• A client on a mobile device where bandwidth and throughput are limited has a very large roster which cause connections to take an unacceptable amount of time. With entity versioning, connections after the first connection do not take as long, and use less bandwidth.
• A client often wants to view the list of multi-user chat rooms available on a servers MUC service. However, the list is very long and takes a long time to download. After enabling entity versioning the client can fetch the list, and then poll for changes at a later date without re-requesting the entire list.
• A client wishes to cache the features supported by servers of the contacts in their roster since their disco items is not likely to change often.

• A server is running in an environment where storing multiple versions of each users roster may put too much pressure on the storage backend. After enabling entity versioning, they only have to store a small token per user and can calculate the diffs to send to the client afterwards.
• A server maintains an out-of-band HTTP API for fetching information about MUC rooms to display on their web page. They wish to use a reverse proxy to cache API requests based on etags. Instead of attempting to check if the backend page has changed and generate etags, the room's entity version token is used as a weakly-validated ETag.

If a server supports entity versioning, it MUST inform the connecting client when returning stream features during the stream negotiation process. This is done by including a <ver/> element, qualified by the 'urn:xmpp:entityver:0' namespace. At the latest, this SHOULD be done when informing a client that resource binding is required. For example:

The entity versioning stream feature is merely informative and therefore is never mandatory-to-negotiate.

Version tokens are short case-sensitive strings which are generated by the server. Their format is not defined in this spec, but a recommendation may be found in the Implementation Notes. Version tokens are akin to a weakly-validated etag for the entity in question.

Servers that implement this protocol must assign such a version token to each entity that is controlled by the server. The server MUST then update this version every time any mutable property of the entity changes (eg. when the subscription status of a user changes). The server MAY choose to update this token at any time (to force the clients to invalidate their cached representation fo the object). This version token MUST then be included with every object representation of that entity sent down in the stream. This is done by including a sub-node called "version" qualified by the entity versioning XML namespace defined in this document. Similarly, clients MAY also add version nodes for each version token they possess to the request for a list (not specifying a version token will force the server to send information on that entity to the client). If a server sends up a list of version tokens, the server MUST then check to see if those tokens correspond to any entity which it knows about, and not send down any entities with matching version tokens in the response.

For example, a roster request might look like:

Note that in this case there may be three roster items total (and the client only knows about two of them), or there may be two total roster items and the server is informing the client about a change to "bill@shakespeare.lit". Version tokens MUST also be present in roster pushes:

A disco request for rooms (as defined in &xep0045;) might look like:

In this example coven@chat.shakespeare.lit has been modified (eg. the room name might have been changed), but inverness@chat.shakespeare.lit has not changed, therefore no update is sent down.

Clients that implement this protocol SHOULD then cache the entity in question when a version token is received.

While the version token approach to caching does not require a great deal of state to be stored on the client or the server, it does require a lot more information to be sent by the client when requesting a list of entities. For a very large list which is not likely to have changed, it may be useful know in advance if the roster has changed or not (so that we can avoid sending the large request entirely). To do this, we can request an aggregate version token from the server. This aggregate token is calculated by constructing a string of comma separated "bare JID:version" pairs sorted in byte-wise order, and taking the MD5 hash of the constructed string. For example, if the server is calculating the aggregate version token for a roster, it might end up with the following string:

Which results in the aggregate token:

The actual request is an IQ sent to the server, or entity handling the versioned list which contains a query that specifies the namespace of the list we want to fetch. Eg. to fetch the aggregate token for the roster one would query the server:

Similarly, to fetch the aggregate token for a list of MUC rooms, one would query the MUC component directly:

Because aggregate tokens are OPTIONAL to implement, clients MUST fall back to a normal request if any error is returned in response to an aggregate token IQ.

Clients are also NOT REQUIRED to check aggregate tokens. However, clients MAY wish to check aggregate tokens before making a roster or MUC request when the cached roster or MUC list is very large. When to check aggregate tokens is left up to the clients.

This specification defines the following XML namespace:

• urn:xmpp:entityver:0

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;.