CoRE Working Group A. Castellani
Internet-Draft University of Padova
Intended status: Informational S. Loreto
Expires: October 8, 2016 Ericsson
A. Rahman
InterDigital Communications, LLC
T. Fossati
Alcatel-Lucent
E. Dijk
Philips Research
April 6, 2016
Guidelines for HTTP-CoAP Mapping Implementations
draft-ietf-core-http-mapping-09
Abstract
This document provides reference information for implementing a proxy
that performs translation between the HTTP protocol and the CoAP
protocol, focusing on the reverse proxy case. It describes how a
HTTP request is mapped to a CoAP request and how a CoAP response is
mapped back to a HTTP response. Furthermore, it defines a template
for URI mapping and provides a set of guidelines for HTTP to CoAP
protocol translation and related proxy implementations.
Status of This Memo
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provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on October 8, 2016.
Copyright Notice
Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Reverse HTTP-CoAP Proxy . . . . . . . . . . . . . . . . . . . 5
4. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5. URI Mapping . . . . . . . . . . . . . . . . . . . . . . . . . 7
5.1. URI Terminology . . . . . . . . . . . . . . . . . . . . . 8
5.2. Default Mapping . . . . . . . . . . . . . . . . . . . . . 8
5.2.1. Optional Scheme Omission . . . . . . . . . . . . . . 9
5.2.2. Encoding Caveats . . . . . . . . . . . . . . . . . . 9
5.3. URI Mapping Template . . . . . . . . . . . . . . . . . . 9
5.3.1. Simple Form . . . . . . . . . . . . . . . . . . . . . 10
5.3.2. Enhanced Form . . . . . . . . . . . . . . . . . . . . 11
5.4. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 13
5.4.1. Examples . . . . . . . . . . . . . . . . . . . . . . 13
6. Media Type Mapping . . . . . . . . . . . . . . . . . . . . . 15
6.1. Overview . . . . . . . . . . . . . . . . . . . . . . . . 15
6.2. 'application/coap-payload' Media Type . . . . . . . . . . 17
6.3. Loose Media Type Mapping . . . . . . . . . . . . . . . . 17
6.4. Media Type to Content Format Mapping Algorithm . . . . . 18
6.5. Content Transcoding . . . . . . . . . . . . . . . . . . . 19
6.5.1. General . . . . . . . . . . . . . . . . . . . . . . . 19
6.5.2. CoRE Link Format . . . . . . . . . . . . . . . . . . 20
6.5.3. Diagnostic Messages . . . . . . . . . . . . . . . . . 20
7. Response Code Mapping . . . . . . . . . . . . . . . . . . . . 20
8. Additional Mapping Guidelines . . . . . . . . . . . . . . . . 23
8.1. Caching and Congestion Control . . . . . . . . . . . . . 23
8.2. Cache Refresh via Observe . . . . . . . . . . . . . . . . 23
8.3. Use of CoAP Blockwise Transfer . . . . . . . . . . . . . 24
8.4. CoAP Multicast . . . . . . . . . . . . . . . . . . . . . 25
8.5. Timeouts . . . . . . . . . . . . . . . . . . . . . . . . 25
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 25
9.1. New 'core.hc' Resource Type . . . . . . . . . . . . . . . 25
9.2. New 'coap-payload' Internet Media Type . . . . . . . . . 26
10. Security Considerations . . . . . . . . . . . . . . . . . . . 27
10.1. Traffic Overflow . . . . . . . . . . . . . . . . . . . . 28
10.2. Handling Secured Exchanges . . . . . . . . . . . . . . . 28
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10.3. URI Mapping . . . . . . . . . . . . . . . . . . . . . . 29
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 29
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 30
12.1. Normative References . . . . . . . . . . . . . . . . . . 30
12.2. Informative References . . . . . . . . . . . . . . . . . 31
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . 31
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 34
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 Representational State Transfer (REST) based
architectures such as the Web. The latter goal has led to defining
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,
[RFC7252] focuses primarily on specifying the forward proxy scenario,
and leaves many aspects of the reverse proxy scenario for future
definition. Therefore, a primary goal of this informational document
is to define a consistent set of guidelines that an HTTP-to-CoAP
reverse proxy implementation should adhere to. The main reason for
adhering to such guidelines is to reduce variation between proxy
implementations, thereby increasing interoperability between an HTTP
endpoint and a CoAP endpoint independent of the reverse proxy that
implements the cross-protocol mapping. (For example, a reverse proxy
conforming to these guidelines made by vendor A can be easily
replaced by a reverse proxy from vendor B that also conforms to the
guidelines.)
This document is organized as follows:
o Section 2 describes terminology to identify proxy types, mapping
approaches and proxy deployments;
o Section 3 introduces the reverse HTTP-CoAP proxy;
o Section 4 lists use cases in which HTTP clients need to contact
CoAP servers;
o Section 5 introduces a default and advanced HTTP-to-CoAP URI
mapping syntax;
o Section 6 describes how to map HTTP media types to CoAP content
formats and vice versa;
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o Section 7 describes how to map CoAP responses to HTTP responses;
o Section 8 describes additional mapping guidelines related to
caching, congestion, timeouts and CoAP blockwise
[I-D.ietf-core-block] transfers;
o Section 10 discusses possible security impact of HTTP-CoAP
protocol mapping.
2. Terminology
The keywords "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].
HC Proxy: a proxy performing a cross-protocol mapping, in the context
of this document a HTTP-CoAP mapping. A Cross-Protocol Proxy can
behave as a Forward Proxy, Reverse Proxy or Interception Proxy. In
this document we focus on the Reverse Proxy case.
Forward Proxy (or Forward HC 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/de-referencing agent for a predefined subset of the
URI space. In [RFC7230] this is called a Proxy. [RFC7252] defines
Forward-Proxy similarly.
Reverse Proxy(or Reverse HC Proxy): as in [RFC7230], a receiving
agent that acts as a layer above some other server(s) and translates
the received requests to the underlying server's protocol. A Reverse
HC Proxy 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"
(Section 5.3.1 of [RFC7230]) as a request-target URI.
Interception Proxy (or Interception HC Proxy) [RFC3040]: a proxy that
receives inbound traffic flows through the process of traffic
redirection; transparent to the client.
Placement terms: a Server-Side proxy is placed in the same network
domain as the server; conversely a Client-Side proxy is placed in the
same network domain as the client. In any other case, the proxy is
said to be External.
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Note that a Reverse Proxy appears to a client as an origin server
while a Forward Proxy does not, so, when communicating with a Reverse
Proxy a client may be unaware it is communicating with a proxy at
all.
3. Reverse HTTP-CoAP Proxy
A reverse HC proxy is accessed by clients only supporting HTTP, and
handles their HTTP requests by mapping these to CoAP requests, which
are forwarded to CoAP servers; mapping back received CoAP responses
to HTTP responses. 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" (Section 5.3.1 of [RFC7230]) as
a request target.
See Figure 1 for an example deployment scenario. Here a reverse HC
proxy is placed server-side, at the boundary of the Constrained
Network domain, to avoid sending any HTTP traffic into the
Constrained Network and to avoid any (unsecured) CoAP multicast
traffic outside the Constrained Network. A DNS server (not shown) is
used by the HTTP Client to resolve the IP address of the reverse HC
proxy and optionally also used by the reverse HC proxy to resolve IP
addresses of CoAP servers.
Constrained Network
.-------------------.
/ .------. \
/ | CoAP | \
/ |server| \
|| '------' ||
|| ||
.--------. HTTP Request .-----------. CoAP Req .------. ||
| HTTP |----------------->| HTTP-CoAP |----------->| CoAP | ||
| Client |<-----------------| Proxy |<-----------|Server| ||
'--------' HTTP Response '-----------' CoAP Resp '------' ||
|| ||
|| .------. ||
|| | CoAP | ||
\ |server| .------. /
\ '------' | CoAP | /
\ |server| /
\ '------' /
'-----------------'
Figure 1: Reverse Cross-Protocol Proxy Deployment Scenario
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Other placement options for the reverse HC proxy are client-side (not
shown), which is in the same domain as the HTTP Client; or external,
which is both outside the HTTP Client's domain and the CoAP servers'
domain.
Normative requirements on the translation of HTTP requests to CoAP
requests and of the CoAP responses back to HTTP responses are defined
in Section 10.2 of [RFC7252]. However, [RFC7252] only considers the
case of a Forward HC Proxy in which a client explicitly indicates it
targets a request to a CoAP server. This document provides
guidelines and more details for the implementation of a Reverse HC
Proxy, which should be followed in addition to the normative
requirements. Note that most of the guidelines also apply to an
Intercepting HC Proxy.
4. Use Cases
To illustrate in which situations HTTP to CoAP protocol translation
may be used, three use cases are described below.
1. Legacy building control application without CoAP: A building
control application that uses HTTP but not CoAP, can check the status
of CoAP sensors and/or actuators via a reverse HC proxy.
2. Making sensor data available to 3rd parties on the Web: For
demonstration or public interest purposes, a reverse HC proxy may be
configured to expose the contents of a CoAP sensor to the world via
the web (HTTP and/or HTTPS). Some sensors might only handle secure
'coaps' requests, therefore the proxy is configured to translate any
request to a 'coaps' secured request. The reverse HC proxy is
furthermore configured to only pass through GET requests in order to
protect the constrained network. In this way even unattended HTTP
clients, such as web crawlers, may index sensor data as regular web
pages.
3. Smartphone and home sensor: A smartphone can access directly a
CoAP home sensor using an authenticated 'https' request, if its home
router contains a reverse HC proxy. An HTML5 application on the
smartphone can provide a friendly UI to the user using standard
(HTTP) networking functions of HTML5.
A key point in the above use cases is the expected nature of the URI
to be used by the HTTP client initiating the HTTP request to the
reverse HC proxy. Specifically, in use case #1, there will be no
"coap" or "coaps" related information embedded in the HTTP URI as it
is a legacy HTTP client sending the request. So, the HTTP request
will follow the processing steps described in all later sections of
this document except for the one defined in section Section 5 (i.e.,
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related to embedded "coap" or "coaps" URI processing). Use case #2
is also expected to be similar.
In contrast, in use case #3, it is expected that the HTTP client will
specifically embed "coap" or "coaps" related information in the HTTP
URI of the HTTP request to the reverse HC proxy. In this case, the
HTTP request will follow the processing steps described in all later
sections of this document including the one defined in section
Section 5 (i.e., related to embedded "coap" or "coaps" URI
processing).
5. URI Mapping
Though, in principle, a CoAP URI could be directly used by a HTTP
client to de-reference a CoAP resource through a reverse HC proxy,
the reality is that all major web browsers, networking libraries and
command line tools do not allow making HTTP requests using URIs with
a scheme "coap" or "coaps".
Thus, there is a need for web applications to embed or "pack" a CoAP
URI into a HTTP URI so that it can be (non-destructively) transported
from the HTTP client to the reverse HC proxy. The reverse HC proxy
can then "unpack" the CoAP 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 into an HTTP URI so that:
o the requesting HTTP client can handle it;
o the receiving reverse 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 CoRE Link Format [RFC6690]
through which clients of a reverse HC proxy can dynamically
discover information about the supported URI Mapping Template(s),
as well as the URI where the reverse HC proxy function is
anchored.
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5.1. URI Terminology
In the remainder of this section, the following terms will be used
with a distinctive meaning:
HC Proxy URI:
URI which refers to the reverse HC proxy function. It
conforms to syntax defined in Section 2.7 of [RFC7230].
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, its scheme is either "coap" or
"coaps".
Hosting HTTP URI:
URI that conforms to syntax in Section 2.7 of [RFC7230]. Its
authority component refers to a reverse HC proxy, whereas
path (and query) component(s) embed the information used by a
reverse HC proxy to extract the Target CoAP URI.
5.2. Default Mapping
The default mapping is for the Target CoAP URI to be appended as-is
to the HC Proxy URI, to form the Hosting HTTP URI. This is the URI
that will then be sent by the HTTP client in the HTTP request to the
reverse HC proxy.
For example: given a HC Proxy 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.
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
unambiguously from the Hosting HTTP URI. Also, it is 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 HC Proxy 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 SHOULD be implemented and activated
by default in a reverse HC proxy, unless there are valid reasons,
e.g. application specific, to use a different mapping function.
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5.2.1. Optional Scheme Omission
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 [RFC6570] used by
a reverse HC proxy to inform its clients about the expected syntax
for the Hosting HTTP URI. This will then be used by the HTTP client
to construct the URI to be sent in the HTTP request to the reverse HC
proxy.
When instantiated, an URI Mapping Template is always concatenated to
a HC Proxy URI provided by the reverse 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 templates [RFC6570] to take
care of the expansion of values that are allowed to include reserved
URI characters. The syntax of all URI formats is specified in this
section in Augmented Backus-Naur Form (ABNF) [RFC5234].
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5.3.1. Simple Form
The simple form MUST be used for mappings where the Target CoAP URI
is going to be copied (using rules of Section 5.2.2) 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 as in Section 5.2.1 apply, in that the CoAP
scheme may be omitted from the Hosting HTTP URI.
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 HC Proxy URI. Note that these
examples all define mapping templates that deviate from the default
template of Section 5.2 to be able to illustrate the use of the above
template variables.
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
3. Target CoAP URI as a query argument of the Hosting HTTP URI:
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?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. "coap" URI is a query argument of the Hosting HTTP URI; client
decides to omit scheme because a default scheme is agreed
beforehand between client and proxy:
?coap_uri={+cu}
coap://s.example.com/light
http://p.example.com/hc?coap_uri=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.
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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
qq = [ "?" query ] ; qq is empty iff 'query' is empty
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 HC Proxy URI.
1. Target CoAP URI components in path segments, and optional query
in query component:
{+s}{+hp}{+p}{+qq}
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:
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?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 reverse HC proxy SHOULD
publish information related to the location and syntax of the reverse
HC proxy function using the CoRE Link Format [RFC6690] interface.
To this aim a new Resource Type, "core.hc", is defined in this
document. It can be used as the value for the "rt" attribute in a
query to the /.well-known/core in order to locate the URI where the
reverse HC proxy function is anchored, i.e. the HC Proxy URI.
Along with it, the new target attribute "hct" is defined in this
document. This attribute MAY be returned in a "core.hc" link to
provide the URI Mapping Template associated to the mapping resource.
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 a
"hct" attribute is present in the returned link, then a client MUST
use it to create the Hosting HTTP URI.
The URI mapping SHOULD be discoverable (as specified in [RFC6690]) on
both the HTTP and the CoAP side of the reverse 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. For example, in use case #3
in section Section 4, the smartphone may discover the public
reverse HC proxy before leaving the home network. Then when
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outside the home network, the smartphone will be able to query the
appropriate home sensor.
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 reverse 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 serialized 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 as defined
in [I-D.ietf-core-links-json]:
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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 A reverse HC proxy may expose two different HC Proxy 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","hct":"{+cu}"},
{"href":"/hc/secure","rt":"core.hc","hct":"{+su}"}
]
6. Media Type Mapping
6.1. Overview
A reverse HC proxy needs to translate HTTP media types
(Section 3.1.1.1 of [RFC7231]) and content encodings (Section 3.1.2.2
of [RFC7231]) into CoAP content formats (Section 12.3 of [RFC7252])
and vice versa.
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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 both the
Content-Type and Content-Encoding HTTP headers into a single CoAP
Content-Format option, whereas GET needs to map Accept and Accept-
Encoding HTTP headers into a single CoAP Accept option. To generate
the HTTP response, the CoAP Content-Format option is mapped back to a
suitable HTTP Content-Type and Content-Encoding combination.
An HTTP request carrying a Content-Type and Content-Encoding
combination which the reverse HC proxy is unable to map to an
equivalent CoAP Content-Format, SHALL elicit a 415 (Unsupported Media
Type) response by the reverse HC proxy.
On the content negotiation side, failure to map Accept and Accept-*
headers SHOULD be silently ignored: the reverse HC proxy SHOULD
therefore forward as a CoAP request with no Accept option. The
reverse HC proxy thus disregards the Accept/Accept-* header fields by
treating the response as if it is not subject to content negotiation,
as mentioned in Sections 5.3.* of [RFC7231]. However, a reverse HC
proxy implementation is free to attempt mapping a single Accept
header in a GET request to multiple CoAP GET requests, each with a
single Accept option, which are then tried in sequence until one
succeeds. Note that an HTTP Accept */* MUST be mapped to a CoAP
request without Accept option.
While the CoAP to HTTP direction has always a well defined mapping
(with the exception examined in Section 6.2), 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 reverse HC proxy could implement different
media type mappings.
When tightly coupled, the reverse 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 reverse 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 case, the "loose" media type mapping detailed in
Section 6.3 MAY be implemented.
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The latter grants more 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 network boundary of the communication path.
Therefore, when applied, other forms of access control must be set in
place to avoid unauthorized users to deplete or abuse systems and
network resources.
6.2. 'application/coap-payload' Media Type
If the reverse HC proxy receives a CoAP response with a Content-
Format that it does not recognize (e.g. because the value has been
registered after the proxy has been deployed, or the CoAP server uses
an experimental value which is not registered), then the reverse HC
proxy SHALL return a generic "application/coap-payload" media type
with numeric parameter "cf" as defined in Section 9.2.
For example, the CoAP content format '60' ("application/cbor") would
be represented by "application/coap-payload;cf=60", if the reverse HC
Proxy doesn't recognize the content format '60'.
A HTTP client may use the media type "application/coap-payload" as a
means to send a specific content format to a CoAP server via a
reverse HC Proxy if the client has determined that the reverse HC
Proxy does not directly support the type mapping it needs. This case
may happen when dealing for example with newly registered, yet to be
registered, or experimental CoAP content formats.
6.3. 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
specializations of a more generic media type can be aliased to their
super-class and then mapped (if possible) to one of the 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-
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stream" can be used as a fallback when no better alias is found for a
specific media type.
Table 1 defines the default lookup table for the "loose" media type
mapping. Given an input media type, the table returns its best
generalized media type using the most specific match i.e. the table
entries are compared to the input in top to bottom order until an
entry matches.
+---------------------+--------------------------+
| Internet media type | Generalized media type |
+---------------------+--------------------------+
| application/*+xml | application/xml |
| application/*+json | application/json |
| text/xml | application/xml |
| text/* | text/plain |
| */* | application/octet-stream |
+---------------------+--------------------------+
Table 1: Media type generalization lookup table
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 generalizations allowed.
6.4. Media Type to Content Format Mapping Algorithm
This section defines the algorithm used to map an HTTP Internet media
type to its correspondent CoAP content format.
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
can be used if the implementation chooses so.
Note that the algorithm may have side effects on the associated
representation (see also Section 6.5).
In the following:
o C-T, C-E, and C-F stand for the values of the Content-Type (or
Accept) HTTP header, Content-Encoding (or Accept-Encoding) HTTP
header, 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
generalized table.
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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
6.5. Content Transcoding
6.5.1. General
Payload content transcoding (e.g. 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.
As noted in Section 6.4, the process of mapping the media type can
have side effects on the forwarded entity body. This may be caused
by the removal or addition of a specific content encoding, or because
the reverse HC proxy decides to transcode the representation to a
different (compatible) format. The latter proves useful when an
optimized version of a specific format exists. For example an XML-
encoded resource could be transcoded to Efficient XML Interchange
(EXI) format, or a JSON-encoded resource into CBOR [RFC7049],
effectively achieving compression without losing any information.
However, it should be noted that in certain cases, transcoding can
lose information in a non-obvious manner. For example, encoding an
XML document using schema-informed EXI encoding leads to a loss of
information when the destination does not know the exact schema
version used by the encoder, which means that whenever the reverse HC
proxy transcodes an application/XML to application/EXI in-band
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metadata could be lost. Therefore, the implementer should always
carefully verify such lossy payload transformations before triggering
the transcoding.
6.5.2. CoRE Link Format
The CoRE Link Format [RFC6690] is a set of links (i.e., URIs and
their formal relationships) which is carried as content payload in a
CoAP response. These links usually include CoAP URIs that might be
translated by the reverse HC proxy to the correspondent HTTP URIs
using the implemented URI mapping function (see Section 5). Such a
process would inspect the forwarded traffic and attempt to re-write
the body of resources with an application/link-format media type,
mapping the embedded CoAP URIs to their HTTP counterparts. Some
potential issues with this approach are:
1. The client may be interested to retrieve original (unaltered)
CoAP payloads through the reverse HC proxy, not modified
versions.
2. Tampering with payloads is incompatible with resources that are
integrity protected (although this is a problem with transcoding
in general).
3. The reverse HC proxy needs to fully understand [RFC6690] syntax
and semantics, otherwise there is an inherent risk to corrupt the
payloads.
Therefore, CoRE Link Format payload should only be transcoded at the
risk and discretion of the proxy implementer.
6.5.3. Diagnostic Messages
CoAP responses may, in certain error cases, contain a diagnostic
message in the payload explaining the error situation, as described
in Section 5.5.2 of [RFC7252]. If present, the CoAP response
diagnostic payload SHOULD be copied in the HTTP response body. The
CoAP diagnostic message MUST NOT be copied into the HTTP reason-
phrase, since it potentially contains CR-LF characters which are
incompatible with HTTP reason-phrase syntax.
7. Response Code Mapping
Table 2 defines the HTTP response status codes to which each CoAP
response code SHOULD be mapped. Multiple appearances of a HTTP
status code in the second column indicates multiple equivalent HTTP
responses are possible based on the same CoAP response code,
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depending on the conditions cited in the Notes (third column and text
below table).
+-----------------------------+-----------------------------+-------+
| 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 | 403 Forbidden | 5 |
| 4.02 Bad Option | 400 Bad Request | 6 |
| 4.02 Bad Option | 500 Internal Server Error | 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 Ent. Too Large | 413 Request Repr. Too 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 2: CoAP-HTTP Response Code Mappings
Notes:
1. A CoAP server may return an arbitrary format payload along with
this response. If present, this payload MUST 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 entity. (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/PUT, and code 204 if not. Hence, a
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proxy MUST transfer any CoAP payload contained in a CoAP 2.02
response to the HTTP client using a 200 OK response.
3. HTTP code 304 (Not Modified) is sent if the HTTP client performed
a conditional HTTP request and the CoAP server responded with
2.03 (Valid) to the corresponding CoAP validation request. Note
that Section 4.1 of [RFC7232] puts some requirements on header
fields that must be present in the HTTP 304 response.
4. A 200 response to a CoAP 2.03 occurs only when the reverse HC
proxy, for efficiency reasons, is running a local cache. An
unconditional HTTP GET which produces a cache-hit, could trigger
a re-validation (i.e. a conditional GET) on the CoAP side. The
proxy receiving 2.03 updates the freshness of its cached
representation and returns it to the HTTP client.
5. A HTTP 401 Unauthorized (Section 3.1 of [RFC7235]) response is
not applicable because there is no equivalent in CoAP of WWW-
Authenticate which is mandatory in a HTTP 401 response.
6. If the proxy has a way to determine that the Bad Option is due to
the straightforward mapping of a client request header into a
CoAP option, then returning HTTP 400 (Bad Request) is
appropriate. In all other cases, the proxy MUST return HTTP 500
(Internal Server Error) stating its inability to provide a
suitable translation to the client's request.
7. A CoAP 4.05 (Method Not Allowed) response SHOULD normally be
mapped to a HTTP 400 (Bad Request) code, because the HTTP 405
response would require specifying the supported methods - which
are generally unknown. In this case the reverse HC Proxy SHOULD
also return a HTTP reason-phrase in the HTTP status line that
starts with the string "CoAP server returned 4.05" in order to
facilitate troubleshooting. However, if the reverse HC proxy has
more granular information about the supported methods for the
requested resource (e.g. via a Resource Directory
([I-D.ietf-core-resource-directory])) then it MAY send back a
HTTP 405 (Method Not Allowed) with a properly filled in "Allow"
response-header field (Section 7.4.1 of [RFC7231]).
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 (Section 5.7 of [RFC7252])
to execute some, or all, of its CoAP requests.
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8. Additional Mapping Guidelines
8.1. Caching and Congestion Control
A reverse HC proxy should cache CoAP responses, and reply whenever
applicable with a cached representation of the requested resource.
If the HTTP client drops the connection after the HTTP request was
made, a reverse HC proxy should wait for the associated CoAP response
and cache it if possible. Subsequent requests to the reverse 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 request.
According to [RFC7252], a proxy must limit the number of outstanding
requests to a given CoAP server to NSTART. To limit the amount of
aggregate traffic to a constrained network, the reverse HC proxy
should also put a limit on 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.
Highly volatile resources that are being frequently requested may be
observed [RFC7641] by the reverse HC proxy to keep their cached
representation fresh while minimizing the amount of CoAP traffic in
the constrained network. See Section 8.2.
8.2. Cache Refresh via Observe
There are cases where using the CoAP observe protocol [RFC7641] 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 CoAP nodes -- with possibly high variance
in requests' distribution -- which would greatly benefit from a
server driven cache update mechanism. Ideal candidates for CoAP
observe are also 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 refreshing 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 T_C be the mean time between two representation changes of R, and
let M_R be the mean number of CoAP messages per second exchanged to
and from resource R. If we assume that the initial cost for
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establishing the observation is negligible, an observation on R
reduces M_R iff T_R < 2*T_C with respect to using ETag validation,
that is iff the mean arrival rate of requests for resource R is
greater than half the change rate of R.
When observing the resource R, M_R is always upper bounded by 2/T_C.
8.3. Use of CoAP Blockwise Transfer
A reverse 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 constrained networks,
and to cope with small datagram buffers in CoAP end-points as
described in [RFC7252] Section 4.6.
A reverse 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 reverse HC proxy SHOULD attempt to use blockwise transfer
when sending a CoAP PUT or POST request message that is larger than
BLOCKWISE_THRESHOLD bytes. The value of BLOCKWISE_THRESHOLD is
implementation-specific, for example it can be:
o calculated based on a known or typical UDP datagram buffer size
for CoAP end-points, or
o set to N times the known size of a link-layer frame in a
constrained network where e.g. N=5, or
o preset to a known IP MTU value, or
o set to a known Path MTU value.
The value BLOCKWISE_THRESHOLD, or the parameters from which it is
calculated, should be configurable in a proxy implementation. The
maximum block size the proxy will attempt to use in CoAP requests
should also be configurable.
The reverse HC proxy SHOULD detect CoAP end-points not supporting
blockwise transfers. This can be done by checking for a 4.02 (Bad
Option) response returned by an end-point in response to a CoAP
request with a Block* Option, and subsequent absence of the 4.02 in
response to the same request without Block* Options. This allows the
reverse HC proxy to be more efficient, not attempting repeated
blockwise transfers to CoAP servers that do not support it.
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8.4. CoAP Multicast
A reverse HC proxy MAY support CoAP multicast. If it does, the
reverse HC proxy sends out a multicast CoAP request if the Target
CoAP URI's authority is a multicast IP literal or resolves to a
multicast IP address; assuming the proper security measures are in
place to mitigate security risks of CoAP multicast (Section 10). If
the security policies do not allow the specific CoAP multicast
request to be made, the reverse HC proxy SHOULD respond 403
(Forbidden).
If a reverse HC proxy does not support CoAP multicast, it SHOULD
respond 403 (Forbidden) to any valid HTTP request that maps to a CoAP
multicast request.
Details related to supporting CoAP multicast are currently out of
scope of this document since in a reverse proxy scenario a HTTP
client typically expects to receive a single response, not multiple.
However, a reverse HC proxy that implements CoAP multicast may
include application-specific functions to aggregate multiple CoAP
responses into a single HTTP response. We suggest using the
"application/http" internet media type (Section 8.3.2 of [RFC7230])
to enclose a set of one or more HTTP response messages, each
representing the mapping of one CoAP response.
8.5. Timeouts
If the CoAP server takes a long time in responding, 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 reverse HC proxy MUST define an internal timeout for each pending
CoAP request, because the CoAP server may silently die before
completing the request. Assuming the Proxy uses confirmable CoAP
requests, such timeout value T SHOULD be at least
T = MAX_RTT + MAX_SERVER_RESPONSE_DELAY
where MAX_RTT is defined in [RFC7252] and MAX_SERVER_RESPONSE_DELAY
is defined in [RFC7390].
9. IANA Considerations
9.1. New 'core.hc' Resource Type
This document registers a new Resource Type (rt=) Link Target
Attribute, 'core.hc', in the "Resource Type (rt=) Link Target
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Attribute Values" subregistry under the "Constrained RESTful
Environments (CoRE) Parameters" registry.
Attribute Value: core.hc
Description: HTTP to CoAP mapping base resource.
Reference: See Section 5.4.
9.2. New 'coap-payload' Internet Media Type
This document defines the "application/coap-payload" media type with
a single parameter "cf". This media type represents any payload that
a CoAP message can carry, having a content format that can be
identified by a CoAP Content-Format parameter (an integer in range
0-65535). The parameter "cf" is the integer defining the CoAP
content format.
Type name: application
Subtype name: coap-payload
Required parameters:
cf - CoAP Content-Format integer in range 0-65535 denoting the
content format of the CoAP payload carried.
Optional parameters: None
Encoding considerations:
The specific CoAP content format encoding considerations for the
selected Content-Format (cf parameter) apply.
Security considerations:
The specific CoAP content format security considerations for the
selected Content-Format (cf parameter) apply.
Interoperability considerations:
Published specification: (this I-D - TBD)
Applications that use this media type:
HTTP-to-CoAP Proxies.
Fragment identifier considerations: N/A
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Additional information:
Deprecated alias names for this type: N/A
Magic number(s): N/A
File extension(s): N/A
Macintosh file type code(s): N/A
Person and email address to contact for further information:
Esko Dijk ("esko@ieee.org")
Intended usage: COMMON
Restrictions on usage:
An application (or user) can only use this media type if it has to
represent a CoAP payload of which the specified CoAP Content-Format
is an unrecognized number; such that a proper translation directly to
the equivalent HTTP media type is not possible.
Author: CoRE WG
Change controller: IETF
Provisional registration? (standards tree only): N/A
10. Security Considerations
The security concerns raised in Section 9.2 of [RFC7230] also apply
to the reverse HC proxy scenario.
A reverse proxy deployed at the boundary of a constrained network is
an easy single point of failure for reducing availability. As such,
special care should be taken in designing, developing and operating
it, keeping in mind that, in most cases, it has fewer limitations
than the constrained devices it is serving.
The following sub paragraphs categorize and discuss a set of specific
security issues related to the translation, caching and forwarding
functionality exposed by a reverse HC proxy.
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10.1. Traffic Overflow
Due to the typically constrained nature of CoAP nodes, particular
attention should be given to the implementation of traffic reduction
mechanisms (see Section 8.1), because inefficient proxy
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 which is
mapped to a CoAP multicast resource, as discussed in Section 11.3 of
[RFC7252].
The risk likelihood 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 which disrupts availability of the
targeted CoAP server can be limited by access controlling the exposed
multicast resources, so that only known/authorized users can access
such URIs.
10.2. Handling Secured Exchanges
An HTTP request can be sent to the reverse HC proxy over a secured
connection. However, there may not always exist a secure connection
mapping to CoAP. For example, a secure distribution method for
multicast traffic is complex and may not be implemented (see
[RFC7390]).
A reverse HC proxy should implement rules for security context
translations. For example all "https" unicast requests are
translated to "coaps" requests, or "https" requests are translated to
unsecured "coap" requests. 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 the HC
proxy operates. These rules should be configurable.
It is RECOMMENDED that, by default, accessing a "coaps" URI is only
allowed from a corresponding "https" URI.
By default, a reverse 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 means. Assuming that CoAP nodes are
isolated behind a firewall as in the server-side reverse HC proxy
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deployment shown in Figure 1, the reverse HC proxy may be configured
to translate the incoming HTTPS request using plain CoAP (NoSec
mode).
10.3. URI Mapping
The following risks related to the URI mapping described in Section 5
and its use by HC proxies have been identified:
DoS attack on the constrained/CoAP network.
Mitigation: by default deny any Target CoAP URI whose authority is
(or maps to) a multicast address. Then explicitly white-list
multicast resources/authorities that are allowed to be de-
referenced. See also Section 8.4.
Leaking information on the constrained/CoAP network resources and
topology.
Mitigation: by default deny any Target CoAP URI (especially
/.well-known/core is a resource to be protected), and then
explicitly white-list resources that are allowed to be seen from
outside.
The internal CoAP Target resource is totally transparent from
outside.
Mitigation: implement a HTTPS-only interface, which makes the
Target CoAP URI totally opaque to a passive attacker.
11. Acknowledgements
An initial version of Table 2 in Section 7 has been provided in
revision -05 of the CoRE CoAP I-D. 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.
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12. References
12.1. Normative References
[I-D.ietf-core-block]
Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
draft-ietf-core-block-16 (work in progress), October 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.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[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.
[RFC7232] Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
(HTTP/1.1): Conditional Requests", RFC 7232, 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.
[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>.
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Internet-Draft HTTP-CoAP Mapping April 2016
12.2. Informative References
[I-D.ietf-core-links-json]
Li, K., Rahman, A., and C. Bormann, "Representing CoRE
Formats in JSON and CBOR", draft-ietf-core-links-json-04
(work in progress), November 2015.
[I-D.ietf-core-resource-directory]
Shelby, Z. and C. Bormann, "CoRE Resource Directory",
draft-ietf-core-resource-directory-02 (work in progress),
November 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.
[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.
[RFC7390] Rahman, A. and E. Dijk, "Group Communication for the
Constrained Application Protocol (CoAP)", RFC 7390,
October 2014.
Appendix A. Change Log
[Note to RFC Editor: Please remove this section before publication.]
Changes from ietf-08 to ietf-09:
o Clean up requirments language as per Klaus' comment.
Changes from ietf-07 to ietf-08:
o Addressed WGLC review comments from Klaus Hartke as per the
correspondence of March 9, 2016 on the CORE WG mailing list.
Changes from ietf-06 to ietf-07:
o Addressed Ticket #384 - Section 5.4.1 describes briefly
(informative) how to discover CoAP resources from an HTTP client.
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o Addressed Ticket #378 - For HTTP media type to CoAP content format
mapping and vice versa: a new draft (TBD) may be proposed in CoRE
which describes an approach for automatic updating of the media
type mapping. This was noted in Section 6.1 but is otherwise
outside the scope of this draft.
o Addressed Ticket #377 - Added IANA section that defines a new HTTP
media type "application/coap-payload" and created new Section 6.2
on how to use it.
o Addressed Ticket #376 - Updated Table 2 (and corresponding note 7)
to indicate that a CoAP 4.05 (Method Not Allowed) Response Code
should be mapped to a HTTP 400 (Bad Request).
o Added note to comply to ABNF when translating CoAP diagnostic
payload to reason-phrase in Section 6.5.3.
Changes from ietf-05 to ietf-06:
o Fully restructured the draft, bringing introductory text more to
the front and allocating main sections to each of the key topics;
addressing Ticket #379;
o Addressed Ticket #382, fix of enhanced form URI template
definition of q in Section 5.3.2;
o Addressed Ticket #381, found a mapping 4.01 to 401 Unauthorized in
Section 7;
o Addressed Ticket #380 (Add IANA registration for "core.hc"
Resource Type) in Section 9;
o Addressed Ticket #376 (CoAP 4.05 response can't be translated to
HTTP 405 by HC proxy) in Section 7 by use of empty 'Allow' header;
o Removed details on the pros and cons of HC proxy placement
options;
o Addressed review comments of Carsten Bormann;
o Clarified failure in mapping of HTTP Accept headers (Section 6.3);
o Clarified detection of CoAP servers not supporting blockwise
(Section 8.3);
o Changed CoAP request timeout min value to MAX_RTT +
MAX_SERVER_RESPONSE_DELAY (Section 8.6);
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o Added security section item (Section 10.3) related to use of CoAP
blockwise transfers;
o Many editorial improvements.
Changes from ietf-04 to ietf-05:
o Addressed Ticket #366 (Mapping of CoRE Link Format payloads to be
valid in HTTP Domain?) in Section 6.3.3.2 (Content Transcoding -
CORE Link Format);
o Addressed Ticket #375 (Add requirement on mapping of CoAP
diagnostic payload) in Section 6.3.3.3 (Content Transcoding -
Diagnostic Messages);
o Addressed comment from Yusuke (http://www.ietf.org/mail-
archive/web/core/current/msg05491.html) in Section 6.3.3.1
(Content Transcoding - General);
o Various editorial improvements.
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
HTTP-CoAP 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:
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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.
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
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Esko Dijk
Philips Research
High Tech Campus 34
Eindhoven 5656 AE
The Netherlands
Email: esko.dijk@philips.com
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