TCP Connection Release
As you read this section, make sure you are able to identify and explain the two types of connection release.
4.3.2 TCP connection release
TCP, like most connection-oriented transport protocols, supports two types of connection release:
- graceful connection release, where each TCP user can release its own direction of data transfer
- abrupt connection release, where either one user closes both directions of data transfer or one TCP entity is forced to close the connection (e.g. because the remote host does not reply anymore or due to lack of resources)
The abrupt connection release mechanism is very simple and relies on a single segment having the RST bit set. A TCP segment containing the RST bit can be sent for the following reasons:
- a non-SYN segment was received for a non-existing TCP connection RFC 793
- by extension, some implementations respond with an RST segment to a segment that is received on an existing connection but with an invalid header RFC 3360. This causes the corresponding connection to be closed and has caused security attacks RFC 4953
- by extension, some implementations send an RST segment when they need to close an existing TCP con- nection (e.g. because there are not enough resources to support this connection or because the remote host is considered to be unreachable). Measurements have shown that this usage of TCP RST was widespread [AW05]
When an RST segment is sent by a TCP entity, it should contain the current value of the sequence number for the connection (or 0 if it does not belong to any existing connection) and the acknowledgement number should be set to the next expected in-sequence sequence number on this connection.
Note: TCP RST wars
TCP implementers should ensure that two TCP entities never enter a TCP RST war where host A is sending a RST segment in response to a previous RST segment that was sent by host B in response to a TCP RST segment sent by host A... To avoid such an infinite exchange of RST segments that do not carry data, a TCP entity is never allowed to send a RST segment in response to another RST segment.
The normal way of terminating a TCP connection is by using the graceful TCP connection release. This mecha- nism uses the FIN flag of the TCP header and allows each host to release its own direction of data transfer. As for the SYN flag, the utilisation of the FIN flag in the TCP header consumes one sequence number. The figure FSM for TCP connection release shows the part of the TCP FSM used when a TCP connection is released.
Figure 4.42: FSM for TCP connection release
Starting from the Established state, there are two main paths through this FSM.
The first path is when the host receives a segment with sequence number x and the FIN flag set. The utilisation of the FIN flag indicates that the byte before sequence number x was the last byte of the byte stream sent by the remote host. Once all of the data has been delivered to the user, the TCP entity sends an ACK segment whose ack field is set to ( x + 1) mod 232 to acknowledge the FIN segment. 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. At this point, the TCP connection enters the CLOSE_WAIT state. In this state, the host can still send data to the remote host. Once all its data have been sent, it sends a FIN segment and enter the LAST_ACK state. In this state, the TCP entity waits for the acknowledgement of its FIN segment. It may still retransmit unacknowledged data segments e.g. if the retransmission timer expires. Upon reception of the acknowledgement for the FIN segment, the TCP connection is completely closed and its TCB can be discarded.
The second path is when the host decides first to send a FIN segment. In this case, it enters the FIN_WAIT1 state. It this state, it can retransmit unacknowledged segments but cannot send new data segments. It waits for an acknowledgement of its FIN segment, but may receive a FIN segment sent by the remote host. In the first case, the TCP connection enters the FIN_WAIT2 state. In this state, new data segments from the remote host are still accepted until the reception of the FIN segment. The acknowledgement for this FIN segment is sent once all data received before the FIN segment have been delivered to the user and the connection enters the TIME_WAIT state. In the second case, a FIN segment is received and the connection enters the Closing state once all data received from the remote host have been delivered to the user. In this state, no new data segments can be sent and the host waits for an acknowledgement of its FIN segment before entering the TIME_WAIT state.
The TIME_WAIT state is different from the other states of the TCP FSM. A TCP entity enters this state after having sent the last ACK segment on a TCP connection. This segment indicates to the remote host that all the data that it has sent have been correctly received and that it can safely release the TCP connection and discard the corresponding TCB. After having sent the last ACK segment, a TCP connection enters the TIME_WAIT and remains in this state for 2 ∗ M SL seconds. During this period, the TCB of the connection is maintained. This ensures that the TCP entity that sent the last ACK maintains enough state to be able to retransmit this segment if this ACK segment is lost and the remote host retransmits its last FIN segment or another one. The delay of 2 ∗ M SL seconds ensures that any duplicate segments on the connection would be handled correctly without causing the transmission of an RST segment. Without the TIME_WAIT state and the 2 ∗ M SL seconds delay, the connection release would not be graceful when the last ACK segment is lost.
Note: TIME_WAIT on busy TCP servers
The 2 ∗ M SL seconds delay in the TIME_WAIT state is an important operational problem on servers having thousands of simultaneously opened TCP connections [FTY99]. Consider for example a busy web server that processes 10.000 TCP connections every second. If each of these connections remain in the TIME_WAIT state for 4 minutes, this implies that the server would have to maintain more than 2 million TCBs at any time. For this reason, some TCP implementations prefer to perform an abrupt connection release by sending a RST segment to close the connection [AW05] and immediately discard the corresponding TCB. However, if the RST segment is lost, the remote host continues to maintain a TCB for a connection no longer exists. This optimisation reduces the number of TCBs maintained by the host sending the RST segment but at the potential cost of increased processing on the remote host when the RST segment is lost.
Source: Olivier Bonaventure, https://s3.amazonaws.com/saylordotorg-resources/wwwresources/site/wp-content/uploads/2012/02/Computer-Networking-Principles-Bonaventure-1-30-31-OTC1.pdf
This work is licensed under a Creative Commons Attribution 3.0 License.