Why Do TIME_WAIT Connections Surge in High‑Concurrency Scenarios and How to Fix Them

1. Problem Description

In simulated high‑concurrency scenarios, a batch of TCP connections enter the TIME_WAIT state.

After a short time the TIME_WAIT connections disappear as they are reclaimed, and services resume normally; thus TIME_WAIT is a normal phenomenon under high load.

In production, continuous high load may cause some TIME_WAIT connections to be reclaimed while new ones are created, and in extreme cases a large number of TIME_WAIT connections appear.

Impact: each TIME_WAIT occupies a local port (max 65535). When many connections are in TIME_WAIT, new TCP connections can fail with “address already in use: connect”.

2. Problem Analysis

The root causes of massive TIME_WAIT connections are:

Numerous short‑lived connections.

When the HTTP Connection header is set to close, the server actively closes the connection.

The TCP four‑handshake termination keeps the socket in TIME_WAIT for twice the Maximum Segment Lifetime (MSL) to ensure ACK retransmission and discard delayed packets.

TIME_WAIT is the state of the side that actively closes the connection; it lasts for 2 × MSL (typically 4 minutes, as MSL is 2 minutes).

3. Solutions

General ways to reduce the impact of excessive TIME_WAIT:

Client side: set HTTP Connection to keep-alive to keep connections alive.

Server side:

Enable reuse of sockets in TIME_WAIT (e.g., set SO_REUSEADDR).

Reduce TIME_WAIT duration to 1 MSL (≈2 minutes) via kernel parameters.

Reference: https://www.cnblogs.com/yjf512/p/5327886.html

4. Appendices

Appendix A – Query TCP connection states

On macOS, use:

# Query TIME_WAIT connections
$ netstat -nat | grep TIME_WAIT
# Count connections by state
$ netstat -n | awk '/^tcp/ {++S[$NF]} END {for(a in S) print a, S[a]}'

Appendix B – MSL (Maximum Segment Lifetime)

MSL is the maximum time a packet can exist in the network; RFC 793 defines it as 2 minutes, though implementations may use 30 seconds, 1 minute, or 2 minutes.

Appendix C – TCP three‑way handshake and four‑way termination

Illustrations and detailed explanations can be found in external resources.

TCP TIME_WAIT is essential for reliable connection termination and for discarding delayed packets; without it, ACK loss or stray packets could corrupt new connections.