CoRE Working Group                                         A. CastellaniInternet-Draft                                      University of PadovaIntended status: Informational                                 S. LoretoExpires: January 21, 2017                                       Ericsson                                                               A. Rahman                                        InterDigital Communications, LLC                                                              T. Fossati                                                                   Nokia                                                                 E. Dijk                                                        Philips Lighting                                                           July 20, 2016          Guidelines for HTTP-to-CoAP Mapping Implementations                    draft-ietf-core-http-mapping-13Abstract   This document provides reference information for implementing a   cross-protocol network proxy that performs translation from the HTTP   protocol to the CoAP protocol.  This will enable a HTTP client to   access resources on a CoAP server through the proxy.  This document   describes how a HTTP request is mapped to a CoAP request, and then   how a CoAP response is mapped back to a HTTP response.  This includes   guidelines for URI mapping, media type mapping and additional proxy   implementation issues.  This document covers the Reverse, Forward and   Interception cross-protocol proxy cases.Status of This Memo   This Internet-Draft is submitted in full conformance with the   provisions of BCP 78 and BCP 79.   Internet-Drafts are working documents of the Internet Engineering   Task Force (IETF).  Note that other groups may also distribute   working documents as Internet-Drafts.  The list of current Internet-   Drafts is at http://datatracker.ietf.org/drafts/current/.   Internet-Drafts are draft documents valid for a maximum of six months   and may be updated, replaced, or obsoleted by other documents at any   time.  It is inappropriate to use Internet-Drafts as reference   material or to cite them other than as "work in progress."   This Internet-Draft will expire on January 21, 2017.Castellani, et al.      Expires January 21, 2017                [Page 1]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016Copyright Notice   Copyright (c) 2016 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject to BCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   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.  HTTP-to-CoAP Proxy  . . . . . . . . . . . . . . . . . . . . .   5   4.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   6   5.  URI Mapping . . . . . . . . . . . . . . . . . . . . . . . . .   7     5.1.  URI Terminology . . . . . . . . . . . . . . . . . . . . .   8     5.2.  Null Mapping  . . . . . . . . . . . . . . . . . . . . . .   8     5.3.  Default Mapping . . . . . . . . . . . . . . . . . . . . .   8       5.3.1.  Optional Scheme Omission  . . . . . . . . . . . . . .   9       5.3.2.  Encoding Caveats  . . . . . . . . . . . . . . . . . .   9     5.4.  URI Mapping Template  . . . . . . . . . . . . . . . . . .  10       5.4.1.  Simple Form . . . . . . . . . . . . . . . . . . . . .  10       5.4.2.  Enhanced Form . . . . . . . . . . . . . . . . . . . .  11     5.5.  Discovery . . . . . . . . . . . . . . . . . . . . . . . .  13       5.5.1.  Examples  . . . . . . . . . . . . . . . . . . . . . .  13   6.  Media Type Mapping  . . . . . . . . . . . . . . . . . . . . .  15     6.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  15     6.2.  'application/coap-payload' Media Type . . . . . . . . . .  16     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  . . . . . . . . . . . . . . . . . . . . . . . .  25Castellani, et al.      Expires January 21, 2017                [Page 2]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016   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.  Multicast  . . . . . . . . . . . . . . . . . . . . . . .  27     10.2.  Traffic Overflow . . . . . . . . . . . . . . . . . . . .  28     10.3.  Handling Secured Exchanges . . . . . . . . . . . . . . .  28     10.4.  URI Mapping  . . . . . . . . . . . . . . . . . . . . . .  29   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  29   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  30     12.1.  Normative References . . . . . . . . . . . . . . . . . .  30     12.2.  Informative References . . . . . . . . . . . . . . . . .  31   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  32   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  351.  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 on the basic mapping of request methods and simple   response code mapping between HTTP and CoAP, and it leaves many   details of the cross-protocol proxy for future definition.   Therefore, a primary goal of this informational document is to define   a consistent set of guidelines that an HTTP-to-CoAP proxy   implementation should adhere to.  The key benefit to adhering to such   guidelines is to reduce variation between proxy implementations,   thereby increasing interoperability between an HTTP client and a CoAP   server independent of the proxy that implements the cross-protocol   mapping.  (For example, 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 document describes HTTP mappings that apply to protocol elements   defined in the base CoAP specification [RFC7252].  It is up to CoAP   protocol extensions (new methods, response codes, options, content-   formats) to describe their own HTTP mappings, if applicable.   This document is organized as follows:   o  Section 2 defines proxy terminology;Castellani, et al.      Expires January 21, 2017                [Page 3]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016   o  Section 3 introduces the HTTP-to-CoAP proxy;   o  Section 4 lists use cases in which HTTP clients need to contact      CoAP servers;   o  Section 5 introduces a null, 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;   o  Section 7 describes how to map CoAP responses to HTTP responses;   o  Section 8 describes additional mapping guidelines related to      caching, congestion, timeouts, etc.;   o  Section 10 discusses possible security impact of HTTP-to-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 an HTTP-to-CoAP (HC) mapping.  Specifically, the HC   proxy acts as an HTTP server and a CoAP client.  The HC Proxy can   take on the role of a Forward, Reverse or Interception Proxy.   Forward Proxy (or Forward HC Proxy): a message forwarding agent that   is selected by the HTTP 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 from   the HTTP client.  The HTTP client uses the "origin-form"   (Section 5.3.1 of [RFC7230]) as a request-target URI.Castellani, et al.      Expires January 21, 2017                [Page 4]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016   Interception Proxy (or Interception HC Proxy) [RFC3040]: a proxy that   receives inbound HTTP traffic flows through the process of traffic   redirection; transparent to the HTTP client.   Note that a Reverse Proxy appears to an HTTP 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.  HTTP-to-CoAP Proxy   A HC proxy is accessed by an HTTP client which wants to access a   resource on a CoAP server.  The HC proxy handles the HTTP request by   mapping it to the equivalent CoAP request, which is then forwarded to   the appropriate CoAP server.  The received CoAP response is then   mapped to an appropriate HTTP response and finally sent back to the   originating HTTP client.   See Figure 1 for an example deployment scenario.  Here a HC proxy is   located 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 HC proxy and optionally also used by   the HC proxy to resolve IP addresses of CoAP servers.Castellani, et al.      Expires January 21, 2017                [Page 5]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016                                               Constrained Network                                              .-------------------.                                             /      .------.       \                                            /       | CoAP |        \                                           /        |server|         \                                          ||        '------'         ||                                          ||                         ||     .--------.  HTTP Request   .------------.  CoAP Req  .------.   ||     |  HTTP  |---------------->|HTTP-to-CoAP|----------->| CoAP |   ||     | Client |<----------------|   Proxy    |<-----------|Server|   ||     '--------'  HTTP Response  '------------'  CoAP Resp '------'   ||                                          ||                         ||                                          ||   .------.              ||                                          ||   | CoAP |              ||                                           \   |server|  .------.    /                                            \  '------'  | CoAP |   /                                             \           |server|  /                                              \          '------' /                                               '-----------------'             Figure 1: HTTP-To-CoAP Proxy Deployment Scenario   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] focuses on the   basic mapping of request methods and simple response code mapping   between HTTP and CoAP, and leaves many details of the cross-protocol   HC proxy for future definition.  This document provides additional   guidelines and more details for the implementation of a HC Proxy,   which should be followed in addition to the normative requirements.   Note that the guidelines apply to all forms of an HC proxy (i.e.   Reverse, Forward, Intercepting) unless explicitly otherwise noted.4.  Use Cases   To illustrate the 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 control actuators via a HC proxy.   2.  Making sensor data available to 3rd parties on the Web: For   demonstration or public interest purposes, a 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 may only accept secure   'coaps' requests, therefore the proxy is configured to translateCastellani, et al.      Expires January 21, 2017                [Page 6]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016   request to those devices accordingly.  The HC proxy is furthermore   configured to only pass through GET requests in order to protect the   constrained network.   3.  Smartphone and home sensor: A smartphone can access directly a   CoAP home sensor using a mutually authenticated 'https' request,   provided its home router runs a HC proxy and is configured with the   appropriate certificate.  An HTML5 application on the smartphone can   provide a friendly UI using the 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 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.  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 HC proxy.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 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 HC proxy.  The 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 term used in the document to describe 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 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;Castellani, et al.      Expires January 21, 2017                [Page 7]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016   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 HC proxy can dynamically discover      information about the supported URI Mapping Template(s), as well      as the 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:   HC Proxy URI:           URI which refers to the 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 HC proxy, whereas path (and           query) component(s) embed the information used by a HC proxy           to extract the Target CoAP URI.5.2.  Null Mapping   The null mapping is the case where there is no Target CoAP URI   appended to the HC Proxy URI.  In other words, it is a "pure" HTTP   URI that is sent to the HC Proxy.  This would typically occur in   situations like Use Case #1 described in Section 4, and the Proxy   would typically be a Reverse Proxy.  In this scenario, the HC Proxy   will determine through its own proprietary algorithms what the Target   CoAP URI should be.5.3.  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   HC proxy.Castellani, et al.      Expires January 21, 2017                [Page 8]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016   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 5.5.   The default URI mapping function SHOULD 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.5.3.1.  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.5.3.2.  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.Castellani, et al.      Expires January 21, 2017                [Page 9]Internet-Draft            HTTP-to-CoAP Mapping                 July 20165.4.  URI Mapping Template   This section defines a format for the URI template [RFC6570] used by   a 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 HC proxy.   When instantiated, an URI Mapping Template is always concatenated to   a HC Proxy URI provided by the HC proxy via discovery (see   Section 5.5), or by other means.   A simple form (Section 5.4.1) and an enhanced form (Section 5.4.2)   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].5.4.1.  Simple Form   The simple form MUST be used for mappings where the Target CoAP URI   is going to be copied (using rules of Section 5.3.2) at some fixed   position into the Hosting HTTP URI.   The "tu" template variable is intended to be used in a template   definition to represent a Target CoAP URI:       tu = coap-URI / coaps-URI  ; from [RFC7252], Section 6.1 and 6.2   The same considerations as in Section 5.3.1 apply, in that the CoAP   scheme may be omitted from the Hosting HTTP URI.5.4.1.1.  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 5.3 to be able to illustrate the use of the above   template variables.   1.  Target CoAP URI is a query argument of the Hosting HTTP URI:Castellani, et al.      Expires January 21, 2017               [Page 10]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016   ?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   2.  Target CoAP URI in the path component of the Hosting HTTP URI:   forward/{+tu}   coap://s.example.com/light   http://p.example.com/hc/forward/coap://s.example.com/light   or   coaps://s.example.com/light   http://p.example.com/hc/forward/coaps://s.example.com/light   3.  "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={+tu}   coap://s.example.com/light   http://p.example.com/hc/?coap_uri=s.example.com/light5.4.2.  Enhanced Form   The enhanced form can be used to express more sophisticated mappings   of the Target CoAP URI into the Hosting HTTP URI, i.e., mappings that   do not fit into the simple form.Castellani, et al.      Expires January 21, 2017               [Page 11]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016   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 if and only if 'query' is empty   The qq form is used when the path and the (optional) query components   are to be copied verbatim from the Target CoAP URI into the Hosting   HTTP URI, i.e. as "{+p}{+qq}".  Instead, the q form is used when the   query and path are mapped as separate entities, e.g. as in   "coap_path={+p}&coap_query={+q}".5.4.2.1.  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:Castellani, et al.      Expires January 21, 2017               [Page 12]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016       ?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=on5.5.  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 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   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 5.3, 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 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.5.5.1.  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 HC      proxy before leaving the home network.  Then when outside the homeCastellani, et al.      Expires January 21, 2017               [Page 13]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016      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 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]:Castellani, et al.      Expires January 21, 2017               [Page 14]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016       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"6.  Media Type Mapping6.1.  Overview   A 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.   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 HC proxy is unable to map to an equivalent CoAP   Content-Format, SHALL elicit a 415 (Unsupported Media Type) response   by the HC proxy.   On the content negotiation side, failure to map Accept and Accept-*   headers SHOULD be silently ignored: the HC proxy SHOULD therefore   forward as a CoAP request with no Accept option.  The 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 HC proxy implementation isCastellani, et al.      Expires January 21, 2017               [Page 15]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016   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 HC proxy could implement different media type   mappings.   When tightly coupled, the HC proxy knows exactly which content   formats are supported by the applications, and can be strict when   enforcing its forwarding policies in general, and the media type   mapping in particular.   On the other side, when the HC proxy is a general purpose application   layer gateway, being too strict could significantly reduce the amount   of traffic that it would be able to successfully forward.  In this   case, the "loose" media type mapping detailed in Section 6.3 MAY be   implemented.   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 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 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 HC Proxy   doesn't recognize the content format '60'.Castellani, et al.      Expires January 21, 2017               [Page 16]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016   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 HC   Proxy if the client has determined that the 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-   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 tableCastellani, et al.      Expires January 21, 2017               [Page 17]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016   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.Castellani, et al.      Expires January 21, 2017               [Page 18]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016           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 26.5.  Content Transcoding6.5.1.  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 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 HC proxy   transcodes an application/XML to application/EXI in-band metadataCastellani, et al.      Expires January 21, 2017               [Page 19]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016   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 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 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 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,   depending on the conditions cited in the Notes (third column and text   below table).Castellani, et al.      Expires January 21, 2017               [Page 20]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016   +-----------------------------+-----------------------------+-------+   | 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       proxy MUST transfer any CoAP payload contained in a CoAP 2.02       response to the HTTP client using a 200 OK response.Castellani, et al.      Expires January 21, 2017               [Page 21]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016   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 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 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 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.Castellani, et al.      Expires January 21, 2017               [Page 22]Internet-Draft            HTTP-to-CoAP Mapping                 July 20168.  Additional Mapping Guidelines8.1.  Caching and Congestion Control   A 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 HC proxy should wait for the associated CoAP response and   cache it if possible.  Subsequent 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   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 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 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   establishing the observation is negligible, an observation on RCastellani, et al.      Expires January 21, 2017               [Page 23]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016   reduces M_R if and only if T_R < 2*T_C with respect to using ETag   validation, that is if and only if 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 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 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   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 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 HC proxy to   be more efficient, not attempting repeated blockwise transfers to   CoAP servers that do not support it.Castellani, et al.      Expires January 21, 2017               [Page 24]Internet-Draft            HTTP-to-CoAP Mapping                 July 20168.4.  CoAP Multicast   A HC proxy MAY support CoAP multicast.  If it does, the 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.  If   the 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 proxy scenario a HTTP client   typically expects to receive a single response, not multiple.   However, a 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.   For further considerations related to the handling of multicast   requests, see Section 10.1.8.5.  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 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 Considerations9.1.  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   Attribute Values" subregistry under the "Constrained RESTful   Environments (CoRE) Parameters" registry.   Attribute Value: core.hcCastellani, et al.      Expires January 21, 2017               [Page 25]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016   Description: HTTP to CoAP mapping base resource.   Reference: See Section 5.5.9.2.  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 an integer in range 0-65535 corresponding to a CoAP   Content-Format parameter ([RFC7252], Section 12.2).  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   Additional information:      Deprecated alias names for this type: N/ACastellani, et al.      Expires January 21, 2017               [Page 26]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016      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/A10.  Security Considerations   The security concerns raised in Section 9.2 of [RFC7230] also apply   to the HC proxy scenario.   A HC 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 HC proxy.10.1.  Multicast   Multicast requests impose a non trivial cost on the constrained   network and endpoints, and might be exploited as a DoS attack vector   (see also Section 10.2).  From a privacy perspective, they can be   used to gather detailed information about the resources hosted in the   constrained network.  For these reasons, it is RECOMMENDED that   requests to multicast resources are access controlled with a default-Castellani, et al.      Expires January 21, 2017               [Page 27]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016   deny policy.  It is RECOMMENDED that the requestor of a multicast   resource is strongly authenticated.  If privacy is a concern, for   example whenever the HTTP request transits through the public   Internet, the request SHOULD be transported over a mutually   authenticated and encrypted TLS connection.10.2.  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.10.3.  Handling Secured Exchanges   An HTTP request can be sent to the 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 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.Castellani, et al.      Expires January 21, 2017               [Page 28]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016   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 means.  Assuming that CoAP nodes are isolated behind   a firewall as in the HC proxy deployment shown in Figure 1, the HC   proxy may be configured to translate the incoming HTTPS request using   plain CoAP (NoSec mode).10.4.  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 Abhijan Bhattacharyya, Brian Frank, Carsten Bormann,   Christian Amsuess, Christian Groves, Cullen Jennings, Dorothy   Gellert, Francesco Corazza, Hannes Tschofenig, Jaime Jimenez, Kepeng   Li, Kerry Lynn, Klaus Hartke, Linyi Tian, Michele Rossi, Michele   Zorzi, Nicola Bui, Peter Saint-Andre, Zach Shelby 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.Castellani, et al.      Expires January 21, 2017               [Page 29]Internet-Draft            HTTP-to-CoAP Mapping                 July 201612.  References12.1.  Normative References   [I-D.ietf-core-block]              Bormann, C. and Z. Shelby, "Block-wise transfers in CoAP",              draft-ietf-core-block-21 (work in progress), July 2016.   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate              Requirement Levels", BCP 14, RFC 2119,              DOI 10.17487/RFC2119, March 1997,              <http://www.rfc-editor.org/info/rfc2119>.   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform              Resource Identifier (URI): Generic Syntax", STD 66,              RFC 3986, DOI 10.17487/RFC3986, January 2005,              <http://www.rfc-editor.org/info/rfc3986>.   [RFC5234]  Crocker, D., Ed. and P. Overell, "Augmented BNF for Syntax              Specifications: ABNF", STD 68, RFC 5234,              DOI 10.17487/RFC5234, January 2008,              <http://www.rfc-editor.org/info/rfc5234>.   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,              and D. Orchard, "URI Template", RFC 6570,              DOI 10.17487/RFC6570, March 2012,              <http://www.rfc-editor.org/info/rfc6570>.   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,              <http://www.rfc-editor.org/info/rfc6690>.   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer              Protocol (HTTP/1.1): Message Syntax and Routing",              RFC 7230, DOI 10.17487/RFC7230, June 2014,              <http://www.rfc-editor.org/info/rfc7230>.   [RFC7231]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer              Protocol (HTTP/1.1): Semantics and Content", RFC 7231,              DOI 10.17487/RFC7231, June 2014,              <http://www.rfc-editor.org/info/rfc7231>.   [RFC7232]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer              Protocol (HTTP/1.1): Conditional Requests", RFC 7232,              DOI 10.17487/RFC7232, June 2014,              <http://www.rfc-editor.org/info/rfc7232>.Castellani, et al.      Expires January 21, 2017               [Page 30]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016   [RFC7235]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer              Protocol (HTTP/1.1): Authentication", RFC 7235,              DOI 10.17487/RFC7235, June 2014,              <http://www.rfc-editor.org/info/rfc7235>.   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained              Application Protocol (CoAP)", RFC 7252,              DOI 10.17487/RFC7252, June 2014,              <http://www.rfc-editor.org/info/rfc7252>.   [RFC7641]  Hartke, K., "Observing Resources in the Constrained              Application Protocol (CoAP)", RFC 7641,              DOI 10.17487/RFC7641, September 2015,              <http://www.rfc-editor.org/info/rfc7641>.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-06              (work in progress), July 2016.   [I-D.ietf-core-resource-directory]              Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE              Resource Directory", draft-ietf-core-resource-directory-08              (work in progress), July 2016.   [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,              DOI 10.17487/RFC2616, June 1999,              <http://www.rfc-editor.org/info/rfc2616>.   [RFC3040]  Cooper, I., Melve, I., and G. Tomlinson, "Internet Web              Replication and Caching Taxonomy", RFC 3040,              DOI 10.17487/RFC3040, January 2001,              <http://www.rfc-editor.org/info/rfc3040>.   [RFC4732]  Handley, M., Ed., Rescorla, E., Ed., and IAB, "Internet              Denial-of-Service Considerations", RFC 4732,              DOI 10.17487/RFC4732, December 2006,              <http://www.rfc-editor.org/info/rfc4732>.   [RFC7049]  Bormann, C. and P. Hoffman, "Concise Binary Object              Representation (CBOR)", RFC 7049, DOI 10.17487/RFC7049,              October 2013, <http://www.rfc-editor.org/info/rfc7049>.Castellani, et al.      Expires January 21, 2017               [Page 31]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016   [RFC7390]  Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for              the Constrained Application Protocol (CoAP)", RFC 7390,              DOI 10.17487/RFC7390, October 2014,              <http://www.rfc-editor.org/info/rfc7390>.Appendix A.  Change Log   [Note to RFC Editor: Please remove this section before publication.]   Changes from ietf-12 to ietf-13 (Christian Amsuss' comments):   o  More missing slashes in URI mapping template examples.   Changes from ietf-11 to ietf-12 (2nd WGLC):   o  Addressed a few editorial issues (including a clarification on      when to use qq vs q in the URI mapping template).   o  Fixed missing slash in one template example.   o  Added para about the need for future CoAP protocol elements to      define their own HTTP mappings.   Changes from ietf-10 to ietf-11 (Chair review):   o  Removed cu/su distinction from the URI mapping template.   o  Addressed a few editorial issues.   Changes from ietf-09 to ietf-10:   o  Addressed Ticket #401 - Clarified that draft covers not only      Reverse HC Proxy but that many parts also apply to Forward and      Interception Proxies.   o  Clarified that draft concentrates on the HTTP-to-CoAP mapping      direction (i.e. the HC proxy is a HTTP server and a CoAP client).   o  Clarified the "null mapping" case where no CoAP URI information is      embedded in the HTTP request URI.   o  Moved multicast related security text to the "Security      Considerations" to consolidate all security information in one      location.   o  Removed references to "placement" of proxy (e.g. server-side vs      client-side) as is confusing and provides little added value.Castellani, et al.      Expires January 21, 2017               [Page 32]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016   o  Fixed version numbers on references that were corrupted in last      revision due to outdated xml2rfc conversion tool local cache.   o  Various editorial improvements.   Changes from ietf-08 to ietf-09:   o  Clean up requirements 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.   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;Castellani, et al.      Expires January 21, 2017               [Page 33]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016   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);   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;Castellani, et al.      Expires January 21, 2017               [Page 34]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016   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:   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.comCastellani, et al.      Expires January 21, 2017               [Page 35]Internet-Draft            HTTP-to-CoAP Mapping                 July 2016   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   Nokia   3 Ely Road   Milton, Cambridge  CB24 6DD   UK   Email: thomas.fossati@nokia.com   Esko Dijk   Philips Lighting   High Tech Campus 34   Eindhoven  5656 AE   The Netherlands   Email: esko.dijk@philips.comCastellani, et al.      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