<abstract>This specification defines a Jingle application type for transferring files between two entities. The protocol provides a modular framework that enables the exchange of information about the file to be transferred as well as the negotiation of parameters such as the transport to be used.</abstract>
<remark><p>Initial published version.</p></remark>
</revision>
<revision>
<version>0.0.3</version>
<date>2008-02-29</date>
<initials>psa</initials>
<remark><p>Corrected use of content-replace action; specified that the In-Band Bytestreams transport method is mandatory-to-implement but must have the lowest preference order.</p></remark>
</revision>
<revision>
<version>0.0.2</version>
<date>2008-02-28</date>
<initials>psa</initials>
<remark>Modified negotiation flow to use new content-replace action.</remark>
</revision>
<revision>
<version>0.0.1</version>
<date>2008-01-29</date>
<initials>psa</initials>
<remark>First draft.</remark>
</revision>
</header>
<section1topic='Introduction'anchor='intro'>
<p>&xep0096; defines the current XMPP protocol extension for file transfer. However, that protocol has several drawbacks, most related to the &xep0095; protocol on which it depends:</p>
<li>It does not enable a true, bidirectional negotiation; instead, the initiator sets the terms for the file transfer and the responder either accepts the terms or cancels the negotiation.</li>
<li>It is the only technology in the Jabber/XMPP protocol "stack" that uses <cite>XEP-095: Stream Initiation</cite>. More modern technologies such as voice and video session negotiation use &xep0166;, and it would be helpful if implementors could use the same code for all negotiation use cases.</li>
</ol>
<p>To overcome these drawbacks, this specification defines a file transfer negotiation method that meets the following requirements:</p>
<ul>
<li>Reuse the session negotiation semantics from <cite>XEP-0166</cite>.</li>
<li>Reuse the file description format from <cite>XEP-0096</cite>.</li>
<li>Define a clear upgrade path from <cite>XEP-0096</cite> to this specification.</li>
<p>Jingle file transfer is only as reliable as the transports on which it depends. In particular, SOCKS5 Bytestreams ("S5B") does not always result in NAT or firewall traversal. To work around that problem, this specification requires all implementations to support In-Band Bytestreams ("IBB"), which tends to result in a successful (if slow) file transfer. A future version of this specification will also recommend implementation of a Jingle transport method that emulates the IETF's ICE-TCP technology, which is currently a work in progress (see &ice-tcp;).</p>
<p>First, the party that wishes to initiate the file transfer determines the responder's capabilities (via &xep0030; or &xep0115;). Here we assume that the responder supports the following service discovery features (note: these features may not reflect final namespace assignments):</p>
<li>An application type of "urn:xmpp:tmp:jingle:apps:file-transfer" &NSNOTE;. In particular, the <description/> element contains an <offer/> or <request/> element that in turn contains a <file/> element qualified by the existing 'http://jabber.org/protocol/si/profile/file-transfer' namespace from <cite>XEP-0096</cite>.</li>
<li>An appropriate transport method. Because the existing transport methods used in <cite>XEP-0096</cite> (i.e., &xep0065; and &xep0047;) are not yet defined as Jingle transport methods, this specification registers those definitions.</li>
<p>In this example, the initiator is <kingclaudius@shakespeare.lit>, the responder is <laertes@shakespeare.lit>, and the initiation request specifies a file offer and a transport method of bytestreams (i.e., XEP-0065).</p>
<p>If the responder is willing to accept the bytestream, it MUST attempt to open a standard TCP socket on the network address of the StreamHost communicated by the initiator. If the initiator provides more than one StreamHost, the responder SHOULD try to connect to them in the order they occur.</p>
<p>If the responder is able to open a TCP socket on a StreamHost, it MUST utilize the SOCKS5 protocol specified in &rfc1928; to establish the connection with the StreamHost.</p>
<examplecaption='Responder Connects to StreamHost'><![CDATA[
<p><cite>XEP-0096</cite> enables the initiator to offer more than one transport and for the receiving party to choose its desired transport. This flow can be emulated in Jingle negotiation if the initiator includes more than one <content/> element (each with a different proposed transport method) and the responder removes all but its desired transport method before the session is accepted.</p>
<p>The responder then sends a Jingle content-remove (deleting SOCKS5 Bytestreams) in order to choose the desired transport, which in this case is IBB.</p>
<p>Currently, <cite>XEP-0096</cite> does not enable the parties to fall back to a second method (e.g., In-Band Bytestreams) if the first method tried (e.g., SOCKS5 Bytestreams) does not work. This problem is addressed by Jingle. Such a fallback scenario is especially helpful when re-using the existing SOCKS5 Bytestreams method, since that method does not necessarily result in NAT or firewall traversal and therefore often results in a failed attempt at setting up the initial transport. However, because In-Band Bytestreams almost always succeeds (except if the parties violate rate-limiting policies at their servers), it provides a reliable transfer method of last resort. To provide seamless fallback, the initiator or responder can counter-propose IBB if S5B setup fails.</p>
<p>Note: Currently, <cite>XEP-0166</cite> prohibits use of the content-replace action during the PENDING state. That restriction may be removed in the future.</p>
<p>If the responder is willing to accept the bytestream, it MUST attempt to open a standard TCP socket on the network address of the StreamHost communicated by the initiator. If the initiator provides more than one StreamHost, the responder SHOULD try to connect to them in the order they occur.</p>
<p>If the responder tries but is unable to connect to any of the StreamHosts and it cannot or does not wish to attempt a connection from its side, it returns a ¬found; error to the initiator.</p>
<examplecaption='Target Is Unable to Connect to Any StreamHost and Wishes to End Transaction'><![CDATA[
<p>At this point the file transfer has failed using SOCKS5 Bytestreams. However, all is not lost, because the parties can attempt to fall back to In-Band Bytestreams. Therefore the responder sends a content-replace action including a transport of IBB and a <em>request</em> for the file originally offered (note the use of the <request/> element instead of the <offer/> element, since the responder wants the initiator to send the previously-offered file).</p>
<p>All implementations MUST support the In-Band Bytestreams transport method as a reliable method of last resort. An implementation SHOULD support other transport methods as well.</p>
<p>Support for Jingle file transfer can be determined through discovery of the 'urn:xmpp:tmp:jingle:apps:file-transfer' namespace &NSNOTE;, via either service discovery (<cite>XEP-0030</cite>) or entity capabilities (<cite>XEP-0115</cite>). If the initiator knows that the responder supports Jingle file transfer, it SHOULD first attempt negotiation using XEP-0166 rather than XEP-0095.</p>
<p>In order to secure the data stream, implementations SHOULD use encryption methods appropriate to the transport method being used. For details, refer to the specifications for those transport methods.</p>
<p>Until this specification advances to a status of Draft, its associated namespaces shall be:</p>
<ul>
<li>urn:xmpp:tmp:jingle:apps:file-transfer</li>
</ul>
<p>Upon advancement of this specification, the ®ISTRAR; shall issue permanent namespaces in accordance with the process defined in Section 4 of &xep0053;.</p>
<p>The following namespaces are requested, and are thought to be unique per the XMPP Registrar's requirements:</p>
<p>The XMPP Registrar shall include "file-transfer" in its registry of Jingle application formats. The registry submission is as follows:</p>
<code><![CDATA[
<application>
<name>file-transfer</name>
<desc>Jingle sessions for the transfer of a file</desc>
<transport>reliable</transport>
<doc>XEP-xxxx</doc>
</application>
]]></code>
</section2>
<section2topic='Jingle Transport Methods'anchor='registrar-transports'>
<p>The XMPP Registrar shall add to its registry of Jingle transport methods definitions for the reliable transport methods defined in <cite>XEP-0047</cite> and <cite>XEP-0065</cite>. The registry submissions are as follows:</p>
<code><