This XEP introduces unique and stable IDs for messages, which are beneficial in various ways. For example, they can be used together with Message Archive Management (XEP-0313)  to uniquely identify a message within an archive. They are also useful in the context of Multi-User Chat (XEP-0045)  conferences, as they allow to identify a message reflected by a MUC service back to the originating entity.
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 RFC 4122  to generate the IDs.
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.
The server or component MAY add a <stanza-id/> element. In that case, it MUST preserve the content of the <origin-id/> element.
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.
The value of the 'id' attribute should not provide any further information besides the opaque ID itself. Entities observing the value MUST NOT be able to infer any information from it, e.g. the size of the message archive. The value of 'id' MUST be considered a non-secret value.
Before processing the stanza ID of a message and using it for deduplication purposes or for MAM catchup, the receiving entity SHOULD ensure that the stanza ID could not have been faked, by verifying that the entity referenced in the by attribute does annouce the 'urn:xmpp:sid:0' namespace in its disco features.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
This specification defines the following XML namespaces:
The XMPP Registrar  shall include the foregoing namespaces in its registry of protocol namespaces (see <https://xmpp.org/registrar/namespaces.html>) and in its disco features registry (<https://xmpp.org/registrar/disco-features.html>) as defined in Service Discovery (XEP-0030) .
Thanks to Thijs Alkemade and Georg Lukas for providing feedback.
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Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Add ability to discover support
Rename client-id element to origin-id.
Minor fixes (typos, s/JID/XMPP Address, etc.)
Initial published version approved by the XMPP Council.