core?rt=core-rd
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
Finland
Email: bilhanan.silverajan@tut.fi
Teemu Savolainen
Nokia
Hermiankatu 12 D
FI-33720 Tampere
Finland
Email: teemu.savolainen@nokia.com
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