5. Clients
Applications that run on computers
Rely on servers for
◦ Files
Clients are Applications
◦ Devices
◦ Processing power
Example: E-mail client
◦ An application that enables you to send and
receive e-mail
6. Servers
Computers or processes that manage
network resources
◦ Disk drives (file servers) Servers Manage
◦ Printers (print servers) Resources
◦ Network traffic (network servers)
Example: Database Server
◦ A computer system that processes database
queries
8. Client–Server Computing
Process takes place
◦ on the server and
◦ on the client Client-Server
Servers Computing Optimizes
Computing Resources
◦ Store and protect data
◦ Process requests from clients
Clients
◦ Make requests
◦ Format data on the desktop
9. Application Functions
Software application
functions are separated into
three distinct parts
Server:
Data Management
Client: Presentation & Application Logic
10. Application Components
3 Data Management 2 Client Types
2 Application Logic
Fat
Thin Client
1 Presentation Client
3 Logical Tiers
Database Applications:
Most common use of client-server architectures
11. Middleware
Software that connects two
otherwise separate applications
Example: Middleware product linking Database Server:
a database system to a Web server Manages Data
Middleware Links
Applications
Web Server:
Presents Dynamic Pages
Client: Requests Data via Web
12. Types of Servers
From A to Z
Application Servers
List Servers
Audio/Video Servers
Mail Servers
Chat Servers
News Servers
Fax Servers
Proxy Servers
FTP Servers
Telnet Servers
Groupware Servers
Web Servers
IRC Servers
Z39.50 Servers
Source: http://webopedia.lycos.com
13. ADVANTAGES OF CLIENT-
SERVER
Advantages often cited include:
◦ Centralization - access, resources, and data
security are controlled through the server
◦ Scalability - any element can be upgraded when
needed
◦ Flexibility - new technology can be easily
integrated into the system
◦ Interoperability - all components (clients,
network, servers) work together
14. DISADVANTAGES OF CLIENT-
SERVER
Disadvantages often cited include:
◦ Dependability - when the server goes down,
operations cease
◦ Higher than anticipated costs
◦ Can cause network congestion
15. CLIENT-SERVER
ARCHITECTURES
There are basically two types of client-server
architectures
◦ Two tier architectures
◦ Three tier architectures
The choice between the two should be
made based on combination of:
◦ Schedule for project implementation
◦ Expected system changes and enhancements
16. TWO-TIER ARCHITECTURES
Application components are
distributed between the
Server server and client software
In addition to part of the
application software, the
server also stores the data,
Network and all data accesses are
through the server.
The presentation (to the user)
PC PC PC
is handled strictly by the client
Clients software.
17. TWO-TIER ARCHITECTURES
(cont.)
The PC clients assume the bulk of the
responsibility for the application logic.
The server assumes the bulk of the
responsibility for data integrity checks, query
capabilities, data extraction and most of the
data intensive tasks, including sending the
appropriate data to the appropriate clients.
18. TWO-TIER ARCHITECTURES,
ADVANTAGES
The commonly cited advantages of two-tier
systems include:
◦ Fast application development time
◦ Available tools are robust and lend themselves to
fast prototyping to insure user needs a met
accurately and completely.
◦ Conducive to environments with homogeneous
clients, homogeneous applications, and static
business rules.
19. TWO-TIER ARCHITECTURES,
DISADVANTAGES
The commonly cited disadvantages of two-
tier systems include:
◦ Not suitable for dispersed, heterogeneous
environments with rapidly changing business
rules.
◦ Because the bulk of the application logic is on the
client, there is the problem of client software
version control and new version redistribution.
◦ Security can be complicated because a user may
require separate passwords for each SQL server
accessed.
20. THREE-TIER ARCHITECTURES
3-tier architectures attempt
to overcome some of the
Server Server limitations of the 2-tier
architecture by separating
presentation, processing, and
data into 3 separate and
Network distinct entities.
The software in the client
PC PC PC
handles the presentation (to
the user) using similar tools
Clients as in the 2-tier architecture.
21. THREE-TIER ARCHITECTURES
(cont.)
When data or processing are required by
the presentation client, a call is made to the
middle-tier functionality server.
This tier performs calculations, does reports,
and makes any needed client calls to other
servers (e.g.. a data base server).
22. THREE-TIER ARCHITECTURES
(cont.)
Middle tier servers are usually coded in a
highly portable, non-proprietary language
such as C or C++.
Middle tier servers may be multithreaded
and can be accessed by multiple clients.
The calling mechanism from client to server
and from server to server is by means of
RPC’s.
23. THREE-TIER ARCHITECTURES,
ADVANTAGES (cont.)
Commonly cited advantages include:
◦ Having separate functionality servers allows for
the parallel development of individual tiers by
application specialists.
◦ Provides for more flexible resource allocation.
Can reduce network traffic by having the
functionality servers strip data to the precise
structure needed before sending it to the clients.
24. THREE-TIER ARCHITECTURES,
DISADVANTAGES
Often cited disadvantages of 3-tier
architectures include:
◦ Creates an increased need for network traffic
management, server load balancing, and fault
tolerance.
◦ Current tools are relatively immature and are
more complex.
◦ Maintenance tools are currently inadequate for
maintaining server libraries. This is a potential
obstacle for simplifying maintenance and
promoting code reuse throughout the
organization.
26. What is Peer-to-Peer?
A model of communication where every
node in the network acts alike.
As opposed to the Client-Server model,
where one node provides services and other
nodes use the services.
27. Advantages of P2P Computing
No central point of failure
◦ E.g., the Internet and the Web do not have a central point
of failure.
◦ Most internet and web services use the client-server
model (e.g. HTTP), so a specific service does have a
central point of failure.
Scalability
◦ Since every peer is alike, it is possible to add more peers
to the system and scale to larger networks.
28. Disadvantages of P2P Computing
Decentralized coordination
◦ How to keep global state consistent?
◦ Need for distributed coherency protocols.
All nodes are not created equal.
◦ Computing power, bandwidth have an impact on
overall performance.
Programmability
◦ As a corollary of decentralized coordination.
39. Modes of connection
Circuit-switched
◦ dedicated path
◦ guaranteed (fixed) bandwidth
◦ [almost] constant latency
Packet-switched
◦ shared connection
◦ data is broken into chunks called packets
◦ each packet contains destination address
◦ available bandwidth ≤ channel capacity
◦ variable latency
40. What’s in the data?
For effective communication
◦ same language, same conventions
For computers:
◦ electrical encoding of data
◦ where is the start of the packet?
◦ which bits contain the length?
◦ is there a checksum? where is it?
how is it computed?
◦ what is the format of an address?
◦ byte ordering
42. Protocols
Exist at different levels
understand format of humans vs. whales
address and how to different wavelengths
compute checksum
versus
request web page French vs. Hungarian
43. Layering
To ease software development and maximize
flexibility:
◦ Network protocols are generally organized in
layers
◦ Replace one layer without replacing surrounding
layers
◦ Higher-level software does not have to know
how to format an Ethernet packet
… or even know that Ethernet is being used
44. Layering
Most popular model of guiding
(not specifying) protocol layers is
OSI reference model
Adopted and created by ISO
7 layers of protocols
45. OSI Reference Model: Layer 1
Transmits and receives
raw data to
communication medium.
Does not care about
contents.
voltage levels, speed,
connectors
1 Physical
Physical
Examples: RS-232, 10BaseT
46. OSI Reference Model: Layer 2
Detects and corrects errors.
Organizes data into packets
before passing it down.
Sequences packets (if
necessary).
Accepts acknowledgements
from receiver.
2 Data Link
Data Link
1 Physical
Physical
Examples: Ethernet MAC, PPP
47. OSI Reference Model: Layer 3
Relay and route
information to
destination.
Manage journey of packets
and figure out
intermediate hops (if
3 Network
Network
needed).
2 Data Link
Data Link
1 Physical
Physical
Examples: IP, X.25
48. OSI Reference Model: Layer 4
Provides a consistent
interface for end-to-end
(application-to-
application)
communication. Manages
4 Transport
Transport flow control.
3 Network
Network Network interface is
similar to a mailbox.
2 Data Link
Data Link
1 Physical
Physical
Examples: TCP, UDP
49. OSI Reference Model: Layer 5
Services to coordinate
dialogue and manage data
exchange.
5 Session
Session Software implemented
Transport switch.
4 Transport
Manage multiple logical
3 Network
Network connections.
2 Data Link
Data Link Keep track of who is
talking: establish & end
1 Physical
Physical Examples: HTTP 1.1, SSL,
communications.
NetBIOS
50. OSI Reference Model: Layer 6
Data representation
6 Presentation
Presentation
Concerned with the
5 Session
Session meaning of data bits
4 Transport
Transport Convert between
3 Network
Network machine
representations
2 Data Link
Data Link
1 Physical
Physical Examples: XDR, ASN.1,
MIME, MIDI
51. OSI Reference Model: Layer 7
7 Application
Application Collection of application-
Presentation specific protocols
6 Presentation
5 Session
Session
4 Transport
Transport
3 Network
Network
2 Data Link
Data Link Examples:
email (SMTP, POP, IMAP)
1 Physical
Physical file transfer (FTP)
directory services (LDAP)
53. Local Area Network (LAN)
Communications network
◦ small area (building, set of buildings)
◦ same, sometimes shared, transmission medium
◦ high data rate (often): 1 Mbps – 1 Gbps
◦ Low latency
◦ devices are peers
any device can initiate a data transfer with any other
device
Most elements on a LAN are workstations
◦ endpoints on a LAN are called nodes
55. Connecting nodes to LANs
network computer
Adapter
◦ expansion slot (PCI, PC Card, USB dongle)
◦ usually integrated onto main board
Network adapters are referred to as
Network Interface Cards (NICs) or
adapters
56. Media
Wires (or RF, IR) connecting together the devices
that make up a LAN
Twisted pair
◦ Most common:
STP: shielded twisted pair
UTP: unshielded twisted pair
(e.g. Telephone cable, Ethernet 10BaseT)
Coaxial cable
◦ Thin (similar to TV cable)
◦ Thick (e.g., 10Base5, ThickNet)
Fiber
Wireless
57. Hubs, routers, bridges
Hub
◦ Device that acts as a central point for LAN cables
◦ Take incoming data from one port & send to all other ports
Switch
◦ Moves data from input to output port.
◦ Analyzes packet to determine destination port and makes a virtual
connection between the ports.
Concentrator or repeater
◦ Regenerates data passing through it
Bridge
◦ Connects two LANs or two segments of a LAN
◦ Connection at data link layer (layer 2)
Router
◦ Determines the next network point to which a packet should be
forwarded
◦ Connects different types of local and wide area networks at
network layer (layer 3)
63. Clients and Servers
Send messages to applications
◦ not just machines
Client must get data to the desired process
◦ server process must get data back to client process
To offer a service, a server must get a
transport address for a particular service
◦ well-defined location
65. Transport provider
Layer of software that accepts a network
message and sends it to a remote machine
Two categories:
connection-oriented protocols
connectionless protocols
67. Connection-oriented Protocols
analogous to phone call
1. establish connection dial phone number
2. [negotiate protocol] [decide on a language]
3. exchange data speak
4. terminate connectionhang up
virtual circuit service
◦ provides illusion of having a dedicated circuit
◦ messages guaranteed to arrive in-order
◦ application does not have to address each message
vs. circuit-switched service
69. Connectionless Protocols
analogous to mailbox
- no call setup
- send/receive data drop letter in mailbox
(each packet addressed) (each letter addressed)
- no termination
datagram service
◦ client is not positive whether message arrived at
destination
◦ no state has to be maintained at client or server
◦ cheaper but less reliable than virtual circuit
service
70. Ethernet
Layers 1 & 2 of OSI model
◦ Physical (1)
Cables: 10Base-T, 100Base-T, 1000Base-T, etc.
◦ Data Link (2)
Ethernet bridging
Data frame parsing
Data frame transmission
Error detection
Unreliable, connectionless communication
71. Ethernet
48-byte ethernet address
Variable-length packet
◦ 1518-byte MTU
18-byte header, 1500 bytes data
Jumbo packets for Gigabit ethernet
◦ 9000-byte MTU
dest addr src addr
frame
data (payload) CRC
type
6 bytes 6 bytes 2 46-1500 bytes 4
18 bytes + data
72. IP – Internet Protocol
Born in 1969 as a research network of 4 machines
Funded by DoD’s ARPA
Goal:
build an efficient fault-tolerant network
that could connect heterogeneous
machines and link separately connected
networks.
73. Internet Protocol
Connectionless protocol designed to handle the
interconnection of a large number of local and
wide-area networks that comprise the internet
IP can route from one physical network to
another
74. IP Addressing
Each machine on an IP network is assigned a
unique 32-bit number for each network
interface:
◦ IP address, not machine address
A machine connected to several physical
networks will have several IP addresses
◦ One for each network
75. IP Address space
32-bit addresses → >4 billion addresses!
Routers would need a table of 4 billion
entries
Design routing tables so one entry can
match multiple addresses
◦ hierarchy: addresses physically close will share
a common prefix
76. IP Addressing: networks & hosts
cs.rutgers.edu remus.rutgers.edu
128.6.4.2 128.6.13.3
80 06 04 02 80 06 0D 03
network # host #
first 16 bits identify Rutgers
external routers need only one entry
◦ route 128.6.*.* to Rutgers
77. IP Addressing: networks & hosts
IP address
◦ network #: identifies network machine
belongs to
◦ host #: identifies host on the network
use network number to route packet to
correct network
use host number to identify specific
machine
78. IP Addressing
Expectation:
◦ a few big networks and many small ones
◦ create different classes of networks
◦
class use leading bits to identifynet #
leading bits bits for network bits for host
A 0 7 (128) 24 (16M)
B 10 14 (16K) 16 (64K)
C 110 21 (2M) 8 (256)
To allow additional networks within an organization:
use high bits of host number for a
“network within a network” – subnet
80. Running out of addresses
Huge growth
Wasteful allocation of networks
◦ Lots of unused addresses
Every machine connected to the internet
needed a worldwide-unique IP address
Solutions: CIDR, NAT, IPv6
81. IP Special Addresses
All bits 0
◦ Valid only as source address
◦ “all addresses for this machine”
◦ Not valid over network
All host bits 1
◦ Valid only as destination
◦ Broadcast to network
All bits 1
◦ Broadcast to all directly connected networks
Leading bits 1110
◦ Class D network
127.0.0.0: reserved for local traffic
◦ 127.0.0.1 usually assigned to loopback device
82. IPv6 vs. IPv4
IPv4
◦ 4 byte (32 bit) addresses
IPv6:
◦ 16-byte (128 bit) addresses
3.6 x 1038 possible addresses
8 x 1028 times more addresses than IPv4
◦ 4-bit priority field
◦ Flow label (24-bits)
83. Network Address Translation
(NAT)
External IP address
24.225.217.243
Internal
IP address
192.168.1.x .1 .2 .3 .4 .5
84. Getting to the machine
IP is a logical network on top of multiple
physical networks
OS support for IP: IP driver
receive data send data
IP driver
IP driver
receive packet send packet
network driver
network driver
from wire to wire
85. IP driver responsibilities
Get operating parameters from device
driver
◦ Maximum packet size (MTU)
◦ Functions to initialize HW headers
◦ Length of HW header
Routing packets
◦ From one physical network to another
Fragmenting packets
Send operations from higher-layers
Receiving data from device driver
Dropping bad/expired data
86. Device driver responsibilities
Controls network interface card
◦ Comparable to character driver
top half bottom half
Processes interrupts from network
interface
◦ Receive packets
◦ Send them to IP driver
Get packets from IP driver
◦ Send them to hardware
◦ Ensure packet goes out without collision
87. Network device
Network card examines packets on wire
◦ Compares destination addresses
Before packet is sent, it must be enveloped
for the physical network
device
IP header IP data
header
payload
88. Device addressing
IP address → ethernet address
Address Resolution Protocol (ARP)
1. Check local ARP cache
2. Send broadcast message requesting ethernet
address of machine with certain IP address
3. Wait for response (with timeout)
89. Transport-layer protocols over IP
IP sends packets to machine
◦ No mechanism for identifying sending or
receiving application
Transport layer uses a port number to
identify the application
TCP – Transmission Control Protocol
UDP – User Datagram Protocol
90. TCP – Transmission Control
Protocol
Virtual circuit service
(connection-oriented)
Send acknowledgement for each received
packet
Checksum to validate data
Data may be transmitted simultaneously in
both directions
91. UDP – User Datagram Protocol
Datagram service (connectionless)
Data may be lost
Data may arrive out of sequence
Checksum for data but no retransmission
◦ Bad packets dropped
92. IP header
device TCP/UDP
IP header IP data
header header
payload
vers hlen svc type (TOS) total length
fragment identification flags fragment offset
20 bytes
TTL protocol header checksum
source IP address
destination IP address
options and pad
93. Headers: TCP & UDP
device TCP/UDP
IP header IP data
header header
payload
TCP header UDP header
src port dest port src port dest port
seq number seg length checksum
20 bytes
ack number
hdr 8 bytes
- flags window
len
checksum urgent ptr
options and pad
94. Device header (Ethernet II)
device TCP/UDP
IP header IP data
header header
payload
frame
dest addr src addr data CRC
type
6 bytes 6 bytes 2 46-1500 bytes 4
18 bytes + data
95. Quality of Service Problems in IP
Too much traffic
◦ Congestion
Inefficient packet transmission
◦ 59 bytes to send 1 byte in TCP/IP!
◦ 20 bytes TCP + 20 bytes IP + 18 bytes ethernet
Unreliable delivery
◦ Software to the rescue – TCP/IP
Unpredictable packet delivery
97. Sockets
IP lets us send data between machines
TCP & UDP are transport layer protocols
◦ Contain port number to identify transport
endpoint (application)
One popular abstraction for transport layer
connectivity: sockets
◦ Developed at Berkeley
98. Sockets
Attempt at generalized IPC model
Goals:
◦ communication between processes should not
depend on whether they are on the same
machine
◦ efficiency
◦ compatibility
◦ support different protocols and naming
conventions
99. Socket
Abstract object from which messages are sent
and received
◦ Looks like a file descriptor
◦ Application can select particular style of
communication
Virtual circuit, datagram, message-based, in-order
delivery
◦ Unrelated processes should be able to locate
communication endpoints
Sockets should be named
Name meaningful in the communications domain
101. Step 1
Create a socket
int s = socket(domain, type, protocol)
AF_INET SOCK_STREAM useful if some
SOCK_DGRAM families have
more than one
protocol to
support a given
service
102. Step 2
Name the socket (assign address, port)
int error = bind(s, addr, addrlen)
socket Address structure length of
struct sockaddr* address
structure
103. Step 3a (server)
Set socket to be able to accept connections
int error = listen(s, backlog)
socket queue length for
pending connections
104. Step 3b (server)
Wait for a connection from client
int snew = accept(s, clntaddr, &clntalen)
socket pointer to address length of
structure address
new socket structure
for this session
105. Step 3 (client)
Connect to server
int error = connect(s, svraddr, svraddrlen)
socket Address structure length of
struct sockaddr* address
structure
107. Step 5
Close connection
shutdown(s, how)
how:
0: can send but not receive
1: cannot send more data
2: cannot send or receive (=0+1)
108. Sockets in Java
java.net package
Two major classes:
◦ Socket: client-side
◦ ServerSocket: server-side
109. Step 1a (server)
Create socket and name it
ServerSocket svc =
new ServerSocket(port)
110. Step 1b (server)
Wait for connection from client
Server req = svc.accept()
new socket for client session
111. Step 1 (client)
Create socket and name it
Socket s = new Socket(address, port);
obtained from:
getLocalHost, getByName,
or getAllByName
Socket s =
new Socket(“cs.rutgers.edu”, 2211);
112. Step 2
Exchange data
obtain InputStream/OutputStream from
Socket object
BufferedReader in =
new BufferedReader(
new InputStreamReader(
s.getInputStream()));
PrintStream out =
new PrintStream(s.getOutputStream());