Exit Zen
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../../protocols/tcp.rst:42
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the port chosen by the server
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../../protocols/tcp.rst:44
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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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../../protocols/tcp.rst:49
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the `FIN` flag is used during connection release
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../../protocols/tcp.rst:50
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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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../../protocols/tcp.rst:99
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The rest of this section is organized as follows. We first explain the establishment and the release of a TCP connection, then we discuss the mechanisms that are used by TCP to provide a reliable bytestream service. We end the section with a discussion of network congestion and explain the mechanisms that TCP uses to avoid congestion collapse.
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../../protocols/tcp.rst:107
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TCP connection establishment
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../../protocols/tcp.rst:111
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A TCP connection is established by using a three-way handshake. The connection establishment phase uses the `sequence number`, the `acknowledgment number` and the `SYN` flag. When a TCP connection is established, the two communicating hosts negotiate the initial sequence number to be used in both directions of the connection. For this, each TCP entity maintains a 32 bits counter, which is supposed to be incremented by one at least every 4 microseconds and after each connection establishment [#ftcpclock]_. When a client host wants to open a TCP connection with a server host, it creates a TCP segment with :
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../../protocols/tcp.rst:113 • ../../protocols/tcp.rst:118
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the `SYN` flag set
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../../protocols/tcp.rst:114
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the `sequence number` set to the current value of the 32 bits counter of the client host's TCP entity
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../../protocols/tcp.rst:116
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Upon reception of this segment (which is often called a `SYN segment`), the server host replies with a segment containing :
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