View Source: TcpStates - Waikato Linux Users Group

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States of a [TCP] connection. These can be seen with <tt>netstat -at</tt>

<tt>CLOSED</tt>:
  A theoretical state where a [TCP] connection doesn’t exist yet.
<tt>LISTEN</tt>:
  A state where [TCP] connection is waiting for a connection to be made to it.
<tt>SYN_RCVD</tt>:
  A [SYN] has been received, a [SYN] [ACK] has been sent, and we are waiting for an [ACK].
<tt>SYN_SENT</tt>:
  A [SYN] has been sent to start a connection, the [SYN] [ACK] hasn’t yet been received.
<tt>ESTABLISHED</tt>:
  A connection has been established
<tt>CLOSE_WAIT</tt>:
  After a [TCP] connection has been closed, the spec says you should make sure that the connection is not reopened for a few minutes to prevent packets that were delayed in the Internet causing the connection to be reopened. These will disappear by themselves after a while. This can cause problems on servers that have lots of very short connections (such as a web server) by filling up the OS’s connection table. See also [EADDRINUSE]
<tt>LAST_ACK</tt>:
  A [FIN] has been received, we’ve sent our own FIN, but we are waiting on the [ACK] before we can move to the closed state.
<tt>FIN_WAIT_1</tt>:
  A [FIN] has been sent, waiting for a [ACK] or [FIN]
<tt>FIN_WAIT_2</tt>:
  A [FIN] has been sent, and an [ACK] has been received. More data can be received. This connection is half open.
<tt>CLOSING</tt>:
  A [FIN] has been sent, and one received, the [FIN] hasn’t been [ACK]ed yet.
<tt>TIME_WAIT</tt>:
  The connection has closed, and is waiting for 2MSL

!! Tuning your system’s settings

The amount of time that a connection waits in various stages is controlled by the OperatingSystem. Some [OS]es let you tune these settings. For example, for [Linux]-based [OS]es, look at the control files in <tt>/proc/sys/net/ipv4</tt>. According to <tt>/usr/src/linux/Documentation/filesystems/proc.txt</tt>:

<tt>tcp_syn_retries</tt>:
  Number of times initial [SYN]s for a [TCP] connection attempt will be retransmitted. Should not be higher than 255. This is only the timeout for outgoing connections, for incoming connections the number of retransmits is defined by <tt>tcp_retries1</tt>.
<tt>tcp_fin_timeout</tt>:
  The length of time in seconds it takes to receive a final [FIN] before the socket is always closed. This is strictly a violation of the [TCP] specification, but required to prevent denial-of-service attacks.
<tt>tcp_retries1</tt>:
  Defines how often an answer to a [TCP] connection request is retransmitted before giving up.
<tt>tcp_retries2</tt>:
  Defines how often a [TCP] packet is retransmitted before giving up.
4 pages link to TcpStates:
Last edited on Thursday, May 26, 2005 9:23:01 pm by AristotlePagaltzis
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