This document describes how to establish Jabber communications over local networks using zero-configuration networking.
WARNING: This Standards-Track JEP is Experimental. Publication as a Jabber Enhancement Proposal does not imply approval of this proposal by the Jabber Software Foundation. Implementation of the protocol described herein is encouraged in exploratory implementations, but production systems should not deploy implementations of this protocol until it advances to a status of Draft.
Type: Standards Track
Last Updated: 2006-02-09
JIG: Standards JIG
Approving Body: Jabber Council
Dependencies: XMPP Core, RFC 3927, draft-cheshire-dnsext-dns-sd, draft-cheshire-dnsext-multicastdns
Superseded By: None
Short Name: N/A
Wiki Page: <http://wiki.jabber.org/index.php/Link-Local Messaging (JEP-0174)>
This Jabber Enhancement Proposal is copyright 1999 - 2006 by the Jabber Software Foundation (JSF) and is in full conformance with the JSF's Intellectual Property Rights Policy <http://www.jabber.org/jsf/ipr-policy.shtml>. This material may be distributed only subject to the terms and conditions set forth in the Creative Commons Attribution License (<http://creativecommons.org/licenses/by/2.5/>).
The preferred venue for discussion of this document is the Standards-JIG discussion list: <http://mail.jabber.org/mailman/listinfo/standards-jig>.
Given that this JEP normatively references IETF technologies, discussion on the JSF-IETF list may also be appropriate (see <http://mail.jabber.org/mailman/listinfo/jsf-ietf> for details).
The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 3920) and XMPP IM (RFC 3921) specifications contributed by the Jabber Software Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this JEP has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.
The keywords "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119.
XMPP as defined in RFC 3920  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 authentication with a server, and the client is not granted access to the network if authentication fails for any reason. If a client attempts to communicate directly with another client, such communication will fail because all XMPP communications are sent through a server 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 addresses 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. Note well that such communications are 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 "client mesh" is most accurately referred to as an XMPP-like system that exists outside the context of existing XMPP networks.
Such local, peer-to-peer Jabber meshes 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 local Jabber communication has been built into 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 local Jabber communication.
|Bonjour||Apple Computer's implementation of zero-configuration networking, formerly known as Rendezvous. See <http://www.apple.com/macosx/features/bonjour/>.|
|DNS-SD||A convention for naming and structuring DNS SRV records (see RFC 2782 ) such that a client can dynamically discover a domain for a service using only standard DNS queries. See DNS-Based Service Discovery . For a full list of registered DNS-SD records, see <http://www.dns-sd.org/ServiceTypes.html>.|
|Link-local address||An IPv4 or IPv6 address that is valid for communication with other devices connected to the same physical or logical link. See RFC 3927 .|
|Multicast DNS||A technology that provides the ability to perform DNS-like operations on the local link in the absence of any conventional unicast DNS server. See Multicast DNS .|
|Zero-configuration networking||A set of technologies that enable the use of the Internet Protocol for local communications. See <http://www.zeroconf.org/>.|
In order to advertise its availability on a local network, a client MUST advertise a DNS-SD service of "_presence._tcp.machine-name.local.". For example, if the machine is "roundabout", the advertised service would be "_presence._tcp.roundabout.local.". The IPv4 and IPv6 addresses associated with that machine will vary depending on the local network to which the machine is connected. For example, on an Ethernet connection the physical address might be as follows:
However, when the machine is connected to a wireless network at a conference, its physical address might change to the following:
The JID shown to other entities on the network SHOULD be of the form "user@machine-name". For instance, in the foregoing examples, the user's address would be shown as "stpeter@roundabout".
When the _presence._tcp service is used, presence is exchanged via the format described in the TXT Keys section of this document. In particular, presence information is not pushed as in XMPP (see RFC 3921 ). Instead, clients poll for the presence of other local entities by periodically sending new DNS SRV requests for the _presence._tcp service. When the number of local entities is large, polling can result in a large number of request and response packets being sent over the local network. Therefore, care should be taken to minimize the number of polls (e.g., polling no more than once a minute on networks with a small number of entities and no more than once every several minutes on networks with a large number of entities).
In order to exchange Jabber communications, the sender specifies 'to' and 'from' addresses using the logical local addresses:
<message to='hildjj@wolfram' from='stpeter@roundabout'> <body>hey, testing out link-local communications</body> </message>
DNS-SD enables service definitions to include various TXT keys that specify parameters to be used in the context of the service type. The TXT keys defined for the _presence._tcp service are as follows:
|1st||The first name of the user.|
|The email address of the user.|
|jid||The Jabber ID of the user.|
|last||The last name of the user.|
|msg||Natural-language text describing the user's state. This is equivalent to the XMPP <status/> element.|
|phsh||The SHA-1 hash of the user's avatar icon or photo. |
|port.p2pj||The port for peer-to-peer Jabber communications. The port SHOULD be 5298.|
|status||The presence availability of the user. Allowable values are "avail" and "dnd".|
|txtvers||The version of the TXT fields supported by the client. This document describes txtvers "1".|
|vc||A flag advertising the user's ability to engage in video conferencing. Allowable values are "C!".|
XMPP networks depend on TLS (RFC 2246 ) for channel encryption, SASL (RFC 2222 ) for authentication, and the Domain Name System (RFC 1034 ) for validation of server hostnames; these technologies help to ensure the identity of sending entities. By contrast, zero-configuration networking uses dynamic discovery and asserted machine names as the basis of sender identity. Because of the extremely different nature of full XMPP networks and localized client meshes, 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. However, a client on a local peer-to-peer 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).
The p2pj port of 5298 is not included in the IANA Port Numbers Registry  maintained by the Internet Assigned Numbers Authority (IANA) . The author will investigate whether the port needs to be registered.
This JEP requires no interaction with the Jabber Registrar .
Thanks to Jens Alfke for his assistance.
3. DNS-Based Service Discovery <http://files.dns-sd.org/draft-cheshire-dnsext-dns-sd.txt>. Work in progress.
5. Multicast DNS <http://files.multicastdns.org/draft-cheshire-dnsext-multicastdns.txt>. Work in progress.
7. 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 caches it. Implementations should also include logic for expiring avatar icons.
12. The Internet Assigned Numbers Authority (IANA) is the central coordinator for the assignment of unique parameter values for Internet protocols, such as port numbers and URI schemes. For further information, see <http://www.iana.org/>.
13. The Jabber Registrar maintains a list of reserved Jabber protocol namespaces as well as registries of parameters used in the context of protocols approved by the Jabber Software Foundation. For further information, see <http://www.jabber.org/registrar/>.