Transcript Computer Network Basics

Computer
Network Basics
Components of Any Computer
Computer
Processor Memory
(active) (passive)
Control
(“brain”)
(where
programs,
Datapath data live
(“brawn”) when
running)
Devices
Input
Output
Keyboard,
Mouse
Disk,
Network
Display,
Printer
Communication Devices
 Synchronous communication uses a clock
signal separate from the data signalcommunication can only happen during the
‘tick’ of the timing cycle
 Asynchronous communication does not use
a clock signal- rather, it employs a start
and stop bit to begin and end the irregular
transmission of data
Connecting to Networks (and
Other I/O)
 Bus - shared medium of communication that
can connect to many devices
 Hierarchy of Buses in a PC
Operating Systems
Developer or manufacturer
Operating system
Apple Computers Inc.
Mac OS 8/9/X
AT&T Bell Laboratories
Unix
Be Inc.
beOS
Berkeley University
BSD, FreeBSD
Carnegie-Mellon University
Mach 3.0
Cisco Systems Inc.
IOS
HP
HP-UX
IBM
AIX and OS/2
Linus Thorvald
Linux
Microsoft
Windows XP, Vista
Novell
NetWare
Santa Cruz Operation Inc. (SCO) SCO XENIX, SCO UNIX, SCO MPX
Siemens
SINIX
Silicon Graphics
IRIX
Sun Microsystems
Solaris, SunOS, JavaOS
Operating Systems Developed for
Portable Devices
Developer or manufacturer
Operating system
Microsoft
Windows CE
Microsoft
Windows Mobile 6.0
Palm
PalmOS
Symbian
Symbian OS
RIM (Research In Motion Limited)
RIM
A Closer Look at Network Structure
 network edge:
applications and
hosts
 network core:
 routers
 network of
networks
General Architecture of Computer
Networks
External
nodes
(or stations)
Cloud
Internal nodes
(swithing devices)
The Network Core
 mesh of interconnected
routers
 the fundamental
question: how is data
transferred through net?
 circuit switching:
dedicated circuit per
call: telephone net
 packet-switching:
data sent thru net in
discrete “chunks”
Connection of Networks
router or
gateway
networks or subnetworks
node
(host,
station)
Network Topology
a) bus, b) star, c) ring, d) tree structure
a)
b)
c)
d)
Classification of the networks according
to the connection establishing
 Line switched network
 Packet switched network
 Radiating/data disseminating systems
 Point-to-point connected networks
Wired Media
 Telephone line
 Thin Coax
 Thick Coax
 Unshielded Twisted Pair (UTP)
 Shielded Twisted Pair (STP)
 Fibre
(Data) Reliability
 A network service is (data) reliable
if the sender application can rely on
the error-free and ordered delivery
of the data to the destination
 In the Internet the reliability can
obtained mainly by
acknowledgements and
retransmission
 In such a way the losses in the
underlying layers can be retrieved
Flow-control and Congestion
Prevention
 Flow-control: to protect the
receiver against the overload
I.e.: the sender (source) sends more
data than the receiver can process
 it is mainly necessary in link and
transport level

 Congestion prevention: to prevent
the intermediate nodes against the
overload

it is mainly necessary in network level
Overload and Congestion
 Overload: Too many packets occur in a
subnetwork in the same time, which
prevent each other and in such a way
the throughput decreases
 Congestion: the queues in the routers
are too long, the buffers are full.

As a consequence some packages are
dropped if the buffers of the routers are
overloaded
 In extreme case: grid-lock, lock-up
Deadlock
 Deadlock: the most serious situation of the
congestion, the routers wait for each other
 Direct store and forward deadlock: the
buffers of two neighbouring routers are
full with the packets to be sent to the
other router
 Indirect store and forward deadlock: the
deadlock occurred not between two
neighbouring routers but in a subnetwork,
where any of the routers has not free
buffer space for accepting packets
Networking Definitions
 Network: physical connection that allows two computers to
communicate
 Packet: unit of transfer, bits carried over the network
 Network
carries packets from on CPU to another
 Destination gets interrupt when packet arrives
 Protocol: agreement between two parties as to how
information is to be transmitted
 Broadcast Network: Shared Communication Medium
 Delivery: How does a receiver know who packet is for?
 Put
header on front of packet: [ Destination | Packet ]
 Everyone gets packet, discards if not the target
 Arbitration: Act of negotiating use of shared medium
 Point-to-point network: a network in which every physical
wire is connected to only two computers
 Switch: a bridge that transforms a shared-bus
(broadcast) configuration into a point-to-point network
 Router: a device that acts as a junction between two
networks to transfer data packets among them
The Need for a Protocol Architecture
 Procedures to exchange data between
devices can be complex
 High degree of cooperation required
between communicating systems
 destination
addressing, path
 readiness to receive
 file formats, structure of data
 how commands are sent/received and
acknowledged
 etc.
Layered Protocol Architecture
 Modules arranged in a vertical stack
 Each layer in stack:
 Performs related functions
 Relies on lower layer for more primitive
functions
 Provides services to next higher layer
 Communicates with corresponding peer layer of
neighboring system using a protocol
Network Layering
 Layering: building complex services from simpler ones

Each layer provides services needed by higher layers by utilizing services
provided by lower layers
 The physical/link layer is pretty limited
Packets are of limited size (called the “Maximum Transfer Unit or MTU:
often 200-1500 bytes in size)
 Routing is limited to within a physical link (wire) or perhaps through a
switch

 Our goal in the following is to show how to construct a secure, ordered,
message service routed to anywhere:
Physical Reality: Packets
Abstraction: Messages
Limited Size
Arbitrary Size
Unordered (sometimes)
Ordered
Unreliable
Reliable
Machine-to-machine
Process-to-process
Only on local area net
Routed anywhere
Asynchronous
Synchronous
Insecure
Secure
Key Features of a Protocol
 Set of rules or conventions to exchange
blocks of formatted data
 Syntax: data format
 Semantics: control information
(coordination, error handling)
 Timing: speed matching, sequencing
 Actions: what happens when an event
occurs
Operation of Protocols
Host
Host
(n-1). layer
protocol entity
(n-1). layer
protocol entity
n. layer
protocol entity
n. layer
protocol entity
(n+1). layer
protocol entity
(n+1). layer
protocol entity
...
...
Physical connection
(interlayer) protocol
layerprotocol
The OSI Model
 Physical Layer
 (Data) Link Layer
 Network Layer
 Transport Layer
 Session Layer
 Presentation Layer
 Application Layer
Physical Layer
 Transmission of energy onto the
medium
Collection of energy from the medium
 This layer is concerned with the physical
transmission of raw bits
 This bits are transmitted through
mechanical, electrical, and procedural
interfaces which include

• interface card standard
• modem standards
• certain portions of the ISDN and LAN MAN
standards
(Data) Link Layer
 Transmission of frames over one link or network
 Often subdivided into the MAC and LLC
 It receives bits from the physical layer, converting bits
to frames

frame boundaries
 Using protocols (e.g. HDLC), this layer corrects errors
that might have occurred during transmission across a link
 In addition this layer provides an “error-free”
transmission channel to the next layer known as the
network layer: error control


ARQ
duplicates
 Flow control
Network Layer I
 The previous two layers were concerned with getting
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error-free data across a link
The network layer establishes connections between nodes,
routes data packets through the network, and accounts for
them
End-to-end transmission of packets (possibly over multiple
links)
Controls the operation of the subnet
Routing


static
dynamic
 Congestion control
 At this stage, there may be congestion due to many packets waiting
to be routed
 Some packets may be lost during congestion
Network Layer II
 Accounting
 packets
 bytes
 etc.
 Internetworking
 This layer is also concerned with internetworking
where there is ‘talking’ between technologies, such as
the traditional Internet connected to ATM
 segmentation
 addressing
 sequencing
 accounting
 Broadcast subnets: thin network layer
Transport Layer I
 This layer presumes the ability to pass
through a network and provides additional
services to end-users, such as and-to-and
packet reliability
 End-to-end delivery of a complete message
(end-to-end communication path, usually
reliable)
 Isolation from “hardware”
 Multiplexing/demultiplexing
 Divide message into packets
 Reassemble (possibly out of order packets)
into the original message of the distant end
Transport Layer II
 End-to-end flow control
 Acknowledgments
 Types of service
error-free, point-to-point, in sequence,
flow controlled
 no correctness guarantees
 no sequencing

 Establishing/terminating connections
 naming/addressing
 intra-host addressing (process, ports)
Session Layer
 This layer enables users to establish sessions across a
network between machines
 In addition, it offers session management services
 Set up and management of end-to-end conversation
 Establish and terminate sessions

superset of connections
 Assignment of logical ports
 Dialogue control
 Token management
 for critical operations
 Synchronization
 checkpoints/restarts
Presentation Layer
 This layer is concerned with the syntax and semantics of
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messages, code conversions between machines, and other
data conversion services
Some of these services are data compression and data
encryption
Interface between lower layers and application
Formatting
Syntax & semantics of messages
Data encoding (e.g.: ASCII to EBCDIC)
Compression
Encryption/Decryption
Authentication
Application Layer
 This layer provides support for the user's network
applications
 Some application layer services have been standardized,
e.g.:
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

File Transfer and Management (FTAM)
Message Handling Services for electronic mail (X.400)
Directory Services (X.500)
Electronic Data Interchange (EDI)
 Program you’re running,applications
 file transfer, access & management
 e-mail
 virtual terminals
 WWW
The OSI Protocol Stack
Endsystem
Operation
of the
model
Intermediate
Intermediate
Endsystem
Application
layer entity
Application
layer entity
Presentation
layer entity
Presentation
layer entity
Session layer
entity
Session layer
entity
Transport
layer entity
Transport
layer entity
Network
layer entity
Network
layer entity
Network
layer entity
Network
layer entity
Datalink
layer entity
Datalink
layer entity
Datalink
layer entity
Datalink
layer entity
Physical layer
entity
Physical layer
entity
Physical layer
entity
Physical layer
entity
Physical medium
Virtual
transmission
Real data
transmission
Names of the Nodes, Connections and
Data Units
Layer name
Node
Connection
Data unit
Application layer
application
network service
e.g. file (ADU)
Presentation layer
host
session
data structure (PPDU)
Session layer
host
transport connection
message (SPDU)
Transport layer
host
network path
message (TPDU)
Network layer
host, router
line
(data)packet (NPDU)
(Data)link layer
station
(physical) channel
(data)frame (LLC PDU)
Physical layer
switch
physical transmission
medium
bit
Communication among the layers
 Connection
oriented network service
(virtual circuits, eg. ATM)
• Reliable transport service
• Unreliable transport service
 Connectionless
network service
(datagram service, eg. IP)
• Reliable transport service (eg. TCP)
• Unreliable transport service (eg. UDP)
Network Tools
 Repeater: connects network segments
logically to one network
 Hub: multiport repeater
 Bridge: datalink level connection of two
networks
 Switch: multiport bridge
 Router: connects networks that are
compatible in transport level

subnetworks are connected to the interfaces of
the repeater
 Gateway (proxy server): router between
two individual network. The “Way Out”
Physical Layer Devices
 Repeater
 Hub
 “dumb”
 level-1 hub
 multi-port repeater
Data Link Layer Devices
 Bridge
 Cascaded vs. Backbone
 Single
 Multiple
 Switch (switched hub)
Routers
 Provide link between networks
 Accommodate network differences:
 Addressing schemes
 Maximum packet sizes
 Hardware and software interfaces
 Network reliability

Congestion/Traffic Management
Devices of the Network Connection
Application layer
Presentation layer
Session layer
Transport layer
Application layer
Gateway
or
Proxy server
Presentation layer
Session layer
Transport layer
Network layer
Router or Gateway
Network layer
Datalink layer
Bridge or Switch
Datalink layer
Physical layer
Repeater or Hub
Physical layer
Architectural Implementation of the
LANs
 Ethernet
(IEEE 802.3)
 FDDI
 Gigabit
Ethernet
 Token Bus (IEEE 802.4)
 Token Ring (IEEE 802.5)
Characteristics of High-Speed LANs
Data Rate
Transmission Mode
Access Method
Supporting
Standard
Fast Ethernet
Gigabit Ethernet
Fibre Channel
Wireless LAN
100 Mbps
1 Gbps, 10 Gbps
100 Mbps – 3.2
Gbps
1 Mbps – 2 Gbps
UTP,STP, Optical
Fiber
UTP, shielded
cable, optical fiber
Optical fiber,
coaxial cable, STP
2.4 GHz, 5 GHz
Microwave
CSMA/CD
CSMA/CD
Switched
CSMA/CA Polling
IEEE 802.3
IEEE 802.3
Fibre Channel
Association
IEEE 802.11
Wide Area Network Connections
 Solutions for connecting LANs to the
Internet
Ethernet (ring or star topology)
 Managed Leased Line Network (MLLN)
 ATM (Asynchronous Transfer Mode)
 Switched line
 ISDN line

Soft and Hard States
 State: the data collection, which are necessary for
keeping the connection between two protocol entities
 Hard state



If the connection is established once, it is never timed out, even
if it is not in usage
To cancel the connection one of the participants of the connection
must explicitly close it
The history of the state is stored
 Soft state
 To keep the connection the participants must send occasionally
keep-alive messages, since without keep-alive message the state
information is timed out after a certain period
 The state is called as “soft” since in the ordinary operation the
state can change easily
 The history of the state is not stored
Packet switching versus circuit switching
Is packet switching best in every case?
 Great for bursty data
resource sharing
 no call setup (less start-up delay)

 However…
 Packets can experience delays, so not for “real-time”
applications
 excessive congestion leads to packet delay and loss
• protocols (like TCP) are needed for reliable data
transfer, and congestion control
Performance Considerations
 Before continue, need some performance metrics
 Overhead: CPU time to put packet on wire
 Throughput: Maximum number of bytes per second
• Depends on “wire speed”, but also limited by slowest router (routing
delay) or by congestion at routers
 Latency:
time until first bit of packet arrives at receiver
• Raw transfer time + overhead at each routing hop
Router
LW1
LR1
Router
LW2
LR2
 Contributions to Latency
 Wire latency: depends on speed of light on wire
• about 1–1.5 ns/foot
 Router
latency: depends on internals of router
• Could be < 1 ms (for a good router)
Lw3
Delay in packet-switched networks
packets experience delay
on end-to-end path
 four sources of delay
at each hop
transmission
A
 Nodal processing:
 check bit errors
 determine output link
 Queueing:
 time waiting at output
link for transmission
 depends on congestion
level of router
propagation
B
nodal
processing
queueing
Delay in packet-switched networks
Transmission delay:
 R=link bandwidth (bps)
 L=packet length (bits)
 time to send bits into
link = L/R
Propagation delay:
 d = length of physical link
 s = propagation speed in
medium (~2x108 m/sec)
 propagation delay = d/s
transmission
A
propagation
B
nodal
processing
queueing
Queueing delay
 R=link bandwidth (bps)
 L=packet length (bits)
 a=average packet
arrival rate
traffic intensity = La/R
 La/R ~ 0: average queueing delay small
 La/R -> 1: delays become large
 La/R > 1: more “work” arriving than can be
serviced, average delay infinite!
Internet protocol stack
 Application: supporting network
applications

ftp, smtp, http
 Transport: host-host data transfer

tcp, udp
 Network: routing of datagrams from
source to destination

ip, routing protocols
 Network access: data transfer between
neighboring network elements

ppp, ethernet
 Physical: bits “on the wire”
Layering: logical communication
E.g.: transport
 take data from app
 add addressing,
reliability check
info to form
“datagram”
 send datagram to
peer
 wait for peer to
ack receipt
 analogy: post
office
data
application
transport
transport
network
link
physical
application
transport
network
link
physical
ack
data
network
link
physical
application
transport
network
link
physical
data
application
transport
transport
network
link
physical
Layering: physical communication
data
application
transport
network
link
physical
application
transport
network
link
physical
network
link
physical
application
transport
network
link
physical
data
application
transport
network
link
physical
Protocol layering and data
Each layer takes data from above
 adds header information to create new data unit
 passes new data unit to layer below
source
M
Ht M
Hn Ht M
Hl Hn Ht M
application
transport
network
link
physical
destination
application
Ht
transport
Hn Ht
network
Hl Hn Ht
link
physical
M
message
M
segment
M
M
datagram
frame
IP over ATM
 ATM Adaptation
Layer (AAL):
interface to upper
layers


end-system
segmentation/rea
ssembly
 ATM Layer: cell
switching
 Physical
application
TCP/UDP
IP
AAL5
ATM
physical
application
TCP/UDP
IP
AAL5
ATM
physical
ATM
physical
application
TCP/UDP
IP
AAL5
ATM
physical
application
TCP/UDP
IP
AAL5
ATM
physical
The Internet Protocol Stack
Application
Application
Presentation
Sockets
TCP
Session
UDP
IP
Network Access
Transport
Network
Data Link
Physical
Network Protocols
 Protocol: Agreement between two parties as to how
information is to be transmitted
 Example:
system calls are the protocol between the operating
system and application
 Networking examples: many levels
• Physical level: mechanical and electrical network (e.g. how are 0 and 1
represented)
• Link level: packet formats/error control (for instance, the CSMA/CD
protocol)
• Network level: network routing, addressing
• Transport Level: reliable message delivery
 Protocols on today’s Internet:
NFS
Transport
RPC
UDP
Network
Physical/Link
WWW
e-mail
ssh
TCP
IP
Ethernet
ATM
Packet radio
Building a messaging service
 Process to process communication
 Basic routing gets packets from machinemachine
 What we really want is routing from processprocess
• Example: ssh, email, ftp, web browsing
 Several
IP protocols include notion of a “port”, which is
a 16-bit identifiers used in addition to IP addresses
• A communication channel (connection) defined by 5 items:
[source address, source port, dest address, dest port, protocol]
 UDP: The User Datagram Protocol
 UDP layered on top of basic IP (IP Protocol 17)
• Unreliable, unordered, user-to-user communication
IP Header
(20 bytes)
16-bit source port
16-bit UDP length
16-bit destination port
16-bit UDP checksum
UDP Data
Building a messaging service (con’t)
 UDP: The Unreliable Datagram Protocol
 Datagram: an unreliable, unordered, packet sent from
source user  dest user (Call it UDP/IP)
 Important aspect: low overhead!
• Often used for high-bandwidth video streams
• Many uses of UDP considered “anti-social” – none of the “wellbehaved” aspects of (say) TCP/IP
 But we need ordered messages
 Create ordered messages on top of unordered ones
• IP can reorder packets! P0,P1 might arrive as P1,P0
 How
to fix this? Assign sequence numbers to packets
• 0,1,2,3,4…..
• If packets arrive out of order, reorder before delivering to
user application
• For instance, hold onto #3 until #2 arrives, etc.
 Sequence
numbers are specific to particular connection
TCP/IP packet, Ethernet frame
 Application sends message
Ethernet Hdr
 TCP breaks into 64KB
segments, adds 20B header
 IP adds 20B header, sends
to network
 If Ethernet, broken into
1500B frames with headers,
trailers (24B)
 All Headers, trailers have
length field, destination, ...
IP Header
TCP Header
EHIP Data
TCP data
Message
Ethernet Hdr