What is internet, www, TCP/IP, HTTP protocol, SSL (Secure socket layer),
HTTPS, HTML, Web server, Web client, Proxy server, Caching, Cookies,
Application server, Thin client, Daemon, Client side scripting, Server side
scripting, CGI, Dynamic web pages, Digital certificates and list of HTTP status
codes.
• Internet
– A global network connecting millions of computers.
• World Wide Web (the Web)
– An information sharing model that is built on top of the Internet,
utilizes HTTP protocol and browsers (such as Internet Explorer) to
access Web pages formatted in HTML that are linked via hyperlinks
and the Web is only a subset of the Internet (other uses of the
Internet include email (via SMTP), Usenet, instant messaging and
file transfer (via FTP)
• URL (Uniform Resource Locator)
– The address of documents and other content on the Web. It is
consisting of protocol, domain and the file. Protocol can be either
HTTP, FTP, Telnet, News etc., domain name is the DNS name of
the server and file can be Static HTML, DOC, Jpeg, etc., . In other
words URLs are strings that uniquely identify resources on internet.
• TCP/IP
– TCP/IP protocol suite used to send data over the Internet. TCP/IP
consists of only 4 layers - Application layer, Transport layer,
Network layer & Link layer
Internet Protocols:
Application Layer - DNS, TLS/SSL, TFTP, FTP, HTTP, IMAP, IRC,
NNTP, POP3, SIP, SMTP, SNMP, SSH, TELNET, BitTorrent, RTP,
rlogin.
Transport Layer- TCP, UDP, DCCP, SCTP, IL, RUDP,
Network Layer - IP (IPv4, IPv6), ICMP, IGMP, ARP, RARP, …
Link Ethernet Layer- Wi-Fi, Token ring, PPP, SLIP, FDDI, ATM,
DTM, Frame Relay, SMDS,
• TCP (Transmission Control Protocol)
– Enables two devices to establish a connection and exchange data.
– In the Internet protocol suite, TCP is the intermediate layer
between the Internet Protocol below it, and an application above it.
Applications often need reliable pipe-like connections to each other,
whereas the Internet Protocol does not provide such streams, but
rather only unreliable packets. TCP does the task of the transport
layer in the simplified OSI model of computer networks.
– It is one of the core protocols of the Internet protocol suite. Using
TCP, applications on networked hosts can create connections to one
another, over which they can exchange data or packets. The
protocol guarantees reliable and in-order delivery of sender to
receiver data. TCP also distinguishes data for multiple, concurrent
applications (e.g. Web server and e-mail server) running on the
same host.
• IP
– Specifies the format of data packets and the addressing protocol.
The Internet Protocol (IP) is a data-oriented protocol used for
communicating data across a packet-switched internet work. IP is a
network layer protocol in the internet protocol suite. Aspects of IP
are IP addressing and routing. Addressing refers to how end hosts
become assigned IP addresses. IP routing is performed by all hosts,
but most importantly by internetwork routers
• IP Address
– A unique number assigned to each connected device, often assigned
dynamically to users by an ISP on a session-by-session basis –
dynamic IP address. Increasingly becoming dedicated, particularly
with always-on broadband connections – static IP address.
Packet
– A portion of a message sent over a TCP/IP Network. It contains
content and destination
• HTTP (Hypertext Transfer Protocol)
– Underlying protocol of the World Wide Web. Defines how messages
are formatted and transmitted over a TCP/IP network for Web
sites. Defines what actions Web servers and Web browsers take in
response to various commands.
– HTTP is stateless. The advantage of a stateless protocol is that hosts
don't need to retain information about users between requests, but
this forces the use of alternative methods for maintaining users'
state, for example, when a host would like to customize content for
a user who has visited before. The common method for solving this
problem involves the use of sending and requesting cookies. Other
methods are session control, hidden variables, etc
– example: when you enter a URL in your browser, an HTTP
command is sent to the Web server telling to fetch and transmit the
requested Web page
o HEAD: Asks for the response identical to the one that
would correspond to a GET request, but without the
response body. This is useful for retrieving metainformation
written in response headers, without
having to transport the entire content.
o GET : Requests a representation of the specified
resource. By far the most common method used on
the Web today.
o POST : Submits user data (e.g. from a HTML form) to
the identified resource. The data is included in the
body of the request.
o PUT: Uploads a representation of the specified
resource.
o DELETE: Deletes the specified resource (rarely
implemented).
o TRACE: Echoes back the received request, so that a
client can see what intermediate servers are adding or
changing in the request.
o OPTIONS:
o Returns the HTTP methods that the server supports.
This can be used to check the functionality of a web
server.
o CONNECT: For use with a proxy that can change to
being an SSL tunnel.
HTTP pipelining
– appeared in HTTP/1.1. It allows clients to send multiple requests at
once, without waiting for an answer. Servers can also send multiple
answers without closing their socket. This results in fewer
roundtrips and faster load times. This is particularly useful for
satellite Internet connections and other connections with high
latency as separate requests need not be made for each file. Since it
is possible to fit several HTTP requests in the same TCP packet,
HTTP pipelining allows fewer TCP packets to be sent over the
network, reducing network load. HTTP pipelining requires both the
client and the server to support it. Servers are required to support it
in order to be HTTP/1.1 compliant, although they are not required
to pipeline responses, just to accept pipelined requests.
• HTTP-Tunnel
– technology allows users to perform various Internet tasks despite
the restrictions imposed by firewalls. This is made possible by
sending data through HTTP (port 80). Additionally, HTTP-Tunnel
technology is very secure, making it indispensable for both average
and business communications. The HTTP-Tunnel client is an
application that runs in your system tray acting as a SOCKS server,
managing all data transmissions between the computer and the
network.
• HTTP streaming
– It is a mechanism for sending data from a Web server to a Web
browser in response to an event. HTTP Streaming is achieved
through several common mechanisms. In one such mechanism the
web server does not terminate the response to the client after data
has been served. This differs from the typical HTTP cycle in which
the response is closed immediately following data transmission.
The web server leaves the response open such that if an event is
received, it can immediately be sent to the client. Otherwise the
data would have to be queued until the client's next request is made
to the web server. The act of repeatedly queing and re-requesting
information is known as a Polling mechanism. Typical uses for
HTTP Streaming include market data distribution (stock tickers),
live chat/messaging systems, online betting and gaming, sport
results, monitoring consoles and Sensor network monitoring.
HTTP referrer
– It signifies the webpage which linked to a new page on the Internet.
By checking the referer, the new page can see where the request
came from. Referer logging is used to allow websites and web
servers to identify where people are visiting them from, for
promotional or security purposes. Since the referer can easily be
spoofed (faked), however, it is of limited use in this regard except
on a casual basis. A dereferer is a means to strip the details of the
referring website from a link request so that the target website
cannot identify the page which was clicked on to originate a request.
Referer is a common misspelling of the word referrer. It is so
common, in fact that it made it into the official specification of
HTTP – the communication protocol of the World Wide Web – and
has therefore become the standard industry spelling when
discussing HTTP referers.
• SSL (Secure Sockets Layer)
– Protocol for establishing a secure connection for transmission, it
uses the HTTPS convention
– SSL provides endpoint authentication and communications privacy
over the Internet using cryptography. In typical use, only the server
is authenticated (i.e. its identity is ensured) while the client remains
unauthenticated; mutual authentication requires public key
infrastructure (PKI) deployment to clients. The protocols allow
client/server applications to communicate in a way designed to
prevent eavesdropping, tampering, and message forgery.
– SSL involves a number of basic phases:
o Peer negotiation for algorithm support
o Public key encryption-based key exchange and
certificate-based authentication
o Symmetric cipher-based traffic encryption
o During the first phase, the client and server negotiate
which cryptographic algorithms will be used. Current
implementations support the following choices:
o for public-key cryptography: RSA, Diffie-Hellman,
DSA or Fortezza;
o for symmetric ciphers: RC2, RC4, IDEA, DES, Triple
DES or AES;
o For one-way hash functions: MD5 or SHA.
HTTPS
– is a URI scheme which is syntactically identical to the http: scheme
normally used for accessing resources using HTTP. Using an https:
URL indicates that HTTP is to be used, but with a different default
port and an additional encryption/authentication layer between
HTTP and TCP. This system was invented by Netscape
Communications Corporation to provide authentication and
encrypted communication and is widely used on the Web for
security-sensitive communication, such as payment transactions.
• HTML (Hypertext Markup Language)
– The authoring language used to create documents on the World
Wide Web
– Hundreds of tags can be used to format and layout a Web page’s
content and to hyperlink to other Web content.
• Hyperlink
– Used to connect a user to other parts of a web site and to other web
sites and web-enabled services.
• Web server
– A computer that is connected to the Internet. Hosts Web content
and is configured to share that content.
– Webserver is responsible for accepting HTTP requests from clients,
which are known as Web browsers, and serving them Web pages,
which are usually HTML documents and linked objects (images,
etc.).
• Examples:
o Apache HTTP Server from the Apache Software
Foundation.
o Internet Information Services (IIS) from Microsoft.
o Sun Java System Web Server from Sun Microsystems,
formerly Sun ONE Web Server, iPlanet Web Server,
and Netscape Enterprise Server.
o Zeus Web Server from Zeus Technology
• Web client
– Most commonly in the form of Web browser software such as
Internet Explorer or Netscape
– Used to navigate the Web and retrieve Web content from Web
servers for viewing.
• Proxy server
– An intermediary server that provides a gateway to the Web (e.g.,
employee access to the Web most often goes through a proxy)
– Improves performance through caching and filters the Web
– The proxy server will also log each user interaction.
• Caching
– Web browsers and proxy servers save a local copy of the
downloaded content – pages that display personal information
should be set to prohibit caching.
• Web form
– A portion of a Web page containing blank fields that users can fill in
with data (including personal info) and submits for Web server to
process it.
• Web server log
– Every time a Web page is requested, the Web server may
automatically logs the following information:
o the IP address of the visitor
o date and time of the request
o the URL of the requested file
o the URL the visitor came from immediately before
(referrer URL)
o the visitor’s Web browser type and operating system
• Cookies
– A small text file provided by a Web server and stored on a users PC
the text can be sent back to the server every time the browser
requests a page from the server. Cookies are used to identify a user
as they navigate through a Web site and/or return at a later time.
Cookies enable a range of functions including personalization of
content.
• Session vs. persistent cookies
– A Session is a unique ID assigned to the client browser by a web
server to identify the state of the client because web servers are
stateless.
– A session cookie is stored only while the user is connected to the
particular Web server – the cookie is deleted when the user
disconnects
– Persistent cookies are set to expire at some point in the future –
many are set to expire a number of years forward
• Socket
– A socket is a network communications endpoint.
• Application Server
– An application server is a server computer in a computer network
dedicated to running certain software applications. The term also
refers to the software installed on such a computer to facilitate the
serving of other applications. Application server products typically
bundle middleware to enable applications to intercommunicate
with various qualities of service — reliability, security, nonrepudiation,
and so on. Application servers also provide an API to
programmers, so that they don't have to be concerned with the
operating system or the huge array of interfaces required of a
modern web-based application. Communication occurs through the
web in the form of HTML and XML, as a link to various databases,
and, quite often, as a link to systems and devices ranging from huge
legacy applications to small information devices, such as an atomic
clock or a home appliance.
– An application server exposes business logic to client applications
through various protocols, possibly including HTTP. the server
exposes this business logic through a component API, such as the
EJB (Enterprise JavaBean) component model found on J2EE (Java
2 Platform, Enterprise Edition) application servers. Moreover, the
application server manages its own resources. Such gate-keeping
duties include security, transaction processing, resource pooling,
and messaging
– Ex: JBoss (Red Hat), WebSphere (IBM), Oracle Application Server
10g (Oracle Corporation) and WebLogic (BEA)
• Thin Client
– A thin client is a computer (client) in client-server architecture
networks which has little or no application logic, so it has to depend
primarily on the central server for processing activities. It is
designed to be especially small so that the bulk of the data
processing occurs on the server.
• Thick client
– It is a client that performs the bulk of any data processing
operations itself, and relies on the server it is associated with
primarily for data storage.
• Daemon
– It is a computer program that runs in the background, rather than
under the direct control of a user; they are usually instantiated as
processes. Typically daemons have names that end with the letter
"d"; for example, syslogd is the daemon which handles the system
log. Daemons typically do not have any existing parent process, but
reside directly under init in the process hierarchy. Daemons usually
become daemons by forking a child process and then making the
parent process kill itself, thus making init adopt the child. This
practice is commonly known as "fork off and die." Systems often
start (or "launch") daemons at boot time: they often serve the
function of responding to network requests, hardware activity, or
other programs by performing some task. Daemons can also
configure hardware (like devfsd on some Linux systems), run
scheduled tasks (like cron), and perform a variety of other tasks.
• Client-side scripting
– Generally refers to the class of computer programs on the web that
are executed client-side, by the user's web browser, instead of
server-side (on the web server). This type of computer
programming is an important part of the Dynamic HTML
(DHTML) concept, enabling web pages to be scripted; that is, to
have different and changing content depending on user input,
environmental conditions (such as the time of day), or other
variables.
– Web authors write client-side scripts in languages such as
JavaScript (Client-side JavaScript) or VBScript, which are based on
several standards:
o HTML scripting
o HTTP
o Document Object Model
• Client-side scripts are often embedded within
an HTML document, but they may also be
contained in a separate file, which is referenced
by the document (or documents) that use it.
Upon request, the necessary files are sent to
the user's computer by the web server (or
servers) on which they reside. The user's web
browser executes the script, then displays the
document, including any visible output from
the script. Client-side scripts may also contain
instructions for the browser to follow if the
user interacts with the document in a certain
way, e.g., clicks a certain button. These
instructions can be followed without further
communication with the server, though they
may require such communication.
• Server-side Scripting
– It is a web server technology in which a user's request is fulfilled by
running a script directly on the web server to generate dynamic
HTML pages. It is usually used to provide interactive web sites that
interface to databases or other data stores. This is different from
client-side scripting where scripts are run by the viewing web
browser, usually in JavaScript. The primary advantage to serverside
scripting is the ability to highly customize the response based
on the user's requirements, access rights, or queries into data
stores.
o ASP: Microsoft designed solution allowing various
languages (though generally VBscript is used) inside a
HTML-like outer page, mainly used on Windows but
with limited support on other platforms.
o ColdFusion: Cross platform tag based commercial
server side scripting system.
o JSP: A Java-based system for embedding code in
HTML pages.
o Lasso: A Datasource neutral interpreted programming
language and cross platform server.
o SSI: A fairly basic system which is part of the common
apache web server. Not a full programming
environment by far but still handy for simple things
like including a common menu.
o PHP : Common opensource solution based on
including code in its own language into an HTML
page.
o Server-side JavaScript: A language generally used on
the client side but also occasionally on the server side.
o SMX : Lisplike opensource language designed to be
embedded into an HTML page.
• Common Gateway Interface (CGI)
– is a standard protocol for interfacing external application software
with an information server, commonly a web server. This allows the
server to pass requests from a client web browser to the external
application. The web server can then return the output from the
application to the web browser.
• Dynamic Web pages:
– can be defined as: (1) Web pages containing dynamic content (e.g.,
images, text, form fields, etc.) that can change/move without the
Web page being reloaded or (2) Web pages that are produced onthe-
fly by server-side programs, frequently based on parameters in
the URL or from an HTML form. Web pages that adhere to the first
definition are often called Dynamic HTML or DHTML pages.
Client-side languages like JavaScript are frequently used to produce
these types of dynamic web pages. Web pages that adhere to the
second definition are often created with the help of server-side
languages such as PHP, Perl, ASP/.NET, JSP, and languages. These
server-side languages typically use the Common Gateway Interface
(CGI) to produce dynamic web pages.
• Digital Certificates
In cryptography, a public key certificate (or identity certificate) is a certificate
which uses a digital signature to bind together a public key with an identity —
information such as the name of a person or an organization, their address, and
so forth. The certificate can be used to verify that a public key belongs to an
individual.
In a typical public key infrastructure (PKI) scheme, the signature will be of a
certificate authority (CA). In a web of trust s
"endorsements"). In either case, the signatures on a certificate are attestations by
the certificate signer that the identity information and the public key belong
together.
Certificates can be used for the large-scale use of public-key cryptography.
Securely exchanging secret keys amongst users becomes impractical to the point
of effective impossibility for anything other than quite small networks. Public key
cryptography provides a way to avoid this problem. In principle, if Alice wants
others to be able to send her secret messages, she need only publish her public
key. Anyone possessing it can then send her secure information. Unfortunately,
David could publish a different public key (for which he knows the related private
key) claiming that it is Alice's public key. In so doing, David could intercept and
read at least some of the messages meant for Alice. But if Alice builds her public
key into a certificate and has it digitally signed by a trusted third party (Trent),
anyone who trusts Trent can merely check the certificate to see whether Trent
thinks the embedded public key is Alice's. In typical Public-key Infrastructures
(PKIs), Trent will be a CA, who is trusted by all participants. In a web of trust,
Trent can be any user, and whether to trust that user's attestation that a
particular public key belongs to Alice will be up to the person wishing to send a
message to Alice.
In large-scale deployments, Alice may not be familiar with Bob's certificate
authority (perhaps they each have a different CA — if both use employer CAs,
different employers would produce this result), so Bob's certificate may also
include his CA's public key signed by a "higher level" CA2, which might be
recognized by Alice. This process leads in general to a hierarchy of certificates,
and to even more complex trust relationships. Public key infrastructure refers,
mostly, to the software that manages certificates in a large-scale setting. In X.509
PKI systems, the hierarchy of certificates is always a top-down tree, with a root
certificate at the top, representing a CA that is 'so central' to the scheme that it
does not need to be authenticated by some trusted third party.
A certificate may be revoked if it is discovered that its related private key has
been compromised, or if the relationship (between an entity and a public key)
embedded in the certificate is discovered to be incorrect or has changed; this
might occur, for example, if a person changes jobs or names. A revocation will
likely be a rare occurrence, but the possibility means that when a certificate is
trusted, the user should always check its validity. This can be done by comparing
it against a certificate revocation list (CRL) — a list of revoked or cancelled
certificates. Ensuring that such a list is up-to-date and accurate is a core function
in a centralized PKI, one which requires both staff and budget and one which is
therefore sometimes not properly done. To be effective, it must be readily
available to any who needs it whenever it is needed and must be updated
frequently. The other way to check a certificate validity is to query the certificate
authority using the Online Certificate Status Protocol (OCSP) to know the status
of a specific certificate.
Both of these methods appear to be on the verge of being supplanted by XKMS.
This new standard, however, is yet to see widespread implementation.
A certificate typically includes:
The public key being signed.
A name, which can refer to a person, a computer or an organization.
A validity period.
The location (URL) of a revocation center.
The most common certificate standard is the ITU-T X.509. X.509 is being
adapted to the Internet by the IETF PKIX working group.
Classes
Verisign introduced the concept of three classes of digital certificates:
Class 1 for individuals, intended for email;
Class 2 for organizations, for which proof of identity is required; and
Class 3 for servers and software signing, for which independent verification and
checking of identity and authority is done by the issuing certificate authority (CA)
• List of HTTP status codes
1xx Informational
Request received, continuing process.
100: Continue
101: Switching Protocols
2xx Success
The action was successfully received, understood, and accepted.
200: OK
201: Created
202: Accepted
203: Non-Authoritative Information
204: No Content
205: Reset Content
206: Partial Content
3xx Redirection
The client must take additional action to complete the request.
300: Multiple Choices
301: Moved Permanently
302: Moved Temporarily (HTTP/1.0)
302: Found (HTTP/1.1)
see 302 Google Jacking
303: See Other (HTTP/1.1)
304: Not Modified
305: Use Prox+Many HTTP clients (such as Mozilla and Internet Explorer) don't correctly
handle responses with this status code.
306: (no longer used, but reserved)
307: Temporary Redirect
4xx Client Error
The request contains bad syntax or cannot be fulfilled.
400: Bad Request
401: Unauthorized
Similar to 403/Forbidden, but specifically for use when authentication is possible
but has failed or not yet been provided. See basic authentication scheme and
digest access authentication.
402: Payment Required
403: Forbidden
404: Not Found
405: Method Not Allowed
406: Not Acceptable
407: Proxy Authentication Required
408: Request Timeout
409: Conflict
410: Gone
411: Length Required
412: Precondition Failed
413: Request Entity Too Large
414: Request-URI Too Long
415: Unsupported Media Type
416: Requested Range Not Satisfiable
417: Expectation Failed
5xx Server Error
The server failed to fulfill an apparently valid request.
500: Internal Server Error
501: Not Implemented
502: Bad Gateway
503: Service Unavailable
504: Gateway Timeout
505: HTTP Version Not Supported
509: Bandwidth Limit Exceeded
