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TIME\_WAIT on busy TCP servers
The :math:`2*MSL` 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 remains 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 :term:`TCB`. However, if the `RST` segment is lost, the remote host continues to maintain a :term:`TCB` for a connection that no longer exists. This optimization 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.
Footnotes
A detailed presentation of all standardization documents concerning TCP may be found in :rfc:`4614`
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
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`).
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.
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.
Sending a packet with a different source IP address than the address allocated to the host is called sending a :term:`spoofed packet`.
The full list of all TCP options may be found at http://www.iana.org/assignments/tcp-parameters/
In practice, only the `SYN` segment do not have their `ACK` flag set.
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.
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).
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.
See http://fasterdata.es.net/tuning.html for more information on how to tune a TCP implementation
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`
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]_
As a TCP client often establishes several parallel or successive connections with the same server, :rfc:`2140` has proposed to reuse for a new connection some information that was collected in the TCB of a previous connection, such as the measured rtt. However, this solution has not been widely implemented.

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locale/pot/protocols/tcp.pot, string 196