A TCP/IP primer

Networking has become key to many business
processes. Keeping that network running smoothly falls into the domain
of mission-critical functions. Now that the Internet has exploded onto
the front pages of USA Today and has references sprinkled
through the Wall Street Journal, the amount of work created
for system administrators who run sites, fix configuration problems,
and resolve user complaints has also increased in magnitude.

This month, we're going to look at the business of making connections
between TCP/IP end-points. It's a small subset of the broader
network-administration problems of performance, security, and
controlled-growth management, but it's also the most fundamental. If
you can't get connected or you can't keep unwanted network entities
from connecting to you, then the big picture gets obscured by the pile
of complaints reading "the network is down."

Making TCP/IP connections appears to be fairly simple: You identify
the other host, specify a service you want to use, and blast data down
the wire, hoping that you'll enjoy some semblance of good performance
and reliable security all the while. Meeting those expectations is
what makes TCP/IP connection management an interesting problem. We'll
review the mechanics of TCP/IP connections and service location and
go through the details of linking two end-points over a socket.

Junction introduction: Getting linked

The first requirement for establishing a TCP/IP connection is the name
of the other end. That name consists of two parts: the remote IP
address and a port number. IP addresses are obtained through the Domain
Name Service (DNS), Network Information Service (NIS), or the local
/etc/hosts file, all of which map hostnames to IP addresses.
The port number is a bit trickier because it depends on the service
being used. Each host numbers ports starting at 1, with a separate set
of ports for TCP and the User Datagram Protocol (UDP). Since each IP
packet contains information about the higher-level protocol it is
carrying, the two port sets cannot be confused. Port numbers below 1024
are known as reserved ports and can be opened only by
processes running as root; port numbers 1024 and above are
unrestricted.

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