Corrected several DNS errors in the text and examples; added friendly How It Works section; updated to reflect version 1.5 of XEP-0115.
First draft.
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.
-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.
-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 Security Considerations regarding the ability to "forward" messages from a local mesh to an XMPP network or vice-versa).
-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. 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.
+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.
+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.
+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 Security Considerations regarding the ability to "forward" messages from a local mesh to an XMPP network or vice-versa).
+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. 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.
+This section provides a friendly introduction to link-local messaging.
+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:
+The meaning of these records is as follows:
+Your chat client also wants to advertise some information about you (subject to your control so that you don't divulge private information), so it invokes the mDNS daemon to also stores some DNS TXT records (see &rfc1464;):
+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.
+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.
+
+ ]]>
+ Your client then responds with a response stream header (perhaps subject to user approval -- it's not always safe to chat with strangers!).
+
+ ]]>
+ Romeo then sends you an XMPP message.
+
+ Hey, just saying hi!
+
+ ]]>
+ You chat with Romeo for a while, then your client closes the stream.
+
+ ]]>
+ And Romeo's client does the same.
+
+ ]]>
+ Finally you decides to head home so your mDNS daemon sends a Multicast DNS "Goodbye" packet for the 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.
+| DNS-SD | -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 &dnssd;. For a full list of registered DNS-SD records, see <http://www.dns-sd.org/ServiceTypes.html>. | +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 draft-cheshire-dnsext-dns-sd. For a full list of registered DNS-SD records, see <http://www.dns-sd.org/ServiceTypes.html>. |
| Link-local address | @@ -161,7 +240,7 @@||
| Multicast DNS (mDNS) | -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 &mdns;. | +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 draft-cheshire-dnsext-multicastdns. |
| Zero-configuration networking | @@ -169,9 +248,16 @@
In order to advertise its availability for link-local messaging, a client MUST publish four different kinds of DNS records:
A PTR record of the following form:
+An address ("A" or "AAAA") record of the following form:
An "SRV" record (see &rfc2782;) of the following form:
+An SRV record of the following form:
IN SRV port-number username@machine-name.local.
+_presence._tcp SRV port-number machine-name.local.
]]>
A "PTR" record (see &rfc2317; and &rfc1886;) of the following form:
-Optionally, various "TXT" records (see &rfc1464;) of the following form, as further described in to the TXT Records section of this document:
+Optionally, various TXT records of the following form, as further described in the TXT Records section of this document:
IN TXT "txtvers=1"
IN TXT "1st=user-first-name"
IN TXT "email=user-email-address"
- IN TXT "ext=optional-list-of-extensions"
IN TXT "jid=user-jabber-id"
IN TXT "last=user-last-name"
IN TXT "msg=freeform-availability-status"
@@ -204,56 +284,61 @@ _presence._tcp.local. port-number IN PTR username@machine-name._presence._tcp.lo
IN TXT "phsh=sha1-hash-of-avatar"
IN TXT "port.p2pj=5562"
IN TXT "status=avail-away-or-dnd"
- IN TXT "txtvers=1"
IN TXT "vc=capabilities-string"
IN TXT "ver=application-version"
]]>
+ Note: In accordance with Section 6.7 of draft-cheshire-dnsext-dns-sd, the "txtvers" record should be the first record specified.
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.
-So, for example, if the machine name is "roundabout", the username is "stpeter", 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:
+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:
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 "10.2.1.187" but when the machine is connected to a wireless network, its physical address might change to "10.10.1.179".
-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., "roundabout-1"), adding the character "-" and digit "2" if the resulting machine name is already taken (e.g., "roundabout-2"), and similarly incrementing the digit until a unique machine name is constructed.
-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., "stpeter-1"), adding the character "-" and digit "2" if the resulting username is already taken (e.g., "stpeter-2"), and similarly incrementing the digit until a unique username is constructed.
+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".
+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.
+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.
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; shall maintain a registry of TXT records for use with the _presence._tcp service type, as specified in the XMPP Registrar Considerations section of this document.
-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.
-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).
-Note: See the Security Considerations 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).
-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; shall maintain a registry of TXT records for use with the _presence._tcp service type, as specified in the XMPP Registrar Considerations section of this document.
+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.
+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).
+Note: See the Security Considerations 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).
+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.
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.
When the _presence._tcp service is used, presence is exchanged via the format described in the TXT Records 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 Multicast DNS.
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 and ver TXT records (and OPTIONAL to include the ext TXT record). The values of these records MUST be the same as the values for the 'node', 'ver', and 'ext' attributes that are advertised for the application in XMPP presence (if any) via the Entity Capabilities protocol as described in XEP-0115.
+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 XEP-0115.
In order to exchange link-local messages, the initiator and recipient MUST first establish XML streams between themselves, as is familiar from RFC 3920.
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:
@@ -261,8 +346,8 @@ psa IN TXT "ver=524"Note: If the initiator supports stream features and the other stream-related aspects of XMPP 1.0 as specified in RFC 3920, then it SHOULD include the version='1.0' flag as shown in the previous example.
@@ -271,8 +356,8 @@ psa IN TXT "ver=524"If both the initiator and recipient included the version='1.0' flag, the recipient SHOULD also send stream features as specified in RFC 3920:
@@ -283,14 +368,16 @@ psa IN TXT "ver=524" ]]>The exchange of stream headers results in an unencrypted and unauthenticated channel between the two entities. See the Security Considerations section of this document regarding methods for authenticating and encrypting the stream.
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:
To end the chat, either party closes the XML stream:
Both parties then MUST close the TCP connection between them.
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.
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.
@@ -319,11 +408,13 @@ _presence._tcp.local. IN NULL raw-binary-data-hereIf 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.
RFC 1035 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.
Although RFC 2317 and RFC 2782 do not allow characters outside the US-ASCII character range in PTR and SRV records respectively, Section 4.1 of DNS-Based Service Discovery 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 DNS-Based Service Discovery. However, as mentioned above, the "machine-name" portion of the <Instance> portion MUST NOT contain characters outside the US-ASCII range.
Although RFC 1464 does not allow characters outside the US-ASCII character range in TXT records, Section 6.5 of DNS-Based Service Discovery 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 DNS-Based Service Discovery.
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.
@@ -344,6 +435,7 @@ _presence._tcp.local. IN NULL raw-binary-data-hereThe 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).
DNS-SD service type names are not yet managed by &IANA;. Section 19 of DNS-Based Service Discovery 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 <http://www.dns-sd.org/ServiceTypes.html> regarding registration of service type names for DNS-SD.
Before this specification was written, there was an existing registration for the "presence" service type, with registration information as follows:
@@ -367,6 +459,7 @@ _presence._tcp.local. IN NULL raw-binary-data-hereThe ®ISTRAR; maintains a registry of TXT records advertised in the context of link-local messaging (see &LINKLOCAL;).
@@ -412,8 +505,17 @@ _presence._tcp.local. IN NULL raw-binary-data-hereThanks to Jens Alfke, Justin Karneges, Marc Krochmal, Eric St. Onge, and Sjoerd Simons for their input.
+Thanks to Emanuele Aina, Jens Alfke, Marco Barisione, Justin Karneges, Marc Krochmal, Eric St. Onge, and Sjoerd Simons for their input.