[Standards] XEP-0073: Question about service discovery

[Remko Tronçon]

> Then what are service discoveries (XEP-0030) and entity capabilities
> (XEP-0115) for? I would assume that the other client does not support
> anything beyond RFC3921 if it doesn't implement XEP-0115.

Service discovery was initially only done on specific occasions, and
almost exclusively on request of the user. If a contact didn't support
a protocol, you were told by trying it and getting back an
'unsupported' error.

It became clear that you want have the information from disco#info
available for all your contacts at all time (amongst others to make
better UIs), but it is unreasonable to query each one of your contacts
every time you or your contact logs on. This is why XEP-0115 is there:
it describes a way to cache disco#info from  everybody, such that you
have the information when you need it, and don't need to query it
explicitly anymore when you need it.

cheers,
Remko