Internet DRAFT - draft-mietz-coap-state-option
draft-mietz-coap-state-option
core R. Mietz
Internet-Draft University of Luebeck
Intended status: Standards Track June 7, 2013
Expires: December 9, 2013
CoAP High-Level State Option Extension
draft-mietz-coap-state-option-00
Abstract
CoAP is a RESTful application protocol for constrained devices which
are often equipped with sensors measuring a physical phenomenon such
as temperature on a precise scale. These sensor values are made
available by a resource on the CoAP endpoint. However, for many
applications it is not necessary to have the full precision a sensor
can provide. It's often even enough to only have some high-level
states instead of raw values. This document presents a new option
for CoAP to dynamically create new resources for a sensor which
provides user-defined high-level states instead of raw sensor values.
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
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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 December 9, 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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publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
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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
1.1. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
2. High-Level State Option Extension . . . . . . . . . . . . . . 4
2.1. High-level State Option Definition . . . . . . . . . . . . 4
2.2. Using the High-level State Option . . . . . . . . . . . . 5
2.2.1. Creating State Resources . . . . . . . . . . . . . . . 5
2.2.2. Querying State Resources . . . . . . . . . . . . . . . 9
2.2.3. Deleting States Resources . . . . . . . . . . . . . . 10
3. Examples of Usage . . . . . . . . . . . . . . . . . . . . . . 10
3.1. Example 1 . . . . . . . . . . . . . . . . . . . . . . . . 10
3.2. Example 2 . . . . . . . . . . . . . . . . . . . . . . . . 13
4. Security Considerations . . . . . . . . . . . . . . . . . . . 15
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.1. Normative reference . . . . . . . . . . . . . . . . . . . 16
7.2. Informative Reference . . . . . . . . . . . . . . . . . . 16
Appendix A. XML Schema for XML Serialization of One State
Resources . . . . . . . . . . . . . . . . . . . . . . 16
Appendix B. XML Schema for XML Serialization of Multiple
State Resources . . . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19
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1. Introduction
This document adds a new option to the Constrained Application
Protocol (CoAP): High-Level State.
1.1. Motivation
The Constrained Application Protocol [I-D.ietf-core-coap] (CoAP) is a
lightweight efficient variant of the well-known Hypertext Transfer
Protocol specifically designed for devices with limited resources
such as small memory, little processing power, and constrained energy
capacities. The main area of operation of CoAP is on wireless sensor
nodes, i.e., wireless devices equipped with sensors to monitor
environmental parameters such as temperature, air quality, or
humidity. Hence, not only static metadata but also sensor values are
retrieved by users via CoAP. The change frequency of measured sensor
values depends on the one hand on the accuracy of the sensor but on
the other hand on the (physical) property the sensor measures. It
therefore may vary from milliseconds up to minutes, hours, or even
days. A user interested in a parameter needs to request the current
value periodically to keep track of changes. However, periodic
querying can consume a good portion of the total amount of energy
available and results in the quick depletion of a device's energy.
Additionally, many requests might be unnecessary because the sensor
value did not change compared to the last request. For that reason,
CoAP observe [I-D.ietf-core-observe] introduces a mechanism to
register interest in a resource much like with publish-subscribe
systems. A CoAP server then only sends a response whenever the
sensor value changes. As a result, only a reduced number of messages
are required to keep track of the sensor readings.
Although the CoAP observe option can save resources, it might happen
that the number of messages and thus, resource consumption even
increases. This happens if the sensor value changes very often
resulting in frequent update messages. Besides that, some clients
may not be interested in raw precise sensor values but in a range a
sensor values falls into. We call this range a high-level state
because it categorizes the sensor value. So, instead of being
interested if a room has 21.9 oC or 22.1 oC, the user might only want
to know if it is "warm" in that room. The number of states for an
environmental parameter is typically low. Accordingly, states change
with much lower frequency as the underlying raw sensor values.
Consequently, this leads to fewer responses when using CoAP observe.
The High-Level State option allows creating, querying and deleting
high-level state resources for sensor resources on CoAP servers with
the known CoAP request methods GET, POST, and DELETE. The user can
define which sensor values are mapped to which high-level state.
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Additionally, he can retrieve descriptions of already existing high-
level state resources to reuse them.
1.2. Terminology
The keywords "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 uses terms of the Constrained Application Protocol as
defined in the terminology section of [I-D.ietf-core-coap].
Additionally, this specification defines the following terms:
High-level state:
In contrast to a raw sensor reading a high-level state combines a
number of sensor outputs under a new descriptive term given as a
string.
High-level state resource:
A CoAP resource, which returns different high-level states. State
resource will be used synonymously with the term high-level state
resource.
2. High-Level State Option Extension
2.1. High-level State Option Definition
+------+---+---+---+---+------------+--------+----------+---------+
| Type | C | U | N | R | Name | Format | Length | Default |
+------+---+---+---+---+------------+--------+----------+---------+
| TBD | - | - | - | x | High-Level | (see | 1-257 B | (none) |
| | | | | | State | below) | | |
+------+---+---+---+---+------------+--------+----------+---------+
Figure 1: High-Level State Option Definition
The High-Level State Option is "elective" and "proxy-safe". It is
"repeatable". Hence, the High-Level State Option can occur more than
once. The use of repetition will be described in the following
sections.
This Option can only be present in requests. Additionally, it has
different semantics when used with different request methods. These
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are described in the following sections.
The value carried in the Option has the following general format:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| T | - |
+-+-+-+-+-+-+-+-+
| Optional |
+ Values +
...
+ (0-257 bytes) +
| |
+-+-+-+-+-+-+-+-+
Figure 2: General format
T (TYPE): The value of the TYPE field is a 2 bit integer. If used in
POST requests, it indicates the value format after the first byte.
If used in GET requests, it specifies what representation a client
expects in response to that request. Further details are discussed
in Section 2.2.1 and Section 2.2.2 about the creation respectively
querying of state resources.
The bits 2-7 are unused and MUST be ignored by the server.
2.2. Using the High-level State Option
The semantics of the Option depend on the used CoAP request method.
In short, POST and DELETE create and remove state resources while GET
is used to retrieve the state or a description of existing state
resources.
2.2.1. Creating State Resources
To create a new state resource for a sensor resource, the client has
to POST a request to the Uri-Path of the sensor. The server MUST
create the state resource as a subresource of the sensor resource.
The output of most sensors is on a continuous numerical scale.
However, some sensors output string or Boolean data types (true/
false respectively 1/0). The client should be able to map each of
these data types to high-level states. For numerical values, the
client should be able to specify mappings from intervals to states
and for strings it should be able to map one or several different
strings to a state. Boolean values can be easily mapped by using
either the string mapping if the output is true or false or the
numerical mapping if the output is 1 or 0. Hence, two formats for
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the Option are available.
The used format is indicated by the TYPE field. Figure 3 shows the
data types of a sensor output along with the integer used for the
TYPE field and the format which is assumed to follow. Boolean is not
listed because, as argued before, it can be mapped with the integer
or string type.
+-----------+------+--------+
| Data type | TYPE | Format |
+-----------+------+--------+
| integer | 0 | 1 |
| float | 1 | 1 |
| string | 2 | 2 |
+-----------+------+--------+
Figure 3: Format types
The number of bytes used for values in the different Option formats
is given in Figure 4. Floats are encoded in Single-precision
floating-point format as defined in [IEEE754]. One Option defines
one state. Therefore, by repeating the High-Level State Option
several states can be defined.
+-----------+----------------+
| Data type | Length (bytes) |
+-----------+----------------+
| integer | 2 |
| float | 4 |
| string | 0-128 |
+-----------+----------------+
Figure 4: Data type lengths
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Format 1 is used for a mapping of an interval of numerical values to
a state. It consists of a numerical lower and a numerical upper
bound as well as a string giving the state name. The lower bound is
inclusive while the upper bound is exclusive. This allows defining
consecutive, continuous, non-intersecting intervals.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| T | - |
+-+-+-+-+-+-+-+-+
| Lower Bound |
+ (incl.) +
...
+ (2-4 bytes) +
| |
+-+-+-+-+-+-+-+-+
| Upper Bound |
+ (excl.) +
...
+ (2-4 bytes) +
| |
+-+-+-+-+-+-+-+-+
| |
+ State +
...
+ (0-128 bytes) +
| |
+-+-+-+-+-+-+-+-+
Figure 5: Format 1: Used for an interval mapping
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Format 2 is to map a string to a state. Whenever the string output
of the sensor matches the string which is given as the first
parameter the string given as the second parameter is the current
state.
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
| T | - |
+-+-+-+-+-+-+-+-+
| |
+ Output +
...
+ (0-128 bytes) +
| |
+-+-+-+-+-+-+-+-+
| |
+ State +
...
+ (0-128 bytes) +
| |
+-+-+-+-+-+-+-+-+
Figure 6: Format 2: Used for a string mapping
The server MUST ignore payload enclosed in the request.
If the server successfully created the state resource serving the
defined states, it MUST send a response with response code 2.01
(Created) and the Location-Path Option which gives the relative Uri-
Path for the newly created resource. The Uri-Path of the created
resource can be an arbitraly allowed string. However, it is
RECOMMENDED to use short Uri-Paths.
As already mentioned, by repeating the Option, several states can be
defined. However, if the Option is repeated with different values
for the TYPE field, the server MUST NOT process the request, and MUST
send a response with response code 4.02 (Bad Option).
If the upper bound of an interval is smaller than or equal the lower
bound of that interval, the server MUST NOT process the request, and
MUST send a response with response code 4.02 (Bad Option).
Due to ambiguity, it is forbidden to define different states for the
same value. Hence, if a numerical mapping is used and if at least
one intersection of any two intervals is non-empty, the server MUST
NOT process the request, and MUST send a response with response code
4.02 (Bad Option). The same holds, if a string mapping is used and
the same string is mapped to different states.
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In the case that a client requests creation of a state resource with
exactly same mappings and states as an already existing one, the
server SHOULD send a response with response code 2.05 (Content) with
the Uri-Path of the existing state resource as payload instead of
creating another resource with the same semantics. Implementers must
be aware that state resources can be deleted anytime. Accordingly,
if a state resource is used by several clients and one deletes it,
the other clients are not aware of that. Contrary, creating state
resources with same semantics for each client, consumes more
resources.
If the server is not able to create the state resource for the given
sensor resource, e.g., because of insufficient resources, it MUST
send a response with response code 5.03 (Service Unavailable). The
server SHOULD include a payload indicating the reason for not
creating the state resource.
If a client requests to create a state resource for a non-sensor
resource, the server MUST NOT process the request and MUST send a
response with response code 4.03 (Forbidden).
If the TYPE of a request is not matching the data type outputted by
the sensor, the server SHOULD reject the request and SHOULD send a
response with response code 4.02 (Bad Option).
2.2.2. Querying State Resources
The current state of a state resource can be retrieved by a normal
GET-request without the High-Level State Option present in the
request. However, by including the Option, the client can control
the data which should be returned. The type of data that the server
should return is indicated by the TYPE field in the request.
Figure 7 gives an overview of all available T values.
+--------------+-----+
| T | No. |
+--------------+-----+
| State | 0 |
| State Number | 1 |
| Description | 2 |
+--------------+-----+
Figure 7: Response types
If T = 0, the server MUST return the current state. With T = 1 an
integer representing the state MUST be returned by the server. The
integer is determined as follows: in each state resource creation
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process upon a POST-request the states MUST be enumerated starting
with 0 by their order of appearance in the High-Level State Options.
If T = 2 the server MUST return a description of the state resource
describing the mappings and the appropriate states. An XML Schema
for the XML format is given in Appendix A. If there is no defined
state for a sensor value, i.e., there is no mapping for this value,
the server MUST return the state "undefined" if T = 0 and -1 if T =
1.
A list of all available state resource of a sensor resources can be
retrieved by sending a GET-request with High-Level-State Option to
the sensor resource with T = 2 included. The xml format the server
MUST return is defined by the XML Schema in Appendix B. All other
requests with the High-Level State Option to a sensor resource SHOULD
be ignored by the server.
2.2.3. Deleting States Resources
If a client wants to remove a state resource, it has to send a DELETE
message to the state resource. The server MUST delete the state
resource and a "2.02 (Deleted) response code SHOULD be used on
success or in case the resource did not exist before the request" as
stated in Section 5.8.4 in [I-D.ietf-core-coap].
3. Examples of Usage
In the following sections two examples show how the high-level state
option is used to create, query, and delete state resources.
3.1. Example 1
Consider four users who want to retrieve data from a temperature
sensor. However, they are not interested in the raw values but high-
level states. Hence, they want to create state resources on the
server. The parameters of the High-Level State Option in the
examples are T and the appropriate optional values as described in
Section 2.2.
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User 1 is the first to communicate with the CoAP server. He wants to
have two states, namely a "cold" and a "warm" state, where the first
is defined as temperature between -50 oC and 20 oC and the second all
values which are between 20 oC and 50 oC. For that, a POST-request
with the bounds is send to the server:
Client Server
| |
| |
+----->| Header: POST
| POST | Token: 0x06
| | Uri-Path: "temp"
| | High-Level State: "1 -50.0 20.0 cold"
| | High-Level State: "1 20.0 50.0 warm"
| |
|<-----+ Header: 2.01 Created
| 2.01 | Token: 0x06
| | Location-Path: x42y
| |
Figure 8: User 1 creating a state resource with two states
The server answers with a response code of 2.01 (Created) and a
Location-Path indicating that a state resource, which can serve the
two desired high-level states, is now available under the given path.
Second, the next user wants to have four states ("cold", "moderate",
"warm", and "hot") with bounds of -50 oC, 0 oC, 10 oC, 25 oC, and 50
oC. The communication to create these states looks as follows:
Client Server
| |
| |
+----->| Header: POST
| POST | Token: 0x09
| | Uri-Path: "temp"
| | High-Level State: "1 -50.0 0.0 cold"
| | High-Level State: "1 0.0 10.0 moderate"
| | High-Level State: "1 10.0 25.0 warm"
| | High-Level State: "1 25. 50 hot"
| |
|<-----+ Header: 2.01 Created
| 2.01 | Token: 0x09
| | Location-Path: 1ee7
| |
Figure 9: User 2 creating another state resource
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Afterwards, the server has two state resources for the sensor
resource. The one serves two states and the other one which serves
four states.
Also the third user wants to create a high-level resource. By
chance, he requests the same states and bounds as the first user
which can be seen in the POST-request:
Client Server
| |
| |
+----->| Header: POST
| POST | Token: 0x19
| | Uri-Path: "temp"
| | High-Level State: "1 -50.0 20.0 cold"
| | High-Level State: "1 20.0 50.0 warm"
| |
|<-----+ Header: 2.05 Content
| 2.05 | Token: 0x19
| | Location-Path: x42y
| |
Figure 10: Creation of a State Resource with Same Semantics as
Already Existing State Resource
Consequently, the response is different too the one sent to user 1.
The response code is 2.04 (Changed) and the Location-Path Option
gives the path of the already existing resource.
The last user finally sends his POST-request to create another
resource with three states:
Client Server
| |
| |
+----->| Header: POST
| POST | Token: 0x83
| | Uri-Path: "temp"
| | High-Level State: "1 -60.0 12.3 cold"
| | High-Level State: "1 12.3 21.9 medium"
| | High-Level State: "1 21.9 72.0 warm"
| |
|<-----+ Header: 5.03 Service Unavailable
| 5.03 | Token: 0x83
| | Payload: "Already too many resources"
| |
Figure 11: Failing of State Resource Creation due to Low Resources
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Unfortunately, the server rejected to create a new resource due to
insufficient resources which is indicated by the response. Because
of that the user sent another request to retrieve a list of already
available state resources:
Client Server
| |
| |
+----->| Header: GET
| GET | Token: 0x84
| | Uri-Path: "temp"
| | Accept: application/json
| | High-Level State: 2
| |
| |
|<-----+ Header: 2.05 Content
| 2.05 | Token: 0x84
| | Payload: "{res:{r:[{
| | p:'x42y',
| | num:[{l:-50,h:20,s:'cold'},
| | {l:20,h:50,s:'warm'}]},{
| | p:'1ee7',
| | num:[{l:-50,h:0,s:'cold'},
| | {l:0,h:10,s:'moderate'},
| | {l:10,h:25,s:'warm'},
| | {l:25,h:50,s:'hot'}]}]}}"
| |
Figure 12: Retrieving List of Existing State Resources
With this list of JSON-encoded state resource the user has the
ability to decide if he wants to use one of the existing state
resources.
3.2. Example 2
In the second example, we consider a sensor which outputs strings
describing the current weather and a user who first wants to use a
string mapping to create a state resource for the weather sensor.
Afterwards, he queries the state resource and finally deletes it.
The output range of the sensor is "rainy", "cloudy", "sunny", and
"foggy".
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The client sends a POST-request with a string mapping to create the
two states "home" and "beach".
Client Server
| |
| |
+----->| Header: POST
| POST | Token: 0x06
| | Uri-Path: "weather"
| | High-Level State: "2 rainy home"
| | High-Level State: "2 cloudy home"
| | High-Level State: "2 foggy home"
| | High-Level State: "2 sunny beach"
| |
|<-----+ Header: 2.01 Created
| 2.01 | Token: 0x06
| | Location-Path: mr21
| |
Figure 13: The User Creates a State Resource
After creation of the state resource, he retrieves the current state
by sending a GET-request.
Client Server
| |
| |
+----->| Header: GET
| GET | Token: 0x09
| | Uri-Path: "weather/mr21"
| | High-Level State: "0"
| |
|<-----+ Header: 2.05 Content
| 2.05 | Token: 0x83
| | Payload: "beach"
| |
Figure 14: The User Retrieves the Current State
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Because it is good weather, the user decides to go to the beach. But
before, he releases the resources on the server by deleting the state
resource.
Client Server
| |
| |
+------->| Header: DELETE
| DELETE | Token: 0x09
| | Uri-Path: "weather/mr21"
| |
|<-------+ Header: 2.02 Deleted
| 2.05 | Token: 0x83
| |
Figure 15: The User Retrieves the Current State
4. Security Considerations
PUT operations, often used for updates, in conjunction with the High-
Level State Option are forbidden. Hence, the server MUST NOT process
such requests and MUST respond with a response code of 4.05 (Method
not allowed). Updates of state resources can lead to unexpected
behavior of clients if several clients use the same state resource.
If one client is updating mappings or states of a state resource,
other clients which are not aware of the update could end up in
abnormal behavior because they cannot handle the unexpected results.
For the same reason, a server SHOULD NOT reuse Uri-Paths of deleted
state resources.
Depending on the implementation and the remaining resources of the
server, creation of state resources can consume a considerable amount
of resources. However, this is true for all resource creations and
not limited to the presented new High-Level State Option. Anyway,
implementers SHOULD be aware of this fact and consider
countermeasures such as limiting the number of state resources which
can be created, limiting the number of state resource creation
requests per client, or to introduce a duration after a successful
state resource creation in which further requests are rejected.
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5. IANA Considerations
The IANA is requested to add the following "CoAP Option Numbers"
entry as per Section 12.2 of [I-D.ietf-core-coap]:
+--------+------------------+-----------------------------+
| Number | Name | Reference |
+--------+------------------+-----------------------------+
| TBD | High-Level State | Section 2 of this document |
+--------+------------------+-----------------------------+
6. Acknowledgements
Thanks to Lukas Ruge, Dennis Pfisterer, Kay Roemer and Philipp
Abraham for proof-reading, helpful comments and discussions that have
helped to shape this document.
7. References
7.1. Normative reference
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., and C. Bormann, "Constrained
Application Protocol (CoAP)", draft-ietf-core-coap-16
(work in progress), May 2013.
[IEEE754] Institute of Electrical and Electronics Engineers (IEEE),
"754-2008 - IEEE Standard for Floating-Point
Arithmetic", August 2008.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
7.2. Informative Reference
[I-D.ietf-core-observe]
Hartke, K., "Observing Resources in CoAP",
draft-ietf-core-observe-08 (work in progress),
February 2013.
Appendix A. XML Schema for XML Serialization of One State Resources
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<?xml version="1.0" encoding="utf-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns="http://www.example.com/state-option"
targetNamespace="http://www.example.com/state-option"
elementFormDefault="qualified"
attributeFormDefault="unqualified">
<!-- To describe a state resources -->
<xs:element name="r" type="resource"/>
<!-- To describe the mappings of a state resource along
with the uri-path-->
<xs:complexType name="resource">
<xs:sequence>
<xs:choice>
<xs:element name="num" type="num_map" maxOccurs="unbounded"/>
<xs:element name="str" type="string_map" maxOccurs="unbounded"/>
</xs:choice>
</xs:sequence>
</xs:complexType>
<!-- To describe a numerical mapping with lower bound,
upper bound and state -->
<xs:complexType name="num_map">
<xs:sequence>
<xs:sequence maxOccurs="unbounded">
<xs:element name="l" type="xs:float"/>
<xs:element name="h" type="xs:float"/>
</xs:sequence>
<xs:element name="s" type="xs:string"/>
</xs:sequence>
</xs:complexType>
<!-- To describe a string mapping with
one or more strings and state -->
<xs:complexType name="string_map">
<xs:sequence>
<xs:element name="str" type="xs:string" maxOccurs="unbounded"/>
<xs:element name="s" type="xs:string"/>
</xs:sequence>
</xs:complexType>
</xs:schema>
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Appendix B. XML Schema for XML Serialization of Multiple State
Resources
<?xml version="1.0" encoding="utf-8"?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
xmlns="http://www.example.com/state-option"
targetNamespace="http://www.example.com/state-option"
elementFormDefault="qualified"
attributeFormDefault="unqualified">
<xs:element name="res" type="res"/>
<!-- To describe one or more state resources -->
<xs:complexType name="res">
<xs:sequence>
<xs:element name="r" type="resource" maxOccurs="unbounded"/>
</xs:sequence>
</xs:complexType>
<!-- To describe the mappings of a state resource along
with the uri-path-->
<xs:complexType name="resource">
<xs:sequence>
<xs:element name="p" type="xs:normalizedString"/>
<xs:choice>
<xs:element name="num" type="num_map" maxOccurs="unbounded"/>
<xs:element name="str" type="string_map" maxOccurs="unbounded"/>
</xs:choice>
</xs:sequence>
</xs:complexType>
<!-- To describe a numerical mapping with lower bound,
upper bound and state -->
<xs:complexType name="num_map">
<xs:sequence>
<xs:sequence maxOccurs="unbounded">
<xs:element name="l" type="xs:float"/>
<xs:element name="h" type="xs:float"/>
</xs:sequence>
<xs:element name="s" type="xs:string"/>
</xs:sequence>
</xs:complexType>
<!-- To describe a string mapping with
one or more strings and state -->
<xs:complexType name="string_map">
<xs:sequence>
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<xs:element name="str" type="xs:string" maxOccurs="unbounded"/>
<xs:element name="s" type="xs:string"/>
</xs:sequence>
</xs:complexType>
</xs:schema>
Author's Address
Richard Mietz
University of Luebeck
Ratzeburger Allee 160
Luebeck, Schleswig-Holstein 23562
DE
Phone: +49 451 500 5984
Email: mietz@iti.uni-luebeck.de
URI: http://www.iti.uni-luebeck.de/
Mietz Expires December 9, 2013 [Page 19]
