When an XMPP instant messaging client connects to its server, it typically retrieves a great deal of information, such as a user's roster, privacy lists, and service discovery information. However, this information may not have changed since the client last retrieved the data. In order to improve client start-up time or conserve bandwidth in certain environments, it would be desirable if the client could cache the information and retrieve it only if it has changed.

Such a mechanism exists as part of &rfc2616; in the form of the Entity Tags, which are included in the ETag and If-None-Match headers used for HTTP caching. Since &xep0131; enables an XMPP entity to communicate any HTTP header, it should be possible to re-use existing Entity Tag semantics for caching information sent over an XMPP network. This document defines best practices for such functionality, which could be used between any two XMPP entities that support SHIM headers (though it is envisioned to be most useful for clients that retrieve information from servers and services).

This document addresses the following requirements:

1. Enable caching of information sent over an XMPP network (i.e., retrieval of information only if it has changed since it was last retrieved).
2. Re-use existing HTTP Entity Tag semantics.

In HTTP, an "entity" is the information transferred as the payload of a request or response, and an "Entity Tag" is an opaque string that uniquely identifies that payload. For example, when an HTTP server sends an entity in an HTTP response, it can include an ETag header that identifies the payload as cacheable, and the client can cache that entity; in future requests, the client can include the same value in an If-None-Match header and the server will either return an HTTP 304 ("Not Modified") error if the entity has not changed or return the entity in the HTTP response if it has changed. (Note: For information about the semantics of Entity Tags, the ETag header, and the If-None-Match header, refer to Sections 3.11, 14.19, and 14.26 respectively of RFC 2616.)

The basic concept behind XMPP Entity Tag use is semantically equivalent to the use in HTTP (although we use the term "data object" to refer to the payload); this is made possible by a straightforward application of SHIM headers as specified in XEP-0131. In the context of an IQ request-response interaction, the responding entity will include an ETag SHIM header in its IQ response (indicating that the data object can be cached), the requesting entity will include that identical value in an If-None-Match SHIM header when it queries the server for the same data object, and the responding entity will return either an IQ result with the changed data object or an IQ error indicating that the data object has not changed.

Here is an example of such a request-response flow (the example is that of roster retrieval):

If the responding entity does not understand the If-None-Match header or does not handle Entity Tags for the namespace in the request, it MUST ignore the header and return whatever information it would have returned if the header had not been present.

If the responding entity determines that the requested information has not changed since it was last retrieved by the requesting entity, then it MUST return a <not-modified/> error corresponding to the HTTP 304 error returned by HTTP entities that support the ETag header:

Note: The <not-modified/> error condition is not specified as a stanza error condition in &rfc3920; and an error code of 304 was not included in the older Jabber error codes (see &xep0086;). However, the <not-modified/> error condition is included in &xmppcore;.

Note: In HTTP, an Entity Tag may be either "strong" or "weak" (see Section 13.3.3 of RFC 2616); Entity Tags as used in XMPP extensions MUST be considered strong rather than weak.

Note: The ETag and If-None-Match headers SHOULD be used only in &IQ; stanzas, although they MAY be used in &MESSAGE; stanza interactions if IQ request-response semantics are not appropriate, for example in &xep0072; and in certain applications that use &xep0004;.

If a malicious entity gains access to a user's credentials or is able to masquerade as another entity on the network (e.g., as a man in the middle), it could force retrieval of information before it has changed. However, such access would compromise communications in a more serious fashion and corruption of the Entity Tags functionality is insignificant in comparison.

This proposal requires no interaction with &IANA;.

This proposal requires no interaction with the ®ISTRAR;.

This document describes best practices for use of XEP-0131 and therefore does not require a dedicated schema.

Is the ETag tied to a bare JID or full JID?