This document adds a new 'ver' attribute to the &QUERY; element qualified by the 'jabber:iq:roster' namespace, defined as follows:

Definition: The 'ver' attribute is a strictly increasing sequence number that is increased (but not necessarily incremented-by-one) with any modification to the roster data. The value of the attribute MUST be a non-negative 64-bit integer, MUST be changed only by the server, and MUST be treated by the client as opaque. The server MUST ensure that each change to the roster data will result in a different sequence number and that the sequence number associated with a given roster modification will be greater than the sequence number associated with any previous roster modification. (Note: This rule effectively disallows the use of the system clock as a sequence number, since if the system time changes, e.g. because of an adjustment based on an NTP &rfc0958; update, the next generated value might be less than the previous one.)

For purposes of this specification, a "roster modification" is any change to the roster data that would result in a roster push to a connected client. Therefore internal states related to roster processing within the server that do not result in a roster push do not necessitate a change to the sequence number.

If a client supports roster versioning, it SHOULD include the 'ver' element in its request for the roster, where the 'ver' attribute is set to the sequence number associated with its last cache of the roster.

If the client has not yet cached the roster or the cache is lost or corrupted, but the client wishes to bootstrap the use of roster versioning, it SHOULD set the 'ver' attribute to a value of zero (0).

Naturally, if the client does not support roster versioning or does not wish to bootstrap use of roster versioning, it will behave like an RFC-3921-compliant client by not including the 'ver' attribute.

If the roster has not changed since the version enumerated by the client, the server MUST return an empty IQ-result.

If the roster has changed since the version enumerated by the client, the server MUST return a &QUERY; element that includes the latest version number.

The &QUERY; element MUST either contain the complete roster (including the version number to indicate that the roster has changed) or be empty (indicating that roster changes will be sent as interim roster pushes).

In general, if returning the complete roster would use less bandwidth than sending individual roster pushes to the client (e.g., if the roster contains only a few items), the server SHOULD return the complete roster.

However, if returning the complete roster would use more bandwidth than sending individual roster pushes to the client (e.g., if the roster contains many items, only a few of which have changed), the server SHOULD return an empty &QUERY; element, then send individual roster pushes.

The interim roster pushes can be understood as follows:

1. Imagine that the client had an active presence session for the entire time between its cached roster version (in this case, 305) and the new roster version (317).
2. During that time, the client might have received roster pushes related to roster version numbers 306, 307, 310, 311, 313, 314, 315, and 317 (the version numbers must be strictly increasing but there is no requirement that the sequence shall be continuous).
3. However, some of those roster pushes might have contained intermediate updates to the same roster item (e.g., changes in the subscription state for bill@shakespeare.lit from "none" to "to" and from "to" to "both").
4. The interim roster pushes would not include all of the intermediate steps, only the final result of all changes applied while the client was in fact offline.

The client can determine when the interim roster pushes have ended by comparing the version number it received on the empty &QUERY; element against the version number it receives in roster pushes. The client MUST NOT process any of the interim roster pushes until it has processed all of them (this helps to prevent partial processing if the client loses its connection to the server before it has received all of the interim roster pushes).

When the server sends subsequent roster pushes to the client, it MUST include the updated roster version number. Roster pushes MUST occur in sequence order. The version number contained in a roster push MUST be unique. A "change to the roster" is any addition of, update to, or removal of a roster item that would result in a roster push, including changes in subscription states, as described in RFC 3921 or rfc3921bis.

Until this specification advances to a status of Draft, the associated namespace for its stream feature shall be "urn:xmpp:features:rosterver". Upon advancement of this specification, the ®ISTRAR; shall issue a permanent namespace in accordance with the process defined in Section 4 of &xep0053;; the requested namespace is "urn:xmpp:seq", which is thought to be unique per the XMPP Registrar's requirements.