<abstract>This specification defines a proposed modification to the XMPP roster protocol that enables versioning of rosters such that the server will not send the roster to the client if the roster has not changed, thus saving bandwidth during session establishment.</abstract>
<remark><p>Defined protocol solely in terms of full rosters and roster pushes (no more roster diffs); added implementation notes; clarified server behavior if cached version is unavailable.</p></remark>
<remark><p>Initial published version; per Council consensus, removed optionality regarding semantics of the version attribute.</p></remark>
</revision>
<revision>
<version>0.0.3</version>
<date>2008-03-05</date>
<initials>psa</initials>
<remark><p>Corrected semantics of version attribute (should be a strictly increasing sequence number but may be any unique identifier).</p></remark>
</revision>
<revision>
<version>0.0.2</version>
<date>2008-03-04</date>
<initials>psa</initials>
<remark><p>Clarified description of roster diff; added diff attribute and specified its use in roster results; specified use of version attribute in roster pushes.</p></remark>
<p>Although XMPP rosters can become quite large, they change infrequently. Therefore it can be inefficient for the server to send the roster to the client during session establishment if the roster has not changed. This document defines a small modification to the XMPP roster protocol specified in &xmppim; that enables "versioning" of roster information.</p>
<p>The basic model is that if the client specifies a version number when it requests the roster, the server returns an empty IQ-result. If the roster has changed, the server sends numbered roster pushes for each roster item that has been touched in any way since the version specified by the client. The client processes each roster push as it normally would, incrementing its local version number with each roster push. Therefore the client can receive only the items that have been modified, not the entire roster.</p>
<pclass='def'><strong>Definition:</strong> The <strong>'ver' attribute</strong> 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 for this session.</p>
<p>An XMPP server cannot guarantee that a timestamp will be strictly increasing (e.g., the system time might change because of an adjustment based on an update triggered by the user of the Network Time Protocol (&rfc0958;); therefore, because the 'ver' attribute is defined as a strictly increasing sequence number, server implementations are advised to employ a method other than timestamps for generation of the 'ver' attribute.</p>
<pclass='def'><strong>Definition:</strong> A <strong>"roster modification"</strong> 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 would not result in a roster push do not necessitate a increase in the sequence number. If a series of roster modifications result in a roster item that does not differ from the version cached by the client (e.g., a change to the item's 'name' attribute and then a change back to the original value), the server MUST consider the item to have been modified and therefore MUST send the item to the client (typically via a roster push as described below).</p>
<p>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.</p>
<p>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 MUST set the 'ver' attribute to a value of zero (0).</p>
<p>Naturally, if the client does not support roster versioning or does not wish to bootstrap the use of roster versioning, it will behave like an RFC-3921-compliant client by not including the 'ver' attribute.</p>
<p>Whether or not the roster has changed since the version enumerated by the client, the server MUST either return the complete roster as described in RFC 3921 or return an empty IQ-result (thus indicating that any roster modifications will be sent via roster pushes, as described below). In general, unless 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 send an empty IQ-result and then send the modifications (if any) via roster pushes. In addition, if the client signals a sequence number that is greater than the sequence number currently on file at the server for that JID, the server MUST return the whole current roster as if client announced its version to be zero, thus bootstrapping the client's local cache.</p>
<p>If the roster has not changed since the version enumerated by the client, the server will simply not send any roster pushes to the client (until and unless some relevant event triggers a roster push during the lifetime of the client's session).</p>
<p>If the roster has changed since the version enumerated by the client, the server MUST then send one roster push to the client for each roster item that has been modified since the version enumerated by the client.</p>
<li>Imagine that the client had an active presence session for the entire time between its cached roster version (say, 299) and the new roster version (say, 315).</li>
<li>During that time, the client might have received roster pushes related to roster version numbers 301, 303, 305, 307, 309, 311, 313, and 315 (the version numbers must be strictly increasing but there is no requirement that the sequence shall be continuous).</li>
<li>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").</li>
<li>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 (say, 303, 307, 311, and 315).</li>
<p>The client MUST handle an "interim roster push" in the same way it handles any roster push (indeed, from the client's perspective it cannot tell the difference between an "interim" roster push and a "live" roster push). If the client's session ends before it receives all of the interim roster pushes, when requesting the roster after reconnection it SHOULD request the version associated with the last roster <em>push</em> it received during the session that was disconnected, not the version associated with the roster <em>result</em> it received at the start of the session that was disconnected.</p>
<p>When roster versioning is enabled, the server MUST include the updated roster sequence number with each roster push. Roster pushes MUST occur in sequence order and the sequence number contained in a roster push MUST be unique.</p>
<p>This section provides a detailed scenario that illustrates the use of roster versioning. In this example the client gets disconnected before the server has had a chance to send all of its roster pushes, but this is immaterial to the synchronization process.</p>
<p>If a server supports roster versioning, it MUST inform the connecting entity when returning stream features during the stream negotiation process; at the latest, when informing a client that resource binding is required. This is done by including a <ver/> element qualified by the 'urn:xmpp:features:rosterver' namespace.</p>
<p>It is possible that client-side caching of roster information across sessions (rather than holding them in memory only for the life of a session) could introduce new vulnerabilities, such as misuse by malware. Implementations are advised to appropriately protect cached roster data.</p>
<p>This specification defines the following XML namespace:</p>
<ul>
<li>urn:xmpp:features:rosterver</li>
</ul>
<p>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 &STREAMFEATURES;, as described in Section 4 of &xep0053;.</p>
<p>Thanks to Dave Cridland, Richard Dobson, Leonid Evdokimov, Fabio Forno, Alexander Gnauck, Juha Hartikainen, Joe Hildebrand, Justin Karneges, Sachin Khandelwal, Curtis King, Pedro Melo, Matthew Wild, and Jiří Zárevúcký for their comments. The definition of a sequence number borrows some concepts from the definition of the CONDSTORE extension to IMAP as provided in &rfc4551;</p>
