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../../protocols/tcp.rst:32
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TCP header format
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A TCP header contains the following fields :
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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 :
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../../protocols/tcp.rst:39
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the address of the client
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the address of the server
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the port chosen by the client
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the port chosen by the server
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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`
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../../protocols/tcp.rst:45
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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]_
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../../protocols/tcp.rst:46
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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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../../protocols/tcp.rst:48
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the `SYN` flag is used during connection establishment
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the `FIN` flag is used during connection release
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the `RST` is used in case of problems or when an invalid segment has been received
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../../protocols/tcp.rst:51
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when the `ACK` flag is set, it indicates that the `acknowledgment` field contains a valid number. Otherwise, the content of the `acknowledgment` field must be ignored by the receiver
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../../protocols/tcp.rst:52
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the `URG` flag is used together with the `Urgent pointer`
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../../protocols/tcp.rst:53
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the `PSH` flag is used as a notification from the sender to indicate to the receiver that it should pass all the data it has received to the receiving process. However, in practice TCP implementations do not allow TCP users to indicate when the `PSH` flag should be set.
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../../protocols/tcp.rst:55
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the `checksum` field contains the value of the Internet checksum computed over the entire TCP segment and a pseudo-header as with UDP
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../../protocols/tcp.rst:56
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the `Reserved` field was initially reserved for future utilization. It is now used by :rfc:`3168`.
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../../protocols/tcp.rst:57
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the `TCP Header Length` (THL) or `Data Offset` field is a four bits field that indicates the size of the TCP header in 32 bit words. The maximum size of the TCP header is thus 64 bytes.
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../../protocols/tcp.rst:58
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the `Optional header extension` is used to add optional information to the TCP header. Thanks to this header extension, it is possible to add new fields to the TCP header that were not planned in the original specification. This allowed TCP to evolve since the early eighties. The details of the TCP header extension are explained in sections :ref:`TCPOpen` and :ref:`TCPReliable`.
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