<abstract>This specification defines an XMPP protocol extension for active management of an XML stream between two XMPP entities, including features for stanza acknowledgements and stream resumption.</abstract>
<remark><p>Mark 'h' element as xs:unsignedInt in the schema too, it is already specified to be a 32-bit wrapping unsigned integer in the text.</p></remark>
<remark><p>Simplification based on implementation experience: removed acking per number of stanzas exchanged because either entity can request an ack at any time; moved throttling feature to a separate specification; removed 'stanzas' attribute from <enable/> element; added 'location' attribute to <enabled/> element; clarified several implementation issues in the text; fixed several examples; versioned the XML namespace from urn:xmpp:sm:2 to urn:xmpp:sm:3.</p></remark>
<remark><p>Corrected value of 'h' so that zero means no stanzas have yet been handled; clarified distinction between a cleanly closed stream and an unfinished stream.</p></remark>
<li>Added <t/> element for throttling notifications, including 'stanzas' attribute for dynamic adjustment of the stanzas window.</li>
<li>Simplified protocol by making the sequence number increment per stanza and removing the u attribute (which is now unnecessary).</li>
<li>Removed 'h' attribute from <r/> element.</li>
<li>Added 'max' and 'stanzas' attributes to both <enable/> and <enabled/> elements, and removed those attributes from the <sm/> element.</li>
<li>Incremented the protocol version from 1 to 2.</li>
<remark><p>Removed pings (use XEP-0199, whitespace pings, or TCP keepalives instead); removed section on throttling, since it is unworkable.</p></remark>
<remark><p>Added support for session resumption; re-organized the document; changed name to stream management; changed provisional namespace.</p></remark>
<p>&xmppcore; defines the fundamental streaming XML technology used by XMPP (i.e., stream establishment and termination including authentication and encryption). However, the core XMPP specification does not provide tools for actively managing a live XML stream.</p>
<p>The basic concept behind stream management is that the initiating entity (either a client or a server) and the receiving entity (a server) can exchange "commands" for active management of the stream. The following stream management features are of particular interest because they are expected to improve network reliability and the end-user experience:</p>
<p>Stream management implements these features using short XML elements at the root stream level. These elements are not "stanzas" in the XMPP sense (i.e., not &IQ;, &MESSAGE;, or &PRESENCE; stanzas as defined in <cite>RFC 6120</cite>) and are not counted or acked in stream management, since they exist for the purpose of managing stanzas themselves.</p>
<p>Stream management is used at the level of an XML stream. To check TCP connectivity underneath a given stream, it is RECOMMENDED to use whitespace keepalives (see <cite>RFC 6120</cite>), &xep0199;, or TCP keepalives. By contrast with stream management, &xep0079; and &xep0184; define acks that are sent end-to-end over multiple streams; these facilities are useful in special scenarios but are unnecessary for checking of a direct stream between two XMPP entities.</p>
<p>Note: Stream Management can be used for server-to-server streams as well as for client-to-server streams. However, for convenience this specification discusses client-to-server streams only. The same principles apply to server-to-server streams.</p>
<p>The server returns a stream header to the client along with stream features, where the features include an <sm/> element qualified by the 'urn:xmpp:sm:3' namespace &VNOTE;.</p>
<p>Note: The client cannot negotiate stream management until it has authenticated with the server and has bound a resource; see below for specific restrictions.</p>
<p>If the client wants to be allowed to resume the stream, it includes a boolean 'resume' attribute, which defaults to false &BOOLEANNOTE;. For information about resuming a previous session, see the <linkurl='#resumption'>Resumption</link> section of this document.</p>
<p>The <enable/> element MAY include a 'max' attribute to specify the client's preferred maximum resumption time in seconds.</p>
<p>Upon receiving the enable request, the server MUST reply with an <enabled/> element or a <failed/> element qualified by the 'urn:xmpp:sm:3' namespace. The <failed/> element indicates that there was a problem establishing the stream management "session". The <enabled/> element indicates successful establishment of the stream management session.</p>
<p>The <enabled/> element MAY include a 'max' attribute to specify the server's preferred maximum resumption time.</p>
<p>The <enabled/> element MAY include a 'location' attribute to specify the server's preferred IP address or hostname (optionally with a port) for reconnection, in the form specified in Section 4.9.3.19 of <cite>RFC 6120</cite> (i.e., "domainpart:port", where IPv6 addresses are enclosed in square brackets "[...]" as described in &rfc5952;); if reconnection to that location fails, the standard XMPP connection algorithm specified in <cite>RFC 6120</cite> applies.</p>
<p>The client MUST NOT attempt to negotiate stream management until it is authenticated; i.e., it MUST NOT send an <enable/> element until after authentication (such as SASL, &xep0078; or &xep0220;) has been completed successfully.</p>
<p>For client-to-server connections, the client MUST NOT attempt to enable stream management until after it has completed Resource Binding <em>unless it is resuming a previous session</em> (see <linkurl='#resumption'>Resumption</link>).</p>
<p>The server SHALL enforce this order and return a <failed/> element in response if the order is violated (see <linkurl='#errors'>Error Handling</link>).</p>
<p>Note that a client SHALL only make at most one attempt to enable stream management. If a server receives a second <enable/> element it SHOULD respond with a stream error, thus terminating the client connection.</p>
<p>After enabling stream management, the client or server can send ack elements at any time over the stream. An ack element is one of the following:</p>
<li>The 'h' attribute identifies the last <strong>handled</strong> stanza (i.e., the last stanza that the server will acknowledge as having received).</li>
<pclass='def'><strong>Definition:</strong> Acknowledging a previously-received ack element indicates that the stanza(s) sent since then have been "handled" by the server. By "handled" we mean that the server has accepted responsibility for a stanza or stanzas (e.g., to process the stanza(s) directly, deliver the stanza(s) to a local entity such as another connected client on the same server, or route the stanza(s) to a remote entity at a different server); until a stanza has been affirmed as handled by the server, that stanza is the responsibility of the sender (e.g., to resend it or generate an error if it is never affirmed as handled by the server).</p>
<p>Receipt of an <r/> element does not imply that new stanzas have been transmitted by the peer; receipt of an <a/> element only indicates that new stanzas have been processed if the 'h' attribute has been incremented.</p>
<p>The value of 'h' starts at zero at the point stream management is enabled or requested to be enabled (see note below). The value of 'h' is then incremented to one for the first stanza handled and incremented by one again with each subsequent stanza handled. In the unlikely case that the number of stanzas handled during a stream management session exceeds the number of digits that can be represented by the unsignedInt datatype as specified in &w3xmlschema2; (i.e., 2<spanclass='super'>32</span>), the value of 'h' SHALL be reset from 2<spanclass='super'>32</span>-1 back to zero (rather than being incremented to 2<spanclass='super'>32</span>).</p>
<pclass='box'>Note: Each entity maintains two counters for any given stream: a counter of stanzas it has sent, and a counter of stanzas it has received and handled ('h'). The counter for an entity's own sent stanzas is set to zero and started after sending either <enable/> or <enabled/>. The counter for the received stanzas ('h') is set to zero and started after receiving either <enable/> or <enabled/>.</p>
<p>When an <r/> element ("request") is received, the recipient MUST acknowledge it by sending an <a/> element to the sender containing a value of 'h' that is equal to the number of stanzas handled by the recipient of the <r/> element. The response SHOULD be sent as soon as possible after receiving the <r/> element, and MUST NOT be withheld for any condition other than a timeout. For example, a client with a slow connection might want to collect many stanzas over a period of time before acking, and a server might want to throttle incoming stanzas. The sender does not need to wait for an ack to continue sending stanzas.</p>
<p>Either party MAY send an <a/> element at any time (e.g., after it has received a certain number of stanzas, or after a certain period of time), even if it has not received an <r/> element from the other party.
It is RECOMMENDED that initiating entities (usually clients) send an <a/> element right before they gracefully close the stream, in order to inform the peer about received stanzas. Otherwise it can happen that stanzas are re-sent (usually by the server) although they were actually received.
<p>When a party receives an <a/> element, it SHOULD keep a record of the 'h' value returned as the sequence number of the last handled outbound stanza for the current stream (and discard the previous value).</p>
<p>If a stream ends and it is not resumed within the time specified in the original <enabled/> element, the sequence number and any associated state MAY be discarded by both parties. Before the session state is discarded, implementations SHOULD take alternative action regarding any unhandled stanzas (i.e., stanzas sent after the most recent 'h' value received):</p>
<li>A server SHOULD treat unacknowledged stanzas in the same way that it would treat a stanza sent to an unavailable resource, by either returning an error to the sender, delivery to an alternate resource, or committing the stanza to offline storage. (Note that servers SHOULD add a delay element with the original (failed) delivery timestamp, as per &xep0203;).</li>
<li>A user-oriented client SHOULD try to silently resend the stanzas upon reconnection or inform the user of the failure via appropriate user-interface elements. Clients SHOULD also add a delay element with the original (failed) send timestamp, so that the original send date is preserved. Otherwise receiving clients could only display the reconnection timestamp of the sending client, which may confuse users.</li>
<p>Because unacknowledged stanzas might have been received by the other party, resending them might result in duplicates; there is no way to prevent such a result in this protocol, although use of the XMPP 'id' attribute on all stanzas can at least assist the intended recipients in weeding out duplicate stanzas.</p>
<p>It can happen that an XML stream is terminated unexpectedly (e.g., because of network outages). In this case, it is desirable to quickly resume the former stream rather than complete the tedious process of stream establishment, roster retrieval, and presence broadcast.</p>
<p>In addition, this protocol exchanges the sequence numbers of the last received stanzas on the previous connection, allowing entities to establish definitively which stanzas require retransmission and which do not, eliminating duplication through replay.</p>
<p>To request that the stream will be resumable, when enabling stream management the client MUST add a 'resume' attribute to the <enable/> element with a value of "true" or "1" &BOOLEANNOTE;.</p>
<p>If the server will allow the stream to be resumed, it MUST include a 'resume' attribute set to "true" or "1" on the <enabled/> element and MUST include an 'id' attribute that specifies an identifier for the stream.</p>
<pclass='def'><strong>Definition:</strong> The 'id' attribute defines a unique identifier for purposes of stream management (an "SM-ID"). The SM-ID MUST be generated by the server. The client MUST consider the SM-ID to be opaque and therefore MUST NOT assign any semantic meaning to the SM-ID. The server MAY encode any information it deems useful into the SM-ID, such as the full JID &LOCALFULL; of a connected client (e.g., the full JID plus a nonce value). Any characters allowed in an XML attribute are allowed. The SM-ID MUST NOT be reused for simultaneous or subsequent sessions (but the server need not ensure that SM-IDs are unique for all time, only for as long as the server is continuously running). The SM-ID SHOULD NOT be longer than 4000 bytes.</p>
<p>As noted, the <enabled/> element MAY include a 'location' attribute that specifies the server's preferred location for reconnecting (e.g., a particular connection manager that hold session state for the connected client).</p>
<examplecaption='Server prefers reconnection at a particular location'><