XMPP Core [1] 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 [2]. 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 [3]:
_service._proto.name. TTL class SRV priority weight port target.
XMPP Core [1] defines SRV records only where 'service' is 'xmpp-client' and 'xmpp-server'. This document specifies two 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.
If the _xmpps-client (or _xmpps-server) target is set to . (dot), this indicates as per RFC 2782 [3] that the service is not provided for the given domain. In this context, this means that Direct TLS is not supported. In this case, the initiating party SHOULD look up _xmpp-client (or _xmpp-server) records. The initiating party MUST NOT perform A/AAAA fallback as per RFC 6120 [1] (since the service provider has already indicated that the SRV protocol is supported).
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 [5] 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 [4]) requires registration of new Protocol IDs. This document specifies two Protocol IDs:
Protocol: XMPP jabber:client namespace
Identification Sequence:
0x78 0x6d 0x70 0x70 0x2d 0x63 0x6c 0x69 0x65 0x6e 0x74 ("xmpp-client")
Reference: [SRV records for XMPP over TLS (XEP-0368) [6]]
Protocol: XMPP jabber:server namespace
Identification Sequence:
0x78 0x6d 0x70 0x70 0x2d 0x73 0x65 0x72 0x76 0x65 0x72 ("xmpp-server")
Reference: [SRV records for XMPP over TLS (XEP-0368) [6]]
The ALPN registry is currently located here.
Issues with direct TLS ports mentioned in RFC 2595 [7] Section 7 do not apply here for these reasons:
This document requires no interaction with the XMPP Registrar [8].
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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.
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.
Given that this XMPP Extension Protocol normatively references IETF technologies, discussion on the <xsf-ietf@xmpp.org> list might also be appropriate.
Errata can be sent to <editor@xmpp.org>.
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".
1. RFC 6120: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc6120>.
2. RFC 6186: Use of SRV Records for Locating Email Submission/Access Services <http://tools.ietf.org/html/rfc6186>.
3. RFC 2782: A DNS RR for specifying the location of services (DNS SRV) <http://tools.ietf.org/html/rfc2782>.
4. RFC 7301: Transport Layer Security (TLS) Application-Layer Protocol Negotiation Extension <https://tools.ietf.org/html/rfc7301>.
5. RFC 7590: Use of Transport Layer Security (TLS) in the Extensible Messaging and Presence Protocol (XMPP) <http://tools.ietf.org/html/rfc7590>.
6. XEP-0368: SRV records for XMPP over TLS <https://xmpp.org/extensions/xep-0368.html>.
7. RFC 2595: Using TLS with IMAP, POP3 and ACAP <http://tools.ietf.org/html/rfc2595>.
8. 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/>.
Note: Older versions of this specification might be available at http://xmpp.org/extensions/attic/
Describe how to fall back if _xmpps-server/_xmpps-client records cannot be found.
Fix trivial 'to' vs. 'two' typo.
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.
First draft.
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