XEP-0288: Bidirectional Server-to-Server Connections

2.1 Stream Feature¶

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.

<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
  <bidi xmlns='urn:xmpp:features:bidi'/>
</stream:features>

If the initiating entity chooses to use TLS, STARTTLS negotiation MUST be completed before enabling bidirectionality.

2.2 Negotiation¶

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.

<!-- Client -->
<bidi xmlns='urn:xmpp:bidi'/>

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.

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.

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 [5].

6.1 Protocol Namespaces¶

The XMPP Registrar [7] includes 'urn:xmpp:bidi' in its registry of protocol namespaces (see <https://xmpp.org/registrar/namespaces.html>).

6.2 Stream Features¶

The XMPP Registrar includes 'urn:xmpp:features:bidi' in its registry of stream features (see <https://xmpp.org/registrar/stream-features.html>).

7.1 Bidi¶

<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='urn:xmpp:bidi'
    xmlns='urn:xmpp:bidi'
    elementFormDefault='qualified'>

  <xs:annotation>
    <xs:documentation>
      The protocol documented by this schema is defined in
      XEP-0288: http://www.xmpp.org/extensions/xep-0288.html
    </xs:documentation>
  </xs:annotation>

  <xs:element name='bidi' type='empty'/>

  <xs:simpleType name='empty'>
    <xs:restriction base='xs:string'>
      <xs:enumeration value=''/>
    </xs:restriction>
  </xs:simpleType>

</xs:schema>

7.2 Stream Feature¶

<?xml version='1.0' encoding='UTF-8'?>

<xs:schema
    xmlns:xs='http://www.w3.org/2001/XMLSchema'
    targetNamespace='urn:xmpp:features:bidi'
    xmlns='urn:xmpp:features:bidi'
    elementFormDefault='qualified'>

  <xs:element name='bidi' type='empty'/>

  <xs:simpleType name='empty'>
    <xs:restriction base='xs:string'>
      <xs:enumeration value=''/>
    </xs:restriction>
  </xs:simpleType>

</xs:schema>

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1. RFC 3920: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc3920>.

2. RFC 6120: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc6120>.

3. In constrained environments, bidirectional server-to-server connections exhibit a reduced packet round trip time, see <http://www.isode.com/whitepapers/xmpp-performance-constrained.html>.

4. XEP-0220: Server Dialback <https://xmpp.org/extensions/xep-0220.html>.

5. 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.

6. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.

7. The XMPP Registrar maintains a list of reserved protocol namespaces as well as registries of parameters used in the context of XMPP extension protocols approved by the XMPP Standards Foundation. For further information, see <https://xmpp.org/registrar/>.