CoRE Working Group A. Castellani
Internet-Draft University of Padova
Intended status: Informational S. Loreto
Expires: January 5, 2015 Ericsson
A. Rahman
InterDigital Communications, LLC
T. Fossati
Alcatel-Lucent
E. Dijk
Philips Research
July 4, 2014
Guidelines for HTTP-CoAP Mapping Implementations
draft-ietf-core-http-mapping-04
Abstract
This draft provides reference information for HTTP-CoAP protocol
translation proxy implementation, focusing on the reverse proxy case.
It details deployment options, defines a template for URI mapping,
and provides a set of guidelines and considerations related to
protocol translation.
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 January 5, 2015.
Copyright Notice
Copyright (c) 2014 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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Cross-Protocol Usage of URIs . . . . . . . . . . . . . . . . 4
4. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 5
5. URI Mapping . . . . . . . . . . . . . . . . . . . . . . . . . 5
5.1. URI Terminology . . . . . . . . . . . . . . . . . . . . . 6
5.2. Default Mapping . . . . . . . . . . . . . . . . . . . . . 6
5.2.1. Optional scheme . . . . . . . . . . . . . . . . . . . 7
5.2.2. Encoding Caveats . . . . . . . . . . . . . . . . . . 7
5.3. URI Mapping Template . . . . . . . . . . . . . . . . . . 7
5.3.1. Simple Form . . . . . . . . . . . . . . . . . . . . . 8
5.3.2. Enhanced Form . . . . . . . . . . . . . . . . . . . . 9
5.4. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 10
5.4.1. Examples . . . . . . . . . . . . . . . . . . . . . . 11
6. HTTP-CoAP Reverse Proxy . . . . . . . . . . . . . . . . . . . 12
6.1. Proxy Placement . . . . . . . . . . . . . . . . . . . . . 13
6.2. Response Code Translations . . . . . . . . . . . . . . . 14
6.3. Media Type mapping . . . . . . . . . . . . . . . . . . . 16
6.3.1. Loose Media Type Mapping . . . . . . . . . . . . . . 18
6.3.2. Internet Media Type to Content Format Mapping
Algorithm . . . . . . . . . . . . . . . . . . . . . . 18
6.3.3. Content Transcoding . . . . . . . . . . . . . . . . . 19
6.4. Caching and Congestion Control . . . . . . . . . . . . . 20
6.5. Cache Refresh via Observe . . . . . . . . . . . . . . . . 21
6.6. Use of CoAP Blockwise Transfer . . . . . . . . . . . . . 21
6.7. Security Translation . . . . . . . . . . . . . . . . . . 22
6.8. Other guidelines . . . . . . . . . . . . . . . . . . . . 22
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 23
8. Security Considerations . . . . . . . . . . . . . . . . . . . 23
8.1. Traffic overflow . . . . . . . . . . . . . . . . . . . . 24
8.2. Handling Secured Exchanges . . . . . . . . . . . . . . . 24
8.3. URI Mapping . . . . . . . . . . . . . . . . . . . . . . . 25
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 25
10. References . . . . . . . . . . . . . . . . . . . . . . . . . 25
10.1. Normative References . . . . . . . . . . . . . . . . . . 25
10.2. Informative References . . . . . . . . . . . . . . . . . 26
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
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1. Introduction
CoAP [RFC7252] has been designed with the twofold aim to be an
application protocol specialized for constrained environments and to
be easily used in REST architectures such as the Web. The latter
goal has led to define CoAP to easily interoperate with HTTP
[RFC7230] through an intermediary proxy which performs cross-protocol
conversion.
Section 10 of [RFC7252] describes the fundamentals of the CoAP-to-
HTTP and the HTTP-to-CoAP cross-protocol mapping process. However,
implementing such a cross-protocol proxy can be complex, and many
details regarding its internal procedures and design choices require
further elaboration. Therefore a first goal of this document is to
provide more detailed information to proxy designers and
implementers, to help implement proxies that correctly inter-work
with other CoAP and HTTP client/server implementations that adhere to
the HTTP and CoAP specifications.
The second goal of this informational document is to define a
consistent set of guidelines that a HTTP-to-CoAP proxy implementation
MAY adhere to. The main reason of adhering to such guidelines is to
reduce variation between proxy implementations, thereby increasing
interoperability. (As an example use case, a proxy conforming to
these guidelines made by vendor A can be easily replaced by a proxy
from vendor B that also conforms to the guidelines.)
This draft is organized as follows:
o Section 2 describes terminology to identify proxy types, mapping
approaches and proxy deployments;
o Section 3 discusses how URIs refer to resources independent of
access protocols;
o Section 4 briefly lists use cases in which HTTP clients need to
contact CoAP servers;
o Section 5 introduces a default HTTP-to-CoAP URI mapping syntax;
o Section 6 describes the properties of the HTTP-to-CoAP reverse
proxy;
o Section 8 discusses possible security impact related to HTTP-CoAP
protocol mapping.
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2. Terminology
This document assumes readers are familiar with the terms Reverse
Proxy as defined in [RFC7230] and Interception Proxy as defined in
[RFC3040]. In addition, the following terms are defined:
HC Proxy: is a proxy performing a cross-protocol mapping, in the
context of this document a HTTP-CoAP (HC) mapping. A Cross-Protocol
Proxy can behave as a Forward Proxy, Reverse Proxy or Interception
Proxy. Note: In this document we focus on the Reverse Proxy mode of
the Cross-Protocol Proxy.
Forward Proxy: a message forwarding agent that is selected by the
client, usually via local configuration rules, to receive requests
for some type(s) of absolute URI and to attempt to satisfy those
requests via translation to the protocol indicated by the absolute
URI. The user decides (is willing to) use the proxy as the
forwarding/dereferencing agent for a predefined subset of the URI
space.
Reverse Proxy: a receiving agent that acts as a layer above some
other server(s) and translates the received requests to the
underlying server's protocol. It behaves as an origin (HTTP) server
on its connection towards the (HTTP) client and as a (CoAP) client on
its connection towards the (CoAP) origin server. The (HTTP) client
uses the "origin-form" [RFC7230] as a request-target URI.
Reverse and Forward proxies are technically very similar, with main
differences being that the former appears to a client as an origin
server while the latter does not, and that clients may be unaware
they are communicating with a proxy.
Placement terms: a server-side (SS) proxy is placed in the same
network domain as the server; conversely a client-side (CS) proxy is
in the same network domain as the client. In any other case than SS
or CS, the proxy is said to be External (E).
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119].
3. Cross-Protocol Usage of URIs
A Uniform Resource Identifier (URI) provides a simple and extensible
method for identifying a resource. It enables uniform identification
of resources via a separately defined extensible set of naming
schemes [RFC3986].
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URIs are formed of at least three components: scheme, authority and
path. The scheme often corresponds to the protocol used to access
the resource. However, as noted in Section 1.2.2 of [RFC3986] the
scheme does not imply that a particular protocol is used to access
the resource. So, we can define the same resource to be accessible
by different protocols i.e. the resource can have cross-protocol URIs
referring to it.
HTTP clients only support 'http' and 'https' schemes and cannot
directly access CoAP servers (which support 'coap' and/or 'coaps').
In this situation, communication is enabled by a HC Proxy, as shown
in Figure 1, supporting URI mapping features. Such features are
discussed in Section 5.
4. Use Cases
To illustrate in which situations HTTP to CoAP request mapping may be
used, three use cases are briefly described.
1. Smartphone and home sensor: Any smartphone can access directly a
home sensor using an authenticated 'https' request, if its home
router contains a HTTP-CoAP proxy. For this use-case an HTML5
application can be built providing a friendlier UI to the user.
2. Legacy building control application without CoAP: A building
control application that uses HTTP but not CoAP, can check the status
of sensors and/or actuators via a HTTP-CoAP proxy.
3. Making sensor data available to 3rd parties: For demonstration or
public interest purposes, a HTTP-CoAP proxy may be configured to
expose the contents of a sensor to the world via the web (HTTP and/or
HTTPS). The sensor can only handle secure 'coaps' requests,
therefore the proxy is configured to translate any request to a
'coaps' secured request. The proxy is furthermore configured to only
pass through GET requests. In this way even unattended HTTP clients,
such as web crawlers, may index sensor data as regular web pages.
5. URI Mapping
Though, in principle, a CoAP URI could be directly used by a HTTP
user agent to de-reference a CoAP resource through a HC Proxy, the
reality is that all major web browsers and command line tools do not
allow making HTTP requests using URIs with a scheme different from
"http" or "https".
Thus, there is a need for web applications to "pack" a CoAP URI into
a HTTP URI so that it can be (non-destructively) transported from the
user agent to the HC Proxy. The HC Proxy can then "unpack" the CoAP
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URI and finally de-reference it via a CoAP request to the target
Server.
URI Mapping is the process through which the URI of a CoAP resource
is transformed in to an HTTP URI so that:
o the requesting HTTP user agent can handle it;
o the receiving HC Proxy can extract the intended CoAP URI
unambiguously.
To this end, the remainder of this section will identify:
o the default mechanism to map a CoAP URI into a HTTP URI;
o the URI template format to express a class of CoAP-HTTP URI
mapping functions;
o the discovery mechanism based on [RFC6690] through which clients
of a HC Proxy can dynamically discover information about the
supported URI Mapping Template(s), as well as the base URI where
the HC Proxy function is anchored.
5.1. URI Terminology
In the remainder of this section, the following terms will be used
with a distinctive meaning:
Target CoAP URI:
URI which refers to the (final) CoAP resource that has to be
de-referenced. It conforms to syntax defined in section 6 of
[RFC7252]. Specifically, it has a scheme of "coap" or
"coaps".
Hosting HTTP URI:
URI that conforms to syntax in section 2.7 of [RFC7230]. Its
authority component refers to an HC Proxy, whereas path (and
query) component(s) embed the information used by an HC Proxy
to extract the Target CoAP URI.
5.2. Default Mapping
The default is for the Target CoAP URI to be appended as-is to a base
URI provided by the HC Proxy to form the Hosting HTTP URI.
For example: given a base URI http://p.example.com/hc and a Target
CoAP URI coap://s.example.com/light, the resulting Hosting HTTP URI
would be http://p.example.com/hc/coap://s.example.com/light.
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Provided a correct Target CoAP URI, the Hosting HTTP URI resulting
from the default mapping is always syntactically correct.
Furthermore, the Target CoAP URI can always be extracted in an
unambiguous way from the Hosting HTTP URI. Also worth noting that,
using the default mapping, a query component in the target CoAP
resource URI is naturally encoded into the query component of the
Hosting URI, e.g.: coap://s.example.com/light?dim=5 becomes
http://p.example.com/hc/coap://s.example.com/light?dim=5.
There is no default for the base URI. Therefore it is either known
in advance, e.g. as a configuration preset, or dynamically discovered
using the mechanism described in Section 5.4.
The default URI mapping function is RECOMMENDED to be implemented and
activated by default in a HC Proxy, unless there are valid reasons,
e.g. application specific, to use a different mapping function.
5.2.1. Optional scheme
When found in a Hosting HTTP URI, the scheme (i.e. "coap" or
"coaps"), the scheme component delimiter (":"), and the double slash
("//") preceding the authority MAY be omitted. In such case, a local
default - not defined by this document - applies.
So, http://p.example.com/hc/s.coap.example.com/foo could either
represent the target coap://s.coap.example.com/foo or
coaps://s.coap.example.com/foo depending on application specific
presets.
5.2.2. Encoding Caveats
When the authority of the Target CoAP URI is given as an IPv6address,
then the surrounding square brackets MUST be percent-encoded in the
Hosting HTTP URI, in order to comply with the syntax defined in
Section 3.3. of [RFC3986] for a URI path segment. E.g.:
coap://[2001:db8::1]/light?on becomes
http://p.example.com/hc/coap://%5B2001:db8::1%5D/light?on.
Everything else can be safely copied verbatim from the Target CoAP
URI to the Hosting HTTP URI.
5.3. URI Mapping Template
This section defines a format for the URI template used by a HC Proxy
to inform its clients about the expected syntax for the Hosting HTTP
URI.
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When instantiated, an URI Mapping Template is always concatenated to
a base URI provided by the HC Proxy via discovery (see Section 5.4),
or by other means.
A simple form (Section 5.3.1) and an enhanced form (Section 5.3.2)
are provided to fit different users' requirements.
Both forms are expressed as level 2 URI template's to take care of
the expansion of values that are allowed to include reserved URI
characters.
5.3.1. Simple Form
The simple form MUST be used for mappings where the Target CoAP URI
is going to be copied verbatim at some fixed position into the
Hosting HTTP URI.
The following template variables MUST be used in mutual exclusion in
a template definition:
cu = coap-URI ; from [RFC7252], Section 6.1
su = coaps-URI ; from [RFC7252], Section 6.2
tu = cu / su
The same considerations done in Section 5.2.1 apply.
5.3.1.1. Examples
All the following examples (given as a specific URI mapping template,
a Target CoAP URI, and the produced Hosting HTTP URI) use
http://p.example.com/hc as the base URI.
1. "coap" URI is a query argument of the Hosting HTTP URI:
?coap_target_uri={+cu}
coap://s.example.com/light
http://p.example.com/hc?coap_target_uri=coap://s.example.com/light
2. "coaps" URI is a query argument of the Hosting HTTP URI:
?coaps_target_uri={+su}
coaps://s.example.com/light
http://p.example.com/hc?coaps_target_uri=coaps://s.example.com/light
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3. Target CoAP URI as a query argument of the Hosting HTTP URI:
?target_uri={+tu}
coap://s.example.com/light
http://p.example.com/hc?target_uri=coap://s.example.com/light
or
coaps://s.example.com/light
http://p.example.com/hc?target_uri=coaps://s.example.com/light
4. Target CoAP URI in the path component of the Hosting HTTP URI
(i.e. the default URI Mapping template):
/{+tu}
coap://s.example.com/light
http://p.example.com/hc/coap://s.example.com/light
or
coaps://s.example.com/light
http://p.example.com/hc/coaps://s.example.com/light
5.3.2. Enhanced Form
The enhanced form can be used to express more sophisticated mappings,
i.e. those that do not fit into the simple form.
There MUST be at most one instance of each of the following template
variables in a template definition:
s = "coap" / "coaps" ; from [RFC7252], Sections 6.1 and 6.2
hp = host [":" port] ; from [RFC3986] Sections 3.2.2 and 3.2.3
p = path-abempty ; from [RFC3986] Section 3.3.
q = [ "?" query ] ; from [RFC3986] Section 3.4
5.3.2.1. Examples
All the following examples (given as a specific URI mapping template,
a Target CoAP URI, and the produced Hosting HTTP URI) use
http://p.example.com/hc as the base URI.
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1. Target CoAP URI components in path segments, and optional query
in query component:
{+s}{+hp}{+p}{+q}
coap://s.example.com/light
http://p.example.com/hc/coap/s.example.com/light
or
coap://s.example.com/light?on
http://p.example.com/hc/coap/s.example.com/light?on
2. Target CoAP URI components split in individual query arguments:
?s={+s}&hp={+hp}&p={+p}&q={+q}
coap://s.example.com/light
http://p.example.com/hc?s=coap&hp=s.example.com&p=/light&q
or
coaps://s.example.com/light?on
http://p.example.com/hc?s=coaps&hp=s.example.com&p=/light&q=on
5.4. Discovery
In order to accommodate site specific needs while allowing third
parties to discover the proxy function, the HC Proxy SHOULD publish
information related to the location and syntax of the HC Proxy
function using the CoRE Link Format [RFC6690] interface.
To this aim a new Resource Type, "core.hc", is associated with a base
URI, and can be used as the value for the "rt" attribute in a query
to the /.well-known/core in order to locate the base URI where the HC
Proxy function is anchored.
Along with it, the new target attribute "hct" MAY be returned in a
"core.hc" link to provide the associated URI Mapping Template. The
default template given in Section 5.2, i.e. {+tu}, MUST be assumed if
no "hct" attribute is found in the returned link. If an "htc"
attribute is present in the returned link, then a compliant client
MUST use it to create the Hosting HTTP URI.
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Discovery SHOULD be available on both the HTTP and the CoAP side of
the HC proxy, with one important difference: on the CoAP side the
link associated to the "core.hc" resource needs an explicit anchor
referring to the HTTP origin, while on the HTTP interface the link
context is already the HTTP origin carried in the request's Host
header, and doesn't have to be made explicit.
5.4.1. Examples
o The first example exercises the CoAP interface, and assumes that
the default template, {+tu}, is used:
Req: GET coap://[ff02::1]/.well-known/core?rt=core.hc
Res: 2.05 Content
</hc>;anchor="http://p.example.com";rt="core.hc"
o The second example - also on the CoAP side of the HC Proxy - uses
a custom template, i.e. one where the CoAP URI is carried inside
the query component, thus the returned link carries the URI
template to be used in an explicit "hct" attribute:
Req: GET coap://[ff02::1]/.well-known/core?rt=core.hc
Res: 2.05 Content
</hc>;anchor="http://p.example.com";rt="core.hc";hct="?uri={+tu}"
On the HTTP side link information can be serialised in more than one
way:
o using the 'application/link-format' content type:
Req: GET /.well-known/core?rt=core.hc HTTP/1.1
Host: p.example.com
Res: HTTP/1.1 200 OK
Content-Type: application/link-format
Content-Length: 18
</hc>;rt="core.hc"
o using the 'application/link-format+json' content type:
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Req: GET /.well-known/core?rt=core.hc HTTP/1.1
Host: p.example.com
Res: HTTP/1.1 200 OK
Content-Type: application/link-format+json
Content-Length: 31
[{"href":"/hc","rt":"core.hc"}]
o using the Link header:
Req: GET /.well-known/core?rt=core.hc HTTP/1.1
Host: p.example.com
Res: HTTP/1.1 200 OK
Link: </hc>;rt="core.hc"
o An HC Proxy may expose two different base URIs to differentiate
between Target CoAP resources in the "coap" and "coaps" scheme:
Req: GET /.well-known/core?rt=core.hc
Host: p.example.com
Res: HTTP/1.1 200 OK
Content-Type: application/link-format+json
Content-Length: 111
[
{"href":"/hc/plaintext","rt":"core.hc","htc":"{+cu}"},
{"href":"/hc/secure","rt":"core.hc","htc":"{+su}"}
]
6. HTTP-CoAP Reverse Proxy
A HTTP-CoAP Reverse Cross-Protocol Proxy is accessed by web clients
only supporting HTTP, and handles their requests by mapping these to
CoAP requests, which are forwarded to CoAP servers; and mapping back
the received CoAP responses to HTTP. This mechanism is transparent
to the client, which may assume that it is communicating with the
intended target HTTP server. In other words, the client accesses the
proxy as an origin server using the "origin-form" [RFC7230] as a
Request Target.
Normative requirements on the translation of HTTP requests to CoAP
and of the CoAP responses back to HTTP responses are defined in
Section 10.2 of [RFC7252]. However, that section only considers the
case of a HTTP-CoAP Forward Cross-Protocol Proxy in which a client
explicitly indicates it targets a request to a CoAP server, and does
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not cover all aspects of proxy implementation in detail. The present
section provides guidelines and more details for the implementation
of a Reverse Cross-Protocol Proxy, which MAY be followed in addition
to the normative requirements.
Translation of unicast HTTP requests into multicast CoAP requests is
currently out of scope since in a reverse proxy scenario a HTTP
client typically expects to receive a single response, not multiple.
However a HC Proxy MAY include custom application-specific functions
to generate a multicast CoAP request based on a unicast HTTP request
and aggregate multiple CoAP responses into a single HTTP response.
Note that the guidelines in this section also apply to an HTTP-CoAP
Intercepting Cross-Protocol Proxy.
6.1. Proxy Placement
Typically, a Cross-Protocol Proxy is located at the edge of the
constrained network. See Figure 1. The arguments supporting server-
side (SS) placement are the following:
Caching: Efficient caching requires that all request traffic to a
CoAP server is handled by the same proxy which receives HTTP
requests from multiple source locations. This maximally reduces
the load on (constrained) CoAP servers.
Multicast: To support CoAPs use of local-multicast functionality
available in a constrained network, the Cross-Protocol Proxy
requires a network interface directly attached to the constrained
network.
TCP/UDP: Translation between HTTP and CoAP requires also TCP/UDP
translation; TCP may be the preferred way for communicating with
the constrained network due to its reliability or due to
intermediate gateways configured to block UDP traffic.
Arguments against SS placement, in favor of client-side (CS), are:
Scalability: A solution where a single SS proxy has to manage
numerous open TCP/IP connections to a large number of HTTP clients
is not scalable. (Unless multiple SS proxies are employed with a
load-balancing mechanism, which adds complexity.)
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+------+
| |
| DNS |
| |
+------+ Constrained Network
--------------------
/ \
/ /-----\ /-----\ \
/ CoAP CoAP \
/ server server \
|| \-----/ \-----/ ||
+------+ HTTP Request +----------+ ||
|HTTP |------------------------>| HTTP-CoAP| Req /-----\ ||
|Client| | Cross- |------->| CoAP ||
| |<------------------------| Proxy |<-------|server ||
+------+ HTTP Response +----------+ Resp \-----/ ||
|| ||
|| /-----\ ||
|| CoAP ||
\ server /
\ \-----/ /
\ /-----\ /
\ CoAP /
\ server /
\ \-----/ /
----------------
Figure 1: Reverse Cross-Protocol Proxy Deployment Scenario
6.2. Response Code Translations
Table 1 defines all possible CoAP responses along with the HTTP
response to which each CoAP response SHOULD be translated. This
table complies with the Section 10.2 requirements of [RFC7252] and is
intended to cover all possible cases. Multiple appearances of a HTTP
status code in the second column indicates multiple equivalent HTTP
responses are possible, depending on the conditions cited in the
Notes (third column).
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+-----------------------------+-----------------------------+-------+
| CoAP Response Code | HTTP Status Code | Notes |
+-----------------------------+-----------------------------+-------+
| 2.01 Created | 201 Created | 1 |
| 2.02 Deleted | 200 OK | 2 |
| | 204 No Content | 2 |
| 2.03 Valid | 304 Not Modified | 3 |
| | 200 OK | 4 |
| 2.04 Changed | 200 OK | 2 |
| | 204 No Content | 2 |
| 2.05 Content | 200 OK | |
| 4.00 Bad Request | 400 Bad Request | |
| 4.01 Unauthorized | 400 Bad Request | 5 |
| 4.02 Bad Option | 400 Bad Request | 6 |
| 4.03 Forbidden | 403 Forbidden | |
| 4.04 Not Found | 404 Not Found | |
| 4.05 Method Not Allowed | 400 Bad Request | 7 |
| 4.06 Not Acceptable | 406 Not Acceptable | |
| 4.12 Precondition Failed | 412 Precondition Failed | |
| 4.13 Request Entity Too | 413 Request Repr. Too Large | |
| Large | | |
| 4.15 Unsupported Media Type | 415 Unsupported Media Type | |
| 5.00 Internal Server Error | 500 Internal Server Error | |
| 5.01 Not Implemented | 501 Not Implemented | |
| 5.02 Bad Gateway | 502 Bad Gateway | |
| 5.03 Service Unavailable | 503 Service Unavailable | 8 |
| 5.04 Gateway Timeout | 504 Gateway Timeout | |
| 5.05 Proxying Not Supported | 502 Bad Gateway | 9 |
+-----------------------------+-----------------------------+-------+
Table 1: HTTP-CoAP Response Mapping
Notes:
1. A CoAP server may return an arbitrary format payload along with
this response. This payload SHOULD be returned as entity in the
HTTP 201 response. Section 7.3.2 of [RFC7231] does not put any
requirement on the format of the payload. (In the past,
[RFC2616] did.)
2. The HTTP code is 200 or 204 respectively for the case that a CoAP
server returns a payload or not. [RFC7231] Section 5.3 requires
code 200 in case a representation of the action result is
returned for DELETE, POST and PUT and code 204 if not. Hence, a
proxy SHOULD transfer any CoAP payload contained in a 2.02
response to the HTTP client in a 200 OK response.
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3. A CoAP 2.03 (Valid) response only (1) confirms that the request
ETag is valid and (2) provides a new Max-Age value. HTTP 304
(Not Modified) also updates some header fields of a stored
response. A non-caching proxy may not have enough information to
fill in the required values in the HTTP 304 (Not Modified)
response, so it may not be advisable for a non-caching proxy to
provoke the 2.03 (Valid) response by forwarding an ETag. A
caching proxy will fill the information out of the cache.
4. A 200 response to a CoAP 2.03 occurs only when the proxy is
caching and translated a HTTP request (without validation
request) to a CoAP request that includes validation, for
efficiency. The proxy receiving 2.03 updates the freshness of
the cached representation and returns the entire representation
to the HTTP client.
5. The HTTP code 401 Unauthorized MUST NOT be used, as long as in
CoAP there is no equivalent defined of the required WWW-
Authenticate header (Section 3.1 of [RFC7235]).
6. In some cases a proxy receiving 4.02 may retry the request with
less CoAP Options in the hope that the server will understand the
newly formulated request. For example, if the proxy tried using
a Block Option which was not recognised by the CoAP server it may
retry without that Block Option.
7. The HTTP code "405 Method Not Allowed" MUST NOT be used since
CoAP does not provide enough information to determine a value for
the required "Allow" response-header field.
8. The value of the HTTP "Retry-After" response-header field is
taken from the value of the CoAP Max-Age Option, if present.
9. This CoAP response can only happen if the proxy itself is
configured to use a CoAP Forward Proxy to execute some, or all,
of its CoAP requests.
6.3. Media Type mapping
A HC Proxy translates HTTP media types (Section 3.1.1.1 of [RFC7231])
and content encodings (Section 3.1.2.1 of [RFC7231]) into CoAP
content formats (Section 12.3 of [RFC7252]).
Media type translation can happen in GET, PUT or POST requests going
from HTTP to CoAP, and in 2.xx (i.e. successful) responses going from
CoAP to HTTP. Specifically, PUT and POST need to map the Content-
Type and Content-Encoding HTTP headers into a CoAP Content-Format
option, whereas GET needs to map Accept and Accept-Encoding HTTP
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headers into a CoAP Accept option. On the way back, the CoAP
Content-Format option is renormalised into a suitable HTTP Content-
Type and Content-Encoding combination.
An HTTP request carrying a Content-Type and Content-Encoding
combination which the HC Proxy is unable to map to an equivalent CoAP
Content-Format, SHALL elicit a 415 (Unsupported Media Type) response
by the HC Proxy.
If the HC Proxy receives a CoAP response with a Content-Format that
it does not recognise (for example because the value has been
registered after the proxy has been deployed), then it is allowed to
either return a HTTP entity without a Content-Type header, or examine
the data to determine its type on the fly.
On the content negotiation side, failing to map Accept and Accept-
Encoding headers SHOULD be silently ignored: the HC Proxy SHOULD
therefore forward the request with no Accept option.
While the CoAP to HTTP direction has always a well defined mapping,
the HTTP to CoAP direction is more problematic because the source
set, i.e., potentially 1000+ IANA registered media types, is much
bigger than the destination set, i.e. the mere 6 values initially
defined in Section 12.3 of [RFC7252].
Depending on the tight/loose coupling with the application(s) for
which it proxies, the HC Proxy could implement different media-type
mappings.
When tightly coupled, the HC Proxy knows exactly which content
formats are supported by the applications, and can be strict when
enforcing its forwarding policies in general, and the media-type
mapping in particular.
On the other side, when the HC Proxy is a general purpose application
layer gateway, being too strict could significantly reduce the amount
of traffic that it'd be able to successfully forward. In this cases,
the "loose" media-type mapping detailed in Section 6.3.1 MAY be
implemented.
The latter grants unconstrained evolution of the surrounding
ecosystem, at the cost of allowing more attack surface. In fact, as
a result of such strategy, payloads would be forwarded more liberally
across the unconstrained/constrained boundary of the communication
path. Therefore, when applied, other forms of access control must be
set in place to avoid unauthorised users to deplete or abuse systems
and network resources.
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6.3.1. Loose Media Type Mapping
By structuring the type information in a super-class (e.g. "text")
followed by a finer grained sub-class (e.g. "html"), and optional
parameters (e.g. "charset=utf-8"), Internet media types provide a
rich and scalable framework for encoding the type of any given
entity.
This approach is not applicable to CoAP, where Content Formats
conflate an Internet media type (potentially with specific
parameters) and a content encoding into one small integer value.
To remedy this loss of flexibility, we introduce the concept of a
"loose" media type mapping, where media-types that are
specialisations of a more generic media-type can be aliased to their
super-class and then mapped (if possible) to one of CoAP content
formats. For example, "application/soap+xml" can be aliased to
"application/xml", which has a known conversion to CoAP. In the
context of this "loose" media type mapping, "application/octet-
stream" can be used as a fall back when no better alias is found for
a specific media-type.
Table 2 defines the default lookup table for the "loose" media-type
mapping. Given an input media-type, the table returns its best
generalised media-type using longest prefix match.
+---------------------+--------------------------+
| Internet media-type | Generalised media-type |
+---------------------+--------------------------+
| application/*+xml | application/xml |
| application/*+json | application/json |
| text/xml | application/xml |
| text/* | text/plain |
| */* | application/octet-stream |
+---------------------+--------------------------+
Table 2: Media type generalisation
The "loose" media-type mapping is an OPTIONAL feature.
Implementations supporting this kind of mapping SHOULD provide a
flexible way to define the set of media-type generalisations allowed.
6.3.2. Internet Media Type to Content Format Mapping Algorithm
This section defines the algorithm used to map an Internet media type
to its correspondent CoAP content format.
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The algorithm uses the mapping table defined in Section 12.3 of
[RFC7252] plus, possibly, any locally defined extension of it.
Optionally, the table and lookup mechanism described in Section 6.3.1
can be used if the implementation chooses so.
Note that the algorithm may have side effects on the associated
representation (see also Section 6.3.3).
In the following:
o C-T, C-E, and C-F stand for the values of the Content-Type (or
Accept), Content-Encoding (or Accept-Encoding) HTTP headers, and
Content-Format CoAP option respectively.
o If C-E is not given it is assumed to be "identity".
o MAP is the mandatory lookup table, GMAP is the optional
generalised table.
INPUT: C-T and C-E
OUTPUT: C-F or Fail
1. if no C-T: return Fail
2. C-F = MAP[C-T, C-E]
3. if C-F is not None: return C-F
4. if C-E is not "identity":
5. if C-E is supported (e.g. gzip):
6. decode the representation accordingly
7. set C-E to "identity"
8. else:
9. return Fail
10. repeat steps 2. and 3.
11. if C-T allows a non-lossy transformation into \
12. one of the supported C-F:
13. transcode the representation accordingly
14. return C-F
15. if GMAP is defined:
16. C-F = GMAP[C-T]
17. if C-F is not None: return C-F
18. return Fail
Figure 2
6.3.3. Content Transcoding
As noted in Section 6.3.2, the process of mapping the type of the
resource can have side effects on the forwarded entity body.
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This may be caused by the removal or addition of a specific content
encoding, or because the HC Proxy decides to transcode the
representation to a different (compatible) format. The latter proves
useful when an optimised version of a specific format exists. For
example an XML-encoded resource could be transcoded to EXI, or a
JSON-encoded resource into CBOR [RFC7049], effectively achieving
compression without losing any information.
Payload transcoding (see steps 11-14 of Figure 2) is an OPTIONAL
feature. Implementations supporting this feature SHOULD provide a
flexible way to define the set of transcodings allowed.
6.4. Caching and Congestion Control
A HC Proxy SHOULD limit the number of requests to CoAP servers by
responding, where applicable, with a cached representation of the
resource.
Duplicate idempotent pending requests by a HC Proxy to the same CoAP
resource SHOULD in general be avoided, by duplexing the response to
the requesting HTTP clients without duplicating the CoAP request.
If the HTTP client times out and drops the HTTP session to the HC
Proxy (closing the TCP connection) after the HTTP request was made, a
HC Proxy SHOULD wait for the associated CoAP response and cache it if
possible. Further requests to the HC Proxy for the same resource can
use the result present in cache, or, if a response has still to come,
the HTTP requests will wait on the open CoAP session.
According to [RFC7252], a proxy MUST limit the number of outstanding
interactions to a given CoAP server to NSTART. To limit the amount
of aggregate traffic to a constrained network, the HC Proxy SHOULD
also pose a limit to the number of concurrent CoAP requests pending
on the same constrained network; further incoming requests MAY either
be queued or dropped (returning 503 Service Unavailable). This limit
and the proxy queueing/dropping behavior SHOULD be configurable. In
order to efficiently apply this congestion control, the HC Proxy
SHOULD be SS placed.
Resources experiencing a high access rate coupled with high
volatility MAY be observed [I-D.ietf-core-observe] by the HC Proxy to
keep their cached representation fresh while minimizing the number
CoAP messages. See Section 6.5.
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6.5. Cache Refresh via Observe
There are cases where using the CoAP observe protocol
[I-D.ietf-core-observe] to handle proxy cache refresh is preferable
to the validation mechanism based on ETag as defined in [RFC7252].
Such scenarios include, but are not limited to, sleepy nodes -- with
possibly high variance in requests' distribution -- which would
greatly benefit from a server driven cache update mechanism. Ideal
candidates would also be crowded or very low throughput networks,
where reduction of the total number of exchanged messages is an
important requirement.
This subsection aims at providing a practical evaluation method to
decide whether the refresh of a cached resource R is more efficiently
handled via ETag validation or by establishing an observation on R.
Let T_R be the mean time between two client requests to resource R,
let F_R be the freshness lifetime of R representation, and let M_R be
the total number of messages exchanged towards resource R. If we
assume that the initial cost for establishing the observation is
negligible, an observation on R reduces M_R iff T_R < 2*F_R with
respect to using ETag validation, that is iff the mean arrival time
of requests for resource R is greater than half the refresh rate of
R.
When using observations M_R is always upper bounded by 2*F_R: in the
constrained network no more than 2*F_R messages will be generated
towards resource R.
6.6. Use of CoAP Blockwise Transfer
A HC Proxy SHOULD support CoAP blockwise transfers
[I-D.ietf-core-block] to allow transport of large CoAP payloads while
avoiding excessive link-layer fragmentation in LLNs, and to cope with
small datagram buffers in CoAP end-points as described in [RFC7252]
Section 4.6.
A HC Proxy SHOULD attempt to retry a payload-carrying CoAP PUT or
POST request with blockwise transfer if the destination CoAP server
responded with 4.13 (Request Entity Too Large) to the original
request. A HC Proxy SHOULD attempt to use blockwise transfer when
sending a CoAP PUT or POST request message that is larger than a
value BLOCKWISE_THRESHOLD. The value of BLOCKWISE_THRESHOLD MAY be
implementation-specific, for example calculated based on a known or
typical UDP datagram buffer size for CoAP end-points, or set to N
times the size of a link-layer frame where e.g. N=5, or preset to a
known IP MTU value, or set to a known Path MTU value. The value
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BLOCKWISE_THRESHOLD or parameters from which it is calculated SHOULD
be configurable in a proxy implementation.
The HC Proxy SHOULD detect CoAP end-points not supporting blockwise
transfers by checking for a 4.02 (Bad Option) response returned by an
end-point in response to a CoAP request with a Block* Option. This
allows the HC Proxy to be more efficient, not attempting repeated
blockwise transfers to CoAP servers that do not support it. However
if a request payload is too large to be sent as a single CoAP request
and blockwise transfer would be unavoidable, the proxy still SHOULD
attempt blockwise transfer on such an end-point before returning 413
(Request Entity Too Large) to the HTTP client.
For improved latency a HC Proxy MAY initiate a blockwise CoAP request
triggered by an incoming HTTP request even when the HTTP request
message has not yet been fully received, but enough data has been
received to send one or more data blocks to a CoAP server already.
This is particularly useful on slow client-to-proxy connections.
6.7. Security Translation
A HC proxy SHOULD implement explicit rules for security context
translations. A translation may involve e.g. applying a rule that
any "https" request is translated to a "coaps" request, or e.g.
applying a rule that a "https" request is translated to an unsecured
"coap" request. Another rule could specify the security policy and
parameters used for DTLS connections. Such rules will largely depend
on the application and network context in which a proxy is applied.
To enable widest possible use of a proxy implementation, these rules
SHOULD be configurable in a HC proxy.
If a policy for access to 'coaps' URIs is configurable in a HC proxy,
it is RECOMMENDED that the policy is by default configured to
disallow access to any 'coaps' URI by a HTTP client using an
unsecured (non-TLS) connection. Naturally, a user MAY reconfigure
the policy to allow such access in specific cases.
6.8. Other guidelines
For long delays of a CoAP server, the HTTP client or any other proxy
in between MAY timeout. Further discussion of timeouts in HTTP is
available in Section 6.2.4 of [RFC7230].
A HC Proxy MUST define an internal timeout for each pending CoAP
request, because the CoAP server may silently die before completing
the request. The timeout value SHOULD be approximately less than or
equal to MAX_RTT defined in [RFC7252].
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When the DNS protocol is not used between CoAP nodes in a constrained
network, defining valid FQDN (i.e., DNS entries) for constrained CoAP
servers, where possible, MAY help HTTP clients to access the
resources offered by these servers via a HC proxy.
HTTP connection pipelining (section 6.2.2.1 of [RFC7230]) MAY be
supported by the proxy and is transparent to the CoAP network: the HC
Proxy will sequentially serve the pipelined requests by issuing
different CoAP requests.
It is expected that the HC function will often be implemented in
software on the proxy. Many different software approaches are
possible, including using CGI [RFC3875] as an interface between the
HTTP layer and the protocol translation engine.
7. IANA Considerations
This memo includes no request to IANA.
8. Security Considerations
The security concerns raised in Section 15.7 of [RFC2616] also apply
to the HC Proxy scenario. In fact, the HC Proxy is a trusted (not
rarely a transparently trusted) component in the network path.
The trustworthiness assumption on the HC Proxy cannot be dropped.
Even if we had a blind, bi-directional, end-to-end, tunneling
facility like the one provided by the CONNECT method in HTTP, and
also assuming the existence of a DTLS-TLS transparent mapping, the
two tunneled ends should be speaking the same application protocol,
which is not the case. Basically, the protocol translation function
is a core duty of the HC Proxy that can't be removed, and makes it a
necessarily trusted, impossible to bypass, component in the
communication path.
A reverse proxy deployed at the boundary of a constrained network is
an easy single point of failure for reducing availability. As such,
a special care should be taken in designing, developing and operating
it, keeping in mind that, in most cases, it could have fewer
limitations than the constrained devices it is serving.
The following sub paragraphs categorize and argue about a set of
specific security issues related to the translation, caching and
forwarding functionality exposed by a HC Proxy module.
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8.1. Traffic overflow
Due to the typically constrained nature of CoAP nodes, particular
attention SHOULD be posed in the implementation of traffic reduction
mechanisms (see Section 6.4), because inefficient implementations can
be targeted by unconstrained Internet attackers. Bandwidth or
complexity involved in such attacks is very low.
An amplification attack to the constrained network may be triggered
by a multicast request generated by a single HTTP request mapped to a
CoAP multicast resource, as considered in Section TBD of [RFC7252].
The impact of this amplification technique is higher than an
amplification attack carried out by a malicious constrained device
(e.g. ICMPv6 flooding, like Packet Too Big, or Parameter Problem on
a multicast destination [RFC4732]), since it does not require direct
access to the constrained network.
The feasibility of this attack, disruptive in terms of CoAP server
availability, can be limited by access controlling the exposed HTTP
multicast resource, so that only known/authorized users access such
URIs.
8.2. Handling Secured Exchanges
It is possible that the request from the client to the HC Proxy is
sent over a secured connection. However, there may or may not exist
a secure connection mapping to the other protocol. For example, a
secure distribution method for multicast traffic is complex and MAY
not be implemented (see [I-D.ietf-core-groupcomm]).
By default, a HC Proxy SHOULD reject any secured client request if
there is no configured security policy mapping. This recommendation
MAY be relaxed in case the destination network is believed to be
secured by other, complementary, means. E.g.: assumed that CoAP
nodes are isolated behind a firewall (e.g. as the SS HC proxy
deployment shown in Figure 1), the HC Proxy may be configured to
translate the incoming HTTPS request using plain CoAP (i.e. NoSec
mode.)
The HC URI mapping MUST NOT map to HTTP (see Section 5) a CoAP
resource intended to be accessed only using HTTPS.
A secured connection that is terminated at the HC Proxy, i.e. the
proxy decrypts secured data locally, raises an ambiguity about the
cacheability of the requested resource. The HC Proxy SHOULD NOT
cache any secured content to avoid any leak of secured information.
However in some specific scenario, a security/efficiency trade-off
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could motivate caching secured information; in that case the caching
behavior MAY be tuned to some extent on a per-resource basis.
8.3. URI Mapping
The following risks related to the URI mapping described in Section 5
have been identified:
DoS attack on the internal network.
Default deny any Target CoAP URI whose authority is (or maps to) a
multicast address. Then explicitly whitelist multicast resources
that are allowed to be de-referenced.
Leaking information on the internal network resources and topology.
Default deny any Target CoAP URI (especially /.well-known/core is
the resource to be protected), and then explicit whitelist
resources that are allowed to be seen from outside.
Reduced privacy due to the mechanics of the URI mapping.
The internal CoAP Target resource is totally transparent from
outside: an HC Proxy implementing a HTTPS-only interface makes the
Target CoAP URI totally opaque to a passive attacker.
9. Acknowledgements
An initial version of the table found in Section 6.2 has been
provided in revision -05 of [RFC7252]. Special thanks to Peter van
der Stok for countless comments and discussions on this document,
that contributed to its current structure and text.
Thanks to Carsten Bormann, Zach Shelby, Michele Rossi, Nicola Bui,
Michele Zorzi, Klaus Hartke, Cullen Jennings, Kepeng Li, Brian Frank,
Peter Saint-Andre, Kerry Lynn, Linyi Tian, Dorothy Gellert, Francesco
Corazza for helpful comments and discussions that have shaped the
document.
The research leading to these results has received funding from the
European Community's Seventh Framework Programme [FP7/2007-2013]
under grant agreement n. [251557].
10. References
10.1. Normative References
[I-D.ietf-core-block]
Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
draft-ietf-core-block-12 (work in progress), June 2013.
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[I-D.ietf-core-observe]
Hartke, K., "Observing Resources in CoAP", draft-ietf-
core-observe-14 (work in progress), June 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.
[RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570, March 2012.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, August 2012.
[RFC7230] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Message Syntax and Routing", RFC 7230, June
2014.
[RFC7231] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Semantics and Content", RFC 7231, June 2014.
[RFC7235] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Authentication", RFC 7235, June 2014.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252, June 2014.
10.2. Informative References
[I-D.ietf-core-groupcomm]
Rahman, A. and E. Dijk, "Group Communication for CoAP",
draft-ietf-core-groupcomm-19 (work in progress), June
2014.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3040] Cooper, I., Melve, I., and G. Tomlinson, "Internet Web
Replication and Caching Taxonomy", RFC 3040, January 2001.
[RFC3875] Robinson, D. and K. Coar, "The Common Gateway Interface
(CGI) Version 1.1", RFC 3875, October 2004.
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[RFC4732] Handley, M., Rescorla, E., and IAB, "Internet Denial-of-
Service Considerations", RFC 4732, December 2006.
[RFC7049] Bormann, C. and P. Hoffman, "Concise Binary Object
Representation (CBOR)", RFC 7049, October 2013.
Appendix A. Change Log
[Note to RFC Editor: Please remove this section before publication.]
Changes from ietf-03 to ietf-04:
o Expanded use case descriptions in Section 4;
o Fixed/enhanced discovery examples in Section 5.4.1;
o Addressed Ticket #365 (Add text on media-type conversion by HTTP-
CoAP proxy) in new section 6.3.1 (Generalized media-type mapping)
and new section 6.3.2 (Content translation);
o Updated HTTPBis WG draft references to recently published RFC
numbers.
o Various editorial improvements.
Changes from ietf-02 to ietf-03:
o Closed Ticket #351 "Add security implications of proposed default
HC URI mapping";
o Closed Ticket #363 "Remove CoAP scheme in default HTTP-CoAP URI
mapping";
o Closed Ticket #364 "Add discovery of HTTP-CoAP mapping
resource(s)".
Changes from ietf-01 to ietf-02:
o Selection of single default URI mapping proposal as proposed to WG
mailing list 2013-10-09.
Changes from ietf-00 to ietf-01:
o Added URI mapping proposals to Section 4 as per the Email
proposals to WG mailing list from Esko.
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Authors' Addresses
Angelo P. Castellani
University of Padova
Via Gradenigo 6/B
Padova 35131
Italy
Email: angelo@castellani.net
Salvatore Loreto
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: salvatore.loreto@ericsson.com
Akbar Rahman
InterDigital Communications, LLC
1000 Sherbrooke Street West
Montreal H3A 3G4
Canada
Phone: +1 514 585 0761
Email: Akbar.Rahman@InterDigital.com
Thomas Fossati
Alcatel-Lucent
3 Ely Road
Milton, Cambridge CB24 6DD
UK
Email: thomas.fossati@alcatel-lucent.com
Esko Dijk
Philips Research
High Tech Campus 34
Eindhoven 5656 AE
The Netherlands
Email: esko.dijk@philips.com
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