Transcript NET331_Ch8+Ch20

Kingdom of Saudi Arabia
Prince Norah bint Abdul Rahman University
College of Computer Since and Information System
NET331
NETWORK LAYER (1)
T.Najah AlSubaie
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Introduction
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The physical and data link layers of a network
operate locally.
These two layers are jointly responsible for data
delivery on the network from one node to the next.
Links between two hosts
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Network layer in an internetwork
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Need for Network Layer
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To solve the problem of delivery through several
links, the network layer was designed.
The network layer is responsible for host-to-host
delivery and for routing the packets through the
routers.
Need for Network Layer
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The network layer at the router is responsible for
routing the packet.
When a packet arrives, the router consults its routing
table and finds the link from which the packet must be
sent.
The packet, after some changes in the header, with the
routing information is passed to the data link layer
again.
The network layer at the destination is responsible for
address verification; it makes sure that the destination
address on the packet is the same as the address of the
host.
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Network layer at the source, router,
and destination
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Network layer at the source, router,
and destination
Internet as a Datagram Network
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The Internet, at the network layer, is a packet
switched network.
In the packet-switched network, the message
needs to be divided into packets of fixed or
variable size.
The size of the packet is determined by the network
and the governing protocol.
When a switch receives a packet, no matter what is
the source or destination, the packet must wait if
there are other packets being processed.
Packet Switching Networks
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Packet switching uses either the virtual circuit
approach or the datagram approach.
Datagram Networks
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In a datagram network, each packet is treated
independently of all others. Even if a packet is part
of a multi packet transmission, the network treats it
as though it existed alone.
Packets in this approach are referred to as
datagrams.
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A datagram network with four switches
(routers)
Routing Table
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In this type of network, each switch has a routing
table which is based on the destination address.
The routing tables are dynamic and are updated
periodically.
The destination addresses and the corresponding
forwarding output ports are recorded in the tables.
Routing table in a datagram network
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Destination Address
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Every packet in a datagram network carries a
header that contains, among other information, the
destination address of the packet.
When the switch receives the packet, this destination
address is examined; the routing table is consulted
to find the corresponding port through which the
packet should be forwarded.
This address remains the same during the entire
journey of the packet.
VIRTUAL-CIRCUIT NETWORKS
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a source and destination need to go through three
phases: setup, data transfer, and teardown.
In the setup phase, the source and destination use
their global addresses to help switches make table
entries for the connection.
In the teardown phase, the source and destination
inform the switches to delete the corresponding
entry.
Data transfer occurs between these two phases
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Switch and tables in a virtual-circuit
network
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Source-to-destination data transfer in a
virtual-circuit network
VIRTUAL-CIRCUIT NETWORKS
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Setup Phase:
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Data Transfer Phase:
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In the setup phase, a switch creates an entry for a virtual circuit.
All switches need to have a table entry for this virtual circuit. The
table, has four columns for each virtual circuit that is already set
up.
Teardowil Phase:
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In this phase, source A, after sending all frames to B, sends a
special frame called a teardown request.
Destination B responds with a teardown confirmation frame.
All switches delete the corresponding entry from their tables.
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Setup request in a virtual-circuit
network
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Setup acknowledgment in a virtualcircuit network
Datagram VS. Virtual circuit networks
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Efficiency
In datagram network, resources are allocated only when
there are packets to be transferred.
 In virtual-circuit network, all packets belonging to the same
source and destination travel the same path;
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Delay:
In datagram network, each packet may experience a wait
at a switch before it is forwarded. In addition, the delay is
not uniform for the packets of a message.
 In a virtual-circuit network, there is a one-time delay for
setup and a one-time delay for teardown. If resources are
allocated during the setup phase, there is no wait time for
individual packets.
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Delay in Datagram Network
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Delay in Virtual Circuit Network
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Network Layer: Internet Protocol
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In the Internet model, the main network protocol is
the Internet Protocol (IP)
The Internet Protocol version 4 (IPv4) is the delivery
mechanism used by the TCP/IP protocols.
Position ofIPv4 in TCPIIP protocol suite
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IPv4
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IPv4 is an unreliable and connectionless datagram
protocol-a best-effort delivery service.
The term best-effort means that IPv4 provides no
error control or flow control(except for error
detection on the header).
IPv4 assumes the unreliability of the underlying
layers and does its best to get a transmission
through to its destination, but with no guarantees.
IPv4
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IPv4 is also a connectionless protocol for a packet
switching network that uses the datagram approach.
This means that each datagram is handled
independently, and each datagram can follow a
different route to the destination.
This implies that datagrams sent by the same source to
the same destination could arrive out of order. Also,
some could be lost or corrupted during transmission.
IPv4 relies on a higher-level protocol to take care of
all these problems.
IPv4 datagram format
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IPv4 datagram format
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A datagram is a variable-length packet consisting of two parts:
header and data.
The header is 20 to 60 bytes in length and contains information
essential to routing.
Source address. This 32-bit field defines the IPv4 address of the
source.
This field must remain unchanged during the time the IPv4 datagram
travels from the source host to the destination host.
Destination address. This 32-bit field defines the IPv4 address of
the destination.
This field must remain unchanged during the time the IPv4 datagram
travels from the source host to the destination host.
IPv4 datagram format
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Version (VER). This 4-bit field defines the version of the
IPv4 protocol.
Header length (HLEN). This 4-bit field defines the total
length of the datagram header in 4-byte words.
Services: an 8-bit field called as service type
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The first 3 bits are called precedence bits.
The precedence defines the priority of the datagram in issues such
as congestion.
 If a router is congested and needs to discard some datagrams,
those datagrams with lowest precedence are discarded first.
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The next 4 bits are called type of service (TOS) bits,
IPv4 datagram format
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TOS bits is a 4-bit subfield with each bit having a
special meaning.
A bit can be either 0 or 1, and only one of the bits
can have the value of 1 in each datagram
Identification. This field is used in fragmentation
Service Type
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Default types of service
IPv4 datagram format
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Total length: This is a n-bit field that defines the
total length (header plus data) of the IPv4
datagram in bytes.
To find the length of the data coming from the
upper layer, subtract the header length from the
total length.
The header length can be found by multiplying the
value in the HLEN field by 4