CoRE Working Group B. Silverajan
Internet-Draft Tampere University of Technology
Intended status: Informational March 10, 2016
Expires: September 11, 2016
CoAP Protocol Negotiation
draft-silverajan-core-coap-protocol-negotiation-02
Abstract
CoAP has been standardised as an application-level REST-based
protocol. This document introduces a way forward for CoAP clients
and servers to exchange resource representations when multiple
transports exist at an endpoint, by agreeing upon alternate locations
as well as transport and protocol configurations.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Rationale . . . . . . . . . . . . . . . . . . . . . . . . . . 2
3. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. New Link Attribute and Relation types . . . . . . . . . . . . 4
5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 4
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . 6
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 7
A.1. From -01 to -02 . . . . . . . . . . . . . . . . . . . . . 7
A.2. From -00 to -01 . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 7
1. Introduction
In the Constrained Application Protocol (CoAP) [RFC7252], resources
are uniquely represented by Uniform Resource Identifiers (URIs).
Using URIs, CoAP endpoints, such as clients, origin servers and
proxies, are able to exchange representations using REST-based
methods. A URI in CoAP serves two purposes. Firstly, it functions
as a locator, by specifying the network location of the endpoint
hosting the resource, and the underlying transport used by CoAP for
accessing the resource representation. Secondly, it identifies the
name of the specific resource found at that endpoint together with
its namespace, or resource path.
This draft proposes a new link format attribute as well as a new link
relation type that together enable an origin server to serve a
resource from other protocol configuratons or endpoints. CoAP
clients then interact with an origin server's CoRE resource discovery
interface to obtain a set of links describing alternate locations of
resources.
2. Rationale
Ongoing activity and discussion in CoRE has revealed the need to
convey CoAP messages over not just UDP and DTLS, but also over
alternative transports such as SMS [I-D.becker-core-coap-sms-gprs],
TCP [I-D.ietf-core-coap-tcp-tls] and WebSockets
[I-D.savolainen-core-coap-websockets]. The underlying transport to
be used by CoAP is identified by the scheme component of a new URI
format, as described in
[I-D.silverajan-core-coap-alternative-transports].
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Working group discussions and feedback for the new URI format's
design criteria indicated eventually that two sets of requirements
for CoAP over alternative transports were deemed to be important: The
first focuses on how to express location information within the new
URI format in order to reach the origin server hosting the CoAP
resource, the alternative transport used as well as the path and
resource name that uniquely identifies the specific resource within
the server. The scope of
[I-D.silverajan-core-coap-alternative-transports] is focused towards
this first set of requirements, as well as an analysis of transport
layer properties.
The second set of requirements pertains to accessing CoAP resources
when multiple transports are present at a CoAP endpoint, by
separating endpoint location information from the identification of a
CoAP resource. By doing so, both CoAP clients can better discern if
the same CoAP resource representation can continue to be retrieved
from a CoAP server over other transports. The multiple transport
problem cannot be directly solved by simply introducing a new URI
format. Therefore, [I-D.silverajan-core-coap-alternative-transports]
provides a categorization of CoAP nodes based on their ability to use
multiple transports to convey CoAP messages, whilst its main emphasis
is in providing guidance for implementing support for CoAP over an
alternative transport. Instead, the issue of multiple transports for
a CoAP resource is addressed in this document.
3. Goals
Should an origin server wish to serve a resource over multiple
transports, a single CoAP URI cannot be used to express the identity
of the resource independently of alternate underlying transports or
protocol configurations. Similarly, if the server wishes to serve
representations of the resource from a different endpoint and path,
the URI mechanism is incapable of capturing the relationship between
these alternate representations or locations.
However, providing a way to express such relationships would be
useful in the following cases:
1. CoAP clients interacting with Type T1 or T2 CoAP origin servers
(see Section 3 of
[I-D.silverajan-core-coap-alternative-transports]) either before
or during an ongoing transaction to communicate using CoAP over a
different protocol configuration or alternative transport.
2. Avoiding URI aliases [WWWArchv1], where a single resource is
represented with multiple URIs, without describing relations
among the alternate representations.
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3. Allowing intermediate nodes such as CoAP-based proxies to
intelligently cache and respond to CoAP clients with the same
resource representation requested over alternative transports or
server endpoints.
4. Ability to separate the CoAP resource paths from web-based CoAP
endpoint path in a URI.
4. New Link Attribute and Relation types
A CoAP server wishing to allow interactions with resources from
multiple locations or transports can do so by specifying the
Transport Type "tt" link attribute, which is an opaque string.
Multiple transport types can be included in the value of this
parameter, each separated by a space. In such cases, transport types
appear in a prioritised list, with the most preferred transport type
by the CoAP server specified first and the lowest priority transport
type last.
At the same time, each transport type supported by the server is also
described with an "altloc" link relation type. The "altloc" relation
type specifices a URI (containing the URI scheme, authority and
optionally path) providing an alternate endpoint location up to but
not including the resource path of a representation.
Both "tt" and "altloc" are optional CoAP features. If supported,
they occur at the granularity level of an origin server, ie. they
cannot be applied selectively on some resources only. Therefore
"altloc" is always anchored at the root resource ("/").
Additionally, the "tt" link attribute and "altloc" relation type can
be ignored by unsupported CoAP clients.
(TBD: As type T1 nodes may not have all transports active at all
times, should a lifetime value be reflected in server responses?)
5. Examples
Example 1 shows a CoAP server returning all transport types and the
alternate resource locations to a CoAP client performing a CoAP
Request to ./well-known/core
In this case, the server supplies two different locations to interact
with resources using CoAP over TCP. At the same time, the path to
the WebSocket endpoint is provided in addition to the FQDN of the
server, for using CoAP over WebSockets.
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REQ: GET /.well-known/core
RES: 2.05 Content
;ct=40;title="Sensor Index", tt="tcp ws sms",
;rt="temperature-c";if="sensor",
;rt="light-lux";if="sensor",
;rel="altloc",
;rel="altloc",
;rel="altloc",
;rel="altloc"
Figure 1: Example of Server response
Example 2 shows a CoAP client actively soliciting a CoAP server for
all supported transport types and protocol configurations.
REQ: GET /.well-known/core?tt=*
RES: 2.05 Content
;tt="tcp sms ws"
;rel="altloc",
;rel="altloc",
;rel="altloc",
;rel="altloc"
Figure 2: CoAP client discovering transports supported by a CoAP
server.
Example 3 shows a CoAP client explicitly soliciting support for a
specific transport type using a query filter parameter.
REQ: GET /.well-known/core?tt=sms
RES: 2.05 Content
;tt="tcp sms ws"
;rel="altloc"
Figure 3: CoAP client looking for a specific transport to use with a
CoAP server.
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6. IANA Considerations
New link attributes and link relations need to be registered.
7. Security Considerations
Probably lots. (TBD)
8. Acknowledgements
Thanks to Klaus Hartke for comments and reviewing this draft, and
Teemu Savolainen for initial discussions about protocol negotations
and lifetime values.
9. References
9.1. Normative References
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
.
9.2. Informative References
[I-D.becker-core-coap-sms-gprs]
Becker, M., Li, K., Kuladinithi, K., and T. Poetsch,
"Transport of CoAP over SMS", draft-becker-core-coap-sms-
gprs-05 (work in progress), August 2014.
[I-D.ietf-core-coap-tcp-tls]
Bormann, C., Lemay, S., Technologies, Z., and H.
Tschofenig, "A TCP and TLS Transport for the Constrained
Application Protocol (CoAP)", draft-ietf-core-coap-tcp-
tls-01 (work in progress), November 2015.
[I-D.savolainen-core-coap-websockets]
Savolainen, T., Hartke, K., and B. Silverajan, "CoAP over
WebSockets", draft-savolainen-core-coap-websockets-05
(work in progress), October 2015.
[I-D.silverajan-core-coap-alternative-transports]
Silverajan, B. and T. Savolainen, "CoAP Communication with
Alternative Transports", draft-silverajan-core-coap-
alternative-transports-09 (work in progress), December
2015.
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[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
.
[WWWArchv1]
http://www.w3.org/TR/webarch/#uri-aliases, "Architecture
of the World Wide Web, Volume One", December 2004.
Appendix A. Change Log
A.1. From -01 to -02
Document lifetime extension, references updated
A.2. From -00 to -01
Reworked "Introduction" section, added "Rationale", and "Goals"
sections.
Author's Address
Bilhanan Silverajan
Tampere University of Technology
Korkeakoulunkatu 10
FI-33720 Tampere
Finland
Email: bilhanan.silverajan@tut.fi
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