Plug it and Forget it

You walk into a room with a MacOS X laptop and Wi-Fi card, and instantly see the shared files of other Mac or even Windows users? Sounds interesting? Such a setup can be based on Rendezvous, an IP networking, zero-configuration networking standard from Apple, which is included in the latest version of Mac OS X (Jaguar). 

With Rendezvous, you can create an instant network of computers, devices and software apps automatically over any IP network, such as Ethernet or 802.11-based wireless networks. You can access the services and capabilities of any Rendezvous-enabled device connected to the network without the need of a third-party server. Such auto configuration is useful when the networks are small or lack dedicated personnel for administration or when devices are connected on ad-hoc basis.

Generally, protocols such as AppleTalk (for Macs) and NetBIOS (for Windows) are used for network configuration and auto discovery of services running on the computers connected to it. But these two are incompatible, making things go haywire when you connect a Mac to a Windows-based computer. Moreover, these computers need to use IP to communicate over the Internet. This means that a developer needs to incorporate support for both protocols at both the local level and IP for the Internet. The need for a solution to such a problem is one of the driving forces behind Rendezvous. 

A problem with IP is that it lacks auto configuration and auto discovery features, which are present in Aplpletalk and NetBIOS. So, on an IP-based network, DNS and DHCP servers are required to maintain the sanity of the network. Rendezvous achieves automatic discovery without the need of such servers. The features that have been implemented in Rendezvous are based on the IETF’s (Internet Engineering Task Force) recommendation on ZeroConfiguration (www.zero conf. org). 

In an IP-based system, the DHCP server acts as a central repository of IP addresses and distributes them to computers asking for an IP address. In Rendezvous, there is no central administration. The nodes here use link-local addressing, in which a computer or any device connected to the network randomly selects an IP address from a predefined range of addresses and assigns that to itself. Then it broadcasts a message to other nodes to check if some other node is using this IP or not; if yes, then the computer again randomly selects an IP address, and so on. Once it assigns itself an address, it is on the network and can send and receive data. 

To perform naming services (to correlate between English names and IP addresses), a DNS (Domain Name Service) server is generally used. But, Rendezvous uses a slight variation called mDNS-SD (Multicast Domain Name Service—Service Discovery) to advertise its name and the services it offers. This notification includes essential information like name, IP and port addresses. Each device connected to the network reads this advertisement and stores the information in a lightweight personal DNS server. The apps running then query the personal DNS server for information, for example, apps that need printing capability query this DNS server on the same computer and then display the result on their own interface.

On the downside, Mac OS 9 users will not be able to use the Rendezvous because of lack of an upgrade option. Also, zero-config- uration networks can be breached because of inherent security issues in a multicast network setup. However, Rendezvous has measures to restrict access of your shared data to a particular subset of user(s). 

Ankit Khare