<abstract>This document defines an XMPP protocol extension for broadcasting and dynamically discovering client, device, or generic entity capabilities in a way that minimizes network impact.</abstract>
<remark><p>Specified that inclusion of hash attribute is required, removed default value of sha-1, and added reference to IANA registry; mentioned pre-image attack and added reference to RFC 4270; clarified meaning and construction of caps node attribute and disco node attribute; specified that node attribute shall be included in disco#info request for backwards-compatibility; further specified security considerations; clarified handling of the legacy format to assist developers; defined optional v attribute for the software version; added service discovery feature for caps optimization to prevent confusion regarding server support of caps vs. caps optimization.</p></remark>
<remark><p>In response to persistent security concerns over caps poisoning, redefined ver attribute to be a hash of the service discovery identity and features in a way that is backwards-compatible with the legacy format.</p></remark>
<remark><p>Added developer-friendly introduction; specified that ext names must be stable across application versions; further clarified examples; added stream feature use case; removed message example (send directed presence instead).</p></remark>
<remark><p>Added several items to the Security Considerations; clarified naming requirements regarding 'node', 'ver', and 'ext' attributes.</p></remark>
<remark><p>Specified that the protocol can be used whenever presence is used (e.g., by gateways); improved the XML schema; made several editorial adjustments.</p></remark>
<remark><p>Add more clarifying assumptions and requirements, make it clear that clients don't have to send capabilities every time if the server is optimizing.</p></remark>
<p>It is often desirable for an 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:</p>
<p>In the past, after logging in some Jabber clients sent one &xep0030; and one &xep0092; request to each entity from which they received presence. That "disco/version flood" resulted in an excessive use of bandwidth and was impractical on a larger scale, particularly for users with large rosters. Therefore this document defines a more robust and scalable solution: namely, a presence-based mechanism <note>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.</note> 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.</p>
<p>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 to its presence packets a <c/> element with special attributes. As a result, your client receives the following presence packet:</p>
<p>The 'node' attribute represents the client software Romeo is using. The optional 'v' attribute represents the specific version of that client software (it is only an "FYI" and is not used further in entity capabilities). The 'ver' attribute is a specially-constructed string that represents the identity (see &DISCOCATEGORIES;) and supported features (see &DISCOFEATURES;) of the entity.</p>
<p>At this point, your client has no idea what the capabilities are of someone with a version string '8RovUdtOmiAjzj+xI7SK5BCw3A8='. Your client therefore sends a service discovery query to Romeo, asking what his client can do.</p>
<p>At this point, your client knows that anyone advertising a version string of '8RovUdtOmiAjzj+xI7SK5BCw3A8=' has a client that can do &xep0045; and the other features returned by Romeo's client (the string can be relied upon because of how it is generated and checked, as explained later in this document). Your client remembers this information, so that it does not need to explicitly query the capabilities of a contact with the same version string. For example, Benvolio may send you the following presence:</p>
<p>... you have no information about what this contact's client is capable of unless you have cached previous entity capabilities information; therefore you need to query for capabilities explicitly again via service discovery.</p>
<li>Members of a community tend to cluster around a small set of clients with a small set of capabilities. More specifically, multiple people in my roster use the same client, and they upgrade versions relatively slowly (commonly a few times a year, perhaps once a week at most, certainly not once a minute).</li>
<li>Clients must be able to participate even if they support only &xmppcore;, &xmppim;, and <cite>XEP-0030</cite>.</li>
<li>Clients must be able to participate even if they are on networks without connectivity to other XMPP servers, services offering specialized XMPP extensions, or HTTP servers.<note>These first two requirements effectively eliminated &xep0060; as a possible implementation of entity capabilities.</note></li>
<li>It must be possible to write a XEP-0045 server implementation that passes the given information along.</li>
<li>It must be possible to publish a change in capabilities within a single presence session.</li>
<li>Server infrastructure above and beyond that defined in <cite>XMPP Core</cite> and <cite>XMPP IM</cite> must not be required for this approach to work, although additional server infrastructure may be used for optimization purposes.</li>
<p>Entity capabilities are encapsulated in a <c/> element qualified by the 'http://jabber.org/protocol/caps' namespace. The attributes of the <c/> element are as follows.</p>
<td>A set of nametokens specifying additional feature bundles; this attribute is deprecated (see the <linkurl='#legacy'>Legacy Format</link> section of this document).</td>
<td>The hashing algorithm used to generate the 'ver' attribute; expected values are sha-1 and sha-256, although other values may be used (such values SHOULD be as registered in the &ianahashes;).</td>
<p>* Note: It is RECOMMENDED for the value of the 'node' attribute to be an HTTP URL at which a user could find further information about the software product, such as "http://psi-im.org/" for the Psi client; this enables a processing application to also determine a unique string for the generating application, which it could maintain in a list of known software implementations (e.g., associating the name received via the disco#info reply with the URL found in the caps data).</p>
<p>*** Note: Before version 1.4 of this specification, the 'ver' attribute was used to specify the released version of the software; while the values of the 'ver' attribute that result from use of the algorithm specified herein are backwards-compatible, applications SHOULD appropriately handle the <linkurl='#legacy'>Legacy Format</link>.</p>
<p>In order to help prevent poisoning of entity capabilities information, the value of the 'ver' attribute MUST be generated according to the following method.</p>
<p>Note: All sorting operations MUST be performed using "i;octet" collation as specified in Section 9.3 of &rfc4790;.</p>
<li>Sort the service discovery identities <note>A registry of service discovery identities is located at &DISCOCATEGORIES;.</note> by category and then by type (if it exists), formatted as 'category' '/' 'type'.</li>
<li>Compute ver by hashing S using the algorithm specified in in the 'hash' attribute (e.g., SHA-1 as defined in &rfc3174;). The hashed data MUST be generated with binary output and encoded using Base64 as specified in Section 4 of &rfc4648; (note: the Base64 output MUST NOT include whitespace and MUST set padding bits to zero). <note>The OpenSSL command for producing such output with SHA-1 is is "echo -n 'S' | openssl dgst -binary -sha1 | openssl enc -nopad -base64".</note></li>
<p>For example, consider an entity whose service discovery category is "client", whose service discovery type is "pc", and whose supported features are "http://jabber.org/protocol/disco#info", "http://jabber.org/protocol/disco#items", and "http://jabber.org/protocol/muc". Using the SHA-1 algorightm, the value of the 'ver' attribute would be generated as follows:</p>
<p>Each time a generating entity sends presence, it annotates that presence with an entity identifier ('node' attribute) and identity and feature identifier ('ver' attribute). So that servers can remember the last presence for use in responding to probes, a client SHOULD include entity capabilities with every presence notification it sends.</p>
<p>If the supported features change during a generating entity's presence session (e.g., a user installs an updated version of a client plugin), the application MUST recompute the 'ver' attribute and SHOULD send a new presence broadcast.</p>
<examplecaption='Presence with recomputed ver attribute'><