network protocols handbook.pdf

35Protocols GuideTCP/IP - Application Layer ProtocolsProtocol NameSLP: Service Location ProtocolProtocol DescriptionThe Service Location Protocol (SLP) provides a scalable frameworkfor the discovery and selection of network services. Usingthis protocol, computers using the Internet no longer needso much static configuration for network services for networkbasedapplications. This is especially important as computersbecome more portable and users less tolerant or less able tofulfill the demands of network system administration.Traditionally, users find services by using the name of a networkhost (a human readable text string), which is an alias for a networkaddress. SLP (Service Location Protocol) eliminates theneed for a user to know the name of a network host supportinga service. Rather, the user names the service and supplies a setof attributes, which describe the service. SLP (Service LocationProtocol) allows the user to bind this description to the networkaddress of the service.SLP (Service Location Protocol) provides a dynamic configurationmechanism for applications in local area networks. It isnot a global resolution system for the entire Internet; rather itis intended to serve enterprise networks with shared services.Applications are modeled as clients that need to find serversattached to the enterprise network at a possibly distant location.For cases where there are many different clients and/or servicesavailable, the protocol is adapted to make use of nearby DirectoryAgents that offer a centralized repository for advertisedservices.The basic operation in SLP is that a client attempts to discoverthe location for a service. In small installations, each service isconfigured to respond individually to each client. In larger installations,each service will register its service with one or moredirectory agents and clients contact the directory agent to fulfilla request for service location information. This is intended to besimilar to URL specifications and make use of URL technology.Protocol StructureService Location Protocol HeaderFunction Value Message Type Abbreviation1 Service Request SrvReq2 Service Reply SrvRply3 Service Registration SrvReg4 Service Deregister SrvDereg5 Service Acknowledge SrvAck6 Attribute Request AttrRgst7 Attribute Reply AttrRply8 DA Advertisement DAADvert9 Service Type Request SrvTypeRqst10 Service Type Reply SrvTypeRplyLength Number of bytes in the message including theService location header.O The overflow bit.M The monolingual bit.U RL Authentication bit present.A Attribute authentication bit present.F If the F bit is set in a Service Acknowledgement,the directory agent has registered theservice as a new entry.Rsvd These bits are reserved and must have a valueof 0.Dialect To be use by future versions of the SLP. Mustbe set to zero.Language Code The language encoded in this fieldindicates the language in which the remainderof the message should be interpreted.Character Encoding The characters making up stringswithin the remainder of this message may beencoded in any standardized encodingXID Transaction Identifier. Allows matching repliesto individual requests.Related protocolsTCP, UDP, DHCPSponsor SourceSLP is defined by IETF (http://www.ietf.org) in RFC 2165.Referencehttp://www.javvin.com/protocol/rfc2165.pdfService Location Protocol8 16 32bitVersion Function LengthO M U A F rsvd Dialect Language CodeChar encodingXIDVersion The current version is version 1Function The function field describes the operation ofthe Service location datagram. The followingmessage types exist: