Bài giảng Mạng máy tính - Chapter 1: Introduction (Part I) - Hồ Phương Đắc
Part I: Introduction
Chapter goal:
Overview:
get context,
overview, “feel” of
networking
what’s the Internet
what’s a protocol?
network edge
more depth, detail
network core
later in course
access net, physical media
performance: loss, delay
protocol layers, service models
backbones, NAPs, ISPs
history
approach:
descriptive
use Internet as
example
ATM network
1: Introduction
1
What’s the Internet: “nuts and bolts” view
router
workstation
millions of connected
server
computing devices: hosts,
end-systems
mobile
local ISP
pc’s workstations, servers
PDA’s phones, toasters
running network apps
communication links
regional ISP
fiber, copper, radio,
satellite
routers: forward packets
(chunks) of data thru
network
company
network
1: Introduction
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What’s the Internet: “nuts and bolts” view
router
server
protocols: control sending,
receiving of msgs
workstation
mobile
e.g., TCP, IP, HTTP, FTP, PPP
local ISP
Internet: “network of
networks”
loosely hierarchical
regional ISP
public Internet versus
private intranet
Internet standards
RFC: Request for comments
IETF: Internet Engineering
Task Force
company
network
1: Introduction
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What’s the Internet: a service view
communication
infrastructure enables
distributed applications:
WWW, email, games, e-
commerce, database.,
voting,
more?
communication services
provided:
connectionless
connection-oriented
cyberspace [Gibson]:
“a consensual hallucination
experienced daily by billions of
operators, in every nation, ...."
1: Introduction
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What’s a protocol?
human protocols:
network protocols:
“what’s the time?”
“I have a question”
introductions
machines rather than
humans
all communication
activity in Internet
governed by protocols
… specific msgs sent
… specific actions taken
when msgs received,
or other events
protocols define format,
order of msgs sent and
received among network
entities, and actions
taken on msg
transmission, receipt
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What’s a protocol?
a human protocol and a computer network protocol:
Hi
TCP connection
req.
Hi
TCP connection
reply.
Got the
time?
2:00
<file>
time
Q: Other human protocol?
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A closer look at network structure:
network edge:
applications and
hosts
network core:
routers
network of
networks
access networks,
physical media:
communication links
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The network edge:
end systems (hosts):
run application programs
e.g., WWW, email
at “edge of network”
client/server model
client host requests, receives
service from server
e.g., WWW client (browser)/
server; email client/server
peer-peer model:
host interaction symmetric
e.g.: teleconferencing
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Network edge: connection-oriented service
Goal: data transfer
TCP service [RFC 793]
between end sys.
reliable, in-order byte-
handshaking: setup
stream data transfer
(prepare for) data
loss: acknowledgements
and retransmissions
transfer ahead of time
Hello, hello back human
flow control:
protocol
sender won’t overwhelm
set up “state” in two
receiver
communicating hosts
congestion control:
TCP - Transmission
senders “slow down sending
rate” when network
congested
Control Protocol
Internet’s connection-
oriented service
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Network edge: connectionless service
Goal: data transfer
between end systems
same as before!
App’s using TCP:
HTTP (WWW), FTP
(file transfer), Telnet
(remote login), SMTP
(email)
UDP - User Datagram
Protocol [RFC 768]:
Internet’s
connectionless service
App’s using UDP:
streaming media,
teleconferencing,
Internet telephony
unreliable data
transfer
no flow control
no congestion control
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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”
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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
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”
frequency division
time division
1: Introduction
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Network Core: Packet Switching
each end-end data stream
resource contention:
divided into packets
aggregate resource
demand can exceed
amount available
user A, B packets share
network resources
each packet uses full link
congestion: packets
bandwidth
queue, wait for link use
resources used as needed, store and forward:
packets move one hop
at a time
Bandwidth division into “pieces”
transmit over link
Dedicated allocation
wait turn at next
Resource reservation
link
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Network Core: Packet Switching
10 Mbs
Ethernet
C
A
statistical multiplexing
1.5 Mbs
B
queue of packets
waiting for output
link
45 Mbs
D
E
Packet-switching versus circuit switching: human
restaurant analogy
other human analogies?
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Network Core: Packet Switching
Packet-switching:
store and forward behavior
1: Introduction
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Packet switching versus circuit switching
Packet switching allows more users to use network!
1 Mbit link
each user:
100Kbps when “active”
active 10% of time
N users
circuit-switching:
1 Mbps link
10 users
packet switching:
with 35 users,
probability > 10 active
less that .004
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Packet switching versus circuit switching
Is packet switching a “slam dunk winner?”
Great for bursty data
resource sharing
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 6)
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Packet-switched networks: routing
Goal: move packets among routers from source to
destination
we’ll study several path selection algorithms (chapter 4)
datagram network:
destination address determines next hop
routes may change during session
analogy: driving, asking directions
virtual circuit network:
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
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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
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