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Service Location

Volume Number: 15 (1999)
Issue Number: 10
Column Tag: Network Management

Service Location Protocol

by Alan B. Oppenheimer, Open Door Networks, Inc.

Making IP as easy to use as AppleTalk


Due to the tremendous popularity of the Internet, Internet Protocol (IP) is rapidly becoming the standard for communications over organizations' internal networks (intranets) as well as over the Internet itself. As Macintosh administrators migrate their traditionally AppleTalk networks to IP, they are discovering that much of the ease of use which their customers have previously taken for granted is being lost. Unlike AppleTalk, IP was simply not designed with a focus on the end user. Luckily an effort started nearly ten years ago by Apple Computer is finally starting to pay off, and easy-to-use IP may be just around the corner.

The Service Location Protocol (SLP) is an emerging Internet standard for automatic resource discovery on IP networks. Work on SLP, which brings AppleTalk's point-and-click service location features to IP networks, was initiated by Apple in the late 1980's. As usual, Apple was a bit ahead of its time, and the work progressed slowly for years. The recent explosion of interest in the Internet, coupled with the migration of internal networks to IP, has rekindled interest in the protocol.

Apple's recent inclusion of SLP as part of the new Network Services Location (NSL) Manager in Mac OS 8.5 continues the momentum that is building behind the protocol. As with any new protocol, however, limitations and issues exist which will need to be addressed as the protocol, its implementations, and products that use it mature.

SLP Objectives

According to the SLP specification (RFC 2165), SLP "provides a scalable framework for the discovery and selection of network services" and "eliminates the need for a user to know the name of a network host supporting a service." Sound familiar? To those of us using AppleTalk and the Chooser for the past 14 years, another way of stating this objective is "to enable the creation of a Chooser-like application that works with Internet rather than AppleTalk protocols."

An additional objective of SLP is to address AppleTalk's limitations in the naming area. Specifically, AppleTalk naming services are viewed (correctly or not) as difficult to scale to large network environments. SLP attempts to overcome these limitations by allowing, but not requiring, centralized name servers.

SLP History

As the Macintosh's native networking system, AppleTalk was designed with the Macintosh's principal focus in mind: the end user. Specifically, Apple built a dynamic naming and service location system into AppleTalk. Through the Name Binding Protocol (NBP), services could dynamically register on the network. And through the Macintosh Chooser and equivalent third-party functionality, such services could be browsed for and accessed through Macintosh-standard point-and-click technology. Although originally just used for printers, the Chooser rapidly grew into the standard for finding and accessing other AppleTalk devices, such as AppleShare and compatible file servers.

In the mid 1980s, due mainly to the Mac's success in university environments, Apple realized that a Macintosh implementation of the TCP/IP protocol suite was needed. TCP/IP, designed by and for the research community, had functionality and scalability as its principal goals and paid little attention to ease-of-use. As Apple began implementing and distributing TCP/IP on the Macintosh (MacTCP), it concluded that the TCP/IP protocol suite would need to be enhanced to meet the overall ease-of-use requirements of Macintosh users.

In the late 1980s, Apple approached the Internet Engineering Task Force (IETF), the body responsible for developing Internet-standard protocols. A working group was begun on the subject of finding services on IP networks, using the techniques developed in AppleTalk and NBP as a basis. Although the group made some progress, there did not seem to be enough interest in making IP easy to use, and the SLP effort moved very slowly. It is important to remember that, at that time, the domain name system (DNS) was just getting started, and no one had even heard of a URL.

With the explosion of interest in the Internet and Internet protocols over the past few years, work on SLP has been reinvigorated. SLP version 2 has been proposed and a Service Location Web site has been established <>. A number of vendors have implemented SLP and "connectathons" have taken place to test SLP implementations and enhance interoperability between vendors. Novell recently included an SLP implementation with NetWare version 5. In this area, things are accelerating rapidly.

SLP Details

SLP is based on concepts developed by Apple as part of the Name Binding Protocol (NBP) and the Zone Information Protocol (ZIP). In its simplest form (and the only form currently commercially available), SLP uses the same distributed database ideas used in NBP. That is, a service, wishing to be available by name to clients on its IP internetwork (that is, on its intranet), makes a call to its local SLP implementation. The service provides that implementation (known as a Service Agent, or SA) with its URL and various of its attributes (such as its name and the fact that it's, for instance, a Web or AFP server). The SA stores the passed information in its portion of the distributed naming database, and is from that point on responsible for advertising the service on the network. Note that, unlike with NBP, the SA does not seem to confirm that the service's name is unique on the network.

A client application, wishing to obtain a list of available services of a particular type (or other attribute), calls its local SLP implementation (known as a User Agent, or UA). The UA is responsible for finding services of particular types by looking through the distributed database maintained by SA's throughout the network. To look through the database, the UA sends (via a multicast address) a series of request packets, which are received by all the SA's on the intranet. The requests contain details of the specific attributes of the service being looked for. Any SA that contains information in its portion of the database that matches the request responds directly to the requesting UA with the information. In this way the UA can accumulate the desired information and pass that information back to its client.

The above scheme is essentially identical to NBP's. Additionally, both NBP and SLP contain provisions for looking for services in a hierarchical fashion. In NBP, hierarchy is introduced through the concept of zones and the Zone Information Protocol. In SLP, the equivalents of zones are called "scopes." Scopes allow services to be searched for within administrative domains, just as with zones. Unlike with zones, however, there is currently no specific protocol for maintaining scopes, which must be statically configured into various components of the system.

An area where SLP tries to improve on NBP is through the optional use of centralizing name servers, called Directory Agents or DA's. DA's eliminate the need for UA's to send out multicast packets when looking for services. Multicast packets traverse entire intranets and can result in significant traffic in large network environments (although there are many people of the opinion that the significance of this traffic is quite overrated). In the presence of DA's, UA's send lookup requests to their local DA, which maintains the list of all registered names within the DA's scope. Multicast is not needed. The DA responds directly to the UA. Similarly, SA's send names which have been registered to their local DA, rather than maintaining those names in their part of a distributed database. The address of the DA associated with any particular machine (SA or UA) is obtained in much the same way as the machine's router or DNS, for instance through DHCP or static configuration.

Figure 1. Service Location Protocol Operation.
(with and without Directory Agent)

SLP and Mac OS 8.5

When Apple shipped Mac OS 8.5, it brought the SLP effort it started nearly ten years earlier to fruition. Mac OS 8.5 includes the new Network Services Location (NSL) Manager, an API which enables services to register through protocols like SLP and client-side applications to browse for and initiate access to such services. NSL provides a plug-in architecture for service location. Mac OS 8.5 includes an SLP version 1 plug-in, along with a plug-in that utilizes static DNS-based configuration. It is expected that Apple and third parties will provide additional plug-ins in the future.

As with many of its early "enabling technologies," Apple seems to be counting on third parties to utilize the NSL technology to provide end user solutions. The only use of NSL within Mac OS 8.5 itself is that Personal Web Sharing registers with the NSL Manager. Neither the Chooser nor the new Network Browser, nor Navigation Services utilize NSL to present a list of registered services to the end user. Additionally, no end-user documentation discusses NSL or SLP in any way. The only information available is from Apple's developer Web site <>.

SLP Limitations

Like any new technology, a number of limitations exist with SLP in its current state. Most of these limitations relate to the scalability of the system. SLP adopted many of the best features of AppleTalk, while at the same time trying to improve upon AppleTalk's perceived limitations in large network environments. Many of SLP's scalability features, however, have not yet been fully implemented.

Just as AppleTalk defined the concept of a "zone", in which services could be searched for in a hierarchical manner, SLP defines the very similar concept of a "scope." Scopes, however, are generally statically configured into servers, end nodes and, when available, Directory Agents (DA's). Due to this limitation, Apple chose to base scopes (called "neighborhoods" in NSL) on domain names in their Mac OS 8.5 implementation. Use of domain names for scoping creates various problems in intranet environments. For instance many intranets use only one domain name, without subdomains. Due to these current limitations with scopes, many third-party products, such as Open Door's ShareWay IP, will be unable to take advantage of scoping, and will have to present services in a non-hierarchical list for the time being.

SLP also attempted to mimic AppleTalk's ability to provide dynamic naming services without need for any centralized name servers or other agents. SLP uses IP multicast for this purpose, just as NBP uses a zone-wide broadcast. As with zone-wide broadcast, IP multicast requires the cooperation of IP routers, the devices which connect IP subnets together to form intranets. Most current IP routers implement IP multicast, which is used for such features as IP-based audio and video broadcasting and video conferencing. However IP multicasting may not be completely implemented across some intranets. In the absence of IP multicasting, SLP name lookups will only work within the subnet on which they are performed, or within the groups of subnets over which IP multicast is supported. Additionally it appears that Apple's Mac OS 8.5 implementation of SLP artificially limits the radius of any SLP search to three subnets from the initial requestor.

In large environments, SLP attempts to improve upon NBP by optionally allowing centralizing Directory Agents. DA's minimize the need to use IP multicast, and can result in significantly less traffic than a completely distributed system like NBP. They can also enhance the protocol's hierarchical scope concept, and provide other services. Unfortunately, at the current time, no commercially supported DA is available, so IP multicast must continue to be used. As SLP momentum continues, however, it is expected that DA's will be commercially available in the near future.

An additional problem with SLP seems to be that it does not attempt to prevent duplicate name registration. In the dynamic environments which Macintosh users are used to, it is only a matter of time before this piece of NBP functionality is missed.

As developers, vendors and users learn from SLP experience, changes to the protocol will undoubtedly be made. Work is proceeding on SLP version 2, which enhances use of IP multicast to achieve greater efficiencies, addresses other limitations and provides new features. SLP v2 is not yet commercially available however.

Despite its current limitations, SLP remains very useable for specific tasks, such as dynamic service location on all but the largest of intranets. Even within large intranets, SLP should function well for workgroup-specific service location. And as both Apple and third-parties enhance their implementations, most current limitations should soon be eliminated, providing an IP service location system that is just as easy to use as AppleTalk's.

SLP Futures

Trying to guess how anything related to the Internet is going to evolve is asking for trouble. Trying to guess how anything related to Apple and the Internet is going to evolve is asking for even more trouble. It seems likely, however, that Apple will integrate more and more SLP technology directly into the Mac OS. Both the Network Browser and Navigation Services (the Standard File replacement) seem like natural places to integrate SLP browsing functionality. Both of these utilities can already browse AppleTalk-based servers through NBP, so browsing through SLP definitely seems like an appropriate next step. On the other hand, it appears as if these technologies are intended to eventually replace the Chooser, which has always been AppleTalk- and printer-centric. Thus SLP browsing may well not be integrated into the Chooser.

It would also be a relatively trivial matter for Web browsers and FTP and other Internet clients to add support for SLP browsing, in much the same way as Open Door added such support to AFP Engage! With SLP now built into the Mac OS, we could quickly see the appearance of application-specific browsers. A chicken-and-egg issue exists with this scenario however. Client application developers want servers to use SLP before they add SLP support, and server developers want clients to do the same. One of the reasons Apple built SLP registration into Mac OS 8.5's Personal Web Sharing, and one of the reasons Open Door is pushing the technology, to is to get around this chicken-and-egg problem.

Other NSL plug-ins may also come to market, either from Apple or third parties. In particular, the Lightweight Directory Access Protocol (LDAP) is becoming popular in certain environments, and could be used as the basis of various service location schemes, although additional work would be needed on how to use directories for service location. Gateways between SLP and LDAP are also envisioned.

Another area for potential future development is SLP proxies. Envisioned by the SLP specification, an SLP proxy is a device that registers (acts as a User Agent) for another device that cannot do so. Through SLP proxies, SLP support can be added to legacy devices that don't support SLP, such as printers and older Web servers. Finally, SLP includes a rich set of features which are currently not being taken advantage of, such as the ability to associate any number of attributes with any device, and to search by attribute. It is very much hoped that we see implementation and use of these advanced features in the near future.


After a number of years of inattention, Apple's effort to bring AppleTalk's ease-of-use to Internet protocols is beginning to pay off. The Service Location Protocol shows promise for providing a much-needed means of dynamically browsing for and selecting services in intranet environments. As with most new protocols, limitations and issues remain, and third party implementations are scarce, but the inclusion of SLP in Mac OS 8.5, coupled with efforts such as Open Door's ShareWay IP 2.0 should serve to further enhance the momentum behind SLP and lead to a much easier to use Internet.

SLP in Open Door's ShareWay IP and AFP Engage! 2.0 Products

Open Door Networks has accepted Apple's challenge and is providing the first end-user products to take advantage of NSL and SLP. Open Door's president (and author of this article) Alan Oppenheimer was involved in both the design of AppleTalk and Apple's initial SLP efforts. Open Door's ShareWay IP 2.0 product (see sidebar), which provides AppleTalk Filing Protocol (AFP) file service using IP protocols, registers with NSL to make itself visible through SLP. And Open Door's AFP Engage! 2.0 product acts as a "Chooser" for SLP-registered AFP servers, displaying a dynamically updating, browseable list from which users can select the server they wish to initiate access to. Through Open Door's ShareWay product family and SLP, Macintosh file sharing services, including Personal File Sharing, become as easy to use over IP as over AppleTalk.

When Apple told us about SLP, it became clear that ShareWay IP (figure 2) was a perfect product to which to add SLP functionality. the product provides any AppleTalk-based AFP server with the ability to deliver AFP file service over IP. Although best known for its ability to add IP capabilities to the Mac's built-in personal file sharing, ShareWay IP also lets older versions of AppleShare, Windows NT, Novell NetWare and Unix AFP servers provide IP-based service.

ShareWay IP 2.0 uses SLP to register the URL to its targeted server on the network. Since Apple's implementation is somewhat limited in terms of its support for SLP attributes, ShareWay IP includes all the relevant attributes in the URL registered. Specifically, the URL registered is of the form:

  • afp://IP-address/?NAME=TargetMachineName&ZONE=ZoneName
  • IP-address is the IP address of the ShareWay IP machine
  • TargetMachineName is the target server’s machine name
  • ZoneName is the name of the AppleTalk zone, if any, in which the target server resides

As we began to investigate Apple's SLP implementation, it also became clear to us that we were going to have to write a client-side browser application. Despite the obvious opportunities for integrating SLP with both the Chooser and the new Network Browser (let alone Navigation Services), Apple's 8.5 implementation provides no client-side application for browsing SLP-registered services. Although we expect Apple to provide such integration in the future, we didn't want to wait for what could be an indeterminate amount of time. We also already have a somewhat appropriate client-side application. Our AFP Engage! product had been serving as a helper app for AFP URLs for over a year, enabling users to click on AFP URLs in Web browsers, email readers and other Internet applications. The addition of an SLP browser for AFP servers thus seemed like a natural addition.

Figure 2. Browsing IP-based AFP servers using SLP and AFP Engage!

AFP Engage! 2.0 includes a new, Chooser-like window (figure 2) that lists all SLP-registered AFP servers. AFP Engage! simply calls the NSL Manager to obtain a list of any servers registered with AFP URLs, and displays the associated names of those servers in the window. When the user double-clicks on one of the servers, AFP Engage! calls the AppleShare client to mount the associated server, just as it has done previously for AFP URLs. Due to limitations in Apple's implementation of SLP scopes, scoping is ignored by both ShareWay IP and AFP Engage!, and servers are displayed in a single, non-hierarchical list. We expect to add scoping in the future.

The combination of ShareWay IP 2.0 and AFP Engage! 2.0 thus serve to provide AppleTalk-like ease-of-use for ShareWay-supported AFP servers. In fact, with AFP Engage's ability to process AFP URLs, the combination provides a solution which in some ways is even more flexible than AppleTalk's - users can access servers either the AppleTalk way, through dynamic browsing and point-and-click, or the IP way, through URLs.

Open Door is hopeful that these two products are just a start, and that we see such ease-of-use and flexibility added over time to more and more network products. Apple has set the stage. Now it's time for both third parties and end users to start acting!

Alan Oppenheimer is president and founder of Open Door Networks Inc., a Macintosh Internet tools and services company. Before founding Open Door in January 1995, Alan worked at Apple Computer for 11 years, where he was one of the creators of the AppleTalk network system.


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