XMPP Core  specifies the use of xmpp-client/xmpp-server SRV records as the method of discovering how to connect to an XMPP server. This XEP extends that to include new xmpps-client/xmpps-server SRV records pointing to direct TLS ports and combine priorities and weights as if they were a single SRV record similar to RFC 6186 . It also provides an easy way for clients to bypass restrictive firewalls that only allow HTTPS, for servers to host multiple protocols/services on a single port, and for servers and clients to take advantage of less round trips and existing direct TLS loadbalancers.
The following format for DNS SRV resource records is specified in RFC 2782 :
XMPP Core  defines SRV records only where 'service' is 'xmpp-client' and 'xmpp-server'. This document specifies to additionally look up records where 'service' is 'xmpps-client' and 'xmpps-server'. This document specifies that the following additional rules apply:
For server operators, this provides a way to host multiple services on the same port, especially when SNI or ALPN extensions are used. Servers could even host xmpp-client and xmpp-server services on the same TLS port. For clients, this provides a virtually zero overhead way to bypass restrictive firewalls that only allow HTTP over port 80 and HTTPS over port 443, as XMPP-over-TLS is indistinguishable from HTTP-over-TLS when ALPN is not used. For clients and servers, direct TLS saves round trips vs STARTTLS.
The only overhead is the single additional SRV lookup. All clients that support STARTTLS already have support for direct TLS.
Server operators should not expect multiplexing (via ALPN) to work in all scenarios and therefore should provide additional SRV record(s) that do not require multiplexing (either standard STARTTLS or dedicated direct XMPP-over-TLS). This is a result of relying on ALPN for multiplexing, where ALPN might not be supported by all devices or may be disabled by a user due to privacy reasons.
Direct TLS provides AT LEAST the same level of security as STARTTLS, and more privacy without ALPN as using STARTTLS leaks that the underlying protocol is XMPP, while any direct TLS stream should be indistinguishable from any other direct TLS stream. Direct TLS provides more security than STARTTLS if RFC 7590  is not followed, as it isn't subject to STARTTLS stripping. All security setup and certificate validation code SHOULD be shared between the STARTTLS and direct TLS logic as well. All SRV-based connection methods are subject to DNS modification/stripping/spoofing of SRV records in the absence of DNSSEC.
ALPN (RFC 7301 ) requires registration of new Protocol IDs. This document specifies two Protocol IDs:
Protocol: XMPP jabber:client namespace
0x78 0x6d 0x70 0x70 0x2d 0x63 0x6c 0x69 0x65 0x6e 0x74 ("xmpp-client")
Reference: [SRV records for XMPP over TLS (XEP-0368) ]
Protocol: XMPP jabber:server namespace
0x78 0x6d 0x70 0x70 0x2d 0x73 0x65 0x72 0x76 0x65 0x72 ("xmpp-server")
Reference: [SRV records for XMPP over TLS (XEP-0368) ]
The ALPN registry is currently located here.
Issues with direct TLS ports mentioned in RFC 2595  Section 7 do not apply here for these reasons:
This document requires no interaction with the XMPP Registrar .
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Advance to draft as approved by the XMPP Council.
Implement more last call comments, editorial changes, mixing SRV records is now SHOULD.
Implement last call comments, editorial changes, SNI is now MUST.
Initial published version approved by the XMPP Council.
tls proto to tcp, change service instead. Set SNI correctly.