1. 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)

2. EEC-484/584: Computer Networks
Outline
Protocol layers, reference models
Network standards
Internet history
Application layer
Principles of networked applications
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EEC-484/584: Computer Networks

3. Protocol “Layers” Networks are complex! Question: 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?
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EEC-484/584: Computer Networks

4. Organization of Air Travel
ticket (purchase)
baggage (check)
gates (load)
runway takeoff
airplane routing
ticket (complain)
baggage (claim)
gates (unload)
runway landing
A series of steps
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EEC-484/584: Computer Networks

5. Layering of Airline Functionality
ticket (purchase)
baggage (check)
gates (load)
runway (takeoff)
airplane routing
departure
airport
arrival
intermediate air-traffic
control centers
ticket (complain)
baggage (claim)
gates (unload)
runway (land)
ticket
baggage
gate
takeoff/landing
Layers: each layer implements a service
Via its own internal-layer actions
Relying on services provided by layer below
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EEC-484/584: Computer Networks

6. EEC-484/584: Computer Networks
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
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EEC-484/584: Computer Networks

7. Internet Protocol Stack
Application: supporting network applications
HTTP, DNS, SMTP
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
Physical: bits “on the wire”
Application
Transport
Network
Link
Physical
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EEC-484/584: Computer Networks

8. ISO/OSI Reference Model
Presentation: allow applications to interpret meaning of data, e.g., encryption, compression, machine-specific 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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EEC-484/584: Computer Networks

9. EEC-484/584: Computer Networks
Encapsulation
source
message M
application
transport
network
link
physical
segment Ht M Ht
datagram Ht Hn M Hn
frame Ht Hn Hl M
link
physical
switch
destination
network
link
physical Ht Hn M Ht Hn Hl M M
application
transport
network
link
physical Ht Hn M Ht M Ht Hn M
router Ht Hn Hl M
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10. Network Standardization
Why standard?
Only way to achieve interoperability
Standards also increase the market for products adhering to them
Two kinds of standards
De facto – from the fact (standards that just happened)
De jure – by law (formal, legal standards adopted by authorized organization)
Each vendor/supplier has its own ideas of how things should be done, the only way out is to agree on some network standards
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EEC-484/584: Computer Networks

11. Treaty Organization between Nations
United Nations
ITU - International Telecommunications Union
CCITT/ITU-T – telephone and data communications
Their standards are called recommendations for political reasons
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EEC-484/584: Computer Networks

12. 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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EEC-484/584: Computer Networks

13. EEC-484/584: Computer Networks
IEEE 802 Standards
Standard might not be always successful
The 802 working groups. The important ones are marked with *. The ones marked with  (e.g., ) are hibernating. The one marked with (e.g., 802.8) gave up and disbanded itself.
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EEC-484/584: Computer Networks

14. Internet Standard Body
Internet Society (used to be Internet Architecture Board)
Internet Research Task Force (IRTF)
Concentrate on long term research
Internet Engineering Task Force (IETF)
Deal with short term engineering issues
Standardization process
Proposed standard: request for comments (RFCs)
Draft standard: after >= 4 month test by >= 2 sites
Internet standard: if convinced the idea is sound
3000+ of RFCs in ietf Web site
To advance to the Draft Standard stage, a working implementation must have been rigorously tested by at least two independent sites for at least 4 months. If the IAB is convinced that the idea is sound and the software works, it can declare the RFC to be an Internet Standard.
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EEC-484/584: Computer Networks

15. EEC-484/584: Computer Networks
Internet History
: Early packet-switching principles
1961: Kleinrock - queueing theory shows effectiveness of packet-switching
1964: Baran - packet-switching in military nets
1967: ARPAnet conceived by Advanced Research Projects Agency
1969: first ARPAnet node operational
1972:
ARPAnet public demonstration
NCP (Network Control Protocol) first host-host protocol
first program
ARPAnet has 15 nodes
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EEC-484/584: Computer Networks

16. EEC-484/584: Computer Networks
Internet History
: Internetworking, new and proprietary nets
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
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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EEC-484/584: Computer Networks

17. EEC-484/584: Computer Networks
Internet History
: new protocols, a proliferation of networks
1983: deployment of TCP/IP
1982: SMTP protocol defined
1983: DNS defined for name-to-IP-address 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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EEC-484/584: Computer Networks

18. EEC-484/584: Computer Networks
Internet History
1990, 2000’s: commercialization, the Web, new apps
Early 1990’s: ARPAnet decommissioned
1991: NSF lifts restrictions on commercial use of NSFnet (decommissioned, 1995)
Early 1990s: Web
Hypertext [Bush 1945, Nelson 1960’s]
HTML, HTTP: Berners-Lee
1994: Mosaic, later Netscape
Late 1990’s: commercialization of the Web
Late 1990’s – 2000’s:
More killer apps: instant messaging, P2P file sharing
Network security to forefront
Est. 50 million host, 100 million+ users
Backbone links running at Gbps
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EEC-484/584: Computer Networks

19. EEC-484/584: Computer Networks
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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20. 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
You now have:
Context, overview, “feel” of networking
More depth, detail to follow!
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21. Application Layer Protocols
Principles of networked applications
Client server model
Sockets
Addressing
Protocol
What do we need from transport layer?
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22. Creating a Network Application
Write programs that
run on different end systems and
communicate over a network
No need to write code for devices in subnet
Subnet devices do not run user application code
application on end systems allows for rapid app development, propagation
application
transport
network
data link
physical
e.g., Web: Web server software communicates with browser software
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23. Inter-Process Communications
Process: program running within a host
Processes in different hosts communicate by exchanging messages
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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EEC-484/584: Computer Networks

24. EEC-484/584: Computer Networks
Sockets
process
TCP with
buffers,
variables
socket
host or
server
process
TCP with
buffers,
variables
socket
host or
server
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
Controlled by
app developer
Socket analogous to door
sending process shoves message out door
sending process relies on transport infrastructure on other side of door which brings message to socket at receiving process
Internet
Controlled
by OS
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25. EEC-484/584: Computer Networks
Addressing
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?
Answer: NO, many processes can be running on same host
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EEC-484/584: Computer Networks

26. EEC-484/584: Computer Networks
Addressing
Identifier includes both IP address and port numbers (16-bit) associated with process on host
Example port numbers:
HTTP server: 80
SSH server: 22
To send HTTP request to academic.csuohio.edu Web server:
IP address:
Port number: 80
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27. Application Layer Protocol Defines
Types of messages exchanged
e.g., request, response
Message syntax
what fields in messages & how fields are delineated
Message semantics
meaning of information in fields
Rules for when and how processes send & respond to messages
Public-domain protocols:
defined in RFCs
allows for interoperability
e.g., HTTP, SMTP
Proprietary protocols:
e.g., KaZaA
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28. 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
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
Timing
some apps (e.g., Internet telephony, interactive games) require low delay to be “effective”
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EEC-484/584: Computer Networks

29. EEC-484/584: Computer Networks
Exercise
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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EEC-484/584: Computer Networks