Download Packet Switching

Computer Networks
张辉
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Computer
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Networking: A Top-Down Approach Featuring
the Internet by Kurose and Ross
计算机网络教程
谢希仁 人民邮电出版社（2002年5月）
1: Introduction
1
What Will We Cover?
 网络简介
 网络体系结构
 网络物理层（传输媒介、接口、信号）
 数据链路层（网络检错、同步、HDLC、PPP）
 局域网技术（Ethernet、Token Ring、Token bus)
 网络层（IP 编址、subnetting、VLSM、CIDR）
 路由原理（RIP、OSPF、BGP）
 传输层（TCP 、UDP）
 流量控制、拥塞控制及网络性能
 应用层（SMTP、ftp、Web、DNS等）
 网络安全及网络管理
 网络新技术(MPLS、IPv6、Multicasting等)
1: Introduction
2
Part I: Introduction
Chapter goal:
 get context,
overview, “feel” of
networking
 more depth, detail
later in course
 approach:
 descriptive
 use Internet as
example
Overview:
 what’s the Internet
 what’s a protocol?
 network edge
 network core
 access net, physical media
 performance: loss, delay
 protocol layers, service models
 backbones, NAPs, ISPs
 history
 ATM network
1: Introduction
3
What’s the Internet: “nuts and bolts” view
 millions of connected
computing devices: hosts,
end-systems


pc’s workstations, servers
PDA’s phones
router
server
mobile
local ISP
running network apps
 communication links

workstation
regional ISP
fiber, copper, radio,
satellite
 routers: forward packets
of data thru network
company
network
1: Introduction
4
What’s the Internet: “nuts and bolts” view
 protocols: control sending,
receiving of msgs

e.g., TCP, IP, HTTP, FTP, PPP
 Internet: “network of
router
server
workstation
mobile
local ISP
networks”


loosely hierarchical
public Internet versus
private intranet
 Internet standards
 RFC: Request for comments
 IETF: Internet Engineering
Task Force
regional ISP
company
network
1: Introduction
5
What’s the Internet: a service view
 communication
infrastructure enables
distributed applications:


WWW, email, games, ecommerce, 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
6
What’s a protocol?
human protocols:
 “what’s 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
1: Introduction
7
What’s a protocol?
a human protocol and a computer network protocol:
Hi
TCP connection
req.
Hi
TCP connection
reply.
Got the
time?
Get http://gaia.cs.umass.edu/index.htm
2:00
<file>
time
Q: Other human protocol?
1: Introduction
8
Who is Who on the Internet ?
 Internet Engineering Task Force (IETF): The IETF is
the protocol engineering and development arm of the
Internet. Subdivided into many working groups, which
specify Request For Comments or RFCs.
 IRTF (Internet Research Task Force): The Internet
Research Task Force is a composed of a number of
focused, long-term and small Research Groups.
 Internet Architecture Board (IAB): The IAB is
responsible for defining the overall architecture of the
Internet, providing guidance and broad direction to the
IETF.
 The Internet Engineering Steering Group (IESG): The
IESG is responsible for technical management of IETF
activities and the Internet standards process.
Standards. Composed of the Area Directors of the
IETF working groups.
1: Introduction
9
Internet Standardization Process
 All standards of the Internet are published as
RFC (Request for Comments). But not all RFCs
are Internet Standards !
 available:
http://www.ietf.org
 A typical (but not only) way of standardization
is:
 Internet
 RFC
Drafts
 Proposed
Standard
 Draft Standard (requires 2 working implementation)
 Internet Standard (declared by IAB)
 David Clark, MIT, 1992: "We reject: kings,
presidents, and voting. We believe in: rough
consensus and running code.”
1: Introduction
10
A closer look at network structure:
 network edge:
applications and
hosts
 network core:
 routers

network of
networks
 access networks,
physical media:
communication links
1: Introduction
11
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
1: Introduction
12
Network edge: connection-oriented service
Goal: data transfer
between end sys.
 handshaking: setup
(prepare for) data
transfer ahead of time


Hello, hello back human
protocol
set up “state” in two
communicating hosts
 TCP - Transmission
Control Protocol

Internet’s connectionoriented service
TCP service [RFC 793]
 reliable, in-order byte-
stream data transfer

loss: acknowledgements
and retransmissions
 flow control:
 sender won’t overwhelm
receiver
 congestion control:
 senders “slow down sending
rate” when network
congested
1: Introduction
13
Network edge: connectionless service
Goal: data transfer
between end systems

same as before!
 UDP - User Datagram
Protocol [RFC 768]:
Internet’s
connectionless service
 unreliable data
transfer
 no flow control
 no congestion control
App’s using TCP:
 HTTP (WWW), FTP
(file transfer), Telnet
(remote login), SMTP
(email)
App’s using UDP:
 streaming media,
teleconferencing,
Internet telephony
1: Introduction
14
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”
1: Introduction
15
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
1: Introduction
16
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
17
Circuit Switching
 Three phases
1.
2.
3.
circuit establishment
data transfer
circuit termination
 If circuit not available: “Busy signal”
 Examples
 Telephone networks
 ISDN (Integrated Services Digital Networks)
1: Introduction
18
Circuit Switching
 A node (switch) in a circuit switching network
incoming links
Node
outgoing links
1: Introduction
19
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
 transmit over link
 wait turn at next
link
1: Introduction
20
Network Core: Packet Switching
10 Mbs
Ethernet
A
B
statistical multiplexing
C
1.5 Mbs
queue of packets
waiting for output
link
D
45 Mbs
E
Packet-switching versus circuit switching: human
restaurant analogy
 other human analogies?
1: Introduction
21
Packet Switching
 Data are sent as formatted bit-sequences, so-
called packets.
 Packets have the following structure:
Header
Data
Trailer
• Header and Trailer carry control information (e.g.,
destination address, check sum)
 Each packet is passed through the network from
node to node along some path (Routing)
 At each node the entire packet is received, stored
briefly, and then forwarded to the next node
(Store-and-Forward Networks)
 Typically no capacity is allocated for packets
1: Introduction
22
Packet Switching
 A node in a packet switching network
incoming links
Node
outgoing links
Memory
1: Introduction
23
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
 circuit-switching:
 10 users
N users
1 Mbps link
 packet switching:
 with 35 users,
probability > 10 active
less that .004
1: Introduction
24
Packet switching versus circuit switching
Is packet switching a “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

1: Introduction
25
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
1: Introduction
26
Packet Switching
A
Source
B
R2
R1
R3
Destination
R4





It’s the method used by the Internet.
Each packet is individually routed packet-by-packet, using
the router’s local routing table.
The routers maintain no per-flow state.
Different packets may take different paths.
Several packets may arrive for the same output link at the
same time, therefore a packet switch has buffers.
1: Introduction
27
Why does the Internet usepacket switching?
1.
Efficient use of expensive links:



2.
The links are assumed to be expensive and scarce.
Packet switching allows many, bursty flows to share the
same link efficiently.
“Circuit switching is rarely used for data networks, ...
because of very inefficient use of the links” - Gallager
Resilience to failure of links & routers:

”For high reliability, ... [the Internet] was to be a datagram
subnet, so if some lines and [routers] were destroyed,
messages could be ... rerouted” - Tanenbaum
Source: Networking 101
1: Introduction
28
Packet Switching
A
B
R2
Source
R1
Destination
R3
R4
Host A
TRANSP1
TRANSP2
R1
“Store-and-Forward” at each
Router
PROP1
R2
TRANSP3
PROP2
TRANSP4
R3
Host B
PROP3
PROP4
Minimum end to end latency   (TRANSPi  PROPi )
i
1: Introduction
29
Packet Switching
Why not send the entire message in one packet?
M/R
M/R
Host A
Host A
R1
R1
R2
R2
R3
R3
Host B
Latency   ( PROPi  M / Ri )
i
Host B
Latency  M / Rmin   PROPi
i
Breaking message into packets allows parallel transmission across
all links, reducing end to end latency. It also prevents a link from
being “hogged” for a long time by one message.
1: Introduction
30
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
31
Residential access: point to point access
 Dialup via modem
 up
to 56Kbps direct access to
router (conceptually)
 ISDN: intergrated services
digital network: 128Kbps alldigital connect to router
 ADSL: asymmetric digital
subscriber line
 up to 1 Mbps home-to-router
 up to 8 Mbps router-to-home
1: Introduction
32
Residential access: cable modems
 HFC: hybrid fiber coax
 asymmetric: up to 10Mbps
upstream, 1 Mbps
downstream
 network of cable and fiber
attaches homes to ISP
router

shared access to router
among home
 deployment: available via
cable companies, e.g.,
MediaOne
1: Introduction
33
Institutional access: local area networks
 company/univ local area
network (LAN) connects
end system to edge router
 Ethernet:
 shared or dedicated
cable connects end
system and router
 10 Mbs, 100Mbps,
Gigabit Ethernet
 deployment: institutions,
home LANs soon
1: Introduction
34
Wireless access networks
 shared wireless access
network connects end
system to router
 wireless LANs:


radio spectrum replaces
wire
e.g., Lucent Wavelan 10
Mbps
router
base
station
 wider-area wireless
access

CDPD: wireless access to
ISP router via cellular
network
mobile
hosts
1: Introduction
35
Physical Media
 physical link:
transmitted data bit
propagates across link
 guided media:

signals propagate in
solid media: copper,
fiber
 unguided media:
 signals propagate
freelye.g., radio
Twisted Pair (TP)
 two insulated copper
wires


Category 3: traditional
phone wires, 10 Mbps
ethernet
Category 5 TP:
100Mbps ethernet
1: Introduction
36
Physical Media: coax, fiber
Coaxial cable:
 wire (signal carrier)
within a wire (shield)


baseband: single channel
on cable
broadband: multiple
channel on cable
 bidirectional
 common use in 10Mbs
Fiber optic cable:
 glass fiber carrying
light pulses
 high-speed operation:


100Mbps Ethernet
high-speed point-to-point
transmission (e.g., 5 Gps)
 low error rate
Ethernet
1: Introduction
37
Physical media: radio
 signal carried in
electromagnetic
spectrum
 no physical “wire”
 bidirectional
 propagation
environment effects:



reflection
obstruction by objects
interference
Radio link types:
 microwave
 e.g. up to 45 Mbps channels
 LAN (e.g., waveLAN)
 2Mbps, 11Mbps
 wide-area (e.g., cellular)
 e.g. CDPD, 10’s Kbps
 satellite
 up to 50Mbps channel (or
multiple smaller channels)
 270 Msec end-end delay
 geosynchronous versus
LEOS
1: Introduction
38
Delay in packet-switched networks
packets experience delay
on end-to-end path
 four sources of delay
at each hop
transmission
A
 nodal processing:
 check bit errors
 determine output link
 queueing
 time waiting at output
link for transmission
 depends on congestion
level of router
propagation
B
nodal
processing
queueing
1: Introduction
39
Delay in packet-switched networks
Transmission delay:
 R=link bandwidth (bps)
 L=packet length (bits)
 time to send bits into
link = L/R
transmission
A
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 quantitites!
propagation
B
nodal
processing
queueing
1: Introduction
40
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!
1: Introduction
41
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?
1: Introduction
42
Organization of air travel
ticket (purchase)
ticket (complain)
baggage (check)
baggage (claim)
gates (load)
gates (unload)
runway takeoff
runway landing
airplane routing
airplane routing
airplane routing
 a series of steps
1: Introduction
43
Organization of air travel: a different view
ticket (purchase)
ticket (complain)
baggage (check)
baggage (claim)
gates (load)
gates (unload)
runway takeoff
runway landing
airplane routing
airplane routing
airplane routing
Layers: each layer implements a service
 via its own internal-layer actions
 relying on services provided by layer below
1: Introduction
44
Layered air travel: services
Counter-to-counter delivery of person+bags
baggage-claim-to-baggage-claim delivery
people transfer: loading gate to arrival gate
runway-to-runway delivery of plane
airplane routing from source to destination
1: Introduction
45
ticket (purchase)
ticket (complain)
baggage (check)
baggage (claim)
gates (load)
gates (unload)
runway takeoff
runway landing
airplane routing
airplane routing
arriving airport
Departing airport
Distributed implementation of layer functionality
intermediate air traffic sites
airplane routing
airplane routing
airplane routing
1: Introduction
46
Why layering?
Dealing with complex systems:
 explicit structure allows identification,
relationship of complex system’s pieces
 layered reference model for discussion
 modularization eases maintenance, updating of
system
 change of implementation of layer’s service
transparent to rest of system
 e.g., change in gate procedure doesn’t affect
rest of system
 layering considered harmful?
1: Introduction
47
An Example: No Layering
Application
Transmission
Media
Telnet
coaxial
cable
FTP
fiber
optic
HTTP
packet
radio
 No layering: each new application has to be
re-implemented for every network
technology!
1: Introduction
48
An Example: Benefit of Layering
 Solution: introduce an intermediate layer that
provides a common abstraction for various
network technologies
Application
Telnet
HTTP
FTP
Transport
& Network
Transmission
Media
coaxial
cable
fiber
optic
packet
radio
1: Introduction
49
ISO OSI Reference Model
 Seven layers
 lower
three layers are hop-by-hop
 next four layers are end-to-end
Application
Presentation
Session
Transport
Network
Datalink
Physical
Network
Datalink
Physical
Physical medium
Application
Presentation
Session
Transport
Network
Datalink
Physical
1: Introduction
50
Internet Layering and OSI Layering
 OSI: conceptually define: service, interface,
protocol
 Internet: provide a successful implementation
Application
Presentation
Session
Transport
Network
Datalink
Physical
Application
Transport
Internet
Host-tonetwork
Telnet
FTP DNS
TCP
UDP
IP
LAN
Packet
radio
1: Introduction
51
Internet protocol stack
 application: supporting network
applications

ftp, smtp, http
 transport: host-host data transfer
 tcp, udp
 network: routing of datagrams from
source to destination

ip, routing protocols
 link: data transfer between
neighboring network elements

ppp, ethernet
application
transport
network
link
physical
 physical: bits “on the wire”
1: Introduction
52
Layering: logical communication
Each layer:
 distributed
 “entities”
implement
layer functions
at each node
 entities
perform
actions,
exchange
messages with
peers
application
transport
network
link
physical
application
transport
network
link
physical
network
link
physical
application
transport
network
link
physical
application
transport
network
link
physical
1: Introduction
53
Layering: logical communication
E.g.: transport
 take data from app
 add addressing,
reliability check
info to form
“datagram”
 send datagram to
peer
 wait for peer to
ack receipt
 analogy: post
office
data
application
transport
transport
network
link
physical
application
transport
network
link
physical
ack
data
network
link
physical
application
transport
network
link
physical
data
application
transport
transport
network
link
physical
1: Introduction
54
Layering: physical communication
data
application
transport
network
link
physical
application
transport
network
link
physical
network
link
physical
application
transport
network
link
physical
data
application
transport
network
link
physical
1: Introduction
55
Protocol layering and data
Each layer takes data from above
 adds header information to create new data unit
 passes new data unit to layer below
source
M
Ht M
Hn Ht M
Hl Hn Ht M
application
transport
network
link
physical
destination
application
Ht
transport
Hn Ht
network
Hl Hn Ht
link
physical
M
message
M
segment
M
M
datagram
frame
1: Introduction
56
Internet structure: network of networks
 roughly hierarchical
 national/international
local
ISP
backbone providers (NBPs)


e.g. BBN/GTE, Sprint,
AT&T, IBM, UUNet
interconnect (peer) with
each other privately, or at
public Network Access Point
(NAPs)
 regional ISPs
 connect into NBPs
 local ISP, company
 connect into regional ISPs
regional ISP
NBP B
NAP
NAP
NBP A
regional ISP
local
ISP
1: Introduction
57
National Backbone Provider
e.g. BBN/GTE US backbone network
1: Introduction
58
Internet History
1961-1972: Early packet-switching principles
 1961: Kleinrock - queueing
theory shows
effectiveness of packetswitching
 1964: Baran - packetswitching in military nets
 1967: ARPAnet conceived
by Advanced Reearch
Projects Agency
 1969: first ARPAnet node
operational
 1972:




ARPAnet demonstrated
publicly
NCP (Network Control
Protocol) first hosthost protocol
first e-mail program
ARPAnet has 15 nodes
1: Introduction
59
A Brief History of the Internet
 1969

ARPANET commissioned: 4 nodes, 50kbps
1: Introduction
60
Initial Expansion of the ARPANET
Dec. 1969
July 1970
Apr. 1972
March 1971
Sep. 1972
1: Introduction
61
Internet History
1972-1980: Internetworking, new and proprietary nets
 1970: ALOHAnet satellite





network in Hawaii
1973: Metcalfe’s PhD thesis
proposes Ethernet
1974: Cerf and Kahn architecture for
interconnecting networks
late70’s: proprietary
architectures: DECnet, SNA,
XNA
late 70’s: switching fixed
length packets (ATM
precursor)
1979: ARPAnet has 200 nodes
Cerf and Kahn’s
internetworking principles:
 minimalism, autonomy no internal changes
required to
interconnect networks
 best effort service
model
 stateless routers
 decentralized control
define today’s Internet
architecture
1: Introduction
62
Internet History
1980-1990: new protocols, a proliferation of networks
 1983: deployment of




TCP/IP
1982: smtp e-mail
protocol defined
1983: DNS defined
for name-to-IPaddress translation
1985: ftp protocol
defined
1988: TCP congestion
control
 new national networks:
Csnet, BITnet,
NSFnet, Minitel
 100,000 hosts
connected to
confederation of
networks
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63
Internet History
1990’s: commercialization, the WWW
 Early 1990’s: ARPAnet
decomissioned
 1991: NSF lifts restrictions
on commercial use of NSFnet
(decommissioned, 1995)
 early 1990s: WWW
 hypertext [Bush 1945,
Nelson 1960’s]
 HTML, http: Berners-Lee
 1994: Mosaic, later
Netscape
 late 1990’s:
commercialization of the
Late 1990’s:
 est. 50 million
computers on Internet
 est. 100 million+ users
 backbone links runnning
at 1 Gbps
WWW
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64
Internet主干结构
1: Introduction
65
1: Introduction
66
租用至亚洲的
的线路
美国主干
地区网
租用横穿大西
洋的线路
隧道
欧洲主干
国家网
1
IP令牌总线局域网
2
IP令牌环局域网
IP以太局域网
1: Introduction
67
Internet Physical Infrastructure
Local/Regional
ISP
 Residential
Access



Modem
DSL
Cable modem
 Campus
network access



Ethernet
FDDI
Wireless
 Access to ISP,
Backbone
transmission


Local/Regional
ISP
Backbone:
National ISP
 Internet Service
T1/T3, OC-3, OC-12
ATM, SONET, WDM
Providers

Point of Presence
(POP)
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68
Growth of the Internet
in Terms of Number of Hosts
 Number of Hosts on
the Internet:
Aug. 1981
213
Oct. 1984
1,024
Dec. 1987
28,174
Oct. 1990
313,000
Oct. 1993 2,056,000
Apr. 1995 5,706,000
Jul. 1997 19,540,000
Jul. 2000 93,047,000
Jul. 2001 125,888,000
1,000,000,000
100,000,000
10,000,000
1,000,000
100,000
10,000
1,000
100
10
1
1981 1984 1987 1990 1993 1996 1999
1: Introduction
69
Chapter 1: Summary
Covered a “ton” of
material!
 Internet overview
 what’s a protocol?
 network edge, core,





access network
performance: loss, delay
layering and service
models
backbones, NAPs, ISPs
history
ATM network
You now hopefully have:
 context, overview,
“feel” of networking
 more depth, detail
later in course
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70