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0.4

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Peter Saint-Andre 2007-01-29 16:19:27 +00:00
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xep-0156.xml
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@ -10,7 +10,7 @@
<abstract>This document defines a DNS TXT Resource Record format for use in specifying methods of connecting to an XMPP server.</abstract>
&LEGALNOTICE;
<number>0156</number>
<status>Deferred</status>
<status>Experimental</status>
<type>Standards Track</type>
<sig>Standards</sig>
<approver>Council</approver>
@ -23,6 +23,12 @@
<shortname>N/A</shortname>
&hildjj;
&stpeter;
<revision>
<version>0.4</version>
<date>2007-01-29</date>
<initials>psa</initials>
<remark><p>Added _xmpp-client-tcpssl for old-style SSL connections; discussed potential for using U-NAPTR.</p></remark>
</revision>
<revision>
<version>0.3</version>
<date>2005-05-16</date>
@ -61,15 +67,16 @@
</revision>
</header>
<section1 topic='Introduction' anchor='intro'>
<p>Although &rfc3920; specifies the use of TCP as the method of connecting to an XMPP server, other connection methods are possible. These include the older &xep0025; method, the more recent &xep0124; method, and less common methods such as &wap;. For some of these methods, it is necessary to discover further parameters before connecting, such as the HTTP URL of an alternative connection manager. Currently, if a client application needs to discover connection methods before connecting to an XMPP service, the relevant information must be provided manually by a human user, which is cumbersome and error-prone. Thankfully, there are several potential ways to complete this pre-connection service discovery in an automated fashion:</p>
<p>Although &rfc3920; specifies the use of TCP as the method of connecting to an XMPP server, other connection methods are possible. These include the &xep0124; method, the &xep0025; method (now deprecated), and less common methods such as &wap;. For some of these methods, it is necessary to discover further parameters before connecting, such as the HTTP URL of an alternative connection manager. Currently, if a client application needs to discover connection methods before connecting to an XMPP service, the relevant information must be provided manually by a human user, which is cumbersome and error-prone. Thankfully, there are several potential ways to complete this pre-connection service discovery in an automated fashion:</p>
<ol>
<li><p>Specify a &w3wsdl; definition (or other XML file format) and a canonical URL for that definition at a domain that offers XMPP services. Unfortunately, this approach requires access to the HTTP server for the domain (and quite possibly to the root directory thereof), which may be difficult for XMPP server administrators to arrange. In addition, it requires a client to retrieve the relevant file via HTTP before performing DNS lookups and XMPP connection; it would be more efficient to use recognized DNS methods since DNS lookups are already required by <cite>RFC 3920</cite>.</p></li>
<li><p>Specify a way to define the required service discovery information as part of the existing DNS SRV records (see &rfc2782;) for a domain that offers XMPP services. While this approach sounds promising, it is not feasible since the DNS SRV Target field can be used only to specify domain names and cannot be used to specify full URIs (such as the URL for an HTTP connection manager).</p></li>
<li><p>Specify a way to define the required service discovery using the "straightforward NAPTR" (S-NAPTR) profile of the Dynamic Delegation Discovery System (see &rfc3958; and &rfc3401;). Unfortunately, S-NAPTR also does not allow inclusion of full URIs, and thus does not meet the requirements for discovery of XMPP connection methods.</p></li>
<li><p>Specify a way to define the required service discovery using a new profile of the Dynamic Delegation Discovery System, which would be nearly identical to S-NAPTR except that it would specify only one app-service (most likely "xmpp") and allow inclusion of the DDDS "U" flag (see &rfc3404;), so that the output of a DDDS rule could be a URI. While this is a valid approach that is worth pursuing, the authors are concerned about the deployability of such an approach, especially for client-side applications (the main focus of this specification).</p></li>
<li><p>Specify a way to define the required service discovery using the "URI-enabled NAPTR" (U-NAPTR) profile of the Dynamic Delegation Discovery System (see &unaptr;). While this is a valid approach that is worth pursuing, the authors are concerned about the deployability of such an approach given the rarity of support for DDDS and U-NAPTR, especially in client-side applications (the main focus of this specification).</p></li>
<li><p>Specify a way to define the required service discovery information via properly-formatted DNS TXT records (see &rfc1464;). While this approach requires an update to the DNS records for the server domain, that is usually necessary in order to establish XMPP services in the first place. Furthermore, although there are some perils to be avoided in the use of DNS TXT records (e.g., wildcards), the technology is well understood and widely deployed (e.g., it is used by the <link url='http://www.openspf.org/'>Sender Policy Framework</link> and <link url='http://www.microsoft.com/mscorp/safety/technologies/senderid/default.mspx'>SenderID</link> email server verification technologies).</p></li>
</ol>
<p>Therefore this document defines a way to encapsulate information about XMPP connection methods in DNS TXT resource records.</p>
<p>Therefore this document defines a way to encapsulate information about XMPP connection methods in DNS TXT resource records. However, the authors will monitor the availability of DDDS-based systems and may request that the methods defined in this document be deprecated at some point in the future.</p>
<p>The discovery mechanism specified herein MAY also be used to define the ports at which standard XMPP TCP services are offered; this is especially useful when XMPP service administrators do not have access to SRV records (e.g., because many DNS services do not yet support SRV records). However, any values (e.g., port numbers) presented via SRV records MUST be taken as canonical and MUST supersede values provided via TXT records.</p>
</section1>
<section1 topic='Record Format' anchor='format'>
<p>The following format for DNS TXT resource records is specified in <cite>RFC 1464</cite>:</p>
@ -88,18 +95,18 @@
</section1>
<section1 topic='Business Rules' anchor='bizrules'>
<p>The following business rules apply:</p>
<ol>
<ol start='1'>
<li>TXT lookups MUST be used only as a fallback after the methods specified in <cite>RFC 3920</cite> have been exhausted.</li>
<li>A domain SHOULD NOT present information in DNS TXT records that is available via the DNS SRV records defined in <cite>RFC 3920</cite>. <note>However, a domain MAY present such information in DNS TXT records if SRV records are not supported or accessible. Any values (e.g., port numbers) presented via SRV records MUST be taken as canonical and MUST supersede values provided via TXT records.</note></li>
<li>A domain SHOULD NOT present information in DNS TXT records that is available via the DNS SRV records defined in <cite>RFC 3920</cite>.</li>
<li>The order of DNS TXT records SHOULD NOT be interpreted as significant by the presenting domain or the receiving entity.</li>
</ol>
</section1>
<section1 topic='Examples' anchor='examples'>
<p>The following examples show three DNS TXT resource records: the first indicates support for the httpbind connection method defined in XEP-0124 including the appropriate URL, the second indicates support for the httppoll connection method defined in XEP-0025 including the appropriate URL, and the third indicates support for WAP connections including the appropriate URL.</p>
<p>The following examples show three DNS TXT resource records: the first indicates support for the httpbind connection method defined in XEP-0124 including the appropriate URL, the second indicates support for the httppoll connection method defined in XEP-0025 including the appropriate URL, and the third indicates support for WAP connections including the appropriate URL, and the fourth indicates support for the older, SSL-only connection method for clients.</p>
<example caption='TXT Resource Records'><![CDATA[
_xmppconnect IN TXT "_xmpp-client-httpbind=https://webconnect.jabber.org:8080/bind.cgi"
_xmppconnect IN TXT "_xmpp-client-httppoll=https://webconnect.jabber.org:8081/poll.cgi"
_xmppconnect IN TXT "_xmpp-client-httpbind=https://web.jabber.org:8080/bind.cgi"
_xmppconnect IN TXT "_xmpp-client-wap=http://wap.jabber.org/connector.cgi"
_xmppconnect IN TXT "_xmpp-client-tcpssl=5223"
]]></example>
</section1>
<section1 topic='Security Considerations' anchor='security'>
@ -146,6 +153,13 @@ _xmppconnect IN TXT "_xmpp-client-wap=http://wap.jabber.org/connector.cgi"
<doc>RFC 3920</doc>
</attribute>
<attribute>
<name>_xmpp-client-tcpssl</name>
<desc>old-style SSL-encrypted client-to-server TCP connection method</desc>
<value>should contain the port number (traditionally 5223)</value>
<doc>N/A</doc>
</attribute>
<attribute>
<name>_xmpp-server-tcp</name>
<desc>standard server-to-server TCP connection method (usually on port 5269)</desc>