Transcript Introduction to computer communication networks

Introduction to Network
Computing
Bill Chu
The World-Wide-Web
revolution
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Virtually all the general purpose computers are connected in the
past five years.
Killer applications
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Visical
World Wide Web
Email
E-commerce / On-line trading
Napster
E-Business
The Internet has changed the way we
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Understand computing
Use/write applications
Deliver application
Maintain applications
Sample network-based
applications
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Hotmail
Yahoo
Amazon.com
Tax return
Games
E-bay
Cracking of DES
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The network has
become the
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computer
storage
library
market
Introduction to computer
communication networks
Bei-Tseng (Bill) Chu
Protocol
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A protocol is a series of steps, involving two or more
parties, designed to accomplish a task
Everyone involved in the protocol must know the
protocol and all of the steps to follow in advance
Everyone in the protocol must agree to follow it
The protocol must be unambiguous; each step must
be well defined and there must be no chance of a
misunderstanding.
The protocol must be complete; there must be a
specified action for every possible situation.
Communication Protocol Model
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A template to describe a protocol
It has three components
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Address: naming
Format: messages
Behavior: rules
Protocol Model
Address:
- how to name a partner
Format:
- specify the message formats
Rules:
- specify the behaviors of the
protocol
- what should be done when
something happens
Protocol Example: Phone
Phone conversation Protocol
Address:
phone number xxx-xxxx
Format:
English
Rules:
- dial when initiating
- pick up phone when
ringing
- ...
Can a single protocol do it all?
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Computer communication has to deal all sorts of problems
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Electrical/Optical signals/noise
Errror detection and recovery
medium control access
message boundary
routing, fragmentation
flow control (net congestion)
loss and duplicated messages
synchronization
representation
application specific
Yes. It can be done but
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how to develop in timely fashion (debug, verify and low cost)
how to maintain it
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how to extend and evolve it
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Layered Approach
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Divide and conquer
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partition into multiple layers of software
each layer has clear programming interfaces
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each layer solves a limited set of problems
each layer encapsulates the related details
Pros
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each interface provides a service to adjacent layers
reduce complexity, isolate changes, promote manageability
Cons
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efficiency
ISO Reference Model
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Layer Number
Layer Name
Problems to Be Solved
7
6
5
4
3
2
1
Application layer
Presentation layer
Session layer
Transport layer
Network layer
Data link layer
Physical layer
application specific (Lab #2)
data representation (XDR)
synchronization & dialog (client/server)
reliable delivery of messages (sockets)
routing & fragmentation
medium access control & framing
signaling, physical connections
Not every layer is created equal
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Some functions occurs in
multiple layers:
+ Error handling may be in
every layer
+ Flow control can be in
multiple layers
physical and data link are hardware heavy
network and transport are software heavy
session and presentation are typically light layers
ISO vs. TCP/IP
7.Application
Application
6.Presentation
5.Session
4.Transport
Transport
3.Network
Internet
2.Data Link
Host to
network
1. Physical
Physical layer protocols
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Highly physical network technology dependent
Main tasks
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define the signaling protocol
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what is the meaning of 1s or 0s
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what is bad signals
define the physical connections required
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voltages or frequencies
RS232 connectors for RS232 serial line communication
RJ45 or BNC connectors for Ethernet
define the communication media
define the network topology
Physical Network Technologies
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Circuit-switched network (CS)
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connection-oriented network
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establish connection before communication
once communication established, a circuit line is reserved for
the communicating partners
example: telephone network
Packet-switched network (PS)
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store-forward based network
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packet sent from a node to another node
the intermediate node stores the packet and decides to forward
to another node towards the destination
no circuit line is reserved
example: Ethernet
Circuit connection
Routing table
In
Out
2
4
Host B
in
out
Routing table
Host A
Routing table
In
Out
3
4
2
1
Routing table
In
Out
2
4
In
Out
2
3
2
4
Comparisons: CS and PS
Circuit-switched
line resource
performance
cost
adaptive routing
switch device
reliability
utilization
dedicated
guaranteed
expensive
not easy
highly complex
high
low
Packet-switched
shared
averaged
less
easy
simple
higher
higher
Network Types by Scope
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WAN
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MAN
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wide area network
cross large span of space (continental)
typically heterogeneous and low speed
example: Internet
metro-area network
regional scope (city-wide)
LAN
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local area network
limited scope (a couple of buildings)
typically homogeneous & high speed
example: Ethernet & Token ring
Network Transmission
Medium
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Open air
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Optical
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radio, microwaves, satellites, infrared
noise signals, collision
clear signals, low power and high rate (Gbps)
Copper wire
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Lower cost interfaces
Bi-directional
Bus Network Topology
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Every nodes tap into a common medium
Signals may collide with each other
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need to arbitrate who will get the bus
capable of broadcasting message (one send & many listen)
the common medium is the bottleneck
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single node failure causes no network failure
the medium failure brings down the network
Example: (old, 10BASE2, 10BASE5) Ethernet
common medium
Cable Modem
Star Network Topology
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One node at the center as the master node
Other nodes linked to the master as slaves
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slaves communicate via master
easy to arbitrate among slaves (master decides)
not scalable (the master is the bottleneck)
normally for small networks or that requires predictable
performance
master failure shutdowns the whole net
Example: Ethernet, DSL
slave
Master
slave
slave
slave
slave
Ring Network Topology
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Nodes are arranged in a ring
One node receives from its predecessor & sends to
its successor
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arbitrate who can access the ring
messages forwarded by each node
sender deletes its messages from the ring
the common ring is the single point of failure (complicated
connectors needed)
Mash Network Topology
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Nodes are arranged in grids
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each node can talk to its neighbors directly
non-neighbor nodes needs store-andforward for communication
Hyper Network Topology
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No restrictions on how to link the nodes
Topology can adapt to individual organization needs
slave
Master
slave
slave
slave
slave
Data Link Layer Protocols
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Main tasks
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medium access control
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framing
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arbitrate who can use the transmission medium
define the boundaries of a packet
Physical technology dependent (like physical layer)
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typically implemented in hardware or firmware
when you buy a network card, you get both physical and
data link layer protocols in the card
Introduction to Internet and
TCP/IP
Bei-Tseng (Bill) Chu
Motivations for Internet
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Observations:
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Different physical networks everywhere
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Limited connectivity
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connection between limited hosts
different needs for different technologies
legacy networks
each physical technology has its limits
Different technologies use different “languages”
Need for a uniform virtual network
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universal connectivity
every host speaks the same language independent of physical
networks
Virtual Network over Physical Networks
Internet
Virtual network
Physical network
Internet Protocol (Network layer)
Ethernet
Token Ring
…...
hosts
ATM
Design Goals of Internet
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A virtual (global) network
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independent of physical technologies
independent of locations
universal language
Universal connectivity
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every host is equal no matter of
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Scalable
growth without limits
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its architecture and system origin
its physical network attachment
Robust
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no single point of failure
Problems to be Solved
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Universal naming
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Routing
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how to route IP packages among different local networks
to reach the destination?
Fragmentation
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how to translate the universal name to local name used
by local physical network?
different physical networks use different package sizes.
how this should be handled?
Error handling
Internet Architecture
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Internet: ‘‘the mother of all networks’’
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THE network of (interconnected) networks
Physical networks interconnected via gateways
Gateway(router) is a host glues nets together
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Other nets
attached to multiple nets
forward IP packages between nets
Gateway
Gateway
Token Ring
Ethernet
IP Packet Format
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IP packet consists of header and data portions
IP header
IP data portion
IP Header Format
V HL type
total length
Identification
F frag. offset
TTL
prot
header cksum
Source IP address
Destination IP address
options if any
IP Address
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IP address specifies a connection to a network instead of a host
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Dotted notation
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IP address == NetID + HostID
each byte in an IP address represented as a decimal
bytes are separated with a period (.)
example: 152.15.35.44
Classes
0
8
16
24
class A
0
class B
10
class C
110
class D
1110
Multicast address
class E
11110
reserved for future use
NetID
31
HostID
NetID
HostID
NetID
HostID
How do computers talk to each
other on an ethernet bus
8.2.1.1
00550DA2F5D82
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8.2.1.2
02550JA2F5D82
8.2.1.3
02950JX2F5Y82
Each computer on the internet as a unique IP address.
Each network interface (e.g. ethernet card) has a unique address
8.2.1.1 wants to send a message 8.2.1.3
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It yells “yaahoo, who is 8.1.2.3”
Each computer listens to messages on the bus (collision detection and
resolution) for “yaahoo”
8.2.1.3 replies to 00550DA2F5D82 “I am 02950JX2F5Y82”
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8.2.1.1 sends the message to 8.2.1.3.
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IP Routing
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Principles
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Route packages according to their destination IP net ID
Forward packages hop by hop
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each gateway has the routing knowledge of its nearby neighbors
hosts route packages to gateway and gateway does the rest.
Routing types
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direct routing
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If the destination net ID is the same as the local net ID, no routing to
gateway is needed and send the package via underlying physical
network
indirect routing
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if the destination net ID is different from the local net ID, send the
package to an appropriate gateway.
IP Rules
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Fragmentation rules
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if underlying net size < packet size & not final dest
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if final dest is reached & packets are fragmented
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break packet into small packets and send them
reassemble fragmental packets into original size
Error rules
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if IP header is erroneous, drop/report it
Table-Driven IP Routing
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Each IP host has a IP routing table
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each entry associates a destination net ID with a forwarding
gateway
each route has a performance metrics
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number of hops to reach the destination
Route table for 152.15.36.9
Dest. Net ID
Next hop host
NIF
163.29
152.15.254.2
54
e0
163.29.x.x
ncsu
152.15.36.9
163.29.10.88
152.15.254.254
e0
Default
152.15.35.1
e0
152.15.x.x
uncc
net3
152.15.35.1
Routes in an IP Routing Table
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Next-hop routes
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Host-specific routines
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the destination is an IP net ID: a packages addressed to
any host in the net, send to the associated gateway
the destination is a complete IP address: route all packages
to the specified host via the associated gateway
Default routes
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the catch all routing: all packages not specified by the
above, send the gateway associated with the default route.
the gateway associated with the default route is called
default gateway
Topology of the internet
9.1.1.2
9.1.1.3
9.1.1.1
b
c
net: 9.1.1.x
9.1.1.4
8.2.1.1
8.2.1.2
8.2.1.3
2
3
a
1
d
4
net: 8.2.1.x
hardware addresses
net 3.1.1.x
8.2.1.4
net 1.1.1.x
net 2.1.1.x
Sub-netting
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All hosts in a network must have the
same net work number
As the number of networks grow, so
does the need for net work numbers
Solution: subnet, divide the host name
portion of the IP address into subnet id
and the host
Subnet mask and routing table.
Intranet example: home
networking
Broadband
provider
Internet
Intranet IP
address
PC
Internet IP
Address
PC
PC
Router
Intranet IP
address
Intranet IP
address
Intranet IP
address
PPP protocol (phone dialin)
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Computer (client) dials to a modem.
Computer on the other (ISP server) end
is on the internet.
The ISP server assigns an ip address for
the dialing computer
All messages send from the client are
routed by the isp host to the rest of the
internet.
DHCP protocol (most ethernet)
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Client sends a message (on the local
bus) to a DHCP server requesting an IP
address for the session
DHCP server assigns an ip address
IP Fragmentation
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IP packages are broken to fit underlying physical
network when a package is sent
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locations of fragmentation
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original package sender
gateways that forward the package
IP packages are reassembled at the final destination
gateway
fragment
assemble
net 1
net 2
IP Fragmentation Example
Original IP datagram
IP header
800 bytes
800 bytes
fragmentation
Fragment 1 (offset 0)
IP header
(fragment 1)
800 bytes
Fragment 2 (offset 800)
IP header
(fragment 2)
800 bytes
Fragment 3 (offset 1600)
IP header
(fragment 3)
250 bytes
250 bytes
ISO vs. TCP/IP
7.Application
Application
6.Presentation
5.Session
4.Transport
Transport
3.Network
Internet
2.Data Link
Host to
network
1. Physical
Transport Layer
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Why do we need a transport layer?
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Network layer provides delivery only
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Users want to transport data
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from a host to a host
in a best effort fashion
from application to application
in a reliable delivery
Transport layer fits the gap between user needs and
IP messaging
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provide communication endpoint for applications
deliver messages reliably
Problems to Be Solved in Transportation layer
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Missing packets
Duplicated packets
Out of order packets
Flow control
Synchronization
Reliable Delivery via Unreliable
Networks
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Missing packet
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Duplicate messages
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sequencing packets
Flow control
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sequencing packets
Out of order messages
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acknowledge and timeout
retransmission
wait and stop
window-sliding
Synchronization
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hand shaking
Transport Protocols in TCP/IP
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User Datagram Protocol (UDP)
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provide communication endpoint for applications
best effort delivery of messages (packets)
message boundary is observed
the protocol embedded in Internet Datagram sockets
Transmission Control Protocol (TCP)
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provide communication endpoint for applications
reliable delivery via connection-based communication
no message boundary between packages
the protocol embedded in Internet Stream sockets
Establish a TCP Connection
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Three way handshaking
application 1
TCPpack p;
p.codebit=SYN;
p.seq_no = x; p.winsize=S1;
send (p);
tcp_state=SYNSENT;
receive(p);
application 2
syn+x
syn+ack+y
p.ack = p.seq_no + 1;
p.seq_no = x++;
send(p);
tcp_state=ESTABLISHED
ack
TCPpack pack;
tcp_state=LISEN;
receive(pack);
pack.ack=pack.seq_no+1
pack.seq_no = y; pack.winsize=S2;
send(pack);
tcp_state=ESTABLISHED;
Denial of service attacks
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Exploits the TCP session establishment
protocol.
An attacker will send syn, but never
sends ack. This type of attack is also
called “sync flood”.
Synchronized attacks launched on
multiple (often victim) machines.
TCP Retransmission
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Sender
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is free to divide user stream data in packets
expects an ack for each packet sent
starts a timer when a packet is sent
upon an ack reception, advances seq_no expected
upon a timer expiration, resends the packet
Receiver
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sends an ack whenever a packet is received
deletes the packet if it duplicated
is free to pass acknowledged packets to user
is forced to pass to user the data when receives a PUSH