Website Testing
TCP Connection Test: 3-Way Handshake Latency
Every TCP connection begins with a 3-way handshake (SYN, SYN-ACK, ACK) that costs one full round trip before any data flows. The TCP Connection Test measures this handshake time for any host, isolating connection setup from any application or TLS overhead.
What It Measures
This tool measures TCP connection establishment time in milliseconds, separating the SYN to SYN-ACK time (server reachability and round trip) from the SYN-ACK to ACK time (your local processing). It also reports any retransmissions or failures that suggest path or server issues.
How It Works
- Initiates a TCP connection to the target host and port
- Records the time from SYN sent to SYN-ACK received
- Records the time from SYN-ACK to ACK sent (small, mostly local)
- Reports total handshake time, retransmissions, and any failures
Why It Matters
Slow TCP handshakes contribute to slow page loads, slow API calls, and slow connections in general. If TCP handshake to a server is much higher than your ICMP ping to the same host, the server may be overloaded with SYN backlog, blocked by a firewall, or located behind extra network layers (NAT, load balancers). Knowing the breakdown helps target the right fix.
Understanding Your Results
TCP handshake should track within 5ms of your ICMP ping to the same host. Significantly higher handshake time indicates server side queue saturation or middlebox processing. Zero retransmissions during the handshake is ideal. Repeated handshake failures mean the port is filtered, the server is down, or you are being rate limited.
Frequently Asked Questions
Why is TCP handshake higher than ping?
ICMP ping bypasses application layer processing on the server. TCP handshake requires the server's TCP stack to allocate resources (a SYN backlog slot, then a full socket on ACK). If the server is overloaded, has a small SYN queue, or sits behind a firewall doing SYN inspection, handshake time exceeds ping. The difference is a useful diagnostic.
What is TCP Fast Open?
TCP Fast Open (TFO) is an extension that allows sending data in the SYN packet for repeat connections, eliminating one round trip on subsequent connections. It is supported in Linux, macOS, modern Windows, and most modern web servers, but some firewalls and middleboxes drop SYN packets carrying data. When supported end to end, TFO reduces handshake overhead significantly.
What does a slow SYN-ACK indicate?
Slow SYN-ACK (the server's response to your SYN) usually means server side: SYN queue full (often the start of a SYN flood attack), application layer load balancer slow to allocate, or the server's network stack is busy. If multiple users see the same slow SYN-ACK to a server, the issue is likely server side rather than network side.
How does this differ from the website latency test?
TCP Connection Test focuses purely on the handshake. Website Latency includes DNS, TCP, TLS, and HTTP request stages, giving you the breakdown of total page load latency. Use TCP Connection Test for low level network diagnosis. Use Website Latency for end to end user perceived latency.
Related Tools
Browse all tools
Explore all 50+ network tools, from speed and latency to DNS, routing, and CDN diagnostics. Free, instant, no installation required.
See all network tools