This document summarizes a lecture on network layer concepts including link state routing, hierarchical routing, multicasting routing protocols, and subnetting. It discusses key topics like how link state routing works by discovering neighbors, measuring link costs, building link state packets, and distributing them. It also covers hierarchical routing to reduce routing table sizes, and different approaches to multicasting like source-based trees and group-shared trees. Subnetting is introduced as a way to divide networks into smaller subnets.

1. Lecture Title: Network Layer
Lecture Name: Dr. Tamer Omar
Lecture Date:
Year: 3rd Year
Spec.:
Subject: MSE 4532 Computer Communication Networks

2. Objectives
Dr. Tamer Omar
❑Link State Routing
❑Hierarchical Routing
❑Multicasting Routing Protocols

3. Dr. Tamer Omar
Link State Routing
(an adaptive routing algorithm)
1. Discover your neighbors and learn their
addresses.
2. Measure the cost (delay) to each neighbor.
3. Construct a packet containing all this
information
4. Send this packet to all other routers.
5. Compute the shortest path to every other
router.

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1. Discovering Your Neighbors
❑ Send “Hello” packet on each point-to-
point line. Destination node replies with
its address.

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1. Discovering Your Neighbors
Nine routers and a broadcast LAN.

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1. Discovering Your Neighbors
A graph model of previous slide.

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2. Measuring Line Cost
❑ Send an “ECHO” packet over the line.
❑ Destination is required to respond to “ECHO”
packet immediately.
❑ Measure the time required for this operation.
❑ Question: Should we measure just the time it
takes to transmit the packet, or should we
include the time that the packet waits in the
queue?

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3. Build Link State Packets

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4. Distributing the Link State Packets
❑Use selective flooding
❑Sequence numbers prevent duplicate
packets from being propagated
❑Lower sequence numbers are rejected
as obsolete

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4. Distributing the Link State Packets
The packet buffer for router B in previous slide
▪ the link state packet from A arrives directly, so it must be sent
to C and F and acknowledged to A, as indicated by the flag
bits. Similarly, the packet from F has to be forwarded to A and
C and acknowledged to F.

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Hierarchical Routing
❑ As a network size grows, routing table grows.
❑ Divide routers into regions
❑ Each router knows all the details about routing
within its region and knows nothing about the
internal structure of other region.
❑ Advantage: less storage requirement
❑ Penalty: longer path.

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Hierarchical Routing
❑ As a network size grows, routing table grows.
❑ Divide routers into regions
❑ Each router knows all the details about routing
within its region and knows nothing about the
internal structure of other region.
❑ Works like telephone routing
❑ Advantage: less storage requirement
❑ Penalty: longer path.

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Hierarchical Routing

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Unicast Routing Protocols
❑ In unicast communication, there is one source and one
destination. The relationship between the source and the
destination is one-to-one.
❑ A unicast packet starts from the source S1 and passes through
routers to reach the destination D1.
❑ In unicast routing, each router in the domain has a table that
defines a shortest path tree to possible destinations.

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Unicast Routing Protocols

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Multicasting Routing Protocols
❑ In multicast communication, there is one source and a group of
destinations. The relationship is one-to-many. In this type of
communication, the source address is a unicast address, but the
destination address is a group address, which defines one or
more destinations.
A multicast packet starts from the
source S1 and goes to all
destinations that
belong to group G1. In multicasting,
when a router receives a packet, it
may forward it
through several of its interfaces

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Multicasting Routing Protocols
❑ A multicast packet may have destinations in more than
one network. Forwarding of a single packet to
members of a group requires a shortest path tree. If we
have n groups, we may need n shortest path trees. We
can imagine the complexity of multicast routing. Two
approaches have been used to solve the problem:
source-based trees and group-shared trees.
❑ Source-Based Tree. In the source-based tree approach,
each router needs to have one shortest path tree for
each group. The shortest path tree for a group defines
the next hop for each network that has loyal
member(s) for that group.

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Source-based trees
If router Rl receives a packet with destination address G1, it needs to send a copy of
the packet to the attached network, a copy to router R2, and a copy to router R4 so
that all members of G1 can receive a copy. In this approach, if the number of groups is
m, each router needs to have m shortest path trees, one for each group. We can
imagine the complexity of the routing table if we have hundreds or thousands of
groups. H

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Group-Shared Tree
❑In the group-shared tree approach, instead of each
router having m shortest path trees, only one
designated router, called the center core, takes the
responsibility of distributing multicast traffic.
❑The core has m shortest path trees in its routing table.
The rest of the routers in the domain have none. If a
router receives a multicast packet, it encapsulates the
packet in a unicast packet and sends it to the core
router. The core router removes the multicast packet
from its capsule, and consults its routing table to
route the packet.

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Group-Shared Tree

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Multicast Routing
❑ Multicast routing is a networking method for efficient
distribution of one-to-many traffic. A multicast source,
such as a live video conference, sends traffic in one
stream to a multicast group. The multicast group
contains receivers such as computers, devices, and IP
phones.
• Voice over IP (VOIP)
• Video on demand (VOD)
• Video conferencing
• IP television (IPTV)

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Broadcasting
❑ In broadcast communication, the relationship between
the source and the destination is one-to-all. There is
only one source, but all the other hosts are the
destinations

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Congestion Control Algorithms
❑ Congestion Control is a mechanism that
controls the entry of data packets into the
network, enabling a better use of a shared
network infrastructure and avoiding
congestive collapse. Congestive-Avoidance
Algorithms (CAA) are implemented at the TCP
layer as the mechanism to avoid congestive
collapse in a network.

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General Principles of Congestion Control
❑ Open loop solutions attempt to solve the
problem by good design
❑ Closed loop solutions are based on the concept of a
feedback loop. This approach has three parts when
applied to congestion control:
1. Monitor the system to detect when and where
congestion occurs.
2. Pass this information to places where action can be
taken.
3. Adjust system operation to correct the problem.

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Policies that affect congestion.
❑ Retransmission Policy :
It is the policy in which retransmission of the
packets are taken care of. If the sender feels
that a sent packet is lost or corrupted, the
packet needs to be retransmitted. This
transmission may increase the congestion in
the network.
▪ To prevent congestion, retransmission
timers must be designed to prevent
congestion and also able to optimize
efficiency.

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Policies that affect congestion.
❑ Window Policy :
The type of window at the sender’s side may
also affect the congestion. Several packets in
the Go-back-n window are re-sent, although
some packets may be received successfully at
the receiver side. This duplication may
increase the congestion in the network and
make it worse.
❑ Therefore, Selective repeat window should be
adopted as it sends the specific packet that
may have been lost.

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Policies that affect congestion.
❑ Discarding Policy :
A good discarding policy adopted by the
routers is that the routers may prevent
congestion and at the same time partially
discard the corrupted or less sensitive
packages and also be able to maintain the
quality of a message.
▪ In case of audio file transmission, routers
can discard less sensitive packets to
prevent congestion and also maintain the
quality of the audio file.

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Policies that affect congestion.
❑ Acknowledgment Policy :
Since acknowledgements are also the part of
the load in the network, the acknowledgment
policy imposed by the receiver may also affect
congestion. Several approaches can be used
to prevent congestion related to
acknowledgment.
▪ The receiver should send
acknowledgement for N packets rather
than sending acknowledgement for a single
packet. The receiver should send an
acknowledgment only if it has to send a

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Policies that affect congestion.
❑ Admission Policy :
In admission policy a mechanism should be
used to prevent congestion. Switches in a flow
should first check the resource requirement of
a network flow before transmitting it further. If
there is a chance of a congestion or there is a
congestion in the network, router should deny
establishing a virtual network connection to
prevent further congestion.

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Logical Addressing
❑ IP address
The Internet addresses are 32 bits in length;
this gives us a maximum of 232addresses.
These addresses are referred to as IPv4 (IP
version 4) addresses or simply IP addresses if
there is no confusion.
❑ A new design of the IP layer called the new generation
of IP or IPv6 (lP version 6). In this version, the
Internet uses 128-bit addresses that give much greater
flexibility in address allocation. These addresses are
referred to as IPv6 (IP version 6) addresses.
❑ In this lecture, we first discuss IPv4 addresses, which
are currently being used in the Internet.

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IPv4 ADDRESSES
❑ An IPv4 address is a 32-bit address that uniquely and
universally defines the connection of a device (for example, a
computer or a router) to the Internet.
❑ An identifier for a computer or device on a TCP/IP network.
Networks using the TCP/IP protocol route messages based on
the IP address of the destination.
❑ The IPv4 addresses are universal in the sense that the
addressing system must be accepted by any host that wants to
be connected to the Internet
❑ Address Space
• An address space is the total number of addresses used by
the protocol. If a protocol uses N bits to define an address,
the address space is 2N because each bit can have two
different values (0 or 1) and N bits can have 2N values

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IPv4 ADDRESSES
❑ IPv4 uses 32-bit addresses, which means that the
address space is 232 or 4,294,967,296 (more than 4
billion). This means that, theoretically, if there were no
restrictions, more than 4 billion devices could be
connected to the Internet. We will see shortly that the
actual number is much less because of the restrictions
imposed on the addresses.
❑ Notations
• There are two prevalent notations to show an IPv4
address: binary notation and dotte ddecimal
notation.

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Binary Notation
❑ In binary notation, the IPv4 address is displayed as
32 bits. Each octet is often referred to as a byte. So
it is common to hear an IPv4 address referred to as
a 32-bit address or a 4-byte address. The following
is an example of an IPv4 address in binary notation:
01110101 10010101 00011101 0000001

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Dotted-Decimal Notation
❑ To make the IPv4 address more compact and easier
to read, Internet addresses are usually written in
decimal form with a decimal point (dot) separating
the bytes. The following is the dotted~decimal
notation of the above address:
117.149.29.2

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Classful Addressing
❑ Each IP address breaks down into the following sections:
▪ Net ID—Identifies a network. The first several bits of a
net ID, known as the class field or class bits, identify the
class of the IP address.
▪ Host ID—Identifies a host on a network.
❑ In classful addressing, the address space is divided into five
classes: A, B, C, D, and E. Each class occupies some part of
the address space.
❑ We can find the class of an address when given the address
in binary notation or dotted-decimal notation. If the address
is given in binary notation, the first few bits can
immediately tell us the class of the address. If the address is
given in decimal-dotted notation, the first byte defines the
class.

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Classful Addressing
• Class A - 168.212.226.204
• supports 16 million hosts on each of 127 networks
• Class B - 168.212.226.204
• supports 65,000 hosts on each of 16,000 networks
– NDUS has two Class B addresses
• 134.129.xxx.xxx Eastern ND
• 134.234.xxx.xxx Western ND
• Class C - 168.212.226.204
• supports 254 hosts on each of 2 million networks

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Classful Addressing

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Classful Addressing
Class Address range Remarks
A 0.0.0.0 to 127.255.255.255 The IP address 0.0.0.0 is used by a host at
startup for temporary communication. This
address is never a valid destination
address. Addresses starting with 127 are
reserved for loopback test. Packets
destined to these addresses are processed
locally as input packets rather than sent to
the link.
B 128.0.0.0 to 191.255.255.255 N/A
C 192.0.0.0 to 223.255.255.255 N/A
D 224.0.0.0 to 239.255.255.255 Multicast addresses.
E 240.0.0.0 to 255.255.255.255 Reserved for future use, except for the
broadcast address 255.255.255.255.

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Classes and Blocks
❑ One problem with classful addressing is that each class is divided into a
fixed number
❑ When an organization requested a block of addresses, it was granted one
in class A, B, or C. Class A addresses were designed for large
organizations with a large number of attached hosts or routers. Class B
addresses were designed for midsize organizations with tens of
thousands of attached hosts or routers. Class C addresses were designed
for small organizations with a small number of attached hosts or routers

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Classes and Blocks
❑ We can see the flaw in this design. A block in class A
address is too large for almost any organization. This means
most of the addresses in class A were wasted and were not
used. A block in class B is also very large, probably too
large for many of the organizations that received a class B
block. A block in class C is probably too small for many
organizations. Class D addresses were designed for
multicasting. Each address in this class is used to define one
group of hosts on the Internet. The Internet authorities
wrongly predicted a need for 268,435,456 groups. This
never happened and many addresses were wasted here too.
And lastly, the class E addresses were reserved for future
use; only a few were used, resulting in another waste of
addresses

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Subnetting
◼ Divide a network to smaller networks
(subnets)
◼ Reasons
◼ Reduce network traffic
◼ Improve network performance
◼ Management
◼ Allocate host bits for subnet bits
◼ Make use of subnet masks

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Mask
❑ Although the length of the netid and hostid (in bits) is
predetermined in classful addressing, we can also use a 32-
bit number called the default mask.
❑ The mask can help us to find the netid and the hostid. For
example, the mask for a class A address has eight 1s, which
means the first 8 bits of any address in class A define the
netid; the next 24 bits define the hosted

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Subnet Mask
◼ A 32-bit string
◼ All 1s and then all 0s, they never mix
◼ It specifies number of bits for network
ID in an IP address

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Subnet Mask (Example I)
◼ IP address: 190.138.23.45
◼ Subnet Mask: 255.255.0.0
◼ Network ID: 190.138.0.0
◼ Host ID: 23.45

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Subnet Mask (Example II)
◼ IP Address: 201.100.26.171
◼ Subnet Mask: 255.255.255.192
11111111.11111111.11111111.11000000
◼ First 26 bits as network ID, therefore
◼ Network ID: 201.100.26.128
◼ Host ID: 43

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Subnet Mask (Example III)
◼ Design a subnet mask for a Class B
network with 25 subnets
◼ Number of bits for subnet = lg(25+2) = 5
◼ Number of bits for network ID = 16+5=21
◼ Subnet mask is
11111111.11111111.11111000.00000000
255.255.248.0

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Subnet Mask (Example IV)
◼ Given IP address 203.59.43.134 and
subnet mask 255.255.255.192, find
valid host range in this subnet
◼ Network ID: 203.59.43.128
◼ All 0 and all 1 host IDs are reserved,
therefore
◼ 203.59.43.129 – 203.59.43.190
◼ Number of hosts 62
◼ To verify: 62 = 26-2

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Subnet Mask (Example VI)
◼ A block of addresses is granted to a small
organization. We know that one of the
addresses is 205.16.37.39/28.
◼ Find the first address
◼ Address: 11001101 00010000 00100101 00100111
AND
◼ Mask 11111111 11111111 11111111 11110000
◼ First address 11001101 00010000 00100101 00100000
◼ Find the last address
◼ Address: 11001101 00010000 00100101 00100111
OR
◼ Mask Complement 00000000 00000000 00000000 00001111
◼ Last address 11001101 00010000 00100101 00101111
◼ The number of addresses
◼ Mask Complement+1=1111+1=16