Re: [Fwd: local device discovery - api, demo and source code]

On Wed, 2011-04-13 at 15:08 +0200, Jean-Claude Dufourd wrote:
> On 12/4/11 17:17 , Dave Raggett wrote:
> > Forwarding at Francois's suggestion.
> >
> > (see also the W3C blog entry).
> JCD: Thank you for the information. This triggers many questions.
> - in your list of discovery technologies, why no UPnP ?

The discovery protocol for UPnP *is* SSDP, which is covered. It uses a
combination of a multicast search probe, unicast responses, and
multicast notifications.

> - are you saying that the discovery plugin, or web introducers, or both, 
> are relevant to this task force ?

Both as they play different roles, see explanation in:

> - how is this discovery plugin connected to Web Introducers ? I see the 
> complementarity, but not the connection.

In principle, a third party could implement a web introducer for local
devices on top of the local discovery API, however, it isn't clear just
how much value this gives over a more specific wrapper library.

> About the complete system using both for letting a page communicate with 
> a device:
> - do you manage the (same) device disappearing and reappearing on the 
> network ? or is there user intervention required for each connection ?

This is problematic as the discovery protocols are widely deployed
(hence hard to get changed), yet don't offer the same semantics.
Ideally, we would support an event or callback when a device is added or
removed from the network, but that isn't going to be practical in
general, although possible in specific contexts.

> - do you have a notion of authorized device (or paired device) ?

Yes, that happens when you want to access a device/service, and isn't
part of the current demo.

> - how does a device communicate with a page ? is there a sort of 
> postMessage from the device to the page ? or do you have a "ghost page" 
> per device to handle that ?

There are multiple possible approaches, e.g. call backs, DOM events,
HTTP or Web Sockets, and so forth.

> Do we need the confidentiality of Web Introducers ? I do not see why.

On local networks you can see the devices if they enable discovery, but
it may require authentication before you can access the services they
provide. Users should be asked for permission before an application is
given such access, and this brings in the usual challenges of

> With UPnP, messages are structured (not just a string). How would this 
> be dealt with in a message from a device to a page ?

The current demo retrieves the XML device description and passes it to
JavaScript. This is a general question of layering. Third party
component developers will want low level access, whilst everyday web
developers want something that insulates them from what they see as
unnecessary details. This is where a market for third party components

> Thanks
> JC

 Dave Raggett <>

Received on Wednesday, 13 April 2011 13:35:04 UTC