Architecture 14-740: Fundamentals of Computer Networks Bill Nace - - PowerPoint PPT Presentation

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Architecture 14-740: Fundamentals of Computer Networks Bill Nace - - PowerPoint PPT Presentation

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Architecture 14-740: Fundamentals of Computer Networks Bill Nace Material from Computer Networking: A Top Down Approach, 7 th edition. J.F. Kurose and K.W. Ross Review What is the internet? A Series of Tubes Nuts-n-bolts

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Material from Computer Networking: A Top Down Approach, 7th edition. J.F. Kurose and K.W. Ross

Architecture

14-740: Fundamentals of Computer Networks Bill Nace

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SLIDE 2

Review

  • What is the internet?
  • “A Series of Tubes”
  • Nuts-n-bolts
  • Service oriented
  • Network Edge
  • Hosts, clients, servers
  • Network Core
  • Routers, packet-switched, store-and-forward

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SLIDE 3

Game Plan

  • Delay and loss in packet-switched nets
  • Layered network architecture
  • ISO OSI seven-layer model

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SLIDE 4

Sources of delay/loss

  • Packets queue in router buffers
  • store and forward
  • packet arrival rate may exceed output

capacity

  • hopefully only for a short time

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10 Mbps Ethernet 1.5 Mbps T1 line queue of packets waiting for output link (delay) packet being transmitted (delay) free buffers Arriving packets dropped (loss) if no buffers are free

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SLIDE 5

Four sources of packet delay

1)Nodal processing

  • Time spent in the router
  • check for bit errors
  • determine output link

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  • 3. Transmission
  • 4. Propagation
  • 2. Queueing
  • 1. Nodal processing
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SLIDE 6

2)Queueing

  • Time waiting for clear output link
  • depends on congestion level of router
  • 3. Transmission
  • 4. Propagation
  • 2. Queueing
  • 1. Nodal processing
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SLIDE 7

3)Transmission Delay

  • L = packet length (bits)
  • R = link bandwidth (bps)
  • time to send bits into the link = L / R
  • 3. Transmission
  • 4. Propagation
  • 2. Queueing
  • 1. Nodal processing
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SLIDE 8

4)Propagation Delay

  • d = length of physical link
  • s = propagation speed in physical

medium (~2x108 m / sec)

  • Note: very different quantity from R
  • propagation delay = d / s
  • 3. Transmission
  • 4. Propagation
  • 2. Queueing
  • 1. Nodal processing
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SLIDE 9

Nodal delay

dnodal = dproc + dqueue + dtrans + dprop

  • dproc = processing delay
  • typically a few microseconds
  • dqueue = queuing delay
  • depends on congestion, 0 to a few milliseconds
  • dtrans = transmission delay
  • L / R, significant for low-speed links
  • dprop = propagation delay
  • d/s, a few microseconds to hundreds of milliseconds

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SLIDE 10

Queuing delay (again)

  • L = packet length (bits/pkt)
  • R = link bandwidth (bps)
  • λ = avg packet arrival rate (pkt/s)
  • traffic intensity = L λ / R
  • L λ / R ~ 0: average queueing delay small
  • L λ / R ➙ 1: delays become large
  • Small (temporary) increase in traffic substantially

increases delay

  • L λ / R > 1: more “work” arriving than can be
  • accomplished. Average delay is infinite!

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average queuing delay Lλ / R 1.0

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SLIDE 11

Packet loss

  • Queue (aka buffer) preceding link in

router has finite capacity

  • When packet arrives to full queue, packet

is dropped (aka lost)

  • Lost packet may be retransmitted by

previous node, by source end system, or not retransmitted at all

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SLIDE 12

“Real” delays

  • What do “real” internet delay & loss look like?
  • Tool: traceroute provides delay

measurements for each hop in the path

  • For all routers i
  • send 3 packets that will reach router i on

path towards destination

  • router i will return packets to sender
  • measure transmission and reply interval

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SLIDE 13

Traceroute algorithm

  • For all routers i
  • send 3 packets that will reach router i
  • n path towards destination
  • router i will return packets to sender
  • measure transmission and reply interval

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3 probes 3 probes 3 probes

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SLIDE 14

Example

14 >> traceroute www.cmuj.jp traceroute to cmuj.jp (222.146.60.103), 64 hops max, 40 byte packets 1 HH130-ROUTER (128.2.130.254) 0.971 ms 0.382 ms 0.378 ms 2 POD-A-WEH-ECE.GW.CMU.NET (128.2.35.217) 0.578 ms 0.519 ms 0.433 ms 3 * * * 4 POD-I-CYH-VL986.GW.CMU.NET (128.2.0.250) 0.613 ms 0.658 ms 0.507 ms 5 sl-st21-pit-1-1-0.sprintlink.net (144.223.26.89) 0.604 ms 0.667 ms 0.555 ms 6 sl-st20-pit-1-0-1.sprintlink.net (144.232.2.90) 0.600 ms 0.669 ms 0.603 ms 7 sl-bb23-rly-15-0.sprintlink.net (144.232.20.216) 6.560 ms 6.647 ms 6.443 ms 8 sl-bb21-rly-9-0.sprintlink.net (144.232.14.133) 6.519 ms 6.489 ms 6.333 ms 9 sl-crs2-rly-0-4-0-0.sprintlink.net (144.232.2.56) 6.579 ms 6.774 ms 6.447 ms 10 sl-bb21-dc-12-0-0.sprintlink.net (144.232.9.212) 7.062 ms 7.066 ms 7.057 ms 11 sl-st21-ash-10-0-0.sprintlink.net (144.232.20.148) 8.576 ms 8.602 ms 8.619 ms 12 p16-2-0-0.r20.asbnva02.us.bb.gin.ntt.net (129.250.8.45) 8.818 ms 8.652 ms 8.782 ms 13 as-2.r21.lsanca03.us.bb.gin.ntt.net (129.250.5.24) 67.614 ms 88.863 ms 75.028 ms 14 ae-0.r20.lsanca03.us.bb.gin.ntt.net (129.250.3.33) 68.163 ms 71.246 ms 67.811 ms 15 as-2.r20.osakjp01.jp.bb.gin.ntt.net (129.250.3.202) 177.412 ms 177.707 ms 177.814 ms 16 ae-4.r20.tokyjp01.jp.bb.gin.ntt.net (129.250.4.209) 176.502 ms 168.719 ms 172.205 ms 17 129.250.11.114 (129.250.11.114) 189.410 ms 186.481 ms 184.967 ms 18 210.254.187.185 (210.254.187.185) 176.460 ms 176.414 ms 176.382 ms 19 60.37.27.131 (60.37.27.131) 187.155 ms 118.23.168.3 (118.23.168.3) 176.118 ms 172.646 ms 20 210.254.188.138 (210.254.188.138) 173.078 ms 169.601 ms 182.398 ms 21 222.146.54.178 (222.146.54.178) 171.101 ms 176.199 ms 176.551 ms 22 220.111.40.14 (220.111.40.14) 195.458 ms 187.703 ms 188.072 ms 23 222.146.54.10 (222.146.54.10) 191.032 ms 187.826 ms 190.441 ms 24 cmuj.jp (222.146.60.103) 175.449 ms 183.011 ms 167.049 ms

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SLIDE 15

Improvements

  • Try mtr instead (Matt's Traceroute)
  • Parallel and continuous sending of all

probes

  • Better stats
  • Try WinMTR on Windows

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SLIDE 16

Game Plan

  • Delay and loss in packet-switched nets
  • Layered network architecture
  • ISO OSI seven-layer model

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SLIDE 17

Networks are complex!

  • Many components:
  • Hosts, routers and links of various media
  • Applications and protocols
  • Many players:
  • Apps / components created by different

corporations

  • Individual networks run by different
  • rganizations

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SLIDE 18

Fundamental Question

  • How should a network be organized?
  • or at least, our understanding and

discussion of networks?

  • Lots of requirements: connectivity,

scalability, robustness, efficiency, manageability, accountability, etc

  • What architecture should be used?

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SLIDE 19

Layered Network Architecture

  • Layering is the grouping of 


functions into related and 
 manageable sets ...

  • in such a way that each layer:
  • provides a service to the layer above
  • defined in a protocol
  • specifies an interface for accessing the service
  • uses services of the layer directly below
  • Network architecture defines all layers and the

design of protocols / interfaces of each layer

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SLIDE 20
  • Monolithic non-layered architectures are costly,

inflexible, and soon obsolete

  • Provides a structured way to understand components
  • Protocol in each layer can be developed separately

from those in other layers

  • Implementation in a layer can change without affecting
  • ther layers, as long as interface remains the same
  • Details of lower layers are not needed
  • Simplifies design, implementation and testing of

network technology

Why layering?

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SLIDE 21

Why not layering?

  • Duplication of lower layer functions
  • Reliability at link layer as well as transport
  • Information hiding affects performance
  • Strict adherence to layering principle is

not always good

  • Might need information from a lower layer

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SLIDE 22

Game Plan

  • Delay and loss in packet-switched nets
  • Layered network architecture
  • ISO OSI seven-layer model

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SLIDE 23
  • ISO: International Standard Organization
  • In 1970s, computer vendors had developed many

proprietary network architectures -- without interoperability

  • Open Systems Interconnection (OSI) was a

reference framework to enable multi-vendor interconnection

  • Provides a unified view of layers, services and

protocols, which is still in use today

  • Started 1978; first standard 1979
  • TCP/IP pre-empted deployment of OSI protocols
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SLIDE 24

Network Data Link Physical Application Presentation Session Transport Network Data Link Physical

ISO OSI Model

SERVICE USER SERVICE PROVIDER

END-TO-END PROTOCOLS

Application Presentation Session Transport Network Data Link Physical Network Data Link Physical

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SLIDE 25

Physical Layer

  • Transfers bits across link
  • Definition & specification of the physical aspects
  • Mechanical: cable, plugs, pins...
  • Electrical: modulation, voltage levels, timing…
  • Procedural: activation/deactivation...
  • Ethernet, DSL, cable modem, phone modem …
  • Twisted-pair, coaxial, optical, wireless, IR, …

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Physical

bits

Physical

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SLIDE 26

Data Link Layer

  • Transfers frames across direct connections between 2

nodes

  • A frame is a sequence of bits, or blocks of information
  • Inserts framing info to denote frame boundaries
  • Inserts control, addressing and error correction info in

header

  • Detects transmission errors on link. May retransmit frames
  • Activation, maintenance, & deactivation of link connection
  • Examples: Ethernet, PPP

, HDLC

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Data Link

frames

Data Link Physical

bits

Physical

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SLIDE 27

Sidebar: data “link” layer

  • Local Area Networks (LANs) generally use

broadcast transmissions, e.g. Ethernet

  • The notion of “link” includes the case of multiple

nodes connected to a broadcast medium

  • A Media Access Control (MAC) protocol

coordinates use of media among multiple machines

  • Flat addressing space (MAC address)
  • Hosts listen & recognize frames destined for them
  • Collisions are avoided or detected/recovered

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SLIDE 28

Network Layer

  • Transfers packets across multiple links
  • Addressing must scale to large networks
  • usually hierarchically
  • Routing protocol determines best paths

across the network

  • Model’s most complex layer
  • A distributed system!

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Network

packets

Network Data Link

frames

Data Link Physical

bits

Physical

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SLIDE 29

Transport Layer

  • End-to-end transfer of messages
  • Breaks long messages into shorter segments
  • Port numbers enable multiplexing
  • Message segmentation and reassembly
  • Connection setup, maintenance, and release

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Transport

segments

Transport

Network

packets

Network

packets

Network

packets

Network Data Link

frames

Data Link

frames

Data Link

frames

Data Link Physical

bits

Physical

bits

Physical

bits

Physical

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SLIDE 30

Transport layer services

  • Connection-oriented service
  • Reliable delivery of byte stream
  • Error detection and recovery
  • Congestion control
  • Flow control
  • Connectionless service
  • Best-effort delivery

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SLIDE 31

Application and upper layers

  • Application: Provides services that are

frequently required by applications: DNS, web access, file transfer, email…

  • Presentation: machine-independent data

representation

  • Session: dialog management, error recovery, …

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Application

messages

Application

Transport

segments

Transport

Network

packets

Network

packets

Network

packets

Network Data Link

frames

Data Link

frames

Data Link

frames

Data Link Physical

bits

Physical

bits

Physical

bits

Physical

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SLIDE 32

Lesson Objectives

  • Now, you should be able to:
  • read, comment upon and review

research papers with a strong process to store and search the knowledge gained from the paper

  • calculate delay in a packet-switched

network as a sum of nodal processing delay, queueing delay, transmission delay and propagation delay

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SLIDE 33

Now, you should be able to:

  • describe the algorithm used by traceroute to

measure delay in real networks. Additionally, be able to use traceroute and interpret its output

  • argue the benefits of a layered architecture (as

compared to monolithic)

  • describe the internet's layered architecture

according to the OSI model, including the mission of each layer, the scope of the layer, the type of data transferred by the layer and summary of the execution steps accomplished

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SLIDE 34

Wrap-up

  • Delay and loss in packet-switched nets
  • Layered network architecture
  • ISO OSI seven-layer model
  • Next lesson: TCP / IP Architecture

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