<abstract>This document defines Mediated Information eXchange (MIX), an XMPP protocol extension for the exchange of information among multiple users through a mediating service. The protocol can be used to provide human group communication and communication between non-human entities using channels, although with greater flexibility and extensibility than existing groupchat technologies such as Multi-User Chat (MUC). MIX uses Publish-Subscribe to provide flexible access and publication, and uses Message Archive Management (MAM) to provide storage and archiving. </abstract>
<p>The Mediated Information eXchange (MIX) protocol is intended as a replacement for Multi-User Chat (MUC). MUC is a major application of XMPP that was developed in 2002 and standardized in &xep0045;. MIX implements the same basic MUC patterns in a more flexible and extensible way in order to address requirements that have emerged since MUC was developed. MIX supports all of the core chatroom features that are familiar from MUC, such as discussion topics and invitations. Like MUC, it also defines a strong access control model, including the ability to kick and ban users, to name administrators, and to require membership in order to participate in channels.</p>
<li>Experience has shown that it is difficult to use MUC to build several kinds of communication applications (such as a multimedia conference focus) without undesirable adaptations.</li>
<li>In the years after MUC was designed, both &xep0060; and &xep0313; have been developed and it is desirable to reuse these building blocks (e.g., MAM can be used for message history) rather than using the less robust methods defined in &xep0045;.</li>
<p>If a server wishes to expose both MUC and MIX representations of chatrooms, it is recommended to do so by serving MUC and MIX on different domains, but a server MAY serve them on the same domain. The MIX service SHOULD include a reference to the MUC mirror, so that clients understanding both protocols can choose to show only one copy of the service.</p>
<li>A user's participation in a channel persists and is not modified by the user's client going online and offline.</li>
<li>Multiple devices associated with the same account can share the same nick in the channel, with well-defined rules making each client individually addressable.</li>
<li>Channels are not required to support or reflect presence for participants.</li>
<li>A reconnecting client can quickly resync with respect to messages and presence.</li>
<li>A user may (subject to configuration) receive messages from a channel as an invisible observer.</li>
<li>Configuration can be observed externally to the channel (e.g., list of participants, access control rights, etc.).</li>
<li>MIX services should provide mechanisms to prevent JIDs from being harvested.</li>
<li>MIX and Message Archive Management (MAM) should work well together.</li>
<li>A user can determine which channels they participate in.</li>
<li>Provide extensibility regarding data formats that can be sent within a channel (files, structured data, indications about media sources in multimedia conferences, etc.) as well as flexibility regarding which data formats a user wants to receive.</li>
<li>Enable federation of a channel across multiple servers, to provide a service equivalent to "federated MUC" &xep0289;.</li>
<li>MIX channels (roughly equivalent to MUC rooms) are hosted on one or more MIX domains, (examples: 'mix.example.com'; 'conference.example.com'; 'talk.example.com'), which are discoverable through &xep0030;. Each channel on a MIX service may then be discovered and queried.</li>
<li> In MIX each channel (e.g., 'channel@mix.example.com') is a pubsub service. This is based on the model from &xep0163; where every user JID (e.g., 'user@example.com') is its own pubsub service. </li>
<li>A channel's pubsub service contains a number of nodes for different event types or data formats. As described below, this document defines several standard nodes; however, future specifications or proprietary services can define their own nodes for extensibility.</li>
<li>Affiliations with the nodes are managed by the MIX service by channel level operations, so that the user does not have to separately manage affiliations with the individual PubSub nodes.
<li>&xep0313; (MAM) is used for all history access, with each node being individually addressable for MAM queries. This simplifies implementation compared to MUC (which had a specialized and rather limited history retrieval mechanism).</li>
<li>A client can achieve a 'quick resync' of a node by requesting just those changes it has not yet received, using standard MAM protocol. This solves the old MUC issue of either receiving duplicate messages when rejoining a room or potentially missing messages.</li>
<li>Because MAM is used for history, only those nodes that have a 'current value' need to store any items in them — e.g., 'urn:xmpp:mix:nodes:presence' and 'urn:xmpp:mix:nodes:subject' would store their current values (with older values being queryable through MAM), while 'urn:xmpp:mix:nodes:messages' would store no items.</li>
<li>A user's participation in a channel outlives their client sessions. A client which is offline will not share presence within the channel, but the associated user will still be listed as an participant. </li>
<li>Each participant is addressable by a single bare JID, which is a proxy JID (not the user's real JID) to make it straightforward to hide the user's real JID from other channel participants. Full JIDs comprised of this bare JID plus a resource are then constructed, allowing visibility into the number of online resources participating in a channel.</li>
<li>MIX requires client support and server support from the server providing the MIX service. Although some protocol is shared with MUC, MUC clients are not interoperable with MIX servers. This means that where a user chooses to use MIX, all of the users clients need to support MIX.</li>
<p>MIX is based upon domains providing a MIX service, such as 'mix.shakespeare.example'. Note that although PubSub communication is used, a domain used for MIX is a MIX domain and not a standard &xep0060; domain. Like MUC, there is no requirement on the naming of these domains; the label 'mix' and the fact that it is a subdomain of a 'shakespeare.example' service are purely for example.</p>
<p>Every MIX channel is an addressable PubSub service (with additional MIX semantics) that will be addressed using a bare JID by other XMPP entities, for example coven@mix.shakespeare.example. While &xep0060; is used as the basis for the MIX model, MIX uses standard presence and groupchat messages to provide an interface to the MIX service that does not expose PubSub protocol for many of the more common functions.
<p> Historical data (such as the messages sent to the channel) is stored in an archive supporting Message Archive Management (MAM) so that clients can subsequently access this data with MAM. Each node can be archived separately (e.g., the presence node or the configuration node). MIX clients can retrieve archived information with MAM in order to quickly resync messages with regard to a channel, and can do so without providing presence information.</p>
The primary 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. The user will receive messages from the channel and/or access channel presence from time to time with one or more clients.
Where a user has no clients active, the approach is that all messages will be archived by the MIX channel (on arrival at the MIX channel) so that when clients come online they will use MAM to communicate to the MIX channel to access messages that have not been delivered to the client. Following the rules of &rfc6121;, arriving MIX messages (which will be of type=groupchat) and presence information will be discarded if all clients are offline. Offline messages are not used with MIX. This means that a MIX client needs to resynchronize with the MIX service when it comes online.
</p>
<p>
MIX delivery does not require any special support from the XMPP server to which the MIX client connects. When using a basic XMPP server for this service, all online clients for a user will receive the same set of messages and presence information, as MIX sends this information to the bare JID.
It may be desirable in some situations to provide different service to different clients. For example, a mobile client may participate in a smaller set of MIX channels than a desktop client. This needs support from the server to which the client connects, so that MIX client and the connected server can negotiate which channels to send. This is not supported by the core MIX specification, but it is anticipated that this will be supported by another specification.
MIX channels store presence in the presence node using the proxy JID of each online client for a user. User presence may be included for all or selected clients of a given user, based on client choice to publish presence. Where a user publishes presence for multiple clients, this information is available to all users subscribing to the channel presence.
</p>
<p>
External XMPP entities can direct stanzas to clients publishing their presence by sending them to the appropriate full proxy JID in the presence node. These stanzas MAY be routed to the client by the MIX channel. The choice to do this is dependent on MIX channel policy. For example, a disco request MAY be routed through the MIX channel to the client.
Presence updates are distributed by a channel to the bare JID of subscribers and then the subscriber's server will distribute to each of the subscriber's currently online clients.
Private messages MAY be sent to channel participants if allowed by channel policy. Private messages are
addressed to the user's bare proxy JID determined from the participants node, and routed by the MIX to the user's bare real JID, where standard distribution rules will apply.
</p>
<p>
Private messages MAY also be sent to specific clients identified by the full proxy JID of the client in the participants list, if allowed by channel policy.
<tr><td>'JID Visible'</td><td>In these channels, some or all participant JIDs are visible to all channel members.</td></tr>
<tr><td>'JID Hidden'</td><td>In these channels, no participant JIDs are visible to channel members, but they are visible to channel administrators.</td></tr>
<tr><td>'No JID Visibility Preference'</td><td>The users JID will be visible in JID Visible channels and hidden in JID Hidden channels.</td></tr>
<tr><td>'Prefer Hidden'</td><td>The user's JID will be hidden in JID Visible channels if the channel policy allows this.</td></tr>
<tr><td>'Enforce Hidden'</td><td>The user's JID will never be shown and the user will be removed from channel if channel administrator enforces visibility.</td></tr>
<tr><td>'Enforce Visible'</td><td>The users JID will always be shown and the user will be removed from channel if mode is changed to 'JID Hidden'.</td></tr>
The primary representation of a participant in a MIX channel is a proxy JID, which is an anonymized JID using the same domain as the channel and with the name of the channel encoded, using the format "<channel>+<generated identifier>@<mix domain>". They generated identifier MUST NOT contain the '+' characters. The Channel name MAY contain the '+' character. For example in the channel 'coven@mix.shakespeare.example', the user 'hag66@shakespeare.example' might have a proxy JID of 'coven+123456@mix.shakespeare.example'. The reason for the proxy JID is to support JID Hidden channels. Proxy JIDs are also used in JID Visible channels, for consistency and to enable switching of JID visibility. The "urn:xmpp:mix:nodes:jidmap" node maps from proxy JID to real JID. While a user is a participant in a Channel, the mapping between real JID and proxy JID MUST NOT be changed,
The full JIDs held in the presence nodes are anonymized using a similar mechanism. First the bare JID is mapped to a bare proxy JID, using the mapping held in the "urn:xmpp:mix:nodes:jidmap" node for the bare JID. Then the resource is replaced with a generated value. For example, 'hag66@shakespeare.example' in the channel coven might have a proxy JID of 'coven+123456@mix.shakespeare.example/789'. There is no client visible mapping of proxy full JIDs maintained as this is not needed. The MIX channel will need to maintain a mapping, to support directly addressing clients, such as for client to client disco#info queries. While an anonymized JID is held in the presence node, the mapping to real JID MUST NOT be changed. When an anonoymized full JID is added to the presence node, mapping to a real full JID that was previously in the presence node, the same anonymized JID as the previous time MAY be used or a different anonymized JID MAY be used.
<tr><td>'urn:xmpp:mix:nodes:messages'</td><td>For publishing messages. Each item of this node will contain a message sent to the channel.</td></tr>
<tr><td>'urn:xmpp:mix:nodes:subject'</td><td>For publishing the subject of the channels</td></tr>
<tr><td>'urn:xmpp:mix:nodes:participants'</td><td>For publishing the list of participants (identified by anonymized bare proxy JID), and identifying the nick. This is a list of users who would receive messages (by subscription to the messages node) and/or would receive presence (by subscription to the presence node).</td></tr>
<tr><td>'urn:xmpp:mix:nodes:jidmap'</td><td>For publishing a list of anonymized bare JIDs from the participants node with a 1:1 mapping to the corresponding real JIDs.</td></tr>
<tr><td>'urn:xmpp:mix:nodes:presence'</td><td>For publishing information about the availability status of online participants, which may include multiple clients for a single participant.</td></tr>
<tr><td>'urn:xmpp:mix:nodes:acl'</td><td>For storing information about access control lists (such as the list of owners and administrators). This information will generally be restricted to authorized users.</td></tr>
jidmap is the only node that will always be present for a MIX channel and all other nodes are optional. All channels will have either a message node, a presence node or both, because a channel will always share messages and/or presence. MIX provides mechanisms to allow users to conveniently subscribe to a chosen set of nodes and to unsubscribe to all nodes with a single operation.
</p>
<p>
The structure of each of the standard nodes is now considered in more detail
<p>Items in this node will contain a message identified by a unique ID. A MIX implementation SHOULD NOT make messages available for retrieval from this node using pubsub. The recommended approach is that zero history is held in the messages node, and that this node is used for publication only. The recommended approach to retrieve message history is MAM. Users subscribe to this node to receive messages.</p>
<p>Private Messages are not stored in the messages node.</p>
<p>The subject node publishes the current subject of channel. Subject history is stored in MAM. A single item is stored in this node at a time which MUST contain a "text" element and MAY contain additional text elements. Where multiple text elements are provided, each MUST posses an xml:lang attribute that describes the natural language of the subject. Users subscribe to this node to receive subject updates. The following example shows the format of a item in the subject node.</p>
<p>Each channel participant is represented as an item of the 'urn:xmpp:mix:nodes:participants' channel node. Each item is named by the bare proxy JID of the participant. For example 'coven+123456@mix.shakespeare.example' might name the node item associated with participant 'hag66@shakespeare.example'. The nick associated with the user is mandatory and is stored in the item. The nick for each channel participant MUST be different to the nick of other participants.
This node may be subscribed to for jid-visible channels that permit subscription to this node - this will allow users to see changes to channel membership.
</p>
<examplecaption="Value of Participants Node"><![CDATA[
Each item is identified by proxy bare JID, mapping to the real bare JID. This node is used to give administrator access to real JIDs and participant access to real JIDs in jid-visible channels.
In JID visible channels, all participants may subscribe to this node. In JID hidden channels, only administrators can subscribe. </p>
The presence node contains the presence value for clients belonging to participants that choose to publish presence to the channel. A MIX channel MAY require that all participants publish presence. Each item in the presence node is identified by the full proxy JID, and contains the current presence value for that JID. The presence is encoded in the same way as data that would be sent in a presence stanza. The full JID is always used in this node. In MIX it is possible to have a 'presence-less channel' by not using this node. Access Control may be set to enforce that for each of the full JIDs in this list, the bare JID MUST be in the participants list.
<p>QUESTION: The current specification allows channels to be configured so that node subscription is not restricted to participants (although this restriction is the default). Is this flexibility desirable, or should we restrict to participants?</p>
<p>
This node may be subscribed to by users using bare JID. So presence of online clients is sent to all users subscribed to this node, noting that presence is always sent using standard presence protocol.
</p>
<examplecaption="Value of Presence Node"><![CDATA[
<p>The Configuration node holds the configuration of the channel as a single item, named by the date-time of the last update to the configuration. A single item is stored in the node at a time, with previous configuration history accessed by MAM. Users subscribed to the configuration node will enable notification of configuration change. The configuration node is optional for a MIX channel. For example, configuration choices could be fixed and not exposed. A subset of the defined configuration options may be used and additional non-standard configuration options may be added. If configuration options to control functionality of the nature described here are provided, the options defined in this standard MUST be used. The following configuration options are provided:</p>
<ul>
<li>'Date of Configuration Change'. Encoded as the item ID.</li>
<li>'Last Change Made By'. Bare JID of the user making the last change.</li>
<li>'Name'. The name of the channel</li>
<li> 'Members'. List of JIDs with member rights. The rights of members are configured in the ACL node. </li>
<li>'Outcast'. List of JIDs banned from subscribing to any nodes in the MIX channel. </li>
</ul>
<p>TEMPORARY NOTE AND QUESTION: This is currently work in progress. Suggestions for other items to be included in configuration are welcome.</p>
<p>The format of the Configuration node follows &xep0004;. This allows configuration to be updated by MIX defined commands and that the results of update commands can be directly written to the PubSub node. Updates to the Configuration may use these commands or direct writing to the PubSub node.</p>
<p>The ACL node is closely related to the configuration node, and contains more information that will generally be more restricted as to who can access and modify. An anticipated configuration reflected in the defaults has ACL node configured so that it can be modified by channel owners and read only by channel owners and administrators. The default for the configuration node is update by owners or administrators and visibility to list members. Split of functionality has been made on the basis of this model. </p>
<p>TEMPORARY NOTE AND QUESTION: The split of configuration/acl is arbitrary. It would be possible to merge them, or to split into more nodes, giving finer control granularity. Input is solicited on the split an detailed assignment of items to nodes.</p>
<p>The ACL node holds access control related configuration of the channel as a single item, named by the date-time of the last update to the ACL. History MUST be set to 1. Previous ACL history is accessed by MAM. Users may subscribe to the ACL node if allowed using bare JID. The ACL node is optional for a MIX channel. For example, ACL choices could be fixed and not exposed. A subset of the defined ACL options may be used and additional non-standard ACL options may be added. If configuration options to control functionality of the nature described here are provided, the options defined in this standard MUST be used. The following ACL options are provided:</p>
<ul>
<li>'Date of Configuration Change'. Encoded as the item ID.</li>
<li>'Last Change Made By'. Bare JID of the user making the last change.</li>
<li>'Owners'. List of bare JIDs with Owner rights as defined in ACL node. When a list is created, the JID creating the list is configured as an owner, unless this attribute is explicitly configured to another value. Owners will generally have full rights to make changes to channel configuration and access control.</li>
<li>'Administrators'. List of bare JIDs with Administrator rights. An Administrator has rights to make changes to channel access control and configuration as defined in ACL node. An Administrator also has operational right on a channel, such a kicking users out of the channel. These rights are analogous to moderator rights defined in <cite>Multi-User Chat</cite>.</li>
<li>'End of Life'. The date and time at which the channel will be automatically removed by the server. If this is not set, the channel is permanent.</li>
<li>'Nodes Present'. Specifies which nodes are present: "participants"; "presence"; "subject"; "acl". Note that "config" is implicit, and "jidmap" is implied by "participants". </li>
<li>'Message Node Subscription'. Controls who can subscribe to messages node: "members-only"; "participants-only"; "anyone". Users with banned affiliation may not subscribe. Default "participants-only".</li>
<li>'Participants Node Subscription'. Controls who can subscribe to participants node: "members-only"; "participants-only"; "anyone"; "nobody". Users with banned affiliation may not subscribe. Default "participants-only".</li>
<li>'Participants Node Subscription'. Controls who can subscribe to participants node: "members-only"; "participants-only"; "anyone"; admins-and-owners" "nobody". Users with banned affiliation may not subscribe. Default "participants-only". Note that this value needs to be restricted to "admins-and-owners" for channels with JID visibility set to a value other than "jid-mandatory-visible".</li>
<li>'Presence Node Subscription'. Controls who can subscribe to presence node: "members-only"; "participants-only"; "anyone"; "nobody". Users with banned affiliation may not subscribe. Default "participants-only".</li>
<li>'Subject Node Subscription'. Controls who can subscribe to subject node: "members-only"; "participants-only"; "anyone"; "nobody". Users with banned affiliation may not subscribe. Default "participants-only".</li>
<li>'Configuration Node Access'. Controls who can subscribe to and has read access configuration node: "members-only"; "participants-only"; "admins-and-owners"; "owners-only"; "anyone"; "nobody". Default "admins-and-owners".</li>
<li>'ACL Node Access'. Controls who can subscribe to and has read access acl node: "members-only"; "admins-and-owners"; "owners-only"; "anyone"; "nobody". Default "admins-and-owners".</li>
<li>'Configuration Node Update'. Controls who can modify configuration node: "owners-only"; "admins-and-owners". Default "admins-and-owners".</li>
<li>'ACL Node Update'. Controls who can modify acl node: "owners-only"; "admins-and-owners". Default "owners-only".</li>
<li>'JID Visibility'. Choice of "jid-hidden", "jid-optionally-visible", "jid-mandatory-visible". </li>
<li>'Open Presence'. If selected, any client may register presence. If not selected, only clients with bare JID in the participants list may register presence.</li>
<li>'Allow Message Retraction'. If this option is selected users will be able to retract messages sent to the MIX channel.</li>
<li>'Membership Extension by Invitation'. This option extends a members only channel so that an invitation from a channel member will add the invited user to the members list.
</li>
<li>'No Private Messages'. If this option is selected, private messages may not be used with the channel.</li>
</ul>
<p>TEMPORARY NOTE: This is currently work in progress. Suggestions for other items to be included in ACL are welcome.</p>
<p>The format of the ACL node follows &xep0004;. </p>
<p>The MIX service then MUST return its identity and the features it supports, which MUST include the 'urn:xmpp:mix:0' feature, and the identity MUST have a category of 'conference' and a type of 'text'. </p>
<p>
QUESTION: Is there a need for a different type than text? Provisional answer: no.
<p>A MIX service MUST NOT advertise support for &xep0313;, as MAM is supported by the channels and not by the service. A MIX service MUST NOT advertise support for generic &xep0060;, as although MIX makes use of PubSub it is not a generic PubSub service.</p>
<section2topic='Discovering the Channels on a Service'anchor='disco-channel-list'>
<p>The list of channels supported by a MIX service is obtained by a disco command.</p>
<p>AUTHOR'S NOTE: This version contains a number of AUTHOR NOTES, which are reminder instructions to the author as to editing actions to be taken, and also indicate where changes are anticipated in future versions.</p>
<p>
AUTHOR'S NOTE: Need to add example. This standard disco approach is going to replace the earlier proposed "urn:xmpp:mix:nodes:channels", unless there are objections.
<p>The channel MUST return its identity and the features it supports. Note that a MIX channel MUST support MAM and so the response will always include both MIX and MAM support.</p>
<section2topic='Discovering Nodes at a Channel'anchor='disco-channel-nodes'>
<p>Use disco#items to find the nodes associated with a channel. The MIX server MUST only return nodes that exist and that the user making the query has rights to subscribe to.</p>
<examplecaption='Entity Queries for Nodes at a Channel'><![CDATA[
<section2topic='Discovering Participants in a Channel'anchor='disco-channel-participants'>
<p>
Online clients in the channel are determined from the presence node. Participants in the channel are determined from the "urn:xmpp:mix:nodes:participants" node which will give proxy JID and nick. The jidmap node is used to map to real JIDs. </p>
<section3topic='Joining a Channel'anchor='usecase-user-join'>
<p>A user joins a channel by sending a MIX "join" command. There is no default set of nodes, so user MUST specify the set of nodes to be subscribed to. To achieve the equivalent service to MUC, a user would subscribe to messages, presence and subject.
This will lead to the server subscribing the user to each of the requested nodes associated with the channel. The MIX server will also add the user to the participant list by injecting a new item into the "urn:xmpp:mix:nodes:participants" node automatically.
</p>
<p>The default MIX model is that only channel participants may subscribe to nodes. A MIX channel MAY allow non-participant subscription. This will be handled by clients directly subscribing to the desired PubSub nodes.</p>
<p>The channel must process the join atomically. The channel responds with an IQ-result. This stanza includes the nodes to which the user has been successfully subscribed, as well as the bare JID that will be used for the user in this channel and added to the participant list. If a user cannot be subscribed to one or more of the requested nodes (e.g., because the node does not exist), but can be subscribed to some the response simply lists the nodes successfully subscribed. If none of the nodes requested are successfully subscribed to, and error response is sent indicating the reason that the first node requested was not subscribed to. This response will also include other nodes requested where subscription failed for the same reason. A user may subsequently request subscription to nodes in a channel to which the user was not initially subscribed. </p>
<p>As noted, the participant might not be subscribed to all nodes associated with the channel (in this case only messages, participants, and subject).</p>
<examplecaption="Channel Processes Join With Modifications"><![CDATA[
<p>The user that has been added to the channel is identified by the item id of the item added to the pubsub node, which is the proxy JID of the new channel participant. Each <participant> element will include the nick of the user being added, which will be how the user will typically be shown in the channel.</p>
<p>
Following the MIX server side processing, the user's server will usually add the MIX channel to the user's roster using one way presence. This means that the MIX channel will get presence information from the user. This roster entry will lead to correct handling of the user's presence in the MIX channel. If the user does not wish to publish presence and the channel permits this, then this roster addition does not happen. If the channel requires presence and the user removes the channel from the user's roster, the channel MAY remove the user as a channel participant.
</p>
</section3>
<section3topic="User Preferences and Additional Information"anchor="usecase-visibilty-pref">
<p>A channel may store additional user preferences and parameters. Where the channel requires a value to be explicitly a &xep0004; form will be returned in response to the join request with mandatory and optional fields. If parameters are optional, the user may request to set them. </p>
<p>
A user may set their JID visibility preference to one of the following values:
</p>
<ol>
<li>'Use Channel Default'. In this setting, the channel default value will be used. This value is used if another value is not explicitly set.</li>
<li>'Never Show JID'. If this is set, JID will never be shared and user will be removed from the channel if its mode changes to JID-visible-mandatory.</li>
<li>'Prefer Not Show JID. If this is set, JID will only be shared if mode is JID-visible-mandatory.</li>
<p>Users generally remain in a channel for an extended period of time. In particular membership of the channel does not change when the user goes offline as happens with &xep0045;. So, leaving the channel is a permanent action for a user across all clients, not just a matter of telling the channel that the user is not currently available or for a single client. In order to leave a channel, a user sends a MIX "leave" command to the channel. When a user leaves the channel, the user's client will remove the channel from the user's roster.</p>
<p>When the user leaves the channel, the MIX service is responsible for unsubscribing the user from all nodes in the channel and for removing the user from the participants and presence list. If the user has online presence when the user leaves the channel, the change of presence status caused by removing the user's entry or entries from the presence node will ensure that subscribers to the presence node are correctly updated on presence status.
Deletion from the participants and presence functions as if the item (channel member) had been deleted using the PubSub retract mechanism with notification set to true. Notification of the deletion is sent to clients subscribed to the participants PubSub nodes, as shown in the example below.
</p>
<examplecaption="Reporting when User Leaves a Channel"><![CDATA[
<section3topic="Setting a Nick"anchor="usecase-setting-nick"></section3>
<p>
Each member of a channel may have a nick, which is how other users in the channel will see the user. In some cases a nick is not needed, for example where a user reads messages in a channel but does not send messages or share presence information. A nick MUST be present for a user to send a message to a channel or for a user's presence to be shared on a channel. There are four ways that a user's nick can be obtained. The choice of mechanism or mechanisms is dependent on channel policy:
</p>
<ol>
<li>The nick is registered with the user account in some way, for example as part of user provisioning with nick configured as an attribute in a directory service. This may be chosen by corporate services that wish to ensure consistent nick values for a set of users and channels.</li>
<li>The nick is registered with the MIX service, as described in <linkurl='#usecase-user-register'> Registering a Nick </link>.</li>
<li>The user explicitly sets the nick, as described in this section.</li>
A user will typically set a nick when joining a channel and may update this nick from time to time. The user does this by sending a command to the channel to set the nick. If the user wishes the channel to assign a nick (or knows that the channel will assign a nick) the nick field can be left blank, so that the user can see what is assigned in the result.
</p>
<examplecaption="User sets Nick on Channel"><![CDATA[
The channel will return the nick that is to be used, noting that this may be different to the requested nick. MIX services SHOULD apply the "nickname" profile of the PRECIS OpaqueString class, as defined in &rfc7700;.
<section3topic='Registering a Nick'anchor='usecase-user-register'>
<p>A user can register a nick with the MIX service. Nick registration can be used ensure that a user is able to use the same nick in all channels in the service and to prevent other users from using a registered nick. This can help achieve a consistent experience across a set of channels and prevent user confusion. Support for nick registration by a MIX service is optional. Where nick registration is supported, nick registration may be optional or mandatory.
Where a user has registered a Nick with the MIX service, it may be used by each channel according to policy for the channel. A nick is associated with the user's bare JID.
The response will be a list of features of the MIX channel. If Nick Registration is supported, then the result set will include <feature var="mix_nick_register"/>.
</p>
<p>
To register a nick with the MIX service the user sends
<p>On success, the service informs the user of its nick. The nick that is issued might be different from the nick that was requested, for example if the service completes normalization of nicknames for purposes of internationalization.</p>
<p>If the register request does not contain a <nick/> element, then the MIX service assigns one. It is recommended that the assigned nick is a UUID following &rfc4122;.
<section3topic='Setting User Presence'anchor='usecase-user-presence'>
<p>
A user joins a channel over an extended period, and membership in a channel does not generally change when user goes online or offline. The users membership of the channel is reflected by the user's bare JID in the participant node. All messages to the channel are sent to this JID.
</p>
<p>
A user may share presence information with the channel, for one or more online clients. Where a user shares presence information with a channel, the channel is entered by the user's server into the user's roster when the user subscribes 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. The MIX channel will update the "urn:xmpp:mix:nodes:presence" node with any change of status and the updated presence information and then share this updated presence with users subscribed to this node, as described below. When the user sets an explicit status, this is used to modify the presence node in the channel. When a client being used by the user goes offline, the associated server will send presence status "unavailable" to the MIX channel, which will cause the full JID for that client to be removed from the presence node.
</p>
<p>
A channel may require that all channel members share presence information with the channel, which is represented in the "urn:xmpp:mix:nodes:presence" node. If sharing presences is required by the channel, an XMPP client conforming to this specification SHALL share presence with the channel by including the channel in the user's roster. Note that the MIX server cannot generally enforce this, but it can require and enforce that when a message is sent to the channel, that the sender of the message is in the presence list.
Presence status and availability is set in a MIX channel by standard presence stanzas sent to the MIX channel by the user's server. User's wishing to receive presence updates will subscribe to the MIX channel presence node. Presence updates are sent out to subscribing using standard XEP-0045 compatible presence stanzas.
A user setting status is now used as an example. Unlike in &xep0045; 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 server then publishes the user's availability to the "urn:xmpp:mix:nodes:presence" node. If there is not an item named by the full JID of the client with updated presence status, this item is created. If there is not an item named by the full JID of the client with updated presence status, then an item is created.</p>
<p>The user's presence information is then published by the service to the "urn:xmpp:mix:nodes:presence" node, with the 'publisher' attribute set to the user's participant identifier (the proxy JID. The MIX channel then broadcasts the presence change to all users who are subscribed to the "urn:xmpp:mix:nodes:presence" node. The presence stanza is sent from the proxy (anonymized) full JID of the user. </p>
The presence is distributed to those subscribing to the MIX channel presence node using a standard XMPP presence stanza. The presence change is recorded on the "urn:xmpp:mix:nodes:presence" node in the item for the full JID of the client to which the presence relates. The history of this node will be held as PubSub format in the MAM archive, so that presence history may be viewed.
The presence information for a channel is stored in the urn:xmpp:mix:nodes:presence node and distributed using standard presence stanzas to users subscribing to this presence node. The user's server will then pass this presence information on to all online clients. When a client goes offline, it will lose synchronization with the presence node. When the client comes online, the clients server will send a directed presence stanza to the MIX channel. This will happen as a consequence of the MIX channel being in the user's roster and the MIX channel sending a presence update to the MIX channel. When the MIX channel adds or modifies presence for a client to the roster, this presence change will be distributed to all users subscribing to the presence node.
When the full JID of a client is added to the MIX channel presence node and that full JID is not in the presence list, the MIX channel will send to the client presence for all of the items in the presence node using standard presence stanzas. This will give the client full current presence.
<section3topic="Determining Real JIDs"anchor="usecase-real-jids">
<p>
Presence information will provide a MIX client with the nicks and anonymized proxy JIDs for participants. For JID visible rooms, the user may look up the real JID using the "urn:xmpp:mix:nodes:jidmap" node. The items in this node are identified by proxy JID, and so the real JID can be retrieved using a straightforward PubSub query. While a user may subscribe to the jidmap node, it is more likely to be used to directly look up JIDs as and when needed.
A MIX client will typically display message history of the channel to the user. When a client comes online it will need to obtain this message history from the MAM archive associated with the channel. There are three basic approaches a client will take:
</p>
<ol>
<li>If the client has previously displayed message history and has been offline for a reasonably small time, the client will wish to retrieve all messages since the last one displayed to the user.</li>
<li>The client may wish to display a fixed number of messages, perhaps finding more messages if the client subsequently requests.</li>
<li>The client may wish to display messages from a recent time period, perhaps finding more messages if the client subsequently requests.</li>
<p>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. Handling by the MIX channel is slightly different.</p>
<examplecaption="Client Goes Offline in the Channel"><![CDATA[
<p>The MIX channel will retract (remove) the item in the presence node of the MIX channel identified by the client's full JID. The MIX channel will notify subscribers to the presence node of the user going offline by sending a presence stanza to the full proxy JID of each client.</p>
There is the possibility that the message associated with the user going offline will be lost. If this happens, "ghost" entries will appear in the presence node. A MIX server may take steps to address this, for example by probing client with a disco for presence items that remain unchanged for a long period.
</p>
<p>
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 &xep0198; to maintain an XMPP client connection in the event of short term TCP failure.
<p>The message comes from the channel as a standard groupchat message, compatible with MUC.</p>
<p>AUTHOR NOTE AND QUESTION: Message id coming back is different in example. This is because the reflected message uses MAM ID, which seems helpful. However, it makes it harder for sender to correlate reflections. Need to be explicit as to compliant behaviour. Input as to whether the ID should change is solicited. A more complex option would be to include both IDs in some way.</p>
<section3topic="Retracting a Message"anchor="usecase-retract">
<p>
A MIX channel MAY support message retraction, where the sender of a messages deletes a messages from the message history and optionally replace it with another message. This retraction mechanism will be based on the &xep0060; retract operation. A client looking at message history may choose to look at "current state" which will show status after the retraction or "full history" which will include the message that was retracted.
</p>
<p>
AUTHOR'S NOTE: Define new protocol to support this and add example.
A user may add a proxy JID from the participants node of a MIX channel to the user's roster. This will enable convenient communication with the user through the MIX channel, without the user knowing the real JID of the channel participant.
</p>
<p> AUTHOR'S NOTE: To be expanded considerably. </p>
All nodes of each MIX channel are archived using MAM. Access to history in these nodes uses standard MAM protocol to access the channel nodes and to retrieve information from the nodes. Information retrieved using MAM will be of the format stored on the node and will include &xep0060; PubSub wrapping. MAM is the MIX mechanism to get at this information, in preference to direct use of PubSub.
</p>
<p>
AUTHOR'S NOTE: Add example of message retrieved using MAM, showing XEP-0060 wrapping
</p>
<p>
There are XEPs that extend MAM and in some cases modify core MAM requirements. Such XEPs may be used in conjunction with MAM, provided that they are supported by the MIX channel. If such XEPs are used, the requirements specified MUST be followed.
</p>
<p>
PubSub provides the ability to modify the state of the node, and distinguishes between current state and full history. It is anticipated that MAM will be extended with an option to choose between "current state" and "full history". This capability will be useful for and used by MIX.
</p>
<p>
AUTHOR'S NOTE: This specification will be updated to reference this MAM extension when it is written.
<section3topic='Checking For Permission To Create a Channel'anchor='usecase-admin-check-create'>
<p>
MIX does not standardize an access control model for creating and deleting MIX channels. The choice is left to the MIX implementer, and could be a very simple or complex approach.
</p>
<p>
AUTHOR'S NOTE: MIX Protocol to be added to create channel; check for permission to create channel; delete channel
<section3topic='Creating a Channel for Ad Hoc User'anchor='usecase-admin-create-adhoc'>
<p>
Rooms may be created for ad hoc use between a set of users. Channels of this nature will have channel name created by the server and will not be addressable or discoverable.
</p>
<examplecaption=""><![CDATA[
]]></example>
</section3>
<section3topic='Configuring a Channel'anchor='usecase-admin-'>
<p>
Channel jid is set on channel creation and may not be changed. All other channel may be changed if channel configuration allows.
</p>
<p>
AUTHOR'S NOTE: Allow configuration by direct writes to 'urn:xmpp:mix:nodes:config' and ACL node. Also specify MIX XEP-0004 commands to achieve this.
<section3topic='Destroying a Channel'anchor='usecase-admin-destroy'>
<p>
MIX channels are always explicitly destroyed by an owner of the channel or by a server operator. There is no concept of temporary channel, equivalent to &xep0045; temporary room which is automatically destroyed by the server when the users leave. However, channels may be configured with an explicit lifetime, after which the channel MUST be removed by the MIX server. Where a channel is created for ad hoc use, it may be desirable to keep the channel for history reference or for re-use by the same set of users.
</p>
<p>Note that the owner of the channel does not need to have presence registered in the channel in order to destroy it.</p>
<section3topic='Server Destroying a Channel'anchor='usecase-server-destroy'>
<p>
Servers may destroy channels which have no participants and/or presence according to local policy. There will often be good reasons to not destroy rooms in these scenarios.
<section1topic="Supporting MIX and MUC together"anchor="mix-and-muc-together">
<p>
MIX is specified as a service that can be used independent of MUC and a MIX service may be implemented without MUC. If both MIX and MUC are implemented, three approaches are noted.
</p>
<ol>
<li>Entirely separate MIX and MUC implementation, with MIX and MUC using separate domains and MIX channels being completely separate from MUC rooms.</li>
<li>Fully integrated MIX and MUC implementation, with MIX and MUC sharing a single domain and rooms/channels equivalent.</li>
<li>Partially integrated MIX and MUC implementation, with MIX and MUC using separate domains, but rooms/channel are common. This means that each MIX channel will have MUC room of the same name and common membership.</li>
</ol>
<p>The fully integrated approach would be transparent to clients. Disco of a room or channel would show support for both MUC and MIX, which would enable a client to choose whether to use MUC or MIX. The following example shows how a MIX service that also supported MUC would respond:</p>
<examplecaption="Service responds with Disco Info result showing MIX and MUC support"><