LPI Linux Certification in a Nutshell (59 page)

TCP/IP is a suite of Internet protocols, including the
Transmission Control Protocol (TCP), Internet Protocol (IP), User
Datagram Protocol (UDP), and Internet Control Message Protocol (ICMP),
among others. Some protocols use
handshaking
(the exchange of control
information among communicating systems) to establish and maintain a
connection. Such a protocol is said to be connection-oriented and
reliable, because the protocol itself is responsible for handling
transmission errors, lost packets, and packet arrival order. A protocol
that does not exchange control information is said to be connectionless
and unreliable. In this context, “unreliable” simply means that the
protocol doesn’t handle transmission problems itself; they must be
corrected in the application or system libraries. Connectionless
protocols are simpler and have fewer overheads than connection-oriented
protocols. TCP/IP is a
stack
of protocols because protocols
are built in a hierarchy of
layers
. Low-level protocols are used
by higher-level protocols on adjacent layers of the protocol
stack:

When an inbound network request is made, such as that from
a web browser or FTP client, it is sent to the IP address of the server.
In addition, the request carries inside it a
port number
(or just
port
), which is a 16-bit value placed near the
beginning of a network packet. The port number defines the type of
server software that should respond to the request. For example, by
default, web browsers send requests en
coded
for port 80. Web servers “listen” to
port 80 and respond to incoming requests. The encoded port can be
considered part of the address of a request. While the IP address
specifies a particular interface (or host), the port specifies a
specific service available on that host. Many port numbers are
predefined, and the list is expanded as needed to accommodate new
technologies. The official list of port number assignments is managed by
the Internet Assigned Numbers Authority (IANA). The ports known by your
system are listed in
/etc/services
.

Port numbers 1 through 1023 are often referred to as
privileged ports
because the services
that use them often run with superuser authority. Many of these, such as
ports used for FTP (21), Telnet (23), and HTTP (80), are often referred
to as
well-known ports
because they are
standards. Port numbers from 1024 through 65535 (the maximum) are
unprivileged ports
and can be used by
applications run by ordinary system users.

During the initial contact, the client includes a local (randomly
selected) unprivileged port on the client machine for the server to use
when responding to the request. Client-to-server communications use the
well-known port, and the server-to-client communications use the
randomly selected port. This Objective requires you to be familiar with
the privileged port numbers detailed in
Table 19-4
.

This list is a tiny fraction of the many well-known ports, but it
may be necessary for you to know those in the list both by name and by
number.

The following popular applications, although not strictly
a part of TCP/IP, are usually provided along with a TCP/IP
implementation.

-i

Turn off interactive prompting during multiple file
transfers (also see the
prompt
command in
the next list).

-v

Set verbose mode; display server responses and transfer
statistics.

ascii
,
binary

Establish the transfer mode for files.
ASCII mode is provided to correctly transfer
text among computer architectures where character encoding
differs.

get
file


Receive a single
file
from the server.

mget
files


Receive multiple
files
from the server.
files
can be specified using normal
file glob
patterns
.

ls
[
files
]

Obtain a directory listing from the server,
optionally listing
files
.

put
file


Send a single
file
to
the server.

mput
files


Send multiple
files
to the server.

prompt

Toggle on and off interactive prompting during
mget
and
mput
(also
see the
-i
option in the previous
list).

pwd

Print the working remote directory.

quit, exit

Cleanly terminate the FTP session.