Protocol Layering

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Protocol Layering

• Chapter 10

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• Looked at
• Architectural foundations of internetworking
• Forwarding of datagrams
• Mapping IP addresses to physical addresses
• This chapter
• Considers host and router software structure
• Presents principle of layering
• Shows layering makes IP software easier to
  understand and build
• Traces path of datagrams thru protocol software

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Need for Multiple Protocols

• Protocols ?? Computer communication Programming
  languages ?? Computation
• Lower level interact with physical
• Higher level specify higher-level abstraction
• Need protocol family or protocol suite to perform
  complex data communication tasks

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• Possible problems
• Hardware failure
• Network congestion
• Packet delay or loss
• Data corruption
• Data duplication or inverted arrival
• Together, problems are overwhelming
• Partitioning into subproblems allows us to
  conquer the complexity

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Conceptual Layers

• Modules of protocol SW stacked as layers
• Each layer handles a part of the problem
• Sending message between applications
• Transfer down layers on sending machine
• Forward across network
• Transfer up layers on receiving machine
• More complex than simple layering model suggests

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Figure 10.1
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Figure 10.2
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Figure 10.3
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Functionality of the Layers

• What functionality goes in each layer?
• Two main models dominate the field
• ISO 7-layer reference model
• By International Organization for Standardization
• TCP/IP 5-layer reference model
• Did not come from standards committee
• Resulted from research that led to TCP/IP
  protocol suite

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TCP/IP and OSI
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X.25 and ISO Model

• ISO has been basis for several protocol
  implementations
• One is X.25 protocol suite
• Has been adopted by public data networks
• Especially popular in Europe
• X.25 view
• Network much like telephone system
• Net has complex packet switches with smarts to
  route

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• Physical layer
• Specifies standard for physical interconnection
• Procedures for transferring packets
• Electrical characteristics of voltage and current
• Data link layer
• How data travels between host and packet switch
• Uses frame as unit of data that is passed
• Also specifies checksum and acknowledgements
• One protocol High Level Data Link Comm. (HDLC)
• Network layer
• Defines basic unit of transfer across network
• Packets here may be larger than frames
• Responds to congestion problems

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• Transport layer
• Provides end-to-end reliability
• Double checks lower level checks
• Session layer
• Handles remote terminal access
• Special computer PAD with dial-up access
• Presentation layer
• Includes functions application programs need
• Text compression graphics conversion
• Application layer
• Applications that use the network
• Email file transfer

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TCP/IP 5-Layer Model

• TCP/IP software organized into five conceptual
  layers
• Four software layers
• Built on fifth layer of hardware

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• Layers and objects passed

Application
Messages or Streams
Transport
Transport Protocol Packets
Internet
IP Datagrams
Network Interface
Network-specific Frames
Hardware
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• Application layer
• Users invoke application programs that access
  services available across a TCP/IP internet
• Applications choose style of transport needed
• Sequence of individual messages
• Continuous stream of bytes
• Transport layer
• Provide end-to-end communication
• Communication from one application to another
• Provides the reliability
• Divides data into packets to be passed down

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• Internet layer
• Handles communication from one machine to another
• Encapsulates packet in IP datagram
• Uses routing algorithm to route datagram
• Handles incoming datagrams
• Checks validity routing functions
• For local machine, deletes header and passes up
  using appropriate transport protocol
• Sends and receives ICMP error control messages
• Network interface layer
• Accepts IP datagrams and transmits over a
  specific network
• Can be a device driver or a complex subsystem
  that uses own data link protocol

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Locus of Intelligence

• ISO
• Network is complex, independent system
• Relatively simple hosts attach
• TCP/IP
• Hosts participate in most all network protocols
• Implements end-to-end error detection and
  recovery
• Choose routers
• Handle ICMP control messages
• Simple packet delivery with intelligent hosts
  attached

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Protocol Layering Principle

• Layering Principle
• Layered protocols are designed so that layer n at
  the destination receives exactly the same object
  sent my layer n at the source
• Design one layer at a time
• Figure other layers will perform their functions

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Figure 10.6
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• TCP/IP layering
• Two types of transfers
• From source to ultimate destination
• Across multiple networks
• Higher layers
• Layering principle applies across end-to-end
  transfers
• Lowest layer
• Applies to single machine transfers
• Internet layer (?)
• Since some things change (TTL counter), layering
  principle only applies to single machine transfers

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Figure 10.7
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Layering in The Presence of Network Substructure
(10.8)

• Skip.

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Boundaries in the TCP/IP Model

• Two boundaries that may not be obvious
• Protocol address boundary
• Separates high-level and low-level addressing
• Operating systems boundary
• Separates the system from application programs

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(No Transcript)
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• Protocol Address Boundary
• Application programs and all protocol software
• From Internet layer upward use only IP addresses
• Network interface layer handles physical
  addresses
• Operating System Boundary
• Software at the application layer is not part of
  the operating system
• Software at other layers generally is

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Disadvantage of Layering

• Strict layering can produce inefficient SW
• Transport layer divides stream into packets
• Can optimize transfer if knows largest packet
  size that will fit in one frame
• If strict layering is used, transport layer will
  not know the datagram or frame formats, or number
  of header octets
• Most implementation relax strict layering
• Allow info like route selection, MTU, etc. to
  propagate upward

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Multiplexing Demultiplexing

• Occurs throughout the layered hierarchy
• Multiplexing
• When sending message, extra bits included that
  encode message type, originating program,
  protocols used
• Demultiplexing
• At destination, extra information is used to
  guide processing

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RARP Module

IP Module
ARP Module
Demultiplexing Based on Frame Type
Frame Arrives
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ICMP Protocol
UDP Protocol
TCP Protocol
IP Module
Datagram Arrives
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Summary

• Protocols specify
• Data is represented when transferred
• How the transfer occurs
• How error are detected
• How acknowledgements are passed
• Each protocol solves one of the communications
  subproblems

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• Layering provides a conceptual frame-work for
  protocol design
• Each layer handles a part of the problem
• Layering Principle
• Software implementing a layer at destination
  machine receives exactly what the software
  implementing that layer on the source machine
  sends
• X.25 protocols
• Follow ISO reference model
• Example of reliable communications service
• TCP/IP protocols use different layering scheme
• Multiplexing/demultiplexing are used to
  distinguish among multiple protocols at a layer