This document makes several assumptions:

• The type of client I am using is of interest to the people in my roster.
• Clients for the people on my roster might want to make user interface decisions based on my capabilities.
• Different clients may support the same capabilities.
• 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).
• Some clients are running against servers without server-to-server connectivity enabled, and without access to the Internet via HTTP.
• Conversations are possible between users who are not on each other's rosters.
• Client capabilities may change over the course of a presence session, due to features being enabled and disabled.

The protocol defined herein addresses the following requirements:

1. Clients must be able to participate even if they support only &xmppcore;, &xmppim;, and XEP-0030.
2. 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.These first two requirements effectively eliminated &xep0060; as a possible implementation of entity capabilities.
3. Clients must be able to retrieve information without querying each user.
4. Since presence is normally broadcasted to many users, the byte size of the proposed extension must be as small as possible.
5. It must be possible to write a XEP-0045 server implementation that passes the given information along.
6. It must be possible to publish a change in capabilities within a single presence session.
7. Server infrastructure above and beyond that defined in XMPP Core and XMPP IM must not be required for this approach to work, although additional server infrastructure may be used for optimization purposes.

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.

* Note: It is RECOMMENDED for the value of the 'node' attribute to identify both the software product and the released version in the form "ProductURL;SoftwareVersion", such as "http://psi-im.org/;0.11" This enables a processing application to strip off everything after the ";" character and thereby determine a unique string for the generating application, which it could maintain in a list of known products or (if the string is a URL) which it could use to find more detailed information about the generating application.. In any case, the value of the 'node' attribute MUST NOT include the "#" character, which is used as a separator character in the Discovering Capabilities use case.

** 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 backward-compatible, applications SHOULD appropriately handle the Legacy Format.

In order to help prevent poisoning of entity capabilities information, the value of the 'ver' attribute MUST be generated according to the following method.

Note: All sorting operations MUST be performed using "i;octet" collation as specified in Section 9.3 of &rfc4790;.

1. Initialize an empty string S.
2. Sort the service discovery identities by category and then by type (if it exists), formatted as 'category' '/' 'type'.
3. For each identity, append the 'category/type' to S, followed by the '<' character.
4. Sort the supported features.
5. For each feature, append the feature to S, followed by the '<' character.
6. Compute ver by hashing S using the SHA-1 algorithm as specified in &rfc3174; (with binary output) and encoding the hash using Base64 as specified in Section 4 of &rfc4648; (note: the Base64 output MUST NOT include whitespace and MUST set padding bits to zero). The OpenSSL command for producing such output is "echo -n 'S' | openssl dgst -binary -sha1 | openssl enc -nopad -base64".

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". The value of the 'ver' attribute would be generated as follows:

1. S = ''
2. Only one identity: "client/pc"
3. S = 'client/pc<'
4. Sort the features: "http://jabber.org/protocol/disco#info", "http://jabber.org/protocol/disco#items", "http://jabber.org/protocol/muc".
5. S = 'client/pc<http://jabber.org/protocol/disco#info<http://jabber.org/protocol/disco#items<http://jabber.org/protocol/muc<'
6. ver = 8RovUdtOmiAjzj+xI7SK5BCw3A8=

A server that is managing an entity's presence session MAY choose to optimize traffic through the server. In this case, the server MAY strip off redundant capabilities annotations. Because of this, receivers of annotations MUST NOT expect an annotation on every presence packet they receive. If the server wants to perform this traffic optimization, it MUST ensure that the first presence each subscriber receives contains the annotation. The server MUST also ensure that any changes in the annotation (e.g., an updated 'ver' attribute) are sent to all subscribers.

A client MAY query the server using disco#info to determine if the server supports the 'http://jabber.org/protocol/caps' feature. If so, the server MUST perform the optimization delineated above, and the client MAY choose to send the capabilities annotation only on the first presence packet, as well as whenever its capabilities change.

If two entities exchange messages but they do not normally exchange presence (i.e., via presence subscription), the entities MAY choose to send directed presence to each other, where the presence information SHOULD be annotated with the same capabilities information as each entity sends in broadcasted presence. If capabilities information has not been received from another entity, an application MUST assume that the other entity does not support capabilities.

An application that accepts entity capabilities information SHOULD cache associations between the 'ver' attribute and discovered features within the scope of one presence session and MAY cache such associations across sessions. This obviates the need for extensive service discovery requests within a session or at the beginning of a session.

Use of the protocol specified in this document might make some client-specific forms of attack slightly easier, since the attacker could more easily determine the type of client being used. However, since most clients respond to Service Discovery and Software Version requests without performing access control checks, there is no new vulnerability. Entities that wish to restrict access to capabilities information SHOULD use &xep0016; to define appropriate communications blocking (e.g., an entity MAY choose to allow IQ requests only from "trusted" entities, such as those with whom it has a subscription of "both").

Adherence to the algorithm defined in the Generation of ver Attribute section of this document for both generation and checking of the 'ver' attribute helps to guard against poisoning of entity capabilities information by malicious or improperly implemented entities.

If the value of the 'ver' attribute is a hash as defined herein (i.e., if the 'ver' attribute is not generated according to the legacy format), inclusion of the 'algo' attribute is required. Knowing explicitly that the value of the 'ver' attribute is a hash enables the recipient to avoid spurious notification of invalid hashes.

The 'name' attribute of the service discovery <identity/> element is not included in the hash generation method. The primary reason for excluding it is that it is human-readable text and therefore may be provided in different localized versions. As a result, its inclusion would needlessly multiply the number of possible hash values and thus the time and resources required to validate values of the 'ver' attribute.

This document requires no interaction with &IANA;.

Before Version 1.4 of this specification, the 'ver' attribute was generated differently, the 'ext' attribute was used more extensively, and the 'algo' attribute was absent. For historical purposes, Version 1.3 of this specification is archived at <http://www.xmpp.org/extensions/attic/xep-0115-1.3.html>. For backward-compatibility with the legacy format, the 'node' attribute is REQUIRED and the 'ext' attribute MAY be included.

An application can determine if the legacy format is in use by checking for the presence of the 'algo' attribute, which is REQUIRED in the current format.

In the legacy format, the value of the 'ver' attribute is not a hash of the service discovery identity and supported features. Therefore, a processing entity cannot validate the identity and features by checking the hash. If the processing entity supports the legacy format, it SHOULD check the 'node', 'ver', and 'ext' combinations as specified in the archived version 1.3 of this specification, and MAY cache the results. If not, the processing entity SHOULD ignore the 'ver' value entirely (since it cannot be verified) and SHOULD NOT cache it.

Thanks to Rachel Blackman, Dave Cridland, Richard Dobson, Sergei Golovan, Justin Karneges, Jacek Konieczny, Ian Paterson, Kevin Smith, Tomasz Sterna, Michal Vaner, and Matt Yacobucci for comments and suggestions.