XEP-XXXX: Namespace Versioning in urn:xmpp

This document describes the common practise of namespace versioning for the urn:xmpp URN namespace, and how this affects (and does not affect) the protocols which have such namespaces.
  • Dave Cridland
  • Peter Saint-Andre
© 1999 – 2020 XMPP Standards Foundation. SEE LEGAL NOTICES.


WARNING: This Informational document is Experimental. Publication as an XMPP Extension Protocol does not imply approval of this proposal by the XMPP Standards Foundation. Implementation of the best practice or protocol profile described herein is encouraged in exploratory implementations, although production systems are advised to carefully consider whether it is appropriate to deploy implementations of this protocol before it advances to a status of Draft.
0.0.2 (2015-04-07)
Document Lifecycle
  1. Experimental
  2. Proposed
  3. Active

1. Introduction

Over the years that the XMPP registrar has been issuing namespaces for protocols, a number of namespace formats and issuance strategies have been attempted. As of the time of writing, the current one, defined in XEP-0053 Section 4, has proven successful, but many newcomers to the XSF are surprised to see version numbers in namespaces and have often misconstrued these to be in violation of XML namespace handling.

This document attempts to provide rationale behind why these were adopted, why they are successful, and why they are not in violation of the XML namespace rules.

2. Rationale

2.1 Before The Dawn Of Time

In the beginning, namespaces were simply minted, but relatively early on, it was observed that experimental protocols were often deployed, leading to pollution and devaluation of the namespace. If two incompatible variants of a protocol used the same namespace, then this could easily lead to a situation whereby Draft or Final stage protocols had difficulty deploying because they'd fail to interoperate, and often "choke", when faced with older variants of the protocol.

In order to combat this, during the switchover to the "urn:xmpp" URN namespace, URNs of the form "urn:xmpp:tmp:foo" were used for Experimental stage protocols. Only on their advancement to Draft would the "tmp" be removed, meaning that non-tmp forms were sure to be the Draft variant.

2.2 Namespace Versioning

Although this prevented Draft/Final protocol implementations from being choked by Experimental protocols, and therefore retained the value of the Draft/Final namespace, it left Experimental protocols with weak interoperability. In particular, since a namespace was certain to change at Draft, implementors were discouraged from implementing Experimental protocols to some extent, and it was observed that the benefit of early implementation experience was harmed.

Namespace versioning was therefore introduced. Each new XEP is, in effect, allocated an arc or tree of the URN namespace, and any time the protocol is changed such that it would fail to interoperate with older versions of the protocol, a new namespace from this arc is minted. To avoid reuse, limit the creativity needed, and provide some indication to implementors of which variant is newer, numbers are used.

However, each new namespace is indeed an entirely new namespace, and not only is no assertion of compatibility made, but it is only when there is either known (or high risk of) incompatibility that a new namespace is minted, so in effect there is an assertion made that the two variants are incompatible. This assertion is, of course, precisely in conformance with the definition (and even spirit) of XML namespaces.

3. Misconceptions

3.1 Namespaces can't be versioned!

With alarming frequency, someone new to the process usually suggests that the XSF has as a body failed to understand that a different namespace - however similar - is an entirely different namespace and thus denotes a different protocol.

It is important to realise that this is precisely the desired effect. When a new namespace is minted by this method, the protocol has been changed in an incompatible way, and implementations cannot make any assumption about what that change might consist of.

3.2 Versions happen on every change

Many changes to specifications do not result in an incompatibility between versions. This can even occur when the wire protocol has actually changed to some degree, if those changes are not observable without optional negotiation for example.

XEP-0053 Section 4, point 2, stipulates that new namespaces are only minted if the new revision is not backwards-compatible with the old.

3.3 Namespaces change on advancement

There is no particular significance to a namespace ending with ':0', and as such, namespaces do not require a change to a ':1' on advancement to Draft. In fact, a specification requiring such a change (due to incompatible changes introduced during Last Call) might be questionable.

3.4 Authors are responsible for namespaces

While it is typically left to authors to change namespaces, this is technically an XMPP Registrar function. How the Registrar decides whether to create a new namespace by incrementing the version number is unspecified; they might choose to consult with Council or the community.

4. Internationalization Considerations

Namespace versions are an internal protocol feature, therefore i18n has no impact.

5. Security Considerations

Any attempt to handle unknown protocol namespaces by heuristically treating them as another protocol namespace - including especially other versions of the same protocol namespace root - may expose an attack surface.

6. IANA Considerations

This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [1].

7. XMPP Registrar Considerations

The XMPP Registrar [2] MAY sleep easier, knowing that people understand these things finally.


Appendix A: Document Information

XMPP Standards Foundation
Last Updated
Approving Body
XMPP Council
Superseded By
Short Name
Source Control

This document in other formats: XML  PDF

Appendix B: Author Information

Dave Cridland
Peter Saint-Andre


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.

Disclaimer of Warranty

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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.

IPR Conformance

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).

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Appendix D: Relation to XMPP

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.

Appendix E: Discussion Venue

The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.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 <editor@xmpp.org>.

Appendix F: Requirements Conformance

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".

Appendix G: Notes

1. 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/>.

2. 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/>.

Appendix H: Revision History

Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/

  1. Version 0.0.2 (2015-04-07)

    Noticed two conversations in two weeks. Must be time to reify this one. Added two new misconceptions; the author vs registrar one was mine.

  2. Version 0.0.1 (2011-05-19)

    After the third person in a single month thought that namespace versioning is a bad idea, wrote this background informational XEP to explain the rationale behind XEP-0053§4.