During its formalization of the core Jabber protocols, the IETF's XMPP WG introduced the concept of XML stream features. While the order in which features shall be negotiated is clearly defined for the features specified in RFC 3920 [1] and RFC 3921 [2], the number of possible features is open-ended (which is why the XMPP Registrar [3] maintains a registry of stream features). This document specifies the recommended order for negotiation of various stream features.
The XMPP RFCs define an ordering for the features defined therein, namely:
That order MUST be followed if no other stream features are negotiated.
Stream Compression (XEP-0138) [4] is negotiated when it is not possible to set up TLS compression for whatever reason. It seems safest to negotiate stream compression after negotiation of both TLS (to safely complete the negotiation) and SASL (to prevent certain denial-of-service attacks caused by consumption of server resources for compression before the connecting entity is authenticated). Therefore the following order is RECOMMENDED:
The In-Band Registration (XEP-0077) [5] protocol can be used to establish an account before logging in. That step would be completed before SASL because an entity cannot authenticate if it does not first create an account. Therefore the following order is RECOMMENDED:
If both stream compression and in-band registration are negotiated, the following order is RECOMMENDED:
The XMPP RFCs define an ordering for the features defined therein, namely:
That order MUST be followed if no other stream features are negotiated.
RFC 3920 requires SASL negotiation after TLS negotiation. When the certificate presented by the initiating entity makes reference to a trusted root certification authority, SASL negotiation provides meaningful authentication. In that case, the order shown above is recommended.
However, it is possible that the initiating entity will present a self-signed certificate or a certificate whose associated root certification authority is not trusted by the receiving entity. In this situation, service provisioning policies at the receiving entity may dictate the use of server dialback in order to provide a stronger level of trust for the server-to-server stream (where such trust is essentially trust in the underlying Domain Name System), even though server dialback explicitly does not provide authentication. In this case, the following order is RECOMMENDED:
Stream Compression (XEP-0138) [4] is negotiated when it is not possible to set up TLS compression for whatever reason. It seems safest to negotiate stream compression after negotiation of both TLS (to safely complete the negotiation) and SASL (to prevent certain denial-of-service attacks). Therefore the following order is RECOMMENDED:
If stream compression is negotiated in addition to TLS and dialback, it is RECOMMENDED to negotiate it after both TLS and dialback:
The order of negotiated stream features has security implications and may be security-critical. In particular, TLS MUST be negotiated first.
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [6].
This document requires no interaction with the XMPP Registrar.
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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 <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>.
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".
1. RFC 3920: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc3920>.
2. RFC 3921: Extensible Messaging and Presence Protocol (XMPP): Instant Messaging and Presence <http://tools.ietf.org/html/rfc3921>.
3. 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/>.
4. XEP-0138: Stream Compression <https://xmpp.org/extensions/xep-0138.html>.
5. XEP-0077: In-Band Registration <https://xmpp.org/extensions/xep-0077.html>.
6. 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/
Per a vote of the XMPP Council, advanced status to Active.
Per Council feedback, removed section on deprecated jabber:iq:auth protocol.
Modified recommended order of for server-to-server negotiations if service provisioning requires dialback after TLS negotiation.
Split into client-to-server and server-to-server sections; specified recommended order of server-to-server negotiations.
Changed order of SASL and stream compression to reflect list discussion.
Initial version.
First draft.
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