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<!DOCTYPE html PUBLIC "-//w3c//dtd html 4.0 transitional//en"><html><head> <meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> <meta name="GENERATOR" content="Mozilla/4.76C-CCK-MCD Caldera Systems OpenLinux [en] (X11; U; Linux 2.4.2 i686) [Netscape]"> <title>An Introduction to SLP</title></head><body alink="#ff0000" bgcolor="#ffffff" link="#0000ee" text="#000000" vlink="#551a8b"><h1>An Introduction to the Service Location Protocol (SLP)<br></h1><h2><b>Whitepaper <br></b></h2><b>Original Draft: <a href="http://www.calderasystems.com">CalderaSystems, Inc</a></b><b><br>Final Revision: <a href="http://www.novell.com">Novell, Inc</a></b><hr width="100%"><h3>What Is Service Location Protocol?</h3><p>The <span style="font-style: italic;">Service Location Protocol</span>(SLP) was originally an <span style="font-style: italic;">InternetEngineering Task Force</span> (IETF) standards track protocol thatprovides a framework to allow networking applications to discover theexistence,location, and configuration of networked services in enterprisenetworks.Traditionally, in order to locate services on the network, users ofnetworkapplications have been required to supply the host name or networkaddressof the machine that provides a desired service. Ensuring thatusersand applications are supplied with the correct information has, in manycases, become an administrative nightmare.</p><p>Protocols that support service location are often taken for granted,mostly because they are already included (without fanfare) in manynetworkoperating systems. For example, without Microsoft's SMB servicelocationfacilities, "Network Neighborhood" could not discover servicesavailablefor use on the network and Novell NetWare would be unable to locateeDirectory trees. Nevertheless, an IETF sponsored protocol for servicelocation was notstandardizeduntil the advent of SLP. Because it is not tied to a proprietarytechnology, SLP provides a service location solution that could becomeextremely important (especially on Unix) platforms.<br></p><h3>About this Document</h3>Like all IETF standards, SLP is described in great detail by documentscalled RequestFor Comments (RFC's). IETF RFCs are usually lengthy, very detailed,and written using precise language and notations. Thiswhitepaper, on the other hand, was written to give a general overviewofSLP. This document will, by nature, omit important details that wouldbeinterestingto the more technical reader. For these details, the reader is referredthefollowing RFCs:<blockquote><a href="../../rfc/rfc2165.txt">RFC 2165</a> - ServiceLocationProtocol, Version 1<br> <a href="../../rfc/rfc2608.txt">RFC 2608</a> - Service LocationProtocol,Version 2<br> <a href="../../rfc/rfc2609.txt">RFC 2609</a> - Service Templates andService Schemes<br> <a href="../../rfc/rfc2614.txt">RFC 2614</a> - An API for ServiceLocationProtocol</blockquote><h3>SLP for Users and Administrators</h3>SLP can eliminate the need for users to know the names of networkhosts. With SLP, the user only needs to know the description of theservice heis interested in. Based on this description, SLP is then able toreturn the URL of the desired service.<p>Consider the following example of a new employee setting up hisworkstationto use one of the department printers. Here's a dialog between this newemployee (<span style="font-style: italic;">Newbie</span>), and hiscoworker (<span style="font-style: italic;">Stan</span>), who's beenaround for a while: <br></p><blockquote><b><u>Traditional</u></b> <br> <span style="font-style: italic;">Newbie</span>: "Hey Stan, the setupprogram is asking me for the nameof our printer. What should I type in?"<br> <span style="font-style: italic;">Stan</span>: "Which printer do youwant?"<br> <span style="font-weight: bold;"></span><span style="font-style: italic;">Newbie</span>: "The big one by the copier."<br> <span style="font-weight: bold;"></span><span style="font-style: italic;">Stan</span>: "I've heard it doesn't workwell with postscriptapplications. You'll have to use the one down the hall."<br> <span style="font-weight: bold;"></span><span style="font-style: italic;">Newbie</span>: "Ok. What should I type in."<br> <span style="font-weight: bold;"></span><span style="font-style: italic;">Stan</span>: "Actually, I don't know; Iuse the one by the copier. You'llprobably have to call the IS help desk."<br> <span style="font-weight: bold;"></span><span style="font-style: italic;">Newbie</span>: <groan> ... <p><b><u>With SLP</u></b> <br>A setup program uses SLP to display to the user the description(includinglocation) of the printers that work with postscript. The user selectsonethat is close to his office. The SLP service returns all necessaryaddressing information directly to his device setup application.<span style="font-weight: bold;"></span></p></blockquote><h3>SLP for Software Developers</h3>In many cases, SLP can eliminate the need for software applications toprompt users for host names, or to read host names from configurationfiles.<p>Consider the following example of a software engineer who is writingan LDAP-enabled application. <br></p><blockquote><b><u>Traditional</u></b> <br>Currently, the only way to know the hostname of the LDAP server touse in the call to ldap_init() is to read it from a configurationfile.The configuration file must be created at install time and updated asthelocation of the LDAP server changes. Keeping this configurationfileup to date becomes a real problem, especially when the LDAP applicationis installed on a laptop that connects to various networks. <p><b><u>With SLP</u></b> <br>The developer uses SLP to obtain the host names and attributes of LDAPservers that are displayed to the user at install time, and again iftheuser desires to connect to a different LDAP server.<br> </p></blockquote><p>As illustrated in the example above, SLP does not always eliminatetheneed to prompt users for information. However, it does allow thesoftware developer to present a descriptive list of services that canbeunderstood by the user.</p><blockquote></blockquote><h3>Agents</h3>In order to understand the rest of this document (as well as all otherSLP documentation), you will need to know about SLP <span style="font-style: italic;">agents</span>. InSLPan agent is a software entity that processes SLP protocolmessages.There are three types of SLP agents: <br><blockquote><b>User Agent (UA)</b> <br>The SLP <span style="font-style: italic;">User Agent</span> is asoftware entity that is looking for thelocationof one or more services. Usually implemented (at least partially), as alibrary to which client applications link, it provides clientapplications with a simple interface for accessing SLP registeredservice information. <p><b>Service Agent (SA)</b> <br>The SLP <span style="font-style: italic;">Service Agent</span> is asoftware entity that advertises the locationof one or more services. SLP advertisement is designed to be bothscalable and effective, minimizing the use of network bandwidth throughthe use of targeted multi-cast messages, and uni-cast responses toqueries. </p> <p><b>Directory Agent(DA)</b> <br>The SLP <span style="font-style: italic;">Directory Agent</span> is asoftware entity that acts as a centralizedrepository for service location information. Both Service Agents andUser Agents make it a priority to discover available Directory Agents,as using a Directory Agent minimizes the amount of multi-cast messagessent by the protocol on the network.<br> </p></blockquote><h3>Messages</h3>In order to be able to provide a "framework" for service location, SLPagents communicate with each other using eleven (11) different types ofmessages. The dialog between agents is usually limited to verysimpleexchanges of request and reply messages.<blockquote><b>Service Request (SrvRqst)</b> <br>Message sent by UA's to SA's and DA's to request the location of aservice. <p><b>Service Reply (SrvRply)</b> <br>Message sent by SA's and DA's in response to a SrvRqst message. TheSrvRplycontains the URL of the requested service. </p> <p><b>Service Registration (SrvReg)</b> <br>Message sent by SA's to DA's containing information about a servicethatis available. </p> <p><b>Service Deregister (SrvDeReg)</b> <br>Message sent by SA's to inform DA's that a service is no longeravailable. </p> <p><b>Service Acknowledge (SrvAck)</b> <br>A generic acknowledgment that is sent by DA's to SA's in response toSrvRegand SrvDeReg messages. </p> <p><b>Attribute Request (AttrRqst)</b> <br>Message sent by UA's to request the attributes of a service. </p> <p><b>Attribute Reply (AttrRply)</b> <br>Message sent by SA's and DA's in response to a AttrRqst. The AttrRplycontains the list of attributes that were requested. </p> <p><b>Service Type Request (SrvTypeRqst)</b> <br>Message sent by UA's to SA's and DA's requesting the types of servicesthat are available. </p> <p><b>Service Type Reply (SrvTypeRply)</b> <br>Message by SA's and DA's in response to a SrvTypeRqst. The SrvTypeRplycontains a list of requested service types. </p> <p><b>DA Advertisement (DAAdvert)</b> <br>Message sent by DA's to let SA's and UA's know where they are. </p> <p><b>SA Advertisement (SAAdvert)</b> <br>Message sent by SA's to let UA's know where they are. </p></blockquote><h3>Unicast, Multicast and Broadcast</h3>SLP is a unicast and multicast protocol. This means that themessagesdescribed in the previous section can be sent tooneagent at a time (unicast) or to all agents (that are listening) at thesame time (multicast). A multicast is not a broadcast. Intheory,broadcast messages are "heard" by <i>every</i> node on thenetwork. Multicast differs from broadcast because multicast messagesare only"heard"by the nodes on the network that have "joined the multicast group" - bydefinition, those that are interested in the information.<br><p>For obvious reasons, network routers filter almost all broadcasttraffic. This means that broadcasts that are generated on one subnetwill not beforwarded, or "routed" to any of the other subnets connected to therouter(from the router's perspective, a subnet is all machines connected tooneof its ports). Multicast messages, on the other hand, <span style="font-style: italic;">are</span> forwarded byrouters. Multicast traffic from a given group is forwarded by routersto allsubnetsthat have at least one machine that is interested in receiving themulticastfor that group. <br></p><h3>Agent Dialog Examples</h3>Agent Initialization<br><img src="AgentInit.jpg" height="40" width="237"><p>Service Registration<br><img src="ServiceReg.jpg" height="40" width="237"></p><p>Service Request/Reply<br><img src="ServiceRqst.jpg" height="40" width="237"> <br></p><h3>SLP Application Programmer's Interface</h3>One of the most important parts of the SLP specification is thestandard<span style="font-style: italic;">Application Programmer's Interface</span>(API). The SLP API is a library interfacethat allows programmers to use SLP in their applications to locateservices. Without the API, SLP would be little more than aspecification. Withthe API, developers can add easily add SLP based features to theirprograms. The SLP API provides applications with the same sort ofinterface to service information that the LDAP client API provides toLDAP enabled applications. <br><p>The following is a list of the major SLP API function calls (moreinformationcan be found in the <a href="../ProgrammersGuide/index.html">OpenSLPProgrammer'sGuide</a> or <a href="../../rfc/rfc2614.txt">RFC 2614</a>): <br></p><blockquote> <b>SLPReg()</b> <br>Registers a service URL and service attributes with SLP. <p><b>SLPDeReg()</b> <br>Deregisters a previously registered service. </p> <p><b>SLPFindSrvs()</b> <br>Finds services based on service type or attributes. </p> <p><b>SLPFindAttrs()</b> <br>Obtains a list of attributes for services registered with SLP. </p> <p><b>SLPFindSrvTypes()</b> <br>Obtains a list of the types of services that have been registered withSLP. </p></blockquote><h3>Additional Information</h3>Technical readers probably have additional questions that are beyondthescope of this document.<p><b>Security</b><br>SLPv2 has been designed to be a secure protocol. When properlyimplemented, SLPv2 can ensure integrity and authenticity of data beingtransmittedbetween SLP agents. See <a href="../../rfc/rfc2608.txt">RFC2608</a>, section 9.2 for more information.</p><p><b>Scalability</b><br>SLPv2 was designed to be a scalable solution for enterprise servicelocation. It is not intended to be a solution for the globalInternet. However, as an enterprise solution, SLP can be configured to use"scopes"(see <a href="../../rfc/rfc2608.txt">RFC 2608</a> section 11) and SLPDirectory Agentsinways that should allow it to scale well in very larger networks. Moreconcrete evidence of SLPv2 scalability will become available when SLPismore widely used.</p><p><b>Implementations</b><br>The following is a list of known SLP implementations:</p><blockquote><a href="http://www.openslp.org">OpenSLP</a> <br>An OpenSource project that aims to provide a full SLPv2 implementation <p><a href="http://research.sun.com/slp">Sun Microsystems</a> <br>Offers a "reference implementation" of SLPv2 that is available underthe Sun Community License </p> <p><a href="http://www.novell.com">Novell NetWare</a> <br>SLPv2 is implemented by Novell NetWare servers in NetWare versions 5through 7.<br> </p> <p><a href="http://www.axis.com">Axis Communications</a> <br>Uses SLP in its thin server products </p></blockquote></body></html>⌨️ 快捷键说明
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