1. Communication Protocols
Protocol: a standard set of rules for communicating
Standards evolve over time
International agreements make Internet possible
Internet Society makes standards and promotes research:
Protocol hierarchy/protocol stack
layers of protocols
physical transmission to end application rules and standards
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3. Communication Protocols Physical Layer
Physical layer protocols
Rules for exchange of binary data across physical channel (fiber-optic, twisted-pair, wireless, etc.)
How to know when a bit is present on the line
How much time the bit will remain on the line
Whether the bit is digital or analog in form
What physical quantities represent 0 and 1
Shape of the connector between computer and transmission line
Create abstract “bit pipe” for higher layers to use
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5. Communication Protocols Data Link Layer
Data Link protocols
Ensure reliable transmission of bits
Error detection and correction: notice failures in transmission and fix them
Framing: determine which bits belong to one message
Two parts:
Layer 2a: Medium Access Control
Layer 2b: Logical Link Control
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6. Communication Protocols Data Link Layer
Medium Access Control protocols
Rules for communicating on shared lines
Ethernet: Contention-based protocol
When node wants to send a message
Listen to the line and wait until it is free
Begin transmitting as soon as it is free
If collision results, wait a random amount of time
Repeat
Advantage: distributed, no master bottleneck
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8. Communication Protocols Data Link Layer
Logical Link Control protocols
Rules for detecting and correcting errors
ARQ algorithm (Automatic Repeat Request)
Sender:
Transmit packet and wait for ACK or time out
If receive ACK, go on to next packet
Otherwise, repeat on current packet
Receiver:
If no error, return acknowledgement message (ACK)
Otherwise, return nothing
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9. Communication Protocols Data Link Layer
Packet contains:
Markers for start and end of packet (SOP and EOP)
Sequence number for packet (e.g., 2 of 5)
Packet data
Error-checking bits (e.g., parity)
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10. Communication Protocols Data Link Layer
Purpose of Data Link layer
Create virtual error-free message pipe
Messages go in one end
Come out the other correct and in the right order
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11. Communication Protocols Network Layer
Network layer protocols
Transmit message across multiple nodes in a network
“Good faith” transmission
Requirements:
Standard for addressing all network nodes
Routing method for finding route from any node to any other node
Internet network layer: IP (Internet Protocol)
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12. Communication Protocols Network Layer
Addressing
Host name: human-friendly name for node
IP address: unique numerical address used by computer,
Domain Name Service (DNS): map host names to IP addresses
Symbolic host name goes to local DNS server
If it has no record, goes to remote servers until one has the host name and retrieves the IP address
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13. Communication Protocols Network Layer
Routing
Picking a path through network from source to destination
Seek shortest/best path: fastest travel
Massive network requires efficient path-seeking
Networks are dynamic: nodes come online and go offline all the time. Routing must adapt quickly
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14. Communication Protocols Transport Layer
Transport layer protocols
Application-to-application, reliable packet delivery
Port number: unique identifier for program
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15. Communication Protocols Transport Layer
Application types have standard port numbers
Web server: port 80
Domain Name Service: port 42
SMTP, sending port 25
TCP (Transport Control Protocol)
Ensures no errors
Establishes ordered delivery of packets
Another version of ARQ algorithm
Virtual direct, quality connection between programs
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17. Communication Protocols Application Layer
Application layer protocols
Handle formatted data transmitted between application programs
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18. Communication Protocols Application Layer
Hypertext Transfer Protocol (HTTP)
Web page/service identified by unique URL (Uniform Resource Locator)
protocol://host name/page
Multiple protocols: http, mailto, news, ftp
Web browser uses TCP to send formatted messages to Web server, and vice versa
TCP uses network layer (IP), data link layer, and physical layer
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19. Communication Protocols Application Layer
Process:
Browser reads protocol, extracts host name (and requests IP address from DNS server)
Sends a connect message to port 80 on that machine
After connection established, sends “Get” message with page information
Server responds with message containing page contents, size, and indicates connection closes at end of message
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21. Network Services and Benefits
Interpersonal Communications
Electronic mail ( )
Send message to be read at recipient’s convenience
Fast, multimedia, broadcast medium
Bulletin board system (BBS)
Public forum for shared communications
Evolved into Internet forums, chat rooms
Instant messaging and texting
Social networking
Online social groups designed for rich interaction
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22. Network Services and Benefits
Resource sharing
Print server serves all computers on a LAN
File server provides storage to all users
Client/server computing
Some nodes provide services, other nodes use those services
Distributed databases and data warehouses
Massive data stored in various sites online
Groupware or wiki
Support collaborative knowledge/data construction
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24. Network Services and Benefits
Electronic commerce (e-commerce)
Early applications
Automatic paycheck deposit
ATMs
Checkout scanners and inventory systems
Current applications
Online stores for everything
Electronic bill payment
Online payment systems (Paypal)
Future?
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25. A Brief History of the Internet and the World Wide Web
“A Brief History of the Internet”, 1997, by some of the founders
Early years
Licklider’s “Galactic Network”, 1962
ARPA-funded ARPANET, 1966
, 1972
Many networks (e.g., HEPNet, DECNet) 1970s/80s
Internetworking: standards for communication
Gateway: device for translating between networks
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27. A Brief History of the Internet and the World Wide Web
Middle years
TCP/IP established standard
Telnet, FTP (File Transfer Protocol)
NSFNet, broadens access (1984)
ARPANET only open to ARPA grant recipients
NSFNet open to universities, government agencies, libraries, museum, schools
Networks begin to connect, late 1980s
ARPANET ceases to be as separate network
NSFNet turns over to private providers, 1995
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29. A Brief History of the Internet and the World Wide Web
High-energy physicist at CERN: Tim Berners-Lee
Wanted user-friendly information and data exchange
Hypertext: documents containing links to other documents
Web protocols made public; rapid expansion
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31. Summary
Computing devices can communicate through various wired and wireless media
Computer networks vary in size and form, including LANs, WANs, WLANs, and WWANs
LANs are configured differently from WANs, and use different communication methods
The Internet is a WAN of WANs
Protocols are necessary to standardize communications across different media and among different computers
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32. Summary
The protocol hierarchy breaks down network communications into different layers of abstraction
physical, data link, network, transport, and application
Protocols like the ARQ algorithm and TCP/IP provide rules for the transfer of information
The Internet has permitted new kinds of connections among people: , e-commerce, resource sharing
The Internet and Web grew from ARPAnet and NSFNet as new network applications developed
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