XMPP Core [1] and XMPP IM [2] 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 [2]). 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, romeo@montague.net) sends a message to an intended recipient (juliet@capulet.com).
<message from='romeo@montague.net/orchard' to='juliet@capulet.com'>
<body>
O blessed, blessed night! I am afeard.
Being in night, all this is but a dream,
Too flattering-sweet to be substantial.
</body>
</message>
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.
<presence from='juliet@capulet.com/balcony'> <priority>1</priority> </presence>
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) [6] extension to indicate that the message was stored offline).
<message from='romeo@montague.net/orchard' to='juliet@capulet.com'>
<body>
O blessed, blessed night! I am afeard.
Being in night, all this is but a dream,
Too flattering-sweet to be substantial.
</body>
<delay xmlns='urn:xmpp:delay'
from='capulet.com'
stamp='2002-09-10T23:08:25Z'>Offline Storage</delay>
</message>
Message stanzas SHOULD be handled by a server as follows (based on the values of the 'type' attribute specified in RFC 6121 [2]):
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) [7] 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) [8] room.
headline -- Messages with a 'type' attribute whose value is "headline" SHOULD NOT be stored offline, since such messages are usually time-sensitive.
error -- Messages with a 'type' attribute whose value is "error" SHOULD NOT be stored offline, although a server MAY store Advanced Message Processing (XEP-0079) [9] error messages offline.
If a server supports offline message handling as described herein, it SHOULD return a "msgoffline" feature in response to Service Discovery (XEP-0030) [10] information requests:
<iq from='juliet@capulet.com/chamber' to='capulet.com'> <query xmlns='http://jabber.org/protocol/disco#info'/> </iq>
<iq from='capulet.com' to='juliet@capulet.com/chamber'>
<query xmlns='http://jabber.org/protocol/disco#info'>
...
<feature var='msgoffline'/>
...
</query>
</iq>
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) [11] 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 Delayed Delivery (XEP-0203) [6] for security considerations regarding the inclusion and processing of delayed delivery notations.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [12].
The XMPP Registrar [13] includes "msgoffline" in its registry of service discovery features (see <https://xmpp.org/registrar/disco-features.html>).
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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 <standards@xmpp.org> discussion list.
Discussion on other xmpp.org discussion lists might also be appropriate; see <https://xmpp.org/community/> for a complete list.
Errata can be sent to <editor@xmpp.org>.
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".
1. RFC 6120: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc6120>.
2. RFC 6121: Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence <http://tools.ietf.org/html/rfc6121>.
3. This document does not discuss IQ or presence stanzas, handling of which is described in RFC 6120 [2] and RFC 6121.
4. As specified in RFC 6121 [3], available resources that have specified a negative presence priority shall never receive message stanzas addressed to <node@domain>.
5. XEP-0013: Flexible Offline Message Retrieval <https://xmpp.org/extensions/xep-0013.html>.
6. XEP-0203: Delayed Delivery <https://xmpp.org/extensions/xep-0203.html>.
7. XEP-0085: Chat State Notifications <https://xmpp.org/extensions/xep-0085.html>.
8. XEP-0045: Multi-User Chat <https://xmpp.org/extensions/xep-0045.html>.
9. XEP-0079: Advanced Message Processing <https://xmpp.org/extensions/xep-0079.html>.
10. XEP-0030: Service Discovery <https://xmpp.org/extensions/xep-0030.html>.
11. XEP-0116: Encrypted Session Negotiation <https://xmpp.org/extensions/xep-0116.html>.
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 https://xmpp.org/extensions/attic/
Use the correct namespace in the Service Discovery examples; fix some formatting.
@report{saint-andre2005msgoffline,
title = {Best Practices for Handling Offline Messages},
author = {Saint-Andre, Peter},
type = {XEP},
number = {0160},
version = {1.0.1},
institution = {XMPP Standards Foundation},
url = {https://xmpp.org/extensions/xep-0160.html},
date = {2005-09-27/2016-10-07},
}END