INTRO TO NETWORK LAYER

1. Section 1

2.  Definition of Layer
 Layer Services
1- packetizing
2- routing
3- forwarding
 Other Services

3.  The network layer is responsible for the host-
to-host delivery of datagrams.
 It provides services to the transport layer
 receives services from the data-link layer

4.  The network layer is involved at:
Source host.
Destination host.
All routers in the path.

5. At the source host the network layer:
 accepts a packet from a transport layer.
 encapsulates the packet in a datagram.
 delivers the packet to the data-link layer.

6. At the destination:
 The datagram is decapsulated.
 The packet is extracted.
 Delivered to the corresponding transport
layer.

7.  the routers use three layers if they are
routing packets only;
 however, they may need the transport and
application layers for control purposes.
 Routers in the path
is shown with
two data-link layers and two physical layers,

8. Encapsulating the payload in a network-layer packet at
the source and decapsulating the payload from the
network-layer packet at the destination called
Packetizing.
The source host receives the payload from an upper-
layer protocol, adds a header that contains the
source and destination addresses and some other
information that is required by the network-layer
protocol and delivers the packet to the data-link
layer.
The destination host receives the network-layer packet
from its data-link layer,decapsulates the packet, and
delivers the payload to the corresponding upper-layer
protocol.

9.  Why does the network-layer protocol need to
provide packetizing service to the transport
layer?
 The network and The transport layer
communication is between two hosts.
 This means that each layer has a different
source/destination address pair; each layer
needs a different header to accommodate
these pair of addresses.

10.  If the packet is fragmented at the source or at
routers along the path, the network layer is
responsible for reassembling them.
 The routers allowed to decapsulate the packets
they received if the packets need to be
fragmented.
 The routers are not allowed to change source
and destination addresses either. They just
inspect the addresses for the purpose of
forwarding the packet to the next network on
the path.
 For packet is fragmented, the header needs to
be copied to all fragments .

11.  A physical Network is a combination of LANs ,
WANs and routers that connect them.
 Therefore, there is more than one route from
the source to the destination.
 The network layer is responsible for finding the
best one among these possible routes that called
routing.
 The network layer have some specific strategies
for defining the best route called routing
protocols
 routing protocols help the routers coordinate
their knowledge about the neighborhood and to
come up with consistent tables to be used when
a packet arrives.

12.  Why is routing the responsibility of the
network layer?
 The network layer is the most essential layer
 It deals with the data transferring from on to
another place
 It is responsible for end to end (source to
destination) packet delivery

13.  When a router receives a packet from one of
its attached networks, it needs to forward the
packet to another attached network.
 forwarding can be defined as the action
applied by each router when a packet arrives
at one of its interfaces based on routing table.
 To make this decision, the router uses a piece
of information in the packet header, which can
be the destination address or a label.

14.  Network layer provide limited error control
but no follow control.
 Congestion may occur if the number of
datagrams sent by source computers is
beyond the capacity of the network or
routers.
 In this situation, some routers may drop
some of the datagrams this called Congestion
Control.

15.  The network layer provide better quality of
service to support applications such as
multimedia communication.
 The network layer provide IPSec to make a
connection between networks more secure