# SOME DESCRIPTIVE TITLE. # Copyright (C) 2019 Olivier Bonaventure # This file is distributed under the same license as the Computer networking # : Principles, Protocols and Practice package. # FIRST AUTHOR , 2019. # #, fuzzy msgid "" msgstr "" "Project-Id-Version: Computer networking : Principles, Protocols and " "Practice 3\n" "Report-Msgid-Bugs-To: \n" "POT-Creation-Date: 2019-10-09 12:39+0000\n" "PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n" "Last-Translator: FULL NAME \n" "Language-Team: LANGUAGE \n" "MIME-Version: 1.0\n" "Content-Type: text/plain; charset=utf-8\n" "Content-Transfer-Encoding: 8bit\n" "Generated-By: Babel 2.7.0\n" #: ../../exercises/ex-sharing.rst:6 msgid "" "This is an unpolished draft of the third edition of this e-book. If you " "find any error or have suggestions to improve the text, please create an " "issue via https://github.com/CNP3/ebook/issues?milestone=3 or help us by " "providing pull requests to close the existing issues." msgstr "" #: ../../exercises/ex-sharing.rst:11 msgid "Sharing resources" msgstr "" #: ../../exercises/ex-sharing.rst:15 msgid "Medium Access Control" msgstr "" #: ../../exercises/ex-sharing.rst:17 msgid "" "To understand the operation of Medium Access Control algorithms, it is " "often interesting to use a geometric representation of the transmission " "of frames on a shared medium. This representation is suitable if the " "communicating devices are attached to a single cable. Consider a simple " "scenario with a host connected at one end of a cable. For simplicity, let" " us consider a cable that has a length of one kilometer. Let us also " "consider that the propagation delay of the electrical signal is five " "microseconds per kilometer. The figure below shows the transmission of a " "2000 bits frame at 100 Mbps by host A on the cable." msgstr "" #: ../../exercises/ex-sharing.rst:42 msgid "" "If the transmitting host is located at another position on the shared " "medium than one of the edges, then the geometrical pattern that " "represents the transmission of a frame is slightly different. If the " "transmitting host is placed in the middle of the cable, then the signal " "is transmitted in both directions on the cable. The figure below shows " "the transmission of one 100 bits frame at 100 Mbps by host C on the same " "cable." msgstr "" #: ../../exercises/ex-sharing.rst:66 msgid "" "In a shared medium, a collision may happen if two hosts transmit at " "almost the same time as shown in the example below." msgstr "" #: ../../exercises/ex-sharing.rst:92 msgid "" "Consider the following scenario for the ALOHA medium access control " "algorithm. Three hosts are attached to a one-kilometer long cable and " "transmit 1000 bits frames at 1 Mbps. Each arrow represents a request to " "transmit a frame on the corresponding host. Each square represents 250 " "microseconds in the figure. Represent all the transmitted frames and list" " the frames that collide." msgstr "" #: ../../exercises/ex-sharing.rst:119 msgid "" "Same question as above, but now consider that the hosts transmit 1000 " "bits frames at 100 Mbps. The cable has a length of 2 kilometers. C is in " "the middle of the cable. Each square in the figure below corresponds to " "10 microseconds." msgstr "" #: ../../exercises/ex-sharing.rst:150 msgid "" "In ALOHA, the hosts rely on acknowledgments to detect whether their frame" " has been received correctly by the destination. Consider a network " "running at 100 Mbps where the host exchange 1000 bits frames and " "acknowledgments of 100 bits. Draw the frames sent by hosts A and B in the" " figure below. Assume that a square corresponds to 10 microseconds and " "that the cable has a length of 2 kilometers." msgstr "" #: ../../exercises/ex-sharing.rst:175 msgid "" "Same question as above, but now assume that the retransmission timer of " "each host is set to 50 microseconds." msgstr "" #: ../../exercises/ex-sharing.rst:203 msgid "" "In practice, hosts transmit variable length frames. Consider a cable " "having a bandwidth of 100 Mbps and a length of 2 kilometers." msgstr "" #: ../../exercises/ex-sharing.rst:231 msgid "" "With CSMA, hosts need to listen to the communication channel before " "starting their transmission. Consider again a 2 kilometers long cable " "where hosts send frames at 100 Mbps. Show in the figure below the correct" " transmission of frames with CSMA." msgstr "" #: ../../exercises/ex-sharing.rst:260 msgid "" "CSMA/CD does not use acknowledgments but instead assumes that each host " "can detect collisions by listening while transmitting. Consider a 2 " "kilometers long cable running at 10 Mbps. Show in the figure below the " "utilization of the communication channel and the collisions that would " "occur. For this exercise, do not attempt to retransmit the frames that " "have collided." msgstr "" #: ../../exercises/ex-sharing.rst:290 msgid "" "Consider again a network that uses CSMA/CD. This time, the bandwidth is " "set to 1 Gbps and the cable has a length of two kilometers. When a " "collision occurs, consider that the hosts B and C retransmit immediately " "while host A waits for the next slot." msgstr "" #: ../../exercises/ex-sharing.rst:316 msgid "" "An important part of the CSMA/CD algorithm is the exponential backoff. To" " illustrate the operation of this algorithm, let us consider a cable that" " has a length of one kilometer. The bandwidth of the network is set to 10" " Mbps. Assume that when a collision occurs, host A always selects the " "highest possible random delay according to the exponential backoff " "algorithm while host B always selects the shortest one. In this network, " "the slot time is equal to the time required to transmit 100 bits. We " "further assume that a host can detect collision immediately (i.e. as soon" " as the other frame arrives)." msgstr "" #: ../../exercises/ex-sharing.rst:343 msgid "Fairness and congestion control" msgstr "" #: ../../exercises/ex-sharing.rst:345 msgid "" "Consider the network below. Compute the max-min fair allocation for the " "hosts in this network assuming that nodes `Sx` always send traffic " "towards node `Dx`. Furthermore, link `R1-R2` has a bandwidth of 10 Mbps " "while link `R2-R3` has a bandwidth of 20 Mbps." msgstr "" #: ../../exercises/ex-sharing.rst:382 msgid "" "To understand congestion control algorithms, it can also be useful to " "represent the exchange of packets by using a graphical representation. As" " a first example, let us consider a very simple network composed of two " "hosts interconnected through a switch." msgstr "" #: ../../exercises/ex-sharing.rst:399 msgid "" "Suppose now that host A uses a window of three segments and sends these " "three segments immediately. The segments will be queued in the router " "before being transmitted on the output link and delivered to their " "destination. The destination will reply with a short acknowledgment " "segment. A possible visualization of this exchange of packets is " "represented in the figure below. We assume for this figure that the " "router marks the packets to indicate congestion as soon as its buffer is " "non-empty when its receives a packet on its input link. In the figure, a " "`(c)` sign is added to each packet to indicate that it has been " "explicitly marked." msgstr "" #: ../../exercises/ex-sharing.rst:432 msgid "" "In practice, a router is connected to multiple input links. The figure " "below shows an example with two hosts." msgstr "" #: ../../exercises/ex-sharing.rst:491 msgid "" "In general, the links have a non-zero delay. This is illustrated in the " "figure below where a delay has been added on the link between `R` and " "`C`." msgstr "" #: ../../exercises/ex-sharing.rst:538 ../../exercises/ex-sharing.rst:613 msgid "Consider the network depicted in the figure below." msgstr "" #: ../../exercises/ex-sharing.rst:558 msgid "" "In this network, compute the minimum round-trip-time between `A` (resp. " "`B`) and `D`. Perform the computation if the hosts send segments " "containing 1000 bits." msgstr "" #: ../../exercises/ex-sharing.rst:559 msgid "" "How is the maximum round-trip-time influenced if the buffers of router " "`R1` store 10 packets ?" msgstr "" #: ../../exercises/ex-sharing.rst:560 msgid "" "If hosts `A` and `B` send to `D` 1000 bits segments and use a sending " "window of four segments, what is the maximum throughput that they can " "achieve ?" msgstr "" #: ../../exercises/ex-sharing.rst:561 msgid "" "Assume now that `R1` is using round-robin scheduling instead of a FIFO " "buffer. One queue is used to store the packets sent by `A` and another " "for the packets sent by `B`. `A` sends one 1000 bits packet every second " "while `B` sends packets at 10 Mbps. What is the round-trip-time measured " "by each of these two hosts if each of the two queues of `R1` can store 5 " "packets ?" msgstr "" #: ../../exercises/ex-sharing.rst:564 msgid "" "When analyzing the reaction of a network using round-robin schedulers, it" " is sometimes useful to consider that the packets sent by each source are" " equivalent to a fluid and that each scheduler acts as a tap. Using this " "analogy, consider the network below. In this network, all the links are " "100 Mbps and host `B` is sending packets at 100 Mbps. If A sends at 1, 5," " 10, 20, 30, 40, 50, 60, 80 and 100 Mbps, what is the throughput that " "destination `D` will receive from `A`. Use this data to plot a graph that" " shows the portion of the traffic sent by host `A` which is received by " "host `D`." msgstr "" #: ../../exercises/ex-sharing.rst:582 msgid "Compute the max-min fair bandwidth allocation in the network below." msgstr "" #: ../../exercises/ex-sharing.rst:588 msgid "Simple network topology" msgstr "" #: ../../exercises/ex-sharing.rst:591 msgid "Consider the simple network depicted in the figure below." msgstr "" #: ../../exercises/ex-sharing.rst:609 msgid "" "In this network, a 250 Kbps link is used between the routers. The " "propagation delays in the network are negligible. Host `A` sends 1000 " "bits long segments so that it takes one msec to transmit one segment on " "the `A-R1` link. Neglecting the transmission delays for the " "acknowledgments, what is the minimum round-trip time measured on host `A`" " with such segments ?" msgstr "" #: ../../exercises/ex-sharing.rst:610 msgid "" "If host `A` uses a window of two segments and needs to transmit five " "segments of data. How long does the entire transfer lasts ?" msgstr "" #: ../../exercises/ex-sharing.rst:611 msgid "" "Same question as above, but now host `A` uses the simple DECBIT " "congestion control mechanism and a maximum window size of four segments." msgstr "" #: ../../exercises/ex-sharing.rst:630 msgid "" "Hosts `A` and `B` use the simple congestion control scheme described in " "the book and router `R1` uses the DECBIT mechanism to mark packets as " "soon as its buffers contain one packet. Hosts `A` and `B` need to send " "five segments and start exactly at the same time. How long does each " "hosts needs to wait to receive the acknowledgment for its fifth segment ?" msgstr "" #: ../../exercises/ex-sharing.rst:633 msgid "Discussion questions" msgstr "" #: ../../exercises/ex-sharing.rst:639 msgid "" "In a deployed CSMA/CD network, would it be possible to increase or " "decrease the duration of the slotTime ? Justify your answer" msgstr "" #: ../../exercises/ex-sharing.rst:641 msgid "" "Consider a CSMA/CD network that contains hosts that generate frames at a " "regular rate. When the transmission rate increases, the amount of " "collisions increases. For a given network load, measured in bits/sec, " "would the number of collisions be smaller, equal or larger with short " "frames than with long frames ?" msgstr "" #: ../../exercises/ex-sharing.rst:643 msgid "" "Slotted ALOHA improves the performance of ALOHA by dividing the time in " "slots. However, this basic idea raises two interested questions. First " "how would you enforce the duration of these slots ? Second, should a slot" " include the time to transmit a data frame or the time to transmit a data" " frame and the corresponding acknowledgment ?" msgstr "" #: ../../exercises/ex-sharing.rst:645 msgid "" "Like ALOHA, CSMA relies on acknowledgments to detect where a frame has " "been correctly received. When a host senses an idle channel, if should " "transmit its frame immediately. How should it react if it detects that " "another host is already transmitting ? Consider two options :" msgstr "" #: ../../exercises/ex-sharing.rst:647 msgid "" "the host continues to listen until the communication channel becomes " "free. It transmits as soon as the communication channel becomes free." msgstr "" #: ../../exercises/ex-sharing.rst:648 msgid "" "the host stops to listen and waits for a random time before sensing again" " the communication channel to check whether it is free." msgstr ""