It is often desirable for a Jabber/XMPP application (commonly but not necessarily a client) to take different actions depending on the capabilities of another application from which it receives presence information. Examples include:

• Showing a different set of icons depending on the capabilities of other clients.
• Not sending &xep0071; content to plaintext clients such as cell phones.
• Allowing the initiation of Voice over IP (VoIP) sessions only to clients that support VoIP.
• Not showing a "Send a File" button if another user's client does not support &xep0096;.

Some older Jabber clients send one &xep0030; and one &xep0092; request to each entity from which they received presence after login. That "disco+version flood" results in an excessive use of bandwidth and is impractical on a larger scale, particularly for users or applications with large rosters. Therefore this document proposes a more robust and scalable solution: namely, a presence-based mechanism This proposal is not limited to clients, and can be used by any entity that exchanges presence with another entity, e.g., a gateway. However, this document uses the example of clients throughout. for exchanging information about entity capabilities. Clients SHOULD NOT engage in the older "disco+version flood" behavior and instead SHOULD use Entity Capabilities as specified herein.

This section provides a friendly, non-normative introduction to the workings of entity capabilities.

Imagine that you are a Shakespearean character named Juliet and one of your contacts, a handsome fellow named Romeo, becomes available. His client wants to publish its capabilities, and does this by adding a <c/> element to its presence packets. As a result, your client receives the following presence packet:

The 'node' attribute represents the client Romeo is using, and the 'ver' attribute represents the specific version of this client. At this point, your client has no idea what the capabilities are of someone with a client string 'http://exodus.jabberstudio.org/caps' and a version string '0.9'. Your client therefore sends a query to Romeo, asking what his client version can do (using service discovery):

The response is:

At this point, your client knows that anyone using the client string 'http://exodus.jabberstudio.org/caps' and the version string '0.9' has a client that can do MUC. Your client remembers this information, such that it does not need to explicitly query the capabilities of a contact with the exact same client and version string. For example, Benvolio may send you the following presence:

Now your client automatically knows that Benvolio can do MUC, without needing to ask him explicitly via service discovery.

On the other hand, for a person with the following presence ...

... or the following presence ...

... you have no information about what this contact's client is capable of (as he is using a different client/version), and you therefore need to query his capabilities explicitly again.

So that is how the 'node' and 'ver' attributes work. But most clients also allow certain features to be turned on or off at run time. For example, clients can allow users to turn off the exchange of chat states (for privacy reasons). For such a client, this means that it cannot just advertise its capabilities as was done before, since this capabilitiy depends on whether or not the user has turned the option on or off (and capabilities for the same client/version are assumed to be identical for all contacts). Such capabilities are advertised using "extensions". For example:

In this case, Benvolio is using an extension that his client calls 'csn'. Again, your client has no clue what 'csn' means, as it is a name arbitrarily chosen by Benvolio's client. It therefore queries Benvolio's client to find out what a client 'http://exodus.jabberstudio.org/caps' means when it says it supports 'csn'. It does this by sending a disco#info request to the node#ext (not the node#ver as was done to find the base capabilities):

The response is:

Now, your client knows that Benvolio's client (and anyone using the same client node as Benvolio and extension 'csn', since the 'ext' values MUST be stable across client versions) supports chat state notifications. The sum total of what Benvolio's client supports is the set of features advertised in disco#info responses to the node#ver (base features) and the set of features advertised in disco#info responses to each node#ext (extension features).

However, suppose Bill logs on:

Although you know that 'csn' meant "chat state notifications" for Benvolio (and for everyone using the same client as Benvolio), you do not know what this means for Bill, because he is using a different client! So you need to query his client:

and the response ...

... reveals that this client uses 'csn' to denote the capability of "stanza session negotiation" (formerly known as "chat session negotiation"). So although the 'ext' values are stable for each client (and for all versions thereof), they are not stable across different clients.

Each time a conformant client sends presence, it annotates that presence with an element that specifies the client type, the version of that client, and which feature bundles (if any) are currently enabled. Unless the server optimizations shown later are being used, the client MUST send this with every presence change (except for unavailable presence) to enable existing servers to remember the last presence for use in responding to probes. The client MUST send the 'node' and 'ver' attributes.

In addition, the client MAY send an 'ext' attribute (short for "extensions") if it has one or more feature bundles to advertise. A feature bundle is any non-standard addition or extension to the core application, such as a client plugin. If more than one feature bundle is advertised, the value of the 'ext' attribute MUST be a space-separated list of bundle names. Each extension name MUST be of type NMTOKEN, where multiple extension names are separated by the white space character #x20, resulting in a tokenized attribute type of NMTOKENS (see Section 3.3.1 of &w3xml;). The client MUST NOT send an 'ext' attribute if there are no interesting non-core features enabled. The names of the feature bundles MUST NOT be used for semantic purposes: they are merely opaque identifiers that will be used in other use cases. However, a client MUST ensure that the same 'ext' value refers to the same feature bundle across client versions (i.e., different values of the 'ver' attribute). If bundles are added or substracted during an entity's session (e.g., a user plugs in a video camera), the entity SHOULD update the value of the 'ext' attribute to reflect the changed capabilities and send a new presence broadcast. If a feature bundle itself changes in any way (e.g., a user installs an updated version of a client plugin), the application MUST change the bundle name and SHOULD send a new presence broadcast.

Note: The values of the 'node', 'ver', and 'ext' attributes MUST NOT contain the '#' character, since that character is used as a separator in the Discovering Capabilities use case.

Once someone on my roster knows what client I am using, they need to be able to figure out what features are supported by that client. In the deprecated "disco flood" approach, this has been done by sending one "disco#info" request to each entity in a user's roster. Entity capabilities makes that unnecessary through the use of annotated presence. In particular, a client that receives the annotated presence sends a disco#info request (as defined in XEP-0030: Service Discovery) to exactly one of the users that sent a particular combination of node and ver. If the requestor has received the same annotation from multiple JIDs, the requestor SHOULD pick a random JID from that list to which the requestor will send the disco#info request.

The disco#info request is sent to a JID + node combination that consists of the chosen <user@host/resource> JID and a service discovery node that is constructed as follows: concatenate (1) the value of the caps 'node' attribute, (2) the "#" character, and (3) the version number specified in the caps 'ver' attribute.

The random user then returns all of the capabilities supported by the base installation of the application without plugins or other add-ons:

Subsequent requests MAY be made to determine the supported features associated with each extension. These requests MUST be sent to a random <user@host/resource> JID that sent a caps annotation that included a particular node/ext combination. The disco#info request shall be sent to a JID + node combination that consists of the chosen JID and a service discovery node that is constructed as follows: concatenate (1) the value of the caps 'node' attribute, (2) the "#" character, and (3) the extension name specified by one of the space-separated names in the caps 'ext' attribute. The requestor SHOULD try to use different JIDs for each of these requests, as well as for the first request.

Note: The set of features that a given entity advertises in response to a "client#version" request and all "client#extension" requests MUST be equivalent to the response it gives to a disco#info request with no 'node' attribute:

All of the responses to the disco#info queries SHOULD be cached. If a particular entity cannot store the responses, it SHOULD NOT make the requests. An entity SHOULD NOT make the service discovery requests unless the information is required for some local functionality. An entity MUST NOT ever make a request to another entity that has the same version of the same application as the requesting entity, except for extensions that are not supported by the requestor's installation (e.g., one "Exodus 0.9" client MUST NOT query another "Exodus 0.9" client unless the second client has advertised an extension or plugin that the first client does not have).

A server MAY include its own entity capabilities in a stream feature element so that connecting clients and peer servers do not need to send service discovery requests each time they connect: