<abstract>This specification defines a protocol for using server-to-server connections in a bidirectional way such that stanzas are sent and received on the same TCP connection.</abstract>
<p>&rfc3920; restricted server-to-server communication in such a way that a server had to use one TCP connection for XML stanzas sent from the server to the peer, and another TCP connection (initiated by the peer) for stanzas from the peer to the server, for a total of two TCP connections. &rfc6120; allows two servers to send stanzas in a bidirectional way, but does not define methods for explicitly signalling the usage thereof. This is accomplished herein.</p>
<p>While this may seem like a mere optimization that decreases the number of sockets used by an implementation or increases the performance of the server-to-server connection<note>In constrained environments, bidirectional server-to-server connections exhibit a reduced packet round trip time, see <<linkurl='http://www.isode.com/whitepapers/xmpp-performance-constrained.html'>http://www.isode.com/whitepapers/xmpp-performance-constrained.html</link>>.</note>, it actually removes some of the practical barriers for the implementation of Multiplexing in &xep0220;.</p>
<p>If a server supports bidirectional server-to-server streams, it should inform the connecting entity when returning stream features during the stream negotiation process (both before and after TLS negotiation). This is done by including a <bidi/> element qualified by the 'urn:xmpp:features:bidi' namespace.</p>
<p>To enable bidirectional communication, the connecting server sends a <bidi/> element qualified by the 'urn:xmpp:bidi' namespace. This SHOULD be done before either SASL negotiation or Server Dialback.</p>
<examplecaption="Connecting Server Requests Bidirectionality"><![CDATA[
C: <bidixmlns='urn:xmpp:bidi'/>
]]></example>
<p>After enabling bidirectionality, the connecting server continues to authenticate via SASL or requests to send stanzas for a domain pair with Server Dialback. The receiving server MUST NOT send stanzas to the peer before it has authenticated via SASL, or the peer's identity has been verified via Server Dialback. Note that the receiving server MUST NOT attempt to verify a dialback key on the same connection where the corresponding request was issued.</p>
<p>Also note that the receiving server MUST only send stanzas for which it has been authenticated - in the case of TLS/SASL based authentication, this is the value of the stream's 'to' attribute, whereas in the case of Server Dialback this is the inverse of any domain pair that has been used in a dialback request.</p>
<p>Finally, once bidirectionality is enabled, the receiving server MAY initiate Server Dialback authentications for other domains it hosts to any domain authenticated to be hosted by the connecting server. In particular, it may initiate Target Piggybacking for any target domain that has successfully been used as a source domain by the connecting server. Note that this implies that a connecting server that uses bidi and dialback MUST support dialback error conditions as defined in XEP 0220<note>Ideally, support for dialback errors would be signalled by a proper extension mechanism such as <stream:features/>. However, these are currently only sent from the receiving server to the connecting server and can therefore not be used for signalling support for dialback errors in the other direction.</note>.</p>
Note that when using Server Dialback, a server must be very careful when receiving a <db:result/> of type 'valid' without having sent a corresponding request to add the domain pair given by the 'from' and 'to' attributes. In particular it MUST NOT route stanzas to the domain given in the elements 'from' attribute over this XML stream without further proof of the peers identity.</p>
<p>Upon advancement of this specification from a status of Experimental to a status of Draft, the ®ISTRAR; shall add the foregoing namespace to the registry located at &NAMESPACES;, as described in Section 4 of &xep0053;.</p>
<p>Upon advancement of this specification from a status of Experimental to a status of Draft, the XMPP Registrar shall add 'urn:xmpp:features:bidi' in its registry of stream features at &STREAMFEATURES;.</p>
