In XMPP, rosters and presence subscriptions have been used to date only among IM users (see &xmppim;). However, nothing prevents the application of these concepts to other XMPP entities, such as components and servers. Given that a presence subscription typically indicates some level of trust in a peer, server deployments can use the sharing of XMPP presence information as a way to indicate that a given server has a trust relationship with a peer server (informally, we can say that the two servers consider each other "buddies"). The server might then share certain kinds of additional information only with its trusted peers, for example &xep0268;.

To establish such a trust relationship with a peer, a server sends a presence subscription request to the peer, just as is done between XMPP users.

A server MUST NOT send such a presence subscription request unless explicitly requested to do so by a server administrator (see below).

Upon receiving such a presence subscription request, the XMPP server software running at the peer shall either prompt the server administrator to approve the request or (if explicitly configured to accept subscriptions requests) automatically approve it. (A future version of this specification might define an approval method based on &xep0004;.)

If the request is approved, the peer server then informs the originating server that the request has been approved.

The peer SHOULD also send a subscription request to the originating server.

The originating server would then approve that subscription request.

If an XMPP server implementation supports this usage of presence subscriptions, it MUST keep a list of approved entities, which we denote a "server roster". The implementation MAY use that roster for access control purposes defined in other specifications.

This section defines an &xep0050; node scoped by the &xep0068; FORM_TYPE specified in &xep0133;. Upon advancement of this specification to Draft, this section ought to be moved to XEP-0133.

The command node for this use case SHOULD be "http://jabber.org/protocol/admin#server-buddy".

A sample protocol flow for this use case is shown below.

Unless an error occurs (see the "Error Handling" section of XEP-0133), the service SHOULD return the appropriate form.

Note: In virtual hosting environments, the server can determine the the domain name from which to send the presence subscription based on the 'to' address of the &IQ; stanza.

Notification of completion MAY include the processed data in a data form of type "result".

To advertise its support for the server buddy feature, when replying to service discovery information ("disco#info") requests a server MUST return a URNs "urn:xmpp:server-presence".

Because server "buddies" might be granted greater privileges than unknown peers, care should be taken in sending or approving presence subscription requests. In particular, a server MUST NOT send a presence subscription request unless explicitly requested to do so by a server administrator.

This document requires no interaction with &IANA;.

Thanks to Kim Alvefur, Waqas Hussain, and Tobias Markmann for their feedback.