Exit Zen
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../../protocols/tcp.rst:237
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Concerning the TCP options, the robustness principle implies that a TCP implementation should be able to accept TCP options that it does not understand, in particular in received `SYN` segments, and that it should be able to parse any received segment without crashing, even if the segment contains an unknown TCP option. Furthermore, a server should not send in the `SYN+ACK` segment or later, options that have not been proposed by the client in the `SYN` segment.
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../../protocols/tcp.rst:243
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TCP reliable data transfer
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../../protocols/tcp.rst:245
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The original TCP data transfer mechanisms were defined in :rfc:`793`. Based on the experience of using TCP on the growing global Internet, this part of the TCP specification has been updated and improved several times, always while preserving the backward compatibility with older TCP implementations. In this section, we review the main data transfer mechanisms used by TCP.
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../../protocols/tcp.rst:247
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TCP is a window-based transport protocol that provides a bi-directional byte stream service. This has several implications on the fields of the TCP header and the mechanisms used by TCP. The three fields of the TCP header are :
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../../protocols/tcp.rst:249
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`sequence number`. TCP uses a 32 bits sequence number. The `sequence number` placed in the header of a TCP segment containing data is the sequence number of the first byte of the payload of the TCP segment.
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../../protocols/tcp.rst:250
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`acknowledgment number`. TCP uses cumulative positive acknowledgments. Each TCP segment contains the `sequence number` of the next byte that the sender of the acknowledgment expects to receive from the remote host. In theory, the `acknowledgment number` is only valid if the `ACK` flag of the TCP header is set. In practice, almost all [#fackflag]_ TCP segments have their `ACK` flag set.
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../../protocols/tcp.rst:251
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`window`. a TCP receiver uses this 16 bits field to indicate the current size of its receive window expressed in bytes.
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../../protocols/tcp.rst:255
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The Transmission Control Block
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../../protocols/tcp.rst:257
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For each established TCP connection, a TCP implementation must maintain a Transmission Control Block (:term:`TCB`). A TCB contains all the information required to send and receive segments on this connection :rfc:`793`. This includes [#ftcpurgent]_ :
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../../protocols/tcp.rst:259
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the local IP address
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../../protocols/tcp.rst:260
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the remote IP address
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../../protocols/tcp.rst:261
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the local TCP port number
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the remote TCP port number
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the current state of the TCP FSM
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../../protocols/tcp.rst:264
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the `maximum segment size` (MSS)
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../../protocols/tcp.rst:265
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`snd.nxt` : the sequence number of the next byte in the byte stream (the first byte of a new data segment that you send uses this sequence number)
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../../protocols/tcp.rst:266
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`snd.una` : the earliest sequence number that has been sent but has not yet been acknowledged
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../../protocols/tcp.rst:267
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`snd.wnd` : the current size of the sending window (in bytes)
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../../protocols/tcp.rst:268
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`rcv.nxt` : the sequence number of the next byte that is expected to be received from the remote host
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`rcv.wnd` : the current size of the receive window advertised by the remote host
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