RFC 3920 introduced the concept of binding a resource to an XML stream (this concept superseded part of the obsolete jabber:iq:auth protocol described in Non-SASL Authentication (XEP-0078) ). As defined in RFC 3920, resource binding enables a client to bind one resource to a stream but does not enable a client to unbind a resource and leaves underspecified what a server and client should do if a client binds more than one resource to a stream. Because the ability to bind multiple resources to a stream is desirable in certain environments (e.g., for devices that are unable to open more than one TCP connection or when a machine runs a daemon process that is used by multiple applications), this document proposes improvements to resource binding in order to address these shortcomings.
Note: The recommendations from this document were NOT incorporated into RFC 6120  and this document is Obsolete.
In order to properly manage the resources associated with an XML stream, a client must be able to unbind resources. This shall be completed by sending an IQ-set with a child element of <unbind/> qualified by the 'urn:ietf:params:xml:ns:xmpp-bind' namespace, which in turn has a child element of <resource/> whose XML character data specifies the resource to be unbound:
If the server does not understand the <unbind/> element, it MUST return an error of <bad-request/>. Otherwise, if there is no such resource, the server MUST return an error of <item-not-found/>. When the client unbinds the only resource associated with the stream, the server SHOULD close the stream and terminate the TCP connection. The 'from' address included in bind and unbind stanzas SHOULD be the full JID <email@example.com/resource> associated with the resource in question; however, the server MUST NOT accept further stanzas from that full JID after successfully processing the unbind request.
A server SHOULD advertise its support for the 'urn:ietf:params:xml:ns:xmpp-bind' namespace and the unbind functionality by returning an appropriate stream feature as shown below:
When a client binds multiple resources to the same stream, proper management of 'from' addresses is imperative. In particular, a client MUST specify a 'from' address on every stanza it sends over a stream to which it has bound multiple resources, where the 'from' address is the full JID <firstname.lastname@example.org/resource> associated with the relevant resource. If a client does not specify a 'from' address on a stanza it sends over a stream to which it has bound multiple resources (or if it specifies as the 'from' address a full JID other than one of the bound resources), the server MUST return the stanza to the client with an <unknown-sender/> stanza error. 
Naturally, the existing rules from RFC 3920 regarding validation of asserted 'from' addresses still apply.
In instant messaging and presence applications, an XMPP server manages a session on behalf of a connected client. A server MUST create and manage a separate session for each distinct resource, even if there are multiple resources associated with a given XML stream. In particular:
The following examples show a possible flow of resource binding and unbinding (stanzas prefixed by "C:" are sent by the client, stanzas prefixed by "S:" are sent by the server).
First, the client binds an initial resource to the stream.
Now the client sends some stanzas, making sure to set its 'from' address:
Now the client binds a second resource to the stream.
If the server does not allow entities to bind multiple resources to the stream, it MUST return a <not-allowed/> error as described in RFC 3920.
Now the client sends more stanzas.
In handling the last stanza shown above, the server returns an <unknown-sender/> error to the sender because the sender did not include a 'from' address.
Now the client binds a third resource to the stream.
Now the client unbinds its initial resource.
Now the client unbinds another resource.
Now the client unbinds its last remaining resource.
The server now SHOULD close the stream and terminate the underlying TCP connection.
If properly implemented, the modifications described herein do not introduce any new security concerns above and beyond those defined in RFC 3920. However, care must be taken to properly manage 'from' addresses in order to avoid the delivery of stanzas from an unintended resource (which may, for example, leak presence information).
No interaction with the Internet Assigned Numbers Authority (IANA)  is required as a result of this document.
No interaction with the XMPP Registrar  is required as a result of this document, since the 'urn:ietf:params:xml:ns:xmpp-bind' namespace is already registered (see <https://xmpp.org/registrar/namespaces.html>).
Note: The following provisional schema is intended to replace the existing schema for the Resource Binding stream feature.
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This XMPP Extension Protocol is copyright © 1999 – 2020 by the XMPP Standards Foundation (XSF).
Permission is hereby granted, free of charge, to any person obtaining a copy of this specification (the "Specification"), to make use of the Specification without restriction, including without limitation the rights to implement the Specification in a software program, deploy the Specification in a network service, and copy, modify, merge, publish, translate, distribute, sublicense, or sell copies of the Specification, and to permit persons to whom the Specification is furnished to do so, subject to the condition that the foregoing copyright notice and this permission notice shall be included in all copies or substantial portions of the Specification. Unless separate permission is granted, modified works that are redistributed shall not contain misleading information regarding the authors, title, number, or publisher of the Specification, and shall not claim endorsement of the modified works by the authors, any organization or project to which the authors belong, or the XMPP Standards Foundation.
## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. ##
In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall the XMPP Standards Foundation or any author of this Specification be liable for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if the XMPP Standards Foundation or such author has been advised of the possibility of such damages.
This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which can be found at <https://xmpp.org/about/xsf/ipr-policy> or obtained by writing to XMPP Standards Foundation, P.O. Box 787, Parker, CO 80134 USA).
The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.
The primary venue for discussion of XMPP Extension Protocols is the <email@example.com> discussion list.
Discussion on other xmpp.org discussion lists might also be appropriate; see <http://xmpp.org/about/discuss.shtml> for a complete list.
Errata can be sent to <firstname.lastname@example.org>.
The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".
3. Currently there is no <unknown-sender/> stanza defined in RFC 3920. The author will work to add such an error condition (with a type of "modify") to the document that revises RFC 3920.
5. 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/>.
6. 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/>.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Per a vote of the XMPP Council, changed status from Deprecated to Obsolete.
Per a vote of the XMPP Council, changed specification to Deprecated because the recommendations described in this document were not incorporated into RFC 6120.
Per a vote of the XMPP Council, advanced specification to Draft.
Per Council feedback, modified error handling when client does not include a from address; also, clarified rules regarding which full JID to include on bind and unbind requests.
Required client to include from address on every stanza it sends over a stream to which it has bound multiple resources; specified server handling of sessions.
Added stream feature for unbind.