CoRE Working Group B. Silverajan
Internet-Draft Tampere University of Technology
Intended status: Informational T. Savolainen
Expires: August 29, 2013 Nokia
February 25, 2013
CoAP Communication with Alternative Transports
draft-silverajan-core-coap-alternative-transports-01
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. While UDP remains an optimal solution
for use in IP-based constrained environments and nodes, M2M
communication using non-IP networks, NAT and firewall traversal
issues and possibly mechanisms incurring a lower overhead to CoAP/
HTTP 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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://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 August 29, 2013.
Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Rationale and Benefits . . . . . . . . . . . . . . . . . . . . 4
3. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. CoAP Transport URI . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Transport in URI scheme name . . . . . . . . . . . . . . . 6
4.2. Transport in URI path or query component . . . . . . . . . 7
4.3. Expressing transport in the URI in other ways . . . . . . 7
4.3.1. Transport in the URI authority component . . . . . . . 8
4.3.2. Transport as part of a 'service:' URL scheme . . . . . 8
4.4. Other Considerations . . . . . . . . . . . . . . . . . . . 8
5. Alternative Transport Analysis and Requirements . . . . . . . 9
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
8. Security Considerations . . . . . . . . . . . . . . . . . . . 11
9. Informative References . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12
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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 that provides 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.
CoAP's functionality and packet sizes have been specified in order to
allow constrained nodes the ability to execute a simple application
protocol to set and retrieve resources using a REST-based approach.
To allow for very low communication overhead as well as the
unreliability of constrained environments, CoAP is bound to UDP with
optional reliability, to support unicast and multicast communication.
Security is provided by means of the Datagram Transport Layer
Security (DTLS). Interworking with web is being standardized by
means of stateless HTTP mapping.
Owing to its simplicity, CoAP is an attractive option for all manner
of uses. In addition to simple end-to-end communication between CoAP
end-points as well as between CoAP and HTTP-based end-points, it is
being used towards resource discovery and lookups, group-based
communication, proxying and mirroring resources on behalf of sleeping
nodes.
As the heterogeneity of interconnected networks and nodes continues
increasing, alternative modes of transporting CoAP packets, in
addition to UDP should be considered. This allows, for instance,
retrieval of resource values and attributes of sensor nodes in non-IP
networks and the ability of nodes to overcome firewall and NAT
traversal issues. As the Internet of Things takes shape and begins
integrating with new kinds of networks and services, it is important
to understand the relevance of extending CoAP towards new transport
protocols in order to have a uniform method of lightweight retrieval
and modification of resources on constrained end-points and M2M
communication. Not all constrained nodes might have the ability to
take advantage of IP. At the same time, not all nodes with the
ability to run CoAP over UDP will be confined to just one type of
networking technology. As an example, the Lightweight M2M protocol
being drafted by the Open Mobile Alliance uses CoAP, and as
transports, specifies both UDP binding as well as Short Message
Service (SMS) bindings [OMALWM2M]. The whys and hows of running CoAP
over SMS, USSD and GPRS is an ongoing work, expanded upon in
[I-D.becker-core-coap-sms-gprs]
This document generalizes the CoAP Unique Resource Identifier (URI),
specified in [I-D.ietf-core-coap] and further expanded in
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[I-D.becker-core-coap-sms-gprs]. These drafts describe CoAP using
the "coap:", "coaps:" as well as "coap+tel:" URI schemes. In this
draft we explore how the URI can be further extended towards
specifying usable and alternative transports without imposing
incompatibilities with current practices. The mechanisms introduced
should remain in conformance to practices stipulated in [RFC4395].
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.
2. Rationale and Benefits
The variety of alternative transports is large. These include IETF
specified transport protocols such as TCP and Websockets, Disruption
Tolerant technologies such as the Bundle Protocol, non-IP transports
based on Bluetooth Low Energy and Near-Field Communications (NFC).
[I-D.ietf-core-coap] acknowledges that CoAP can be used in
conjunction with XMPP and SIP and [I-D.becker-core-coap-sms-gprs]
documents ongoing work on letting CoAP work with SMS. It is
nevertheless important to understand the relevance of extending CoAP
towards new transport protocols in order to have a uniform method of
lightweight retrieval and modification of resources on constrained
end-points by exploiting the underlying native characteristics of
such networks and their transports without necessarily having to rely
on an IP adaptation layer.
CoAP over alternative transports allows 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 networks, whereby a node can continue relying on the same
REST-based API changing its end point identifier and transport
protocol, when for example, its network technology migrates from a
non-IP transport to an IP and UDP-based transport. This might be the
case in a ZigBee or BLE node having CoAP over a proprietary network
layer but subsequently supporting UDP/IP adaptation.
3. Use Cases
CoAP [I-D.ietf-core-coap] has been designed to work on top of UDP/IP,
that is, on top of transport that can lose, reorder, and duplicate
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packets. UDP has been chosen as the transport protocol over IP due
its lightweight nature and connectionless characteristics allowing
functions such as multicast and group communications
[I-D.ietf-core-groupcomm]. As part of these design choices, CoAP
uses the exponential backoff mechanism as a simple form of congestion
control.
While the nature of UDP/IP transport for CoAP is well suited for
constrained node communications [RFC6568], there are use cases where
alternative transports would be better suited, or where UDP/IP is
simply not available. In this section we discuss about a set of use
cases where different transport channels could be useful.
A host with a CoAP client may reside behind a NAT or a firewall, and
would like to talk to a CoAP server, possibly by using CoAP Observe-
functionality [I-D.ietf-core-observe]. However, the host would wish
to conserve resources, such as energy, and avoid NAT keepalives
required to maintain NAT/firewall mappings. Furthermore, the
application on the host may need to use HTTP for (initial)
communications, but would preferably avoid use of HTTP/CoAP proxy,
especially with "long polling" feature, required to be able to
receive data from the CoAP server.
For the sake of simplicity, an application would like to communicate
with constrained nodes using CoAP without using IP-based transport
channels. For example, the application would like to use SMS
[I-D.becker-core-coap-sms-gprs] or Bluetooth Low-Energy [BTCorev4.0]
for communications. Furthermore, an application may be communicating
via Delay-Tolerant Networks [RFC4838] using Bundle Protocol
[RFC5050], and would like to transport CoAP formatted messages. In
all of these cases it is not a given that UDP or IP are supported by
a transport channel.
4. 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. When a local end-
application supplies the URI to its own CoAP client implementation,
it is parsed before the appropriate header values are encoded into
the CoAP request. At the same time, the scheme specified in the URI
is verified to determine whether plain UDP should be used ('coap') or
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whether the CoAP client should use DTLS instead ('coaps') to initiate
communication with a CoAP origin server. Secondly, the CoAP client
decomposes the URI into a set of options, namely Uri-Host, Uri-Port,
Uri-Path and Uri-Query that are encoded into the CoAP packet sent to
the CoAP origin server to specify the target resource. An origin
server receiving such a packet can reconstruct the original CoAP URI
from the option values.
A COAP URI used in this way can allow an end-application to notify
its CoAP implementation of the transport mechanism to be used. The
CoAP URI can also be used to distinguish the transport being used
between communicating CoAP entities.
How the transport protocol is identified as a distinguishable
component within the URI without violating [RFC3986], whilst ensuring
little or no impact to [I-D.ietf-core-coap] is the challenge. We
envision several alternatives for the CoAP URI based on available
existing practices, each having its strengths and limitations.
4.1. Transport in URI scheme name
For a URI that is expressed generically as
URI = scheme ":" "//" authority path-abempty [ "?"query ]
The transport protocol can be expressed as part of the scheme name.
According to [RFC3986] scheme names consist of a sequence of
characters beginning with letter and followed by any combination of
letters, digits, plus ("+"), period ("."), or hyphen ("-").
[I-D.ietf-core-coap] uses "coaps" instead of "coap" to specify DTLS
as the transport, while the preferred form used by
[I-D.becker-core-coap-sms-gprs] is "coap+tel". Using alternative
transports would therefore invoke new scheme names, such as "coap+
sip", "coap+tcp", "coap+l2cap" and so on. The authority component of
the URI would invariably then be the end point identifier specific
for that transport required, be it an IP-enabled endpoint or not.
Expressing the transport this way conforms to [RFC4395]. When such a
URI is provided from an end-application to its CoAP implementation,
URL parsing to retrieve the transport type and endpoint identifier is
trivial. It is also expected that CoAP implementations not
recognising new scheme names may simply discard the request or
response procedure.
As the usage of each such scheme name results in an entirely new
scheme, IANA intervention is required for the registration of each
scheme name. Consequently such a registration process must conform
to the guidelines stipulated in [RFC4395], particularly where
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permanent URI scheme registration is concerned. Care must therefore
be taken to ensure the scheme is well-defined and unambiguous in the
transport description.
4.2. 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 [RFC3588] 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.
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
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.
4.3. Expressing transport in the URI in other ways
Other means of indicating the transport are also possible, and while
these schemes might be incompatible with existing practices, they are
presented for the sake of completeness.
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4.3.1. 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.
4.3.2. 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.
4.4. Other Considerations
This section outlines miscellaneous considerations concerning
transport bindings with the CoAP URI.
1. When CoAP communication over an alternative transport is desired,
a clear, unambiguous name should be used. As an example, both
Bluetooth Low Energy and Classic Bluetooth carry traffic over
L2CAP. A "coap+l2cap" scheme name would therefore raise
ambiguity.
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2. 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 would then need to be registered.
5. Alternative Transport Analysis and Requirements
In this section we take a general look at alternative protocols for
CoAP and the requirements CoAP imposes on underlying layer in order
to successfully support various kinds of functionality. CoAP factors
lossiness, unreliability, small packet sizes and connection
statelessness into its protocol logic. We discuss general transport
differences and requirements to carry CoAP packets here. Note that
Reqs 1, 2, and 3 are related.
REQ1: Ability to provide unique 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.
REQ2: 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.
REQ3: 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.
REQ4: Binary encoding support.
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
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therefore be met with a transport offering binary encoding support,
although techniques to exist in allowing binary payloads to be
transferred over text-based transport protocols
REQ5: 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.
REQ6: 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.
REQ7: 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 sleeping CoAP 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.
6. Acknowledgements
7. IANA Considerations
This memo includes no request to IANA.
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8. 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.
9. 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.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., Bormann, C., and B. Frank,
"Constrained Application Protocol (CoAP)",
draft-ietf-core-coap-13 (work in progress), December 2012.
[I-D.ietf-core-groupcomm]
Rahman, A. and E. Dijk, "Group Communication for CoAP",
draft-ietf-core-groupcomm-05 (work in progress),
February 2013.
[I-D.ietf-core-observe]
Hartke, K., "Observing Resources in CoAP",
draft-ietf-core-observe-07 (work in progress),
October 2012.
[IANA-paparazzi-uri]
https://www.iana.org/assignments/uri-schemes/prov/
paparazzi, "Resource Identifier (RI) Scheme name:
paparazzi", September 2012.
[OMALWM2M]
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Open Mobile Alliance (OMA), "Lightweight Machine to
Machine Technical Specification", 2013.
[RFC2609] Guttman, E., Perkins, C., and J. Kempf, "Service Templates
and Service: Schemes", RFC 2609, June 1999.
[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J.
Arkko, "Diameter Base Protocol", RFC 3588, September 2003.
[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.
[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.
Authors' Addresses
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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