cnp3-ebook/protocols/tcp — English @ Weblate

The translation has come to an end.

Exit Zen

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
54 ../../protocols/tcp.rst:None

	English Denial of Service attacks 25/10000 · 25
55 ../../protocols/tcp.rst:192

	English When a TCP entity opens a TCP connection, it creates a Transmission Control Block (:term:`TCB`). The TCB contains the entire state that is maintained by the TCP entity for each TCP connection. During connection establishment, the TCB contains the local IP address, the remote IP address, the local port number, the remote port number, the current local sequence number and the last sequence number received from the remote entity. Until the mid 1990s, TCP implementations had a limit on the number of TCP connections that could be in the `SYN RCVD` state at a given time. Many implementations set this limit to about 100 TCBs. This limit was considered sufficient even for heavily load http servers given the small delay between the reception of a `SYN` segment and the reception of the `ACK` segment that terminates the establishment of the TCP connection. When the limit of 100 TCBs in the `SYN Rcvd` state is reached, the TCP entity discards all received TCP `SYN` segments that do not correspond to an existing TCB. 1019/10000 · 1019
56 ../../protocols/tcp.rst:194

	English This limit of 100 TCBs in the `SYN Rcvd` state was chosen to protect the TCP entity from the risk of overloading its memory with too many TCBs in the `SYN Rcvd` state. However, it was also the reason for a new type of Denial of Service (DoS) attack :rfc:`4987`. A DoS attack is defined as an attack where an attacker can render a resource unavailable in the network. For example, an attacker may cause a DoS attack on a 2 Mbps link used by a company by sending more than 2 Mbps of packets through this link. In this case, the DoS attack was more subtle. As a TCP entity discards all received `SYN` segments as soon as it has 100 TCBs in the `SYN Rcvd` state, an attacker simply had to send a few 100 `SYN` segments every second to a server and never reply to the received `SYN+ACK` segments. To avoid being caught, attackers were of course sending these `SYN` segments with a different address than their own IP address [#fspoofing]_. On most TCP implementations, once a TCB entered the `SYN Rcvd` state, it remained in this state for several seconds, waiting for a retransmission of the initial `SYN` segment. This attack was later called a `SYN flood` attack and the servers of the ISP named panix were among the first to `be affected <http://memex.org/meme2-12.html>`_ by this attack. 1287/10000 · 1287
57 ../../protocols/tcp.rst:200

	English To avoid the `SYN flood` attacks, recent TCP implementations no longer enter the `SYN Rcvd` state upon reception of a `SYN segment`. Instead, they reply directly with a `SYN+ACK` segment and wait until the reception of a valid `ACK`. This implementation trick is only possible if the TCP implementation is able to verify that the received `ACK` segment acknowledges the `SYN+ACK` segment sent earlier without storing the initial sequence number of this `SYN+ACK` segment in a TCB. The solution to solve this problem, which is known as `SYN cookies <http://cr.yp.to/syncookies.html>`_ is to compute the 32 bits of the `ISN` as follows : 635/10000 · 635
58 ../../protocols/tcp.rst:202

	English the high order bits contain the low order bits of a counter that is incremented slowly 86/10000 · 86
59 ../../protocols/tcp.rst:203

	English the low order bits contain a hash value computed over the local and remote IP addresses and ports and a random secret only known to the server 142/10000 · 142
60 ../../protocols/tcp.rst:205

	English The advantage of the `SYN cookies`_ is that by using them, the server does not need to create a :term:`TCB` upon reception of the `SYN` segment and can still check the returned `ACK` segment by recomputing the `SYN cookie`. The main disadvantage is that they are not fully compatible with the TCP options. This is why they are not enabled by default on a typical system. 370/10000 · 370
61 ../../protocols/tcp.rst:None

	English Retransmitting the first `SYN` segment 38/10000 · 38
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

Shortcut	Action
`?`	Open available keyboard shortcuts.
`Alt` + `Home`	Navigate to the first translation in the current search.
`Alt` + `End`	Navigate to the last translation in the current search.
`Alt` + `PageUp` or `Ctrl` + `↑` or `Alt` + `↑` or `Cmd` + `↑` or	Navigate to the previous translation in the current search.
`Alt` + `PageDown` or `Ctrl` + `↓` or `Alt` + `↓` or `Cmd` + `↓` or	Navigate to the next translation in the current search.
`Ctrl` + `Enter` or `Cmd` + `Enter`	Submit current form; this works the same as pressing Save and continue while editing translation.
`Ctrl` + `Shift` + `Enter` or `Cmd` + `Shift` +`Enter`	Unmark translation as Needing edit and submit it.
`Alt` + `Enter` or `Option` + `Enter`	Submit the string as a suggestion; this works the same as pressing Suggest while editing translation.
`Ctrl` + `E` or `Cmd` + `E`	Focus on translation editor.
`Ctrl` + `U` or `Cmd` + `U`	Focus on comment editor.
`Ctrl` + `M` or `Cmd` + `M`	Shows Automatic suggestions tab.
`Ctrl` + `1` to `Ctrl` + `9` or `Cmd` + `1` to `Cmd` + `9`	Copies placeable of a given number from source string.
`Ctrl` + `M` followed by `1` to `9` or `Cmd` + `M` followed by `1` to `9`	Copy the machine translation of a given number to current translation.
`Ctrl` + `I` followed by `1` to `9` or `Cmd` + `I` followed by `1` to `9`	Ignore one item in the list of failing checks.
`Ctrl` + `J` or `Cmd` + `J`	Shows the Nearby strings tab.
`Ctrl` + `S` or `Cmd` + `S`	Focus on search field.
`Ctrl` + `O` or `Cmd` + `O`	Copy the source string.
`Ctrl` + `Y` or `Cmd` + `Y`	Toggle the Needs editing checkbox.
`→`	Browse the next translation string.
`←`	Browse the previous translation string.