Comptia Security Plus Mock Test Q20

Which the following flags are used to establish a TCP connection? (Select TWO).

A. PSH
B. ACK
C. SYN
D. URG
E. FIN

Correct Answer: B,C
Section: Network Security

Explanation:
To establish a TCP connection, the three-way (or 3-step) handshake occurs:
SYN: The active open is performed by the client sending a SYN to the server. The client sets the segment’s sequence number to a random value A.
SYN-ACK: In response, the server replies with a SYN-ACK. The acknowledgment number is set to one more than the received sequence number i.e. A+1, and the sequence number that the server chooses for the packet is another random number, B.
ACK: Finally, the client sends an ACK back to the server. The sequence number is set to the received acknowledgement value i.e. A+1, and the acknowledgement number is set to one more than the received sequence number i.e. B+1.

Incorrect Answers:
A: The PSH flag tells the TCP stack to flush all buffers and send any outstanding data up to and including the data that had the PSH flag set.

D: URG indicates that the urgent pointer field has a valid pointer to data that should be treated urgently and be transmitted before non-urgent data.

E: FIN is used to indicate that the client will send no more data.

References:
http://linuxpoison.blogspot.com/2007/11/what-are-tcp-control-bits.html