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Proposed Stream Feature Improvements This document proposes improvements to the XML stream features definition for inclusion in the specification that supersedes RFC 3920. &LEGALNOTICE; 0192 Draft Standards Track Standards XMPP Core None None N/A &stpeter; 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 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, and the recommendations described herein have been incorporated into &rfc6120;.

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 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 ¬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 &xep0170;. (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, support for the server dialback protocol is currently adverised through inclusion of a dialback namespace prefix in the stream header:

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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:

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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.

No interaction with &IANA; is required as a result of this document.

The ®ISTRAR; includes a dialback stream feature of 'urn:xmpp:features:dialback' in its registry of stream features (see &STREAMFEATURES;).

The submission is as follows:

urn:xmpp:features:dialback dialback dialback Support for Server Dialback XEP-0220 ]]>

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

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

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

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Thanks to Ralph Meijer and Joe Hildebrand for their comments.