Protocol Layering

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Protocol Layering

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Motivation for Protocol Layering - The
Programming Language Analogy

• Computer programming
• Program -gt Compiler -gt Assembly Code -gt
  Assembler -gt Machine Code -gt Hardware
• Internet programming
• Application -gt Transport Protocol -gt
  Connectionless Datagram Delivery Protocol -gt
  Physical Network

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The Internet Protocol Suite

• Internetworking does not use a single protocol to
  transmit data
• Imagine how hard it would be to write one that
• Ran on all underlying network technologies
• Transmitted and routed all traffic on the
  internet
• Handled hardware failure, network congestion,
  delays, data loss, data corruption, data
  duplication, sequencing errors
• Internetworking uses a set of cooperating
  protocols (each with its own job)

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Benefits of Protocol Layering

• Complexity hidden from application-level
  programmer
• Physical network details hidden in low-level
  protocols
• Each layer of software can be written, tested,
  and modified independently

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Protocol Layering

• Each layer provides services to the layer above
  and utilizes services from the layer below

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Protocol Layering

• The path of a datagram

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Functionality of the Layers

• ISO 7-Layer Reference Model

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ISO 7-Layer Reference Model

• Physical Layer (ISO Layer 1)
• Service transmission of a raw bit stream over a
  communication channel ensuring a reliable
  delivery of 0s and 1s
• Functions conversion of bits into electrical or
  optical signals
• Examples X.21, RS-232-C

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ISO 7-Layer Reference Model

• Data Link Layer (ISO Layer 2)
• Service reliable transfer of frames over a link
• Functions synchronization, error control, flow
  control
• Examples HDLC, CCITT, LAP-D
• Network Layer (ISO Layer 3)
• Service provide switching and routing functions
  to transfer data between hosts
• Functions routing, addressing, switching,
  congestion control
• Examples IP, X.25, CLNP

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ISO 7-Layer Reference Model

• Transport Layer (ISO Layer 4)
• Service control delivery of messages between
  hosts
• Functions connection management, error control,
  flow control, multiplexing
• Examples TCP, UDP, ISO TP0 - TP4
• Session Layer (ISO Layer 5)
• Service support communication between
  cooperating application programs
• Functions session management, synchronization,
  recovery
• Examples ISO session protocol, RPC

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ISO 7-Layer Reference Model

• Presentation Layer (ISO Layer 6)
• Service handle compatibility issues
• Functions virtual device support, syntax
  conversion, cryptography
• Examples ASN.1, ISO presentation protocol
• Application Layer (ISO Layer 7)
• Service provide network access to application
  programs
• Functions (application specific)
• Examples Telnet, FTP, E-mail, WWW

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ISO 7-Layer Reference Model

• Designed by committee - International
  Organization for Standardization (ISO)
• Reference model, not an implementation
• Implemented by a set of protocols known as X.25

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TCP/IP Internet Layering Model

• Functionality of the layers

Objects Passed Between Layers
Conceptual Layer
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TCP/IP Internet Layering Model

• Network Interface Layer (TCP/IP Layer 1)
• Service accepts IP datagrams and transmits them
  over a specific network
• Functions conversion of datagrams to physical
  frames
• Examples device drivers
• Internet Layer (TCP/IP Layer 2)
• Service connectionless datagram delivery
• Functions inter-host communication, routing
• Examples IP

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TCP/IP Internet Layering Model

• Transport Layer (TCP/IP Layer 3)
• Service provide reliable end-to-end
  communication
• Functions flow control, sequencing,
  acknowledgments
• Examples TCP
• Application Layer (TCP/IP Layer 4)
• Service (application specific)
• Functions (application specific)
• Examples Telnet, FTP, E-mail, WWW

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Internet Model vs. ISO Model

• Similarities
• Differences

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The Protocol Layering Principle

• Layered protocols are designed so that layer N
  at the destination receives exactly the same
  object sent by layer N at the source.

Application
Identical message
Identical packet
Identical datagram
Identical frame
Network
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Boundaries in the TCP/IP Model

Boundary
Software outside the operating system
Software inside the operating system
Only IP addresses used
Physical addresses used
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Protocol Layering

• Advantages
• Disadvantages