HPE6-A85 · Question #59
Drag and Drop Question What is the correct order of the TCP 3-Way Handshake sequence? Answer:
The correct answer is The initiating host sends a packet with no data to the target host with a SEQ=1 and sets the SYN flag to 1.; The target host responds with a packet with ACK=2, SEQ=9, and the SYN and ACK flags set to 1.; The initiating host sends a packet with SEQ=2, ACK=9, and the ACK flag set to 1.; A normal-controlled connection is established.. TCP 3-Way Handshake - Explanation The TCP 3-Way Handshake establishes a reliable, synchronized connection before any data is exchanged. The sequence numbers coordinate each side's "starting point" for tracking data. --- Step 1 - SYN (Client Initiates) > "The initiating host sends
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Drag and Drop Question What is the correct order of the TCP 3-Way Handshake sequence? Answer:
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Correct arrangement
- The initiating host sends a packet with no data to the target host with a SEQ=1 and sets the SYN flag to 1.
- The target host responds with a packet with ACK=2, SEQ=9, and the SYN and ACK flags set to 1.
- The initiating host sends a packet with SEQ=2, ACK=9, and the ACK flag set to 1.
- A normal-controlled connection is established.
Explanation
TCP 3-Way Handshake - Explanation
The TCP 3-Way Handshake establishes a reliable, synchronized connection before any data is exchanged. The sequence numbers coordinate each side's "starting point" for tracking data.
Step 1 - SYN (Client Initiates)
"The initiating host sends a packet with no data to the target host with a SEQ=1 and sets the SYN flag to 1."
Why first: The connection must be initiated by one side. The SYN flag signals "I want to connect." SEQ=1 is the client's Initial Sequence Number (ISN) - the baseline for tracking future data. No data is sent because no connection exists yet; this is purely a handshake packet.
Step 2 - SYN-ACK (Server Responds)
"The target host responds with a packet with ACK=2, SEQ=9, and the SYN and ACK flags set to 1."
Why second: The server must do two things simultaneously:
- Acknowledge the client's SYN →
ACK=2means "I received your SEQ=1, send me byte 2 next" (ISN + 1) - Declare its own ISN →
SEQ=9is the server's starting sequence number, also requesting synchronization
Both SYN and ACK flags are set because this packet serves dual purpose. This is the only step where both flags are active.
Step 3 - ACK (Client Confirms)
"The initiating host sends a packet with SEQ=2, ACK=9, and the ACK flag set to 1."
Why third: The client acknowledges the server's SYN-ACK:
SEQ=2- client continues from where it left off (server told it to start here)ACK=9- acknowledges the server'sSEQ=9, confirming the server's ISN was received- Only the ACK flag is set - the client is not initiating a new sync, just confirming
Step 4 - Connection Established
"A normal-controlled connection is established."
Why last: This is the result, not a packet. Once both sides have exchanged and acknowledged each other's ISNs, the connection is fully synchronized and data transfer can begin. It cannot come before the handshake completes.
Common Mistakes & Misconceptions
| Mistake | Reality |
|---|---|
| Placing "connection established" after Step 1 or 2 | The connection is only established after both sides confirm each other's ISNs - requires all 3 steps |
| Confusing which host sends which packet | Step 1 and 3 = client (initiator); Step 2 = server (target) |
| Thinking SYN-ACK is two separate packets | It's one packet with both flags set simultaneously |
| Assuming ACK numbers equal SEQ numbers | ACK = received SEQ + 1 (signals the next expected byte, not a repeat of what was received) |
| Forgetting the handshake is for both directions | The server also needs its ISN acknowledged - that's why there are 3 steps, not 2 |
Memory aid: Think of it like a phone call - "Can you hear me?" (SYN) → "Yes I hear you, can you hear me?" (SYN-ACK) → "Yes!" (ACK) → conversation begins.
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