Automatic Discovery of IP Network Services As computers become more AUTOCONFIGURATION SERVICE LOCATION PROTOCOL portable and networks larger and more pervasive the need to automate the location E RIK G UTTMAN Sun Microsystems and client T he complexity of configuring every element in the network clients servers peers and infrastructure is a key problem facing network technology s advance As long as configuration remains difficult network administration will be expensive tedious and troublesome and users will be unable to take advantage of the full range of capabilities networked systems could provide The Service Location Protocol1 is an Internet Engineering Task Force standard for enabling network based applications to automatically discover the location including address or domain name and other configuration information of a required service Clients can connect to and make use of services using SLP Currently without SLP service locations must be manually configured or entered into a configuration file SLP provides for fully decentralized operation and scales from small unadministered networks to large enterprise networks with policies dictating who can discover which resources This article describes SLP s operation and how it adapts to conditions where infrastructure is not available where administration is minimal or where network administrators simply wish to reduce workload BACKGROUND The Service Location Protocol SVRLOC working group has been active in the IETF for several years In 1997 the group published SLP Version 1 as a Proposed Standard RFC 1 In June 1999 the Internet Engineering Steering Group announced that Version 2 and its related documents were promoted to Proposed Standard RFCs as well 2 SLPv2 which updates and replaces SLPv1 is the subject of this article It removes several of the originally imposed requirements provides protocol extensibility new options can be added without modifying the base protocol adheres to new IESG protocol recommendations improves security and eliminates a number of inconsistencies in the SLPv1 specification IEEE INTERNET COMPUTING 1089 7801 9 9 10 00 1999 IEEE configuration for network services also increases The Service Location Protocol is an IETF standard that provides a scalable framework for automatic resource discovery on IP networks http computer org internet JULY AUGUST 1999 71 A U T O C O N F I G U R A T I O N Active DA discovery Service request DA DA advertisement UA SA Passive DA discovery DA DA advertisement UA SA Figure 1 Methods of DA discovery In active discovery User Agents and Service Agents multicast requests to locate Directory Agents on the network whereas in passive discovery UAs and SAs learn of DAs via periodic multicast advertisements 72 JULY AUGUST 1999 SRVLOC working group sought to correct this problem by minimizing the impact of service discovery on the network SLP uses multicast and Dynamic Host Configuration Protocol3 to initialize its scalable service discovery framework without the need for configuring individual SLP agents SLP can operate in networks ranging from a single LAN to a network under a common administration also known as an enterprise network These networks can be quite large potentially tens of thousands of networked devices Neither multicast discovery nor DHCP scales to the Internet since these protocols must be configured and administered Moreover the Internet lacks a common centralized administration To the extent that SLP relies on either multicast discovery or DHCP for its own configuration SLP does not scale to the Internet Backward compatibility with SLPv1 depends on whether the Directory Agent described in the next section supports both versions For example Sun has successfully implemented a backwardly compatible SLP DA Otherwise backward compatibility requires a Service Agent also described below to implement both versions of the protocol Current SLP Implementations Sun Microsystems Novell IBM Apple Axis Communications Lexmark Madison River Technologies and Hewlett Packard have adopted SLPv1 and increasingly SLPv2 for products There are also two reference implementations of SLPv2 available from http www srvloc org Problems with Earlier Protocols Prior to SLP service discovery protocols allowed users to discover services only by type For instance both Apple and Microsoft offered networking protocols that could discover instances of printers and file servers and users had to then select from the list to meet their needs From the beginning the SVRLOC working group sought a solution that would allow network software to discover services according to their characteristics as well as type Thus clients would be able to explicitly discover services that met their requirements and software could automatically obtain the service location without bothering users On the other hand since services are advertised along with their characteristics SLP also enables rich user interaction SLP enables browser operations since the protocol includes a set of directorylike functions Thus clients using SLP can browse all the available types of service These clients may also request the attributes of a class of service which aids in formulating interactive requests Finally SLP makes it possible to look up the attributes of a particular service once it has been discovered Another problem with the proprietary protocols was their notorious lack of scalability The PROTOCOL OVERVIEW http computer org internet SLP establishes a framework for resource discovery that includes three agents that operate on behalf of the network based software User Agents UA perform service discovery on behalf of client software Service Agents SA advertise the location and attributes on behalf of services Directory Agents DA aggregate service information into what is initially a stateless repository Figure 1 illustrates the two different methods for DA discovery active and passive In active discovery UAs and SAs multicast SLP requests to the network In passive discovery DAs multicast advertisements for their services and continue to do this periodically in case any UAs or SAs have failed to receive the initial advertisement UAs and SAs can also learn the locations of DAs by using the DHCP options for Service Location SLP DA Option 78 4 DHCP servers configured by network administrators can use DHCP Option 78 to distribute the addresses of DAs to hosts that request them SLP agents configured in this man IEEE INTERNET COMPUTING S E ner