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Explain Operating System Interface OSI Layer 3 – Network Layer

Layer number 3 in the OSI model is the network layer.
– This is the one that provides procedural as well as functional means required for the transfer of data sequences with varying lengths from and to different hosts in different networks.
– Unlike data link layer that interconnects the hosts within a network, the network layer interconnects different networks.
– It also maintains the quality of the service as requested by the transport layer.
– Performing the following functions is also the responsibility of the network layer:
1. Routing functions
2. Fragmentation
3. Reassembly
4. Reporting the delivery errors

– This is the layer at which the routers operate and make inter-networking actually possible.
– The logical addressing scheme is not hierarchical and can be chosen by the network engineer.

Sub Layers of Network Layer

There are 3 sub – layers of this layer:
1. Sub – network access: Protocols dealing with the interface with the networks for example, X.25 are considered by this layer.
2. Sub – network – independent convergence: The transfers across a number of networks are handled by this layer.
3. Sub – network – dependent convergence: This layer comes in to play when it is required to equalize the levels of the networks on either side.

– An example of the second sub–layer is given by the case of IPv6 ISO 8473 or CLNP.
– Its purpose is to take care of the connection less transfers of the data with only one hop at a time from the following:
a) End system to ingress router
b) Router to router
c) Egress router to end system at the destination

– However, this layer does not takes guarantee for whether the delivery of the data will be reliable or not.
– It can only detect the data packets that are erroneous and discards them.
– As per this scheme, X.25 is classed with the subnet access protocols which are IPv4 and IPv6.
– This has to be done necessarily because these protocols carry two addresses i.e., the node addresses and the interface addresses.
– The following verily belong to the network:
a) Routing protocols
b) Multi – cast group management
c) Network layer information
d) Network layer errors
e) Network layer address assignment.

– All these belong to the network because of the payload and not because of the protocol carrying them.
– The packets are forwarded via the immediate routers.
– Usually the networks are divided in to a number of sub networks.
– These networks connect to other networks in order to establish communication over a wide area.
– Routers or the gateways are the specialized hosts that are used by these networks for forwarding the data packets across the networks.
– This concept is related to mobile applications also where a user might move from one place to another.
– For such situations it is important that the messages intended for him must follow him.
– IPv4 did not have any such feature but IPv6 does offers a solution.
– There are a number of protocols operating in this layer namely:
a) Internet protocol (IPv4/ IPv6
b) Distance vector multicast routing protocol (DVMRP)
c) Internet control message protocol (ICMP)
d) Internet group multicast protocol (IGMP)
e) Internet protocol security (IPsec)
f) Protocol independent multicast sparse mode (PIM – SM)
g) Protocol independent multicast dense mode (PIM – DM)
h) Inter-network packet exchange (IPX)
i) Data gram delivery protocol (DDP)
j) Address resolution protocol (ARP)
k) Routing information protocol (RIP)
l) Routed – SMLT (RSMLT)
m) Shortest path bridging

– The protocols used in inter networking are all described by the TCP/ IP model.
– In this model a layer called the link layer above which another layer called the internet layer is located.
– Some references consider this layer and the network layer to be equivalent of each other.
– But since the division of the characteristics of the protocols (such as type: whether connection less or connection – oriented) in different layers is different for the two models i.e., the TCP/ IP model and the OSI model, this comparison is considered to be incorrect.

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