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Chapter 2
Communicating Over the
Network
CIS 81 Networking Fundamentals
Rick Graziani
Cabrillo College
[email protected]
Last Updated: 2/17/2008
This Presentation
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 Email: [email protected]
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The Platform for Communications
Elements of Communication
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Communicating the Messages
Continuous stream of bits
00101010100101010101010101010101010
I have to wait…
 Theoretically, single communication, such as a music video or an e-mail
message, could be sent across a network from a source to a destination as
one massive continuous stream of bits.
 No other device would be able to send or receive messages on the same
network.
 Results in significant delays.
 Inefficient use of channel or link.
 Any loss in data, entire message would have to be resent.
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Communicating the Messages
Segmentation
 Better approach – segmentation.
 Division of the data stream into smaller pieces is called segmentation.
 Segmentation has two benefits…
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Communicating the Messages
Segmentation
Benefits of segmentation:
 Multiplexing:
 Different conversations can be interleaved on the network.
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Communicating the Messages
X
 Reliability
 Increase the reliability of network communications.
 Separate pieces of each message can travel across different paths to
destination.
 Path fails or congested, alternate path can be used.
 Part of the message fails to make it to the destination, only the missing
parts need to be retransmitted.
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Disadvantage
of
Segmentation
 Disadvantage – added level of complexity.
 Like sending a 100 page letter one page at a time.
 All of the separate envelopes needed
 Need to label the pages with a sequence number.
 This extra overhead is handled by protocols used to format and address these
messages (later).
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Components of the Network
 Devices (hardware)
 End devices, switch, router, firewall, hub
 Media (wired, wireless)
 Cables, wireless mediums
 Services (software)
 Network applications, routing protocols, processes, algorithms
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End devices
 End devices:
 Computers (work stations, laptops, file servers, web servers)
 Network printers
 VoIP phones
 Security cameras
 Mobile handheld devices (such as wireless barcode scanners, PDAs)
 End devices are referred to as hosts.
 A host device is either the source or destination of a message.
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Source Address: 209.67.102.55
Destination Address: 107.16.4.21
209.67.102.55
107.16.4.21
 Each host on a network is identified by an address.
 IP (Internet Protocol) address (later)
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Servers and Clients
Client
Server
 A host can act as a client, a server, or both.
 Software installed on the host determines the role.
 Servers are hosts that have software installed that enables them to provide
information and services, like e-mail or web pages, to other hosts on the
network.
 Clients are hosts that have software installed that enables them to request
and display the information obtained from the server.
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Intermediary
Devices
switch
or hub
switch
or hub
routers
 Intermediary devices:
 Provide connectivity to the network (switches/hubs)
 Connect individual networks (routers)
 Connect segments (links) within the same network (switches/hubs)
 Examples:
 Network Access Devices (Hubs, switches, and wireless access points)
 Internetworking Devices (routers)
 Communication Servers and Modems
 Security Devices (firewalls)
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Network
Media
 Network media: The medium provides the channel over which the message
travels from source to destination.
 Metallic wires - encoding into patterns of electrical impulses.
 Fiber optics – encoding into pulses of light (infrared or visible light ranges)
 Wireless – encoding patterns of electromagnetic waves.
 (Later: OSI Physical Layer)
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Network
Media
 Different media considerations:
 Distance it can carry the signal
 Environment it works in
 Bandwidth
 Cost of medium and installation
 Cost of connectors and equipment
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Local Area Network (LAN)
 Local Area Network (LAN)
 An individual network usually spans a single geographical area, providing
services and applications to people within a common organizational
structure, such as a single business, campus or region.
 LAN devices
 Switches (and hubs)
 Routers
 Multilayer switches
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Wide Area Network (WAN)




T1, DS3, OC3
PPP, HDLC
Frame Relay, ATM
ISDN, POTS
 Wide Area Networks (WANs)
 Leased connections through a telecommunications service provider
network.
 Networks that connect LANs in geographically separated locations
 Telecommunications service provider (TSP) interconnect the LANs at the
different locations.
 TSPs transported voice and data communications on separate networks.
 Providers are now offering converged information network services to their
subscribers.
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The Internet –
A Network of
Networks




ISPs (Internet Service Providers) are often also TSPs.
Connect their customers to the Internet.
The Internet is created by the interconnection of networks belonging to ISPs.
ISPs cooperate with other ISPs and TSPs to make sure their customers have
access to all Internet networks.
 BGP peering and routing is used.
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CENIC – ISP for K-12, Community Colleges, CSU and UC
 ISPs route traffic within their own group of networks (autonomous system).
 ISPs connect their networks to other ISPs networks.
 Within the ISP’s networks are both WANs and customer LANs
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Network
Representations
 Network Interface Card (NIC) - Provides the physical connection to the
network at the PC or other host device.
 Physical Port - A connector or outlet on a networking device where the media
is connected to a host or other networking device.
 Interface - Specialized ports on an internetworking device that connect to
individual networks.
 Because routers are used to interconnect networks, the ports on a router
are referred to network interfaces.
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Protocols
Protocol
 Protocol – Rules that govern communications.
 Protocol suite - A group of inter-related protocols that are necessary to
perform a communication function.
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Multiple protocols (encapsulated)
HTTP
Header
Protocols
Frame Header
IP Header
Data
App
TCP Header Header
Frame Trailer
Data
 The message received by the host usually contains multiple protocols, plus the
actual data.
 Note: Application Header (HTTP) may or may not exist. Typically Application
Header or Data. (later)
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Multiple protocols (encapsulated)
HTTP
Header
Protocols
Frame Header
IP Header
Data
App
TCP Header Header
Frame Trailer
Data
 Encapsulation – Process of adding a header to the data or any previous set of
headers.
 Decapsulation – Process of removing a header.
 More later.
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Example: Protocol – IPv4
Frame Header
IP Header
TCP Header HTTP
Header
Frame Trailer
Data
 Example of IPv4
 More later
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209.67.102.55
Frame Header
107.16.4.21
IP Header
TCP Header HTTP
Header
Frame Trailer
Data
209.67.102.55
107.16.4.21
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Protocols
 Networking protocols suites describe processes such as:
 The format or structure of the message
 The process by which networking devices share information about
pathways with other networks
 How and when error and system messages are passed between devices
 The setup and termination of data transfer sessions
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Protocol Suites and Standards
 Early days – proprietary network equipment and protocols.
 Now – Industry standards
 Institute of Electrical and Electronics Engineers (IEEE)
 Develops standards in telecommunications, information technology and
power generation.
 Examples: 802.3 (Ethernet), 802.11 (WLAN)
 Internet Engineering Task Force (IETF)
 Internet standards
 RFCs (Request for Comments)
 Example: TCP, IP, HTTP, FTP
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Interaction of
Protocols
Hypertext Transfer Protocol (HTTP)
 Common protocol that governs interaction between web server and a web
client.
 Defines the content and formatting of the requests and responses between the
client and server.
 Both the client and the web server software implement the HTTP application.
 HTTP relies on other protocols to govern how the messages are transported
between client and server.
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Interaction
of
Protocols
segment
Transmission Control Protocol (TCP)
 Transport protocol that manages the individual conversations between servers
and clients (not just web servers and web clients)
 TCP divides the HTTP messages into smaller pieces, called segments
 Responsible for controlling the information exchanged between the server and
the client:
 Size of data
 Flow control – how much is sent and received
 Reliability – Sequence numbers in case lost or missing
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packet
Internetwork Protocol (IP)
 Responsible for taking the formatted segments from TCP, encapsulating them
into packets.
 Assigns the appropriate source and destination addresses,
 Original source address of host
 Final destination address of host
 Used by routers in selecting the best path to the destination host.
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Interaction
of
Protocols
frame
Network access protocols (Data link and Physical layer protocols)
 Physical transmission of data on the media.
 Take the packets from IP and format them to be transmitted over the media.
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Interaction of Protocols
209.67.102.55
107.16.4.21
Network access protocols (Data link and Physical layer protocols)
 Responsible for addressing and sending the IP packet between two devices on
the same network.
 Host to router
 Router to router
 Router to host
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Technology-Independent Protocols
Frame Header
IP Header
TCP Header HTTP
Header
Frame Trailer
IP Packet
IP Packet
Ethernet
Ethernet




T1, DS3, OC3
PPP, HDLC
Frame Relay, ATM
ISDN, POTS
 Protocols are not dependent upon any specific technology.
 For example:
 Our IP Packet (IP + TCP + HTTP + Data) can be delivered over various
types of networks using a variety of data link frames.
 More later! – Herding cats.
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Using Layered Protocols
Layered
Model

Layered Models separate the functions of specific protocols.
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Benefits of
a Layered
Model
 Using a layered model:
 Have defined information that they act upon and a defined interface to the
layers above and below.
 Fosters competition because products from different vendors can work
together.
 Prevents technology or capability changes in one layer from affecting other
layers above and below.
 Provides a common language to describe networking functions and
capabilities.
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Protocol and Reference Models
 A protocol model provides a model that closely matches the structure of a
particular protocol suite.
 A reference model provides a common reference for maintaining consistency
within all types of network protocols and services.
 Not intended to be an implementation specification.
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Protocol and Reference Models
 The Open Systems Interconnection (OSI) model is the most widely known
internetwork reference model.
 OSI is also a protocol suite or protocol model.
 OSI lost out to TCP/IP as the protocol suite of the Internet.
 OSI protocol suite includes layers 3 through 7
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TCP/IP Model
 TCP/IP Model and Protocol Suite is an open standard.
 No one company controls it.
 Governed by IETF Working Groups with standards proposed using Request for
Comments (RFCs).
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Example: RFC 791 IPv4
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The Communication Process - Encapsulation
 Encapsulation – Process of adding control information as it passes down
through the layered model.
Data Link
Header
IP
Header
TCP
Header
HTTP
Header
Data
Data Link
Trailer
Server
HTTP Data
 Note: Application Header (HTTP) may or may not exist. (later)
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The Communication Process - Decapsulation
 Decapsulation – Process of removing control information as it passes
upwards through the layered model.
Data Link
Header
IP
Header
TCP
Header
HTTP
Header
Data
Data Link
Trailer
Client
HTTP Data
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Wireshark will let us examine protocols!
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The
Communication
Process
 Protocol Data Unit (PDU) - The form that a piece of data takes at any layer.
 At each stage of the process, a PDU has a different name to reflect its new
appearance.
 PDUs are named according to the protocols of the TCP/IP suite.
 Data - The general term for the PDU used at the Application layer
 Segment - Transport Layer PDU
 Packet - Internetwork Layer PDU
 Frame - Network Access Layer PDU
 Bits - A PDU used when physically transmitting data over the medium
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ISO and the OSI Model
 The International Organization for Standardization (ISO) released the
OSI reference model in 1984, was the descriptive scheme they created.
 “ISO. A network of national standards institutes from 140
countries working in partnership with international
organizations, governments, industry, business and
consumer representatives. A bridge between public and
private sectors.” www.iso.ch
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OSI Model
 It breaks network communication into
smaller, more manageable parts.
 It standardizes network components to
allow multiple vendor development and
support.
 It allows different types of network
hardware and software to communicate
with each other.
 It prevents changes in one layer from
affecting other layers.
 It divides network communication into
smaller parts to make learning it easier to
understand.
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OSI Model
 Presentation and Session layers are not
commonly referred to in most instances.
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Comparing OSI and TCP/IP Models
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Network Addressing
 Layer 3 addresses are primarily designed to move data from one local network
to another local network within an internetwork.
 Layer 2 addresses are only used to communicate between devices on a single
local network,
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Layer 2 Addresses (Data Link Layer)
 Includes the host physical address.
 Layer 2 is concerned with the delivery of messages on a single local network.
 The Layer 2 address is unique on the local network and represents the address
of the end device on the physical media.
 In a LAN using Ethernet, this address is called the Media Access Control (MAC)
address.
 When two end devices communicate on the local Ethernet network, the frames
that are exchanged between them contain the destination and source MAC
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addresses. (later)
Ethernet LAN - Multiaccess
Frame Header
Who
are
you?
IP Header
TCP Header HTTP
Header
Frame Trailer
Destination Address
Source Address
Internet
 On an Ethernet LAN there are usually more
than just two devices.
 There can be hundreds, even thousands of
devices on a single LAN.
 Need a way to send it to a specific device on
the LAN.
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Ethernet LAN - Multiaccess
Frame Header
IP Header
TCP Header HTTP
Header
Frame Trailer
Destination Address
aaa
Internet
Source Address
987
bbb
These Ethernet
addresses are
abbreviated for brevity.
999
111
333
777
888
eee
222
555
666
ccc
ddd
444
 Ethernet LANs are multiaccess networks.
 Multiple devices can access the network
(even at the same time).
 Ethernet NICs have unique 48 bit MAC
addresses. (much more later)
000
123
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Sending an IP Packet to a device within the LAN
Frame Header
IP Header
TCP Header HTTP
Header
Frame Trailer
Destination Address: 888
aaa
Internet
Source Address: aaa
987
bbb
These Ethernet
addresses are
abbreviated for brevity.
999
111
333
777
888
eee
222
555
666
ccc
ddd
444
 Layer 2 addresses, including Ethernet MAC
addresses are used to get the IP packet from
one device to another device on the same
network.
000
123
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Sending an IP packet outside the LAN
Frame Header
IP Header
TCP Header HTTP
Header
Frame Trailer
Destination Address: 987
aaa
Internet
Source Address: aaa
987
bbb
These Ethernet
addresses are
abbreviated for brevity.
999
111
333
777
888
eee
222
555
666
ccc
ddd
444
 The Layer 2 destination address is always a
layer 2 address within that network.
 Disregarding using proxies which are
uncommon.
000
123
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What is the Address on my Ethernet NIC?
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Serial vs Multiaccess Network
Serial (PPP)
Multiaccess (Ethernet)
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Serial point-to-point networks





PPP – Point-to-Point Protocol (later)
Only two devices on this network.
No need for unique layer 2 address.
Can use anything.
PPP uses an 8 bit broadcast address – FF - Hex (all 1’s binary)
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MAC: BB-BB-BB-22-22-22
PPP
A
W
X
IP: 172.16.35.5
MAC: AA-AA-AA-11-11-11
What are the frame and packet addresses at every
point from Host A to Host D?
PPP
MAC: CC-CC-CC-33-33-33
PPP
D
Y
Z
IP: 192.168.4.10
MAC: DD-DD-DD-44-44-44
L3 Destination Address?
L3 Source Address?
Frame Header
IP Header
TCP Header HTTP
Header
Frame Trailer
L2 Destination Address?
L2 Source Address?
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Layer 3 Addresses (Network Layer)
 Layer 3 addresses are primarily designed to move data from one local network to
another local network within an internetwork.
 At the boundary of each LAN a router, decapsulates the frame to read the
destination host address contained in the header of the packet.
 Routers use the Layer 3 destination address to determine which path to use to
reach the destination host.
 Once the path is determined, the router encapsulates the packet in a new frame
and sends it on its way toward the destination end device.
 When the frame reaches its final destination, the frame and packet headers are
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removed and the data moved up to Layer 4.
Layer 2 Data Link Frame
Dest.
Dest.Add
MAC
MAC
0B-31
FF-FF
0B-20
00-10
Source Add
MAC
0A-10
00-20
0C-22
Layer 3 IP Packet
Type
800
Dest. IP
192.168.4.10
Source IP
192.168.1.10
IP
fields
Data
Trailer
 The sending host builds message with multiple encapsulations.
Data Link
Header
IP
Header
TCP
Header
HTTP
Header
Data
Data Link
Trailer
 The receiving host receives the message with multiple decapsulations.
Data Link
Header
IP
Header
TCP
Header
HTTP
Header
Data
Data Link
Trailer
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Getting Data to the Right Application
 Layer 4 (TCP/UDP) contains a port number which represents the application
or service carried in the IP packet.
 Destination port – destination application
 Source port – source application
 More later.
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Getting Data to the Right Application
 Destination port number tells the OS (TCP/IP) stack which application to hand
the data to.
 Examples:
 80 = HTTP (www)
 23 = Telnet
 20, 21 = FTP
 25 = SMTP
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Chapter 2
Communicating Over the
Network
CIS 81 Networking Fundamentals
Rick Graziani
Cabrillo College
[email protected]
Last Updated: 2/17/2008