msgid "" msgstr "" "Project-Id-Version: English (cnp3-ebook)\n" "Report-Msgid-Bugs-To: \n" "POT-Creation-Date: 2026-04-18 21:16+0200\n" "PO-Revision-Date: YEAR-MO-DA HO:MI+ZONE\n" "Last-Translator: FULL NAME \n" "Language-Team: English \n" "Language: en\n" "MIME-Version: 1.0\n" "Content-Type: text/plain; charset=UTF-8\n" "Content-Transfer-Encoding: 8bit\n" "Plural-Forms: nplurals=2; plural=n != 1;\n" "X-Generator: Weblate 5.14.3\n" #: ../../principles/referencemodels.rst:8 #, read-only 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 "" "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." #: ../../principles/referencemodels.rst:12 #, read-only msgid "The reference models" msgstr "The reference models" #: ../../principles/referencemodels.rst:14 #, read-only 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 "" "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]_." #: ../../principles/referencemodels.rst:19 #, read-only msgid "The five layers reference model" msgstr "The five layers reference model" #: ../../principles/referencemodels.rst:21 #, read-only msgid "" "Our reference model is divided into five layers, as shown in the figure " "below." msgstr "" "Our reference model is divided into five layers, as shown in the figure " "below." #: ../../principles/referencemodels.rst:42 #, read-only msgid "The Physical layer" msgstr "The Physical layer" #: ../../principles/referencemodels.rst:46 #, read-only 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 "" "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." #: ../../principles/referencemodels.rst:48 #, read-only msgid "" "An important point to note about the Physical layer is the service that it " "provides. This service is usually an unreliable 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 "" "An important point to note about the Physical layer is the service that it " "provides. This service is usually an unreliable 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 :" #: ../../principles/referencemodels.rst:50 #, read-only msgid "" "the Physical layer may change, e.g. due to electromagnetic interference, the " "value of a bit being transmitted" msgstr "" "the Physical layer may change, e.g. due to electromagnetic interference, the " "value of a bit being transmitted" #: ../../principles/referencemodels.rst:51 #, read-only msgid "" "the Physical layer may deliver `more` bits to the receiver than the bits " "sent by the sender" msgstr "" "the Physical layer may deliver `more` bits to the receiver than the bits " "sent by the sender" #: ../../principles/referencemodels.rst:52 #, read-only msgid "" "the Physical layer may deliver `fewer` bits to the receiver than the bits " "sent by the sender" msgstr "" "the Physical layer may deliver `fewer` bits to the receiver than the bits " "sent by the sender" #: ../../principles/referencemodels.rst:72 #, read-only msgid "The Datalink layer" msgstr "The Datalink layer" #: ../../principles/referencemodels.rst:74 #, read-only 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 "" "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." #: ../../principles/referencemodels.rst:76 #, read-only 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 "" "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." #: ../../principles/referencemodels.rst:96 #, read-only msgid "The Network layer" msgstr "The Network layer" #: ../../principles/referencemodels.rst:100 #, read-only 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 "" "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." #: ../../principles/referencemodels.rst:131 #, read-only msgid "The Transport layer" msgstr "The Transport layer" #: ../../principles/referencemodels.rst:133 #, read-only msgid "" "The network layer enables hosts to reach each others. However, different " "communication flows can take place between the same hosts. These " "communication flows might have different needs " "(some require reliable delivery, other not) and need to be distinguished. " "Ensuring an identification of a communication flow between two given hosts " "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 "" "The network layer enables hosts to reach each others. However, different " "communication flows can take place between the same hosts. These " "communication flows might have different needs " "(some require reliable delivery, other not) and need to be distinguished. " "Ensuring an identification of a communication flow between two given hosts " "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." #: ../../principles/referencemodels.rst:135 #, read-only 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 "" "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." #: ../../principles/referencemodels.rst:164 #, read-only msgid "The Application layer" msgstr "The Application layer" #: ../../principles/referencemodels.rst:168 #, read-only 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 "" "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." #: ../../principles/referencemodels.rst:201 #, read-only 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 "" "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. :" #: ../../principles/referencemodels.rst:203 #, read-only msgid "physical layer entities exchange bits" msgstr "physical layer entities exchange bits" #: ../../principles/referencemodels.rst:204 #, read-only msgid "datalink layer entities exchange *frames*" msgstr "datalink layer entities exchange *frames*" #: ../../principles/referencemodels.rst:205 #, read-only msgid "network layer entities exchange *packets*" msgstr "network layer entities exchange *packets*" #: ../../principles/referencemodels.rst:206 #, read-only msgid "transport layer entities exchange *segments*" msgstr "transport layer entities exchange *segments*" #: ../../principles/referencemodels.rst:207 #, read-only msgid "application layer entities exchange *SDUs*" msgstr "application layer entities exchange *SDUs*" #: ../../principles/referencemodels.rst:212 #, read-only msgid "Reference models" msgstr "Reference models" #: ../../principles/referencemodels.rst:215 #, read-only 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 "" "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." #: ../../principles/referencemodels.rst:219 #, read-only msgid "The TCP/IP reference model" msgstr "The TCP/IP reference model" #: ../../principles/referencemodels.rst:221 #, read-only 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 "" "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 :" #: ../../principles/referencemodels.rst:223 #, read-only msgid "the Application layer" msgstr "the Application layer" #: ../../principles/referencemodels.rst:224 #, read-only msgid "the Transport layer" msgstr "the Transport layer" #: ../../principles/referencemodels.rst:225 #, read-only msgid "" "the Internet layer which is equivalent to the network layer of our reference " "model" msgstr "" "the Internet layer which is equivalent to the network layer of our reference " "model" #: ../../principles/referencemodels.rst:226 #, read-only msgid "" "the Link layer which combines the functions of the physical and datalink " "layers of our five-layer reference model" msgstr "" "the Link layer which combines the functions of the physical and datalink " "layers of our five-layer reference model" #: ../../principles/referencemodels.rst:228 #, read-only 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 "" "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." #: ../../principles/referencemodels.rst:233 #, read-only msgid "The OSI reference model" msgstr "The OSI reference model" #: ../../principles/referencemodels.rst:235 #, read-only 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 "" "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 " ":" #: ../../principles/referencemodels.rst:237 #, read-only 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 establishing a session-" "connection, to support in-order data exchange (including mechanisms that " "allow recovering from the abrupt release of an underlying transport " "connection), and to release the connection in an orderly manner." msgstr "" "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 establishing a session-" "connection, to support in-order data exchange (including mechanisms that " "allow recovering from the abrupt release of an underlying transport " "connection), and to release the connection in an orderly manner." #: ../../principles/referencemodels.rst:238 #, read-only 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 " "provides 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 "" "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 " "provides 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." #: ../../principles/referencemodels.rst:239 #, read-only 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 "" "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." #: ../../principles/referencemodels.rst:275 #, read-only msgid "Footnotes" msgstr "Footnotes" #: ../../principles/referencemodels.rst:276 #, read-only msgid "" "An interesting historical discussion of the OSI-TCP/IP debate may be found " "in [Russel06]_" msgstr "" "An interesting historical discussion of the OSI-TCP/IP debate may be found " "in [Russel06]_" #: ../../principles/referencemodels.rst:278 #, read-only 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 "" "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."