cnp3-ebook/protocols/tcp — English @ Weblate

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34 ../../protocols/tcp.rst:113 • ../../protocols/tcp.rst:118

	English the `SYN` flag set 18/10000 · 18
35 ../../protocols/tcp.rst:114

	English the `sequence number` set to the current value of the 32 bits counter of the client host's TCP entity 101/10000 · 101
36 ../../protocols/tcp.rst:116

	English Upon reception of this segment (which is often called a `SYN segment`), the server host replies with a segment containing : 123/10000 · 123
37 ../../protocols/tcp.rst:119

	English the `sequence number` set to the current value of the 32 bits counter of the server host's TCP entity 101/10000 · 101
38 ../../protocols/tcp.rst:120 • ../../protocols/tcp.rst:126

	English the `ACK` flag set 18/10000 · 18
39 ../../protocols/tcp.rst:121

	English the `acknowledgment number` set to the `sequence number` of the received `SYN` segment incremented by 1 :math:`\pmod{2^{32}}`. When a TCP entity sends a segment having `x+1` as acknowledgment number, this indicates that it has received all data up to and including sequence number `x` and that it is expecting data having sequence number `x+1`. As the `SYN` flag was set in a segment having sequence number `x`, this implies that setting the `SYN` flag in a segment consumes one sequence number. 495/10000 · 495
40 ../../protocols/tcp.rst:124

	English This segment is often called a `SYN+ACK` segment. The acknowledgment confirms to the client that the server has correctly received the `SYN` segment. The `sequence number` of the `SYN+ACK` segment is used by the server host to verify that the `client` has received the segment. Upon reception of the `SYN+ACK` segment, the client host replies with a segment containing : 370/10000 · 370
41 ../../protocols/tcp.rst:127

	English the `acknowledgment number` set to the `sequence number` of the received `SYN+ACK` segment incremented by 1 :math:`\pmod{2^{32}}` 129/10000 · 129
42 ../../protocols/tcp.rst:129

	English At this point, the TCP connection is open and both the client and the server are allowed to send TCP segments containing data. This is illustrated in the figure below. 167/10000 · 167
43 ../../protocols/tcp.rst:135

	English Establishment of a TCP connection 33/10000 · 33
44 ../../protocols/tcp.rst:137

	English In the figure above, the connection is considered to be established by the client once it has received the `SYN+ACK` segment, while the server considers the connection to be established upon reception of the `ACK` segment. The first data segment sent by the client (server) has its `sequence number` set to `x+1` (resp. `y+1`). 327/10000 · 327
45 ../../protocols/tcp.rst:141

	English Computing TCP's initial sequence number 39/10000 · 39
46 ../../protocols/tcp.rst:143

	English In the original TCP specification :rfc:`793`, each TCP entity maintained a clock to compute the initial sequence number (:term:`ISN`) placed in the `SYN` and `SYN+ACK` segments. This made the ISN predictable and caused a security issue. The typical security problem was the following. Consider a server that trusts a host based on its IP address and allows the system administrator to log in from this host without giving a password [#frlogin]_. Consider now an attacker who knows this particular configuration and is able to send IP packets having the client's address as source. He can send fake TCP segments to the server, but does not receive the server's answers. If he can predict the `ISN` that is chosen by the server, he can send a fake `SYN` segment and shortly after the fake `ACK` segment confirming the reception of the `SYN+ACK` segment sent by the server. Once the TCP connection is open, he can use it to send any command to the server. To counter this attack, current TCP implementations add randomness to the `ISN`. One of the solutions, proposed in :rfc:`1948` is to compute the `ISN` as :: 1109/10000 · 1109
47 ../../protocols/tcp.rst:147

	English where `M` is the current value of the TCP clock and `H` is a cryptographic hash function. `localhost` and `remotehost` (resp. `localport` and `remoteport` ) are the IP addresses (port numbers) of the local and remote host and `secret` is a random number only known by the server. This method allows the server to use different ISNs for different clients at the same time. `Measurements <http://lcamtuf.coredump.cx/newtcp/>`_ performed with the first implementations of this technique showed that it was difficult to implement it correctly, but today's TCP implementation now generate good ISNs. 594/10000 · 594
48 ../../protocols/tcp.rst:151

	English A server could, of course, refuse to open a TCP connection upon reception of a `SYN` segment. This refusal may be due to various reasons. There may be no server process that is listening on the destination port of the `SYN` segment. The server could always refuse connection establishments from this particular client (e.g. due to security reasons) or the server may not have enough resources to accept a new TCP connection at that time. In this case, the server would reply with a TCP segment having its `RST` flag set and containing the `sequence number` of the received `SYN` segment incremented by one as its `acknowledgment number`. This is illustrated in the figure below. We discuss the other usages of the TCP `RST` flag later (see :ref:`TCPRelease`). 759/10000 · 759
49 ../../protocols/tcp.rst:157

	English TCP connection establishment rejected by peer 45/10000 · 45
50 ../../protocols/tcp.rst:159

	English TCP connection establishment can be described as the four state Finite State Machine shown below. In this FSM, `!X` (resp. `?Y`) indicates the transmission of segment `X` (resp. reception of segment `Y`) during the corresponding transition. `Init` is the initial state. 269/10000 · 269
51 ../../protocols/tcp.rst:177

	English A client host starts in the `Init` state. It then sends a `SYN` segment and enters the `SYN Sent` state where it waits for a `SYN+ACK` segment. Then, it replies with an `ACK` segment and enters the `Established` state where data can be exchanged. On the other hand, a server host starts in the `Init` state. When a server process starts to listen to a destination port, the underlying TCP entity creates a TCP control block and a queue to process incoming `SYN` segments. Upon reception of a `SYN` segment, the server's TCP entity replies with a `SYN+ACK` and enters the `SYN RCVD` state. It remains in this state until it receives an `ACK` segment that acknowledges its `SYN+ACK` segment, with this it then enters the `Established` state. 739/10000 · 739
52 ../../protocols/tcp.rst:179

	English Apart from these two paths in the TCP connection establishment FSM, there is a third path that corresponds to the case when both the client and the server send a `SYN` segment to open a TCP connection [#ftcpboth]_. In this case, the client and the server send a `SYN` segment and enter the `SYN Sent` state. Upon reception of the `SYN` segment sent by the other host, they reply by sending a `SYN+ACK` segment and enter the `SYN RCVD` state. The `SYN+ACK` that arrives from the other host allows it to transition to the `Established` state. The figure below illustrates such a simultaneous establishment of a TCP connection. 624/10000 · 624
53 ../../protocols/tcp.rst:185

	English Simultaneous establishment of a TCP connection 46/10000 · 46

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