Transcript Communication Protocols - Computer and Information Science

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: www.isoc.org
• Protocol hierarchy/protocol stack
– layers of protocols
– physical transmission to end application rules and
standards
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Communication Protocols
Physical Layer
Physical layer protocols
• Rules for exchange of binary data across physical
channel (fiber-optic, twisted-pair, wireless, etc.)
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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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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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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
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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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Communication Protocols
Data Link Layer
Logical Link Control protocols
• Rules for detecting and correcting errors
• ARQ algorithm (Automatic Repeat Request)
Receiver:
If no error, return acknowledgement
message (ACK)
Otherwise, return nothing
Sender:
Transmit packet and wait for ACK or
time out
If receive ACK, go on to next packet
Otherwise, repeat on current packet
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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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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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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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Communication Protocols
Network Layer
Addressing
• Host name: human-friendly name for node
• IP address: unique numerical address used
by computer, 141.140.1.5
• 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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Communication Protocols
Network Layer
Routing
• Picking a path through network from source
to destination
• Seek shortest/best path: fastest travel
• Massive network requires efficient pathseeking
• Networks are dynamic: nodes come online
and go offline all the time. Routing must
adapt quickly
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Communication Protocols
Transport Layer
Transport layer protocols
• Application-to-application, reliable packet
delivery
• Port number: unique identifier for
program
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Communication Protocols
Transport Layer
• Application types have standard port
numbers
– Web server: port 80
– Domain Name Service: port 42
– SMTP, sending e-mail: port 25
• TCP (Transport Control Protocol)
– Ensures no errors
– Establishes ordered delivery of packets
– Another version of ARQ algorithm
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direct, quality connection between
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Communication Protocols
Application Layer
Application layer protocols
• Handle formatted data transmitted
between application programs
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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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Communication Protocols
Application Layer
Process: http://hostname/page
• 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
•Invitation
Server
responds with message containing
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page contents, size, and indicates
Network Services and Benefits
Interpersonal Communications
• Electronic mail (e-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
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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
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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)
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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
– E-mail, 1972
– Many networks (e.g., HEPNet, DECNet)
1970s/80s
• Internetworking: standards for
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communication
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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,
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A Brief History of the Internet
and the World Wide Web
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
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expansion
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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
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• Protocols are necessary to standardize
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
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