XEP-xxxx: Instant Stream Resumption

This specification introduces a mechanism for instant stream resumption, based on Stream Management (XEP-0198), allowing XMPP entities to instantaneously resume an XMPP stream.
Florian Schmaus
© 1999 – 2020 XMPP Standards Foundation. SEE LEGAL NOTICES.


WARNING: This document has not yet been accepted for consideration or approved in any official manner by the XMPP Standards Foundation, and this document is not yet an XMPP Extension Protocol (XEP). If this document is accepted as a XEP by the XMPP Council, it will be published at <http://xmpp.org/extensions/> and announced on the <standards@xmpp.org> mailing list.
Standards Track
0.0.2 (2016-03-11)
Document Lifecycle
  1. Experimental
  2. Proposed
  3. Draft
  4. Final

1. Introduction

This XEP specifies an instant stream resumption mechanism based on Stream Management (XEP-0198) [1], allowing XMPP entities to instantaneously resume an XMPP stream. This can be seen as the complementary part to XMPP Quickstart (XEP-0305) [2] 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 just a secure key to resume the stream.

2. Glossary

Instant Stream Resumption.
Instant Stream Resumption Key (ISR Key)
A key, represented as string, which contains at least 128 bits of entropy generated by a cryptographically secure random number generator.
Transport Layer Security (RFC 5246 [3]).

3. Obtaining a Instant Stream Resumption Key

If an entity supports ISR, then the <enabled/> Nonza [4], which is send as positive reply upon a request to enable Stream Management, MUST contain an 'key' attribute qualified by the 'urn:xmpp:isr:0' namespace containing a ISR Key. 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.

Example 1. An <enabled/> Nonza with a ISR key
Example 2. An <enabled/> Nonza with a ISR key and location

4. Instant Stream Resumption

In order to instantaneously resume an XMPP stream the initiating entity, which is either an XMPP client or server, must posses a valid ISR key. After it has obtained the ISR key, using the process described in the previous section, it first determines the host for resumption, and after that, tries to perform the instant stream resumption.

4.1 Determing the Host for Resumption

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

  1. The host provided in the optional 'location' attribute qualified by the 'urn:xmpp:isr:0' namespace found in the <enabled/> element of XEP-0198 (the "isr:location").
  2. The hosts determined by means of SRV records for XMPP over TLS (XEP-0368) [5].
  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 TLS from the beginning, i.e. <starttls/> MUST NOT be used, instead the TLS handshake is performed right after establishing the connection.

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

4.2 Performing Instant Stream Resumption

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

  1. establishing a TLS session right away, or
  2. performing STARTTLS (RFC 6120 [6] § 5).

Next, 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 previous stream identifier, the XEP-0198 "SM-ID", in the 'previd' attribute, the sequence number of the last by Stream Management handled stanza in the 'h' attribute and the initiator-hmac as value of at least one <hash/> element as specified by Use of Cryptographic Hash Functions in XMPP (XEP-0300) [7], which are put as child elements under the <hmac/> element.

The initiator-hmac is defined as follows:

initiator-hamc = Base64(HMAC(key, "Initiator" || tls-server-end-point))

The function defined in RFC 2104 [8] is used to compute the HMAC using the hash algorithm specified in the 'algo' attribute of the <hash/> element as the cryptographic hash function H. The ISR Key is used as key of the HMAC. And the bytewise concatnation of the ASCII String "Initiator" and the bytes from tls-server-end-point, which a TLS Channel Binding defined in RFC 5929 [9] § 4, is used a the HMAC text. The resulting bytes of the HMAC function are encoded using Base64 as defined in RFC 4648 [10] § 4 and resulting string is used as text value of the <hash/> element.

Note that the initiating entity SHOULD pipeline the instant stream resumption request together with then initial <stream> open element. The initiating entity is able to do so since it already knows that the service supports ISR because it announced an ISR key. Servers MUST nevertheless announce that they support ISR by including an <isr/> element qualified by the 'urn:xmpp:isr:0' namespace in their stream features.

Example 3. Initiating entity requests instant stream resumption
    <hash xmlns='urn:xmpp:hashes:1' algo='sha256'>

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.

4.2.1 Successful Stream Resumption

On success the server replies with <inst-resumed/> Nonza which MUST contain a new ISR Key found in the 'key' attribute, the sequence number of the last by Stream Mangement handled stanza in the 'h' attribute and the 'responder-hmac' as value of the <hash/> element being a child of the <hamc/> element.

The responder-hmac is defined as follows:

responder-hmac = Base64(HMAC(key, "Responder" || tls-server-end-point))

That is, it is the same as the initiator-hamc, but instead of using the ASCII string "Initiator", the ASCII string "Responder" is used.

The initiating entity is required to verify the responder-hmac achieve mutual authentication.

Example 4. Server acknowledges instant stream resumption
 <isr xmlns='urn:xmpp:isr:0'/>
    <hash xmlns='urn:xmpp:hashes:1' algo='sha-256'>

After the <inst-resumed/> was received and has been verified both entities MUST consider the resumed stream to be re-established. This includes all previously negotiated stream features like Stream Compression (XEP-0138) [11]. 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.

4.2.2 Failed Stream Resumptino

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.

Example 5. Server indicates instant stream resumption failure

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

Example 6. Server indicates failure with handled stanzas count

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.

5. Security Considerations

It is of vital importance that the Instant Stream Resumption Key is generated by a cryptographically secure random generator. See RFC 4086 [12] for more information about Randomness Requirements for Security

6. IANA Considerations

This document requires no interaction with the Internet Assigned Numbers Authority (IANA) [13].

7. XMPP Registrar Considerations

The XMPP Registrar [14] includes 'urn:xmpp:isr:0' in its registry of protocol namespaces (see <https://xmpp.org/registrar/namespaces.html>).

8. XML Schema

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

9. Acknowledgements

Thanks to Jonas Wielicki and Thijs Alkemade for their feedback.


Appendix A: Document Information

XMPP Standards Foundation
Standards Track
Last Updated
Approving Body
XMPP Council
XMPP Core, XEP-0198, XEP-0300
Superseded By
Short Name

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Appendix B: Author Information

Florian Schmaus


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.

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## 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. ##

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IPR Conformance

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Appendix D: Relation to XMPP

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.

Appendix E: Discussion Venue

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>.

Appendix F: Requirements Conformance

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".

Appendix G: Notes

1. XEP-0198: Stream Management <https://xmpp.org/extensions/xep-0198.html>.

2. XEP-0305: XMPP Quickstart <https://xmpp.org/extensions/xep-0305.html>.

3. RFC 5246: The Transport Layer Security (TLS) Protocol Version 1.2 <http://tools.ietf.org/html/rfc5246>.

4. XEP-0360: Nonzas (are not Stanzas) <https://xmpp.org/extensions/xep-0360.html>.

5. XEP-0368: SRV records for XMPP over TLS <https://xmpp.org/extensions/xep-0368.html>.

6. RFC 6120: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc6120>.

7. XEP-0300: Use of Cryptographic Hash Functions in XMPP <https://xmpp.org/extensions/xep-0300.html>.

8. RFC 2104: HMAC: Keyed-Hashing for Message Authentication <http://tools.ietf.org/html/rfc2104>.

9. RFC 5929: Channel Bindings for TLS <http://tools.ietf.org/html/rfc5929>.

10. RFC 4648: The Base16, Base32, and Base64 Data Encodings <http://tools.ietf.org/html/rfc4648>.

11. XEP-0138: Stream Compression <https://xmpp.org/extensions/xep-0138.html>.

12. RFC 4086: Randomness Requirements for Security <http://tools.ietf.org/html/rfc4086>.

13. 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/>.

14. 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/>.

Appendix H: Revision History

Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/

  1. Version 0.0.2 (2016-03-11)

    Second draft.

  2. Version 0.0.1 (2016-02-12)

    First draft.