[Standards] Combining features in disco#info

Maxime Buquet pep at bouah.net
Thu Aug 1 22:51:35 UTC 2019

Hi standards,

With edhelas we've been looking at changing XEP-0413: Order-By a bit,
for it to be allowed in disco#items requests.

Order-By is currently mostly used with PubSub (if it is implemented at
all). That is, a PubSub service would advertize it in disco#info, thus
meaning that it's possible to use <order/> when making a PubSub request.

We would want to also be able to say "it's possible to disco#items using
<order/> on this service".

This brings us to a more generic subject, since RSM (0059) has the
same issue.

Side-note: MAM does require RSM so it shouldn't be necessary to
advertize it alongside, but it is usually done.

MAM on PubSub is a thing, as well as RSM on PubSub, which could lead to
MAM and RSM being advertized on a PubSub service. Does that mean this
PubSub service supports RSM or not?

We would like to be able to say in disco#info "Order-By is an option of
disco#items", or "RSM is an option of PubSub".

Ge0rG proposed to use the following:

  <feature var='http://jabber.org/protocol/disco#items'>
    <feature var='urn:xmpp:order-by:0'/>

I kind of like it as it reflects the meaning really well, but it seems
0030's schema doesn't allow it so it could get rejected by parsers.

Another solution would be to declare features per combination. I don't
find it pretty, but it would certainly do the job.

Any more suggestions? Has there been any discussion around this in the

Flow also pointed out that it might not be necessary to advertize RSM
for some services, as one could just send a request including it and if
they get back RSM then it means the service supports it. (Still
annoying when I don't want to get 18k? MUCs in a single result, see

This might work for RSM but it won't for Order-By, as we want to have
guarantees on the result.

Happy Hacking!

Maxime “pep” Buquet
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