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	English
	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.
	transport layer entities exchange *segments*
	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.
	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.
	The Transport layer
	the Transport layer
	The TCP/IP reference model
	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.
	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.
	The reference models
	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.
	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.
	the Physical layer may deliver `more` bits to the receiver than the bits sent by the sender
	the Physical layer may deliver `fewer` bits to the receiver than the bits sent by the sender
	the Physical layer may change, e.g. due to electromagnetic interference, the value of a bit being transmitted
	The Physical layer
	The OSI reference model
	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.
	The Network layer
	the Link layer which combines the functions of the physical and datalink layers of our five-layer reference model