<abstract>This specification defines how to establish XMPP-like communications over local networks based on zero-configuration networking. This method uses DNS-based Service Discovery and Multicast DNS to discover entities that support the protocol, including their IP addresses and preferred ports. Any two entities can then negotiate a serverless connection using standard XML streams in order to exchange XMPP message and IQ stanzas.</abstract>
<remark><p>Clarified order of publishing and querying DNS records; corrected several DNS errors in the text and examples; added friendly How It Works section; added implementation note about port choice; updated TXT records to reflect version 1.5 of XEP-0115; updated registry.</p></remark>
<remark><p>Per a vote of the XMPP Council, advanced status to Draft; XMPP Registrar assigned linklocal shortname and created appropriate registry.</p></remark>
<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>
<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>
<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>
<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>
<remark><p>Updated the security considerations to recommend either TLS+SASL at the stream layer or encrypted sessions at the messaging layer.</p></remark>
<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>
<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 <linkurl='#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>
<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>
<li>The SRV record (see &rfc2782;) maps the presence service instance "juliet@pronto" to the machine "pronto.local." on port 5562.</li>
<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>
<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>
<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 <linkurl='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". To do this, his client essentially reverses the order of DNS record publication (explained above) by asking for pointers to presence services (i.e., PTR records that match "_presence._tcp.local."), querying each service for its service instance and port (i.e., SRV record), mapping each service instance to an IP address (i.e., A record), and finding out additional information about the entity using the service (i.e., TXT records). As a result, Romeo's client will discover any number of local presence services, among them a service named "juliet@pronto" (with some intriguing TXT records) at IP address 10.2.1.187 and port 5562. Being a romantic fellow, he then initiates a chat with you by opening an XML stream to the advertised IP address and port.</p>
<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>
<td>Apple Computer's implementation of zero-configuration networking, formerly known as Rendezvous. See <<linkurl='http://www.apple.com/macosx/features/bonjour/'>http://www.apple.com/macosx/features/bonjour/</link>>.</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 <<linkurl='http://www.dns-sd.org/ServiceTypes.html'>http://www.dns-sd.org/ServiceTypes.html</link>>.</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>
<td>A set of technologies that enable the use of the Internet Protocol for local communications. See <<linkurl='http://www.zeroconf.org/'>http://www.zeroconf.org/</link>>.</td>
<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>
<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>
<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 ®ISTRAR; maintains a registry of TXT records for use with the _presence._tcp service type, as specified in the <linkurl='#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 <linkurl='#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>
<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 <Instance> 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>
<p>When the _presence._tcp service is used, presence is exchanged via the format described in the <linkurl='#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>
<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', 'n', 'os', 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>
<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>
<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>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>
<p>The exchange of stream headers results in an unencrypted and unauthenticated channel between the two entities. See the <linkurl='#security'>Security Considerations</link> section of this document regarding methods for authenticating and encrypting the stream.</p>
<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 <Instance> portion of the Service Instance Name).</note></p>
<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>
<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>
<section2topic='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>
<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>
<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 <Instance> 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 <Instance> 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>
<section2topic='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>
<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>
<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>
<section2topic='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>
<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>
<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 <<linkurl='http://www.dns-sd.org/ServiceTypes.html'>http://www.dns-sd.org/ServiceTypes.html</link>> 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>
<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>
<p>Thanks to Emanuele Aina, Jens Alfke, Marco Barisione, Justin Karneges, Marc Krochmal, Eric St. Onge, and Sjoerd Simons for their input. Some of the explanatory concepts were loosely borrowed from &sipbonjour;.</p>
