The Mediated Information eXchange (MIX) protocol framework and core capabilities are specified in Mediated Information eXchange (MIX) (XEP-0369)  (MIX-CORE). MIX-CORE and Mediated Information eXchange (MIX): Presence Support. (XEP-0403)  define behaviour of a MIX server supporting MIX channels. In order for a MIX system to operate correctly, the XMPP server connecting MIX clients MUST follow the rules set out in this specification to achieve correct MIX operation. This specification also sets out requirements for XMPP clients, so a MIX client MUST follow both the rules of XMPP-CORE and this specification.
A MIX channel does not send messages and presence directly to the MIX client of a channel participant, but addresses them to the participant using the participant's bare JID. The participant's server MUST then handle these messages and pass them on to zero, one or multiple clients. To enable MIX to work, this behaviour is necessary and so the server of every MIX client MUST follow the rules set out in this specification. This approach enables flexible support of multiple clients for a MIX channel participant. The MIX model is that a user will join a channel over an extended period, and that the user (not a specific client used by the user) joins the channel. The primary subscription is with the client's bare JID. There are a number of MIX requirements on behaviour of the MIX Participant's server, which are summarized here:
Messages from a MIX channel to a MIX participant (which will be of type=groupchat), presence information, and other information sent as a result of MIX channel subscription are sent to the participant's server using the participant's bare JID. This means that the MIX participant's server MUST implement the modification of the standard RFC 6121  message processing rules specified here.
This section defines behaviour REQUIRED by MIX for servers supporting MIX participants. This functionality MUST be provided by servers used by clients that participate in MIX channels. In future, this specification MAY be incorporated into Pubsub Account Management (XEP-0376)  (PAM) which follows a similar model.
A MIX User's server MUST determine which online clients support MIX. This will enable the server to send MIX traffic to all MIX capable clients, but not to other clients. A MIX capable client MAY choose to come online and not advertise MIX capability. The mechanism for a server to discover client capability is described in Discovering Client MIX Capability.
Messages from a MIX channel will usually be handled by the participant's server. The only exception to this is where the MIX channel is responding directly to messages from the client. Messages and presence distributed by a MIX channel will always be sent to the participant's server and addressed to the user's bare JID. The participant's server will archive the message in MAM and send on the messages from the channel to each of the user's online clients which advertise MIX capability. If there are no such clients activated, the message is not sent to any clients.
Messages sent to the participant's sever will always be addressed to the user's bare JID. The participant’s server will modify the recipient to the full JID of each client to which the message is forwarded. The following example, repeated from Mediated Information eXchange (MIX) (XEP-0369) , shows a message distributed by a MIX channel and directed to the participant’s server with the participant's bare JID.
The server receiving the message will then deliver the messages to all online clients. Messages are delivered to all available online resources irrespective of status and resource priority. The following example shows how the participant's server modifies the inbound message to replace the bare JID in the 'to' with a full JID for each of two active MIX clients.
Messages sent by a MIX channel participant to the MIX channel are always sent from a MIX client directly to the channel. The participant's server relays the message but does not modify the messages.
Servers supporting this specification MUST advertise this to clients for which they wish to support this specification. A client wishing to use MIX MUST check for this capability in the local server before using MIX, by verifying support for the client's account. A client wishing to use the channels PEP node MUST verify the support of 'urn:xmpp:mix:pam:channels:0' on the client's account first. The capability is represented by the 'urn:xmpp:mix:pam:2' feature. In addition to this the server MAY advertise the 'urn:xmpp:mix:pam:2#archive' feature, which shows that the local server archives MIX messages.
Most interaction between a MIX client and a MIX channel is directly between the client and the channel. The participant's server relays the message but does not modify the messages. In particular configuration management and discovery is direct. Interaction will be direct, unless explicitly stated otherwise in this specification.
Channel Join and Leave functions operate indirectly through the participant's server. The reason for this is that where a channel shares user presence, the channel is included in a private PEP node of the user which is managed on the local server. The Join and Leave functions lead to PubSub notifications and so they MUST go through the participant's server. To achieve this, this specification wraps the operations so that the server can correctly route messages.
A user joins a channel by sending a MIX "client-join" command from one of the user's clients, which wraps the "join" command specified in Mediated Information eXchange (MIX) (XEP-0369) . Mediated Information eXchange (MIX) (XEP-0369)  specifies how the join command works, and so this specification considers only the wrapping and client actions. The <client-join/> is a child element of <iq/> element. The <client-join/> element is qualified by the 'urn:xmpp:mix:pam:2' namespace. The channel being joined is specified by a 'channel' attribute in the <client-join/> element, which is used by the server to correctly address the join. The <join> is specified in Mediated Information eXchange (MIX) (XEP-0369)  and is a child element of <client-join/>.
The information in this message is used to derive the message sent to the MIX channel. The 'from' is the bare JID of the user. The 'to' is the channel from the client join 'channel' attribute. The 'id' is taken from the client join message. The <join> is taken from the client join message. This is shown in the following example, repeated from the earlier specification of channel behaviour.
The MIX service will process this request and respond as specified by Mediated Information eXchange (MIX) (XEP-0369) . An example response taken from Mediated Information eXchange (MIX) (XEP-0369)  is shown below.
The user's server will then make modifications to the user's private 'urn:xmpp:mix:pam:channels' PEP node as set out in a later section of this specification. After making these changes, the user's server will send the client-join response containing the server's join response back to the client that made the join request. This has the 'from' set to the user's bare JID and the 'to' set to the client's full JID. This is illustrated below:
Users generally remain in a channel for an extended period of time. The process for leaving a MIX channel is specified in Mediated Information eXchange (MIX) (XEP-0369) . When a user desires to leave a channel, it will issue a client-leave request to the local server. The <client-leave/> is a child element of <iq/> element. The <client-leave/> element is qualified by the 'urn:xmpp:mix:pam:2' namespace. The channel being left is specified by a 'channel' attribute in the <client-leave/> element, which is used by the server to correctly address the leave. The <leave> is specified in Mediated Information eXchange (MIX) (XEP-0369)  and is a child element of <client-leave/>. This is shown in the following example.
After receiving the confirmation that the user has left the MIX channel, the user's server will remove the MIX channel entry from the user's 'urn:xmpp:mix:pam:channels' PEP node and follow other processing as specified below. The user's server will then notify the client with the servers response. This wraps the response from the server with a client-leave, with the 'from' set to the user's bare JID and the 'to' set to the client's full JID. This is illustrated below:
The user's presence status is shared with the channel and channel participants subscribing to presence. The user's server will need to record the subscribed channels in the user's channels PEP node in order to correctly handle MIX processing. This roster entry will lead to the user's server correctly sending user's presence from all the user's MIX clients to the MIX channel. Where the user wishes to share presence, the roster subscription is configured with one way presence, as presence is sent to the MIX channel but no presence information about the MIX channel is sent to the user.
The participant's server MUST ensure that only presence information from clients that advertise MIX capability is sent to the MIX channel. So, if a user has two online clients, but only one is MIX capable, then the channel will only receive presence information relating to the MIX client.
If the user does not wish to publish presence information to the channel, the user's server will add the roster entry with mode subscription=none. The roster entry will be present to record that the user has joined the channel, but it will not send presence information to the channel. The user's server MUST do this when the user has chosen Presence preference of 'not share' as specified in Mediated Information eXchange (MIX): JID Hidden Channels. (XEP-0404) . If the user changes the value of the preference, the server MUST modify subscription mode to reflect this.
The user's server MUST remove the user's roster entry when the user leaves the channel.
A user joins a channel over an extended period, and participation in a channel does not generally change when a user clients go online or offline. The user's participation in a channel is reflected by the user's bare JID in the participant node. When a user subscribes to presence as specified in Mediated Information eXchange (MIX): Presence Support. (XEP-0403) , all presence messages are sent to this JID. Presence updates are sent out to subscribing participants using standard presence stanzas.
A user MAY share presence information with the channel, for the user's online clients. This is achieved by a roster entry for the channel configured with one way presence, which will cause all presence changes for the user's MIX clients to be sent to the channel. When an XMPP client comes online or changes presence status, this will be communicated by the user to the user's server using broadcast presence. The user's XMPP server is then responsible to share this presence information to each entry in the roster and in particular to each MIX channel in the roster.
A user setting status is now used as an example. Unlike in Multi-User Chat (XEP-0045)  where coming online is a special action, coming online in MIX is implicit when presence status is set. Going offline is a achieved by setting presence status to unavailable, which removes the client full JID entry from the presence node. When a user sets a presence status, the user's server sends updated presence to the MIX channel, and the MIX service then publishes the user's availability to the presence node. If there is not an item named by the full JID of the client with updated presence status, this item is created. The sequence is shown in the following examples, starting with a client setting presences status on the connected server.
The server then sends the presence information to roster entries. The following example then shows the presence stanza from the client's server to the MIX channel. The presence is then handled as specified in Mediated Information eXchange (MIX) (XEP-0369) .
A MIX channel will send out presence information to participants that subscribe to the presence node, as specified in Mediated Information eXchange (MIX): Presence Support. (XEP-0403) . An example is shown below:
The user's server will then pass this on to all online clients, with 'from' unchanged and 'to' set to the client receiving presence. An example is shown below:
The user's local server will receive a flow of all presence updates for the user. It will pass this presence information on to all online clients. This ensures that an online client is kept updated with presence. When a client goes offline, it will cease getting presence updates. Presence updates will continue to flow to the user's local server, and so the local server is able maintain up to date presence state for the channel, even when there are no online clients. When a user had no online clients the user's server MAY continue to maintain MIX presence status for the user or MAY discard inbound MIX presence information.
When the client comes online, it will activate use of the MIX. When the user's server has been maintaining MIX presence, it will then send full presence status to the client using standard presence messages. This will enable the client to update presence information for the subscribed MIX channels. Note that this does not need any interaction with MIX servers.
There are two situations where a MIX participant's server will need to get presence status from the channel, before it can send presence to the client. The first time is when a user joins the channel as a participant and subscribes to presence. Upon this subscription the MIX channel will send to the participant's server (using the user's bare JID) presence for all of the items in the presence node using standard presence stanzas. This will give the participant's full current presence for the channel.
The second scenario is when the MIX participant's server needs to load or refresh presence status for a channel. This will be needed when the participant's server is started or when the server chooses to not maintain presence for a user when all clients go offline. This MIX participant's server requests presence update by sending a directed presence stanza to the MIX channel from the user's bare JID. The MIX channel can distinguish this from a presence update, which will always be sent from the clients full JID. This will cause the MIX channel to send a full presence update for the channel.
When a client goes offline, this presence update is sent by the client's server to the MIX channel. From the client perspective, this is the same as any other presence change. Going online and offline will simply be presence updates.
It is desirable to prevent clients from going offline briefly and then coming back online again, as this will lead to "flapping presence". The RECOMMENDED approach to achieve this is use of Stream Management (XEP-0198)  to maintain an XMPP client connection in the event of short term TCP failure.
As part of the channel joining process, the user's server MUST add the MIX channel to the user's private channels PEP node. This PEP node enables the user's client to quickly determine which channels the user has joined and which stable participant ids it has in those channels. The node MUST only be edited by the server and MUST only be accesible to the user, not other contacts subscribed to the user's presence.
To retrieve all currently joined channels a PubSub query on the user's own virtual PubSub service is made. The used PEP node is 'urn:xmpp:mix:pam:channels'.
Upon that the server returns all currently joined channels. Each channel is represented by one PubSub item. The id of the item MUST be the channel JID. Each item MUST have a <channel/> element qualified by the 'urn:xmpp:mix:pam:channels:0' namespace. The <channel/> element MUST also include a 'participant-id' attribute that is the stable ID of the client. This facilitates the client to match messages that reference this stable ID.
A server MUST only return the joined channels to the user, e.g. it MUST NOT return the channels joined by Bob to Alice.
It is important for other clients to know when a channel was joined or left, so they can correctly display the channel to the user. To receive those notifications, the client needs to subscribe to its private PEP node 'urn:xmpp:mix:pam:channels'. This is done by adding the 'urn:xmpp:mix:pam:channels+notify' feature to the client's Service Discovery (XEP-0030)  features and including Entity Capabilities (XEP-0115)  in the client's presence.
In case the server has not cached the features for the 'ver' attribute, it will need to request the client's features. The client needs to include the 'urn:xmpp:mix:pam:channels+notify' feature, if it wants to receive join and leave notifications.
When another client joins or leaves a channel, the server MUST send notifications to all subscribed resources. It MUST NOT send notifications to other users.
Archive of MIX channel messages MAY be performed by the participant's server. When this is done, the capability is advertized to MIX clients using the 'urn:xmpp:mix:pam:2#archive' feature. If archive is provided it MUST always be used, so that where a message is sent to the participant's server and discarded because there are no active clients, it will still be archived. This means that when archiving is provided, the messages will be available in the local archive and can be picked up by clients when they come online.
This section describes an issue that needs to be addressed by servers that provide blocking capabilities based on JID. Messages distributed by a MIX channel come from JIDs containing the bare JID of the channel. For presence stanzas (specified in MIX-PRESENCE), IQ stanza relay (specified in MIX-PRESENCE), and private messages (specified in MIX-ANON) use an encoded JID, where the local part of the bare JID contains both the channel name and the senders Stable Participant ID, for example 'firstname.lastname@example.org'. A server implementing blocking and MIX-PAM needs to recognize this encoding, to prevent blocking these messages when this is not desired.
See considerations in Mediated Information eXchange (MIX) (XEP-0369) .
See considerations in Mediated Information eXchange (MIX) (XEP-0369) .
When converting a 1:1 conversation to a channel there is potential to expose sensitive information and to present invalid information.
The urn:xmpp:mix namespace needs to be registered.
To be supplied when MIX progresses to proposed standard.
See Mediated Information eXchange (MIX) (XEP-0369)  for a list of contributors to the MIX Family of specifications.
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Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Add section on Blocking;
Align to MIX-CORE and MIX-PRESENCE changes;
Split out from MIX 0.10.0;