1. More on Discovery and Advertisement

2. Discovery and advertisement protocols allow clients to find interesting services and service to make their existence known (dynamically) to clients.
A discovery-enabled client is able to power on and immediately discover available services, provided that the services in the area use a compatible service discovery protocol.
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3. Unicast Discovery
Unicast discovery is the simplest form of service discovery protocol.
A client configured statically with the location of one or more service catalogs or services can contact the needed resources directly.
Unicast discovery protocols typically use TCP/IP or another reliable, stream-oriented transport protocol.
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4. 9/20/2017

5. In Jini, clients and services can contact service catalogs whose locations have been statically configured using the Unicast Discovery Protocol.
A TCP connection is established to the service catalog, followed by transfer of an instance of Service Registrar to the client or service.
The Service Registrar instance exposes methods that can be used to control the service catalog.
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6. Multicast Discovery
Multi cast discovery allows dynamic discovery of interesting services.
All current discovery frameworks provide a dynamic multicast-based protocol, and most are built on top of unreliable UDP multicast.
This limits the scope of dynamic service discovery to the local network segment or, more accurately to the multicast radius of the local network.
This radius, in turn, is determined by the design of the local network and administrative decisions that affect the flow of multicast traffic.
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7. Advertisement
Service advertisement is the converse of discovery, allowing services entering or leaving a network to advertise their availability or unavailability.
In addition, services periodically advertise their presence for the benefit of clients that have just entered the network.
Rather than attempting discovery of needed services periodically.
clients might register their interest in the availability of needed services and rely on a mechanism that provides asynchronous notification when service advertisements arrive.
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8. Generally, advertisements contain an expiration date, which is an informal promise that the advertisement remains valid for a certain period of time.
As with dynamic discovery protocols, service advertisements are based on multicast and are thus constrained to the administratively controlled multicast radius.
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9. No Directory Agent
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10. In a system with no directory agents (DAs), a user agent (UA) will typically send multicast requests to find appropriate services.
The request contains a service type and essential characteristics of the needed service.
A service agent (SA) which provides a matching service will unicast a reply containing service location and other contact-related information.
Service agents also spontaneously advertise their presence through multicast advertisement messages.
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11. At least 1DA
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12. There are a variety of options for discovering DAs:
In systems with at least one DA, SAs register available services with the DA and UAs query the DA for the locations of available services.
There are a variety of options for discovering DAs:
the locations of DAs may be statically configured, DHCP may be used, or multicast discovery can be used.
The interactions between UAs, SAs, and DAs can be constrained through the use of “scopes” (not pictured), which allow service groups to be formed.
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