SIFT supports the following features.

If a server supports the SIFT protocol, it MUST advertise that fact in its responses to &xep0030; information ("disco#info") requests by returning a feature of "urn:xmpp:sift:1":

This response enables the client to know that the server supports SIFT in general, but particular SIFT features. In order to discover which SIFT features are supported, a client sends an IQ-get containing a <features/> element qualified by the 'urn:xmpp:sift:1' namespace.

The server then returns a list of the particular SIFT features it supports.

The foregoing IQ-result indicates that montague.net supports only message and presence sifting, allows the client to filter those kinds of stanzas only from all entities or from other entities (but not from local entities, remote entities, or itself), allows the client to filter those kinds of stanzas only to both the bare JID and full JID (but not to the bare JID or the full JID alone), and does not support filtering based on payload.

Naturally, the server could return the typical XMPP error conditions, such as &unavailable; if the server does not support the SIFT protocol or the version specified by the client.

To enable sifting of stanzas, the client sends an IQ-set to the server containing a <sift/> child element that in turn contains an <iq/> element, a <message/> element, a <presence/> element, or some combination of those elements. Each of these elements MAY include a 'recipient' attribute whose value is "all", "bare", or "full" (defaulting to "all"). Each of these elements MAY also include a 'sender' attribute whose value is "all", "local", "others", "remote", or "self" (defaulting to "all").

Note: The last SIFT request sent from the client to the server overrides all previous SIFT requests; SIFT requests are not cumulative. Therefore, each SIFT request needs to contain all the SIFT rules that the client wishes the server to enforce, not a delta from the previous request.

The foregoing IQ-set means "sift messages from others and presence from all senders, no matter if the recipient is my bare JID or my full JID".

Each of the child elements &IQ;, &MESSAGE;, and &PRESENCE; MAY also contain one or more <allow/> children whose 'name' attribute specifies the element name and whose 'ns' attribute specifies the XML namespace of stanza payloads the client would like to allow. If no <allow/> elements are included, then sifting of that kind of stanza is completed without reference to the payload.

The foregoing IQ-set means "filter out inbound IQ stanzas except if the payload matches <jingle xmlns='urn:xmpp:jingle:1'/> or <query xmlns='http://jabber.org/protocol/disco#info'/>".

In XMPP, an IQ stanza can contain only one payload element, so the filtering logic is straightforward. However, a message or presence stanza can contain multiple payload elements (cf. &xep0226;). Therefore, filtering for message and presence stanzas means that if the stanza contains the defined payload or payloads (perhaps in addition to other payloads), the server shall deliver it to the client.

For instance, the following example shows how a client would filter inbound messages and IQs to only receive SOAP payloads as specified in &xep0072;.

Similarly, the following example shows how a client would filter inbound presence notifications to only receive notifications that contain entity capabilities data as specified in &xep0115;.

Naturally, the server could return the typical XMPP error conditions, such as &unavailable; if the server does not support the SIFT protocol or the version specified by the client, &feature; if the server does not support a particular feature (e.g., &IQ; sifting) requested by the client, &badrequest; if the request is malformed, &internalserver; if the server experiences a malfunction while attempting to process the request, and so on.

When the client indicates that it wishes to receive inbound presence notifications, the server SHOULD send outbound presence probes on the client's behalf. Responses to these presence probes are addressed to the bare JID of the account and then broadcasted to all of the resources that have expressed interest in receiving inbound presence notifications.

If the client subsequently indicates that it wants the server to intercept inbound presence notifications, the server MUST NOT deliver to the client presence notifications that are addressed to the bare JID or full JID as defined by the 'recipient' attribute.

If the client then indicates again that it wishes to receive inbound presence notifications, the server shall resynchronize the client regarding the presence states of its contacts (how it does so is implementation-specific, e.g. whether it queues received presence notifications or re-probes the user's contacts).

When a client indicates that it wishes to receive messages, the server SHOULD deliver to the client all messages in the offline message queue and MUST deliver to the client any subsequent messages that would normally be delivered to the client in accordance with the rules defined in &xmppcore; and &xmppim;.

If the client subsequently indicates that it wants the server to intercept inbound messages (and there are no other connected or available resources that have expressed interest in receiving inbound messages), the server SHOULD treat messages as if there were no connected or available resources (e.g., storing them offline for later delivery); if the client then indicates again that it wishes to receive inbound messages, the server SHOULD send those queued messages to the client so that it can get back in sync regarding messages received from its contacts.

If the client does not request filtering of inbound IQ stanzas, the server MUST pass through to the client all IQ stanzas that are addressed to the full JID of the client (subject to appropriate security controls as defined in the relevant RFCs and XEPs).

If the client requests filtering of inbound IQ stanzas, for unfiltered payload name+namespace combinations the server MUST pass through to the client all IQ stanzas that are addressed to the full JID of the client (subject to appropriate security controls as defined in the relevant RFCs and XEPs), whereas for filtered payload name+namespace combinations the server MUST respond to all IQ stanzas in a way consistent with the specification for the given payload namespace (if defined) or as specified in &xmppcore; and &xmppim; for IQs where no full JID &LOCALFULL; matches; typically that means returning a &unavailable; error.

Naturally, if the server advertises support for the SIFT protocol but the client does not send any IQ-set stanzas containing SIFT payloads, the server MUST proceed as it normally would in accordance with the core XMPP specifications.

In order to be invisible at the start of a session, a client can register for (i.e., not request interception of) inbound messages and presence notifications without sending initial presence.

The server would then probe the user's contacts and return the resulting presence notifications to the client, as well as allow inbound message and IQ stanzas.

If the user wants to "go visible", the client will send initial presence.

The user can later go invisible again by sending presence of type "unavailable" without modifying the SIFT rules or closing the stream.

RFC 3921 defines the concept of negative values for the presence <priority/> element, where a negative value instructs the server to not deliver to the client any messages that are directed to the bare JID of the user. This behavior can be emulated using SIFT by asking the server to intercept inbound message stanzas for the bare JID, but not presence notifications or IQ stanzas.

If a client requests message sifting, but sends presence, it SHOULD specify a negative priority as a hint to contacts.

Because inbound presence notifications can be "chatty", mobile clients and other entities with limited battery life might want to "hush" the presence session by asking the server to intercept inbound presence notifications but not message stanzas.

This specification defines the following XML namespace:

• urn:xmpp:sift:1

Upon advancement of this specification from a status of Experimental to a status of Draft, the ®ISTRAR; shall add the foregoing namespace to the registry located at &NAMESPACES;, as described in Section 4 of &xep0053;.