The Service Location Protocol (SLPv2) is an IETF proposed standard for service discovery in IP network. There are three different entities in an SLP system: UA, SA and DA.

• User Agent (UA): A UA initiates service discovery on behalf of one or more applications. It sends queries to all SAs via multicast or to a DA via unicast.
  • send service type request (SrvTypeRqst), get service type reply (SrvTypeRply).
  • send service request (SrvRqst), get service reply (SrvRply).
  • send attribute request (AttrRqst), get attribute reply (AttrRply).
• Service Agent (SA): An SA works on behalf of one or more services. It responds directly to UA queries. If DAs exist, it registers the services with DAs.
  • send service registration (SrvReg), get service acknowledgment (SrvAck).
  • send service deregistration (SrvDeReg), get service acknowledgment (SrvAck).
• Directory Agent (DA): A DA serves as a centralized information repository in an SLP system. It accepts SA registrations and answers UA queries. DA support is optional in an SLP system; it is introduced for performance and scalability considerations.

• Service Location: SLP is a method for locating service. How to access the service is another issue. Some service discovery systems combine the service location and access together, e.g. Jini.
• URL: In SLP, the URL means unique service ID as well as service location. Here is a discussion on this issue in SLP mailing list.
• Service Scope: SLP uses service scope to achieve scalability. All SLP entities (DA/SA/UA) should be configured with proper scopes, and all SLP messages should set scope properly.
• Service Type: Each service registration in SLP must have a service type. It is a classification for services, and it is used as a wildcard for service searching. A standardized service type has a template.
• Naming Authority: A standardized service type should be registered with a naming authority (IANA or private enterprise numbers).
• Language Tag: Each service can be registered in several different languages (e.g., English, Chinese), and thus result in several different entries in the registration database.