Both Service Discovery (XEP-0030)  and Publish-Subscribe (XEP-0060)  refer to "nodes" in relation to XMPP entities. However, the concept of an XMPP node has never been clearly specified (e.g., in RFC 3920  or RFC 6120 ). This document attempts to remedy that oversight.
To clarify the nature of a node, it is first helpful to describe the architecture of XMPP systems.
Because XMPP is a client-server technology that relies on the Domain Name System, the fundamental building block of XMPP systems is the "domain". The idea of an Internet domain is borrowed from the real world, where a domain is an area of physical territory over which an individual or organization has control (e.g., the United States of America). Similarly, an Internet domain (e.g., jabber.org or xmpp.org) is a virtual space or area that is controlled by an individual or organization (e.g., Jeremie Miller or the XMPP Standards Foundation). Given the workings of the Domain Name System, it is also possible to have "subdomains" such as planet.jabber.org or interop.xmpp.org, which can be seen as the virtual equivalent of administrative subdivisions in the real world (e.g., a particular state within the USA, such as Colorado). In any case, a domain identifier is the primary portion of a JabberID (e.g., "jabber.org" in the JID "firstname.lastname@example.org"), and can stand alone as a complete JabberID.
A given physical domain contains particular points or places. Similarly, a given virtual domain can contain particular points or entities. These entities are often thought of as accounts (e.g., the URI mailto:email@example.com represents an email account and the URI xmpp:firstname.lastname@example.org represents an XMPP account), but other entity types are possible (e.g., email@example.com happens to be a Multi-User Chat (XEP-0045)  room. Confusingly, the part of a JabberID that identifies an account or entity within the scope of an XMPP domain is called a node (e.g., the string "stpeter" in the JabberID firstname.lastname@example.org is called a "node identifier"). Unfortunately, this usage collides with the term "node" as used in Service Discovery and Publish-Subscribe. Therefore we suggest the term "localpart" for a particular point or entity in an XMPP domain. A localpart identifier is an optional secondary portion of a JabberID (e.g., "stpeter" in the JID "email@example.com").
A given domain or localpart can have various assets associated with it; in XMPP these assets are called "resources". In the case of an account registered with an XMPP service, such resources are typically devices or connections. In the case of a multi-user chat room, such resources are usually room occupants. And so on. A resource identifier is an optional tertiary portion of a JabberID (e.g., "roundabout" in the JID "firstname.lastname@example.org/roundabout" or "psa" in the JID "email@example.com/psa").
The Service Discovery and Publish-Subscribe extensions to XMPP use an optional quaternary identifer called a "node", which identifies a particular facet or aspect of an XMPP domain, localpart, or resource. The exact nature of a node depends on the protocol in use:
XEP-0030: Service Discovery -- Here a node is typically that facet or aspect of an XMPP entity that handles requests and provides responses related to a particular protocol or feature bundle. For example, a service discovery information request to the node "http://jabber.org/protocol/commands" may return all of the Ad-Hoc Commands (XEP-0050)  supported by the responding entity, a service discovery information request to the node "http://jabber.org/protocol/muc#rooms" may return all of the multi-user chat rooms in which an entity is an occupant, and a service discovery information request to the node "pubsub/nodes" may return all of the pubsub nodes that are hosted by a publish-subscribe service (thus this node acts as a kind of "meta-node").
XEP-0060: Publish-Subscribe -- Here a node is a particular "channel" or "feed" hosted at a publish-subscribe service, where the nature of the channel or feed is defined in part by the characteristic payload published at the node (e.g., Atom notifications) and in part by the focus or topic of such payloads (e.g., Atom notifications related triggered by new entries at the Planet Jabber weblog aggregation site).
As shown in the following examples, a node is not encapsulated in the JabberID but instead is communicated in protocol to indicate that the interaction is directed to or from a specific facet of a domain, a localpart, or a resource.
As authorized by XMPP URI Query Components (XEP-0147) , the XMPP Registrar  maintains a registry of queries and key-value pairs for use in XMPP URIs at <https://xmpp.org/registrar/querytypes.html>.
The "disco" and "pubsub" querytypes are already registered, including the "node" key.
In order to specify a node "foo" as a quaternary identifier without specifying a "disco" querytype or "pubsub" querytype, an XMPP URI can be constructed as follows:
This URI can be parsed as follows:
An XMPP node can include any UTF-8 character.
This specification introduces no security considerations above and beyond those discussed in RFC 6120, XEP-0030, XEP-0060.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
This document requires no interaction with the XMPP Registrar .
This document in other formats: XML PDF
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 <firstname.lastname@example.org> 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 <email@example.com>.
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".
8. 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/>.
9. 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/>.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Initial published version; per Council discussion changed local to localpart.
Added more examples.