<title>Discovering Alternative Connection Methods</title>
<abstract>This document defines a DNS TXT Resource Record format for use in discovering alternative methods of connecting to an XMPP server.</abstract>
<remark><p>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).</p></remark>
<remark><p>Clarified order of lookups; restored _xmpp-client-tcp and added _xmpp-server-tcp as optional records if SRV is not supported or accessible.</p></remark>
</revision>
<revision>
<version>0.2</version>
<date>2005-12-05</date>
<initials>psa</initials>
<remark><p>Removed _xmpp-client-tcp from TXT records (belongs in SRV records only).</p></remark>
<p>Although &rfc3920; specifies the use of TCP as the method of connecting to an XMPP server, alternative connection methods exist, including 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 alternative 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>
<li><p>Define 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>Define a way to specify alternative connection methods 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>Define a way to specify alternative connection methods 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 alternative connection methods.</p></li>
<li><p>Define a way to specify alternative connection methods 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>Define a way to specify alternative connection methods 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 <linkurl='http://www.openspf.org/'>Sender Policy Framework</link> and <linkurl='http://www.microsoft.com/mscorp/safety/technologies/senderid/default.mspx'>SenderID</link> email server verification technologies).</p></li>
<p>This document defines a way to encapsulate information about alternative connection methods in DNS TXT resource records. However, the authors will monitor the availability of DDDS-based systems supporting U-NAPTR, and may request deprecation of the DNS TXT methods defined in this document at some point in the future.</p>
<li>The attribute name SHOULD begin with the string "_xmpp-client-" or "_xmpp-server-" and SHOULD be registered as described in the <linkurl='#registrar'>XMPP Registrar Considerations</link> section of this document.</li>
<li>If the txt-data field contains only an attribute name (i.e., no unquoted "=" character followed by additional characters), the receiving application SHOULD interpret it as indicating the presence of the attribute or feature with no defined value.</li>
<li>If the txt-data field contains an unquoted "=" character, it MUST also contain an attribute value.</li>
<p>The following examples show two DNS TXT resource records: the first indicates support for the httpbind connection method defined in XEP-0124 and the second indicates support for WAP connections (both include appropriate URLs).</p>
<p>It is possible that advertisement of alternative connection methods may 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 must be taken in determining which alternative connection methods are appropriate to advertise.</p>
