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Instant Stream Resumption This specification introduces an mechanism for instant stream resumption, based on Stream Management (XEP-0198), allowing XMPP entities to instantaneously resume an XMPP stream. This XMPP Extension Protocol is copyright (c) 1999 - 2016 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 shall the XMPP Standards Foundation or the authors of this Specification be liable for any claim, damages, or other liability, whether in an action of contract, tort, or otherwise, arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification. ## 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 out of the use or inability to use 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 may be found at <http://xmpp.org/extensions/ipr-policy.shtml> or obtained by writing to XSF, P.O. Box 1641, Denver, CO 80201 USA). xxxx ProtoXEP Standards Track Standards Council XMPP Core XEP-0198 isr Florian Schmaus flo@geekplace.eu flo@geekplace.eu 0.0.1 2016-02-12 fs

First draft.

This XEP specifies an instant stream resumption mechanism based on &xep0198;, allowing XMPP entities to instantaneously resume an XMPP stream. This can be seen as the complementary part to &xep0305; allowing for fast XMPP session (re-)establishment.

Compared to the existing stream resumption mechanism of XEP-0198 § 5, the approach defined herein reduces the round trips required to resume a stream to exactly one (besides the round trips required by the TLS handshake). This is achieved by using only a secure token to resume the stream.
ISR
Instant Stream Resumption.
Instant Stream Resumption Token (ISR Token)
A string token with at least 128 bits of entropy generated by a cryptographically secure random number generator.

If an entity supports ISR, then the <enabled/> NonzaXEP-0360: Nonzas (are not Stanzas) <https://xmpp.org/extensions/xep-0360.html>., which is send as positive reply upon a request to enable Stream Management, MUST contain an 'tok' attribute qualified by the 'urn:xmpp:isr:0' namespace containing a ISR Token. The Nonza MAY also contain a 'location' attribute qualified by the 'urn:xmpp:isr:0' namespace which specifies the preferred IP address or hostname, and a TCP port number of the host which should be used for instant stream resumption.

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In order to instantaneously resume an XMPP stream the entity trying to do so must posses a valid ISR token. If it then needs to perform ISR, it first determines the host for resumption, and after that, tries to perform the instant stream resumption.

The lookup mechanism order to determine host candidates for ISR resumption is as follows:

  1. The host specified in the optional 'location' attribute qualified by the 'urn:xmpp:isr:0' namespace found in the <enabled/> element of XEP-0198.
  2. The hosts determined by means of &xep0368;.
  3. The host announced in the 'location' attribute of the <enabled/> Nonza defined in XEP-0198.
  4. Standard host lookup mechanisms.

The host candidates retrieved by those mechanisms SHOULD be tried by the initiating entity in this order.

Note that the hosts announced by the 'location' attribute qualified by the 'urn:xmpp:isr:0' namespace MUST be connected to using Transport Layer Security (TLS, see &rfc5246;) from the beginning, i.e. <starttls/> MUST NOT be used, instead the TLS Handshake is performed right after establishing the connection.

The order prefers hosts which allow connections where TLS is enabled from the beginning. This is desirable in order to reduce the required round trips by skipping the <starttls/> step.

After the host on which the instant stream resumption should be performed was determined, the entity connects to, and establishes TLS by either

  1. establishing a TLS session right away, or
  2. performing STARTTLS (&rfc6120; § 5).

After the connection has been secured, the initiating entity sends an XMPP <stream> open element followed by a <instant-resume/> Nonza qualified by the 'urn:xmpp:isr:0' namespace which MUST contain the ISR token in the 'tok' attribute and the sequence number of the last by Stream Management handled stanza in the 'h' attribute.

Note that the initiating entity SHOULD pipeline the instant stream resumption request together with then initial <stream> open element since it already has determined that the service supports this feature. Servers MUST announce that they support ISR by including an <isr/> element qualified by the 'urn:xmpp:isr:0' namespace in their stream features.

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ISR MUST only be performed over TLS secured sessions. What follows is that the ISR feature MUST only be announced after STARTTLS has been successfully performed or on streams where TLS was established from the beginning.

On success the server replies with <inst-resumed/> Nonza which MUST contain a new ISR Token found in the 'tok' attribute and the sequence number of the last by Stream Mangement handled stanza in the 'h' attribute.

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After the <inst-resumed/> was received both entities MUST consider the resumed stream to re-established. This includes all previously negotiated stream features like &xep0138;. It does however not include the specific state of the features: For example in case of stream compression, the dictionary used by the compression mechanism of the resumed stream MUST NOT be considered to be restored after instant stream resumption.

If the server is unable to resume the stream instantly it MUST reply with a <failed/> Nonza qualified by the 'urn:xmpp:isr:0' namespace.

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The server MAY also include a 'h' attribute in the <failed/> element indicating the number of handled stanzas.

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Instant stream resumption errors SHOULD be considered recoverable, the initiating entity MAY continue with normal session establishment; however, misuse of stream management MAY result in termination of the stream.

It is of vital importance that the Instant Stream Resumption Token is generated by a cryptographically secure random generator. See &rfc4086; for more information about Randomness Requirements for Security

This document requires no interaction with &IANA;.

The ®ISTRAR; includes 'urn:xmpp:isr:0' in its registry of protocol namespaces (see &NAMESPACES;).

TODO: Add after the XEP leaves the 'experimental' state.

Thanks to Jonas Wielicki for his feedback.