Network Services The Network Layer

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4.1. Network Services

At the outset, the network layer provides a set of services to the transport layer. These services characterize the interface between the two layers and isolate the network details lower three layers from the network users upper four layers. Network service content is therefore of considerable importance. Network services are defined in terms of network service primitives. Figure 4.35 summarizes the primitives together with their possible types and parameters. Please note that these primitives only serve as a modeling tool and do not imply anything about how network services are implemented. Addresses refer to Network Service Access Points NSAP; these denote entities at the network layer that act as the interface to service users. Quality Of Service QOS denotes a set of parameters such as error rate, delays, cost, failure likelihood, throughput which collectively describe the quality of the network service. User data refers to actual user data provided by service users for transfer by the service provider. Options denote a variety of options that may be activated with some commands e.g., whether data transfers can be confirmed. 54 Communication Networks Copyright © 2005 PragSoft Figure 4.35 Network service primitives. Primitive Types Parameters Purpose N-CONNECT request indicate response confirm addresses, options, quality of service, user data Used for initiating a connection connections are always initiated by service users. N-DATA request indicate user data, confirm option Used for normal data transfer with or without confirmation. N-EXPEDITED-DATA request indicate user data Used for high priority data transfer. N-ACK-DATA request indicate Used for confirmation of data transfer. N-RESET request indicate response confirm Used by the service user or provider for resetting the network service. N-DISCONNECT request indicate addresses, reason, user data Used by the service user or provider for disconnecting a connection. Figure 4.36 Sample scenario of network services. service user A Network service provider Network service user B Network N-CONNECT request N-CONNECT indication N-CONNECT confirm N-CONNECT response N-DATA request N-DATA indication N-ACK-DATA indication N-ACK-DATA request N-EXPEDITED-DATA request N-EXPEDITED-DATA indication N-RESET indication N-RESET indication N-RESET response N-RESET response N-DISCONNECT indication N-DISCONNECT request Figure 4.36 illustrates the use of the network services in a sample scenario. Network service user A first requests a connection, which is indicated to network service user B by the service provider. B responds to the request and the network confirms with A. Then A expedites some data which is indicated to B by the network. A normal data transfer from A to B follows which includes confirmation. www.pragsoft.com Chapter 4: The Network Layer 55 Then the network encounters an error and simultaneously sends reset indications to A and B, which both respond to. Finally, B requests a disconnection, which is indicated to A, and terminates the connection.

4.2. Switching Methods