Stream Initiation (XEP-0095)  defines a protocol to initiate a data stream between two Jabber/XMPP entities (e.g., for the purpose of SI File Transfer (XEP-0096) ). However, the sender is still responsible for informing potential receivers about the existence of a given stream. This document provides an automated way for a sender to announce the availability of a stream without initiating the data transfer. The purpose is to provide a "pull" protocol that enables a receiver to then request initiation of the stream from the sender.
This proposal addresses the following requirements:
A stream owner uses the <sipub/> element to announce that it can perform a specific SI request. This element can be sent to a publish-subscribe (XEP-0060) node, or sent directly to potential recipients within a <message/> stanza.
The format of the <sipub/> element is as follows:
This format is nearly identical to that for the stream initiation <si/> element (see XEP-0095). The major difference is the lack of the feature negotiation for the stream methods, and the addition of a 'from' attribute.
The 'from' attribute SHOULD be present, and MUST be present if the stanza containing the <sipub/> is not from the stream owner (e.g., if the stream is advertised at a publish-subscribe node). If present, this attribute MUST be the valid JID for the stream owner.
The 'id' attribute is an opaque identifier. This attribute MUST be present, and MUST be a valid non-empty string. It uniquely identifies the published request at the potential sender.
As with stream initiation, the 'profile' attribute MUST be present, and MUST be the namespace URI governing the profile information. It identifies the format for the SI profile.
As with stream initiation, the 'mime-type' attribute SHOULD be present, and MUST be an IANA-registered content type.  It provides the receiver with additional information about what the data stream will be.
The <sipub/> element MUST contain an element qualified by the namespace specified by the 'profile' attribute (e.g., <file xmlns='http://jabber.org/protocol/si/profile/file-transfer'/> for file transfer). This is the additional information about the data stream.
The <sipub/> information is typically provided via pubsub:
The <sipub/> element MAY also be included directly within a <message/> stanza sent to another entity (or multiple entities, e.g., in Multi-User Chat (XEP-0045)  or via Extended Stanza Addressing (XEP-0033) ). This can be especially useful for informing an offline entity about an available stream.
One of the goals of sipub is to integrate Stream Initiation with Data Forms to provide a "file upload" capability. This is accomplished via the datatypes specified in Data Forms Validation (XEP-0122) . Each datatype is specific to the profile desired.
For example the datatype "sipub:file-transfer" is used to identify the file upload field(s) corresponding to XEP-0096:
When submitting such a form, a field's value(s) MUST be the <sipub/> identifier(s). Also, the submitter MUST provide an <sipub/> element within the data form for each file to be uploaded:
The form processor will use this to retrieve the file(s) to be uploaded.
A potential receiver starts the stream initiation session by sending an IQ-get to the sender, using the <start xmlns='http://jabber.org/protocol/sipub'/> element. This element contains the 'id' attribute to specify which published stream to retrieve:
If the sender accepts the request, it responds with an IQ-result containing a <starting/> element. This element indicates the stream initiation identifier to be used:
Then the sender begins the stream initiation negotiation:
If the requested identifier is not valid, the sender SHOULD respond with a <not-acceptable/> error:
If the receiver does not have permission to request the data stream, the sender SHOULD respond with a <forbidden/> error:
When publishing a stream via the <sipub/> element, the identifier SHOULD NOT be used as-is for the <si/> element, since a single publication will likely result in multiple <si/> requests, possibly from the same receiver.
This document introduces no security concerns beyond those specified in XEP-0060 and the relevant Stream Initiation profile in use.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
The XMPP Registrar includes 'sipub:' in its registry of Data Forms Validation Datatype Prefixes.
Normally, each SI profile that wishes to be considered for use with Data Forms MUST register its own datatype qualified by the "sipub:" prefix. However, this document provides an initial seed, based on the currently accepted SI profiles. The following datatypes shall be registered for use with Data Forms Validation:
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Deprecated per a vote of the XMPP Council.