Establishing an XMPP session requires a fairly large number of round trips between the initiating entity and the receiving entity. In many deployment scenarios, it would be helpful to reduce the number of round trips and, in general, the time needed to establish a session. This document defines protocols and best practices to do just that.

Note: Various parts of this document might be moved to separate documents at some point.

In accordance with &rfc6120;, before attempting to establish a stream the initiating entity needs to determine the IP address and port at which to connect, usually by means of DNS lookups as described in Section 3.2 of RFC 6120. Implementations SHOULD cache the results of DNS lookups in order to avoid this step whenever possible.

XMPP applications SHOULD cache whatever information they can about the peer, especially stream features data and &xep0030; information. To facilitate such caching, servers SHOULD include &xep0115; data in stream features as shown in Section 6.3 of XEP-0115. Note that for maximum benefit the server MUST include all of the stream features it supports in its replies to "disco#info" queries (i.e., not advertise such features only during stream establishment).

XMPP clients SHOULD cache roster information, and servers SHOULD make such caching possible, using &xep0237; as subsequently included in Section 2.1.1 of &rfc6121;.

One method of speeding the connection process is pipelining of requests, as in &rfc2920; and the QUICKSTART extension proposed for SMTP (&smtpquickstart;). The application of similar principles to XMPP was originally suggested by Tony Finch in February 2008 <http://mail.jabber.org/pipermail/standards/2008-February/017966.html>.

In essence, pipelining relies on two assumptions:

1. The parties to a stream can proactively send multiple XMPP-related "commands" in a single TCP packet (as one simple example, the receiving entity can send both the response stream header and stream features in a single packet).
2. The features that the receiving entity supports (e.g., stream features and SASL mechanisms) are stable over time, which means the initiating entity can assume support for certain features and send certain XMPP-related commands without discovering again that the receiving entity supports them.

Together, these assumptions enable the parties to reduce the number of round trips needed to complete the stream negotiation process.

If an XMPP server supports pipelining, it MUST advertise a stream feature of <pipelining xmlns='urn:xmpp:features:pipelining'/>.

If both parties support pipelining, they can proceed as follows (the examples use the XML from Section 9.1 of RFC 6120 for the client-server stream establishment, but the same principles apply to server-to-server streams).

In Step 1, the client assumes that the server supports the XMPP STARTTLS extension so it pipelines its initial stream header, the <starttls/> command, and the TLS ClientHello message.

In Step 2, the server pipelines its response stream header, stream features advertisement, STARTTLS <proceed/> response, and TLS ServerHello messages (which might include ServerHello, Certificate, ServerKeyExchange, CertificateRequest, and ServerHelloDone -- see &rfc5246; for details).

In Step 3, the parties complete the TLS negotiation.

In Step 4, the server knows that the client will need to restart the stream so it proactively attaches its response stream header and stream features after the TLS Finished message.

In Step 5, the client pipelines its initial stream header with the command for initiating the SASL authentication process (including SASL "initial response" data as explained in Section 6.3.10 of RFC 6120 to reduce the number of round trips).

At this point the client and server might exchange multiple SASL-related messages, depending on the SASL mechanism in use. This specification does not attempt to reduce the number of round trips involved in the challenge-response sequence.

When the client suspects that it is sending its final SASL response, it SHOULD append an initial stream header and resource binding request.

In Step 8, the server informs the client of SASL success (including "additional data with success" as explained in Section 6.3.10 of RFC 6120 to reduce the number of round trips), sends a response stream header and stream features, and informs the client of successful resource binding.

The XMPP stream negotiation process in RFC 6120 required at least 19 round trips (including 4 for TLS negotiation). With pipelining, the number of round trips is reduced to 8.

Naturally, for typical client-to-server sessions, additional round trips are needed so that the client can gather service discovery information, retrieve the roster, etc. As noted, these steps can be reduced or eliminated by using entity capabilities and roster versioning.

The pain of multiple round trips is magnified if the initiating entity needs to reconnect frequently (e.g., because of intermittent network outages). Although &xep0124; can be used to mitigate the pain, BOSH is not appropriate for all scenarios and is not currently used in others (e.g., server-to-server streams).

The minimize the speed of reconnection, implementations are strongly encouraged to support TLS Session Resumption (&rfc5077;) in addition to the technologies already mentioned.

Reconnection can be further enhanced by using the stream resumption feature described in &xep0198;. XEP-0198 does not legislate exactly when it is safe for the server to allow the client to send the <resume/> request. Clearly, sending it before the stream is encrypted would increase the possibility of replay attacks. However, sending it after TLS negotiation (Step 4 above) but before SASL authentication and resource binding (Steps 5 through 8) would enable the client to begin sending stanzas more quickly. It is a matter of server policy whether to advertise the SM feature after TLS negotiation or only after SASL negotiation.

To follow.

This document requires no interaction with &IANA;.

This specification defines the following XML namespace:

• urn:xmpp:features:pipelining

Upon advancement of this specification from a status of Experimental to a status of Draft, the ®ISTRAR; shall add the foregoing namespace to the registry located at &STREAMFEATURES;, as described in Section 4 of &xep0053;.

Special thanks to Tony Finch for suggesting this work and for providing the initial outline of how pipelining would work. Thanks also to Waqas Hussain, Jehan Pagès, and Kevin Smith for their feedback.