Java Tech - Chapter 13 : Networking Basics

The IP layer communicates using packets called datagrams. Datagrams contain headers of 20-60 bytes and data payloads of up to 65K bytes.

Datagrams contain the basic IP source and destination addresses, where the IP (Internet Protocol) address consists of four bytes as in 130.237.217.62. The left most byte is the highest order addressing and so could represent a region or country. The lower order bytes narrow the address down until the final byte indicates, for example, a particular computer on a LAN (Local Area Network).

There are a number of special addresses. For example, any address beginning with 127 such as 127.0.0.1 will translate as a loop back address. This means that any packets sent with this destination address will automatically return to the source computer.

Users typically deal with text addresses called hostnames that are easier to remember than the numerical addresses. For example, java.sun.com is a hostname. Special nodes on the Internet called Domain Name Servers (DNS) translate hostnames into the corresponding numerical IP addresses. A hostname is composed of a top level domain such as com, se, or edu. These domains are then divided into second level subdomains such as sun.com , kth.se, or ucr.edu. The systems at these locations can divide a domain further such as java.sun.com, gluon.particle.kth.se, or physics.ucr.edu.

TCP - UDP

Transmission Control Protocol (TCP) and the User Datagram Protocol (UDP) are two separate Internet protocols within the Transport Layer.

Datagrams from the same message can travel completely different paths as the routers dynamically choose paths for the same destination address so as to avoid loading down any one link. Thus datagrams may become lost or arrive out of order from how they were sent.

The Transport Layer attempts to smooth over the problems of the IP layer. The Transport Layer can place packets into the proper order and request retransmission of missing packets (that is, packets not arriving within a given time).

TCP guarantees bytes in correct order. UDP does not guarantee correct order but has lower overhead than TCP.

For some applications, such as audio and video transmission, the loss of a few bytes is not noticeable and so the UDP serves very well since it is faster than TCP.

Application Layer

The Application Layer involves all the user related programs like web browsers and ftp. The application layer programs use various protocols:

HTTP - Hyper Text Transfer Protocol
FTP - File Transfer Protocol
Telnet - the telnet protocol provides for console sessions.
SMTP - Simple Mail Transport Protocol - email.

These protocols require TCP since data can not be randomly dropped from source files and web pages. Other application layer programs like ping, which sends test packets to a given IP address, can use the simpler UDP. Most Java network programs only deal with this application layer.

References & Web Resources