1. Principles of Network Applications

2. Some network apps e-mail web text messaging remote login
P2P file sharing
multi-user network games
streaming stored video (YouTube, Hulu, Netflix)
voice over IP (e.g., Skype)
real-time video conferencing
social networking
search …

3. Application architectures
transport
network
data link
physical
possible structure of applications:
client-server
peer-to-peer (P2P)
Hybrid of client-server and P2P

4. Client-Server Architecture
always-on host
permanent IP address
data centers for scaling
clients:
communicate with server
may be intermittently connected
may have dynamic IP addresses
do not communicate directly with each other
client/server

5. P2P Architecture no always-on server
arbitrary end systems directly communicate
peers request service from other peers, provide service in return to other peers
self scalability – new peers bring new service capacity, as well as new service demands
peers are intermittently connected and change IP addresses
complex management
peer-peer

6. Hybrid of Client-Server and P2P
Skype
Voice-Over-IP (VoIP) P2P application
Centralized server: finding address of remote party:
Client-Client connection: direct (not through server)
Instant Messaging
Chatting between two users is P2P
Centralized service: client presence detection/location
user registers its IP address with central server when it comes online
user contacts central server to find IP addresses of buddies (friends)
peer-peer

7. Processes Communicating
clients, servers
process: program running within a host
within same host, two processes communicate using:
inter-process communication (IPC) (defined by OS)
processes in different hosts communicate by exchanging messages
client process: process that initiates communication server process: process that waits to be contacted
Himm: applications with P2P architectures have client processes & server processes

8. Sockets process sends/receives messages to/from its socket
socket analogous to door
sending process shoves message out door
sending process relies on transport infrastructure on other side of door to deliver message to socket at receiving process
application
application
socket
controlled by
app developer
process
process
transport
transport
network
network
controlled
by OS
link
Internet
link
physical
physical

9. Addressing Processes to receive messages, process must have identifier
host device has unique 32-bit IP address
Q: does IP address of host on which process runs suffice for identifying the process?
identifier includes both IP address and port numbers associated with process on host.
example port numbers:
HTTP server: 80
mail server: 25
to send HTTP message to just.edu.jo web server:
IP address:
port number: 80
A: no, many processes can be running on the same host

10. Network Applications/1
Most Net Applications use the Client-Server architecture,
In which two processes (applications) communicate with each other to exchange some information.
One of the two processes acts as a client process, and the other one acts as a server

11. Network Applications/2
Client Process
Typically makes a request for information.
After getting the response, it may terminate or may do some other processing (or even other requests).
Example:
Internet Browser works as a client application,
It sends a request to the Web Server to get one HTML webpage.

12. Network Applications/3
Server Process
Takes/Receives a request from clients
When it gets a request from a client, the server process will
perform the required processing,
gather the requested information, and
send it to the requestor client.
Once done, it becomes ready to serve another client.
Server processes are always alert and ready to serve incoming requests.

13. Network Applications/4
Example:
Web Server keeps waiting for requests from Internet Browsers
As soon as it gets any request from a browser, it picks up a requested HTML page and sends it back to that Browser.
Note that:
A client needs to know the address of the server: (IP and Port), but
The server does not need to know the address or even the existence of the client prior to the connection being established/requested.
Once a connection is established,
both sides (client & server know about each others) and can send and receive information back and forth between them.

14. System Architecture
The architecture of a computer system is the high-level (most general) design on which the system is based
Architectural features include:
Components
Collaborations (how components interact)
Connectors (how components communicate, communication methods)

15. Client-Server Architecture
Each component of a Client-Server system has the role of either client or server
Client: a component that makes requests clients are active initiators of transactions
Server: a component that satisfies requests servers are passive and react to client requests

16. Centralized / Distributed
The client-server architecture can be thought of as a median between
Centralized Processing: computation is performed on a central platform, (a set of central servers)
Distributed Processing: computation is performed on platforms located with the user
Distributed Systems: physically separate computers working together
Challenge: coordination is more difficult when performed over a network
Centralized
Client / Server
Distributed

17. Client-Server Architecture
The Web is a client-server system
Web browsers act as clients, and make requests to web servers
Web servers respond to requests with requested information and/or computation
Client
Client
Server
Client
Server
request
Client
Client
Server

18. Tiered Web Architectures
Web applications are usually implemented with:
2-tier,
3-tier,
or multitier (N-tier) architectures
Each tier is a platform (client or server) with a unique responsibility

19. 2-Tier Architecture Tier 1: Client platform, hosting a web browser
Tier 2: Server platform, hosting all server software components

20. 2-Tier Characteristics
Advantage:
Inexpensive (single platform)
Disadvantages
Interdependency (coupling) of components
No redundancy
Limited scalability
Typical application
users
Small company or organization, e.g.,
law office,
medical practice,
local non-profit

21. 3-Tier Architecture
Tier 3 takes over part of the server function from tier 2, typically data management, etc.

22. 3-Tier Characteristics
Advantages
Improved performance, from specialized hardware
Decreased coupling of software components
Improved scalability
Disadvantages
No redundancy
Typical Application
users
Small business or regional organization, e.g.,
specialty retailer,
small college and universities

23. Multitier C/S Architecture
A multitier (N-tier) architecture is an expansion of the 3-tier architecture,
The expansion can be in one of several different possible ways
Replication of the function of a tier
Specialization of function within a tier
Portal services, focusing on handling incoming web traffic

24. Replication Application and data servers are replicated
Servers share the total workload

25. Specialization Servers are specialized
Each server handles a designated part of the workload, by function

26. Portal Services
Portal servers handle incoming traffic, reducing application server load
e.g., firewall, load balancer, transaction processing manager

27. Multi-Tier Characteristics
Advantages
Decoupling of software components
Flexibility to add/remove platforms in response to load
Scalability
Redundancy
Disadvantages
Higher costs (maintenance, design, electrical load, cooling)
Typical Application
1000+ users
Large business or organization

28. Characteristics Summary
2-Tier
3-Tier
N-Tier 10
100
1000
users
large e-commerce, business, or organization
small e-commerce, regional business or organization
local business or organization
Capacity, scalability, redundancy, & cost

29. Summary 2-Tier vs. 3-Tier 2-Tier Architecture
The client directly interacts with the server.
This may have some security holes and performance problems.
For example:
Internet Explorer and Web Server work on 2-tier architecture
Here security problems are resolved using:
Secure Socket Layer (SSL) or
the new version is Transport Layer Security (TLS)
i.e. HTTPS uses port # 443 (also called HTTP over TLS, HTTP over SSL, or HTTP Secure)

30. Summary 2-Tier vs. 3-Tier 3-Tier Architecture Middleware
In this architecture, one more software sits in between the client and the server.
This middle software is called ‘middleware’.
Middleware
Used to perform all the security checks and load balancing in case of heavy load.
For example: It may takes all the requests from the client
after performing the required authentication, it passes that request to the server.
Then the server does the required processing and sends the response back to the middleware and
finally the middleware passes this response back to the client.
If you want to implement a 3-tier architecture,
then you can keep any middleware like Web Logic or WebSphere software in between your Web Server and Web Browser.

31. End