Exit Zen
176
../../protocols/tcp.rst:570
English
The normal way of terminating a TCP connection is by using the graceful TCP connection release. This mechanism 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 utilization of the `FIN` flag in the TCP header consumes one sequence number. The figure :ref:`fig-tcprelease` shows the part of the TCP FSM used when a TCP connection is released.
0/4240 · 424
177
../../protocols/tcp.rst:603
English
Starting from the `Established` state, there are two main paths through this FSM.
0/810 · 81
178
../../protocols/tcp.rst:605
English
The first path is when the host 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 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 :math:`(x+1) \pmod{2^{32}}` 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 acknowledgment of its `FIN` segment. It may still retransmit unacknowledged data segments, e.g., if the retransmission timer expires. Upon reception of the acknowledgment for the `FIN` segment, the TCP connection is completely closed and its :term:`TCB` can be discarded.
0/11330 · 1133
180
../../protocols/tcp.rst:609
English
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 :term:`TCB`. After having sent the last `ACK` segment, a TCP connection enters the `TIME\_WAIT` and remains in this state for :math:`2*MSL` 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 :math:`2*MSL` 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 :math:`2*MSL` seconds delay, the connection release would not be graceful when the last `ACK` segment is lost.
0/11020 · 1102
181
../../protocols/tcp.rst:611
English
TIME\_WAIT on busy TCP servers
0/300 · 30
183
../../protocols/tcp.rst:620
English
Footnotes
0/100 · 9
184
../../protocols/tcp.rst:621
English
A detailed presentation of all standardization documents concerning TCP may be found in :rfc:`4614`
0/990 · 99
185
../../protocols/tcp.rst:623
English
Several researchers have analyzed the utilization of TCP and UDP in the global Internet. Most of these studies have been performed by collecting all the packets transmitted over a given link during a period of a few hours or days and then analyzing their headers to infer the transport protocol used, the type of application, ... Recent studies include http://www.caida.org/research/traffic-analysis/tcpudpratio/, https://research.sprintlabs.com/packstat/packetoverview.php or http://www.nanog.org/meetings/nanog43/presentations/Labovitz_internetstats_N43.pdf
0/5590 · 559
186
../../protocols/tcp.rst:625
English
This 32 bits counter was specified in :rfc:`793`. A 32 bits counter that is incremented every 4 microseconds wraps in about 4.5 hours. This period is much larger than the Maximum Segment Lifetime that is fixed at 2 minutes in the Internet (:rfc:`791`, :rfc:`1122`).
0/2650 · 265
187
../../protocols/tcp.rst:627
English
On many departmental networks containing Unix workstations, it was common to allow users on one of the hosts to use ``rlogin`` :rfc:`1258` to run commands on any of the workstations of the network without giving any password. In this case, the remote workstation "authenticated" the client host based on its IP address. This was a bad practice from a security viewpoint.
0/3710 · 371
188
../../protocols/tcp.rst:634
English
Of course, such a simultaneous TCP establishment can only occur if the source port chosen by the client is equal to the destination port chosen by the server. This may happen when a host can serve both as a client as a server or in peer-to-peer applications when the communicating hosts do not use ephemeral port numbers.
0/3210 · 321
189
../../protocols/tcp.rst:636
English
Sending a packet with a different source IP address than the address allocated to the host is called sending a :term:`spoofed packet`.
0/1340 · 134
190
../../protocols/tcp.rst:638
English
The full list of all TCP options may be found at http://www.iana.org/assignments/tcp-parameters/
0/960 · 96
191
../../protocols/tcp.rst:640
English
In practice, only the `SYN` segment do not have their `ACK` flag set.
0/690 · 69
192
../../protocols/tcp.rst:642
English
A complete TCP implementation contains additional information in its TCB, notably to support the `urgent` pointer. However, this part of TCP is not discussed in this book. Refer to :rfc:`793` and :rfc:`2140` for more details about the TCB.
0/2390 · 239
193
../../protocols/tcp.rst:646
English
This TCP segment is then placed in an IP header. We describe IPv6 in the next chapter. The minimum size of the IPv6 (resp. IPv4) header is 40 bytes (resp. 20 bytes).
0/1650 · 165
194
../../protocols/tcp.rst:649
English
A precise estimation of the maximum bandwidth that can be achieved by a TCP connection should take into account the overhead of the TCP and IP headers as well.
0/1590 · 159
195
../../protocols/tcp.rst:651
English
See http://fasterdata.es.net/tuning.html for more information on how to tune a TCP implementation
0/970 · 97
196
../../protocols/tcp.rst:653
English
In theory, a TCP implementation could store the timestamp of each data segment transmitted and compute a new estimate for the round-trip-time upon reception of the corresponding acknowledgment. However, using such frequent measurements introduces a lot of noise in practice and many implementations still measure the round-trip-time once per round-trip-time by recording the transmission time of one segment at a time :rfc:`2988`
0/4290 · 429
197
../../protocols/tcp.rst:655
English
Some security experts have raised concerns that using the real-time clock to set the `TSval` in the timestamp option can leak information such as the system's up-time. Solutions proposed to solve this problem may be found in [CNPI09]_
0/2340 · 234