RE: DAP-ISSUE-131: Support UPnP device discovery by Device Type? [Network Service Discovery]

> -----Original Message-----
> From: ext Rich Tibbett [mailto:richt@opera.com]
> Sent: Friday, July 19, 2013 8:57 PM
> To: Device APIs Working Group
> Subject: Re: DAP-ISSUE-131: Support UPnP device discovery by Device Type?
> [Network Service Discovery]
>
> On Tue, Jul 9, 2013 at 7:20 PM, Device APIs Working Group Issue Tracker
> <sysbot+tracker@w3.org> wrote:
> > DAP-ISSUE-131: Support UPnP device discovery by Device Type? [Network
> > Service Discovery]
>
> > 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.
>
> Herein lies the rub. The objective of this API is to be able to _communicate
> with obtained networked services_, not simply to be able to detect their
> existence in the network and supply defunct objects back to a web app.
> Being able to detect (potentially proprietary) "add-on" services is only 
> really
> useful if the web app is capable of communicating with that given service
> type.
>
> The current API allows a developer to obtain such "add-on" services if they
> know that service's type up-front. Knowing the service's type before
> obtaining that service is step 1 on the path to being able to interact with 
> that
> service. Step 2 is understanding what API that service supports and step 3 
> is
> actually writing code to interact with that service when it is detected and
> returned and thus being able to interact with that service and thus 
> fulfilling
> the original objective when obtaining that service in the first place.
>
> It's important to understand here that the Network Service Discovery API
> does not support service description (SCPD) lookups as a core requirement.
> That is, in my opinion, a largely defunct part of the UPnP Services
> Architecture specification. Either a developer knows how to communicate
> with a UPnP service up-front or they don't. Learning about the actions that 
> a
> UPnP service supports in real-time, as the app is running, is largely uselss
> until/unless the developer knows what do with those actions and how to
> plug those actions in to their web app in real-time. That does not seem to
> reflect how developers interact with APIs on the web today (though I hold
> out hope that intelligent self-describing and self-organising API 
> interaction is
> just around the corner).
>
> The alternative, and the process supported today, is for the web app to
> declare the services it expects to utilize _and is capable of communicating
> with_ up-front in the initial getNetworkServices call.
> >From there, the concept of a 'device' can be re-constituted by
> comparing the 'deviceId' attribute, if it is available, on each 
> NetworkService
> object.
>
> Obtaining a bunch of potentially unknown networked services by device type
> that a web app is then not able to communicate with is perfectly useless.
> That seems to be the main benefit proposed by this request:
> searching for X number of services by device type, each with a service type
> that is potentially unknown to the current web app.

No that's not what is being proposed by this request. The heart of the request 
is to be able to discover standard/well-known devices that are known to 
contain certain standard/well-known services _which work together to get a job 
done_. Take the UPnP MediaRenderer device, which is defined to contain an 
AVTransport service and a RenderingControl service, and optionally a 
ConnectionManager service. To play back media on such a device, an app would 
make use of both the AVTransport service to initiate playback, and the 
RenderingControl services to control certain aspects of the playback such as 
volume control. (If the ConnectionManager service is also supported, then it 
can also utilize that to achieve some other functionalities.) Thus, an app is 
usually interested in the device as a whole, not the individually services. 
With the current API, if a web app were to look for MediaRenderer devices, it 
would do a getNetworkServices call with AVTransport, RenderingControl and 
ConnectionManager services as the requested types. When it receives a bunch of 
service objects, it would try to figure out which of them belong to the same 
device and then use them accordingly. The proposal to perform discovery by 
device type would allow the web app to do a getNetworkServices call with 
MediaRenderer as the requested type. The results it gets back would be 
well-composed devices with coordinating services that are ready to use.

> That may be great for
> getting a bunch of JavaScript objects, each one representing a networked
> service, but if you can't read or write to the provide URL endpoint in each
> object, what is the point in providing them to the web app in the first 
> place?
> On the other hand, if the web app requests services explicitly because it
> understands how to interact with them then we have ourselves a useful API.
>
> br/ Rich

> I ended up skipping the following sentence, which makes the reference
> provided below useful :)
> Zeroconf-based NetworkService objects do not provide a 'deviceId'
> attribute since there is no concept of a 'device' in that technology [1]. 
> This
> proposed change only seems to account for UPnP technology.

I'm not sure which "proposed change" this comment applies to. Can you clarify?

Regards, Cathy.

>
> [1] Though there are ways in which this can be done by e.g. comparing the
> host, port and base path of Zeroconf-based NetworkService objects against
> each other. If they match then they may be considered as coming from the
> same physical device (note: that is a different definition from a UPnP
> 'device').
>
> > 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.)
> >
> > 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.
> >
> > 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.
> >
> > Regards, Cathy.
> >
> >
> > [1]
> > http://lists.w3.org/Archives/Public/public-device-apis/2012Nov/0101.ht
> > ml [2]
> > http://lists.w3.org/Archives/Public/public-device-apis/2013Feb/0012.ht
> > ml [3]
> > http://lists.w3.org/Archives/Public/public-device-apis/2013Mar/0003.ht
> > ml