cnp3-ebook/protocols/tcp — French @ Weblate: For each established TCP connection, a TCP implementation must maintain …

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	the first byte indicates the `type` of the option.
	the second byte indicates the total length of the option (including the first two bytes) in bytes
	the last bytes are specific for each type of option
	:rfc:`793` defines the Maximum Segment Size (MSS) TCP option that must be understood by all TCP implementations. This option (type 2) has a length of 4 bytes and contains a 16 bits word that indicates the MSS supported by the sender of the `SYN` segment. The MSS option can only be used in TCP segments having the `SYN` flag set.
	:rfc:`793` also defines two special options that must be supported by all TCP implementations. The first option is `End of option`. It is encoded as a single byte having value `0x00` and can be used to ensure that the TCP header extension ends on a 32 bits boundary. The `No-Operation` option, encoded as a single byte having value `0x01`, can be used when the TCP header extension contains several TCP options that should be aligned on 32 bit boundaries. All other options [#ftcpoptions]_ are encoded using the TLV format.
	The robustness principle
	The handling of the TCP options by TCP implementations is one of the many applications of the `robustness principle` which is usually attributed to `Jon Postel`_ and is often quoted as `"Be liberal in what you accept, and conservative in what you send"` :rfc:`1122`.
	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.
	TCP reliable data transfer
	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.
	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 :
	`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.
	`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.
	`window`. a TCP receiver uses this 16 bits field to indicate the current size of its receive window expressed in bytes.
	The Transmission Control Block
	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]_ :
	the local IP address
	the remote IP address
	the local TCP port number
	the remote TCP port number
	the current state of the TCP FSM
	the `maximum segment size` (MSS)
	`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)
	`snd.una` : the earliest sequence number that has been sent but has not yet been acknowledged
	`snd.wnd` : the current size of the sending window (in bytes)
	`rcv.nxt` : the sequence number of the next byte that is expected to be received from the remote host
	`rcv.wnd` : the current size of the receive window advertised by the remote host
	`sending buffer` : a buffer used to store all unacknowledged data
	`receiving buffer` : a buffer to store all data received from the remote host that has not yet been delivered to the user. Data may be stored in the `receiving buffer` because either it was not received in sequence or because the user is too slow to process it
	The original TCP specification can be summarized as a transport protocol that provides a byte stream service and uses `go-back-n` with a `selective-repeat` reception strategy.
	To send new data on an established connection, a TCP entity performs the following operations on the corresponding TCB. It first checks that the `sending buffer` does not contain more data than the receive window advertised by the remote host (`rcv.wnd`). If the window is not full, up to `MSS` bytes of data are placed in the payload of a TCP segment. The `sequence number` of this segment is the sequence number of the first byte of the payload. It is set to the first available sequence number : `snd.nxt` and `snd.nxt` is incremented by the length of the payload of the TCP segment. The `acknowledgment number` of this segment is set to the current value of `rcv.nxt` and the `window` field of the TCP segment is computed based on the current occupancy of the `receiving buffer`. The data is kept in the `sending buffer` in case it needs to be retransmitted later.

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../../protocols/tcp.rst:257

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4 years ago

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2 months ago

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locale/fr/LC_MESSAGES/protocols/tcp.po, string 82

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