Section 3: Protocols and

Models

Introduction to Networks v7.0

3.1 The Rules

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The Rules
Communications Fundamentals
Networks can vary in size and complexity. It is not enough to have a connection,
devices must agree on “how” to communicate.
There are three elements to any communication:
• There will be a source (sender).
• There will be a destination (receiver).
• There will be a channel (media) that provides for the path of communications to
occur.

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The Rules
Communications Protocols
• All communications are governed by protocols.

• Protocols are the rules that communications will follow.

• These rules will vary depending on the protocol.

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The Rules
Rule Establishment
• Individuals must use established rules or agreements to govern the conversation.

• The first message is difficult to read because it is not formatted properly. The second shows
the message properly formatted

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The Rules
Rule Establishment (Cont.)
Protocols must account for the following requirements:
• An identified sender and receiver
• Common language and grammar
• Speed and timing of delivery
• Confirmation or acknowledgment requirements

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The Rules
Network Protocol Requirements
Common computer protocols must be in agreement and include the following
requirements:
• Message encoding
• Message formatting and encapsulation
• Message size
• Message timing
• Message delivery options

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The Rules
Message Encoding
• Encoding is the process of converting information into another acceptable form for
transmission.
• Decoding reverses this process to interpret the information.

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The Rules
Message Formatting and Encapsulation
• When a message is sent, it must use a specific format or structure.

• Message formats depend on the type of message and the channel that is used to
deliver the message.

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The Rules
Message Size
Encoding between hosts must be in an appropriate format for the medium.
• Messages sent across the network are converted to bits
• The bits are encoded into a pattern of light, sound, or electrical impulses.
• The destination host must decode the signals to interpret the message.

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The Rules
Message Timing
Message timing includes the following:
Flow Control – Manages the rate of data transmission and defines how much information
can be sent and the speed at which it can be delivered.
Response Timeout – Manages how long a device waits when it does not hear a reply from
the destination.
Access method - Determines when someone can send a message.
• There may be various rules governing issues like “collisions”. This is when more than one
device sends traffic at the same time and the messages become corrupt.
• Some protocols are proactive and attempt to prevent collisions; other protocols are
reactive and establish a recovery method after the collision occurs.

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The Rules
Message Delivery Options
Message delivery may one of the following methods:
• Unicast – one to one communication
• Multicast – one to many, typically not all
• Broadcast – one to all

Note: Broadcasts are used in IPv4 networks, but are not an option for IPv6. Later we will also
see “Anycast” as an additional delivery option for IPv6.

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The Rules
A Note About the Node Icon
• Documents may use the node icon , typically a circle, to represent all devices.

• The figure illustrates the use of the node icon for delivery options.

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3.2 Protocols

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Protocols
Network Protocol Overview
Network protocols define a
Protocol Type Description
common set of rules.
• Can be implemented on
devices in: Network enable two or more devices to communicate over
Communications one or more networks
• Software
• Hardware Network Security secure data to provide authentication, data
integrity, and data encryption
• Both
• Protocols have their own: Routing enable routers to exchange route information,
compare path information, and select best path
• Function
• Format Service used for the automatic detection of devices or
• Rules Discovery services

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Protocols
Network Protocol Functions
• Devices use agreed-upon protocols
to communicate .
• Protocols may have may have one
or functions.

Function Description
Addressing Identifies sender and receiver
Reliability Provides guaranteed delivery
Flow Control Ensures data flows at an efficient rate
Sequencing Uniquely labels each transmitted segment of data
Error Detection Determines if data became corrupted during transmission
Application Interface Process-to-process communications between network applications
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Protocols
Protocol Interaction
• Networks require the use of several
protocols.
• Each protocol has its own function and format.

Protocol Function
Hypertext Transfer  Governs the way a web server and a web client interact
Protocol (HTTP)  Defines content and format
Transmission Control  Manages the individual conversations
Protocol (TCP)  Provides guaranteed delivery
 Manages flow control
Internet Protocol (IP) Delivers messages globally from the sender to the receiver
Ethernet Delivers messages from one NIC to another NIC on the same Ethernet Local
Area Network (LAN)
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3.3 Protocol Suites

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Protocol Suites
Network Protocol Suites
Protocols must be able to work with other
protocols.
Protocol suite:
• A group of inter-related protocols
necessary to perform a communication
function
• Sets of rules that work together to help
solve a problem
The protocols are viewed in terms of layers:
• Higher Layers
• Lower Layers- concerned with moving
data and provide services to upper
layers
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Protocol Suites
Evolution of Protocol Suites
There are several protocol suites.
• Internet Protocol Suite or TCP/IP- The
most common protocol suite and maintained
by the Internet Engineering Task Force
(IETF)

• Open Systems Interconnection (OSI)

protocols- Developed by the International
Organization for Standardization (ISO) and
the International Telecommunications Union
(ITU)

• AppleTalk- Proprietary suite release by

Apple Inc.

• Novell NetWare- Proprietary suite

developed by Novell Inc.

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Protocol Suites
TCP/IP Protocol Example
• TCP/IP protocols operate at the
application, transport, and
internet layers.
• The most common network
access layer LAN protocols are
Ethernet and WLAN (wireless
LAN).

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Protocol Suites
TCP/IP Protocol Suite
• TCP/IP is the protocol suite used by
the internet and includes many
protocols.

• TCP/IP is:

• An open standard protocol suite

that is freely available to the public
and can be used by any vendor
• A standards-based protocol suite
that is endorsed by the networking
industry and approved by a
standards organization to ensure
interoperability

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Protocol Suites
TCP/IP Communication Process
• A web server encapsulating and sending a • A client de-encapsulating the web page for
web page to a client. the web browser

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3.4 Standards Organizations

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Standards Organizations
Open Standards
Open standards encourage:
• interoperability

• competition

• innovation

Standards organizations are:

• vendor-neutral

• non-profit organizations

• established to develop and promote the

concept of open standards.

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Standards Organizations
• Internet Society (ISOC) - Promotes
Internet Standards the open development and evolution of
internet
• Internet Architecture Board (IAB) -
Responsible for management and
development of internet standards
• Internet Engineering Task Force
(IETF) - Develops, updates, and
maintains internet and TCP/IP
technologies
• Internet Research Task Force
(IRTF) - Focused on long-term
research related to internet and
TCP/IP protocols
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Standards Organizations
Internet Standards (Cont.)
Standards organizations involved with the
development and support of TCP/IP
• Internet Corporation for Assigned
Names and Numbers (ICANN) -
Coordinates IP address allocation, the
management of domain names, and
assignment of other information
• Internet Assigned Numbers Authority
(IANA) - Oversees and manages IP
address allocation, domain name
management, and protocol identifiers
for ICANN

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3.5 Reference Models

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Reference Models
The Benefits of Using a Layered Model
Complex concepts such as how a
network operates can be difficult to
explain and understand. For this
reason, a layered model is used.
Two layered models describe network
operations:
• Open System Interconnection (OSI)
Reference Model
• TCP/IP Reference Model

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Reference Models
The Benefits of Using a Layered Model (Cont.)
These are the benefits of using a layered model:
• Assist in protocol design because protocols that operate at a specific layer have
defined information that they act upon and a defined interface to the layers above
and below
• Foster competition because products from different vendors can work together

• Prevent technology or capability changes in one layer from affecting other layers
above and below
• Provide a common language to describe networking functions and capabilities

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Reference Models
The OSI Reference Model
OSI Model Layer Description
7 - Application Contains protocols used for process-to-process communications.
Provides for common representation of the data transferred between application
6 - Presentation
layer services.

5 - Session Provides services to the presentation layer and to manage data exchange.

Defines services to segment, transfer, and reassemble the data for individual
4 - Transport
communications.

3 - Network Provides services to exchange the individual pieces of data over the network.

2 - Data Link Describes methods for exchanging data frames over a common media.

1 - Physical Describes the means to activate, maintain, and de-activate physical connections.

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Reference Models
The TCP/IP Reference Model
TCP/IP Model
Description
Layer
Application Represents data to the user, plus encoding and dialog control.

Transport Supports communication between various devices across diverse networks.

Internet Determines the best path through the network.

Network Access Controls the hardware devices and media that make up the network.

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Reference Models
OSI and TCP/IP Model Comparison

• The OSI model divides the network

access layer and the application
layer of the TCP/IP model into
multiple layers.
• The TCP/IP protocol suite does not
specify which protocols to use when
transmitting over a physical medium.
• OSI Layers 1 and 2 discuss the
necessary procedures to access the
media and the physical means to
send data over a network.

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3.6 Data Encapsulation

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Data Encapsulation
Segmenting Messages Segmenting is the process of breaking up
messages into smaller units. Multiplexing is
the processes of taking multiple streams of
segmented data and interleaving them
together.
Segmenting messages has two primary
benefits:
• Increases speed - Large amounts of
data can be sent over the network
without tying up a communications link.
• Increases efficiency - Only segments
which fail to reach the destination need to
be retransmitted, not the entire data
stream.

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Data Encapsulation
Sequencing

Sequencing messages is the process of

numbering the segments so that the
message may be reassembled at the
destination.
TCP is responsible for sequencing the
individual segments.

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Data Encapsulation Encapsulation is the process where protocols
Protocol Data Units add their information to the data.
• At each stage of the process, a PDU has a
different name to reflect its new functions.
• There is no universal naming convention for
PDUs, in this course, the PDUs are named
according to the protocols of the TCP/IP
suite.
• PDUs passing down the stack are as
follows:
1. Data (Data Stream)
2. Segment
3. Packet
4. Frame
5. Bits (Bit Stream)
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Data Encapsulation
Encapsulation Example
• Encapsulation is a top down
process.
• The level above does its
process and then passes it
down to the next level of the
model. This process is
repeated by each layer until
it is sent out as a bit stream.

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Data Encapsulation
De-encapsulation Example
• Data is de-encapsulated as it moves up
the stack.
• When a layer completes its process,
that layer strips off its header and
passes it up to the next level to be
processed. This is repeated at each
layer until it is a data stream that the
application can process.
1. Received as Bits (Bit Stream)
2. Frame
3. Packet
4. Segment
5. Data (Data Stream)
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3.7 Data Access

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Data Access
Addresses
Both the data link and network layers use addressing to deliver data from source to
destination.
Network layer source and destination addresses - Responsible for delivering the IP
packet from original source to the final destination.
Data link layer source and destination addresses – Responsible for delivering the data
link frame from one network interface card (NIC) to another NIC on the same network.

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Data Access
Layer 3 Logical Address

The IP packet contains two IP

addresses:
• Source IP address - The IP
address of the sending device,
original source of the packet.
• Destination IP address - The IP
address of the receiving device,
final destination of the packet.
These addresses may be on the same
link or remote.

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Data Access
Layer 3 Logical Address (Cont.)
An IP address contains two parts:
• Network portion (IPv4) or Prefix (IPv6)
• The left-most part of the address indicates
the network group which the IP address is
a member.
• Each LAN or WAN will have the same
network portion.
• Host portion (IPv4) or Interface ID
(IPv6)
• The remaining part of the address identifies
a specific device within the group.
• This portion is unique for each device on
the network.
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Data Access
Devices on the Same Network

When devices are on the same

network the source and destination will
have the same number in network
portion of the address.
• PC1 – 192.168.1.110
• FTP Server – 192.168.1.9

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Data Access
Role of the Data Link Layer Addresses: Same IP Network
When devices are on the same Ethernet
network the data link frame will use the
actual MAC address of the destination
NIC.
MAC addresses are physically embedded
into the Ethernet NIC and are local
addressing.
• The Source MAC address will be that of
the originator on the link.
• The Destination MAC address will
always be on the same link as the
source, even if the ultimate destination
is remote.
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Data Access
Devices on a Remote Network
• What happens when the actual
(ultimate) destination is not on the
same LAN and is remote?
• What happens when PC1 tries to
reach the Web Server?
• Does this impact the network and data
link layers?

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Data Access
Role of the Network Layer Addresses
When the source and destination have
a different network portion, this means
they are on different networks.
• PC1 – 192.168.1
• Web Server – 172.16.1

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Data Access
Role of the Data Link Layer Addresses: Different IP Networks
When the final destination is remote, Layer
3 will provide Layer 2 with the local default
gateway IP address, also known as the
router address.
• The default gateway (DGW) is the router
interface IP address that is part of this
LAN and will be the “door” or “gateway” to
all other remote locations.
• All devices on the LAN must be told about
this address or their traffic will be confined
to the LAN only.
• Once Layer 2 on PC1 forwards to the
default gateway (Router), the router then
can start the routing process of getting the
information to actual destination.
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Data Access
Role of the Data Link Layer Addresses: Different IP Networks
(Cont.)
• The data link addressing is local
addressing so it will have a source and
destination for each link.
• The MAC addressing for the first
segment is :
• Source – AA-AA-AA-AA-AA-AA
(PC1) Sends the frame.
• Destination – 11-11-11-11-11-11 (R1-
Default Gateway MAC) Receives
the frame.
Note: While the L2 local addressing will
change from link to link or hop to hop, the
L3 addressing remains the same.
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Data Access
Data Link Addresses
• Since data link addressing is local addressing, it will have a source and destination for
each segment or hop of the journey to the destination.
• The MAC addressing for the first segment is:
• Source – (PC1 NIC) sends frame
• Destination – (First Router- DGW interface) receives frame

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Data Access
Data Link Addresses (Cont.)
The MAC addressing for the second hop is:
• Source – (First Router- exit interface) sends frame
• Destination – (Second Router) receives frame

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Data Access
Data Link Addresses (Cont.)
The MAC addressing for the last segment is:
• Source – (Second Router- exit interface) sends frame
• Destination – (Web Server NIC) receives frame

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Data Access
Data Link Addresses (Cont.)
• Notice that the packet is not modified, but the frame is changed, therefore the L3 IP
addressing does not change from segment to segment like the L2 MAC addressing.
• The L3 addressing remains the same since it is global and the ultimate destination is still
the Web Server.

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