CCNA Cisco Certified Network Associate Study Guide - FTP Server

TCP/IP and the DoD Model 113
A telephone analogy might help you understand how TCP works. Most of
us know that before you speak to someone on a phone, you must first establish
a connection with that other person—wherever they might be. This is
like a virtual circuit with the TCP protocol. If you were giving someone
important information during your conversation, you might ask, “Did you
get that?” A query like that is similar to a TCP acknowledgment. From time
to time, for various reasons, people also ask, “Are you still there?” They end
their conversations with a “goodbye” of some kind, putting closure on the
phone call. TCP also performs these types of functions.
Alternately, using UDP is like sending a postcard. To do that, you don’t
need to contact the other party first. You simply write your message, address
the postcard, and mail it. This is analogous to UDP’s connectionless orientation.
Since the message on the postcard is probably not a matter of life or
death, you don’t need an acknowledgment of its receipt. Similarly, UDP does
not involve acknowledgments.
Port Numbers
TCP and UDP must use port numbers to communicate with the upper layers.
Port numbers keep track of different conversations crossing the network
simultaneously. Originating-source port numbers are dynamically assigned
by the source host, which will be some number starting at 1024. 1023 and
below are defined in RFC 1700, which discusses what is called well-known
port numbers.
Virtual circuits that do not use an application with a well-known port
number are assigned port numbers randomly chosen from within a specific
range instead. These port numbers identify the source and destination host
in the TCP segment.
Figure 3.5 illustrates how both TCP and UDP use port numbers.
FIGURE 3.5 Port numbers for TCP and UDP
Application
layer
Port
numbers
Transport
layer
FTP Telnet Doom DNS TFTP POP3
21
23
TCP
666
53
69
UDP
110
News
144