cnp3-ebook/exercises/tcp-2 — French @ Weblate: We can now explore how TCP's retransmission techniques interact with …

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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`.

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	The complete packetdrill_ script is available from :download:`/exercises/packetdrill_scripts/connect.pkt`.
	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.
	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.
	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`.
	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`.
	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`.
	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`.
	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`.
	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.
	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`.
	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`.
	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`.
	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`.
	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`.
	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`.
	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`.
	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`.
	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`.
	Open questions
	Unless otherwise noted, we assume for the questions in this section that the following conditions hold.
	the sender/receiver performs a single :manpage:`send(3)` of `x` bytes
	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.
	the delay required to transmit a single TCP segment containing MSS bytes is small and set to 1 milliseconds, independently of the MSS size
	the transmission delay for a TCP acknowledgment is negligible
	the initial value of the congestion window is one MSS-sized segment
	the value of the duplicate acknowledgment threshold is fixed and set to 3
	TCP always acknowledges each received segment
	To understand the operation of the TCP congestion control, it is often useful to write time-sequence diagrams for different scenarios. The example below shows the operation of the TCP congestion control scheme in a very simple scenario. The initial congestion window (``cwnd``) is set to 1000 bytes and the receive window (``rwin``) advertised by the receiver (supposed constant for the entire connection) is set to 2000 bytes. The slow-start threshold (``ssthresh``) is set to 64000 bytes.
	Can you explain why the sender only sends one segment first and then two successive segments (the delay between the two segments on the figure is due to graphical reasons) ?
	Can you explain why the congestion window is increased after the reception of the first acknowledgment ?
	How long does it take for the sender to deliver 3 KBytes to the receiver ?

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