Understanding the network.pdf - Back to Home

Internet Society, a nonprofit organization that coordinates the development of the
Internet.
TCP/IP and UNIX
In the early days of computing, operating systems were something the user built.
Early computer systems came with a limited set of tools, which provided the
capability to load, compile program routines, and perform basic I/O tasks. The
"operating system" was developed by the programmer/users because everything
was written in the assembler language specific to that computer (there was no Plug
and Play in the early days). In the late 1960s, two fellows at Bell Labs, Dennis
Ritchie and Ken Thompson, started working on a different approach to computer
operating systems, known as UNIX. The goal was to develop a core operating
system (the kernel) to manage the various hardware elements of the computer
(CPU processing, memory management, storage I/O, and so on), plus an extensive
set of tools that were easily ported to any hardware platform.
Although UNIX was the property of Bell Labs, it was made available to universities
and research facilities for noncommercial use with no support. As a result of its wide
distribution, a lab project became an operating system that was improved by a large
community of academic programmers who saw its potential. The notion that "the
documentation is the code" and that "programs should be small, and do one thing
and do it well" came out of this giant collaborative effort. In 1974, some of the
people involved with the development of ARPAnet and TCP/IP saw a similar synergy
between UNIX and TCP/IP. Along with the integration of TCP/IP protocols into
ARPAnet, an effort was started at the University of California at Berkeley to
integrate TCP/IP into the UNIX implementation. This beta (experimental) TCP/IP
stack was introduced in 1981 as the Berkeley Software Distribution (BSD) 4.1a
release, and the production stack was BSD 4.2. The result of this integration
enabled the majority of ARPAnet sites to connect through TCP/IP. Along with its
status as a government standard, TCP/IP's wide use in the UNIX operating system
ensured its place as the de facto networking standard.
Layer 3: IP Protocol
The IP protocol is responsible for the delivery of datagrams. A datagram is a block
of data, "packaged" by the upper layer protocols for transmission. This data is
"packaged" by the Upper Layer Protocol (ULP). The contents of this data can be TCP
or UDP transport data or the transport layer data of a different networking protocol,
such as AppleTalk or IPX. IP is uninterested in the type of data it is providing
delivery for and can accommodate any Layer 2 packet size limitation, which it
handles by fragmenting the datagram into the supported Layer 2 frame size. The
datagram fragments are then transmitted to the destination host where they are