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CSN200 Introduction to Telecommunications,
Winter 2000


Lecture
-
07

Network Layer

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Network Layer
:


The network layer sits between the application layer and the data link layer. The network layer
accepts messages from the application layer and formats and addresses them for transmission by the
data link layer.


The network layer also acc
epts individual messages from the data link layer and organizes them into
coherent messages that it passes to the application layer.

layer sits between the


The
network layer

performs three important functions:



Addressing
-


determines the correct netwo
rk layer and data link layer addresses.



Routing
-


determines where the message
should be send next

on its way to its final
destination.



Packetizing
-


breaking long messages into smaller packets for transmission by the data link
layer and reassembles the
m at the receiver's end.


Network Protocols
:


Network layer protocols are the set of rules, it should follow to communicates with the
application layer and the data link layer.

There are many different network layer protocols. Each protocol performs essen
tially the same
functions, but each is incompatible with others unless there is a special device to translate
between them.

The four most commonly used network protocols are:



TCP/IP







IPX/SPX



X.25








SNA

TCP/IP
-

The Transmission Control Protocol/ Inte
rnet Protocol (TCP/IP) was developed for the
U.S. Dept of Defense’s Advanced Research Project Agency Network (ARPANET) in
1974.

TCP/IP allows reasonable efficient and error
-
free transmission.

TCP/IP has two parts:



TCP
-

performs packetizing: breaking the d
ata into smaller packets, numbering
them, ensuring each packet is reliably delivered, and putting them into the proper
order at the destination. TCP is only active at the sender and receiver.



IP
-

performs routing and addressing.

Data transmission using
TCP/IP and Ethernet:

Ethernet

packet header

IP

Packet

TCP

Packet

HTTP

Packet

User Data

Ethernet

packet trailer


CSN200 Introduction to Telecommunications,
Winter 2000


Lecture
-
07

Network Layer

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A typical TCP packet has 192
-
bit header of control information.

1

2

3

4

5

6

7

8

9

10

11

User Data

1

Source ID


16 bits

2

Destination ID


16 b
its

3

Sequence number

32 bits

4

ACK number


32 bits

5

Header length


4 bits

6

Unused


6 bits

7

Flags



6 bits

8

Flow control


16 bits

9

CRC 16


16 bits

10

Urgent pointer


16 bits

11


Options


16 bits

Two forms of IP are currently in use:



IPv4 also has a 19
2
-
bit header.



IPv6 has a 320
-
bit header.

The primary reason for the increase in packet size is an increase in the address size from 32 bits
to 128 bits, due to the dramatic growth in the usage of the Internet.

The size of the message field depends on the d
ata link layer protocol used. TCP/IP is commonly
combined with Ethernet.

1

Version number

4 bits

2

Header length


4 bits

3

Type of Service

8 bits

4

Total length


16 bits

5

Identifiers


16 bits

6

Flags



3 bits

7

Packet offset


13 bits

8

Hop limit


8 bits

9

Protocol


8 bits

10


CRC 16


16 bits

11

Source address

32 bits

12

Destination Address

32 bits

13

Options


varies

14

User data


varies

15

Flow name


24 bits

16


Next header


8 bits


IP Packet version :

IP4
-

(4 bytes)

1

2

3

4

5

6

7

8

9

10

11

12

13

14 Us
er Data


IP6
-

(16 bytes)

1

15

4

16

8

11(16 bytes = 128 bits)

12 (16 bytes = 128 bits)

14 User Data

CSN200 Introduction to Telecommunications,
Winter 2000


Lecture
-
07

Network Layer

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IPX/SPX
-

Internet Packet Exchange /Sequenced Packet Exchange (IPX/SPX) is based on a
routing protocol developed by Xerox in the 1970s, and is the primar
y network protocol
used by Novell Netware, although Novell plans to replace IPX/SPX with TCP/IP.

IPX/SPX has two parts:



SPX performs the same packetizing functions of TCP: breaking the data into smaller
packets, numbering them, ensuring each packet is reli
ably delivered, and putting
them into the proper order at the destination.



IPX performs the same routing and addressing functions as IP.

IPX/SPX is similar to TCP/IP in concept, but different in structure.


X.25
-

is a standard developed by ITU
-
ISS for us
e in wide area networks.

X.25 also has two parts:



Packet layer protocol (PLP) the routing protocol that performs routing and
addressing functions similar to IP



X.3 performs the packetizing functions of TCP.


SNA
-

System Network Architecture (SNA) is an a
pproach to networking developed by IBM in
1974. SNA is used only on IBM and compatible mainframes.

As with the other network layer protocols, SNA has two parts. The transmission control
layer performs functions similar to TCP, and the path control layer
performs functions
similar to IP. SNA typically uses SDLC as its data link layer protocol.


Addressing
:

Before you can send a message, you must know the destination address.

When the users work with application software, they typically use the
application

layer address.

The network layer translates this application layer address into a
network layer address
.

The network layer then determines the best route through the network to the final destination.

Based on this routing, the network layer identifies the

data link layer address

of the next
computer to which the message should be sent.

Address

Example Software

Example Address

Application Layer

Web browser

www.cba.uga.edu

Network Layer

TCP/IP

128.192.98.5

Data Link Layer

Ethernet

00
-
0C
-
00
-
F5
-
03
-
5A


CSN200 Introduction to Telecommunications,
Winter 2000


Lecture
-
07

Network Layer

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Assi
gning Addressing
:

In general, the data link layer address is permanently encoded in each network card, and as part
of the hardware cannot be changed.

Network layer addresses are generally assigned by software. Every network layer software
package usually
has a configuration file that specifies the network layer address for that
computer.

Application layer addresses (or server addresses) are also assigned by a software configuration
file.
Virtually all servers have an application layer address
, but most cl
ient computers do
not.

Network layer addresses and application layer addresses go hand in hand. (www.uga.edu
-
application means 192.128.98.53 at the network layer.


On the Internet, InterNIC is responsible for network layer addresses (IP addresses) and
app
lication layer addresses or domain names (www.uga.edu).

There are five classes of Internet addresses.

Classes A, B, and C are presented in Figure 6
-
7

Class D and E are reserved for special purposes and are not assigned to organizations.


Class

Available to

User

Address Structure

Example


Class A

16 million

First byte fixed

50.x.x.x

Organization assigns last
three bytes

Class B

16,000

First two bytes fixed

128.192.x.x

Organization assigns last
two bytes

Class C

250

First three bytes fixed

192.1.56.x

Organ
ization assigns last
byte


One of the problems with the current address class system is that the Internet is quickly running
out of addresses. Although there are more than 1 billion possible addresses, the fact that
they are assigned in sets (or groups)
significantly restricts the number of usable addresses.

The IP address shortage was one of the reasons behind the IPv6, providing in theory, 3.2 x 10
38
possible addresses.


CSN200 Introduction to Telecommunications,
Winter 2000


Lecture
-
07

Network Layer

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CSN200 Introduction to Telecommunications,
Winter 2000


Lecture
-
07

Network Layer

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Subnet
:

Each organization must assign the IP addresses it has received to specif
ic computers on its
networks. In general, IP addresses are assigned so that all computers on the same local
area network have a similar address.

Each LAN that is logically grouped together by IP number is called a TCP/IP
subnet
.

While it is customary to u
se the last byte of the IP address to indicate different subnets it is not
required. Any portion of the IP address can be designated as a subnet by using a
subnet
mask
. IP addresses are binary numbers, so partial bytes can be used as subnets.


Dynamic Ad
dressing:

Every computer knows its network layer address from a configuration file that is installed when
the computer is first attached to the network. However, each time the computer is moved,
or its network is assigned a new address, the software on ea
ch individual computer must be
updated.

The easiest way around this problem is dynamic addressing. With this approach, a server is
designated to supply a network layer address to a computer each time the computer
connects to the network.

Two standards for

dynamic addressing are commonly used in TCP/IP networks:



Bootstrap Protocol (bootp)

Dynamic Host Control Protocol (DHCP)

The boot or DHCP server can be configured to assign the same network layer address to the
computer each time it requests an address or

it can lease the address to the computer by
picking the “next available” network layer address from a list of authorized addresses.

Dynamic addressing greatly simplifies network management in non
-
dial
-
up networks too.


Address Resolution:

In order to sen
d a message, the sender must be able to translate the application layer address (or
server name) of the destination into a network layer address and in turn translate that into a
data link layer address.

This process is called
address resolution
.

TCP/IP us
es two different approaches, one for resolving application layer addresses into IP
addresses and one for resolving IP addresses into data link layer addresses.


CSN200 Introduction to Telecommunications,
Winter 2000


Lecture
-
07

Network Layer

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Server Name Resolution:

Server name resolution is the translation of application layer addresse
s into network layer
addresses. This is done using the Domain Name Service (DNS).

Whenever you receive a set of Internet addresses, you must inform InterNIC of the name and IP
addresses of the name server that will provide DNS information for all addresse
s in that
Class.

When TCP/IP needs to translate an application layer address into an IP address, it sends a special
TCP
-
level packet to the nearest DNS server.

This packet asks the DNS server to send the requesting computer the IP address that matches the
Internet address provided. This is why it sometimes takes a long time to access certain
sites. IP addresses are then temporarily stored in a server address table.


Data Link Layer Address Resolution:

In order to actually send a message, the network layer

software must know the data link layer of
the destination computer.


In the case of a distant computer, the network layer would route the message by selecting a path
through the network that would ultimately lead to the destination.

To send a message to
a computer in its network, a computer must know the correct data link
layer address. In this case, TCP/IP software sends a
broadcast message

(using
Address
-
Resolution
-
Protocol

or ARP) to all computers in its subnet requesting the data link layer
address.


Routing:

In many networks, there are many possible routes or paths a message can take to get from one
computer to another.

Routing

is the process of determining the route or path through the network that a message will
travel from the sender to the receiv
er. Every computer that performs routing has a
routing
table

developed by the network manager that specifies how message will travel through the
network.

Destination

Route

A

A

C

C

D

A

E

E

F

E

G

C

CSN200 Introduction to Telecommunications,
Winter 2000


Lecture
-
07

Network Layer

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There are two fundamental approaches t
o routing:



Centralized routing
-

all routing decisions are made by one central computer.



Decentralized Routing
-

allows all computers in the network to make their own
routing decisions following a formal routing protocol. Most decentralized routing
prot
ocols are self
-
adjusting, adapting to changes in the network configurations.

Types of Routing:


There are two types of dynamic routing:



Static Routing

-

where the routing table developed by the network manager, and changes
made only when computers are adde
d or removed from network.



Dynamic Routing

(adaptive routing)
-

is used when there are multiple routes through a
network and it is important to select the best (or fastest) route, in order to route messages
away from traffic on busy circuits.

An initial rou
ting table is developed by the network manager, but is continuously updated by the
computers themselves to reflect changing network conditions, such as network traffic.

There are three commonly used dynamic routing protocols



Routing Information Protocol

(R
IP)
-

used by the network manager to develop the
routing table. Used by both TCP/IP and IPX/SPX.



Internet Control Message Protocol

(ICMP)
-
used on the internet with TCP/IP.

CSN200 Introduction to Telecommunications,
Winter 2000


Lecture
-
07

Network Layer

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Open Shortest Path First

(OSPF) uses the number of computers in a route as well as

network traffic and error rates to select the best route.

OSPF is more efficient than RIP because it normally doesn’t use broadcast messages. Instead it
selectively sends status update messages directly to selected computes (used by TCP/IP).

Two drawback
s to Dynamic Routing.



It requires more processing by each computer in the network than centralized or
static routing.



The transmission of status information “wastes” network capacity.


Connectionless vs. Connection
-
Oriented Routing
:

Some messages or blocks

of application data are small enough that they can be transmitted in
one packet or frame at the data link layer. In other cases the application data in one
message is too large and must be broken into several packets.

There are two ways these sets of pac
kets can be routed through a network.



Connectionless routing

means each packet is treated separately and makes its own
way through the network.



Connection
-
Oriented routing

Sets up a
virtual circuit

between the sender and
receiver. Appears to use point
-
to
-
point circuit
-
switching, but actually uses store
-
and
-
forward. Has greater overhead than connectionless, due to the routing
information.

TCP/IP can operate as connection
-
oriented or connectionless.

When connection
-
oriented routing is needed, both TCP and

IP are used. TCP establishes the
virtual circuit and IP routes the messages.

When connectionless routing is desired, only IP is needed, and the TCP packet is replaced with a
User Datagram Protocol (UDP) packet.

IPX/SPX can also operate as either as conne
ction
-
oriented or connectionless.

When connection
-
oriented routing is needed, both IPX and SPX are used. SPC establishes the
virtual circuit and IPX routes the messages.

When connectionless routing is desired, only IPX is needed.

Quality of Service

(QoS
) routing is a special type of connection
-
oriented dynamic routing in
which different messages or packets are assigned different priorities.

With QoS routing, different classes of service are defined each with different priorities.

QoS routing is common in

certain kinds of networks (e.g. ATM).

CSN200 Introduction to Telecommunications,
Winter 2000


Lecture
-
07

Network Layer

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Unicast, Multicast and Broadcast:


The most common type of message in a network is the usual transmission between two
computers. This is called a
unicast message
.

A
broadcast message

can also be sent to all computer
s on a LAN or subnet.

A third type of message, called a
multicast message

is used to send the same message to a group
of computers.


Computers wishing to participate in a multicast end a message to the sending computer or some
other computer performing rou
ting along the way using a special type of TCP
-
level packet
called
Internet Group Management Protocol.

Each multicast group is temporarily assigned a special Class D IP address to identify the group,
thus allowing a restricted broadcast of messages to this

specific group.