IEEE Communications Magazine
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`December 2001
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`Copyright © 2001 IEEE. Reprinted from IEEE Communications Magazine,
`December 2001 Vol. 39 No. 12.
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`This material is posted here with permission of the IEEE. Such permission of the IEEE
`does not in any way imply IEEE endorsement of any of UPnP Forum's products or
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`By choosing to view this document, you agree to all provisions of the copyright laws
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`December 2001
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`Home Networking with Universal Plug
`and Play
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`Brent A. Miller, IBM Corporation
`Toby Nixon, Microsoft Corporation
`Charlie Tai, Intel Corporation
`Mark D. Wood, Eastman Kodak Company
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`Abstract
`In this paper, we present an overview of the Universal Plug and Play (UPnPTM)
`technology and the UPnP Forum, the multi-company organization that develops parts of
`the architecture. A technical description of the technology is presented, followed by three
`illustrative usage cases where it could be applied in home networking environments.
`Finally the authors describe the benefits that UPnP technology can provide in home
`networking and briefly discuss potential future work in this area.
`1 Introduction
`All sorts of devices—PCs, mobile phones, cameras, handheld computers and so on—are
`increasingly connecting to networks, and they are using a multitude of connectivity
`methods to do so. This trend increases the need for self-configuring networks that allow
`devices to easily and automatically join and leave networks and to learn about other
`connected devices. Home networks, automotive networks and similar environments
`demand new technologies that can automate device and service discovery and control,
`obviating the need to administer these networks.
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`The Universal Plug and Play (UPnP) architecture enables pervasive peer-to-peer network
`connectivity of PCs of all form factors, intelligent appliances and wireless devices. It is a
`distributed, open networking architecture that leverages TCP/IP and Web technologies to
`enable seamless proximity networking in addition to control and data transfer among
`networked devices in the home, office, and everywhere in between. Figure 1 depicts an
`example UPnP networking topology, illustrating multiple device types and multiple
`connectivity methods.
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`'
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`an I I .1 I’;
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`FDA
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`Internet
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`Point
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`amen
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`Cable Modem
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`PD‘
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`3 w
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`iredLAN
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`j
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`E
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`1394
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`ACPower
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`Figure 1. Example UPnP topology.
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`What is universal about the UPnP architecture?
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`It uses common protocols rather than vendor-specific device drivers.
`It is independent of the underlying physical media and transports.
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`0
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`0 UPnP devices can be implemented using any programming language, and on any
`operating system.
`0 The UPnP architecture leverages HTTP and other Internet technologies such as
`XML and SOAP.
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`0 UPnP technology enables vendor control over device user interface and
`interaction via a browser.
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`It also enables conventional application programmatic control.
`0 Vendors agree on UPnP control protocols on a per-device—class basis.
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`0 Vendors can unilaterally extend the basic control protocols as needed.
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`UPnP architecture supports zero-configuration networking and automatic discovery of
`devices. Network infrastructure such as DHCP and DNS servers are optional; they may
`be used if available on the network but are not required. Furthennore, a device can leave
`a network smoothly and automatically without unwanted state information remaining
`behind. The UPnP architecture learns from the Intemet’s success and heavily leverages
`its components, including IP, TCP, UDP, H'ITP, SOAP and X1VIL.
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`2 The UPnP Forum
`Universal Plug and Play is not only a technology, it is also a cross-industry initiative. The
`UPnP Forum is the embodiment of that initiative, and its primary mission is to develop
`device control protocols (DCPs) that describe standard methods for device interaction.
`For more information about the UPnP Forum, see [FORUM99].
`
`Formed in 1999, the Forum now has more than 350 member companies from many
`industries, including consumer electronics, home automation and security, computers and
`peripherals, networking, appliances, semiconductors, and others.1 General membership
`requires completion of a membership agreement2 but is free of charge. Forum members
`receive a license to the intellectual property necessary to implement the UPnP
`specifications and may participate in the development of those specifications.
`
`Working committees produce the DCPs. In 2001 working committees were developing
`DCPs for audio-visual devices, home automation and security equipment, appliances,
`printers, cameras and other imaging devices and Internet gateways. The steering
`committee governs the overall business of the Forum, establishing working committees,
`ratifying DCPs and guiding the Forum in general. Steering committee members are
`elected by the Forum’s members.3
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`For UPnP version 1, the Forum has completed or will complete dozens of DCPs for
`various classes of UPnP devices. As DCPs are ratified, they are published on the Forum’s
`web site at http://www.upnp.org.
`3 Technical Description
`
`3.1 Overview
`The UPnP Device Architecture [DEVARCH00] defines the protocols for communication
`between UPnP control points and devices. These protocols are illustrated in Figure 2,
`taken from [FORUMWP], and described further in following sections. The architecture
`specifies six phases of interaction:
`
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`• Addressing, by which devices obtain an IP address;
`• Discovery, by which control points become aware of the existence of devices;
`• Description, by which control points learn details about devices and their
`services;
`• Control, in which control points invoke service actions;
`• Eventing, by which devices notify control points of changes in state; and
`• Presentation, by which devices can present Web pages to control points allowing
`for status and control interactions.
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`1 A complete current membership list is available at http://www.upnp.org/forum/members.htm.
`2 See http://www.upnp.org/membership htm for UPnP Forum membership information.
`3 The Steering Committee consists of up to 20 member companies. A complete list of current Steering
`Committee members can be found at http://www.upnp.org/forum/memberstatements htm.
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`UPnP Vendor Defined
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`UPnP Fomm Woflting Committee Defined
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`TOP
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`PnP Device Architecture Defined
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`U
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`Figure 2. UPnP protocol stack.
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`3.2 Addressing
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`Every UPnP device incorporates a Dynamic Host Configuration Protocol [DHCP] client
`and searches for a DHCP server when initially connected to the network. A device first
`requests an IP address via DHCP. If a response is received before a prescribed timeout,
`the device uses the dynamically assigned address.
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`Ifno DHCP server responds, the device uses automatic IP addressing [Auto-IP]. It
`randomly chooses an address in the 169.254/16 range, and tests it using an Address
`Resolution Protocol [ARP] probe to determine if it is already in use. Ifso, another
`address is chosen and tested until an unused address is found. It then periodically checks
`for the existence of a DHCP server; if a server responds, the device uses the assigned
`address, and stops using the address selected by Auto-IP after a period ofparallel use to
`complete interactions in progress.
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`In addition to numeric IP addresses, names can be used to access devices if they are
`identified in a Domain Name Service (DNS) server [RFCl034, RFCl035]. Device names
`could be registered manually, dynamically according to [RFC2l36] or by the DHCP
`server.
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`3.3 Discovery
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`Devices advertise their services to control points on the network using the UPnP
`discovery protocol, which is based on the Simple Service Discovery Protocol [SSDP].
`Control points also may use SSDP to search for devices of interest on the network. The
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`fundamental exchange in both cases is a discovery message containing a few specific
`attributes of the device or one of its services: its type, a unique identifier, and a pointer to
`more detailed information.
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`Devices multicast, using a well-known address and port, several [GENA] NOTIFY
`messages advertising the availability of embedded devices and services. Control points
`monitor this standard multicast address for notifications that new capabilities are
`available.
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`Advertisement messages indicate the lifetime of the advertisement. If a device remains
`available, it retransmits its advertisements well before the lifetime expiration. If a device
`or service becomes unavailable and an orderly shutdown is possible, previous
`advertisements are cancelled by sending cancellation messages; otherwise,
`advertisements eventually expire on their own. The lifetime for advertisements is selected
`to balance the need to minimize network traffic with the desire to maximize the currency
`of device status.
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`Control points may search for devices or services of interest by multicasting an SSDP M-
`SEARCH message. Responses from devices contain information essentially identical to
`advertisement messages, but responses to search requests are unicast directly to the
`requesting control point.
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`The time to live (TTL) field in the IP header of multicast discovery messages is set to a
`small value to reduce propagation of UPnP messages beyond the boundaries of the local
`network, hence limiting the scope of advertisement.
`3.4 Description
`After a control point has discovered a device, it uses a URL contained in the discovery
`message to request a UPnP description from the device. The description includes a device
`description and one or more service descriptions.
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`The device description includes vendor-specific information such as the model name and
`number, serial number, and manufacturer name. For each service included in the device,
`the device description lists the service type, name, and URLs for the service description,
`control, and eventing. A device description may include descriptions of embedded
`devices and a URL to access a presentation page.
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` service description includes a list of actions the service responds to, arguments for each
`action and a list of state variables. The variables model the state of the service at run time
`and are described in terms of their data type, range and event characteristics.
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`UPnP descriptions use [XML] syntax and are based on a standard UPnP device template
`or service template. Device and service templates are produced by the UPnP Forum and
`are derived from the UPnP template language. The template language itself is written in
`XML syntax and is derived from an XML schema language [XDR]. Because the template
`language, templates, and descriptions are all machine-readable, automated tools can be
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`used to ensure that the latter two have all required elements, are correctly nested and have
`values of the correct data types.
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`UPnP vendors can differentiate their devices by extending services, embedding other
`devices and including additional UPnP services, actions, or state. When a control point
`retrieves a device’s description, these added features are exposed to the control point for
`control and eventing.
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`Retrieving a UPnP device description is simple: the control point issues an [HTTP] GET
`request on the URL in the discovery message and the device returns the device
`description in the body of an HTTP response. Retrieving a UPnP service description is a
`similar process that uses a URL within the device description.
`3.5 Control
`Having obtained knowledge of a device and its services, a control point can ask those
`services to invoke actions. Invoking actions is a form of remote procedure call; a control
`point sends the action to the device’s service, and when the action has completed (or
`failed), the service returns any results or errors.
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`Actions are invoked by sending a message to the control URL for the service. Control
`messages are expressed in XML using the Simple Object Access Protocol [SOAP] and
`sent via HTTP requests; results and errors are received via HTTP responses. The effects
`of the action, if any, are modeled by changes in the variables that describe the run-time
`state of the service and may be reflected in event messages.
`3.6 Eventing
`A UPnP service description includes a list of actions that the service responds to and a list
`of variables that model the state of the service at run time. The service publishes updates,
`in the form of event messages, when these variables change and a control point may
`subscribe to receive this information.
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`To subscribe to event notification, a control point sends a subscription message to the
`subscription URL specified in the device description and provides a delivery URL to
`which event notifications should be sent. The subscription message is a request to receive
`all event messages; no mechanism is provided to subscribe to a subset of evented state
`variables. All subscribed control points receive all event messages regardless of why the
`state variable changed.
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`If the subscription is accepted, the device responds with a unique identifier for the
`subscription and the duration of the subscription. The device then sends an initial event
`message to the control point that includes the names and current values for all evented
`variables.
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`When an evented state variable changes, the service notifies subscribed control points by
`sending event messages. These messages are GENA NOTIFY messages, sent using
`HTTP, that contain within their body an XML structure that specifies the names of one or
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`more state variables and the new values of those variables. Event messages are sent as
`soon as possible after the state changes so that control points receive accurate and timely
`information about the service, allowing them to display a responsive user interface.
`Control points acknowledge receipt of event messages by responding with an HTTP OK
`message. Event notifications contain sequence numbers to allow detection of lost or out-
`of-order messages.
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`Some state variable values might change too rapidly for eventing to be useful. Event
`notifications for such state variables may be filtered; devices send notifications only
`when the degree of change exceeds a limit or when a specified amount of time has
`elapsed since the last notification. The parameters for these filters are specified in the
`service description.
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`To keep a subscription active, the control point sends a message to renew the subscription
`before it expires. The renewal message is sent to the same URL as the subscription
`message, and specifies the subscription identifier received when the subscription was
`accepted. The response for a renewal message is the same as one for a subscription
`message, updating the lifetime of the subscription. Should a subscription expire, the
`device stops sending event messages to that control point; any attempt to renew the
`expired subscription is rejected.
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`When a control point no longer needs event notifications from a particular service, it
`cancels its subscription by sending an appropriate message to the subscription URL.
`3.7 Presentation
`In addition to the functional interface provided by control and eventing, devices also may
`provide a presentation URL for a “Web” interface, accessible from a standard Web
`browser. If such a URL is provided in the device description, the control point can use an
`HTTP GET request to retrieve an HTML page from this URL, load the page into a
`browser and allow a user to control the device and view device status. The degree to
`which control and status monitoring can be accomplished depends on the specific
`capabilities of the presentation page and device.
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`Unlike the UPnP device and service descriptions, the contents of a presentation page are
`completely specified by the UPnP vendor. A presentation page need not be present, but if
`present it must be an HTML page and should be written in HTML version 3.0 or later.
`The architecture does not define further requirements for presentation pages, but certain
`design characteristics are recommended.
`4 Usage Scenarios
`Consumers are interested in the usage cases enabled by the UPnP architecture. As home
`networks become more widespread, consumers increasingly will appreciate the value of
`sharing and accessing many devices via a home network. UPnP technology permits
`devices to be used anywhere within a home network. With the advent of low-cost
`wireless communication methods such as IEEETM 802.11 and BluetoothTM technologies,
`the network’s reach might extend to the entire house and yard. Devices can be located
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`where it is most convenient and can be controlled from any UPnP control point, including
`PCs, Internet appliances, mobile devices and others.
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`Ease of use is critical to the success of the home network. Home networking is intended
`to make life easier for consumers, enabling them to share resources and access devices
`from any location. These benefits are realized only if devices are easy to use; the success
`of home networking could be severely limited if device and network configuration is
`required. To foster ease of use, the UPnP Forum is actively defining DCPs for more than
`a dozen devices, enabling several popular usage cases. Here we describe some usage
`cases that are likely to benefit from the use of UPnP technology.
`4.1 Usage Case 1: UPnP Internet Gateways
`With the proliferation of multiple-PC households and the wide deployment of broadband
`services, Internet gateways are becoming popular as consumers seek to share their high-
`speed Internet connection among multiple devices.
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`Internet gateways typically employ Network Address Translation (NAT) function to
`allow multiple home PCs and other Internet devices to share the external IP address that
`is assigned by the Internet service provider. The UPnP Internet gateway can help to solve
`a problem that often confronts home and small business users: peer-to-peer applications
`such as video-conferencing, IP telephony and on-line gaming don’t with NAT because
`devices that share a single IP address cannot be identified uniquely over the Internet.
`
`Consider a hypothetical user Paula who initially has a PC in the den with a 56K modem
`connection to the Internet. Paula recently purchased a second PC to browse the Web and
`run other Internet applications while she is in the kitchen. She installs a wireless home
`network and an Internet gateway to allow both PCs to share a new ADSL broadband
`connection. Paula discovers that the on-line gaming and peer-to-peer applications that
`worked well with only one PC no longer work. Even after calling the technical support
`hotline, Paula, like most consumers, still could not figure out how to manually set up the
`gateway’s port mapping table to make the non-UPnP gateway work as she would like.
`When Paula learns that her on-line gaming and peer-to-peer applications already have
`UPnP capability, she installs a new UPnP Internet gateway and her applications run as
`expected, without any configuration.
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`UPnP technology automates the processes of discovering the Internet gateway and
`configuring the port mapping function that maps applications running in the home to
`unique external port numbers that are visible to external peers. A UPnP Internet gateway
`also can generate event notifications when the connection speed changes or when a
`connection is lost. Applications might take advantage of this information and adjust
`themselves accordingly. The UPnP architecture allows control points to work seamlessly
`with Internet gateways without user intervention. Hence, Paula can enjoy Internet
`browsing and on-line gaming with friends and get the most out of her home network and
`high-speed Internet connection.
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`4.2 Usage Case 2: The Printer
`Another device approaching final standardization in the UPnP Forum is the printer. The
`standard UPnP printer definition enables a control point to print to any UPnP printer
`without prior configuration. Traditionally, a user wishing to share a printer among
`different PCs needs to install the appropriate printer driver on each PC. If a printer is a
`network printer, then prior knowledge of either the printer or a print server’s identity is
`required. UPnP technology automates the discovery and configuration processes.
`
`For example, suppose Paula has a printer attached to her wireless network. She comes
`home from work one day and turns on the television to watch the news. During a
`commercial, she wishes to print an e-mail from her PDA. UPnP technology enables
`Paula’s PDA to discover the available printers in the home. Paula selects one and prints
`her document. The UPnP architecture provides a standard control protocol for interacting
`with the printer and alerts her to abnormal conditions such as paper jams. If Paula’s home
`network had consisted of a mixture of wired and wireless networks, UPnP technology
`would allow her PDA to find the printer regardless of the physical network it is attached
`to.
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`To enable complete interoperability, agreement on data formats is required for devices
`such as printers. The UPnP architecture does not prescribe a specific data transfer format;
`however, the UPnP printer working committee has standardized basic formats to ensure
`interoperability. All UPnP printers must support an XHMTL-based page description
`language and JPEG-encoded images. Other document and image formats may be
`supported at the manufacturer’s discretion.
`4.3 Usage Case 3: Multimedia Applications
`An exciting range of multimedia applications for consumer electronic devices, PCs and
`even some mobile devices could be enabled by specifications being cooperatively
`developed by the UPnP Forum’s audio, audio-visual, camera, and electronic picture
`frame working committees. The goal is to enable users to store, play and view audio and
`video content across this range of device classes.
`
`Consider again our hypothetical user Paula and suppose that she purchases a UPnP
`camera with a networked docking station. When Paula returns home from her son Ben’s
`soccer match and docks the camera, her PC-based image management application notes
`that the camera is now available on the network. The application queries the camera,
`finds new content and automatically retrieves the new soccer match pictures. Later that
`night when Paula checks her e-mail, she already has at her fingertips the favorite photos
`of the day, which she then e-mails to Ben’s grandfather. She also programs her electronic
`picture frame to include one particular favorite in its repeating slideshow for the next
`week.
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`While checking her e-mail, Paula learns that a new recording by her favorite artist is
`available. Paula visits the recording label’s Web site, purchases a copy of one of the
`songs and initiates a download of the song to her new audio player in the family room.
`Later that night Paula decides to listen to the song on her home entertainment system.
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`Using her remote control, she selects the new recording from her audio player and plays
`it on her stereo.
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`Several key factors empower these usage cases and many others. Beyond the basic
`discovery, descriptive and control capabilities provided by the UPnP architecture, work is
`in progress to define a standard model for a multimedia storage service, leveraging
`emerging XML-based frameworks such as MPEG-21. Moreover, by developing common
`mechanisms to establish and manage data transfer, different multimedia devices could
`interoperate. In addition to supporting IP-based protocols such as HTTP and RTSP,
`working committees are defining support for non-IP-based mechanisms such as IEEE
`1394 and direct analog connections. Although control information by necessity uses an
`IP-based network, data transfer need not do so.
`
`Note that some devices could act as both UPnP controlled devices that implement one or
`more UPnP DCPs and as UPnP control points. For example, a digital camcorder might
`control other UPnP devices. Vendors are free to implement whatever control
`functionality is appropriate; a camcorder might allow a user to send video directly from
`the camcorder to a picture frame or digital television. A camcorder also might allow the
`user to print still snapshots on a printer. Although UPnP control points can discover any
`UPnP device, they can only interact with or control those devices or services for which
`they have prior knowledge of the interaction model.
`5 UPnP Technology Benefits for Home Networking
`
`5.1 Standards-Based
`Universal Plug and Play builds upon existing protocols and technology (including IP,
`TCP, UDP, HTTP, HTML, SOAP, and XML) whenever practical. The Internet provides
`highly proven, well understood and open networking technology that is easy to use. IP
`internetworking is well suited for Universal Plug and Play because it has proven its
`ability to span different physical media, enable real-world multiple-vendor interoperation
`and achieve synergy with the Internet and many home and office intranets. Mixed-media,
`multi-vendor networks are likely to become common in the future, and the UPnP
`architecture has been designed explicitly for these environments.
`5.2 Universality
`By leveraging Internet protocols, UPnP technology is ideally suited for cross-device,
`cross-network deployment. It can work with any underlying networking technology that
`supports IP traffic, but it does not require all devices to include an IP stack. Cost,
`technology or legacy might preclude IP traffic for some media and devices. By using
`UPnP protocol bridges, devices that communicate with non-IP protocols can participate
`in UPnP networks.
`
`Furthermore, the UPnP architecture is based on the notion of sending only data, not
`executable code, between control points and devices. Data is transmitted in a platform-
`independent manner, allowing vendors to select the most appropriate operating system
`and language for device and control point implementation.
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`Although the UPnP architecture provides a mechanism for universal control, it does not
`stipulate universality of data transfer. Instead, common data formats within certain
`domains and usage cases may be agreed upon, such as the previously described example
`of XHTML and JPEG formats for UPnP printers. In some scenarios, the control point and
`controlled device might use UPnP control protocols at run time to negotiate mutually
`supported formats for data transfer.
`6 Future Work
`UPnP version 1 is well on its way toward completion, with version 1 of the Device
`Architecture publicly available and DCPs for many device classes approaching finality.
`Several potential areas of future work are discussed next.
`
`UPnP version 1 does not directly specify any countermeasures for security threats, but
`instead leverages security features found in the standards-based protocols upon which the
`UPnP architecture is built. Among these are network isolation (firewalls), MAC layer
`encryption and physical access control. Because these measures alone are not sufficient
`to address all scenarios, a security working committee has been formed within the Forum
`to investigate potential future security enhancements.
`
`To help foster interworking with other technologies, the UPnP Forum surveys other
`standards-based initiatives with potential synergy. For example, the Forum is
`investigating possible future exploitation of common elements of the UPnP architecture
`and Web services4, which today share some protocols. Another example is work
`underway within the Bluetooth Special Interest Group5 to develop a UPnP Bluetooth
`profile. Of course, because the UPnP architecture is based on IP, future use of IPv66 must
`be considered.
`
`Although the version 1 DCPs address many device classes, numerous other devices types
`could be candidates for future DCPs. The UPnP Forum actively solicits new members
`with expertise in devices that could benefit from UPnP technology and welcomes
`proposals for new DCPs.
`7 Summary
`In this paper we introduced Universal Plug and Play, an architecture for easy and robust
`connectivity of many sorts of devices in homes, offices and elsewhere. The UPnP Forum
`is a multi-company organization that develops the UPnP DCPs.
`
`The theory of operation for UPnP technology is based on the UPnP Device Architecture
`version 1.0. Important aspects of the architecture are addressing, discovery, description,
`control, eventing and presentation. Together these elements enable seamless networking
`
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`4 See http://www-4.ibm.com/software/solutions/webservices/pdf/WSCA.pdf and
`http://msdn microsoft.com/webservices/ for information about Web services.
`5 See http://www.bluetooth.com for information about the Bluetooth SIG.
`6 See http://www.ietf.org/rfc/rfc2460.txt?number=2460 for information about IPv6.
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`December 2001
`
`of multiple classes of devices, such as printers, Internet gateways and multimedia
`devices.
`
`Universal Plug and Play offers advantages for home networking applications, including:
`•
`leveraging existing Internet standards that are already widely deployed with proven
`benefits; and
`• universality (the “U” in UPnP) that allows many different physical networks to
`connect various types of devices using any platform and any operating system.
`
`
`Future directions for this technology might entail security enhancements, coexistence
`with other standards and additional or expanded DCPs.
`
`The open UPnP architecture is supported by many companies from multiple industries.
`These companies are developing UPnP products and participating in the definition and
`refinement of the technology. The architecture builds upon existing standards and
`emphasizes ease of use. As such, we believe that UPnP technology is an important one
`for home networking applications.
`Cited References
`[ARP] Plummer, Internet Engineering Task Force RFC 826, An Ethernet Address
`Resolution Protocol, 1982, http://www.ietf.org/rfc/rfc0826.txt?number=826 .
`
`[Auto-IP] Internet Engineering Task Force draft, Automatically Choosing an IP Address
`in an Ad-Hoc IPv4 Network, http://search.ietf.org/internet-drafts/draft-ietf-dhc-ipv4-
`autoconfig-05.txt.
`
`[DEVARCH00] Microsoft Corp., Universal Plug and Play Device Architecture, version
`1.0, June 2000, http://www.upnp.org/download/UPnPDA10_20000613.htm.
`
`[DHCP] Droms, Internet Engineering Task Force RFC 2131, Dynamic Host
`Configuration Protocol, 1997, http://www.ietf.org/rfc/rfc2131.txt.
`
`[FORUM99] Universal Plug and Play Forum, About the Universal Plug and Play Forum,
`1999, http://www.upnp.org/forum/default.htm.
`
`[FORUMWP] Universal Plug and Play Forum, Understanding Universal Plug and Play,
`2000, http://www.upnp.org/download/UPNP_UnderstandingUPNP.doc.
`
`