Commenting is a popular activity on the Internet. Users leave comments on just about anything: blog posts, news articles, product reviews, photos, status updates, etc. Existing commenting solutions often involve proprietary access methods and authentication, and are silo'd off from other services. This specification proposes an open and federated way of commenting. A conversation exists as a set of Publish-Subscribe (XEP-0060)  nodes, containing comment items or other activity, and any user with a JID may leave a comment there (per conversation access rules). The protocol is designed to be modern, social, and extensible. Additionally, while the protocol is described in XMPP terms, the core concept is meant to translate easily to the HTTP-based Social Web.
The following features are required:
Publish-Subscribe (XEP-0060)  contains an example of how to retrieve items with Result Set Management (XEP-0059) , but the specification is light on details. This section gives a more complete description of how RSM is used in conjunction with PubSub.
Clients MAY provide an RSM section in the iq request:
The server MAY provide an RSM section in the response. This is already documented in XEP-0060. The server can do this regardless of whether or not the client has made the request with RSM.
Use of RSM implies that there is a natural ordering of the items at a node. The criteria used for ordering is to be determined by the node.
RSM and max_items do not mix. If the client provides both, then the server MUST prefer RSM. If the server does not support RSM, then it may honor max_items and return items ordered by newest first (which may not necessarily be the same as the ordering used by RSM).
A conversation exists across a set of PubSub nodes, some of which are dynamic:
|Node||Natural Sort Order||Description|
|"info"||N/A||Singleton node containing information about the conversation.|
|"comments" (dynamic)||modified-ascending||Contains comment items only. This node is primarily used for presenting conversations to the user. It is essentially a subset of the activity node.|
|"activity" (dynamic)||modified-ascending||Contains all activity items in the conversation, including comments. This node is primarily used for submitting comments and receiving mention events. Item persistence is OPTIONAL. Advanced implementations may choose to maintain full activity history of a conversation and expose it in this node.|
The comments and activity nodes share item data. Comments are added to the conversation by publishing to the activity node, yet the comment will also appear in the comments node as a result. In fact, since the activity node is not required to offer item persistence, it is possible that the comment might only be retrievable through the comments node. Implementations of this protocol will therefore require tight association between the comments and activity nodes. It is not possible to implement this protocol using a "generic" PubSub service.
A dynamic node accepts additional parameters by appending the parameters to the node name using a "query"-like notation. Parameters and values in the query string MUST be percent-encoded.
|Name||Allowed Values||Applies To||Example|
|"order"||"-created" (sort items by created time, descending)||comments||Node name "comments?order=-created" would present comments in created-descending order.|
|"parent_ids"||Comma-separated list of parent comment IDs to filter by. An empty value means to include top-level comments.||comments||Node name "comments?order=-created&parent_ids=1%2C5a%2Co19g%2C" (note last value is empty) would present items in created-descending order, filtered to only include comments that have parent ID "1", "5a", "o19g", or are top-level.|
The "activity" node is defined as dynamic to allow for future expansion, even though no dynamic node parameters are defined in this document that apply to it.
Before utilizing additional nodes or parameters not defined in this document, the client SHOULD first determine support via Service Discovery (XEP-0030)  or other discovery mechanism. If the server does not support or understand a parameter or value, it SHOULD reject the request. Attempting to service a request by ignoring unsupported parameters will most likely result in incorrect or undesired behavior.
A conversation is accessible through a JID and optionally a node prefix. The prefix is prepended to the desired node name, separated by a '/' character. For example, if the "Coffee Talk" conversation is said to be accessible at the JID "firstname.lastname@example.org" without a node prefix, then PubSub interactions are made using the node names defined above (i.e. "info", "comments", etc). If that conversation is instead said to be accessible at the JID "comments.example.com" with node prefix "coffeetalk", then PubSub interactions are made using node names like "coffeetalk/info", "coffeetalk/comments", etc. This allows conversations to be provisioned as either one per JID or many per JID.
Information about a conversation can be obtained by requesting the item from the info node.
It is also possible to subscribe to the info node to track changes to the conversation information.
The process for retrieving the first "page" of a conversation and listening for further updates can be summarized as follows:
First, the client subscribes to the comments node. A temporary subscription is recommended, so that it is removed if the client goes offline.
Next, the client retrieves past comments. For simplicity, the example below will fetch the 50 most recently created comments. This would be useful for displaying the comments in a flat list.
Comments are stored as Activity Streams items in Atom format.
Comments are submitted by publishing the comment to the activity node.
Upon receiving the request, the server MUST sanitize the item as necessary before accepting it. In particular, the author information MUST be confirmed or replaced with the information of the user that submitted the comment. If the item is not formatted as a valid Activity Streams Comment, then the request MUST be rejected.
Next, the server SHOULD ensure it has the submitter's user information. This is so when the comment is served to other clients, name and avatar information can be provided as well. This is done by requesting the VCard of the submitter's bare JID using vcard-temp (XEP-0054) . If the server skips this step, the author name SHOULD be the bare JID of the submitter.
Conversations MAY support submission of activity items other than comments. A client SHOULD first determine support for other item types before attempting to submit them. If a server does not support an item type, it should reject it:
If the server deems a submitted comment to be relevant to a user who is not subscribed to the activity node, it SHOULD send an unsolicited event to that user anyway. This way, users can "tag" or "mention" users not involved in a conversation, so that they may be notified about it.
A comment is considered relevant to a user if one the following are true:
If a comment is deleted, it SHOULD remain in the past items of the node(s), but with its content cleared out and replaced with bogus author data and no activity:object. This change should also cause the comment item to be pushed out again to subscribers and relevant users. This way, entities that are tracking the conversation for changes can be informed of deletes. Even after a network failure, the deleted items can be discovered by retrieving past items.
In order for the user information that gets saved with comments to not become stale over time, servers SHOULD have ways of refreshing this information by refetching user vcards.
To load a conversation intended for display with nesting, the following algorithm is RECOMMENDED:
As noted when handling comment submission above, the server MUST replace author information with that of the user performing the submission. This is essential to prevent author spoofing.
Care SHOULD be taken to prevent "mention spam." If the server determines a user is acting maliciously, then it MUST NOT send unsolicited events as a result of a submission. If a user service receives a mention event from a comment author that it has determined to be malicious, then it MUST NOT process the event further.
No interaction with the Internet Assigned Numbers Authority (IANA)  is required as a result of this document.
This specification defines the following XML namespace:
Upon advancement of this specification from a status of Experimental to a status of Draft, the XMPP Registrar  shall add the foregoing namespace to the registry located at <https://xmpp.org/registrar/namespaces.html>, as described in Section 4 of XMPP Registrar Function (XEP-0053) .
If the protocol defined in this specification undergoes a revision that is not fully backwards-compatible with an older version, the XMPP Registrar shall increment the protocol version number found at the end of the XML namespaces defined herein, as described in Section 4 of XEP-0053.
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