1
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Copyrights @ 2013 by John Wiley & Sons, Inc
Most chapter references are related to Behrouz A Forouzan ed4
Jozef Goetz, 2012
expanded by Jozef Goetz, 2012
2
Objectives
Exam Objective Matrix
Technology Skill Covered
Exam Objective
Exam Objective Number
Introduction to the OSI Model
Compare the layers of the OSI and TCP/IP
models.
OSI model:
• Layer 1—Physical
• Layer 2—Data Link
• Layer 3—Network
• Layer 4—Transport
• Layer 5—Session
• Layer 6—Presentation
• Layer 7—Application
Classify how applications, devices, and
protocols relate to the OSI model layers.
• IP address
• Frames
• Packets
• Cable
1.1
Jozef Goetz, 2012
1.2
3
Objectives
Exam Objective Matrix
Technology Skill Covered
Exam Objective
Exam Objective Number
TCP/IP Model
Compare the layers of the OSI and TCP/IP
models.
TCP/IP model:
• Network Interface Layer
• Internet Layer
• Transport Layer
• Application Layer
• (Also described as: Link Layer, Internet
Layer, Transport Layer, Application Layer)
1.1
Jozef Goetz, 2012
2-1 LAYERED TASKS
•We use the concept of layers in our daily life.
•As an example, let us consider two friends who
communicate through postal mail.
•The process of sending a letter to a friend would be
complex if there were no services available from the
post office.
Topics discussed in this section:
Sender, Receiver, and Carrier
Hierarchy
Jozef Goetz, 2012
4
Sending a letter:
5
The concept of layers
with the sender and receiver address
Home layer
Mailbox layer
sorted and
PO layer
and sorted
Note: each layer at the sending site uses
the services of the layer immediately below it.
There is a hierarchy (order) of different asks:
•e.g. at the sender site.
•The letter must be written and dropped in the mailbox before being picked up by the letter carrier and
Jozef Goetz, 2012delivered to PO
6
Jozef Goetz, 2012
7
Jozef Goetz, 2012
8
Jozef Goetz, 2012
9
Jozef Goetz, 2012
2-2 THE OSI MODEL
•Established in 1947, the International Standards
Organization (ISO) is a multinational body dedicated to
worldwide agreement on international standards.
•An ISO standard that covers all aspects of network
communications is the Open Systems Interconnection
(OSI) model.
•It was first introduced in the late 1970s.
Topics discussed in this section:
Layered Architecture
Peer-to-Peer Processes
Encapsulation
Jozef Goetz, 2012
10
11
Note
ISO is the organization.
OSI is the model.
Jozef Goetz, 2012
12
Figure 2.1 Seven layers of the OSI model
Jozef Goetz, 2012
Mnemonics:
Top to bottom
All People Seem To Need Data Processing
Bottom to top
Please Do Not Throw Sausage Pizza Away
13
Figure 2.2
Jozef Goetz, 2012
Internet layers
14
Figure 2.3
Jozef Goetz, 2012
Peer-to-peer processes
Protocol Hierarchies

Layers:

Networks are organized as a stack of levels or layers to reduce their design
complexity.





Each layer perform a specific collection of well–understood related
functions
We build each layer on the one below it.
Each layer talks to the ones above & below it
Each layer hides underlying details from the one above it – sort of a virtual
machine, offering certain services (operations) to the layer above it
Layers differ in number and function from one network to another.
Jozef Goetz, 2012
15
interfaces and protocols

Between each pair of
adjacent layers is an
interface.

defines what information
and services the lower layers
makes available to the upper
one.

Well defined interfaces
and layers provide
modularity to a network


Layers or their services
(functions) are replaceable
The corresponding layers on
different machines are called
peers

The peers communicate by
using protocols
Jozef Goetz, 2012
16
Services to Protocols Relationship

Services is a set primitives (operations) that the layer provides
layer above it


What functions does this layer provide? Services like an
OOD - object oriented design
to the
abstract data type in
A protocol is a set of rules governing the format and meaning of
the packets and/or msgs that are exchange by the peers entities within
a layer.

17

A protocol relates to the implementation of the service and as such

Entities use protocols to implement their service definition
is not visible to the user of the service
The services and the protocols are completely decoupled
Jozef Goetz, 2012
18
Figure 2.3 The interaction between layers in the OSI model
A communications protocol
is a system of digital
message formats and rules
for exchanging those
messages in or between
computing systems
Jozef Goetz, 2012
Protocols
Protocols:
1.
2.

Jozef Goetz, 2012
Define data types for transmission
Provide different services (a set of
operations)

Initiate a connection

Transmit data

Terminate a connection
Are layered to form architectures
19
20
Jozef Goetz, 2012
21
Figure 2.4 An exchange using the OSI model
Each layer in the sending device
•adds its own info (header) to the msg it receives from the layer just above it and
•passes the whole package to the layer below
Header contains
information that describes
what each layer of the OSI
Model should do with the
data
At the receiving device, the msg is unwrapped layer by layer
(the msg moves back up throught the OSI layers):
•i.e. the headers and trailers (T2) attached to it at the corresponding sending layer
Jozef Goetz, 2012 are removed, and actions appropriate to that layer are taken
Data as It Moves Through OSI Layers,
Sent by One Computer and Received by
Another
Jozef Goetz, 2012
22
23
Figure 2.4 An exchange using the Internet model
Encapsulation concept: level N – 1 is not aware which part of the packet is data and which part is
the header
•A packet at level 5 is encapsulated in packet at level 4.
•Level 4 is not aware which part of the encapsulated packet is data and which part
•is the header.
•The packet in level 4 is encapsulated in packet at level 3, and so on.
Jozef Goetz, 2012
Protocol encapsulation
user X
“Hello”
user Y
e-mail client
“Hello”
e-mail server
TCP server
“Hello”
TCP server
IP server
“Hello”
IP server
“Hello”
ethernet
driver/card
ethernet
driver/card
Jozef Goetz, 2012
24
Encapsulation/De-encapsulation


The process of moving data between
layers of the OSI Model
Encapsulation:
Data > segment >
packet > frame >
bits

De-encapsulation:
Bits > frame >
packet > segment
> data
Jozef Goetz, 2012
25
How Data Is Referred to in the OSI Model
Data
Segment
• Transport layer
Packet
• Networking layer
Frame
• Data Link layer
Bits
Jozef Goetz, 2012
• Application, Presentation, and Session
layers
• Physical layer
26
Physical Layer PL 1
Note:
1. The physical layer is responsible for
transmitting individual bits from one
node to the next.
Jozef Goetz, 2012
27
Physical layer
28
•Defines physical means that are used to transmit data:
•Cable standards – e.g. copper wiring
•Wireless standards – e.g. radio frequencies
•Fiber optic standards – e.g. light pulses
•Concerned with the characteristics of the physical medium
to transfer row bits over a communication channel.
•Deals with
•mechanical,
•electrical,
•functional,
•procedural and
•timing interfaces, and
•the physical medium itself.
Jozef Goetz, 2012
29
Physical layer duties
1.
2.
3.
4.
5.
6.
Jozef Goetz, 2012
Physical characteristics of interfaces and media
•
Plug types, materials etc.
Representation (encoding) of bits
•
E.g. TTL 5v = 1, 0 = 0v, RS232C 1 = -12v, 0 = +12v
•
Sensitivity of receivers, e.g. minimum voltage for 1 in RS232C is -3v
Data Rate/Transmission Rate
•
How many bits per second
Synchronization of bits
•
Not only the bit rate but when does a char or frame/packet starts
and ends
Transmission mode: simplex, half-duplex, or full-duplex
Physical topology: mesh, star, bus, ring, or hybrid
Data Link Layer DDL 2
Shows relationship of it to other layers
Jozef Goetz, 2012
30
Figure 2.7
Node (hop)-to-node (hop) delivery by the DLL
31
The data link layer is responsible for transmitting frames from one node to the next (of the same network).
Jozef Goetz, 2012


Data Link Layer DDL 2
- point-to-point connection
Deals with transforming a raw line into a line
that appears free of errors to the network
layer.
32
It accomplishes with breaking up the data
into data frames, transmit the frames
sequentially, and then receiving
acknowledgements back.
This layer also deals with different hardware speeds
by slowing some transmissions if the other end can’t
keep up.
 How much buffer space the receiver has?
Jozef Goetz, 2012
 How the error is handled?

Data link layer duties:
•Framing
• the data link layer DLL divides the stream of bits into
manageable data units called frames
• Physical Addressing
• adds a header to the frame containing sender and/or receiver
address
• Flow Control
• Controls how fast a transmitter can send to a receiver
• Error Control
• Detection and recovery from error conditions or lost frames
•Achieved through a trailer T2 added to the end of the frame
• (Medium) Access Control
• who has control over the link at any given time
•Fair access to shared data link networks e.g. Ethernet
Jozef Goetz, 2012
33
Data Link Layer of OSI Model




Can move frames from one adjacent
computer to another, cannot move frames
across routers
Encapsulation = frame
Requires a unique MAC address. or physical
address e.g. 11-00-68-BF-F8-EA
Protocols defined include Ethernet Protocol
and Point-to-Point Protocol (PPP)
Jozef Goetz, 2012
34
Data Link Layer of OSI Model


Device example: Switch
Two sublayers:


Logical Link Control (LLC) and
the Media Access Control (MAC)
Jozef Goetz, 2012
35
LLC and MAC Sublayers

Logical Link Control (LLC)





Data Link layer addressing,
flow control,
address notification,
error correction
Media Access Control (MAC)


Determines which computer has access to the
network media at any given time
Determines where one frame ends and the next
one starts, called frame synchronization
Jozef Goetz, 2012
36
37
Figure 2.19 Example 1
•In Figure below a node with physical address 10 sends a frame to
a node with physical address 87. The two nodes are connected by a
link. At the data link level this frame contains physical addresses
in the header.
•These are the only addresses needed. The rest of the header
contains other information needed at this level.
•The trailer usually contains extra bits needed for error
detection
Jozef Goetz, 2012
Network Layer NL 3
Shows relationship of it to other layers
Jozef Goetz, 2012
38
39
Figure 2.9
Source-to-destination delivery across different networks
The network layer is responsible for the delivery of packets from the original
source to the final destination (across different network).
Jozef Goetz, 2012
Network Layer 3


Controls the operation of the subnet.
A key design issue is - determining how packets
are routed.

Packet routing may be



40
static (based on static tables ex. determined from a terminal
session) or
dynamic (determined for each packet), depending on the
type of network.
Other issues:



Jozef Goetz, 2012
congestion control,
quality of service: (delay, transit time etc.),
overcome protocol differences
Network Layer of OSI Model

Responsible for moving packets (data)
from one end of the network to the
other, called end-to-end
communications

Device example: Router

Jozef Goetz, 2012
Routing is the ability of various network
devices and their related software to move
data packets from source to destination
41
Network layer duties
42
1. Source to Destination Delivery
• Beyond the same link
• Interconnected basic different networks
2.
Logical Addressing such as IP e.g. 192.45.0.120
• if packet passes the network boundary we need to distinguish
the source and destination network by adding header with the
logical sender and receiver addresses
• so we separate from physical addressing at data link
• logical addresses need to be assigned to every computer on a
network
• the logical addresses doesn’t change
3.
Routing/Internetworking
• determine how to get from the source to the destination
• connecting devices (routers or switches ) are used to route to
Jozef Goetz, 2012
43
Figure 2.20 Example 3
•In Figure we want to send data from a
node with network address A and
physical address 10, located on one
LAN, to a node with a network address
P and physical address 95, located on
another LAN.
•Because the two devices are located
on different networks, we cannot use
physical addresses only;
•the physical addresses only
have local jurisdiction.
•What we need here are universal
addresses that can pass through the
LAN boundaries?
•The network (logical) addresses have
this characteristic.
Jozef Goetz, 2012
Transport layer TL 4
44
Note: Network L. deliver individual packets, it doesn't recognize any relationship
between packets which belonged to some segments.
The transport layer is responsible for
delivery of a segment (message) from
one process to another.
Jozef Goetz, 2012
Figure 2.11
Jozef Goetz, 2012
45
Reliable process-to-process delivery of a message
Transport Layer of OSI Model



46
Takes data from higher levels of OSI Model
and breaks it into segments that can be sent
to lower-level layers for data transmission
Conversely, reassembles data segments into
data that higher-level protocols and
applications can use
Also puts segments in correct order (called
sequencing ) so they can be reassembled in
correct order at destination
Jozef Goetz, 2012
Transport Layer TL 4


Serves as the interface between the Session and
Network layers.
The Transport Layer is responsible for




47
breaking up larger data (messages) from the session layer
into (smaller messages) segments and then
sending them across the network layer
making sure the all pieces arrive correctly to the destination
The transport layer is a true end-to-end layer, all
the way from the source to the destination


Jozef Goetz, 2012
A program on the source machine carries on a conversation with
the similar program on the destination machine
In the lower layers, the protocols are between each machine and
its immediate neighbors, not between the ultimate source and
destination.
Transport layer duties
1. Port addressing
• Process (running program) on one computer
delivers a message (msg) to a specific process on
the other.
• To identify the process the port address is
provided to the TL
2. Segmentation and reassembly control
• Sending large amounts of data needs to be
managed by breaking down into transmissible
segments
• Each segment contains a sequence # in order to
reassemble data (message) at the destination
Jozef Goetz, 2012
48
Transport layer duties
49
3. Connection control
• a phone call like service (connection oriented service
- such as TCP to ensure destination received segments)
or
• a postcard like service without verification
(connectionless service - such as UDP to send
segments without assurance of delivery ) – e.g. VoIP,
video – data-streaming apps
4. Flow control
• End to End control at a process level
5. Error control
• End to End error control rather than a single link
Jozef Goetz, 2012
50
Figure. Example 3. Internet Model
Figure shows an example of transport layer communication.
Data coming from the upper layers have port addresses j and k
(j is the address of the sending process, and k is the address of
the receiving process).
•Since the data size is larger than the network layer can
handle, the data (message) are split into two segments,
each segment retaining the port addresses (j and k).
Then in the network layer, network addresses (A and P) are
added to each packet.
Jozef Goetz, 2012
Session layer SL 5
The session layer is responsible for dialog
control and synchronization.
Jozef Goetz, 2012
51
Session Layer SL 5


52
Allows users on different machines to establish a
session, manage and terminate session.
Session services provide:

dialog control (who transmits next)
• provides communications between 2 Ps in simplex, half-duplex, or
full-duplex

synchronization (checkpointing long transactions so they
can continue after a crash), adjournment, termination and
restart or recovery
• SL allows to add checkpoints, or synchronization points, to a
stream data in order if a crash happens during the retransmission
send one from the last checkpoint
• e.g. 2000 pages are sent, but a crash happens at page 123,
so pages are needed to resend are 101 to 125

token management (preventing the same critical
operation at the same time).
Jozef Goetz, 2012
Presentation Layer 6

Handles the syntax and
semantics of the info
exchanged between systems.
The presentation layer is responsible for translation,
compression, and encryption.
Jozef Goetz, 2012
53
Presentation Layer PL 6
1. Manages translation


Allows data to move from one machine to
another while retaining the appropriate
format.
allows higher-level data structure (e.g.,
banking records), to be defined into a
common format at the sender and send it
to the receiver which converts into its receiverdependent format.
Jozef Goetz, 2012
54
Presentation Layer PL 6
2. Encryption



The sender transforms the orginal info to
another form (to protect from interception or
eavesdropping) and sends msg out over the
network
Decryption reverses to its orginal form at the
receiver
e.g. Secure Socket Layer (SSL) and IPSec (ure)
3. Compression

Reduces the # of bits

Jozef Goetz, 2012
Important for sending text, audio, and video
55
Application Layer 7
The application layer is responsible
for providing network services (access
to the network) to the user.
Jozef Goetz, 2012
56
Application Layer



Contains protocols that are often needed by
users, such as HTTP.
The application layer is the most abstract
layer and is closest to the user.
The user is most likely to interact with the
application layer than any other layer in
the model.
Jozef Goetz, 2012
57
Application Layer

58
Provided services:

network virtual terminal


file transfer, access and mgmnt




for e-mail forwarding and storage
E-mail program may use POP3 (Post Office Protocol version 3) to
read e-mails and SMTP (Simple Mail Transport Protocol) to send
e-mails
directory services


access to a remote host to make changes and read data
retrieve/send files from/to a remote host
mail services


to logon to a remote host
database access for global info about services etc.
Firefox web browser uses HTTP (Hyper-Text Transport Protocol)
Jozef Goetz, 2012
59
Summary of layers !
Jozef Goetz, 2012
The OSI Reference Model

This is the “International Standards
Organization Open Systems
Interconnection Reference Model”, or the
ISO OSI Reference Model.

We will call it the OSI model.

There is a protocol defined for each layer by the ISO.

60
The protocols are rarely used anymore, but the model is very
relevant.
Jozef Goetz, 2012
Principles behind the model.
1.
2.
3.
4.
5.
• Layers should be created where abstraction is
needed.
• Each layer should perform a well-defined function.
• The function of the layer should be chosen while
defining protocols.
• The layer boundaries should be chosen to minimize
the information flow across the interfaces.
• There should not be too many or too few layers.
Jozef Goetz, 2012
61
OSI



The OSI itself is not a network
architecture because it does not
specify exact services and protocols
to be used in each layer.
It just tells what each layer should do
However ISO produced standards for
all the layers, although these are not
part of the reference model itself
Jozef Goetz, 2012
62
Reference Models.
The OSI
reference
model.
Jozef Goetz, 2012
63
64
2-4 TCP/IP PROTOCOL SUITE (Internet Model)
•The layers in the TCP/IP protocol suite do not exactly
match those in the OSI model.
•The original TCP/IP protocol suite was defined as
having four layers: host-to-network, internet, transport,
and application.
•However, when TCP/IP is compared to OSI, we can say
that the TCP/IP protocol suite is made of five layers:
physical, data link, network, transport, and application.
Topics discussed in this section:
Physical and Data Link Layers
Network Layer
Transport Layer
Application Layer
Jozef Goetz, 2012
The TCP/IP Reference Model



– the model itself is not much use but the
protocols are widely used
This data communication model was first defined in
1974 when the internet was still part of ARPANET.
Major Goals:





65
The Army-driven design was meant to allow the computer network
to communicate even if a part of the network was down.
This would allow the network to continue to work in the case of a
war.
The military was also worried about lost connections.
They did not want a connection to die as long as the source and
destination machines were alive, even if some of the machines in the
middle were lost.
This put further requirements on any protocol that
might be created.
Jozef Goetz, 2012
66




The TCP/IP reference model.
The host-to-network layer is undefined in this model.
They leave out all of the underlying layers, leaving the
implementation up to whoever creates the network.
The TCP/IP internet layer is similar in functionality to the
OSI network layer.
Jozef Goetz, 2012
67
Figure 2.16 TCP/IP and OSI model p.43-44
•The 5th layer deals with the Internet
services provided by various applications.
•The first 4 layers deal with the
communication between hosts.
•Layers 3 and 4 are fully implemented in
the operating systems kernel on most
existing systems.
•The first 2 layers are network hardware
specific, the others are work independently
of the physical layer
•Most of the 1st layer is handled by
hardware (communication medium used,
attachments of hosts to the medium).
•The rest of the 1st layer and all the 2nd
layer is handled by the (Network Interface
Card) NIC card in a host.
Jozef Goetz, 2012
68
Network Models
with approximate mapping between the two
•The 5th layer deals with the Internet services provided by
various applications.
•The first 4 layers deal with the communication between
hosts.
•Layers 3 and 4 are fully implemented in the operating
systems kernel on most existing systems.
•The first 2 layers are network hardware specific, the others
are work independently of the physical layer
•Most of the 1st layer is handled by hardware
(communication medium used, attachments of hosts to the
medium).
•The rest of the 1st layer and all the 2nd layer is handled by
the (Network Interface Card) NIC card in a host.
Jozef Goetz, 2012
Internet Layer.


The requirements resulted in a complex network led to choice
of a packet-switching network based on a
connectionless internetwork layer.
The Internet (Network) Layer is responsible




for injecting packets into any network and
have them travel to their destination.
Order of arrival is not important to the internet layer –
they packets can be sorted out later by the higher layers.
The internet layer defines an official packet format for and
protocol called Internet Protocol (IP).
the major issue is packet routing to avoid
congestion.
Jozef Goetz, 2012

69
Transport Layer.
70
The transport layer is intended to allow two machines
to carry on a conversation, just like the OSI
transport layer.


There are two end-to-end protocols defined for this:
– TCP (Transmission Control Protocol)
 reliable, byte stream connection-oriented


for application without errors – file transfer
– UDP (User Datagram Protocol)
 unreliable, connectionless


Jozef Goetz, 2012
for applications that don’t need TCP’s sequence or flow control and
wish to provide their own
used for client/server request reply queries in which prompt
delivery is more important than accurate delivery,
 such as a transmitting speech or video
TCP/IP Model and its Relation to
Protocols of the TCP/IP Suite

The Application layer contains all of the
higher-level protocols
71
Network Interface Layer has Link Layer synonym
– FTP
– file transfer TFTP - Trivial File Transfer Protocol - used for automated transfer of configuration or
boot files between machines in a local environment
– SMTP – e-mail
– DNS - Domain Name System
– NNTP - Network News Transfer Protocol
– HTTP - Hypertext Transfer Protocol
Jozef Goetz, 2012
The TCP/IP Protocol Suite

IGMP - Internet Group
Management Protocol

handles multicasting
ICMP - Internet Control
Message Protocol





Handles errors and
control messages.
Protocol is used to
forward information,
primarily error messages.
To see if a computer is
running, the `ping'
program sends an echo
request, which is part of
ICMP.
ARP -Address Resolution
Protocol is a protocol for
mapping an Internet
Protocol IP address to a
physical machine address
that is recognized in the local
network.

For example, in IP
Version 4, the most
common level of IP in use
today, an address is 32
bits long.

In an Ethernet LAN,
however, addresses for
attached devices are 48
bits
Jozef Goetz, 2012

RARP - Reverse Address Resolution Protocol is a protocol by
which a physical machine in a LAN can request to learn its IP
address from a gateway server's Address Resolution Protocol
(ARP) table or cache.

A network administrator creates a table in a LAN’s
gateway router that maps the physical machine
(addresses) into IP addresses.
72
Comparing OSI and TCP/IP Models

Both are:



based on the concepts of a stack of
independent protocols
the layers up through transport layers
provide an end-to-end networkindependent transport service
the layers above transport are
application-oriented users
Jozef Goetz, 2012
73
Comparing OSI and TCP/IP Models
74
• The differences between the TCP/IP (4 layers) and
OSI (7 layers) models.
• OSI model clearly defined the distinction between
services, interfaces (specifies what the parameters
are and what result to expect) and protocols, where
the TCP/IP model does not.
• Protocols can easily be replaced in the OSI
model (as long as it provides the offered services),
but not in the TCP/IP model.



Jozef Goetz, 2012
This fits in nicely with OO programming concepts as the layer
(an object) hide information and has a set of methods (services)
that processes outside the object can invoke
The code internal to the object is its protocol and it is not visible
outside the object
Abstraction hides details of one layer from others
Comparing OSI and TCP/IP Models
75
• The OSI model has no biases for protocols,
but the OSI model was overly general and
did not help with the creation of new
protocols.
• OSI supports:


connectionless and connection-oriented in the
network layer, but only
connection-oriented in the transport layer, providing
no choice to the user.
• TCP/IP supports:


Jozef Goetz, 2012
only connectionless in the network layer,
but both in the transport layer, giving user a choice
(TCP, UDP).
Comparing OSI and TCP/IP Models


The OSI reference model was
devised before the corresponding
protocols were invented
With TCP/IP the protocols came
first, and the model was just a
description of the existing
protocols

Jozef Goetz, 2012
So protocols fit perfectly
76
A Critique of the OSI Model and Protocols


Why OSI did not take over the world
Bad timing


Bad technology



7 layers choice more political than technical, layer 5
and 6 nearly empty, 2 and 3 are overfull
Functions: addressing, flow control, error control are
in each layer

instead error control must be done in the
highest layer
Bad implementations



Competing TCP/IP was already there
Initially because of the enormous complexity
In contrast TCP/IP was part of Berkeley UNIX and
free
Bad politics
Jozef Goetz, 2012
77
Bad Timing

The apocalypse of the two elephants.
Jozef Goetz, 2012
78
A Critique of the Models and Protocols
• OSI
1. – Arrived late on the scene
2. – Model was too complex and the protocols were flawed
3.
(some stuff appears in multiple levels)
4. – Poor implementation (no good example)
5. – Poorly marketed
• TCP/IP
1. – model is just an explanation of what was already
implemented
2. – model is not useful for describing any other protocol
3. – model does not clearly define layers
4. – model is not complete
5. – lead to the introduction of other poorly implemented
protocols
Jozef Goetz, 2012
79
A Critique of the TCP/IP Reference Model

1.
Problems:
Service, interface, and protocol not
distinguished

2.
3.
Not a general model
Host-to-network “layer” not really a layer
•
•
4.
in the normal sense of the term as used in the context of layered
protocols
It is an interface between layer 2 and 3
Doesn’t distinguish the physical and data link
layers.

5.
what is required by good software engineering practice to
distinguish between the specification and the implementation
These are completely different. They are not a separate layers.
Minor protocols deeply firmly established
(entrenched), hard to replace
Jozef Goetz, 2012
80
A Critique of the Models and Protocols.
In summary
OSI
 – The model, minus the session and presentation
layers, is exceptionally useful.
TCP/IP
 – The protocol is in wide use and is accepted
virtually everywhere.
The Rest of the Course:
• Focus on the useful layers of the OSI model while
ignoring the ISO protocols.

Jozef Goetz, 2012
81
2-5 ADDRESSING
Four levels of addresses are used in an internet
employing the TCP/IP protocols: physical, logical, port,
and specific.
Topics discussed in this section:
Physical Addresses
Logical Addresses
Port Addresses
Specific Addresses
Jozef Goetz, 2012
82
83
Figure 2.17 Addresses in TCP/IP
Jozef Goetz, 2012
84
Figure 2.18 Relationship of layers and addresses in TCP/IP
e.g. URL
Universal
Resource
Locator
Jozef Goetz, 2012
85
Figure 2.19 Physical addresses
•In Figure 2.19 a node with physical address 10 sends a
frame to a node with physical address 87.
•The two nodes are connected by a link (bus topology
LAN).
•As the figure shows, the computer with physical address
10 is the sender, and the computer with physical address
87 is the receiver.
Jozef Goetz, 2012
86
Example 2.2
Most local-area networks use a 48-bit (6-byte) physical
address (or MAC addresses) written as 12 hexadecimal
digits; every byte (2 hexadecimal digits) is separated by
a colon, as shown below:
07:01:02:01:2C:4B
A 6-byte (12 hexadecimal digits) physical address.
Jozef Goetz, 2012
Wide Area Networks
87
Relation between hosts on LANs and the subnet.
 The subnet consists of routers and transmission
lines.


Jozef Goetz, 2012
A router is a specialized piece of switching hardware
that is responsible for determining the direction that
data packets should be sent.
Routers are responsible for directing data down
transmission lines from one LAN to another.
Wide Area Networks


88
A stream of packets from sender to receiver.
A subnet is organized according to the principle called Storeand-forward or Packet-switched subnet
– The message is broken down into smaller packets to send.
– Each packet is sent out onto the network.
– As a packet arrives at a router, it is stored there until the outgoing line
is free. It is then sent on it’s way.
– All the packets make take the same or different routes depending on if
they are individually routed or not.
Jozef Goetz, 2012
89
Figure: IP addresses
•Figure shows a part of an
internet with two routers
connecting three LANs.
•Each device (computer or
router) has a pair of
addresses
(logical
and
physical)
for
each
connection.
•In this case, each computer
is connected to only one
link and therefore has only
one pair of addresses.
•Each router, however, is
connected to 3 networks
(only two are shown in the
figure).
•So each router has 3
pairs of addresses, one
for each connection.
Jozef Goetz, 2012
90
Figure 2.21 Port addresses
•Figure 2.21 shows two
computers communicating via
the Internet.
•The sending computer is
running three processes at this
time with port addresses a, b,
and c.
•The receiving computer is
running two processes at this
time with port addresses j and
k.
•Process a in the sending
computer
needs
to
communicate with process j in
the receiving computer.
•Note that although physical
addresses change from hop to
hop,
logical
and
port
addresses remain the same
from the source to destination.
Jozef Goetz, 2012
91
Example 2.5
As we will see in Chapter 23, a port address is a 16-bit
(2 byte) address represented by one decimal number as
shown.
753
A 16-bit port address represented
as one single number.
Jozef Goetz, 2012
92
Note
The physical addresses change from hop to hop,
but the logical and port addresses usually remain the same.
Jozef Goetz, 2012
Summary: Difference between UDP and
TCP internet protocol (IP) traffic
93
http://www.cyberciti.biz/faq/key-differences-between-tcp-and-udp-protocols/
Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) is
a transportation protocol that is one of the core protocols of the Internet
protocol suite. Both TCP and UDP work at transport layer TCP/IP model and both
have very different usage.
Difference between TCP and UDP
TCP: Reliability: TCP is connection-oriented protocol. When a file or message send
it will get delivered unless connections fails. If connection lost, the server will request
the lost part. There is no corruption while transferring a message.
UDP: Reliability: UDP is connectionless protocol. When you a send a data or
message, you don't know if it'll get there, it could get lost on the way. There may be
corruption while transferring a message.
TCP: Ordered: If you send two messages along a connection, one after the
other, you know the first message will get there first. You don't have to worry
about data arriving in the wrong order.
UDP: Ordered: If you send two messages out, you don't know what order they'll
arrive in i.e. no ordered
Jozef Goetz, 2012
Summary: Difference between UDP and
TCP internet protocol (IP) traffic
94
TCP: Heavyweight: - when the low level parts of the TCP "stream" arrive in the
wrong order, resend requests have to be sent, and all the out of sequence
parts have to be put back together, so requires a bit of work to piece together.
UDP: Lightweight: No ordering of messages, no tracking connections, etc. It's
just fire and forget! This means it's a lot quicker, and the network card / OS
have to do very little work to translate the data back from the packets.
TCP: Streaming: Data is read as a "stream," with nothing distinguishing where
one packet ends and another begins. There may be multiple packets per
read call.
UDP: Datagrams: Packets are sent individually and are guaranteed to be whole
if they arrive. One packet per one read call.
TCP: Examples: World Wide Web (Apache TCP port 80), e-mail (SMTP TCP port 25
Postfix MTA), File Transfer Protocol (FTP port 21) and Secure Shell (OpenSSH
port 22) etc.
UDP: Examples: Domain Name System (DNS UDP port 53), streaming media
applications such as IPTV or movies, Voice over IP (VoIP), Trivial File
Transfer Protocol (TFTP) and online multiplayer games etc
Jozef Goetz, 2012
Summary
The physical layer is responsible for transmitting a bit stream over a physical
medium. It is concerned with
a. physical characteristics of the media
b. representation of bits
c. type of encoding
d. synchronization of bits
e. transmission rate and mode
f. the way devices are connected with each other and to the links
The data link layer is responsible for
a. framing data bits
b. providing the physical addresses of the sender/receiver
c. data rate control
d. detection and correction of damaged and lost frames
The network layer is concerned with delivery of a packet across multiple networks;
therefore its responsibilities include
a. providing host-to-host addressing
b. Routing
The transport layer oversees the process-to-process delivery of the entire data (message).
It is responsible for
a. dividing the message into manageable segments
b. reassembling it at the destination,
c. flow and error control
The application layer services include, file transfer, remote access, shared database
management, and mail services
Jozef Goetz, 2012
95
Summary
Addresses


The physical address is the local address of a node; it is used by the data
link layer to deliver data from one node to another within the same
network.
The logical address defines the sender and receiver at the network layer
and is used to deliver messages across multiple networks.


The port address (service-point) identifies the application process on
the station.
The application layer services include





file transfer,
remote access,
shared database
management, and
mail services
The application, presentation, and session layers of the OSI model are
represented by the application layer in the Internet model.

Jozef Goetz, 2012
The lowest four layers of OSI correspond to the Internet model layers.
96
Physical Mail
CEO X
“Lunch?”
97
CEO Y
Secretary X
“Lunch?”
Secretary Y
Mail Room X
“Lunch?”
Mail Room Y
Postman X
“Lunch?”
Postman Y
Post office X
“Lunch?”
Post office Y
Jozef Goetz, 2012
Protocol stack:
packet forwarding
Host A
98
Host B
Router R
Router W
HTTP
HTTP
TCP
TCP
IP
ethernet
Jozef Goetz, 2012
IP
ethernet link
IP
link ethernet
IP
ethernet
OSI units
Address Layer
Data Unit
Delivery
7 ApplicationLayer
6 PresentationLayer
5 SessionLayer
port #'s
4 TransportLayer
log.address IP 3 NetworkLayer
MAC phAddress 2 DataLinkLayer
1 PhysicalLayer
Data (Messages)
Data (Messages)
Data (Messages)
TCP Segments, UDP diagrams Process To Process //End to End
Packet/Datagram
Source Host to Destination Host
Frames
Point to Point //Hop to Hop
<=The TCP/IP model
reference to be used in
our lectures
Jozef Goetz, 2012
99