The TIME_WAITs on Mac OS X

No TIME_WAITs on Mac OS X

Normally, when a TCP connection is closed, the socket on the side where close() is called first is left in the TIME_WAIT state.

When one of the peers is a Mac OS X (Lion) machine, no TIME_WAIT is listed by netstat -an on the Mac if close() is called first on the Mac side. However, it seems that the socket is actually in TIME_WAIT state, because trying to call listen() again (without using the socket option SO_REUSEADDR) causes listen() to fail.

Waiting for 2*MSL (Maximum Segment Lifetime which is 15 seconds on Mac OS X Lion as reported by sysctl net.inet.tcp.msl) clears the TIME_WAIT state, and listen() can be called again without error.

Why can't I see the socket in TIME_WAIT?

Testing

Here are two simple test programs in Python.

Server

#!/usr/bin/env python

import socket

HOST = ''
PORT = 50007
l = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
l.bind((HOST, PORT))
l.listen(1)
print("Listening on %d" % PORT)
(s, _) = l.accept()
print("Connected")
raw_input("Press <enter> to close...")
l.close()
s.close()
print("Closed")

Client

#!/usr/bin/env python

import socket
import sys

HOST = sys.argv[1]
PORT = 50007

print("Opening connection to server")
s = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
s.connect((HOST, PORT))
raw_input("Press <enter> to close...")
s.close()
print("Closed")

When running both the server and the client on two different Linux machines, the peer that press <enter> to call close() first gets a TIME_WAIT as expected:

$ ./server-timewait.py 
Listening on 50007
Connected
Press <enter> to close...
Closed
$ netstat -an | grep 50007
tcp        0      0 172.16.185.219:50007    172.16.185.42:49818     TIME_WAIT  
$ 

When one of the peers is a Mac (running OS X Lion) I never see a TIME_WAIT when running netstat -an | grep 50007 after closing first on the Mac.