Ccna routing and Switching Complete Study Guide

Key Concepts of Host-to-Host Protocols

Since you’ve now seen both a connection-oriented (TCP) and connectionless (UDP) proto-

col in action, it’s a good time to summarize the two here. Table 3.1 highlights some of the 

key concepts about these two protocols for you to memorize.

Ta b l e   3 .1     Key features of TCP and UDP

TCP

UDP

Sequenced

Unsequenced

Reliable

Unreliable

Connection-oriented

Connectionless

Virtual circuit

Low overhead

Acknowledgments

No acknowledgment

Windowing flow control

No windowing or flow control of any type

And if all this isn’t quite clear yet, a telephone analogy will really 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 no matter where they are. This is akin to 

establishing a virtual circuit with the TCP protocol. If you were giving someone important 

information during your conversation, you might say things like, “You know? or “Did you 

get that?” Saying things like this is a lot like a TCP acknowledgment—it’s designed to get 

you verification. From time to time, especially on mobile phones, people ask, “Are you still 

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Chapter 3 

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  Introduction to TCP/IP 

there?” People end their conversations with a “Goodbye” of some kind, putting closure on 

the phone call, which you can think of as tearing down the virtual circuit that was created 

for your communication session. TCP performs these types of functions.

Conversely, using UDP is more 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 

send it off. 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.

Let’s take a look at another figure, one that includes TCP, UDP, and the applications 

associated to each protocol: Figure 3.14 (discussed in the next section).

f I g u r e   3 .14     Port numbers for TCP and UDP

FTP

Telnet

POP3

DNS

TFTP

BootPS

TCP

UDP

Application

layer

Port numbers

Transport

layer

21

23

110

53

69

67