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ccna study guide pdf aaron balchunas.UDP uses a best-effort transmission system, which is similar to the U.S. Postal System's Plain Mail service. When you mail a piece of flat mail, you write the recipient'saddress and return address on an envelope, drop the letter into the mailbox, and hope that it will reach the recipient. When using this service, there is no guarantee that the letter will reach the recipient, but in most cases it will. However, if the letter does not reach the recipient, it is the responsibility of the letter writer to remail it; the post office does not do this task for the letter writer.UDP is similar in that it does not guarantee that the transport layer data segment will reach its destination.TCP provides a reliable connection, while UDP provides an unreliable connection. udp does not establish a connection through three handshakes, but sends data directly. Also, UDP does not check whether the target device has received the data segment that has been sent out; in other words, it performs an acknowledgement. If an acknowledgement is necessary and the transport layer (UDP) does not provide such a feature, it will usually be up to the application layer application to provide this verification.Despite these drawbacks, UDP has one advantage over TCP: less overhead. For example, if only one data segment needs to be sent and communication ends when an answer is received, it would be unreasonable to send and receive two more data segments via three handshakes to establish a connection because it would be too inefficient.DNS queries are a good example of where UDP would be appropriate. Two other applications that often use UDP are voice and video; but only if the path to the target device is so reliable that no large number of packets are dropped, and even if a packet is lost from time to time, the caller and video viewer may not be aware of it. Of course, if a large amount of data is to be sent to the target device and it needs to be verified that all the data has been received, TCP would be the better transmission mechanism.Exam Essentials UDP is more efficient than TCP because it has less overhead.When transmitting a UDP data segment, the value of the protocol field in the IP header is 17 Table 8.2 lists the components of a UDP data segment. Note UDPdata segments differ significantly from TCP data segments. First, because UDP is connectionless, it does not require a sequence number or an acknowledgment number. Second, because there is no flow control, the window size field is not required. As you can see, compared to TCP, UDP is much simpler and more efficient. Like TCP, UDP contains a checksum field, which is the only field associated with reliability. The checksum field allows the target device's UDP to discover corrupted data segments and thus discard them.UDP does not implement any of the control and other reliability features required for a session; these will be provided by the application layer.Table 8.2 Components of UDP Data Segments UDP Field Name Length (bits) Description.Source Port 16 Identifies the application sending the messageDestination Port 16 Identifies the application that received the message