<abstract>This specification defines how to communicate over local or wide-area networks using the principles of zero-configuration networking for endpoint discovery and the syntax of XML streams and XMPP messaging for real-time communication. 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 XML streams in order to exchange XMPP message and IQ stanzas.</abstract>
<remark><p>Per a vote of the XMPP Council, advanced specification to Final; corrected TXT record format in conformance with draft-cheshire-dnsext-dns-sd; clarified a number of issues related to DNS-SD and Multicast DNS; as an optimization, recommended sending of disco#info data in stream features; removed reference to Encrypted Sessions specification from the security considerations.</p></remark>
<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; generalized text to handle usage of wide-area DNS-SD.</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 can 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; encouraged implementations to send XMPP stream features.</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>The Extensible Messaging and Presence Protocol (XMPP) as defined in &xmppcore; 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 after it has authenticated with a server, and the server will 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 unless it first authenticates with a server.</p>
<p>However, it is possible to establish an XMPP-like communication system on a local (or even wide-area) network using the principles of 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 serverless communication using the _presence._tcp DNS SRV service type. Once discovery has been completed, the clients are able to negotiate an XML stream between themselves and then exchange messages and other structured data using the XMPP &MESSAGE; and &IQ; stanzas.</p>
<p>Serverless messaging is typically restricted to a local network (or ad-hoc wide-area network) because of how zero-configuration networking works. It is impossible for clients that communicate via this serverless mode to insert messages into an XMPP network, which is why this kind of "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 serverless mesh to an XMPP network or vice-versa).</p>
<p>Such a "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 serverless messaging has been a feature of Apple's iChat client when operating in Bonjour (formerly Rendezvous) mode since 2002. Because it is desirable for other Jabber/XMPP clients to support such functionality, this document describes how to use zero-configuration networking as the basis for serverless communication, mainly for use on local links (although the protocol can also be used on ad-hoc wide-area networks).</p>
<p>This section provides a friendly introduction to serverless messaging. The examples show usage on a local link using dynamically configured link-local addresses as described in &rfc3927; (see the <linkurl='#impl-wide'>Wide-Area Networks</link> section of this document regarding non-local usage).</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 serverless 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 &rfc6763; and Multicast DNS ("mDNS") as defined in &rfc6762;. As a result, the daemon (1) publishes the following DNS records to the multicast DNS address 224.0.0.251 (or FF02::FB for IPv6) and (2) listens for multicast DNS queries requesting these records:</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 publish a single DNS TXT record (see &rfc1464;) that encapsulates some strings of information, where the record name is the same as the SRV record and the record value follows the format described in the <linkurl='#txt'>TXT Record</link> section of this document. The strings are typically key-value pairs such as the following:</p>
<p>Other people at the hotspot can 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 multicast DNS 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 serverless 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 multicast DNS queries for 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 record parameters). <note>As explained in the DNS-SD specification, these queries might all be returned in the same answer.</note> 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 record parameters) 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 or wide-area communications. See <<linkurl='http://www.zeroconf.org/'>http://www.zeroconf.org/</link>>.</td>
<p>A TXT record whose name is the same as the SRV record and whose value follows the format described in the <linkurl='#txt'>TXT Record</link> section of this document, consisting of a set of strings that typically represent a series of key-value pairs such as the following:</p>
<p>Note: The DNS-SD specification stipulates that the TXT record MUST be published, but that it MAY contain no more than a single zero byte (e.g., if the user does not wish to publish any personal information).</p>
<p>The "machine" is the name of the computer, the "user" is the system username of the principal currently logged into the computer, the "port" can 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 plausibly associated with a certain Shakespearean character, the DNS records would be as follows:</p>
<p>The IPv4 and IPv6 addresses associated with a machine might 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". See <cite>RFC 3927</cite> for details.</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 a TXT record that specifies parameters to be used in the context of the relevant service type. The name of the TXT record is the same as that of the SRV record (i.e., "user@machine._presence._tcp.local."). The value of the TXT record is one or more strings, where each string is a parameter that usually takes the form of a key-value pair.</p>
<p>The ®ISTRAR; maintains a registry of the parameters that can be used in the TXT record value for the _presence._tcp service type, as specified in the <linkurl='#registrar'>XMPP Registrar Considerations</link> section of this document. Those parameters are not listed here. It is OPTIONAL to include any of these TXT record parameters, and an implementation MUST NOT fail (i.e., MUST enable serverless messaging) even if none of the parameters are provided by another entity. However, as mentioned the TXT record MUST be published (although its value MAY be a single zero byte).</p>
<p>Most of the registered TXT record parameters relate to human users, in which context certain parameters are of greater interest than others, e.g. "msg", "nick", and "status"; however, serverless 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 can have an impact on personal privacy (e.g., the "1st", "last", "nick", "email", and "jid" parameters).</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 serverless messaging. <note>The replies will include a record corresponding to the client itself; the client MUST filter out this result.</note> In accordance with Section 13 of the DNS-SD specification, these replies can include the SRV, A/AAAA, and TXT records in the Additional Section of the DNS message (subject to the size limits described in Section 19 of the Multicast DNS specification).</p>
<p>The client MAY then find out detailed information about each machine by sending SRV and TXT queries <note>These questions MAY all be sent in one DNS query packet.</note> to "user@machine.local." for each machine; however, to preserve bandwidth, the client SHOULD NOT send these queries unless it is about to initiate communication with the other user, and it MUST cancel the queries after it has received a response).
<p>When the _presence._tcp service is used, presence is exchanged via the format described in the <linkurl='#txt'>TXT Record</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 entities on the local link or wide-area network. Recommended rates for sending updates can be found in the Multicast DNS specification.</p>
<p>In order to exchange serverless messages, the initiator and recipient MUST first establish XML streams between themselves, as is familiar from <cite>RFC 6120</cite>.</p>
<p>First, the initiator opens a TCP connection at the IP address and port discovered via the DNS lookup for an 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 6120</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 6120</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 addresses: <note>The to and from addresses MUST be of the form "user@machine" as discovered via SRV (this is the <Instance> portion of the Service Instance Name).</note></p>
<p>The closing party (i.e., the party that sent the first closing stream tag) then MUST close the TCP connection between them.</p>
<p>Note: The closing party might receive additional stanzas from the other party after sending its closing stream tag and before receiving a closing stream tag from the other party (e.g., because of network latency or because the other party has messages queued up for delivery when it receives the closing party's closing stream tag). Therefore, the closing party needs to be prepared to handle such messages, which it SHOULD do by presenting them to the controlling user (if any).</p>
<p>In order to go offline, a link-local entity MUST send a Multicast DNS "Goodbye" packet for the user's PTR record as described in Section 11.2 of the Multicast DNS specification. 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>Because serverless 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 record parameters (and OPTIONAL to include the ext parameter). The values of these parameters MUST be the same as the values for the 'node', 'hash', 'ver', and 'ext' 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>As with Entity Capabilities over native XMPP networks, a client might not know the &xep0030; features associated with the 'ver' value advertised by another entity. However, in the case of serverless messaging there is no way for the client to discover the entity's supported features without initiating an XML stream to that entity and then sending a Service Discovery information ("disco#info") request over the negotiated stream.</p>
<p>Unfortunately, full stream negotiation (including TLS and SASL if appropriate) can require a large number of packets. Therefore, as an optimization, it is RECOMMENDED for the receiving entity in a serverless XML stream negotiation to include its disco#info data (including node) as a stream feature, as shown in the following examples.</p>
<examplecaption="Initiator Opens a Stream"><