<abstract>This specification defines an XMPP protocol extension that enables any two entities to establish a one-to-one bytestream between themselves, where the data is broken down into smaller chunks and transported in-band over XMPP.</abstract>
<remark><p>Per a vote of the XMPP Council, advanced to Final; in addition, specified that the 'block-size' attribute defines the size of the data chunk before instead of after base64-encoding, to accord with existing implementations.</p></remark>
<remark><p>Encouraged use of IQ stanzas rather than message stanzas for sending data; clarified bidirectional nature of IBB; more clearly specified data handling, error conditions, XML syntax, and attribute datatypes; added optional stanza attribute as in XEP-0261.</p></remark>
<p>This document describes In-Band Bytestreams (IBB), an XMPP protocol extension that enables two entities to establish a virtual bytestream over which they can exchange Base64-encoded chunks of data over XMPP itself. Because IBB provides a generic bytestream, its usage is open-ended. To date it has been used as a fallback method for sending files (see &xep0096; and &xep0234;) when out-of-band methods such as &xep0065; are not available. However, IBB could also be useful for any kind of relatively low-bandwidth activity, such as games, shell sessions, or encrypted text.</p>
<p>In order to set up an in-band bytestream, the initiator sends an IQ stanza of type "set" containing an <open/> element qualified by the 'http://jabber.org/protocol/ibb' namespace. This element possesses three attributes:</p>
<p>Note: It is RECOMMENDED to send IBB data using IQ stanzas instead of message stanzas because IQ stanzas provide feedback to the sender regarding delivery to the recipient).</p>
<p>If the responder informs the initiator that it wishes to proceed with the session, the initiator can begin to send data over the bytestream (in addition, because the bytestream is bidirectional, the responder can also send data; see the <linkurl='#bidi'>Bidirectionality</link> section of this document for details).</p>
<p>Each chunk of data is contained in a <data/> element qualified by the 'http://jabber.org/protocol/ibb' namespace. The data element SHOULD be sent in an IQ stanza to enable proper tracking and throttling, but instead MAY be sent in a message stanza. The data to be sent, prior to base64-encoding and prior to any wrapping in XML, MUST NOT be larger than the 'block-size' determined in the bytestream negotiation.</p>
<p>Each chunk of data is included as the XML character data of the <data/> element after being encoded as Base64 as specified in Section 4 of &rfc4648;. Each block MUST be a valid base64 block with padding at the end if needed.</p>
<p>The <data/> element MUST possess a 'seq' attribute; this is a 16-bit unsigned integer that acts as a counter for data chunks sent in a particular direction within this session. The 'seq' value starts at 0 (zero) for each sender and MUST be incremented for each packet sent by that entity. Thus, the second chunk sent has a 'seq' value of 1, the third chunk has a 'seq' value of 2, and so on. The counter loops at maximum, so that after value 65535 (2<spanclass='super'>15</span> - 1) the 'seq' MUST start again at 0.</p>
<p>The sender of a data chunk need not wait for these acknowledgements before sending further stanzas. However, it is RECOMMENDED that the sender does wait in order to minimize the potential for rate-limiting penalties or throttling.</p>
<p>It is possible that delivery of the stanza might fail, in which case the sender's or recipient's server shall return an appropriate error:</p>
<ol>
<li>Because the recipient has gone offline, the recipient's server returns a &recipient; error with a type of 'wait'.</li>
<li>Because a server-to-server link has gone down, the sender's server returns a &timeout; error with a type of 'wait'.</li>
<p>Upon receiving an error related to delivery of a &MESSAGE; or &IQ; stanza, the sender SHOULD temporarily suspend the stream but either (1) retry sending at a later time or (2) renegotiate the higher-level session (if any) such as a session controlled via &xep0234;.</p>
<p>It is also possible that there is a problem with the data packet itself, in which case the recipient shall return an appropriate error:</p>
<ol>
<li>Because the session ID is unknown, the recipient returns an ¬found; error with a type of 'cancel'.</li>
<li>Because the sequence number has already been used, the recipient returns an &unexpected; error with a type of 'cancel'.</li>
<li>Because the data is not formatted in accordance with Section 4 of <cite>RFC 4648</cite>, the recipient returns a &badrequest; error with a type of 'cancel'.</li>
</ol>
<p>Upon receiving an error related to the data packet, the sender MUST close the bytestream as described under <linkrel='#close'>Closing the Bytestream</link>.</p>
<p>Data packets MUST be processed in the order they are received. If an out-of-sequence packet is received for a particular direction within a bytestream (determined by checking the 'seq' attribute), then this indicates that a packet has been lost. The recipient MUST NOT process the data of such an out-of-sequence packet, nor any that follow it within the same bytestream; instead, the recipient MUST close the bytestream as described in the next section.</p>
<p>The receiving party then acknowledges that the bytestream has been closed by returning an IQ-result (however, the receiving party might wait until it has had a chance to send any remaining data in the other direction, if the bytestream is bidirectional; in this case, the party that sent the original <close/> element SHOULD wait to receive the IQ response from the receiving party before considering the bytestream to be closed).</p>
<p>It is possible that the recipient of the close notification does not know about the bytestream, in which case it would return an ¬found; error.</p>
<examplecaption='Recipient does not know about the IBB session'><