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

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
34 ../../principles/referencemodels.rst:225
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — the Internet layer which is equivalent to the network layer of our reference model 82/10000 · 82
35 ../../principles/referencemodels.rst:226
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — the Link layer which combines the functions of the physical and datalink layers of our five-layer reference model 113/10000 · 113
36 ../../principles/referencemodels.rst:228
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 142/10000 · 142
37 ../../principles/referencemodels.rst:233
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — The OSI reference model 23/10000 · 23
38 ../../principles/referencemodels.rst:235
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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 : 302/10000 · 302
39 ../../principles/referencemodels.rst:237
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 721/10000 · 721
40 ../../principles/referencemodels.rst:238
	English ↹ ↵ NBS … “ ” ‘ ’ ﹙-﹚ ‐ – — 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. 762/10000 · 762