An Overview of Networking
A
network is a collection of computers that can communicate with each
other. Depending on how wide the coverage is, a network can be referred
to as a local area network (LAN) or a wide area network (WAN). A LAN is
normally confined to a limited geographic area, such as a building, and
comprises from as few as three to as many as hundreds of computers. A
WAN, by contrast, is a combination of multiple LANs that are
geographically separate. The largest network of all is, of course, the
Internet.
The communication medium
within a network can be cables, telephone lines, high-speed fiber,
satellites, and so on. As the wireless technology gets more and more
mature and inexpensive, a wireless local area network (WLAN) is becoming
more commonplace nowadays.
Just like two people use a
common language to converse, two computers communicate by using a common
'language' both agreed on. In computer jargon, this 'language' is
referred to as protocol. What's confusing is that there are several
layers of protocols. This is because at the physical layer two computer
communicate by exchanging bitstreams, which are collections of 1s and
0s. This is too hard to be understood by applications and humans.
Therefore, there is another layer that translates bitstreams into
something more tangible and vice versa.
The easiest protocols are
those at the application layer. Writing applications require you to
understand protocols in the application layer. There are several
protocols in this layer: HTTP, FTP, telnet, etc.
Application layer
protocols use the protocols in the transport layer. Two popular ones at
the transport layer are TCP and UDP. In turn transport layer protocols
utilize the protocols at the layer below it. The diagram in Figure 1 shows some of these layers.
Thanks to this strategy,
you don't have to worry about protocols in other layers than the
application layer. Java even goes the extra mile to provide classes that
encapsulate application layer protocols. For example, with Java, you do
not need to understand the HTTP to be able to send a message to an HTTP
server.
Another thing that you should
know is that a network employs an addressing system to distinguish a
computer from another, just like your house has an address so that the
mailman can deliver your mail. The equivalent of the street address on
the Internet is the IP address. Each computer is assigned a unique IP
address.
The IP
address is not the smallest unit in the network addressing system. The
port is. The analogy is an apartment building that share the same street
address but has many units, each with its own suite number.
The Hypertext Transfer Protocol (HTTP)
The
HTTP is the protocol that allows Web servers and browsers to send and
receive data over the Internet. It is a request and response protocol.
The client requests a file and the server responds to the request. HTTP
uses reliable TCP connections—by default on TCP port 80. The first
version of HTTP was HTTP/0.9, which was then overridden by HTTP/1.0.
Replacing HTTP/1.0 is the current version of HTTP/1.1, which is defined
in Request for Comments (RFC) 2616 and downloadable from
http://www.w3.org/Protocols/HTTP/1.1/rfc2616.pdf.
In the HTTP, it is always the
client who initiates a transaction by establishing a connection and
sending an HTTP request. The Web server is in no position to contact a
client or make a callback connection to the client. Either the client or
the server can prematurely terminate a connection. For example, when
using a Web browser you can click the Stop button on your browser to
stop downloading a file, effectively closing the HTTP connection with
the Web server.
HTTP Requests
An HTTP request consists of three components:
The following is an example of an HTTP request:
POST /examples/default.jsp HTTP/1.1
Accept: text/plain; text/html
Accept-Language: en-gb
Connection: Keep-Alive
Host: localhost
User-Agent: Mozilla/5.0 (Macintosh; U; Intel Mac OS X 10.5; en-US;
rv:1.9.2.6) Gecko/20100625 Firefox/3.6.6
Content-Length: 33
Content-Type: application/x-www-form-urlencoded
Accept-Encoding: gzip, deflate
lastName=Franks&firstName=Michael
The method—URI—protocol version appears as the first line of the request.
POST /examples/default.jsp HTTP/1.1
where POST is the request method, /examples/default.jsp the URI and HTTP/1.1 the Protocol/Version section.
Each HTTP request can use
one of the many request methods as specified in the HTTP standards. The
HTTP 1.1 supports seven types of request: GET, POST, HEAD, OPTIONS, PUT,
DELETE, and TRACE. GET and POST are the most commonly used in Internet
applications.
The URI specifies an
Internet resource. It is usually interpreted as being relative to the
server's root directory. Thus, it should always begin with a forward
slash /. A Uniform Resource Locator (URL) is actually a type of URI (See http://www.ietf.org/rfc/rfc2396.txt). The protocol version represents the version of the HTTP protocol being used.
The request header contains
useful information about the client environment and the entity body of
the request. For example, it could contain the language the browser is
set for, the length of the entity body, and so on. Each header is
separated by a carriage return/linefeed (CRLF) sequence.
Between the headers and the
entity body, there is a blank line (CRLF) that is important to the HTTP
request format. The CRLF tells the HTTP server where the entity body
begins.
In the previous HTTP request, the entity body is simply the following line:
lastName=Franks&firstName=Michael
The entity body can easily become much longer in a typical HTTP request.
HTTP Responses
Similar to an HTTP request, an HTTP response also consists of three parts:
The following is an example of an HTTP response:
HTTP/1.1 200 OK
Server: Apache-Coyote/1.1
Date: Thu, 12 Aug 2010 13:13:33 GMT
Content-Type: text/html
Last-Modified: Thu, 5 Aug 2010 13:13:12 GMT
Content-Length: 112
<html>
<head>
<title>HTTP Response Example</title>
</head>
<body>
Welcome to Brainy Software
</body>
</html>
The first line of the
response header is similar to the first line of a request header. The
first line tells you that the protocol used is HTTP version 1.1, the
request succeeded (200 is the success code), and that everything went
okay.
The
response headers contain useful information similar to the headers in
the request. The entity body of the response is the HTML content of the
response itself. The headers and the entity body are separated by a
sequence of CRLFs.
