cnp3-ebook/protocols/tcp — English @ Weblate

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62 ../../protocols/tcp.rst:210

	English As IP provides an unreliable connectionless service, the `SYN` and `SYN+ACK` segments sent to open a TCP connection could be lost. Current TCP implementations start a retransmission timer when they send the first `SYN` segment. This timer is often set to three seconds for the first retransmission and then doubles after each retransmission :rfc:`2988`. TCP implementations also enforce a maximum number of retransmissions for the initial `SYN` segment. 453/10000 · 453
63 ../../protocols/tcp.rst:215

	English As explained earlier, TCP segments may contain an optional header extension. In the `SYN` and `SYN+ACK` segments, these options are used to negotiate some parameters and the utilization of extensions to the basic TCP specification. 231/10000 · 231
64 ../../protocols/tcp.rst:219

	English The first parameter which is negotiated during the establishment of a TCP connection is the Maximum Segment Size (:term:`MSS`). The MSS is the size of the largest segment that a TCP entity is able to process. According to :rfc:`879`, all TCP implementations must be able to receive TCP segments containing 536 bytes of payload. However, most TCP implementations are able to process larger segments. Such TCP implementations use the TCP MSS Option in the `SYN`/`SYN+ACK` segment to indicate the largest segment they are able to process. The MSS value indicates the maximum size of the payload of the TCP segments. The client (resp. server) stores in its :term:`TCB` the MSS value announced by the server (resp. the client). 722/10000 · 722
65 ../../protocols/tcp.rst:221

	English Another utilization of TCP options during connection establishment is to enable TCP extensions. For example, consider :rfc:`1323` (which is discussed in :ref:`TCPReliable`). :rfc:`1323` defines TCP extensions to support timestamps and larger windows. If the client supports :rfc:`1323`, it adds a :rfc:`1323` option to its `SYN` segment. If the server understands this :rfc:`1323` option and wishes to use it, it replies with a :rfc:`1323` option in the `SYN+ACK` segment and the extension defined in :rfc:`1323` is used throughout the TCP connection. Otherwise, if the server's `SYN+ACK` does not contain the :rfc:`1323` option, the client is not allowed to use this extension and the corresponding TCP header options throughout the TCP connection. TCP's option mechanism is flexible and it allows the extension of TCP while maintaining compatibility with older implementations. 879/10000 · 879
66 ../../protocols/tcp.rst:223

	English The TCP options are encoded by using a Type Length Value format where : 71/10000 · 71
67 ../../protocols/tcp.rst:225

	English the first byte indicates the `type` of the option. 50/10000 · 50
68 ../../protocols/tcp.rst:226

	English the second byte indicates the total length of the option (including the first two bytes) in bytes 97/10000 · 97
69 ../../protocols/tcp.rst:227

	English the last bytes are specific for each type of option 51/10000 · 51
70 ../../protocols/tcp.rst:229

	English :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. 329/10000 · 329
71 ../../protocols/tcp.rst:231

	English :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. 523/10000 · 523
72 ../../protocols/tcp.rst:233

	English The robustness principle 24/10000 · 24
73 ../../protocols/tcp.rst:235

	English 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`. 266/10000 · 266
74 ../../protocols/tcp.rst:237

	English 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. 466/10000 · 466
75 ../../protocols/tcp.rst:243

	English TCP reliable data transfer 26/10000 · 26
76 ../../protocols/tcp.rst:245

	English 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. 375/10000 · 375
77 ../../protocols/tcp.rst:247

	English 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 : 227/10000 · 227
78 ../../protocols/tcp.rst:249

	English `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. 206/10000 · 206
79 ../../protocols/tcp.rst:250

	English `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. 388/10000 · 388
80 ../../protocols/tcp.rst:251

	English `window`. a TCP receiver uses this 16 bits field to indicate the current size of its receive window expressed in bytes. 119/10000 · 119
81 ../../protocols/tcp.rst:255

	English The Transmission Control Block 30/10000 · 30

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