<p>This XEP introduces unique and stable IDs for messages, which are beneficial in various ways. For example, they can be used together with &xep0313; to uniquely identify a message within an archive. They are also useful in the context of &xep0045; conferences, as they allow to identify a message reflected by a MUC service back to the originating entity.</p>
<p>In order to create a &stanza-id; extension element, the creating XMPP entity generates and sets the value of the 'id' attribute, and puts its own XMPP address as value of the 'by' attribute. The value of the 'id' attribute must be unique and stable, i.e. it MUST NOT change later for some reason within the scope of the 'by' value. Thus the IDs defined in this extension MUST be unique and stable within the scope of the generating XMPP entity. It is RECOMMENDED that the ID generating service uses UUID and the algorithm defined in &rfc4122; to generate the IDs.</p>
Some use cases require the originating entity, e.g. a client, to generate the stanza ID. In this case, the client MUST use the &origin-id; element extension element qualified by the 'urn:xmpp:sid:0' namespace. Note that originating entities often want to conceal their XMPP address and therefore the &origin-id; element has no 'by' attribute.
<li>Stanza ID generating entities, which encounter a &stanza-id; element where the 'by' attribute matches the 'by' attribute they would otherwise set, MUST delete that element even if they are not adding their own stanza ID.</li>
<li>Entities, which are routing stanzas, SHOULD NOT strip any elements qualified by the 'urn:xmpp:sid:0' namespace from message stanzas unless the preceding rule applied to those elements.</li>
<li>Stanzas MUST possess, in the direct child level of the stanza, at most one &stanza-id; extension element with the same XMPP address as value of the 'by' attribute.</li>
<li>Every &stanza-id; and &origin-id; extension element MUST always possess an 'id' attribute and MUST NOT have any child elements or text content.</li>
<li>The value of the 'by' attribute MUST be the XMPP address of the entity assigning the unique and stable stanza ID. For one-on-one messages the assigning entity is the account. In groupchats the assigning entity is the room. Note that XMPP addresses are normalized as defined in &rfc6122;.</li>
<p>An entity that follows the business rules, especially the rule on overriding the ID in elements where the by atttribute matches the 'by' attribute they would otherwise set, SHOULD announce the 'urn:xmpp:sid:0' namespace in its disco features allowing other entities to verify that those business rules are properly enforced.</p>
<examplecaption='Client sends service discovery request to the room'><