Client-Server vs Peer-to-Peer

Definitions and History

• The World Wide Web is what most people think of in relation to "the internet", but it is only a subset of it

Short History of the Internet

• The early ARPANET network was funded and controlled by the United States government
• It was used exclusively for academic and scientific purposes
• The early network started with a few connected campuses in 1969 and grew to a few hundred by the early 1980s.
• In 1981 new networks adopted the the TCP/IP (Transmission Control Protocol / Internet Protocol)

Tim Berners-Lee

• The invention of the WWW is usually attributed to the British Tim Berners-Lee
• British Tim Berners-Lee, along with the Belgian Robert Cailliau, published a proposal in 1990 for a hypertext system while both were working in in Switzerland.

Core Features of the Web

• Shortly after that initial proposal Berners-Lee developed the main features of the web:
• A URL to uniquely identify a resource on the WWW.
• The HTTP protocol to describe how requests and responses operate.
• Software that responds to HTTP requests, known as web server software.
• HTML to publish documents.
• Browsers that make HTTP requests from URLs and that can display the HTML it receives.

Intranet

• An "intranet" refers to an internet network that is local to an organization or business
• Intranet resources are often private, so only employees or authorized external parties have access
• The "Internet" is a broader term that encompasses both private (intranet) and public networked resources
• Intranets are typically protected from unauthorized external access via security feature such as firewalls or private IP ranges
• It is difficult to get an accurate gauge of web pages that exist within Intranets, but some estimate that almost half of all web resources are hidden in private intranets.
• Awareness of intranets is important when one considers market usage of different web technologies
• On the public internet, PHP, MySQL, and WordPress are dominant
• In corporate intranets, other technologies such as ASP.NET, JSP, SharePoint, Oracle, SAP, and IBM WebSphere, are just as important

Static Web Sites

• In the earliest days of the web, a webmaster would publish web pages, and periodically update them.
• Skills needed to create a web site were pretty basic: one needed knowledge of the HTML markup language and familiarity with editing and creating images.
• A static web site consists only of HTML pages that look identical for all users at all times.

Dynamic Web Sites

• Within a few years of the invention of the web, sites began to get more complicated as sites began to use programs running on web servers to generate content dynamically.
• Dynamic web sites have server-based programs that read content from databases, interface with existing enterprise computer systems, communicate with financial institutions, and then output HTML that are sent back to the users' browsers.
• Dynamic web pages are created at run-time by a program created by a programmer; this page content can vary for user to user.

Web 2.0 and Beyond

• Web 2.0 had two meanings, one for users and one for developers.
• Web 2.0 refers to an interactive experience where users can contribute and consume web content, creating a more user-driven web experience
• Web 2.0 refers to a change in the paradigm of how dynamic web sites are created
• Programming logic which previously existed only on the server, began to migrate to the browser, requiring learning Javascript, a programming language that runs in the browser, as well as mastering the rather difficult programming techniques involved in asynchronous communication

Internet Protocols

• The internet exists because of a suite of interrelated communications protocols.
• A protocol is a set of rules that partners in communication use when they communicate.

A Layered Architecture

• The TCP/IP Internet protocols were originally abstracted as a four-layer stack
• Later abstractions subdivide it further into five or seven layers
• The earliest and simplest model is the four-layer network model.

Four Layer Network Model

• The first layer is the Application Layer containing higher protocols that allow applications to interact with the transport layer: HTTP, FTP, POP etc
• Second is the Transport Layer which ensures transmissions arrive in order and without error: TCP, UDP
• Third is the Internet Layer which establishes connection, routing, and addressing: IPv4, IPv6
• The final layer is the Link Layer responsible for physical transmission of raw bits MAC

IP Addresses

• There are two types of IP Addresses.
• IPv4 addresses are the IP addresses from the original TCP/IP protocol
• IPv4 uses 12 numbers, implemented as four 8-bit integers, written with a dot between each integer.
• Since an unsigned 8-bit integer's maximum value is 255, four integers together can encode approximately 4.2 billion unique IP addresses
• IPv6 was created to future proof the Internet against theIPv4 4.2 billion limit
• IPv6, it uses eight 16-bit integers which provides for 2128 unique addresses, over a billion billion times the number in IPv4
• 16-bit integers are normally written in hexadecimal, due to their longer length.
• An IP address is generally assigned by a Internet Service Provider (ISP).
• In organizations, purchasing extra IP addresses from the ISP is not cost effective
• On a local network, computers can share a single IP address between them.

TCP Packets

• Messages are broken into packets with a sequence number.
• For each TCP packet sent, an ACK (acknowledgement) must be received back.
• The sender will resend any packets that did not get an ACK back.
• The Message is reassembled from packets and ordered according to their sequence numbers.

Client-Server Model

• The web is client-server model of communications.
• In the client-server model, there are two types of actors: clients and servers.
• The server is a computer agent that is normally active 24 hours a day, 7 days a week (24/7), listening for queries from any client who make a request.
• client is a computer agent that requests and receives responses from the server, in the form of response codes, images, text files, and other data.
• Within the client-server model, the request-response loop is the basic mechanism on the server for receiving requests and transmitting data in response.
• The client initiates a request to a server which creates a response that could include a HTML file, an image or some data.

The Peer-to-Peer Alternative

• Each computer is functionally identical in a peer-to-peer model
• Each node can send and receive directly with one another.
• In the model each peer acts as both a client and server able to upload and download information.

Server Types

• Clients make requests for resources from a URL, to the client, the server is a single machine so it is common to split the functionality of a web site between several different types of server like; Data, Application, Email, Web, Media and Authentication servers.
• Not only are there different types of servers, there is often replication of each of the different server types
• Globally popular sites such as Facebook receive millions of requests a second.

Server Farms

• A single web server that is also acting as an application or database server will struggle handle more than a few hundred requests a second
• The usual strategy for busier sites is to use a server farm.
• The goal behind server farms is to distribute incoming requests between clusters of machines so that any given web or data server is not excessively overloaded, load balancers are used.
• it is not uncommon to still use a server farm even if the server can handle the load because it provides redundancy.
• Server hardware and hard drives are stacked on top of each other in server racks
• A typical server farm consists of many server racks, each containing many servers .

Data Centers

• Server farms are typically housed in special facilities called data centers
• Data centers that prevent the potential for site down times provide mirrored data centers in different parts of the country, or even the world
• Costs for multiple redundant data centers are quite high, so most web companies lease space from a third-party data center.
• It is common for a single server machine may host multiple sites
• Commercial web hosting companies host hundreds or thousands of sites on mirrored servers.

Where is the Internet?

• Our experience of the hardware is in our homes though Fiber junction boxes, fiber optic cables, ISP head end, Cable modem termination system (CMTS) & Cable modem
• Your ISP has to pass on your requests for Internet packets to other networks
• This intermediate step typically involves one or more regional network hubs.
• Your ISP may have a large national network with optical fiber connecting most of the main cities in the country
• Some countries have multiple national or regional networks, each with their own optical network.
• International Internet communication will need to travel underwater in Undersea fiber optic cable.

Routers and Routing Tables

• The routing table specifies a host of destination networks and sub-networks through which a certain router can connect.
• Every router on the internet has its own routing table

DNS

• Instead of IP addresses, we use the Domain Name System (DNS)
• Domain name address resolution facilitates your request from your computer to the correct one on the other side of the word

Domain Levels

• There are four levels: top, second, third and fourth: www.example.com

###DNS Address Resolution

• While domain names are certainly an easier way for users to reference a web site, eventually, your browser needs to know the IP address of the web site in order to request any resources from it.
• The Domain Name System provides a mechanism for software to discover this numeric IP address.
• This process is referred to here as address resolution: requesting from the address www.funwebdev.com

Uniform Resource Locators

• In order to allow clients to request particular resources from the server, a naming mechanism is required so that the client knows how to ask the server for the file.
• Uniform resource locator (URL): http://www.funwebdev.com/index.php?page=1#article Where protocol is http, domain is www.funwebdev.com, path is /index, Query String is page=1 and Fragment is #article"

Query String

• They are the way of passing information such as user form input from the client to the server.
• In URL's they are encoded as key-value pairs delimited by "&" symbols and preceded by the "?" symbol.
• ie: ?username=john&password=abcdefg

Hypertext Transfer Protocol

• The Hypertext Transfer Protocol establishes a TCP connection on port 80 (by default).
• The server waits for the request, and then responds with a response code, headers and an optional message (which can include files).

Web Requests

• The experience of seeing a single web page is facilitated by the client's browser which requests the initial HTML then parses it to find all the resources from within it, like images, style sheets and scripts.
• The page is only fully loaded for the user when the files have been retrieved.

HTTP Request Methods

• The HTTP protocol defines several different types of requests, each with a different intent and characteristics.
• The most common requests types are GET and POST request, along with the HEAD request.
• Other requests, such as PUT, DELETE, CONNECT, TRACE and OPTIONS are seldom used.

Web Servers

• A web server is, nothing more than a computer that responds to HTTP requests.
• To run a website one must choose an application stack
• LAMP software stack: Linux OS, Apache web server, MySQL database, and PHP scripting language
• WISA software stack: Windows OS, IIS web server, SQL Server database, and the ASP.NET server-side development technologies.