xeps/xep-0174.xml

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<!DOCTYPE xep SYSTEM 'xep.dtd' [
  <!ENTITY % ents SYSTEM 'xep.ent'>
%ents;
]>
<?xml-stylesheet type='text/xsl' href='xep.xsl'?>
<xep>
<header>
  <title>Link-Local Messaging</title>
  <abstract>This document describes how to establish XMPP-like communications over local networks using zero-configuration networking.</abstract>
  &LEGALNOTICE;
  <number>0174</number>
  <status>Draft</status>
  <type>Standards Track</type>
  <sig>Standards</sig>
  <approver>Council</approver>
  <dependencies>
    <spec>XMPP Core</spec>
    <spec>XMPP IM</spec>
    <spec>RFC 3927</spec>
    <spec>draft-cheshire-dnsext-dns-sd</spec>
    <spec>draft-cheshire-dnsext-multicastdns</spec>
  </dependencies>
  <supersedes/>
  <supersededby/>
  <shortname>linklocal</shortname>
  <registry/>
  &stpeter;
  <revision>
    <version>1.1pre2</version>
    <date>in progress, last updated 2007-12-20</date>
    <initials>psa</initials>
    <remark><p>Corrected several DNS errors in the text and examples; added friendly How It Works section; updated to reflect version 1.5 of XEP-0115; added implementation note about port choice; updated registry.</p></remark>
  </revision>
  <revision>
    <version>1.0</version>
    <date>2007-06-12</date>
    <initials>psa</initials>
    <remark><p>Per a vote of the XMPP Council, advanced status to Draft; XMPP Registrar assigned linklocal shortname and created appropriate registry.</p></remark>
  </revision>
  <revision>
    <version>0.16</version>
    <date>2007-05-30</date>
    <initials>psa</initials>
    <remark><p>Updated the definition of port.p2pj TXT record so that it is not hardcoded to 5298 but instead tracks the port advertised via SRV; updated the IANA considerations to reflect acceptance of the proposed registration change by the maintainers of the http://www.dns-sd.org/ServiceTypes.html website.</p></remark>
  </revision>
  <revision>
    <version>0.15</version>
    <date>2007-05-14</date>
    <initials>psa</initials>
    <remark><p>Updated IANA Considerations to reflect proposed modifications to DNS-SD registration.</p></remark>
  </revision>
  <revision>
    <version>0.14</version>
    <date>2007-05-11</date>
    <initials>psa</initials>
    <remark><p>Specified that email and jid TXT values may contain a space-separated list of addresses; clarified roster display rules; clarified rules for handling presence name collisions; added security consideration about lack of validation for TXT record data.</p></remark>
  </revision>
  <revision>
    <version>0.13</version>
    <date>2007-03-28</date>
    <initials>psa</initials>
    <remark><p>Clarified handling of stream version.</p></remark>
  </revision>
  <revision>
    <version>0.12</version>
    <date>2007-03-26</date>
    <initials>psa</initials>
    <remark><p>Specified creation of registry for TXT records and hardcoded txtvers record value at 1.</p></remark>
  </revision>
  <revision>
    <version>0.11</version>
    <date>2007-03-14</date>
    <initials>psa</initials>
    <remark><p>Added section on capabilities discovery; added TXT records corresponding to the node, ver, and ext attributes from XEP-0115; changed textvers value to 2.</p></remark>
  </revision>
  <revision>
    <version>0.10</version>
    <date>2007-03-13</date>
    <initials>psa</initials>
    <remark><p>Added nick TXT key; added note about use of AAAA records with IPv6; specified that from and to addresses are recommended for stream headers; specified that stream features should be sent.</p></remark>
  </revision>
  <revision>
    <version>0.9</version>
    <date>2006-12-22</date>
    <initials>psa</initials>
    <remark><p>Updated the security considerations to recommend either TLS+SASL at the stream layer or encrypted sessions at the messaging layer.</p></remark>
  </revision>
  <revision>
    <version>0.8</version>
    <date>2006-07-31</date>
    <initials>psa</initials>
    <remark><p>Recommended use of TLS and SASL for stream security.</p></remark>
  </revision>
  <revision>
    <version>0.7</version>
    <date>2006-06-06</date>
    <initials>psa</initials>
    <remark><p>Further clarified internationalization considerations.</p></remark>
  </revision>
  <revision>
    <version>0.6</version>
    <date>2006-06-05</date>
    <initials>psa</initials>
    <remark><p>Clarified internationalization considerations and use of mDNS in unicast mode for avatar retrieval.</p></remark>
  </revision>
  <revision>
    <version>0.5</version>
    <date>2006-04-14</date>
    <initials>psa</initials>
    <remark><p>Specified presence name conflict resolution procedure, offline procedure, use of DNS NULL record for icons, and handling of multiple network interfaces.</p></remark>
  </revision>
  <revision>
    <version>0.4</version>
    <date>2006-03-16</date>
    <initials>psa</initials>
    <remark><p>Corrected PTR format and client discovery process.</p></remark>
  </revision>
  <revision>
    <version>0.3</version>
    <date>2006-02-23</date>
    <initials>psa</initials>
    <remark><p>Added more details about DNS setup and stream initiation; specified internationalization considerations.</p></remark>
  </revision>
  <revision>
    <version>0.2</version>
    <date>2006-02-22</date>
    <initials>psa</initials>
    <remark><p>Corrected information about Service Instance Name format, p2pj port, and presence discovery process.</p></remark>
  </revision>
  <revision>
    <version>0.1</version>
    <date>2006-02-09</date>
    <initials>psa</initials>
    <remark><p>Initial version; changed title to Link-Local Messaging.</p></remark>
  </revision>
  <revision>
    <version>0.0.1</version>
    <date>2006-02-07</date>
    <initials>psa</initials>
    <remark><p>First draft.</p></remark>
  </revision>
</header>

<section1 topic='Introduction' anchor='intro'>
  <section2 topic='Motivation' anchor='motivation'>
    <p>XMPP as defined in &rfc3920; does not support direct client-to-client interactions, since it requires authentication with a server: an XMPP client is allowed access to the network only once it has authenticated with a server, and the server must not grant access if authentication fails for any reason. If an unauthenticated client attempts to communicate directly with another client, such communication will fail because all XMPP communications are sent through one or more servers and a client cannot inject messages onto the network without first authenticating with a server.</p>
    <p>However, it is possible to establish an XMPP-like communications system on a local network using zero-configuration networking. In this situation, the clients obviate the XMPP requirement for authentication with a server by relying on zero-configuration networking to establish link-local communication using the _presence._tcp DNS SRV service type. Once discovery has been completed, the clients are then able to exchange messages and other structured data using the XMPP &MESSAGE; and &IQ; stanzas.</p>
    <p>Link-local messaging is restricted to the local network because of how zero-configuration networking works. It is impossible for clients that communicate via link-local addresses to insert messages into an XMPP network, which is why this kind of local "mesh" is most accurately referred to as an XMPP-like system that exists outside the context of existing XMPP networks (though see the <link url='#security'>Security Considerations</link> regarding the ability to "forward" messages from a local mesh to an XMPP network or vice-versa).</p>
    <p>Such a local "mesh" can be quite valuable in certain circumstances. For instance, participants in a trade show or conference, users of the same wifi hotspot, or employees on the same local area network can communicate without the need for a pre-configured server. For this reason, support for link-local messaging has been a feature of Apple's iChat client when operating in Bonjour (formerly Rendezvous) mode for many years. Because it is desirable for other Jabber clients to support such functionality, this document describes how to use zero-configuration networking as the basis for link-local communication.</p>
  </section2>
  <section2 topic='How It Works' anchor='howitworks'>
    <p>This section provides a friendly introduction to link-local messaging.</p>
    <p>Imagine that you are a Shakespearean character named Juliet. You are are using your laptop computer (a machine named "pronto") at a wifi hotspot in downtown Verona and you want to find other people to chat with on an ad-hoc basis (i.e., not people in your normal XMPP roster). Therefore your chat client advertises a link-local address of "juliet@pronto" so that other people can dynamically find you at the hotspot. Your client does this by invoking a daemon on your machine that supports DNS-based Service Discovery ("DNS-SD") as defined in &dnssd; and Multicast DNS ("mDNS") as defined in &mdns;. As a result, the daemon stores the following DNS records and listens for multicast DNS queries asking for them:</p>
    <code><![CDATA[
_presence._tcp.local. PTR juliet@pronto._presence._tcp.local.

juliet@pronto._presence._tcp.local. SRV 5562 pronto.local. 

pronto.local. A 10.2.1.187
    ]]></code>
    <p>The meaning of these records is as follows:</p>
    <ul>
      <li>The PTR ("pointer") record (see &rfc2317; and &rfc1886;) says that there is a service of type "presence" on the local subnet (".local.") called "juliet@pronto" and that the service communicates over TCP.</li>
      <li>The SRV record (see &rfc2782;) maps the service instance "juliet@pronto" to the machine "pronto.local." on port 5562.</li>
      <li>The A record specifies the IP address 10.2.1.187 at which the "pronto" machine will listen for connections.</li>
    </ul>
    <p>Your chat client also wants to advertise some information about you (subject to your control so that you don't divulge private information). Therefore it invokes the mDNS daemon to also store some DNS TXT records (see &rfc1464;):</p>
    <code><![CDATA[
juliet IN TXT "txtvers=1"
juliet IN TXT "1st=Juliet"
juliet IN TXT "email=juliet@capulet.lit"
juliet IN TXT "hash=sha-1"
juliet IN TXT "jid=juliet@capulet.lit"
juliet IN TXT "last=Capulet"
juliet IN TXT "msg=Hanging out downtown"
juliet IN TXT "nick=JuliC"
juliet IN TXT "node=http://www.adiumx.com"
juliet IN TXT "phsh=a3839614e1a382bcfebbcf20464f519e81770813"
juliet IN TXT "port.p2pj=5562"
juliet IN TXT "status=avail"
juliet IN TXT "v=1.1.4"
juliet IN TXT "vc=CA!"
juliet IN TXT "ver=66/0NaeaBKkwk85efJTGmU47vXI="
    ]]></code>
    <p>Other people at the hotspot may also advertise similar DNS records for use on the local link. Essentially, the mDNS daemons running on all of the machines at the hotspot collectively manage the ".local." domain, which has meaning only at the hotspot (not across the broader Internet). Queries and responses for services on the local link occur via link-local multicast over UDP port 5353 instead of via normal DNS unicast over UDP port 53. When a new machine joins the local link, it can send out queries for any number of service types, to which the other machines will reply. For the purpose of link-local messaging we are interested only in the "presence" service, but many other services could exist on the local link (see <link url='http://www.dns-sd.org/'>dns-sd.org</link> for a complete list).</p>
    <p>Now let us imagine that a fine young gentleman named Romeo joins the hotspot and that his chat client (actually his mDNS daemon) sends out link-local multicast queries for link-local services of type "presence" (i.e., PTR records that match "_presence._tcp.local.", followed by appropriate SRV, A, and TXT record queries to discover detailed information about those services). His client will then discover (among others) a service named "juliet@pronto" at IP address 10.2.1.187 and port 5562, with some intriguing TXT records. Being a romantic fellow, he then initiates a chat with you by opening an XML stream to the advertised IP address and port.</p>
    <code><![CDATA[
<stream:stream 
        xmlns='jabber:client' 
        xmlns:stream='http://etherx.jabber.org/streams'
        from='juliet@pronto'
        to='romeo@forza'
        version='1.0'>
    ]]></code>
    <p>Your client then responds with a response stream header (perhaps subject to user approval -- it's not always safe to chat with strangers!).</p>
    <code><![CDATA[
<stream:stream 
        xmlns='jabber:client' 
        xmlns:stream='http://etherx.jabber.org/streams'
        from='romeo@forza'
        to='juliet@pronto'
        version='1.0'>
    ]]></code>
    <p>Romeo then sends you an XMPP message.</p>
    <code><![CDATA[
<message to='romeo@forza' from='juliet@pronto'>
  <body>Hey, just saying hi!</body>
</message>
    ]]></code>
    <p>You chat with Romeo for a while, then your client closes the stream.</p>
    <code><![CDATA[
</stream:stream>
    ]]></code>
    <p>And Romeo's client does the same.</p>
    <code><![CDATA[
</stream:stream>
    ]]></code>
    <p>Finally you decide to head home so your mDNS daemon sends a Multicast DNS "Goodbye" packet for your PTR record. As a result, everyone else at the hotspot receives a Multicast DNS "Remove" event, at which point they cancel any outstanding A, SRV, TXT, or NULL record queries related to your presence service.</p>
  </section2>
</section1>

<section1 topic='Glossary' anchor='glossary'>
  <table caption='Terminology'>
    <tr>
      <th>Term</th>
      <th>Description</th>
    </tr>
    <tr>
      <td>Bonjour</td>
      <td>Apple Computer's implementation of zero-configuration networking, formerly known as Rendezvous. See &lt;<link url='http://www.apple.com/macosx/features/bonjour/'>http://www.apple.com/macosx/features/bonjour/</link>&gt;.</td> 
    </tr>
    <tr>
      <td>DNS-SD</td>
      <td>A convention for naming and structuring DNS SRV records such that a client can dynamically discover a domain for a service using only standard DNS queries. See <cite>draft-cheshire-dnsext-dns-sd</cite>. For a full list of registered DNS-SD records, see &lt;<link url='http://www.dns-sd.org/ServiceTypes.html'>http://www.dns-sd.org/ServiceTypes.html</link>&gt;.</td> 
    </tr>
    <tr>
      <td>Link-local address</td>
      <td>An IPv4 or IPv6 address that is valid for communication with other devices connected to the same physical or logical link. See &rfc3927;.</td>
    </tr>
    <tr>
      <td>Multicast DNS (mDNS)</td>
      <td>A technology that provides the ability to perform DNS-like operations on a local link in the absence of any conventional unicast DNS server. See <cite>draft-cheshire-dnsext-multicastdns</cite>.</td>
    </tr>
    <tr>
      <td>Zero-configuration networking</td>
      <td>A set of technologies that enable the use of the Internet Protocol for local communications. See &lt;<link url='http://www.zeroconf.org/'>http://www.zeroconf.org/</link>&gt;.</td> 
    </tr>
  </table>
</section1>

<section1 topic='DNS Records' anchor='dns'>
  <p>In order to advertise its availability for link-local messaging, a client MUST publish four different kinds of DNS records:</p>
  <ol>
    <li>
      <p>A PTR record of the following form:</p>
      <code><![CDATA[
_presence._tcp.local. PTR username@machine-name._presence._tcp.local.
      ]]></code>
    </li>
    <li>
      <p>An address ("A" or "AAAA") record of the following form: <note>The IP address MAY be either an IPv4 address or an IPv6 address.</note></p>
      <code><![CDATA[
machine-name.local. A ip-address
      ]]></code>
    </li>
    <li>
      <p>An SRV record of the following form:</p>
      <code><![CDATA[
_presence._tcp <ttl> SRV <priority> <weight> port-number machine-name.local. 
      ]]></code>
    </li>
    <li>
      <p>Optionally, various TXT records of the following form, as further described in the <link url='#txt'>TXT Records</link> section of this document:</p>
      <code><![CDATA[
<owner> IN <ttl> TXT "txtvers=1"
<owner> IN <ttl> TXT "1st=user-first-name"
<owner> IN <ttl> TXT "email=user-email-address"
<owner> IN <ttl> TXT "hash=entity-capabilities-algorithm"
<owner> IN <ttl> TXT "jid=user-jabber-id"
<owner> IN <ttl> TXT "last=user-last-name"
<owner> IN <ttl> TXT "msg=freeform-availability-status"
<owner> IN <ttl> TXT "nick=user-nickname"
<owner> IN <ttl> TXT "node=application-identifier"
<owner> IN <ttl> TXT "phsh=sha1-hash-of-avatar"
<owner> IN <ttl> TXT "port.p2pj=5562"
<owner> IN <ttl> TXT "status=avail-away-or-dnd"
<owner> IN <ttl> TXT "v=entity-capabilities-version"
<owner> IN <ttl> TXT "vc=capabilities-string"
<owner> IN <ttl> TXT "ver=entity-capabilities-identity"
      ]]></code>
      <p>Note: In accordance with Section 6.7 of <cite>draft-cheshire-dnsext-dns-sd</cite>, the "txtvers" record should be the first record specified.</p>
    </li>
  </ol>
  <p>The "machine-name" is the name of the computer, the "username" is the system username of the principal currently logged into the computer, the "port" may be any unassigned port number, and the "ip-address" is the physical address of the computer on the local network.</p>
  <p>So, for example, if the machine name is "pronto", the username is "juliet", the chosen port is "5562", the IP address is "10.2.1.187", and the personal information is that associated with the author of this document, the DNS records would be as follows:</p>
  <code><![CDATA[
_presence._tcp.local. PTR juliet@pronto._presence._tcp.local.

juliet@pronto._presence._tcp.local. SRV 5562 pronto.local. 

pronto.local. A 10.2.1.187

juliet IN TXT "txtvers=1"
juliet IN TXT "1st=Juliet"
juliet IN TXT "email=juliet@capulet.lit"
juliet IN TXT "hash=sha-1"
juliet IN TXT "jid=juliet@capulet.lit"
juliet IN TXT "last=Capulet"
juliet IN TXT "msg=Hanging out downtown"
juliet IN TXT "nick=JuliC"
juliet IN TXT "node=http://www.adiumx.com"
juliet IN TXT "phsh=a3839614e1a382bcfebbcf20464f519e81770813"
juliet IN TXT "port.p2pj=5562"
juliet IN TXT "status=avail"
juliet IN TXT "v=1.1.4"
juliet IN TXT "vc=CA!"
juliet IN TXT "ver=66/0NaeaBKkwk85efJTGmU47vXI="

  ]]></code>
  <p>The IPv4 and IPv6 addresses associated with a machine may vary depending on the local network to which the machine is connected. For example, on an Ethernet connection the physical address might be "192.168.0.100" but when the machine is connected to a wireless network the physical address might change to "10.10.1.187".</p>
  <p>If the machine name asserted by a client is already taken by another machine on the network, the client MUST assert a different machine name, which SHOULD be formed by adding the character "-" and digit "1" to the end of the machine name string (e.g., "pronto-1"), adding the character "-" and digit "2" if the resulting machine name is already taken (e.g., "pronto-2"), and similarly incrementing the digit until a unique machine name is constructed.</p> 
  <p>If the username asserted by a client is already taken by another application on the machine, the client MUST assert a different username, which SHOULD be formed by adding the character "-" and digit "1" to the end of the username string (e.g., "juliet-1"), adding the character "-" and digit "2" if the resulting username is already taken (e.g., "juliet-2"), and similarly incrementing the digit until a unique username is constructed.</p> 
  <section2 topic='TXT Records' anchor='txt'>
    <p>DNS-SD enables service definitions to include various TXT records that specify parameters to be used in the context of the relevant service type. The &REGISTRAR; maintains a registry of TXT records for use with the _presence._tcp service type, as specified in the <link url='#registrar'>XMPP Registrar Considerations</link> section of this document.</p>
    <p>It is OPTIONAL to include any of these TXT records, and an implementation MUST NOT fail (i.e., MUST enable link-local messaging) even if none of the TXT records are provided by another local entity.</p>
    <p>Most of the registered TXT records relate to human users, in which context certain records are of greater interest than others, e.g. "msg", "nick", and "status"; however, link-local messaging can be used by non-human entities (e.g., devices).</p>
    <p>Note: See the <link url='#security'>Security Considerations</link> section of this document regarding the inclusion of information that may have an impact on personal privacy (e.g., the "1st", "last", "nick", "email", and "jid" records).</p>
  </section2>
</section1>

<section1 topic='Discovering Other Users' anchor='disco'>
  <p>In order to discover other users, a client sends an mDNS request for PTR records that match "_presence._tcp.local.". The client then receives replies from all machines that advertise support for link-local messaging. <note>The replies will include a record corresponding the client itself; the client MUST filter out this result.</note> The client MAY then find out detailed information about each machine by sending SRV and TXT queries to "machine-name.local." for each machine (however, to preserve bandwidth, the client SHOULD NOT send these queries unless it is about to initiate communications with the other user, and it MUST cancel the queries after it has received a response). Note: The presence name to be used for display in a link-local "roster" SHOULD be obtained from the &lt;Instance&gt; portion of the received PTR record for each user; however, the client MAY instead display a name or nickname derived from the TXT records if available.</p>
</section1>

<section1 topic='Exchanging Presence' anchor='presence'>
  <p>When the _presence._tcp service is used, presence is exchanged via the format described in the <link url='#txt'>TXT Records</link> section of this document. In particular, presence information is not pushed as in XMPP (see &rfc3921;). Instead, clients listen for presence announcements from other local entities. Recommended rates for sending updates can be found in <cite>Multicast DNS</cite>.</p>
</section1>

<section1 topic='Discovering Capabilities' anchor='caps'>
  <p>Because link-local communication does not involve the exchange of XMPP presence, it is not possible to use &xep0115; for capabilities discovery. Therefore, it is RECOMMENDED to instead include the node, hash, and ver TXT records (and OPTIONAL to include the ext and v TXT records). The values of these records MUST be the same as the values for the 'node', 'hash', 'ver', 'ext', and 'v' attributes that are advertised for the application in normal XMPP presence (if any) via the Entity Capabilities protocol as described in <cite>XEP-0115</cite>.</p>
</section1>

<section1 topic='Initiating a Messaging Session' anchor='initiate'>
  <p>In order to exchange link-local messages, the initiator and recipient MUST first establish XML streams between themselves, as is familiar from <cite>RFC 3920</cite>.</p>
  <p>First, the initiator opens a TCP connection at the IP address and port discovered via the DNS lookup for a local entity and opens an XML stream to the recipient, which SHOULD include 'to' and 'from' address:</p>
  <example caption="Opening a Stream"><![CDATA[
<stream:stream 
        xmlns='jabber:client' 
        xmlns:stream='http://etherx.jabber.org/streams'
        from='juliet@pronto'
        to='romeo@forza'
        version='1.0'>
  ]]></example>
  <p>Note: If the initiator supports stream features and the other stream-related aspects of XMPP 1.0 as specified in <cite>RFC 3920</cite>, then it SHOULD include the version='1.0' flag as shown in the previous example.</p>
  <p>The recipient then responds with a stream header as well:</p>
  <example caption="Stream Header Response"><![CDATA[
<stream:stream 
        xmlns='jabber:client' 
        xmlns:stream='http://etherx.jabber.org/streams'
        from='romeo@forza'
        to='juliet@pronto'
        version='1.0'>
  ]]></example>
  <p>If both the initiator and recipient included the version='1.0' flag, the recipient SHOULD also send stream features as specified in <cite>RFC 3920</cite>:</p>
  <example caption="Recipient Sends Stream Features"><![CDATA[
<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
</stream:features>
  ]]></example>
  <p>The exchange of stream headers results in an unencrypted and unauthenticated channel between the two entities. See the <link url='#security'>Security Considerations</link> section of this document regarding methods for authenticating and encrypting the stream.</p>
</section1>

<section1 topic='Exchanging Messages' anchor='exchange'>
  <p>Once the streams are established, either entity then can send XMPP message or IQ stanzas by specifying 'to' and 'from' addresses using the logical local addresses: <note>The to and from addresses MUST be of the form "username@machine-name" as discovered via SRV (this is the &lt;Instance&gt; portion of the Service Instance Name).</note></p>
  <example caption="Sending a Message"><![CDATA[
<message to='romeo@forza' from='juliet@pronto'>
  <body>Hey, just saying hi!</body>
</message>
  ]]></example>
</section1>

<section1 topic='Ending a Messaging Session' anchor='end'>
  <p>To end the chat, either party closes the XML stream:</p>
  <example caption="Ending the Chat"><![CDATA[
</stream:stream>
  ]]></example>
  <p>The other party then closes the stream in the other direction as well:</p>
  <example caption="Closing the Stream"><![CDATA[
</stream:stream>
  ]]></example>
  <p>Both parties then MUST close the TCP connection between them.</p>
</section1>

<section1 topic='Going Offline' anchor='offline'>
  <p>In order to go offline, a client MUST send a Multicast DNS "Goodbye" packet for the user's PTR record. As a result, all other entities on the local network will receive a Multicast DNS "Remove" event, at which point they MUST cancel any outstanding TXT, SRV, or NULL record queries for the offline user.</p>
</section1>

<section1 topic='Implementation Notes' anchor='impl'>
  <section2 topic='Multiple Network Interfaces' anchor='impl-network'>
    <p>Devices that use link-local messaging may have multiple network interfaces. As a result, it is possible to discover the same entity multiple times. Even if a client discovers the same presence name on multiple network interfaces, it MUST show only one entity in the local roster. In addition, because local IP addresses can be dynamically re-assigned, the client SHOULD NOT store the IP address to be used for communications when it discovers that address in the initial DNS lookup phase; instead, it SHOULD delay sending the Multicast DNS query until the client is ready to communicate with the other entity.</p>
  </section2>
  <section2 topic='Buddy Icons' anchor='impl-icons'>
    <p>If an entity has an associated icon (e.g., a user avatar or photo), its client SHOULD publish the raw binary data for that image via a DNS NULL record of the following form:</p>
    <code><![CDATA[
_presence._tcp.local. IN NULL raw-binary-data-here
    ]]></code>
    <p>Note: In accordance with &rfc1035;, the data MUST be 65535 octets or less.</p>
    <p>After retrieving the "phsh" value from a Buddy's TXT record, a client SHOULD search its local picture database to learn the last recorded picture hash value for an entity and then compare it to the "phsh" value in the TXT record. If the values are equal, the client SHOULD use the local copy of the icon. If the picture hash values are not equal, the client SHOULD issue a Multicast DNS NULL record query to retrieve the new icon. After retrieving the NULL record, the client SHOULD replace the old "phsh" value in the picture database with the new "phsh" value and save the icon to disk. If the client needs to send a Multicast DNS query in order to retrieve the icon, it MUST cancel the NULL record query immediately after receiving a response containing the new picture data.</p>
    <p>If a user changes their picture, the user's client MUST update the NULL record with the contents of the new picture, calculate a new picture hash, and then update the "phsh" value in the TXT record with the new hash value. Since all users logged into local presence are monitoring for TXT record changes, they will see that the "phsh" value was changed; if they wish to view the new icon, their clients SHOULD issue a new Multicast DNS query to retrieve the updated picture.</p>
  </section2>
  <section2 topic='Port' anchor='impl-port'>
    <p>The port used for link-local messaging MAY be any unassigned port number, as determined by the messaging application on the device. The chosen port MUST be specified in the SRV record and applications MUST use the port specified in the SRV record. However, the chosen port SHOULD also be specifed in the "port.p2pj" TXT record for backwards-compatibility with older implementations, and if included the port specified in the TXT record MUST be the same as the port specified in the SRV record.</p>
  </section2>
</section1>

<section1 topic='Internationalization Considerations' anchor='i18n'>
  <p><cite>RFC 1035</cite> does not allow characters outside the &ascii; character range in DNS A records. Therefore the "machine-name" portion of an A record as used for link-local messaging MUST NOT contain characters outside the US-ASCII character range.</p>
  <p>Although <cite>RFC 2317</cite> and <cite>RFC 2782</cite> do not allow characters outside the US-ASCII character range in PTR and SRV records respectively, Section 4.1 of <cite>DNS-Based Service Discovery</cite> recommends support for UTF-8-encoded Unicode characters in the &lt;Instance&gt; portion of Service Instance Names, which in link-local messaging is the "username@machine-name" portion of the PTR or SRV record. This document adheres to the recommendation in <cite>DNS-Based Service Discovery</cite>. However, as mentioned above, the "machine-name" portion of the &lt;Instance&gt; portion MUST NOT contain characters outside the US-ASCII range.</p>
  <p>Although <cite>RFC 1464</cite> does not allow characters outside the US-ASCII character range in TXT records, Section 6.5 of <cite>DNS-Based Service Discovery</cite> mentions support for UTF-8-encoded Unicode characters in text record values (e.g., values of the TXT "msg" name). This document adheres to the recommendation in <cite>DNS-Based Service Discovery</cite>.</p>
</section1>

<section1 topic='Security Considerations' anchor='security'>
  <section2 topic='Authentication and Encryption' anchor='security-auth'>
    <p>XMPP networks use TLS (&rfc2246;) for channel encryption, SASL (&rfc4422;) for authentication, and the Domain Name System (&rfc1034;) for weak validation of server hostnames; these technologies help to ensure the identity of sending entities and to encrypt XML streams. By contrast, zero-configuration networking uses dynamic discovery and asserted machine names as the basis of sender identity. Therefore, link-local messaging does not result in authenticated identities in the same way that XMPP itself does, nor does it provide for an encrypted channel between local entities.</p>
    <p>There are two potential solutions to this problem:</p>
    <ol>
      <li>Negotiate the use of TLS and SASL for the XML stream as described in <cite>RFC 3920</cite>.</li>
      <li>Negotiate encryption with identity checking for the message exchange using &xep0116;.</li>
    </ol>
    <p>It is RECOMMENDED to use one of these methods to secure communications between local entities. However, subject to client configuration and local service policies, two entities MAY accept an unauthenticated and unencrypted channel; but a client SHOULD warn the human user that the channel is unauthenticated and unencrypted.</p>
  </section2>
  <section2 topic='Stanza Injection' anchor='security-inject'>
    <p>Because of fundamental differences between a true XMPP network and a localized XMPP client "mesh", local entities MUST NOT attempt to inject local traffic onto an XMPP network and an XMPP server MUST reject communications until an entity is properly authenticated in accordance with the rules defined in <cite>RFC 3920</cite>. However, a client on a local mesh MAY forward traffic to an XMPP network after having properly authenticated on such a network (e.g., to forward a message received on a local client mesh to a contact on an XMPP network).</p>
  </section2>
  <section2 topic='TXT Record Data' anchor='security-txt'>
    <p>Because there is no mechanism for validating the information that is published in DNS TXT records, it is possible for clients to "poison" this information (e.g., by publishing email addresses or Jabber IDs that are controlled by or associated with other users).</p>
  </section2>
  <section2 topic='Private Information' anchor='security-priv'>
    <p>The TXT records optionally advertised as part of this protocol MAY result in exposure of privacy-sensitive information about a human user (such as full name, email address, and Jabber ID). A client MUST allow a user to disable publication of this personal information (e.g., via client configuration).</p>
  </section2>
</section1>

<section1 topic='IANA Considerations' anchor='iana'>
  <p>DNS-SD service type names are not yet managed by &IANA;. Section 19 of <cite>DNS-Based Service Discovery</cite> proposes an IANA allocation policy for unique application protocol or service type names. Until the proposal is adopted and in force, Section 19 points to &lt;<link url='http://www.dns-sd.org/ServiceTypes.html'>http://www.dns-sd.org/ServiceTypes.html</link>&gt; regarding registration of service type names for DNS-SD.</p>
  <p>Before this specification was written, there was an existing registration for the "presence" service type, with registration information as follows:</p>
  <div class='indent'>
    <ol>
      <li>Short name: presence</li>
      <li>Long name: iChat AV</li>
      <li>Responsible person: Jens Alfke &lt;jens at apple.com&gt;</li>
      <li>Defined TXT keys: txtvers, port.p2pj, phsh, vc, 1st, AIM, msg, status, last</li>
    </ol>
  </div>
  <p>On 2007-05-14, the XMPP Registrar submitted the following proposed modification to the existing registration, which was accepted on 2007-05-30:</p>
  <div class='indent'>
    <ol>
      <li>Short name: presence</li>
      <li>Long name: Link-Local Messaging</li>
      <li>Responsible person: XMPP Registrar &lt;registrar at xmpp.org&gt;</li>
      <li>Protocol URL: http://www.xmpp.org/extensions/xep-0174.html</li>
      <li>Primary transport protocol: _tcp</li>
      <li>TXT keys URL: http://www.xmpp.org/registrar/linklocal.html</li>
    </ol>
  </div>
</section1>

<section1 topic='XMPP Registrar Considerations' anchor='registrar'>
  <section2 topic='Link-Local Messaging TXT Records Registry' anchor='registrar-linklocal'>
    <p>The &REGISTRAR; maintains a registry of TXT records advertised in the context of link-local messaging (see &LINKLOCAL;).</p>
    <section3 topic='Registration Process' anchor='registrar-linklocal-process'>
      &REGPROCESS;
      <code><![CDATA[
<record>
  <name>The attribute name of the TXT record.</name>
  <desc>A natural-language description of the record.</desc>
  <status>
    The requirements status of the record. Should be one of: 
      - required
      - recommended 
      - optional
      - deprecated
      - obsolete
 </status>
</record>
      ]]></code>
      <p>The registrant may register more than one TXT record at a time, each contained in a separate &lt;record/&gt; element.</p>
    </section3>
    <section3 topic='Initial Registration' anchor='registrar-linklocal-reg'>
      <p>The following submission registers TXT records in use as of June 2007. Refer to the registry itself for a complete and current list of TXT records (this specification may or may not be revised when new TXT records are registered).</p>
      <code><![CDATA[

<record>
  <name>1st</name>
  <desc>The given or first name of the user.</desc>
  <status>optional</status>
</record>

<record>
  <name>email</name>
  <desc>
    The email address of the user; may contain a space-separated list 
    of more than one email address.
  </desc>
  <status>optional</status>
</record>

<record>
  <name>ext</name>
  <desc>
    A space-separated list of extensions; the value of this record MUST 
    be the same as that provided via normal XMPP presence (if applicable) 
    in the 'ext' attribute specified in Entity Capabilities (XEP-0115).
  </desc>
  <status>optional</status>
</record>

<record>
  <name>hash</name>
  <desc>
    The hashing algorithm used to generated the 'ver' attribute in 
    Entity Capabilities (XEP-0115) and therefore the ver TXT record
    in Link-Local Messaging.
  </desc>
  <status>recommended</status>
</record>

<record>
  <name>jid</name>
  <desc>
    The Jabber ID of the user; may contain a space-separated list of 
    more than one JID.
  </desc>
  <status>recommended</status>
</record>

<record>
  <name>last</name>
  <desc>The family or last name of the user.</desc>
  <status>optional</status>
</record>

<record>
  <name>msg</name>
  <desc>
    Natural-language text describing the user's state. This is 
    equivalent to the XMPP &lt;status/&gt;; element.
  </desc>
  <status>optional</status>
</record>

<record>
  <name>nick</name>
  <desc>A friendly or informal name for the user.</desc>
  <status>recommended</status>
</record>

<record>
  <name>node</name>
  <desc>
    A unique identifier for the application; the value of this record MUST 
    be the same as that provided via normal XMPP presence (if applicable) 
    in the 'node' attribute specified in Entity Capabilities (XEP-0115).
  </desc>
  <status>recommended</status>
</record>

<record>
  <name>phsh</name>
  <desc>
    The SHA-1 hash of the user's avatar icon or photo. This SHOULD be 
    requested using mDNS in unicast mode by sending a DNS query to the 
    mDNS multicast address (224.0.0.251 or its IPv6 equivalent FF02::FB).
    The client should keep a local cache of icons keyed by hash. If the 
    phsh value is not in the cache, the client should fetch the unknown 
    icon and then cache it. Implementations should also include logic for 
    expiring avatar icons.
  </desc>
  <status>optional</status>
</record>

<record>
  <name>port.p2pj</name>
  <desc>
    The port for link-local communications. This MUST be the same as the
    value provided for SRV lookups. Clients MUST use the port discovered 
    via SRV lookups and MUST ignore the value of this TXT record. However, 
    clients SHOULD advertise this TXT record if it is important to ensure
    backwards-compatibility with some existing implementations. (Note: In
    some existing implementations this value was hardcoded to "5298".)
  </desc>
  <status>deprecated</status>
</record>

<record>
  <name>status</name>
  <desc>
    The presence availability of the user. Allowable values are "avail", 
    "away", and "dnd", which map to mere XMPP presence (the user is 
    available) and the XMPP &lt;show/&gt; values of "away" and "dnd", 
    respectively; if the status record is not included, the status should 
    be assumed to be "avail".
  </desc>
  <status>recommended</status>
</record>

<record>
  <name>txtvers</name>
  <desc>
    The version of the TXT records supported by the client. For backwards 
    compatibility this is hardcoded at "1". This TXT record should be the
    first one provided, in accordance with the DNS-SD specification.
  </desc>
  <status>deprecated</status>
</record>

<record>
  <name>v</name>
  <desc>
    The application version; the value of this record MUST be the same 
    as that provided via normal XMPP presence (if applicable) in the 'v' 
    attribute specified in Entity Capabilities (XEP-0115).
  </desc>
  <status>optional</status>
</record>

<record>
  <name>vc</name>
  <desc>
    A flag advertising the user's ability to engage in audio or video 
    conferencing. If the user is able to engage in audio conferencing, 
    the string MUST include the "A" character. If the user is able to 
    engage in video conferencing, the string MUST include the "V" 
    character. If the user is able to engage in conferencing with more 
    than one participant, the string MUST include the "C" character. If 
    the user is not currently engaged in an audio or video conference, 
    the string MUST include the "!" character. The order of characters 
    in the string is immaterial. NOTE: This flag is included only for
    backwards-compatibility; implementations SHOULD use the node, ver, 
    and ext records for more robust capabilities discovery as described 
    in the Discovering Capabilities section of XEP-0174.
  </desc>
  <status>optional</status>
</record>

<record>
  <name>ver</name>
  <desc>
    A hashed string that defines the XMPP service discovery (XEP-0030) 
    identity of the application and the XMPP service discovery features 
    supported by the application; the value of this record MUST be the 
    same as that provided via normal XMPP presence (if applicable) in 
    the 'ver' attribute specified in Entity Capabilities (XEP-0115).
  </desc>
  <status>recommended</status>
</record>

      ]]></code>
    </section3>
  </section2>
</section1>

<section1 topic='Acknowledgements' anchor='ack'>
  <p>Thanks to Emanuele Aina, Jens Alfke, Marco Barisione, Justin Karneges, Marc Krochmal, Eric St. Onge, and Sjoerd Simons for their input.</p>
</section1>

</xep>