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Part I: Introduction
Chapter goal: get context, overview, "feel" of networking more depth, detail later in course approach: r descriptive r use Internet as example Overview: what' the Internet s what' a protocol? s network edge network core access net, physical media performance: loss, delay protocol layers, service models backbones, NAPs, ISPs history ATM network
1: Introduction 1
What' the Internet: "nuts and bolts" view s
millions of connected computing devices: hosts, end-systems
r r
router server local ISP
workstation mobile
running network apps communication links
r
pc' workstations, servers s PDA' phones, toasters s
regional ISP
routers: forward packets (chunks) of data thru network
fiber, copper, radio, satellite
company network
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What' the Internet: "nuts and bolts" view s
protocols: control sending, receiving of msgs
r
router server local ISP
workstation mobile
Internet: "network of networks"
r r
e.g., TCP, IP, HTTP, FTP, PPP
Internet standards
r r
loosely hierarchical public Internet versus private intranet RFC: Request for comments IETF: Internet Engineering Task Force
regional ISP
company network
1: Introduction 3
What' the Internet: a service view s
communication infrastructure enables distributed applications:
r
r
communication services provided:
r r
WWW, email, games, ecommerce, database., voting, more?
connectionless connection-oriented
cyberspace [Gibson]:
"a consensual hallucination experienced daily by billions of operators, in every nation, ...."
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What' a protocol? s
human protocols: "what' the time?" s "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
1: Introduction 5
What' a protocol? s
a human protocol and a computer network protocol: Hi Hi
Got the time?
TCP connection req. TCP connection reply.
Get http:// gaia.cs.umass.edu/index. htm
2:00
time Q: Other human protocol?
<file>
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A closer look at network structure:
network edge:
applications and hosts network core:
routers r network of networks
r
access networks,
physical media: communication links
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The network edge:
end systems (hosts):
r r r
run application programs e.g., WWW, email at "edge of network" client host requests, receives service from server e.g., WWW client (browser)/ server; email client/server host interaction symmetric
client/server model
r
r
peer-peer model:
r r
e.g.: teleconferencing
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Network edge: connection-oriented service
Goal: data transfer
between end sys. handshaking: setup (prepare for) data transfer ahead of time
r
TCP service [RFC 793]
reliable, in-order bytestream data transfer
r
r
Hello, hello back human protocol set up "state" in two communicating hosts
flow control:
r
loss: acknowledgements and retransmissions sender won' overwhelm t receiver senders "slow down sending rate" when network congested
1: Introduction 9
TCP - Transmission Control Protocol
r
congestion control:
r
Internet' connections oriented service
Network edge: connectionless service
Goal: data transfer
r
between end systems UDP - User Datagram Protocol [RFC 768]: Internet' s connectionless service r unreliable data transfer r no flow control r no congestion control
same as before!
App' using TCP: s
HTTP (WWW), FTP (file transfer), Telnet (remote login), SMTP (email)
App' using UDP: s
streaming media, teleconferencing, Internet telephony
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The Network Core
mesh of interconnected routers the fundamental question: how is data transferred through net? r circuit switching: dedicated circuit per call: telephone net r packet-switching: data sent thru net in discrete "chunks"
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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
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Network Core: Circuit Switching (Multiplexing)
network resources (e.g., bandwidth) divided into "pieces"
pieces allocated to calls resource piece idle if not used by owning call (no sharing) dividing link bandwidth into "pieces" r frequency division r time division
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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 r transmit over link r wait turn at next link
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Network Core: Packet Switching
A B
10 Mbs Ethernet statistical multiplexing 1.5 Mbs
queue of packets waiting for output link
C
45 Mbs
D
E
Packet-switching versus circuit switching: human restaurant analogy other human analogies?
1: Introduction 15
Network Core: Packet Switching
Packet-switching: and store forward behavior
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Packet switching versus circuit switching
Packet switching allows more users to use network! 1 Mbit link each user:
r r
100Kbps when "active" active 10% of time
circuit-switching:
r
N users 1 Mbps link
packet switching:
r
10 users
with 35 users, probability > 10 active less that .004
1: Introduction 17
Packet switching versus circuit switching
Is packet switching a "slam dunk winner?" Great for bursty data r resource sharing r no call setup Excessive congestion: packet delay and loss r protocols needed for reliable data transfer, congestion control Q: How to provide circuit-like behavior? r bandwidth guarantees needed for audio/video apps still an unsolved problem (chapter 6)
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Packet-switched networks: routing
Goal: move packets among routers from source to destination
r
datagram network:
r r r
we' study several path selection algorithms (chapter 4) ll destination address determines next hop routes may change during session analogy: driving, asking directions each packet carries tag (virtual circuit ID), tag determines next hop fixed path determined at call setup time, remains fixed thru call routers maintain per-call state
1: Introduction 19
virtual circuit network:
r
r
r
Network Taxonomy
Telecommunication networks
Circuit-switched networks Connection-oriented?
Packet-switched networks
FDM
TDM
Networks with VCs
Datagram networks
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Access networks and physical media
Q: How to connection 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?
1: Introduction 21
Residential access: point to point access
Dialup via modem r up to 56Kbps direct access to router (conceptually) ISDN: intergrated services digital network: 128Kbps alldigital connect to router ADSL: asymmetric digital subscriber line r up to 1 Mbps home-to-router r up to 8 Mbps router-to-home r DSL/ADSL deployment: SBC, Covad
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Residential access: cable modems
HFC: hybrid fiber coax
r
asymmetric: up to 10Mbps upstream, 1 Mbps downstream
network of cable and fiber attaches homes to ISP router
r
r
deployment: available via cable companies, e.g., Road Runner, @Home, MediaOne
shared access to router among home issues: congestion, dimensioning
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Institutional access: local area networks
company/univ local area network (LAN) connects end system to edge router Ethernet: r shared or dedicated cable connects end system and router r 10 Mbs, 100Mbps, Gigabit Ethernet deployment: institutions, home LANs soon LANs: chapter 5
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Wireless access networks
shared wireless access network connects end system to router wireless LANs:
r
router base station
r
wider-area wireless access
r
radio spectrum replaces wire e.g., Lucent Wavelan 10 Mbps
CDPD: wireless access to ISP router via cellular network
mobile hosts
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Physical Media
physical link: transmitted data bit propagates across link guided media:
r
Twisted Pair (TP) two insulated copper wires
r
unguided media:
r
signals...