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Typus
BauernOpfer
Bearbeiter
Hindemith
Gesichtet
Yes.png
Untersuchte Arbeit:
Seite: 25, Zeilen: 1ff (komplett)
Quelle: Bishaj 2007
Seite(n): 2, 3, 4, Zeilen: 2: l. Spalte: 45ff - r. Spalte 1-3.22-30; 3: r. Spalte 56-58; 4: l. Spalte: 1ff
[The] discovery protocol is used when a device looks for a lookup service to register with. The join protocol is used when a service has located the lookup service and wants to join it. The lookup protocol is used when a client needs to locate and invoke a service. Discovery/join is how a service is added to the system. The service provider multicasts a request for local lookup services to identify themselves (discovery). After this, a service object is loaded into the lookup service (joining). This service object has the Java interface for the service including the methods of the interface to be invoked for using the service. There can also be other descriptive attributes. The lookup process ensures that a copy of the service object is loaded into the client. The client can now use the service. Java Remote Method Invocation (RMI) can be used for the communication between services. RMI basically implements the remote procedure call mechanisms in Java. It allows data as well as objects to be passed through the network.

With Jini, administration is still needed, because when a device is introduced to the network, it needs to be assigned an IP. Jini supports distributed events: an object can register its interest in events of another object and be notified whenever these events happen. This adds reliability guarantees to the architecture. Interfaces to devices have to be implemented for the Jini architecture to work. This is a disadvantage for Jini, since other service discovery protocols have them already available for consumer electronics.

2.5.3 SLP

SLP [58] is an IETF standard for service discovery. It enables the discovery and selection of a wide range of services accessible through an IP network. With SLP, a user needs only to search for a particular type of service, and optionally for attributes associated with it. It has three major software entities: User Agent (UA), Service Agent (SA), and Directory Agent (DA).

The SA advertises the location and attributes of one or more services on behalf of them by using broadcasts. It also replies with IP unicast to requests for these services. The services register and deregister with the SA. Each registrations has a lifetime and reregistration is required periodically. It is the same leasing concept already discussed in previous protocols.

The UA receives requests from a client application and forwards multicast requests to SAs. Hence, there is little network overhead for SA discovery. The SA unicasts a response back with the service URL and possible attributes if there is a match with the registered services.

The DA is a central information repository and it is optional. Its main function is to improve the performance of SLP. It can be thought of as a tier between the UAs and the SAs, which communicate with the DA instead of with each other. DAs relieve the network traffic from many multicast requests; the effect is more visible in large network, where multicast traffic can increases sharply because there are many SAs and UAs. A DA keeps the SA advertisements, and responds to UA requests. An SA registers itself with a DA. The registration contains the URL for the services, the lifetime, and descriptive attributes of the service. The registration should be refreshed periodically. A UA sends a request message to the DA, which in turn sends a message containing the URL of the service matched against the UA needs. SLP can work both with and without a DA: (1) with DA, information about the services is kept in the DA, so that the UA sends a query there for services; (2) without a DA, the UA multicasts service requests to the network, and a SA offering the service would reply back. In small networks, there may be no real need for a DA.

SLP scales well in large networks; the reason is the minimal use of multicast messages [and the fact that it can have multiple DAs.]


[58] Choonhwa Lee, Sumi Helal, Protocols for Service Discovery in Dynamic and Mobile Networks. International Journal of Computer Research, 2002.

[Seite 2]

The discovery protocol is used when a device looks for a lookup service to register with. The join protocol is used when a service has located the lookup service and wants to join it. The lookup protocol is used when a client needs to locate and invoke a service. Discovery/join is how a service is added to the system. The service provider multicasts a request for local lookup services to identify themselves (discovery). After this, a service object is loaded into the lookup service (joining). This service object has the Java interface for the service including the methods of the interface to be invoked for using the service. There can also be other descriptive attributes. The lookup process ensures that a copy of the service object is loaded into the client. The client can now use the service.

Java Remote Method Invocation (RMI) can be used for the communication between services [8, 11]. RMI basically implements the remote procedure call mechanisms in Java. It allows data as well as objects to be passed through the network.

[...]

With Jini, administration is still needed, because when a device is introduced to the network, it needs to be assigned an IP. Jini supports distributed events: an object can register its interest in events of another object and be notified whenever these events happen. This adds reliability guarantees to the architecture. Interfaces to devices have to be implemented for the Jini architecture to work. This is a disadvantage for Jini, since other SDPs have them already available for consumer electronics.

[Seite 3]

2.4 SLP

SLP [5] is an IETF standard for service discovery. It enables the discovery and selection of a wide range of services ac-

[Seite 4]

cessible through an IP network. With SLP, a user needs only to search for a particular type of service, and optionally for attributes associated with it.

It has three major software entities [5]: User Agent (UA), Service Agent (SA), and Directory Agent (DA).

The SA advertises the location and attributes of one or more services on behalf of them by using broadcasts. It also replies with IP unicasts to requests for these services. The services register and deregister with the SA. Each registrations has a lifetime and reregistration is required periodically. It is the same leasing concept already discussed in previous protocols.

The UA receives requests from a client application and forwards multicast requests to SAs. Hence, there is little network overhead for SA discovery. The SA unicasts a response back with the service URL and possible attributes if there is a match with the registered services.

DA is a central information repository and it is optional. Its main function is to improve the performance of SLP. It can be thought of as a tier between the UAs and the SAs, which comminicate [sic] with the DA instead of with each-other [sic]. DAs relieve the network traffic from many multicast requests; the effect is more visible in large network, where multicast traffic can increases sharply because there are many SAs and UAs. A DA keeps the SA advertisements, and responds to UA requests. An SA registers itself with a DA. The registration contains the URL for the services, the lifetime, and descriptive attributes of the service. The registration should be refreshed periodically. A UA sends a request message to the DA, which in turn sends a message containig the URL of the service matched against the UA needs. SLP can work both with and without a DA: (1) with DA, information about the services is kept in the DA, so that the UA sends a query there for services; (2) without a DA, the UA multicasts service requests to the network, and a SA offering the service would reply back [5]. In small networks, there may be no real need for a DA.

SLP scales well in large networks; the reason is the minimal use of multicast messages and the fact that it can have multiple DAs.


[5] Choonhwa Lee and Sumi Helal. Protocols for Service Discovery in Dynamic and Mobile Networks. International Journal of Computer Research, 2002.

[8] Sun Microsystems. Jini Architecture Specification, version 1.2, December 2001. http: //www.sun.com/software/jini/specs/jini1.2html/jini-title.html, 09 April 2007.

[11] Jim Waldo. The Jini architecture for network-centric computing. Communications of the ACM, July 1999.

Anmerkungen

Ein Verweis auf die Quelle findet sich am Anfang des Kapitels auf Seite 23. Eine wörtliche Übernahme des gesamten Kapitels wird durch diesen aber in keiner Weise gekennzeichnet.

Sichter
(Hindemith), Graf Isolan

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