Although XMPP Core [1] specifies the use of TCP as the method of connecting to an XMPP server, alternative connection methods exist, including the BOSH (XEP-0124) [2] method (for which XMPP Over BOSH (XEP-0206) [3] is the XMPP profile) and the websocket subprotocol specified in RFC 7395 [4]. For some of these methods, it is necessary to discover further parameters before connecting, such as the HTTP URL of an alternative connection manager. Without ways to auto-discover alternative connection methods, the relevant information would need to be provided manually by a human user (which is cumbersome and error-prone) or hard-coded into XMPP software applications (which is brittle and not interoperable).
This document defines a way to encapsulate information about alternative connection methods for auto-discovery via Link entries in a server's "host-meta" file.
The HTTP lookup method uses Web Host Metadata RFC 6415 [5] to categorize and list the URIs of alternative connection methods. It is primarily intended for use by clients in environments where the ability to perform DNS queries is restricted, such as in web browsers.
Each alternative connection method is specified in the host-meta (XRD) file using a distinctive link relation RFC 5988 [6]. This specification defines several extension relation types:
The following business rules apply:
The following examples show two host-meta link records: the first indicates support for the XMPP Over BOSH connection method defined in XEP-0124 and XEP-0206 and the second indicates support for the XMPP Over WebSocket connection method defined in RFC 7395 [4].
<?xml version='1.0' encoding='utf-8'?>
<XRD xmlns='http://docs.oasis-open.org/ns/xri/xrd-1.0'>
...
<Link rel="urn:xmpp:alt-connections:xbosh"
href="https://web.example.com:5280/bosh" />
<Link rel="urn:xmpp:alt-connections:websocket"
href="wss://web.example.com:443/ws" />
...
</XRD>
It is possible to use additionally a JSON-based format for host-meta information. The JSON representation of the host metadata is named JRD and specified in Appendix A of RFC 6415 [5]. The above XRD example would be presented in JRD as:
{
...
"links": [
...
{
"rel": "urn:xmpp:alt-connections:xbosh",
"href": "https://web.example.com:5280/bosh"
},
{
"rel": "urn:xmpp:alt-connections:websocket",
"href": "wss://web.example.com:443/ws"
}
]
}
To make connection discovery work in web clients (including those hosted on a different domain) the host service SHOULD set appropriate CORS headers for Web Host Metadata files. The exact headers and values are out of scope of this document but may include: Access-Control-Allow-Origin, Access-Control-Allow-Methods and Access-Control-Allow-Headers.
Due care has to be exercised in limiting the scope of Access-Control-Allow-Origin response header to Web Host Metadata files only.
Access-Control-Allow-Origin: *
Access-Control-Allow-Origin header with a value of * allows JavaScript code running on a different domain to read the content of Web Host Metadata files. Special value * ensures that the request will only succeed if it is invoked without user credentials (e.g. cookies, HTTP authentication).
It is possible that advertisement of alternative connection methods can introduce security vulnerabilities, since a connecting entity (usually a client) might deliberately seek to connect using the method with the weakest security mechanisms (e.g., no channel encryption or relatively weak authentication). Care needs to be taken in determining which alternative connection methods are appropriate to advertise or implement in your lookup.
Entities that use these connection methods MUST only fetch host-meta over Transport Layer Security (TLS), and MUST only use 'https' or 'wss' URLs that are protected using TLS.
A previous version of this XEP defined a DNS method to look up this info using a TXT _xmppconnect record, this was insecure and has been removed.
Because the link relations specified here are extension relation types rather than registered relation types (see Section 4 of RFC 5988), this document requires no interaction with the Internet Assigned Numbers Authority (IANA) [7].
The XMPP Registrar [8] shall include 'urn:xmpp:alt-connections' in its registry of protocol namespaces (see <https://xmpp.org/registrar/namespaces.html>).
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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 <https://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. XEP-0124: Bidirectional-streams Over Synchronous HTTP <https://xmpp.org/extensions/xep-0124.html>.
3. XEP-0206: XMPP Over BOSH <https://xmpp.org/extensions/xep-0206.html>.
4. RFC 7395: An Extensible Messaging and Presence Protocol (XMPP) Subprotocol for WebSocket <http://tools.ietf.org/html/rfc7395>.
5. RFC 6415: Web Host Metadata <http://tools.ietf.org/html/rfc6415>.
6. RFC 5988: Web Linking <http://tools.ietf.org/html/rfc5988>.
7. 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/>.
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 https://xmpp.org/extensions/attic/
Remove DNS _xmppconnect method due to security vulnerability.
Include all described formats in the abstract, instead of just mentioning the TXT record format.
Fix reference to RFC 6415 and organize requirements more clearly. This raises the JSON requirement from MAY (OPTIONAL) to SHOULD (effectively), to accustom web-based software.
Add information about CORS header usage and requirements
Make JSON example less error-prone.
Fix XML header on example.
Replace references to draft-ietf-xmpp-websocket with RFC7395 (XMPP over WebSocket).
Fix a small typo in one of the examples (UTF-9 encoding).
Defined HTTP lookup methods using well-known URIs as specified in RFC 5785.
Per a vote of the XMPP Council, advanced status to Draft; XMPP Registrar assigned alt-connections shortname and created appropriate registry.
Updated to reflect renaming of HTTP Binding to XMPP Over BOSH.
Finally and definitively removed _xmpp-client-tcp and _xmpp-server-tcp attributes since clients and servers should use either SRV records or standard XMPP ports (5222 or 5269).
Removed _xmpp-client-tcpssl attribute since use of the old-style SSL-only port is discouraged.
Added _xmpp-client-tcpssl for old-style SSL connections; added discussion of IETF U-NAPTR technology.
Clarified order of lookups; restored _xmpp-client-tcp and added _xmpp-server-tcp as optional records if SRV is not supported or accessible.
Removed _xmpp-client-tcp from TXT records (belongs in SRV records only).
Initial version.
More fully specified the rationale for using DNS TXT records.
Added security considerations and registrar considerations.
First draft.
@report{hildebrand2005alt-connections,
title = {Discovering Alternative XMPP Connection Methods},
author = {Hildebrand, Joe and Saint-Andre, Peter and Stout, Lance},
type = {XEP},
number = {0156},
version = {1.4.0},
institution = {XMPP Standards Foundation},
url = {https://xmpp.org/extensions/xep-0156.html},
date = {2005-08-23/2022-02-10},
}END