This XEP specifies an instant stream resumption mechanism based on Stream Management (XEP-0198) , allowing XMPP entities to instantaneously resume an XMPP stream. This can be seen as the complementary part to XMPP Quickstart (XEP-0305)  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.
If an entity supports ISR, then the <enabled/> Nonza , 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.
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.
The lookup mechanism order to determine host candidates for ISR resumption is as follows:
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.
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
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) , 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  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  § 4, is used a the HMAC text. The resulting bytes of the HMAC function are encoded using Base64 as defined in RFC 4648  § 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.
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 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.
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) . 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.
The server MAY also include a 'h' attribute in the <failed/> element indicating the number of stanzas it has handled so far.
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 Key is generated by a cryptographically secure random generator. See RFC 4086  for more information about Randomness Requirements for Security
This document requires no interaction with the Internet Assigned Numbers Authority (IANA) .
The XMPP Registrar  includes 'urn:xmpp:isr:0' in its registry of protocol namespaces (see <https://xmpp.org/registrar/namespaces.html>).
TODO: Add after the XEP leaves the 'experimental' state.
Thanks to Jonas Wielicki and Thijs Alkemade for their feedback.
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