The RECOMMENDED approach is for the sender to include the cid when communicating with the recipient. The recipient SHOULD then check its cache of data to determine if the data identified by that cid is cached. If the data is cached, the recipient would then load its cached data. If the data is not cached, the recipient would then retrieve the data by sending an IQ-get to the sender (or potentially some other entity) containing an empty <data/> element whose 'cid' attribute specifies the data to be retrieved, to which the sender would reply with an IQ-result containing a <data/> element whose XML character data provides the binary data.

The <data/> element MUST be used only to encapsulate small bits of binary data and MUST NOT be used for large data transfers. Naturally the definitions of "small" and "large" are rather loose. In general, the data SHOULD NOT be more than 8 kilobytes, and dedicated file transfer methods (e.g., &xep0065; or &xep0047;) SHOULD be used for exchanging blobs of data larger than 8 kilobytes. However, implementations or deployments MAY impose their own limits.

If the data to be shared is particularly small (e.g., less than 1k), then the sender MAY send it directly by including a <data/> element directly in a &MESSAGE;, &PRESENCE;, or &IQ; stanza. The following rules apply:

1. When the <data/> element is directly included in an XMPP &MESSAGE; or &PRESENCE; stanza, it SHOULD be a first-level child of the stanza.
2. When the <data/> element is directly included in an XMPP &IQ; stanza, it MUST be a child of the appropriate first-level child (since the IQ stanza must not include more than one first-level child).
3. When the <data/> element is used to retrieve the data from the sender as described under Retrieving Uncached Data, it MUST be a first-level child of the stanza.

The sender can refer to data that it hosts by including a cid in the data it sends. The following example shows how to include the cid in &xep0071; but any appropriate format can be used, such as &xep0221;.

The recipient can then retrieve the data from the sender as described in the next section.

Data is requested and transferred using the XMPP &IQ; stanza type by making reference to the cid. In particular, the recipient requests the binary data by sending an IQ-get containing an empty <data/> element with a 'cid' attribute that matches the cid URI previously communicated.

The recipient then would either return an error (e.g., ¬found; if it does not have data matching the Content-ID) or return the data.

It is RECOMMENDED for the recipient to cache data; however, the recipient MAY opt not to cache data, for example because it runs on a device that does not have sufficient space for data storage.

The default behavior is for the recipient to cache the data only for the life of the entity's application session (not a client's presence session with the server or the controlling user's communication session with the contact from whom the user received the data); that is, the recipient would clear the cache when the application is terminated or restarted.

As a hint regarding the suggested period for caching the data, the sender MAY include a 'max-age' attribute whenever it sends a <data/> element. The meaning of the 'max-age' attribute exactly matches that of the Max-Age attribute from RFC 2965.

If it is not suggested to cache the data (e.g., because it is ephemeral), the value of the 'max-age' attribute MUST be "0" (the number zero).

A recipient SHOULD cache data based on the hash of the data as encapsulated in the cid. However, if a hash cannot be extracted from the cid, if the recipient does not support the hashing algorithm used, or the recipient does not support hashes, then the recipient SHOULD cache based on the JID of the sender.

To exchange binary data, the data is encapsulated as the XML character data of a <data/> element qualified by the 'urn:xmpp:bob' namespace, where the data MUST be encoded as Base64 in accordance with Section 4 of &rfc4648; (note: the Base64 output MUST NOT include whitespace and MUST set the number of pad bits to zero).

The following attributes are defined for the <data/> element.

The following example illustrates the format (line endings are provided for readability only).

The ®ISTRAR; includes "urn:xmpp:bob" in its registry of protocol namespaces (see &NAMESPACES;).