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Overview

The UPnP architecture allows peer-to-peer networking of PCs, networked appliances, and wireless devices. It is a distributed, open architecture based on TCP/IP, UDP and HTTP.

UPnP enables communication between any two devices under the command of any control device on the network (LAN). Among its features are:

• Media and device independence. UPnP technology can run on many media including phone lines, power lines (PLC), Ethernet, IR (IrDA), RF (Wi-Fi, Bluetooth), and FireWire. No device drivers are used; common protocols are used instead.
• User interface (UI) Control. UPnP architecture enables vendor control over device user interface and interaction using the web browser.
• Operating system and programming language independence. Any operating system and any programming language can be used to build UPnP products. UPnP does not specify or constrain the design of an API for applications running on control points; OS vendors may create APIs that suit their customer's needs. UPnP enables vendor control over device UI and interaction using the browser as well as conventional application programmatic control.
• Internet-based technologies. UPnP technology is built upon IP, TCP, UDP, HTTP, and XML, among others.
• Programmatic control. UPnP architecture also enables conventional application programmatic control.
• Extensibility. Each UPnP product can have device-specific services layered on top of the basic architecture.

The UPnP architecture supports zero-configuration, "invisible networking" and automatic discovery for many device categories from a range of vendors; any device can dynamically join a network, obtain an IP address, announce its name, convey its capabilities upon request, and learn about the presence and capabilities of other devices. DHCP and DNS servers are optional and are only used if they are available on the network. Devices can leave the network automatically without leaving any unwanted state information behind.

The foundation for UPnP networking is IP addressing. Each device must have a Dynamic Host Configuration Protocol (DHCP) client and search for a DHCP server when the device is first connected to the network. If no DHCP server is available, that is, the network is unmanaged, the device must assign itself an address. If during the DHCP transaction, the device obtains a domain name, for example, through a DNS server or via DNS forwarding, the device should use that name in subsequent network operations; otherwise, the device should use its IP address.

Protocol

Discovery

Given an IP address, the first step in UPnP networking is discovery. When a device is added to the network, the UPnP discovery protocol allows that device to advertise its services to control points on the network. Similarly, when a control point is added to the network, the UPnP discovery protocol allows that control point to search for devices of interest on the network. The fundamental exchange in both cases is a discovery message containing a few, essential specifics about the device or one of its services, for example, its type, identifier, and a pointer to more detailed information. The UPnP discovery protocol is based on the Simple Service Discovery Protocol (SSDP).

Description

The next step in UPnP networking is description. After a control point has discovered a device, the control point still knows very little about the device. For the control point to learn more about the device and its capabilities, or to interact with the device, the control point must retrieve the device's description from the URL provided by the device in the discovery message. The UPnP description for a device is expressed in XML and includes vendor-specific, manufacturer information like the model name and number, serial number, manufacturer name, URLs to vendor-specific web sites, etc. The description also includes a list of any embedded devices or services, as well as URLs for control, eventing, and presentation. For each service, the description includes a list of the commands, or actions, to which the service responds, and parameters, or arguments, for each action; the description for a service also includes a list of variables; these variables model the state of the service at run time, and are described in terms of their data type, range, and event characteristics.

Control

The next step in UPnP networking is control. After a control point has retrieved a description of the device, the control point can send actions to a device's service. To do this, a control point sends a suitable control message to the control URL for the service (provided in the device description). Control messages are also expressed in XML using the Simple Object Access Protocol (SOAP). Like function calls, in response to the control message, the service returns any action-specific values. The effects of the action, if any, are modeled by changes in the variables that describe the run-time state of the service.

Event notification

The next step in UPnP networking is event notification, or "eventing". A UPnP description for a service includes a list of actions the service responds to and a list of variables that model the state of the service at run time. The service publishes updates when these variables change, and a control point may subscribe to receive this information. The service publishes updates by sending event messages. Event messages contain the names of one or more state variables and the current value of those variables. These messages are also expressed in XML and formatted using the General Event Notification Architecture (GENA). A special initial event message is sent when a control point first subscribes; this event message contains the names and values for all evented variables and allows the subscriber to initialize its model of the state of the service. To support scenarios with multiple control points, eventing is designed to keep all control points equally informed about the effects of any action. Therefore, all subscribers are sent all event messages, subscribers receive event messages for all "evented" variables that have changed, and event messages are sent no matter why the state variable changed (either in response to a requested action or because the state the service is modeling changed).

Presentation

The final step in UPnP networking is presentation. If a device has a URL for presentation, then the control point can retrieve a page from this URL, load the page into a web browser, and depending on the capabilities of the page, allow a user to control the device and/or view device status. The degree to which each of these can be accomplished depends on the specific capabilities of the presentation page and device.

UPnP AV (Audio and Video) standards

UPnP AV stands for UPnP Audio and Video, and is a grouping within the UPnP standards supervised by the DLNA (Digital Living Network Alliance), (formely: Digital Home Working Group), which is a forum of vendors and manufacturers who work in the home entertainment industry, and offer a "DLNA CERTIFIED™" branding for those products which follow their Networked Device Interoperability Guidelines. The DLNA forum members "share a vision of a wired and wireless interoperable network of Personal Computers (PC), Consumer Electronics (CE) and mobile devices in the home enabling a seamless environment for sharing and growing new digital media and content services", and is "DLNA is focused on delivering an interoperability framework of design guidelines based on open industry standards to complete the cross-industry digital convergence". On the 12th of July 2006 the UPnP Forum announced the release of 'Enhanced AV Specifications', this release was version 2 of the UPnP Audio and Video specifications (UPnP AV v2), with new MediaServer version 2.0 and MediaRenderer version 2.0 classes. These enhancements are created by adding capabilities to the UPnP AV MediaServer and MediaRenderer device classes that allow a higher level of interoperability between MediaServers and MediaRenderers from different manufacturers. Some of the early device complying to these standards where marketed by Philips under the Streamium brand name.

UPnP AV components

• UPnP MediaServer DCP - which is the UPnP-server (a 'slave' device) that share/stream media-data (like audio/video/picture/files) to UPnP-clients on the network).
• UPnP MediaServer ControlPoint - which is the UPnP-client (a 'master' device) that can auto-detect UPnP-servers on the network to browse and stream media/data-files from them.
• UPnP MediaRenderer DCP - which is a 'slave' device that can render content.
• UPnP RenderingControl DCP - control MediaRenderer settings; volume, brightness, RGB, sharpness, and more).
• UPnP Remote User Interface (RUI) client/server - which sends/receives control-commands between the UPnP-client and UPnP-server over network, (like record, schedule, play, pause, stop, etc.).
  • Web4CE (CEA 2014) for UPnP Remote UI - CEA-2014 standard designed by Consumer Electronics Association's R7 Home Network Committee. Web-based Protocol and Framework for Remote User Interface on UPnP Networks and the Internet (Web4CE). This standard allows a UPnP-capable home network device to provide its interface (display and control options) as a web page to display on any other device connected to the home network. That means that you can control a home networking device through any web-browser-based communications method for CE devices on a UPnP home network using ethernet and a special version of HTML called CE-HTML.
• QoS (Quality of Service) - is an important (but not mandatory) service function for use with UPnP AV (Audio and Video). QoS (Quality of Service) refers to control mechanisms that can provide different priority to different users or data flows, or guarantee a certain level of performance to a data flow in accordance with requests from the application program. Since UPnP AV is mostly to deliver streaming media that is often near real-time or real-time audio/video data which it is critical to be delivered within a specific time or the stream is interrupted. QoS (Quality of Service) guarantees are especially important if the network capacity is limited, for example public networks, like the internet.
  • QoS (Quality of Service) for UPnP consist of Sink Device (client-side/front-end) and Source Device (server-side/back-end) service functions. With classes such as; Traffic Class that indicates the kind of traffic in the traffic stream, (for example, audio or video). Traffic Identifier (TID) which identifies data packets as belonging to a unique traffic stream. Traffic Specification (TSPEC) which contains a set of parameters that define the characteristics of the traffic stream, (for example operating requirement and scheduling). Traffic Stream (TS) which is a unidirectional flow of data that originates at a source device and terminates at one or more sink device(s).

NAT traversal

UPnP comes with a solution for NAT (Network Address Translation) traversal: IGD (Internet Gateway Device) protocol. NAT traversal for UPnP enables UPnP packages to pass through a router or firewall without problems and without user interaction, (that is if that router or firewall supports NAT).

Problems with UPnP

• UPnP assumes that all local systems and their users are completely trustworthy, and that no local system is infected with any worm or trojan.
  If either of these assumptions are not true then UPnP can be used to totally defeat a firewall by allowing incoming connections to arbitrary local systems on any port.
• UPnP uses HTTP over UDP (known as HTTPU and HTTPMU for unicast and multicast), even though this is not standardized and is specified only in an Internet-Draft that expired in 2001. [1]
• UPnP does not have a lightweight authentication protocol, while the available security protocols are complex. As a result, many UPnP devices ship with UPnP turned off by default as a security measure.

Future developments

The standard DPWS is a candidate successor for UPnP. It solves many of the problems of UPnP. A DPWS client is included in Microsoft Windows Vista as part of the Windows Rally technologies.

Another alternative, NAT-PNP, is an IETF draft introduced by Apple Inc in 2005.

Referenties

• UPnP™ Forum Universal Plug and Play Device Standards
• Golden G. Richard: Service and Device Discovery : Protocols and Programming, McGraw-Hill Professional, ISBN 0-07-137959-2
• Michael Jeronimo, Jack Weast: UPnP Design by Example: A Software Developer's Guide to Universal Plug and Play, Intel Press, ISBN 0-9717861-1-9