On the Wire Autoconfiguration for IP Networking: Enabling Local Communication Erik Guttman Sun Microsystems, Germany It would be ideal if a host implementation of the devices via IP. In this tutorial, I examine the back- Internet protocol suite could be entirely self-con- ground, current status, and future prospects for figuring. This would allow the whole suite to be zero configuration networking. implemented in ROM or cast into silicon, it would simplify diskless workstations, and it would be an Zero Configuration Networking immense boon to harried LAN administrators as Automatic configuration parameters have different well as system vendors. We have not reached this properties from those assigned by static and ideal; in fact, we are not even close. —RFC 11221 dynamic configuration. They are ephemeral; they will likely be different each time they are obtained P hosts and network infrastructure have histor- and might even change at any time. Automatical- ically been difficult to configure—requiring net- ly configured hosts actively participate in assign- I work services and relying on highly trained net- ing and maintaining their configuration parame- work administrators—but emerging networking ters, which have only local significance. Autonomy protocols promise to enable hosts to establish IP net- from network services implies that hosts must net- works without prior configuration or network ser- work configuration. vices. Even very simple devices with few computing In direct contrast, normal IP configuration is resources will be able to communicate via standards- persistent (especially for servers), or at the very track protocols wherever they are attached. Current least, stable. The IP protocol suite aims at scala- IETF standardization efforts, such as those in the bility, especially with respect to configuration. Zeroconf working group, aim to make this form of Addresses and names often have global signifi- networking simple and inexpensive. cance, which has proven essential for enabling Hosts that are permanently connected to an Internet growth. Obtaining and managing global administered network are usually assigned static addresses and names requires a great deal of network configurations by network administrators. administrative work, however. These processes are Other hosts are attached to administered networks not at all automatic and likely never will be. (such as corporate local-area networks or dial-in Despite these differences, the essential zero con- accounts) using dynamic network configuration. figuration networking protocols really imply In these, all necessary parameters are assigned to changes to only the lower layers of IP-enabled the host by a network configuration service, which devices. (See the sidebar “IP Host Layering” for an also requires configuration. In many situations— introduction to the terminology required for dis- impromptu meetings, administered network mis- cussing automatic configuration.) configurations, or network service failures, for Existing network-based applications will work example—establishing an IP network is desirable, without modification over enhanced network ser- but administering it can be impractical or impos- vice and application layers using standard interfaces. sible. In these cases, automatic network configu- Indeed, users should not even be aware that the net- ration of parameters is valuable for hosts. The IETF work service layer has been configured automati- Zeroconf WG’s real goal is to enable direct com- cally rather than statically or dynamically. munications between two or more computing Four functions will benefit from zero configu- IEEE INTERNET COMPUTING 1089-7801/01/$10.00©2001 IEEE http://computer.org/internet/ MAY • JUNE 2001 81 Tutorial IP Host Layering features. Adopting emerging zero configuration Layering provides the foundation for numerous, stable extensible comput- protocol standards will let us retire proprietary net- ing platforms. Figure A depicts the pervasive layered architecture, often working—a move that has broad support. Even net- called the IP stack, which is used for Internet hosts.This roughly corre- work equipment vendors uniformly accept that sponds to the OSI seven-layer model.1 The figure excludes the OSI pre- proprietary network protocols have seen their day sentation and session layers. IP applications implement data presentation and should be replaced by IP standards. functions themselves. Session features such as encryption, compression, or For reasons of scalability and reducing impact persistence between protocol transactions are added in an ad hoc fashion on existing networks, the zero configuration pro- at various layers. tocols’ effect on the overall network must be lim- Each layer provides services to the layer above it through standard inter- ited. The algorithms used for zero configuration faces. If lower layers provide the same functionality using the same inter- protocols generally use multicast. In practice, these faces, services can be implemented in different ways; new mechanisms protocols are limited to either a single network link defined at the network services layer can thus support unmodified exist- (that is, routers do not forward these protocol mes- ing applications.Avoiding changes in upper layers eases the adoption of new sages) or to a set of networks (where some routers Internet technologies. Network service layer enhancements that require are configured as boundaries, over which protocol client applications (such as e-mail readers and Web browsers) to be upgrad- messages are not forwarded). ed are not broadly adopted. Each layer could be automati- Defining an Approach cally configured. In practice, the less Those working on IETF zero configuration proto- configuration required, the better, Application col standardization (currently in the Zeroconf, Ser- because simpler technology works vice Location Protocol, DNS Extensions, and IPng Transport service more predictably and eases deploy- working groups) have considered two main ment. The transport service, link Network service approaches to overcoming the differences between control, and media access layers configured and automatic operation. rarely require configuration in Link control The first strategy requires transitions between Internet hosts. By contrast, the local and global configuration and has been application and network service explored through consumer-oriented operating layers nearly always require config- Physical network system software since 1998. This strategy implies uration in order to operate at all. that hosts would support automatic configuration only for as long as they lacked global configura- References Figure A. Internet host layering. Zero tion. The two modes are exclusive, and the pres- 1. A.Tanenbaum, Computer Networks, Sec- configuration protocols will be implemented ence of a dynamic configuration service requires ond Edition, Prentice-Hall, Englewood at the application and network service a transition from automatic (local) to dynamic Cliffs, N. J., 1989. layers of the Internet protocol stack. (global) configuration. An example of this transition strategy is the network interface autoconfiguration protocol ration protocols, in the context of both IPv4 and adopted for desktop operating systems from Apple IPv6. With no modification to existing interfaces, and Microsoft. This protocol (which the IETF has zero configuration protocols will improve name- not yet standardized) enables a host to simply to-address translation (at the application level) and choose an unassigned IP address from a reserved IP interface configuration (at the network level). range. The host then attempts to obtain (global) IP Functions previously unavailable to IP hosts will configuration parameters from the network via the introduce new interfaces: service discovery at the Dynamic Host Configuration Protocol.2 The host application layer and multicast address allocation issues periodic DHCP requests, which will eventu- at the network layer. ally succeed in reaching a DHCP server if one ever These additional services will not disrupt exist- becomes available on the network. Once a DHCP ing applications. They will “raise the bar” by pro- server responds and offers IP configuration para- viding additional features long absent from the meters, these replace automatic configuration. Internet protocol suite, but (in the case of service This mechanism works fine for clients discovery) available in proprietary network proto- employing common client-server protocols col suites from Apple, Microsoft, and Novell. (See because very few make use of long-duration the sidebar “Early Autoconfiguration Efforts”, next connections. Individual network application page). These proprietary protocols continue to be operations result in distinct transactions even used only because of their ease-of-configuration when connections fail. If the client host experi- 82 MAY • JUNE 2001 http://computer.org/internet/ IEEE INTERNET COMPUTING On the Wire Early Autoconfiguration Efforts ences network reconfiguration, applications sim- ply establish new connections. The Internet Protocol suite emerged as the data communication standard for If a server’s configuration changes, however, a network run by and for researchers.Their design goals were primarily inter- recovery is not so easy: Client applications cease operability, extensibility, and scalability. IP networks achieved growth and to function if they cannot find a server. Servers enabled global communication by using unique