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TCP/IP and the DoD Model

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Notice that low overhead! Try to find the sequence number, ack number,

and window size in the UDP segment. You can't (I hope) because they just
aren't there!

Key Concepts of Host-to-Host Protocols

Since we've seen both a connection-oriented (TCP) and connectionless
(UDP) protocol in action, it would be good to summarize the two here. The
following list highlights some of the key concepts that you should keep in
mind regarding these two protocols. Memorize Table 3.1.

A telephone analogy could 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--wherever they are. This
is like a virtual circuit with the TCP protocol. If you were giving someone
important information during your conversation, you might say, "You
know?" or ask, "Did you get that?" Saying something like this is a lot like
a TCP acknowledgment--it's designed to get you verification. From time to
time (especially on cell phones), 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 orien-
tation. Since the message on the postcard is probably not a matter of life or

T A B L E 3 . 1

Key Features of TCP and UDP

TCP

UDP

Sequenced

Unsequenced

Reliable

Unreliable

Connection-oriented

Connectionless

Virtual circuit

Low overhead

Three-way handshake

No acknowledgment

Windowing flow control

No windowing or flow control

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