Networking principles involve the transport layer and application layer. The transport layer handles basic communication and reliability, while the application layer provides abstractions like files and services. The network transfers bits and operates at the application's request, while applications determine what to send, when to send it, where to send it, and the meaning of the bits. All network applications use a client-server paradigm, where a server waits passively for contact from an active client. A client initiates communication with one server at a time, while a server can handle multiple clients simultaneously. Concurrency allows servers to handle multiple requests without clients waiting.

1. Networking Principles
IT1 Course Slide
Instructor:
Majid Taghiloo

2. Functionality
• Transport layer and layers below
– B i communication
Basic i ti
– Reliability
• Application layer
– Abstractions
• Files
• Services
• Databases
– Names
2

3. Dichotomy of Duties
• Network
– Transfers bits
– Operates at application s request
application’s
• Applications determine
– What to send
– When to send
– Where to send
– Meaning of bits
3

4. Important Point
Although an internet system provides a
, p
basic communication service, the protocol
software cannot initiate contact with, or
p f , p
accept contact from, a remote computer.
Instead, two application programs must
participate in any communication: one
participate in any communication: one
application initiates communication and
the other accepts it.
the other accepts it.
4

5. How Two Application Programs Make
Contact
• One application
– Begins execution first
g
– Waits passively at prearranged location
• A th application
Another li ti
– Begins execution later
– Actively contacts first program
• Called client‐server interaction
5

6. Client‐Server
Client Server Paradigm
• Used by all network applications
• Passive program called a server
• Active program called a client
6

7. Internet Communication
All network applications use a form of
communication known as the client‐server
paradigm. A server application waits
p
passively for contact, while a client
yf ,
application initiates communication
actively.
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8. Characteristics of a Client
• Arbitrary application program
• Becomes client temporarily
• Can also perform other computations
• Invoked directly by user
• Runs locally on user’s computer
user s
• Actively initiates contact with a server
• Contacts one server at a time
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9. Characteristics of a Server
• Special‐purpose, privileged program
• Dedicated to providing one service
• Can handle multiple remote clients simultaneously
• Invoked automatically when system boots
y y
• Executes forever
• Needs powerful computer and operating system
• Waits passively for client contact
• Accepts requests from arbitrary clients
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10. Terminology
• Server
– An executing program that accepts contact over
gp g p
the network
• Server class computer
Server‐class
– Hardware sufficient to execute a server
• Informally
– Term server often applied to computer
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11. Direction of Data Flow
• Data can fl
flow
– From client to server only
– From server to client only
– In both directions
• Application protocol determines flow
• Typical scenario
– Client sends request(s)
– Sever sends response(s)
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12. Key Idea
Although the client initiates contact,
information can flow in either or both
information can flow in either or both
directions between a client and server.
Many services arrange for the client to send
one or more requests and the server to
return one response for each request.
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13. Clients, Servers,
Clients Servers and Other Protocols
Clients and servers are application programs •
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14. Server CPU Use
• Facts
– Server operates like other applications
• Uses CPU to execute instructions
• Performs I/O operations
– Waiting for data to arrive over a network does not
require CPU time
• Consequence
– Server program only uses CPU when servicing a
p g y g
request
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15. Multiple Services
• Can have multiple servers on single computer
• Servers only use processor when handling a
request
• P
Powerful h d
f l hardware required to h dl many
i d handle
services simultaneously
15

16. Illustration of Multiple Servers
• Each server offers one service
• One server can handle multiple clients
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17. Identifying a Service
• Protocol port number used
l b d
• Each service given unique p number, P
g q port
• Server
– Informs OS it is using port P
– Waits for requests to arrive
• Cli
Client
– Forms request
– Send request to port P on server computer
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18. The Point About Ports
Transport protocols assign each service a
unique port identifier. A server must specify
unique port identifier A server must specify
the identifier when it begins execution. A
client must specify the identifier when it
client must specify the identifier when it
requests transport protocol software to
contact a server. Protocol software on the
server computer uses the identifier to direct
an incoming request to the correct server.
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19. In Theory
• Port numbers are merely integers
• Any server could use any port number
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20. In Practice
• P t l port numbers used as service
Protocol t b d i
identifiers
• Need uniform numbering
– To allow arbitrary client to contact server on
arbitrary machine
– To avoid inventing “directory assistance”
mechanism
• Port numbers
– Uniform throughout Internet
– Set by standards bodies
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21. Terminology
• Sequential program
– Typical of most programs
yp p g
– Single thread of control
• C
Concurrent program
t
– Multiple threads of control
– Execution proceeds “in parallel”
– More difficult to create
21

22. Servers and Concurrency
• Sequential server
– Also called iterative
– Handles one request at a time
• C
Concurrent server
t
– Can handle multiple requests at a time
– No waiting
22

23. Delay in Servers
• Concurrent server
– Server creates new thread of control to handle each request
– Client only waits for its request to be processed
• Sequential server
– Client waits for all previous requests to be processed as well
as f it request t b processed
for its t to be d
– Unacceptable to user if long request blocks short requests
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24. Concurrency in Servers
Concurrent execution is fundamental to
servers because concurrency permits
servers because concurrency permits
multiple clients to obtain a given service
without having to wait for the server to
without having to wait for the server to
finish previous requests. In a concurrent
server, the main server thread creates a
new service thread to handle each client.
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25. Protocol Ports and Concurrent Servers
• Apparent problem
– One port number assigned to each service
p g
– Concurrent server has multiple copies (threads)
running
– Client and server may interact
– Messages sent to server’s port must be delivered
’ b d li d
to correct copy
25

26. Protocol Ports and Concurrent Servers
(continued)
• Solution to problem: use i f
l i bl information about
i b
client as well as server to deliver incoming
packets
• TCP uses four items to identify connection
y
– Server’s IP address
– Server’s protocol port number
Server s
– Client’s IP address
– Client’s protocol port number
Client s
26

27. Demultiplexing in a Concurrent Server
Transport protocols assign an identifier to
Transport protocols assign an identifier to
each client as well as to each service.
Protocol software on the server s machine
Protocol software on the server’s machine
uses the combination of client and server
identifiers to choose the correct copy of a
identifiers to choose the correct copy of a
concurrent server.
27

28. Variations on a Theme
• A server can use
– Connectionless transport (UDP)
– Connection‐oriented transport (TCP)
– Both for a single service
g
• A single server can offer multiple services
– Often used for “trivial” services
trivial
– Server uses multiple port numbers simultaneously
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29. Variations on a Theme (cont)
• A server can
– Maintain interaction with a client for days or hours
y
– Send a short response and terminate interaction
– Perform I/O on the local computer
– Become a client for another service (potential
cycle problem)
l bl )
29

30. Example of Circularity
• Time server
i
– Returns time of day
• File server
– Allows client to read or write a file
– Calls time server when generating time stamp for
file
• Suppose programmer modifies time server to
log requests to a file
30

31. Interacting with Protocol Software
g
• Client or server uses transport protocols
• Protocol software inside OS
• Applications outside OS
• Mechanism needed to bridge the two
– Called Application Program Interface (API)
31

32. Application Program Interface
• Part of operating system
• Permits application to use protocols
• Defines
– Operations allowed
– Arguments for each operation
g p
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