CS计算机代考程序代写 distributed system assembly PowerPoint Presentation

PowerPoint Presentation

Computer Systems
Introduction to Networks (Part II)

Dr. Mian M. Hamayun
m.m. .uk

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Lecture Objective

The objective of this lecture is to develop a basic
understanding of different types of network
delays, protocol layering and OSI model.

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Lecture Outline

 Packet Loss and Delays
 What is a Protocol?
 Protocol Layers and Encapsulation
 Open Systems Interconnection Model
 Large Messages and Message Consistency
 Ports and Addressing
 Summary

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How do loss and delay occur?

 Packets queue in router buffers
 packet arrival rate to link (temporarily) exceeds output link

capacity

 packets queue, wait for turn

A

B

packet being transmitted (delay)

packets queueing (delay)

free (available) buffers: arriving packets
dropped (loss) if no free buffers

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How do loss and delay occur?

 d
proc

: nodal processing

 check bit errors

 determine output link

 typically < msec A B propagation transmission nodal processing queueing dnodal = dproc + dqueue + dtrans + dprop  d queue : queueing delay  time waiting at output link for transmission  depends on congestion level of router Slide #6 of 36 How do loss and delay occur?  d trans : transmission delay  L: packet length (bits)  R: link bandwidth (bps)  d trans = L/R  d prop : propagation delay  d: length of physical link  s: propagation speed in medium (~2x108 m/sec)  d prop = d/sdtrans and dprop very different A B propagation transmission nodal processing queueing dnodal = dproc + dqueue + dtrans + dprop Slide #7 of 36 Caravan Analogy  cars “propagate” at 100 km/hr  toll booth takes 12 sec to service car (bit transmission time)  car~bit; caravan ~ packet  Q: How long until caravan is lined up before 2nd toll booth? toll booth toll booth ten-car caravan 100 km 100 km  time to “push” entire caravan through toll booth onto highway = 12*10 = 120 sec  time for last car to propagate from 1st to 2nd toll both: 100km/(100km/hr)= 1 hr  A: 62 minutes Slide #8 of 36 Caravan Analogy (more)  suppose cars now “propagate” at 1000 km/hr  and suppose toll booth now takes one min to service a car  Q: Will cars arrive to 2nd booth before all cars serviced at first booth?  A: Yes! after 7 min, 1st car arrives at second booth; three cars still at 1st booth. toll booth toll booth ten-car caravan 100 km 100 km Slide #9 of 36 Packet Loss  queue (aka buffer) preceding link in buffer has finite capacity  packet arriving to full queue dropped (aka lost)  lost packet may be retransmitted by previous node, by source end system, or not at all A B packet being transmitted packet arriving to full buffer is lost buffer (waiting area) Slide #10 of 36 Throughput  throughput: rate (bits/time unit) at which bits transferred between sender/receiver  instantaneous: rate at given point in time  average: rate over longer period of time server sends bits (fluid) into pipe pipe that can carry fluid at rate Rs bits/sec) pipe that can carry fluid at rate Rc bits/sec) server, with file of F bits to send to client link capacity Rs bits/sec link capacity Rc bits/sec Slide #11 of 36 Throughput (more)  R s < R c What is average end-end throughput? Rs bits/sec Rc bits/sec  R s > R

c
What is average end-end throughput?

Rs bits/sec Rc bits/sec

link on end-end path that constrains end-end throughput

bottleneck link

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What is a Protocol?

 To effectively communicate, network devices use
protocols
 Set of guidelines

 Must specify:
 The sequence of messages to be exchanged

 The format of the data in the messages

 Implemented by pair of software modules in sending
and receiving computer

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Protocol – Example 1

 Transport protocol: transmits message of a length
from a sending process to a receiving process.

 A process wishing to send message passes it to
transport protocol module

 Transport software divides message into packets

 These are then transmitted using network protocol

 Inverse operations are performed at other end

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Protocol – Example 2

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Message: Frame

10101010 1010101 etc…

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Message: Frame

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Inside a Frame: Packet

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Protocol Layers

 Network software arranged in hierarchy of layers

 Each layer presents interface to layer above

 Every computer in network should have these layers

 Layers communicate with those above and below
through procedure calls

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Conceptual Layering of Protocol Software

Layer n

Layer 2

Layer 1

Message sent Message received

Communication
medium

Sender Recipient

Instructor’s Guide for Coulouris, Dollimore, Kindberg and Blair,

Distributed Systems: Concepts and Design Edn. 5
© 2012

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Protocol Layers

 Each layer accepts items of data in a specified format
from layer above

 Transformations applied to encapsulate data

 Then data passed to layer below

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Protocol Layers – Example

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Encapsulation – in Layered Protocols

 Complete set of protocol layers is a protocol suite or
protocol stack

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Open Systems Interconnection (OSI) Model

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OSI Protocol Summary

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Pros and Cons of Protocol Layering

 Simplifies and generalizes software interfaces
accessing communication services

 Downside: performance costs.
 N layers = N control transfers

 N copies of the data due to encapsulation

 Hence, data transfer rate much lower than available
network bandwidth

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What does the Internet Use?

 Differs slightly from OSI model
 Application, presentation and session layers not clearly

distinguished in Internet protocol stack

► Applications decide how to treat these

 Session layer integrated with transport layer

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Large Messages

 Problem: entire message may need more than one
frame

 An Ethernet frame can only hold 1500 bytes of data
 Any large messages will have to be broken down

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Message Consistency

 Reliable data transfer
 Sender & Receiver communicate using TCP
 If there are errors in the packets, the receiver

notifies sender and the sender resends those
packets again.

 If it was one large packet, there could be issues
due to poor connection

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How could we define what the
packet order should be?

Each packet contains a sequence number!

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Ports

 Transport layer’s task is to provide a network-
independent message transfer service between pairs of
network ports

 Software defined destination points at a host computer
 Port 25 for email…

 Attached to processes

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Addressing

 Transport layer must deliver messages to destinations
with transport addresses

 Composed of network address and a port number
 Numeric identifier

 In the Internet, every host computer is assigned an IP number

 Identifies it and the subnet to which it is connected

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Datagram Packet Delivery

 Datagram – like a letter/telegram
 Delivery of each packet is a one-shot process

 Network retains no info once delivered

 Sequence of packets may take different routes from host to
destination, so may arrive out of sequence

 Datagrams contain:
 Full network address of source and destination

 Internet’s network layer based on datagram delivery

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Hosts, Routers, and Link Layer Switches

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Summary

In this lecture we have been introduced to:
 The different sources of delays in networks

 The notion of a protocol and protocol layering

 The OSI Model

 Packet Assembly, Ports & Addressing

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References / Links

 Chapter #1: Computer Networks and the Internet,

Computer Networking: A Top-Down Approach (7th

edition)
by Kurose & 1
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