Existing Jabber protocols provide a strong foundation for the controlled, permissions-based sharing of files between Jabber entities, e.g., to enable shared workspaces among ad-hoc workgroups and the attachment of files to Multi-User Chat (XEP-0045)  rooms.
This document defines several additional building blocks (a simple request protocol along with well-known service discovery nodes) that tie together existing protocols to enable the sharing of files between Jabber entities.
The protocol defined herein assumes that the offering entity is the file owner. The primary reason for this approach is that most likely the file owner (e.g., a chatroom or end user) knows best which files are available and which requesting entities have permission to view which files. Furthermore, for the purposes of this protocol it is unnecessary to assume that dedicated file-hosting services will exist on the Jabber network or that existing Jabber servers will offer file-hosting functionality (though see the File-Hosting Services section of this document).
If an entity directly supports the protocol defined herein, it SHOULD include a feature of "http://jabber.org/protocol/files" in its response to a Service Discovery (XEP-0030)  information request. The protocol flow is shown in the following example (an end user querying a chatroom):
This document stipulates that communications regarding files MUST occur by sending stanzas to the well-known service discovery node "files" (or sub-nodes thereof as defined below). Therefore, even if (as in the foregoing example) the file owner directly supports the protocol defined herein, the requesting entity MUST send subsequent file-related service discovery requests to the node "files" (or sub-nodes thereof). The file owner also SHOULD list that node in its response to a service discovery items request, as shown in the following example:
It is possible that the file owner does not directly support the protocol defined herein and therefore that the offering entity has a JID different from that of the file owner. In this case, the file owner MUST NOT include a feature of "http://jabber.org/protocol/files" in its response to a service discovery information request, as shown in the following example (an end user querying another end user):
However, in this case the file owner SHOULD still include the offering entity (e.g., a hosting service) in its response to a service discovery items request:
If an offering entity has files to share, it SHOULD reply positively when a requesting entity sends a service discovery items request to the well-known service discovery node "files":
If the requesting entity is not allowed to view the offering entity's files (the requesting entity is not an occupant of a chatroom, is not registered with the offering entity, is not a contact in a user's roster, etc.) or the offering entity has no files to share, the offering entity SHOULD return an empty <query/> element:
If the requesting entity is allowed to view the offering entity's files and the offering entity has files to share, the offering entity SHOULD return a list of items:
Note: The NodeID MUST begin with the string 'files' followed by the '/' character followed the name of the directory or file; further subdirectories or files within a directory MUST follow the same pattern (e.g., "files/somedir/anotherfile"). Thus the protocol defined herein enforces semantic meaning on NodeIDs; this is OPTIONAL within Service Discovery but REQUIRED by this document.
If the offering entity has only a few files to share, it may be appropriate to make them available via service discovery only, thus requiring the requesting entity to "walk the tree" of directories and files as described in the Finding All Files via Service Discovery section. However, if the offering entity has a larger number of files to share, the number of service discovery requests and responses required to "walk the tree" of all directories and files might result in excessive amounts of traffic between the requesting entity and the offering entity; in this case, the offering entity SHOULD provide a "tree file" that defines the hierarchy of directories and files in the standardized format specified in the Retrieving the Tree File section. The number of files that counts as "large" is not defined herein and is left up to the implementation or deployment; in practice, it is RECOMMENDED for the offering entity to provide a tree file if it has more than five (5) files to share.
If the offering entity does not provide a tree file, the requesting entity will need to "walk the tree" via service discovery in order to find all the files shared by the offering entity. The previous example showed an offering entity that had two items available: a directory and a file. In order to determine if an item is a directory or a file, the requesting entity MUST send a disco#info request to the relevant node:
If the item is a directory, the offering entity SHOULD return information about the directory, including an identity whose category is "filesys" and whose type is "directory":
If the requesting entity wants information about every item, it MUST query each item individually:
If the item is a file, the offering entity SHOULD return information about the file, including an identity whose category is "filesys" and whose type is "file":
Note: The offering entity MAY also include detailed information about the file, as described in the Determining File Details section of this document.
If the requesting entity wants to find all files, it needs to send a disco#items query to the directory:
The offering entity will then return a list of files and directories contained within the queried directory:
The requesting entity then needs to send further disco#info and disco#items requests to the offering entity, specifying the appropriate service discovery nodes...
In this scenario, we can reconstruct the file tree as follows:
Obviously, finding all files via service discovery is a tedious process. Therefore, it is RECOMMENDED that the offering entity provide a "tree file" if it has more than five (5) files to share. The format of the tree file is defined by the 'http://jabber.org/profile/si/profile/tree-transfer' namespace that is specified in Tree Transfer Stream Initiation Profile (XEP-0105) . The tree file MUST be named "tree.xml" and MUST be available at the well-known service discovery node "tree.xml". The offering entity MAY create a different tree file for each requesting entity (depending on the requesting entity's permissions to view certain directories and files); for this reason, the tree file SHOULD NOT be contained in the root "files" directory itself (note that its NodeID is "tree.xml", not "files/tree.xml").
If the offering entity provides a tree file, it MUST communicate that fact in the disco#items result it returns to the requesting entity in response to the initial request:
If the offering entity includes a service discovery item whose NodeID is "tree.xml", the requesting entity SHOULD retrieve that file (using the protocol specified in the Retrieving a File section) before sending any further service discovery requests to the offering entity.
The following example shows the exact format of the tree file that would represent the file and directory hierarchy discovered via service discovery in the preceding section:
If the offering entity provides a tree file, it is RECOMMENDED (but not required) for the offering entity to also make information about its files discoverable via Service Discovery as described in the following section.
As is evident from the foregoing examples, neither "walking the tree" via Service Discovery nor retrieving the tree file will yield the kind of detailed information about a file (MIME type, file size, descriptive text, etc.) that can help a user or application decide whether to retrieve the file.
To address the felt need for more detailed information about files, an offering entity MAY provide such information in response to disco#info requests sent to a specific NodeID (file or directory) by including extended information structured according to Service Discovery Extensions (XEP-0128) . The following examples illustrate this usage.
The fields shown are RECOMMENDED, and are specified more fully in the XMPP Registrar Considerations section of this document.
In order to retrieve a file, the requesting entity sends a retrieval request to the JID+NodeID of the relevant item:
Note: If the requested file was found by means of the tree file rather than service discovery, the NodeID of the retrieve request MUST be constructed according to the rules specified above for service discovery NodeIDs (i.e., 'files' followed by the '/' character followed by the name of the directory or file, followed by additional '/' characters and subdirectory or file names as needed).
If the offering entity agrees to share the file with the requesting entity, it MUST return an IQ result to the requesting entity and then immediately initiate a file transfer to the requesting entity following the protocol defined in SI File Transfer (XEP-0096) :
The value of the <si/> element's 'id' attribute MUST be the same as the value of the 'sid' attribute communicated in the tree file or the 'name' attribute communicated via service discovery; for this reason, the service discovery 'name' attribute is REQUIRED for NodeIDs that correspond to files, and its value MUST follow the rules for the 'sid' attribute specified in XEP-0105.
Upon receiving the file transfer initiation from the offering entity, the requesting entity SHOULD check the SI 'id' in order to correlate the file transfer with the request; if there is a match, the requesting entity SHOULD silently accept the file transfer and not require intervention by a human before proceeding.
If the offering entity does not agree to share the file with the requesting entity, it MUST return an appropriate IQ error to the requesting entity, such as "Not Authorized", "Forbidden", "Payment Required", "Registration Required", or "Not Found" (see Error Condition Mappings (XEP-0086)  regarding error syntax).
A dedicated file-hosting service may agree to host files on behalf of a user or other entity, in which case the hosting service (or, to be precise, a specific resource of the hosting service) becomes the offering entity in the use cases defined herein. While the nature of such hosting services is outside the scope of this document, the following guidelines may be helpful to implementers.
First, a file-hosting service SHOULD provide a distinct JID for each account on the service in order to enable communications between the requesting entity and the hosting service. For example, let us suppose that <files.shakespeare.lit> is a file-hosting service; specific accounts on the service could be structured as JIDs of the form <firstname.lastname@example.org> or <files.shakespeare.lit/account>, in which case the requesting entity would communicate directly with that JID and treat that JID as the offering entity. The file-hosting service SHOULD enable the file owner (e.g., an end user whose JID is <email@example.com>) to upload files to the service using standard Internet protocols (such as HTTP, FTP, scp, or XEP-0096), control who can view or retrieve files, and otherwise configure the offering entity. The file owner SHOULD also list the JID of the "offering entity" in response to service discovery items requests sent to the user's bare JID, so that requesting entities can find files hosted by the service on the file owner's behalf.
It is possible that some Jabber server deployments would choose to offer file-hosting capabilities for their users (if supported in the underlying server implementation), so that the offering entity would have the same <user@host> address as the file owner. In this case, the server itself can be considered a file-hosting service.
Managing access to files and directories is the responsibility of the offering entity. However, the offering entity SHOULD NOT share files with requesting entities that are not known to it via presence subscription, prior registration, room occupancy, or some similar mechanism.
No interaction with the Internet Assigned Numbers Authority (IANA)  is required as a result of this document.
Upon advancement of this document to a status of Draft, the XMPP Registrar shall add 'files' and 'tree.xml' to its registry of well-known service discovery nodes.
Upon advancement of this document to a status of Draft, the XMPP Registrar shall add a category of 'filesys' to its registry of service discovery identities, with two associated types: 'directory' and 'file'.
Field Standardization for Data Forms (XEP-0068)  defines a process for standardizing the fields used within Data Forms scoped by a particular namespace. This document reserves the FORM_TYPE "http://jabber.org/protocol/files" as well as specific fields for use within the context of that FORM_TYPE, as specified in the following registry submission.
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