The &xep0124; protocol defines how arbitrary XML elements may be transported efficiently and reliably over HTTP in both directions between a client and server. This document defines some minor extensions to that protocol that enable XMPP streams (as specified in &rfc3920; and &rfc3921;) to be bound to HTTP.

If the BOSH <body/> wrapper is not empty, then it SHOULD contain one of the following:

• A complete <stream:features/> element (in which case the BOSH <body/> element MUST include the namespace xmlns:stream='http://etherx.jabber.org/streams').
• A complete element used for SASL negotiation and qualified by the 'urn:ietf:params:xml:ns:xmpp-sasl' namespace.
• XML elements extracted from a <stream:error/> element and wrapped in a BOSH <body/> element that features the 'remote-stream-error' terminal error condition. Each such element MUST be qualified either by the namespace 'urn:ietf:params:xml:ns:xmpp-streams' or by an application-specific namespace.
• One or more complete <message/>, <presence/>, and/or <iq/> elements qualified by the 'jabber:client' namespace.

Note: Many existing XMPP-specific implementations of BOSH clients and connection managers do not specify the namespace of <message/>, <presence/>, or <iq/> elements, since that allows them to forward stanzas without modification (the XMPP <stream:stream/> wrapper element used with TCP typically sets the default namespace to 'jabber:client'). They instead simply assume that the full content of the 'jabber:client' namespace is a subset of the 'http://jabber.org/protocol/httpbind' namespace.

Note: Inclusion of TLS negotiation elements is allowed but is NOT RECOMMENDED. The definition of how TLS might be implemented over BOSH is currently beyond the scope of this document.

The client SHOULD include a 'xmpp:version' attribute qualified by the 'urn:xmpp:xbosh' namespace in its session creation request. This attribute corresponds to the 'version' attribute of the XMPP <stream:stream/> element as defined in RFC 3920. The connection manager SHOULD forward the value to the XMPP server accordingly.

Note: Unlike the protocol defined in &xep0025;, an opening <stream:stream> tag is not sent to the connection manager (since BOSH <body/> elements MUST not contain partial XML elements). Any XML streams between the connection manager and an XMPP server are the responsibility of the connection manager (and beyond the scope of this document).

The connection manager SHOULD include a 'xmpp:version' attribute (qualified by the 'urn:xmpp:xbosh' namespace) and a <stream:features/> element (qualified by the 'http://etherx.jabber.org/streams' namespace) in a response as soon as they are available, either in its session creation response, or (if it has not yet received them from the XMPP server) in any subsequent response. If no <stream:features/> element is included in the connection manager's session creation response, then the client SHOULD send empty request elements until it receives a response containing a <stream:features/> element.

Note: The same procedure applies to the deprecated XMPP-specific 'authid' attribute of the BOSH <body/> element which contains the value of the XMPP stream ID generated by the XMPP server. This value is needed only by legacy XMPP clients in order to complete digest authentication using the deprecated &xep0078; protocol. Separate 'sid' and 'authid' attributes are required because the connection manager is not necessarily part of a single XMPP server (e.g., it may handle HTTP connections on behalf of multiple XMPP servers).

Note: The client SHOULD ignore any Transport Layer Security (TLS) feature since BOSH channel encryption SHOULD be negotiated at the HTTP layer.

TLS compression (as defined in RFC 3920) and Stream Compression (as defined in &xep0138;) are NOT RECOMMENDED since compression SHOULD be negotiated at the HTTP layer and using the 'accept' attribute of the BOSH session creation response. TLS compression and Stream Compression SHOULD NOT be used at the same time as HTTP content encoding.

Note: The 'xmpp:version' attribute SHOULD also be included on the request and response when adding new streams to a session.

A success case for authentication, resource binding, and IM session establishment using the XMPP protocols is shown below. For detailed specification of these protocols (including error cases), refer to RFC 3920 and RFC 3921.

Upon receiving the <success/> element, the client SHOULD then ask the connection manager to restart the stream. It does this by setting to "true" the 'xmpp:restart' attribute (qualified by the 'urn:xmpp:xbosh' namespace) of the BOSH <body/> element. Note: The client SHOULD also include 'to' and 'xml:lang' attributes in its request:

Upon receiving any request with the 'xmpp:restart' attribute set to "true" the connection manager SHOULD consider the original stream with the XMPP server to be closed (it is not necessary to send a closing </stream:stream> tag). It SHOULD then initiate a new stream by sending an opening <stream:stream> tag over the same TCP connection to the XMPP server. Upon receiving the response from the XMPP server, it SHOULD forward any available features (or an empty element) to the client:

The content of the <body/> element is a copy of the content of the <stream:error/> element received from the XMPP server:

It is possible that a connection manager will receive a stanza for delivery to a client even though the client connection is no longer active (e.g., before the connection manager is able to inform the XMPP server that the connection has died). In this case, the connection manager would return an error to the XMPP server; it is RECOMMENDED that the connection manager proceed as follows, since the situation is similar to that addressed by point #2 of Section 11.1 of RFC 3921:

1. If the delivered stanza was &PRESENCE;, silently drop the stanza and do not return an error to the sender.
2. If the delivered stanza was &IQ;, return a &unavailable; error to the sender.
3. If the delivered stanza was &MESSAGE;, return a &recipient; error to the sender.

When an XMPP server receives a &MESSAGE; stanza of type "error" containing a &recipient; condition from a connection manager, it SHOULD store the message for later delivery if offline storage is enabled, otherwise route the error stanza to the sender.

This protocol does not introduce any new security considerations beyond those specified in BOSH.

This document requires no interaction with &IANA;.