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
                                                                 E. Dijk
                                                        Philips Research
                                                            July 4, 2014

            Guidelines for HTTP-CoAP Mapping Implementations


   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

   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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   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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

   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

   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

   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",
   "OPTIONAL" in this document are to be interpreted as described in

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

   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

   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

   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 and a Target
   CoAP URI coap://, the resulting Hosting HTTP URI
   would be

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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:// becomes

   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, could either
   represent the target coap:// or
   coaps:// depending on application specific

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

   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

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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

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.  Examples

   All the following examples (given as a specific URI mapping template,
   a Target CoAP URI, and the produced Hosting HTTP URI) use as the base URI.

   1.  "coap" URI is a query argument of the Hosting HTTP URI:



   2.  "coaps" URI is a query argument of the Hosting HTTP URI:



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   3.  Target CoAP URI as a query argument of the Hosting HTTP URI:





   4.  Target CoAP URI in the path component of the Hosting HTTP URI
       (i.e. the default URI Mapping template):





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  Examples

   All the following examples (given as a specific URI mapping template,
   a Target CoAP URI, and the produced Hosting HTTP URI) use as the base URI.

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   1.  Target CoAP URI components in path segments, and optional query
       in query component:





   2.  Target CoAP URI components split in individual query arguments:





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

   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

   On the HTTP side link information can be serialised in more than one

   o  using the 'application/link-format' content type:

       Req:  GET /.well-known/core?rt=core.hc HTTP/1.1

       Res:  HTTP/1.1 200 OK
             Content-Type: application/link-format
             Content-Length: 18


   o  using the 'application/link-format+json' content type:

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       Req:  GET /.well-known/core?rt=core.hc HTTP/1.1

       Res:  HTTP/1.1 200 OK
             Content-Type: application/link-format+json
             Content-Length: 31


   o  using the Link header:

       Req:  GET /.well-known/core?rt=core.hc HTTP/1.1

       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

       Res:  HTTP/1.1 200 OK
             Content-Type: application/link-format+json
             Content-Length: 111


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

   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


   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 of [RFC7231])
   and content encodings (Section 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

   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

   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

   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

   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

   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
   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 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

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

   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

   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

              Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
              draft-ietf-core-block-12 (work in progress), June 2013.

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              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

   [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

              Rahman, A. and E. Dijk, "Group Communication for CoAP",
              draft-ietf-core-groupcomm-19 (work in progress), June

   [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

   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

   o  Closed Ticket #364 "Add discovery of HTTP-CoAP mapping

   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


   Salvatore Loreto
   Hirsalantie 11
   Jorvas  02420


   Akbar Rahman
   InterDigital Communications, LLC
   1000 Sherbrooke Street West
   Montreal  H3A 3G4

   Phone: +1 514 585 0761

   Thomas Fossati
   3 Ely Road
   Milton, Cambridge  CB24 6DD


   Esko Dijk
   Philips Research
   High Tech Campus 34
   Eindhoven  5656 AE
   The Netherlands


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