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RE: [discovery-api] Draft spec updates

From: <Cathy.Chan@nokia.com>
Date: Tue, 2 Oct 2012 20:17:04 +0000
To: <richt@opera.com>, <public-device-apis@w3.org>
Message-ID: <A46437648ECB3D4F852B077AFF9099F518E2FB14@008-AM1MPN1-061.mgdnok.nokia.com>
Below please find my comments on the updated spec.

1. At the beginning of Section 7, there's discussion on when the UA might
perform the service discovery mechanisms.
[[It is expected that user agents will perform these service discovery
mechansisms asynchronously and periodically update the list of networked
devices as required. The timing of any service discovery mechanisms is an
implementation detail left to the discretion of the implementer (e.g. once
on user agent start-up, every X seconds during user agent execution or on
invocation of this API from a Web page).]]
It's unclear to me what exactly "service discovery mechanism" encompasses,
especially in the UPnP case. Does it refer only to issuing a search request
for UPnP root devices? Or does it also include the *continuous* monitoring of 
standard UPnP address and port for incoming requests and responses? To me,
it doesn't make sense for a UA to issue a search at start-up but not follow
up by continuously monitoring the standard address+port for new/updated
announcements and disconnection messages (byebye messages). Such an
implementation is bound to return either stale or useless information to web
pages on invocations of the getNetworkServices API as time goes by. This
also affects whether the definition of the list of available service records
as " track all the services that have been discovered and are available in
the current network *at any given time*" is accurate or not.
Secondly, if the UA only performs discovery on invocation of the
getNetworkServices API, does it still issue the search looking for all
device types (using "_services._dns-sd._udp" with Zeroconf or
"upnp:rootdevice" with UPnP, as described in 7.1 and 7.2), or should it
issue the search based on the requested types?
Also, typo in this paragraph: s/mechansisms/mechanisms/

2. With respect to the "list of authorized service records", "each record in
the list of authorized service records is associated with a services manager
object that is assigned as part of the getNetworkServices() algorithm". Is
it possible for a record to be associated with multiple Services Manager
objects? I would imagine that if two separate web pages both invoked the
getNetworkServices API with the same service type, there would only be one
record for each matching service, and each of those records would be
associated with both Service Manager objects.
Thus, s/a services manager object that is/services manager objects that are/

3. In the rule for "adding an available service"...
3.1 In step 4.2, if multiple matching active records are associated with a 
Manager object, the servicesAvailable attribute of the Services Manager object
would incorrectly be incremented multiple times. For example, if a Services
Manager object was associated with two UPnP ContentDirectory service records,
and a new UPnP ContentDirectory service is being added, the servicesAvailable
attribute of the Services Manager would be incremented by 2, and two
serviceavailable events would be dispatched. Thus, the algorithm needs to be
modified to ensure that step 4.2 is only done once for each Services Manager
3.2. Step 4.3 is missing the necessary conditions: 1) if the new service 
flag is set to false, 2) the id of the "network service record" (i.e. the one 
added) matches that of the current active service object, and 3) the online
attribute of the current active service object was previously false (before
being replaced in step 2.3).
Both of these comments also apply to the rule for "removing an available 

5. In 7.1, step 2.7, why is an expiration time of 120 seconds mandated? My
understanding is that DNS-SD/mDNS allows the expiration time to be specified
in either the PTR or TXT record. Even if it's not available from there, this
value should be left up to the implementation.

6. In 7.2, "advertisement for UPnP root device" should be "search request
for UPnP root device" throughout.

7. In 7.2, "The user agent must listen for incoming requests and process any
incoming responses to any advertisement for UPnP root devices on the
standard UPnP address and port, on all current local network interface
addresses with the port 1900, according to the rules defined in this
section." This is not completely accurate. Incoming *requests* arrive on the
standard UPnP address and port, but incoming *responses* arrive on
whatever port the UA used to send the initial search request, which *must
not* be 1900. The sentence should be split into two, one addressing requests
and one addressing responses. Additionally, for readability and clarity, I
would put the sentence about incoming requests to just before the steps that
begin with "For each HTTP Request received ...".
The user agent must listen for and process any incoming responses to any
search request for UPnP root devices according to the rules defined in this
For each HTTP Response following ...
The user agent must listen for and process incoming requests on the standard
UPnP address and port, on all current local network interface addresses with
the port 1900, according to the rules defined in this section.
For each HTTP Request received ...

8. In 7.2, in the steps for processing HTTP Responses...
8.1 We can add that the UA may stop listening after the max wait time has been
8.2 In Step 5, the value of the CACHE-CONTROL header begins with the string
"max-age=". It needs to be removed before passing the content as the device
expiry argument.
Thus, s/ CACHE-CONTROL from ssdp device/CACHE-CONTROL from ssdp device
(minus the leading string of "max-age=")/

9. In 7.2, in the steps for processing HTTP Requests...
9.1 In Step 1, s/return a HTTP 200 OK response,//.
The UDA expressly states that "No responses are sent for messages with
method NOTIFY" (at the end of section 1.2.2 of UDA11).
9.2 Step 3 applies only to ssdp:alive messages. It should start with "If the
value of the NTS entry of the ssdp device is 'ssdp:alive'". Also, s/return a
400 Bad Request response,//.
9.3 In step 4, there is no need to process ssdp:update messages, which are
bound to be followed by ssdp:alive messages. (Besides, ssdp:update messages
do not have the LOCATION header.) I brought up ssdp:update messages in my
previous comments because in the previous drafts, anything that is not an
ssdp:alive message is assumed to be an ssdp:byebye message and triggers
device removal. In the new draft, device removal is explicitly associated
with ssdp:byebye messages (instead of "non ssdp:alive" messages), so my
concern has been addressed.
9.4 In Step 4, same as 8.2 above.

10. In 7.2, in the rule for obtaining a UPnP Device Description File...
10.1 Step 2 "root device descriptor file" should be "device descriptor file".
10.2 Step 4 describes processing of embedded devices. Since embedded devices
may further contain a deviceList of embedded devices, this step should be
moved to the end of the rule for processing a UPnP Device Description File.
That way, processing of embedded devices is done recursively without extra

11. In 7.2, in the rule for processing a UPnP Description File...
11.1 The algorithm uses the USN header value from the SSDP message as the
device identifier. The device identifier is then used for two purposes - in
2.2 for creating a <unique> id attribute for the service (by combining the
device identifier with the UPnP serviceId), and in 2.3 for subsequently
identifying services that belong to the device in the device removal process.
The identifier works fine for the second purpose. However, for the first
purpose, if a root device contains embedded devices, this could easily result
in duplicate ids among service records. For example, the MediaServer spec
requires that a ContentDirectory service always has a serviceId of
"urn:upnp-org:serviceId:ContentDirectory". If a device contains two embedded
MediaServer devices (which might be odd but entirely legitimate, and may
enable some special use cases), then the algorithm would result in two service
records with the same id attribute. My suggestion is to use the UDN element
(a unique ID for each device, and each embedded devices) within the (current)
device description file in composing the id attribute to ensure uniqueness.

12. In the steps for setting up a UPnP Events Subscription...
12.1 In Step 5.4.4, with a valid 200 OK response, in addition to updating
callback ID, it is also necessary to update the timeout date, and return to
Step 5.4. (It might be simpler to just go straight back to Step 5.1 if the
response is a 200 OK.)
12.2 In Step 5.5, there also needs to be a step to send a 200 OK response back
to the event publisher. (UDA11 Section 4.3.2)

I'd add that I really like the organization changes and clarifications made to
this revision. Kudos!

Regards, Cathy.

> -----Original Message-----
> From: ext Rich Tibbett [mailto:richt@opera.com]
> Sent: Tuesday, September 25, 2012 7:43 AM
> To: W3C Device APIs WG
> Subject: [discovery-api] Draft spec updates
> A new version of the Networking and Service Discovery & Messaging
> specification is now available as an ED update.
> This update incorporates all of the (excellent) feedback previously
> to this list by Cathy [1] [2]. The net result is a major rewrite and
clarification of
> the underlying rules and algorithms included in the draft specification.
> update does not affect the operation of the API interfaces previously
> included but is instead more targeted at clarifying low-level
> details of working with different network discovery protocols.
> The latest draft of this specification is available in the usual place:
> http://dvcs.w3.org/hg/dap/raw-file/tip/discovery-api/Overview.html
> Any further feedback you have would be very helpful in moving this
> specification forward.
> Best regards,
> Rich
> [1]
> http://lists.w3.org/Archives/Public/public-device-apis/2012Aug/0017.html
> [2] http://lists.w3.org/Archives/Public/public-device-
> apis/2012Aug/0095.html

Received on Tuesday, 2 October 2012 20:18:09 GMT

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