CoRE Working Group A.P. Castellani
Internet-Draft University of Padova
Intended status: Informational S. Loreto
Expires: July 07, 2013 Ericsson
A. Rahman
InterDigital Communications, LLC
T. Fossati
E. Dijk
Philips Research
January 03, 2013

Best Practices for HTTP-CoAP Mapping Implementation


This draft describes advanced features for HTTP-CoAP proxy implementors. It details deployment options, discusses possible approaches for URI mapping, and provides useful considerations related to protocol translation.

Status of This Memo

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Table of Contents

1. Terminology and Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

This document assumes readers are familiar with the terms and concepts that are used in [I-D.ietf-core-coap] . In addition, this document defines the following terminology:

A device providing cross-protocol HTTP-CoAP mapping is called an HTTP-CoAP cross-protocol proxy (HC proxy).

At least two different kinds of HC proxies exist:

2. Introduction

RESTful protocols, such as HTTP [RFC2616] and CoAP [I-D.ietf-core-coap], can interoperate through an intermediary proxy which performs cross-protocol mapping.

A base reference for the mapping process is provided in [I-D.ietf-core-coap]. However, depending on the involved application, deployment scenario, or network topology, such mapping can be realized using a wide range of intermediaries.

Moreover, the process of implementing such a proxy can be complex, and details regarding its internal procedures and design choices deserve further discussion, which is provided in this document.

This draft itself is an evolution of the mapping features covered in [I-D.castellani-core-http-mapping].

3. Use Case: HTTP/IPv4-CoAP/IPv6 Proxy

This section covers the expected common use case regarding an HTTP/IPv4 client accessing a CoAP/IPv6 resource.

While HTTP and IPv4 are today widely adopted communication protocols in the Internet, a pervasive deployment of constrained nodes exploiting the IPv6 address space is expected: enabling direct interoperability of such technologies is a valuable goal.

An HC proxy supporting IPv4/IPv6 mapping is said to be a v4/v6 proxy.

An HC v4/v6 proxy SHOULD always try to resolve the URI authority, and SHOULD prefer using the IPv6 resolution if available. The authority part of the URI is used internally by the HC proxy and SHOULD NOT be mapped to CoAP.

Figure 1 shows an HTTP client on IPv4 (C) accessing a CoAP server on IPv6 (S) through an HC proxy on IPv4/IPv6 (P). The DNS has an A record for "" resolving to the IPv4 address of the HC proxy, and an AAAA record with the IPv6 address of the CoAP server.

C     P     S
|     |     |
|     |     |  Source: IPv4 of C
|     |     |  Destination: IPv4 of P
+---->|     |  GET /foo HTTP/1.1
|     |     |  Host:
|     |     |  ..other HTTP headers ..
|     |     |
|     |     |  Source: IPv6 of P
|     |     |  Destination: IPv6 of S
|     +---->|  CON GET
|     |     |  URI-Path: foo
|     |     |
|     |     |  Source: IPv6 of S
|     |     |  Destination: IPv6 of P
|     |<----+  ACK
|     |     |
|     |     |  ... Time passes ...
|     |     |
|     |     |  Source: IPv6 of S
|     |     |  Destination: IPv6 of P
|     |<----+  CON 2.00
|     |     |  "bar"
|     |     |
|     |     |  Source: IPv6 of P
|     |     |  Destination: IPv6 of S
|     +---->|  ACK
|     |     |
|     |     |  Source: IPv4 of P
|     |     |  Destination: IPv4 of C
|<----+     |  HTTP/1.1 200 OK
|     |     |  .. other HTTP headers ..
|     |     |
|     |     |  bar
|     |     |


Figure 1: HTTP/IPv4 to CoAP/IPv6 Mapping

The proposed example shows the HC proxy operating also the mapping between IPv4 to IPv6 using the authority information available in any HTTP 1.1 request. This way, IPv6 connectivity is not required at the HTTP client when accessing a CoAP server over IPv6 only, which is a typical expected use case.

When P is an interception HC proxy, the CoAP request SHOULD have the IPv6 address of C as source (IPv4 can always be mapped into IPv6).

The described solution takes into account only the HTTP/IPv4 clients accessing CoAP/IPv6 servers; this solution does not provide a full fledged mapping from HTTP to CoAP.

In order to obtain a working deployment for HTTP/IPv6 clients, a different HC proxy access method may be required, or Internet AAAA records should not point to the node anymore (the HC proxy should use a different DNS database pointing to the node).

When an HC interception proxy deployment is used this solution is fully working even with HTTP/IPv6 clients.

4. Multiple Message Exchanges Mapping

This section discusses the mapping of the multicast and observe features of CoAP, which have no corresponding primitive in HTTP, and as such are not immediately translatable.

The mapping, which must be considered in both the arrow directions (H->C, C->H) may involve multi-part responses, as in the multicast use case, asynchronous delivery through HTTP bidirectional techniques, and HTTP Web Linking in order to reduce the semantics lost in the translation.

4.1. Relevant Features of Existing Standards

Various features provided by existing standards are useful to efficiently represent sessions involving multiple messages.

4.1.1. Multipart Messages

In particular, the "multipart/*" media type, defined in Section 5.1 of [RFC2046], is a suitable solution to deliver multiple CoAP responses within a single HTTP payload. Each part of a multipart entity SHOULD be represented using "message/http" media type containing the full mapping of a single CoAP response as previously described.

4.1.2. Immediate Message Delivery

An HC proxy may prefer to transfer each CoAP response immediately after its reception. This is possible thanks to the HTTP Transfer-Encoding "chunked", that enables transferring single responses without any further delay.

A detailed discussion on the use of chunked Transfer-Encoding to stream data over HTTP can be found in [RFC6202]. Large delays between chunks can lead the HTTP session to timeout, more details on this issue can be found in [I-D.thomson-hybi-http-timeout].

An HC proxy MAY prefer (e.g. to avoid buffering) to transfer each response related to a multicast request as soon as it comes in from the server. One possible way to achieve this result is using the "chunked" Transfer-Encoding in the HTTP response, to push individual responses until some trigger is fired (timeout, max number of messages, etc.).

An example showing immediate delivery of CoAP responses using HTTP chunks will be provided in Section 4.4, while describing its application to an observe session.

4.1.3. Detailing Source Information

Under some circumstances, responses may come from different sources (i.e. responses to a multicast request); in this case details about the actual source of each CoAP response MAY be provided to the client. Source information can be represented using HTTP Web Linking as defined in [RFC5988], by adding the actual source URI into each response using Link option with "via" relation type.

4.2. Multicast Mapping

In order to establish a multicast communication such a feature should be offered either by the network (i.e. IP multicast, link-layer multicast, etc.) or by a gateway (i.e. the HC proxy). Rationale on the methods available to obtain such a feature is out-of-scope of this document, and extensive discussion of group communication techniques is available in [I-D.ietf-core-groupcomm].

Additional considerations related to handling multicast requests mapping are detailed in the following sections.

4.2.1. URI Identification and Mapping

In order to successfully handle a multicast request, the HC proxy MUST successfully perform the following tasks on the URI:

The HC proxy MUST understand whether the requested URI identifies a group of nodes.
The HC proxy MUST know how to distribute the multicast request to involved servers; this process is specific of the group communication technology used.

When using IPv6 multicast paired with DNS, the mapping to IPv6 multicast is simply done using DNS resolution. If the group management is performed at the proxy, the URI or part of it (i.e. the authority) can be mapped using some static or dynamic table available at the HC proxy. In Section 3.5 of [I-D.ietf-core-groupcomm] discusses a method to build and maintain a local table of multicast authorities.

4.2.2. Request Handling

When the HC proxy receives a request to a URI that has been successfully identified and mapped to a group of nodes, it SHOULD start a multicast proxying operation, if supported by the proxy.

Multicast request handling consists of the following steps:

Multicast TX:
The HC proxy sends out the request on the CoAP side by using the methods offered by the specific group communication technology used in the constrained network;
Collecting RXs:
The HC proxy collects every response related to the request;
The HC proxy has to pay special attention in multicast timing, detailed discussion about timing depends upon the particular group communication technology used;
Distributing RXs to the client:
The HC proxy can distribute the responses in two different ways: batch delivering them at the end of the process or on timeout, or immediately delivering them as they are available. Batch requires more caching and introduces delays but may lead to lower TCP overhead and simpler processing. Immediate delivery is the converse. A trade-off solution of partial batch delivery may also be feasible and efficient in some circumstances.

4.2.3. Examples

Figure 2 shows an HTTP client (C) requesting the resource "/foo" to a group of CoAP servers (S1/S2/S3) through an HC proxy (P) which uses IP multicast to send the corresponding CoAP request.

C     P     S1    S2    S3
|     |     |     |     |
+---->|     |     |     |  GET /foo HTTP/1.1
|     |     |     |     |  Host:
|     |     |     |     |  .. other HTTP headers ..
|     |     |     |     |
|     +---->|---->|---->|  NON GET
|     |     |     |     |  URI-Path: foo
|     |     |     |     |
|     |<----------+     |  NON 2.00
|     |     |     |     |  "S2"
|     |     |     |     |
|     | X---------------+  NON 2.00
|     |     |     |     |  "S3"
|     |     |     |     |
|     |<----+     |     |  NON 2.00
|     |     |     |     |  "S1"
|     |     |     |     |
|     |     |     |     |  ... Timeout ...
|     |     |     |     |
|<----+     |     |     |  HTTP/1.1 200 OK
|     |     |     |     |  Content-Type: multipart/mixed;
|     |     |     |     |                boundary="response"
|     |     |     |     |  .. other HTTP headers ..
|     |     |     |     |
|     |     |     |     |  --response
|     |     |     |     |  Content-Type: message/http
|     |     |     |     |
|     |     |     |     |  HTTP/1.1 200 OK
|     |     |     |     |  Link: <>;
|     |     |     |     |        rel=via
|     |     |     |     |
|     |     |     |     |  S2
|     |     |     |     |
|     |     |     |     |  --response
|     |     |     |     |  Content-Type: message/http
|     |     |     |     |
|     |     |     |     |  HTTP/1.1 200 OK
|     |     |     |     |  Link: <>;
|     |     |     |     |        rel=via
|     |     |     |     |
|     |     |     |     |  S1
|     |     |     |     |
|     |     |     |     |  --response--
|     |     |     |     |


Figure 2: Unicast HTTP to Multicast CoAP Mapping

The example proposed in the above diagram does not make any assumption on which underlying group communication technology is available in the constrained network. Some detailed discussion is provided about it along the following lines.

C makes a GET request to This domain name MAY either resolve to the address of P, or to the IPv6 multicast address of the nodes (if IP multicast is supported and P is an interception proxy), or the proxy P is specifically known by the client that sends this request to it.

To successfully start multicast proxying operation, the HC proxy MUST know that the destination URI involves a group of CoAP servers, e.g. the authority is known to identify a group of nodes either by using an internal lookup table, using DNS paired with IPv6 multicast, or by using some other special technique.

A specific implementation option is proposed to further explain the proposed example. Assume that DNS is configured such that all subdomain queries to, such as, resolve to the address of P. P performs the HC URI mapping by removing the 'coap' subdomain from the authority and by switching the scheme from 'http' to 'coap' (result: "coap://"); "" is resolved to an IPv6 multicast address to which S1, S2 and S3 belong. The proxy handles this request as multicast and sends the request "GET /foo" to the multicast group .

4.3. Multicast Response Caching

We call perfect caching when the proxy uses only the cached representations to provide a response to the HTTP client. In the case of a multicast CoAP request, perfect caching is not adequate. This section updates the general caching and congestion control guidelines of with specific guidelines for the multicast use case.

Due to the inherent unreliable nature of the NON messages involved and since nodes may have dynamic membership in multicast groups, responding only with previously cached responses without issuing a new multicast request is not recommended. This perfect caching behaviour leads to miss responses of nodes that later joined the multicast group, and/or to repeatedly serve partial representations due to message losses. Therefore a multicast CoAP request SHOULD be sent by a HC proxy for each incoming request addressed to a multicast group.

Caching of multicast responses is still a valuable goal to pursue reduce network congestion, battery consumption and response latency. Some considerations to be performed when adopting a multicast caching behaviour are outlined in the following paragraph.

Caching of multicast GET responses MAY be implemented by adopting some technique that takes into account either knowledge about dynamic characteristics of group membership (occurrence or frequency of group changes) or even better its full knowledge (list of nodes currently part of the group).

When using a technique exploiting this knowledge, valid cached responses SHOULD be served from cache.

4.4. Observe Mapping

By design, and certainly not without a good rationale, HTTP lacks a publish-subscriber facility. This implies that the mapping of the CoAP observe semantics has to be created ad hoc, perhaps by making use of one of the well-known HTTP techniques currently employed to establish an HTTP bidirectional connection with the target resource - as documented in [RFC6202].

In the following sections we will describe some of the approaches that can be used to identify an observable resource and to create the communication bridging needed to set up an end to end HTTP-CoAP observation.

4.4.1. Identification

In order to appropriately process an observe request, the HC proxy needs to know whether a given request is intended to establish an observation on the target resource, instead of triggering a regular request-response exchange.

At least two different approaches to identify such special requests exist, as discussed below. Observable URI Mapping

An URI is said to be observable whenever every request to it implicitly requires the establishment of an HTTP bidirectional connection to the resource.

Such subscription to the resource is always paired, if possible, to a CoAP observe session to the actual resource being observed. In general, multiple connections that are active with a single observable resource at the same time, are multiplexed to the single observe session opened by the intermediary. Its notifications are then de-multiplexed by the HC proxy to every HTTP subscriber.

An intermediary MAY pair a couple of distinct HTTP URIs to a single CoAP observable resource: one providing the usual request-response mediated access to the resource, and the other that always triggers a CoAP observe session. Discovery

As shown in Figure 3, in order to know whether an URI is observable, an HTTP UA MAY do a pre-flight request to the target resource using the HTTP OPTIONS method (see section 6.2 of [I-D.ietf-httpbis-p2-semantics]) to discover the communication options available for that resource.

If the resource supports observation, the proxy adds a Link Header [RFC5988] with the "obs" attribute as link-param (see Section 7 of [I-D.ietf-core-observe]).