9/13/2010 Computer Network? The “nuts and bolts” view of the Internet

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9/13/2010 Computer Network? The “nuts and bolts” view of the Internet
9/13/2010
Computer Network?
The “nuts and bolts” view of the Internet
q millions of connected
q “interconnected collection of autonomous
❍
❍
❍
❍
Email, chat, Web “surfing”, streaming media
q
❍
regional ISP
fiber, copper, radio, satellite
links have different
capacities (bandwidth)
routers: forward packets
q packet: piece of a message
q Internet ≠ Web
mobile
local ISP
running network apps
communication links
❍
q
PCs, workstations, servers
PDAs, phones, toasters
workstation
server
hosts or end-systems
computers connected by a communication
technology”
q What is the Internet?
“network of networks”
❍ “collection of networks interconnected by
routers”
❍ “a communication medium used by millions”
router
computing devices called
(basic unit of transfer)
company
network
TDDD36: Fundamentals
TDDD36: Fundamentals
“internetworking”?
Layered Architecture: Why?
q internetwork – interconnection of
q Networks are complex with many pieces
networks – also called an “internet”
q subnetwork – a constituent of an internet
q intermediate system – a device used to
connect two networks allowing hosts of the
networks to correspond with each other
Bridge
❍ Router
❍
❍ Hosts,
routers, links, applications,
protocols, hardware, software
q Can we organize it, somehow?
q Let’s consider a Web page request:
Browser requests Web page from server
Server should determine if access is privileged
❍ Reliable transfer page from server to client
❍ Physical transfer of “bits” from server to client
❍
❍
q Internet is an example of an internetwork.
TDDD36: Fundamentals
Motivation Continued …
TDDD36: Fundamentals
Motivation Continued …
Dealing with complex systems:
Application logic
Application
Services
Communication
Service
Network
Services
Web Server
Application
Services
Reliable delivery
Transfer “bits”
Communication
Service
Network
Services
q explicit structure allows identification, relationship
of complex system’s pieces
❍ layered reference model for discussion
q modularization eases maintenance, updating of system
❍ change of implementation of layer’s service
transparent to rest of system
❍ e.g., change in network technology doesn’t affect
rest of system
q layering considered harmful? (design vs implemention)
Web Client
TDDD36: Fundamentals
TDDD36: Fundamentals
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9/13/2010
Layers, Protocols, Interfaces
Application
Services
Application logic
protocol
Application
Services
Communication
Service
Reliable delivery
protocol
Communication
Service
Network
Services
Transfer “bits”
protocol
Network
Services
Layer
Interface
Layer
Interface
Layered Architecture
q Networks organized as a stack of layers?
❍ The purpose of a layer is to offer services to
the layer above it using a well-defined interface
(programming language analogy: libraries hide
details while providing a service)
❍ Reduces design complexity
q Protocols: “horizontal” conversations at any
Web Server
Web Client
layer n (i.e., between peer layers)
q Data Transfer: each layer passes data &
control information to the layer below;
eventually physical medium is reached.
TDDD36: Fundamentals
Layered Architecture (cont’d)
TDDD36: Fundamentals
Reference Models
q A set of layers & protocols is called a
Network Architecture. These
specifications enable hardware/software
developers to build systems compliant with
a particular architecture.
❍
E.g., TCP/IP, OSI
TDDD36: Fundamentals
TDDD36: Fundamentals
The Application Layer
The Transport Layer
q Residence of network applications and
q Concerned with end-to-end data transfer
their application control logic
q Examples include:
q Transmission unit is called segment
between end systems (hosts)
HTTP (Hyper-Text Transfer Protocol)
FTP (File Transfer Protocol)
❍ Telnet
❍ SMTP (Simple Mail Transfer Protocol)
❍ DNS (Domain Name Service)
❍
❍
TDDD36: Fundamentals
q TCP/IP networks such as the Internet
provides two types of services to
applications
“connection-oriented” service – Transmission
Control Protocol (TCP)
❍ “connectionless” service - User Datagram
Protocol (UDP)
❍
TDDD36: Fundamentals
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9/13/2010
TCP: Connection-oriented Service
UDP: Connectionless Service
q Handshaking between client & server
q No handshaking
programs
q Send whenever and however you want
Parameters for ensuing exchange
❍ Maintain connection-state
❍
q Packet switches do not maintain any
connection-state; state is at end systems
❍
q A “best effort” service
❍ No reliability
❍ No congestion & flow control services
q Useful for network applications that
hence “connection-oriented”
q Similar to a phone conversation
q TCP is bundled with reliability, congestion
control, and flow control.
prefer quick delivery of most packets
rather than guaranteed (slow) delivery of
all packets (e.g., VOIP, video streaming)
TDDD36: Fundamentals
Internet protocol stack
Protocol layering and data
q application: supporting network
applications
❍
FTP, SMTP, STTP
application
q transport: host-host data transfer
❍ TCP, UDP
transport
q network: routing of datagrams from
network
source to destination
❍
IP, routing protocols
q link: data transfer between
neighboring network elements
❍
TDDD36: Fundamentals
link
physical
PPP, Ethernet
Each layer takes data from above
q adds header information to create new data unit
q passes new data unit to layer below
source
M
Ht M
Hn Ht M
Hl Hn Ht M
destination
application
transport
network
link
physical
application
Ht
transport
Hn Ht
network
Hl Hn Ht
link
physical
M
message
M
segment
M
M
datagram
frame
q physical: bits “on the wire”
TDDD36: Fundamentals
Wireless, mobility: impact on higher layer protocols
q logically, impact
router
TDDD36: Fundamentals
Something for next class
Question to think/read about ...
should be minimal …
best effort service model remains unchanged
TCP and UDP can (and do) run over wireless, mobile
q … but performance-wise:
❍ packet loss/delay due to bit-errors (discarded
packets, delays for link-layer retransmissions), and
handoffs from mobility and transient connectivity
❍ TCP interprets loss as congestion, will decrease
congestion window un-necessarily
❍ delay impairments for real-time traffic
❍ limited bandwidth of wireless links
❍
❍
TDDD36: Fundamentals
q Which technology is the fastest?
❍ 10 Mbps Ethernet
❍ 11 Mbps 802.11
TDDD36: Fundamentals
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