[Standards] XEP-0115 Entity Capabilities - clarifications

Alban Crequy alban.crequy at collabora.co.uk
Mon Aug 16 11:11:24 UTC 2010


Le Sun, 15 Aug 2010 23:05:49 +0100,
Martin Morrison <martin.morrison at gmail.com> a écrit :

> I've recently been implementing XEP-0115 Entity Capabilities, both the
> latest and the legacy versions, and have a few issues that I feel
> should be clarified in the latest version of the spec:
> 
> 1. The legacy version of the spec explicitly says (in 4.2, below
> example 8):
[...]

> 2. The spec only uses the word "SHOULD" when specifying how the Disco
> 'node' attribute is formed. A receiving entity that supports both
> Entity Capabilities and has multiple disco#items nodes thus has
> somewhat of a dilemma in deciding how to respond to a disco#info
> request for an unknown node. Should it return an <item-not-found/>
> error, or assume that the remote entity has used some other mechanism
> to construct the 'node' attribute in the request, and return the base
> capabilities as if the node was empty?

I think <item-not-found/> is best: sending an empty capability in this
case is bad because the hash will not match and the client will discard
the reply.

> 3. Related to item 2, the following race condition can occur:
> 
> - romeo at shakespeare.lit sends Presence to juliet at shakespeare.lit with
> an Entity Capabilities hash
> - In response, juliet sends a disco#info request with the "node#hash"
> as the 'node' attribute
> - Meanwhile, romeo changes the feature set of his client (e.g. turns
> on his camera)
> - Upon receiving the disco#info request, what does romeo do?
> 
> As the 'node' attribute has been formed using the recommended method,
> Romeo can establish that the hash doesn't match his current
> capabilities. Should he return an error, or ignore the contents of the
> 'node' attribute completely and just return his current capabilities
> (which will be accepted, since he will already have pushed an updated
> hash via Presence)? Either way, I think it would help if the spec
> specified what was expected.

For this race, it was suggested to just send <item-not-found/> in this
thread: http://mail.jabber.org/pipermail/standards/2008-May/018713.html

Sending <item-not-found/> will work: Juliet will receive the new hash
a bit later and she will send a new disco request with the new hash.

Alternatively, Telepathy-Gabble keeps a cache mapping
hash->capabilities. So with this implementation, Romeo will send the
*previous* capabilities corresponding with the requested hash. Juliet
will ask the capabilities again anyway when she receives the new hash
if she wants to know the new capabilities.

Disco queries in XEP-0115 are not "what are your current caps?" but
"what are the caps corresponding to this hash?". It is up to the
clients to keep track of hashes and to make new disco requests when
they receive a new hash if they want to know the capabilities.

Alban




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