XMPP Core  and XMPP IM  specify general rules for handling XML stanzas, but explicitly do not address how to handle message stanzas sent to recipients (e.g., IM users or other nodes) that are offline, except to say that a server MUST return a <service-unavailable/> error if offline message storage or message forwarding is not enabled (see RFC 6121). This document fills the gap by specifying best practices for storage and delivery of so-called "offline messages".
The RECOMMENDED process flow is as follows:
This flow is described more fully below.
First, the sender (in this example, firstname.lastname@example.org) sends a message to an intended recipient (email@example.com).
Next, the recipient's server determines if there are any available resources that have sent non-negative presence priority. If there are, the server immediately delivers the message stanza to the resource that it determines to be most available (based on its own algorithm).
Next, the recipient's server determines if offline messages can be stored on behalf of the intended recipient. If not (e.g., because the recipient's offline message queue is full), the server returns a <service-unavailable/> error to the sender. If so, the server stores the message for later delivery.
Now the recipient authenticates with the server and sends initial presence (with a non-negative priority) to the server.
The recipient's server now delivers the offline message to that resource (it is RECOMMENDED for the server to add a Delayed Delivery (XEP-0203)  extension to indicate that the message was stored offline).
Message stanzas SHOULD be handled by a server as follows (based on the values of the 'type' attribute specified in RFC 6121):
normal -- Messages with a 'type' attribute whose value is "normal" (or messages with no 'type' attribute) SHOULD be stored offline.
chat -- Messages with a 'type' attribute whose value is "chat" SHOULD be stored offline, with the exception of messages that contain only Chat State Notifications (XEP-0085)  content (such messages SHOULD NOT be stored offline).
groupchat -- Messages with a 'type' attribute whose value is "groupchat" SHOULD NOT be stored offline, since by definition a user without online presence cannot be in a Multi-User Chat (XEP-0045)  room.
headline -- Messages with a 'type' attribute whose value is "headline" SHOULD NOT be stored offline, since such messages are usually time-sensitive.
If a server supports offline message handling as described herein, it SHOULD return a "msgoffline" feature in response to Service Discovery (XEP-0030)  information requests:
A message stored offline may not be readable by the recipient if the message was encrypted using a session-based encryption method such as Encrypted Session Negotiation (XEP-0116)  or if the key used in object encryption is revoked after the message was sent but before it is read.
In certain countries, offline storage of message stanzas may introduce legal requirements or privacy vulnerabilities that do not apply to messages that are delivered immediately and never stored on an intermediate server.
See XEP-0203 for security considerations regarding the inclusion and processing of delayed delivery notations.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
The XMPP Registrar  includes "msgoffline" in its registry of service discovery features (see <https://xmpp.org/registrar/disco-features.html>).
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This XMPP Extension Protocol is copyright © 1999 – 2018 by the XMPP Standards Foundation (XSF).
Permission is hereby granted, free of charge, to any person obtaining a copy of this specification (the "Specification"), to make use of the Specification without restriction, including without limitation the rights to implement the Specification in a software program, deploy the Specification in a network service, and copy, modify, merge, publish, translate, distribute, sublicense, or sell copies of the Specification, and to permit persons to whom the Specification is furnished to do so, subject to the condition that the foregoing copyright notice and this permission notice shall be included in all copies or substantial portions of the Specification. Unless separate permission is granted, modified works that are redistributed shall not contain misleading information regarding the authors, title, number, or publisher of the Specification, and shall not claim endorsement of the modified works by the authors, any organization or project to which the authors belong, or the XMPP Standards Foundation.
## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. ##
In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall the XMPP Standards Foundation or any author of this Specification be liable for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if the XMPP Standards Foundation or such author has been advised of the possibility of such damages.
This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which can be found at <https://xmpp.org/about/xsf/ipr-policy> or obtained by writing to XMPP Standards Foundation, P.O. Box 787, Parker, CO 80134 USA).
The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.
The primary venue for discussion of XMPP Extension Protocols is the <firstname.lastname@example.org> discussion list.
Discussion on other xmpp.org discussion lists might also be appropriate; see <http://xmpp.org/about/discuss.shtml> for a complete list.
Errata can be sent to <email@example.com>.
The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".
3. This document does not discuss IQ or presence stanzas, handling of which is described in RFC 6120 and RFC 6121.
4. As specified in RFC 6121, available resources that have specified a negative presence priority shall never receive message stanzas addressed to <node@domain>.
12. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.
13. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <https://xmpp.org/registrar/>.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Use the correct namespace in the Service Discovery examples; fix some formatting.