1. HUYNH Cong Phap, PhD
hcphap@gmail.com, hcphap@dut.udn.vn
Network programming
Lesson 1

2. Client – Server Computing

3. Client Server Architecture
 A network architecture
in which each
computer or process
on the network is
either a client or a
server.

4. Components
 Clients
 Servers
 Communication Networks
Server
Client

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

7. Communication Networks
Networks Connect
Clients and
Servers

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.

25. Peer to Peer Computing

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.

29. P2P Computing Applications
 File sharing
 Process sharing
 Collaborative environments

30. P2P File Sharing Applications
 Improves data availability
 Replication to compensate for failures.
 E.g., Napster, Gnutella, Freenet, KaZaA
(FastTrack), your DFS project.

31. P2P Process Sharing Applications
 For large-scale computations
 Data analysis, data mining, scientific
computing
 E.g., SETI@Home, Folding@Home,
distributed.net, World-Wide Computer

32. P2P Collaborative Applications
 For remote real-time human collaboration.
 Instant messaging, virtual meetings, shared
whiteboards, teleconferencing, tele-presence.
 E.g., talk, IRC, ICQ, AOL Messenger, Yahoo!
Messenger, Jabber, MS Netmeeting, NCSA
Habanero, Games

33. Types of P2P
 Pure P2P
 Hybrid P2P

34. Napster

35. Gnutella
But introduces aanew single point of failure!
But introduces new single point of failure!

36. KaZaA/Morpheus

37. Distributed systems
Data Networking &
Client-Server Communication

38. Distributed systems
Independent machines work cooperatively
without shared memory
 They have to talk somehow
 Interconnect is the network

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

41. Protocols
These instructions and conventions
are known as protocols

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)

52. Some networking
terminology

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

54. Connecting nodes to LANs
network computer
?

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)

58. Networking Topology
Bus Network

59. Networking Topology
Tree Network

60. Networking Topology
Star Network

61. Networking Topology
Ring Network

62. Networking Topology
Mesh Network

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

64. Machine address
versus
Transport address

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

66. Connection-oriented Protocols
1. establish connection
2. [negotiate protocol]
3. exchange data
4. terminate connection

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

68. Connectionless Protocols
- no call setup
- send/receive data
(each packet addressed)
- no termination

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

79. IP Addressing
IBM: 9.0.0.0 – 9.255.255.255
00001001 xxxxxxxx xxxxxxxx xxxxxxxxx
network # host #
8 bits 24 bits
Subnet within IBM (internal routers only)
00001001 10101010 11 xxxxxx xxxxxxxxx
network # host #
18 bits 14 bits

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

96. Programming Interfaces

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

100. Programming with
sockets

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

106. Step 4
Exchange data
Connection-oriented
read/write
recv/send (extra flags)
Connectionless
sendto, sendmsg
recvfrom, recvmsg

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());

113. Step 3
Terminate connection
close streams, close socket
in.close();
out.close();
s.close();

114. The end.