Network Service Model
The network service model defines the characteristics of end to end transport of data between one edge of the network to the other that is between sending and receiving end systems.
Lets now consider some possible services that the network layer can provide. In the sending host when the transport layer passes a packet to the network layer specific services that could be provided by network layer include:
a.This service guarantees that the packet will eventually arrive at tits destination.
b.Offers uniform services various types of sub network . the sub network may be defined as a set of one or more intermediate nodes which are used for establishing network connection between end systems and provide routing and relaying of frames thought it.
c.Offers uniform addressing and services to LAN and WAN the network layer defines network addresses which are being used by transport enmities to access network services. Each transport entity is a unique network connection end point identifier defined by the network layer which may be independent of addressing required by underlying layer. The network entities define network connection and may also include intermediate nodes which usually provide relaying. The network connection over intermediate subnets is known as a subnet connection and is usually handled by underlying protocols.
d.Establishes control and maintains a logical connection between the transport layer entities for exchanging the data across the network. The network connection offers point to point connection and more than one network connection may be defined between the same pair of network addresses. The network service data units are transferred transparently between transport entities over the network connection and generally do not have upper limit on their size.
e.The transport layer can adapt to different types of sub network ( it is independent of the characteristics of sub network e, g routing strategies considered topologies and type of the sub network).
f.Offers an acceptable quality of services during the network connection which in turn is based on parameters such as residual error rate service availability throughput reliability etc. It also reports any undetectable error to the transport layer.
g.Provides sequencing for NDSU over the network connection when requested by transport entities.
h.Provides both connection oriented and connectionless services to the users.
In connection oriented services a logical connection is set up between two transport entities including agreement for the type of service desired parameters cost priorities transfer of data in both direction with appropriate flow control and finally the termination of the connection.
On the other hand in connectionless service network layer primitives SEND and RECEIVE are being used to send and receive the packet across the networks. The users are responsible for flow control and error control on their hosts.