4.1.6 Segmentation and Reassembly - Divide and Conquer Page 1:

A previous chapter explained how PDUs are built by passing data from an application down through the various protocols to create a PDU that is then transmitted on the medium. At the destination host, this process is reversed until the data can be passed up to the application.

Some applications transmit large amounts of data - in some cases, many gigabytes. It would be impractical to send all of this data in one large piece. No other network traffic could be transmitted while this data was being sent. A large piece of data could take minutes or even hours to send. In addition, if there were any error, the entire data file would have to be lost or resent. Network devices would not have memory buffers large enough to store this much data while it is transmitted or received. The limit varies depending on the networking technology and specific physical medium being in use.

Dividing application data into pieces both ensures that data is transmitted within the limits of the media and that data from different applications can be multiplexed on to the media.

TCP and UDP Handle Segmentation Differently.

In TCP, each segment header contains a sequence number. This sequence number allows the Transport layer functions on the destination host to reassemble segments in the order in which they were transmitted. This ensures that the destination application has the data in the exact form the sender intended.

Although services using UDP also track the conversations between applications, they are not concerned with the order in which the information was transmitted, or in maintaining a connection. There is no sequence number in the UDP header. UDP is a simpler design and generates less overhead than TCP, resulting in a faster transfer of data.

Information may arrive in a different order than it was transmitted because different packets may take different paths through the network. An application that uses UDP must tolerate the fact that data may not arrive in the order in which it was sent.

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In this activity, you will "look inside" packets to see how DNS and HTTP use port numbers.

Click the Packet Tracer icon to launch the Packet Tracer activity.

4.2 The tcp Protocol - Communicating with Reliability

4.2.1 TCP - Making Conversations Reliable Page 1:

The key distinction between TCP and UDP is reliability. The reliability of TCP communication is performed using connection-oriented sessions. Before a host using TCP sends data to another host, the Transport layer initiates a process to create a connection with the destination. This connection enables the tracking of a session, or communication stream between the hosts. This process ensures that each host is aware of and prepared for the communication. A complete TCP conversation requires the establishment of a session between the hosts in both directions.

After a session has been established, the destination sends acknowledgements to the source for the segments that it receives. These acknowledgements form the basis of reliability within the TCP session. As the source receives an acknowledgement, it knows that the data has been successfully delivered and can quit tracking that data. If the source does not receive an acknowledgement within a predetermined amount of time, it retransmits that data to the destination.

Part of the additional overhead of using TCP is the network traffic generated by acknowledgements and retransmissions. The establishment of the sessions creates overhead in the form of additional segments being exchanged. There is also additional overhead on the individual hosts created by the necessity to keep track of which segments are awaiting acknowledgement and by the retransmission process.

This reliability is achieved by having fields in the TCP segment, each with a specific function, as shown in the figure. These fields will be discussed later in this section.