On 14.11.2016 10:03, Peter Waher wrote:
>> The concept of friendship is a fundamental building block of the IoT
>> XEPs, yet it is nowhere formal specified what friendship actually is in
>> terms of XMPP. The XEPs appear map friendship between things to
>> subscription states between XMPP entities, but they do not specify if
>> friendship is a symmetric relation or an asymmetric one. XMPP
>> subscription states are asymmetric: Just because I'm subscribed to your
>> presence, it doesn't mean that your are subscribed to mine. I believe
>> that the IoT XEPs assume that an thing is a friend if both are
>> subscribed to each others presence. But do we really want that?
>
> The provisioning server keeps track of friendship relationsships. This
> is symmetric in the sense, that both can subscribe to the presence of
> the other, if they want to. They don’t need to.
>
> I’ve added a note to add a description, or definition of the friendship
> relationship, and how it relates to presence subscriptions.

The question still remains: Do the XEPs envision friendship to be
defined as a full presence subscription between the two involved
entities. Which I think would very problematic because unsolicited
presence stanzas should be avoided (and have caused a lot of trouble in
XMPP's past). Usually a thing is not interested in the presence status
of its friends, Therefore I suggest to define friendship as follows:

"E is a friend of T, iff E is subscribed to T's presence" (where E is an
entity, and T is a thing).

Note that this does not disallow T to subscribe to E's presence. It is
just not required for E to be a friend of T.

>> While implementing we also discovered that the XEPs do not discuss an
>> important protocol flow. One of the test scenarios I implemented in
>> Smack consists of a provisioning server (PS), a thing, a owner and an
>> XMPP entity trying to become a friend of the thing (the 'requestor').
>> The requestor sends a subscription request to the thing, in order to
>> befriend the thing. Now the thing asks the PS whether or not the
>> requestor is a friend and the PS will immediately return that the
>> requestor is not a friend, because there was no decision from the owner
>> yet. Here ends the story in the XEPs. In our scenario, the owner will
>> eventually accept or reject the friendship request in the PS's web
>> interface. But how does the requestor get notified about the owners
>> decision? Possible XEP-0324 ? 3.2.4 ? but do we want the requestor to
>> act on recommendations send from arbitrary JIDs? We also need to discuss
>> this.
>
> If no rules are defined, rejection is returned by default to the device,
> since immediate feedback is always assumed. It is also assumed the owner
> is notified of the action. But since the owner might respond to the
> action at a much later stage, that response is seen as a separate event.
> If the owner accepts the request, this event is informed asynchronously,
> as described in §3.2.4, as you mentioned.

The XEPs should explicit mention that friendship recommendations are
used by the provisioning server (PS) to deliver the owners decision to
the thing. And how do you envision the requestor (R) to be notified?
I've implemented it in Smack so that the thing forwards the friendship
recommendation to R. Because I don't want R to act on friendship
recommendations send from the PS, as it's not clear for R which PS is
authoritative.

But even with this unspecified mechanism, I need nine stanzas to
complete friendship and a cache of the last pending friendship requests
in R.

My suggestion would be to have PS sending a friendship-decision-pending
response to T, on which T simply does nothing. This reduces the required
stanzas to complete friendship to six and removes the client from the
burden to keep track of pending friendship requests.

Please note that this and the previous point (definition of friendship)
are two of my main concerns about the current XEPs that I'd like to
address first. I'd like to use the upcoming Summit (and SIG meetings) to
discuss them.

>> My general impression is that the current IoT XEPs are to large and to
>> complex. It reminds me of the XEP-0136 Messaging Archiving situation,
>> where this big and complex XEP got not much traction because it is so
>> heavyweight and hard to implement. And now we have XEP-0313 Message
>> Archive Management, which is simple, covers most uses cases and is easy
>> to extend, thus allowing the missing use cases to be added on top. We
>> should think big, but write simple and modular XEPs.
>
> And yet much smaller than pubsub, which to many seems to be one of the
> cornerstones of XMPP.

True, but there are efforts to split PubSub into smaller XEPs.

>> The XEPs are written in style which possible assumes that examples are
>> normative: Some sections consist mostly of figures and examples. The IoT
>> XEPs need more normative texts.
>
> More normative text, and less examples? Many claim the XEPs are too
> long. But as you say, most of the text consists of examples.

The XEPs are very long, which makes it very hard to digest. I think from
the ones I've looked at (Discovery, Provisioning, Data, Control), half
of what is written in there could be moved into an add-on XEP containing
the advanced features. The other half would then be a minimal baseline
profile which would make it easy to get started with, solve probably 80%
of the use cases and gave implementers a quicker "success" experience.

>> The data read scheme out should follow the scheme we re-introduced i
>> XEP-0313: Message Archive Management: IQ-request ? data1 ? ? ? dataN ?
>> IQ-result. This would allow to remove the 'done' attribute and this
>> scheme is a little bit easier to implement. I going to repeat myself,
>> because I said this in MAM thread years ago: I believe we always should
>> use this very scheme when requesting data which we expect to be
>> delivered by multiple stanzas.
>
> Are you suggesting that using data-forms would be “easier” for
> transporting sensor data?

No.

> And that constant polling to extract
> asynchronously retrieved data would be simpler?

No.

I was trying to describe the ideal pattern/scheme in XMPP used to
retrieve N data, where every datum is packed in a single stanza: The
requester sends the IQ-request, on which the sender starts sending data1
to dataN in message stanzas, and after sending dataN, the IQ-result is
send. This pattern is also used in MAM.

Currently the data xep does:
IQ-request → IQ-result → data1 → … → dataN-withFin


> It also presupposes
> knowledge about the amount of data to be retrieved,

No it does not.

>> Timestamp is not using XEP-0082: XMPP Date and Time Profiles.
>
> Time zones are described in §6.1.

Ok, but why not simply re-use XEP-0082 as existing building block?

Now I have to implement yet another data/time parser for XEP-0323 just
because it varies slightly from a widely established XEP.

>> Why does it not re-use XSD data types? (SenML [2] also seems like a
>> potential candidate. Haven had time to look into it though.)
>
> What XSD data types are you referring to?

https://www.w3.org/2001/XMLSchema-datatypes

Have a look at XEP-0122's 'datatype' attribute. Ideally the IoT Data XEP
would re-use the same attribute and its definition.

>> XEP-0324 - Provisioning
>> ====================
>>
>> 3.1.1 "if the provisioning server is not available in the roster of
>> the device?"
>> This falsely mixes the existence of an roster entry with presence
>> subscription. But the XMPP Roster and Presence Subscription are two
>> (mostly) orthogonal concepts. You can have a presence subscription
>> established without a related roster item, and you can have a roster
>> item without being subscribed.
>
> I agree on the second, but not the first statement. RFC 6121 §2.5.2.
> clearly states that removing a roster item automatically unsubscribes to
> any presence.

But unsubscribing does not remove the roster entry. But yes, there are
some points where roster and presence interact. Never the less the
sentence should be

"if the device is not subscribed to the provisioning server, then the
device should send a subscription request"

>> 3.2.2 "Any resource information in the JID must be ignored by the
>> provisioning server."
>> Some XEPs of the IoT XEPs go with a different approach of explicitly
>> disallowing the JID value to be a JID with resourcepart. I suggest
>> consistency and I would favor the approach of explicitly forbidding the
>> JID to contain a resourcepart instead of ignoring the resourcepart.
>
> First, it seems to be Note 2, §3.2.1. But, it seems to be a
> misunderstanding. What is meant is that rules, in the provisioning
> server, must be based on the bare JID, not the full JID, of the device.

Ahh, ok.

> The resource part is assumed to be a random value that can change over time.

That's a good design approach I support! I suggest to make it clear that
a thing is defined by it's bare JID, and never by its full JID, or in
other words, there can never be two or more things with the same bare JID.

>> 3.2.4 "isFriend(jid) optional, for security"
>> There is no gain in security if the Device asks the provisioning server
>> if it really should add the JID as friend. If an attacker is able to
>> spoof messages with a 'from' JID of the provisioning server, then he is
>> very likely also able to spoof IQ replies too.
>
> Very likely is not the same as guaranteed.

Possible, but everything breaks down if you assume the possibility of
spoofed XMPP addresses.

It sure is possible if something is not configured correctly, especially
considering a federation between multiple independent parties. But if
you need absolute confidentially, then use crypto.

>> XEP-0325 - Control
>> =================
>
>> Why should we us this instead of XEP-0050: Ad-Hoc Commands? I've seen
>> people controlling their 2015 home automation system (Homematic) with an
>> very old Psi version using XEP-0050. Ad-Hoc Commands and Data Forms are
>> a key strength of XMPP.
>
> There are several reasons. While having access to ad-hoc commands and
> data forms might be a strength, if you use a full-stack XMPP
> implementation, it’s not, if you have a light weight implementation,
> something which is expected, especially within IoT.

I wouldn't call XEP-0325 lightweight.

> So, the simplest use
> cases do not use data forms at all.

Not sure about that. Smack has pretty good support for data forms. IoT
control would have been easier to implement if I would have based it on
data forms and not had to implement everything from scratch.

> It’s also possible to map control
> parameters and sensor data.

And that's not possible with data forms? Especially if both data and
control would be based on them?

> Another reason is the possibility to execute
> commands on multiple nodes at the same time, as well as executing
> multiple control commands in the same request,

Ahh, now it gets interesting. First, please consider putting that
rationale for not using data forms in XEP-0325. Otherwise be prepared to
get asked that very same question over and over again.

And secondly: Was it ever evaluated if data forms could be extended by
that functionality (executing commands on multiple nodes, or multiple
commands in the same request)? I could imagine that such a data forms
add-on feature could be beneficial in other areas too.

> as well as the added
> semantics available for interoperability.

Care to elaborate?

>> XEP-0347 - Discovery
>> ====================
>
>>? 3.9: Claiming a thing: Why are 5 values required to claim a thing,
>> when a tuple of ID and Key would be sufficient and provide the same
>> security guarantees? Do we really want the user to enter 5 strings,
>> every one adding another error source, in order to claim a thing? Or is
>> it up to the registry/thing/whatever to decide what is sufficient to
>> claim a thing?
>> Claiming should be as simple as possible. I've claimed over a hundert
>> devices in the past months and was always annoyed that I had to enter 5
>> values.
>
> It’s up to the manufacturer to decide, what is required. And you should
> not have to enter anything. Instead, another transport method of the
> conceptual information from device/manufacturer to owner is suggested.
> The XEP mentions the use of QR-codes, for instance. The point here, is
> that the number of parameters used in the claim is variable, and defined
> by the manufacturer. All the XEP does, is require the owner to specify
> the same set of parameters, in the claim.

Then please make it clear in the XEP that the PS will require the
potential-soon-to-be-owner to enter the exact same values used by the
thing to register itself with the PS. But what if the thing wants to
provide more metadata values (there a dozens different types already
defined), but only have the owner enter one or two?

I think the default on-boarding procedure could be much simplified by
keeping the same security: Simply let the thing generate a secure token
and have the owner present that token as proof of ownership. How the
bytes of the token are transported and retrieved from the thing (QR
code, via an app, …) can be decided by the manufacturer.

(This reminds me a bit of the easy-onboarding efforts in XMPP:
https://wiki.xmpp.org/web/Easy_Onboarding )

- Florian


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