2. cnp3-ebook
3. principles/referencemodels
4. English
5. Zen

14 ../../principles/referencemodels.rst:76
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 812/10000 · 812
15 ../../principles/referencemodels.rst:96
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The Network layer 17/10000 · 17
16 ../../principles/referencemodels.rst:100
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 637/10000 · 637
17 ../../principles/referencemodels.rst:131
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The Transport layer 19/10000 · 19
18 ../../principles/referencemodels.rst:133
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 639/10000 · 639
19 ../../principles/referencemodels.rst:135
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 272/10000 · 272
20 ../../principles/referencemodels.rst:164
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The Application layer 21/10000 · 21
21 ../../principles/referencemodels.rst:168
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 325/10000 · 325
22 ../../principles/referencemodels.rst:201
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. : 215/10000 · 215
23 ../../principles/referencemodels.rst:203
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — physical layer entities exchange bits 37/10000 · 37
24 ../../principles/referencemodels.rst:204
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — datalink layer entities exchange *frames* 41/10000 · 41
25 ../../principles/referencemodels.rst:205
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — network layer entities exchange *packets* 41/10000 · 41
26 ../../principles/referencemodels.rst:206
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — transport layer entities exchange *segments* 44/10000 · 44
27 ../../principles/referencemodels.rst:207
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — application layer entities exchange *SDUs* 42/10000 · 42
28 ../../principles/referencemodels.rst:212
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — Reference models 16/10000 · 16
29 ../../principles/referencemodels.rst:215
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 168/10000 · 168
30 ../../principles/referencemodels.rst:219
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The TCP/IP reference model 26/10000 · 26
31 ../../principles/referencemodels.rst:221
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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 : 352/10000 · 352
32 ../../principles/referencemodels.rst:223
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — the Application layer 21/10000 · 21
33 ../../principles/referencemodels.rst:224
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — the Transport layer 19/10000 · 19