TCP Connection Release

Starting from the Established state, there are two main paths through this FSM.

Receiving a FIN

Throughout the rest of this lesson we’ll refer to the two hosts as client and server. In the case of this path, the client receives a FIN segment. Let’s trace it.

1. The client receives a segment with sequence number $x$ and the FIN flag set. The utilization of the FIN flag indicates that the byte before sequence number $x$ was the last byte of the byte stream sent by the server. The FIN segment is subject to the same retransmission mechanisms as a normal TCP segment. In particular, its transmission is protected by the retransmission timer that we’ll look at in the next few lessons.
2. Once all of the data has been delivered to the application layer entity, the TCP entity sends an ACK segment to acknowledge the FIN segment it received in step (1), whose acknowledgment number field is set to $(x+ 1)\ mod\ 2^{32}$.
3. At this point, the TCP connection enters the CLOSE_WAIT state. In this state, the client can still send data to the server.
4. Once the client has sent all the data that it was supposed to, it sends a FIN segment and enters the LAST_ACK state. In this state, the client waits for the acknowledgment of its FIN segment. It may still retransmit unacknowledged data segments, e.g. if the retransmission timer expires.
5. Upon reception of the acknowledgment for the FIN segment, the TCP connection is completely closed and its TCB can be discarded.

Tracing through the FSM

Here are some slides tracing this path through the FSM.

Sending a FIN

The second path is when the client decides first to send a FIN segment.

1. Upon sending the FIN, the connection enters the FIN_WAIT1 state. In this state, the client can retransmit unacknowledged segments, but cannot send new data segments. There are two paths that this one can split into after this:

   • A. The client receives an ACK segment in response to its FIN. The TCP connection enters the FIN_WAIT2 state in which new data segments from the server are still accepted until the reception of a FIN segment. The acknowledgment for this segment is sent once all the data before the FIN segment has been delivered to the client. After this, the connection enters the TIME_WAIT state.

   • B. In the second case, a FIN segment is received from the server. The connection enters the Closing state once all the data from the server has been delivered to the client. In this state, no new data segments can be sent and the client waits for an acknowledgment of its FIN segment before entering the TIME_WAIT state.

2. A TCP connection enters the TIME_WAIT state after the client sends the last ACK segment to a server. This segment indicates to the server that all the data that it’s sent has been correctly received and that it can safely release the TCP connection and discard the corresponding TCB.

3. The connection remains in the TIME_WAIT state for 2 $\times$ a certain length of time called the maximum segment lifetime (MSL) seconds. The TCP standard defines MSL as 120 seconds (2 minutes). Although, this value is flexible in modern implementations. During the $2\times$MSL period, the TCB of the connection is maintained on both ends to:

   • allow retransmission of the sent ACK segments if any are lost.
   • handle duplicate segments on the connection correctly without causing the transmission of a RST segment

Slides of Path 1A

Quick Quiz! #

Now that we’re done with what connection release looks like in TCP, let’s move on to efficient data transmission with TCP using Nagle’s algorithm in the next lesson!