1 Chapter Introduction
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5th edition. Jim Kurose, Keith Ross Addison-Wesley, April 2009.
Computer Networking: A Top Down Approach ,
Introduction
1-1
Chapter 1: Introduction
Our goal:
get feel and terminology more depth, detail later in course approach: use Internet as example
Overview:
whats the Internet? whats a protocol? network edge; hosts, access net, physical media network core: packet/circuit switching, Internet structure performance: loss, delay, throughput protocol layers, service models
Introduction
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Chapter 1: roadmap
1.1 What is the Internet? 1.2 Network edge
end systems, access networks, links
1.3 Network core
circuit switching, packet switching, network structure
1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models
Introduction
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Whats the Internet: nuts and bolts view
PC server
wireless laptop cellular handheld
hosts = end systems running network apps communication links
millions of connected computing devices:
Mobile network Global ISP
Home network Regional ISP
access points wired links
router
fiber, copper, radio, satellite transmission rate = bandwidth routers: forward packets (chunks of data)
Institutional network
Introduction
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Cool internet appliances
Web-enabled toaster + weather forecaster IP picture frame http://www.ceiva.com/
Worlds smallest web server http://www-ccs.cs.umass.edu/~shri/iPic.html
Internet phones
Introduction 1-5
Whats the Internet: nuts and bolts view
protocols control sending,
receiving of msgs
Mobile network Global ISP
e.g., TCP, IP, HTTP, Skype, Ethernet RFC: Request for comments IETF: Internet Engineering Task Force
Internet standards
Home network Regional ISP
networks
Internet: network of
loosely hierarchical public Internet versus private intranet
Institutional network
Introduction
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Whats the Internet: a service view
communication
distributed applications: Web, VoIP, email, games, e-commerce, file sharing communication services provided to apps: reliable data delivery from source to destination best effort (unreliable) data delivery
Introduction 1-7
infrastructure enables
Whats a protocol?
a human protocol and a computer network protocol: Hi Hi
Got the time?
TCP connection request TCP connection response
Get http://www.awl.com/kurose-ross
2:00 time Q: Other human protocols?
<file>
Introduction
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Whats a protocol?
human protocols: whats the time? I have a question introductions specific msgs sent specific actions taken when msgs received, or other events network protocols: machines rather than humans all communication activity in Internet governed by protocols
protocols define format, order of msgs sent and received among network entities, and actions taken on msg transmission, receipt
Introduction 1-9
Chapter 1: roadmap
1.1 What is the Internet? 1.2 Network edge
end systems, access networks, links
1.3 Network core
circuit switching, packet switching, network structure
1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models
Introduction
1-10
A closer look at network structure:
network edge:
applications and hosts access networks, physical media: wired, wireless communication links network core:
interconnected routers network of networks
Introduction
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The network edge:
end systems (hosts):
client/server model
run application programs e.g. Web, email at edge of network
peer-peer
peer-peer model:
client host requests, receives service from always-on server client/server e.g. Web browser/server; email client/server minimal (or no) use of dedicated servers e.g. Skype, BitTorrent
Introduction 1-12
Access networks and physical media
Q: How to connect end systems to edge router?
residential access nets institutional access networks (school, company) mobile access networks
Keep in mind:
bandwidth (bits per second) of access network? shared or dedicated?
Introduction 1-13
Dial-up Modem
central office telephone network Internet
home PC
home dial-up modem
ISP modem (e.g., AOL)
Uses existing telephony infrastructure Home is connected to central office up to 56Kbps direct access to router (often less) Cant surf and phone at same time: not always on
Digital Subscriber Line (DSL)
home phone
Existing phone line: 0-4KHz phone; 4-50KHz upstream data; 50KHz-1MHz downstream data
Internet
DSLAM
splitter DSL modem home PC central office
telephone network
Also uses existing telephone infrastruture up to 1 Mbps upstream (today typically < 256 kbps) up to 8 Mbps downstream (today typically < 1 Mbps) dedicated physical line to telephone central office
Residential access: cable modems
Does not use telephone infrastructure
Instead uses cable TV infrastructure
HFC: hybrid fiber coax asymmetric: up to 30Mbps downstream, 2 Mbps upstream network of cable and fiber attaches homes to ISP router homes share access to router unlike DSL, which has dedicated access
Introduction
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Residential access: cable modems
Diagram: http://www.cabledatacomnews.com/cmic/diagram.html
Introduction
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Cable Network Architecture: Overview
Typically 500 to 5,000 homes
cable headend cable distribution network (simplified) home
Introduction
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Cable Network Architecture: Overview
server(s)
cable headend cable distribution network home
Introduction
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Cable Network Architecture: Overview
cable headend cable distribution network (simplified) home
Introduction
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Cable Network Architecture: Overview
FDM (more shortly):
V I D E O 1 V I D E O 2 V I D E O 3 V I D E O 4 V I D E O 5 V I D E O 6 D A T A 7 D A T A 8 C O N T R O L 9
Channels
cable headend cable distribution network home
Introduction
1-21
Fiber to the Home
ONT Internet optical fiber optical fibers ONT
OLT
central office
optical splitter ONT
Optical links from central office to the home Two competing optical technologies:
Passive Optical network (PON) Active Optical Network (AON)
Much higher Internet rates; fiber also carries television and phone services
Ethernet Internet access
100 Mbps Ethernet switch 100 Mbps Institutional router To Institutions ISP
1 Gbps 100 Mbps
Typically used in companies, universities, etc 10 Mbs, 100Mbps, 1Gbps, 10Gbps Ethernet Today, end systems typically connect into Ethernet switch
server
Wireless access networks
shared wireless access network connects end system to router
via base station aka access point 802.11b/g (WiFi): 11 or 54 Mbps provided by telco operator ~1Mbps over cellular system (EVDO, HSDPA) next up (?): WiMAX (10s Mbps) over wide area
router base station
wireless LANs:
wider-area wireless access
mobile hosts
Introduction
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Home networks
Typical home network components: DSL or cable modem router/firewall/NAT Ethernet wireless access point
to/from cable headend cable modem router/ firewall Ethernet wireless laptops wireless access point
Introduction 1-25
Physical Media
Bit: propagates between transmitter/rcvr pairs physical link: what lies between transmitter & receiver guided media:
Twisted Pair (TP) two insulated copper wires
signals propagate in solid media: copper, fiber, coax signals propagate freely, e.g., radio
Category 3: traditional phone wires, 10 Mbps Ethernet Category 5: 100Mbps Ethernet
unguided media:
Introduction
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Physical Media: coax, fiber
Coaxial cable:
Fiber optic cable:
glass fiber carrying light pulses, each pulse a bit high-speed operation:
two concentric copper conductors bidirectional baseband:
broadband:
single channel on cable legacy Ethernet multiple channels on cable HFC
high-speed point-to-point transmission (e.g., 10s100s Gps)
low error rate: repeaters spaced far apart ; immune to electromagnetic noise
Introduction
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Physical media: radio
signal carried in electromagnetic spectrum no physical wire bidirectional propagation environment effects:
Radio link types:
terrestrial microwave
e.g. up to 45 Mbps channels 11Mbps, 54 Mbps 3G cellular: ~ 1 Mbps Kbps to 45Mbps channel (or multiple smaller channels) 270 msec end-end delay geosynchronous versus low altitude
Introduction 1-28
LAN (e.g., Wifi)
wide-area (e.g., cellular)
reflection obstruction by objects interference
satellite
Chapter 1: roadmap
1.1 What is the Internet? 1.2 Network edge
end systems, access networks, links
1.3 Network core
circuit switching, packet switching, network structure
1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models
Introduction
1-29
The Network Core
mesh of interconnected routers the fundamental question: how is data transferred through net? circuit switching: dedicated circuit per call: telephone net packet-switching: data sent thru net in discrete chunks
Introduction 1-30
Network Core: Circuit Switching
End-end resources reserved for call
link bandwidth, switch capacity dedicated resources: no sharing circuit-like (guaranteed) performance call setup required
Introduction
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Network Core: Circuit Switching
network resources (e.g., bandwidth) divided into pieces
pieces allocated to calls resource piece idle if not used by owning call
dividing link bandwidth into pieces frequency division time division
(no sharing)
Introduction
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Circuit Switching: FDM and TDM
Example: FDM 4 users frequency time TDM
frequency time
Introduction
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Numerical example
How long does it take to send a file of
640,000 bits from host A to host B over a circuit-switched network?
All links are 1.536 Mbps Each link uses TDM with 24 slots/sec 500 msec to establish end-to-end circuit
Lets work it out!
Introduction
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Network Core: Packet Switching
each end-end data stream divided into packets user A, B packets share network resources each packet uses full link bandwidth resources used as needed
Bandwidth division into pieces Dedicated allocation Resource reservation
resource contention: aggregate resource demand can exceed amount available congestion: packets queue, wait for link use store and forward: packets move one hop at a time
Node receives complete packet before forwarding
Introduction
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Packet Switching: Statistical Multiplexing
A B
100 Mb/s Ethernet
statistical multiplexing
1.5 Mb/s
C
queue of packets waiting for output link
D
E
Sequence of A & B packets does not have fixed pattern, bandwidth shared on demand statistical multiplexing. TDM: each host gets same slot in revolving TDM frame.
Introduction 1-36
Packet-switching: store-and-forward
L R R R
takes L/R seconds to transmit (push out) packet of L bits on to link at R bps
entire packet must arrive at router before it can transmitted be on next link delay = 3L/R (assuming zero propagation delay)
store and forward:
Example: L = 7.5 Mbits R = 1.5 Mbps transmission delay = 15 sec
more on delay shortly
Introduction 1-37
Packet switching versus circuit switching
Packet switching allows more users to use network!
1 Mb/s link each user:
100 kb/s when active active 10% of time
circuit-switching:
N users 1 Mbps link
packet switching:
10 users
with 35 users, probability > 10 active at same time is less than .0004
Q: how did we get value 0.0004?
Introduction
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Packet switching versus circuit switching
Is packet switching a slam dunk winner? great for bursty data resource sharing simpler, no call setup excessive congestion: packet delay and loss protocols needed for reliable data transfer, congestion control Q: How to provide circuit-like behavior? bandwidth guarantees needed for audio/video apps still an unsolved problem (chapter 7)
Q: human analogies of reserved resources (circuit switching) versus on-demand allocation (packet-switching)?
Introduction 1-39
Internet structure: network of networks
roughly hierarchical at center: tier-1 ISPs (e.g., Verizon, Sprint, AT&T, Cable and Wireless), national/international coverage treat each other as equals
Tier-1 providers interconnect (peer) privately
Tier 1 ISP Tier 1 ISP
Tier 1 ISP
Introduction
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Tier-1 ISP: e.g., Sprint
POP: point-of-presence
to/from backbone peering
.
to/from customers
Introduction
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Internet structure: network of networks
Tier-2 ISPs: smaller (often regional) ISPs
Connect to one or more tier-1 ISPs, possibly other tier-2 ISPs
Tier-2 ISP pays tier-1 ISP for connectivity to rest of Internet tier-2 ISP is customer of tier-1 provider
Tier-2 ISP
Tier-2 ISP
Tier 1 ISP Tier 1 ISP
Tier-2 ISP
Tier-2 ISPs also peer privately with each other.
Tier 1 ISP
Tier-2 ISP
Tier-2 ISP
Introduction
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Internet structure: network of networks
Tier-3 ISPs and local ISPs
last hop (access) network (closest to end systems)
local ISP Tier 3 ISP Tier-2 ISP local ISP
Local and tier3 ISPs are customers of higher tier ISPs connecting them to rest of Internet
local ISP
local ISP Tier-2 ISP
Tier 1 ISP Tier 1 ISP
Tier-2 ISP local ISP
Tier 1 ISP
Tier-2 ISP local ISP
Introduction 1-43
Tier-2 ISP local local ISP ISP
Internet structure: network of networks
a packet passes through many networks!
local ISP
Tier 3 ISP Tier-2 ISP
local ISP
local ISP Tier-2 ISP
local ISP
Tier 1 ISP Tier 1 ISP
Tier-2 ISP local ISP
Tier 1 ISP
Tier-2 ISP local local ISP ISP
Tier-2 ISP local ISP
Introduction 1-44
Chapter 1: roadmap
1.1 What is the Internet? 1.2 Network edge
end systems, access networks, links
1.3 Network core
circuit switching, packet switching, network structure
1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models
Introduction
1-45
How do loss and delay occur?
packets queue in router buffers
packet arrival rate to link exceeds output link capacity packets queue, wait for turn
packet being transmitted (delay)
A B
packets queueing (delay) free (available) buffers: arriving packets dropped (loss) if no free buffers
Introduction
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Four sources of packet delay
1. nodal processing:
2. queueing
check bit errors determine output link
time waiting at output link for transmission depends on congestion level of router
A B
transmission propagation
nodal processing
queueing
Introduction 1-47
Delay in packet-switched networks
3. Transmission delay: R=link bandwidth (bps) L=packet length (bits) time to send bits into link = L/R 4. Propagation delay: d = length of physical link s = propagation speed in medium (~2x108 m/sec) propagation delay = d/s Note: s and R are very different quantities!
propagation
A B
transmission
nodal processing
queueing
Introduction
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Caravan analogy
100 km ten-car caravan toll booth toll booth 100 km
cars propagate at 100 km/hr toll booth takes 12 sec to service car (transmission time) car~bit; caravan ~ packet Q: How long until caravan is lined up before 2nd toll booth?
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
Introduction 1-49
Caravan analogy (more)
100 km ten-car caravan toll booth toll booth 100 km
Cars now propagate at 1000 km/hr Toll booth now takes 1 min to service a car Q: Will cars arrive to 2nd booth before all cars serviced at 1st booth?
Yes! After 7 min, 1st car at 2nd booth and 3 cars still at 1st booth. 1st bit of packet can arrive at 2nd router before packet is fully transmitted at 1st router!
See Ethernet applet at AWL Web site
Introduction 1-50
Nodal delay
d nodal d proc d queue d trans d prop
dproc = processing delay
typically a few microsecs or less depends on congestion = L/R, significant for low-speed links a few microsecs to hundreds of msecs
dqueue = queuing delay
dtrans = transmission delay
dprop = propagation delay
Introduction
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Queueing delay (revisited)
R=link bandwidth (bps) L=packet length (bits) a=average packet arrival rate
traffic intensity = La/R La/R ~ 0: average queueing delay small La/R -> 1: delays become large La/R > 1: more work arriving than can be serviced, average delay infinite!
Introduction 1-52
Real Internet delays and routes
What do real Internet delay & loss look like? Traceroute program: provides delay measurement from source to router along end-end Internet path towards destination. For all i:
sends three packets that will reach router i on path towards destination router i will return packets to sender sender times interval between transmission and reply.
3 probes 3 probes 3 probes
Introduction
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Real Internet delays and routes
traceroute: gaia.cs.umass.edu to www.eurecom.fr
Three delay measurements from gaia.cs.umass.edu to cs-gw.cs.umass.edu
1 cs-gw (128.119.240.254) 1 ms 1 ms 2 ms 2 border1-rt-fa5-1-0.gw.umass.edu (128.119.3.145) 1 ms 1 ms 2 ms 3 cht-vbns.gw.umass.edu (128.119.3.130) 6 ms 5 ms 5 ms 4 jn1-at1-0-0-19.wor.vbns.net (204.147.132.129) 16 ms 11 ms 13 ms 5 jn1-so7-0-0-0.wae.vbns.net (204.147.136.136) 21 ms 18 ms 18 ms 6 abilene-vbns.abilene.ucaid.edu (198.32.11.9) 22 ms 18 ms 22 ms 7 nycm-wash.abilene.ucaid.edu (198.32.8.46) 22 ms 22 ms 22 ms trans-oceanic 8 62.40.103.253 (62.40.103.253) 104 ms 109 ms 106 ms link 9 de2-1.de1.de.geant.net (62.40.96.129) 109 ms 102 ms 104 ms 10 de.fr1.fr.geant.net (62.40.96.50) 113 ms 121 ms 114 ms 11 renater-gw.fr1.fr.geant.net (62.40.103.54) 112 ms 114 ms 112 ms 12 nio-n2.cssi.renater.fr (193.51.206.13) 111 ms 114 ms 116 ms 13 nice.cssi.renater.fr (195.220.98.102) 123 ms 125 ms 124 ms 14 r3t2-nice.cssi.renater.fr (195.220.98.110) 126 ms 126 ms 124 ms 15 eurecom-valbonne.r3t2.ft.net (193.48.50.54) 135 ms 128 ms 133 ms 16 194.214.211.25 (194.214.211.25) 126 ms 128 ms 126 ms 17 * * * * means no response (probe lost, router not replying) 18 * * * 19 fantasia.eurecom.fr (193.55.113.142) 132 ms 128 ms 136 ms
Introduction 1-54
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
buffer (waiting area) packet being transmitted
packet arriving to full buffer is lost
Introduction 1-55
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
link capacity server, with server sends bits pipe that can carry Rs bits/sec fluid at rate file into pipe (fluid) of F bits Rs bits/sec) to send to client
link capacity pipe that can carry Rfluid at rate c bits/sec Rc bits/sec)
Introduction 1-56
Throughput (more)
Rs
< Rc What is average end-end throughput?
Rs bits/sec Rc bits/sec
Rs
> Rc What is average end-end throughput?
Rs bits/sec Rc bits/sec
bottleneck link
link on end-end path that constrains end-end throughput
Introduction 1-57
Throughput: Internet scenario
per-connection
Rs Rs R Rc Rc 10 connections (fairly) share backbone bottleneck link R bits/sec
Introduction 1-58
end-end throughput: min(Rc,Rs,R/10) in practice: Rc or Rs is often bottleneck
Rs
Rc
Chapter 1: roadmap
1.1 What is the Internet? 1.2 Network edge
end systems, access networks, links
1.3 Network core
circuit switching, packet switching, network structure
1.4 Delay, loss and throughput in packet-switched networks 1.5 Protocol layers, service models
Introduction
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Protocol Layers
Networks are complex! many pieces: hosts routers links of various media applications protocols hardware, software
Question:
Is there any hope of organizing structure of network? Or at least our discussion of networks?
Introduction
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Organization of air travel
ticket (purchase) baggage (check) gates (load) runway takeoff airplane routing ticket (complain) baggage (claim) gates (unload) runway landing airplane routing airplane routing
a series of steps
Introduction 1-61
Layering of airline functionality
ticket (purchase) baggage (check) gates (load) runway (takeoff) airplane routing
departure airport
ticket (complain) baggage (claim gates (unload) runway (land) airplane routing airplane routing airplane routing
arrival airport
ticket baggage gate takeoff/landing airplane routing
intermediate air-traffic control centers
Layers: each layer implements a service via its own internal-layer actions relying on services provided by layer below
Introduction
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Why layering?
Dealing with complex systems:
explicit structure allows identification, relationship of complex systems pieces layered reference model for discussion modularization eases maintenance, updating of system change of implementation of layers service transparent to rest of system e.g., change in gate procedure doesnt affect rest of system layering considered harmful?
Introduction 1-63
Internet protocol stack
application: supporting network applications
FTP, SMTP, HTTP
application transport network link physical
transport: process-process data transfer
TCP, UDP
network: routing of datagrams from source to destination
IP, routing protocols
link: data transfer between neighboring network elements
PPP, Ethernet
Introduction 1-64
physical: bits on the wire
ISO/OSI reference model
presentation: allow applications to interpret meaning of data, e.g., encryption, compression, machinespecific conventions session: synchronization, checkpointing, recovery of data exchange Internet stack missing these layers! these services, if needed, must be implemented in application needed?
application presentation session transport network link physical
Introduction
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source
datagram Hn Ht message segment Ht
M M M M
frame Hl Hn Ht
application transport network link physical
Encapsulation
link physical switch
destination
M Ht Hn Ht Hl Hn Ht M M M
application transport network link physical
Hn Ht Hl Hn Ht
M M
network link physical
Hn Ht
M
router
Introduction
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Introduction: Summary
Covered a ton of material! Internet overview whats a protocol? network edge, core, access network packet-switching versus circuit-switching Internet structure performance: loss, delay, throughput layering, service models You now have: context, overview, feel of networking more depth, detail to
follow!
Introduction
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