One document matched: draft-silverajan-core-coap-protocol-negotiation-04.txt
Differences from draft-silverajan-core-coap-protocol-negotiation-03.txt
CoRE Working Group B. Silverajan
Internet-Draft TUT
Intended status: Informational M. Ocak
Expires: May 4, 2017 Ericsson
October 31, 2016
CoAP Protocol Negotiation
draft-silverajan-core-coap-protocol-negotiation-04
Abstract
CoAP has been standardised as an application-level REST-based
protocol. When multiple transport protocols exist for exchanging
CoAP resource representations, this document introduces a way forward
for CoAP endpoints as well as intermediaries to agree upon alternate
transport and protocol configurations as well as URIs for CoAP
messaging, using the CoRE Resource Directory.
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 May 4, 2017.
Copyright Notice
Copyright (c) 2016 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
(http://trustee.ietf.org/license-info) in effect on the date of
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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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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Aim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. Overcoming Middlebox Issues . . . . . . . . . . . . . . . 4
2.2. Better resource caching and serving in proxies . . . . . 5
3. Node Types based on Transport Availability . . . . . . . . . 6
4. New Resource Directory Parameters . . . . . . . . . . . . . . 7
4.1. The 'at' RD parameter . . . . . . . . . . . . . . . . . . 7
4.2. The 'tt' RD parameter . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Security Considerations . . . . . . . . . . . . . . . . . . . 9
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 9
8. References . . . . . . . . . . . . . . . . . . . . . . . . . 10
8.1. Normative References . . . . . . . . . . . . . . . . . . 10
8.2. Informative References . . . . . . . . . . . . . . . . . 10
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 10
A.1. From -03 to -04 . . . . . . . . . . . . . . . . . . . . . 10
A.2. From -02 to -03 . . . . . . . . . . . . . . . . . . . . . 11
A.3. From -01 to -02 . . . . . . . . . . . . . . . . . . . . . 11
A.4. From -00 to -01 . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
The Constrained Application Protocol (CoAP) [RFC7252] allows clients,
origin servers and proxies, to exchange and manipulate resource
representations using REST-based methods over UDP or DTLS. CoAP
messaging is however being extended to use other alternative
underlying transports. These include reliable transports such as
TCP, WebSockets and TLS. In addition, the use of SMS as a CoAP
transport remains a possibility for simple communication in cellular
networks.
When CoAP-based endpoints and proxies possess the ability to perform
CoAP messaging over multiple transports, significant benefits can be
obtained if communicating client endpoints can discover that multiple
transport bindings may exist on an origin server over which CoAP
resources can be retrieved. This allows a client to understand and
possibly subsitute a different transport protocol configuration for
the same CoAP resources on the origin server, based on the
preferences of the communicating peers. Inevitably, if two CoAP
endpoints reside in distinctly separate networks with orthogonal
transports, a CoAP proxy node is needed between the two networks so
that CoAP Requests and Responses can be exchanged properly.
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A URI in CoAP, however, serves two purposes simultaneously. It
firstly 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. It secondly
identifies the name of the specific resource found at that endpoint
together with its namespace, or resource path. A single CoAP URI
cannot be used to express the identity of the resource independently
of alternate underlying transports or protocol configuration.
Multiple URIs can result for a single CoAP resource representations
if:
o the authority components of the URI differ, owing to the same
physical host exposing several network endpoints. For example,
"coap://example.org/sensors/temperature" and
"coap://example.net/sensors/temperature"
o the scheme components of the URI differ, owing to the origin
server exposing several underlying transport alternatives. For
example, "coap://example.org/sensors/temperature" and
"coap+tcp://example.org/sensors/temperature"
o the path components of the URI differ, should an origin server
also allow alternative transport endpoint such as the WebSocket
protocol, to be expressed using the path. For example,
"coap://example.org/sensors/temperature" and
"coap+ws://example.org/ws-endpoint/sensors/temperature"
Without a priori knowledge, clients would be unable to ascertain if
two or more URIs provided by an origin server are associated to the
same representation or not. Consequently, a communication mechanism
needs to be conceived to allow an origin server to properly capture
the relationship between these alternate representations or locations
and then subsequently supply this information to clients. This also
goes some way in limiting URI aliasing [WWWArchv1].
In order to support CoAP clients, proxies and servers wishing to use
CoAP over multiple transports, this draft proposes the following:
o An ability for servers to register supported CoAP transports to a
CoRE Resource Directory [I-D.ietf-core-resource-directory] with
optional registration lifetime values
o A means for CoAP clients to interact with a CoRE resource
directory interface for requesting and discovering alternative
transports and locations of CoAP resources
o New Resource Directory parameter types enabling the above-
mentioned features.
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(Note: Although previous versions of this draft provided a mechanism
for CoAP clients to directly interact with, discover, use and
possibly even negotiate an alternative transport for CoAP-based
communication directly with an origin server, discussions at the CoRE
Working Group yielded new insights about problems with the proposed
approach [CoREWG96]. The current version consequently adopts the
usage of the CoRE Resource Directory. Future work is planned on
performing discovery and negotiation without the RD as well.)
2. Aim
The following simple scenarios aim to better portray how CoAP
protocol negotiation benefits communicating nodes
2.1. Overcoming Middlebox Issues
Discovering which transports are available is important for a client
to determine the optimal alternative to perform CoAP messaging
according to its needs, particularly when separated from a CoAP
server via a NAT. It is well-known that some firewalls as well as
many NATs, particularly home gateways, hinder the proper operation of
UDP traffic. NAT bindings for UDP-based traffic do not have as long
timeouts as TCP-based traffic.
+-----------+
| Resource |
+--4-->| Directory |
| +-----------+
+---+ | ^
+----4--->| |<---+ +---1----+
+-------------+--V--+ | | +-V-----------------+
| | |--2-->| |--2-->| | |
| | UDP | | N | | UDP | |
| | |<--3--| |<--3--| | |
| CoAP Client +-----+ | A | +-----+ CoAP Server |
| | |--5-->| |--5-->| | |
| | TCP | | T | | TCP | |
| | |<--6--| |<--6--| | |
+-------------+-----+ +---+ +-----+-------------+
Figure 1: CoAP Client initially accesses CoAP Server over UDP and
then switching to TCP
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Figure 1 depicts such a scenario. Step 1 depicts the CoAP Server
registering its transports to a Resource Directory. A CoAP client
uses UDP initially for accessing a CoAP Server in Step 2 and receives
a response in Step 3. Subsequently a CoAP client, residing behind a
NAT, performs a lookup on the Resource Directory in Step 4 to
discover alternative transports offered by the server. Steps 5 and 6
illustrate the client then deciding to use TCP for CoAP messaging
instead of UDP to set up an Observe relationship for a resource at
the CoAP Server, in order to avoid incoming packets containing
resource updates being discarded by the NAT.
2.2. Better resource caching and serving in proxies
Figure 2 outlines a more complex example of intermediate nodes such
as CoAP-based proxies to intelligently cache and respond to CoAP or
HTTP clients with the same resource representation requested over
alternative transports or server endpoints. As with the earlier
example, the CoAP Server registers its transports to a Resource
Directory (This is assumed to be performed beforehand and not
depicted in the figure, for brevity)
In this example, a CoAP over WebSockets client successfully obtains a
response from a CoAP forward proxy to retrieve a resource
representation from an origin server using UDP, by supplying the CoAP
server's endpoint address and resource in a Proxy-URI option. Arrow
1 represents a GET request to "coap+ws://proxy.example.com" which
subsequently retrieves the resource from the CoAP server using the
URI "coap://example.org/sensors/temperature", shown as arrow 2.
+---------+
| CoAP+WS | +--------+-------+---+ +-----+---------+
| Client |<-1->| Web | | |<-2->| | |
+---------+ | Socket | CoAP | U | | UDP | CoAP |
+---------+ +--------+ Proxy | D | +-----+ Server |
| HTTP |<-3->| HTTP | | P | | TCP | |
| Client |<-4->| | | | | | |
+---------+ +--------+-------+---+ +-----+---------+
Figure 2: Proxying and returning a resource's alternate cached
representations to multiple clients
Subsequently, assume an HTTP client requests the same resource, but
instead specifies a CoAP over TCP alternative URI instead. Arrow 3
represents this event, where the HTTP client performs a GET request
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to "http://proxy.example.com/coap+tcp://example.org/sensors/
temperature". When the proxy receives the request, instead of
immediately retrieving the temperature resource again over TCP, it
first verifies from the Resource Directory whether the cached
resource retrieved over UDP is a valid equivalent representation of
the resource requested by the HTTP client over TCP. Upon
confirmation, the proxy is able to supply the same cached
representation to the HTTP client as well (arrow 4).
3. Node Types based on Transport Availability
In [RFC7228], Tables 1, 3 and 4 introduced classification schemes for
devices, in terms of their resource constraints, energy limitations
and communication power. For this document, in addition to these
capabilities, it seems useful to also identify devices based on their
transport capabilities.
+-------+----------------------------+
| Name | Transport Availability |
+-------+----------------------------+
| T0 | Single transport |
| | |
| T1 | Multiple transports, with |
| | one or more active at any |
| | point in time |
| | |
| T2 | Multiple active and |
| | persistent transports |
| | at all times |
+-------+----------------------------+
Table 1: Classes of Available Transports
Type T0 nodes possess the capability of exactly 1 type of transport
channel for CoAP, at all times. These include both active and sleepy
nodes, which may choose to perform duty cycling for power saving.
Type T1 nodes possess multiple different transports, and can retrieve
or expose CoAP resources over any or all of these transports.
However, not all transports are constantly active and certain
transport channels and interfaces could be kept in a mostly-off state
for energy-efficiency, such as when using CoAP over SMS.
Type T2 nodes possess more than 1 transport, and multiple transports
are simultaneously active at all times in a persistent manner. CoAP
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proxy nodes which allow CoAP endpoints from disparate transports to
communicate with each other, are a good example of this.
4. New Resource Directory Parameters
In order to allow resource interactions between clients and servers
with multiple locations or transports, the registration, update and
lookup interfaces of the CoRE Resource Directory need to be extended.
In this section two new RD parameters, "at" and "tt" are introduced.
Both 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. When absent, it is assumed that
the server does not support multiple transports or locations.
4.1. The 'at' RD parameter
A CoAP server wishing to advertise its resources over multiple
transports does so by using a new "at" parameter to register a list
of CoAP alternative transport URIs during registration with a
Resource Directory. Such a URI would contain the schemes, addresses
as well as any ports or paths at which the server is available.
+-----------+-------+---------------+-------------------------------+
| Name | Query | Validity | Description |
+-----------+-------+---------------+-------------------------------+
| CoAP | at | URI | Comma separated list of URIs |
| Transport | | | (scheme, address, port, and |
| URI List | | | path) available at the server |
+-----------+-------+---------------+-------------------------------+
Table 2: The "at" RD parameter
The "at" parameter extends the Resource Directory's Registration and
Update interfaces.
The following example shows a type T1 endpoint registering its
resources and advertising its ability to use TCP as an alternative
transport:
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Req: POST coap:/rd.example.com/rd
?ep=node1&at=coap+tcp://server.example.com,coap+ws://server.example.com:5683/ws/
Content-Format: 40
Payload:
</sensors/temp>;ct=41;rt="temperature-f";if="sensor",
</sensors/door>;ct=41;rt="door";if="sensor"
Res: 2.01 Created
Location: /rd/4521
The next example shows the same endpoint updating its registration
with a new lifetime and the availability of a single alternative
transport for CoAP (in this case WebSockets):
Req: POST /rd/4521?lt=600&at=coap+ws://server.example.com:5683/ws/
Content-Format: 40
Payload:
</sensors/temp>;ct=41;rt="temperature-f";if="sensor",
</sensors/door>;ct=41;rt="door";if="sensor"
Res: 2.04 Changed
4.2. The 'tt' RD parameter
A CoAP client wishing to perform a look-up on the Resource Directory
for CoAP servers supporting multiple transports does so by using a
new "tt" parameter to query for CoAP alternative transport URIs.
+-----------+-------+---------------+-------------------------------+
| Name | Query | Validity | Description |
+-----------+-------+---------------+-------------------------------+
| CoAP | tt | | Transport type |
| Transport | | | requested by |
| Type | | | the client |
+-----------+-------+---------------+-------------------------------+
Table 3: The "tt" RD parameter
The "tt" parameter extends the Resource Directory's rd-lookup
interface.
The following example shows a client performing a lookup for
endpoints supporting TCP:
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Req: GET /rd-lookup/ep?tt=tcp
Res: 2.05 Content
<coap+tcp://[FDFD::123]:61616>;ep="node5",
<coap+tcp://[FDFD::123]:61616>;ep="node7"
The next example shows a client performing a lookup for all
transports supported by a specific endpoint:
Req: GET /rd-lookup/ep?ep=node5&tt=*
Res: 2.05 Content
<coap+tcp://[FDFD::123]:61616>;ep="node5",
<coap+ws://[FDFD::123]:61616>;ep="node5"
5. IANA Considerations
This document requests the registration of new RD parameter types
"at" and "tt".
6. Security Considerations
When multiple transports, locations and representations are used,
some obvious risks are present both at the origin server as well as
by requesting clients.
When a client is presented with alternate URIs for retrieving
resources, it presents an opportunity for attackers to mount a series
of attacks, either by hijacking communication and masquerading as an
alternate location or by using a man-in-the-middle attack on TLS-
based communication to a server and redirecting traffic to an
alternate location. A malicious or compromised server could also be
used for reflective denial-of-service attacks on innocent third
parties. Moreover, clients may obtain web links to alternate URIs
containing weaker security properties than the existing session.
7. Acknowledgements
Thanks to Klaus Hartke for comments and reviewing this draft, and
Teemu Savolainen for initial discussions about protocol negotations
and lifetime values. Zach Shelby provided significant suggestions on
how the Resource Directory can be employed and extended in place of
link attributes and relation types.
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8. References
8.1. Normative References
[I-D.ietf-core-resource-directory]
Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE
Resource Directory", draft-ietf-core-resource-directory-08
(work in progress), July 2016.
[RFC7228] Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained-Node Networks", RFC 7228,
DOI 10.17487/RFC7228, May 2014,
<http://www.rfc-editor.org/info/rfc7228>.
[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>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<http://www.rfc-editor.org/info/rfc7641>.
8.2. Informative References
[CoREWG96]
https://www.ietf.org/proceedings/96/minutes/minutes-
96-core, "IETF96 CoRE minutes", July 2016.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[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 -03 to -04
Removed previously introduced link attribute and relation types
Initial foray with Resource Directory support
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A.2. From -02 to -03
Added new author
Rewrite of "Introduction" section
Added new Aims Section
Added new Section on Node Types
Introduced "al" Active Lifetime link attribute
Added new Section on Observing transports and resources
Security and IANA considerations sections populated
A.3. From -01 to -02
Freshness update.
A.4. From -00 to -01
Reworked "Introduction" section, added "Rationale", and "Goals"
sections.
Authors' Addresses
Bilhanan Silverajan
Tampere University of Technology
Korkeakoulunkatu 10
FI-33720 Tampere
Finland
Email: bilhanan.silverajan@tut.fi
Mert Ocak
Ericsson
Hirsalantie 11
02420 Jorvas
Finland
Email: mert.ocak@ericsson.com
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