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

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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
43 ../../exercises/tcp-2.rst:637

	English Open questions 14/10000 · 14
44 ../../exercises/tcp-2.rst:639

	English Unless otherwise noted, we assume for the questions in this section that the following conditions hold. 103/10000 · 103
45 ../../exercises/tcp-2.rst:641

	English the sender/receiver performs a single :manpage:`send(3)` of `x` bytes 69/10000 · 69
46 ../../exercises/tcp-2.rst:642

	English the round-trip-time is fixed and does not change during the lifetime of the TCP connection. We assume a fixed value of 100 milliseconds for the round-trip-time and a fixed value of 200 milliseconds for the retransmission timer. 227/10000 · 227
47 ../../exercises/tcp-2.rst:643

	English the delay required to transmit a single TCP segment containing MSS bytes is small and set to 1 milliseconds, independently of the MSS size 138/10000 · 138

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