Chapter 9

List three of the main features of TCP.

Answer: The three main features of TCP are segmentation, error recovery, and using port numbers to allow delivery to the correct application program. Also, TCP helps reduce complexity in applications by hiding the data delivery details from the application, as well as providing encapsulation to deliver the data across a network.

Imagine that you visit a website, and as a result, you download a graphics image called logo.gif. The logo is 4000 bytes. How many segment(s) will the server send back to your browser to send you the logo? Assume an mss of 1480 bytes. Explain your answer.

Answer: TCP will segment the data into pieces, typically no larger than 1460 bytes of data. (An mss of 1480 minus the 20 bytes of TCP header leaves 1460 bytes for the data in each segment.) To send 4000 bytes, 3 segments would be required.

Imagine that a web server sent 3 TCP segments to a browser on Keith's PC, each 1000 bytes long, with sequence numbers 4304, 5304, and 6304, respectively. Assuming no errors had occurred, explain how Keith's PC could acknowledge receipt of the data.

Answer: In the next TCP segment that Keith sends to the web server, his TCP software will set the acknowledgement field to 7304, the next byte that he expects to receive from the server.

Imagine that a web server sent 3 TCP segments to a browser on Keith's PC, each 1000 bytes long, with sequence numbers 4304, 5304, and 6304, respectively. Assuming that the third segment had errors in it, explain how the data would be recovered.

Answer: In the next TCP segment that Keith sends to the web server, his TCP software will set the acknowledgement field to 6304, the next byte that he expects to receive from the server. That tells the server that the third segment Keith had sentthe one with the sequence number field set to 6304was not received. So, the server re-sends the third segment.

Define the word encapsulation, using TCP encapsulation as an example.

Answer: When TCP receives data from an application layer protocol on the same computer, TCP adds a TCP header to the data. That process is called encapsulation.

Imagine that Keith brings up a browser on his PC and browses a website. He then brings up another browser, browsing a second website. When Keith receives the next TCP segment, how does his PC know to which browser to give the data?

Answer: TCP on Keith's PC looks at the destination port number field in the received TCP segment's header. Each browser uses a different TCP port number, so Keith's PC can identify which browser should receive the data.

How many TCP segments flow between two computers to establish a TCP connection?

Answer: Three

What is the name of the other TCP/IP transport layer protocol?

Answer: User Datagram Protocol (UDP)

A TCP header, along with the application data, is called what?

Answer: A TCP segment

Define the term forward acknowledgment.

Answer: Forward acknowledgment refers to the process of acknowledging data by setting the acknowledgment field to the next byte of data that should be received, as opposed to setting the acknowledgment field to the last byte of data that was received.

Define the term well-known port.

Answer: A well-known port is a port number that a server purposefully uses for a particular application so that when client computers want to use that service, they know what destination port to put into the TCP destination port field.