cnp3-ebook/exercises/tcp-2 — English @ Weblate

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17 ../../exercises/tcp-2.rst:95

	English This TCP segment is sent immediately by the stack. The ``SYN`` flag is set and the dot next to the ``S`` character indicates that the ACK flag is also set. The SYN+ACK segment does not contain any data but its acknowledgment number is set to 1 (relative to the initial sequence number). For outgoing packets, packetdrill_ does not verify the value of the advertised window. In this line, it also accepts any TCP options (``<...>``). 432/10000 · 432
18 ../../exercises/tcp-2.rst:98

	English The third segment of the three-way handshake is sent by packetdrill_ after a delay of 0.1 seconds. The connection is now established and the accept system call will succeed. 173/10000 · 173
19 ../../exercises/tcp-2.rst:105

	English The :manpage:`accept` system call returns a new file descriptor, in this case value ``4``. At this point, packetdrill_ can write data on the socket or inject packets. 166/10000 · 166
20 ../../exercises/tcp-2.rst:113

	English packetdrill_ writes 10 bytes of data through the :manpage:`write` system call. The stack immediately sends these 10 bytes inside a segment whose ``Push`` flag is set [#fpush]_. The payload starts at sequence number ``1`` and ends at sequence number ``10``. packetdrill_ replies by injecting an acknowledgment for the entire data after 100 milliseconds. 352/10000 · 352
21 ../../exercises/tcp-2.rst:115

	English packetdrill_ can also inject data that will be read by the stack as shown by the lines below. 93/10000 · 93
22 ../../exercises/tcp-2.rst:123

	English In the example above, packetdrill_ injects a segment containing two bytes. This segment is acknowledged and after that the :manpage:`read` system call succeeds and reads the available data with a buffer of 1000 bytes. It returns the amount of read bytes, i.e. ``2``. 266/10000 · 266
23 ../../exercises/tcp-2.rst:125

	English We can now close the connection gracefully. Let us first issue inject a segment with the ``FIN`` flag set. 106/10000 · 106
24 ../../exercises/tcp-2.rst:133

	English packetdrill_ injects the ``FIN`` segment and the instrumented kernel returns an acknowledgment. If packetdrill_ issues the :manpage:`close` system call, the kernel will send a ``FIN`` segment to terminate the connection. packetdrill_ injects an acknowledgment to confirm the end of the connection. 297/10000 · 297
25 ../../exercises/tcp-2.rst:142

	English The complete packetdrill_ script is available from :download:`/exercises/packetdrill_scripts/connect.pkt`. 106/10000 · 106
26 ../../exercises/tcp-2.rst:145

	English Another interesting features of packetdrill_ is that it is possible to inspect the state maintained by the Linux kernel for the underlying connection using the ``TCP_INFO`` socket option. This makes it possible to retrieve the value of some variables of the TCP control block. 276/10000 · 276
27 ../../exercises/tcp-2.rst:147

	English Let us first explore how a TCP connection can be established. In the previous script, we have injected the segments that a client would send to a server. We can also use the Linux stack as a client and inject the segments that a server would return. Such a client process would first create its :manpage:`socket`` and then issue the :manpage:`connect` system call. At this point, the stack sends a ``SYN`` segment. To simplify the scripts, we have configured the stack to use a ``MSS`` of 1000 bytes and disabled the TCP extensions (the details of this configuration may be found at the beginning of the script). The server replies with a ``SYN+ACK`` and the stack sends acknowledges it to finish the three-way-handshake. 721/10000 · 721
28 ../../exercises/tcp-2.rst:165

	English The ``tcpi_state`` variable used in this script is returned by ``TCP_INFO`` [#ftcpinfo]_. It tracks the state of the TCP connection according to TCP's finite state machine [#fstates]_. This script is available from :download:`/exercises/packetdrill_scripts/client.pkt`. 269/10000 · 269
29 ../../exercises/tcp-2.rst:167

	English Another example is the simultaneous establishment of a TCP connection. The TCP stack sends a ``SYN`` and receives a ``SYN`` in response instead of a ``SYN+ACK``. It then acknowledges the received ``SYN`` and retransmits its own ``SYN``. The connection becomes established upon reception of the fourth segment. This script is available from :download:`/exercises/packetdrill_scripts/dual.pkt`. 392/10000 · 392
30 ../../exercises/tcp-2.rst:191

	English Another usage of packetdrill_ is to explore how a TCP connection ends. The scripts below show how a TCP stack closes a TCP connection. The first example shows a local host that connects to a remote host and then closes the connection. The remote host acknowledges the ``FIN`` and later closes its direction of data transfer. This script is available from :download:`/exercises/packetdrill_scripts/local-close.pkt`. 414/10000 · 414
31 ../../exercises/tcp-2.rst:213

	English As for the establishment of a connection, it is also possible for the two communicating hosts to close the connection at the same time. This is shown in the example below where the remote host sends its own ``FIN`` when acknowledging the first one. This script is available from :download:`/exercises/packetdrill_scripts/local-close2.pkt`. 339/10000 · 339
32 ../../exercises/tcp-2.rst:234

	English A third scenario for the termination of a TCP connection is that the remote hosts sends its ``FIN`` first. This script is available from :download:`/exercises/packetdrill_scripts/remote-close.pkt`. 197/10000 · 197
33 ../../exercises/tcp-2.rst:259

	English Another very interesting utilization of packetdrill_ is to explore how a TCP stack reacts to acknowledgments that would correspond to lost or reordered segments. For this analysis, we configure a very large initial congestion window to ensure that the connection does not start with a slow-start. 296/10000 · 296
34 ../../exercises/tcp-2.rst:261

	English Let us first use packetdrill_ to explore the evolution of the TCP retransmission timeout. The value of this timeout is set based on the measured round-trip-time and its variance. When the retransmission timer expires, TCP doubles the retransmission timer. This exponential backoff mechanism is important to ensure that TCP slowdowns during very severe congestion periods. We use the ``tcpi_rto`` variable from ``TCP_INFO`` to track the evolution of the retransmission timer. This script is available from :download:`/exercises/packetdrill_scripts/rto.pkt`. 556/10000 · 556
35 ../../exercises/tcp-2.rst:293

	English We can use a similar code to demonstrate that the TCP stack performs a fast retransmit after having received three duplicate acknowledgments. This script is available from :download:`/exercises/packetdrill_scripts/frr.pkt`. 223/10000 · 223
36 ../../exercises/tcp-2.rst:326

	English A TCP stack uses both the fast retransmit technique and retransmission timers. A retransmission timer can fire after a fast retransmit when several segments are lost. The example below shows a loss of two consecutive segments. This script is available from :download:`/exercises/packetdrill_scripts/frr-rto.pkt`. 312/10000 · 312

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