CoRE Working Group B. Silverajan
Internet-Draft Tampere University of Technology
Intended status: Informational T. Savolainen
Expires: January 4, 2018 Nokia Technologies
July 3, 2017
CoAP Communication with Alternative Transports
draft-silverajan-core-coap-alternative-transports-10
Abstract
The aim of this document is to provide a way forward to best decide
upon how alternative transport information can be expressed in a CoAP
URI. This draft examines the requirements for a new URI format for
representing CoAP resources over alternative transports. Various
potential URI formats are presented. Benefits and drawbacks of
embedding alternative transport information in various ways within
the URI components are also discussed. From all listed formats, the
document finds scheme-based model to be the most technically
feasible.
Status of This Memo
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Conformance and Design Considerations . . . . . . . . . . . . 4
3. Situating Transport Information in CoAP URIs . . . . . . . . 5
3.1. Using the URI scheme component . . . . . . . . . . . . . 5
3.1.1. Analysis . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Using the URI authority component . . . . . . . . . . . . 6
3.2.1. Analysis . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Using the URI path component . . . . . . . . . . . . . . 7
3.3.1. Analysis . . . . . . . . . . . . . . . . . . . . . . 7
3.4. Using the URI query component . . . . . . . . . . . . . . 7
3.4.1. Analysis . . . . . . . . . . . . . . . . . . . . . . 8
4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . 9
Appendix A. Expressing transport in the URI in other ways . . . 10
A.1. Transport information as part of the URI authority . . . 10
A.1.1. Usage of DNS records . . . . . . . . . . . . . . . . 11
A.2. Making CoAP Resources Available over Multiple Transports 11
A.3. Transport as part of a 'service:' URL scheme . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
The Constrained Application Protocol (CoAP) [RFC7252] is a
lightweight, binary application layer protocol designed for
constrained environments. Owing to its operating environment, CoAP
uses UDP and DTLS as its underlying transports between communicating
endpoints. However, with an increase in deployment experiences as
well as its popularity, compelling reasons exist for extending CoAP
messaging to work over alternative transports such as TCP, TLS,
WebSockets and SMS. These allow CoAP to better address firewall and
NAT traversal issues, to operate in Web browser-based and HTML5
applications as well as for energy-constrained M2M communication in
cellular networks.
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CoAP uses a REST-based model similar to HTTP, where URIs are used to
identify resources at servers. An important factor of allowing CoAP
communication over alternative transports, is to express not only the
resource identifier, but also the alternative transport information
in the URI.
CoAP URIs contain information, such as the endpoint address as well
as the location of the resource hosted at the endpoint. CoAP URIs
beginning with "coap://" are using UDP, while those beginning with
"coaps://" are using DTLS.
coap :// server.example.org /sensors/temperature
\___/ \______ ________/ \______ _________/
| \/ \/
URI scheme URI authority URI path
Figure 1: A CoAP URI
Figure 1 shows the structure of a simple example CoAP URI, in which
the various URI components can beinterpreted as follows:
o The URI scheme component (e.g. "coap") contains an application-
level identifier which typically identifies the protocol being
used as well as its transport and network level protocol
configurations. Such configurations are defined by convention or
standardisation of the protocol using the scheme.
o The URI authority component ("server.example.com") contains the
endpoint identification, which is typically a fully qualified
domain name or a network-level host address.
o The URI path component ("/sensors/temperature") contains a
parameterised resource identifier providing the location and
identity of the resource at the endpoint.
In addition to these URI components, Figure 2 shows how specific
queries on resource representations are provided by CoAP clients to
servers, by specifying one or more URI query components in the URI.
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coap :// server.example.org /sensors/temperature ?u=cel
\___/
|
URI query
Figure 2: A CoAP URI with query
This document focuses on how CoAP URIs can be extended to contain
information about alternative transports. For deriving the new URI
format, the main design considerations are presented in the next
section. Following that, various potential URIs are presented.
These URIs provide examples of how transport identifiers can be
situated in the URI scheme, authority, path or query components. The
proposed URIs are analysed to select feasible formats while
disqualifying those not meeting the design criteria.
2. Conformance and Design Considerations
In order to understand which URI formats are best suited for
expressing transport information, certain considerations firstly need
to be taken into account. Doing so eliminates URI formats that do
not meet or conform to the stated requirements. The main criteria
are:
1. Conformance to the generic syntax for a URI described in
[RFC3986]. A URI format needs to be described in which each URI
component clearly meets the syntax and percent-encoding rules
described.
2. Alignment with best practices for URI design, as described in
[RFC7320]. This is particularly important when it pertains to
establishing or standardising the structure and usage of URIs
with respect to the various URI components.
3. Request messages sent to a CoAP endpoint using a CoAP Transport
URI may be responded to with a relative URI reference. [RFC3986]
provides an algorithm to establish how relative references can be
resolved against a base URI to obtain a target URI. Given this
algorithm, a URI format needs to be described in which relative
reference resolution does not result in a target URI that loses
its transport-specific information
4. The URI can be supplied as a Proxy-Uri option by a CoAP end-point
to a CoAP forward proxy. This allows communication with a CoAP
end-point residing in a network using a different transport.
Section 6.4 of [RFC7252] provides an algorithm for parsing a
received URI to obtain the request's options. Conformance to
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[RFC3986] is also necessary in order for the parsing algorithm to
be successful.
In addition to the above mentioned requirements, where possible, the
following considerations need to be borne in mind:
1. The URI format is able to represent a resource and the transport
information for use in constrained environments, without
requiring the presence of a naming infrastructure, such as DNS or
a directory/lookup service.
2. Alternative transport information can be easily retrieved by
computationally constrained nodes. In other words, the URI
format does not result in unneccessarily complex code or logic in
such nodes to parse and extract the transport to be used, nor the
endpoint address.
3. URIs are designed to uniquely identify resources. When a single
resource is represented with multiple URIs, URI aliasing
[WWWArchv1] occurs. Avoiding URI aliasing is considered good
practice.
4. CoAP URIs do not support fragment identifiers.
3. Situating Transport Information in CoAP URIs
The following subsections aim to describe potential URI formats in
which the alternative transport information is placed in various URI
components.
3.1. Using the URI scheme component
Expressing the transport information in the URI scheme component can
be achieved by using new schemes. These can conform to an agreed-
upon convention such as "coap+alternative_transport_name" for each
new alternative transport and/or "coaps+alternative_transport_name"
for its secure counterpart.
Examples of such URIs are:
o coap+tcp://server.example.org/sensors/temperature for using CoAP
over TCP
o coap+sms://0015105550101/sensors/temperature for using CoAP over
SMS with the endpoint identifier being a telephone subscriber
number
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o coaps+tcp://server.example.org/sensors/temperature for using CoAP
over TLS
3.1.1. Analysis
Expressing transport information in the URI scheme delivers a URI
which is human-readable and computationally as easy to parse as
standard CoAP URIs, to extract transport identification information.
The URI syntax conforms to [RFC3986], and relative URI resolution
does not result in the loss of transport identification information.
However, each new alternative transport requires minting new schemes,
and IANA intervention is required for the registration of each scheme
name. The registration process follows the guidelines stipulated in
[RFC7595]. Additionally, should a CoAP server wish to expose its
resources over multiple transports (such as both UDP and TCP) , URI
aliasing can occur if the URI scheme components of these multiple
URIs differ in describing the same resource.
3.2. Using the URI authority component
Expressing the transport information within the authority component
can result in two possible URI formats.
The first approach is to structure the URI authority's host sub-
component with a transport prefix to the endpoint identifier and a
delimiter, such as "<transport-name>-endpoint_identifier".
Examples of resulting URIs are:
o coap://tcp-server.example.org/sensors/temperature for using CoAP
over TCP
o coap://sms-0015105550101/sensors/temperature for using CoAP over
SMS
The second approach is to hint at the alternative transport
information, by explicitly specifying using the URI authority's port
sub-component, thereby differentiating them from standard CoAP URIs.
Examples of resulting URIs are:
o coap://server.example.org:5684/sensors/temperature for using CoAP
over TLS
o coap://server.example.org:80/sensors/temperature for using CoAP
over WebSockets
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3.2.1. Analysis
Embedding the transport information in the host would violate the
guidelines for the structure of URI authorities in section 2.2 of
[RFC7320]. Consequently, the host in a URI authority component
cannot be used as a basis for a new CoAP URI for alternative
transports.
Embedding the transport information in the port, on the other hand,
would not violate the guidelines for the structure of URI authorities
in section 2.2 of [RFC7320]. It would result in a CoAP URI that is
less human-readable, but URI aliasing is minimised.
On the other hand, if a CoAP request message using a CoAP Transport
URI of this form elicits a CoAP Response containing a relative URI,
for example, of the form "//server2.example.org/path/to/another/
resource", relative URI resolution rules of [RFC3986] would result in
the loss of transport identification information. Consequently,
using the URI authority component cannot be used as a basis for a new
CoAP URI for alternative transports.
3.3. Using the URI path component
Should the URI path component be used, then special characters or
keywords need to be supplied in the path to make the transport
explicit. Here, many proposals can exist. In general however, this
will result in a URI format such as:
o coap://server.example.org/sensors/temperature;tcp for using CoAP
over TCP, by appending the transport information at the end of the
URI.
3.3.1. Analysis
Embedding the transport information in the URI path directly results
in a URI that is human-readable. However, if a CoAP request message
using a CoAP Transport URI of this form elicits a CoAP Response
containing a relative URI, for example, of the form
"../../path/to/another/resource", relative URI resolution rules of
[RFC3986] would result in the loss of transport identification
information. Consequently, using the URI path component cannot be
used as a basis for a new CoAP URI for alternative transports.
3.4. Using the URI query component
The alternative transport information, should URI query components be
used, would result in a URI format such as:
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o coap://server.example.org/sensors/temperature?alternative-
transport=wss for using CoAP over secure WebSockets.
3.4.1. Analysis
Embedding the transport information in a URI query also results in a
URI that is human-readable. However, if a CoAP request message using
a CoAP Transport URI of this form elicits a CoAP Response containing
a relative URI, for example, of the form "../../path/to/another/
resource", relative URI resolution rules of [RFC3986] would result in
the loss of transport identification information. Consequently,
using the URI query component cannot be used as a basis for a new
CoAP URI for alternative transports.
4. Discussion
Based on the analysis of the various options for embedding
alternative transport information in a CoAP URI, the most technically
feasible option is to use the URI scheme component, as described in
Section 3.1. To date, this has also been the WG consensus.
A discussion with IESG members during review of
[I-D.ietf-core-coap-tcp-tls] revealed however, that using the URI
scheme to express transport information is not desirable, to avoid
the proliferation of new URI schemes for the same application-layer
protocol. A strategy was instead proposed to preserve the existing
CoAP URI and reuse it for alternative transports, by employing a
combination of UDP Confirmable messages and timeouts to determine the
eventual correct transport to use between a client and server
[IESG-feedback]. The undertaken strategy would have obvious
implications regarding interoperability, application and protocol
logic, resource usage, for both new CoAP and existing CoAP
implementations and deployments. Although URI aliasing can
theoretically be avoided with this approach, at the time of writing,
its technical feasibility over using the simpler strategy of using
URI schemes, has yet to be validated. An obvious drawback is
therefore that implementers and other SDOs may choose to
provisionally or permanently register new URI schemes with IANA, for
CoAP over alternative transports anyway, as was done by the Open
Connectivity Foundation (OCF) [CoAP-TCP-TLS-registration].
5. IANA Considerations
This memo includes no request to IANA.
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6. Security Considerations
New security risks are not envisaged to arise from the guidelines
given in this document, for describing a new URI format containing
transport identification within the URI scheme component. However,
when specific alternative transports are selected for implementing
support for carrying CoAP messages, risk factors or vulnerabilities
can be present. Examples include privacy trade-offs when MAC
addresses or phone numbers are supplied as URI authority components,
or if specific URI path components employed for security-specific
interpretations are accidentally encountered as false positives.
While this document does not make it mandatory to introduce a
security mode with each transport, it recommends ascribing meaning to
the use of "coap+" and "coaps+" prefixes in the scheme component,
with the "coaps+" prefix used for secure transports for CoAP
messages.
7. Acknowledgements
Email discussions, comments and ideas from Thomas Fossati, Akbar
Rahman, Klaus Hartke, Martin Thomson, Mark Nottingham, Dave Thaler,
Graham Klyne, Carsten Bormann and Markus Becker greatly helped
previous versions of this draft.
8. References
8.1. Normative References
[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,
<http://www.rfc-editor.org/info/rfc3986>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<http://www.rfc-editor.org/info/rfc7252>.
[RFC7320] Nottingham, M., "URI Design and Ownership", BCP 190,
RFC 7320, DOI 10.17487/RFC7320, July 2014,
<http://www.rfc-editor.org/info/rfc7320>.
8.2. Informative References
[CoAP-TCP-TLS-registration]
https://www.iana.org/assignments/uri-schemes/prov/coap+tcp
, https://www.iana.org/assignments/uri-schemes/prov/
coaps+tcp, , June 2017.
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[I-D.ietf-core-coap-tcp-tls]
Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
Silverajan, B., and B. Raymor, "CoAP (Constrained
Application Protocol) over TCP, TLS, and WebSockets",
draft-ietf-core-coap-tcp-tls-09 (work in progress), May
2017.
[IESG-feedback]
https://www.ietf.org/mail-archive/web/core/current/
msg08768.html, , May 2017.
[RFC2609] Guttman, E., Perkins, C., and J. Kempf, "Service Templates
and Service: Schemes", RFC 2609, DOI 10.17487/RFC2609,
June 1999, <http://www.rfc-editor.org/info/rfc2609>.
[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,
<http://www.rfc-editor.org/info/rfc7595>.
[WWWArchv1]
http://www.w3.org/TR/webarch/#uri-aliases, "Architecture
of the World Wide Web, Volume One", December 2004.
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 not meeting the design considerations listed, or are
incompatible with existing practices outlined in [RFC7252]. They are
however, retained in this section for historical documentation and
completeness.
A.1. Transport information as part of the URI authority
A single URI scheme, "coap-at" can be introduced, as part of an
absolute URI which expresses the transport information within the
authority component. One approach is to structure the component with
a transport prefix to the endpoint identifier and a delimiter, such
as "<transport-name>-endpoint_identifier".
Examples of resulting URIs are:
o coap-at://tcp-server.example.com/sensors/temperature
o coap-at://sms-0015105550101/sensors/temperature
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An implementation note here is that some generic URI parsers will
fail when encountering a URI such as "coap-at://tcp-
[2001:db8::1]/sensors/temperature". Consequently, an equivalent, but
parseable URI from the ip6.arpa domain needs to be formulated
instead. For [2001:db8::1] using TCP, this would result in the
following URL:
coap-at://tcp-1.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0
.1.0.0.2.ip6.arpa:5683/sensors/temperature
Usage of an IPv4-mapped IPv6 address such as [::ffff.192.100.0.1] can
similarly be expressed with a URI from the ip6.arpa domain.
This URI format allows the usage of a single scheme to represent
multiple types of transport end-points. Consequently, it requires
consistency in ensuring how various transport-specific endpoints are
identified, as a single URI format is used. Attention must be paid
towards the syntax rules and encoding for the URI host component.
Additionally, against a base URI of the form "coap-at://tcp-
server.example.com/sensors/temperature", resolving a relative
reference, such as "//example.net/sensors/temperature" would result
in the target URI "coap-at://example.net/sensors/temperature", in
which transport information is lost.
A.1.1. Usage of DNS records
DNS names can be used instead of IPv6 address literals to mitigate
lengthy URLs referring to the ip6.arpa domain, if usage of DNS is
possible.
DNS SRV records can also be employed to formulate a URL such as:
coap-at://srv-_coap._tcp.example.com/sensors/temperature
in which the "srv" prefix is used to indicate that a DNS SRV lookup
should be used for _coap._tcp.example.com, where usage of CoAP over
TCP is specified for example.com, and is eventually resolved to a
numerical IPv4 or IPv6 address.
A.2. Making CoAP Resources Available over Multiple Transports
The CoAP URI used thus far is as follows:
URI = scheme ":" hier-part [ "?" query ]
hier-part = "//" authority path-abempty
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A new URI format could be introduced, that does not possess an
"authority" component, and instead defining "hier-part" to instead
use another component, "path-rootless", as specified by RFC3986
[RFC3986]. The partial ABNF format of this URI would then be:
URI = scheme ":" hier-part [ "?" query ]
hier-part = path-rootless
path-rootless = segment-nz *( "/" segment )
The full syntax of "path-rootless" is described in [RFC3986]. A
generic URI defined this way would conform to the syntax of
[RFC3986], while the path component can be treated as an opaque
string to indicate transport types, endpoints as well as paths to
CoAP resources. A single scheme can similarly be used.
A constrained node that is capable of communicating over several
types of transports (such as UDP, TCP and SMS) would be able to
convey a single CoAP resource over multiple transports. This is also
beneficial for nodes performing caching and proxying from one type of
transport to another.
Requesting and retrieving the same CoAP resource representation over
multiple transports could be rendered possible by prefixing the
transport type and endpoint identifier information to the CoAP URI.
This would result in the following example representation:
coap-at:tcp://example.com?coap://example.com/sensors/temperature
\_______ ______/ \________________ __________________/
\/ \/
Transport-specific CoAP Resource
Prefix
Figure 3: Prefixing a CoAP URI with TCP transport
Such a representation would result in the URI being decomposed into
its constituent components, with the CoAP resource residing within
the query component as follows:
Scheme: coap-at
Path: tcp://example.com
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Query: coap://example.com/sensors/temperature
The same CoAP resource, if requested over a WebSocket transport,
would result the following URI:
coap-at:ws://example.com/endpoint?coap://example.com/sensors/temperature
\___________ __________/ \_______________ ___________________/
\/ \/
Transport-specific CoAP Resource
Prefix
Figure 4: Prefixing a CoAP URI with WebSocket transport
While the transport prefix changes, the CoAP resource representation
remains the same in the query component:
Scheme: coap-at
Path: ws://example.com/endpoint
Query: coap://example.com/sensors/temperature
The URI format described here overcomes URI aliasing [WWWArchv1] when
multiple transports are used, by ensuring each CoAP resource
representation remains the same, but is prefixed with different
transports. However, against a base URI of this format, resolving
relative references of the form "//example.net/sensors/temperature"
and "/sensor2/temperature" would again result in target URIs which
lose transport-specific information.
Implementation note: While square brackets are disallowed within the
path component, the '[' and ']' characters needed to enclose a
literal IPv6 address can be percent-encoded into their respective
equivalents. The ':' character does not need to be percent-encoded.
This results in a significantly simpler URI string compared to
section 2.2, particularly for compressed IPv6 addresses.
Additionally, the URI format can be used to specify other similar
address families and formats, such as Bluetooth addresses.
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
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"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
Bilhanan Silverajan
Tampere University of Technology
Korkeakoulunkatu 10
FI-33720 Tampere
Finland
Email: bilhanan.silverajan@tut.fi
Teemu Savolainen
Nokia Technologies
Hatanpaeaen valtatie 30
FI-33100 Tampere
Finland
Email: teemu.savolainen@nokia.com
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