Discover Web Things via mDNS

[benfrancis]

As a user I want to search for things which are broadcasting their URL on the local network so I can add them to my gateway

Let's experiment with discovering things using mDNS to broadcast their URL.

[mrstegeman]

webthing-python and webthing-node have been implemented to broadcast an mDNS service with type _wot._tcp and a url field in the TXT record.

[hobinjk]

I was using var browser = mdns.createBrowser(mdns.tcp('http', 'webthing')); locally which I think is a bit more semantic (_http._tcp,_webthing, "a tcp service providing http that uses the webthing api"). I took this approach from http://agnat.github.io/node_mdns/user_guide.html#service_types

[hobinjk]

I was also using href instead of url but your use of url is much better and more semantic so I'll modify accordingly.

[mrstegeman]

@hobinjk I'm fine with that. Is webthing supposed to be a sub-type? If so, I believe it should be mapping to _webthing._sub._http._tcp, not _http._tcp,_webthing. Maybe that's just what mdns uses internally?

[mrstegeman]

Yep, that works.

[hobinjk]

Oh, that's interesting, it should be _webthing._http._tcp, it looks like the comma syntax is just what mdns and the dns-sd utility use internally

[mrstegeman]

According to this book, you use the comma format when browsing, but on the network, a subtype actually maps to an additional PTR record with the format _webthing._sub._http._tcp.

[hobinjk]

Oh wow this is much more confusing and convoluted than I thought it would be. Thanks for the reference! Do you think it makes more sense as _webthing._sub._http._tcp or as _webthing._tcp?

[mrstegeman]

It makes more sense to me to do the sub-type. It's a bit easier to do it the other way, but this seems more proper.

[benfrancis]

See also https://tools.ietf.org/html/rfc6763

An example in the RFC is _printer._sub._http._tcp.<Domain> so _webthing._sub._http._tcp seems reasonable.

Do you think a _webthing subtype is something we could propose as a standard in a "Thing Discovery" section of our Web Thing API spec? Presumably that would include defining the field in the TXT record where you can find the URL of the thing description.

There appears to be an IANA registry for service types but it doesn't seem to include any sub-types. It doesn't even include http.

[mrstegeman]

I definitely think the subtype would be good to include, along with the Eddystone beacon format, if that's something we want to push forward.

[mrstegeman]

And yes, HTTP is included: https://www.iana.org/assignments/service-names-port-numbers/service-names-port-numbers.xhtml?&page=2

But I don't see any subtypes in there.

[mrstegeman]

This is supported by https://github.com/mozilla-iot/thing-url-adapter