W3C

Network Service Discovery

W3C Working Draft 04 October 2012

This version:
http://www.w3.org/TR/2012/WD-discovery-api-20121004/
Latest published version:
http://www.w3.org/TR/discovery-api/
Latest editor's draft:
http://dvcs.w3.org/hg/dap/raw-file/tip/discovery-api/Overview.html
Previous version:
http://www.w3.org/TR/2012/WD-discovery-api-20120807/
Editors:
Rich Tibbett, Opera Software ASA
Clarke Stevens, CableLabs

Copyright © 2012 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark and document use rules apply.

Abstract

This specification defines a mechanism for an HTML document to discover and subsequently communicate with HTTP-based services advertised via common discovery protocols within the current network.

Status of This Document

This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.

This document represents the early consensus of the group on the scope and features of the proposed API.

This document was published by the Device APIs Working Group as a Working Draft. This document is intended to become a W3C Recommendation. If you wish to make comments regarding this document, please send them to public-device-apis@w3.org (subscribe, archives). All feedback is welcome.

Publication as a Working Draft does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.

This document was produced by a group operating under the 5 February 2004 W3C Patent Policy. W3C maintains a public list of any patent disclosures made in connection with the deliverables of the group; that page also includes instructions for disclosing a patent. An individual who has actual knowledge of a patent which the individual believes contains Essential Claim(s) must disclose the information in accordance with section 6 of the W3C Patent Policy.

Table of Contents

1. Introduction

This section is non-normative.

To enable Web pages to connect and communicate with Local-networked Services provided over HTTP, this specification introduces the NavigatorNetworkService interface.

Using this API consists of requesting a well-known service type, known by developers and advertised by Local-networked Devices. User authorization, where the user connects the web page to discovered services, is expected before the web page is able to interact with any Local-networked Services.

A web page creates a request to obtain connectivity to services running in the network by specifying a well-known discovery service type that it wishes to interact with.

The user agent, having captured all advertised services on the network from the service discovery mechanisms included in this recommendation, attempts to match the requested service type to a discovered service according to the processing described herein.

If a service connectivity request is successful then the Web page is provided with the necessary information to communicate with the authorized Local-networked Service. If the request fails then the Web page will receive an error callback containing an error code describing the cause of Local-networked Service connectivity failure.

Once connected to a Local-networked Service the Web page can send requests and receive responses to the Local-networked Service via the messaging format and appropriate channel inferred from the service type authorized via the provided API. The Web page, once connected, can also receive service-pushed events, in the messaging format supported by the Local-networked Device, if such event subscription functionality is provided by the connected Local-networked Service.

Example of requesting a DNS-SD advertised service: 

function showServices( services ) {
  // Show a list of all the services provided to the web page
  for(var i = 0, l = services.length; i < l; i++) console.log( services[i].name );
}

navigator.getNetworkServices('zeroconf:_boxee-jsonrpc._tcp', showServices);

Example of requesting a UPnP advertised service, also handling error conditions: 

function showServices( services ) {
  // Show a list of all the services provided to the web page
  for(var i = 0, l = services.length; i < l; i++) console.log( services[i].name );
}

function error( e ) {
  console.log( "Error occurred: " + e.code );
}

navigator.getNetworkServices('upnp:urn:schemas-upnp-org:service:ContentDirectory:1', showServices, error);

Example of requesting either a DNS-SD or UPnP advertised service: 

function showServices( services ) {
  // Show a list of all the services provided to the web page (+ service type)
  for(var i = 0, l = services.length; i < l; i++)
     console.log( services[i].name + '(' + services[i].type + ')' );
}

navigator.getNetworkServices([
  'zeroconf:_boxee-jsonrpc._tcp',
  'upnp:urn:schemas-upnp-org:service:ContentDirectory:1'
], showServices);

For more detailed examples see the Examples section.

2. Conformance

As well as sections marked as non-normative, all authoring guidelines, diagrams, examples, and notes in this specification are non-normative. Everything else in this specification is normative.

The key words must, must not, required, should, should not, recommended, may, and optional in this specification are to be interpreted as described in [RFC2119].

Requirements phrased in the imperative as part of algorithms (such as "strip any leading space characters" or "return false and abort these steps") are to be interpreted with the meaning of the key word ("must", "should", "may", etc) used in introducing the algorithm.

Some conformance requirements are phrased as requirements on attributes, methods or objects. Such requirements are to be interpreted as requirements on user agents.

Conformance requirements phrased as algorithms or specific steps may be implemented in any manner, so long as the end result is equivalent. (In particular, the algorithms defined in this specification are intended to be easy to follow, and not intended to be performant.)

The only conformance class defined by this specification is a user agent.

User agents may impose implementation-specific limits on otherwise unconstrained inputs, e.g. to prevent denial of service attacks, to guard against running out of memory, or to work around platform-specific limitations.

When support for a feature is disabled (e.g. as an emergency measure to mitigate a security problem, or to aid in development, or for performance reasons), user agents must act as if they had no support for the feature whatsoever, and as if the feature was not mentioned in this specification. For example, if a particular feature is accessed via an attribute in a Web IDL interface, the attribute itself would be omitted from the objects that implement that interface - leaving the attribute on the object but making it return null or throw an exception is insufficient.

2.1 Dependencies

This specification relies on several other underlying specifications.
HTML
Many fundamental concepts from HTML are used by this specification. [HTML5]
WebIDL
The IDL blocks in this specification use the semantics of the WebIDL specification. [WEBIDL]

3. Terminology

The construction "a Foo object", where Foo is actually an interface, is sometimes used instead of the more accurate "an object implementing the interface Foo".

The term DOM is used to refer to the API set made available to scripts in Web applications, and does not necessarily imply the existence of an actual Document object or of any other Node objects as defined in the DOM Core specifications. [DOM4]

An IDL attribute is said to be getting when its value is being retrieved (e.g. by author script), and is said to be setting when a new value is assigned to it.

A valid service type is a string that begins with upnp: or zeroconf: followed by one or more characters in the ranges U+0021, U+0023 to U+0027, U+002A to U+002B, U+002D to U+002E, U+0030 to U+0039, U+0041 to U+005A, U+005E to U+007E.

A valid service type provided in the type attribute of the getNetworkServices() method will be matched against the services currently contained in the list of available service records according to the algorithms defined in this specification.

A user-agent generated callback url is a Local-network accessible URL endpoint that a user agent generates and maintains for receiving HTTP NOTIFY requests from UPnP Event sources. It is only required when the user agent implements UPnP Service Discovery as defined in this specification.

4. Requesting networked services 

[Supplemental, NoInterfaceObject]
interface NavigatorNetworkService {
  // Obtain a Local-networked Service
  void getNetworkServices( in any type,
                           in NavigatorNetworkServiceSuccessCallback successCallback,
                           in optional NavigatorNetworkServiceErrorCallback errorCallback );
};
Navigator implements NavigatorNetworkService;

[Callback=FunctionOnly, NoInterfaceObject]
interface NavigatorNetworkServiceSuccessCallback {
  void handleEvent( in NetworkServices services );
};

[NoInterfaceObject]
interface NavigatorNetworkServiceError {
  const unsigned short PERMISSION_DENIED_ERR = 1;
  const unsigned short UNKNOWN_TYPE_PREFIX_ERR = 2;
  readonly attribute unsigned short code;
};

[Callback=FunctionOnly, NoInterfaceObject]
interface NavigatorNetworkServiceErrorCallback {
  void handleEvent( in NavigatorNetworkServiceError error );
};

4.1 Methods

window . navigator . getNetworkServices ( type , successCallback [, errorCallback ] )

Prompts the user to select discovered network services that have advertised support for the requested service type.

The type argument contains one or more valid service type tokens that the web page would like to interact with.

If the user accepts, the successCallback is invoked, with zero or more NetworkService objects as its argument.

If the user declines, the errorCallback (if any) is invoked.

When the getNetworkServices(type, successCallback[, errorCallback]) method is called, the user agent must run the following steps:

  1. Let requested control types be initially set to an empty array.
  2. If type is an array consisting of one or more valid service type tokens, then let requested control types by the value of type, removing any non-valid service type tokens from the resulting array.
  3. If type is a string consisting of one valid service type token, then let requested control types be an array containing one item with a value of type.
  4. If requested control types is an array that contains at least one or more valid service type tokens then continue to the step labeled process below. Otherwise, the user agent must queue a task to invoke errorCallback, if it is provided and is an object of type Function, with a new NavigatorNetworkServiceError object whose code attribute has the numeric value 2 (UNKNOWN_TYPE_PREFIX_ERR) as its argument, abort any remaining steps and return.
  5. Process: Let services found be an empty array.
  6. For each available service in the list of available service records run the following steps:
    1. For each requested control type in requested control types: If available service's type attribute equals the requested control type then let matched service equal the value of available service and continue at the step labeled attach below.
    2. Continue at the next available service.
    3. Attach: If matched service is not empty then run the following steps:
      1. Let new service object be a new NetworkService object, mapping the parameters of matched service to this new object where possible.
      2. Append new service object to the services found array.
  7. Return, and run the remaining steps asynchronously.
  8. Optionally, e.g. based on a previously-established user preference, for security reasons, or due to platform limitations, the user agent may queue a task to invoke errorCallback, if it is provided and is an object of type Function, with a new NavigatorNetworkServiceError object whose code attribute has the numeric value 1 (PERMISSION_DENIED_ERR) as its argument, abort any remaining steps and return.
  9. If services found is not an empty array then the user agent must prompt the user in a user-agent-specific manner for permission to provide the entry script's origin with an array of NetworkService objects representing the user-authorized subset of services found.

    If the user grants permission to access one or more networked services then the user agent should include an "ongoing local-network communication" indicator.

    If the user denies permission, then the user agent must queue a task to invoke errorCallback, if it is provided and is an object of type Function, with a new NavigatorNetworkServiceError object whose code attribute has the numeric value 1 (PERMISSION_DENIED_ERR) as its argument, abort any remaining steps and return.

    If the user never responds, this algorithm stalls on this step.

  10. Let services be an empty array.
  11. If services found is not an empty array then set services to be an array of one or more NetworkService objects for which the user granted permission above - known as the current objects user-authorized services.
  12. For each Object service in services, if any, run the following sub-steps:
    1. Add the service's url parameter to the entry script origin's URL whitelist.
    2. If service's type parameter begins with the DOMString "upnp:" and the service's eventsUrl parameter is not empty then setup a UPnP Events Subscription for service.
  13. Let services manager be a new NetworkServices object.
  14. Set services manager's servicesAvailable attribute to the number of items currently found in the list of available service records whose type property matches any of the tokens requested in requested control types.
  15. Add services, if any, to the services manager object as its collection of indexed properties. If services is an empty array then the services manager does not have any indexed properties.
  16. Set services manager's length attribute to the number of items in services.
  17. Add services manager to the list of active service managers.
  18. The user agent must queue a task to invoke successCallback with services manager as its argument.

The task source for these tasks is the user interaction task source.

When a NetworkService object is provided to a Web page, the user agent must add the url property to the entry script origin's URL whitelist. This list enables the Web page to override and initiate cross-site resource requests towards these URLs, and any sub-resources of these URLs, within the current entry script's origin via various existing mechanisms (e.g. Web Sockets, Server-Sent Events, Web Messaging, XMLHttpRequest).

If the user navigates away from the entry script's origin then the user agent must remove all previously whitelisted urls from the entry script origin's URL whitelist. There is no persistence to network service selections provided to a web page. It is not possible to access a previously white-listed networked service without the necessary user authorization in all of the following cases:

  • If the current script is reloaded at any point in the same or different window.
  • if the current script reinvokes the getNetworkServices() method at any point in its execution.
  • If the user navigates forward or back in their history to reload the current page.
  • If a script is running in a different origin.

4.2 Error Handling

error . code

Returns the current error's error code. At the current time, this will be 1 or 2, for which the corresponding error constants PERMISSION_DENIED_ERR and UNKNOWN_TYPE_PREFIX_ERR are defined.

The code attribute of a NavigatorNetworkServiceError object must return the code for the error, which will be one of the following:

PERMISSION_DENIED_ERR (numeric value 1)
The user or user agent denied the page permission to access any services.
UNKNOWN_TYPE_PREFIX_ERR (numeric value 2)
No valid service type tokens were provided in the method invocation.

5. Obtaining networked services

The NetworkServices interface represents a collection of zero or more indexed properties that are each a user-authorized NetworkService object.

A NetworkServices object is the top level success callback parameter from a call to getNetworkServices(). 

[NoInterfaceObject]
interface NetworkServices {
  readonly attribute unsigned long    length;
  getter NetworkService (unsigned long index);
  NetworkService? getServiceById(DOMString id);

  readonly attribute unsigned long    servicesAvailable;

  // event handler attributes
           attribute EventHandler     onserviceavailable;
           attribute EventHandler     onserviceunavailable;

};

NetworkServices implements EventTarget;

5.1 Attributes

length

Returns the current number of items in the current object's collection of NetworkService objects.

servicesAvailable

Returns the current number of items matching one of the app-requested valid service type tokens in the list of available service records.

The length attribute must return the number of NetworkService objects represented by the collection.

The servicesAvailable attribute must return the number of services in the list of available service records whose type attribute matches any of the valid service type tokens that was initially used to create the current NetworkServices object.

5.2 Methods

services [ index ]

Returns the specified NetworkService object.

services . getServiceById ( id )

Returns the NetworkService object with the given identifier, or null if no service has that identifier.

A NetworkServices object represents the current collection of zero or more NetworkService objects - its indexed properties. A NetworkServices object is immutable meaning that indexed properties cannot be added and indexed properties cannot be removed for the lifetime of a NetworkServices object.

Note

Each service in a NetworkServices object thus has an index; the first has the index 0, and each subsequent service is numbered one higher than the previous one.

The supported property indices of NetworkServices objects at any instant are the numbers from zero to the number of the NetworkService objects in the collection minus one.

To determine the value of an indexed property for a given index index in a NetworkServices object the user agent must return the NetworkService object that represents the indexth item in the collection.

The getServiceById(id) method must return the first NetworkService object in the collection whose id attribute is equal to the value of the id argument provided. When no NetworkService objects match the given argument, the method must return null.

Services available within the local network can connect and disconnect at different times during the execution of a web page. A user agent can inform a web page when the state of networked services matching the requested valid service type change. Web pages can use this information to enable in-page experiences for communicating the state of networked services with the ability to change the particular service or set of services the page is connected to by re-invoking the getNetworkServices() method.

5.3 Events

The following are the event handlers (and their corresponding event handler event types) that must be supported, as IDL attributes, by all objects implementing the NetworkServices interface:

Event handler Event handler event type
onserviceavailable serviceavailable
onserviceunavailable serviceunavailable

6. Communicating with a networked service

The NetworkService interface is used to provide a set of connection information for an HTTP service endpoint and if available, service events, running on a networked device. 

[NoInterfaceObject]
interface NetworkService {
  readonly attribute DOMString        id;
  readonly attribute DOMString        name;
  readonly attribute DOMString        type;
  readonly attribute DOMString        url;
  readonly attribute DOMString        config;

  readonly attribute boolean          online;

  // event handler attributes
           attribute EventHandler     onserviceonline;
           attribute EventHandler     onserviceoffline;

           attribute EventHandler     onnotify;
};

NetworkService implements EventTarget;

6.1 Attributes

service . id

A unique identifier for the given user-selected service instance.

service . name

The name of the user-selected service.

service . type

The valid service type token value of the user-selected service.

service . url

The control URL endpoint (including any required port information) of the user-selected control service that has been added to the entry script origin's URL whitelist.

service . config

The configuration information associated with the service depending on the requested service type.

The id attribute is a unique identifier for the service. The same service provided at different times or on different objects must have the same id value.

The name attribute represents a human-readable title for the service.

The type attribute reflects the value of the valid service type of the service.

The url attribute is an absolute URL pointing to the root HTTP endpoint for the service that has been added to the entry script origin's URL whitelist. Web pages can subsequently use this value for implicit cross-document messaging via various existing mechanisms (e.g. Web Sockets, Server-Sent Events, Web Messaging, XMLHttpRequest).

The config attribute provides the raw configuration information extracted from the given network service.

6.2 States

service . online

Returns true if the service is reporting that it is accessible on the local network or false if the service is reporting that it is no longer accessible (temporarily or permanently) on the local network.

The online attribute indicates whether the service is reporting itself as being either online, and therefore accessible on the local network, in which case this attribute will return true or, offline, and therefore not accessible on the local network, either temporarily or permanently, in which case this attribute will return false. This attribute must default to true.

6.3 Events

The following are the event handlers (and their corresponding event handler event types) that must be supported, as IDL attributes, by all objects implementing the NetworkService interface:

Event handler Event handler event type
onnotify notify
onserviceonline serviceonline
onserviceoffline serviceoffline

7. Service Discovery

A user agent conforming to this specification may implement SSDP [UPNP-DEVICEARCH11] and Zeroconf [DNS-SD] + [MDNS] service discovery mechanisms - the requirements detailed in this section of the specification - to enable Web pages to request and connect with HTTP services running on networked devices, discovered via either mechanism, through this API. When a user agent implements either of these service discovery mechanisms, then it must conform to the corresponding algorithms provided in this section of the specification.

This section presents how the results of these two service discovery mechanisms will be matched to requested service types, how the user agent stores available and active services and how their properties are applied to any resulting NetworkService objects.

It is expected that user agents will perform these service discovery mechanisms asynchronously and periodically update the list of available service records as required. The timing of any service discovery mechanisms is an implementation detail left to the discretion of the implementer (e.g. by continuously monitoring the network as a background process or on invocation of this API from a Web page).

The list of available service records is a single dynamic internal lookup table within user agents that is used to track all the services that have been discovered and are available in the current network at any given time. At any point during the running of either of the two service discovery mechanisms then existing entries within this table can be updated, entries can be added and entries can be removed as the status of networked services changes according to the rules defined in this specification.

The list of active service managers is an internal list within user agents that is used to track all NetworkServices objects currently being shared with any web pages at the current time within the user agent. Each NetworkServices object in the list of active service managers represents a collection of zero or more NetworkService objects - known as its indexed properties. NetworkService objects are attached as the indexed properties of a NetworkServices object as part of the getNetworkServices() algorithm.

The rule for adding an available service is the process of adding a new service or updating an existing service in the list of available service records that is generally available on the user's current network. This rule takes one argument, network service record, and consists of running the following steps:

  1. Let new service registration flag be true.
  2. For each existing service record in the current list of available service records, run the following sub-steps:
    1. If the existing service record's id property does not equal network service record's id property then abort any remaining sub-steps and continue at the next available existing service record.
    2. Set new service registration flag to false.
    3. Replace the value of existing service record in the current list of available service records with the value of network service record.
  3. If new service registration flag is set to true then add network service record to the list of available service records as a new item.
  4. For each service manager in the list of active service managers run the following steps:
    1. Let service type in current service manager flag be false.
    2. For each active service in service manager run the following steps:
      1. If network service record's type property does not equal active service's type attribute then abort any remaining sub-steps for this active service and continue at the next available active service.
      2. Set the service type in current service manager flag to true.
      3. If the new service registration flag is set to false, the network service record's id property equals the active service's id attribute and active service's online attribute is currently set to false then set active service's online attribute to true and then queue a task to dispatch a newly created event with the name serviceonline that uses the Event interface, which does not bubble, is not cancellable, and has no default action, at the current active service object.
    3. If the new service registration flag is set to true and the service type in current service manager flag is also set to true then increment service manager's servicesAvailable attribute by 1 and then queue a task to dispatch a newly created event with the name serviceavailable that uses the Event interface, which does not bubble, is not cancellable, and has no default action, at the current service manager object.

The rule for removing an available service is the general process of removing a service from the list of available service records that has left the user's current network or has otherwise expired. This rule takes one argument, service identifier, and consists of running the following steps:

  1. For each existing service record in the current list of available service records, run the following sub-steps:
    1. If the existing service record's id property does not match service identifier then skip any remaining sub-steps for the current existing service record and continue at the next available existing service record.
    2. If the existing service record's type property begins with the DOMString "upnp:" and existing service record's eventsURL property is set then run the rule to terminate an existing UPnP Events Subscription, if one is currently active (as a result of having previously called setup a UPnP Events Subscription against the current existing service record).
    3. For each service manager in the list of active service managers run the following steps:
      1. Let service type in current service manager flag be false.
      2. For each active service in service manager run the following steps:
        1. If existing service record's type property does not equal the active service's type attribute then abort any remaining sub-steps for this active service and continue at the next available active service.
        2. Set the service type in current service manager flag to true.
        3. If existing service record's id property equals the active service's id attribute and active service's online attribute is currently set to true then set active service's online attribute to false and then queue a task to dispatch a newly created event with the name serviceoffline that uses the Event interface, which does not bubble, is not cancellable, and has no default action, at the current active service.
      3. If the service type in current service manager flag is set to true then decrement service manager's servicesAvailable attribute by 1 and then queue a task to dispatch a newly created event with the name serviceunavailable that uses the Event interface, which does not bubble, is not cancellable, and has no default action, at the current service manager object.
    4. Remove existing service record from the current list of available service records.

User agents should expire a service record from the list of available service records when its expiryTimestamp attribute exceeds the current UTC timestamp. When this condition is met the user agent should run the rule for removing an available service, passing in the expired service record's id attribute as the only argument.

7.1 Zeroconf (mDNS + DNS-SD)

For each DNS response received from a user-agent-initiated Multicast DNS Browse for PTR records with the name _services._dns-sd._udp on the resolved recommended automatic browsing domain [MDNS], the user agent must run the following steps:

  1. Let service mDNS responses be an array of PTR records received by issuing a Multicast DNS Browse for PTR records with the name of the current discovered service type.
  2. For each Object service mDNS response in service mDNS responses, run the following steps:
    1. Let network service record be an Object consisting of the following empty properties: id, name, type, url, config, expiryTimestamp.
    2. Set network service record's id property to the value of the full PTR Service Instance Name [MDNS].
    3. Set network service record's name property to the value of the PTR Service Instance Name's Instance component [MDNS].
    4. Set network service record's type property to the concatenation of the string zeroconf: followed be the value of the PTR Service Instance Name's Service component [MDNS].
    5. Set network service record's url property to the resolvable Service URL obtained from performing an DNS-SD Lookup [DNS-SD] of the current service from the PTR record provided [MDNS].
    6. Set network service record's config property to the string value of the contents of the first DNS-SD TXT record associated with the service mDNS response as defined in [DNS-SD].
    7. Set network service record's expiryTimestamp property to the value of the current date, in UTC timestamp format, plus a value of 120 seconds.
    8. Run the general rule for adding an available service, passing in the current network service record as the only argument.

7.2 Simple Service Discovery Protocol (SSDP)

A user agent that implements UPnP service discovery must issue a search request for UPnP root devices against the user's current local network according to the full normative text and timing provided in 'Section 1.3.2: Search request with M-SEARCH' detailed in [UPNP-DEVICEARCH11].

The user agent must issue all search requests for UPnP root devices with a HTTP request line equal to M-SEARCH * HTTP/1.1, with a HOST header equal to the reserved multicast address and port of 239.255.255.250:1900, a MAN header equal to ssdp:discover, an ST header equal to upnp:rootdevice and a user-agent defined MX header equal to a maximum UPnP advertisement response wait time value between 1 and 5 seconds.

The user agent must listen for any incoming responses to any search request for UPnP root devices.

For each HTTP Response following an initial search request for UPnP root devices sent on a standard UPnP address and port the user agent must run the following steps:

  1. If the HTTP Response is not a HTTP 200 OK response then this response is invalid and the user agent must discard this response, abort any remaining steps and return. The user agent may issue a new search request for UPnP root devices as a result of this error occurring.
  2. If the maximum UPnP advertisement response wait time has been exceeded since the initial search request for UPnP root devices was sent then the HTTP Response is invalid and the user agent must discard this response, abort any remaining steps and return. The user agent may stop listening for responses from the current search request for UPnP root devices as a result of this error occurring. Equally, the user agent may issue a new search request for UPnP root devices as a result of this error occurring.
  3. Let ssdp device be an Object with a property for each HTTP header received in the HTTP Response, with each key being the name of a HTTP response header and each value being that HTTP response header's value.
  4. If ssdp device does not contain at least one CACHE-CONTROL entry, at least one USN entry, at least one ST entry and at least one LOCATION entry or the value of its ST entry is not upnp:rootdevice, then the HTTP Response is invalid and the user agent must discard this response, abort any remaining steps and return.
  5. The user agent must run the rule for obtaining a UPnP Device Description File passing in the first occurrence of LOCATION from ssdp device as the device descriptor URL argument and the first occurrence of USN from ssdp device as the device identifier argument and the first occurrence of CACHE-CONTROL from ssdp device (minus the leading string of max-age=) as the device expiry argument.

The user agent must listen for incoming requests on the standard UPnP address and port on all current local network interface addresses with the port 1900.

For each HTTP Request received on a standard UPnP address and port the user agent must run the following steps:

  1. If the HTTP Request is not a HTTP NOTIFY request then it is not a valid UPnP Request and the user agent must discard this request, abort any remaining steps and return.
  2. Let ssdp device be an Object with a property for each HTTP header received in the HTTP Request, with each key being the name of a HTTP header and each value being that HTTP header's value.
  3. If ssdp device does not contain at least one CACHE-CONTROL entry, at least one USN entry, at least one NT entry, at least one NTS entry and at least one LOCATION entry or the value of its NT entry is not upnp:rootdevice, then the HTTP Request is a malformed UPnP Request and the user agent must discard this request, abort any remaining steps and return.
  4. If ssdp device's NTS entry is equal to ssdp:alive then the user agent must run the rule for obtaining a UPnP Device Description File passing in the first occurrence of LOCATION from ssdp device as the device descriptor URL argument and the first occurrence of USN from ssdp device as the device identifier argument and the first occurrence of CACHE-CONTROL from ssdp device (minus the leading string of max-age=) as the device expiry.

    Otherwise, if ssdp device's NTS entry is equal to ssdp:byebye then the user agent must run the rule for removing all services from a registered UPnP Device passing in the first occurrence of USN from ssdp device as the device identifier argument.

The rule for obtaining a UPnP Device Description File is the process of obtaining the contents of a standard UPnP Device Description [UPNP-DEVICEARCH11] from a URL-based resource. This rule takes three arguments - device descriptor URL, device identifier and device expiry - and when called the user agent must run the following steps:

  1. Let device descriptor file contain the contents of the file located at the URL provided in device descriptor URL obtained according to the rules defined in 'Section 2.11: Retrieving a description using HTTP' in [UPNP-DEVICEARCH11].
  2. If the value provided in device descriptor URL cannot be resolved as a reachable URL on the current network or the device descriptor file remains empty then it is invalid and the user agent must abort any remaining steps and return.
  3. Run the rule for processing a UPnP Device Description File, passing in the current device descriptor file, device identifier and device expiry arguments.

The rule for processing a UPnP Device Description File is the process of parsing the contents of a standard UPnP Device Description [UPNP-DEVICEARCH11] and registering the UPnP services contained therein within the list of available service records.

The rule for processing a UPnP Device Description File takes three arguments - device descriptor file, device identifier and device expiry - and when called the user agent must run the following steps:

  1. Let advertised services be a list of all advertised services obtained from the device descriptor file containing the value of the first occurrence of the <serviceList> element as it is defined in 'Section 2.3: Device Description' in [UPNP-DEVICEARCH11].
  2. For each <service> element - known as an advertised service - in advertised services run the following steps:
    1. Let network service record be a new Object consisting of the following empty properties: id, deviceId, name, type, url, eventsUrl, config, expiryTimestamp.
    2. Set network service record's id property to the concatenated string value of the first occurrence of the <UDN> element in the device descriptor file with the advertised service's <serviceId> sub-element.
    3. Set network service record's deviceId property to the value of device identifier.
    4. Set network service record's name property to the string value of the first occurrence of the advertised service's <serviceId> sub-element.
    5. Set network service record's type property to the concatenation of the string upnp: followed by the string value of the first occurrence of the advertised service's <serviceType> sub-element.
    6. Set network service record's url property to the string value of the first occurrence of the advertised service's <controlURL> sub-element.
    7. Set network service record's config property to the string value of the contents of the first occurrence of the <device> element in the device descriptor file.
    8. If advertised service's <eventSubURL> sub-element is not empty, then set network service record's eventsUrl property to the string value of the first occurrence of the advertised service's <eventSubURL> sub-element. Otherwise, do not set network service record's eventsUrl property.
    9. Set network service record's expiryTimestamp property to the value of the current date, in UTC timestamp format, plus the value of device expiry.
    10. Run the general rule for adding an available service, passing in the current network service record as the only argument.
  3. If device descriptor file contains a <deviceList> element then for each <device> element within <deviceList> - herein known as an embedded device descriptor file - the user agent must run the rule for processing a UPnP Device Description File, passing in the current embedded device descriptor file as the device descriptor file argument, along with the current device identifier and device expiry arguments.

The rule for removing all services from a registered UPnP Device is the process of removing all services associated with a device from the list of available service records that has left the user's current network or has otherwise timed out or expired. This rule takes one argument, device identifier, and consists of running the following steps:

  1. For each existing service record in the current list of available service records, run the following sub-steps:
    1. If the existing service record's deviceId property does not match device identifier then skip any remaining sub-steps for the current existing service record and continue at the next available existing service record.
    2. Run the general rule for removing an available service passing in existing service record's id property as the only argument.

When the user agent is to setup a UPnP Events Subscription, it is to run the following steps with the current network service record object as defined in 'Section 4.1.2: SUBSCRIBE with NT and CALLBACK' in [UPNP-DEVICEARCH11]:

  1. If network service record's eventsUrl property is empty then the user agent must abort these steps.
  2. Let callback URL be the value of creating a new user-agent generated callback url.
  3. Send a HTTP SUBSCRIBE request with a NT header with a string value of upnp:event, a TIMEOUT header with a user-agent defined timeout value (in the form Second-XX where XX is the user-agent defined timeout value in seconds) and a CALLBACK header with a string value of callback URL towards the network service record's eventsUrl property.
  4. If a non-200 OK response is received from the HTTP SUBSCRIBE request then the user agent must abort these steps.
  5. On receiving a valid 200 OK response, run the following steps:
    1. Let callback ID equal the string value of the first included SID header, if it exists.
    2. Let timeout date equal the sum of the current UTC date value plus the integer value of the first included TIMEOUT header (minus the leading string of Second-), if it exists.
    3. Run the following steps asynchronously and continue to the step labeled listen below.
    4. Refresh Subscription: Run the following steps at a set interval (X) within the user agent:
      1. Let current date equal the current UTC date.
      2. If current date is less than the timeout date then continue to the step labeled refresh subscription above.
      3. Send a HTTP SUBSCRIBE request with a SID header with the string value of callback ID and a user-agent defined TIMEOUT header (in the form Second-XX where XX is the user-agent defined timeout value in seconds) towards the network service record's eventsUrl property.
      4. On receiving a valid 200 OK, update callback ID with the string value of the first included SID header and set timeout date to the sum of the current UTC date value plus the integer value of the first included TIMEOUT header (minus the leading string of Second-), if it exists. If the current date is greater than or equal to timeout date then the user agent should continue from the step labeled refresh subscription above. For all non 200 OK responses the user agent should continue from the step labeled refresh subscription above.
    5. Listen: For each HTTP NOTIFY request received at the callback URL the user agent is to run the following steps:
      1. Let content clone be the result of obtaining the message body of the HTTP NOTIFY request. If content clone is empty, then the user agent must abort these steps.
      2. Let notification event be a new simple event that uses the Event interface with the name notify, which does not bubble, is not cancellable, and has no default action.
      3. Let the data attribute of notification event have the DOMString value of content clone.
      4. Queue a task to dispatch notification event at the current NetworkService object.
      5. Return a HTTP 200 OK response to the sender of the HTTP NOTIFY request.

A user agent can terminate an existing UPnP Events Subscription at any time for a network service record by sending an HTTP UNSUBSCRIBE request - as defined in 'Section 4.1.4: Cancelling a subscription with UNSUBSCRIBE' in [UPNP-DEVICEARCH11] - with a HOST header set to that active service's eventsUrl property and a SID header set to the callback ID obtained when the initial setup a UPnP Events Subscription action occurred.

7.3 Network Topology Monitoring

When the user agent detects that the user has dropped from their connected network then, for each existing service record in the list of available service records, the user agent must run the general rule for removing an available service passing in each existing service record's id property as the only argument for each call.

When the user agent detects that the user has connected to a new network or reconnected to an existing network, then it should restart its discovery mechanisms as defined in the Service Discovery section of this specification, maintaining the existing list of active service managers currently in use.

8. Events Summary

The following events are dispatched on the NetworkServices and/or NetworkService objects:

Event name Interface Dispatched when...
serviceavailable Event When a new service that matches one of the requested type tokens is found in the current network.
serviceunavailable Event When an existing service that matches one of the requested type tokens gracefully leaves or expires from the current network.
serviceonline Event When a current service renews its service registration within the current network.
serviceoffline Event When a current service gracefully leaves or otherwise expires from the current network.
notify Event When a valid UPnP Events Subscription Message is received on a user-agent generated callback url for a current service. This event never fires for Zeroconf-based services.

9. Garbage collection

A user agent must only garbage collect a NetworkServices object and remove its entry from the list of active service managers when the user has navigated away from the entry script's origin in which the current NetworkServices object was provided.

A user agent must not garbage collect individual NetworkService objects until their parent NetworkServices object has been garbage collected.

A user agent must garbage collect the NetworkService indexed properties of a NetworkServices object when that NetworkServices object itself has been garbage-collected.

10. Use Cases and Requirements

This section covers what the requirements are for this API, as well as illustrates some use cases.

A. Examples

This section is non-normative.

This sample code exposes a button. When clicked, this button is disabled and the user is prompted to offer a network service. The user may also select multiple network services. When the user has authorized a network service to be connected to the web page then the web page issues a simple command to get a list of all the albums stored on the connected media player service.

The button is re-enabled only when the connected network service disconnects for whatever reason (the service becomes unavailable on the network, the user disconnects from their current network or the user revokes access to the service from the current web page). At this point the user can re-click the button to select a new network service to connect to the web page and the above steps are repeated.

The provided service type identifier and service interaction used in this example is based on the well-defined service type and messaging format supported by the XBMC Media Server. 

<input type="button" value="Start" onclick="start()" id="startBtn"/>
<div id="debugconsole"></div>

<script>
 var startBtn = document.getElementById('startBtn'),
     debug = document.getElementById('debugconsole');

 function start() {
   if(navigator.getNetworkServices) {
      navigator.getNetworkServices('zeroconf:_xbmc-jsonrpc._tcp', gotXBMCService, error);
      startBtn.disabled = true;
   } else {
      debug.innerHTML += "<br>Service Discovery not supported!";
   }
 }

 function gotXBMCService(services) {

// Listen for service disconnect messages

   services[0].addEventListener('serviceoffline', function ( e ) {
       debug.innerHTML += "<br>" + services[0].name + " disconnected.";
       startBtn.disabled = false;
   }, false);

// Send a service message to get albums list (and process the service response)

   var svcXhr = new XMLHttpRequest();
   svcXhr.open("POST", services[0].url + "/getAlbums"); // services[0].url and its sub-resources have been
                                                        // whitelisted for cross-site XHR use in this
                                                        // current browsing context.

   svcXhr.setRequestHeader('Content-Type', 'application/json-rpc');

   svcXhr.addEventListener('readystatechange', function ( response ) {
     if( response.readyState != 4 || response.status != 200 )
        return;
     debug.innerHTML += "<br>" + services[0].name + " response received: ";
     debug.textContent += JSON.parse(response.responseText);
   }, false);

   var svcMsg = [
     { "jsonrpc": "2.0", "method": "AudioLibrary.GetAlbums", "params": { "genreid": -1,
         "artistid": -1, "start": -1, "end": -1 }, "id": "1" }
   ];

   svcXhr.send(JSON.stringify(svcMsg));
   debug.innerHTML += "<br>" + services[0].name + " request sent: ";
   debug.textContent += JSON.stringify(svcMsg);

 }

 function error( err ) {
   debug.innerHTML += "<br>An error occurred obtaining a local network service.";
   startBtn.disabled = false;
 }
</script>

This sample exposes a drop-down list containing a number of common Home-based audio devices. When the user selects an audio device from the list provided, they are prompted to authorize a network service based on the service type requested. The user may also select multiple network services matching the selected service type. In this example, the user selects their make as being Sony and their model as being Bravia S1000 from which the Web page can derive a service type (urn:schemas-upnp-org:service:RenderingControl:1).

Once the user has authorized the device, the web page sends a simple mute command according to the messaging format supported by the device. 

<select name="make" id="make">
  <option selected="selected" disabled="disabled">Select make</option>
  <option>Sony</option>
  <option>Philips</option>
  <option>Alba</option>
</select>
<select name="model" id="model"></select>
<div id="debugconsole"></div>

<script>
  var debug = document.getElementById('debugconsole');

  var models = {
    "Sony": [
      {"name": "Bravia TV S1000", "type": "upnp", "service": "urn:schemas-upnp-org:service:RenderingControl:1" },
      {"name": "Bravia TV S2000", "type": "zeroconf", "service": "_mediarenderer._http._tcp" },
      {"name": "HiFi WD10", "type": "upnp", "service": "urn:schemas-upnp-org:service:RenderingControl:1" }
    ],
    "Philips": [ /* ... */ ],
    "Alba": [ /* ... */ ]
  };

  var makeEl = document.getElementById("make"),
      modelEl = document.getElementById("model");

  makeEl.addEventListener('change', function() {
    modelEl.innerHTML = ""; // reset
    var defaultOption = document.createElement("option");
    defaultOption.textContent = "Select model";
    defaultOption.setAttribute("disabled", "disabled");
    defaultOption.setAttribute("selected", "selected");
    modelEl.appendChild(defaultOption);
    for(var i = 0, l = models[makeEl.value].length; i < l; i++) {
      var option = document.createElement("option");
      option.textContent = models[makeEl.value][i]["name"];
      option.setAttribute("value", models[makeEl.value][i]["type"] + ":" + models[makeEl.value][i]["service"]);
      modelEl.appendChild(option);
    }
  }, false);

  modelEl.addEventListener('change', function() {
    if(navigator.getNetworkServices &&
         modelEl.value == "upnp:urn:schemas-upnp-org:service:RenderingControl:1") {
      navigator.getNetworkServices(modelEl.value, successCallback, errorCallback);
    } else if (modelEl.value == "zeroconf:_mediarenderer._http._tcp") {
      debug.innerHTML += "<br>Service type is not implemented by this application.";
    } else {
      debug.innerHTML += "<br>Service Discovery is not supported!";
    }
  }, false);
</script>

<script>
  function successCallback( services ) {

  // Listen for service push notification messages

    services[0].addEventListener('notify', function ( msg ) {
         debug.innerHTML += "<br>" + services[0].name + " event received: ";
         debug.textContent += msg.data;
    }, false);

 // Send a control signal to mute the service audio

    var svcXhr = new XMLHttpRequest();
    svcXhr.open("POST", services[0].url); // services[0].url and its
                                          // sub-resources have been whitelisted for
                                          // cross-site XHR use in this current
                                          // browsing context.

    svcXhr.setRequestHeader('SOAPAction', 'urn:schemas-upnp-org:service:RenderingControl:1#SetMute');
    svcXhr.setRequestHeader('Content-Type', 'text/xml; charset="utf-8";');

    svcXhr.onreadystatechange = function ( response ) {
      if( response.readyState != 4 || response.status != 200 )
        return;
      debug.innerHTML += "<br>" + services[0].name + " response received: ";
      debug.textContent += response.responseXML;
    }

    // Service messaging to mute the provided service
    var svcMsg = '<?xml version="1.0" encoding="utf-8"?>' +
                 '<s:Envelope s:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/" ' +
                   'xmlns:s="http://schemas.xmlsoap.org/soap/envelope/">' +
                   '<s:Body>' +
                     '<u:SetMute xmlns:u="urn:schemas-upnp-org:service:RenderingControl:1">' +
                       '<InstanceID>0</InstanceID>' +
                       '<Channel>Master</Channel>' +
                       '<DesiredMute>true</DesiredMute>' +
                     '</u:SetMute>' +
                   '</s:Body>' +
                 '</s:Envelope>';

    svcXhr.send(svcMsg);
    debug.innerHTML += "<br>" + services[0].name + " request sent: ";
    debug.textContent += svcMsg;
  }

  function errorCallback( error ) {
    debug.innerHTML += "<br>An error occurred: " + error.code;
  }
</script>

B. Acknowledgements

Thanks are expressed by the editor to the following individuals for their feedback on this specification to date (in alphabetical order):

Gar Bergstedt, Lars-Erik Bolstad, Cathy Chan, Hari G Kumar, Bob Lund, Giuseppe Pascale, Marcin Simonides, Clarke Stevens, Christian Söderström, Mark Vickers.

Thanks are also expressed by the editor to the following organizations and groups for their support in producing this specification to date (in alphabetical order):

CableLabs, Opera Software ASA, W3C Device APIs Working Group, W3C Web and TV Interest Group.

C. References

C.1 Normative references

[DNS-SD]
S. Cheshire; M. Krochmal. DNS-Based Service Discovery. 27 February 2011. IETF Draft. URL: http://files.dns-sd.org/draft-cheshire-dnsext-dns-sd.txt
[DOM4]
Anne van Kesteren; Aryeh Gregor; Ms2ger. DOM4. URL: http://dvcs.w3.org/hg/domcore/raw-file/tip/Overview.html/
[HTML5]
Ian Hickson; David Hyatt. HTML5. 29 March 2012. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/html5
[MDNS]
S. Cheshire; M. Krochmal. Multicast DNS. 14 February 2011. IETF Draft. URL: http://files.multicastdns.org/draft-cheshire-dnsext-multicastdns.txt
[RFC2119]
S. Bradner. Key words for use in RFCs to Indicate Requirement Levels. March 1997. Internet RFC 2119. URL: http://www.ietf.org/rfc/rfc2119.txt
[UPNP-DEVICEARCH11]
UPnP Device Architecture 1.1. 15 October 2008. UPnP Forum. PDF document. URL: http://upnp.org/specs/arch/UPnP-arch-DeviceArchitecture-v1.1.pdf
[WEBIDL]
Cameron McCormack. Web IDL. 27 September 2011. W3C Working Draft. (Work in progress.) URL: http://www.w3.org/TR/2011/WD-WebIDL-20110927/