Computer Networks — Test 29

Correct Answer: B. B. 123
Explanation: The Network Time Protocol is a clock synchronization network protocol implemented by using UDP port number 123 to send and receive time stamps.

Correct Answer: A. A. 8 bytes
Explanation: The fixed size of the UDP packet header is 8 bytes. It contains four two-byte fields: Source port address, Destination port address, Length of packet, and checksum.

Correct Answer: D. D. Client
Explanation: Port numbers from 1025 to 5000 are used as ephemeral port numbers in Windows Operating System. Ephemeral port numbers are short-lived port numbers which can be used for clients in a UDP system where there are temporary clients all the time.

Correct Answer: D. D. Udp header plus data
Explanation: Total length is the 16 bit field which contains the length of UDP header and the data. The maximum value of the Total length field and the maximum size of a UDP datagram is 65,535 bytes (8 byte header + 65,527 bytes of data).

Correct Answer: A. A. Udp length = ip length €? ip header’s length
Explanation: A user datagram is encapsulated in an IP datagram. There is a field in the IP header that defines the total length of the IP packet. There is another field in the IP header that defines the length of the header. So if we subtract the length of the IP header that is encapsulated in the IP packet, we get the length of UDP datagram.

Correct Answer: B. B. Checksum
Explanation: Checksum field is used to detect errors over the entire user datagram. Though it is not as efficient as CRC which is used in TCP, it gets the job done for the UDP datagram as UDP doesn’t have to ensure the delivery of the packet.

Correct Answer: B. B. Esp transport mode
Explanation: ESP transport mode should be used to ensure the integrity and confidentiality of data that is exchanged within the same LAN. ESP tunnel mode is comparatively more secure and should be used to assure the security of the data within different LANs.

Correct Answer: D. D. Both sha and md5
Explanation: SHA or MD5 can be used. Kerberos V5 is an authentication protocol, not an encryption protocol; therefore, answer A is incorrect. Certificates are a type of authentication that can be used with IPsec, not an encryption protocol; therefore, answer B is incorrect.

Correct Answer: C. C. Both ah tunnel mode and esp tunnel mode
Explanation: The AH and ESP tunnel mode IPSec should be used for data transfer purpose, option d is for integrity & confidentiality purpose. Tunnel mode provides security for the entire original IP packet unlike transport mode which is not as secure as it only encrypts the data portion and not the whole packet.

Correct Answer: A. A. Ah
Explanation: The Authentication Header (AH) authenticates the origin of data, and guarantees the integrity of the information that’s being sent using IPSec. It also provides anti-reply security.