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Re: [discovery-api] Supporting UPnP devices

From: Rich Tibbett <richt@opera.com>
Date: Tue, 19 Mar 2013 06:05:12 +0100
Message-ID: <5147F208.6030404@opera.com>
To: Cathy.Chan@nokia.com
CC: public-device-apis@w3.org
Rich Tibbett wrote:
> Cathy.Chan@nokia.com wrote:
>> There has been on and off discussion on whether the discovery API should
>> support UPnP device discovery by device type. See e.g. [1], [2], [3].
>> At the
>> March 6 DAP call we had consensus to explore supporting UPnP devices
>> in the
>> discovery API, and I agreed to take a deeper look at the implications
>> of such
>> a change and make a proposal. Below are some key findings that I think
>> are
>> worth discussing.
>>
>> The first thing is I'd assume that we want to continue supporting
>> searching
>> for individual UPnP services, in addition to searching for UPnP
>> devices which
>> contain UPnP services. By this I mean that a web app would continue to
>> be able
>> to search for say a ContentDirectory service and obtain a NetworkService
>> object that represents the service, regardless of what UPnP device that
>> service resides in. In addition to that, a web app would also be able to
>> search for a say MediaServer device and obtain an object that
>> represents the
>> UPnP device, which in turn contains a ContentDirectory service. I
>> believe this
>> is important as there are "add-on" UPnP services that are not tied to any
>> particular device types. Besides, the flexibility may come in handy
>> for some
>> web app developers.
>>
>> The biggest impact in adding UPnP device level support is on the
>> object model.
>> The NetworkService interface currently consists of the following
>> attributes:
>> * id
>> * name
>> * type
>> * url
>> * config
>> * online
>> and the following event handlers
>> * onserviceonline
>> * onserviceoffline
>> * onnotify
>>
>> While this works well for both mDNS and individual UPnP services (and
>> DIAL,
>> which is a special case of UPnP), it isn't particularly suited for
>> representing a UPnP device. For one thing, a UPnP device does not have a
>> single url associated with it to send messages. Instead, each of the
>> services
>> inside it would have one such url. Similarly, a UPnP device itself
>> does not
>> receive event messages. The underlying services do. Thus, a UPnP
>> device does
>> not need/utilize the url attribute and onnotify handler. Instead, it
>> needs an
>> array of objects that represents the underlying services. So, the
>> difference
>> between the desired interface to represent a UPnP device and the current
>> NetworkService interface would be
>> - url
>> - onnotify
>> + services[]
>>
>> Now, if we look at the services objects to be included in a UPnP device
>> object, they need a little less than what is provided by the
>> NetworkService
>> interface, as some of those would now belong to the parent device object.
>> Having them also at the service level would only make it more
>> confusing. Most
>> notably, the online attribute and the associated online/offline events
>> belong
>> to the parent device. The difference between the desired interface to
>> represent a UPnP service *residing under a UPnP device object* and the
>> current
>> NetworkService interface would be
>> - id
>> - name
>> - config
>> - online
>> - onserviceonline
>> - onserviceoffline
>> (In other words, the desired interface for a UPnP service needs only
>> type, url
>> and onnotify.)
>
> Interesting analysis.
>
>>
>> My proposal would be to expand the NetworkService interface to add an
>> optional
>> attribute for the services array and allow the url attribute to be
>> optional/nullable, with the caveat that the onnotify handler would be
>> entirely
>> unused when the object represents a UPnP device. For representing the
>> UPnP
>> service objects, I would propose introducing a separate interface with
>> only
>> the necessary attributes/event handlers instead of reusing the
>> NetworkService
>> interface to minimize confusion. The name of the interface would need
>> some
>> serious thinking/bikeshedding though.
>
> We could detach the url, type and onnotify attributes completely from
> NetworkService objects and supply all 'access point'-like info (type,
> url and onnotify) via a set of indexed properties on each NetworkService
> object?
>
> So a NetworkService object itself can represent zero or n number of
> indexed properties each providing some separate access point info that
> could have been obtained from either a UPnP or Zeroconf service or
> device. We would also likely add a few helper methods to the
> NetworkService interface e.g. length, getServiceByType.
>
> navigator.getNetworkServices('upnp:urn:schemas-upnp-org:device:MediaServer:4',
> function(services) {
> for(var i = 0; i < services.length; i++) {
> var service = services[i];
> for(var j = 0; j < service.length; j++) {
> var accessInfo = service[j];
> console.log(accessInfo.type + ": " + accessInfo.url);
>
> // Do something with the access point info
> // ... e.g. ...
> var xhr = new XMLHttpRequest();
> xhr.open('POST', accessInfo.url, false);
> // ...
> // xhr.send(someRequestBody);
> }
> }
> }, function(err) { console.dir(err); });
>
> This makes the design of the NetworkService interface similar to how the
> NetworkServices interface currently works in the spec. That's probably a
> Good Thing.
>
>>
>> Once we agree on the object model, most of the changes to add UPnP device
>> support should be rather straightforward. However, I do expect to see
>> some
>> sub-steps in various algorithms that would look significantly
>> different for
>> UPnP
>> devices compared with the existing services.
>
> I expect we're looking at a significant amount of effort to rewrite the
> UPnP related algorithms but I think this change gives us more
> flexibility in how we ultimately implement the specification.
>
> Each 'service' object (if that is still the correct terminology) can
> have 0, 1 or n access points. That means we may also be able to provide
> access point information for things in the device description file
> (assuming we can bind them to 'type' names that make sense).

Just to clarify, I mean we now have the possibility of providing 
additional 'asset' information via the indexed properties such as icon 
url endpoints and description file url endpoints (e.g. SCPD urls) if we 
can come up with a good way to describe their 'type'.

e.g.

// adding to the example code provide above
if( service[j].type === 'upnp:icon' ) { // for example
   someImgElement.src = service[j].url;
}

>
> Thanks for kicking this off, Cathy. It is always great to discuss around
> the spec itself.
>
> - Rich
>
>>
>> Regards, Cathy.
>>
>>
>> [1]
>> http://lists.w3.org/Archives/Public/public-device-apis/2012Nov/0101.html
>> [2]
>> http://lists.w3.org/Archives/Public/public-device-apis/2013Feb/0012.html
>> [3]
>> http://lists.w3.org/Archives/Public/public-device-apis/2013Mar/0003.html
>>
>>
Received on Tuesday, 19 March 2013 05:05:49 UTC

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