cnp3-ebook/principles/transport

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86 ../../principles/transport.rst:547

	English In the previous sections, we have described services that are provided by the transport layer. However, it is important to note that the notion of service is more general than in the transport layer. As explained earlier, the network layer also provides a service, which in most networks is an unreliable connectionless service. There are network layers that provide a connection-oriented service. Similarly, the datalink layer also provides services. Some datalink layers will provide a connectionless service. This will be the case in Local Area Networks for examples. Other datalink layers, e.g. in public networks, provide a connection oriented service. 657/10000 · 657
87 ../../principles/transport.rst:554

	English The transport layer entity interacts with both a user in the application layer and the network layer. It improves the network layer service to make it usable by applications. From the application's viewpoint, the main limitations of the network layer service come from its unreliable service: 292/10000 · 292
88 ../../principles/transport.rst:556

	English the network layer may corrupt data; 35/10000 · 35
89 ../../principles/transport.rst:557

	English the network layer may loose data; 33/10000 · 33
90 ../../principles/transport.rst:558

	English the network layer may not deliver data in-order; 48/10000 · 48
91 ../../principles/transport.rst:559

	English the network layer has an upper bound on maximum length of the data; 67/10000 · 67
92 ../../principles/transport.rst:560

	English the network layer may duplicate data. 37/10000 · 37
93 ../../principles/transport.rst:562

	English To deal with these issues, the transport layer includes several mechanisms that depend on the service that it provides. It interacts with both the applications and the underlying network layer. 193/10000 · 193
94 ../../principles/transport.rst:568

	English Interactions between the transport layer, its user, and its network layer provider 82/10000 · 82
95 ../../principles/transport.rst:570

	English We have already described in the datalink layers mechanisms to deal with data losses and transmission errors. These techniques are also used in the transport layer. 164/10000 · 164
96 ../../principles/transport.rst:574

	English Connectionless transport 24/10000 · 24
97 ../../principles/transport.rst:576

	English The simplest service that can be provided in the transport layer is the connectionless transport service. Compared to the connectionless network layer service, this transport service includes two additional features : 217/10000 · 217
98 ../../principles/transport.rst:578

	English an `error detection` mechanism that allows detecting corrupted data 67/10000 · 67
99 ../../principles/transport.rst:579

	English a `multiplexing technique` that enables several applications running on one host to exchange information with another host 122/10000 · 122
100 ../../principles/transport.rst:583

	English To exchange data, the transport protocol encapsulates the SDU produced by its user inside a `segment`. The `segment` is the unit of transfer of information in the transport layer. Transport layer entities always exchange segments. When a transport layer entity creates a segment, this segment is encapsulated by the network layer into a packet which contains the segment as its payload and a network header. The packet is then encapsulated in a frame to be transmitted in the datalink layer. 491/10000 · 491
101 ../../principles/transport.rst:611

	English A `segment` also contains control information, usually stored inside a `header` and the payload that comes from the application. To detect transmission errors, transport protocols rely on checksums or CRCs like the datalink layer protocols. 240/10000 · 240
102 ../../principles/transport.rst:613

	English Compared to the connectionless network layer service, the transport layer service allows several applications running on a host to exchange SDUs with several other applications running on remote hosts. Let us consider two hosts, e.g. a client and a server. The network layer service allows the client to send information to the server, but if an application running on the client wants to contact a particular application running on the server, then an additional addressing mechanism is required. The network layer address identifies a host, but it is not sufficient to differentiate the applications running on a host. `Port numbers` provides this additional addressing. When a server application is launched on a host, it registers a `port number`. This `port number` will be used by the clients to contact the server process. 829/10000 · 829
103 ../../principles/transport.rst:615

	English The figure below shows a typical usage of port numbers. The client process uses port number `1234` while the server process uses port number `5678`. When the client sends a request, it is identified as originating from port number `1234` on the client host and destined to port number `5678` on the server host. When the server process replies to this request, the server's transport layer returns the reply as originating from port `5678` on the server host and destined to port `1234` on the client host. 507/10000 · 507
104 ../../principles/transport.rst:634

	English To support the connection-oriented service, the transport layer needs to include several mechanisms to enrich the connectionless network-layer service. We discuss these mechanisms in the following sections. 206/10000 · 206
105 ../../principles/transport.rst:638

	English Connection establishment 24/10000 · 24

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