# 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/reliability.rst:7 msgid "Reliable transfer" msgstr "" #: ../../exercises/reliability.rst:11 msgid "Open questions" msgstr "" #: ../../exercises/reliability.rst:13 msgid "" "Consider a `b` bits per second link between two hosts that has a " "propagation delay of `t` seconds. Derive a formula that computes the time" " elapsed between the transmission of the first bit of a `d` bytes segment" " from a sending host and the reception of the last bit of this segment on" " the receiving host." msgstr "" #: ../../exercises/reliability.rst:16 msgid "" "Transmission links have sometimes different upstream and downstream " "bandwidths. A typical example are access networks that use ADSL " "(Asymmetric Digital Subscriber Lines). Consider two hosts connected via " "an ADSL link having an upstream bandwidth of 1 Mbps and a downstream " "bandwidth of 50 Mbps. The propagation delay between the two hosts is 10 " "milliseconds. What is the maximum throughput, expressed in frames/second," " that the alternating bit protocol can obtain on this link if each data " "frame has a length of 125 bytes and acknowledgments are 25 bytes long. " "Same question if the protocol is modified to support 1500 bytes long data" " frames." msgstr "" #: ../../exercises/reliability.rst:19 msgid "" "How would you set the duration of the retransmission timer in the " "alternating bit protocol ?" msgstr "" #: ../../exercises/reliability.rst:22 msgid "" "A version of the Alternating Bit Protocol supporting variable length " "frames uses a header that contains the following fields :" msgstr "" #: ../../exercises/reliability.rst:24 msgid "a `number` (0 or 1)" msgstr "" #: ../../exercises/reliability.rst:25 msgid "a `length` field that indicates the length of the data" msgstr "" #: ../../exercises/reliability.rst:26 msgid "a Cyclic Redundancy Check (`CRC`)" msgstr "" #: ../../exercises/reliability.rst:28 msgid "" "To speedup the transmission of the frames, a student proposes to compute " "the CRC over the data part of the segment but not over the header. What " "do you think of this proposed solution ?" msgstr "" #: ../../exercises/reliability.rst:30 msgid "" "Derive a mathematical expression that provides the `goodput`, i.e. the " "amount of payload bytes that have been transmitted during a period of " "time, achieved by the Alternating Bit Protocol assuming that :" msgstr "" #: ../../exercises/reliability.rst:32 msgid "Each frame contains `D` bytes of data and `c` bytes of control information" msgstr "" #: ../../exercises/reliability.rst:33 msgid "Each acknowledgment contains `c` bytes of control information" msgstr "" #: ../../exercises/reliability.rst:34 msgid "" "The bandwidth of the two directions of the link is set to `B` bits per " "second" msgstr "" #: ../../exercises/reliability.rst:35 msgid "The delay between the two hosts is `s` seconds in both directions" msgstr "" #: ../../exercises/reliability.rst:36 msgid "there are no transmission errors" msgstr "" #: ../../exercises/reliability.rst:38 msgid "" "Consider a go-back-n sender and a go-back receiver that are directly " "connected with a 10 Mbps link that has a propagation delay of 100 " "milliseconds. Assume that the retransmission timer is set to three " "seconds. If the window has a length of 4 segments, draw a time-sequence " "diagram showing the transmission of 10 segments (each segment contains " "10000 bits):" msgstr "" #: ../../exercises/reliability.rst:40 msgid "when there are no losses" msgstr "" #: ../../exercises/reliability.rst:41 msgid "when the third and seventh segments are lost" msgstr "" #: ../../exercises/reliability.rst:42 msgid "when every second acknowledgment is discarded due to transmission errors" msgstr "" #: ../../exercises/reliability.rst:44 msgid "" "Same question when using selective repeat instead of go-back-n. Note that" " the answer is not necessarily the same." msgstr "" #: ../../exercises/reliability.rst:50 msgid "Practice" msgstr "" #: ../../exercises/reliability.rst:52 msgid "" "Reliable protocols depend on error detection algorithms to detect " "transmission errors. The following questions will reinforce your " "understanding of these algorithms." msgstr "" #: ../../exercises/reliability.rst:54 msgid "" "Reliable protocols rely on different types of checksums to verify whether" " frames have been affected by transmission errors. The most frequently " "used checksums are :" msgstr "" #: ../../exercises/reliability.rst:56 msgid "" "the Internet checksum used by UDP, TCP and other Internet protocols which" " is defined in :rfc:`1071` and implemented in various libraries." msgstr "" #: ../../exercises/reliability.rst:57 msgid "" "the 16 bits or the 32 bits Cyclical Redundancy Checks (CRC) that are " "often used on disks, in zip archives and in datalink layer protocols. See" " http://rosettacode.org/wiki/CRC-32 for CRC-32 implementations in various" " languages." msgstr "" #: ../../exercises/reliability.rst:58 #, python-format msgid "" "the Fletcher checksum [Fletcher1982]_, see " "https://en.wikipedia.org/wiki/Fletcher%27s_checksum for implementation " "details" msgstr "" #: ../../exercises/reliability.rst:60 msgid "" "By using your knowledge of the Internet checksum, can you find a " "transmission error that will not be detected by this checksum ?" msgstr "" #: ../../exercises/reliability.rst:62 msgid "" "The Cyclic Redundancy Checks (CRCs) are efficient error detection codes " "that are able to detect :" msgstr "" #: ../../exercises/reliability.rst:64 msgid "all errors that affect an odd number of bits" msgstr "" #: ../../exercises/reliability.rst:65 msgid "" "all errors that affect a sequence of bits which is shorter than the " "length of the CRC" msgstr "" #: ../../exercises/reliability.rst:67 msgid "" "Implement a small software that computes the CRC-32 for a text file. " "Then, modify the contents of the file to change an even number of bits or" " an odd number of bits inside the file. When modifying the file, remember" " that an ASCII file is composed of 8 bits characters that are encoded by " "using the ASCII table that you can find at : " "http://en.wikipedia.org/wiki/ASCII . You can also write a small program " "that produces binary files that are a small variation of each other." msgstr "" #: ../../exercises/reliability.rst:69 msgid "" "Checksums and CRCs should not be confused with secure hash functions such" " as MD5 defined in :rfc:`1321` or SHA-1 described in :rfc:`4634`. Secure " "hash functions are used to ensure that files or sometimes " "packets/segments have not been modified. Secure hash functions aim at " "detecting malicious changes while checksums and CRCs only detect random " "transmission errors. Use the `shasum " "`_ or `md5sum " "`_ programs on Linux to perform the " "same tests as above." msgstr "" #: ../../exercises/reliability.rst:74 msgid "Discussion questions" msgstr "" #: ../../exercises/reliability.rst:77 msgid "" "Consider two high-end servers connected back-to-back by using a 10 Gbps " "interface. If the delay between the two servers is one millisecond, what " "is the throughput that can be achieved by a reliable protocol that is " "using 10,000 bits frames and a window of" msgstr "" #: ../../exercises/reliability.rst:79 msgid "one frame" msgstr "" #: ../../exercises/reliability.rst:80 msgid "ten frames" msgstr "" #: ../../exercises/reliability.rst:81 msgid "hundred frames" msgstr "" #: ../../exercises/reliability.rst:83 msgid "" "Is it possible for a go-back-n receiver to inter-operate with a " "selective-repeat sender ? Justify your answer." msgstr "" #: ../../exercises/reliability.rst:85 msgid "" "Is it possible for a selective-repeat receiver to inter-operate with a " "go-back-n sender ? Justify your answer." msgstr "" #: ../../exercises/reliability.rst:89 msgid "" "A go-back-n receiver has sent :math:`2^n` data segments. All the segments" " have been received correctly and in-order by the receiver, but all the " "returned acknowledgments have been lost. Show by using a time sequence " "diagram (e.g. by considering a window of four segments) what happens in " "this case. Can you fix the problem on the go-back-n sender ?" msgstr "" #: ../../exercises/reliability.rst:91 msgid "" "Same question as above, but assume now that both the sender and the " "receiver implement selective repeat. Note that the answer can be " "different from the above question." msgstr ""