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Versions: 00 01 02 03 04 05 06 07 draft-ietf-core-http-mapping

CoRE Working Group                                         A. Castellani
Internet-Draft                                      University of Padova
Intended status: Informational                                 S. Loreto
Expires: August 29, 2013                                        Ericsson
                                                               A. Rahman
                                        InterDigital Communications, LLC
                                                              T. Fossati
                                                               KoanLogic
                                                                 E. Dijk
                                                        Philips Research
                                                       February 25, 2013


          Best Practices for HTTP-CoAP Mapping Implementation
                 draft-castellani-core-http-mapping-07

Abstract

   This draft provides reference information for HTTP-CoAP protocol
   translation proxy implementors, focusing primarily on the reverse
   proxy case.  It details deployment options, discusses possible
   approaches for URI mapping, and provides a set of guidelines and
   considerations related to protocol translation.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on August 29, 2013.

Copyright Notice

   Copyright (c) 2013 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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   (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  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Cross-Protocol Usage of URIs . . . . . . . . . . . . . . . . .  4
   4.  HTTP to CoAP URI Mapping . . . . . . . . . . . . . . . . . . .  5
     4.1.  Embedded Mapping . . . . . . . . . . . . . . . . . . . . .  5
     4.2.  Homogeneous Mapping  . . . . . . . . . . . . . . . . . . .  5
     4.3.  Scheme Security Mapping  . . . . . . . . . . . . . . . . .  6
   5.  HTTP-CoAP Reverse Proxy  . . . . . . . . . . . . . . . . . . .  6
     5.1.  Proxy Placement  . . . . . . . . . . . . . . . . . . . . .  7
     5.2.  Response Code Translations . . . . . . . . . . . . . . . .  8
     5.3.  Media Type Translations  . . . . . . . . . . . . . . . . . 10
     5.4.  Caching and Congestion Control . . . . . . . . . . . . . . 11
     5.5.  Cache Refresh via Observe  . . . . . . . . . . . . . . . . 11
     5.6.  Use of CoAP Blockwise Transfer . . . . . . . . . . . . . . 12
     5.7.  Security Translation . . . . . . . . . . . . . . . . . . . 13
     5.8.  Other guidelines . . . . . . . . . . . . . . . . . . . . . 13
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 14
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
     7.1.  Traffic overflow . . . . . . . . . . . . . . . . . . . . . 14
     7.2.  Handling Secured Exchanges . . . . . . . . . . . . . . . . 15
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 16
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 17
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 17















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1.  Introduction

   CoAP [I-D.ietf-core-coap] 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
   [RFC2616] through an intermediary proxy which performs cross-protocol
   conversion.

   Section 10 of [I-D.ietf-core-coap] describes the fundamentals of the
   CoAP-HTTP (and vice-versa) 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 5 analyzes the mapping that allows HTTP clients to contact
      CoAP servers;

   o  Section 7 discusses possible security impact related to HTTP/CoAP
      cross-protocol mapping.


2.  Terminology

   This document assumes readers are familiar with the terms Reverse
   Proxy as defined in [I-D.ietf-httpbis-p1-messaging] and Interception
   Proxy as defined in [RFC3040].  In addition, the following terms are
   defined:



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   Cross-Protocol Proxy (or Cross Proxy): is a proxy performing a cross-
   protocol mapping, in the context of this document a HTTP-CoAP (HC)
   mapping.  A Cross-Protocol Proxy can behave as a Forward Proxy,
   Reverse Proxy or Interception Proxy.  Note: In this document we focus
   on the Reverse Proxy mode of the Cross-Protocol Proxy.

   Forward Proxy: a message forwarding agent that is selected by the
   client, usually via local configuration rules, to receive requests
   for some type(s) of absolute URI and to attempt to satisfy those
   requests via translation to the protocol indicated by the absolute
   URI.  The user decides (is willing to) use the proxy as the
   forwarding/dereferencing agent for a predefined subset of the URI
   space.

   Reverse Proxy: a receiving agent that acts as a layer above some
   other server(s) and translates the received requests to the
   underlying server's protocol.  It behaves as an origin (HTTP) server
   on its connection towards the (HTTP) client and as a (CoAP) client on
   its connection towards the (CoAP) origin server.  The (HTTP) client
   uses the "origin-form" [I-D.ietf-httpbis-p1-messaging] as a request-
   target URI.

   Reverse and Forward proxies are technically very similar, with main
   differences being that the former appears to a client as an origin
   server while the latter does not, and that clients may be unaware
   they are communicating with a proxy.

   Placement terms: a server-side (SS) proxy is placed in the same
   network domain as the server; conversely a client-side (CS) proxy is
   in the same network domain as the client.  In any other case than SS
   or CS, the proxy is said to be External (E).

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in
   [RFC2119].


3.  Cross-Protocol Usage of URIs

   A Uniform Resource Identifier (URI) provides a simple and extensible
   method for identifying a resource.  It enables uniform identification
   of resources via a separately defined extensible set of naming
   schemes [RFC3986].

   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



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   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 typically only support 'http' and 'https' schemes.
   Therefore, they cannot directly access CoAP servers (which support
   'coap' and/or 'coaps').  In this situation, communication is enabled
   by a Cross-Protocol Proxy, as shown in Figure 1, supporting URI
   mapping features.  Such features are discussed in the following
   section.


4.  HTTP to CoAP URI Mapping

   Assume that a HTTP client wants to access a CoAP resource and
   indicates a target resource of "http://node.something.net/foobar" to
   a Forward cross proxy.  A possible URI mapping done by the proxy
   could result in "coap://node.coap.something.net/foo".

   As shown in the above example, in a cross-protocol URI the scheme,
   authority and path parts of the URI may all change.  The process of
   providing cross-protocol URIs may be complex, since a mechanism to
   statically or dynamically (e.g., discovery) map the URI is needed.

   Two simple static URI mapping solutions are proposed in the following
   subsections.  Note that other mapping approaches are possible as
   well.

4.1.  Embedded Mapping

   In an embedded mapping approach, the HTTP URI has embedded inside it
   the authority and path part of the CoAP URI.

   Example: The CoAP resource "//node.coap.something.net/foo" can be
   accessed by an HTTP client by inserting in the request
   "http://hc-proxy.something.net/coap/node.coap.something.net/foo".
   The Cross-Protocol Proxy then maps the URI to
   "coap://node.coap.something.net/foo"

4.2.  Homogeneous Mapping

   In a homogeneous mapping approach, only the scheme portion of the URI
   needs to be mapped.  The rest of the URI (i.e. authority, path, etc.)
   remains unchanged.

   Example: The CoAP resource "coap://node.coap.something.net/foo" can
   be accessed by an HTTP client by requesting



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   "http://node.coap.something.net/foo".  The Cross-Protocol Proxy
   receiving the request is responsible to map the URI to
   "coap://node.coap.something.net/foo"

   Background info: The assumption in this case is that the HTTP client
   would be able to successfully resolve "node.coap.something.net" using
   DNS infrastructure to return the IP address of the HC proxy.  Most
   likely this would be through a two step DNS lookup where the first
   DNS lookup would resolve "something.net" using public DNS
   infrastructure.  Then the second DNS lookup on the subdomain "coap"
   and the host "node" would typically be resolved by a DNS server
   operated by the owner of domain "something.net".  So this domain
   owner can manage its own internal node names and subdomain allocation
   which would correspond to the CoAP namespace

4.3.  Scheme Security Mapping

   In general, regardless of the URI mapping scheme used in the Cross-
   Protocol Proxy, an "https" request SHOULD be translated to a "coaps"
   request.  The exception case being cases where security on the CoAP
   side is not needed because the network is well enough protected
   already by other means (e.g. strong link-layer security, or the CoAP
   network runs inside a firewalled network, etc.).


5.  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"
   [I-D.ietf-httpbis-p1-messaging] 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 [I-D.ietf-core-coap].  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 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



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   client typically expects to receive a single response, not multiple.
   However a Cross-Protocol 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.

5.1.  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 functionalities
      available in a constrained network, the Cross-Protocol Proxy
      requires a network interface directly attached to the constrained
      network.

   TCP/UDP:  Translation between HTTP and CoAP requires also TCP/UDP
      translation; TCP may be the preferred way for communicating with
      the constrained network due to its reliability or due to
      intermediate gateways configured to block UDP traffic.

   Arguments against SS placement, in favor of client-side (CS), are:

   Scalability:  A solution where a single SS proxy has to manage
      numerous open TCP/IP connections to a large number of HTTP clients
      is not scalable.  (Unless multiple SS proxies are employed with a
      load-balancing mechanism, which adds complexity.)















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                            +------+
                            |      |
                            | DNS  |
                            |      |
                            +------+            Constrained Network
                                                --------------------
                                               /                    \
                                              /  /-----\     /-----\ \
                                             /     CoAP       CoAP    \
                                            /    server      server    \
                                           ||    \-----/     \-----/   ||
    +------+       HTTP Request      +----------+                      ||
    |HTTP  |------------------------>| HTTP/CoAP|  Req   /-----\       ||
    |Client|                         |  Cross-  |------->| CoAP        ||
    |      |<------------------------|  Proxy   |<-------|server       ||
    +------+       HTTP Response     +----------+  Resp  \-----/       ||
                                           ||                          ||
                                           ||   /-----\                ||
                                           ||    CoAP                  ||
                                            \    server                /
                                             \  \-----/               /
                                              \         /-----\      /
                                               \         CoAP       /
                                                \        server    /
                                                 \      \-----/   /
                                                  ----------------


        Figure 1: Reverse Cross-Protocol Proxy Deployment Scenario

5.2.  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
   [I-D.ietf-core-coap] and is intended to cover all possible cases.
   Multiple appearances of a HTTP status code in the second column
   indicates multiple equivalent HTTP responses are possible, depending
   on the conditions cited in the Notes (third column).












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   +-----------------------------+-----------------------------+-------+
   | CoAP Response Code          | HTTP Status Code            | Notes |
   +-----------------------------+-----------------------------+-------+
   | 2.01 Created                | 201 Created                 | 1     |
   | 2.02 Deleted                | 200 OK                      | 2     |
   |                             | 204 No Content              | 2     |
   | 2.03 Valid                  | 304 Not Modified            | 3     |
   |                             | 200 OK                      | 4     |
   | 2.04 Changed                | 200 OK                      | 2     |
   |                             | 204 No Content              | 2     |
   | 2.05 Content                | 200 OK                      |       |
   | 4.00 Bad Request            | 400 Bad Request             |       |
   | 4.01 Unauthorized           | 400 Bad Request             | 5     |
   | 4.02 Bad Option             | 400 Bad Request             | 6     |
   | 4.03 Forbidden              | 403 Forbidden               |       |
   | 4.04 Not Found              | 404 Not Found               |       |
   | 4.05 Method Not Allowed     | 400 Bad Request             | 7     |
   | 4.06 Not Acceptable         | 406 Not Acceptable          |       |
   | 4.12 Precondition Failed    | 412 Precondition Failed     |       |
   | 4.13 Request Entity Too     | 413 Request Repr. Too Large |       |
   | Large                       |                             |       |
   | 4.15 Unsupported Media Type | 415 Unsupported Media Type  |       |
   | 5.00 Internal Server Error  | 500 Internal Server Error   |       |
   | 5.01 Not Implemented        | 501 Not Implemented         |       |
   | 5.02 Bad Gateway            | 502 Bad Gateway             |       |
   | 5.03 Service Unavailable    | 503 Service Unavailable     | 8     |
   | 5.04 Gateway Timeout        | 504 Gateway Timeout         |       |
   | 5.05 Proxying Not Supported | 502 Bad Gateway             | 9     |
   +-----------------------------+-----------------------------+-------+

                    Table 1: HTTP-CoAP Response Mapping

   Notes:

   1.  A CoAP server may return an arbitrary format payload along with
       this response.  This payload SHOULD be returned as entity in the
       HTTP 201 response.  Section 7.3.2 of
       [I-D.ietf-httpbis-p2-semantics] 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.  [I-D.ietf-httpbis-p2-semantics]
       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 [I-D.ietf-httpbis-p7-auth]).

   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 recognized 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.

5.3.  Media Type Translations

   A Cross-Protocol Proxy translates a media type string, carried in a
   HTTP Content-Type header in a request, to a CoAP Content-Format
   Option with the equivalent numeric value.  The media types supported
   by CoAP are defined in the CoAP Content-Format Registry.  Any HTTP
   request with a Content-Type for which the proxy does not know an
   equivalent CoAP Content-Format number, MUST lead to HTTP response 415
   (Unsupported Media Type).

   Also, a CoAP Content-Format value in a response is translated back to



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   the equivalent HTTP Content-Type.  If a proxy receives a CoAP
   Content-Format value that it does not recognize (e.g. because the
   value is IANA-registered after the proxy software was deployed), and
   is unable to look up the equivalent HTTP Content-Type on the fly, the
   proxy SHOULD return an HTTP entity (payload) without Content-Type
   header (complying to Section 3.1.1.5 of
   [I-D.ietf-httpbis-p2-semantics]).

5.4.  Caching and Congestion Control

   A Cross-Protocol Proxy SHOULD limit the number of requests to CoAP
   servers by responding, where applicable, with a cached representation
   of the resource.

   Duplicate idempotent pending requests by a Cross-Protocol 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 Cross-
   Protocol Proxy (closing the TCP connection) after the HTTP request
   was made, a Cross-Protocol Proxy SHOULD wait for the associated CoAP
   response and cache it if possible.  Further requests to the Cross-
   Protocol 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 [I-D.ietf-core-coap], 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 Cross-
   Protocol 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 Cross-Protocol 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 Cross-
   Protocol Proxy to keep their cached representation fresh while
   minimizing the number CoAP messages.  See Section 5.5.

5.5.  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
   [I-D.ietf-core-coap].  Such scenarios include, but are not limited



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   to, sleepy nodes -- with possibly high variance in requests'
   distribution -- which would greatly benefit from a server driven
   cache update mechanism.  Ideal candidates would also be crowded or
   very low throughput networks, where reduction of the total number of
   exchanged messages is an important requirement.

   This subsection aims at providing a practical evaluation method to
   decide whether the refresh of a cached resource R is more efficiently
   handled via ETag validation or by establishing an observation on R.

   Let T_R be the mean time between two client requests to resource R,
   let F_R be the freshness lifetime of R representation, and let M_R be
   the total number of messages exchanged towards resource R. If we
   assume that the initial cost for establishing the observation is
   negligible, an observation on R reduces M_R iff T_R < 2*F_R with
   respect to using ETag validation, that is iff the mean arrival time
   of requests for resource R is greater than half the refresh rate of
   R.

   When using observations M_R is always upper bounded by 2*F_R: in the
   constrained network no more than 2*F_R messages will be generated
   towards resource R.

5.6.  Use of CoAP Blockwise Transfer

   A Cross-Protocol 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
   [I-D.ietf-core-coap] Section 4.6.

   A Cross-Protocol 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 Cross-Protocol Proxy SHOULD attempt to use
   blockwise transfer when sending a CoAP PUT or POST request message
   that is larger than a value BLOCKWISE_THRESHOLD.  The value of
   BLOCKWISE_THRESHOLD MAY be implementation-specific, for example
   calculated based on a known or typical UDP datagram buffer size for
   CoAP end-points, or set to N times the size of a link-layer frame
   where e.g.  N=5, or preset to a known IP MTU value, or set to a known
   Path MTU value.  The value BLOCKWISE_THRESHOLD or parameters from
   which it is calculated SHOULD be configurable in a proxy
   implementation.

   The Cross-Protocol 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*



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   Option.  This allows the Cross-Protocol 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 cross 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.

5.7.  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.

5.8.  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 [I-D.ietf-httpbis-p1-messaging].

   A cross 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 [I-D.ietf-core-coap].

   When the DNS protocol is not used between CoAP nodes in a constrained
   network, defining valid FQDN (i.e., DNS entries) for constrained CoAP
   servers, where possible, MAY help HTTP clients to access the
   resources offered by these servers via a HC proxy.

   HTTP connection pipelining (section 6.2.2.1 of
   [I-D.ietf-httpbis-p1-messaging]) MAY be supported by the proxy and is
   transparent to the CoAP network: the HC cross proxy will sequentially
   serve the pipelined requests by issuing different CoAP requests.




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6.  IANA Considerations

   This memo includes no request to IANA.


7.  Security Considerations

   The security concerns raised in Section 15.7 of [RFC2616] also apply
   to the cross proxy scenario.  In fact, the cross proxy is a trusted
   (not rarely a transparently trusted) component in the network path.

   The trustworthiness assumption on the cross 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 cross 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 cross proxy module.

7.1.  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 5.4), 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
   [I-D.ietf-core-coap].

   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



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   access to the constrained network.

   The feasibility of this attack, disruptive in terms of CoAP server
   availability, can be limited by access controlling the exposed HTTP
   multicast resource, so that only known/authorized users access such
   URIs.

7.2.  Handling Secured Exchanges

   It is possible that the request from the client to the cross 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 cross 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 cross proxy
   deployment shown in Figure 1), the cross proxy may be configured to
   translate the incoming HTTPS request using plain CoAP (i.e.  NoSec
   mode.)

   The HC URI mapping MUST NOT map to HTTP (see Section 4) a CoAP
   resource intended to be accessed only using HTTPS.

   A secured connection that is terminated at the cross proxy, i.e. the
   proxy decrypts secured data locally, raises an ambiguity about the
   cacheability of the requested resource.  The cross proxy SHOULD NOT
   cache any secured content to avoid any leak of secured information.
   However in some specific scenario, a security/efficiency trade-off
   could motivate caching secured information; in that case the caching
   behavior MAY be tuned to some extent on a per-resource basis.


8.  Acknowledgements

   An initial version of the table found in Section 5.2 has been
   provided in revision -05 of [I-D.ietf-core-coap].  Special thanks to
   Peter van der Stok for countless comments and discussions on this
   document, that contributed to its current structure and text.

   Thanks to Carsten Bormann, Zach Shelby, Michele Rossi, Nicola Bui,
   Michele Zorzi, Klaus Hartke, Cullen Jennings, Kepeng Li, Brian Frank,
   Peter Saint-Andre, Kerry Lynn, Linyi Tian, Dorothy Gellert, Francesco
   Corazza for helpful comments and discussions that have shaped the
   document.



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   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].


9.  References

9.1.  Normative References

   [I-D.ietf-core-block]
              Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
              draft-ietf-core-block-10 (work in progress), October 2012.

   [I-D.ietf-core-coap]
              Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
              "Constrained Application Protocol (CoAP)",
              draft-ietf-core-coap-13 (work in progress), December 2012.

   [I-D.ietf-core-groupcomm]
              Rahman, A. and E. Dijk, "Group Communication for CoAP",
              draft-ietf-core-groupcomm-05 (work in progress),
              February 2013.

   [I-D.ietf-core-observe]
              Hartke, K., "Observing Resources in CoAP",
              draft-ietf-core-observe-07 (work in progress),
              October 2012.

   [I-D.ietf-httpbis-p1-messaging]
              Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing",
              draft-ietf-httpbis-p1-messaging-22 (work in progress),
              February 2013.

   [I-D.ietf-httpbis-p2-semantics]
              Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Semantics and Content",
              draft-ietf-httpbis-p2-semantics-22 (work in progress),
              February 2013.

   [I-D.ietf-httpbis-p7-auth]
              Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Authentication", draft-ietf-httpbis-p7-auth-22
              (work in progress), February 2013.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.




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   [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.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

9.2.  Informative References

   [I-D.bormann-core-simple-server-discovery]
              Bormann, C., "CoRE Simple Server Discovery",
              draft-bormann-core-simple-server-discovery-01 (work in
              progress), March 2012.

   [I-D.shelby-core-resource-directory]
              Shelby, Z., Krco, S., and C. Bormann, "CoRE Resource
              Directory", draft-shelby-core-resource-directory-04 (work
              in progress), July 2012.

   [RFC3040]  Cooper, I., Melve, I., and G. Tomlinson, "Internet Web
              Replication and Caching Taxonomy", RFC 3040, January 2001.

   [RFC4732]  Handley, M., Rescorla, E., and IAB, "Internet Denial-of-
              Service Considerations", RFC 4732, December 2006.


Authors' Addresses

   Angelo P. Castellani
   University of Padova
   Via Gradenigo 6/B
   Padova  35131
   Italy

   Email: angelo@castellani.net


   Salvatore Loreto
   Ericsson
   Hirsalantie 11
   Jorvas  02420
   Finland

   Email: salvatore.loreto@ericsson.com






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   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
   KoanLogic
   Via di Sabbiuno 11/5
   Bologna  40136
   Italy

   Phone: +39 051 644 82 68
   Email: tho@koanlogic.com


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

   Email: esko.dijk@philips.com
























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