Communication and Networks Assignment
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0_Lesson3InternetNetworking.pptxCommunications and Networks
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Diploma in Information Technology
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Lesson 3: Internet Networking
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Lesson 3 Learning Outcomes
Distinguish between connectionless and connection-oriented communication
Distinguish the role of server and client in a client-server model
Identify the characteristics of a server and client
Understand how multiple clients and servers can be used to work together
Define concurrent servers
Describe the motivation for peer-to-peer interaction on the Internet
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Lesson 3 Learning Outcomes
Describe what is the purpose of sockets
Identify the key functions of sockets
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Lesson 3 Outline
Communication Paradigms
Client Server Model
Network Programming
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Internet Rich Diversity of Services
None of the services is part of underlying communication infrastructure
Internet provides a general-purpose mechanism where:
Individual services are supplied by application programs that run on computers attached to the Internet
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Programming on the Internet
Possible to create Internet applications without knowing how networks operate
However, understanding network protocols and technologies allows them to write efficient and reliable code
Enables applications to scale across many sites
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Internet Communication Paradigms
Source: Douglas, C (2016) Computer Networks and Internets
| Stream Paradigms | Message Paradigms |
| Connection-oriented | Connectionless |
| 1-to-1 communication | Many-to-many communication |
| Sequence of individual bytes | Sequence of individual messages |
| Arbitrary length transfer | Each message limited to 64 Kbytes |
| Used by most applications | Used for multimedia applications |
| Built on TCP protocol | Built on UDP protocol |
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Stream Transport
Stream: sequence of bytes flows from one application program to another
Without attaching meaning to the bytes and without inserting boundaries
Two streams between pair of communicating applications, one in each direction
Accepts input from one, delivers to the other
Sending application can choose to generate one byte at a time or can generate blocks of bytes
Network can choose to combine smaller blocks into one large block or divide a large block into smaller blocks
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Message Transport
Message: the network accepts and delivers messages
Each message delivered corresponds to message transmitted
Never delivers part of a message, nor does it join multiple messages together
Sender places exactly n bytes in outgoing message
Receiver receives n bytes in the incoming message
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Message Transport Types
Unicast
a message can be sent from an application on one computer directly to an application on another, 1-to-1
Multicast
a message can be multicast to some of the computers on a network, 1-to-many
Broadcast
a message can be broadcast to all computers on a given network, 1-to-all
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Message Transport Reliability
Message transport is unreliable
Unreliable means no guarantee:
Lost: never delivered
Duplication: more than one copy arrives
Ordered: out-of-order
Programmers must ensure the application operates correctly even if packets are lost or reordered
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Message vs Stream
Most applications require delivery guarantees
Use stream services
Programmers tend to use stream service except in special situations
E.g. video, where multicast is needed, the application provides support to handle packet reordering and loss
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Connection-oriented Communication
Stream service is connection-oriented:
Applications must request connection be created
Once established, connection allows applications to send data in either direction
Finally, when communication finishes, the applications request the connection be terminated
Source: Douglas, C (2016) Computer Networks and Internets
Algorithm for Stream Paradigm
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Practice 3.1
Distinguish between stream and message communication with TWO differences
Describe the THREE steps in a stream paradigm communication algorithm
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Lesson 3 Outline
Communication Paradigms
Client Server Model
Network Programming
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Client-server Model
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How can a pair of applications that run on two independent computers coordinate to guarantee that they request a connection at the same time?
The answer lies in a form of interaction known as the client-server model
A server starts first and awaits contact
A client starts second and initiates the connection
Application programs known as clients and servers handle all services in the Internet
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Client-Server Differences
Source: Douglas, C (2016) Computer Networks and Internets
| Server Application | Client Application |
| Starts first | Starts second |
| Does not need to know which client will contact it | Must know which server to contact |
| Waits passively and arbitrarily long for contact from a client | Initiates a contact whenever communication is needed |
| Communicates with a client by both sending and receiving data | Communicates with a sever by sending and receiving data |
| Stays running after servicing one client and waits for another | May terminate after interacting with a server |
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Characteristics of Client Software
A client software:
Invoked directly by a user, executes only for one session
Runs locally on a user's personal computer
Actively initiates contact with a server
Can access multiple services as needed, but usually contacts one remote server at a time
Does not require especially powerful computer hardware
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Characteristics of Server Software
A server software:
A special-purpose, privileged program
Dedicated to providing one service that can handle multiple remote clients at the same time
Is invoked automatically when a system boots, and continues to execute through many sessions
Runs on a large, powerful computer
Waits passively for contact from arbitrary remote clients
Accepts contact from arbitrary clients, but offers a single service
Requires powerful hardware and a sophisticated OS
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Server Programs & Server-class Computers
Server: a program that waits passively for communication
Not to the computer on which it executes
When a computer is running server program(s), it is sometimes called a server
Hardware vendors contribute to the confusion as they classify powerful machines as servers
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Server Programs and Server-class Computers
Source: Douglas, C (2016) Computer Networks and Internets
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Requests & Response
Once contact is established, two-way communication is possible
From a client to a server
From a server to a client
A client can send a series of requests
The server issues a series of responses
A database client might allow a user to look up more than one item at a time
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One Computer, Multiple Servers
A computer that operate multiple servers is useful
Hardware can be shared
Has lower system administration overhead than multiple computer systems
Demand for a server is often sporadic:
Server can remain idle for long periods of time and does not use the CPU while waiting for a request to arrive
If demand is low, consolidating servers can reduce cost without reducing performance
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Multiple Clients & Servers
A computer can run:
A single client
A single server
Multiple copies of a client that contact a given server
Multiple clients that each contact a server
Multiple servers, each for a service
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Multiple Clients Benefits
A computer with multiple clients is useful as services can be accessed simultaneously
For example, a user can have three (3) windows open simultaneously running three (3) applications:
Chat service
Web browser
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Server Identification
Internet protocols divide identification to 2 pieces:
Identifier for the computer on which a server runs
Identifier for a service on the computer
Each computer in the Internet is assigned a unique 32-bit identifier known as an Internet Protocol (IP) address
192.168.1.0
It is an Internet layer address
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DNS for Server Identification
A client must specify the server’s IP address
To make server identification easy, each computer is assigned a name and the Domain Name System (DNS)
Thus, a user specifies a name such as www.cisco.com rather than an integer address
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Server Demultiplexing
Each service available in the Internet is assigned a unique 16-bit identifier known as port number
Email: 25, HTTP: 80
It is a Transport layer address
When a server begins, it registers with OS by specifying the port number for its service
A client contacts a server to request service
the request contains a port number
A request arrives at a server
software uses port number to determine which application on the server will handle request
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Server Identification & Demultiplexing
Source: Douglas, C (2016) Computer Networks and Internets
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Server Circular Dependencies
Circular dependencies: a server for one service can act as client for another
Before it can fill in a web page, a web server may need to become a client of a database
A server may also become the client of a security service to verify a client is allowed access to the service
Database
Server
Web Page Server
User
Client of
Client of
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Concurrent Servers
Most servers are concurrent
A server uses more than one thread of control
Concurrent execution depends on the OS being used and the code is divided into 2 pieces
Main program (thread)
Handler
Main thread accepts contact from client and creates a thread of control for the client
Each thread of control interacts with a single client and runs the handler code
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Running Concurrent Servers
After handling one client the thread terminates
Main thread keeps server alive after creating a thread to handle a request
If N clients are simultaneously using a concurrent server, N+1 threads will be running:
N threads for each client
+1 main thread to wait for additional requests
Main Thread
User
User
Web Page Server
Handler 1
Handler 2
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Server Bottlenecks
If a single server provides a given service
the network connection between the server and the Internet can become a bottleneck
User
Internet
Server
User
User
Bottleneck
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Peer-to-Peer Interactions
One way to avoid a bottleneck forms the basis of file sharing known as a peer-to-peer (P2P) architecture
Data is distributed equally among a set of N servers
Each request is sent to the appropriate server
Each server only provides 1/N of the data
User
Internet
Server 1 of N
User
1/N traffic
Server 2 of N
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Practice 3.2
Describe TWO identification methods that Internet Protocols uses to identify servers
Explain what is server circular dependencies and bottleneck.
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Lesson 3 Outline
Communication Paradigms
Client Server Model
Network Programming
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What is an API?
Source: https://www.youtube.com/watch?v=s7wmiS2mSXY
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Socket API
Application Program Interface (API): interface that an application uses to specify communication
Details of an API depend on the OS
de facto standard for software that communicates over the Internet is the socket API
commonly abbreviated sockets
Socket API is available for many OS
Windows, various UNIX, Linux
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Sockets & Descriptors
Originally developed as part of UNIX OS, the socket API is integrated with I/O
When an application creates a socket, OS returns a small integer descriptor that identifies the socket
Application passes the descriptor as an argument
when it calls functions to perform an operation on the socket such as to transfer data across the network or to receive data
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Network I/O
In many OS, socket descriptors are integrated with other I/O descriptors
An application can use the read and write operations for socket I/O or I/O to a file
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Socket Functions
In socket programming an application must specify many details, such as
address, port number, whether the application will act as a client or as a server
To avoid having a single socket function with many parameters, designers of the socket API chose to define many functions
An application creates a socket, and then invokes functions for details
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Pros & Cons of Socket Approach
Advantage of socket approach is that most functions have three or fewer parameters
Disadvantage is that a programmer must remember to call multiple functions when using sockets
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Major Functions in Socket API
Source: Douglas, C (2016) Computer Networks and Internets
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Socket Calls in a Client and Server
This illustrates the sequence of socket calls made by a typical client and server that use a stream connection
In some applications, send and recv are called in the reverse order
Source: Douglas, C (2016) Computer Networks and Internets
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Client Connection Functions
Clients call connect to establish a connection with a specific server
connect (socket, saddress, saddresslen)
Argument socket is the descriptor of a socket to use for the connection
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Sockets in Message Paradigm
Socket functions used to send and receive messages are more complicated than stream paradigm because many options are available
A sender can choose whether to store the recipient’s address in the socket and merely send data or to specify the recipient’s address each time a message is transmitted
One function allows a sender to place the address and message in a structure and pass the address of the structure as an argument, and another function allows a sender to pass the address and message as separate arguments
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Practice 3.3
What is an Application Program Interface (API)?
Discuss one advantage and one disadvantage of using sockets.
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Reading
Douglas, C. (2016). Computer Networks and Internets, Global Edition (6th ed.). Pearson Education. ISBN: 978-1292061177 Chapter 3
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End of Lesson
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