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The Transmission Control Protocol
The Transmission Control Protocol (TCP) was initially defined in :rfc:`793`. Several parts of the protocol have been improved since the publication of the original protocol specification [#ftcpspecs]_. However, the basics of the protocol remain and an implementation that only supports :rfc:`793` should inter-operate with today's implementation.
TCP provides a reliable bytestream, connection-oriented transport service on top of the unreliable connectionless network service provided by :term:`IP`. TCP is used by a large number of applications, including :
Email (:term:`SMTP`, :term:`POP`, :term:`IMAP`)
World wide web ( :term:`HTTP`, ...)
Most file transfer protocols ( :term:`ftp`, peer-to-peer file sharing applications , ...)
remote computer access : :term:`telnet`, :term:`ssh`, :term:`X11`, :term:`VNC`, ...
non-interactive multimedia applications (flash, ...)
On the global Internet, most of the applications used in the wide area rely on TCP. Many studies [#ftcpusage]_ have reported that TCP was responsible for more than 90% of the data exchanged in the global Internet.
To provide this service, TCP relies on a simple segment format that is shown in the figure below. Each TCP segment contains a header described below and, optionally, a payload. The default length of the TCP header is twenty bytes, but some TCP headers contain options.
TCP header format
A TCP header contains the following fields :
the `source and destination ports`. The source and destination ports play an important role in TCP, as they allow the identification of the connection to which a TCP segment belongs. When a client opens a TCP connection, it typically selects an ephemeral TCP port number as its source port and contacts the server by using the server's port number. All the segments that are sent by the client on this connection have the same source and destination ports. The server sends segments that contain as source (resp. destination) port, the destination (resp. source) port of the segments sent by the client (see figure :ref:`fig-tcpports`). A TCP connection is always identified by four pieces of information :
the address of the client
the address of the server
the port chosen by the client
the port chosen by the server
the `sequence number` (32 bits), `acknowledgment number` (32 bits) and `window` (16 bits) fields are used to provide a reliable data transfer, using a window-based protocol. In a TCP bytestream, each byte of the stream consumes one sequence number. Their usage is described in more detail in section :ref:`TCPReliable`
the `Urgent pointer` is used to indicate that some data should be considered as urgent in a TCP bytestream. However, it is rarely used in practice and will not be described here. Additional details about the utilization of this pointer may be found in :rfc:`793`, :rfc:`1122` or [Stevens1994]_
the flags field contains a set of bit flags that indicate how a segment should be interpreted by the TCP entity receiving it :


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locale/pot/protocols/tcp.pot, string 5