CoRE Working Group T. Savolainen
Internet-Draft Nokia
Intended status: Standards Track K. Hartke
Expires: October 2, 2015 Universitaet Bremen TZI
B. Silverajan
Tampere University of Technology
March 31, 2015
CoAP over WebSockets
draft-savolainen-core-coap-websockets-04
Abstract
This document specifies how to retrieve and update CoAP resources
using CoAP requests and responses over the WebSocket Protocol.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on October 2, 2015.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2. CoAP over WebSockets . . . . . . . . . . . . . . . . . . . . 5
2.1. Opening Handshake . . . . . . . . . . . . . . . . . . . . 5
2.2. Message Format . . . . . . . . . . . . . . . . . . . . . 6
2.3. Message Transmission . . . . . . . . . . . . . . . . . . 7
2.4. Connection Health . . . . . . . . . . . . . . . . . . . . 7
2.5. Closing the Connection . . . . . . . . . . . . . . . . . 8
3. CoAP over WebSockets URIs . . . . . . . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
5.1. URI Scheme Registrations . . . . . . . . . . . . . . . . 9
5.2. WebSocket Subprotocol Registration . . . . . . . . . . . 11
5.3. Well-Known URI Suffix Registration . . . . . . . . . . . 11
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
The Constrained Application Protocol (CoAP) [RFC7252] is a web
protocol designed for communications between resource constrained
nodes. By default, CoAP operates on top of UDP or DTLS, but there is
interest in using CoAP also over other types of transports, such as
SMS [I-D.becker-core-coap-sms-gprs].
An interesting transport for CoAP could be the WebSocket Protocol
[RFC6455]. The WebSocket protocol provides two-way communication
between a client and a server after upgrading an HTTP [RFC7230]
connection, and may be available in an environment that does not
allow transportation of CoAP over UDP. This environment can be, for
example, a corporate network with Internet access only via an HTTP
proxy, or a CoAP application running inside a web browser without
access to connectivity means other than HTTP and WebSockets.
This document specifies how to access resources using CoAP requests
and responses over the WebSocket Protocol. This allows connectivity-
limited applications to obtain end-to-end CoAP connectivity either by
communicating CoAP directly with a CoAP server that is accessible
over a WebSocket Connection, or via an intermediary that proxies CoAP
requests and responses between different transports, such as between
WebSockets and UDP.
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+---------------------------------------+
| |
| Application |
| |
+---------------------------------------+
| |
| CoAP |
| Requests and Responses |
| |
+ - - - - - - - - - +-------------------+
| | |
| CoAP | CoAP over |
| Messaging | WebSockets |
| | |
+---------+---------+-------------------+
| | | |
| UDP | DTLS | WebSockets |
| | | |
+---------+---------+-------------------+
Figure 1: Abstract layering of CoAP extended by WebSockets
1.1. Overview
CoAP over WebSockets can be used in a number of configurations. The
most basic configuration is a CoAP client seeking to retrieve or
update a CoAP resource located at a CoAP server that exposes a
WebSocket endpoint (Figure 2). The CoAP client takes the role of the
WebSocket client, establishes a WebSocket Connection and sends a CoAP
request, to which the CoAP server returns a CoAP response. The
WebSocket Connection can be used for any number of requests.
___________ ___________
| | | |
| _|___ requests ___|_ |
| CoAP / \ \ -------------> / / \ CoAP |
| Client \__/__/ <------------- \__\__/ Server |
| | responses | |
|___________| |___________|
WebSocket =============> WebSocket
Client Connection Server
Figure 2: CoAP client (WebSocket client)
accesses CoAP server (WebSocket server)
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The challenge in this configuration is to identify resource in the
namespace of the CoAP server: When the WebSocket Protocol is used by
a dedicated client directly (i.e., not from a web page through a web
browser), the client can connect to any WebSocket endpoint. This
means it is necessary that the client is able to determine both the
WebSocket endpoint (identified by a "ws" or "wss" URI) and the path
and query of the CoAP resource within that endpoint from the same
URI. When the WebSocket Protocol is used from a web page, the
choices are more limited [RFC6454], but the challenge persists.
Section 3 proposes a new "coap+ws" URI scheme that identifies both a
WebSocket endpoint and a resource within that endpoint as follows:
coap+ws://example.org/sensors/temperature?u=Cel
\______ ______/\___________ ___________/
\/ \/
Uri-Path: "sensors"
ws://example.org/.well-known/coap Uri-Path: "temperature"
Uri-Query: "u=Cel"
Figure 3: The "coap+ws" URI Scheme
Another possible configuration is to set up a CoAP forward proxy at
the WebSocket endpoint. Depending on what transports are available
to the proxy, it could forward the request to a CoAP server with a
CoAP UDP endpoint (Figure 4), an SMS endpoint (a.k.a. mobile phone),
or even another WebSocket endpoint. The client specifies the
resource to be updated or retrieved in the Proxy-URI Option.
___________ ___________ ___________
| | | | | |
| _|___ ___|_ _|___ ___|_ |
| CoAP / \ \ ---> / / \ CoAP / \ \ ---> / / \ CoAP |
| Client \__/__/ <--- \__\__/ Proxy \__/__/ <--- \__\__/ Server |
| | | | | |
|___________| |___________| |___________|
WebSocket ===> WebSocket UDP UDP
Client Server Client Server
Figure 4: CoAP Client (WebSocket client) accesses CoAP Server
(UDP server) via a CoAP proxy (WebSocket server/UDP client)
A third possible configuration is a CoAP server running inside a web
browser (Figure 5). The web browser initially connects to a
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WebSocket endpoint and is then reachable through the WebSocket
server. When no connection exists, the CoAP server is not reachable;
it therefore can be considered a Sleepy Endpoint (SEP)
[I-D.dijk-core-sleepy-reqs]. Because the WebSocket server is the
only way to reach the CoAP server, the CoAP proxy should be a Reverse
Proxy.
___________ ___________ ___________
| | | | | |
| _|___ ___|_ _|___ ___|_ |
| CoAP / \ \ ---> / / \ CoAP / / \ ---> / \ \ CoAP |
| Client \__/__/ <--- \__\__/ Proxy \__\__/ <--- \__/__/ Server |
| | | | | |
|___________| |___________| |___________|
UDP UDP WebSocket <=== WebSocket
Client Server Server Client
Figure 5: CoAP Client (UDP client) accesses sleepy CoAP Server
(WebSocket client) via a CoAP proxy (UDP server/WebSocket server)
Further configurations are possible, including those where a
WebSocket Connection is established through an HTTP proxy.
1.2. Terminology
This document assumes that readers are familiar with the terms and
concepts that are used in [RFC6455] and [RFC7252].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. CoAP over WebSockets
CoAP over WebSockets is intentionally very similar to CoAP as defined
over UDP. Therefore, instead of presenting CoAP over WebSockets as a
new protocol, this document specifies it as a series of deltas from
[RFC7252].
2.1. Opening Handshake
Before CoAP requests and responses can be exchanged, a WebSocket
Connection needs to be established as defined in Section 4 of
[RFC6455]. The WebSocket client MUST include the subprotocol name
"coap.v1" in the list of protocols, which indicates support for the
protocol defined in this document. Figure 6 shows an example.
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GET /.well-known/coap HTTP/1.1
Host: example.org
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Sec-WebSocket-Protocol: coap.v1
Sec-WebSocket-Version: 13
HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=
Sec-WebSocket-Protocol: coap.v1
Figure 6: Example of an Opening Handshake
2.2. Message Format
Once a WebSocket Connection has been established, CoAP requests and
responses can be exchanged as WebSocket messages. Since CoAP uses a
binary message format, the messages are transmitted in binary data
frames as specified in Sections 5 and 6 of [RFC6455].
The message format is very similar to the format specified for CoAP
over UDP [RFC7252]. The differences are as follows:
o Since the underlying TCP connection provides retransmissions and
deduplication, there is no need for the reliability mechanisms
provided by CoAP over UDP. This means the "T" and "Message ID"
fields in the CoAP message header can be elided.
o Furthermore, since the CoAP version is already negotiated during
the opening handshake, the "Ver" field can be elided as well.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| R | TKL | Code | Token (TKL bytes) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Options (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1 1 1 1 1 1 1 1| Payload (if any) ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7: CoAP Message Format over WebSockets
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The resulting message format is shown in Figure 7. The four most-
significant bits of the first byte are reserved (R) and MUST be set
to zero. The remaining fields and structure are the same as defined
in [RFC7252].
Requests and response messages can be fragmented as specified in
Section 5.4 of [RFC6455], though typically they are sent unfragmented
as they tend to be small and fully buffered before transmission. The
WebSocket protocol does not provide means for multiplexing; if it is
not desirable for a large message to monopolize the connection,
requests and responses can be transferred in a blockwise fashion as
defined in [I-D.ietf-core-block].
Messages MUST NOT be Empty (Code 0.00), i.e., messages always carry
either a request or a response.
2.3. Message Transmission
CoAP requests and responses are exchanged asynchronously over the
WebSocket Connection, i.e., a CoAP client can send multiple requests
without waiting for a response, and the CoAP server can return
responses in any order. Responses MUST be returned over the same
connection as the originating request. Concurrent requests are
differentiated by the Token, which is scoped locally to the
connection.
The connection is bi-directional, so requests can be sent both by the
entity that established the connection and the remote host.
Retransmission and deduplication of messages is provided by the
WebSocket Protocol. CoAP over WebSockets therefore does not make a
distinction between Confirmable or Non-Confirmable messages, and does
not provide Acknowledgement or Reset messages.
Since the WebSocket Protocol provides ordered delivery of messages,
the mechanism for reordering detection when observing resources
[I-D.ietf-core-observe] is not needed. The value of the Observe
Option in notifications therefore MAY be empty on transmission and
MUST be ignored on reception.
2.4. Connection Health
When a client does not receive any response for some time after
sending a CoAP request (or, similarly, when a client observes a
resource and it does not receive any notification for some time), the
connection between the WebSocket client and the WebSocket server may
be lost or temporarily disrupted without the client being aware of
it.
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To check the health of the WebSocket Connection (and thereby of all
active requests, if any), the client can send a Ping frame or an
unsolicited Pong frame as specified in Section 5.5 of [RFC6455].
2.5. Closing the Connection
The WebSocket Connection is closed as specified in Section 7 of
[RFC6455].
All requests for which the CoAP client has not received a response
yet, are cancelled when the connection is closed. If the client
observes one or more resource over the WebSocket Connection, then the
CoAP server (or intermediary in the role of the CoAP server) MUST
remove all entries associated with the client from the lists of
observers when the connection is closed.
3. CoAP over WebSockets URIs
For the first configuration discussed in Section 1.1, this document
defines two new URIs schemes that can be used for identifying CoAP
resources and providing a means of locating these resources:
"coap+ws" and "coap+wss".
Similar to the "coap" and "coaps" schemes, the "coap+ws" and
"coap+wss" schemes organize resources hierarchically under a CoAP
origin server. The key difference is that the server is potentially
reachable on a WebSocket endpoint instead of a UDP endpoint.
The WebSocket endpoint is identified by a "ws" or "wss" URI that is
composed of the authority part of the "coap+ws" or "coap+wss" URI,
respectively, and the well-known path "/.well-known/coap" [RFC5785].
The path and query parts of a "coap+ws" or "coap+wss" URI identify a
resource within the specified endpoint which can be operated on by
the methods defined by the CoAP protocol.
The syntax of the "coap+ws" and "coap+wss" URI schemes is specified
below in Augmented Backus-Naur Form (ABNF) [RFC5234]. The
definitions of "host", "port", "path-abempty" and "query" are the
same as in [RFC3986].
coap-ws-URI =
"coap+ws:" "//" host [ ":" port ] path-abempty [ "?" query ]
coap-wss-URI =
"coap+wss:" "//" host [ ":" port ] path-abempty [ "?" query ]
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The port component is OPTIONAL; the default for "coap+ws" is port 80,
while the default for "coap+wss" is port 443.
Fragment identifiers are not part of the request URI and thus MUST
NOT be transmitted in a WebSocket handshake or in the URI options of
a CoAP request.
4. Security Considerations
CoAP over WebSockets and CoAP over TLS-secured WebSockets do not
introduce additional security issues beyond CoAP and DTLS-secured
CoAP respectively [RFC7252].
The security considerations of [RFC6455] apply.
5. IANA Considerations
5.1. URI Scheme Registrations
5.1.1. "coap+ws"
This document requests the registration of the Uniform Resource
Identifier (URI) scheme "coap+ws".
URI scheme name.
coap+ws
Status.
Permanent.
URI scheme syntax.
Defined in Section 3.
URI scheme semantics.
The "coap+ws" URI scheme provides a way to identify resources that
are potentially accessible over the Constrained Application
Protocol (CoAP) using the WebSocket Protocol.
Encoding considerations.
The scheme encoding conforms to the encoding rules established for
URIs in [RFC3986], i.e., internationalized and reserved characters
are expressed using UTF-8-based percent-encoding.
Applications/protocols that use this URI scheme name.
The scheme is used by CoAP endpoints to access CoAP resources
using the WebSocket protocol.
Interoperability considerations.
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None.
Security considerations.
See Section 4.
Contact.
IETF Chair <chair@ietf.org>
Author/Change controller.
IESG <iesg@ietf.org>
References.
This document.
5.1.2. "coap+wss"
This document requests the registration of the Uniform Resource
Identifier (URI) scheme "coap+wss".
URI scheme name.
coap+wss
Status.
Permanent.
URI scheme syntax.
Defined in Section 3.
URI scheme semantics.
The "coap+wss" URI scheme provides a way to identify resources
that are potentially accessible over the Constrained Application
Protocol (CoAP) using the WebSocket Protocol secured with
Transport Layer Security (TLS).
Encoding considerations.
The scheme encoding conforms to the encoding rules established for
URIs in [RFC3986], i.e., internationalized and reserved characters
are expressed using UTF-8-based percent-encoding.
Applications/protocols that use this URI scheme name.
The scheme is used by CoAP endpoints to access CoAP resources
using the WebSocket protocol secured with TLS.
Interoperability considerations.
None.
Security considerations.
See Section 4.
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Contact.
IETF Chair <chair@ietf.org>
Author/Change controller.
IESG <iesg@ietf.org>
References.
This document.
5.2. WebSocket Subprotocol Registration
This document requests the registration of the subprotocol name
"coap.v1" in the WebSocket Subprotocol Name Registry.
Subprotocol Identifier.
coap.v1
Subprotocol Common Name.
Constrained Application Protocol (CoAP).
Subprotocol Definition.
This document.
5.3. Well-Known URI Suffix Registration
This document requests the registration of the Well-Known URI suffix
"coap" in the Well-Known URI Registry.
URI suffix.
coap
Change controller.
IETF.
Specification document(s).
This document.
Related information.
None.
6. Acknowledgements
Thanks to Nadir Javed for helpful comments and discussions that have
shaped the document.
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7. References
7.1. Normative References
[I-D.ietf-core-observe]
Hartke, K., "Observing Resources in CoAP", draft-ietf-
core-observe-16 (work in progress), December 2014.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, RFC
3986, January 2005.
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785, April
2010.
[RFC6455] Fette, I. and A. Melnikov, "The WebSocket Protocol", RFC
6455, December 2011.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252, June 2014.
7.2. 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.dijk-core-sleepy-reqs]
Dijk, E., "Sleepy Devices using CoAP - Requirements",
draft-dijk-core-sleepy-reqs-00 (work in progress), June
2013.
[I-D.ietf-core-block]
Bormann, C. and Z. Shelby, "Block-wise transfers in CoAP",
draft-ietf-core-block-17 (work in progress), March 2015.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC6454] Barth, A., "The Web Origin Concept", RFC 6454, December
2011.
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[RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Message Syntax and Routing", RFC 7230, June
2014.
Appendix A. Examples
This section gives examples for the first two configurations
discussed in Section 1.1.
An example of the process followed by a CoAP client to retrieve the
representation of a resource identified by a "coap+ws" URI might be
as follows. Figure 8 below illustrates the WebSocket and CoAP
messages exchanged in detail.
1. The CoAP client obtains the URI <coap+ws://example.org/sensors/
temperature?u=Cel>, for example, from a resource representation
that it retrieved previously.
2. It establishes a WebSocket Connection to the endpoint URI
composed of the authority "example.org" and the well-known path
"/.well-known/coap", <ws://example.org/.well-known/coap>.
3. It sends a single-frame, masked, binary message containing a CoAP
request. The request indicates the target resource with the Uri-
Path ("sensors", "temperature") and Uri-Query ("u=Cel") options.
4. It waits for the server to return a response.
5. The CoAP client uses the connection for further requests, or the
connection is closed.
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CoAP CoAP
Client Server
(WebSocket (WebSocket
Client) Server)
| |
| |
+=========>| GET /.well-known/coap HTTP/1.1
| | Host: example.org
| | Upgrade: websocket
| | Connection: Upgrade
| | Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
| | Sec-WebSocket-Protocol: coap.v1
| | Sec-WebSocket-Version: 13
| |
|<=========+ HTTP/1.1 101 Switching Protocols
| | Upgrade: websocket
| | Connection: Upgrade
| | Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=
| | Sec-WebSocket-Protocol: coap.v1
| |
| |
+--------->| Binary frame (opcode=%x2, FIN=1, MASK=1)
| | +-------------------------+
| | | GET |
| | | Token: 0x53 |
| | | Uri-Path: "sensors" |
| | | Uri-Path: "temperature" |
| | | Uri-Query: "u=Cel" |
| | +-------------------------+
| |
|<---------+ Binary frame (opcode=%x2, FIN=1, MASK=0)
| | +-------------------------+
| | | 2.05 Content |
| | | Token: 0x53 |
| | | Payload: "22.3 Cel" |
| | +-------------------------+
: :
: :
| |
+--------->| Close frame (opcode=%x8, FIN=1, MASK=1)
| |
|<---------+ Close frame (opcode=%x8, FIN=1, MASK=0)
| |
Figure 8: A CoAP client retrieves the representation of a resource
identified by a "coap+ws" URI
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Figure 9 shows how a CoAP client uses a CoAP forward proxy with a
WebSocket endpoint to retrieve the representation of the resource
<coap://[2001:DB8::1]/>. The use of the forward proxy and the
address of the WebSocket endpoint are determined by the client from
local configuration rules. The request URI is specified in the
Proxy-Uri Option. Since the request URI uses the "coap" URI scheme,
the proxy fulfills the request by issuing a Confirmable GET request
over UDP to the CoAP server and returning the response over the
WebSocket connection to the client.
CoAP CoAP CoAP
Client Proxy Server
(WebSocket (WebSocket (UDP
Client) Server) Endpoint)
| | |
+--------->| | Binary frame (opcode=%x2, FIN=1, MASK=1)
| | | +------------------------------------+
| | | | GET |
| | | | Token: 0x7d |
| | | | Proxy-Uri: "coap://[2001:DB8::1]/" |
| | | +------------------------------------+
| | |
| +--------->| CoAP message (Ver=1, T=Con, MID=0x8f54)
| | | +------------------------------------+
| | | | GET |
| | | | Token: 0x0a15 |
| | | +------------------------------------+
| | |
| |<---------+ CoAP message (Ver=1, T=Ack, MID=0x8f54)
| | | +------------------------------------+
| | | | 2.05 Content |
| | | | Token: 0x0a15 |
| | | | Payload: "ready" |
| | | +------------------------------------+
| | |
|<---------+ | Binary frame (opcode=%x2, FIN=1, MASK=0)
| | | +------------------------------------+
| | | | 2.05 Content |
| | | | Token: 0x7d |
| | | | Payload: "ready" |
| | | +------------------------------------+
| | |
Figure 9: A CoAP client retrieves the representation of a resource
identified by a "coap" URI via a WebSockets-enabled CoAP proxy
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Authors' Addresses
Teemu Savolainen
Nokia
Hermiankatu 12 D
Tampere FI-33720
Finland
Email: teemu.savolainen@nokia.com
Klaus Hartke
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63905
Email: hartke@tzi.org
Bilhanan Silverajan
Tampere University of Technology
Korkeakoulunkatu 10
Tampere FI-33720
Finland
Email: bilhanan.silverajan@tut.fi
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