nerdexam
GIAC

GSLC · Question #517

The 3-way handshake method is used by the TCP protocol to establish a connection between a client and the server. It involves three steps: 1. In the first step, a SYN message is sent from a client…

The correct answer is B. 241713112. In a TCP 3-way handshake, the server's SYN/ACK acknowledgement number equals the client's ISN plus one, confirming receipt of the client's SYN segment.

Security Architecture & Engineering

Question

The 3-way handshake method is used by the TCP protocol to establish a connection between a client and the server. It involves three steps: 1. In the first step, a SYN message is sent from a client to the server. 2. In the second step, a SYN/ACK message is sent from the server to the client. 3. In the third step, an ACK (usually called SYN-ACK-ACK) message is sent from the client to the server. At this point, both the client and the server have received acknowledgements of the TCP connection. If the Initial Sequence Numbers of the client and server were 241713111 and 241824111 respectively at the time when the client was sending the SYN message in the first step of the TCP 3-way handshake method, what will be the value of the acknowledgement number field of the server's packet when the server was sending the SYN/ACK message to the client in the second step of the TCP 3-way handshake method?

Options

  • A241824111
  • B241713112
  • C241824112
  • D241713111

How the community answered

(26 responses)
  • A
    15% (4)
  • B
    73% (19)
  • C
    8% (2)
  • D
    4% (1)

Why each option

In a TCP 3-way handshake, the server's SYN/ACK acknowledgement number equals the client's ISN plus one, confirming receipt of the client's SYN segment.

A241824111

241824111 is the server's own Initial Sequence Number placed in the sequence number field of the SYN/ACK, not the acknowledgement number field.

B241713112Correct

When the server receives the client's SYN with ISN 241713111, it must acknowledge that sequence number by incrementing it by 1 (241713111 + 1 = 241713112) in the acknowledgement field of its SYN/ACK packet. This increment signals that the server has received the SYN and is expecting the next byte starting at 241713112. The SYN flag itself consumes one sequence number, which is why the increment is exactly 1.

C241824112

241824112 would be the client's acknowledgement of the server's SYN (server ISN + 1), which appears in step 3 (the client's ACK), not in the server's SYN/ACK.

D241713111

241713111 is the raw client ISN with no increment; the ACK number must be the next expected byte, which requires adding 1 to the received SYN sequence number.

Concept tested: TCP 3-way handshake sequence and acknowledgement numbers

Source: https://www.rfc-editor.org/rfc/rfc793

Topics

#TCP handshake#sequence numbers#network protocol#connection establishment

Community Discussion

No community discussion yet for this question.

Sign up to join the discussion
Full GSLC Practice