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Chapter 2 Communicating Over the Network
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Elements of Communication
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Communicating the Messages
Single communications (e.g. video, e-mail message) could be: Continuous stream of bits Take over (hog) the network. Significant delays Inefficient use Any loss - resend entire message
00101010100101010101010101010101010
I have to wait…
Continuous stream of bits
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Communicating the Messages
Better approach – segmentation.
Multiplexing: Different conversations can be interleaved.
Segmentation
001010
001010
001010
001010
001010
001010
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Communicating the Messages
Benefits Reliability (3)
Different paths Alternate path Only the missing segments need to be retransmitted
X
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Disadvantage of Segmentation
Disadvantage – added level of complexity. Analogy: 100 page letter one page at a time
Separate envelopes Sequencing
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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 or hosts: The source or destination of a message.
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Each host is identified by an address. IP (Internet Protocol) address (later)
Source Address: 209.67.102.55 Destination Address: 107.16.4.21
209.67.102.55 107.16.4.21
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Servers and Clients
A host Client, Server, or both. Software determines the role.
Servers provide information and services to clients e-mail or web pages
Clients request information from the server.
Server
Client
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Intermediary Devices
Intermediary devices: Connectivity to and between networks
Examples (4): Network Access Devices (Hubs, switches, and wireless access points) Internetworking Devices (routers) Communication Servers and Modems Security Devices (firewalls)
routers
switch or hub
switch or hub
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Network Media
Network media: The medium over which the message travels. Metallic wires - electrical impulses. Fiber optics – pulses of light Wireless – electromagnetic waves.
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Network Media
Different media considerations (4): Distance it will carry signal Environment it works in Bandwidth (speed) Cost
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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.
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Wide Area Network (WAN)
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. Voice and data on separate networks or converged networks
T1, DS3, OC3 PPP, HDLC Frame Relay, ATM ISDN, POTS
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The Internet – A Network of Networks
ISPs (Internet Service Providers) Often also TSPs Connect their customers to the Internet.
The Internet – ISPs connected to other ISPs
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Network Representations
Network Interface Card (NIC) Ports and interfaces (used interchangeably)
Physical Port Interface - Connect to individual networks.
Protocols
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Protocol
Protocol – Rules that govern communications. Protocol suite - A group of inter-related protocols - Example: TCP/IP
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Message uses Multiple protocols (encapsulated)
Message: Data Multiple protocols
HTTP Header Data
Frame Header IP Header TCP Header App Header Frame Trailer Data
Protocols
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Multiple protocols (encapsulated)
Encapsulation – Process of adding a header to the data or any previous set of headers.
Decapsulation – Process of removing a header.
HTTP Header Data
Frame Header IP Header TCP Header App Header Frame Trailer Data
Protocols
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Example: Protocol – IPv4
Frame Header IP Header TCP Header Frame Trailer Data HTTP Header
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209.67.102.55 107.16.4.21
Frame Header IP Header TCP Header Frame Trailer
Data
209.67.102.55 107.16.4.21
HTTP Header
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Protocols
Networking protocols suites include rules for: (4) Format Accessing the media Error detection Setup and termination
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Early days – proprietary network equipment and protocols. Now – Industry standards Institute of Electrical and Electronics Engineers (IEEE)
Examples: 802.3 (Ethernet), 802.11 (WLAN) Internet Engineering Task Force (IETF)
Internet standards RFCs (Request for Comments) Example: TCP, IP, HTTP, FTP
Protocol Suites and Standards
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Example: RFC 791 IPv4
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Interaction of Protocols
Hypertext Transfer Protocol (HTTP) Protocol that governs interaction between web server and a web client.
HTTP Header Data
Frame Header IP Header TCP Header App Header Frame Trailer Data
Protocols
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Interaction of Protocols
Transmission Control Protocol (TCP) Responsible for controlling the information exchanged between the server and
the client: Size of data Flow control Reliability
segment
HTTP Header Data Protocols
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Internetwork Protocol (IP) 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
packet
HTTP Header Data Protocols
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Interaction of Protocols
Network access protocols (Data link and Physical layer protocols) Format and physical transmission of data on the media.
frame
HTTP Header Data Protocols
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Technology-Independent Protocols
IP is a protocol that will travel over different types of media.
Frame Header IP Header TCP Header Frame Trailer HTTP Header
T1, DS3, OC3 PPP, HDLC Frame Relay, ATM ISDN, POTS
Ethernet Ethernet
IP Packet IP Packet
data
Using Layered Protocols
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Using a layered model: Fosters competition Prevents changes in one layer from affecting other layers above and
below. Provides a common language to describe networking functions and
capabilities.
Benefits of a Layered Model HTTP Header Data Protocols
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Protocol and Reference Models
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Protocol and Reference Models
The Open Systems Interconnection (OSI) model is the most widely known internetwork reference model.
The International Organization for Standardization (ISO) released the OSI reference model in 1984, was the descriptive scheme they created.
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TCP/IP Model
TCP/IP Model and Protocol Suite is an open standard.
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The Communication Process - Encapsulation
Server
Data HTTP Header
TCP Header
IP Header
Data Link Header
Data Link Trailer
HTTP Data
Encapsulation – Process of adding control information as it passes down through the layered model.
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The Communication Process - Decapsulation Data HTTP
Header TCP Header
IP Header
Data Link Header
Data Link Trailer
Client
HTTP Data
Decapsulation – Process of removing control information as it passes upwards through the layered model.
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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. PDUs are named according to the protocols of the TCP/IP suite.
Data - Application layer PDU 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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The sending host builds message with multiple encapsulations.
Dest. MAC 00-10
Source MAC 0A-10
Type 800
Trailer
Layer 2 Data Link Frame
Dest. IP 192.168.4.10
Source IP 192.168.1.10
IP fields Data
Layer 3 IP Packet
Dest. MAC 0B-31
Source MAC 00-20
Type 800
Trailer Dest. IP 192.168.4.10
Source IP 192.168.1.10
IP fields Data Dest. Add FF-FF
Source Add Type 800
Trailer Dest. MAC 0B-20
Source MAC 0C-22
Type 800
Trailer
Data HTTP Header
TCP Header
IP Header
Data Link Header
Data Link Trailer
Data HTTP Header
TCP Header
IP Header
Data Link Header
Data Link Trailer
The receiving host receives the message with multiple decapsulations.
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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
HTTP Header Data Protocols
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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