Zero Configuration IP

*** Note from Asmodian X : This was written over a year ago but the
information is still viable from an educational standpoint. ***

Zero Configuration IP


Welcome Back! It is good to be writing again after the 5+ year hiatus. The target audience of this article are people who are of intermediate experience with networking. Almost all certificates and IT related programs have TCP/IP as a goodly sized chunk of their curriculum, therefore I should expect that a reader would know what TCP/IP was and how it works. For more general information on TCP/IP see: http://en.wikipedia.org/wiki/TCP/IP.

Shout outs to Axon, Frogman and Methodic.


0x01 ................... Objective
0x02 ................... Definition of Zero Configuration Networking
0x03 ................... How ZCN works
0x04 ................... The who's who of implementations
0x05 ................... Works Cited


----Part 0x01 Objective

The objective for this discussion is to gain familiarity with a part of the TCP/IP implementations called Zero Configuration Networking. Most notably Apple's newest operating system (OS 10) contains Apple's implementation of ZCN called Bonjour (also known as "Rendezvous"). Bonjour allows apple computers or any other computer using the Zero-Conf standard to immediately be able to use a network with out the use of manual network configuration or some form of DHCP http://en.wikipedia.org/wiki/DHCP). Windows XP implements some form of Zero-Conf networking for their wireless applications. Even MacOS 9 had this automatic configuration feature. The goal for Apple was to replace their Appletalk protocol with something that is more scalable. Zero configuration networking also has a specification for service broadcasting using parts of the DNS protocol called multi-cast-DNS and unicast-DNS.

----Part 0x02 Definition of Zero Configuration networking

Zero configuration networking is the ability for an un-administrated network node to be able to auto negotiate a network configuration requiring little or no user configuration. This configuration system is optimal for Ad-Hoc wireless networks, home networks and for emergency relief stations (Williams, 2002). A zero configuration network system should include the ability to configure itself in a fashion that allows it to talk to other similar hosts using the TCP/IP protocol. The randomly chosen addresses are checked to make sure that nodes do not collide with one another on the network. Part of this networking structure is to implement a service location capability. The reasoning for using a service location protocol is that the end user does not know what address the zero configuration system has chosen and therefore cannot easily find shared resources (Guttman, 2001)(Cheshire, Kochmal, 2004). Other networking protocols which require auto configuration are Multicast IP and IPv6 (Octavian, 2002).

----Part 0x03 How ZCN works

ZCN in most cases consists of a default behavior of a network interface. (Though some OS's have a separate utility for doing it which is not a fail-over condition.) The behavior first starts with the interface being in an automatic mode, such as either DHCP being selected or zero Conf mode being enabled. The interface shall then try to configure itself via DHCP. IF that fails then it will default to zero configuration mode that chooses an IP address on the 169.254/16 subnet. The 169.254/16 subnet being a private address space reserved for ZCN. If ARP detects an address collision (using an ARP broadcast) then it will back off and choose another random place on the network until it has found a suitable unused address. Assuming all hosts on this network follow the same procedure every one is now able to talk to each other. If at any time the interface is configured to use a rout-able address it must leave the link-local addressing scheme (Cheshire, et al. 2004). There are, of course, exceptions such as Un*x's virtual networking interface but because this is a link-local addressing system the addresses used are not rout-able unless something like network address translation (NAT) is used.
The catch at this point is that each computer is now able to speak to each other, but no one at this point realizes that any one else is on the network. In order to find resources on this new improvised network requires some form of advertisement protocol. There are many was to do service location, one way to do service location is NetBIOS. Microsoft originally used NetBIOS, to facilitate the creation of local area networks. Apple computers came from a similar desire to make local area networks and they called their protocol set Appletalk. Both NetBIOS and Appletalk broadcast over a subnet to advertise services. Both Apples Appletalk protocol and NetBIOS had issues when scaling into a large network with its high overhead (Cheshire, Stuart. Krochmal, Marc., August 2004). The newest method for ZCN service location consists of each workstation transmitting a special Multicast DNS broadcast to advertise its services to all of the other connected clients. Aggressive caching of these requests and responses keeps the overall service location traffic low (Cheshire, et al. 2004).
Multicast service broadcasting relies on several key items. Each station must have a mDNS responder that listens for requests and responds with a list of applicable services. The response is via Multicast so that all stations on a given subnet may hear and record this service. Unicast DNS can also be used to do this when crossing subnets. Since this system is designed for small implementations the need for routability is not acute. Recently a draft is being worked on by Apple that defines a NAT-Portmaping protocol so that a router can be added in a ZCN environment and all users can auto-magically gain access to that Internet connected device.(Apple, "Network Address Translation ...", 2004)(Apple, "Rendezvous FAQ", 2005).

----Part 0x04 The who's who of implementations

First and foremost the Apple Computer Corporation has an advanced implementation of zero configuration networking called "Bonjour." ZCN is implemented in various forms since Mac-OS 9 in whatever extent it's full implementation may be found as a part of Mac-OS 10.1 and newer. Apple has released C source for use on any other platform including windows.

A sketchy implementation for Linux and the BSD's called "HOWL"
is available for almost all flavors via source.

The sourceforge ZCN implementation for the link-local addressing
portion is the Zero-Conf project at sourceforge. It does not have the
Multicast DNS portions in working order, instead suggesting the use of
OpenSLP. (http://www.openslp.org/) (http://zeroconf.sourceforge.net)

Avahi Multicast DNS client.

One of the stronger Linux/BSD implementations currently implemented in
many distributions including Ubuntu and more.

----Part 0x05 Works Cited

Apple Corp. (December 2004). Rendezvous

Apple Corp. (January 2005). Rendezvous FAQ.

Apple Corp. (July 2004). Network Address Translation Port Mapping
Protocol. http://files.dns-sd.org/draft-nat-port-mapping.txt

Cheshire, Stuart. Aboba, Bernard. Guttman, Erik. (July 2004).
Dynamic Configuration of IPv4 Link-Local Addresses.

Cheshire, Stuart. Krochmal, Marc. (February 2004). DNS-Based Service

Cheshire, Stuart. Krochmal, Marc. (August 2004).
Requirements for a Protocol to Replace AppleTalk NBP.

Guttman, Erik. (July 2001). Zero-Conf Host Profile Applicability

Octavian, Catrina. Thaler, Dave. Aboba, Bernard. Et al. (October 2002).
Zero-Conf Multicast Address Allocation Protocol (ZMAAP).

Open SLP Website. (January 2005).

Porchdog software inc. (January 2005). Howl Project Website.

Williams, A. (September 2002) Zero Configuration Networking.

Zero-Conf sourceforge website. (January 2005).

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