Transcript Ch1. Computer Networks and Internet

Ch 1. Computer Networks and
the Internet
Myungchul Kim
[email protected]
What is the Internet?
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One sentence definition?
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Hosts or end systems
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Internet home appliances
Pervasive computing
Ubiquitous computing
Communication links
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Software and hardware
Services
Bandwidth: bits/second
Routers
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Packet
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Route or path
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Internet Service Providers (ISP)
Protocols
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Specifies the format of the packets that are sent and received among
routers and end systems
TCP (Transmission Control Protocol)
IP (Internet Protocol)
Intranet
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End system -> links and routers -> end system
Packet switching: sharing a path
Private networks
Internet Standards
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IETF (Internet Engineering Task Force): www.ietf.org
RFCs (Request for Comments)
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A service description
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distributed applications: remote login, electronic mail, Web surfing, instant
messaging, audio and video streaming, Internet telephony, distributed games,
peer-to-peer (P2P) file sharing,…
Connection-oriented reliable services and a connectionless unreliable service
Protocols
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Figure 1.2.
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Definition of a Protocol
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Defines the format and the order of messages exchanged between two or more
communicating entities, as well as the actions taken on the transmission and/or
receipt of a message or other event
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The Network Edge
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Host = end system: clients and servers
Peer-to-peer: acts as both a client and a server
Transport layer protocols
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Connection-oriented service: TCP
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Handshaking protocol -> connection
End-to-end
Reliable data transfer, flow control and congestion control
Reliable data transfer
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Connectionless service
Connection-oriented service
Acknowledgement
Flow control
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Force the sending end system to reduce its rate whenever there is a risk
keeping the sending pace. With buffers in the end systems.
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Congestion control
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Congestion
Buffer overflow of routers -> packet loss
Force end systems to decrease the rate at which they send packets into the
network during periods of congestion.
Connectionless Service: UDP
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No handshaking
No reliable data transfer
Internet phone and video conferencing
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The Network Core
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Circuit switching
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Packet switching
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Reserved for the communication session
A circuit: at the guaranteed constant rate
Telephone network
The network resources on demand
Internet
Best effort
Multiplexing in Circuit-switched networks
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The dedicated circuits are idle during silent periods
Frequency-division multiplexing (FDM) or Time-division
multiplexing (TDM)
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Fig 1.6.
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Packet switching
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Message -> packets
Routers = packet switches
Store-and-forward transmission -> delay
Output queue -> delay
Packet loss
Fig 1.7
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Packet switching vs Circuit switching
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Real-time services
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Sharing of network resources
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Implementation
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Message switching
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Message switching: sequential transmission
Packet switching: parallel transmission (pipelining)
Error handling?
Header overhead?
Figure 1.8 and Figure 1.9
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Figure 1.10
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Figure 1.11
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Packet forwarding
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Packet-switched networks: datagram networks and virtual circuit networks
Datagram network: forwards packets according to host destination addresses
Virtual circuit networks: forwards packets according to virtual circuit numbers
Virtual circuit networks
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Virtual circuit: a path + virtual circuit numbers + entries in VC-number translation
tables
Fig 1.12
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Maintain state information
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Datagram networks
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Similar to the postal service
Do not maintain connection-state information in their switches.
Network taxonomy
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Fig 1.13
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Networks with VCs are always connection-oriented.
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Network Access
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Residential access
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Company access
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Dial-up modem
Digital subscriber line (DSL): point-to-point
Hybrid fiber coaxial cable (HFC): shared
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LAN
Ethernet
Mobile access
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Wireless LAN
IEEE 802.11b, Wi-Fi
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ISPs and Internet Backbones
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Tier-1 ISPs
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Internet Backbone
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Tier-2 ISPs
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Fig 1.17
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Delay and loss in Packet-switched
networks
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Fig 1.18
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Processing delay
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Examine the packet’s header and determine where to direct the packet
Check for bit-level errors
Microseconds or less
Queuing delay
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A packet waits to be transmitted onto the link
Depends on the number of earlier-arriving packets that are queued and
waiting for transmission across the link.
Microseconds to milliseconds.
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Transmission delay
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Propagation delay
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Store-and-forward delay
Transmit all of the packet’s bits into the link
L/R where L bits = length of the packet, R = 10 Mbps for a 10 Mbps Ethernet link
Microseconds to milliseconds
Propagation speed of the link
d/s where d = distance and s = the propagation speed of the link
Milliseconds
Comparing transmission and propagation delay
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d nodal = d proc + d queue + d trans + d prop
d prop : hundreds of milliseconds for two routers by a satellite link
d trans : hundreds of milliseconds for low-speed dial-up modem links
d proc : at the max rate of a router
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Queuing delay
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Traffic intensity La/R where a = the average rate of packets arrival at
the queue (packets/sec), L bits of a packet, R = the transmission rate
(bits/sec), and the infinite queue.
If La/R > 1, the queue will tend to increase without bound and the
queuing delay will approach infinity.
If La/R ≤ 1, the nature of the arriving traffic impacts the queuing delay.
Periodically or in bursts or random
Fig 1.19
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Packet loss
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A queue has finite capacity.
Performance of a node = delay + packet loss
End-to-end delay
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d end-end = N (d proc + d trans + d prop) for N-1 routers
where the network is uncongested.
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Traceroute
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Repeats experiment three times
eniac.seas.upenn.edu -> diane.ibp.fr (pp. 48, 49)
Queuing delay
Transatlantic link
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Protocol layers and their service models
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A layered architecture allows us to discuss a well-defined,
specific part of a large and complex system.
N-PDUs
Protocol stack
Service model
– Layer n-1 is said to offer services to layer n
Fig 1.22
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Layer functions
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Error control
Flow control
Segmentation and reassembly
Multiplexing
Connection setup
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Duplicate lower-layer functionality
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The internet protocol stack
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Fig 1.23
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Application layer: HTTP, SMTP, FTP
Transport layer: TCP, UDP
Network layer: IP, routing
Link layer: Ethernet, PPP
Physical layer
Fig 1.24
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