RFC 3920 introduced the concept of stream features. Implementation experience has revealed several shortcomings in the current definition and usage of stream features:

• Because not all stream features include a mechanism for specifying that negotiation of the feature is required, servers and clients cannot know with certainty when the stream negotiation has been completed and therefore when it is acceptable to begin sending XML stanzas over the stream.
• The server dialback protocol does not have a stream feature associated with it.

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.)

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:

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.

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.

Thanks to Ralph Meijer and Joe Hildebrand for their comments.