Proposed Stream Feature Improvements This document proposes improvements to the XML stream features definition for inclusion in the specification that supersedes RFC 3920. This XMPP Extension Protocol is copyright © 1999 – 2024 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). 0192 Obsolete Standards Track Standards Council XMPP Core RFC 6120 N/A Peter Saint-Andre stpeter@stpeter.im stpeter@jabber.org https://stpeter.im/ 1.2 2012-02-08 psa

Per a vote of the XMPP Council, changed status from Deprecated to Obsolete.

 1.1 2011-05-11 psa

Per a vote of the XMPP Council, changed specification to Deprecated because the recommendations described in this document were not incorporated into RFC 6120.

 1.0 2007-01-17 psa

Per a vote of the XMPP Council, advanced specification to Draft; XMPP Registrar assigned urn:xmpp:features:dialback as namespace for dialback stream feature.

 0.2 2006-12-20 psa

Removed session establishment examples and text; specified that namespace for dialback stream feature shall be issued by the XMPP Registrar.

 0.1 2006-08-16 psa

Initial version.

 0.0.1 2006-08-10 psa

First draft.

RFC 3920 RFC 3920: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc3920>. introduced the concept of stream features. Implementation experience has revealed several shortcomings in the current definition and usage of stream features:

Those shortcomings are addressed in this document.

Note: The recommendations from this document were NOT incorporated into RFC 6120 RFC 6120: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc6120>. and this document is Obsolete.

The XMPP stream feature for Transport Layer Security (TLS) includes a <required/> child element that can be used to indicate that negotiation of TLS must be completed before proceeding with the rest of the stream negotiation. However, as defined in RFC 3920 RFC 3920: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc3920>. the remaining stream features do not include the ability to flag that negotiation of the feature is required in order to (1) proceed with the negotiation or (2) begin sending XML stanzas. Because the non-TLS features lack a required flag, it is not possible for the initiating entity to know definitively how to proceed at any given stage in the stream negotiation, and the only way for the initiating entity to know whether it may begin sending XML stanzas is to attempt to send them (the receiving entity will return a <not-authorized/> stream error if not all required features have been negotiated). This state of affairs is suboptimal. Therefore, every stream feature must include the ability to flag the feature as required or not required. When the initiating entity receives a stream features element with no features containing a <required/> element, it knows thatt the receiving party will accept XML stanzas over the stream.

The following examples show a possible flow of stream negotiation between a client and a server, using the required flag for all but one of the features and following the order specified in Recommended Order of Stream Feature Negotiation (XEP-0170) XEP-0170: Recommended Order of Stream Feature Negotiation <https://xmpp.org/extensions/xep-0170.html>.. (This example is more verbose than a typical stream negotiation flow, but is provided here for the sake of completeness.)

S: S: C: S: [TLS negotiation] C: S: S: EXTERNAL DIGEST-MD5 PLAIN C: S: cmVhbG09InNvbWVyZWFsbSIsbm9uY2U9Ik9BNk1HOXRFUUdtMmhoIixxb3A9ImF1dGgi LGNoYXJzZXQ9dXRmLTgsYWxnb3JpdGhtPW1kNS1zZXNzCg== C: dXNlcm5hbWU9InNvbWVub2RlIixyZWFsbT0ic29tZXJlYWxtIixub25jZT0i T0E2TUc5dEVRR20yaGgiLGNub25jZT0iT0E2TUhYaDZWcVRyUmsiLG5jPTAw MDAwMDAxLHFvcD1hdXRoLGRpZ2VzdC11cmk9InhtcHAvZXhhbXBsZS5jb20i LHJlc3BvbnNlPWQzODhkYWQ5MGQ0YmJkNzYwYTE1MjMyMWYyMTQzYWY3LGNo YXJzZXQ9dXRmLTgK S: cnNwYXV0aD1lYTQwZjYwMzM1YzQyN2I1NTI3Yjg0ZGJhYmNkZmZmZAo= C: S: C: S: S: zlib C: zlib S: C: S: S: C: someresource S: somenode@example.com/someresource ]]>

As specified in RFC 3920 RFC 3920: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc3920>., support for the server dialback protocol is currently adverised through inclusion of a dialback namespace prefix in the stream header:

]]>

However, it is not clear if inclusion of the dialback namespace indicates that a server supports the server dialback protocol or that it requires negotiation of server dialback. To make this clear, we define a stream feature for server dialback:

]]>

Consider the following scenario, in which Server1 provides a self-signed certificate. According to Server2's local service policy, it does not consider self-signed certificates to be trustworthy and therefore requires negotiation of server dialback in this case.

S2: S2: S2: [TLS negotiation with self-signed certificate] S1: S2: S2: [Dialback negotiation] ]]>

The improvements described herein do not introduce any new security concerns above and beyond those defined in RFC 3920 RFC 3920: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc3920>..

No interaction with the Internet Assigned Numbers Authority (IANA) 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/>. is required as a result of this document.

As specified in Server Dialback (XEP-0220) XEP-0220: Server Dialback <https://xmpp.org/extensions/xep-0220.html>., the XMPP Registrar 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/>. includes a dialback stream feature of 'urn:xmpp:features:dialback' in its registry of stream features (see <https://xmpp.org/registrar/stream-features.html>).

Note: The following provisional schema is intended to replace the existing schema for the SASL stream feature.

]]>

Note: The following provisional schema is intended to replace the existing schema for the Resource Binding stream feature.

]]>

Note: The following defines a schema for the proposed Server Dialback stream feature.

]]>

Thanks to Ralph Meijer and Joe Hildebrand for their comments.