chapter 4 : tcp/ip and osi business data communications, 4e
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Chapter 4 : TCP/IP and OSI
Business Data Communications, 4e
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What is a Protocol?
Allows entities (i.e. application programs) from different systems to communicate
Shared conventions for communicating information are called protocols
Includes syntax, semantics, and timing
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Why Use Protocol Architecture?
Data communications requires complex procedures Sender identifies data path/receiver Systems negotiate preparedness Applications negotiate preparedness Translation of file formats
For all tasks to occur, high level of cooperation is required
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Modular Approach
Breaks complex tasks into subtasks Each module handles specific subset of tasks Communication occurs
between different modules on the same system between similar modules on different systems
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Advantages of Modularity
Easier application development Network can change without all programs
being modified
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Three-Layer Model Distributed data communications involves three primary components: Networks Computers Applications
Three corresponding layers Network access layer Transport layer Application layer
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Network Access Layer
Concerned with exchange of data between computer and network
Includes addressing, routing, prioritizing, etc Different networks require different software at
this layer Example: X.25 standard for network access
procedures on packet-switching networks
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Transport Layer
Concerned with reliable transfer of information between applications
Independent of the nature of the application Includes aspects like flow control and error
checking
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Application Layer
Logic needed to support various applications Each type of application (file transfer, remote
access) requires different software on this layer
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Addressing
Each computer on a network requires a unique address on that network
Each application requires a unique address within the computer to allow support for multiple applications (service access points, or SAP)
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Data Transmission
Application layer creates data block Transport layer appends header to create PDU
(protocol data unit) Destination SAP, Sequence #, Error-Detection Code
Network layer appends another header Destination computer, facilities (e.g. “priority”)
See figure 4.5 in the book
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Standardized Protocol Architectures Vendors like standards because they make
their products more marketable Customers like standards because they enable
products from different vendors to interoperate Two protocol standards are well-known:
TCP/IP: widely implemented OSI: well-known, less used, still useful for
modeling/conceptualizing
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TCP/IP
Transmission Control Protocol/Internet Protocol
Developed by DARPA No official protocol
standard Can identify five layers
Application Host-to-Host (transport) Internet Network Access Physical
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TCP/IP Physical Layer
Physical interface between a DTE (e.g. computer or terminal) and a transmission medium
Specifies: Characteristics of medium Nature of signals Data rate
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TCP/IP Network Access
Exchange of data between end system and network Address of host and destination Prioritization of transmission Software at this layer depends on network (e.g.
X.25 vs. Ethernet) Segregation means that no other software needs to
be concerned about net specifics
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TCP/IP Internet Layer
An Internet is an interconnection of two or more networks
Internet layer handles tasks similar to network access layer, but between networks rather than between nodes on a network
Uses IP for addressing and routing across networks Implemented in workstations and routers
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TCP/IP Transport Layer
Also called host-to-host layer Reliable exchange of data between
applications Uses TCP protocols for transmission
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TCP/IP Application Layer
Logic needed to support variety of applications
Separate module supports each type of application (e.g. file transfer)
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TCP & UDP Most TCP/IP applications use TCP for transport layer TCP provides a connection (logical association)
between two entities to regulate flow check errors UDP (User Datagram Protocol) does not maintain a
connection, and therefore does not guarantee delivery, preserve sequences, or protect against duplication
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IP and IPv6
IP provides for 32-bit source and destination addresses
IPv6 (1996 standard) provides for 128-bit addresses
Migraqtion to IPv6 will be a very slow process
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TCP/IP Applications
SMTP (Simple Mail Transfer Protocol) Basic e-mail facility, transferring messages among hosts
FTP (File Transfer Protocol) Sends files from one system to another on user command
Telnet Remote login capability, allowing a user to emulate a
terminal on the remote system
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Internetworking
Interconnected networks, usually implies TCP/IP
Can appear to users as a single large network The global Internet is the largest example, but
intranets and extranets are also examples
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Routers
Equipment used to interconnect independent networks
Several essential functions Provide a link between networks Provide routing and delivery of data between processes
on systems from different networks Provide the above functions without requiring
modification of the attached networks
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Router Issues
Addressing schemes Maximum packet size Interfaces Reliability
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TCP Segment (TCP PDU) Source port (16 bits) Destination port (16 bits) Sequence number (32 bits) Acknowledgment number
(32 bits) Data Offset (4 bits) Reserved (6 bits) Flags (6 bits) : URG, ACK, PSH, RST, SYN, FIN
Window (16 bits) Checksum (16 bits) Urgent Pointer (16 bits) Options (variable)
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IPv4 Header Version (4 bits) Internet header length (4
bits) Type of Service (8 bits) Total Length (16 bits) Identification (16 bits) Flags (3 bits Fragment Offset (13 bits)
Time to Live (8 bits) Protocol (8 bits Header Checksum (16 bits) Source Address ( 32 bits) Destination Address (32
bits) Options (variable) Padding (variable)
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Why Study OSI?
Still an excellent model for conceptualizing and understanding protocol architectures
Key points: Modular Hierarchical Boundaries between layers=interfaces
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OSI Open Systems
Interconnection
Developed by ISO
Contains seven layers(see page 358)
Application Presentation Session Transport Network Data Link Physical
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OSI Lower Layers
Physical Data Link Network
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OSI Physical Layer
Responsible for transmission of bits Always implemented through hardware Encompasses mechanical, electrical, and
functional interfaces e.g. RS-232
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OSI Data Link Layer
Responsible for error-free, reliable transmission of data
Flow control, error correction e.g. HDLC
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OSI Network Layer
Responsible for routing of messages through network
Concerned with type of switching used (circuit v. packet)
Handles routing between networks, as well as through packet-switching networks
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OSI Upper Layers
Transport Session Presentation Application
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OSI Transport Layer
Isolates messages from lower and upper layers
Breaks down message size Monitors quality of communications channel Selects most efficient communication service
necessary for a given transmission
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OSI Session Layer
Establishes logical connections between systems
Manages log-ons, password exchange, log-offs
Terminates connection at end of session
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OSI Presentation Layer
Provides format and code conversion services Examples
File conversion from ASCII to EBDIC Invoking character sequences to generate bold,
italics, etc on a printer
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OSI Application Layer
Provides access to network for end-user User’s capabilities are determined by what
items are available on this layer
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OSI in Action: Outgoing File Transfer Program issues command to
Application Layer Application passes it to
Presentation, which may reformat, passes to Session
Session requests a connection, passes to Transport
Transport breaks file into chunks, passes to Network
Network selects the data’s route, passes to Data Link
Data Link adds error-checking info, passes to Physical
Physical transmits data, which includes information added by each layer
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OSI in Action: Incoming File Transfer Physical receives bits, passes to
Data Link Data Link checks for errors,
passes to Network Network verifies routing, passes
to Transport Transport reassembles data,
passes to Session Session determines if transfer is
complete, may end session, passes to Presentation
Presentation may reformat, perform conversions, pass to Application layer
Application presents results to user (e.g. updates FTP program display)