# 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" #: ../../principles/referencemodels.rst:8 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 "" #: ../../principles/referencemodels.rst:12 msgid "The reference models" msgstr "" #: ../../principles/referencemodels.rst:14 msgid "" "Given the growing complexity of computer networks, during the 1970s " "network researchers proposed various reference models to facilitate the " "description of network protocols and services. Of these, the Open Systems" " Interconnection (OSI) model [Zimmermann80]_ was probably the most " "influential. It served as the basis for the standardization work " "performed within the :term:`ISO` to develop global computer network " "standards. The reference model that we use in this book can be considered" " as a simplified version of the OSI reference model [#fiso-tcp]_." msgstr "" #: ../../principles/referencemodels.rst:19 msgid "The five layers reference model" msgstr "" #: ../../principles/referencemodels.rst:21 msgid "" "Our reference model is divided into five layers, as shown in the figure " "below." msgstr "" #: ../../principles/referencemodels.rst:27 msgid "The five layers of the reference model" msgstr "" #: ../../principles/referencemodels.rst:32 #: ../../principles/referencemodels.rst:49 msgid "The Physical layer" msgstr "" #: ../../principles/referencemodels.rst:34 msgid "" "Starting from the bottom, the first layer is the Physical layer. Two " "communicating devices are linked through a physical medium. This physical" " medium is used to transfer an electrical or optical signal between two " "directly connected devices." msgstr "" #: ../../principles/referencemodels.rst:36 msgid "" "An important point to note about the Physical layer is the service that " "it provides. This service is usually an unreliable connection-oriented " "service that allows the users of the Physical layer to exchange bits. The" " unit of information transfer in the Physical layer is the bit. The " "Physical layer service is unreliable because :" msgstr "" #: ../../principles/referencemodels.rst:38 msgid "" "the Physical layer may change, e.g. due to electromagnetic interference, " "the value of a bit being transmitted" msgstr "" #: ../../principles/referencemodels.rst:39 msgid "" "the Physical layer may deliver `more` bits to the receiver than the bits " "sent by the sender" msgstr "" #: ../../principles/referencemodels.rst:40 msgid "" "the Physical layer may deliver `fewer` bits to the receiver than the bits" " sent by the sender" msgstr "" #: ../../principles/referencemodels.rst:54 #: ../../principles/referencemodels.rst:65 msgid "The Datalink layer" msgstr "" #: ../../principles/referencemodels.rst:56 msgid "" "The `Datalink layer` builds on the service provided by the underlying " "physical layer. The `Datalink layer` allows two hosts that are directly " "connected through the physical layer to exchange information. The unit of" " information exchanged between two entities in the `Datalink layer` is a " "frame. A frame is a finite sequence of bits. Some `Datalink layers` use " "variable-length frames while others only use fixed-length frames. Some " "`Datalink layers` provide a connection-oriented service while others " "provide a connectionless service. Some `Datalink layers` provide reliable" " delivery while others do not guarantee the correct delivery of the " "information." msgstr "" #: ../../principles/referencemodels.rst:58 msgid "" "An important point to note about the `Datalink layer` is that although " "the figure below indicates that two entities of the `Datalink layer` " "exchange frames directly, in reality this is slightly different. When the" " `Datalink layer` entity on the left needs to transmit a frame, it issues" " as many `Data.request` primitives to the underlying `physical layer` as " "there are bits in the frame. The physical layer will then convert the " "sequence of bits in an electromagnetic or optical signal that will be " "sent over the physical medium. The `physical layer` on the right hand " "side of the figure will decode the received signal, recover the bits and " "issue the corresponding `Data.indication` primitives to its `Datalink " "layer` entity. If there are no transmission errors, this entity will " "receive the frame sent earlier." msgstr "" #: ../../principles/referencemodels.rst:69 msgid "The Network layer" msgstr "" #: ../../principles/referencemodels.rst:73 msgid "" "The `Datalink layer` allows directly connected hosts to exchange " "information, but it is often necessary to exchange information between " "hosts that are not attached to the same physical medium. This is the task" " of the `network layer`. The `network layer` is built above the `datalink" " layer`. Network layer entities exchange `packets`. A `packet` is a " "finite sequence of bytes that is transported by the datalink layer inside" " one or more frames. A packet usually contains information about its " "origin and its destination, and usually passes through several " "intermediate devices called routers on its way from its origin to its " "destination." msgstr "" #: ../../principles/referencemodels.rst:80 msgid "The network layer" msgstr "" #: ../../principles/referencemodels.rst:85 msgid "The Transport layer" msgstr "" #: ../../principles/referencemodels.rst:87 msgid "" "Most realizations of the network layer, including the internet, do not " "provide a reliable service. However, many applications need to exchange " "information reliably and so using the network layer service directly " "would be very difficult for them. Ensuring the reliable delivery of the " "data produced by applications is the task of the `transport layer`. " "`Transport layer` entities exchange `segments`. A segment is a finite " "sequence of bytes that are transported inside one or more packets. A " "transport layer entity issues segments (or sometimes part of segments) as" " `Data.request` to the underlying network layer entity." msgstr "" #: ../../principles/referencemodels.rst:89 msgid "" "There are different types of transport layers. The most widely used " "transport layers on the Internet are :term:`TCP` ,that provides a " "reliable connection-oriented bytestream transport service, and " ":term:`UDP` ,that provides an unreliable connection-less transport " "service." msgstr "" #: ../../principles/referencemodels.rst:96 msgid "The transport layer" msgstr "" #: ../../principles/referencemodels.rst:99 #: ../../principles/referencemodels.rst:109 msgid "The Application layer" msgstr "" #: ../../principles/referencemodels.rst:103 msgid "" "The upper layer of our architecture is the `Application layer`. This " "layer includes all the mechanisms and data structures that are necessary " "for the applications. We will use Application Data Unit (ADU) or the " "generic Service Data Unit (SDU) term to indicate the data exchanged " "between two entities of the Application layer." msgstr "" #: ../../principles/referencemodels.rst:114 msgid "" "In the remaining chapters of this text, we will often refer to the " "information exchanged between entities located in different layers. To " "avoid any confusion, we will stick to the terminology defined earlier, " "i.e. :" msgstr "" #: ../../principles/referencemodels.rst:116 msgid "physical layer entities exchange bits" msgstr "" #: ../../principles/referencemodels.rst:117 msgid "datalink layer entities exchange *frames*" msgstr "" #: ../../principles/referencemodels.rst:118 msgid "network layer entities exchange *packets*" msgstr "" #: ../../principles/referencemodels.rst:119 msgid "transport layer entities exchange *segments*" msgstr "" #: ../../principles/referencemodels.rst:120 msgid "application layer entities exchange *SDUs*" msgstr "" #: ../../principles/referencemodels.rst:125 msgid "Reference models" msgstr "" #: ../../principles/referencemodels.rst:128 msgid "" "Two reference models have been successful in the networking community : " "the OSI reference model and the TCP/IP reference model. We discuss them " "briefly in this section." msgstr "" #: ../../principles/referencemodels.rst:132 msgid "The TCP/IP reference model" msgstr "" #: ../../principles/referencemodels.rst:134 msgid "" "In contrast with OSI, the TCP/IP community did not spend a lot of effort " "defining a detailed reference model; in fact, the goals of the Internet " "architecture were only documented after TCP/IP had been deployed " "[Clark88]_. :rfc:`1122` , which defines the requirements for Internet " "hosts, mentions four different layers. Starting from the top, these are :" msgstr "" #: ../../principles/referencemodels.rst:136 msgid "the Application layer" msgstr "" #: ../../principles/referencemodels.rst:137 msgid "the Transport layer" msgstr "" #: ../../principles/referencemodels.rst:138 msgid "" "the Internet layer which is equivalent to the network layer of our " "reference model" msgstr "" #: ../../principles/referencemodels.rst:139 msgid "" "the Link layer which combines the functions of the physical and datalink " "layers of our five-layer reference model" msgstr "" #: ../../principles/referencemodels.rst:141 msgid "" "Besides this difference in the lower layers, the TCP/IP reference model " "is very close to the five layers that we use throughout this document." msgstr "" #: ../../principles/referencemodels.rst:146 msgid "The OSI reference model" msgstr "" #: ../../principles/referencemodels.rst:148 msgid "" "Compared to the five layers reference model explained above, the " ":term:`OSI` reference model defined in [X200]_ is divided in seven " "layers. The four lower layers are similar to the four lower layers " "described above. The OSI reference model refined the application layer by" " dividing it in three layers :" msgstr "" #: ../../principles/referencemodels.rst:150 msgid "" "the `Session layer`. The Session layer contains the protocols and " "mechanisms that are necessary to organize and to synchronize the dialogue" " and to manage the data exchange of presentation layer entities. While " "one of the main functions of the transport layer is to cope with the " "unreliability of the network layer, the session's layer objective is to " "hide the possible failures of transport-level connections to the upper " "layer higher. For this, the Session Layer provides services that allow to" " establish a session-connection, to support orderly data exchange " "(including mechanisms that allow to recover from the abrupt release of an" " underlying transport connection), and to release the connection in an " "orderly manner." msgstr "" #: ../../principles/referencemodels.rst:151 msgid "" "the `Presentation layer` was designed to cope with the different ways of " "representing information on computers. There are many differences in the " "way computer store information. Some computers store integers as 32 bits " "field, others use 64 bits field and the same problem arises with floating" " point number. For textual information, this is even more complex with " "the many different character codes that have been used [#funicode]_. The " "situation is even more complex when considering the exchange of " "structured information such as database records. To solve this problem, " "the Presentation layer contains provides for a common representation of " "the data transferred. The :term:`ASN.1` notation was designed for the " "Presentation layer and is still used today by some protocols." msgstr "" #: ../../principles/referencemodels.rst:152 msgid "" "the `Application layer` that contains the mechanisms that do not fit in " "neither the Presentation nor the Session layer. The OSI Application layer" " was itself further divided in several generic service elements." msgstr "" #: ../../principles/referencemodels.rst:158 msgid "The seven layers of the OSI reference model" msgstr "" #: ../../principles/referencemodels.rst:162 msgid "Footnotes" msgstr "" #: ../../principles/referencemodels.rst:163 msgid "" "There is now a rough consensus for the greater use of the Unicode_ " "character format. Unicode can represent more than 100,000 different " "characters from the known written languages on Earth. Maybe one day, all " "computers will only use Unicode to represent all their stored characters " "and Unicode could become the standard format to exchange characters, but " "we are not yet at this stage today." msgstr "" #: ../../principles/referencemodels.rst:165 msgid "" "An interesting historical discussion of the OSI-TCP/IP debate may be " "found in [Russel06]_" msgstr ""