Transcript EEC 484/584 - Academic Server

EEC-484/584
Computer Networks
Lecture 3
Wenbing Zhao
(Part of the slides are based on Drs. Kurose & Ross’s slides
for their Computer Networking book)
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Outline
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Protocol layers, reference models
Network standards
Internet history
Application layer
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Principles of networked applications
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Protocol “Layers”
Networks are complex!
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many “pieces”:
 hosts
 routers
 links of various media
 applications
 protocols
 hardware, software
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Question:
Is there any hope of
organizing structure of
network?
Or at least our discussion of
networks?
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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
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A series of steps
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Layering of Airline Functionality
ticket (purchase)
ticket (complain)
ticket
baggage (check)
baggage (claim)
baggage
gates (load)
gates (unload)
gate
runway (takeoff)
runway (land)
takeoff/landing
airplane routing
airplane routing
airplane routing
departure
airport
airplane routing
airplane routing
intermediate air-traffic
control centers
arrival
airport
Layers: each layer implements a service
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Via its own internal-layer actions
Relying on services provided by layer below
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Why Layering?
Dealing with complex systems:
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Explicit structure allows identification, relationship
of complex system’s pieces
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Layered reference model for discussion
Modularization eases maintenance, updating of
system
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Change of implementation of layer’s service
transparent to rest of system
E.g., change in gate procedure doesn’t affect rest of
system
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Internet Protocol Stack
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Application: supporting network applications
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Transport: process-process data transfer
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IP, routing protocols
Link: data transfer between neighboring network
elements
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Transport
TCP, UDP
Network: routing of datagrams from source to
destination
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Application
HTTP, DNS, SMTP
PPP, Ethernet
Network
Link
Physical
Physical: bits “on the wire”
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ISO/OSI Reference Model
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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
Application
Presentation
Session
Transport
Network
Link
Physical
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source
message
segment
M
Ht
M
datagram Hn Ht
M
frame Hl Hn Ht
M
Encapsulation
application
transport
network
link
physical
link
physical
switch
destination
M
Ht
M
Hn Ht
Hl Hn Ht
M
M
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application
transport
network
link
physical
Hn Ht
Hl Hn Ht
M
M
network
link
physical
Hn Ht
M
router
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Network Standardization
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Why standard?
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Only way to achieve interoperability
Standards also increase the market for products
adhering to them
Two kinds of standards
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De facto – from the fact (standards that just happened)
De jure – by law (formal, legal standards adopted by
authorized organization)
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Treaty Organization between Nations
United Nations
ITU - International Telecommunications Union
CCITT/ITU-T – telephone and data
communications
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Voluntary, Nontreaty Organization
ISO (International Standards Organization)
issues standards on wide range of topics
200 TC (Technical Committees)
TC97 – computers and info processing
SC (Subcommittees)
WG (Working Groups)
ANSI (American National Standards Institute)
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IEEE 802 Standards
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Internet Standard Body
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Internet Society (used to be Internet
Architecture Board)
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Internet Research Task Force (IRTF)
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Internet Engineering Task Force (IETF)
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Concentrate on long term research
Deal with short term engineering issues
Standardization process
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Proposed standard: request for comments (RFCs)
Draft standard: after >= 4 month test by >= 2 sites
Internet standard: if convinced the idea is sound
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Internet History
1961-1972: Early packet-switching principles
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1961: Kleinrock - queueing
theory shows effectiveness
of packet-switching
1964: Baran - packetswitching in military nets
1967: ARPAnet conceived
by Advanced Research
Projects Agency
1969: first ARPAnet node
operational
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1972:
 ARPAnet public demonstration
 NCP (Network Control Protocol)
first host-host protocol
 first e-mail program
 ARPAnet has 15 nodes
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Internet History
1972-1980: Internetworking, new and proprietary nets
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1970: ALOHAnet satellite
network in Hawaii
1974: Cerf and Kahn architecture for interconnecting
networks
1976: Ethernet at Xerox PARC
late70’s: proprietary
architectures: DECnet, SNA,
XNA
late 70’s: switching fixed length
packets (ATM precursor)
1979: ARPAnet has 200 nodes
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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
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Internet History
1980-1990: new protocols, a proliferation of networks
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1983: deployment of
TCP/IP
1982: SMTP e-mail
protocol defined
1983: DNS defined for
name-to-IP-address
translation
1985: FTP protocol
defined
1988: TCP congestion
control
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New national networks:
Csnet, BITnet, NSFnet,
Minitel
100,000 hosts connected to
confederation of networks
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Internet History
1990, 2000’s: commercialization, the Web, new apps
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Early 1990’s: ARPAnet
Late 1990’s – 2000’s:
decommissioned
 More killer apps: instant
1991: NSF lifts restrictions on
messaging, P2P file sharing
commercial use of NSFnet
 Network security to forefront
(decommissioned, 1995)
 Est. 50 million host, 100
Early 1990s: Web
million+ users
 Hypertext [Bush 1945, Nelson
 Backbone links running at
1960’s]
Gbps
 HTML, HTTP: Berners-Lee
 1994: Mosaic, later Netscape
 Late 1990’s: commercialization
of the Web
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Internet History
2007:
 ~500 million hosts
 Voice, Video over IP
 P2P applications: BitTorrent (file sharing), Skype (VoIP),
PPLive (video)
 More applications: youtube, gaming
 Wireless, mobility
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Introduction: Summary
Covered a “ton” of material!
 Internet overview
 What’s a protocol?
 Network edge, core, access
network
 Packet-switching versus
circuit-switching
 Internet structure
 Performance: loss, delay,
throughput
 Layering, reference models
 Networking standards
 History
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You now have:
 Context, overview, “feel”
of networking
 More depth, detail to
follow!
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Application Layer Protocols
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Principles of networked applications
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Client server model
Sockets
Addressing
Protocol
What do we need from transport layer?
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Wenbing Zhao
Creating a Network Application
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Write programs that
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run on different end
systems and
communicate over a
network
application
transport
network
data link
physical
No need to write code
for devices in subnet
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Subnet devices do not run
user application code
application on end systems
allows for rapid app
development, propagation
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application
transport
network
data link
physical
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application
transport
network
data link
physical
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Wenbing Zhao
Inter-Process Communications
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Process: program
running within a host
Processes in different
hosts communicate by
exchanging messages
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Client process: process
that initiates
communication
Server process:
process that waits to be
contacted
More accurately, client and server should be regarded
as the roles played by a process. A process can be
both a client and a server
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Wenbing Zhao
Sockets
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Process sends/receives
messages to/from its socket
For each point-to-point
connection, there are two
sockets, one on each side
API (Application
Programming Interface): (1)
choice of transport protocol;
(2) ability to fix a few
parameters
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host or
server
host or
server
Controlled by
app developer
process
process
socket
socket
TCP with
buffers,
variables
Internet
TCP with
buffers,
variables
Controlled
by OS
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Wenbing Zhao
Addressing
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To receive messages, a process must have an
identifier
Each host device has a unique 32-bit IP address
Question: Does the IP address of the host on which
the process runs suffice for identifying the process?
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Wenbing Zhao
Addressing
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Identifier includes both IP address and port
numbers (16-bit) associated with process on host
Example port numbers:
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HTTP server: 80
SSH server: 22
To send HTTP request to academic.csuohio.edu
Web server:
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IP address: 137.148.49.46
Port number: 80
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Wenbing Zhao
Application Layer Protocol Defines
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Types of messages exchanged
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Message syntax
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what fields in messages & how
fields are delineated
Message semantics
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e.g., request, response
meaning of information in fields
Public-domain protocols:
 defined in RFCs
 allows for interoperability
 e.g., HTTP, SMTP
Proprietary protocols:
 e.g., KaZaA
Rules for when and how processes
send & respond to messages
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Wenbing Zhao
What Transport Service Does an Application
Need?
Data loss
 some apps (e.g., audio) can
tolerate some loss
 other apps (e.g., file transfer,
telnet) require 100% reliable
data transfer
Timing
 some apps (e.g., Internet
telephony, interactive
games) require low delay
to be “effective”
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Bandwidth
• some apps (e.g.,
multimedia) require
minimum amount of
bandwidth to be
“effective”
• other apps (“elastic
apps”) make use of
whatever bandwidth
they get
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Wenbing Zhao
Homework#1 problem1
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A system has an n-layer protocol hierarchy.
Applications generate messages of length M
bytes. At each of the layers, an h-byte header
is added. What fraction of the network
bandwidth is filled with headers?
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