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

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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
37 ../../exercises/tcp-2.rst:364

	English More complex scenarios can be written. The script below demonstrates how the TCP stack behaves when three segments are lost. This script is available from :download:`/exercises/packetdrill_scripts/frr-rto2.pkt`. 211/10000 · 211
38 ../../exercises/tcp-2.rst:411

	English The examples above have demonstrated how TCP retransmits lost segments. However, they did not consider the interactions with the congestion control scheme since the use a large initial congestion window. We now set the initial congestion window to two MSS-sized segments and use the ``tcpi_snd_cwnd`` and ``tcpi_snd_ssthresh`` variables from ``TCP_INFO`` to explore the evolution of the TCP congestion control scheme. Our first script looks at the evolution of the congestion window during a slow-start when there are no losses. This script is available from :download:`/exercises/packetdrill_scripts/slow-start.pkt`. 617/10000 · 617
39 ../../exercises/tcp-2.rst:459

	English Some TCP clients use delayed acknowledgments and send a TCP acknowledgment after after second in-sequence segment. This behavior is illustrated in the script below. This script is available from :download:`/exercises/packetdrill_scripts/slow-start-delayed.pkt`. 261/10000 · 261
40 ../../exercises/tcp-2.rst:502

	English We can now explore how TCP's retransmission techniques interact with the congestion control scheme. The Linux TCP code that combines these two techniques contains several heuristics to improve their performance. We start with a transfer of 8KBytes where the penultimate segment is not received by the remote host. In this case, TCP does not receive enough acknowledgments to trigger the fast retransmit and it must wait for the expiration of the retransmission timer. This script is available from :download:`/exercises/packetdrill_scripts/slow-start-rto2.pkt`. 561/10000 · 561
41 ../../exercises/tcp-2.rst:546

	English Another interesting scenario is when the loss happens early in the data transfer. This is shown in the script below where the second segment is lost. We observe that by triggering transmissions of unacknowledged data, the :rfc:`3042` rule speeds up the recovery since a fast retransmit happens. This script is available from :download:`/exercises/packetdrill_scripts/slow-start-frr2.pkt`. 388/10000 · 388
42 ../../exercises/tcp-2.rst:593

	English Our last scenario is when the first segment sent is lost. In this case, two round-trip-times are required to retransmit the missing segment and recover from the loss. This script is available from :download:`/exercises/packetdrill_scripts/slow-start-frr3.pkt`. 260/10000 · 260

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