Flow control is the process of regulating the amount of data sent by a host to another host on the network. The flow control mechanism controls packet flow so that a sender does not transmit more packets than a receiver can process. TCP uses a sliding window flow control protocol. In each TCP segment, the receiver specifies in the receive window field the amount of additional received data (in bytes) that it is willing to buffer for the connection. The sending host can send only up to that amount of data before it must wait for an acknowledgment and window update from the receiving host.
Answer option A is incorrect. TCP uses a sequence number for identifying each byte of data.
Answer option B is incorrect. TCP timestamp helps TCP to compute the round-trip time between the sender and receiver.
Answer option C is incorrect. Congestion control concerns controlling traffic entry into a telecommunications network, so as to avoid congestive collapse by attempting to avoid oversubscription of any of the processing or link capabilities of the intermediate nodes and networks and taking resource reducing steps, such as reducing the rate of sending packets. It should not be confused with flow control, which prevents the sender from overwhelming the receiver.

In a brute force attack, an attacker uses software that tries a large number of key combinations in order to get a password. To prevent such attacks, users should create passwords that are more difficult to guess, i.e., by using a minimum of six characters, alphanumeric combinations, and lower-upper case combinations.
Answer option D is incorrect. Smurf is an attack that generates significant computer network traffic on a victim network. This is a type of denial-of-service attack that floods a target system via spoofed broadcast ping messages. In such attacks, a perpetrator sends a large amount of ICMP echo request (ping) traffic to IP broadcast addresses, all of which have a spoofed source IP address of the intended victim. If the routing device delivering traffic to those broadcast addresses delivers the IP broadcast to all hosts, most hosts on that IP network will take the ICMP echo request and reply to it with an echo reply, which multiplies the traffic by the number of hosts responding.
Answer option A is incorrect. Buffer overflow is a condition in which an application receives more data than it is configured to accept. It helps an attacker not only to execute a malicious code on the target system but also to install backdoors on the target system for further attacks. All buffer overflow attacks are due to only sloppy programming or poor memory management by the application developers. The main types of buffer overflows are:
Stack overflow
Format string overflow
Heap overflow
Integer overflow
Answer option C is incorrect. A zero-day attack, also known as zero-hour attack, is a computer threat that tries to exploit computer application vulnerabilities which are unknown to others, undisclosed to the software vendor, or for which no security fix is available. Zero-day exploits (actual code that can use a security hole to carry out an attack) are used or shared by attackers before the software vendor knows about the mvulnerability. User awareness training is the most effective technique to mitigate such attacks.