The answer is that the TCP handshake is incomplete because the SYN-ACK has not been received. This is indicated by the proto_state=01 value in the diagnose sys session filter output, which corresponds to TCP state 1, or SYN-SENT, meaning the initial SYN packet was sent but the remote host has not yet responded with a SYN-ACK. The proto=6 confirms it is a TCP session, while the duration and expire fields show the session’s age and remaining time to live, not its establishment status. On the Fortinet NSE 4 Network Security Professional NSE4 exam, interpreting this output tests your ability to diagnose session setup failures, a common troubleshooting scenario for firewall policies and asymmetric routing. A frequent trap is mistaking a high duration for an established session, but the state field is the definitive indicator. Memory tip: think of “01” as “one-way” — only the SYN has gone out, so the handshake is still one-sided.
NSE4 Firewall Policies and NAT Practice Question
This NSE4 practice question tests your understanding of firewall policies and nat. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
You run 'diagnose sys session filter dport 443' and see the following output:
proto=6 proto_state=01 duration=3600 expire=3599
What does this indicate?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The TCP handshake is incomplete; the SYN-ACK has not been received
The output shows a TCP session (proto=6) with proto_state=01, which in Fortinet's session table indicates the session is in the SYN-SENT state (TCP state 1). This means the initial SYN has been sent but the SYN-ACK has not yet been received, so the TCP three-way handshake is incomplete. The duration and expire values reflect the session's age and remaining lifetime, not its establishment status.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
The TCP handshake is incomplete; the SYN-ACK has not been received
Why this is correct
State 01 is SYN_SENT, meaning the SYN has been sent but no SYN-ACK received yet.
Related concept
Read the scenario before looking for a memorised answer.
duration is the total session lifetime, not idle time.
✗
The session is fully established and will expire in 3599 seconds
Why it's wrong here
State 01 indicates SYN_SENT, not established.
Common exam traps
Common exam trap: answer the scenario, not the keyword
The trap here is that candidates see 'expire=3599' and assume the session is established and about to expire, but Fortinet's proto_state field directly reveals the TCP handshake phase, and state 01 specifically means the handshake is incomplete.
Detailed technical explanation
How to think about this question
Fortinet's session table uses proto_state values that map directly to TCP state machine states (RFC 793): 01 = SYN-SENT, 02 = ESTABLISHED, 03 = FIN-WAIT-1, etc. The 'diagnose sys session filter' command is a powerful troubleshooting tool; a high number of sessions in proto_state=01 often indicates a network issue like asymmetric routing, firewall policy blocking return traffic, or a server not responding to SYN packets. The expire counter decrements from the session timeout (default 3600 seconds for TCP) and resets on traffic, so a low expire value with a high duration suggests the session is aging out without completing the handshake.
KKey Concepts to Remember
Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.
TExam Day Tips
→Watch for words such as best, first, most likely and least administrative effort.
→Review why wrong options are wrong, not only why the correct option is correct.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
How this comes up in practice
A practitioner preparing for the NSE4 exam encounters this exact type of scenario on the job. The correct answer here is not the most general option — it is the best answer for the specific constraint described. Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option. Real exam questions reward reading the full scenario before eliminating options, because the constraint defines which answer fits.
What to study next
Got this wrong? Here's your next step.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Firewall Policies and NAT — This question tests Firewall Policies and NAT — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The TCP handshake is incomplete; the SYN-ACK has not been received — The output shows a TCP session (proto=6) with proto_state=01, which in Fortinet's session table indicates the session is in the SYN-SENT state (TCP state 1). This means the initial SYN has been sent but the SYN-ACK has not yet been received, so the TCP three-way handshake is incomplete. The duration and expire values reflect the session's age and remaining lifetime, not its establishment status.
What should I do if I get this NSE4 question wrong?
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
What is the key concept behind this question?
Read the scenario before looking for a memorised answer.
About these practice questions
Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. You run 'diagnose sys session filter dport 443' and see the following output:
proto=6 proto_state=01 duration=3600 expire=3599
What does this indicate?
medium
✓ A.The session is a TCP session that has been established for 1 hour and will expire in about 1 hour
B.The session is a UDP session with destination port 443
C.The session will expire in 3600 seconds
D.The session is a TCP session in SYN_SENT state
Why A: Option A is correct because the output shows `proto=6`, which indicates TCP (protocol number 6), and `proto_state=01` corresponds to TCP state ESTABLISHED. The `duration=3600` means the session has been active for 3600 seconds (1 hour), and `expire=3599` indicates the session will expire in 3599 seconds (approximately 1 hour). This matches the description of a TCP session established for 1 hour with about 1 hour remaining before expiry.
Variation 2. You run 'diagnose sys session filter dport 443' and see the following output:
proto=6 proto_state=01 duration=3600 expire=3599
What does this indicate about the session?
medium
✓ A.The session is in the SYN_SENT state, waiting for a SYN-ACK
B.The session is using UDP protocol
C.The session is fully established and actively transferring data
D.The session is being torn down and will expire soon
Why A: The output shows `proto=6` (TCP) and `proto_state=01`, which in Fortinet's session table corresponds to the TCP state `SYN_SENT` (the session has sent a SYN and is awaiting a SYN-ACK). The `duration=3600` and `expire=3599` indicate the session has been alive for 3600 seconds and will expire in 3599 seconds, but the state itself is not established or closing. Option A is correct because `proto_state=01` specifically maps to the TCP SYN_SENT state in FortiOS session diagnostics.
Variation 3. You run 'diagnose sys session filter dport 443' and see the following output:
proto=6 proto_state=01 duration=3600 expire=3599
What does this indicate?
medium
✓ A.The session is a TCP session that has been active for 3600 seconds and will expire in 3599 seconds
B.The session is a UDP session for DNS
C.The session is an ICMP echo request
D.The session is blocked by a firewall policy
Why A: proto=6 indicates TCP, proto_state=01 is TCP SYN_SENT (or ESTABLISHED depending on FortiOS version, but typically 01 is ESTABLISHED), duration=3600 seconds, expire=3599 seconds remaining. The session is established and about to expire (TTL almost up).
Variation 4. You run 'diagnose sys session filter dport 443' and see the following output:
proto=6 proto_state=01 duration=3600 expire=3599
What does this indicate about the session?
hard
✓ A.The session is a TCP session that has been active for 1 hour and will expire in about 1 hour
B.The session is a TCP session that has been active for 1 hour and will expire immediately
C.The session is a UDP session that has been idle for 3600 seconds
D.The session is an ICMP session with a short timeout
Why A: The output shows a TCP session (proto=6) in state 01 (possibly ESTABLISHED or SYN_SENT), duration 3600 seconds, with 3599 seconds until expiry. It is a normal session.
Last reviewed: Jun 24, 2026
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