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.