CORE G. Moritz
Internet-Draft University of Rostock
Intended status: Informational January 26, 2011
Expires: July 30, 2011
SOAP-over-CoAP Binding
draft-moritz-core-soap-over-coap-00
Abstract
The scope of this document is to provide a basis for a lightweight
SOAP over CoAP binding, to allow usage of SOAP Web services also in
resource constrained networks.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Requirements Language . . . . . . . . . . . . . . . . . . 4
1.3. Terminology and Definitions . . . . . . . . . . . . . . . 4
1.4. Requirements . . . . . . . . . . . . . . . . . . . . . . . 4
2. Use of CoAP . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. CoAP Content-Type . . . . . . . . . . . . . . . . . . . . 5
3. Binding Name . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Transport Layer Binding . . . . . . . . . . . . . . . . . . . 6
4.1. Source Address and Port . . . . . . . . . . . . . . . . . 6
5. Addressing . . . . . . . . . . . . . . . . . . . . . . . . . . 6
5.1. URI Scheme . . . . . . . . . . . . . . . . . . . . . . . . 6
5.2. Destination Addressing . . . . . . . . . . . . . . . . . . 6
6. Message Patterns . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Request response . . . . . . . . . . . . . . . . . . . . . 7
6.2. Retransmission . . . . . . . . . . . . . . . . . . . . . . 8
7. CoAP Header Options . . . . . . . . . . . . . . . . . . . . . 8
7.1. Unicast one-way . . . . . . . . . . . . . . . . . . . . . 8
7.2. Unicast request-response . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
9. Security Considerations . . . . . . . . . . . . . . . . . . . 12
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
10.1. Normative References . . . . . . . . . . . . . . . . . . . 12
10.2. Informative References . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 13
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1. Introduction
The intention of this document is to provide a basic approach for a
SOAP-over-CoAP binding. Readers of this document should be basically
familiar with the CoAP draft [I-D.ietf-core-coap], SOAP
[W3C.REC-soap12-part0-20070427], SOAP HTTP Binding
[W3C.REC-soap12-part1-20070427] and SOAP UDP binding specifications
[SOAP-over-UDP]. Parts of this document are derived from these
existing specifications. This document will not provide a
comprehensive specification, but may be a basis for further
discussions and to identify required changes in the current CoAP
[I-D.ietf-core-coap] protocol design which is on the way to become an
IETF standard. These changes might not be identified directly as the
binding described herein will not exploit from all features of CoAP
but use CoAP as an 'application layer transport protocol' as SOAP
already does with HTTP [W3C.REC-soap12-part1-20070427].
1.1. Motivation
The motivation behind this document is based on the initial I-D
[I-D.moritz-6lowapp-dpws-enhancements] and the resulting discussions.
The described binding herein is a major enabler for the efforts
towards usage of SOAP Web services for highly resource constrained
devices like wireless sensor nodes. As core protocol the OASIS
Devices Profile for Web Services (DPWS) [DPWS] is applied. By
combining DPWS with EXI, message size can be reduced significantly as
presented in [I-D.moritz-6lowapp-dpws-enhancements]. By providing a
binding of SOAP to CoAP, the message overhead can be reduced
significantly. Furthermore, SOAP is not required to use
inappropriate mechanisms like TCP congestion control. Reliably
messaging is guaranteed by CoAP internal mechanisms and thus
expensive retransmissions independent of successful or unsuccessful
delivery as required by the SOAP-over-UDP binding are avoided. In
summary, the major advantages are:
o more compact message format
o probably lower message parsing efforts compared to standard HTTP
headers
o avoid inappropriate TCP usage implied by SOAP-over-HTTP binding
o avoid unreliable nature of unicast SOAP-over-UDP binding
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1.2. Requirements Language
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].
1.3. Terminology and Definitions
Defined below are the basic definitions for the terms used in this
specification.
Receiver
The endpoint terminating a SOAP/CoAP message
Sender
The endpoint originating a SOAP/CoAP message
SOAP/CoAP message
A CoAP message containing a SOAP envelope in the CoAP message
body
This specification uses the constructs [action], [destination],
[message id], [reply endpoint], [address] as defined by in WS-
Addressing [W3C.PR-ws-addr-core-20060321].
The SOAP CoAP Binding is optional and SOAP nodes are not required to
implement it. A SOAP node that correctly and completely implements
the SOAP CoAP Binding may to be said to 'conform to the CoAP
Binding.'
1.4. Requirements
This specification intends to meet the following requirements:
o Support a one-way message-exchange pattern (MEP) where a SOAP
envelope is carried in a CoAP message from Sender to Receiver
only.
o Support a request-response message-exchange pattern (MEP) where
SOAP envelopes are carried in CoAP messages from Sender to
Receiver and back from the origin Receiver to the origin Sender.
Even if reasonable, supporting multicast transmissions of SOAP
envelopes carried in CoAP messages are out of scope of this version
of this document and require further research. This binding supports
SOAP 1.2 [W3C.REC-soap12-part0-20070427] Envelopes. Supporting SOAP
1.1 envelopes is out of scope but might be added in future versions
of this document.
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Messages conforming to either SOAP specification can use this
binding. This specification relies on WS-Addressing. The SOAP
envelope MUST use the mechanisms defined in WS-Addressing
[W3C.PR-ws-addr-core-20060321].
2. Use of CoAP
This binding of SOAP to CoAP is intended to make appropriate use of
CoAP as an application protocol. For example, successful and failure
responses are indicated by the appropriated CoAP response codes (e.g.
2xx, 4xx, 5xx). This binding is not intended to fully exploit the
features of CoAP, but rather to use CoAP specifically for the purpose
of communicating with other SOAP nodes implementing the same binding.
2.1. CoAP Content-Type
Conforming implementations of this binding:
o MAY be capable of sending and receiving SOAP/CoAP messages
serialized using media type 'application/soap+xml'.
o MAY be capable of sending and receiving SOAP/CoAP messages
serialized using media type 'application/exi'.
o MUST send requests and responses using such media types provide
for at least the transfer of SOAP XML Infoset, including
'application/soap+xml' and 'application/exi'.
A SOAP/CoAP message MUST contain the CoAP Content-type Identifies
Option and MUST contain a value which satisfies the three points
above.
Note: CoAP has no header option which corresponds to HTTP Accept.
Thus the web methods known from the HTTP binding is not possible. In
the current CoAP draft only view information are available how to
define own header fields.
3. Binding Name
This binding is identified by the URI (see SOAP 1.2 Part 1
[W3C.REC-soap12-part1-20070427] SOAP Protocol Binding Framework):
http://www.ws4d.org/2011/01/soap/bindings/CoAP/
Note: The binding name is tentative but required to indicate the
corresponding binding e.g. in the WSDL of a service.
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4. Transport Layer Binding
The CoAP binding MUST operate over UDP transport layer.
Note: CoAP defines a maximum message size which refers to the IP
layer. The existing UDP binding instead refers only to UDP and
defines a general maximum packet size independent of IP.
4.1. Source Address and Port
The source address MUST be supplied at the IP packet level and MUST
be the IPv4 address (limited to unicast addresses) or IPv6 address
(limited to unicast addresses) of the sender; the receiver SHOULD
reject IP packets containing a SOAP/CoAP message that have
inappropriate values for the source address.
Even though CoAP is intended to be used on the well-known ports
defined in CoAP-04, the use of CoAP is not limited to these ports.
As a result, it is possible to have a dedicated CoAP server for
handling SOAP processing on a distinct port.
5. Addressing
5.1. URI Scheme
The SOAP CoAP binding defines a base URI according to the rules in
CoAP-04. I.e., the base URI is the CoAP Request-URI or the value of
the CoAP Location option field.
5.2. Destination Addressing
WS-Addressing defines a URI,
'http://www.w3.org/2005/08/addressing/anonymous', that can appear in
the [address] property of an endpoint reference. If the [reply
endpoint] property of a SOAP message transmitted over CoAP has an
[address] property with this value, the UDP source address (and
source port) is considered to be the address to which reply messages
should be sent.
The implied value of the [reply endpoint] property for SOAP messages
transmitted over CoAP is an endpoint reference with an [address]
property whose value is
'http://www.w3.org/2005/08/addressing/anonymous'.
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6. Message Patterns
This specification supports the following message patterns:
o Unicast one-way
o Unicast request, unicast response
as described in the remainder of this section. All SOAP/CoAP
messages MUST use the POST method of CoAP, whereby POST is
interpreted in accordance to RFC2616.
Note: In the current draft of CoAP-04, response Code 2.00 OK is only
possible for GET methods. But GET is not the appropriate method for
this binding.
Note: There is no Action field similar to SOAPAction in HTTP, so
extensions are required to the basic CoAP Options. Hence the web
method feature of the HTTP binding cannot be made possible.
CoAP defines Proxy mechanism for caching of responses. Because the
CoAP binding defined herein is intended for service communication and
not RESTful resource manipulation, caching should be avoided. CoAP
defines the Max-Age option to be default a non-zero value. Hence
each SOAP/CoAP message MUST contain the Max-Age option and the value
MUST be zero.
6.1. Request response
To match a request with a response the CoAP Token Option can be used.
The CoAP Token Option SHOULD be carried in all request-response SOAP/
CoAP messages. WS-Addressing defines the [message id] property to
identify messages in time and space and to match requests with
response. In case of using the CoAP/SOAP binding the [message id]
property is SHOULD be empty and MUST contain a value in case if the
CoAP Token Option is not present.
Note: The intention of this paragraph is to reduce message size. The
[message id] property has a typical size of 45 Bytes and cannot by
compressed by knowledge based encodings like EXI because the value of
this property is unique in each message. The CoAP Token Option may
be much more compact by providing the same functionality.
CoAP defines the feature of deferred responses where a transaction
ACK is sent as response to a CON message to indicate a deferred
response. If the value of the [reply endpoint] of the request is
'http://www.w3.org/2005/08/addressing/anonymous', the ACK message
SHOULD NOT carry any payload (e.g. a SOAP Envelope) in the CoAP
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message body if not required by the application logic. If the value
of the [reply endpoint] is not
'http://www.w3.org/2005/08/addressing/anonymous', the origin Receiver
cannot guarantee that the response is intended to be sent to the same
entity like the client and SHOULD include a SOAP Envelope providing
details of the request for the entity behind the [reply endpoint].
6.2. Retransmission
To avoid repeated packet collisions, any retransmission
implementation SHOULD observe good practices such as using
exponential back-off algorithms and spreading. An implementation
SHOULD use the Confirmable (CON) transaction message mechanism
described in the CoAP specification to avoid unnecessary
retransmissions. For each transmission of such a message, the value
of the [message id] property and the CoAP Token Option MUST be the
same.
Note: WS-Event Delivery should not use CON due to ACK flood at Event
Source. Multicast messages also should use NON messages. Not sure
if such requirements are in scope of this document.
7. CoAP Header Options
In this section, the CoAP header and CoAP header options usage is
described in detail.
7.1. Unicast one-way
The unicast one-way message pattern consists of only one request
without a response. Due to the CoAP binding the request consists of
one CoAP transaction. If CoAP Confirmable messaging is used, an
empty CoAP ACK message must be send back to the origin sender to
indicate success or failure.
The request is formulated according to the table below, but can be
extended for application specific needs.
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+--------------+----------------------------------------------------+
| CoAP | Value |
| field/header | |
| option | |
+--------------+----------------------------------------------------+
| CoAP Method | POST is the only supported method of this binding. |
| Request URI | The request URI confirming a CoAP URI [xxx] and |
| | identifying the WS-Addressing [address] endpoint |
| | property (network address). If the value of the |
| | WS-Addressing [address] endpoint property is |
| | 'http://www.w3.org/2005/08/addressing/anonymous' |
| | (directly set or implied by an empty [address] |
| | property), the CoAP Uri-Path Option and the CoAP |
| | Uri-Query Option are empty. |
| Message ID | If messaging model is CON, carrying value in |
| | accordance to messaging model of CoAP . Otherwise |
| | not included. |
| Content-type | Media type of CoAP message body. |
| Option | |
| Token Option | Token in accordance to CoAP specification |
| | identifying the transaction in time and space. |
| | Only required if message is a CoAP Confirmable |
| | message. |
| CoAP message | SOAP message serialized according to the rules for |
| body | carrying SOAP messages in the media type given by |
| | the Content-type Option. |
+--------------+----------------------------------------------------+
Table 1: Unicast one-way Request
If the request is send as Non-Confirmable CoAP message, no response
is send back to the origin requester. If the request is a
Confirmable CoAP message, the response consists only of a CoAP ACK
without any data in the CoAP message body. In case of a Conformable
CoAP request the response is formulated according to the table below,
but can be extended for application specific needs.
+--------------+----------------------------------------------------+
| CoAP | Value |
| field/header | |
| option | |
+--------------+----------------------------------------------------+
| CoAP Status | Status Code in accordance to codes defined in CoAP |
| Code | and interpreted as described in the SOAP-over-HTTP |
| | binding. Note: CoAP describes only a subset of |
| | HTTP status codes and adds own codes. This must |
| | require further alignment. |
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| Message ID | If messaging model is CON, carrying value in |
| | accordance to messaging model of CoAP . Otherwise |
| | not included. |
| Token Option | Token in accordance to CoAP specification |
| | identifying the transaction in time and space. |
| | Only required if message is a CoAP Confirmable |
| | message. |
+--------------+----------------------------------------------------+
Table 2: Unicast one-way ACK Response
Application developers must be aware that the one-way message
exchange patterns returns neither failure nor success messages if
Non-Confirmable messages are used.
7.2. Unicast request-response
The unicast request-response message pattern consists of one request
and one response. Due to the CoAP binding the request consists of
one or more CoAP transactions.
The request is formulated according to the table below, but can be
extended for application specific needs.
+--------------+----------------------------------------------------+
| CoAP | Value |
| field/header | |
| option | |
+--------------+----------------------------------------------------+
| CoAP Method | POST is the only supported method of this binding. |
| Request URI | The request URI confirming a CoAP URI [xxx] and |
| | identifying the WS-Addressing [address] endpoint |
| | property (network address). If the value of the |
| | WS-Addressing [address] endpoint property is |
| | 'http://www.w3.org/2005/08/addressing/anonymous' |
| | (directly set or implied by an empty [address] |
| | property), the CoAP Uri-Path Option and the CoAP |
| | Uri-Query Option are empty. |
| Message ID | If messaging model is CON, carrying value in |
| | accordance to messaging model of CoAP . Otherwise |
| | not included. |
| Content-type | Media type of CoAP message body. |
| Option | |
| Token Option | Token in accordance to CoAP specification |
| | identifying the transaction in time and space. |
| | Only required if message is a CoAP Confirmable |
| | message. |
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| CoAP message | SOAP message serialized according to the rules for |
| body | carrying SOAP messages in the media type given by |
| | the Content-type Option. |
+--------------+----------------------------------------------------+
Table 3: Unicast request-response Request
If the request is send as Non-Confirmable CoAP message, no CoAP ACK
is send back to the origin requester. In this case, for the
response, the origin Receiver has to initiate a new CoAP transaction
which can be either Non-Confirmable or Confirmable.
If the request is a Confirmable CoAP message, the response consists
of a CoAP ACK which may carry the response SOAP envelope as data in
the CoAP message body. For the response, the origin receiver may
also initiate a new CoAP transaction after sending the CoAP ACK to
the origin Sender, which can be either also Non-Confirmable or
Confirmable. (see deferred responses in CoAP)
+--------------+----------------------------------------------------+
| CoAP | Value |
| field/header | |
| option | |
+--------------+----------------------------------------------------+
| CoAP Status | Status Code in accordance to codes defined in CoAP |
| Code | and interpreted as described in the SOAP-over-HTTP |
| | binding. Note: CoAP describes only a subset of |
| | HTTP status codes and adds own codes. This must |
| | require further alignment. |
| Message ID | If messaging model is CON, carrying value in |
| | accordance to messaging model of CoAP . Otherwise |
| | not included. |
| Token Option | Token in accordance to CoAP specification |
| | identifying the transaction in time and space. |
| | Only required if message is a CoAP Confirmable |
| | message. |
| Content-type | Media type of CoAP message body. |
| Option | |
| CoAP message | SOAP message serialized according to the rules for |
| body | carrying SOAP messages in the media type given by |
| | the Content-type Option. |
+--------------+----------------------------------------------------+
Table 4: Unicast request-response ACK with Response
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8. IANA Considerations
No IANA issues have been identified in this draft.
9. Security Considerations
Will be added in future versions.
10. References
10.1. Normative References
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
"Constrained Application Protocol (CoAP)",
draft-ietf-core-coap-04 (work in progress), January 2011.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[W3C.PR-ws-addr-core-20060321]
Gudgin, M., Hadley, M., and T. Rogers, "Web Services
Addressing 1.0 - Core", W3C PR PR-ws-addr-core-20060321,
March 2006.
[W3C.REC-soap12-part0-20070427]
Mitra, N. and Y. Lafon, "SOAP Version 1.2 Part 0: Primer
(Second Edition)", World Wide Web Consortium
Recommendation REC-soap12-part0-20070427, April 2007,
<http://www.w3.org/TR/2007/REC-soap12-part0-20070427>.
10.2. Informative References
[DPWS] Driscoll and Mensch, "OASIS Devices Profile for Web
Services (DPWS) Version 1.1", 2009,
<http://docs.oasis-open.org/ws-dd/ns/dpws/2009/01>.
[I-D.moritz-6lowapp-dpws-enhancements]
Moritz, G., "DPWS for 6LoWPAN",
draft-moritz-6lowapp-dpws-enhancements-01 (work in
progress), June 2010.
[SOAP-over-UDP]
Jeyaraman, "OASIS SOAP-over-UDP Version 1.1", 2009, <http:
//docs.oasis-open.org/ws-dd/soapoverudp/1.1/os/
wsdd-soapoverudp-1.1-spec-os.html>.
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[W3C.REC-soap12-part1-20070427]
Gudgin, M., Karmarkar, A., Nielsen, H., Hadley, M.,
Mendelsohn, N., Lafon, Y., and J. Moreau, "SOAP Version
1.2 Part 1: Messaging Framework (Second Edition)", World
Wide Web Consortium Recommendation REC-soap12-part1-
20070427, April 2007,
<http://www.w3.org/TR/2007/REC-soap12-part1-20070427>.
Author's Address
Guido Moritz
University of Rostock
18051 Rostock,
Germany
Phone: +49 381 498 7269
Email: guido.moritz@uni-rostock.de
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